Renewable and Sustainable Energy Reviews 45 (2015) 359–378
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
A comparative review of biochar and hydrochar in terms of production,
physico-chemical properties and applications
Harpreet Singh Kambo, Animesh Dutta n
School of Engineering, University of Guelph, Guelph, Canada N1G-2W1
art ic l e i nf o
a b s t r a c t
Article history:
Received 11 January 2014
Received in revised form
8 January 2015
Accepted 12 January 2015
Available online 14 February 2015
Slow-pyrolysis of biomass for the production of biochar, a stable carbon-rich solid by-product, has gained
considerable interest due to its proven role and application in the multidisciplinary areas of science and
engineering. An alternative to slow-pyrolysis is a relatively new process called hydrothermal carbonization
(HTC) of biomass, where the biomass is treated with hot compressed water instead of drying, has shown
promising results. The HTC process offers several advantages over conventional dry-thermal pre-treatments
like slow-pyrolysis in terms of improvements in the process performances and economic efficiency, especially
its ability to process wet feedstock without pre-drying requirement. Char produced from both the processes
exhibits significantly different physiochemical properties that affect their potential applications, which includes
but is not limited to carbon sequestration, soil amelioration, bioenergy production, and wastewater pollution
remediation. This paper provides an updated review on the fundamentals and reaction mechanisms of the
slow-pyrolysis and HTC processes, identifies research gaps, and summarizes the physicochemical characteristics of chars for different applications in the industry. The literature reviewed in this study suggests that
hydrochar (HTC char) is a valuable resource and is superior to biochar in certain ways. For example, it contains
a reduced alkali and alkaline earth and heavy metal content, and an increased higher heating value compared
to the biochar produced at the same operating process temperature. However, its effective utilization would
require further experimental research and investigations in terms of feeding of biomass against pressure;
effects and relationships among feedstocks compositions, hydrochar characteristics and process conditions;
advancement in the production technique(s) for improvement in the physicochemical behavior of hydrochar;
and development of a diverse range of processing options to produce hydrochar with characteristics required
for various industry applications.
& 2015 Elsevier Ltd. All rights reserved.
Keywords:
Biochar
Hydrochar
Hydrothermal
Carbonization
Pyrolysis
Contents
1.
2.
3.
4.
5.
n
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
1.1.
Lignocellulosic composition and combustion properties of biomass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
1.2.
Origin and definition of biochar and hydrochar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
Production of biochar and hydrochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
2.1.
Feedstock for the production of biochar and hydrochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
2.2.
Biochar and hydrochar production technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
2.2.1.
Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
2.2.2.
Dry Torrefaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
2.2.3.
Gasification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
2.2.4.
Hydrothermal carbonization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
Chemical reaction mechanism(s) behind the production of hydrochar and biochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364
Characterization of biochar and hydrochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
4.1.
Solid yield and chemical composition characterization of biochar and hydrochar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
4.2.
Morphological characterization of biochar and hydrochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
4.2.1.
Tailoring the structure of chars via physical and chemical activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368
Potential benefits and applications of biochar and hydrochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
Correspondence to: School of Engineering, 50 Stone Road East, University of Guelph, Guelph, Ontario, N1G 2W1 Canada. Tel.: þ1 519 824 4120x52441; fax: þ 1 519 836 0227.
E-mail address: adutta@uoguelph.ca (A. Dutta).
http://dx.doi.org/10.1016/j.rser.2015.01.050
1364-0321/& 2015 Elsevier Ltd. All rights reserved.
360
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5.1.
Energy production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
5.2.
Carbon sequestration and gas adsorbent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
5.3.
Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
5.4.
Activated carbon adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
5.5.
Bio-refinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
6. Conclusions and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
1. Introduction
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% by the year 2020 and by about
65% by 2040 [1,2]. 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. 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 [3]. Biomass is the
one and only renewable energy resource that can be converted into
any form of fuel including solid, liquid, and gaseous [4]. Its nonedible 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 [5].
1.1. Lignocellulosic composition and combustion properties of
biomass
Typically, biomass (like plants and trees) is composed of three
main components: cellulose, hemicellulose, and lignin as shown in
Fig. 1. These components are strongly intermeshed, chemically
bonded by non-covalent forces, and are cross-linked together,
thereby providing structure and rigidity to the plant. The physical
and chemical properties of these components are discussed in
Table 1.
Even though biomass is a common source of energy, particularly
in developing countries, it is still not regarded as an ideal fuel due to
its inferior physical and chemical properties such as fibrous nature,
low bulk density and low heating value, high moisture content, high
volatile components, and high alkali and alkaline earth metallic
content [6–11]. The seasonal variation affects continuous availability
of biomass feedstock; moreover, the wide diversification in the
physical shape, chemical compositions, and energy densities among
different biomasses results in inefficient handling, transportation,
storage and sizing of feedstock. To overcome these aforementioned
limitations, pre-treatment of biomass is a highly necessary step
before it is utilized as an efficient energy resource. A broad range of
biological and thermochemical pre-treatment processes like torrefaction, pyrolysis, gasification, anaerobic digestion, fermentation,
etc. is available to enhance the combustion properties of biomass
and its conversion to liquid or gaseous biofuels [12,13]. However,
thermochemical pre-treatments are generally preferred over biological pre-treatments, as they offer advantages like short reaction
time and high conversion efficiency [14]. Recent research interests
in reducing greenhouse gases (GHGs) emission by means of carbon
sequestration and simultaneously improving food productivity with
the application of biochar in the soil have resulted in biomass
regaining its attention for developing sustainable energy production
and eco-friendly environment [15].
Fig. 1. Structural representation of lignocellulosic biomass with cellulose, hemicellulose, and building blocks of lignin (adapted from [16] with permission).
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Table 1
Physical and chemical properties of lignocellulosic composition of biomass.
Compound
Cellulose
Hemicellulose
Lignin
Chemical
structure
Lignin
Cellobiose
monomers (a) trans-p-coumaryl alcohol,
(b) coniferyl alcohol, and (c) sinapyl alcohol
(D-glucose) Unit
Molecular
formula
Typical
composition
in biomass
(C6H10O5)n
Structural
formation
A homopolymer of D-glucose subunits.
(i) Hardwood: 39–54%
(ii) Softwood: 41–50%
(iii) Agricultural: 24–50%
Cellulose is linked by β-1,4 glycosidic
bonds forming long chains.
Hydrophobicity
Calorific value
Thermal
stability and
solubility in
water
Applications
Medium
17–18 MJ/kg
Cellulose is non-soluble in water under
standard conditions.
It can be hydrolyzed in subcritical water
around 180 1C and around 300–400 1C in
standard conditions.
Paper manufacturing, textiles, biofuels,
chromatography, binding/composite
materials, etc.
C5H10O5
(a) C9H10O2, (b)C10H12O3, and (c) C11H14O4
(i) Hardwood: 15–36%
(ii) Softwood: 11–27%
(iii) Agricultural :22–35%
(i) Hardwood:17–29%
(ii) Softwood:27–30%
(iii) Agricultural:7–29%
A hetropolymer of Xylose, Mannose, Glucose, and
Galactose.
Xylan, the dominating component in hemicellulose,
is linked by β-(1-4)-glycosidic or α-(1-2)-bonded
4-O-methylglucoronic acids. Also may contain acetyl
group attached to it.
A hetropolymer built up of three different
phenyl-propane monomers groups:
p-coumaryl, coniferyl and sinapyl alcohol.
This complex polymer is oriented by a
different degree of methoxylation of abovementioned monomers forming a large
molecular structure(s).
Low
High
17–18 MJ/kg
23.3–26.6 MJ/Kg
Owing to its amorphous structure, thermal breakdown Lignin is the most thermo-chemically stable
of hemicellulose is relatively easier.
component in wood and highly insoluble in
water.
It can be hydrolyzed in water around 160 1C and
Its degradation/hydrolysis starts in near or
around 200–300 1C under standard conditions.
supercritical water or around 600 1C in
ambient conditions.
Mainly includes animal feed, food packaging, health
Manufacturing of adhesive compounds and
care and bio-refinery industry.
bioenergy.
Refs.: [11,17–31].
1.2. Origin and definition of biochar and hydrochar
The origin of biochar is associated with the soils of Amazon
region, often referred to as “Terra-Preta” soils. These soils have
gained global interest because of their significantly improved crop
productivity compared to the surrounding infertile tropical soils
[32]. Additional detailed research revealed that these soils are
believed to have used biochar as a key component that partly
explains the unique properties of Terra-Preta soil [33]. As a fact,
these influential findings impelled researchers to reveal further
hidden secrets, which resulted in a massive publication of literature on biochar and its application in the soil [34–36]. Thereafter,
biochar has been regarded as a significantly important tool for
developing sustainable energy production and environmental
management [15].
“Biochar” is a recently coined term emerging in conjunction
with the renewable fuel, soil amelioration, and carbon sequestration. Definition(s) of biochar includes char and charcoal, excluding
fossil fuel products, produced by the partial combustion (charring
or smoldering) of carbonaceous organic materials like trees and
plants [37]. In the absence or partial supply of oxygen during
combustion, the process inhibits complete combustion of the
source material. Many other researchers have also defined the
term “biochar”; however, all these definitions are somehow
interrelated to each other in terms of its production and applications [38]. So far the most standardized definition of biochar is
regulated by International Biochar Initiative (IBI) guidelines, which
states that ‘the biochar is a solid material obtained from the
thermochemical conversion of biomass in an oxygen-limited environment’. [39]. It is critically important to differentiate between
terminologies like biochar, charcoal, and hydrochar. The primary
difference between these products lies in their fate. The charcoal is
a carbon-rich solid product prepared via charring biomass and is
used as a fuel source for producing energy where biochar is an
alternative term for charcoal when it is used for a particular
purpose, i.e. soil amendments and CCS [15]. On the other hand,
hydrochar is also a somewhat similar product to biochar but is
produced from a completely different pre-treatment process and
conditions. Typically, biochar is produced as a solid by-product
material in a dry carbonization process like pyrolysis, where
hydrochar is produced as slurry (a two-phase mixture of solid
and liquid) via hydrothermal carbonization (HTC) [35,40–43].
