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Combustion characteristics of different biomass fuels

Ayhan Demirbas*

Department of Chemical Engineering, Selcuk University, Konya, Turkey

Received 15 March 2003; accepted 31 October 2003

Abstract

Biomass energy is one of humanity’s earliest sources of energy particularly in rural areas where it is often the only accessible

and affordable source of energy. Worldwide biomass ranks fourth as an energy resource, providing approximately 14% of the

world’s energy needs all human and industrial processes produce wastes, that is, normally unused and undesirable products of a

specific process. Generation and recovery of solid wastes varies dramatically from country to country and deserves special

mention. The burning velocity of pulverized biomass fuels is considerably higher than that of coals. The use of biomass fuels

provides substantial benefits as far as the environment is concerned. Biomass absorbs carbon dioxide during growth, and emits

it during combustion. Utilization of biomass as fuel for power production offers the advantage of a renewable and CO 2-neutral

fuel. Although the structural, proximate and ultimate analyses results of biomass and wastes differ considerably, some

properties of the biomass samples such as the hydrogen content, the sulfur content and the ignition temperatures changed in anarrow interval.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Biomass; Combustion; Fuel properties; Cofiring

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

1.1. Biomass sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

1.2. Current biomass conversion technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

2. Solid wastes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

3. Gasification of biomass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

3.1. Steam reforming of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

3.2. Gasification systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

4. Pyrolysis of biomass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2225. Fuel properties of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

6. Combustion properties of biomass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

6.1. The chemistry of biomass combustion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

6.2. Some combustion properties of selected biomass samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

6.3. Calculation of higher heating value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

7. Cofiring of biomass and coal blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

7.1. Co-pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

7.2. Mechanism of biomass cofiring with coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

8. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

0360-1285/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pecs.2003.10.004

Progress in Energy and Combustion Science 30 (2004) 219–230

www.elsevier.com/locate/pecs

* Corresponding author. Tel.: þ90-462-230-7831; fax: þ90-462-

248-8508.

E-mail address: [email protected] (A. Demirbas).

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1. Introduction

Biomass can be converted into liquid, solid and gaseous

fuels with the help of some physical, chemical and

biological conversion processes [1,2]. The conversion of

biomass materials has a precise objective to transform a

carbonaceous solid material which is originally difficult to

handle, bulky and of low energy concentration, into the fuels

having physico– chemical characteristics which permit

economic storage and transferability through pumping

systems.

Worldwide biomass ranks fourth as an energy resource,

providing approximately 14% of the world’s energy needs;biomass is the most important source of energy in

developing nations, providing; 235% of their energy [3,4].

The use of biomass fuels provides substantial benefits as

far as the environment is concerned. Biomass absorbs

carbon dioxide during growth, and emits it during combus-

tion. Therefore, biomass helps the atmospheric carbon

dioxide recycling and does not contribute to the greenhouse

effect. Biomass consumes the same amount of CO2 from the

atmosphere during growth as is released during combustion.

1.1. Biomass sources

Biomass fuels potential includes wood, short-rotation

woody crops, agricultural wastes, short-rotation herbaceousspecies, wood wastes, bagasse, industrial residues, waste

paper, municipal solid waste, sawdust, bio-solids, grass,

waste from food processing, aquatic plants and algae animal

wastes, and a host of other materials. Animal wastes are

another significant potential biomass resource for electricity

generation, and like crop residues, have many applications,

especially in developing countries. Biomass is only organic

petroleum substitute which is renewable.

Biomass offers important advantages as a combustion

feedstock due to the high volatility of the fuel and the high

reactivity of both the fuel and the resulting char. However, it

should be noticed that in comparison with solid fossil fuels,

biomass contains much less carbon and more oxygen and

has a low heating value.The burning velocity of pulverized biomass fuels is

considerably higher than that of coals. The pulverized

biomass fuels can be burned in a flame in the same way as

oil or gas fuels and at the same high power output [5].

1.2. Current biomass conversion technologies

Direct combustion is the old way of using biomass.

Biomass thermo-chemical conversion technologies such as

pyrolysis and gasification are certainly not the most

important options at present; combustion is responsible for

over 97% of the world’s bio-energy production.

Some processes such as pyrolysis, gasification,

anaerobic digestion and alcohol production have widelybeen applied to biomass in order to obtain its energy

content. Biomass can be directly fired in dedicated

boilers. However, cofiring biomass and coal has techni-

cal, economical, and environmental advantages over the

other options. Cofiring biomass with coal, in comparison

with single coal firing, helps reduce the total emissions

per unit energy produced. The oldest of all fuels, wood

(or biomass), and the old original fuel of the industrial

revolution, coal, are key to this move to a new mission.

