Training on
Technologies for Converting Waste Agricultural Biomass into Energy
Organized by
United Nations Environment Programme (UNEP DTIE IETC)
23-25 September, 2013
San Jose, Costa Rica
Physical Conversion Technologies
Surya Prakash Chandak
Senior Programme Officer
International environmental Technology Centre
Division of Technology, Industry and Economics
Osaka, Japan
CONTENT

Introduction

Preprocessing Techniques

Pretreatment Techniques

Densification – The Process

Densification – The Mechanism

Densification – The Technology

Briquetting Technologies

Pelletizing Technologies

Other Densification Technologies

Performance Comparison
2
INTRODUCTION
 Challenges of waste agricultural biomass to
energy conversion technologies
– Inherent uneven and troublesome characteristics of
the materials.
– Technology should address the followings:







Low bulk density,
Variable and often high moisture content,
Combustibility,
Affinity to spoilage and infestation
Geographically dispersed and varied material,
Seasonal variations in yield and maturity,
A short window of opportunity for harvest and demands on
labor and machines that often conflict with main crop (grain),
 Local regulations that put limits on utilization, storage,
3
transportation and emissions.
INTRODUCTION
 Technological options for improvements
– Before end-use energy applications, WAB materials
have to convert into some improved secondary forms.
– This basic process of upgrading into a variety of
convenient secondary fuels is known as beneficiation.
BENEFICIATION
Drying
Dewatering
Sizing
Densification
Separation
Baling
Pelletization
Briquetting
Torrefaction
4
INTRODUCTION
 Densification
– WAB materials usually take many shapes and sizes,
while a particular biomass energy conversion
technology (feeding system, conversion reactor and
the conversion process itself) usually could accept a
specific range of physical forms.
– Deviations from the design features could lead to not
only fuel handling and maintenance issues but also
considerable reduction in energy conversion
efficiencies.
– Densification is one of the effective ways of managing
the above issues, in which compaction and
agglomeration of particles occur under pressure.
5
INTRODUCTION
 Densification
– Because of their uniform shape and size, densified
products may be easily handled using standard
handling and storage equipment, and they can be
easily adopted in direct-combustion, gasification,
pyrolysis, and utilized in biochemical conversions.
Storing of
Separation of
WAB Materials
Metals and Sands
Baling
Sieving
Segregation or
Classification
Cooling
Pressing
Chipping or
Shredding
Preheating
Drying
Grinding
(Fine Milling)
Bales
Pellets
Briquettes
6
INTRODUCTION
 Densification
– The process for biomass densification can be
classified mainly into baling, pelletization, and
briquetting.
– Bales are a traditional method of densification
commonly used to harvest crops. A bale is formed
using farm machinery (called a baler) that compresses
the chop.
– Briquetting and pelletization are the most common
processes used for biomass densification for solid
fuel applications.
– These processes can increase the bulk density of
WAB material from an initial bulk density of 40-200
kg/m3 to a final compact density of 600-1200 kg/m3.
7
INTRODUCTION
 Densification
Bales
Briquettes
Pellets
8
INTRODUCTION
 Densification
– The most common raw materials used in densification
of WAB in include
 Wood processing residues, mainly sawdust,
 Loose crop residues such as rice husk, coffee husk, tamarind
seeds, tobacco stems, coir pith and spice waste, and
 Charcoal fines.
– The material is compacted and agglomerated under
pressure.
– Depending on the material, the pressure, and the
speed of densification, additional binders may be
needed to bind the material.
9
INTRODUCTION
 Densification
– Advantages:







Reduces transportation and storage costs,
Improves the handling characteristics,
Enhances net heating value of the material per unit volume,
Produces a fuel having more uniform size and properties,
Reduces biodegradation of residues,
Produces clean and durable fuel,
Increases efficiency and reduces emissions during final
energy conversion process
– Further, the combustion of uniformly sized, densified
WAB can be controlled more precisely than loose, low
bulk density biomass and thus increase energy
conversion efficiency and reduce emissions.
– It also reduces or eliminates the possibility of
10
spontaneous combustion seen with loose materials.
INTRODUCTION
 Densification
– Despite of the significant benefits of densification of
WAB, widespread dissemination and usage of the
technology is hindered by number of issues:
 High investment cost and process energy requirements,
 Undesirable combustion characteristics such as poor
ignitability and smoking due to use of improper process
parameters and lack of process quality control, and
 Tendency of the densified products to loosen when exposed
to water or even high humidity weather.
11
INTRODUCTION
 Densification experience in Asia
– Initial introduction of densification process for WAB
materials showed limited success due to several
issues such as:
 Mismatch of technology, raw material supply and prospective
markets,
 Complexity of the technology and the lack of knowledge to
adapt the technology to suit local conditions,
 Excessive operating costs, especially associated with the
electricity usage and regular maintenance requirements,
 Lack of institutional framework for the information
management (i.e. accumulation and exchange of experiences
in briquette / pellet production in conjunction with advances
in briquetting technology)
12
INTRODUCTION
 Densification experience in Asia
– The most common raw materials used in biomass
densification in Asia include sawdust, rice husk,
coffee husk, tamarind seeds, tea dust, tobacco stems,
coir pith and spice waste.
 Sawdust is the dominant raw material in Malaysia, Philippines,
Thailand and Korea,
 Rice husk is the main raw material used in Bangladesh.
– Main energy applications:
 Industrial process heating and institutional cooking (e.g. in
India),
 Domestic cooking (e.g. in Bangladesh, Thailand).
13
PREPROCESSING TECHNIQUES
 Main processes
– Main preprocessing operations of WAB materials
include sizing (or size reduction), separation (or
sieving) and moisture removal.
– Size reduction and separation processes are aimed at
obtaining more uniform and pre-determined particle
size distribution required for optimum operational
performance of the subsequent stage of the
densification process.
– Moisture removal is an essential unit operation in
WAB to energy conversion processes, as most of the
raw forms of materials contain excessively high
moisture content, resulting many undesirable
characteristics.
14
PREPROCESSING TECHNIQUES
 Size reduction
– Size reduction consists of breaking or cutting a solid
biomass to smaller pieces.
 Cutting mostly involves shearing action, whereas breaking
involves some degree of impact and attrition (friction).
 Depending on the material type and application, size
reduction is achieved by one or more steps; eg. chopping
(coarse materials) followed by grinding (fine materials).
(a) Straw in raw form
(b) Chopped straw
(c) Straw in grounded form
15
PREPROCESSING TECHNIQUES
 Size reduction
– Size reduction consists of breaking or cutting a solid
biomass to smaller pieces.
 Cutting mostly involves shearing action, whereas breaking
involves some degree of impact and attrition (friction).
 Depending on the material type and application, size
reduction is achieved by one or more steps; eg. chopping
(coarse materials) followed by grinding (fine materials).
Upper feed
roller
Moving
blades
Biomass
material in
Swinging
hammer
Biomass
material in
Lower
feed rollerStationary
bottom blades
Ground
particles out
Peripheral
screen
16
PREPROCESSING TECHNIQUES
 Size reduction
– As size reduction is an energy intensive unit
operation, it is important to have information on
specific energy consumption a given technology.
– In general, energy consumption of sizing of biomass
materials depends on initial particle size, moisture
content, material properties, feed rate of the material
and machine variables.
– In particular, the energy required to grind or chop
biomass increases exponentially as desired particle
size decreases.
– Since some conversion processes require small
biomass particles, size reduction technology must
reduce energy requirements and subsequent cost.
17
PREPROCESSING TECHNIQUES
 Size reduction
– Size reduction equipment can also be further
categorized as primary and secondary types.
 Typically, primary reduction equipment is selected to
maximize the amount of processed materials in the desired
size range, while minimizing fines.
 Secondary type provides a ground product of greater
uniformity in sizing.
– Another method of classification is on the basis of
applying fundamental stress on the biomass material
in size reduction as:




Impact,
Attrition,
Shear, and
Compression.
18
PREPROCESSING TECHNIQUES
 Size reduction

– Types
of equipment:
(a) Disc type chipper
(b) Drum type chipper
(c) Straw shredder
19
(d) Hammer mill
(e) Knife mill
(f) Disc mills
PREPROCESSING TECHNIQUES
 Size reduction
– Shredders
• The shredders or choppers are mainly used with stalk forage,
such as rice straw, wheat straw and maize stover.
• Biomass needs to be chopped with a chopper (rotary shear
shredder)/ knife mill/ tub grinder to accommodate bulk flow
and uniformity of feed rate.
(a) Straws, stalks and grasses
(b) Straw bales
20
PREPROCESSING TECHNIQUES
 Size reduction
– Shredders
• A chopper, knife cutter, or knife mill is often used for coarse
size reduction (>50 mm) of stalk, straw, and grass feed
stocks.
• According to the mode of cutting, choppers can be divided
into cylinder or flywheel types. Large and medium size
choppers are generally flywheel types, but the majority of
small choppers are cylinder type.
Upper feed
roller
Moving
blades
Biomass
material in
Biomass
material in
Lower
feed rollerStationary
bottom blades
Ground
particles out
21
PREPROCESSING TECHNIQUES
 Size reduction
oving
ades
ary
m blades
– Hammer Mill
• Hammer mills consist of rotating shafts with fixed or swing
hammers are attached to them.
• The material is fed into a hammer mill from the top and by
gravity falls into the grinding chamber.
• The material is contacted by a series of swinging hammers.
Biomass
material in
Swinging
hammer
Ground
particles out
Peripheral
screen
22
PREPROCESSING TECHNIQUES
 Size reduction
– Specific Energy Consumption
• For a given equipment, SEC is determined critically by the
factors such as properties of the biomass material, feeding or
operating speed, moisture content, initial particle size and
final particle size.
Specific energy consumption of WAB in hammer milling (in kWh/t)
Biomass Material
Wheat straw
Maize stover
Switchgrass
Moisture content
(% on wet basis)
8
12
8
12
8
12
Screen size (mm)
0.8
51.0
45.3
21.1
34.2
63.4
56.6
1.6
37.5
43.5
16.2
19.7
50.2
58.4
3.2
10.7
24.2
6.3
11.1
23.9
26.9
23
PREPROCESSING TECHNIQUES
 Separation
– The raw forms of WAB materials are often
contaminated with items such as sand particles, soil,
stones, metal particles and other foreign materials.
 Presence of such contaminants could damage or increase the
wear of machinery. An increased wear of machinery, in turn,
creates a growth of contaminants, thereby intensifying the
effects.
– In WAB densification, separation or screening is used
at different stages, both before and after the
compaction process
 former for the purpose of removing the contaminants and
segregate into required particle size range, and
 latter for removal of dust and fines from the densified
products (especially in the case of pellets)
24
PREPROCESSING TECHNIQUES
 Separation
– Sieves or screens are used for the separation of
particles according to their sizes (segregation or
classification) or for the production of closely graded
materials.
25
PREPROCESSING TECHNIQUES
 Separation
– The screens are vibrated by means of a mechanical
system. The screen is usually inclined at an angle to
the horizontal; multiple screens are also used.
26
PREPROCESSING TECHNIQUES
 Drying and Dewatering
– Moisture content of WAB is one of the main factors
affecting the performance of densification processes.
– The quality of densified products and successful
operation of the machines is highly sensitive to the
moisture content, which preferably should be <15%.
– Typically, moisture content has to be reduced up to
this level following the size reduction, for which a
dryer is normally used.
– Drying equipment may possibly be eliminated due to
the lower moisture content of many WAB materials,
such as rice husk, coffee husk and groundnut shells.
– In contrast, drying is essential for sawdust, wet coir
pith, bagasse, bagasse pith, mustard stalk, etc.
27
PREPROCESSING TECHNIQUES
 Drying and Dewatering
– Removal of moisture in the WAB materials needs
energy and therefore increases the pre-processing
energy requirement.
– If heat for the dryer is recovered from a waste heat
source, energy efficiency could be improved.
– Wet biomass materials containing considerably high
moisture content can be dewatered prior to drying.
– This process refers to the removal of portion of the
moisture in the feedstock in liquid phase.
– Whereas in drying process, the moisture is removed
as vapor.
– Overall energy efficiency can often be improved by
dewatering wet feed stocks prior to thermal drying.
28
PREPROCESSING TECHNIQUES
 Drying and Dewatering
– Basic dewatering technologies include:






Open air storage,
Filters,
Presses,
Screening devices,
Centrifuges,
Hydro cyclones extrusion and expression process
Belt
washing
Belt
alignment
Belt
Feed
Belt
drive
Belt
tensioning
Linear / peripheral
pressure
Belt alignment
Belt
washing
Residues
29
PREPROCESSING TECHNIQUES
 Drying and Dewatering
– Drying is another essential pre-treatment process
required in biomass energy conversion systems.
– There are many types of dryers that could be used to
dry biomass materials, which could be classified as
Classification
Alternatives
Drying media (i.e. the stream passing Flue gas, hot air or superheated steam
through the material to be dried)
Method of heat transfer
Direct- or indirect-fired
Heat transfer media
Flue gas, hot air, steam, or hot water
Pressure
Atmospheric, vacuum or high pressure
Passive: Open sun, solar dryer,
natural ventilation
Active: Dryer burners, boiler (flue gas or
steam), recovered waste heat from
facility processes
Nature of heat source
30
PREPROCESSING TECHNIQUES
 Drying and Dewatering
– Eg: Pneumatic dryer
Moist air out
Buffer
Cyclone
Drying ducting
Hopper
(Material In)
Hot air
generator
Fan
Dried
Material
Out
Figure 2.10: Pneumatic dryer
31
PRETREATMENT TECHNIQUES
 Main processes
– Pretreatment of biomass improves the binding
characteristics of biomass that is low in lignin content
– Some of the commonly used pretreatment processes
are pre-heating, steam explosion, steam conditioning,
torrefaction and ammonia fiber explosion (AFEX)
– Several other pretreatment processes such as
chemical, physico-chemical (microwave, and radio
frequency heating) and biological pretreatment have
been developed, which are mainly tested and used for
bio-fuel applications than densification.
32
PRETREATMENT TECHNIQUES
 Main processes
– Pre-treating biomass prior to densification improves
properties like durability, bulk and energy density, and
calorific value and reduces the specific energy
consumption.
– Other promising methods of improving the binding
characteristics include addition of natural or synthetic
binders.
– Lignocellulosic biomass, which does not bind easily,
can be improved by adding either natural or
commercial binders like protein or lignosulfonates.
33
PRETREATMENT TECHNIQUES
 Preheating and Steam Conditioning
– Pre-heating biomass before densification is widely
used as it results in a higher quality product.
– Most commercial pellet or briquette producers use
pre-heating to form more stable and dense product.
– Pre-heating could increase the throughput of
densification and reduce the specific energy
requirement for the densification process.
– Steam conditioning is a process where steam is added
to the biomass to make the natural binder, lignin,
more available during densification.
– By disrupting lignocellulosic biomass materials via
steam conditioning will improve the compression
characteristics of the biomass.
34
PRETREATMENT TECHNIQUES
 Steam Explosion
– During this process high pressure saturated steam (~
200 C) is supplied to biomass materials in a reactor
for a short period of time (2 – 10 minutes).
– The substrate is quickly flashed to atmospheric
pressure, and the water inside the substrate vaporizes
and expands rapidly, disintegrating the biomass.
– This process causes great reduction in the particle
size and significant physical, chemical, and structural
changes in the biomass.
– It causes hemicelluloses to become more water
soluble and makes cellulose and lignin more
accessible through depolymerization, and makes
lignin more available for binding during densification.35
PRETREATMENT TECHNIQUES
 Steam Explosion
– The extent of chemical and structural modifications
from steam-explosion pretreatment depends on
residence time, temperature, particle size and
moisture content.
Barley Straw Canola Straw
Oat Straw
Wheat Straw
Un-treated
SteamExploded
36
PRETREATMENT TECHNIQUES
 Ammonia Fiber Explosion
– Uses aqueous ammonia at elevated temperatures and
pressures to produce higher hydrolysis yields for
many herbaceous feedstocks.
– This process reduces lignin and removes some
hemicellulose while decrystallizing cellulose in the
biomass.
– The major advantage of this process is little biomass
degradation.
– Several other chemical pretreatment techniques for
lignocellulosic materials have been developed by
using different chemicals such as acids, alkalis,
oxidizing agents and ozone.
37
PRETREATMENT TECHNIQUES
 Biological Pretreatment
– Biological pretreatment using various types of rot
fungi is a process that does not require high energy
for lignin removal from a lignocellulosic biomass,
despite extensive lignin degradation.
– Biological pretreatments are safe, environmentally
friendly and less energy intensive compared to other
pretreatment methods.
– However, the rate of hydrolysis reaction is very slow
and needs a great improvement to be commercially
applicable.
38
PRETREATMENT TECHNIQUES
 Torrefaction
– Torrefaction is a method of changing the properties of
biomass materials by slowly heating it in an interenvironment to a maximum temperature of 300°C.
– The process is also called a mild pyrolysis as most of
the smoke-producing compounds and other volatiles
are removed resulting in a final product that has
approximately 70% of the initial weight and 80–90% of
the original energy content.
– Thus, treatment yields a solid uniform product with
lower moisture content and higher energy content
compared to the initial biomass.
39
PRETREATMENT TECHNIQUES
 Torrefaction
Biomass residues
before and after
torrefaction
Biomass pellets
and torrefied
biomass pellets
40
PRETREATMENT TECHNIQUES
Torrefaction
 Torrefaction
• Range A
The biomass is dried.
• Range B
Softening of the lignin
• Range C
Depolymerisation occurs and
the shortened polymers
condense within the solid
structure.
• Range D
Limited devolatilisation and
carbonisation of the intact
polymers and the solid
structures formed in the
temperature regimes C.
• Range E