However both the chars (biochar and hydrochar) significantly
differ from each other in terms of their physical and chemical
properties [44,45].
2. Production of biochar and hydrochar
2.1. Feedstock for the production of biochar and hydrochar
Classification of biomass feedstock for the production of char is
significantly important because the selection of pre-treatment
method and its feasibility significantly depends on the type of
feedstock (wet or dry). The categorization of wet and dry biomass
feedstocks is made on the basis of initial moisture content. A
freshly harvested biomass such as vegetable wastes, sewage
sludge, animal wastes, algae, etc. generally has high moisture
content ( 430%) and is thus referred to as ‘wet biomass’, where
biomasses like agricultural residues and few wood species typically have low moisture content (o30%) at the time of harvesting
and are therefore classified as ‘dry biomass’ [46]. A wet biomass
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can be dried to low moisture content feedstock with the supplement drying techniques; however, such techniques are highly
energy intensive and can reduce the system's overall economic
efficiency [47,48].
Wet and dry biomass can be further classified into two
categories: (i) purpose-grown biomass and (ii) waste-biomass
[49]. Purpose-grown energy crops like miscanthus, switchgrass,
etc. have a relatively high yield and energy content, and generally
need very low maintenance when compared to other crops. Both
miscanthus and switchgrass usually have low moisture content
(below 10%) at the time of harvesting and therefore eliminate
supplement drying requirement. However, the time of harvesting
can significantly affect the ash-content of biomass, which may
negatively impact the combustion behavior [50]. Such crops are
potential candidates for sustainable energy production and environmental management, but thus far these crops have primarily
been focused by bio-refinery industries for the production of liquid
biofuels [51]. The second category, waste biomass, is more comprehensive that includes agro-forestry waste, animal manure
waste, organic-food wastes, sewage sludge, and MSW [52]. Using
waste biomass for the production of biochar is a reasonable option
because such types of feedstocks do not have any economic value
and moreover do not compete with the food crops for land
requirement. However, there is no universal consensus on what
constituents are in the definition of waste biomass, because
sometimes crop residuals are often left in the field to regain and
satisfy the specific soil properties [53]. Overuse of such waste byproducts for the sake of bioenergy development can disturb the
overall environmental life cycle. On the contrary, using wastebiomass as a feedstock for the production of biochar and hydrochar is beneficial in terms of maintaining eco-friendly environment and utilization of waste streams [15,41].
biochar. In this review paper, the characterizations of biochar
from slow-pyrolysis and hydrochar from HTC process are mainly
discussed.
2.2.1. Pyrolysis
Pyrolysis is a thermochemical decomposition process during
which biomass is heated at elevated temperature (300–650 1C) in
the absence of oxygen. The process results in the formation of
three main products: carbon-rich solid product (biochar), a volatile
matter which can further be partially condensed to liquid phase
(bio-oil), and the remaining so-called “non-condensable” gases,
like CO, CO2, CH4, and H2 [55,56]. Depending upon the reaction
time, temperature, and heating rate the pyrolysis process is subdivided to four categories: slow, fast, flash, and intermediate
pyrolysis [57–61]. Slow-pyrolysis is regarded as the main pyrolysis
process for the production of biochar because of higher solid yield
(25–35%) compared to the other pyrolysis processes [56,62].
During slow-pyrolysis, biomass is heated in a temperature range
of 300–650 1C with long residence time (few minutes to couple of
hours) and low heating rates (10–30 1C/min) [58]. The reaction
time, temperature, pressure, heating rates, and initial moisture
content of raw material are considered as the main key parameters
affecting percentage yield and physicochemical properties of
biochar [55,57,63,64]. Low operating temperatures and slow
heating rate favor high solid product yield [64,65], where the high
operating temperature and high heating rate show significant
influence on the carbon percentage, HHV, and BET-surface area
of biochar [66]. Under a typical slow-pyrolysis process the three
end products are roughly distributed in the same ratio. Although
the previous research thoughts for the target product in this
technique were the production of biochar, the recovery of liquid
by-products for production of organic acids and biofuels, and the
recirculation of gaseous by-products were achieved to improve the
overall efficiency of slow-pyrolysis process [67].
2.2. Biochar and hydrochar production technologies
Currently, several techniques are available for the production of
biochar; however, depending upon the type of feedstock (wet or
dry) and the desired properties of biochar for its different
applications, the choice of pre-treatment method(s) is very limited. As per definition, under all the thermal pre-treatments,
biochar is generally produced by heating biomass at high temperature in the absence or limited supply of oxygen. Thermal pretreatments are classified based on their operating conditions such
as: severity of process parameters (mainly reaction time and
temperature), pre- and post-processing requirements like shaping,
sizing, drying, cooling, condensation, etc. [12,40,54]. Few common
thermal pre-treatments and their approximate products yield are
discussed in Table 2. The solid products produced under processes
like gasification and dry torrefaction are somewhat similar to
biochar; however, for some reasons (discussed in later sections)
the solid product from these processes is not regarded as an ideal
2.2.2. Dry Torrefaction
Dry torrefaction, also referred to as mild pyrolysis, is a process
during which biomass is heated in an inert atmosphere at
temperatures of about 200–300 1C for residence times of 30 min
to a couple of hours [7,68,69]. This process results in approximately 30% mass loss, with only 10% of the energy contained
within the biomass lost in the form of gases. Therefore the specific
energy density of the torrefied solid product is increased [6,69].
Torrefaction process has gained considerable interest in the field of
bioenergy as an important pre-processing step for improving the
physicochemical properties of biomass for combustion [69]. However, torrefied biomass cannot be referred as a “biochar”, because
torrefaction is just the beginning of the pyrolysis process; therefore the torrefied biomass still contains some volatile organic
compounds (original compounds of biomass). With regard to its
physicochemical properties, torrefied biomass has properties
in-between that of raw biomass and biochar. A wide range of
Table 2
Classification of different thermochemical pre-treatments in terms of operating conditions and product(s) yields. With reference to the literature in Section 2.2.
Pre-treatment
Slow pyrolysis
Gasificationa
Dry-torrefactiona
HTC
Operating temperature (1C)
300–650
600–900
200–300
180–260
Residence time
5 min–12 h
10–20 s
30 min–4 h
5 min–12 h
Heating rate
10–30 1C/min
50–100 1C/s
10–15 1C/min
5–10 1C/min
Typical product yield (%)
Solid
Liquid
Gases
25–35
o 10
60–80
45–70
20–30
o5
–
5–25
25–35
4 85
20–40
2–5
a
Generally solid product from these technologies is not regarded as Biochar, because either the solid yield is very low or the solid does not have the same properties as
those of biochar.
H.S. Kambo, A. Dutta / Renewable and Sustainable Energy Reviews 45 (2015) 359–378
363
literature is available on the torrefaction of woody and agricultural
biomasses [70–79].
2.2.3. Gasification
Gasification is a process of partial combustion of biomass at a
very high temperature range (600–1200 1C) for short residence
time (10–20 s) [43,80]. The primary product of gasification is a
mixture of gases (CO, H2, and CO2), also referred to as Syn gas
(Synthetic gas) or producer gas and is itself a fuel [81–83].
Technically in an ideal gasifier no biochar is produced, because
most of the organic material is converted to gases and ash.
However in real practice, the process ends up with a small yield
(o 10%) of biochar [43]. The biochar produced from gasification
process contains a high amount of alkali and alkaline earth metals
(Ca, K, Si, Mg, etc.) and polyaromatic hydrocarbons (PAHs) that are
highly toxic compounds produced from high-temperature reactions [84]. Therefore, using such toxic by-product as a source of
soil amendment is potentially problematic [85,86]. Moreover, the
biochar for gasification process does not fall under IBI standards,
which states that, “because of the known presence of heavy metals
and organic pollutants in bio-solids, care must be taken during
thermochemical conversion to avoid harmful air emissions as well
as the accumulation of toxicants in the final biochar material” [87].
Hence, no biochar from gasification process was included in the
present study for comparison. However, few recent studies have
been performed with the field applications of biochar from
gasification process and have shown positive effects compared to
non-biochar soils [88–90].
2.2.4. Hydrothermal carbonization
Hydrothermal carbonization (HTC), also referred to as wet
torrefaction, is a thermochemical process of converting organic
feedstock into a high carbon rich solid product. HTC is performed
at the temperature range of 180–260 1C during which biomass is
submerged in water and is heated in a confined system under
pressure (2–6 MPa) for 5–240 min [35,91,92]. 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 was first
proposed by Friedrich Bergius in 1913 to describe the natural
coalification process [93]. Later in the last decades of the 19th
century, the process gained attention as a method of hydrothermal
degradation of the organic materials for the synthesis of important
chemicals along with the recovery of liquid and gaseous fuels
[27,91]. But the recent research activities on HTC are more focused
on the production of solid products (hydrochar) that have several
value-added applications in the industry and environment
[35,42,94–99]. With an increase in the severity of reaction, i.e.
for the temperatures above 260 1C, the hydrothermal process is
further classified into two techniques: (i) hydrothermal liquefaction (HTL) and (ii) hydrothermal vaporization (HTV) or hydrothermal gasification (HTG), or super critical water gasification
(SCWG) [97,98]. Because the mainly desired product in HTL and
HTV is the production of liquid and gaseous fuels instead of solid
fuels, therefore, both these technologies are not discussed in this
study. The classification of the hydrothermal processing of biomass
with respect to temperature and pressure is shown in Fig. 2.
