Energy Densification of Lignocellulosic Biomass via Hydrothermal
Carbonization and Torrefaction
by
Harpreet Singh Kambo
A Thesis
presented to
The University of Guelph
In partial fulfilment of requirements
for the degree of
Masters of Applied Science
in Environmental Engineering
Guelph, Ontario, Canada
© Harpreet S. Kambo, August, 2014
ABSTRACT
Energy Densification of Lignocellulosic Biomass via Hydrothermal Carbonization and
Torrefaction
Harpreet Singh Kambo
Advisor
University of Guelph, 2014
Dr. Animesh Dutta
The work presented in this study demonstrated the potential of hydrothermal
carbonization (HTC) of biomass for the production of carbon-rich solid fuel, known as hydrochar
that has significantly improved combustion characteristics. In comparison, the physicochemical
properties of the solid produced via torrefaction (a conventional thermal pre-treatment) were
considerably lower than the hydrochar samples, even if the reaction time was kept much higher
than HTC. Both raw and pre-treated biomass samples were further examined for densification
characterization. The HTC pellets showed significantly improved durability, mass and energy
density, and hydrophobicity compared to raw and torrefied pellets. The result shows that HTC
narrows the differences in fuel qualities and has potential to replace coal in existing coal-fired
power plants without any significant modifications. The HTC process water contains highquality intermediate compounds that can offer a broad range of benefits and can improve the
system’s overall efficiency via recirculation.
DEDICATION
This thesis is dedicated to my:
Parents: Jugraj Singh and Balwinder Bedi
and
Grandparents: Piara Singh Bedi and Mohinder Kaur
for their endless love, support and encouragement
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ACKNOWLEDGEMENTS
This thesis would not have been possible without the help, support and patience of my advisor
Dr. Animesh Dutta.
I would like to thank him for this valuable opportunity and lessons
throughout my academic career. His mentorship and support allowed me to obtain prestigious
scholarship awards in the field of Environmental Engineering. I would also like to extend my
appreciation towards Dr. Fantahun Defersha, for his support throughout the duration of my
research.
These acknowledgements would not be complete without naming the following individuals for
their supports: Mr. Mahendra Thimmanagari (OMAF-MRA), Mike Speagle, Dr. Leon, Dr. Roy,
Bimal, Tushar, Jamie, Ronil. I’m also thankful to my friends: Amanpreet, Rajpreet, Chitkanwal,
Gurdeep, Aseem, and Evan for their encouragement and being there when I needed them the
most.
Finally, I would like to thank my father Mr. Jugraj Singh, mother Mrs. Balwinder Kaur,
Manpreet and Maninder (my brother and his wife) for their love, support, encouragement, and
guidance throughout my personal and academic career.
iv
TABLE OF CONTENT
CHAPTER-I INTRODUCTION .................................................................................................... 1
1.1.
Background ..................................................................................................................... 1
1.2.
Scope and Objectives ...................................................................................................... 6
CHAPTER-II LITERATURE REVIEW ........................................................................................ 8
2.1.
Overview of Biomass ...................................................................................................... 8
2.1.1.
Lignocellulosic Composition of Biomass ................................................................. 8
2.1.1.1.
Cellulose .......................................................................................................... 10
2.1.1.2.
Hemicellulose: ................................................................................................. 11
2.1.1.3.
Lignin: ............................................................................................................. 11
2.2.
Combustion Properties of Biomass ............................................................................... 12
2.3.
Origin and Definition of Biochar and Hydrochar ......................................................... 14
2.4.
Biomass conversion ...................................................................................................... 16
2.4.1.
Feedstock for Biomass conversion ......................................................................... 16
2.4.2.
Pathways for Biomass Conversion ......................................................................... 18
2.4.2.1.
Conversion of Dry Biomass ............................................................................ 18
2.4.2.2.
Conversion of Wet Biomass ............................................................................ 20
2.4.3.
Benefits of Biomass Conversion ............................................................................. 21
2.5.
Hydrothermal Carbonisation of Biomass...................................................................... 22
2.6.
Properties of Hot Compressed Water ........................................................................... 25
2.6.1.
2.7.
Role and Importance of Water in Hydrothermal Carbonisation ............................. 26
Problem Statement ........................................................................................................ 29
2.7.1.
Research Objectives ................................................................................................ 30
2.7.2.
Methodology ........................................................................................................... 32
CHAPTER-III HYDROTHERMAL CARBONISATION AND TORREFACTION OF
BIOMASS ..................................................................................................................................... 34
3.1.
Experimental Section .................................................................................................... 35
3.1.1.
Materials ................................................................................................................. 35
3.1.1.1.
3.1.2.
Feedstock Preparation...................................................................................... 35
Experimental Methods ............................................................................................ 35
3.1.2.1.
Hydrothermal Carbonisation ........................................................................... 35
v
3.1.2.2.
3.1.3.
3.2.
Torrefaction ..................................................................................................... 38
Analytical Methods for Solid Samples ................................................................... 39
3.1.3.1.
Proximate and Ultimate analysis ..................................................................... 39
3.1.3.2.
Higher Heating Value ...................................................................................... 39
3.1.3.3.
Hydrophobicity ................................................................................................ 40
3.1.3.4.
Grindability...................................................................................................... 41
3.1.3.5.
Fiber Analysis .................................................................................................. 43
3.1.3.6.
Inorganic Metals Analysis ............................................................................... 43
3.1.3.7.
Scanning Electron Microscopy (SEM) Analysis ............................................. 43
3.1.3.8.
Brunauer–Emmett–Teller (BET) Analysis ...................................................... 44
Results and Discussions ................................................................................................ 45
3.2.1.
Effect of different operating parameters on biomass .............................................. 45
3.2.1.1.
Effect on mass yield in HTC ........................................................................... 47
3.2.1.1.1. Optimization of Mass Yield in HTC .......................................................... 48
3.2.1.2.
Effect on HHV and energy yield in HTC ........................................................ 51
3.2.1.2.1. Optimization of HHV and energy yield in HTC ........................................ 54
3.2.1.3.
Comparison of mass yield in HTC and torrefaction process ........................... 59
3.2.1.4.
Comparison of HHV and energy yield in HTC and torrefaction process ....... 60
3.2.2.
Proximate and ultimate analysis of hydrochar samples .......................................... 61
3.2.3.
Effect of HTC reaction temperature on grindability of hydrochars ........................ 65
3.2.3.1.
3.2.4.
Effect of HTC operating temperature on inorganic composition of hydrochars .... 73
3.2.4.1.
3.2.5.
Comparison of inorganic impurities in HTC and torrefied samples ............... 76
Effect of HTC operating temperature on hydrophobicity of hydrochars ................ 77
3.2.5.1.
3.3.
Comparison of grindability for coal, torrefied and HTC pre-treated samples. 70
Comparison of hydrophobicity for HTC and torrefied samples ...................... 80
Conclusions ................................................................................................................... 81
CHAPTER-IV CHARACTERIZATION AND RECIRCULATION OF HTC LIQUID
BYPRODUCT AND EFFECT OF ADDITION OF SALT AND ACID IN HTC PROCESS .... 82
4.1.
Experimental Section .................................................................................................... 83
4.1.1.
Materials ................................................................................................................. 83
4.1.2.
Experimental Methods ............................................................................................ 83
vi
4.1.2.1.
Addition of Salt and Acid to HTC process ...................................................... 83
4.1.2.2.
HTC process water characterization ................................................................ 84
4.1.2.3.
Effect of process water recirculation ............................................................... 84
4.1.3.
Analytical methods for solid samples ..................................................................... 84
4.1.4.
Analytical methods for liquid samples ................................................................... 85
4.2.
4.1.4.1.
pH .................................................................................................................... 85
4.1.4.2.
Inorganic Elemental analysis ........................................................................... 85
4.1.4.3.
Total Organic Carbon (TOC) .......................................................................... 85
4.1.4.4.
Organic Acids and Furfurals............................................................................ 85
Results and discussions ................................................................................................. 86
4.2.1.
Effects of addition of salt and acid ......................................................................... 86
4.2.1.1.
Effects on mass yield of hydrochar samples ................................................... 86
4.2.1.2.
Effects on HHV of hydrochar samples ............................................................ 89
4.2.2.
Characterization of HTC process water .................................................................. 91
4.2.3.
Recirculation of HTC liquid by-product ................................................................. 95
4.3.
4.2.3.1.
Effect of recirculation on pH of process water ................................................ 96
4.2.3.2.
Effect of recirculation on mass yield ............................................................... 97
4.2.3.3.
Effect of recirculation on HHV ....................................................................... 99
Conclusions ................................................................................................................. 101
CHAPTER-V STRENGTH, STORAGE AND COMBUSTION CHARACTERISTICS OF
DENSIFIED LIGNOCELLULOSIC BIOMASS PRODUCED VIA TORREFACTION AND
HYDROTHERMAL CARBONIZATION ................................................................................. 103
5.1.
Materials and Methods ................................................................................................ 104
5.1.1.
Materials ............................................................................................................... 104
5.1.2.
Experimental Setup ............................................................................................... 104
5.1.2.1.
Hydrothermal Carbonisation ......................................................................... 104
5.1.2.2.
Torrefaction ................................................................................................... 104
5.1.2.3.
Densification .................................................................................................. 104
5.1.3.
Analytical Methods ............................................................................................... 106
5.1.3.1.
Mass and Energy Density .............................................................................. 106
5.1.3.2.
Higher Heating Value .................................................................................... 106
vii
5.1.3.3.
Compression Strength.................................................................................... 106
5.1.3.4.
Durability ....................................................................................................... 107
5.1.3.5.
Hydrophobicity .............................................................................................. 107
5.1.3.5.1. Equilibrium Moisture Content ................................................................. 107
5.1.3.5.2. Water Resistance ...................................................................................... 108
5.2.
Results and Discussions .............................................................................................. 109
5.2.1.
Mass and Energy Density ..................................................................................... 109
5.2.2.
Strength and Durability of Pellets ......................................................................... 112
5.2.3.
Hydrophobicity ..................................................................................................... 114
5.3.
Conclusions ................................................................................................................. 116
CHAPTER-VI OVERALL CONCLUSIONS AND RECOMMENDATIONS ......................... 118
6.1.
Conclusions ................................................................................................................. 118
6.1.1.
Wet and Dry Thermal Pre-treatment of Lignocellulosic Biomass........................ 118
6.1.2.
HTC Process Water Characterization and Recirculation ...................................... 119
6.1.3.
Addition of Salt and Acid in HTC Process ........................................................... 120
6.1.4.
Densification of Raw and Pre-treated Biomass .................................................... 120
6.2.
Recommendations for Future Research ...................................................................... 121
6.2.1.
Process Optimization ............................................................................................ 121
6.2.2.
Limitations of the HTC Process ............................................................................ 121
6.2.3.
Integration of HTC and Densification Process ..................................................... 122
6.2.4.
Versatility of Feedstock ........................................................................................ 122
6.2.5.
Products Recovery and Integration of HTC with Anaerobic Digestion ............... 123
6.2.6.
Surface and Reaction Chemistry of Hydrochar .................................................... 124
6.2.7.
Hydrochar in Agriculture ...................................................................................... 125
viii
LIST OF FIGURES
Figure2.1 Structural representation of Lignocellulosic biomass with cellulose, hemicellulose, and
building blocks of lignin ................................................................................................................. 9
Figure2.2 Different conversion routes for dry biomass ................................................................ 20
Figure2.3 Different conversion routes for wet biomass................................................................ 21
Figure2.4 Classification of hydrothermal conversion process with respect to reaction temperature
and pressure .................................................................................................................................. 23
Figure2.5 Physical properties of water with temperature, at 24 MPa ........................................... 26
Figure2.6 Effect of thermal pre-treatments on biomass structure................................................. 27
Figure2.7 Typical degradation pathway of biomass composition during dry thermochemical
conversion (A) and hydrothermal conversion (B) ........................................................................ 28
Figure2.8 Flow Chart of Methodology ......................................................................................... 33
Figure3.1 HTC experimental Setup .............................................................................................. 37
Figure3.2 Temperature and Pressure Profile for HTC-5min experiment. .................................... 37
Figure3.3 Torrefaction experimental Setup .................................................................................. 38
Figure3.4 IKA Bomb Calorimeter ................................................................................................ 40
Figure3.5 Planetary Ball Mill and Accessories (Retsch-PM 100) ................................................ 42
Figure3.6 Sieve-Shaker (Retsch-PM 100) .................................................................................... 42
Figure3.7 Effect of reaction time and temperature on Mass yield of different feedstocks ........... 48
Figure3.8 Model Predicted Cube plot for Mass Yield vs Reaction time, temperature and ratio .. 50
Figure3.9 Response Surface plot for Reaction time and temperature vs Mass Yield at mean solid
load factor (1:9)............................................................................................................................. 50
Figure3.10 Model predicted and actual experimental values plot for the mass yield ................... 51
ix
Figure3.11 Effect of reaction time and temperature on HHV of different feedstocks ................. 52
Figure3.12 Effect of reaction time and temperature on energy yield of different feedstocks ...... 52
Figure3.13 Mean Model for Reaction time, temperature and ratio vs HHV ................................ 55
Figure3.14 Response Surface plot for Reaction time and temperature vs HHV at mean solid load
factor (1:9) .................................................................................................................................... 56
Figure3.15 Predicted and Actual values plot for HHV ................................................................. 56
Figure3.16 Mean Model for Reaction time, temperature and ratio vs Energy Yield ................... 58
Figure3.17 Response Surface plot for Reaction time and temperature vs Energy Yield at mean
solid load factor (1:9) .................................................................................................................... 58
Figure3.18 Predicted and Actual values plot for Energy yield ..................................................... 59
Figure3.19 Comparison of Mass yield, energy yield and HHV in HTC and Torrefaction........... 61
Figure3.20 Atomic H/C-O/C ratios of raw and pretreated miscanthus (M), willow (W) and wheat
straw (WS) samples ...................................................................................................................... 64
Figure3.21 Effect of reaction temperature on grindability of raw and HTC pre-treated samples of
Miscanthus, Wheat straw and Willow .......................................................................................... 67
Figure3.22 SEM images of wheat straw samples: (A) Raw, (B) 190°C, (C) 225°C, and (D)
260°C ............................................................................................................................................ 67
Figure3.23 SEM images of willow samples: (A) Raw, (B) 190°C, (C) 225°C, and (D) 260°C .. 68
Figure3.24 High Resolution SEM images of wheat straw samples: (A) Raw, (B) 190°C, (C)
225°C, and (D) 260°C ................................................................................................................... 69
Figure3.25 High Resolution SEM images of willow samples: (A) Raw, (B) 190°C, (C) 225°C,
and (D) 260°C ............................................................................................................................... 70
x
Figure3.26. Particle size distribution for raw and pretreated miscanthus samples and its
comparison with coal .................................................................................................................... 72
Figure3.27 SEM images of miscanthus samples: (A) Raw, (B) HTC-190°C, (C) HTC-225°C, (D)
HTC-260°C and (E) Torrefied-260°C .......................................................................................... 73
Figure3.28 Effect of HTC operating temperature on inorganic yield of wheat straw samples .... 75
Figure3.29 Effect of HTC operating temperature on inorganic yield of willow samples ............ 76
Figure3.30 Inorganic yield for the HTC and torrefied miscanthus samples ................................. 77
Figure3.31 Effect of HTC operating temperature on hydrophobicity of willow hydrochar samples
....................................................................................................................................................... 79
Figure3.32 Effect of HTC operating temperature on hydrophobicity of wheat straw hydrochar
samples .......................................................................................................................................... 79
Figure3.33 Comparison of hydrophobicity for torrefied and HTC pre-treated miscanthus samples
....................................................................................................................................................... 80
Figure4.1 Effect of acid concentration at different temperatures on mass yield of hydrochar
samples .......................................................................................................................................... 87
Figure4.2 Effect of salt concentration at different temperatures on mass yield of hydrochar
samples .......................................................................................................................................... 88
Figure4.3 Effect of salt concentration at different temperatures on ash content of hydrochar
samples .......................................................................................................................................... 88
Figure4.4 Effect of acid concentration at different temperatures on HHV of hydrochar samples 90
Figure4.5 Effect of salt concentration at different temperatures on HHV of hydrochar samples 90
Figure4.6 Effect of reaction temperature on pH of HTC process water ....................................... 93
xi
Figure4.7 Effect of reaction temperature on concentrations of different intermediate compounds
in HTC process water.................................................................................................................... 94
Figure4.8 Effect of reaction temperature on concentrations of alkali and alkaline metals in HTC
process water ................................................................................................................................. 95
Figure4.9 Effect of recirculation of HTC process water on pH of liquid by-product .................. 97
Figure4.10 Effect of recirculation of HTC process water on mass yield of hydrochar samples .. 98
Figure4.11 Effect of recirculation of HTC process water on HHV of hydrochar samples ........ 100
Figure5.1 (A) Pelletization setup, (B) Pellet formation, (C) Pellet extrusion............................. 106
Figure5.2 Radial compression of pellets to test the compression strength(Nielsen et al., 2009) 107
Figure5.3 Effect of pre-treatment type on durability and strength of pellets.............................. 112
Figure6.1 Integration of HTC and AD plant ............................................................................... 124
xii
LIST OF TABLES
Table2.1 Physical and Chemical Properties of Lignocellulosic composition of Biomass ............. 9
Table2.2 Typical Ultimate and Proximate Analysis of Dry and Wet Biomass ............................ 17
Table3.1 Experimental Design for Miscanthus Biomass Feedstock ............................................ 46
Table3.2 Fiber-analysis of raw miscanthus, wheat straw and willow .......................................... 47
Table3.3 Fiber-analysis of Raw and Pretreated Miscanthus Samples .......................................... 60
Table3.4 Proximate and Ultimate analysis of raw and pretreated biomass .................................. 63
Table3.5 BET-Surface area of raw and HTC pretreated willow and wheat straw ....................... 69
Table3.6 BET-Surface area of HTC and torrefied miscanthus ..................................................... 73
Table5.1 Effect of densification on mass density, HHV and energy density ............................. 110
Table5.2 Effect of pre-treatment on grindability of biomass ...................................................... 111
Table5.3 EMC of raw and pretreated pellets before and after immersion .................................. 115
xiii
List of Abbreviations
BET
Brunauer, Emmett, and Teller
DMF
Di-Methyl-Furfural
EJ/y
Exajoules per year
FC
Fixed Carbon
HHV
Higher Heating Value
HMF
Hydroxy-Methyl-Furfural
HTC
Hydrothermal Carbonization
HTL
Hydrothermal Liquefaction
IBI
International Biochar Initiative
ICP-OES
Inductively coupled plasma atomic emission spectroscopy
MC
Moisture Content
MJ/kg
Mega joules per kilogram
PCB
Polychlorinated Biphenyl
PCBs
Pulverized Coal Boilers
PSD
Particle Size Distribution
RH
Relative Humidity
SCWG
Supercritical Water Gasification
SEM
Scanning Electron Microscopy
TOC
Total Organic Carbon
TOP
Torrefaction and Pelletization
VM
Volatile Matter
xiv
CHAPTER-I INTRODUCTION
1.1.
Background
Current energy crises are the consequence of a rising world total population and
tremendous amount of pressure on demand and consumption of fossil fuels, especially in
industrial countries, for energy generation. The world’s total energy consumption was estimated
at about 524 exajoules per year（EJ/y）and has been predicted to increase by about 27 percent
by the year 2020 and about 65 percent by 2040 (British, 2013; EIA, 2013). Worldwide concerns
about global warming with the emission of greenhouse gases (GHGs), of which carbon dioxide
has major impact, has derived a profound review in developing energy policies. Sulphur oxides
(SOx) emissions that are releasing during combustion of fossil fuels are the primary cause of acid
rain and significantly increasing emission rates of CO2 are possessing climactic disasters. A
continuous rise in these emissions can imply serious threats to the environment like excessive
rainfall, floods, droughts and huge climatological variations. The increase in cost, depletion in
availability and deleterious environmental concerns associated with the use of fossil fuels are the
main topic of debates in energy meetings. One of the most effective approaches for dealing with
these issues would be reducing its consumption by substituting it with a clean-green sustainable
and renewable energy resource.
Recent research interests in biomass and using it as a substitute fuel for coal is gaining
signification attention (Bridgeman et al., 2010; Pimchuai et al., 2010). Biomass is a non-edible
renewable energy resource derived from living or recently living organic matter such as wood,
wood waste and agricultural residues. Biomass is the world’s fourth largest source of energy and
its billion ton availability can meet sustainable energy demand and production. However, owing
to its inferior physical and chemical properties such as low bulk density, hydrophilic nature, low
1
calorific value, poor grindability, and high alkali content in ash results into highly inefficient
handling and combustion properties of biomass (Bridgeman et al., 2010; Demirbas, 2004). Raw
biomass is more likely to adsorb moisture and is thus prone to biological deterioration, limiting
its self-storage time (Acharjee et al., 2011).
