3. Physical Conversion Technologies.ppt

Physical Conversion Technologies

Muhammad Waqas Iqbal

(Assistant Professor)Chemical, Polymer & Composite Materials Engg. Deptt.UET, KSK, Lahore.

CONTENT Introduction

Preprocessing Techniques

Pretreatment Techniques

Densification – The Process

Densification – The Mechanism

Densification – The Technology

Briquetting Technologies

Pelletizing Technologies

Other Densification Technologies

Performance Comparison

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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,

transportation and emissions.3

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.

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BENEFICIATION

Drying Dewatering Sizing Densification Separation Torrefaction

Baling Pelletization Briquetting

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.

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– 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.

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Baling

Bales

Cooling

Briquettes

Grinding

(Fine Milling)

Pellets

Pressing Sieving Preheating

Drying Storing of

WAB Materials

Separation of

Metals and Sands

Segregation or Classification

Chipping or Shredding


– 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.

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Figure 1.3: Densified biomass products – bales, briquettes and pellets

Bales

Briquettes

Pellets




– 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.

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– 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 spontaneous combustion seen with loose materials.

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– 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.

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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)

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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).

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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.

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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).

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Figure 2.2: Physical forms of straw before and after size reduction processes

(a) Straw in raw form (b) Chopped straw (c) Straw in grounded form


– 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).

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Moving bladesUpper feed

roller

Lower

feed rollerStationary bottom blades

Biomass material in

Swinging hammer

Peripheral screen

Biomass material in

Ground particles out


– 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


– 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


– Types of equipment:

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Figure 2.3: Size reduction equipment

(a) Disc type chipper (b) Drum type chipper (c) Straw shredder

(d) Hammer mill (e) Knife mill (f) Disc mills

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.

PREPROCESSING TECHNIQUES

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(a) Straws, stalks and grasses (b) Straw bales

Figure 2.5: Shredding / chopping of WAB materials


– 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.

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roller

Lower


Biomass material in

Swinging hammer

Peripheral screen

Biomass material in



– 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.

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roller

Lower


Biomass material in

Swinging hammer

Peripheral screen

Biomass material in



– 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.

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Biomass MaterialMoisture content (% on wet basis)

Screen size (mm)

0.8 1.6 3.2

Wheat straw8 51.0 37.5 10.712 45.3 43.5 24.2

Maize stover8 21.1 16.2 6.312 34.2 19.7 11.1

Switchgrass8 63.4 50.2 23.912 56.6 58.4 26.9

Specific energy consumption of WAB in hammer milling (in kWh/t)

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


– 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.

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– 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.

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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.

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– 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.

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– Basic dewatering technologies include: Open air storage, Filters, Presses, Screening devices, Centrifuges, Hydro cyclones extrusion and expression process

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Feed

Belt alignment

Belt

Belt drive

Residues

Belt

tensioning

Linear / peripheral pressure

Belt washing

Belt alignment

Belt washing


– 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

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Classification AlternativesDrying media (i.e. the stream passing through the material to be dried)

Flue gas, hot air or superheated steam

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

Nature of heat sourcePassive: Open sun, solar dryer, natural ventilationActive: Dryer burners, boiler (flue gas or

steam), recovered waste heat from facility processes


– Eg: Pneumatic dryer

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Figure 2.10: Pneumatic dryer

Hot air generator Hopper

(Material In)

Buffer

Fan

Cyclone

Dried Material

Out

Drying ducting

Moist air out

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.

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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.

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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.

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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

Steam Explosion

– The extent of chemical and structural modifications from steam-explosion pretreatment depends on residence time, temperature, particle size and moisture content.

Un-treated

Steam-Exploded

PRETREATMENT TECHNIQUES

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Barley Straw Canola Straw Oat Straw Wheat Straw

Torrefaction

– Torrefaction is a method of changing the properties of biomass materials by slowly heating it in an inter-environment 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.


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Torrefaction


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Biomass residues before and after torrefaction

Biomass pellets and torrefied biomass pellets

Torrefaction


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• Range AThe biomass is dried.• Range BSoftening 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.

Overview

– Low bulk density, loose forms and wide variations of particle sizes are common drawbacks of WAB

DENSIFICATION – THE PROCESS

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Figure 3.1: Loose waste agricultural biomass

Overview

– The fuel quality of WAB could be improved by means of compaction into high density and regular shape.


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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.


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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 moisture content is greater than 15% on wet basis.


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– 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 1450 to 1500 kg/m³.


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– 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. Lignin too, at elevated temperatures (>140 °C), acts as a

binder and increases durability.


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– 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.


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– 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.


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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.

DENSIFICATION–THE MECHANISM

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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.


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Overall Mechanism

– The stages of deformation mechanism of powder particles under compression


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Biomass particles

Rearrangement, sliding, stacking–reduced porosity

Compressionmechanism

Fragmentation Bonding

Solid bridges

Intermolecularforce

MechanicalinterlockingElastic

deformationPlastic

deformation

Increasedappliedpressure

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 self-bonding have been proposed, universally accepted model is yet to be established.


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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 the densified biomass fuel.


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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.


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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 8-12% 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).


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– 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


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In particular, particle shape and size distribution play a vital role in the compaction and agglomeration stages of the densification process.


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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


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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


– 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.


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Figure 3.5: Effects of applied compacting p

31 MN/m2 63 MN/m2 159 MN/m2 191 MN/m2 254 MN/m2

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.

DENSIFICATION–THE TECHNOLOGY

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– 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 25 mm in d.iameter and 30 to 50 mm in length.