As the process itself is carried out in the presence of water,
therefore, is not affected by the high moisture content of feedstock.
This unique advantage of the HTC process eliminates the pre-drying
requirement of wet biomass, which is a huge energy intensive
process and a financial load in biomass pre-processing especially
when performed under conventional thermal pre-treatments like
slow-pyrolysis and dry torrefaction [100]. The HTC process results
in the formation of three main products: solid (hydrochar), liquid
Fig. 2. Classification of hydrothermal processing of biomass with reference to the
pressure–temperature phase diagram of water.
(bio-oil mixed with water) and small fractions of gases (mainly
CO2). The properties and percentage distribution of the final
products strongly depend upon the process conditions [101].
Although both reaction time and temperature have been observed
to influence the physicochemical characteristics of products, the
reaction temperature remains the governing process parameter
[102]. Hydrochar is the desired product in the HTC process with
the mass yield of about 40–70% [101]. 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 obtained via torrefaction [101,103–105].
The addition of salts and acids has been recommended by few
researchers to improve the physicochemical properties of hydrochar, and to reduce the reaction pressure and temperature
[106,107]. However, the selection of a catalyst should be made very
carefully as it can cause the pitting of reactor.
When leaving the HTC 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 be used as a fuel. Usually, the
mechanical dewatering of wet materials like sludge and paper
pulp can reduce the moisture content to a range of 70–75%.
However, thermal drying is required to achieve further low
moisture content. Since an HTC process of biomass removes a
fraction of the oxygen from feedstock via decarboxylation and
dehydratization reactions, the moisture content of a 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 thermal drying of hydrochar
[108]. The pulverization of hydrochar is a much less energy
intensive process when compared to that of raw feedstock [109].
Moreover, these pulverized particles exhibit a spherical shape,
which further facilitates the fluidization process during gasification [110]. The surface of hydrochar shows a high degree of
aromatization with the large number of oxygen-containing groups.
The presence of oxygen-containing groups on the surface of
hydrochar explains its affinity for the water; therefore it can be
used to increase the water retention capacity of the soil [111]. The
high conversion efficiency, elimination of pre-drying requirement,
and relatively low operating temperature among other pretreatments make HTC a perfectly suitable conversion technique
for the production of hydrochar, especially from wet biomass
feedstock [100,112–114]. However, there are few challenges associated with the HTC process and are discussed in Section 5.
2.2.4.1. Properties of sub and supercritical water. From the phase
diagram of water, the critical point is at 374 1C and 22.1 MPa.
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Fig. 3. Physical properties of water with temperature, at 24 MPa [117].
Liquid water, below the critical point, is referred to as subcritical
water and above as supercritical water. The HTC pre-treatment of
biomass is carried out in the 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. Water at high
temperature and high pressure has a high degree of ionization
and starts dissociating into acidic hydronium ions (H3O þ ) and
basic hydroxide ions (OH ), therefore, shows both acidic and basic
characteristics [115,116]. Fig. 3 shows the effect of temperature on
the physical properties of water. The dielectric behavior of 200 1C
water is similar to that of ambient methanol, 300 1C water is
similar to ambient acetone, 370 1C water is similar to methylene
chloride, and 50 1C water is similar to ambient hexane [117].
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 room temperature and
subcritical state. At this state the density of water reduces
drastically and is in between that of water vapor and liquid, and
the water shows 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 processes with the aim of
producing hydrogen gas [98]. Both sub- and super-critical water
have unique properties that can be used for the destruction of
hazardous wastes such as polychlorinated biphenyls (PCBs) and
polychlorinated di-benzofurans (PCDFs) [118].
2.2.4.2. Role and importance of water in hydrothermal
carbonization. The primary goal of a thermochemical pretreatment is to break down the rigid structure of the biomass
polymers into small and low molecular weight chains. The rate of
destruction of the polymeric structure of biomass typically depends
upon the reaction time, temperature, and reaction medium. In the
HTC process 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, favoring the rapid
degradation and depolymerization of these polymers into water
soluble products like oligomers and monomers [27,42,119]. As water
is cheap, non-toxic, and is inherently present in the wet biomass, it
is used as a reacting medium in the HTC process. Also, water is an
alternative to the corrosive chemicals and toxic solvents [120,121].
The lignocellulosic biomass has specific amount and different types
of inorganic elements in its structure. These inorganic elements are
the alkali and alkaline earth metals such as calcium (Ca), magnesium
(Mg), phosphorous (P), potassium (K), sodium (Na), sulfur (S), iron
(Fe), etc. that are left behind in the form of ash during biomass
combustion. In ash, these metals occur in their oxide forms [122],
CaO, MgO, P2O5, K2O, Na2O, SO3, Fe2O3, that shows various
undesirable and notorious behavior such as slagging, fouling,
klinker formation, corrosion, etc. during biomass combustion
[123]. In HTC process due to the formation of acetic acid in liquid
by-product stream, acid solvation mechanism would solubilize and
leach out these inorganic elemental compositions, reducing the
overall ash content of the solid product, which is not possible in
case of dry thermal pre-treatments [100,105,106]. Moreover, HTC
process can convert organic chlorine (Cl) into inorganic Cl, thus
reducing the potential for corrosion and dioxin formation occurring
in the combustion of feedstock with high Cl content [113].
The HTC process water has been reported containing some
phenolic, organic, and furan derivative compounds like acetic acid,
formic acid, glycolic acid, levulinic acid, and 2,5-hydroxyl-methylfurfural (HMF) that are formed via the degradation of biomass
polymers [124,125]. Products like levulinic acid are key blocks for
the manufacture of 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 manufacture of 2,5dimethylfuran (DMF), which is a liquid biofuel that is superior to
ethanol in certain ways [126]. Both these products, Levulinic acid
and 2,5-HMF, have been identified as “top 12 value added chemicals
from biomass” by US Department of Energy (DOE) [127]. The
identification, recovery, and characterization of these high-quality
intermediate compounds could act a potential platform for the biorefinery and other chemicals manufacturing industries [124].
Generally, for an HTC process, water is added to the system at a
ratio of 3–0 times the mass of dry biomass [14,91,92,103]. For an
industrial HTC plant, the continuous supply of process water will
be one of the challenges for its operation. Previous experiments
have shown that the hydrochar samples obtained at 260 1C have
coal-like combustion properties [102]. However, to produce such a
type of material, massive amounts of water may be required at
full-scale operation, which may not be an economical process. For
example, producing one metric ton of dry hydrochar from miscanthus at 260 1C assuming a 50% mass yield of hydrochar and a
1:6 solid load ratio of (dry) biomass to water would require 12
metric tons of liquid water [102]. At an industrial scale, this
substantial demand for process water may outweigh the superior
properties of hydrochar in terms of production cost. Hence, for
obvious reasons the recirculation of process water makes more
sense and would increase the system's overall efficiency. Other
important advantages associated with the recirculation of HTC
process water are
(i) Reduction in the wastewater that would also reduce the
wastewater treatment cost associated with HTC plant,
(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 [128],
(iii) the formation of acetic acid in the process water may further
catalyze the process during recirculation and therefore can simultaneously reduce the pressure and temperature requirements,
(iv) the degraded sugar compounds from the biomass polymers
present in the HTC process water might deposit in the porous
structure of hydrochar, which may further augment the
overall energy density of solid, and
(v) the HTC process water can be used for the anaerobic digestion
of certain types of biomass [129].
3. Chemical reaction mechanism(s) behind the production of
hydrochar and biochar
During the production of biochar or hydrochar, biomass undergoes series of chemical reactions mechanism that are highly
complicated and partially understood [35,42]. Most of these
H.S. Kambo, A. Dutta / Renewable and Sustainable Energy Reviews 45 (2015) 359–378
chemical reactions are somehow similar in every thermochemical
conversion processes, i.e. the degradation and depolymerization of
polymeric composition of biomass take place, resulting in the
formation of solid, liquid and gaseous (by-) products. The fundamental difference in different thermochemical treatments lies in
the operating conditions and reaction medium that are used for
the production of biochar and hydrochar. The highest reaction
temperature (HRT) reached during a thermochemical process is
the main parameter controlling the reaction mechanism and the
physicochemical properties of char. Decarboxylation, dehydration,
de-carbonylation, de-methoxylation, intermolecular derangement,
condensation, aromatization, etc. are some of the proposed chemical reactions that take place during the thermochemical conversion of biomass [42]. The reaction temperature significantly
governs which reaction dominates. However, in real practice, it is
difficult to maintain uniform temperature profiles in pyrolysis
reactors; therefore, it is most likely possible that many of the
aforementioned reaction mechanisms take place simultaneously
[35].
The thermal stability of polymeric composition of biomass
significantly depends upon the reaction medium in which the
process is carried out. Under standard pressure conditions like in
pyrolysis and torrefaction the decomposition of hemicellulose
takes place between 200–300 1C, followed by cellulose that
decomposes at higher temperatures (300–400 1C). Lignin in the
most thermos-chemically stable polymer in a lignocellulosic
biomass that decomposes around 600 1C [23]. In contrast, during
HTC the degradation/depolymerization of biomass composition
occurs at significantly low temperatures compared to that in
torrefaction and pyrolysis [103]. The degradation of hemicellulose
and cellulose under HTC process starts at around 160–180 1C,
where most of the lignin still remains stable until near or above
critical point of water [27]. The degradation of lignocellulosic
composition of biomass in the HTC process is controlled by
reaction mechanisms very similar to those in the pyrolysis process.
