OVERVIEW OF BIOMASS BASESD FUELS AND THEIR …shodhganga.inflibnet.ac.in/bitstream/10603/46119/11/11_chapter 3.pdf · B- Cowdung heap, C Straw 3800 Stalk 4650 B C ... Fischer-Tropsch

CHAPTER-3

OVERVIEW OF

BIOMASS

BASESD FUELS

AND THEIR

CONVERSION

TECHNOLOGIES

CHAPTER-3

OVERVIEW OF BIOMASS BASED FUELS AND THEIR CONVERSION

TECHNOLOGIES

3.1- Classification of biomass

The wood is the most common example of the biomass. When burned, it releases the

stored solar energy stored into it through photosynthesis by the tree. Still, the wood is not only

example of best known biomass. Different sources of biomass are agricultural residues like

bagasse from sugarcane, straw, hay, fiber and nutshells; wood wastes like bark, sawdust, mill

scrap and timber blocks and the trash paper and organic materials in municipal waste. Also

energy crops, in the form of fast growing trees and shrubs like poplars, willows, Jatropha and

several wild growing grasses like switchgrass, algae (in ponds, lakes or ocean and municipal

waste water treatment plants), the methane captured from landfills and manure from cattle or

poultry can also be used as viable biomass feedstock.

Biomass, after being processed for converting it into fuel is termed as feedstock. Biomass

can be classified in several groups based on a variety of criteria. They may be grouped on the

basis of their availability, physical properties, conversion processes or types of utilization.

Communally the biomass feedstock can be classified in two distinct ways. First they may be

categorized, depending on either their sources of supply or availability in nature. Another way of

classifying the biomass feedstock is to differentiate them as per their physical characteristics.

Different classes of feedstock depending on their route of sources are enlisted in the table

3.1 along with the representative examples.

3.1.1 - Classification of Biomass feedstock supply sector wise

Table 3.1

Classification of biomass feedstock supply sector wise

Supply Sector

Type of feedstock Example

Agriculture Dry residues

Stalk, straw, bagasse, husk, leaves and roots (tubers and bulbs)

Livestock wastes Solid and liquid manure

Energy crops

Dry woody crops Bamboo, willow, poplar, eucalyptus

Dry non-woody crops

Miscanthus, switch grass, common reed, giant reed

Oil crops Jatropha, Soybean, palm, cottonseed,

sunflower

Starch crops Wheat, potatoes, maize, barley, corn,

sugarcane, agave, cactus

Aquatic plants Hyacinth, algae

Forest

Byproducts Bark, leaves, wood chips, logs from

branches and natural thinning

Bio-energy plantation

Pine, eucalyptus, hybrid poplar, sweetgum, cottonwood

Industry

Wood industry Residues

Bark, wood chips, off-cuts, slabs, sawdust from sawmills/ timber mills

Fibrous waste from pulp and paper production units

Food industry Residues

Wet cellulosic waste parts of vegetable and grains, used cooking oils, oil cakes, tallow,

and slaughter-house waste

Industrial products Organic wastes (solvents and other

compounds) of chemical and pharmaceutical processes

Others Shells and husks from palm, coconut,

almond, olive, walnut

Municipal Parks and Gardens Twigs, leaves, bark from pruning and grass

Organic wastes Wood, landfill gas, sewage sludge, sewage

gas

Roadside wastes Wood, shrubs, grass and hay

The classification of feedstock, based on their physical properties is described in the

following subsection 3.1.2 with some representative examples.

3.1.2 - Classification of Biomass feedstock based on the physical characteristics:

Woody biomass: They include materials from trees, bushes and shrubs. Their characteristics are

high bulk density, high calorific value, low moisture content and low ash content. Woody

biomass is a preferred class of feedstock for biomass to energy conversion processes.

Non-woody biomass: They include grains and non-woody stalks/stems of the residues from

agricultural crop harvest, wastes from poultry and livestock farms, organic parts of municipal

solid wastes (MSW) and animal excreta and residues from the food processing industry. Their

characteristics are lower bulk density, higher moisture content, lower calorific value and higher

ash content,. Although, they are economical than woody biomass but, less preferred due to

various associated drawbacks. Typical energy potentials of some common agricultural crops are

summarized in table 3.2.

Table 3.2

Energy potential of some common non-woody agricultural crop residues

Agricultural Crop

Type of Residue

Calorific Value (Kcal/Kg)

Wheat Straw 3500

Paddy Straw 2400

Paddy Husk 3000

Bajra Stalk 3850

Maize Stalk 3450

Maize Cobs 3800

Groundnut Stalk 4150

Gram (Chana)

Cotton

A

Figure 3.1 A

B- Cowdung heap, C

Gram (Chana) Straw 3800

Stalk 4650

B

C

Figure 3.1 A- Cowdung (Calorific value: 3300 Kcal/Kg),

Cowdung heap, C- Cowdung cakes (Calorific value: 3100 Kcal/Kg)

Cowdung (Calorific value: 3300 Kcal/Kg),

Cowdung cakes (Calorific value: 3100 Kcal/Kg)

Dry lignocellulosic biomass feedstock is used for thermo-chemical conversion like

combustion, gasification and liquefaction processes. The wet lignocellulosic biomass feedstock

is used for biological conversion processes, mostly digestion or fermentation. Most important

technical specifications of solid biofuels developed between 2010 and 2012 are EN 14961 for

specification and classification and EN 15234 for quality assurance.