A high alkali and alkaline earth metal content in biomass hinders its application as a solid
fuel and therefore cannot be used directly. The presence of sodium and potassium along with the
sulphur causes highly undesirable effects in terms of slagging, fouling and tendency to corrode
equipment. Alkali metals react with sulphur to form alkali sulphates which deposits on
combustor surface; affecting heat transfer rates and also resulting in the corrosion of turbine
blades made from highly alloyed steels and super alloys (Saddawi et al., 2011). Silica in biomass
reacts to form alkali silicates, that melts at low temperatures (can be lower than 700°C,
depending on composition) and can cause the agglomeration of particles (Agblevor & Besler,
1996; Baxter et al., 1998). Calcium does not cause corrosion but can form deposits that are very
difficult to remove from turbine blades. When using biomass with high alkali content for
combustion, additions of additives are often required to manage ash behaviour which can
increase the overall operational expenses.
Moreover, the lignocellulosic biomass is highly fibrous in nature and is thus highly
energy consuming process to grind it into small particles, where small fine particles are generally
required during gasification or co-firing with pulverized coal (Bridgeman et al., 2010). Hence, to
overcome all these aforementioned limitations, pre-treatment or preprocessing of biomass are
essential steps to improve its combustion properties and its utilization as an effective energy
source.
2
Densification of biomass into compacted regular shaped product(s) like pellets, briquettes
or cubes is one of the optimal solutions and has aroused a great deal of interest worldwide in
recent years as a most efficient technique in improving logistics of biomass. Densification of
agricultural (straw and grasses) and woody (chips) biomass into pellets can increase the bulk
density from 40-200 to 600-800kg/m3 (Kaliyan & Vance Morey, 2009). Thus, the process can
significantly reduce the overall transportation and handling costs associated with the biomass
processing industry. However, owing to the hydrophilic nature of biomass, the pellets produced
from raw biomass tends to shatter when become in contact with water or are even exposed to the
high relative humidity conditions. The presence of high moisture content in biomass
feedstock/pellets can influence fungal growth and therefore the material will most likely rot
easily with time.
Also, the chemical reactions (generally oxidation) or anaerobic microorganism activity in
biomass feedstock/pellets can produce heat at a sufficient rate that it can cause self-heating of the
biomass stockpile which can lead to self-ignition or other harmful toxic gaseous emissions (Guo,
2013). Therefore, the pellets produced from raw biomass are not meant for longer storage (either
indoor or outdoor) without any special controlled-environment storage structures. The
construction of such structures can increase the overall storage cost associated with biomass
feedstock or pellets.
Densification in combination with the thermal pre-treatments like torrefaction is often
proposed as an alternate to improve the physicochemical properties of biomass (Bergman, 2005;
Pimchuai et al., 2010). During torrefaction, biomass is heated in an inert atmosphere at
temperatures of about 200-320°C for 30min to a couple of hours. The process results in an
approximately 30% of mass loss, where only 10% of the energy contained within the biomass is
3
lost in the form of gases and, therefore, the specific energy density of the solid by-product is
increased (Bergman, 2005). As such, the pellets produced from torrefied biomass are more cost
competitive than the regular “white pellets” as they have greater bulk energy density (i.e. energy
per unit volume).
Other advantages associated with the torrefaction process are reduced moisture content,
improved resistance to water damage and microbial growth, and increased friability which makes
torrefied pellets easier to grind (Bergman et al., 2005). While torrefied pellets represent a
significant improvement over the conventional white pellets, these pellets still fail to meet all
end-user requirements as they are weak in strength and have low durability; thus they break
easily and generate dust upon handling, causing a risk of explosion. The bulk energy density and
grindability of the torrefied pellets is not comparable to that of coal and most importantly the
high inorganic metallic content in ash still remains a huge challenge for biomass combustion
(Shang et al., 2014; van der Stelt et al., 2011). Because torrefaction process is unable to remove
alkali and alkaline metals from biomass ash, the use of torrefied biomass in conventional
pulverized coal boiler system is highly inefficient (Kludze et al., 2013). A study suggested a
maximum of 50% replacement of coal by dry torrefied biomass in a pulverized coal fired boiler
because of lower a Higher heating value (HHV), fouling issues, and low grindability compared
to coal (Wilén et al., 2013).
Supplement addition of binding agents can improve the durability of torrefied pellets.
However the addition of such binders may increase the overall manufacturing cost of the pellets
and may also negatively impact the combustion behaviour. Moreover, as the energy input and the
quality of the end product in the torrefaction process significantly depends upon the initial
moisture content of the feedstock, pre-drying of feedstock may be required prior to torrefaction.
4
The drying methods for biomass are highly energy consuming processes and are a huge financial
load in the torrefaction and pelletization process, ultimately making it an inapplicable technique
when dealing with wet biomass like food waste (Mani et al., 2006a). In order to advance the
densification characterisation of biomass without the expense of binders or adhesives, there is a
need to develop an effective technique that can produce pellets with reasonable durability and
strength, and high energy density that have potential to replace coal at thermal power plants
without any modification to the system.
A comparatively new approach of hydrothermal carbonization (HTC), also referred to
wet torrefaction, is performed at the temperature range of 180-260°C during which biomass is
submerged in water and is heated in a confined system under pressure (2 to 6MPa) for 5 to
240min, could potentially address the limitations of biomass (Libra et al., 2011). As the process
itself is carried out in the presence of water it thus eliminates the pre-drying requirement of
feedstock. The HTC process results in the formation of three different products: solid
(hydrochar), liquid (aqueous soluble) and gaseous (mainly CO2) products. The properties and
percentage distribution of the final products strongly depends upon the process reaction
conditions (Yan et al., 2010). Although both reaction time and temperature have been observed
to influence the physicochemical characteristics of products, the reaction temperature remains
the governing process parameter (Funke & Ziegler, 2010). Hydrochar is the primarily desired
solid end product in the HTC process, which exhibits unique and significantly superior
physicochemical properties compared to biochar (from pyrolysis and torrefaction) and has
several value-added industrial applications (Titirici et al., 2012). Hydrochar is highly
hydrophobic and friable, and has increased the percentage of lignin and aqueous soluble
materials in it. It is expected that using hydrochar for densification purposes can improve the
5
pelletability of solid fuels (Reza et al., 2014b). Secondly, as the process is carried out in the
presence of liquid water, it can demineralize the inorganic impurities by precipitating those in a
liquid by-product stream. Reduction in the alkali and alkaline earth metal content from biomass
would potentially mitigate the challenges like slagging, scaling, and fouling in boilers during
biomass combustion. The lack of energy intensive drying processes, high conversion efficiency,
and relatively low operating temperature and residence time range are the significant advantages
offered in HTC when compared to other conventional thermal pre-treatments like torrefaction
(Reza et al., 2012).
1.2.
Scope and Objectives
Where lignocellulosic biomass has a strong influence in developing a sustainable energy
production, most of the research work has primarily been focused on woody biomass. However,
the purpose grown energy crops like miscanthus represents a significant share in the bioenergy
development, as these crops grow very quickly and requires very less maintenance (Brosse et al.,
2012). Furthermore, using agricultural by-products for the bioenergy signifies a strong potential
in the best ways to utilize waste products, like wheat straw an agricultural biomass remains
highly underutilized due to its high ash content.
Extensive variety of literature is available on torrefaction of woody and agricultural
biomass. However, very few studies exist that have examined the comparative assessment of
such crops for producing high energy dense products via HTC and torrefaction pre-treatments.
The primary goal of the work presented in this thesis is to compare the physicochemical
properties and densification characterisation of raw, torrefied and HTC pretreated biomass in
terms of energy density, proximate and ultimate analysis, hydrophobicity, ash yields, fate of
alkali and alkaline earth metals, grindability, particle size distribution, BET surface area,
6
compression strength, durability and water resistance capacity of pellets from both biochar and
hydrochar. This work might be very useful to investors and researchers in determining the
potential of hydrothermal conversion of biomass for a wide range of industrial applications.
7
CHAPTER-II LITERATURE REVIEW
2.1.
Overview of Biomass
Biomass is a lignocellulosic material derived from the living or recently living organic
materials such as wood and agricultural residuals. In a board vision, non-lignocellulosic
materials, like animal and municipal solid wastes (MSWs), are also termed as biomass
(Demirbaş, 2001). Biomass is a fourth largest source of energy followed by coal, oil, and natural
gas and providing at about 14 percent of the world’s total energy consumption (Saxena et al.,
2009; Tumuluru et al., 2010). Biomass is the one and only renewable energy resource that can be
converted into any form of fuel including solid, liquid, and gaseous (Özbay et al., 2001).
Biomass is widely used to meet wide variety of energy requirements such as heat and electricity
generation, and producing biofuels for fueling vehicles. Moreover, its non-edible nature, ability
to grow relatively-quickly even on unfertile land, and abundant availability on earth nominates it
as a potential energy resource for a sustainable energy production which is the overall goal in the
vision of a bioenergy development (Perlack et al., 2005). Using biomass as a fuel can also be an
opportunity to empower rural communities.
2.1.1. Lignocellulosic Composition of Biomass
Lignocellulosic is a generic term used to represent the chemical composition of biomass.
Typically biomass (like plants and trees) is composed of three main components: cellulose,
hemicellulose, and lignin as shown in the Figure2.1. These components are strongly intermeshed,
chemically bonded by a non-covalent forces, and are cross-linked together providing structure
and rigidity to the plant. The physical and chemical properties of these components are discussed
in Table2.1.
8
Figure2.1 Structural representation of Lignocellulosic biomass with cellulose,
hemicellulose, and building blocks of lignin
(Alonso et al., 2012)
Table2.1 Physical and Chemical Properties of Lignocellulosic composition of Biomass
Compound
Cellulose
Hemicellulose
Chemical Structure
Lignin monomers (a)
trans-p-coumaryl alcohol,
(b) coniferyl alcohol, and
(c) sinapyl alcohol
Cellobiose (D-glucose)
Unit
Molecular Formula
Typical
Composition in
Biomass
Structural
Formation
Lignin
(C6H10O5)n
C5H10O5
(a)C9H10O2, (b)C10H12O3,
and (c)C11H14O4
(i) Hardwood:39-54%
(i) Hardwood:15-36%
(ii) Softwood:41-50%
(ii) Softwood:11-27%
(iii) Agricultural:24-50% (iii) Agricultural:22-35%
(i) Hardwood:17-29%
(ii) Softwood:27-30%
(iii) Agricultural:7-29%
A homopolymer of Dglucose subunits.
A hetropolymer built up of
three different phenylpropane monomers groups;
p-coumaryl, coniferyl and
sinapyl alcohol.
Cellulose is linked by
β-1,4 glycosidic bonds
forming long chains
A hetropolymer of
Xylose, Mannose,
Glucose, and Galactose.
Xylan, the dominating
component in
hemicellulose, is linked
by β-(1→4)-glycosidic or
9
This complex polymer is
oriented by different
α-(1→2)-bonded 4-Omethylglucoronic acids.
Also may contain acetyl
group attached to it.
degree of methoxylation of
above mentioned
monomers forming a large
molecular structure(s).
Hydrophobicity
Medium
Low
High
Calorific Value
17-18 MJ/Kg
17-18 MJ/Kg
23.3-26.6 MJ/Kg
Owing to its amorphous
structure, thermal
breakdown of
hemicellulose is
relatively easier.
Lignin is the most thermochemically stable
component in wood and
highly insoluble in water.
Cellulose in nonsoluble in water under
standard conditions.
Thermal stability
and Solubility in
Water
Applications
It can be hydrolysed in
subcritical water around
180°C and around 300400°C in standard
conditions.
Paper manufacturing,
textiles, biofuels,
chromatography,
binding/composite
materials, etc.
It can be hydrolysed in
water around 160°C and
around 200-300°C under
standard conditions.
Mainly includes animal
feed, food packaging,
health care and biorefinery industry.
Its degradation/hydrolysis
starts in near or
supercritical water or
around 600°C in ambient
conditions.
Manufacturing of adhesive
compounds and bioenergy
References: (Demirbaş, 2005; Ebringerová, 2005; Fengel & Wegener, 1983; Garrote et al.,
1999; Glasser & Sarkanen, 1989; Grønli et al., 2002; Janshekar & Fiechter, 1983; Kumar, 2010;
Saha, 2003a; Sun, 2010; Thielemans et al., 2002)
2.1.1.1.
Cellulose
The cellulosic composition of the woody biomass varies from 39 to 54%; generally
hardwood contains the higher percentage of cellulose content in it compared to softwood and
agricultural biomass (Garrote et al., 1999). A cotton fiber is a purest and naturally occurring form
of cellulose (Kumar, 2010). It is composed of D-glucose (C6H10O5) subunits linked by β-1,4
glycosidic bonds forming long chains (called elemental fibrils) linked together by the hydrogen
bonds and van der Waals forces. The β-linkage linear chains in the cellulose are highly stable
10
and are resistant to chemical attack because of the presence of high degree of intra and
intermolecular hydrogen bonding (Jarvis, 2003). The presence of strong hydrogen bonding
provides a strong and rigid structure to a polymer. In addition, due to the crystalline structure,
the thermal degradation of cellulose starts at a temperature range of 300-400°C (Grønli et al.,
2002; Pérez & Samain, 2010).
2.1.1.2.
Hemicellulose:
Unlike cellulose, hemicellulose is a low molecular weight polymer and is made of 2332% of raw biomass. Hemicellulose has a complex carbohydrate structure consisting of different
type of polymers like pentoses, hexoses, mannose, glucose, and sugar acids. Xylan is the most
dominating component of hemicellulose in hardwood and agricultural plants, like grasses and
straw, where glucomannan is dominant in softwood (Fengel & Wegener, 1983; Saha, 2003b).
Due to the low molecular weight and amorphous structure, the hemicellulose is a highly soluble
polymer in water. Among all three lignocellulosic polymers of biomass, hemicellulose is the
least thermally stable polymer followed by cellulose and lignin. The thermal degradation of
hemicellulose starts at a temperature range of 200-300°C (Grønli et al., 2002), however, its
solubilisation in the water (mainly due to hydrolysis) starts at about 180°C under hydrothermal
conditions (Bobleter, 1994; Garrote et al., 1999).
2.1.1.3.
Lignin:
After cellulose and hemicellulose, lignin is the most abundant polymer in the nature and
is present in the cell wall of biomass. Lignin is a complex, cross-linked, and amorphous
hetropolymer that is made up of the three different phenyl-propane groups: (i) p-coumaryl, (ii)
coniferyl, and (iii) sinapyl alcohol forming a large molecular structure (Hendriks & Zeeman,
2009; Kumar, 2010). Generally softwoods contain higher percentage of lignin in it as compare to
11
hardwoods and agricultural biomass (Garrote et al., 1999). The main function of lignin in plant is
to provide structural strength, impermeability, and resistance against microbial attack (Fengel &
Wegener, 1983). However, some bacteria has been reported that can degrade the lignin content
of biomass, for example the white-rot fungi produces different type of extracellular oxidative
enzymes that can cause the degradation of lignin due to the formation of laccases and highly
redox potential ligninolytic peroxidases compounds (Bugg et al., 2011; Ruiz-Dueñas &
Martínez, 2009). Lignin is an amorphous hetro-polymer and is highly hydrophobic in nature,
hence is relatively soluble in water. Lignin is the most thermally stable polymer and its
degradation starts at about 600°C under atmospheric conditions (Grønli et al., 2002). However,
under hydrothermal conditions its degradation starts at a low temperature at about 220°C
(Bobleter, 1994). The solubility of lignin in the controlled environment conditions such as acidic,
neutral or alkaline highly depends upon the molecular structure of lignin (i.e. p-coumaryl,
coniferyl, and sinapyl alcohol or their combinations) (Grabber, 2005). Lignin in the biomass
shows a glass temperature transition behaviour at a temperature range of 135-165°C where it
melts and acts as a natural binding agent during the densification process (Kaliyan & Morey,
2010; Li & Liu, 2000; Reza et al., 2012).
2.2.
Combustion Properties of Biomass
Although biomass is a common source of energy, particularly in the developing
countries, it is still not regarded as an ideal fuel due to the inferior physicochemical properties
such as fibrous nature, low bulk density and low heating value, high moisture content, high
volatile components, high alkali and alkaline earth metallic content (Bridgeman et al., 2008;
Pimchuai et al., 2010). The high moisture content of biomass reduces the combustion
temperature and efficiency, and increases the carbon monoxide (CO) emissions (Khan et al.,
12
2009). The alkali and alkaline metals present in biomass show good catalytic properties during
the combustion and gasification of biomass but on the down side these metallic species are also
responsible for the various ash related problems like fouling, klinkers, and sintering in boilers,
ultimately reducing the thermal efficiency of a heat exchanger (Yip et al., 2009). Biomass
(especially herbaceous plants like grasses and straws) contains high amount of potassium and
chlorine in it. The high chlorine content in a fuel can cause the high temperature corrosion in
boilers, and can accelerate the foul and slag formation when come in contact with the mobile
potassium species (Demirbas, 2004). Biomass generally has low sulfur content than coal but
have high nitrogen content that may increase the NOx emission. The polymeric composition of
biomass, i.e. percentage amount of hemicellulose, cellulose, and lignin, also influences the
combustion behaviour. For instance, unlike hemicellulose and cellulose, high percentage of
lignin content in biomass shows positive effect on the HHV of feedstock (Demirbaş, 2005),
where significantly increases tar formation during the thermochemical processes; the tar
components are very harmful in terms of pollution (Yu et al., 2014). The tar yield can be
controlled by increasing the process temperature and excess air ratio. The combustion behaviour
and compositions of biomass significantly varies with specie, nature of plant tissue, phase of
development, and growing conditions (Demirbas, 2004). The seasonal variation affects the
continuous availability of biomass feedstock and the wide diversification in physical shape,
chemical compositions, and energy density among different biomasses result inefficient
handling, transportation, storage, and sizing of feedstock.
To overcome these problems, the pre-treatment of biomass is a necessary step to convert
it into a uniform and a sustainable fuel source. A broad range of biological and thermochemical
pre-treatment processes including torrefaction, pyrolysis, gasification, anaerobic digestion, and
13
fermentation are available to improve the combustion properties of biomass or their conversion
to the biofuels (Goyal et al., 2008; Saxena et al., 2009). However, the thermochemical pretreatments are generally preferred over biological pre-treatments, as they offer advantages like
the short reaction time and higher product yield (Liu et al., 2012). The research interests in
reducing the greenhouse gas (GHG) emissions by the mean of carbon sequestration or carbon
capture storage (CCS) and simultaneously improving food productivity with the application of
biochar in soil have made biomass to gain considerable attention for its vital role in developing a
sustainable energy and eco-friendly environment (Lehmann & Joseph, 2009). As an estimate, the
addition of 1 ton of soil carbon pool in a degraded cropland soil may improve the crop yield from
20 to 40 kg/ha for wheat, 10 to 20 kg/ha for maize, and 0.5 to 1 kg/ha for cowpeas. Enhancing
food security with the application of biochar in soil also implies 5-15% reduction in the global
fossil fuel emission or 0.4 to 1.2 Giga tons of carbon per year (Lal, 2004).
2.3.
Origin and Definition of Biochar and Hydrochar
The origin of biochar is associated with the soils of the Amazon region, often referred as
“Terra-Preta” soils. These soils have gained a great deal of global interest because of their
significantly higher crop productivity compare to the surrounding infertile tropical soils (Zech et
al., 1990). More detailed research revealed that these soils are believed to have had used biochar
as a key component which partly explains the unique properties of these soils over other (Glaser
et al., 2001). As a fact, these influential findings impelled researchers to reveal the further hidden
secrets, which resulted in a massive publication of literature on the biochar and its application in
the soil (Amonette & Joseph, 2009; Lehmann, 2009; Libra et al., 2011). Thereafter, biochar is
regarded as a significantly important tool for developing a sustainable energy and environmental
management (Lehmann & Joseph, 2009).
14
“Biochar” is a recently coined term emerging in conjunction with the renewable fuel, soil
amelioration and carbon sequestration. The definition(s) of biochar include char and charcoal
produced by the partial combustion (charring or smoldering) of carbonaceous organic materials
like trees and plants (Warnock et al., 2007). Under the absence or limited supply of oxygen, the
process prevents complete combustion (carbon volatilization and ash production) of the source
material. Some other researchers have also defined the term “biochar” in different ways.
However, almost all these definitions are somehow interrelated to each other in terms of the
production and applications of biochar (Ahmad et al., 2013). So far the most standardized
definition of biochar is regulated as per International Biochar Initiative (IBI) guidelines which
states that, ‘‘biochar is a solid material obtained from the thermochemical conversion of biomass
in an oxygen-limited environment’’ (IBI, 2013). It is critically important to differentiate between
the terminologies like biochar, charcoal, and hydrochar. The primarily difference between these
terminologies lies in their fate. The charcoal is a carbon-rich solid product prepared via charring
of biomass and is used as a fuel source for producing energy, where biochar is an alternative
term for the charcoal when it is used for agricultural applications like soil amendments, CCS and
or sometimes as a fuel (Lehmann & Joseph, 2009). On the other hand, hydrochar is slightly
similar product to biochar but is produced from a completely different pre-treatment process.
Typically biochar is produced as a solid by-product material in dry carbonization processes like
pyrolysis, where hydrochar is produced as a slurry (a two phase mixture of solid and liquid) from
HTC of biomass with the water under subcritical conditions (Brewer et al., 2009; Funke &
Ziegler, 2010; Libra et al., 2011; Manyà, 2012; Sohi et al., 2010). However both the chars
(biochar and hydrochar) significantly differs from each other in terms of their physical and
chemical properties (Fuertes et al., 2010; Wiedner et al., 2013).