However, due to the presence of hot compressed water in the HTC
process the degradation of biomass is primarily initiated by
hydrolysis, resulting in the cleavage of ether and ester bonds
between monomeric sugars by the addition of one molecule of
water [27] and thereby reducing the activation energy levels of
biomass polymers [35]. During HTC, the cellulose and hemicellulose are partially or fully driven off, leaving behind a product
(char) with high lignin content. The HHV of lignin is the highest
among biomass polymers and is around 26 MJ/kg followed by
cellulose and hemicelluloses in between that of 17–18 MJ/kg [11].
Based on the thermal stability and heat of combustion value, a
feedstock with a high ratio of hemicellulose to lignin in it will
show higher mass loss and higher energy densification when
compared to the one with low hemicellulose to lignin ratio. This
effect was clearly observed in the study performed by Yan and coworkers [103] on woody biomass (loblolly pine) under torrefaction
and HTC, and also in the experiments carried out by Demirbas
[130] on agricultural residues (corn cobs, olive husks and tea
waste) under pyrolysis.
During HTC, the degradation of hemicellulose in an aqueous
solution results in the formation of 2,5-HMF [42], a compound
whose HHV (22.06 MJ/kg) is considerably higher than that of the
hemicellulose and cellulose [131]. Concentration of 2,5-HMF in
aqueous phase increases with an increase in the reaction temperature [101]. Moreover, a slightly porous surface with improved
adsorption capacity is often obtained for the hydrochar particles
produced under HTC process [132]. Additional activation steps are
sometimes required to further increase the porosity (refer activation steps described in Section 4.2.1) [95]. Deposition of 2,5-HMF in
the porous structure of hydrochar can augment the overall energy
density of hydrochar. This phenomenon supports the reason
365
behind the high energy density for the hydrochar obtained via
HTC compared to the biochar produced from pyrolysis. Addition of
salts and acids (i.e. reducing pH) in process has been suggested to
catalyze the degradation of hemicellulose and cellulose [106,107].
These chemicals interrupt between the hydrogen bonding of
polymer strands and thus facilitate the solubilization and removal
of organic compounds from biomass structure. An increase in the
addition of acetic acid (10–50%) in the HTC process positively
influences the formation of 2,5-HMF in the liquid phase [133], and
therefore high concentration of 2,5-HMF particles will most likely
precipitate in the porous structure of hydrochar, resulting in an
increase in the overall HHV of hydrochar. The formation of 2,5HMF in the aqueous phase significantly depends upon the process
operating conditions like reaction time, temperature, and nature
and amount of acid used in the process [134]. Too high percentage
addition of acid in the process can rehydrate 2,5-HMF particles to
levulinic acid and formic acid. The HHV of levulinic acid (20.09 MJ/
kg) and formic acid ( 15 MJ/kg) is considerably lower than that of
HMF (22.09 MJ/kg) and therefore can reduce the overall heating
value of hydrochar [135].
4. Characterization of biochar and hydrochar
Different thermochemical pre-treatments have different operating conditions and process parametric requirements, hence
resulting in the formation of a final product with different physical
and chemical characteristics. It is critically important to characterize biochar and hydrochar because their characterization will play
a vital role in determining their importance and application in the
industry and environment. For example, a biochar with low carbon
content and high ash content is not suitable for energy product,
and in the same way a biochar with low surface area and low
adsorption capacity is not meant for agricultural and wastewater
treatment industries.
4.1. Solid yield and chemical composition characterization of
biochar and hydrochar
High alkali and alkaline earth metallic composition of biomass
is a huge challenge for its application in the energy sector. These
metallic compositions show undesirable and notorious behavior
such as slagging, fouling, klinker formation, corrosion, etc. during
biomass combustion [123]. The percentage composition of these
metallic species is directly related to the percentage ash of the
original feedstock. The reduction in ash composition from biochar
or hydrochar would become highly advantageous if used for
producing power. As the HTC process is carried out in the presence
of water, which provides an opportunity to demineralize these
inorganic compositions from biomass to the liquid product stream,
it reduces the overall ash content of solid product. However,
efficient utilization of wastewater produced from HTC process still
remains a challenge for the application of HTC at the industrial
scale. As an alternative, the recirculation of HTC process water
could solve the problem. Advantages of process water recirculation
have been already discussed in Section 2.2.4.2.
Table A1 (see Appendix) shows the proximate and ultimate
analysis of raw and pretreated biomass under slow-pyrolysis and
the HTC process. The hydrochar samples produced via HTC process
shows considerable reduction in the ash content when compared
to the raw biomass feedstock. Unlike hydrochar samples, the
biochar obtained via slow-pyrolysis showed an increase in the
percentage ash compared to that of raw biomass. The primary
difference between these two processes is the presence of compressed liquid in the HTC process that aids in the demineralization
of ash composition from biomass, resulting in the reduction of ash
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content from biomass. Based on the information shown in Table
A1, it can be noticed that there is always a compromise between
the quality (percentage carbon and HHV) and the quantity
(percentage mass yield) of the final solid product. A process
carried out at low temperatures shows high mass yield and low
carbon conversion efficiency, where the one performed at high
temperatures shows completely opposite behavior. In general, a
feedstock with the high ratio of hemicellulose to lignin in it shows
low solid yield and high volatiles yield [36,136]. The difference in
the mass loss of the feedstock in the two different thermal pretreatments is mainly due to the variation in the degradation of
biomass polymers. The extent of degradation of biomass polymers
significantly depends upon the reaction medium in which the
process is carried out [35]. Under hydrothermal conditions, the
presence of subcritical water causes biomass polymers (mainly
hemicellulose) to partially transform into an aqueous phase and
therefore cause a significant mass loss of the solid product.
The chemical characterization of biochar and hydrochar in this
section is made on the basis of proximate analysis (ash and fixed
carbon), elemental analysis (O/C–H/C ratios), and HHV. In both the
processes, the solid yield and H/C–O/C ratio decrease where the
energy density increases with an increase in the reaction time and
temperature. The highest (peak) reaction temperature reached
during the process is the most critically important parameter
controlling the reaction mechanism and the physicochemical
properties of the final product. The HHV of lignin is considerably
higher than that of the hemicellulose [11]; therefore, the removal
of hemicellulose from biomass would yield a product with
increased lignin content and increased energy density. Due to
the presence of subcritical water, the decomposition of hemicellulose is relatively rapid in the HTC process than in slow-pyrolysis
[103]; therefore as expected, the high energy density for hydrochar samples can be observed in Table A1 compared to the biochar
samples produced via slow-pyrolysis. The high HHV observed for
the hydrochar samples is also related to the formation of highquality intermediate compounds like 2,5-HMF that have HHVs
close to that of lignin. The precipitation of such compounds in the
porous structure of hydrochar would increase the overall HHV
(carbon content) of hydrochar.
To compare the variation in elemental compositions, the atomic
ratios of hydrogen to carbon (H/C) and oxygen to carbon (O/C) of
both raw and pre-treated samples are plotted in Fig. 4 (VanKrevelen
diagram [137]). The “Van Krevelen diagram” provides the general
information about the quality and type of fuel. A fuel with a low O/
C–H/C atomic ratio is considered highly favorable because of the
decreased smoke, water vapor and energy losses experienced during
combustion [138]. For comparison, H/C–O/C ratios of bituminous
coal and lignite were also plotted on the same diagram. The values
Fig. 4. Van-krevelen diagram with reference to the data shown in Table A1
(Appendix).
of these coals were used from a study performed on the HTC of
lignocellulosic biomass [139]. Both the chars exhibit low H/C–O/C
ratios than the raw feedstock. However, the H/C–O/C ratios for the
hydrochar samples were found higher and similar to those of natural
coal than for the biochar obtained from slow-pyrolysis. This suggests
that the ratio of reaction rates of decarboxylation to dehydration is
higher in HTC compared to that in the slow-pyrolysis process [35].
4.2. Morphological characterization of biochar and hydrochar
The polymeric composition of biomass feedstock has direct
influence on the physicochemical properties of the char produced
via thermochemical pre-treatment. At temperatures above 160 1C
and 180 1C for HTC and pyrolysis, respectively, the polymeric
composition of biomass undergoes series of chemical reactions,
leaving behind a product with modified surficial structure and
properties [35,140,141]. The fundamental molecular structure of
both chars shows improved surface area, porosity, and extensive
aromatic features; however, the structural arrangements and surface functionalities of both the chars are significantly different
from each other.
The biochar produced via slow-pyrolysis is composed of turbostrategically arranged graphite-like layers, and the spacing (surface
area) between these layer increases with an increase in the reaction
temperature [140]. The biochar samples produced at around 350 1C
are mainly dominated by aromatic (aryl) carbon with small proportions of alkyl-O and alkyl-C. When the reaction temperature is
further increased (4500 1C), these alkyl-O and alkyl-C were completely converted to aryl-C and these chars usually have very low H/
C ratios. Unlike biochar, spherically shaped carbonaceous nanoparticles are generally obtained on the surface for the hydrochar
samples. Since the reaction mechanism for an HTC process takes
place inside a sealed reactor, the reaction chemistry behind the
formation of these carbonaceous particles is highly complex. To date,
two different types of structural models have been proposed
explaining the physicochemical characterization of HTC-derived
carbonaceous materials [95]. The first one was carried out using
glucose, sucrose, and starch as a feedstock with the reaction
temperature at 170–240 1C with the residence time of 0.5–15 h
[44]. The characterization in this model was made based on Fourier
transform infrared spectroscopy (FTIR) and X-ray photoelectron
spectroscopy (XPS), which proposed the structure as a polyaromatic lignin-type matrix with a large number of oxygenfunctional groups. It was suggested that these carbonaceous materials exhibit a shape very similar to that of a core–shell type structure,
composing a hydrophobic-core (containing stable oxygen groups
like ether, quinone, pyrone) and a hydrophilic-shell (containing a
large number of reactive oxygen functional groups like hydroxyl/
phenolic, carbonyl, or carboxylic) [44]. On the other hand the second
model (more recent and viable) was performed using glucose as a
component in the HTC process with a reaction temperature at 180 1C
and with the residence time of 24 h [142]. The characterization in
this study was investigated using solid-state 13C Nuclear-MagneticResonance (NMR) technique, which revealed the fact that about 60%
of the carbon atoms in a hydrochar sample are cross-linked directly
via either sp2 or sp3 type carbon groups, in a similar way to that of a
furan ring structure [142]. Another study performed for the surface
characterization of char samples prepared from swine manure
suggested that the biochar from pyrolysis (produced at 620 1C with
the residence time 2 h) mainly contains aromatic groups on its
surface, where the hydrochar samples from the HTC process
(produced at 250 1C with the residence time of 20 h) were mainly
dominated by alkyl moieties [143]. Similar results were also confirmed by Guiotoku and co-workers via microwave-assisted HTC
process (at 200 1C for 240 min) and pyrolysis (at 500 1C for 10 s);
moreover, the hydrochar particles consist of very tiny aggregate
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microspheres with the diameter of about 2.0 mm [144]. The process
reaction temperature was identified as the primary factor controlling the shape, diameter, particle size distribution, and degree of
aromatization of these derived carbonaceous particles [95].