Figure 3.2 - Aquatic weeds a promising biomass for energy

3.2 - Composition of Biomass

Although, the chemical composition of biomass material varies among species, however

plants consist of mainly carbohydrates or sugars and lignin. The carbohydrate part consists of

several sugar molecules linked together in long chains or as polymers. Cellulose and

hemicellulose are wo larger categories of carbohydrate having significant values. Long cellulose

polymers are used in nature to build the fibers that give strength to a plant. Further, lignin in the

biomass consists of non-sugar molecules and acts like a glue that binds the cellulose fibers

together. The carbon dioxide from the atmosphere is combined with the water in the process of

photosynthesis and produces carbohydrates, which form the bulk of the biomass. Thus, solar

energy is basically stored in the biomass in the form of chemical energy through the process of

photosynthesis. When biomass is burnt, the oxygen from the atmosphere reacts with the carbon

present in the biomass, to generate carbon dioxide and water. This process of extracting the

energy stored in the chemical bonds of the biomass material is cyclic in nature, as carbon dioxide

is again used for producing newer biomass material through the process of photosynthesis.

Wood (largely secondary cell walls) is made up of following major constituents which

also vary with type and age of a particular species:

• Cellulose (C6H10O5)n is major structural component and constitutes a major portion (35-

50%) of plant cell wall. It consists of long chains of polysaccharides.

• Hemicellulose is constituted by sugars which comprises of 20-35% of plant biomass on

dry weight basis. Xylans, a type of hemicellulose, are highly complex short chains of

polysaccharides made from units of xylose which is a pentose sugar. They are found in

cell walls of plants and some algae.

• Lignin (10-25%) complex and high molecular weight polymer. It is formed by de-

hydrogenation of p-hydroxy-cinnamyl alcohols. Lignin is a complex phenolic polymer

which fills the gaps or spaces in the cell wall. It strengthens the cell wall by driving out water.

• Water soluble constituents include sugars, amino acids and aliphatic acids.

• Ethers and alcohol-soluble constituents (e.g. fats, oils, waxes, resin and many pigments).

• Proteins of different chemical compositions (1-5%).

3.3 -Physical Biofuel types

Solid biomass based fuels

The utilization of solid biomass based fuels demands a lower effort for conditioning and

conversion of biomass feedstock. This results in total energy balance in consideration of the

complete utilization chain. Wood is considered to have better efficiency amongst the solid

biomass based energy sources. They are used in decentralized environment of households or

small industries for heat generation in the form of firewood, briquettes and pellets with fine

particulate air filters to reduce atmospheric pollu

biomass based fuels are utilized through cogeneration of heat and power. Cogeneration of heat

and power using Sterling engines at a very small scale are being developed, using wood pellets.

Wood from short-rotation plantations can achieve a high thermal energy yield per acre.

Straw and husks from short-rotation agriculture or plantations are reported with good energy

yield, but requires an expensive treatment

competition in the food production.

the near absence of oxygen. It is

This dual functionality of biochar, to

atmosphere, is very useful for mitigating climate change.


particulate air filters to reduce atmospheric pollution. In centralized larger facilities solid



ation plantations can achieve a high thermal energy yield per acre.

rotation agriculture or plantations are reported with good energy

yield, but requires an expensive treatment and less efficient. However, they

competition in the food production. Biochar is obtained through burning biomass

is a carbon negative energy source and an important

dual functionality of biochar, to enhance soil fertility while holding carbon back from the

is very useful for mitigating climate change.

Figure 3.3 - Fuelwood


tion. In centralized larger facilities solid



ation plantations can achieve a high thermal energy yield per acre.

rotation agriculture or plantations are reported with good energy

they can avoid the

burning biomass feedstock in

important soil builder.

carbon back from the

Liquid biomass based fuels

Liquid biomass based fuels belong to natural vegetable oils,

(BTL) fuels (produced by using

depolymerization) and biodiesel. However, the efficient and economical production of fuels is

feasible only in centralized large production f

economical production is good only when it is used locally, e.g., in tractors or agricultural

machines. The use of natural vegetable oils requires complex modifications in standard diesel.

However, biodiesel may be used

blended with the petroleum diesel. Bioethanol

and can be made from common sugar crops like sugar beet

starch through the microbial fermentation of sugar.

Figure 3.4 - Charcoal

Liquid biomass based fuels belong to natural vegetable oils, bioethanol, biomass to liquid

by using Fischer-Tropsch process, flash pyrolysis

and biodiesel. However, the efficient and economical production of fuels is

feasible only in centralized large production facilities. The decentralized efficient and



e used, in unmodified conventional diesel engines

petroleum diesel. Bioethanol and biobutonol are easy to manufacture

and can be made from common sugar crops like sugar beet and sugar cane or crops containing

through the microbial fermentation of sugar.

bioethanol, biomass to liquid

Tropsch process, flash pyrolysis or Catalytic

and biodiesel. However, the efficient and economical production of fuels is

acilities. The decentralized efficient and



diesel engines, standalone or

easy to manufacture/process

sugar cane or crops containing

Figure 3.5 - Bioethanol from Econol (UK)

Figure 3.6 - Biodiesel Fuel

Gaseous biomass based fuels

Gaseous biomass based fuels from organic waste are obtained through thermal

gasification or biological processes of fermentation and anaerobic digestion. These methods are

energy-efficient even at small scale in households and thus well suited for small plants in rural

and remote areas. These small biogas plants do not convert the gas directly into electricity but

condition the gas to be used locally. Biomethane and Syngas (a mixture of carbon monoxide and

hydrogen gases) are examples of biomass based gaseous fuels. Direct composting of organic

waste has negative impact on climate. Therefore, organic waste from different sources can be

separated and used to produce biofuels using multiple stages of fermentation and composting to

maximize yield along with environmental protection.