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– 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.


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– 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.


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– 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.


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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.


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Working Principles

– Tumble agglomeration


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En

ergy

inp

ut

kW

h/t

Bonding agent addition kg/kg of feed

Working Principles

– Pressure agglomeration


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(a) Ram and punch press Punch‐ and‐die press

(b) Ram extrusion press

(e) Flat die pellet mill with press rollers

(f) Roller press / Double roller press

(d) Pellet mill with ring die and press rollers

Solid Conveying

Melting and Pumping

Pumping

Briquettes

Die Heaters Screw Barrel Hopper

Solid Conveying

Melting and Pumping

Pumping

Briquettes


Solid Conveying

Melting and Pumping

Pumping

Briquettes


(c) Screw extrusion press

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 products.


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– Specific energy consumption (SEC) of different technologies

– SEC for different materials


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Technology Common Throughput Range (kg/h)

SPC (kWh/t)

Product Density (kg/m3)

Piston Press 100 - 1800 50 - 70 300 – 600Roller Press with Circular Die 3000 - 8000 20 - 60 400 – 700Cog-Wheel Pellet Principle 3000 - 7000 20 - 60 400 – 600High Pressure Piston Press 40 - 200 500 - 650 650 – 750

Biomass Material Equipment SEC(kWh/t)

Biomass Material

Equipment SEC(kWh/t)

Sawdust Pellet mill 36.8 Sawdust Piston press 37.4Straws Pellet mill 22 - 55 Straws Screw press 150 - 220Straws + Binders Pellet mill 37 - 64 Grass Piston press 77Switchgrass Pellet mill 74.5 Straws + Binder Ram exruder 60 – 95


– 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.


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Technology Operation

Condition

Raw Material Density

(kg/m3)

SEC

(kWh/t)

Compression In laboratory Sawdust 1000 4.0

Sawdust 1200 6.6

Commercial Sawdust 1200 37.4

Extrusion In laboratory MSW 1000 7.7

Commercial MSW 1000 16.4

Sawdust 1000 36.8

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

the range 30 to 100 mm.

BRIQUETTING TECHNOLOGIES

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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.


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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.


72Nozzle

Briquette

Piston

Hydraulic or mechanical piston drive

Feedstock

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.


73

Solid Conveying

Melting and Pumping

Pumping

Briquettes


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.


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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.


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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.


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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.


77

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.


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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.

PELLETIZING TECHNOLOGIES

79

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.


80

Pelletizing Methods– There are several different pelletizing methods, which

could be broadly categorized into two groups based on roller and die press arrangements as Flat die press and Ring die press.

– Flat Die Press: Disk matrix press consisting of a die in the form of a plane disk and rollers


81

-

Rollers

Flat Circular

Die

Solid Conveying

Melting and Pumping Pumping

Briquettes


Solid Conveying


Briquettes


Solid Conveying


Briquettes


(a) Ram and punch press Punch‐ and‐die press

(b) Ram extrusion press

(c) Screw extrusion press

(f) Roller press / Double roller press (e) Flat die pellet mill with press rollers

(d) Pellet mill with ring die and press rollers

Pelletizing Methods– Ring die press: Ring matrix press consisting of a die

in the form of a ring and inside rollers.


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-

Rollers

Ring Die

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.


83

Pelletizing Methods– Other Varieties:


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Punch Press Cog-wheel pelletization principle

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.

OTHER DENSIFICATION TECHNOLOGIES

85

Rotation

Agglomerated sheet

Agglomerates of accepted sizes to packing

Crusher

Ground biomass to the feeder

Fin

e p

arti

cle

s r

ecy

cled

Screener

Rollers

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.

OTHER DENSIFICATION TECHNOLOGIES

86

Performance comparison of different densification technologies

PERFORMANCE COMPARISON

87

ParameterDensification Technology

Screw pressPiston Press

Roller press Pellet mill Agglomerator

Optimum moisture content of the raw material (%)

4 - 8 10 – 15 10 – 15 10 – 15 -

Particle size (mm) 2 - 6 6 - 12 < 4 < 3 0.05 – 0.25Wear of contact parts High Low High High LowOutput from machine Continuous In strokes Continuous Continuous ContinuousSpecific energy consumption (kWh/t)

37 – 150 37 – 77 30 – 83 16 – 75 -

Through puts (ton/hr) 0.5 2.5 5 – 10 5 -Unit density (g/cm3) 1.0 – 1.4 2.5 0.4 – 0.6 1.1 – 1.2Bulk density (g/cm3) 0.5 – 0.6 < 0.1 - 0.7 – 0.8 0.4 – 0.5Maintenance Low High Low Low LowCombustion performance of briquettes

Very good Moderate Moderate Very good -

Performance comparison of different densification technologies

PERFORMANCE COMPARISON

88

ParameterDensification Technology

Screw pressPiston Press

Roller press Pellet mill Agglomerator

Carbonization of charcoal

Goodcharcoal

Notpossible

Notpossible

Notpossible

Notpossible

Homogeneity of densified biomass Homogenous

Not homogenou

s

Not homogenous

Homogenous Homogenous

Suitability in gasifiers Suitable Suitable Suitable Suitable SuitableSuitability for cofiring Suitable Suitable Suitable Suitable SuitableSuitability for biochemical conversion

Not suitable Suitable Suitable Suitable -

Addition of binderNot required

Not required

Required Not required Required

Shape Cylindrical CylindricalGenerally elliptical

Cylindrical Spherical

The End

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Documents

3. Physical Conversion Technologies.ppt