The distinctions in structural and surficial features of both the
chars are mainly due to the different reaction medium used in the
slow-pyrolysis and HTC process, leading to the different reaction
pathways and formation of different intermediate compounds.
During slow-pyrolysis, the reaction pathway is believed to be a
free-radical process initiated by the homolytic cleavage of bonds
that takes place around 300 1C [145]. With further increase in the
reaction temperature (i.e. above 300 1C and below 500 1C) the tar
components produced from the decomposition and degradation of
cellulose are mainly composed of anhydrosugars (anhydroglucose)
that are less reactive than the free radicals produced during bond
cleavage. These anhydroglucose intermediates (tar vapors) and
inorganic compounds present in the biomass feedstock volatilize,
which later condenses in the porous structure of biochar, accounting for the cross-linkaged crystal layered structure of biochar
[36,146]. In comparison, during HTC process due to the presence
of subcritical water, the reaction mechanism is initiated by hydrolysis of biomacromolecules, resulting in the formation of oligosaccharides, hexoses (glucose and fructose), pentoses (xylose), and
fragments of the lignin [35], followed by dehydration reaction
during which hexose and pentons are converted to furfural
compounds. These aforementioned intermediate compounds
further undergo simultaneous combination of reactions including
dehydration, condensation, polymerization, etc., resulting in the
growth of spherically shaped carbonaceous nano-particles with
the nano-particle sized distribution range of 0.5–5 mm [147,148].
There are many factors that could affect the shape and
structure of char particles. The few operating parameters that
regulate the physical shape and chemical properties of the char
during slow-pyrolysis and HTC are shown in Table 3. Although all
these parameters contribute to affect the physicochemical properties of char, the highest reaction temperature (HRT) remains the
most decisive parameter governing the char's structural properties. This is because the degradation, melting of polymers (hemicellulose, cellulose, and lignin), release of volatiles, formation of
intermediates compounds and their further transformations are all
temperature dependent. The high heating rate (HHR) remains the
second most important parameter as it controls the heat mass
transfer and the rate at which volatiles/intermediates are formed
[140]. HRT and HHR generally cause the melting of cell structure
and therefore can reduce the Brunauer–Emmett–Teller (BET) surface area and porosity. The mass yield of char is significantly
reduced when produced under thermochemical process at very
high HRT and HHR. For example in case of gasification and fast
pyrolysis, running the process at operating conditions above
600 1C (HRT) and 50 1C/min (HHR) results in char yield less than
10–30%.
In an experiment performed on wood species under pyrolysis,
the increase in reaction temperature (200–700 1C) showed a
positive influence on BET surface area (10–500 m2/g); however,
with further increase in reaction temperature (800 1C) the surface
area was significantly reduced (150 m2/g) [149]. In another study,
Table 3
Factor affecting physicochemical properties of char via pyrolysis and HTC, adapted from [15,42,157].
Parameter
BET-surface area and porosity
Solid mass yield (%)
Pyrolysis
Pyrolysis
HTC
Pyrolysis
HTC
Decreases
Decreases
Increase
Increase
HTC
Degree of de-hydration and decarboxylation or C/H–C/O ratios
Typically increases up
Increases up to 500 1C,
further increase negatively to 230 1C, further
increase shows
influence the properties
negative response
–
Generally increases from
5–100 1C/min, further
increase destroys the
porous structure
Increases
Increases
Decreases
–
Increase
–
Decreases
Decreases
Increase
Pressure (P)
Decreases
Catalyst (sub-or/tosupercritical
H2O, CO2, N2, air,
salts, acids, etc.)
Reactor (shape,
orientation,
stirrer)
Moisture content
of feedstock
Highest reaction
temperature
(HRT)
High heating rate
(HHR)
Reaction residence
time (RRT)
Pre-processing
(particle size)
Increase
Increase
–
Increases
Decreases (usually not
controlled)
Increases
Increase (highly
complex reaction
mechanism)
–
Decreases
Decreases
Increase
Increase
–
–
–
–
Homogenize Homogenize the
the reactions reactions
Supplement energy is
required for drying and
negatively affects the
properties
Increases with reduction in
size
Not affected (therefore Decrease
is best suitable for wet
biomass)
Solid load (ratio of –
reaction medium
to feedstock)
Generally not required if
Post-processing
performed above 400 1C
(grinding,
physical and
chemical
activation)
The effect is relatively
much less than
pyrolysis
Decreases
Not affected (the process –
itself required water)
Decreases with reduction Decreases (but the effect
in size
is insignificant)
–
Activation causes
Hydrochar has very
poor surface areas and volatilization of material
and thus would reduce
therefore is required.
the mass yield
Decreases (high ratios,
410:1 are typically used
for bio-oil production)
Activation causes
volatilization of material
and thus would reduce
the mass yield
Increases
The process itself
requires water and is
thus initiated by
hydrolysis reaction
–
–
Decreases
Activated
chars have
high
C-content
Activated chars have
high C-content
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during pyrolysis of safflower seeds with the reaction temperatures
in the range of 400–600 1C, the surface area of the biochar reduced
with an increase in the heating rates (10–50 1C/min) [150]. Similar
results were also reported by other researchers [140]. The primary
reason behind the variation in the morphological properties is due
to the volatilization of organic compounds resulting in the formation of voids within the biochar matrix. However, a very high
reaction temperature and high heating rates can destroy the fine
porous structure and can cause the de-volatilization/condensation
of volatile organic matter in the porous structure of biochar,
resulting in the clogging of pores and reduction in the overall
surface area of biochar [151]. The fate of inorganic compositions
results in a biomass feedstock significantly depending upon HRT. A
very high HRT can cause the melting of alkali and alkaline earth
metallic compositions of biomass in the porous structure of char
and therefore can negatively influence the porosity and surface
area [140].
In contrast, the hydrochar obtained via HTC typically have very
poor surface area and porosity [44,95,152]. The BET-surface area of
biochar and hydrochar samples obtained via pyrolysis (at 550 1C
for 15 min) and HTC (at 250 1C for 4 h) of corn stover was 12 and
4 m2/g, respectively [44]. Another study, using pinewood samples,
reported a surface area of 21 m2/g for the samples prepared via
HTC (at 300 1C for 20 min at the heating rate of 10 1C/min) and
29 m2/g for the same feedstock pre-treated under pyrolysis (at
700 1C for 2 h at the heating rate of 10 1C/min) [153]. The effect of
increasing reaction temperature on the surface area and porosity
in an HTC process is similar to the one under slow-pyrolysis. In a
recent study performed by Parshetti and co-workers [154] on
empty fruit bunches feedstock pre-treated under HTC for 20 min,
they found that with an increase in the reaction temperature from
150 to 250 1C the surface area of hydrochar increased from 6.08 to
8.03 m2/g; however, with the further increase in the reaction
temperature to 350 1C the surface area of hydrochar reduced to
2.04 m2/g. Mumme and co-workers [91] reported that an increase
in the reaction temperature from 230 to 270 1C for cellulose chars
and from 190 to 270 1C for digestate chars shows reduction in the
surface area from 27 m2/g to 8 m2/g and from 12 m2/g to less
than 1 m2/g, respectively. The scanning electron microscope (SEM)
images of hydrochar samples showed the loss of fibrous structure
with an increase in the reaction temperature, resulting in the
formation of a smooth surface. This explains the reason behind the
reduction in surface area with an increase in reaction temperature
[91]. With an increase in the residence time in an HTC process, the
transformations of cellulose and hemicellulose take place, resulting in the formation of micro-spherical shaped particles, which
can positively influence the surface area and porosity [155,156].
4.2.1. Tailoring the structure of chars via physical and
chemical activation
For industrial applications of chars as a low cost adsorbent
material, additional activation steps are often required to increase
the surface area and porosity. Activation of char can significantly
increase the surface area (1000–3000 m2/g) due to the development
and opening of internal porous structure of a biomaterial [158].
Physical and chemical activation methods are the two common
techniques used for the activation of chars [140]. During both the
techniques, char is exposed to elevated temperatures in the presence
of activation agents, which develops and improves the porous
structure. An increase in the activation time and temperature can
increase the burn-off (mass loss) and therefore will increase the
surface area and porosity of the final material [159].
Under physical activation the solid material is subjected to high
temperatures (4 900 1C) with the controlled flow of CO2 or
steam or sometimes with a mixture of both. Activation with
CO2 involves reaction of C-CO2, resulting in the removal of C
atoms by burning-off and the opening of closed pores, producing a highly porous structure. However, when the activation is
performed with steam, it results in the volatilization and partial
de-volatilization of char components, producing a highly
improved crystalline carbon structure [140].