Figure 3.7 - Pre-fabricated biogas plant for domestic use

3.4 - Classification of biofuels based on the sequence of development

Although biomass based fuels are produced in solid, liquid and gas physical forms.

However due to development in conversion techniques over the time they may be classified in

the following categories as per sequence of their advancement with time:

3.4.1 - First Generation Biofuels

First generation biomass based fuels are also termed as conventional biofuels. They may

be solid fuels available in nature in raw form like wood, grass, dried animal wastes and forest

residue. They can also be derived from domestic, agricultural or industrial wastes like household

refuse, sawdust, wood chips, charcoal and biochar. Liquid biofuels are extracted from sugars,

starch and vegetable oils of biomass feedstock and include like bioalcohals (Ethanol, methanol,

butanol), bioethers, biodiesel, green diesel and biofuel gasoline. Gaseous biofuels are produced

by anaerobic digestion or combustion of organic feedstock. Biogas (mostly methane) and Syngas

(mixture of carbon monoxide, hydrogen and hydrocarbons) are examples of these biofuels which

are non-polluting and sustainable.

These fuels are also derived from energy crops and biomass that is part of staple food in

several communities and grown on agricultural land thus endangering the food chain. The

growing demands of biofuels derived from these sources also have serious negative impact on

the sustenance of biodiversity of the region and efficient land use. Therefore first generation

biofuels have problem of continuous feedstock supply and so they cannot be considered as

sustainable.

3.4.2 - Second Generation Biofuels

To overcome the problem of sustainable supply of feedstock associated with first

generation biofuels, efforts have been done to use biomass obtained from inedible waste and

non-food crops and non-invasive grasses/shrubs grown on non-agricultural, marginal and no-

arable lands across the world. These avoid food vs. fuel conflict and proper land use without

disturbing ecology and biodiversity. This ensures the sustainability of feedstock to produce

biofuels with increasing demand. Second generation biofuels are obtained by the conversion of

lignocellulosic parts of biomass or woody material of the plants using advanced complex

chemical and biological conversion processes.

Fischer-Tropsch Diesel, wood diesel, cellulosic ethanol, biomethanol and biohydrogen

are examples of these fuels. Dimethyl ether (DME) and BioDME are produced from

lignocellulosic biomass feedstock and used in internal combustion engines as a substitute of

petroleum diesel and gasoline. They are also used as substitute of propane in liquefied petroleum

gas (LPG) in domestic and industrial applications. They are used in turbines for electricity

generation in place of natural gas. Lignocellulosic biomass based fuels has the capability of

reducing GHG emissions to >88% in comparison to 13% by cellulosic biomass based fuels.

3.4.3 - Third Generation Biofuels

Variety of biomass feedstock, including the first and second generation biofuels, may be

used for better yield with improved performance of the produced fuel at an affordable cost using

only limited area of land. Therefore, third generation biomass based fuels are derived from non-

food and otherwise waste biomass feedstock using complex conversion techniques involving

enzymatic and microbial activities and advanced genetic engineering. As an example, genetically

engineered varieties of Jatropha, grown in a small area of marginal land, are economically viable

source of feedstock to produce biodiesel at competitive cost. Work has been done to produce

myco-diesel from cellulose using Gliocladium roseum, a fungus. Lipids extracted from another

species of fungus Cunninghamella japonica have also been converted economically and

efficiently into biofuels. Bacteria like Clostridium have properties to convert lignocellulosic

biomass into useful biomass.

Several species of algae growing in ponds, lakes and oceans are being considered to

provide biofuels in large amounts at economical scale solving the energy problem. Algae

feedstock provides lipids, proteins and carbohydrates, which can be converted into biodiesel

fuels, methane and ethanol respectively. Algae need nutrients, CO2 and water to grow under

sunlight. So, if grown on commercial scale in ponds and bioreactors, algae can absorb huge

amounts of CO2 emitted from industries and thermal power plants and mitigate GHG problem up

to a large extent. Also non-fuel parts of the biomass can be used as nutrient to further grow more

algae. The yield can be measured in mass per unit of volume instead of mass per unit of area as

extremely large quantities of algal biomass can be cultivated in vertical columns requiring

reduced horizontal space.

Figure 3.8 - Algae as pond scum

Figure 3.9 - Close up view of algae as pond scum

3.4.4 - Fourth Generation Biofuels

In addition to replace fossil fuels, biomass based fuels need to reduce CO2 concentration

in atmosphere. Second and third generation biofuels are helpful in this regard by reaching up to a

level of being carbon-neutral. This is possible through absorbing atmospheric CO2 by

photosynthesis during biomass growing and improvements in the biomass to fuel conversion

processes. However, increasing levels of CO2 due to industrial growth require further steps in

this direction for sustainability. Forth generation biofuels are aimed to be produced using such

processes, which not only reduce the levels by replacing fossil fuels, but also capture large

amounts of CO2 from atmosphere and store into emptied underground gas and oil wells or use in

the industrial processes.

3.5 - Biomass to Fuel Conversion Technologies:

Biomass in general may not be used to get energy directly. It is required to be converted

into suitable types/forms of fuels for being used in real life applications in an efficient and

economical way. This function is achieved by converting biomass into fuel form through

appropriate technology. The important biomass to fuel conversion technologies in practice has

been described in this chapter.