15
2.4.
Biomass conversion
2.4.1. Feedstock for Biomass conversion
The classification of biomass feedstock for the production of char is highly important
because the pre-treatment selection and its applicability are primarily decided based on the type
of feedstock (wet or dry). The categorization of wet and dry biomass is made on the basis of its
initial moisture content. The freshly harvested biomass such as vegetable waste, sewage sludge,
animal waste, and algae generally have the high moisture content (>30%) and thus they are
referred as wet biomass, where the biomass like agricultural residues and few wood species
usually have low moisture content (<30%) at the time of harvesting and therefore are classified
as dry biomass (Knežević, 2009). The proximate and ultimate analysis of the dry and wet
biomass is shown in the Table 2.2. Wet biomass can be dried to low moisture content with the
supplement drying techniques, where such techniques are highly energy intensive and can reduce
the system’s overall economic efficiency (Mani et al., 2006a; Sokhansanj & Fenton, 2006).
Wet and dry biomass can be further classified into two categories: (i) Purpose-grown
biomass, and (ii) waste-biomass (Lehmann et al., 2006). Purpose grown crops like Miscanthus,
Switchgrass have high crop yield, high energy content, and relatively need low maintenance
compare to other crops. Miscanthus and switchgrass usually have low moisture content (below
10%) at the time of harvesting and therefore does not need to be dried, however, the harvesting
time can affect the ash content of biomass which may negatively impact the combustion
behaviour (Kludze et al., 2013). Such crops can acts a potential candidate for an environmental
management, but so far these crops are primarily been focused by the bio-refinery industries for
the production of liquid biofuels (Brosse et al., 2012). The second category, waste biomass, is
more comprehensive and it includes agro-forestry waste, animal manure waste, organic-food
16
waste, sewage sludge, and MSW (Brick, 2010). The use of waste biomass for the production of
biochar seems to be a more reasonable option because it is a highly cost-effective feedstock and
it does not compete with food-crops for the land requirement. However, there is no universal
consensus exists on what constituents in a definition of waste biomass, because sometime crop
residuals are often left in the field to regain and satisfy the specific soil properties (Perlack &
Turhollow, 2003). Over use of such waste by-products for the sake of bioenergy development
can disturb the overall environmental life cycle. On the contrary, using waste-biomass as a
feedstock for the production of biochar and hydrochar will be highly beneficial in terms of
maintaining an eco-friendly environment and efficient utilization of waste streams (Lehmann &
Joseph, 2009; Sohi et al., 2010).
Table2.2 Typical Ultimate and Proximate Analysis of Dry and Wet Biomass
Feedstock
Elemental
Analysis (%)
Carbon
Hydrogen
Nitrogen
Sulphur
Oxygen
Adapted from (Libra et al., 2011)
Dry Biomass
Wet Biomass
Woody Grasses
Manures Sewage Sludge MSW
50-55
5-6
0.1-0.2
0-0.1
39-44
Proximate
5-20*
Moisture Content
analysis (%)
35-60**
Volatile Matter
70-90
Ash
0.1-8
Fixed Carbon
10-30
HHV (MJ/Kg)
19-22
46-51
6-7
0.4-1
<0.02-0.08
41-46
52-60
6-8
6-8
0.7-1.2
41-46
53-54
7.2-7.4
5.3-5.6
2.1-3.2
29-32
27-55
3-9
0.4-1.8
0.04-0.18
22-44
10-20**
21-99
88-95
15-40
75-83
0.1-0.8
10-20
18-21
57-70
19-31
13-20
60-80
25-37.5
5-6
9-14
47-71
15-20
2-14
* Dried
** Freshly Harvested (Typically)
17
2.4.2. Pathways for Biomass Conversion
To date, several techniques are available for the conversion of biomass, however,
depending on the type of feedstock (wet or dry), desired output product(s) (solid, liquid, and
gas), percentage distribution, and properties of the product for different applications the choice
of the pre-treatment selection is very limited. Generally the pre-treatments are classified based on
operating conditions such as severity of process parameters (the reaction time and temperature),
pre and post processing requirements like shaping, sizing, drying, cooling, condensation, and
addition of catalyst (Goyal et al., 2008; Manyà, 2012; Mosier et al., 2005).
2.4.2.1.
Conversion of Dry Biomass
One of the most common methods used for the utilization of dry biomass is the direct
combustion for heat and power production. However, due to the inferior physicochemical
properties, as discussed earlier, the direct combustion of biomass produces smoke and other
harmful gaseous, therefore is not an eco-friendly route for the biomass conversion. Compaction
or densification is the most widely used method during which biomass is compacted in a regular
shaped product like pellets or briquettes (Tumuluru et al., 2010). Densification of biomass can
significantly reduce the transportation and handing costs associated with the biomass and has
gained a great deal of industrial and commercial interest (Mani et al., 2006a). Another
compaction method is used for the extraction of liquid product from the oil rich seeds like
canola, jatropha, sunflower, etc. for the production of biodiesel where the left over solid product
can either be directly combusted or can be further processed. Both the compaction processes,
densification and oil extraction, can be efficiently applied on the farm site and can aid in the
development rural communities.
18
The thermochemical pre-treatments like pyrolysis, gasification, torrefaction are
economical only when carried out at large scale. Pyrolysis, slow and fast, is a thermochemical
decomposition process during which the biomass is heated at a temperature range of 300-650˚C
under the absence of oxygen. The process results in the formation of three main products:
carbon-rich solid product (biochar), a volatile matter which can be condensed to liquid phase
(bio-oil), and the remaining so called “non-condensable” gases like CO, CO2, CH4, H2
(Brownsort, 2009; Mohan et al., 2006).
Torrefaction is a French synonym for the roasting, also referred to mild pyrolysis because
the reaction temperature during the process is comparatively much lower than in a pyrolysis
process. During torrefaction biomass is heated at a temperature range of 200-320°C under the
reduced level of oxygen (Bergman et al., 2005). Torrefaction has gained considerable attention
as an important pre-processing step for the improvement of the physicochemical properties of
biomass (Pimchuai et al., 2010). A torrefied biomass can be further shaped into a densified
product like pellets that which have significantly improved bulk energy density compared to
regular white wood pellets (Bergman, 2005). Solvolysis is an another process during which a
biomass is mixed with the hot compressed water for the recovery of sugar and liquid
intermediate compounds (Mok & Antal, 1992). The different conversion pathways for the dry
biomass are shown in the Figure 2.2.
19
Figure2.2 Different conversion routes for dry biomass
(Knežević, 2009)
2.4.2.2.
Conversion of Wet Biomass
Wet biomass (see figure 2.3) can be transformed into a fuel via biological conversion
methods like anaerobic digestion for the production of biogas (methane enrich) or via
fermentation for the production of alcohols. The conventional biological conversion methods are
limited to the certain carbohydrate fractions of biomass. However, for the so called second
generation conversion processes, the enzymes and pre-treatment options are being developed that
target the lignocellulosic biomass in broader sense (Knežević, 2009). Moreover, the
thermochemical pre-treatments are generally preferred over biological pre-treatments, as they
offer several advantages like the short reaction time and higher product yield (Liu et al., 2012).
Wet biomass can be dried to dry state with the supplement drying processes but this route
highly inefficient method. For the conversion of wet biomass in the processes like HTC, the
biomass is mixed with the water and is heated at elevated temperatures for certain period of time.
20
Presence and requirement of the water in HTC eliminates the pre-drying requirement of wet
biomass which is a huge energy intensive and financial burden for the processing of wet biomass
when performed under conventional dry thermal pre-treatments like slow pyrolysis and dry
torrefaction (Uslu et al., 2008). The HTC process of lignocellulosic biomass is the primary
objective of this present study and is discussed in detail in the section 2.5.
Figure2.3 Different conversion routes for wet biomass
(Knežević, 2009)
2.4.3. Benefits of Biomass Conversion
Biomass is one of the most promising fuel sources in the world for the energy production.
Moreover, biomass is the one and only renewable energy resource that can be converted into any
form of fuel including solid, liquid, and gas (Özbay et al., 2001). It is relatively cheaper compare
to the fossil fuels. However, owing to the inferior combustion properties (discussed in section
2.2) of raw biomass compare to the fossil fuels its application in industry is very limited. Once
21
biomass is pre-treated it becomes superior to fossil fuels in a certain ways. The selection of a pretreatment should be made on such basis that it offers a broad range of benefits like:
(i)
Incorporation of the waste biomass streams like food, MSW, sewage, agricultural
residues,
(ii)
Simultaneous recovery of valuable intermediate and by-products for further applications,
(iii)
Cheaper in cost and has minimal pre-processing requirements like drying, sizing, and
(iv)
Free from the use of any corrosive solvents or chemicals.
2.5.
Hydrothermal Carbonisation of Biomass
HTC, also referred to wet torrefaction, is a thermochemical process for converting
organic feedstock into a high carbon rich solid product. During the process the biomass is mixed
with the water and is heated in a confined system at a temperature range of 180-260°C with the
reaction times from 5mins to 4 hours under the pressure of 2-6MPa (Hoekman et al., 2013; Libra
et al., 2011; Mumme et al., 2011). Usually the reaction pressure is not controlled in the process
and is autogenic with the saturation vapor pressure of water (subcritical-water) corresponding to
the reaction temperature.
The HTC process is not a new area of research and was first proposed by Friedrich
Bergius in 1913 to describe the natural coalification process (Bergius, 1931). Later in the last
decades of 19th century, the process gained attention as a hydrothermal degradation of the
organic materials for the synthesis of important chemicals along with the recovery of liquid and
gaseous fuels (Bobleter, 1994; Mumme et al., 2011). But the recent research activities on HTC
are simultaneously focusing on the production of solid product (hydrochar) that have several
value-added applications in the industry and environment (Avola et al., 2012; Funke & Ziegler,
22
2010; Kruse et al., 2013; Libra et al., 2011; Pavlovič et al., 2013; Peterson et al., 2008; Titirici et
al., 2012).
With an increase in the severity of reaction, i.e. for the temperatures above 260°C, the
hydrothermal process can be further classified into two techniques: (i) hydrothermal liquefaction
(HTL), and (ii) Hydrothermal vaporisation (HTV) or hydrothermal gasification (HTG), or super
critical water gasification (SCWG) (Pavlovič et al., 2013; Peterson et al., 2008). As both the
techniques are primarily used for the production of liquid and gaseous fuel, therefore, are not
discussed in this study. The classification of the hydrothermal process with respect to the
temperature and pressure is shown in the Figure 2.4.
Figure2.4 Classification of hydrothermal conversion process with respect to reaction
temperature and pressure
As the process is carried out in the presence of water and therefore is not affected by the
high moisture content of feedstock. This unique advantage of the HTC pre-treatment eliminates
the pre-drying requirement of wet biomass, which is a huge energy intensive and financial load
23
in the biomass pre-processing when performed under the conventional thermal pre-treatments
like slow pyrolysis and dry torrefaction. The process results in the formation of three main
products: solid (hydrochar), liquid (bio-oil in water) and small fractions of gases (mainly CO2).
The percentage distribution and the physicochemical properties of hydrochar are governed by the
reaction temperature (Yan et al., 2010). The mass yield of hydrochar during the HTC process
typically varies from 40 to 70%. It has been reported that with the equivalent mass yield, the
energy densification ratio (ratio of the HHV of biochar to the HHV of raw biomass) obtained via
HTC is significantly higher than the obtained under torrefaction (Yan et al., 2009; Yan et al.,
2010). The addition of salts and acids has been recommended by few researchers to improve the
chemical properties of hydrochar and to reduce the reaction pressure and temperature (Lynam et
al., 2011; Lynam et al., 2012). However, the selection of a catalyst should be made very carefully
as it can cause the pitting of reactor.
When leaving the process, hydrochar is wet in state and is in the form of slurry, therefore,
it has to pass through series of steps like mechanical dewatering (compressing), filtering, and
solar/thermal drying before it can used for combustion. The mechanical dewatering of wet
materials like sludge and paper pulp can reduce the moisture content to a range of 70-75%. The
moisture content of the hydrothermally pre-treated material can be achieved to a value of less
than 50% just by compression, ultimately reducing the supplement energy and time consumption
in drying of hydrochar (Mensinger, 1980). Pulverization of hydrochar is much less energy
intensive when compared to raw wood. Moreover, these pulverized particles also show spherical
shape which may further facilitate the fluidization process during gasification (Tremel et al.,
2012). The surface of hydrochar shows high degree of aromatization with the large number of
oxygen containing groups. Presence of the oxygen groups on the surface of hydrochar explains
24
its affinity for the water and therefore it can be used to increase the water retention capacity of
the soil (Sevilla & Fuertes, 2011). The high conversion efficiency, elimination of pre-drying
requirement, and relatively low operating temperature among other pre-treatments, makes HTC a
perfectly suitable conversion technique for the production of hydrochar especially from wet
biomass.
2.6.
Properties of Hot Compressed Water
From the phase diagram of water, the critical point is at 374°C and 22.1 MPa. Liquid
water, below the critical point, is subcritical and above is supercritical. The HTC pre-treatment
of biomass is carried out in subcritical water, at which water is still in a liquid phase and acts as a
non-polar solvent enhancing the solubility of organic compounds of biomass. A water at high
temperature and high pressure has high degree of ionization and starts dissociating into acidic
hydronium ions (H3O+) and basic hydroxide ions (OH−), therefore, shows both acidic and basic
characteristics (Kalinichev & Churakov, 1999; Marcus, 1999). The Figure 2.5 shows the effect
of temperature on the physical properties of water. The dielectric behavior of 200°C water is
similar to that of ambient methanol, 300°C water is similar to ambient acetone, 370°C water is
similar to methylene chloride, and 50°C water is similar to ambient hexane (Kritzer & Dinjus,
2001).
Water at the temperature and pressure near or above critical point becomes supercritical
water at which the properties of water are significantly different from that at the room
temperature and subcritical state. At this state the density of water is reduced drastically and is in
between that of the water vapor and liquid, and has a gas like diffusion properties. This state of
water is mainly used for the production of gaseous fuels like H2. Hydrous pyrolysis, supercritical
water oxidation (SCWO), and supercritical water gasification (SCWG) are the same process with
25
an aim of hydrogen gas production (Peterson et al., 2008). Both sub and super-critical water has
unique properties that it can be used for the destruction of hazardous wastes such as
polychlorinated biphenyls (PCBs) and polychlorinated di-benzofurans (PCDFs) (Weber et al.,
2002). The working against continuous flow of sub and supercritical water is a biggest challenge
in the commercial application of HTC.
Figure2.5 Physical properties of water with temperature, at 24 MPa
(Kritzer & Dinjus, 2001)
2.6.1. Role and Importance of Water in Hydrothermal Carbonisation
The primary goal of a thermochemical pre-treatment is to breakdown the rigid structure
of the biomass polymers into a small and low molecular weight chains. The Figure 2.6 shows the
effect of thermal pre-treatment on the physical structure of biomass. The rate of destruction of
the polymeric structure of biomass typically depends upon the reaction time, temperature and
reaction medium. In HTC due to the presence of subcritical water, the reaction mechanism is
initiated by hydrolysis, thus it lowers the activation energy level of hemicellulose and cellulose,
favouring the rapid degradation and depolymerisation of these polymers into the water soluble
products like oligomers and monomers (Bobleter, 1994; Funke & Ziegler, 2010; Yu et al., 2007).
26
As water is a cheap, non-toxic, and is inherently present in the wet biomass therefore is
used as a reacting medium in the HTC process. Also water is an alternate to the corrosive
chemicals and toxic solvents (Hashaikeh et al., 2007). In addition, the inorganic impurities
present in the biomass might migrate in the HTC process water and thus the demineralization can
reduce the overall ash content of the solid product which does not takes place in case of
torrefaction (see figure 2.7).
Figure2.6 Effect of thermal pre-treatments on biomass structure
(Shankar Tumuluru et al., 2011)
27
Figure2.7 Typical degradation pathway of biomass composition during dry
thermochemical conversion (A) and hydrothermal conversion (B)
(Source: Author, with reference to (Brownsort, 2009))
The process water (aqueous by-product) produced during the HTC process has been
reported to contain some phenolic, organic, and furan derivatives compounds like acetic acid,
formic acid, glycolic acid, levulinic acid, and 2,5-hydroxyl-methyl-furfural (HMF) that are
formed via the degradation of biomass polymers (Gullón et al., 2012; Kabyemela et al., 1999).
The products like Levulinic acid are a key block for the manufacturing of highly important
chemicals and materials. The 2,5-HMF is an important versatile chemical that has potential to
replace petroleum based products. It is used for the manufacturing of 2,5-dimethylfuran (DMF)
which is a liquid biofuel that is superior to ethanol in a certain ways (Su et al., 2009). Both these
products, Levulinic acid and 2,5-HMF, has been identified as “top 12 value added chemicals
from biomass” by US Department of Energy (DOE) (Werpy & Petersen, 2004). The
identification, recovery and characterisation of these high quality intermediate compounds can
become a potential platform for the bio-refinery and other chemicals manufacturing industries
(Gullón et al., 2012). Recirculation of the HTC process water can provide a broad range of
benefits like:
28
(i)
Reduction in the waste water that would also reduce the waste water treatment cost
associated with it,
(ii)
Recirculation of the process water has been suggested as the most efficient method of
heat recovery and can reduce the external heat consumption by ten-folds (Stemann et al.,
2013a),
(iii)
Formation of acetic acid in the water may further catalyze the process and therefore can
simultaneously reduce the pressure and temperature requirements,
(iv)
High quality intermediate products like HMF might deposit in the pores of hydrochar,
further augment the HHV of solid product, and
(v)
The HTC process water can be used for the anaerobic digestion of sludge (Wirth &
Mumme, 2013).
2.7.
Problem Statement
The government of Ontario is planning to terminate all the coal-fired power generation
plants in Ontario by the end of year 2014 (EPA, 2007). For the replenishment, Ontario
government is seeking for a sustainable and renewable energy source. Ontario power generation
(OPG) and the ministry of agriculture and food (OMAF) and ministry of rural affairs (MRA)
with the support of the Ministry of energy and infrastructure (MEI), are coordinating analysis of
the feasibility of biomass for the production of combustion energy in Ontario (OMAF, 2013).
With no doubt biomass is an attractive fuel source of energy and is a potential feedstock for the
further energy conversions that can be GHG emissions neutral. However, due to the inferior
physicochemical properties biomass is not regarded as an efficient fuel source and therefore its
industrial adaptability is very limited.
29
Torrefaction and pelletization are often proposed as a prerequisite pre-treatment for
upgrading biomass. During which the few properties like energy density, hydrophobicity, and
grindability are improved to some extent but are still not comparable to that of coal. The high
inorganics in ash still remains a huge challenge for the biomass combustion. Also, these methods
are less efficient when dealing with a feedstock with the high moisture content. Moreover, the
pellets produced from the torrefied biomass are weak in strength thus they tend to shatter and
causes a risk of explosion. As an alternate, HTC a comparatively new technique where the
biomass is treated with the hot compressed water instead of drying used in the conventional
thermal pre-treatments is investigated in this study.
2.7.1. Research Objectives
The main research objective of the present study is to understand and optimize the effect
of HTC on woody and agricultural biomass and its comparison with the conventional pretreatment (torrefaction). The research areas that are specifically focused in this study are:
1. To investigate the effect of reaction time, temperature, and feedstock to water ratio on the
mass yield and energy density of the hydrochar samples,
2. To examine the effect of reaction temperature on the storage properties (hydrophobicity),
grindability, particle size distribution (PSD), O/C-H/C ratios, scanning electron
microscopy (SEM), BET-surface area, polymeric composition (cellulose, hemicellulose
and lignin), alkali and alkaline metal composition of the solid product produced via HTC
and torrefaction,
3. Determine the effect of addition of salts and acids on the mass yield, ash content and
HHV of hydrochar samples,
30
4. To characterize the HTC process water in terms of identification and quantification of the
intermediate components produced at different reaction temperatures,
5. To examine the effect of process water recirculation on pH of liquid by-product and the
effect on the mass yield, ash content and HHV of the hydrochar samples, and
6. To study the densification characterization of raw, torrefied and hydrochar samples in
terms of strength, storage and combustion properties.
31
2.7.2. Methodology
The methodology followed in this thesis is shown in the form of flow chart (Figure2.8).
The different biomass feedstocks, willow (woody), wheat straw (agricultural) and miscanthus
(purpose grown crop), were studied to investigate the effect of HTC on lignocellulosic biomass.
The results from the HTC process were compared with the torrefaction process. The solid
product from both the thermal pre-treatments was then examined for the pelletization process.
The liquid by-product from the HTC process at different reaction temperatures was also
characterized for the chemical composition. The HTC process water was then studied for the
recirculation experiments. The results from this study can help in optimizing the design of a
state-of-the-art HTC reactor.
32
Figure2.8 Flow Chart of Methodology
33
CHAPTER-III HYDROTHERMAL CARBONISATION AND TORREFACTION OF
BIOMASS
In this chapter, the experimental demonstration of hydrothermal carbonisation (HTC) and
torrefaction of biomass is examined. HTC of biomass is a process during which biomass is
mixed with water and the mixture is heated to a temperature range of 180-260°C. Where
torrefaction is a process where the water is evaporated from the feedstock is then heated to a
temperature range of 200-300°C under reduced level of oxygen.