Technical issues that can lead to doubt about of biomass

cofiring with coal are being resolved through testing and

experience [6].

Main current biomass technologies are [7]:

1. Destructive carbonization of woody biomass to

charcoal

2. Gasification of biomass to gaseous products

3. Pyrolysis of biomass and solid wastes to liquid, solid

and gaseous products

4. Supercritical fluid extractions of biomass to liquid

products

5. Liquefaction of biomass to liquid products

6. Hydrolysis of biomass to sugars and ethanol

7. Aneorobic digestion of biomass to gaseous products

8. Biomass power for generating electricity by direct

combustion or gasification and

9. pyrolysis

10. Cofiring of biomass with coal11. Biological conversion of biomass and waste (biogas

production, wastewater treatment)

12. Biomass densification (briquetting, pelleting)

13. Domestic cookstoves and heating appliances of

fuelwood

14. Biomass energy conservation in households and

industry

15. Solar photovoltaic and biomass based rural

electrification

16. Conversion of biomass to a pyrolytic oil (biofuel) for

vehicle fuel

17. Conversion of biomass to methanol and ethanol for

internal combustion engines

In earlier work [8], ground biomass samples have been

converted completely into water insoluble and soluble

chemicals in the presence of anhydrous glycerin with alkali

such as Na2CO3 or KOH. The acetone solubles from

acidification of the liquefaction products was called biofuel

in this study. The solubility of the biofuel in gasoline was

tested as 1.96% by weight. In another work [9], the acetone

solubles of the pyrolysis products from the biomass samples

were added to gasoline.

2. Solid wastes

The waste products of a home include paper, containers,tincans, aluminium cans, and food scraps, as well as sewage.

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The waste products of industry and commerce include

paper, wood, and metal scraps, as well as agricultural waste

products [10,11]. Biodegradable wastes, such as paper fines

and industrial biosludge, into mixed alcohol fuels (e.g.

isopropanol, isobutanol, isopentanol). The wastes are first

treated with lime to enhance reactivity; then they are

converted to volatile fatty acids (VFAs) such as acetic acid,

propionic acid, and butyric acid—using a mixed culture of

microorganisms derived from cattle rumen or anaerobic

waste treatment facilities. Pulp and paper wastes may also

be treated to produce methane. The contents of domestic

solid waste are given in Table 1.There are four major methods for conversion of organic

wastes to synthetic fuels: (1) hydrogenation; (2) pyrolysis;

(3) gasification: and (4) bioconversion [12]. The first three

have been advanced to the pilot-plant stage, while the fourth

has been the subject of only minor research effort, but is a

long term possibility. Typical solid wastes include wood

material, pulp and paper industry residues, agricultural

residues, organic municipal material, sewage, manure, and

food processing by-products. Biomass is considered one of

the main renewable energy resources of the future due to its

large potential, economic viability and various social and

environmental benefits. It was estimated that by 2050

biomass could provide nearly 38% of the world’s direct fuel

use and 17% of the world’s electricity [13]. If biomass isproduced more efficiently and used with modern conversion

technologies, it can supply a considerable range and

diversity of fuels at small and large scales. Municipal

solid waste (MSW) is defined as waste durable goods,

nondurable goods, containers and packaging, food scraps,

yard trimmings, and miscellaneous inorganic wastes from

residential, commercial, and industrial sources.

Generation refers to the amount of material that enters

the waste stream before recovery, composting, or combus-

tion. Recovery refers to materials removed from the waste

stream for the purpose of recycling and/or composting.

Table 2 shows the generation and recovery of MSW in the

US in 1993. The energy content of MSW in the US is

typically from 10.5 to 11.5 MJ/kg. The generation andrecovery of MSW varies dramatically from country to

country and deserves special mention. For example, recent

estimates indicate MSW generation in the UK of about 30

million tons of which 90% is landfilled. In comparison,

Sweden landfilled only 34% of their MSW generation [14].

3. Gasification of biomass

Gasification is a form of pyrolysis, carried out at high

temperatures in order to optimize the gas production. The

resulting gas, known as producer gas, is a mixture of carbon

monoxide, hydrogen and methane, together with carbon

dioxide and nitrogen.