3.5.1 - Mechanical Conversion Techniques:

Different varieties of biomass feedstock can be utilized, through direct combustion, to

generate energy, without converting it into any other fuel form. However, the biomass, as

available in the natural form is seldom useful and requires some kind of conditioning for proper

and efficient utilization. These biomass feedstocks can be converted into convenient forms as

highly useful and economical biofuels using following mechanical conversion processes:

(1) Cutting or chopping

Biomass available in nature is of varying in shape and size ranging from wheat and paddy

stalks of crop residue to branches and trunks from forest logging operations and timber industry.

The odd size or shape of these biomass feedstock pose practical problems in using them directly

into the kitchens stoves or furnaces of boilers in industries and power plants. For direct

combustion in domestic and commercial applications, these materials are required to be

converted into smaller pieces of appropriate shape and size (figure 3.1). This conversion is being

done by mechanical process of cutting or chopping/shredding of the available biomass feedstock

into appropriate size/shape. The sizing in small quantity can be done manually by breaking or

chopping through hand operated tools, by applying manual labor. However, cutting machines

like sawmills are used for sizing at large scale in production plants. Sizing of biomass material is

beneficial from storage and transportation points of view. The process, up to some extent, also

increases active surface area of the biomass feedstock and improves combustion with a reduction

in moisture content. Sizing can also be precursor of other biomass to fuel conversion processes

as a pre-treatment stage.

Figure 3.10 - Firewood blocks

(2) Pulverizing

Pulverizing is a type of downsizing of woody and non-woody biomass into fine granules

or powder. Pulverizing increases the active surface area of the biomass feedstock. This helps in

combustion of the biomass more efficiently generating more heat per unit mass of the fuel.

Finely powdered biomass material, as shown in figure x.x, can be used in stoves directly or

mixed with coal for co-firing in furnaces of the boilers at power plants. Pulverizing can also be

precursor of other biomass to fuel conversion processes as a pre-treatment stage of the main

process.

Figure 3.11 - Pulverized wood for co

(3) Briquetting

Briquetting is the process of compacting the loose strands or chips of biomass material.

It provides economic and compact high density fuel which can be stored in smaller space and

transported easily. Briquetted biomass is also low in GHG emission and is

industrial applications as an alternative to the fossil fuels. They are used in power plants alone

are co-fired with fossil coal for generating electricity. Biomass feedstock, from wastes and

residues of agriculture, forestry and wood

using as fuel.

The biomass feedstock is pulverized to a fine size of 3

to reduce its moisture content to 12

briquetting machines, of type and size, depending on the quantity and variety of the available

feedstock and capacity of the plant. No binders are required as high temperature of briquetting

plasticize the lignin present in the biomass to bind fine wood pa

Pulverized wood for co-firing with coal in a CHP plant



transported easily. Briquetted biomass is also low in GHG emission and is used in domestic and


fired with fossil coal for generating electricity. Biomass feedstock, from wastes and

residues of agriculture, forestry and wood industry sectors, is compressed into briquettes for

The biomass feedstock is pulverized to a fine size of 3-5 mm and dried in a drying system

to reduce its moisture content to 12-15%. This pretreated feedstock is then briquetted using



plasticize the lignin present in the biomass to bind fine wood particles into solid mass. High

firing with coal in a CHP plant



used in domestic and


fired with fossil coal for generating electricity. Biomass feedstock, from wastes and

industry sectors, is compressed into briquettes for

5 mm and dried in a drying system

15%. This pretreated feedstock is then briquetted using



rticles into solid mass. High

energy density and low moisture content (~4%) make biomass briquettes an ideal substitute of

coal and petroleum oil for use in furnaces of industries and power plants. They are also used as

feedstock to produce other biofuels like charcoal.

Source: http://www.cnpelletmachine.com/Related_Product/Biomass_Briquette_Machine.html

Figure 3.12 - Wheat straw (loose straw and briquettes)

(4) Pelletizing

Pelletizing is also a process of compacting of the biomass feedstock, into small

manageable sizes for direct use in stoves and furnaces to produce heat through combustion.

Pellets are basically smaller size briquettes (in the shape of rods or tablets) ranging from 5-12

mm and used as heating fuel in homes and industries. Due to their high energy density, they are

used in power plants for electricity generation. Wood pellets are manufactured from residues and

wastes of sawmills and other wood industries. The feedstock is first compressed into a hammer

mill and then passed through a press. High pressure of press increases the temperature of the

feedstock and makes the lignin as a bonding agent for the wood particles. The process is highly

energy efficient as total energy involved in conversion is <2% of the energy content of the

pellets. Most of the agricultural and forest residues like grass can also be converted into different

grades of pellets depending on their energy density. The energy content of approximately 4.7 –

5.2 MWh/tone for wood pellets have been reported. However, to reduce particulate matter

emission on combustion, filters and precipitators are required in industries and power plants.

Source: http://www.pellet-press.com/News/bamboo-pelletizing-mill.html

Figure 3.13 - Bamboo stalk pellets

3.5.2 - Thermo-chemical Conversion Techniques:

(1) Thermal Decomposition Processes

(a) Carbonization or Charcoal Making

The main objective of carbonization is to increase the calorific value of the biomass

feedstock by converting it into solid product charcoal (figure 3.x). Carbonization is the process

of dry distillation of biomass feedstock such as wood, bark, bamboo, rice husks, etc. by heating

at 400-600°C in the partial or complete absence of air or oxygen. The biomass feedstock is

arranged in a pile (figure 3.x) and burnt after covering it from all sides to prevent air/oxygen.