Extensive devolatilisation and
carbonisation of the polymers
and of the solid products that
were formed in regime D.
41
DENSIFICATION – THE PROCESS
 Overview
– Low bulk density, loose forms and wide variations of
particle sizes are common drawbacks of WAB
42
Figure 3.1: Loose waste agricultural biomass
DENSIFICATION – THE PROCESS
 Overview
– The fuel quality of WAB could be improved by means
of compaction into high density and regular shape.
43
DENSIFICATION – THE PROCESS
 Physical Attributes of Densified WAB
– The quality of densified WAB products such as
briquettes and pellets depends on strength and
durability of the particle bonds.
– The quality is influenced by a number of process
variables, like die dimensions, length to diameter
ratios, die temperature, speed, pressure, binders, and
pre-heating of the biomass materials.
– The two important aspects of densification are:
 The ability of the particles to form densified products with
considerable mechanical strength, and
 The ability of the process to increase density.
44
DENSIFICATION – THE PROCESS
 Quality Parameters of Densified WAB
– Moisture Content
 Moisture is a vital constituent in densified WAB products; its
presence in too much or too low quantities would affect the
quality attributes.
 High moisture content could lead to spoilage due to microbial
decomposition, resulting in significant dry matter loss.
 This reduces the energy content and could also have
negative effect on the final quality where cracks occur.
 Densified products with lower moisture content tend to break
up, creating more fines during storage and transportation.
 The optimum moisture content is primarily dependent on
process conditions like initial moisture content of the
feedstock, temperature, and pressure.
 Higher moisture in the final product results when the initial 45
moisture content is greater than 15% on wet basis.
DENSIFICATION – THE PROCESS
 Quality Parameters of Densified WAB
– Bulk and Unit Density
 Bulk density and unit density are important parameters for
handling, storage and transportation.
 The two parameters are greatly influenced by not only the
material properties such as moisture content and particle size
distribution, but also the process parameters such as
pressure and temperature.
 In general, materials with higher moisture and larger particle
sizes reduce the unit and bulk density, while higher process
temperatures and pressures increase the unit and bulk
density of the final product.
 The maximum apparent density of a densified product from
nearly all materials is to a rough approximation constant; it
will normally vary between 1200-1400 kg/m³ for high pressure
processes. The ultimate limit is for most materials between 46
DENSIFICATION – THE PROCESS
 Quality Parameters of Densified WAB
– Durability Index
 The durability index is a quality parameter defined as the
ability of densified materials to remain intact when handled
during storage and transportation.
 Thus, durability of a densified product is its physical strength
and resistance to being broken up.
 The bonding performance of the particle during densification
process critically determines the durability of the products.
 Moisture increase durability when water soluble compounds,
such as water soluble carbohydrates, lignin, protein, starch
and fat are present in the feed material.
 High starch content acts as a binder and increases durability.
 Protein will plasticize with heat and moisture and act as a
binder to increase the durability of the products.
47
 Lignin too, at elevated temperatures (>140 °C), acts as a
binder and increases durability.
DENSIFICATION – THE PROCESS
 Quality Parameters of Densified WAB
– Fines Content
 The presence of dust particles or fines in the densified
product is an undesirable attribute, which could affect
adversely the end-use energy conversion process, especially
when co-firing with other fossils fuels.
 Fines are generated during transportation and storage by the
breakdown of the densified products.
 Densified products processed under suboptimal conditions,
such as low moisture, low temperature, and with less
desirable chemical compositions or with insufficient die size
and roller speeds, are less durable and can result in more
fines in the final product.
 Presence of fines in higher quantities can lead to
spontaneous combustion and dust explosion problems
during final energy conversion processes.
48
DENSIFICATION – THE PROCESS
 Quality Parameters of Densified WAB
– Heating Value
 The heating (or calorific) value of densified products depends
on process conditions like temperature, particle size, and feed
pretreatment.
 In general, products with higher densities and lower moisture
contents have greater heating values.
 The typical higher heating values (HHVs) of briquettes and
pellets range from 17–18 MJ/kg, which could be enhanced
further up to 20–22 MJ/kg through pretreatment processes
like steam explosion or torrefaction prior to densification.
49
DENSIFICATION–THE MECHANISM
 Overall Mechanism
– Densification of WAB materials through briquetting
and pelletizing basically represents compaction of
grinds in a form of systematic agglomeration
involving pressure.
– Densification essentially involves two parts:
 The compaction under pressure of loose WAB material to
reduce its volume and
 The agglomeration of the WAB material so that the product
remains in the compressed state.
50
DENSIFICATION–THE MECHANISM
 Overall Mechanism
– Further, three basic processing stages could be
recognized during densification of biomass.
 Firstly, the compaction of the materials with low to moderate
pressure (0.2–5.0MN/m2) will reduce the space between
particles and form a closely packed mass where the energy is
dissipated due to inter-particle and particle-to-wall friction.
 Secondly, the particles are forced against each other and
undergo plastic and elastic deformation, which significantly