Biomass gasification technologies have historically been

based upon partial oxidation or partial combustion prin-

ciples, resulting in the production of a hot, dirty, low

calorific value gas that must be directly ducted into boilers

or dryers. In addition to limiting applications and often

compounding environmental problems, these technologies

are an inefficient source of usable energy.

Biomass gasification is the latest generation of biomassenergy conversion processes, and is being used to improve

the efficiency, and to reduce the investment costs of biomass

electricity generation through the use gas turbine technol-

ogy. High efficiencies (up to about 50%) are achievable

using combined-cycle gas turbine systems, where waste

gases from the gas turbine are recovered to produce steam

for use in a steam turbine. Economic studies show that

biomass suffocation plants can be as economical as

conventional coal-fired plants.

3.1. Steam reforming of biomass

Most biomass gasification systems utilize air or oxygen

in partial oxidation or combustion processes These pro-cesses suffer from low thermal efficiencies and low calorific

Table 1

Contents of domestic solid waste (Percentage of total)

Component Lower limit Upper limit

Paper waste 33.2 50.7

Food waste 18.3 21.2

Plastic matter 7.8 11.2

Metal 7.3 10.5

Glass 8.6 10.2

Textile 2.0 2.8

Wood 1.8 2.9

Leather and rubber 0.6 1.0

Miscellaneous 1.2 1.8

Source: Ref. [48].

Table 2

Generation and recovery of MSW in the US, 1993 (million tons)

Material Generation Recovered Discarded Projected

generated

2000

Paper 70.5 24.0 46.6 81.0

Glass 12.4 2.7 9.7 12.7

Metals 15.5 4.7 10.8 17.2

Plastics 17.5 0.6 16.9 20.4

Rubber/leather 5.6 0.4 5.4 6.9

Textiles 5.5 0.6 4.9 5.6

Wood 12.4 1.2 11.2 14.5Food 12.5 0 12.5 12.7

Yard trimmings 29.8 5.9 23.9 20.1

Misc. organic 2.8 0 2.8 3.0

Others 3.0 0.6 2.4 3.2

Total 187.5 40.7 147.1 197.3

Source: Ref. [49].

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gas because of the energy required to evaporate the moisture

typically inherent in the biomass and the oxidation of a

portion of the feedstock to produce this energy.

The prosess of synfuels from biomass will lower the

energy cost, improve the waste management and reduce

harmfull emissions. This triple assault on plant operating

challenges is a proprietary technology that gasifies biomass

by reacting it with steam at high temperatures to form a

clean burning synthesis gas (called as the syngas, hydrogen

and carbon monoxide in a 2 to 1 ratio). The molecules in the

biomass (primarily carbon, hydrogen and oxygen) and the

molecules in the steam (hydrogen and oxygen) reorganize to

form this syngas [7].

3.2. Gasification systems

Gasification for power production involves the

devolatilization and conversion of biomass in an

atmosphere of steam and/or air to produce a medium

or low calorific value gas. If air is present, the ratio of

oxygen to biomass is typically around 0.3. A large

number of variables affect gasification based process

design. Gasification medium is an important variable. Air

blown or directly heated gasifiers, use the exothermic

reaction between oxygen and organics to provide the heat

necessary to devolatilize biomass and to convert residual

carbon-rich chars.

Commercial gasifiers are available in a range of size and

types, and run on a variety of fuels, including wood,

charcoal, coconut shells and rice husks. Power output is

determined by the economic supply of biomass, which is

limited to 80 MW in most regions.

The biomass gasification process is similar to processes

used for many years by chemical and petrochemical

manufacturers, including methanol, ammonia and ethylene

producers. In these chemical processes, natural gas or

another hydrocarbon is ‘reformed’ into a more desirable

gaseous chemical feedstock by reacting it with steam at

elevated temperatures. The hydrogen and oxygen molecules

in the steam are liberated and a series of reactions result in areorganization of the compounds to form synthesis gas

(primarily H2, CO and CO2). This synthesis gas is then

catalytically converted into methanol, ammonia or another

product.

4. Pyrolysis of biomass

Pyrolysis is defined as the thermal destruction of organic

materials in the absence of oxygen. Pyrolysis is the basic

thermochemical process for converting biomass to a more

useful fuel [15]. Biomass is heated in the absence of oxygen,

or partially combusted in a limited oxygen supply, to

produce a hydrocarbon rich gas mixture, an oil-like liquidand a carbon rich solid residue.