Charcoal is produced as the main product which is used as a solid fuel for food-cooking and

space-heating purpose.

Source: http://en.wikipedia.org/wiki/Charcoal

Figure 3.14 - Charcoal

The process is very easy to operate using inexpensive equipments producing efficient

biofuels at reasonable cost. The process also produces tar, pyroligneous acid (used as deodorant),

and combustible gases as by-products. The process of carbonization helps in reducing

environmental degradation by utilizing wastes from several sources. This is very useful process

for producing biomass based fuels and solving waste disposal problem simultaneously.

Source: http://en.wikipedia.org/wiki/Charcoal

Figure 3.15 - Wood pile for charcoal making

(b) Gasification

Direct combustion of biomass, in certain cases, produces slagging residues and corrosive

substances, hindering the operation of the furnaces and boilers during the process. Therefore,

direct combustion or co-firing of biomass is not always a techno-economically feasible option

for producing heat. However, biomass may be converted into a clean gaseous fuel form which

can be easily and efficiently burnt into the furnace to generate heat and electricity. The gaseous

fuel thus produced may be transported to the point of utilization. The process of gasification, as

shown in equation 3.1, is a thermo-chemical process of partial combustion and reduction, which

converts solid biomass feedstock into useful clean gaseous fuels.

C6H10O5 + ½ O2 → 6CO + 5H2 + Energy (3.1)

This is an environment friendly (very low GHG and particulate emissions) and well

proven technology in practice for several decades. There are several types of gasifiers used

depending on the type of feedstock and plant capacity. Producer gas and Syngas are produced

from the biodegradable waste material, solving waste disposal problem. Syngas can be used

without any further processing or modifications, directly in gas engines and turbines. Syngas

may also used for producing hydrogen and methanol or can be converted into synthetic biofuel

through the Fischer–Tropsch process.

Source: https://www.ecn.nl/fileadmin/ecn/units/bio/Overig/pdf/Biomassa_voordelen.pdf

Figure 3.16 - Block diagrams of direct gasification reactors

Gasification of biomass is done by using gasifiers of different configurations. Selection

of a gasifier type depends on the characteristics of biomass feedstock and end products. Block

diagrams four basic fixed bed reactor types for gasification are shown in figure 3.16.

Source: N.L. Panwar et al. (2012)

Figure 3.17 - Fixed bed downdraft biomass gasifier

Updraft gasifiers are very efficient with high conversion rates of gasification. The exit

gas temperatures are in the range of 100-300 0C and consist of mainly product gas. Fixed bed

downdraft gasifiers are less efficient with lower rates of conversion. Conversion rate of

downdraft gasifiers can be increased by supplying oxygen to increase the temperature. But this

increases the cost of conversion. Fluidized bed reactors are used for large scale gasification

operations. Fluidized bed gasifiers are designed as circulating fluidized bed (CFB), bubbling

fluidized bed (BFB) and coupled fluidized bed types.

Conversion efficiency of fluidized bed gasifiers is in the range of 90-98%. The output is

in the form of product gas (N2-free) and flue gases. Entrained bed or flow reactor type gasifier is

used to produce Syngas. The biomass feedstock is required to be pulverized to 1mm size in order

to be used. The operation is accomplished at high temperature and pressure than other three types

of gasifiers.

Source: https://www.ecn.nl/fileadmin/ecn/units/bio/Overig/pdf/Biomassa_voordelen.pdf

Figure 3.18 - Block diagram of an advanced two-step gasification process

(c) Pyrolysis

Pyrolysis is the thermal decomposition of biomass, which occurs in the absence of air or

oxygen. The organic material is heated in a non-reactive atmosphere. This process is composed

of both simultaneous and successive reactions. Thermal decomposition of organic components in

biomass starts at 350 - 550 °C and goes up to 700 - 800 °C in the process. The long chains of

carbon, hydrogen and oxygen compounds in the biomass break down into the molecules, with

smaller chains of carbon, in the form of gases, condensable vapors and solid charcoal. The rate

and extent of decomposition of various components depend on the process parameters of the

reactor. They consists of temperature, biomass heating rate, pressure, reactor configuration and

characteristics of the biomass feedstock. Biomass pyrolysis yields mainly biochar, bio-oil and

gases, which include carbon dioxide, hydrogen, methane and carbon monoxide. Carbonization as

described in the section (a) is also a type of low temperature pyrolysis for producing charcoal, a

solid biofuel. The typical pyrolysis process reaction is shown in equation 3.2.

HEAT C6H12O6 ====� (H2 + CO + CH4 + C2H6 + …..+ C5H12) + (CH3OH + CH3COOH) + H2O + C + Tar

(3.2) Biomass Gas Liquid Water Char

Pyrolysis processes has been broadly classified into three basic types:

Slow pyrolysis process is characterized by the low temperature and pressure process of

very long duration (ranging from few hours to several days). Carbonization as described earlier

to produce charcoal is an example of the slow pyrolysis. Ashes and other residue may have some

commercial value, depending on the species of the biomass used in the process.

Fast pyrolysis (shown in figure 3.9), in contrast to the slow pyrolysis process, occurs at

higher temperatures of 577-977 °C and takes <2 seconds of vapor residence time. This is

characterized by the use of otherwise waste lignocellulosic or hard parts of woody biomass

residue to produce environment friendly biofuels. Pyrolysis plants are best located near the

source of the feedstock to for economic reasons. The transportation costs are reduced to only the

costs of transporting produced fuels. Also, the feedstock needs to be finely grinded for fast

reactions.