The performance of both the pre-treatments was measure in terms of the mass yield,
energy density, scanning electron microscopy (SEM), BET-surface area, inorganic elemental
analysis, grindability, particle size distribution (PSD), oxygen to carbon-hydrogen to carbon
(O/C-H/C) ratios, and fiber analysis.
34
3.1.
Experimental Section
3.1.1. Materials
Three different biomass feedstocks, Hybrid willow (woody), wheat straw (agricultural),
and miscanthus (a purpose grown crop), were considered for this study. The willow wood was
obtained from the University of Guelph research field and was harvested in June 2013. Both, the
wheat straw and miscanthus were collected from a privately owned farm in Ontario and were
harvested in May 2013.
3.1.1.1.
Feedstock Preparation
Prior to the experiments each biomass feedstock was prepared for the uniform. The
willow wood samples were chopped manually to a workable length using band saw. The length
and diameter of the prepared willow samples was 25mm and 10mm, respectively. The
miscanthus samples were chopped manually into the samples of length in a range of 20-25mm.
The wheat straw samples were milled using a commercially available grinder and was then
sieved to a range of 3.35mm to 710µm particles. All the prepared samples were stored in a sealed
plastic bag until treatment.
3.1.2. Experimental Methods
3.1.2.1.
Hydrothermal Carbonisation
HTC of biomass was carried out using Parr 600ml bench top reactor (Moline, IL) fitted
with the glass liner (762HC3) as shown in the Figure 3.1. To analyze the effect of operating
conditions on biomass, the experiments were performed at the different reaction temperatures
(190, 225, 260°C), residence times (5, 15, 30mins) and feedstock-to-water ratios or solid load
(1:6 & 1:12 (w/w)). The temperature of the system was controlled using a Proportional-Integral35
Derivative (PID) temperature controller. The reactor pressure was not controlled in the
experiment and was kept autogenic with the vapor pressure of water at the corresponding
temperature. As indicated by the pressure gauge attached to the reactor, the inside pressure of
reactor ranged from 180 to 720 psig. The temperature and pressure profile in the reactor during
HTC are shown in the Figure 3.2.
Prior to the reaction, 10g of miscanthus samples were mixed with deionized water in the
different ratios and was manually stirred for 2-3min to ensure complete wetting. The reactor was
heated up to the required temperature in 20-30mins and was then maintained for the desired time
period. It should be noted that the pre-heating time for the reactor would also play an important
role in the HTC reaction chemistry, as a higher reaction temperature would also require the
longer pre-heating time. However, in this study for the data analysis purposes, the reaction
temperature was defined as the isothermal holding time period. Later, to further quench the
reactions, the reactor was immersed in the cold water and was cooled down to room temperature
in 5-7 mins. Once the inside temperature of the reactor dropped to room temperature, the
pressure release valve was opened under the fume hood to release gaseous products. The solid
and liquid products were separated using a filter paper (20µm) and were collected for the further
analysis (data not shown here for liquid analysis). The hydrochar samples were dried overnight
at 103°C before analysis.
36
Figure3.1 HTC experimental Setup
Figure3.2 Temperature and Pressure Profile for HTC-5min experiment.
Zone A: heating time, Zone B: reaction time, Zone C: cooling time.
37
3.1.2.2.
Torrefaction
Torrefaction experiments were performed using a macro thermo-gravimetric analyzer
(TGA) (see Figure 3.3) that was designed and fabricated in the machine lab at University of
Guelph. The reactor consist a stainless steel tube heated by four electric heaters of 1.25KW
capacity kept in close contact with the reactor wall. A small perforated basket made of stainless
steel fitted with a ceramic crucible was attached to the balance to hold sample.
For the each run, miscanthus sample of about 5g was placed inside the crucible. The reactor
was then heated to 260°C with the heating rate of 10°C/min and was maintained for 30mins.
During torrefaction experiment, to prevent combustion, the reactor was continuously purged with
the nitrogen gas (10ml/min) to maintain inert atmosphere inside the reactor. The nitrogen gas
was passed through the pre-heater (temperature controlled) before purging inside the reactor.
After the desired reaction period, the sample was cooled down to room temperature with the
continuous flow of nitrogen through the reactor. The remaining solid sample was collected and
placed inside air the sealed bag plastic till analysis.
Figure3.3 Torrefaction experimental Setup
(Acharya & Dutta, 2013)
38
3.1.3. Analytical Methods for Solid Samples
3.1.3.1.
Proximate and Ultimate analysis
The proximate analysis (ash, volatile matter and fixed carbon) of the solid samples was
carried out as per ASTM standards. The measured amount of sample was placed in the muffle
furnace (Thermo Scientific- F48055-60, Waltham, MA) maintained at 103±2°C for at least 16hrs
and was then moved to desiccator, containing silica gel, for cooling. The samples were then reweighed and the change in initial and final weight was expressed as percentage moisture
(ASTM-E871). The dried solid samples were then ignited at 575°C for 5hrs in a muffle furnace
to determine percentage ash (ASTM-E1755). The volatile matter of solid samples was measured
by firing at 950°C for 7mins (ASTM- E872). The remaining percentage was reported as fixed
carbon. The ultimate analysis (carbon (C), hydrogen (H), nitrogen (N), sulphur (S), and Oxygen
(O)) of solid samples was measured using elemental analyzer (Thermo Fisher Flash EA 1112,
Waltham, MA).
3.1.3.2.
Higher Heating Value
The higher heating values (HHV) of raw and pre-treated solid samples were determined
using IKA-C200 bomb calorimeter (Wilmington, NC) (Figure 3.4). All the samples were oven
dried at 105°C for 24hrs prior to analysis. Measured amount of the sample (approximately 0.5g)
was placed in the steel container (decomposition vessel) fitted with a ceramic crucible (sample
holder). The vessel was pressurized with the pure oxygen to 30 bars and the sample was then
ignited with a cotton thread connected to an ignition wire in the decomposition vessel. The
constant value for the temperature of water was recorded, both before and after ignition of
sample. The change in the temperature of water was then used to calculate the HHV of samples
(as shown in equation 1).
39
Where:
m = Mass of the sample
C= Heat capacity of the bomb calorimeter
ΔT= Change in Temperature of water, before and after ignition
QE= Correction value for the heat energy generated by the cotton thread during ignition
Figure3.4 IKA Bomb Calorimeter
3.1.3.3.
Hydrophobicity
Equilibrium moisture content (EMC) is defined as the moisture content of the sample that
is in thermodynamic equilibrium with the moisture of the surrounding atmosphere at a given
relative humidity, temperature, and pressure. To determine EMC of the solid samples in this
study, both raw and pre-treated samples were exposed to the rooms maintained at two different
humidity levels: 48-52% and 74-78% at 22-23°C. After exposure of 24 hours to the controlled
40
environment, the samples were then dried in an oven at 103°C for 16hours. The change is weight
was then expressed as the percentage moisture adsorbed (Equation 2)
Where:
MC= Moisture content of the sample
Mf = Final mass of the sample
Mdry = Mass of the oven dried sample
3.1.3.4.
Grindability
Owing to the high fibrous nature, pulverization of biomass is a highly energy intensive
process. The torrefaction pre-treatment has been widely discussed as a solution to improve the
grindability of biomass. To evaluate the performance of HTC on pulverization, grinding of the
pretreated solid samples was performed using planetary ball mill, Retsch PM-100 (Newtown,
PA). Prior to grinding, the samples were sieved to particle size range of +850µm to -1000µm.
For each run, measured amount of sieved samples were loaded in the grinding bowl with five
stainless steel balls (ø10mm). The bowl was then placed in the ball mill operated at 420 rpm for
5mins.
The milled samples were collected and further evaluated for particle size distribution
(PSD) using Retsch AS-200 (Newtown, PA) sieve shaker, with four different sieves (>500mm,
500–250mm, 250–100mm, and <100mm). The PSD was determined by the percentage weight
retained on each sieve. The higher the percentage weight passes through the smallest sieve, the
easier the grindability. For comparison, raw feedstock samples and commercial coal samples
were also milled using same procedure and were measured for PSD.
41
Figure3.5 Planetary Ball Mill and Accessories (Retsch-PM 100)
Figure3.6 Sieve-Shaker (Retsch-PM 100)
42
3.1.3.5.
Fiber Analysis
Generally, biomass is divided in the five main components: hemicellulose, cellulose,
lignin, aqueous soluble, and ash. A chemical analytical method was used to determine the
percentage of hemicellulose, cellulose, lignin, and aqueous soluble materials in the raw and
pretreated samples (Harper & Lynch, 1981). Similar method was also used by an another study
to examine the torrefaction behavior of agricultural biomass (Sule, 2012). Ash in the pre-treated
samples was determined as per ASTM standard (E1755). Prior to the analysis, each pre-treated
sample was dried overnight at 103°C.
3.1.3.6.
Inorganic Metals Analysis
The presence of few alkali and alkaline earth metals in biomass is the biggest challenge
when using it as a fuel source. To determine the effect of thermal pre-treatment on the alkali and
alkaline earth metal composition of biomass, each sample was analyzed for the inorganic
elemental composition (Anderson, 1996). The testing was performed at University of Guelph
laboratory services using a microwave digestion system, CEM-Mars Xpress (Matthews, NC) and
a Varian-Vistra-pro (Palo Alto, CA) inductively coupled plasma atomic emission spectroscopy
(ICP-OES) system. The samples were homogenized and microwave digested with nitric acid in a
closed vessel. The microwave digested samples were brought to the volume with nano pure
water. The clear extract supernatant was measured by the ICP-OES with a cyclonic spray
chamber and sea-spray nebulizer.
3.1.3.7.
Scanning Electron Microscopy (SEM) Analysis
Both raw and pre-treated samples were mounted onto the surface of a standard aluminum
SEM stub using a carbon tape. The samples were then placed into a Cressington 108Auto sputter
coater where a 10-nm gold film was deposited on the surface of sample to provide a conductive
43
coating. The samples were then placed in a scanning electron microscope, FEI Inspect S50
(Japan). The images were collected at an accelerating voltage of 20 kV with an aperture of 3.5
using an ETD secondary electron detector.
3.1.3.8.
Brunauer–Emmett–Teller (BET) Analysis
Samples were placed in the degassing module of the Quantachrome 4200e Surface Area
and Pore Analyzer. The samples were then vacuum-degassed at the room temperature for at
least 3 hours prior to the analysis. All the samples were reweighed to account for the mass loss
(if there is any).
The degased samples were then placed in the analyzer for multipoint BET analysis. The
tubes were calibrated before each run using helium gas. The adsorption isotherm was recorded
for the pressures ranging from 0.03 < P/Po < 0.4 using nitrogen as an adsorbate gas at liquid at
the liquid nitrogen temperatures. The surface area was then calculated using the BET region of
the adsorption isotherm where the cross-sectional area of the condensed N2 gas was assumed to
be 16.2Å.
44
3.2.
Results and Discussions
3.2.1. Effect of different operating parameters on biomass
Table 3.1 shows the experimental design that was employed to study the effect of three
different operating parameters: reaction time, reaction temperature, and ratio of feedstock to
water during HTC and their effect on the physicochemical properties of miscanthus. The values
reported in the Table 3.1 are the mean values of triplicate experiments with a maximum standard
error value shown in bracket for each parameter. Mass yield (Eq.2), energy densification ratio
(Eq.3), and energy yield (Eq.4) are the three important parameters that were measured in the
study and are expressed as:
……..
(2)
……..
(3)
… (4)
An analysis-of-variance (ANOVA) method was used to analyze the data (Table 3.1) and
modelling the HTC process to examine the effect of operating conditions (reaction temperature
(T), reaction time (t) and solid load (R)) on the mass yield, energy yield, and HHV of hydrochar.
The results are presented in the Table 3.1 using statistical analysis (ANOVA, at 95% confidence
interval), it was found that the reaction temperature remains the most significant parameter
controlling the properties of biomass. Reaction time is the second most affecting parameter.
However, no significant effect was found for varying the solid load from 1:6 to 1:12. The
statistical analysis of the data using a model with the interaction term showed no-significant
effect of the interactions among variables (except for the reaction time and temperature). Due to
45
the significant effect of reaction temperature, the further physicochemical analysis was
performed for the samples produced at different temperatures (190°C, 225°C, and 260°C) for
5mins reaction time and solid load of 1:6.
Table3.1 Experimental Design for Miscanthus Biomass Feedstock
Mass Yield
(%)
Operating
Parameters
HHV
(MJ/kg)
Feedstock to Water
Ratio
Feedstock to Water
Ratio
Energy Yield
Feedstock to Water
Ratio
Temperature
(°C)
Time
(mins)
1:6
1:12
1:6
1:12
1:6
1:12
190
5
83.5 ±0.6
85.8 ±0.7
19.9 ±0.1
19.3 ±0.1
90.2
90.1
190
15
77.8 ±0.7
69.7 ±0.1
20.7 ±0.3
20.6 ±0.3
87.2
78.0
190
30
72.5 ±0.8
66.6 ±0.2
21.2 ±0.3
21.5 ±0.5
83.4
77.2
225
5
66.8 ±1.3
65.5 ±1.6
21.4 ±0.1
21.4 ±0.2
77.6
76.0
225
15
63.9 ±0.6
61.1 ±0.4
23.3 ±0.5
23.0 ±0.8
80.5
76.5
225
30
62.4 ±1.1
57.5 ±0.2
23.8 ±0.7
24.3 ±0.2
80.5
75.9
260
5
47.8 ±0.6
44.9 ±1.1
25.7 ±0.3
25 ±0.2
66.5
60.6
260
15
45.9 ±0.9
44.0 ±0.9
29.4 ±0.6
28.6 ±0.4
73.1
68.2
260
30
44.9 ±0.3
42.8 ±1.2
30.3 ±0.6
31.3 ±0.4
73.7
72.5
Raw Miscanthus: 18.47MJ/kg
Similar results were also reported by an another study (Reza, 2011) during which no
significant effect of the reaction time was reported and therefore, at first glance, contradicts the
findings in this study. This contradiction is most possible due to the difference in levels of
reaction times used in the experiments. The present study was performed using three different
levels of reaction times (5, 15, 30mins) where only two different levels (5 and 20mins) were used
in the experiments performed by Reza, (2011).
46
3.2.1.1.
Effect on mass yield in HTC
Figure 3.7 shows the effect of reaction time and temperature on the mass yield of
different biomass feedstocks during HTC. As expected, the mass yield considerably reduces with
an increase in the reaction time and temperature, of which reaction temperature remained the
most prominent factor. Among all the three biomass feedstocks considered in this study,
miscanthus showed the lowest mass yield. Generally biomass with the high hemicellulose
composition in it show higher mass loss due to least thermal stability among other polymers in
biomass (Garrote et al., 1999). To confirm the validity of aforementioned statement, the fiber
analysis of three biomass feedstocks was performed and the results are shown in the Table 3.2. It
was found that the miscanthus has the highest percentage of hemicellulose and aqueous
composition in it and therefore shows the high mass loss compare to the wheat straw and willow
feedstock.
Table3.2 Fiber-analysis of raw miscanthus, wheat straw and willow
Feedstock
Hemicellulose
(%)
Cellulose
(%)
Lignin
(%)
Hot water
Extractives
(%)
Ash
(%)
Miscanthus
36.30
38.80
11.50
12.60
0.80
Wheat
Straw
26.82
49.09
10.66
8.75
4.67
Willow
31.50
49.05
14.73
3.32
1.40
47
5min
15min
30min
100
Mass Yield (%)
90
80
70
60
50
40
190
225
Miscanthus
260
190
225
260
Wheat Straw
Temperature (°C)
Feedstock
190
225
260
Willow
Figure3.7 Effect of reaction time and temperature on Mass yield of different feedstocks
3.2.1.1.1.
Optimization of Mass Yield in HTC
Using the linear model relationship and ANOVA method for the mass yield data shown
in the Table 3.1 as a function of reaction temperature (T), time (t), and ratio of feedstock to water
(R), the statistical equation (Eq. 5) was derived with a R-square value of 0.948. The applicability
of this derived model is highly consistent with the findings in literature on the HTC of
miscanthus (Reza et al., 2013), however, the results are not in correlation with the literature on
other types of feedstock like corn stover, rice husk, pine, tahoe mix, sugarcane bagasse, coconut
fibre etc. (Hoekman et al., 2013; Liu et al., 2013; Reza et al., 2013; Xiao et al., 2012); suggesting
that the degradation in the HTC process significantly depends upon the type of feedstock. This
consequence is due to the fact that different feedstocks have different percentages of polymeric
composition (i.e. fraction of cellulose, hemicellulose, and lignin) in it. A derivation of a
48
statistical model based on the wide range of biomass feedstock and operating conditions might
be very helpful for an industry in predicting the physicochemical properties of hydrochar for the
different applications.
……….
(5)
The strength of the effect of HTC operating conditions on the mass yield of miscanthus is
better represented by a cube plot (Figure 3.8) and a response surface graph (Figure 3.9) using
model predicted values (Equation 5). The lack-of-fitness between actual experimental mean
values and model predicted values are shown in the Figure 3.10. From the Figure 3.8 it can be
noticed that with an increase in the reaction temperature from 190°C to 260°C, the mass yield of
miscanthus considerably reduces by 31% at the reaction time of 5mins and 30mins. Whereas, on
the other hand (keeping the reaction temperature constant), with an increase in the reaction time
from 5 to 30mins, the mass yield of the miscanthus only reduces by 7.6% at 190°C and 260°C.
No significant effect (<3%) was observed for the mass yield with an increase in the solid load
ratio.
49
Figure3.8 Model Predicted Cube plot for Mass Yield vs Reaction time, temperature and
ratio
Figure3.9 Response Surface plot for Reaction time and temperature vs Mass Yield at mean
solid load factor (1:9)
50
Figure3.10 Model predicted and actual experimental values plot for the mass yield
3.2.1.2.
Effect on HHV and energy yield in HTC
Figure 3.11 and 3.12 shows the effect of reaction time and temperature on the HHV and
energy yield of the different biomass feedstocks under HTC at a solid load ratio of 1:6. As
expected, the HHV of the biomass feedstock increases with an increase in the reaction time and
temperature.
51
5min
15min
30min
32
HHV (MJ/Kg)
30
28
26
24
22
20
18
190
225
260
Miscanthus
190
225
260
190
Wheat Straw
Temperature(°C)
Feedstock
225
260
Willow
Figure3.11 Effect of reaction time and temperature on HHV of different feedstocks
5min
15min
30min
100
Energy Yield (%)
95
90
85
80
75
70
65
60
190
225
Miscanthus
260
190
225
260
Wheat Straw
Temperature (°C)
Feedstock
190
225
260
Willow
Figure3.12 Effect of reaction time and temperature on energy yield of different feedstocks
52
In terms of the HHV of biomass polymers, lignin has the highest heating value and is in
the range of 23.3-26.6 MJ/kg, followed by cellulose and hemicelluloses having HHV of about
17.6-17.9MJ/kg (Demirbaş, 2005). The thermal stability of biomass polymers from least to most
follows the order: Hemicellulose<Cellulose<Lignin (refer section 2.1.1). In the hydrothermal
process, due to the presence of subcritical water, the degradation of hemicellulose is rapid and
takes place at a fairly low temperature compare to in the conventional dry pre-treatments like
torrefaction, pyrolysis, gasification etc.
Moreover, under subcritical water conditions, the degradation of hemicellulose and
cellulose results into the formation of intermediate compounds like monomers, furfurals, and 5Hydroxyl Methyl Furfural (HMF) (Reza et al., 2013). 5-HMF has several value-added
applications in biochemical industry (Funke & Ziegler, 2010; Verevkin et al., 2009). The
formation of 5-HMF particles was confirmed during characterization of HTC process water (data
not shown). The HHV of 5-HMF is about 22 MJ/kg, which is significantly higher than that of
hemicellulose and cellulose, and is close to that of lignin (Reza et al., 2014a). Small porous
structures are often obtained during hydrothermal conversion of biomass and these 5-HMF
particles may precipitate in the pores of hydrochar, further augmenting the energy densification
of hydrochars (Reza et al., 2013). These intermediate particles are characterized as aqueous
soluble materials during fiber analysis.
Hence, a feedstock with the high percentage of hemicellulose composition in it tends to
show high mass loss and high increase in the HHV. This phenomenon explains the reason behind
the unexpected high mass loss and high HHV observed in case of the miscanthus when compare
to the wheat straw and willow wood. The degradation of hemicellulose was found to increase
with an increase in the reaction temperature and almost complete degradation of hemicellulose
53
can be observed in the case of hydrochar produced at 260°C (Table 3.3). In biochemical industry,
5-HMF is used as a product for the production of 2,5 Dimethyl-furan (DMF), whose heating
value is significantly higher (33.7 MJ/kg) than that of ethanol (26.9 MJ/kg) (Zhong et al., 2010).
The HHV of hydrochar samples increases where the energy yield reduces with an
increase in the reaction temperature. An increase in the reaction temperature from 190°C to
260°C, at the residence time of 5min, the HHV of miscanthus samples increased from 19.93 to
25.72 MJ/kg, corresponding to the energy densification ratio of 1.08 to 1.39, respectively.
However, at the same reaction conditions, the energy yield of pre-treated miscanthus samples
reduced from 90.16% to 66.48%. The significant drop in the percentage energy yield of
miscanthus samples is strongly correlated with the considerable reduction in the mass yield from
83.53 (at 190°C) to 47.83% (to 260°C). With the further increase in the severity of operating
conditions, i.e. the highest reaction temperature (260°C) and highest residence time (30mins), the
combustion properties (in terms HHV) of hydrochar were surprisingly improved.