increases the inter-particle contact; particles become bonded
through the intermolecular attractive forces.
 Thirdly, increase of the pressure further will result collapsing
of the cell walls of the cellulose constituent of the material, a
significant reduction in volume results in the density of the
material reaching the true density of the component
ingredients.
51
DENSIFICATION–THE MECHANISM
 Overall Mechanism
– The stages of deformation mechanism of powder
particles under compression
Biomass particles
Rearrangement, sliding, stacking–reduced porosity
Solid bridges
Increased
applied
pressure
Fragmentation
Compression
mechanism
Elastic
deformation
Bonding
Plastic
deformation
Intermolecular
force
Mechanical
interlocking
52
DENSIFICATION–THE MECHANISM
 Bonding Mechanism
– The quality of densified products of WAB is critically
determined by the intensity of bonding or interlocking
between individual particles of the material.
– The densification of biomass under high pressure
results in mechanical interlocking and increased
adhesion/cohesion (adhesive forces at the solid/liquid
interface and cohesion forces within the solid are
used for binding) of the solid particles, which form
intermolecular bonds in the contact area.
– Though several fundamental processes and
mechanisms of attaining and maintaining the selfbonding have been proposed, universally accepted
model is yet to be established.
53
DENSIFICATION–THE MECHANISM
 Bonding Mechanism
– Bonding Agents.
 In case that the inborn binders do not provide the required
quality levels (such as strength, durability, heating values,
dust and fines level), as demanded by the end-user, additives
(i.e. added binders) have to be blended with the raw material.
 Selection of such binders (the type and amount) mainly
depends on the strength of the bonding, the cost and the
environmental friendliness of the material of the binder.
 When strength, durability or heating values of the densified
products do not match with the quality standards or
marketing requirements, additives are added to the feed to
enhance the quality or to minimize the quality variations.
 Bonding agents can also be used in order to reduce wear in
production equipment and increase abrasion-resistance of
54
the densified biomass fuel.
DENSIFICATION–THE MECHANISM
 Bonding Mechanism
– Bonding Agents.
 Many bonding agents have been explored and used in
improving the quality of densified products of WAB materials.
 Some of these bonding agents include starches (e.g. maize,
rice, potato), stalks (e.g. corn), bran (e.g. wheat, rice),
molasses, natural paraffin, plant oil, lignin sulphate and
synthetic agents.
 The binders used in biomass densification could be
categorized as the matrix type and the film type.
55
DENSIFICATION–THE MECHANISM
 Factors Affecting Densification
– Moisture Content.
 Moisture content should be as low as possible, generally in
the range of 10-15 % on wet basis.
 High moisture content will pose problems in grinding and
excessive energy is required for drying.
 Nevertheless, moisture content has an important role to play
as it facilitates heat transfer.
 Too high moisture causes steam formation and could result
an explosion. Most suitable moisture content could be of 812% on wet basis.
 Usually, briquetting process could accommodate relatively
high moisture content (preferably up to 15% on wet basis),
whereas pelletization demands much lower moisture contents
(< 10% on wet basis).
56
DENSIFICATION–THE MECHANISM
 Factors Affecting Densification
– Material Properties
 Although biomass densification technology is well developed,
WAB material preparation and densification equipment are
very sensitive to the specific characteristics of raw materials.
 In terms of the WAB material, following properties play
important role in the densification process:
-
Particle size and shape distribution
-
Flow ability and cohesiveness
-
Surface forces
-
Adhesiveness
-
Hardness
57
DENSIFICATION–THE MECHANISM
 Factors Affecting Densification
– Material Properties
 In particular, particle shape and size distribution play a vital
role in the compaction and agglomeration stages of the
densification process.
58
DENSIFICATION–THE MECHANISM
 Factors Affecting Densification
– Material Properties
 In the case of briquetting, relatively larger particles sizes (> 6
mm) are desirable, leading to better interlocking of the
particles and increasing the durability, whereas in
pelletization, relatively smaller particle sizes are required.
 Optimum particle size distribution for quality pellets
Sieve size (mm)
Material retained on sieve (% mass)
3.0
1
2.0
5
1.0
 20
0.5
 30
0.25
 24
<0.25
 20
59
DENSIFICATION–THE MECHANISM
 Factors Affecting Densification
– Temperature and Pressure
 The compression strength of densified biomass depended on
the temperature at which densification was carried out.
 Maximum strength was achieved at a temperature around 220
C. At a given applied pressure, higher density of the product
was obtained at higher temperature. Both strength and
moisture stability increased with increasing temperature.
 High pressures and temperatures during densification may
develop solid bridges by a diffusion of molecules from one
particle to another at the points of contact.
60
31 MN/m
2
63 MN/m
2
159 MN/m
2
191 MN/m
2
254 MN/m
2
DENSIFICATION–THE TECHNOLOGY
 Main Classifications of Technologies
– The densification technologies use any one of the
following methods in producing densified products:
 Binder-less densification,
 Direct densification of biomass using binders and
 Pyrolyzed densification using a binder.
– Different densification processes and technologies
could be classified based on number of factors such
as
 Type of equipment,
 Operating condition,
 Mode of operation,
 Applied pressure, etc.
61
DENSIFICATION–THE TECHNOLOGY
 Main Classifications of Technologies
– Classification based on type of equipment