Source: N.L. Panwar et al. (2012)

Figure 3.19 - Block diagram of fast pyrolysis process

Flash pyrolysis, as the name suggests, have much shorter vapor residence time than the

fast pyrolysis process. The process is used to produce bio-crude oil from organic waste material.

The conversion takes place at higher temperatures in the range of 750-1000 °C and typical vapor

residence time is 5-10 seconds. The yields have been reported to be up to 70%. Therefore, this

process is considered to be very efficient conversion process for producing high quality and

clean biofuels with very low ash content (Shurong Wang et al. 2005). However, the presence of

pyrolytic water in the bio-crude oil is a problem needed to be solved along with techno-economic

feasibility of using particular biomass feedstock for conversion using flash pyrolysis (Z.W.

Zhong et al. 2010).

Source: Shurong Wang et al. (2005)

Figure 3.20 - Schematic of fluidized bed reactor for flash pyrolysis

Plasma pyrolysis is an emerging technology and uses high energy plasma arc to break

down the biomass feedstock into gases at atomic level and solid residue. Plasma is the 4th state of

matter and the conversion takes place at very high temperature (about 12000 °C) and slightly

negative pressure to remove the produced gas. The vapor residence time is very short (5-10

seconds). High energy arc is produced by applying high voltage across a pair of electrodes inside

the reactor. The high temperatures convert biomass into Syngas as a biofuel product and solid

slag as residue. In case of using organic MSW, the cleanup of produced gas is required to

remove trace contaminating metal elements (H. Huang and L. Tang, 2007). The process has

advantages of converting of large quantities of waste material into useful fuel and solving landfill

problem. However, energy efficiency of the system along with technological challenges of

design and fabrication of appropriate reactor for high temperature and chemically corrosive

environment are the key issues of using this technique. The process has been reported to be

economically viable and energy efficient for pyrolysis of rubber waste materials and is to be

investigated for wet biological wastes (Christopher J. Lupa et al. 2012).

Characteristics of the products depend on the parameters like temperature, pressure,

residence time and rate of heating during the conversion process. The liquid fuels referred as bio-

crude or bio-oil may need further up gradation or refinement to be used as advanced biofuel. The

process of pyrolysis has significance of converting lignocellulosic biomass feedstock, obtained

from residue and waste materials, into useful fuels. This helps in reducing the use of edible

varieties of biomass feedstock for producing biofuels and thus avoiding food conflict in a large

number of communities.

(d) Hydrothermal Liquefaction

In the biomass liquefaction macro-molecule compounds in the feedstock, in the presence

of a suitable catalyst, are decomposed into fragments of light molecules. These fragments, which

are unstable and reactive, at the same time, re-polymerize into oily compounds of proper

molecular weights. In this process, the resulting product is liquid oil or bio-crude oil which can

be stored, transferred through pumping systems and used either in direct combustion furnaces or

as feedstock for treatment leading to specific biofuels (Saqib Sohail Toor et al. 2011).

Source: Kruse A et al. (2007)

Figure 3.21 - Hydrothermal biomass degradation

Hydrothermal liquefaction is used to produce bio-crude oil with higher heating value than

the one produced through pyrolysis. The bio-crude oil can be refined, by the process of

hydrogenation, to get fuels comparable to the ones obtained from fossil crude oil. In this process

wet untreated biomass feedstock is injected into a preheated reactor at ~400 °C under high

pressure (<150 bar). The reaction takes about 15 minutes and then it is cooled down very quickly

to ~70°C. The process is highly efficient (~85-90%) and environment friendly as it does not

require drying of feedstock or any contaminating solvents and heat is recycled between heating

and cooling phases of the process. The product bio-crude oil can directly be used in heavy oil

engines. It can further be refined to get diesel, gasoline and jet fuels for transportation systems. It

can be stored, pumped or transported to remote locations of use.

Some drawbacks of the process are high cost of high pressure reactors and feeding

mechanism of slurry and pre-treatment processes of the feedstock. Therefore, the process needs

to be made economically attractive through research and development work as an appropriate

alternative to polluting and uneconomical petroleum and other fossil fuels.

(e) Hydrothermal Gasification

Hydrothermal gasification of biomass is efficient and environment friendly process to

convert wet biomass such as sewage sludge, manure and other high-moisture containing biomass

and wet organic residues into useful gaseous fuel. The biomass feedstock is fed into a gasifier at

supercritical conditions of higher temperature and pressure (>374 0C and >221 bar). The water in

the reactor at supercritical state acts as non-polar organic solvent and converts organic

compounds of feedstock including lignin parts into gases. The reaction can be described as in the

equation 3.3, shown below.

2C6H12O6 + 7H2O → CO + 15H2 + 2CH4 + 9CO2 (3.3) Biomass Water Synthetic natural gas

Overall efficiency of 70% for synthetic natural gas (SNG) has been reported. SNG or

Bio-methane (mixture of H2, CO2 and CH4) is tar free gas. Short residence time of 30 seconds to

2 minutes with complete transformation of the biomass organic material are benefits over

conventional thermochemical gasification of the biomass as described earlier in section (b) of

this chapter. Produced CO2 is dissolved in water and can be concentrated to be used for carbon

sequestration through agricultural and industrial processes.