In pyrolysis process, biomass converts into liquid (bio-

oil or bio-crude), charcoal and non-condensable gasses,

acetic acid, acetone, and methanol by heating the biomass to

about 750 K in the absence of air. The process can be

adjusted to favor charcoal, pyrolytic oil, gas, or methanol

production with a 95.5% fuel-to-feed efficiency. Pyrolysis

can be used for the production of bio-oil if flash pyrolysis

processes are used and are currently at pilot stage [16].

Some problems in the conversion process and use of the oil

need to be overcome; these include poor thermal stability

and corrosivity of the oil. Upgrading by lowering the oxygen

content and removing alkalis by means of hydrogenation

and catalytic cracking of the oil may be required for certainapplications [12].

Pyrolysis of wood has been studied as a zonal process

[17]. Thermal degradation properties of hemicelluloses,

celluloses and lignin can be summarized as follows [18]:

Thermal degradation of hemicelluloses . of cellulose .

of lignin

Pyrolysis of biomass is thermal decomposition of the

fuel. As with coal, pyrolysis is a relatively slow chemical

reaction occurring at low temperatures. The reaction

mechanisms of biomass pyrolysis are complex but can be

defined in five stages for wood [19]:

1. Moisture and some volatile loss.2. Breakdown of hemicellulose; emission of CO and CO2.

3. Exothermic reaction causing the woody biomass

temperature to rise from 525 to 625 K; emission of

methane, hydrogen and ethane.

4. External energy is now required to continue the

process.

5. Complete decomposition occurs.

5. Fuel properties of biomass

The limitations were primarily due to relying on biomass

as the sole source of fuel, despite the highly variable

properties of biomass. The high moisture and ash contents inbiomass fuels can cause ignition and combustion problems.

The melting point of the dissolved ash can also be low which

causes fouling and slagging problems. Because of the lower

heating values of biomass accompanied by flame stability

problems. It is anticipated that blending biomass with higher

quality coal will reduce flame stability problems, as well as

minimize corrosion effects.

The methods of biomass fuel analyses are given in

Table 3. Biomass offers important advantages as a

combustion feedstock due to the high volatility of the fuel

and the high reactivity of both the fuel and the resulting

char. However, it should be noticed that in comparison with

solid fossil fuels, biomass contains much less carbon and

more oxygen and has a low heating value. Also, chlorinecontents of certain biofuels, like straw can exceed the level


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of coal. In the combustion applications, biomass has been

fired directly either alone or along with a primary fuel.

Chlorine, which is found in certain biomass types, such as

straw, may affect operation by corrosion. The high chlorine

and alkali content of some biomass fuels raise concernsregarding corrosion. The greatest concern focuses on high-

temperature corrosion of super-heater tubes induced by

chlorine on the surface.

Biomass differs from coal in many important ways,

including the organic, inorganic, energy content, and

physical properties. Relative to coal, biomass generally

has less carbon, more oxygen, more silica and potassium,

less aluminum and iron, lower heating value, higher

moisture content, and lower density and friability (Table 4).

The point on the burning profile at which the rate of

weight loss due to combustion is a maximum called as ‘peak

temperature’. The burning profile peak temperature is

usually taken as a measure of the reactivity of the sample.

The peak temperatures for biomass samples generally vary

from 560 to 575 K.

The structural analyses of selected biomass samples are

given in Table 5. The ultimate analyses of typical fuel

samples given in the literature and determined are shown in

Tables 6 and 7, respectively. It is difficult to establish a

representative biomass due to large property variations, buttwo examples are included here for comparison. The

composition variations among biomass fuels are larger

than among coals, but as a class biomass has substantially

more oxygen and less carbon than coal. Less obviously,

nitrogen, chlorine, and ash vary significantly among

biomass fuels. These components are directly related to

NOx emissions, corrosion, and ash deposition. Biomass

generally has relatively low sulfur compared to coal.

The proximate analyses of selected biomass samples

given in the literature and determined as defined by ASTM

are shown in Tables 8 and 9, respectively. The inorganic

properties of selected fuel samples are given in Table 10.