 Three main types: Piston press, Screw press, Pelletizing
 Other: Roller press, Low pressure or manual press.
 Densified biomass can be categorized into two main types:
briquettes and pellets.
 The products formed in the piston and screw presses are
larger in size and known as briquettes.
 Briquettes can have different shapes and are greater in size,
with 50 - 60 mm in diameter and 300 - 400 mm in length.
 The briquettes produced by a piston press are completely
solid, while screw press briquettes usually have a concentric
hole, which give better combustion characteristics
 Pellets have cylindrical shape and are small in size, about 6 to
62
25 mm in d.iameter and 30 to 50 mm in length.
DENSIFICATION–THE TECHNOLOGY
 Main Classifications of Technologies
– Classification based on the operating condition
 Depending on the operating conditions, the WAB
densification technologies could be categorized into two
groups: Hot and high pressure densification; Cold and low
pressure densification
 Hot and high pressure densification:
 Is the most common type of densification, and it is essentially a
process of compaction of biomass under heated condition.
 The heating of the biomass is mostly or totally generated by
friction during compaction.
 Usually no binding agent is required for this type of
densification for producing briquettes, but required to set the
process parameters appropriately to realize the necessary
quality levels.
 The piston presses and screw extrusion machines are the two
main high pressure technologies are used at present.
63
DENSIFICATION–THE TECHNOLOGY
 Main Classifications of Technologies
– Classification based on the operating condition
 Cold and low pressure densification:
 Cold and low pressure densification processes employ relatively
low pressure and temperature.
 In this process, the densification could be carried out with or
without addition of bonding agents.
 In the case of densification using binder, a binding agent is
added to glue together the biomass particles.
 Since there is no need to soften lignin, the temperature and
pressure required are low.
 Low pressure compaction includes manually operated
briquetting presses of different types.
64
DENSIFICATION–THE TECHNOLOGY
 Main Classifications of Technologies
– Classification based on the mode of operation
 Based on mode of operation it falls into two categories: Batch
densification and Continuous densification
 The piston presses usually represent batch type densification
technology, while screw extrusion machines and pelletizing
machines represent continuous densification.
– Classification based on the applied pressure
 On the basis of compaction pressure, the densification
technologies can be divided into the following types: High
pressure compaction, and Low or Medium pressure
compaction with a heating device.
 High pressure densification technologies employ processing
pressures above 100 MN/m2, while medium pressure
technologies between 5 – 100 MN/m2 and low pressure
technologies less than 5 MN/m2.
65
DENSIFICATION–THE TECHNOLOGY
 Working Principles
– Generally, two distinct principles could be
distinguished, which are most widely used for size
enlargement of particulate materials: tumble
agglomeration and pressure agglomeration.
– In tumble agglomeration, agglomerates are formed
during suitable movement of the particulate materials
containing binder in the processing equipment.
– Whereas in pressure agglomeration, high forces are
applied to a mass of particulate materials within a
confined volume to increase the density.
– Pressure agglomeration is accomplished in piston,
roller, and extrusion presses as well as in pelletizing
machines.
66
DENSIFICATION–THE TECHNOLOGY
 Working Principles
Energy input kWh/t
– Tumble agglomeration
Bonding agent addition kg/kg of feed
67
(b) Ram extrusion press
DENSIFICATION–THE TECHNOLOGY
(a) Ram and punch press
Punch‐ and‐die press
(b) Ram extrusion press
(a) Ram and punch press
(b) Ram extrusion press
Punch‐ and‐die press
(a) Ram and punch press
Punch‐ and‐die press
Hopper
 Working Principles
Hopper
Hopper
Barrel
Barrel
Screw Barrel
Heaters
DieHeaters
Screw Die
Screw
Heaters
– Pressure agglomeration
Hopper
Hopper
HopperScrew
Barrel
Screw Barrel
Heaters
DieHeaters
Screw Die
Barrel
Heaters
Hopper
Hopper
Die
Die
HopperScrew
Barrel
Screw Barrel
Heaters
DieHeaters
Screw Die
Barrel
Heaters
Die
Briquettes
Briquettes
Briquettes
Briquettes
(c)
Screwextrusion
extrusion
press
(c) Screw
press
Briquettes
Briquettes
Briquettes
Briquettes
Briquettes
Solid
Solid
Solid
Solid
Conveying
Conveying
Conveying
Conveying
Melting
and Melting
Melting
and
Solidand
Melting
Melting
and
Melting
Solid
and and
(d) Pellet
mill
with ring
Pumping
Pumping
Pumping
(d) Pellet
mill with
ring
Pumping
Pumping
Pumping Pumping
Pumping
Conveying
Pumping Pumping
Pumping
Pumping
Conveying
diepress
androllers
press rollers
die and
(c) Screw extrusion press
Solid
Melting
and
Solid
Melting
Solid and
Melting and
(d) Pellet mill with ring
Pumping
Pumping
Pumping
Conveying
Pumping
Conveying
Pumping
Conveying
Pumping
die and press rollers
(b) Ram extrusion press
(a) Ram and punch press
(a) Ram and punch press
Punch‐ and‐die press
and‐die
press
(a)Punch‐
Ram and
punch
press
(b) Ram extrusion press
(b) Ram extrusion press
Punch‐ and‐die press
Hopper
Hopper
Barrel
Screw
Heaters
DieHeaters
Barrel
ScrewScrew
Barrel
Screw
Heaters
Die
Barrel
Screw
Heaters
DieHeatersDie
Barrel
Barrel
Screw
Heaters
Die
Hopper
Hopper
Hopper
Barrel Screw
Screw Barrel
Heaters
DieHeaters
Screw Die
Die
Barrel
Heaters
Hopper
Hopper
Hopper
Hopper
Die
(e) Flat
die pellet mill with press rollers
Briquettes
Briquettes
Briquettes
Briquettes
Briquettes
Briquettes
Briquettes
BriquettesBriquettes
(e) Flat(e)
dieFlat
pellet
presspress
rollers
die mill
pelletwith
mill with
rollers
(c) Screw extrusion press
Solid (c) Screw
Melting
and
Solid
Melting
and
Solidextrusion
Melting
and
press
(d) Pellet mill with ring
Pumping
Pumping
Pumping
(c)