Source: H. Schmieder et al. (2000)

Figure 3.22 – Schematic of a Hydrothermal Gasification plant

(g) Thermal Catalytic Cracking

Cracking is a thermal process that breaks the heavier and higher boiling-point

hydrocarbon fractions into lighter liquid hydrocarbons. The process provides a range of valuable

products such as petroleum gasoline, gas oils and fuel oil, which can be used in different

convenient ways. Two basic types of cracking processes, thermal cracking and catalytic

cracking, are in practice. Thermal cracking is a technique that uses high temperatures and

pressure to break down the hydrocarbons. Catalytic cracking use catalysts to crack the heavy

hydrocarbons into light molecules. The catalytic cracking of hydrocarbons is not restricted to a

single reaction. In fact, several reactions take place and result in to a variety of products with

different compositions.

(h) Torrefaction

Woody biomass feedstock suffers from some serious inherent drawbacks high levels of

moisture content and lower calorific value (low energy density) in comparison to the herbaceous

biomass. They are hygroscopic in nature and absorb moisture from atmosphere during storage.

They are difficult to pulverize and have serious problem of slagging due to presence of elements

like Si, Ca and K in them, resulting fouling of the furnaces, Also, they have high mass with wide

variations in important parameters related to combustion properties. These characteristics make

woody biomass economically less attractive due to high costs involved in their handling and

transportation. They are difficult to pulverize. All these problems are solved upto a large extent

by the process of torrefaction of woody biomass.

Torrefaction is a low temperature (250-300 °C) thermo-chemical process similar to the

roasting of biomass feedstock in absence of air or oxygen. The process occurs at atmospheric

pressure and the duration varies from 30-120 minutes. Predominant moisture in feedstock, during

the process, evaporates and generates steam. The hemicellulose and some other volatile organic

compounds in the biomass are decomposed and produce H2O, CO and CO2 gases after reaction

with this steam.

Source: M.J.C. van der Stelt et al. (2011)

Figure 3.23 - Schematic of wood torrefaction

Torrefaction is basically a form of mild pyrolysis (Maillard reactions) and a precursor to

many other biomass to fuel conversion processes. Torrefaction alone in general decreases energy

density but allows mechanical densification easier and thus results in increased volumetric

energy density. After torrefaction, due to proportional increase in the binding lignin content,

pelletizing of biomass becomes easier. It reduces transportation costs 40-50% in comparison to

raw biomass. Bio-coal is produced through densification (pelletisation and briquetting) of

torrefied biomass. Yields of 66-75% are reported and depend on the variety of the biomass

feedstock.

The resulting fuel is hydrophobic in nature and can be stored in open air without danger

of wetting by rain and rotting by biological decomposition. Another significant advantage is that

a variety of lignocellulosic biomass feedstock (mostly inedible woody biomass consisting of

cellulose, hemicellulose and lignin polymeric structures) can be converted into a homogenous

fuel compositions. Thus, the process enables the conversion of otherwise useless sources of

biomass like wood chips and a wide variety of forest residues into useful biofuels.

Lignocellulosic biomass conversion also reduces competition with edible or food crops.

Torrefied biomass can be utilized as feedstock to produce transportation liquid fuels by using

Fischer–Tropsch process. Finely ground torrefied wood powder can be compressed and used like

liquefied petroleum gas (LPG). Torrefied biomass fuels can easily be ground and mixed with

fossil fuels for co-firing in power plants. The co-firing of fuels results in the reduced levels of

emissions and helps in prevention of atmospheric pollution.

(i) Transesterification

Vegetable oil as fuel for using in internal combustion engines was demonstrated by Rudolf

Diesel (1858-1913) in 1900 in Paris. His idea was to substitute fossil fuels by biomass fuels and

so he invented ‘Diesel Engine’. He wanted the formers to use the fuel produced by them at

farms.

Source: en.wikipedia.org/wiki/Rudolf

Figure 3.24 – Prototype of Diesel Engine invented by Rudolf Diesel (inset) for using biomass

Rudolf_Diesel

of Diesel Engine invented by Rudolf Diesel (inset) for using biomass

based liquid fuels

of Diesel Engine invented by Rudolf Diesel (inset) for using biomass

Edible and non-edible vegetable oils including used/waste oils from food processing

industry are a very good source of valuable biomass based fuel. A variety of non-edible oil

species can be cultivated on marginal land to get large quantities of liquid fuel. Vegetable oils

are obtained by pressing and extraction technologies. They can be used as fuel in compression

ignition (CI) type internal combustion engines (ICE) to produce mechanical power for industrial

and transportation applications and electricity in distributed generation (DG) plants. Petroleum

diesel fuel is traditionally used in these types of engines and so they are termed as diesel engines.

However, due to their inherent natural chemical composition, vegetable oils can be used directly

in the conventional diesel engines only with certain modifications in the engine mechanism.

These modifications limit use of the vegetable oils in the conventional diesel engines as they

require money and engines can run on only one type of fuel at a time. This problem is solved by

converting vegetable oils into a fuel, having petroleum diesel like characteristics, the biodiesel.

Biodiesel, alone or blends with petroleum diesel can be used directly in diesel engines without

any modifications.

Transesterification is the chemical process used for converting vegetable oils into

biodiesel fuel. Vegetable oils are esters of glycol with three fatty acid chains (triglycerides) and

thus have higher viscosity than petroleum diesel fuel. Therefore, vegetable oil, after filtration, is

converted into an ester with single fatty acid chain by reacting with ethyl or methyl alcohols in

presence of catalyst. The monoesters obtained after conversion are termed as biodiesel having

the characteristics very similar to the characteristics of petroleum diesel fuel.

Vegetable oil → Biodiesel + Glycerin (3.4)

The added advantage is that use of biodiesel significantly lowers SO2 emissions in the

atmosphere and lowers air pollution levels. However, transesterification requires handling of

toxic and dangerous chemicals during the conversion process. Therefore, it is done only at large

scale production facilities and not locally at small scale level.