3.2.1.2.1.
Optimization of HHV and energy yield in HTC
The relationship between the HHV data shown in the Table 3.1 as a function of reaction
time (t), temperature (T) and solid load or ratio of water to feedstock (R), was better represented
by ANOVA model with an interaction term. A statistical equation (Equation 6) was derived with
an R-square value of 0.938. The strength of the operating parameters on the HHV is represented
by a cube plot and response surface graph, shown in the Figure 3.13 and Figure 3.14,
respectively. The lack-of-fitness between the actual experimental mean values and model
predicted HHVs is shown in the Figure 3.15. From the Figure 3.13 it can be observed that with
an increase in the reaction temperature from 190 to 260°C, the HHV of miscanthus hydrochar
samples increases by 31% (19.68 to 25.76 MJ/kg) at the reaction time of 5mins and by 48%
54
(20.46 to 30.26 MJ/kg) at the reaction time of 30mins. On the other hand (keeping the reaction
temperature constant), with an increase in the reaction time from 5 to 30mins, the HHV of solid
samples only increases by 4% (19.68 to 20.46MJ/kg) at 190°C and by 17% (25.76 to 30.26
MJ/kg) at 260°C. No significant effect was observed on the HHV of hydrochar samples with an
increase in the feedstock to water ratio (R). The result from the model suggests that the HHV of
the feedstock is governed by operating parameters in the order of: Temperature > Time> Solid
load.
……….
(6)
Figure3.13 Mean Model for Reaction time, temperature and ratio vs HHV
55
Figure3.14 Response Surface plot for Reaction time and temperature vs HHV at mean
solid load factor (1:9)
Figure3.15 Predicted and Actual values plot for HHV
Using an ANOVA model with the interaction terms for energy yield using data shown in
the Table 3.1 as a function of reaction time (t), temperature (T) and solid load or ratio of water to
56
feedstock (R) a statistical equation (Equation 7) is derived with an R-square value of 0.931.
Using the model predicted values (Equation 7) the strength of the operating parameters on the
energy yield of miscanthus feedstock is represented by a cube plot and response surface graph
shown in the Figure 3.16 and Figure 3.17, respectively. The lack-of-fitness between the actual
experimental mean values and model predicted HHV values are shown in the Figure 3.18. Unlike
the trend in the mass yield and HHV of hydrochar, the energy yield of hydrochar samples
decreases with an increase in the reaction time at low temperature and increases with an increase
in reaction time at high temperature. This shows that the effect of reaction time on the HHV of
hydrochar samples is more prominent at high reaction temperature than at low reaction
temperature. In contradiction to the expected low energy yield with an increase in residence time
at higher temperatures, a longer residence time would increase the process severity and therefore,
significantly augments the overall energy yield of the hydrochar. This phenomenon is due to the
continuing polymerization of aqueous soluble products in the liquid phase which may precipitate
in the porous structure of insoluble hydrochar (Funke & Ziegler, 2010). Similar to the results of
the mass yield and HHV, the energy yield of solid product remains relatively unaffected with
change in the feedstock to water ratio.
……… (7)
57
Figure3.16 Mean Model for Reaction time, temperature and ratio vs Energy Yield
Figure3.17 Response Surface plot for Reaction time and temperature vs Energy Yield at
mean solid load factor (1:9)
58
Figure3.18 Predicted and Actual values plot for Energy yield
3.2.1.3.
Comparison of mass yield in HTC and torrefaction process
For the comparison of HTC with torrefaction, the effect of reaction temperature on the
mass yield of the solid product produced in both the conversion processes is shown in the Figure
3.19. At the same reaction temperature, mass loss of miscanthus feedstock under torrefaction is
considerably lower even if the process was carried out at significantly higher residence time
(30mins) compared to the one used for HTC (5min). The difference in the mass loss of the same
feedstock in the two different pre-treatments is mainly due to the variation in the extent of
degradation of hemicellulose. The extent of degradation of the biomass polymers significantly
depends upon the reaction medium in which the process is carried out (Libra et al., 2011). The
results for the fiber analysis of raw, hydrochar, and torrefied samples are shown in the Table 3.3.
It can be noticed that the degradation of hemicellulose in the HTC process is considerably higher
and faster than under torrefaction process, even if the severity of reaction was kept significantly
higher in torrefaction (260°C-30mins) compared to in HTC (190°C-5mins).
59
Decarboxylation, dehydration, de-carbonylation, de-methoxylation, intermolecular
derangement, condensation and aromatization are some of the proposed chemical reactions that
take place during the thermal pre-treatment of biomass (Funke & Ziegler, 2010). However, in the
HTC process due to the presence of the hot compressed water, the reaction mechanism is
initiated by hydrolysis; resulting into the cleavage of ether and ester bonds between monomeric
sugars by the addition of one molecule of water (Bobleter, 1994) and thereby reducing the
activation energy level of a biomass polymer (Libra et al., 2011). Hence, the degradation of
hemicellulose in the HTC process is relatively higher compare to in the torrefaction process.
Table3.3 Fiber-analysis of Raw and Pretreated Miscanthus Samples
Pre-treatment
Feedstock
Condition
Hemicellulose
(%)
Cellulose
(%)
Lignin
(%)
Hot water
Extractives
(%)
Ash
(%)
-
Raw
36.30
38.80
11.50
12.60
0.80
190°C
5.77
56.94
15.61
21.14
0.54
225°C
5.11
53.38
17.75
23.08
0.68
260°C
0.97
27.50
30.57
39.99
0.97
260°C
21.46
36.21
41.31
6.20
1.02
HTC
Torrefaction
3.2.1.4.
Comparison of HHV and energy yield in HTC and torrefaction process
In the torrefied miscanthus very small percentage of hemicellulose degradation was
observed and significant portion of hemicellulose still remained in the solid product. In addition,
very small percentage of aqueous soluble materials was characterized during the fiber analysis
(Table 3.3). This explains the reason behind the lower mass loss and lower HHV of the solid
product obtained via torrefaction compare to the one via HTC (Figure 3.19). For the same energy
yield the pre-treatment condition requirements, i.e. reaction time and temperature, were
60
considerably lower for HTC (190°C-5mins) when compared to torrefaction (260°C-30mins).
Even though the lignin composition of the torrefied miscanthus is higher than the hydrochar
samples, still its HHV is not high. This could be related to the high hemicellulose and low
aqueous soluble material composition. However, the high pressure requirement, efficient heat
recovery, and the issue of HTC process water still remains the huge challenges for a typical HTC
plant. Recirculation of the HTC process water has been suggested as one of the most optimal
solution to improve system’s overall efficiency (Stemann et al., 2013b).
Energy Yield
HHV
100
27
90
26
25
80
24
70
23
60
22
21
50
HHV (MJ/Kg)
Mass Yield (%) / Energy Yield (%)
Mass Yield
20
40
19
30
18
190
225
260
HTC-5mins
260
Torrefaction30mins
Temperature (°C)
Pre-treatment Process and Reaction Time
Figure3.19 Comparison of Mass yield, energy yield and HHV in HTC and Torrefaction
3.2.2. Proximate and ultimate analysis of hydrochar samples
The proximate and ultimate analysis of raw, hydrochar and torrefied miscanthus samples
are summarized in the Table 3.4. The proximate analysis is a most common method used to
61
determine the quality and rank of a coal and other solid fuels. Biomass generally has a very high
volatile matter content and therefore results in the low combustion efficiency and high harmful
emissions when combusted directly (Khan et al., 2009). Moreover, co-firing of biomass with
coal is highly problematic due to the extreme difference in their bulk and energy density, volatile
matter content, and combustion temperatures (Khan et al., 2009). The result shows that the
fraction of volatile matter decreases and percentage fixed carbon increases with an increase in
the HTC reaction temperature. Hydrochars produced at low temperature (190 and 225°C) shows
similar behavior to that of the raw biomass. Similar performance was also observed for the
torrefied miscanthus, even if the reaction conditions were kept significantly high (260°C for
30mins). However, the hydrochar sample produced at 260°C with the residence time of 5mins
shows considerable reduction in the volatile matter and increase in percentage of fixed carbon.
High inorganic elemental composition in the biomass ash causes challenging issues like
fouling and slagging in the boilers which significantly affects the thermal efficiency of a system.
In addition, the high alkali content can lead to the corrosion of boiler tubes. Ash yield is defined
as the ratio of percentage ash in the hydrochar to the ash in the raw biomass multiplied by the
mass yield at corresponding pre-treatment temperature (Reza et al., 2013). As for the hydrochar
samples, significant reduction in the ash yield was observed with an increase in the reaction
temperature. The reduction in the ash yield is directly correlated with the removal of inorganic
elementals (reduction by 20-70%) from hydrochar into the liquid by-product streams (discussed
in section 3.2.5). These observations are consistent with the findings reported in literature (Liu et
al., 2013; Reza et al., 2013; Yan et al., 2009), and suggests that the hydrochar samples produced
at the high temperature can be co-fired with coal without significant modifications to the system.
62
Table3.4 Proximate and Ultimate analysis of raw and pretreated biomass
Ultimate Analysis
Feedstock
Willow
Wheat
Straw
Material
Time
(mins)
Raw
Proximate Analysis
C
(%)
H
(%)
N
(%)
S
(%)
O
(%)
VM
(%)
FC
(%)
Ash
(%)
-
47.81
6.07
0.52
0.00
44.55
85.64
12.97
1.40
HTC-190°C
5
51.19
5.98
0.17
0.00
41.36
82.73
16.09
1.18
HTC-225°C
5
53.41
5.98
0.15
0.00
39.47
78.96
19.70
1.34
HTC-260°C
5
58.60
5.69
0.50
0.00
29.32
72.66
25.77
1.57
HTC-260°C
30
67.43
5.27
0.55
0.00
24.72
-
-
-
Raw
-
46.33
5.59
0.00
0.00
43.72
80.04
15.30
4.65
HTC-190°C
5
46.37
5.75
0.05
0.00
43.43
79.86
15.36
4.79
HTC-225°C
5
49.57
5.49
0.14
0.00
36.43
73.46
21.05
5.49
HTC-260°C
5
61.73
5.27
0.39
0.00
30.96
61.37
32.51
6.12
HTC-260°C
30
75.18
5.05
0.45
0.00
13.36
-
-
-
Raw
-
46.66
6.00
0.21
0.00
45.34
87.51
11.69
0.80
HTC-190°C
5
48.76
5.96
0.20
0.00
44.70
83.84
15.66
0.51
HTC-225°C
5
49.62
5.92
0.28
0.00
41.83
81.86
17.46
0.69
HTC-260°C
5
61.20
5.27
0.38
0.00
31.57
68.91
30.27
0.82
HTC-260°C
30
81.53
4.85
0.14
0.00
13.96
-
-
-
Torrefaction260°C
30
49.55
5.71
0.12
0.00
42.24
84.81
14.25
0.94
Miscanthus
Ultimate analysis of both raw and pre-treated miscanthus samples is shown in the Table
3.4. As expected, the percentage carbon increases and percentage oxygen decreases with an
increase in the HTC reaction temperature. The ultimate analysis of miscanthus torrefied at 260°C
does not show considerable improvement in the carbon content and similar results were observed
for the hydrochars produced at low HTC temperature (190 and 225°C). On the other hand
63
significant augmentation in the percentage carbon and reduction in percentage oxygen can be
noticed in case of the hydrochar sample produced at 260°C. To analyze the variation in the
elemental composition, the atomic ratios of hydrogen to carbon (H/C) and oxygen to carbon
(O/C) of both raw and pre-treated samples are illustrated in Figure 3.20, the plot called van
Krevelen diagram (van der Stelt et al., 2011).
Figure3.20 Atomic H/C-O/C ratios of raw and pretreated miscanthus (M), willow (W) and
wheat straw (WS) samples
The “van Krevelen diagram” provides the general information about the atomic ratios
(O/C-H/C) of typical fuels like biomass, peat and different types of coals. A fuel with the low
O/C-H/C atomic ratios is considered highly favorable because of the low smoke, low water
vapor, and low energy loss during the combustion (Liu et al., 2013). It is clear that biomass has
low carbon content and high oxygen content compared to coal. Due to the removal of water and
64
carbon dioxide, the pretreated biomass has low O/C-H/C atomic ratios. Depending upon the
degree (i.e. the reaction time and temperature) and the type of a pre-treatment, the chemical
composition and properties of a product can be far different from coal. Like in case of the
torrefied (260°C) and HTC (190°C and 225°C) miscanthus, the O/C-H/C ratios of the fuel are
reduced but are still similar to that of the raw biomass. Whereas for the hydrochar produced at
260°C, the atomic ratios are close to that of lignite. For the hydrochar samples obtained at
extremely severe operating conditions, i.e. the highest reaction temperature (260°C) and
highest reaction time (30mins), the combustion properties were surprisingly improved and their
position on the van Krevelen diagram was found to be in the range of high rank coal. The main
reason behind the strong carbonization in HTC compare to the torrefaction process is due to the
presence of water as a reaction medium, leading to the initiation of hydrolysis reaction
mechanism which exhibits a low activation energy level than the other decomposition reactions
(Libra et al., 2011). Generally the pyrolysis process of a biomass leads to the biochars with a
high degree of carbonization (Kambo & Dutta, 2014a; Libra et al., 2011). However, depending
upon the initial moisture content of a feedstock, the use of dry thermal pre-treatment processes
may be highly uneconomical and senseless (Libra et al., 2011).
3.2.3. Effect of HTC reaction temperature on grindability of hydrochars
A thermal pre-treatment can alter the physical structure of biomass and makes it much easier to
grind compared to raw wood (Bridgeman et al., 2010). Figure 3.21 shows the effect of the HTC
reaction temperature on the grindability of miscanthus, wheat straw, and willow samples. The
result indicates that the pretreated samples are much easier to grind compared to the raw
feedstock, moreover, the ease of the grindability increases with an increase in the HTC reaction
65
temperature. These finding are directly co-related with the results from fiber analysis and SEM
images.
The removal of hemicellulose via degradation and depolymerisation reactions during the
thermal pre-treatment of a biomass results into the loss of structure (Funke & Ziegler, 2010). The
destroyed structure has improved friability. The structures of the raw and hydrochar samples
observed under Scanning electron microscope (SEM) are shown in the Figure 3.22 and Figure
3.23. The images show the variation in the structure a hydrochar with an increase in the process
reaction temperature. For the raw feedstock, a clearly well-defined structure can be observed.
However, in case of the hydrochar samples the unwrapping and rupture of polymer bundles and
opening of pores is clearly noticeable. Almost complete loss of the structure can be observed for
the samples pre-treated at 260°C. The pulverized hydrochar samples has been reported to show
spherical shaped particles that may further facilitate the fluidization process during gasification
of biomass (Tremel et al., 2012).
66
500µ-250µ
190C
260C
250µ-100µ
<100µ
260C
190C
225C
Wheat starw
Raw
Miscanthus
260C
225C
190C
Raw
225C
100
90
80
70
60
50
40
30
20
10
0
Raw
Percentage mass Retained (%)
>500µ
Willow
Temperature (°C)
Feedstock
Figure3.21 Effect of reaction temperature on grindability of raw and HTC pre-treated
samples of Miscanthus, Wheat straw and Willow
Figure3.22 SEM images of wheat straw samples: (A) Raw, (B) 190°C, (C) 225°C, and (D)
260°C
67
Figure3.23 SEM images of willow samples: (A) Raw, (B) 190°C, (C) 225°C, and (D) 260°C
Results for the BET surface area of hydrochar samples produced from wheat straw and
willow are shown in the Table 3.5. As expected, the surface area of the hydrochar samples was
not affected by the HTC process temperature. The pressure inside the HTC reactor causes the
destruction of biomass structure; resulting into a low surface area. However, fine carbon particles
are produced during HTC that explains the slight increase in the surface area of hydrochar
samples (Kambo & Dutta, 2014a). The surface area of raw wheat straw was found significantly
low and is mainly due to the smooth surface as observed under high resolution SEM images.
However, slight rough surface was observed for the wheat straw hydrochar samples (Figure
3.24). Where, for the willow hydrochar samples, similar roughness can be observed even at high
resolution SEM (Figure 3.25).
68
Table3.5 BET-Surface area of raw and HTC pretreated willow and wheat straw
Surface Area (m2/g)
Samples
Wheat Straw
Willow
Raw
0.313
2.647
HTC-190°C
1.034
3.603
HTC-225°C
6.63
3.378
HTC-260°C
5.261
3.236
Figure3.24 High Resolution SEM images of wheat straw samples: (A) Raw, (B) 190°C, (C)
225°C, and (D) 260°C
69
Figure3.25 High Resolution SEM images of willow samples: (A) Raw, (B) 190°C, (C) 225°C,
and (D) 260°C
3.2.3.1.
Comparison of grindability for coal, torrefied and HTC pre-treated samples
Figure 3.26 shows the particle size distribution of the milled raw, hydrochar, and
torrefied miscanthus samples and its comparison with the coal. The data indicates that the
fraction of fine particles of miscanthus increases in both the thermal pre-treatments and therefore
a pretreated sample is easier to grind as compared to the raw feedstock. However, the hydrochar
samples have the pulverization and heating properties significantly higher than the torrefied
miscanthus. Moreover, the properties of the miscanthus hydrochar samples (produced at 260°C)
are similar to that of coal. The grindability (percentage fraction of fine particle) of hydrochar
samples increases with an increase in the HTC reaction temperature.
The process operating conditions and the type of a reaction medium used in the
thermochemical pre-treatment plays significantly important role in determining the dominancy
70
and the rate of reaction mechanism (Libra et al., 2011). Due to the presence of water as a
reacting medium in the HTC process, the reaction mechanism is initiated by hydrolysis. The
activation energy level of hydrolysis is lower than the other dry thermal pre-treatment
decomposition reactions. Therefore, the degradation and depolymerisation of biomass polymers
in the HTC process takes places at fairly low temperature when compared to in the torrefaction
(Libra et al., 2011). This phenomenon explains the reason behind the significant variance in the
grindability of a torrefied and HTC pre-treated miscanthus. The SEM images of raw, torrefied
and HTC pre-treated samples are shown in Figure 3.27. For the raw miscanthus, a clearly welldefined structure can be observed. Where in case of hydrochar samples, the rupture of pores is
noticeable at low temperatures, where almost complete loss of structure can be observed for the
sample obtained at 260°C. However, for the torrefied miscanthus, due to volatilization of
hemicellulose, the size of the pores is slightly increased while the structure still remained very
similar to that of the raw biomass.
71
100-250µm (%)
250-500µm (%)
>500µm (%)
HHV
100
90
80
70
60
50
40
30
20
10
0
28
27
26
25
24
23
22
21
20
19
18
190
Raw
225
HTC-5mins
260
HHV (MJ/kg)
Particle Size Distribution (%)
<100µm (%)
260
Torrefied30mins
Coal
Temperature (°C)
Pretreatment type and operating conditons
Figure3.26. Particle size distribution for raw and pretreated miscanthus samples and its
comparison with coal
72
Figure3.27 SEM images of miscanthus samples: (A) Raw, (B) HTC-190°C, (C) HTC-225°C,
(D) HTC-260°C and (E) Torrefied-260°C
The results for the BET surface area of miscanthus samples are shown in Table 3.6. The
results show that the surface area of hydrochar reduces with an increase in the HTC reaction
temperature, where the surface area is highest for the torrefied miscanthus sample. During dry
thermal pre-treatments, like torrefaction and pyrolysis, the volatilization of hemicellulose and
cellulose takes place leaving behind the small openings on the surface, causing the formation of a
turbostratically arranged layer type structure (Downie et al., 2009). However, for the hydrochar
samples, due to the high pressure inside the reactor, the surface area is usually low but these
hydrochar samples have significantly improved adsorption capacity and are therefore well
suitable for the waste water treatment industries (Kumar et al., 2011; Regmi et al., 2012).
Table3.6 BET-Surface area of HTC and torrefied miscanthus
Miscanthus
Samples
Raw
HTC-190°C
HTC-225°C
HTC-260°C
Torrefied-260°C
Reaction time
(minutes)
5
5
5
30
Surface Area
(m2/g)
11.437
10.724
7.357
4.5
14.189
3.2.4. Effect of HTC operating temperature on inorganic composition of hydrochars
A lignocellulosic biomass has specific amount and different types of inorganic elements
in its structure. Type of soil, growing condition, and harvesting time can significantly affect the
inorganic elemental composition of biomass (Kludze et al., 2013). These inorganic elements are
the alkali and alkaline earth metals such as calcium (Ca), magnesium (Mg), phosphorous (P),
73
potassium (K), sodium (Na), sulfur (S), and iron (Fe) that are left behind in the form of ash
during biomass combustion. In ash, these metals exist in their oxide form (Tortosa Masiá et al.,
2007), CaO, MgO, P2O5, K2O, Na2O, SO3, Fe2O3, that show various undesirable and
notorious effects like slagging, fouling, clinker, and corrosion (Baxter et al., 1998). Generally,
most of the inorganic elements in the lignocellulosic biomass are held in the hemicellulose and
soluble extractives (Miles et al., 1996). Therefore, the removal of hemicellulose from a
lignocellulosic biomass may reduce the inorganic elemental composition.