Inorganic components in coal vary by rank and geographic

Table 3

Methods of biomass fuel analyses

Property Analytical method

Heating value ASTM D 2015, E 711

Particle size distribution ASTM E828

Proximate composition

Moisture ASTM E871

Ash ASTM D1102 (873 K),

ASTM E830 (848 K)

Volatile matter ASTM E 872, ASTM E 897

Fixed carbon by difference

Ultimate elemental

Carbon, hydrogen ASTM E 777

Nitrogen ASTM E 778

Sulfur ASTM E 775

Chlorine ASTM E776

Oxygen By difference

Ash elemental ASTM D3682, ASTM D2795,

ASTM D4278, AOAC 14.7

Table 4

Physical, chemical and fuel properties of biomass and coal fuels

Property Biomass Coal

Fuel density (kg/m3) ,500 ,1300

Particle size ,3 mm ,100 mm

C content (wt% of dry fuel) 42 –54 65 –85

O content (wt% of dry fuel) 35 –45 2 –15

S content (wt% of dry fuel) Max 0.5 0.5 – 7.5

SiO2 content (wt% of dry ash) 23 – 49 40 –60

K 2O content (wt% of dry ash) 4 –48 2 –6

Al2O3 content (wt% of dry ash) 2.4 – 9.5 15 – 25

Fe2O3 content (wt% of dry ash) 1.5 – 8.5 8 – 18

Ignation temperature (K) 418 –426 490 –595

Peak temperature (K) 560–575 –

Friability Low HighDry heating value (MJ/kg) 14 –21 23 –28

Table 5

Structural analyses of selected biomass samples (wt. % daf)

Fuel sample Hemicelluloses Cellulose Lignin Extractive

matter a

Hazelnut shell 30.4 26.8 42.9 3.3

Wheat straw 39.4 28.8 18.6 –

Olive husk 23.6 24.0 48.4 9.4

Beech wood 31.2 45.3 21.9 1.6

Spruce wood 20.7 49.8 27.0 2.5

Corncob 31.0 50.5 15.0 3.5

Tea waste 19.9 30.2 40.0 9.9

Walnut shell 22.7 25.6 52.3 2.8

Almond shell 28.9 50.7 20.4 2.5

Sunflower shell 34.6 48.4 17.0 2.7

Source: Ref. [32].a Alcohol/benzene (1/1, v/v) extractives.

Table 6

Ultimate analyses of typical fuel samples given in the literature (wt

% of dry fuel with ash)

Fuel sample C H N S O (diff.) Reference

Hazelnut shell 52.8 5.6 1.4 0.04 42.6 [32,57]

Sawdust 46.9 5.2 0.1 0.04 37.8 [50]

Corn stover 42.5 5 .0 0 .8 0 .2 42.6 [51]

Poplar 48.4 5.9 0.4 0.01 39.6 [52]

Rice husk 47.8 5.1 0.1 – 38.9 [53]

Cotton gin 42.8 5.4 1.4 0.5 35.0 [54]

Sugarcane bagasse 44.8 5.4 0.4 0.01 39.6 [52]

Peach pit 53.0 5.9 0.3 0.05 39.1 [52]

Alfafa stalk 45.4 5.8 2.1 0.09 36.5 [55]Switchgrass 46.7 5 .9 0 .8 0 .19 3 7.4 [55]


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region. As a class, coal has more aluminum, iron, and

titanium than biomass. Biomass has more silica, potassium,

and some times calcium than coal. The wood and woody

materials tend to be low in nitrogen and ash content while

the agricultural materials can have high nitrogen (Tables 6

and 7) and ash contents (Tables 8 and 9).

Straw may have a high content of chlorine and

potassium, elements which are very undesirable in

power plant fuels. Levels of K 2O and Cl were found as

20.0 and 3.6% in ash, respectively, in wheat straws(Table 10). A pretreatment process to remove potassium

from straw fuel may be based on pyrolysis followed by

char wash. The straw is pyrolyzed at moderate tempera-

tures at which the potassium is retained in the char.

Potassium and residual chlorine are extracted from the

residual char by water [20]. Char and pyrolysis gases may

then be used in a conventional boiler without problems

due to the high straw potassium content. To evaluate this

pretreatment process knowledge about the char wash

process is needed. Alkalis, when reacted with sulfates and

chlorine, may harm thermochemical conversion systems,

fouling heat exchange surfaces, gas-turbine blades, and

other power system components [21].

6. Combustion properties of biomass

In general combustion models of biomass can be

classified as macroscopic or microscopic. The macro-

scopic properties of biomass are given with for

macroscopic analysis, such as ultimate analysis, heatingvalue, moisture content, particle size, bulk density, and

ash fusion temperature. Properties for microscopic

analysis include thermal, chemical kinetic, and mineral

data [22]. Fuel characteristics such as ultimate analysis,

heating value, moisture content, particle size, bulk

density, and ash fusion temperature of biomass have

been reviewed [23]. Fuel properties for the combustion

analysis of biomass can be conveniently grouped into

physical, chemical, thermal, and mineral properties.