Screw
extrusion
press
Conveying
Pumping
Conveying
Pumping
Conveying
Solid
Melting
and Pumping
Solid
Melting
Solid and
Melting and
die Pellet
and press
Solid
Melting
and Pumping
(d)
mill rollers
with ring
Solid
Melting
Pumping
Solid and
Melting and Pumping
Conveying
Pumping
Conveying
Pumping
Conveying
Pumping
(d) Pellet mill with ring
Pumping
Pumping
Pumping
(f) Roller press /
Double roller press
(f) Roller press /
(f) Roller press /
Double
press
Double roller
roller press
68
DENSIFICATION–THE TECHNOLOGY
 Energy Required for Densification
– Energy input for densification process could
constitute a significant fraction of densified biomass
production cost, and could have a significant impact
on the economic viability of the technology.
– Biomass densification systems require energy for the
two main processes normally involved:
 fuel preparation, both preprocessing (sieving, drying, size
reduction) and pretreatment
 the densification process itself.
– The energy input for densification of WAB primarily
depends on the properties of the original raw
materials, and also the final end-use application which
demands for prescribed quality levels of the densified
69
products.
DENSIFICATION–THE TECHNOLOGY
 Energy Required for Densification
– Specific energy consumption (SEC) of different
technologies
Technology
Piston Press
Roller Press with Circular Die
Cog-Wheel Pellet Principle
High Pressure Piston Press
Common Throughput
Range (kg/h)
100 - 1800
3000 - 8000
3000 - 7000
40 - 200
SPC
(kWh/t)
50 - 70
20 - 60
20 - 60
500 - 650
Product Density
(kg/m3)
300 – 600
400 – 700
400 – 600
650 – 750
– SEC for different materials
Biomass Material
Equipment
Sawdust
Straws
Straws + Binders
Switchgrass
Pellet mill
Pellet mill
Pellet mill
Pellet mill
SEC
(kWh/t)
36.8
22 - 55
37 - 64
74.5
Biomass
Material
Sawdust
Straws
Grass
Straws + Binder
Equipment
Piston press
Screw press
Piston press
Ram exruder
SEC
(kWh/t)
37.4
150 - 220
77
60 – 95
70
DENSIFICATION–THE TECHNOLOGY
 Energy Required for Densification
– It is important to recognize the fact that there could be
a significant difference of the SEC estimated through
the laboratory results and commercial systems.
Technology
Operation
Raw Material
Density
SEC
(kg/m3)
(kWh/t)
Sawdust
1000
4.0
Sawdust
1200
6.6
Commercial
Sawdust
1200
37.4
In laboratory
MSW
1000
7.7
Commercial
MSW
1000
16.4
Sawdust
1000
36.8
Condition
Compression
Extrusion
In laboratory
71
BRIQUETTING TECHNOLOGIES
 Overview
– Briquetting is usually performed using hydraulic,
mechanical, or roller presses.
– Briquettes have a density of 800–1200 kg/m3,
compared to 60–180 kg/m3 for loose biomass.
– The major limitation of biomass briquettes is uptake of
moisture during storage, leading to increase in
biological degradation / loss of dimensional stability.
– Compared to pellet mills, briquetting machines can
handle larger-sized particles, wider moisture contents
without the addition of binders and have lower
specific energy consumption.
– However, briquettes have lower mechanical strength.
– The briquettes are usually cylindrical with diameter in
72
the range 30 to 100 mm.
BRIQUETTING TECHNOLOGIES
 Piston Press
– The piston press consists of a reciprocating piston
that forces the raw material falling from the feed
hopper into a tapered die.
– There are two types of piston press: the die and punch
technology; and the hydraulic press.
– Hydraulic press process consists of first compacting
the biomass in the vertical direction and then again in
the horizontal direction. The material is pushed by a
piston press against the frictional force caused by die
taper and is heated to 150-200°C during the process.
– The piston presses are normally provided with a
relatively long channel, which serves to maintain the
shape of the briquettes while they undergoing cooling
after emerging from the die.
73
BRIQUETTING TECHNOLOGIES
 Piston Press
– The capacity of commercial piston presses is in the
range 40 to 1500 kg/hr.
– Mechanical presses are normally driven electrically
and fitted with flywheels. Piston presses with
hydraulic drives employ hydraulic transmission
system, which represent a relatively recent
development.
Feedstock
Briquette
Hydraulic or
mechanical
piston drive
Nozzle
Piston
74
BRIQUETTING TECHNOLOGIES
 Screw Press
– The screw presses work on the principle of the unit
operation referred to as extrusion, which is commonly
used with processing of polymer materials.
Hopper
Barrel
Screw
Heaters
Die
Briquettes
Solid
Conveying
Melting and
Pumping
Pumping
75
BRIQUETTING TECHNOLOGIES
 Screw Press
– During this process, the raw material particles move
from the feed port with the help of a rotating screw,
through the barrel and against a die, resulting in
significant pressure gradient and friction due to
biomass shearing.
– The temperature in the system is increased as the
heat is generated due to combined effects friction.
– Finally, the heated biomass is forced through the
extrusion die to form the briquettes or pellets with the
required shape.
– If the die is tapered, the biomass is further compacted
– If the heat generated within the system is not
sufficient for smooth extrusion, heat is provided from
outside either using band or tape heaters.
76
BRIQUETTING TECHNOLOGIES
 Screw Press
– The die temperature is normally maintained at about
300°C. The raw materials get heated up to about 200
°C during the process, where most of the heating is
caused by friction.
– The biomass materials often get partially pyrolyzed at
the surface causing significant amount of smoke
generation during the process.
– The die cross-section can be circular or square with
rounded corners.
– The briquettes are 5-10 cm in diameter.
– The design of the screw results in the formation of a
central circular hole in the briquette, which acts as an
escape route for steam formed during briquetting.
77
BRIQUETTING TECHNOLOGIES
 Screw Press
– The outer surface of the briquettes obtained through