3.5.3 - Biological Conversion Techniques:

(1) Enzymatic Breakdown

The hydrolysis of lignocelluloses to glucose is an important step in the production

processes of cellulosic biofuel like ethanol from abundant cost effective biomass feedstock. In

nature, some types of microorganisms, especially fungi, have ability to decompose the plant cell

wall using some synergistically active enzymes. Work has been done to genetically improve the

microorganisms which can be utilized to degrade the biomass, efficiently by decomposition of

different cell wall constituents like polysaccharides and cellulose, hemicelluloses, pectin and

lignin (James G Elkins et al. 2010). For making the conversion process to be economically

viable, microorganism should possess suitable characteristics. Genetically engineered microbial

systems like bacteria and fungi can be used for direct and efficient conversion of lignocellulosic

biomass feedstock into valuable biofuels. Pre-treatment of biomass feedstock is done before

enzymatic hydrolysis for efficient and economical conversion of biomass into fuel (Seung-Hwan

Lee et al. 2010).

(2) Fermentation

The process of fermentation has been in practice for long time to produce wine, beer,

yogurt and several other products. However, the process can also be used to convert organic

waste materials into useful liquid and gas biofuels. The fuels, produced by using the process

helps in saving environment by reducing atmospheric GHG concentration and solving problem

of landfills for MSW disposal. Fermentation is the metabolic process in which a microorganism

(typically bacteria) converts a carbohydrate, such as starch or a sugar, into an alcohol. The

specific products resulting from the fermentation are decided by the type of microorganism

involved in the process. As an example, yeast (a fungus) performs fermentation to obtain energy

by converting sugar into alcohol. The chemical formula of the process is shown in equation 3.5.

C6H12O6 → 2 C2H5OH + 2 CO2 (3.5) Glucose Ethanol

Fermentation process is of two basic types as described below depending on the process

being performed in presence or absence of air or oxygen.

(i) Aerobic Fermentation

In the aerobic fermentation, glucose, in presence of oxygen, is converted into ethanol

(Merico et al. 2007). During the process, a microorganism in presence of controlled air or

oxygen converts lignocellulosic biomass feedstock by biological degradation into the important

biofuels. During its own growth the microorganisms utilize, part of the produced ethanol as

carbon and energy source, as part of their metabolism (Zhenguo Lin and Wen-Hsiung Li, 2010).

This process is also a precursor to the anaerobic fermentation for the larger scale fuel production

process. Aerobic fermentation or ‘primary fermentation’ is about 70% of the total fermentation

process, before anaerobic or ‘secondary fermentation’ as described in next section.

(ii) Anaerobic Fermentation

Anaerobic fermentation or digestion is biodegradation of biomass feedstock in near

absence of the air or oxygen by microorganisms. Anaerobic fermentation is used to produce

bioethanol as a substitute or for blending of gasoline fuel. Methane, a useful fuel for generating

heat and electricity, is produced by anaerobic fermentation of organic domestic and municipal

waste materials.

Typical chemical reaction, as shown in equation 3.6, in the process produces biogas

(carbon dioxide + methane) by the digestion of carbohydrates.

C6H10O5 + H2O → 3 (CO2 + CH4) (3.6) Carbohydrates Moisture Biogas

In anaerobic fermentation reaction, typically 30-60% of the solid organic feedstock is

converted into biogas. It is an environment friendly process using a wide range of organic

feedstock, both in solid and liquid forms including MSW and agricultural wastes, converted into

useful fuels by methanogenic bacteria. Anaerobic digestion provides means for potential energy

savings. This is also very stable process for converting medium to high strength organic

effluents into useful fuels.

Source: http://en.wikipedia.org/wiki/Biogas

Figure 3.25 - Block diagram of a typical household biogas plant

Schematic of a typical household biogas generation plant, as shown in figure x.x, is used

for converting waste material into methane rich biogas. Waste-to-Energy (WTE) power plants

are highly efficient in harnessing the untapped renewable energy potential of organic wastes.

They are based on anaerobic digestion of biomass and convert the biodegradable fraction of the

waste into high calorific gaseous fuels, which in turn are used to generate heat and electricity. In

addition to the wastewater treatment, the fuel produced by these biogas plants is used for

domestic and industrial heating and electricity generation.

A combination of one or more conversion techniques can be used for producing different

biofuels. This increases the overall efficiency. This is very advantageous from environmental and

economical and point of view also. Because reduced amounts of residue are generated. For

example, acetic acid is formed by the biological conversion of sugars, the non-lignin cellulosic

part of the biomass feedstock. Lignin, as shown in equation 3.7, is converted into hydrogen by

gasification, which can be used to convert acetic acid into ethanol by hydrogenation.

C6H12O6 + 6H2 → 3CH3CH2OH + 3H2O (3.7)

From from ethanol Cellulose lignin

Challenge are to develop the economically viable and environment friendly conversion

technologies, for producing biogasoline and green diesel from the lignocellulosic, algal biomass

feedstock including organic material from waste-water. The development of these fuels can help

in reducing the consumption of fossil fuels in transportation and electricity generation sectors.

Documents

OVERVIEW OF BIOMASS BASESD FUELS AND THEIR …shodhganga.inflibnet.ac.in/bitstream/10603/46119/11/11_chapter 3.pdf · B- Cowdung heap, C Straw 3800 Stalk 4650 B C ... Fischer-Tropsch