Physical property values vary greatly and properties

such as density, porosity, and internal surface area are

related to biomass species whereas bulk density, particle

size, and shape distribution are related to fuel preparation

methods.Important chemical properties for combustion are the

ultimate analysis, proximate analysis, analysis of pyrolysis

products, higher heating value, heat of pyrolysis, heating

value of the volatiles, and heating value of the char.

Thermal property values such as specific heat, thermal

conductivity, and emissivity vary with moisture content,

temperature, and degree of thermal degradation by one order

of magnitude. Thermal degradation products of biomass

consist of moisture, volatiles, char and ash. Volatiles are

further subdivided into gases such as light hydrocarbons,

carbon monoxide, carbon dioxide, hydrogen and moisture,

and tars. The yields depend on the temperature and heating

rate of pyrolysis. Some properties vary with species,

location within the biomass, and growth conditions. Otherproperties depend on the combustion environment. Where

Table 7

Ultimateanalyses of typical fuel samples (wt % of dry fuel with ash)

Ful sample C H N S Cl O (diff.)

Coal type 1 81.5 4.0 1.2 3.0 – 3.3

Red oak wood 50.0 6.0 0.3 – – 42.4

Wheat straw 41.8 5.5 0.7 – 1.5 35.5

Olive husk 49.9 6.2 1.6 0.05 0.2 42.0

Beech wood 49.5 6.2 0.4 – – 41.2

Spruce wood 51.9 6.1 0.3 – – 40.9

Corncob 49.0 5.4 0.5 0.2 – 44.5

Tea waste 48.0 5.5 0.5 0.06 0.1 44.0

Walnut shell 53.5 6.6 1.5 0.1 0.1 45.4Almond shell 47.8 6.0 1.1 0.06 0.1 41.5

Sunflower shell 47.4 5.8 1.4 0.05 0.1 41.3

Source: Refs. [32,34].

Table 8

Proximate analyses of selected biomass given in the literature (wt %

of dry fuel)

Fuel sample Ash Volatile

matter

Fixed carbon Reference

Hazelnut shell 1.5 76.3 21.2 [32]

Sawdust 2.8 82.2 15.0 [50]

Corn stover 5.1 84.0 10.9 [51]

Poplar 1.3 – 16.4 [52]

Sugarcane bagasse 11.3 – 15.0 [52]

Peach pit 1.0 – 19.9 [52]

Rice husk 22.6 61.0 16.7 [53]

Alfafa stalk 6.5 76.1 17.4 [55]Switchgrass 8.9 76.7 14.4 [55]

Table 9

Proximate analyses of selected biomass samples (wt % of dry fuel)

Fuel sample Ash Volatile matter Fixed carbon

Beech wood bark 5.7 65.0 29.3

Oak wood 0.5 77.6 21.9

Wheat straw 13.7 66.3 21.4

Olive husk 4.1 77.5 18.4

Beech wood 0.5 82.5 17.0

Spruce wood 1.7 80.2 18.1

Corncob 1.1 87.4 11.5

Tea waste 1.5 85.5 13.0

Walnut shell 2.8 59.3 37.9Almond shell 3.3 74.0 22.7

Sunflower shell 4.0 76.2 19.8

Colza seed 6.5 78.1 15.4

Pine one 1.0 7.3 21.7

Cotton refuse 6.6 81.0 12.4

Olive refuse 9.2 66.1 24.7

Source: Refs. [30,32,34].


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the properties are highly variable, the likely range of the

property is given [22].

6.1. The chemistry of biomass combustion

Combustion is a series of chemical reactions by which

carbon is oxidized to carbon dioxide, and hydrogen is

oxidized to water.

In order to understand wood combustion, it is important

to understand the properties of wood which determine its

behavior as a fuel for combustion. Influencing propertiesinclude anatomical structure and pathways for movement of

moisture, moisture content, specific gravity, and holocellu-

lose and lignin.