Under hydrothermal conditions, the degradation and depolymerisation of hemicellulose
and cellulose takes place, causing the solid product with slightly improved porosity (Funke &
Ziegler, 2010). It has been reported that these porous structures may allow the leaching of alkali
and alkaline earth metals that are held in the cross-linked matrix structure of a biomass (Reza et
al., 2013). However, these porous structure of hydrochars have significantly improved adsorption
capacity and might become an alternate to the powered activated carbons (PACs) in the waste
water treatment industries in future (Liu et al., 2010; Regmi et al., 2012; Titirici et al., 2012). To
minimize the effect of the interaction of inorganic elementals, the hydrochar samples were
vigorously washed with acetone and de-ionized water in this study. Furthermore, in the HTC
process due to the formation of acetic acid in the liquid phase, an acid solvation mechanism
would also solubilize and leach out these inorganic elemental compositions (Lynam et al., 2011).
Figure 3.28 and 3.29 shows the effect of HTC reaction temperature on the inorganic yield
of raw and pretreated samples of wheat straw and willow. Inorganic yield is expressed as the
ratio of mass of inorganic content in HTC and torrefied biomass with the mass of inorganic
content in raw feedstock multiplied by the mass yield at a corresponding reaction temperature. It
can be noticed that the HTC process is able to remove the inorganic elemental compositions
74
from raw feedstock by 40-80%, and the removal capacity increases with an increase in the HTC
reaction temperature.
Raw
190 C
225 C
260 C
Magnesium Phosphorous Potassium
Sodium
100
90
Percentage Yield (%)
80
70
60
50
40
30
20
10
0
Calcium
Sulfur
Iron
Figure3.28 Effect of HTC operating temperature on inorganic yield of wheat straw samples
75
Raw
190 C
225 C
260 C
Magnesium Phosphorous Potassium
Sodium
100
90
Percentage Yield (%)
80
70
60
50
40
30
20
10
0
Calcium
Sulfur
Iron
Figure3.29 Effect of HTC operating temperature on inorganic yield of willow samples
3.2.4.1.
Comparison of inorganic impurities in HTC and torrefied samples
Figure 3.30 shows the effect of HTC and torrefaction on the alkali and alkaline earth
metal composition of solid samples. Majority of the inorganic impurities such as Ca, S, P, Mg,
and K in lignocellulosic biomass are present in the hemicellulose. Therefore, the removal of
hemicellulose and leaching of these alkali metals into the liquid phase in the HTC process
explains the demineralization phenomena of hydrochar samples. In comparison, as most of the
hemicellulose remains unaffected in the torrefied miscanthus, therefore most of the alkali and
alkaline metals remained relatively unaffected after torrefaction. However, the concentration
level of alkali and alkaline earth metals were found slightly lower than the expected. This might
be due to the volatile behavior of ash forming elements that plays major role in the gaseous and
aerosol emissions (Obernberger et al., 2006). The unexpected trend observed for the
76
phosphorous may be due to its interaction with the porous structure of hydrochar even after the
vigorous washing. Similar results were observed by another study during the HTC pre-treatment
of corn stover (Reza et al., 2013).
Raw
HTC-190
HTC-225
HTC-260
Torrefied-260
100
90
Inorgnaic Yield (%)
80
70
60
50
40
30
20
10
0
Calcium
Magnesium Phosphorous Potassium
Sodium
Sulfur
Iron
Figure3.30 Inorganic yield for the HTC and torrefied miscanthus samples
3.2.5. Effect of HTC operating temperature on hydrophobicity of hydrochars
Thermal pre-treatments are often proposed to improve the storage behavior or
hydrophobicity of biomass. EMC of a material can be used as an indicator of hydrophobicity.
Presence of high moisture content in feedstock influence the fungal growth and therefore will
most like to rot during storage. Also, the chemical reactions (generally oxidation) or anaerobic
microorganism activity in the biomass feedstock can produce heat at a sufficient rate that it can
cause self-heating of the biomass stockpile which can lead to the self-ignition or other harmful
toxic gaseous emission (Guo, 2013). Therefore, the rate of biodegradation of biomass strongly
depends upon the moisture content and which further depends upon the atmospheric surrounding
77
where it is stored. Tendency to adsorb moisture from air depends upon the lignocellulosic
composition of a biomass (Acharjee et al., 2011).
For a lignocellulosic biomass, the moisture can be adsorbed in two ways: (i) non-bonded
(present in cell wall) and (ii) bonded (hydrogen-bonded to the hydroxyl groups of the cell wall).
The non-bonded moisture increases with an increase in the relative humidity where the bonded
moisture does not, as the structural changes takes place in biomass after thermal pre-treatment.
Among the polymeric composition of biomass, hemicelluloses has the greatest capacity towards
water adsorption; while lignin shows very little tendency for water sorption (Acharjee et al.,
2011). Hence, the removal of hemicellulose (or percentage increase in lignin content) from a
solid product will lower its tendency to adsorb water and will make it a hydrophobic product.
The results for the hydrophobic behavior of raw and pre-treated solid samples of willow
and wheat straw are shown in the Figure 3.31 and Figure 3.32, respectively. As expected, the
hydrophobicity of the HTC pre-treated samples increases with an increase in the reaction
temperature. These finding are strongly correlated with the hemicellulosic composition of
hydrochars characterized under fiber analysis (Table 3.4). Also, the structural loss of solid
samples, as shown in SEM images (Figure 3.24 and Figure 3.25), might have reduced the site
availability for water sorption. A solid material with low moisture content can be stored easily
without possessing any threat to the biological deterioration. The transportation of a such
material will be less expensive as there will be less water content to transport along with the
biomass (Yan et al., 2009).
78
Raw
HTC-190C
HTC-225C
HTC-260C
12
Moisture Content (%)
10
8
6
4
2
0
R.H.=48-52%
R.H.=74-78%
Relative Humidity (R.H.) Level (%)
Figure3.31 Effect of HTC operating temperature on hydrophobicity of willow hydrochar
samples
Raw
HTC-190C
HTC-225C
HTC-260C
12
Moisture Content (%)
10
8
6
4
2
0
R.H.=48-52%
R.H.=74-78%
Relative Humidity (R.H.) Level (%)
Figure3.32 Effect of HTC operating temperature on hydrophobicity of wheat straw
hydrochar samples
79
3.2.5.1.
Comparison of hydrophobicity for HTC and torrefied samples
Figure 3.33 shows the effect of HTC and torrefaction on miscanthus. As expected both the
thermal pre-treatments improved the hydrophobicity of biomass. However, the solid materials
obtained via HTC were more hydrophobic in nature than the one obtained via torrefaction. As
already explained in the above section (3.2.5), the high hydrophobicity for the hydrochar samples
is mainly due to the loss of structure and the degradation hemicellulose composition. However,
unlike to the HTC pretreated miscanthus, due to the volatilization of hemicellulose, slight porous
structures are generally obtained via conventional dry thermal pre-treatments like torrefaction
and pyrolysis (Fuertes et al., 2010), which makes torrefied biomass to adsorb free moisture from
air (non-bonded moisture) rapidly (Stelte et al., 2011b) compare to the hydrochar. Moreover, the
presence of hemicellulose in the torrefied biomass increases tendency to adsorb moisture.
Raw
HTC-190C
HTC-225C
HTC-260C
Torrefied-260C
12
Moisture Content (%)
10
8
6
4
2
0
R.H.=48-52%
R.H.=74-78%
Relative Humidity (R.H.) Level (%)
Figure3.33 Comparison of hydrophobicity for torrefied and HTC pre-treated miscanthus
samples
80
3.3.
Conclusions
Both HTC and torrefaction are the promising methods for upgrading biomass and its
conversion to an energy dense, homogeneous, friable, and hydrophobic solid fuel. However, for
the same mass and energy yield, the operating conditions requirement (i.e. reaction time and
temperature) are significantly lower for HTC when compared to torrefaction. Although both the
thermal pre-treatments reduces the EMC of solid product from that of raw miscanthus, the solid
obtained under HTC process is more hydrophobic in nature and therefore can be stored and
transported easily. Hydrothermally pre-treated biomass is highly friable in nature and is easier to
grind to fine size particles (<100µm) compare to the torrefied biomass. Furthermore, the HTC
process is able to reduce the inorganic elemental composition of raw biomass by 40-80% and
thereby significantly reducing the problems of fouling and slagging during combustion. The
HTC process reaction temperature is the most prominent parameter affecting the extent of energy
densification, grindability, hydrophobicity, and inorganic removal capacity. The results from the
proximate and ultimate analysis shows that the hydrochar samples produced at high reaction
temperature have the combustion properties similar to that of a high rank coal.
Most importantly, as the HTC process is carried out in the presence of water, therefore is
not affected by high moisture content of biomass, it thus eliminates the pre-drying requirement a
feedstock which is a huge energy intensive and financial load in the processes like dry
torrefaction. The result shows that HTC narrows the differences in fuel qualities and converts
biomass to a coal-like product that has potential to replace coal in existing coal-fired power
plants without any further modifications.
81
CHAPTER-IV CHARACTERIZATION AND RECIRCULATION OF HTC LIQUID
BYPRODUCT AND EFFECT OF ADDITION OF SALT AND ACID IN HTC
PROCESS
Previous experiments on HTC of lignocellulosic biomass show highly promising results
and its potential in replacing coal. However, the liquid by-product produced during the HTC
process contains degraded components of biomass polymers that are rich in organic carbon
content and have several valuable applications. Directed disposal of the HTC process water is
not an economical and environmental friendly option.
In this chapter, the liquid by-product from the HTC process was characterized for
different intermediate compounds. The liquid water from HTC was then studied for the process
water recirculation experiments so as to improve the efficiency of a HTC system. Also the effect
of addition of catalyst like salt (calcium chloride) and acid (acetic acid) in the HTC process was
examined. All the outcomes from the recirculation and catalyst addition experiments were
determined in terms of the mass yield, HHV and ash content of hydrochar samples.
82
4.1.
Experimental Section
4.1.1. Materials
Miscanthus feedstock was considered in the present investigation. The samples were
prepared in the similar way as explained in the chapter III.
4.1.2. Experimental Methods
4.1.2.1.
Addition of Salt and Acid to HTC process
Previous results show that the hydrochar samples produced at 260°C have considerably
improved physicochemical properties. However, at such a high temperature the saturation
pressure (720psi) inside the reactor is also high, making the process highly unsafe and expensive.
The addition of catalyst in the HTC process might reduce the temperature requirement.
Therefore, to examine the effect of addition of catalysts in the HTC process at low temperatures
(190 and 225°C), acetic acid (CH3COOH) and calcium chloride (CaCl2) salt was added in the
process. The effect of acetic acid was chosen for investigation, because it is the primary acid
produced in HTC liquid by-product stream (Yan et al., 2010). During coal combustion in the
fluidized bed combustors, calcium based salts are used to capturing sulphur (Laursen et al.,
2001). Hence, to improve the co-firing of hydrochar with coal, CaCl2 salt was used in this
experiment.
For the each run of HTC experiment, the mass ratio was varied as 0 g, 0.25 g, 0.50 g,
0.75 g, and 1 g of acetic acid to every 1g of Miscanthus feedstock. The mass ratio of the calcium
chloride was kept either 0.5 g or 1 g to every 1 g of Miscanthus feedstock. Both calcium chloride
(Anhydrous, ACS certified-100%) and acetic acid (glacial, ACS certified-99.7%) were obtained
from Fisher Scientific. The HTC experiments in this investigation were performed similar to as
explained in the chapter III.
83
4.1.2.2.
HTC process water characterization
To analyze the liquid by-product, the liquid samples from the HTC process produced at
the different reaction temperatures (190, 225, and 260°C) with the residence time of 5min were
collected and stored in the refrigerator until analysis.
4.1.2.3.
Effect of process water recirculation
The results from chapter III indicate that the properties of the hydrochar samples obtained
at 260°C are comparable to that of coal. However, to produce such a material at industrial scale,
the supply of fresh water will be one of the key factors. This demand for fresh water can be
minimized by recirculation of some fraction of the process water after filtration of hydrochar
slurry. Therefore, to analyze the effect of process water recirculation on the mass yield, HHV
and ash content of the hydrochar samples, the liquid collected from the initial run was used in the
next cycle of the HTC experiment. Sufficient amount of makeup water (de-ionized) was added to
bring the total liquid mass to six times the (dry) biomass, according to the procedure described in
the chapter III. An identical procedure was repeated for the 10 recirculation experiments. For
each run, the mass of process water collected was nearly constant, and the makeup water added
in recycle experiments was 17% of the total water required at 260°C.
4.1.3. Analytical methods for solid samples
The physical properties like mass yield, HHV, and ash analysis of the solid samples were
determined in the similar way as described in the Chapter III.
84
4.1.4. Analytical methods for liquid samples
4.1.4.1.
pH
The pH of the liquid samples was measured using a digital probe meter (Thermo Orion
pH meter, model 210A). Each reading was measured twice with an absolute error of ±0.02.
4.1.4.2.
Inorganic Elemental analysis
The inorganic elemental analysis of the liquid samples collected from the HTC
experiments was performed at University of Guelph laboratory services using a CEM-Mars
Xpress microwave digestion system and an inductively coupled plasma atomic emission
spectroscopy (ICP-OES) system.
4.1.4.3.
Total Organic Carbon (TOC)
TOC of liquid samples was measure by a Shimadzu TOC-ASI analyzer at Western
Ontario University, London, Ontario.
4.1.4.4.
Organic Acids and Furfurals
The concentration of organic acids (acetic acid, levulinic acid, glycolic acid, and formic
acid) and HMF was measure by a Dionex Ionic chromatographer system (ICS)-3000 at Western
Ontario University, London, Ontario.
85
4.2.
Results and discussions
4.2.1. Effects of addition of salt and acid
4.2.1.1.
Effects on mass yield of hydrochar samples
Mass yield is defined as the mass ratio of the dried hydrochar to dried raw biomass. The
results obtained from the addition of addition of acid on mass yield of hydrochar are reported in
the Figure 4.1. It can be observed that, at both the reaction temperatures, the mass yield of the
hydrochar sample decrease with an increase in the concentration of acid in water. With the
addition of just 0.25 g of acetic acid per gram of miscanthus, the mass yield of the hydrochar
reduces by 9% at 190°C and 6% at 225°C, respectively. The results were compared to the control
experiments where no acid was added to the HTC process.
The low pH (acidic medium) of a liquid in the HTC process improves the overall rate of
reactions (Titirici et al., 2007). The type and amount of acid used in the process plays significant
role in determining the chemical reaction mechanism. The addition of small amount of Arrhenius
acids in the HTC process can catalyze the dehydration reaction mechanism (Peterson et al.,
2008). The addition of Arrhenius acids in HTC process lowers the activation energy level of
biomass polymers via hydrolysis, causing the degradation of polymers much faster and at a low
temperature compare to under neutral condition (Funke & Ziegler, 2010). The effect of
hydrolysis is limited at the high reaction temperatures because of the reduction in acid
dissociation constant (Funke & Ziegler, 2010). This phenomenon explains the reason behind the
higher effect of acid addition on the mass yield at 190°C compare to at 225°C. Therefore, the
addition of acetic acid at very high reaction temperature (say 260°C) might not be as effective as
at low temperature (190°C). However, as no experiments at 260°C were performed in this study,
further experimental demonstration might reveal the true fact.
86
190C
225C
85
Mass Yield (%)
80
75
70
65
60
55
50
0
0.25
0.5
0.75
1
Concentration (grams of acetic acid per grams of Miscanthus)
Figure4.1 Effect of acid concentration at different temperatures on mass yield of hydrochar
samples
Unlike to the linear effect of addition of acid on the mass yield, an unusual trend was
observed with the addition of calcium chloride in the HTC process. The Figure 4.2 shows the
effect of addition of calcium chloride on the mass yield of hydrochar at 190°C and 225°C. The
mass yield of the hydrochar samples produced at 190°C remained unaffected at low
concentration (0.5:1) of salt but was significantly affected high concentration level (1:1). Similar
results were observed for the samples produced at 225°C.
The effect of addition of calcium chloride on the mass yield of hydrochar samples was
more noticeable than the effect of addition of acetic acid. The presence of chloride ions in the
process can disrupt the hydrogen bonding between the polymers strands, causing the degradation
and removal of polymers from biomass (Lynam et al., 2012). However, the addition of high
amount of salt in the HTC caused the precipitation of salt itself into the pores of hydrochar. This
is reason for the high ash content observed for the hydrochar samples produced with the addition
of calcium chloride (see Figure 4.3).
87
190C
225C
100
Mass Yield (%)
90
80
70
60
50
40
30
0
0.5
1
Concentration (grams of CaCl2 salt per grams of Miscanthus)
Figure4.2 Effect of salt concentration at different temperatures on mass yield of hydrochar
samples
190C
225C
12.1
Ash Content (%)
10.1
8.1
6.1
4.1
2.1
0.1
0
0.5
1
Concentration (grams of CaCl2 salt per grams of Miscanthus)
Figure4.3 Effect of salt concentration at different temperatures on ash content of
hydrochar samples
88
4.2.1.2.
Effects on HHV of hydrochar samples
The Figure 4.4 and Figure 4.5 show the effect of addition of acid and salt, respectively on
the HHV of hydrochar samples. The HHV of the hydrochar sample produced at the low
temperature (190°C) with the low acid concentration (0.25:1) was 19.92MJ/kg. The maximum
HHV value of the hydrochar sample was observed at the highest concentration (1:1) and was
about 20.76M/kg. This shows that the HHV of the hydrochar samples produced at low
temperature (190°C) increase with an increase in acid concentration.
On the other hand, the hydrochar samples produced at higher temperature (225°C) had
the maximum HHV of 21.67MJ/kg at the medium concentration of acid in water (0.5:1). With
the further increase in the concentration of acid in the water at 225°C, the HHV of hydrochar
samples was negatively impacted. Both hemicellulose and cellulose are reported to have the low
HHV (17.6-17.9MJ/kg) as compare to that of lignin (21.54-26.85MJ/kg) (Demirbaş, 2005).
Therefore, the removal of hemicellulose and cellulose will increase the percentage fraction of
lignin in biomass; the fractional change of lignin composition in biomass will augment the
overall HHV. The addition of acid and salt has been suggested to cause the rapid degradation of
cellulose in biomass (Lynam et al., 2011). This explains the reason behind the increase in HHV
of hydrochar samples produced with the addition of acetic acid and calcium chloride.
89
190C
225C
22
21.5
HHV (MJ/kg)
21
20.5
20
19.5
19
18.5
18
0
0.25
0.5
0.75
1
Concentration (grams of acetic acid per grams of Miscanthus)
Figure4.4 Effect of acid concentration at different temperatures on HHV of hydrochar
samples
190C
225C
28
27
HHV (MJ/kg)
26
25
24
23
22
21
20
19
18
0
0.5
1
Concentration (grams of CaCl2 salt per grams of Miscanthus)
Figure4.5 Effect of salt concentration at different temperatures on HHV of hydrochar
samples
90
Increase in addition of acetic acid (10-50%) can positively influence the formation of 2,5HMF in liquid phase (de Souza et al., 2012). This may increase the overall HHV of product if 5HMF is deposited in the porous hydrochar structure. The production of 2,5-HMF in the aqueous
phase strongly depends on the operating conditions like reaction time, temperature, and type and
amount of acid used in the process (Asghari & Yoshida, 2006). Too high percentage addition of
an acid can rehydrate HMF to levulinic acid and formic acid (Asghari & Yoshida, 2006). The
HHV of levulinic acid (20.09 MJ/Kg) and formic acid (~15MJ/Kg) are considerably lower than
that of the HMF (22.09MJ/Kg), therefore, reducing the overall HHV of hydrochar samples
(Nilges et al., 2012). This phenomenon explain the reason behind the low HHV of the hydrochar
samples obtained at 225°C with high concentration level of acetic acid.
The addition of calcium chloride in HTC at 190°C and 225°C significantly increased the
HHV of hydrochar samples as compare to that from control experiments. Moreover, the HHV of
these samples is the same as that at 260°C with no additives. Therefore, the same quality of fuel
can be produced at considerably mild conditions, reducing the costs and hazardous of the HTC
process. However, the advantages of the addition of salts and acids in the HTC process were
highly disregarded by the high ash content of the hydrochar samples and their most likeliness to
cause the pitting of reactor specially when using chloride based salts (Lynam et al., 2011).
Therefore, the selection of salt and acid should be made very carefully. Further in-depth research
on the addition of different combination of salts and acids in the HTC process might be highly
useful for the industrial HTC-plant operators.
4.2.2. Characterization of HTC process water
During HTC, the solid, liquid and gaseous products are formed. The percentage
distribution and chemical properties of these products stream significantly depends on the
91
operating conditions (Yan et al., 2010). Generally both reaction time and temperature have been
observed to influence the physicochemical characteristics of products; the reaction temperature
remains the most governing process parameter. The mass yield of the hydrochar samples from
miscanthus feedstock was found to be 84%, 68%, and 48% at the reaction temperature of 190,
225, and 260°C, respectively. The considerable reduction in the mass yield of solid product with
an increase in the reaction temperature causes the formation of degraded/depolymerized products
in the liquid and gaseous by-product stream. Very small fraction of gaseous products (<5-10%)
are produced under the HTC conditions below 260°C (Libra et al., 2011; Yan et al., 2010), where
high percentage of gaseous products are produced in the processes like HTL and SCWG
(Peterson et al., 2008). Hence, apart from solid product, most of the important intermediate
organic compounds remain in the liquid by-product stream. Therefore, an analytical
characterization of the HTC liquid by-product is critically important for the development of an
industrial HTC plant.