Chemical activation is typically performed using potassium
hydroxide (KOH) as an activating agent. It can be performed
either at room temperature or at elevated temperatures (600–
800 1C). During chemical activation potassium from precursor
chemical separates the lamellae of crystallites that form the C
structure. After rinsing the sample with water, potassium is
washed away, leaving behind a structure with highly improved
surface area and porosity [140]. Chemical activation offers
several advantages such as the following: it involves low
temperatures, high surface area, and fast (single-step) conversion. However the method is not preferred over physical
activation because of secondary pollution issues of disposal
[159].
BET surface area of biochar samples prepared via pyrolysis of
broiler litter increased from 60 m2/g to 900 m2/g after acid
activation at 450 1C for 4 h [160]. The surface area of corn cobs
was found to increase with an increase in the activation temperature and residence time from a minimum value of 405 m2/g (at
1073 K for 20 min) to 1705 m2/g (at 1173 K for 80 min) performed
under CO2 activation and from 568 m2/g (at 1073 K for 20 min) to
1063 m2/g (at 1173 K for 60 min) performed with steam [161]. For
the biochar prepared from pistachios nut shells via pyrolysis,
surface area increased from 800 m2/g to 1946 m2/g with an
increase in the temperature from 500 1C to 800 1C. However with
further increase in the reaction temperature to 900 1C, the BETsurface area reduced to about 1800 m2/g. This reduction in
structural properties with the rise in temperature is mainly due
to the shift of micro-porosity to meso-and macro-porous structure
[162].
On the other hand, hydrochars generally have poor microporosity and low surface area and are therefore activated. Surface
area of physically activated hydrochars produced from horse
manure, grass cutting, beer waste, and bio-sludge was found to
be 749, 841, 622, and 489 m2/g, respectively. However the chemical activation of beer waste hydrochar produced in the same
experiment resulted in a significantly higher surface area of about
1073 m2/g [163]. Another study under physical activation of
hydrochars using air as an activation agent produced from sunflower stem, walnut shells and olive stone feedstock showed an
increase in the surface area from 31, 27, and 22 m2/g to 213, 434,
and 204 m2/g, respectively. However, when the activation was
performed using CO2 as an activating agent for the same hydrochar samples, it resulted in significantly high surface areas of
about 379, 438, and 438 m2/g, respectively. The physical activation
of hydrochar using air produces additional functionalities on the
surface of char that may be very beneficial for soil amendments;
on the other hand if the physical activation is performed using CO2
it reduces the oxygen groups on the surface of char [164]. The
chemical activation (using KOH at 3:1, for one hour at 800 1C) of
hydrochar samples produced from Tapioca flour showed considerably increased surface area (986 m2/g)[165]. The HTC reaction
temperature plays a significant role in determining the surface
area of hydrochars even before and after activation processes. The
hydrochars produced at 240 1C from cellulose, glucose and rye
straw were found to have significantly higher surface area than the
one obtained at 180 1C and 280 1C [166]. This shows that a too low
temperature may not be enough to cause decomposition reactions
and a very high reaction temperature can completely destroy the
H.S. Kambo, A. Dutta / Renewable and Sustainable Energy Reviews 45 (2015) 359–378
structure of hydrochar. These findings suggest a fact that surface
area, porosity, particle size distribution, and surface chemistry of
hydrochars can be easily controlled by adjusting reaction time,
temperature, and a suitable method of activation.
5. Potential benefits and applications of biochar and
hydrochar
Initial studies for the application of biochar were primarily
focused on using it as a source of soil amendments [15]. However,
the recent revolutions brought by the advancement in research
and technology for the field of pyrolysis and HTC have broadened
its applications. There are many applications of both the products
including but not limited to energy production, agriculture, carbon
sequestration, wastewater treatment, bio-refinery, etc.
5.1. Energy production
Both slow-pyrolysis and HTC process convert raw biomass
feedstock to a coal-like material that have improved physicochemical properties; therefore, the final material can be used for energy
production as an alternate to coal [167]. In general, the HHV of
polymeric composition of biomass follows this trend: ash oextractives ohemicellulose ocellulose olignin, reflecting the trend
of increasing carbon content [168]. During thermal pre-treatments
the decomposition, degradation, and depolymerization of polymeric composition of biomass take place [42]. Removal of hemicellulose and cellulose from biomass results in an increase of the
C:O ratio, which ultimately increases the HHV final solid product.
Hemicellulose and cellulose degrade into monomers, furfurals, and
5-HMF under subcritical water conditions. The higher HHV of
5-HMF (22.06 MJ/kg) compared to hemicellulose (17.58 MJ/kg) and
other extractives (glucose is 15.57 MJ/kg) may increase the overall
HHV if 5-HMF is deposited in the porous structure of hydrochar
[42,131,169]. Since 5-HMF is produced with the HTC process at
higher reaction temperatures from the degradation of hemicellulose and cellulose, it may augment HHV in hydrochar [35]. This
phenomenon may explain the higher HHV of hydrochar compared
to biochar from dry thermal pre-treatment at the same temperature with an even longer residence time [102,103,170]. During the
HTC process, complete degradation of hemicellulose was observed,
resulting in a product with increased lignin content, which acts as
a natural binder and facilitates the densification process [30,171].
Moreover, the polymeric composition of biomass has a strong
influence on the hydrophobic behavior. Among the polymeric
composition of biomass, hemicelluloses have the greatest capacity
of water adsorption, while lignin shows little tendency for water
sorption [172]. Hence, the removal of hemicellulose (or the
fractional increase in the lignin content) from a biomass feedstock
will improve its hydrophobicity and therefore allow longer selfstorage without any risk of biodegradation. Hydrophobicity of
biochar and hydrochar increases with an increase in the reaction
time and temperature [102,173].
The depolymerization reactions during thermal pre-treatment
cause shortening of biomass polymers [174]. The temperatures for
degradation of biomass are considerably lower for HTC than for
torrefaction [103]. This variation in the polymeric structure of char
makes it highly friable in nature and therefore easier to grind. The
pulverized particles of hydrochar show spherical shaped structures,
which may facilitate the fluidization process during combustion [110].
Most importantly, the hydrochar shows considerable reduction in the
ash content compared to that of raw feedstock and biochar produced
via slow-pyrolysis. The removal of alkali and alkaline earth metals
from raw biomass eliminates the risk of fouling, scaling, slagging and
369
corrosion during combustion, making hydrochar a very suitable
candidate for energy production [102,105,114,168].
5.2. Carbon sequestration and gas adsorbent
When a biomass feedstock is converted to biochar and is then
stored in a reservoir (soil), the process is called carbon sequestration or carbon capture and storage (CCS). This storage of carbon in
soil is the net removal of carbon from the atmosphere. When the
storage is carried out in a deliberate manner, the process can result
in a carbon-neutral or even carbon-negative environment, thereby
compensating for the effect of anthropogenic CO2 emissions. The
application of biochar in soil has received a great deal of worldwide interest in recent years as a strategy for CO2 mitigation
[49,175–177]. The idea of biochar (carbon) sequestration was
derived from the Terra Petra soils of Amazon region. These soils
are reported to have considerable amount of carbon (in the form of
char) stored in the soil and improved microbial diversity compared
to adjacent soils in the surrounding area [33,34,49]. The biochar
present in the Terra Petra soils is expected from the anthropogenic
activities by indigenous people living in the Amazon basin
between 600 and 8700 years ago [178], which suggests the longterm stability of biochar in the soil. Hence, amendment of biochar
in soil is largely hypothesized for carbon sequestration; however,
the stability of biochar in soil depends on several factors [179]. One
of the most important factors affecting the fate of biochar in the
soil is the method used for the production of char [149].
Slow-pyrolysis is a widely used process for the production of
biochar under dry condition. However, the process has a few
limitations. First, the pyrolysis of biomass releases harmful gases
(CO, CH4, and PAHs) and oils to the atmosphere [180]. The control
over these emissions, their recovery, and management involve the
use of complex pyrolysis equipment, which is considerably more
sophisticated and expensive than the traditional kilns [181].
Second, the chemical reactions (generally oxidation) or anaerobic
microorganism activity in a biochar pile during storage or transportation can produce heat at such a sufficient rate that it can
cause self-heating of the biochar stockpile, which can lead to selfignition of biochar [182]. Third, this process is not suitable for the
production of biochar from wet biomass due to the pre-drying
requirement of the feedstock.
In contrast, HTC is a promising method, which can utilize a
broad range of unconventional biomass feedstocks, such as sewage
sludge, animal, and agricultural wastes [100,114,183], without any
pre-drying of feedstock for char production. Unlike pyrolysis, HTC
does not generate large amounts of harmful gases and the
hydrochar particles are not prone to auto-ignition due to the high
concentration of surface oxygen groups. However, to date very few
studies have investigated the potentials of using hydrochars for
carbon sequestration [149,184–187]. Unfortunately, the results
from these studies are not in favor of hydrochar for carbon
sequestration due to its lesser stability in soil. During field
application of char from corn silage produced via HTC and
pyrolysis, Malghani and co-workers [188] found that HTC-char
decomposed rapidly (50% in 100 days regardless of soil type) and
stimulated emissions of CH4 and CO2 to the atmosphere. Similar
effect on the emissions of CO2, N2O, and CH4 with the application
of hydrochar was also observed by other researchers [189,190].
Higher emissions of GHGs with the application of hydrochar in the
soil are most likely the result of increased microbial activity due to
easy degradability of carbon in the hydrochar. In contrast, the char
from slow-pyrolysis process was found to be more stable in the
soil and showed a consistent effect on GHG emissions. In summary, the adverse effects of hydrochar in soil may outweigh the
advantages of HTC process over slow-pyrolysis. Therefore, when
estimating the carbon sequestration potential of different
370
H.S. Kambo, A. Dutta / Renewable and Sustainable Energy Reviews 45 (2015) 359–378
hydrochars, further in-depth research should be carried out to
optimize the application of hydrochar in the soil with improved
stability, low GHG emissions, and positive effects on the agricultural productivity.