Main combustion reactions are:

Non-reacting solid!Heat; drying! PyrolysisðVolatiles;

SteamÞ! Precombustion reactions! Primary gas phase

combustion! Secondary combustion! Effluent stack gas

6.2. Some combustion properties of selected biomass

samples

Non-isothermal and isothermal thermogravimetric tech-

niques have commonly been used to investigate thereactivities of carbonaceous materials [24–28]. A plot of

the rate of weight loss against temperature while burning a

sample under oxidizing atmosphere is referred to as burning

profile [29]. The burning profiles of sunflower shell and pine

cone samples are shown in Figs. 1 and 2. The first peak,

observed on the burning profiles of the biomass samples

corresponds to their moisture release. After releasing the

moisture,somesmalllossesinthemassofthesampleoccurred

due to the desorption of the adsorbed gases. A sudden loss in

the mass of the samples started at the temperatures between

450–500 K, representing the release of some volatiles and

theirignition. Inthe rapidburning region, therateof mass loss

proceeded so rapidly that it reached to its maximum value.

Rapid loss of mass immediately slowed down at thetemperatures between 600 and 700 K. After then, burning

rateapparentlydecreasedandconsequentlysomesmalllosses

in the mass of the sample continuously went on as long as

temperature was increased up to 1273 K, indicating the slow

burning of the partly carbonized residue. At the end of hold

time at 1273 K, samples reached to the constant weight after

given periods [30].

The most important characteristic temperatures of a

burning profile are ignition temperature and peak tempera-

ture [31]. The ignition temperature corresponds to the point

at which the burning profile underwent a sudden rise. The

ignition temperatures of samples were determined from

their burning profiles. As seen Table 11, the temperatures

were determined as 475 K for sunflower shell, 463 K for

colza seed, 475 K for pine cone, 467 K for cotton refuse and

473 K for olive refuse [30]. The point on the burning profile

at which is the rate of weight loss due to combustion is

maximum known as peak temperature. The burning profile

peak temperature is usually taken as a measure of the

reactivity of the sample. These temperatures were found as

573 K for sunflower shell, as 535 K for colza seed, as 565 K

Fig. 1. Burning profile of sunflower shell (source: Ref. [30]).

Table 10

Inorganic properties of typical fuel samples (wt% of ash)

Fuel sample Si2O Al2O2 TiO2 Fe2O3 CaO MgO Na2O K 2O SO3 P2O5 Cl

Coal type 1 42.0 20.0 1.2 17.0 5.5 2.1 1.4 5.8 5.0 – –

Coal type 2 59.7 19.8 2.1 8.3 2.1 1.8 0.8 2.1 2.0 0.2 –

Coal type 3 51.5 22.6 2.0 14.9 3.3 0.9 1.0 2.0 3.5 0.2 –

Red oak wood 49.0 9.5 – 8.5 17.5 1.1 0.5 9.5 2.6 1.8 0.8

Wheat straw 48.0 3.5 – 0.5 3.7 1.8 14.5 20.0 1.9 3.5 3.6

Walnut shell 23.1 2.4 0.1 1.5 16.6 13.4 1.0 32.8 2.2 6.2 0.1

Almond shell 23.5 2.7 0.1 2.8 10.5 5.2 1.6 48.5 0.8 4.5 0.2

Sunflower shell 29.3 2.9 0.1 2.1 15.8 6.1 1.5 35.6 1.3 4.8 0.2

Olive husk 32.7 8.4 0.3 6.3 14.5 4.2 26.2 4.3 0.6 2.5 0.2Hazelnut shell 33.7 3.1 0.1 3.8 15.4 7.9 1.3 30.4 1.1 3.2 0.1

Source: Ref. [56].


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for pine cone, as 598 K for cotton refuses and as 537 K for

olive refuse (Table 11). The rate of weight loss at the

burning profile peak temperature is called the maximum

combustion rate. The maximum combustion rates of the

sunflower shell, colza seed, pinecone, cotton refuse and

olive refuse was calculated as 5.5, 2.8, 5.2, 3.7 and 3.4 mg/

min, respectively [30].

The weight loss percentages of five different biomasssamples versus temperature are illustrated in Fig. 3. From

the Fig. 3, the weight losses of the samples increased sharply

above 500 K. The weight loss differences between olive

refuse and other samples started to increase above

620 K. Olive refuse has the lowest volatile matter content

and the highest ash content; in other words, olive refuse has

the lowest combustible part. The weight loss percentages of

the sunflower shell, colza seed, pinecone, cotton refuse and

olive refuse at 1273 K were % 95.07, 91.05, 84.80, 86.74

and 78.69, respectively [30].