this process is carbonized, hence takes blackish
colour.
– The compaction ratio of screw presses ranges from
2.5:1 to 6:1 or even more.
– Capacity of this type of presses ranges from 50 to 800
kg/hr.
– The major maintenance problems of these briquetting
machines are due to the wear of the screw and the die.
78
BRIQUETTING TECHNOLOGIES
 Screw Press
– Advantages:
 Continuous output with uniform product size,
 Higher bulk density (1500 kg/m³ against 1200 kg/m³ for the die
and punch technology),
 Carbonized outer surface, facilitating easy ignition and
combustion, and also providing an impervious layer for
protection against moisture ingress,
 Presence of the hollow central core, providing a passage for
supplying the air necessary for combustion,
 Smooth running with no shock loads,
 Light weight, due to the absence of reciprocating parts and
flywheel,
 Absence of alternate suction and pressurization of machine,
reducing the possibility of dust collection.
79
BRIQUETTING TECHNOLOGIES
 Screw Press
– Disadvantages:
 Higher power consumed (compared to the piston press)
 Very high wear rate of the screw
 Limitation on the raw material that can be compacted.
80
PELLETIZING TECHNOLOGIES
 Overview
– Pelletization is a process which is closely related to
the briquetting processes.
– The main difference is that the dies have smaller
diameters (usually up to about 3 cm).
– A pellet press is composed of a die and generally of
two or three rollers.
– The die is arranged as holes bored in a thick steel disk
or ring.
– Loose milled material is fed into the pelletizing cavity.
– The rotation of the die and roller pressure forces
material through the die holes.
– The raw material is frictionally heated.
81
PELLETIZING TECHNOLOGIES
 Overview
– The densified material emerges from the die as
strands of uniform section and cut with knives into the
desired length.
– Pellets are cut off when coming out from the die or
they can be cut with adjustable knives to a desired
length.
– The density of the pellets depends on the frictional
forces which are controlled by the length and the
diameter of the apertures in the die, the condition of
the die and rollers, the roller adjustment and the raw
material properties.
82
PELLETIZING TECHNOLOGIES
Punch‐ and‐die press
 Pelletizing Methods
Hopper
Hopper
HopperScrew
Barrel
Screw Barrel
Heaters
DieHeaters
Screw Die
Barrel
Heaters
Die
– There are several different pelletizing methods, which
could be broadly categorized into two groups based
Briquettes
Briquettes
on roller and die press arrangements as Flat die press
and Ring die press.
(c) Screw extrusion press
Solid
Melting
and
Solid
and
Solid
Melting andof a die in
– Flat Die Press: Disk matrix pressMelting
consisting
(d) Pelle
Pumping
Pumping
Pumping
Conveying
Pumping
Conveying
Pumping
Conveying
Pumping
die an
the
form of a plane disk and rollers
Rollers
Flat
Circular
Die
(
(e) Flat die pellet mill with press rollers
83
PELLETIZING TECHNOLOGIES
 Pelletizing Methods
– Ring die press: Ring matrix press consisting of a die
in the form of a ring and inside rollers.
Rollers
Ring
Die
84
PELLETIZING TECHNOLOGIES
 Pelletizing Methods
– Ring die presses are the most popular in the pellet
industry.
– From the basic method it has been several
developments. Ring die may rotate or be static, and
the power transition becomes either the die or rollers.
– Inner diameters of the rings vary from about 25 cm up
to 100 cm with track surfaces from 500 to 6000 cm².
– The capacities of the above types of palletizing
machines are in the range of few kg/hr to 10 t/hr.
– Power consumption of the pellet mills ranges from 15–
40 kWh/t.
85
PELLETIZING TECHNOLOGIES
 Pelletizing Methods
– Other Varieties:
Punch Press
Cog-wheel pelletization principle
86
OTHER DENSIFICATION
TECHNOLOGIES
 Roller Press
– Densification of WAB using roller presses works on
the principle of pressure and agglomeration, where
pressure is applied between two counter-rotating
rollers with identical diameters and parallel axes.
Ground biomass
to the feeder
Fine particles recycled
Rotation
Rollers
Agglomerated
sheet
Crusher
Screener
Agglomerates
of accepted
sizes to
packing
87
OTHER DENSIFICATION
TECHNOLOGIES
 Manual Presses and Low Pressure Briquetting
– There are different types of manual presses used for
briquetting biomass feed stocks.
– They are used both for raw biomass feedstock or
charcoal.
– The use of a binder is imperative.
88
PERFORMANCE COMPARISON
 Performance comparison of different
densification technologies
Parameter
Optimum moisture
content of the raw
material (%)
Particle size (mm)
Wear of contact parts
Output from machine
Specific energy
consumption (kWh/t)
Through puts (ton/hr)
Unit density (g/cm3)
Bulk density (g/cm3)
Maintenance
Combustion
performance of
briquettes
Screw press
Densification Technology
Piston
Roller press Pellet mill
Press
Agglomerator
4-8
10 – 15
10 – 15
10 – 15
-
2-6
High
Continuous
6 - 12
Low
In strokes
<4
High
Continuous
<3
High
Continuous
0.05 – 0.25
Low
Continuous
37 – 150
37 – 77
30 – 83
16 – 75
-
0.5
1.0 – 1.4
0.5 – 0.6
Low
2.5
2.5
< 0.1
High
5 – 10
0.4 – 0.6
Low
5
1.1 – 1.2
0.7 – 0.8
Low
0.4 – 0.5
Low
Very good
Moderate
Moderate
Very good
89
PERFORMANCE COMPARISON
 Performance comparison of different
densification technologies
Densification Technology
Parameter
Piston
Screw press
Roller press Pellet mill Agglomerator
Press
Good
Not
Not
Not
Not
Carbonization of
charcoal
charcoal
possible
possible
possible
possible
Homogeneity of
Not
Not
densified biomass
Homogenous homogenou
Homogenous Homogenous
homogenous
s
Suitability in gasifiers
Suitable
Suitable
Suitable
Suitable
Suitable
Suitability for cofiring
Suitable
Suitable
Suitable
Suitable
Suitable
Suitability for
Not suitable Suitable
Suitable
Suitable
biochemical
conversion
Addition of binder
Not
Not required
Required Not required
Required
required
Generally
Shape
Cylindrical Cylindrical
Cylindrical
Spherical
elliptical
90
The End
91