Since most of the research work has been primarily focused on the production of solid
fuel using HTC process, very limited literature is available for the information on the liquid byproduct from HTC process. Jan Stemann (Stemann et al., 2013b) is the first one to propose the
recirculation of the HTC process water using Poplar wood chips in his experiments at 220°C for
4 hours. In their study the authors characterized the process water produced at 220°C and showed
slight increase in the HHV of solid product with increase in the number of recirculation. As only
one particular reaction temperature was used in their study, the result does not show the effect of
the reaction temperature on the process water characterization. Here in this present study the
effect of different reaction temperatures on the fractional yield of organic acids, TOC, and pH in
the HTC process water was examined.
92
The Figure 4.6 shows the effect of reaction temperature on the pH of the liquid byproduct. The pH of the deionized water is low because of its tendency to react with the CO2 in air
and thus leading to the formation of carbonic acid. The pH of the process water reduces with an
increase in the HTC reaction temperature. This is due to the formation of acidic compounds via
degradation of biomass polymers. The pH of the liquid sample dropped significantly at 225°C
pH
and remained almost constant with the further increase in temperature.
Deionized water
HTC-190C
HTC-225C
Deionized water
HTC-190C
HTC-225C
Liquid
HTC-260C
4.5
4.3
4.1
3.9
3.7
3.5
3.3
3.1
2.9
2.7
2.5
HTC-260C
Figure4.6 Effect of reaction temperature on pH of HTC process water
The results of the pH of liquid samples are consistent with the characterization of organic
acids during HPLC analysis. The characterization of organic acids, TOC and HMF in the liquid
samples under different reaction temperatures are shown in the Figure 4.7. It can be observed
that at 190°C very low acidic compounds were produced and almost no HMF particles were
formed. This suggests that the 190°C temperature is too low for the production of acid and other
intermediate compounds in liquid phase. However, with the further increase in reaction
temperature, i.e. at 225 and 260°C, the amount of organic acids and HMF increased significantly,
93
among which the acetic acid is the most dominating component. A slight reduction in the
concentration of formic acid was observed at 260°C compare to the level of concentration at
225°C. This might be due to the decomposition of formic acid into the gaseous products (mainly
CO2) under hydrothermal conditions (Yu & Savage, 1998). The concentration of TOC in the
HTC process water produced at 190, 225 and 260°C was 27.5, 23.2 and 28.7 g/L, respectively.
The low concentration of TOC at 225°C might be related to the increased porosity of the
hydrochar sample produced at 225°C that might have interacted with the adsorption of TOC
from process water.
HTC-190C
HTC-225C
HTC-260C
35
Concentration (g/L)
30
25
20
15
10
5
0
Acetic acid
Levulinic
acid
Glycolic acid Formic acid
HMF
TOC
Products in liquid samples
Figure4.7 Effect of reaction temperature on concentrations of different intermediate
compounds in HTC process water
94
The Figure 4.8 shows the effect of reaction temperature on the inorganic elemental
composition in the HTC process water samples. The high concentration of inorganic elements in
the HTC process water sample collected at 260°C suggests that the percentage removal of these
inorganic elements from the hydrochar samples is highest at 260°C. These findings are
consistent with the results obtained for the inorganic elemental analysis of the hydrochar samples
as explained in the section 3.2.4.
1600
Concentration (mg/L)
1400
Liquid at 190
1200
Liquid at 225
1000
Liquid at 260
800
600
400
200
0
Element
Figure4.8 Effect of reaction temperature on concentrations of alkali and alkaline metals in
HTC process water
4.2.3. Recirculation of HTC liquid by-product
A previous experiment on the characterization of HTC process water shows the formation
of organic acids and important intermediate compounds. Moreover, the additions of salt and acid
in the HTC process water have shown improvement in the HHV of hydrochar samples. However,
the supplement addition of organic acids and salts in the HTC process and using fresh water for
95
every run is neither an economical nor environmental-friendly option. Recovery of organic acids
and intermediate compounds from HTC process water significantly depends on the availability
of bio-refinery industry and is highly challenging in terms of processing steps like extraction,
separation, and purification (Funke & Ziegler, 2010). These aforementioned limitations of HTC
process water can be minimized by recycling/recirculating the process water after filtration of
hydrochar slurry. The advantages of recirculation of the HTC process water are discussed earlier
in the section 2.6.1.
4.2.3.1.
Effect of recirculation on pH of process water
The pH of the HTC process water significantly depends on the initial pH of the water
used in the process. The pH of the process water was found to be acidic (pH value 2.5-3.5). The
HTC process water produced at 260°C was chosen for recirculation experiments because of its
high acidity, high HMF, and TOC concentration. All these compounds are expected to improve
the HHV of hydrochar during recirculation. The Figure 4.9 shows the effect of recirculation on
the pH of process water. The pH of the fresh deionized water used in this study was 4.37±0.01
and the value significantly dropped to 2.27±0.01 after the first HTC run. However, during the
recirculation of process water, the pH of the process water remained almost constant with a
negligible fluctuation of 0.05 ±0.025.
96
pH
4.5
4.3
4.1
3.9
3.7
3.5
3.3
3.1
2.9
2.7
2.5
0
2
4
6
8
Reciculation Number (#)
10
12
Figure4.9 Effect of recirculation of HTC process water on pH of liquid by-product
4.2.3.2.
Effect of recirculation on mass yield
Figure 4.10 shows the effect of recirculation on the mass yield of hydrochar. The
recirculation experiments in this study were not repeated. However, the pervious experiments in
the chapter III, using the same setup, showed the standard variance of less than 2%. The mass
yield of hydrochar samples, during the recirculation of process water, increased by 5-10% at 2nd
and 3rd run compare to that of the reference-HTC sample (the initial sample produced in HTC
using fresh deionized water). The mass yield of the hydrochar samples remained almost constant
after 4th recirculation. The results of the mass yield are consistent with the pH value of process
water during the recirculation.
Previous result (section 4.2.2) shows that the considerable amount of aqueous soluble
products and organic acids are produced in the liquid phase during HTC of biomass. The
increased mass yield might have resulted from the deposition of the intermediate compounds
97
during the initial run on the porous surface of hydrochar particles. Alternatively, the recirculated
HTC process water from initial runs might have caused the resistance to diffusion of the
intermediate compounds from the hydrochar surface to the bulk solution (Uddin et al., 2013),
causing an increased mass yield. The stable value of the pH of process water (figure 4.9) and the
mass yield of hydrochar samples (figure 4.10) after 4th run might have been due to the
equilibrium of reaction mechanism during recirculation. Similar results were also reported by
other studies using loblolly pine wood (Uddin et al., 2013) and poplar wood samples (Stemann et
al., 2013b).
60
58
Mass Yield (%)
56
54
52
50
48
46
44
42
40
1
2
3
4
5
6
7
Recirculation Number (#)
8
9
10
Figure4.10 Effect of recirculation of HTC process water on mass yield of hydrochar
samples
98
4.2.3.3.
Effect of recirculation on HHV
Figure 4.11 shows the effect of recirculation on HHV of hydrochar samples. A three
degree polynomial equation was used to fit the curve on effect of recirculation on HHV of
hydrochar samples. The HHV of the hydrochar samples increased until 4th recirculation and then
starts reducing. Due to the degradation of hemicellulose and cellulose in HTC, small fractions of
organic acids (e.g., acetic acid, formic acid, levulinic acid, glycolic acid) are produced in the
process water (results are shown in section 4.2.2). The presence of these acids in the liquid phase
catalyze the reactions like decarboxylation and dehydration (Stemann et al., 2013b; Uddin et al.,
2013), the reactions that are expected to improve the carbon content of biomass, causing an
increase in the HHV of hydrochar samples (Funke & Ziegler, 2010). The increase in the HHV of
hydrochar samples was also observed during the addition of acetic acid in the HTC process
(Section 4.2.1).
However, after 4th recirculation the reduction in the HHV of hydrochar samples might
have been due to the excessive formation of acetic acid, causing reactions to reach an
equilibrium state. The high quality intermediate compounds like 2,5-HMF are produced during
the HTC process, however, the presence of acidic surrounding in HTC rehydrate these 2,5-HMF
particles to the low HHV compounds like levulinic acid and formic acid (Asghari & Yoshida,
2006), causing a reduction in the overall HHV of hydrochar samples. Similar results were
observed in the previous experiments on the addition of acetic acid in HTC at 225°C, high acid
addition reduces the HHV of hydrochar samples. However, as the reaction temperature used in
the recirculation experiment (260°C) is significantly higher than that from the one used during
acid addition (190 and 225°C), the direct relationship of the findings from both experiments
might not be an ethical justification.
99
Another study performed on the recirculation of the HTC process water using loblolly
pine feedstock (Uddin et al., 2013), suggested that the organic acids (like acetic acid, formic
acid, levulinic acid) that are produced during HTC are oxygenated compound and have relatively
low HHV than that the deoxygenated hydrochar carbon particles. Any deposition of these
oxygenated compounds on the surface of hydrochar particles will reduce the overall HHV of
material. As the HHV of hydrochar samples produced during the recirculation remained higher
than that of the reference-HTC sample, the minor reduction (<0.5MJ/kg) in the HHV of samples
produced after 4th recirculation is not a huge concern.
26.8
26.7
HHV (MJ/kg)
26.6
26.5
26.4
26.3
26.2
26.1
26.0
0
2
4
6
8
Recirculation Number (#)
10
12
Figure4.11 Effect of recirculation of HTC process water on HHV of hydrochar samples
The ash content increased significantly from 0.8% to 1.54% for the hydrochar samples
produced at initial and 10th recirculation, respectively. The increase in the ash content of
hydrochar samples produced during recirculation is due to the re-deposition of inorganic
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elements in the porous structure of hydrochar. As the hydrochar and liquid by-product samples
produced during the HTC-recirculation experiment were not analysed for inorganic elemental
analysis, therefore, the rate of removal or deposition of inorganics in the hydrochar pores cannot
be explained evidently.
4.3.
Conclusions
The hydrochar samples produced at 260°C has the physicochemical properties similar to
that of the low rank bituminous coal. However, at such a high temperature, the pressure inside
the reactor is very high and thus it makes the process highly unsafe and expensive. The addition
of catalyst like acetic acid and calcium chloride in the HTC process was examined so as to
reduce the temperature requirement. Both the mass yield and HHV of hydrochar samples were
slightly affected with the addition of acetic acid and calcium chloride. The effect of acetic acid
on the mass yield and HHV was more prominent at 190°C than compare to at 225°C. The HHV
of hydrochar samples produced with the addition of calcium chloride was observed same to that
of the sample produced at 260°C without any catalyst; however the sample had very high ash
content (8-12%).
The liquid by-product produced during HTC at the different reaction temperatures was
characterized in terms of the identification and quantification of the intermediate compounds.
The results show that the HTC process water is highly acidic in nature (with the pH between 2.7
to 3.3), and contain some organic acids of which acetic acid is highest in fraction. Additionally
the process water contains high quality intermediate products like 2,5-HMF that have several
value added applications in the chemical and bio-refinery industry. The process water also found
rich in total organic carbon (TOC). The concentration of organic acids and HMF particles in the
HTC process water increases with an increase in the process reaction temperature.
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The HTC process water produced at 260°C is highly acidic in nature and has high
concentration of acids, HMF particles and TOC in it. All these compounds are expected to
improve the HHV of hydrochar samples. Therefore, to determine the effect of recycling HTC
process water, the recirculation of process water was performed at 260°C. The results show that
the mass yield of hydrochar samples increased by 5-10% during the 2nd and 3rd recirculation, and
then remained almost constant for the successive recirculation runs. The HHV of the hydrochar
samples produced during recirculation remained higher than that of the reference-HTC sample
(26.06MJ/kg). A maximum HHV of the hydrochar was attained for the sample produced at 4th
recirculation (26.64MJ/kg).
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CHAPTER-V STRENGTH, STORAGE AND COMBUSTION CHARACTERISTICS OF
DENSIFIED LIGNOCELLULOSIC BIOMASS PRODUCED VIA TORREFACTION
AND HYDROTHERMAL CARBONIZATION
In this chapter the feasibility of pellets produced from miscanthus via two different
thermal pre-treatments, torrefaction and HTC, followed by densification is examined. The
properties of densified pellets were characterized in terms of the mass density, energy density,
durability, compression strength, hydrophobicity, and resistance against water immersion.
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5.1.
Materials and Methods
5.1.1. Materials
To evaluate the performance of the densification of HTC and torrefaction, miscanthus
feedstock’s was used in the study. The sample collection and preparation was done similar to as
explained in the chapter III.
5.1.2. Experimental Setup
5.1.2.1.
Hydrothermal Carbonisation
HTC of miscanthus was performed in the same way as explained in the chapter III. The
previous experiments on HTC show that the reaction temperature is the most governing
parameter affecting the physicochemical properties of hydrochar (Kambo & Dutta, 2014b).
Therefore, the HTC experiments in this study were performed at the three different reaction
temperatures (190, 225, 260°C) with the fixed residence time of 5min and feedstock to water
ratios of 1:6 (w/w). For details refer section 3.1.2.1.
5.1.2.2.
Torrefaction
The torrefaction of miscanthus was performed similar to as explained in the chapter III.
For details refer section 3.1.2.2.
5.1.2.3.
Densification
The densification of raw and pretreated miscanthus samples was performed using a
similar method used in the another study for the pelletization of loblolly pine (Reza et al., 2012).
The initial moisture content of feedstock plays an important role in the pelletization process. The
moisture content in a feedstock can act as a binder and a lubricant in the densification process
(Kaliyan & Morey, 2010). Both the raw and pre-treated miscanthus samples were exposed to the
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room conditions for at least 24 hours prior to the pelletization so as to bring the samples to
equilibrium moisture content (EMC). The EMC of the raw, HTC-190, HTC-225, HTC-260, and
torrefied-260 samples were 10.95, 7.54, 6.22, 3.52, and 6.32%, respectively.
The pelletization of samples was performed using a Parr single-pellet press (model#2912)
modified in the machine lab at University of Guelph. The pellet press consists a cylindrical die
(1/4 inch in the diameter) made of hardened stainless steel, with load cell and a band heater
attached to it. The temperature of the die, attached to the heater, was controlled using a PID
temperature controller and was set to 100°C. The bottom of the die was closed using a removable
backstop plate. After temperature equilibration, about 0.5g of the sample was manually placed in
the die and a compressive pressure at 1250Psig was applied to the sample by a lever attached to
the pellet press (Figure 5.1(A), 5.1(B)). The sample was held under pressure for 30sec and was
then released. Subsequently to compression, the backstop plate was removed from the die and
the pellet was pressed out of the die channel using the same piston that was used for compression
(figure 5.1(C)). The pellet samples were left undisturbed for 1-2min and were kept in the sealed
plastic bags for further analysis.
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Figure5.1 (A) Pelletization setup, (B) Pellet formation, (C) Pellet extrusion
5.1.3. Analytical Methods
5.1.3.1.
Mass and Energy Density
The mass density of the pellets was expressed as the ratio of the mass to volume. A
stereometric method was used to calculate the density of each pellet. The method is based on the
measurement of dimensions (e.g., diameter and length) of a regular shape product. Each single
pellet sample was weighed to the nearest value of 0.0001g using an analytical balance (Mettler
Toledo-MS204S). The volume of the each pellet sample was determined by measuring the
volume of the pellet (cylinder in this case) using a digital caliper (Mastercraft, 58-6800-4). The
energy density was calculated by multiplying the mass density of the pellet by the HHV of the
pellet.
5.1.3.2.
Higher Heating Value
The HHV of the raw and pre-treated pellets was determined using an IKA-C200 bomb
calorimeter. The samples were oven dried at 105°C for 24hrs prior to analysis.
5.1.3.3.
Compression Strength
The compression strength of pellets was measured by using an Instron machine
(Model#5965). The method was adopted from an another study (Nielsen et al., 2009). Each
single pellet was placed between the two horizontal plates and was compressed with a rate of
25mm/min (Figure 5.2). A data logger connect to the machine recorded the force (F) applied on
the pellet. The maximum force a pellet could withstand before fracture was measured as the
value for the compression strength.
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Figure5.2 Radial compression of pellets to test the compression strength(Nielsen et al.,
2009)
(Adapted with permission from publisher)
5.1.3.4.
Durability
The forces acting on pellets during unloading process from the trucks on the ground or
during feeding from the chutes into bins at the pellets manufacturing plant can be simulated by
measuring the impact resistance or the drop resistance or the shattering resistance test. A drop
resistance method was used in this study to determine the durability of the raw and pretreated
miscanthus pellets (Kaliyan & Vance Morey, 2009). Each pellet sample was dropped from
1.85m height on a metal plate 4 times. The percentage weight retained after dropping was
expressed as the pellet durability.
5.1.3.5.
5.1.3.5.1.
Hydrophobicity
Equilibrium Moisture Content
The thermal pre-treatments are often proposed to improve the storage behaviour or
hydrophobicity of biomass. To confirm the validation of the above statement, the pellets were
tested for equilibrium moisture content (EMC). EMC is defined as the moisture content of the
sample that is in the thermodynamic equilibrium with the moisture in the surrounding
atmosphere at a given relative humidity, temperature, and pressure. To determine the EMC of
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pellets, the samples were exposed to controlled environment (relative humidity: 48-52% at 2223°C). After the exposure of 24 hours the samples were then dried in an oven at 103°C for
16hours. The change in the weight before and after drying was expressed as the percentage
moisture adsorbed or EMC.
5.1.3.5.2.
Water Resistance
Pimchuai and his co-authors (Pimchuai et al., 2010) demonstrated the water resistance
capacity of raw and torrefied biomass by immersing the samples in the water for 2 hours. The
samples were then allowed to dry in the change in the weight was expressed as the percentage
moisture absorption. Using the same method, both raw and pre-treated pellets were immersed in
the water for 2 hours. Later the pellets were removed and the excess water was drained by
placing the pellets on an adsorbent paper. The pellets were then gently placed on the aluminum
sheet and were exposed to the controlled environment (relative humidity: 48-52% at 22-23°C)
for 4 hours. The final material was then re-weighed; the change in weight of pellets was
expressed as the moisture content.
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5.2.
Results and Discussions
5.2.1. Mass and Energy Density
The mass and energy density of pellets are the two important parameters that play
significant role in the transportation and handling economics of the wood pellets. The low bulk
density of a raw biomass result in the highly inefficient logistics and is a huge challenge for the
biomass processing industry (Kaliyan & Vance Morey, 2009). The low bulk density, low carbon
content, and high oxygen content of the raw biomass reduces the overall bulk energy density of
feedstock (Jenkins et al., 1998).
Table 5.1 shows the effect of the densification on the mass density and energy density of
the raw and pretreated miscanthus. The mass density of an oven dried miscanthus sample was
found about 321.09±10 kg/m3. This mass density of raw miscanthus was found to be much
higher compare to the values reported in the literature for the miscanthus (Huisman et al., 1997).
Generally for the woody and agricultural biomass, “bulk density” is the most appropriate term
used at industrial scale when referring to the density of a biomass. While measuring the bulk
density of a biomass, the volume of the voids is also included in the measurement, therefore, the
bulk density of a biomass is always lesser than the mass density a feedstock.
In this study the mass density of raw miscanthus increased by about 2.6 times after
densification. The HHV of the samples was measured before and after densification, and was
found to be unaffected as expected. As most of the polymeric (hemicellulose, cellulose and
lignin) and aqueous soluble composition of the biomass does not start to react until the material
temperature reaches 180°C (Reza et al., 2012; Yan et al., 2009), no compositional change should
take place at low temperature (100°C) densification. Moreover, no external binder material was
used in this experiment for the densification of raw and pre-treated biomass. Therefore, as
109
expected, the HHV of the raw and pretreated biomass remained almost unaffected after
densification. However, it is interesting to note that the overall volumetric energy density
(energy unit per volume) of raw miscanthus increased significantly, from 5.93 to 15.69GJ/m3,
after densification.
Table5.1 Effect of densification on mass density, HHV and energy density
Process/Pretreatment
Reaction
Residence
temperature
Time
(°C)
Raw
100
30sec
Pelletization
190/100
5min/30sec
HTC/Pelletization
225/100
5min/30sec
HTC/Pelletization
260/100
5min/30sec
HTC/Pelletization
260/100
30min/30sec
Torrefaction/
Pelletization
Mass Density
(kg/m3)
HHV
(MJ/kg)
Energy
Density
(GJ/m3)
321.09
834.05
886.87
959.39
1035.99
18.47
18.82
20.19
21.62
25.97
5.93
15.69
17.90
20.74
26.90
819.55
20.34
16.66
The mass density of the pellets produced from hydrothermally pretreated miscanthus
increased with an increase in the reaction temperature. The pellets produced at 260°C has the
maximum mass density (1035.99 kg/m3) which is 24.2% higher than that of the pellet produced
from the raw miscanthus. The previous results from experiment show that the grindability of the
biomass increases with an increase in the HTC reaction temperature, thus it makes the material
highly friable in nature and easy to compress. The pelletability of the biomass is considerably
affected by the small particle size distribution of a feedstock (Mani et al., 2006b), therefore, an
increase in the mass density of pellets produced from HTC pretreated biomass is directly corelated with the improved friability of miscanthus (Table5.2)(Kambo & Dutta, 2014b).