5.3. Agriculture
Depending upon the type of biochar, precursor feedstock,
production process and conditions, application rate, pH, surface
area, porosity, and type of soil (fertile, infertile, sandy, loamy clay)
the crop yield response can be either productive or counterproductive with the application of biochar in soil [41,191–194].
Based on the information available in the literature [35], results
have been reported more profoundly:
In degraded or unfertile soils rather than already fertile soils;
in tropical soils rather than in temperate soils;
in combination with NPK fertilizers or nutrient-releasing substances rather than without extra nutrient supply;
when the chars themselves were sources of nutrients, e.g., biochar
from poultry litter.
The physical structure (high surface area and porosity) of
biochar improves soil aeration and provides an asylum to the
beneficial soil organisms like arbuscular mycorrhiza (AM), a type
of fungus, which aids the supply of minerals and water, and guards
crops against infections by root pathogens [37]. Biochar surface
may contain several functional groups such as hydroxyl, keton,
ester, aldehyde, amino, nitro, phenolic and carboxyl groups. Such
heterogenetic surface characteristic of biochar exhibit hydrophilic/
hydrophobic and acidic/basic properties [36,195]. Generally, a
freshly produced biochar is highly hydrophobic in nature and
contains very few polar functional groups at the surface; however,
when mixed in soil, after exposure to O2 and water present in the
soil, the surface of biochar gets oxidized and forms more carboxylic and phenolic groups [196], and thus it becomes hydrophilic with time. Presence of these groups on the surface of
biochar can significantly enhance the cation exchange capacity
(CEC), nutrient retention capacity (NRT) and water holding capacity (WHC) of soil [35,197–200]. All these features considerably
improve soil health and increase crop productivity.
However, the application of biochar in soil is not always a
prolific strategy; results with ‘no effect’ and even ‘negative effects’
have also been observed by many researchers [201]. Unlike the
biochar from slow-pyrolysis, very little information is available
about the application of hydrochar for soil amendment. Moreover,
both the chars (biochar and hydrochar) significantly differ from
each other in terms of physical and chemical properties [152,155];
therefore, the addition of hydrochar in the soil can considerably
alter the soil ecology and functionality of inhabiting microorganisms [167]. In addition, the physicochemical properties of hydrochar and its stability against microbial degradation significantly
depend on the precursor feedstock and HTC process operating
conditions (reaction time and temperature), and properties of soil.
In an experiment carried out by Rillig and co-workers [187], they
observed positive effects on spore germination and AM root
colonization at high hydrochar application rates (20%). However,
the increasing concentrations of hydrochar in soil (410 vol%)
could be deleterious for plant growth of Taraxacum. The negative
effect of additions of hydrochar in soil on crop yield of Lolium
perenne was also observed by other researchers [190]. The detrimental effect on crops with the addition of hydrochar in the soil
was found alleviated when the soil was pre-incubated with HTCslurry for three months prior to sowing. In another recent study,
the application of hydrochar in the soil decreased the nitrogen
availability (due to nitrogen immobilization) for plant to almost
zero in the first week after hydrochar addition and then a slow rerelease of nitrate was observed in the later weeks, suggesting that
hydrochar should be integrated into soils several weeks prior to
planting/sowing [202].
When biochar and hydrochar are applied in soil, the presence
of heavy metals (Hg, As, Pb, Cd, etc.) or any other toxic compounds
can interrupt the existing food chains, which may possess severe
threat to the environment via soil pollution and toxicity. Hence, an
extensive research is needed to assess the fate of eco-toxicological
compositions of hydrochar and its effects on soil biota to reduce
negative effects on plant growth before applications in the field
are undertaken, particularly at high addition rates.
5.4. Activated carbon adsorbents
Biochars produced via slow-pyrolysis at high temperatures
(600–700 1C) typically have high surface area and porosity with
oxygen functional groups and aromatic surfaces. Depending upon
the field of application, biochars are sometimes activated to
further improve their sorption capacity, and are thus referred to
as ‘activated carbon’ materials. The sorption properties of activated
carbon/biochar are considerably versatile and because of its high
surface-to-volume ratio and strong affinity to nonpolar substances,
biochars have the potential to adsorb a variety of organic pollutants and heavy metals from water. There is a wealth of scientific
literature on the use of biochar as an adsorbent for environmental
pollutants [38,203–206].
However, compared to biochar from slow-pyrolysis, very little
is known about the use of hydrochar for water cleaning purposes.
Hydrochars usually have very low surface area and porosity
compared to the biochars; however, due to the presence of
oxygen-rich functional groups on its surface, the adsorption
capacity of hydrochar has been reported to be considerably higher
than that observed for the biochar [153,207]. Similar results have
been reported in some other recent investigations performed on
HTC for wastewater cleaning application [132,208–213]. The
knowledge on the application of hydrochars for water cleaning
purposes compared to the activated carbon from slow-pyrolysis is
still in the embryonic stage, and therefore, further experimental
validations are required in this field of application.
5.5. Bio-refinery
The research and development in the field of pyrolysis of
biomass for bioenergy production is already well established. But
there is yet a lot more to recognize in the field of HTC. The reaction
chemistry behind the HTC process offers a broad range of benefits.
During HTC, the formation of hydronium ions (from water ionization) catalyzes the breakdown of hemicellulose polysaccharides,
resulting in the formation of intermediate products, which
includes 2,5-HMF, aldehydes (acetic, lactic, propenoic, levulinic,
and formic acids), and other phenolic compounds that can
potentially be used for the manufacture of chemicals in the biorefinery industry [42,92,124]. 2,5-HMF is a highly important
intermediate product which has a long history in the field of
chemical engineering and is used for the manufacture of liquid
biofuels like 2,5-dimethyfuran (DMF), and 2,5 Bis-hydroxymethylfuran via hydrogenation [126,214,215]. Products like HMF and
levulinic acid have been identified as top 12 value added chemicals
from biomass by the US Department of Energy [127]. The formation and percentage yield of all these chemicals can be tailored by
controlling the HTC process conditions [35,42,139]. Although the
HTC chemistry is highly interesting and promising, still further
fundamental research is required to access the feasibility of such
an application. Moreover, the availability of bio-refinery industry
H.S. Kambo, A. Dutta / Renewable and Sustainable Energy Reviews 45 (2015) 359–378
plays a significant role in the recovery of these value added
products.
6. Conclusions and recommendations
The knowledge of biochar production from slow-pyrolysis and
its application has made rapid progress in recent years. However,
the HTC of biomass residuals and waste materials, for the production of hydrochar, is still in its early stage of development and
therefore there are many aspects that require additional research.
The reaction chemistry of slow-pyrolysis and HTC process, physicochemical characteristics of both chars and their applications are
discussed in this paper. The literature review in this study
indicates that the chemical composition, morphological features,
and surficial functionalities of both chars are significantly different
from each other, which makes it a highly versatile tool in a broad
range of industry and environment. The potential applications of
chars include carbon sequestration, soil amelioration, bioenergy
production, and wastewater pollution remediation. In addition to
the benefits associated with the use of hydrochar, there are several
research gaps and challenges identified in this review paper. These
research gaps are listed below
Based on the literature summarized in this paper, the peak
reaction temperature was identified as the primary process
parameter governing the physicochemical properties of hydrochar. Nevertheless, other variables such as the biomass feedstock and its composition (elemental and proximate analysis,
holocellulose and lignin contents, and mineral matter characterization) and the process operating conditions (e.g. reaction
pressure, residence time, particle size distribution, solid load
ratio, 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 of the aforementioned parameters for the HTC
processing of biomass. The inclusion of this valuable information seems to be essential in order to establish the appropriate
process conditions to produce a hydrochar with more suitable
characteristics.
The biomass pre-treated under the HTC process has significantly
improved physicochemical properties and has the potential to
replace coal at power plants. However, being a process with high
conversion efficiency when compared to other thermal pre-treatments, like pyrolysis, gasification, and dry torrefaction, its adaptability at the industrial scale is still not in favor. In HTC process, at
high temperatures, the pressure inside the reactor is substantially
high (45 MPa), which makes the process highly complex, hazardous, and expensive. High pressure requirement and continuous
feeding against pressure increase the overall manufacturing costs
of the HTC process equipment. Moreover, for an economical HTC
plant, running it under ideal conditions like efficient heat recovery,
water recirculation, and recovery of liquid intermediate products
could become very critical. Hence, further investigations are
required in terms of the process design and development. Addition
of chloride salts in the HTC process has been suggested to reduce
371
the temperature requirement; however, additional research on the
different types and combinations of salt and acid is required to
validate the results.
The outstanding advantage of an HTC process is the elimination
of pre-drying requirement of a feedstock. Therefore, nonconventional biomasses like animal wastes, food wastes, greenhouse vegetable wastes, and other high moisture content
feedstocks should be prioritized because of their underutilization and economical inapplicability in conventional dry thermal pre-treatments like pyrolysis. Some of the work reviewed
in this paper has been focused on using dry lignocellulosic
biomass for HTC process and shows promising results; however, HTC is efficient on mass basis only for “wet” biomass. In
addition, the integration of HTC and anaerobic digestion
process has shown potential [129]; however, very little work
has been done on it to evaluate the performance of such a
process. Future research on this topic should focus on optimal
process conditions, post-treatment requirements, and chemical
and morphological compositions of char produced from such
an integrated process.
Biochar when used for agriculture is always used in large
quantities and would therefore be impossible to remove from
the soil if there are any adverse consequences from agricultural
and environmental toxicity points of view. A study estimated
that, in a commercial field trial, approximately 30% (mass) of
the biochar (from slow-pyrolysis) was lost during transport,
handling and application [216]. In addition, the risk of spontaneous combustion or dust explosions in the presence of open
fire has to be considered. On the other hand, when hydrochar
leaves the process it is wet in state and is in the form of slurry;
therefore, the small particles of the hydrochar would agglomerate together and will get mixed in soil, thus reducing the risk
of explosion and loss from erosion. However, some hydrochars
do become hydrophobic when oven- or completely air-dried.
Keeping hydrochar at a suitable moisture content for use in soil
without inducing fungal degradation may be a challenge.
Hence, further in-depth experimental research and field trials
on the comparative evaluation of different types of biochars
and hydrochars – or their blends – in the soil would reveal
realistic information.
Acknowledgment
The authors would like to gratefully acknowledge research
Grants from Natural Sciences and Engineering Research Council
of Canada (NSERC, Grant no. 400495), Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA, Grant no. 200370),
Highly Qualified Program (HQP, Grant no. 299283), and Ministry
of the Environment for Best in Science Program (Project 1314010).
Appendix A
See appendix Table A1 here.
372
Table A1
Characterization of biochar (slow pyrolysis) and hydrochar (HTC).
Process
Feedstock
H/C
O/C
HHV (MJ/Kg)
Temp (1C)
Time
Heating rate (1C/min)
Solid yield (%)
Proximate analysis (%)
H/C
O/C
HHV (MJ/Kg)
Reference(s)
Slow pyrolysis White pine
F.C ¼16.39
Ash ¼0.45
1.40
0.62
18.06n
500
30 min
15
30
F.C ¼81.03
Ash ¼0.94
0.43
0.08
31.41
[217]
Slow pyrolysis Fruit cuttings
F.C ¼21.1
Ash ¼4.6
1.61
0.77
17.27
600
1h
10
37.5
0.45
0.20
26.62
[218]
600
2h
10
37
0.42
0.18
26.94
600
3h
10
38
0.42
0.24
24.49
750
1h
10
32.5
0.26
0.25
23.22
750
2h
10
31
0.11
0.21
24.32
750
3h
10
33
0.27
0.29
21.68
900
1h
10
29
0.12
0.09
29.02
900
2h
10
29
0.12
0.22
23.05
900
3h
10
29.5
F.C ¼75
Ash ¼10.85
F.C ¼76.5
Ash ¼9.3
F.C ¼73.90
Ash ¼10.90
F.C ¼76.6
Ash ¼15.6
F.C ¼79.4
Ash ¼11.15
F.C ¼75.60
Ash ¼13.80
F.C ¼78.55
Ash ¼19.70
F.C ¼79.90
Ash ¼17.59
F.C ¼79.85
Ash ¼17.80
0.12
0.24
22.25
300
2.5 min
5
70.9
1.29
0.57
20.3
300
5 min
5
65.6
1.28
0.55
22.7
400
2.5 min
5
27.3
0.70
0.24
26.6
400
5 min
5
25.3
0.66
0.22
24
500
2.5 min
5
12.4
0.49
0.18
19.6
500
5 min
5
12.3
F.C ¼20.5
Ash ¼6.7
F.C ¼24.7
Ash ¼5.3
F.C ¼53.9
Ash ¼11.8
F.C ¼52.2
Ash ¼14.6
F.C ¼52.6
Ash ¼26.6
F.C ¼54.6
Ash ¼23.8
0.50
0.15
22.4
Slow pyrolysis Switchgrass
F.C ¼15.3
Ash ¼3.7
1.56
0.77
19.5
[219]
Slow pyrolysis Barley straw
Ash ¼4.3
1.45
0.66
17.34n
400
2h
3
31
Ash ¼13.4
0.40
0.11
26.02n
[152]
Slow pyrolysis Safflower seeds
Ash ¼3
FC¼ 14.3
1.68
0.62
24.8
400
30 min
10
34
0.71
0.26
28.15
[150]
400
30 min
30
30
0.60
0.27
28.51
400
30 min
50
28.5
0.64
0.26
28.77
450
30 min
10
31
0.59
0.24
28.86
450
30 min
30
29
0.55
0.25
28.98
450
30 min
50
28
0.54
0.26
29.2
500
30 min
10
29
0.50
0.23
29.39
500
30 min
30
27.5
0.49
0.25
29.59
500
30 min
50
26.6
Ash ¼7.5
FC¼ 67.30
Ash ¼8.40
FC¼ 70.20
Ash ¼8.50
FC¼ 71.70
Ash ¼8.20
FC¼ 71.80
Ash ¼8.50
FC¼ 72.80
Ash ¼8.60
FC¼ 74
Ash ¼8.50
FC¼ 74
Ash ¼8.60
FC¼ 76.20
Ash ¼8.70
0.48
0.23
29.73
H.S. Kambo, A. Dutta / Renewable and Sustainable Energy Reviews 45 (2015) 359–378
Proximate analysis (%)
550
30 min
30
26.75
550
30 min
50
25.25
600
30 min
10
26.5
600
30 min
30
25.5
600
30 min
50
24.5
250
250
2h
2h
4
4
40
37
FC¼77
Ash ¼8.90
FC¼77.20
Ash ¼9.10
FC¼78.60
Ash ¼9.10
FC¼79.50
Ash ¼9.20
FC¼79.20
Ash ¼9.30
FC¼79.90
Ash ¼9.50
FC¼80.70
0.44
0.21
29.71
0.45
0.22
29.97
0.47
0.22
30.12
0.38
0.20
20.06
0.38
0.21
30.17
0.43
0.21
30.27
Ash ¼0.54
Ash ¼0.43
0.81
0.86
0.27
0.21
26.19
27.49
[152]
HTC
Corn stover
Ash ¼2.8
1.62
0.73
16.2
250
4h
4
0.36
2.1
0.94
0.18
27.76
[44]
HTC
Spruce
F.C ¼13.27
Ash ¼0.23
1.49
0.65
19.94
175
30 min
12n
88
1.44
0.62
20.4
[220]
200
30 min
12
80
1.40
0.59
21
225
30 min
12
70
1.24
0.49
22.5
175
30 min
12
1.55
0.67
20.5
200
30 min
12
1.45
0.62
20.6
225
30 min
12
F.C ¼14.7
Ash ¼0.11
F.C ¼15.92
Ash ¼0.12
F.C ¼25.12
Ash ¼0.14
F.C ¼11.34
Ash ¼0.09
F.C ¼14.76
Ash ¼0.09
F.C ¼26.09
Ash ¼0.13
1.24
0.49
22.5
0.68
20.42
Maize silage
Ash ¼11.45
1.58
0.55
22.3
190
190
190
230
230
230
270
270
270
2h
6h
10 h
2h
6h
10 h
2h
6h
10 h
3
3
3
3
3
3
3
3
3
71.8
55.7
65
60.4
49.5
49.3
41.3
43.4
40.2
Ash ¼11.65
Ash ¼8.71
Ash ¼10.41
Ash ¼12.38
Ash ¼13.29
Ash ¼ 13.21
Ash ¼13.10
Ash ¼14.57
Ash ¼14.26
1.37
1.29
1.33
1.26
1.20
1.18
1.13
1.16
1.13
0.39
0.34
0.33
0.24
0.16
0.14
0.12
0.10
0.09
25.4
26.4
27
29.7
32.6
33.3
33.8
35.2
35.7
[91]
HTC
Coconut fiber
Ash ¼8.1
FC¼11.0
1.41
0.71
18.4
220
30 min
5
76.6
1.01
0.37
24.7
[138]
250
30 min
5
65.7
0.93
0.30
26.7
300
30 min
5
65
0.97
0.21
29.4
350
30 min
5
55.78
0.74
0.21
28.7
375
30 min
5
59
0.66
0.15
30.6
220
30 min
5
87.34
1.20
0.38
25.3
250
30 min
5
61.12
1.05
0.37
25
300
30 min
5
61.32
1.05
0.25
28.7
350
30 min
5
47.84
1.01
0.22
29.4
375
30 min
5
42.78
Ash ¼6.2
FC¼24.0
Ash ¼5.0
FC¼27.1
Ash ¼4.3
FC¼42.1
Ash ¼4.9
FC¼38.5
Ash ¼8.6
FC¼48.8
Ash ¼7.3
FC¼20.2
Ash ¼6.9
FC¼23.0
Ash ¼7.1
FC¼31.7
Ash ¼9.9
FC¼33.9
Ash ¼14.2
FC¼42.6
0.80
0.21
28.7
Eucalyptus leaves
Ash ¼10.5
FC¼10.3
1.59
0.72
18.9
373
HTC
H.S. Kambo, A. Dutta / Renewable and Sustainable Energy Reviews 45 (2015) 359–378
28
Ash ¼0.62
Ash ¼4.3
1.56
16.69n
17.34n
10
Eucalyptus sawdust
Barley straw
F.C ¼10.26
Ash ¼0.28
0.71
0.66
30 min
HTC
Birch
1.47
1.45
550
374
28.38
0.22
0.90
38.1
4
n
Wood
Higher heating values (HHV) were calculated using a correlation from Channiwala [221].
4h
250
17.93
0.68
Ash ¼1.31
Corn stalk
HTC
1.65
Ash ¼4.64
Feedstock
1.58
0.65
17.51
250
4h
4
35.48
Ash ¼3.36
FC¼ 54.21
Ash ¼0.41
FC¼ 58.55
0.94
0.17
29.21
[139]
References
Process
Table A1 (continued )
Proximate analysis (%)
H/C
O/C
HHV (MJ/Kg)
Temp (1C)
Time
Heating rate (1C/min)
Solid yield (%)
Proximate analysis (%)
H/C
O/C
HHV (MJ/Kg)
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