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Table5.2 Effect of pre-treatment on grindability of biomass
Particle Size Distribution (%)
Material
Raw Miscanthus
HTC-190°C
HTC-225°C
HTC-260°C
Torrefied-260°C
>500µm
(%)
34.99
17.45
46.17
15.68
11.09
14.75
<100µm
(%)
7.75
52.13
2.82
0.14
12.01
1.44
18.68
26.21
66.49
72.21
3.34
42.18
26.65
27.83
250-500µm (%) 100-250µm (%)
In comparison to the HTC pellets, the mass density of the pellets produced from torrefied
biomass is substantially lower and is similar to that of the raw biomass pellet. While measuring
the diameter, the pellets produced from the torrefied biomass had slightly larger diameter than
that of the raw and HTC pellets. This shows that, compare to the raw and HTC pellets, the
internal bonding between the particles of the torrefied biomass was not strong enough to hold
tight together.
The chemical composition of a material significantly affects the pelletization process and
quality of the pellets (Kaliyan & Vance Morey, 2009). The chemical reaction mechanism that
takes place in the HTC process is highly complicated and is partially understood (Funke &
Ziegler, 2010). The formation of intermediate compounds (aqueous soluble materials) in the
HTC process via condensation and polymerization reactions might have been responsible for the
better mass and energy density of the HTC pellets. The formation of such aqueous soluble
material is relatively very low in the torrefaction process (Yan et al., 2009). The reaction
temperature and the type of reaction medium used in the thermochemical process plays
significantly important role in determining the dominancy and rate of reaction mechanism (Libra
et al., 2011).
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5.2.2. Strength and Durability of Pellets
A durability test is a good tool to predict the strength and quality of the pellets during
their supply chain (i.e. from the manufacturing industry to their end use). Durability is defined as
the ability of pellets to remain integral while handling and is measured by the amount of fines
that are produced from the pellets after subjecting to a mechanical or pneumatic agitation (Gil et
al., 2010). In a similar way, the compression strength of a pellet is a measure of the internal
bonding-strength or the maximum force a pellet could withstand before its rapture during
storage. Results for the durability and strength of the raw and pretreated miscanthus pellets are
shown in the Figure 5.3.
Strength
100
400
95
350
90
300
85
250
80
200
75
150
70
100
65
50
60
Strength (N)
Durability (%)
Durability
0
Raw
HTC-190
HTC-225
HTC-260 Torrefied 260
Pellet Type
Figure5.3 Effect of pre-treatment type on durability and strength of pellets
The impact resistance durability (IRD) of the raw miscanthus pellets is 92.19% and it
increases with an increase in the HTC reaction temperature (i.e. from 190 to 225°C). However,
the durability of the HTC-260°C pellet was found relatively lower (88.80%) than that of the raw
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and other HTC pellets. Because the fine particles (<100µm) are produced in the HTC process,
the sharp edges of the HTC-260°C pellet when strikes the metal plate (during IRD test) tend to
shatter into powder, this explains the reason for the low durability of HTC-260°C pellets. Further
in-depth research on the optimization of pellets shape and the pelletization process conditions
may improve the durability of HTC pellets.
The compressive strength of the pellets decreases with an increase in the reaction
temperature. High percentage of lignin in the material acts as a natural binder during
pelletization, but at the same time can make the pellets highly brittle. Except for the high
durability of HTC pellets, similar results were also obtained in an another study performed on
densification of HTC pretreated loblolly pine wood (Reza et al., 2012). The variation in the
results may be due to the different operating conditions like the type of feedstock, densification
pressure, temperature, and die aspect ratio; the operating conditions have strong influence on the
properties of pellets (Kaliyan & Vance Morey, 2009). On the other hand compare to the raw and
HTC pellets, similar to the findings in literature (Li et al., 2012; Wang et al., 2013), the pellets
produced from the torrefied miscanthus were very week in strength and durability.
The Glass Transition Temperature (Tg), is the temperature range where a thermosetting
polymer changes from a hard, rigid or “glassy” state to a more pliable state. Lignin is the only
component of biomass that shows glass transition behavior (Reza et al., 2012), and its Tg is the
range of 137-157°C (Grāvitis et al., 2010). The moisture content of a feedstock can significantly
affect the glass transition temperature of lignin (Stelte et al., 2011a). For the densification at the
temperatures above the Tg, a feedstock will tend to show inter-diffusion or a solid bridge
between the particles of biomass. However, the low densification temperature (100°C) used in
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this study might have eliminated the formation of a solid bridge between the particles. The low
compression strength of the torrefied pellets may be due to the presence of pores in the structure,
that reduce the resistance towards the deformation and indicates the need for the high
densification pressure may be required to produce strong pellets (Li et al., 2012).
5.2.3. Hydrophobicity
EMC of a material is used as an indicator of the hydrophobicity. The presence of high
moisture content in biomass feedstock/pellets can influence fungal growth and therefore the
material will most likely rot easily with time (Acharjee et al., 2011). Table 5.3 shows the
hydrophobicity of the raw and pre-treated pellets. Both the HTC and torrefaction pre-treatment
improves the hydrophobicity of biomass, however, the pellets obtained via HTC are more
hydrophobic in nature. The hydrophobicity of the HTC pellets increases with an increase in the
reaction temperature. For a lignocellulosic biomass, the moisture can be adsorbed in three
different ways: (i) free water (present in micro and macro porous capillaries but not to specific
sorption sites), (ii) non-freezing bounded water, and (iii) freezing bounded water (both (ii) and
(iii) are hydrogen-bonded to the hydroxyl groups of the cell wall). The non-bounded moisture
increases with an increase in the relative humidity. Unlike to the HTC pretreated miscanthus, due
to the volatilization of hemicellulose, the slight porous structures are obtained via dry thermal
pre-treatments like torrefaction and pyrolysis (Fuertes et al., 2010). The presence of the pores on
the surface of torrefied miscanthus increases the tendency to adsorb moisture content (free water)
high and quickly (Stelte et al., 2011b) as compare to the HTC pellets.
Moreover, the chemical composition of biomass has a strong influence on the
hydrophobic behaviour of a feedstock. Among the polymeric composition of biomass,
hemicelluloses has the greatest capacity towards water adsorption, while lignin shows very little
114
tendency for the water sorption (Acharjee et al., 2011). Hence, the removal of hemicellulose (or
the fractional percentage increase in the lignin content) from a biomass feedstock will improve
its hydrophobicity. All these findings support the reason behind the high hydrophobicity of the
HTC pellets compare to raw and torrefied pellets.
Table5.3 EMC of raw and pretreated pellets before and after immersion
Pellet
Raw
HTC-190
HTC-225
HTC-260
Torrefied -260
EMC before
Immersion in water
(%)
6.38
4.29
3.30
2.27
3.83
EMC after
Immersion in water
(%)
80.96
66.86
39.41
18.60
75.59
To determine the strength of the pellets against water resistance, both the raw and
pretreated pellets were immersed in the water for 2hours. The pellets prepared from the raw
miscanthus rapidly segregated to fine particles within 15-20sec after the immersion in water. The
pellets produced via HTC at 190°C sustained for 5mins and those at 225°C remained whole for
about 30mins, later these pellets started isolating into discs like structure. Where the pellets
produced from HTC of miscanthus at 260°C remained as a whole in pellet shape, even after the
immersion of more than 2hours in water. On the other hand, the pellets produced from the
torrefied miscanthus behaved slightly better than the raw miscanthus pellets, lasting for about 12min. The hydrophobic behaviour of the HTC pellets explains the reason behind the strong
strength against water immersion.
Table 5.3 shows the result of the moisture content of the samples exposed to the room
conditions after immersion of 2hours. The results indicate that the pellets produced from HTC of
miscanthus at 260°C are highly hydrophobic in nature and therefore can be stored easily without
115
possessing any threat to the biological deterioration. Moreover, the transportation of such a
feedstock will be less expensive as there will be less water content to transport along with the
biomass (Yan et al., 2009).
5.3.
Conclusions
The work describes the experimental demonstration and comparison of the raw, HTC and
torrefied miscanthus pellets in terms of the strength, storage, and combustion characteristics. It
was found that combining densification with the thermal pre-treatments like HTC and
torrefaction are the promising methods for upgrading biomass and its conversion to an energy
dense, homogeneous, friable, and hydrophobic solid fuel. The storage and combustion properties
of the HTC pellets are considerably better than the pellets obtained from the torrefied biomass
even if the processing conditions of the torrefaction process were kept very high (260°C for
30mins) than that of the HTC process (190 and 225°C for 5mins). However, it should be noticed
that the high pressure requirement in the HTC process makes it more expensive and hazardous.
The physicochemical properties of the HTC pellets produced at 260°C are comparable to
that of the low rank bituminous coal. The mass and energy density of the HTC pellets increases
with an increase in the reaction temperature. In comparison to the raw and torrefied pellets, HTC
pellets are highly hydrophobic in nature and show strong resistance against the water immersion,
improved grindability, and reduced inorganic elements in the ash. In addition, the HTC pellets
produced at low temperature (190 and 225°C) were found to be highly durable, whereas, the
durability reduces with the further increase in the reaction temperature.
On the other hand, torrefied pellets show low mass density and durability even compare
to the raw miscanthus pellets. Lignin is a naturally occurring polymer in the biomass and acts as
116
a natural binding material during pelletization, however, a high percentage of lignin content in
the pellets makes it highly brittle and weak in strength. Compression strength of the pellets
decreases with an increase in the reaction temperature. Therefore, the extension of this work will
be focused on the manufacturing of spherical shape pellets that will have high bulk energy
density, strength, and durability.
117
CHAPTER-VI OVERALL CONCLUSIONS AND RECOMMENDATIONS
6.1.
Conclusions
Hydrothermal carbonization (HTC) of three different types of crops (miscanthus, willow
and wheat straw) was experimentally studied and analyzed to examine the effect of operating
conditions on the physicochemical properties of biomass. For comparison, torrefaction (a
conventional thermal pre-treatment) was also applied to the miscanthus feedstock. The solid
product from both the thermal pre-treatments was investigated for the densification
characterization. The liquid by-product from the HTC process and its effects of recirculation was
also analyzed in this study. The conclusions in this section are drawn based on the work reported
in the previous chapters and are shown below.
6.1.1. Wet and Dry Thermal Pre-treatment of Lignocellulosic Biomass
Both the HTC (wet) and torrefaction (dry) process are promising methods for upgrading
low quality lignocellulosic biomass. However, for the same quality and quantity of a final
product, the severity of the operating conditions (i.e. the reaction time and temperature) is
significantly low for HTC when compare to the torrefaction process. Hydrochar obtained via
HTC is a highly energy dense, hydrophobic and friable product and therefore can be transported,
stored and pulverized very economically compare to the raw and torrefied biomass. Moreover,
the hydrochar samples have considerable reduction in the alkali and alkaline earth metals
composition. Therefore, it co-firing with coal will significantly reduce the fouling and slagging
concerns in the boiler. Most outstandingly, as HTC process is carried out in the presence of
water, instead of drying used in conventional thermal pre-treatments, the process is a highly cost
effective preprocessing technique especially for the wet biomass. The proximate and ultimate
118
analysis indicates that the hydrochar samples produced at 260°C has significantly reduced
volatile compounds and oxygen content and increased carbon content. Hence, HTC converts
lignocellulosic biomass into a coal-like product that has potential to replace coal in existing coalfired power plants without any modifications to the system.
6.1.2. HTC Process Water Characterization and Recirculation
The liquid by-product produced during HTC is acidic in nature and contains high quality
intermediate chemical compounds. The process water contains organic acids like acetic acid,
levulinic acid, formic acid, and glycolic acid and of these acetic acid remained in highest
proportion. The process water also contains high quality intermediate products like HMF that
have several value added applications in the chemical and bio-refinery industries. The
concentration levels of organic acids and HMF particles in the process water increases with an
increase in the HTC reaction temperature. The HTC process water was also found to be rich in
total organic carbon. The high alkali and alkaline earth metallic content in the HTC process
water explains the tendency of the HTC process to demineralization low quality lignocellulosic
biomass. Due to the presence of such compounds in the HTC process water it can be used as a
fertilizer for agricultural applications.
Recirculation of the HTC process water appears to be the most likely economical feature
for a typical industrial HTC plant. The HTC process water produced at 260°C was studied for
recirculation experiments due to its high acidic nature, 2,5-HMF and TOC content. Due to the
presence of organic acids, recirculation of the HTC process water will provoke the carbonization
reactions mechanism. As a result the recirculation of process water increases the system’s overall
efficiency by augmenting the mass and energy yield of hydrochar.
119
6.1.3. Addition of Salt and Acid in HTC Process
The addition of catalyst like acetic acid and calcium chloride to the HTC process showed
an increase in the energy content of hydrochar. Most importantly, addition of calcium chloride to
the HTC process considerably reduces the high temperature requirement. The HHV of hydrochar
samples produced at low temperature (190 and 225°C) with the addition of calcium chloride was
found to be same as the one obtained at 260°C with no additives. Hence reduction in the
temperature will reduce the high pressure requirement and hazardousness of an HTC process.
However, the hydrochar samples prepared with the addition of calcium chloride have
significantly higher ash content and may cause the pitting of a reactor.
6.1.4. Densification of Raw and Pre-treated Biomass
Densification in combination with the thermal pre-treatments like HTC and torrefaction
are promising methods for upgrading biomass and its conversion to an energy dense,
homogeneous, friable, and hydrophobic solid fuel. The strength, storage and combustion
properties of HTC pellets were found considerably better than the raw and torrefied pellets. Both
the mass and energy density of HTC pellets was found to be significantly affected by the process
reaction temperature. The physicochemical properties of HTC pellets produced from miscanthus
pre-treated at 260°C are comparable to that of low rank bituminous coal. In comparison to the
raw and torrefied pellets, HTC pellets were highly hydrophobic in nature and show strong
resistance against water immersion. Lignin is a naturally occurring polymer in biomass that acts
as a natural binding material during pelletization; however, a high percentage of lignin in pellets
makes it highly brittle and weak in strength. Compression strength of HTC pellets decreases with
an increase in the reaction temperature.
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6.2.
Recommendations for Future Research
6.2.1. Process Optimization
In literature and throughout this study, only the reaction time and temperature were found
to be the main dominating parameters affecting the physicochemical properties of hydrochar.
Nonetheless, other variables such as the reaction pressure, feedstock size, and solid load, acidity
of process water, heating rate, and shape and size of a reactor are not negligible. All these factors
should be considered critically important when designing a large scale industrial HTC plant.
However, limited information is available on the effect aforementioned parameters in an HTC
process. Reaction kinetics and a co-relation between the input parameters (like proximate,
ultimate, fiber analysis and type of feedstock etc.), process operating conditions (time,
temperature, pressure, solid load, etc.) and desired output properties (such as mass and energy
yield) may aid in optimizing the design of a state-of-the-art HTC reactor. A techno-economical
evaluation study in terms of energy input and output of a pilot scale HTC plant can help in
understanding the applicability of an HTC system in the industries.
6.2.2. Limitations of the HTC Process
Results from this study indicates that the biomass pre-treated under the HTC process at
260°C has significantly improved physicochemical properties and has potential to replace coal at
power plant. Nevertheless, being a process with the high conversion efficiency when compare to
other thermal pre-treatments, like pyrolysis, gasification, and dry torrefaction, its adaptability at
industrial scale is still not in favor. At such a high temperature (260°C) the pressure inside the
reactor is substantially high (>5 MPa) and makes the process highly unsafe and expensive. High
pressure requirement, continuous feeding against pressure increases the overall manufacturing
costs of equipment. Moreover, for an economical HTC plant, running it under ideal conditions
121
like efficient heat recovery, water recirculation, and recovery of liquid intermediate products can
be very critical. Hence further investigations are required in terms of the process design and
development. The addition of calcium chloride in the process reduces the temperature
requirement but also increases the ash content of hydrochar. Further research on the different
type and combination of salts and acids should be examined to see their effect on the process and
products.
6.2.3. Integration of HTC and Densification Process
Pellets produced from the HTC process at 260°C shows promise in terms of the high
mass and energy density. However, the durability of those pellets was limited by their
conventional shape. Standard conventional pellets are elongated cylinders with fractured end
which limits their bulk packing density. In addition, due to the fine particles size distribution and
sharp edges of these pellets, they have low durability and create dust upon handling, causing a
risk of explosion. Therefore, further extension of this research focusing on developing a
spherical shape pellets may improve the aforementioned issues.
A single die pellet press with the fixed processing parameters (i.e. compression
temperature and pressure) was used in this study for the densification of raw and pre-treated
miscanthus. However, the densification process at an industrial scale takes place very differently
from the method used in this study. Using a ring-die or a flat-die pellet press to examine the
densification characterization of raw and pre-treated biomass might be more reasonable.
6.2.4. Versatility of Feedstock
The outstanding advantage of an HTC process is the elimination of pre-drying
requirement of a feedstock. Lignocellulosic biomasses with the low moisture content were
122
mainly considered in this study. However, the non-conventional and non-lignocellulosic
biomasses like animal waste, food waste, greenhouse vegetable waste, and other high moisture
content feedstocks should be a priority because of their underutilization and economic
inapplicability in the conventional dry thermal pre-treatments like torrefaction and pyrolysis.
6.2.5. Products Recovery and Integration of HTC with Anaerobic Digestion
The HTC process water contains considerable amount of dissolved organic and inorganic
compounds. Recovery to such high quality intermediate compounds will be very beneficial, as
products like HMF, levulinic acid, acetic acid, formic acid, N-P-K and other metals like Ni, P, B,
Mg, and S have their own market value in the agro-bio-chemical industries. Recirculation of the
process water was found to improve the system’s overall efficiency.
HTC of biomass produces hydrochar that is similar to coal and provides an opportunity
for the production of liquid and gaseous fuels. Due to the presence of organic acids (like acetic
and formic acid) in the HTC process water, the integration of an HTC process with the anaerobic
digestion (AD) system will improve the degradation reaction mechanism by acetogenesis and
acetate-consuming methanogenesis (Wirth & Mumme, 2013). A techno-economic study of
combining HTC with an anaerobic digestion process should be evaluated. The general idea of
combining HTC and AD is to utilize the process waste streams for the production of liquid and
gaseous fuels. Figure 6.1 shows the typical design of the HTC and AD process integration.
123
Figure6.1 Integration of HTC and AD plant
Where:
HTC: Hydrothermal Carbonization
AD: Anaerobic Digestion
SCWG: Supercritical Water Gasification
FTS: Fisher Tropsch Synthesis
H2 : Hydrogen; CH4 :Methane ; CO: Carbon monoxide; CO2 :Carbon dioxide
6.2.6. Surface and Reaction Chemistry of Hydrochar
Few studies in the literature have reported the improved adsorption capacity of the
hydrochar samples obtained via HTC and their application in the waste water treatment industry.
In-depth characterization of hydrochars (via HTC) and biochars (via torrefaction, pyrolysis
processes) produced from the different types of feedstocks and using highly advanced scientific
techniques like FTIR (Fourier transform infrared) and NMR (Nuclear magnetic resonance)
spectroscopy might reveal more information on the surface groups on the hydrochar. Using such
techniques in the combination with qualitative and quantitative software can depict a complete
vision of the reaction chemistry and combustion behaviour of hydrochar particles.
124
6.2.7. Hydrochar in Agriculture
Biochar, from slow pyrolysis, is often used as a product for soil amelioration. When
biochar is used for agriculture, is always used in a large quantities and therefore is impossible to
remove from the soil if there are any adverse outcomes from the agricultural and environmental
toxicity point of view. Inorganic impurities (especially heavy metals) cannot be destroyed during
dry thermal pre-treatments like pyrolysis, since they may have a toxic risk potential, it is
critically important to follow the fate of heavy metals. The accumulation of heavy metals in the
soil can drastically affect the food chain and therefore has to be taken into account. A study
estimated that, in a commercial field trial, approximately 30% of the mass of the biochar (from
pyrolysis) was lost during transport, handling and application (Husk & Major, 2010). In addition,
the risk of spontaneous combustion or dust explosions in the presence of open fire has to be
considered (Libra et al., 2011). Also the small particles of biochar due to the larger surface area
tend to oxidize faster compare to the larger particles.
On the other hand, hydrochars generally have low inorganic impurities compare to the
biochars, but so far no standard application guidelines are available for the application of
hydrochar for agriculture. Hydrochar when leaves the process is wet in state and is in the form of
slurry. Therefore, the smaller particles of the hydrochar agglomerate together and well mix in the
soil, reducing erosion loss and minimize a risk of explosion. However, very high moisture along
with the high lignin content of hydrochar might influence the fungal growth and therefore can
drastically affect the agricultural productivity. A comparative evaluation by conducting field
trials on using different types of biochar and hydrochar in an unfertile soil may reveal the factual
information.
125
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