6.3. Calculation of higher heating value

The higher heating values (HHVs) or gross heat of

combustion include the latent heat of the water vapor

products of combustion because the water vapor was

allowed to condense to liquid water. The HHV (in MJ/kg)

of the biomass fuels as a function of fixed carbon (FC, wt %)

was calculated from Eq. (1) [32]:

HHV ¼ 0:196ðFCÞ þ 14:119 ð1Þ

In earlier works [33,34], formulae were also devel-

oped for estimating the HHVs of fuels from different

lignocellulosic materials, vegetable oils and diesel fuels

using their chemical analysis data. For biomass

fuels such as coal, the HHV was calculated using the

modified Dulong’s formula [33,35] as a function of thecarbon, hydrogen, oxygen, and nitrogen contents from

Eq. (2):

HHV ¼{33:5½CC þ 142:3½HC2 15:4½OC

2 14:5½NC} £ 1022ð2Þ

where (CC) was carbon content (wt %), (HC), hydrogen

content (wt %), (OC) oxygen content (wt %) and (NC),

nitrogen content (wt %)

The heat content is related to the oxidation state of the

natural fuels in which carbon atoms generally dominate and

overshadow small variations of hydrogen content. On the

basis of literature values for different species of wood,

Tillman [36] also found a linear relationship between HHVand carbon content.

The HHVs of extractive-free samples reflect the HHV

of lignin relative to cellulose and hemicelluloses. It was

reported that [37], cellulose and hemicelluloses (holocel-

lulose) have a HHV of 18.60 kJ g21, whereas lignin has

a HHV of 23.26 to 26.58 kJ g21. As discussed by Baker

[38], HHVs reported for a given species reflect only the

samples tested and not the entire population of the

species. The HHV of a lignocellulosic fuel is a function

of i ts l ignin content . In general , the HHVs of

lignocellulosic fuels increase with increase of their lignin

contents and the HHV is highly correlated with lignin

content. For the model including the lignin content, the

regression equation was

HHV ¼ 0:0889ðLCÞ þ 16:8218 ð3Þ

where LC was the lignin content (wt % daf and

extractive-free basis).

Again the heat content, which is a very important

factor affecting utilization of any material as a fuel, is

affected by the proportion of combustible organic

components (called as extractives) present in it [39].

The HHVs of the extractive-free plant parts were found to

be lower than those of the unextracted parts which

indicate a likely positive contribution of extractives

towards the increase of HHV. The Differential higher

heating value (DHHV in MJ/kg) of the biomass samplesas a function of extractive content (EC, wt %) can be

Fig. 2. Burning profile of pine cone (source: Ref. [30]).

Table 11

Some combustion properties of selected biomass samples

Sample Ignition

temperature

(K)

Maximum

combustion

rate (mg/min)

Peak

temperature

(K)

Sunflower shell 417 5.50 573

Colza seed 423 2.80 535

Pine cone 463 5.20 565

Cotton refuse 423 3.70 598

Olive refuse 438 3.40 537

Source: Ref. [30].


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(i.e. temperature-time history), heating rate, atmosphere,

pressure and reactor configuration. Co-pyrolysis of car-

bonaceous material divided into a hydrogen-rich volatile

fraction, consisting of gases, vapors, and tar-components,

and a carbon-rich solid residue. The pyrolysis process

consists of a very complex set of reactions involving the

formation of radicals. The radicals are very reactive andcan undergo secondary reactions like cracking and carbon

deposition, both inside and outside the particle. Stabiliz-

ation of a radical, primarily via hydrogen addition, leads to

a volatile component. The evolution of tar is controlled by

mass transport in which the tar molecules evaporate into

the light gas species and are carried out the particle at rates

proportional to their vapor pressure and the volume of light

gas. High pressure reduces the volume of light gases and

hence reduces the yield of heavy molecules with low vapor

pressure. Polymerization and condensation reactions,

occurring via recombination of both volatile and non-

volatile radical components, result in the formation of the

solid char particle. High pyrolysis heating rates produced

chars with large macroporosites, more open pore structures,and larger macropore surface areas. Tar formation

increases with increasing heating rate. The co-pyrolysis

of the lignite sample and the biomass sample was resulted

that the addition of lignite gave a slight synergistic effect in

terms of increasing the oil yield from the hazelnut shell and

also reduced the molecular weights of the resultant oil

considerably.

7.2. Mechanism of biomass cofiring with coal

A comparison of pyrolysis, ignition and combustion of

coal and biomass particles reveals the following:

1. Pyrolysis starts earlier for biomass fuels compared tocoal fuels.

2. The volatile material (VM) content of biomass

(,40% ) is higher compared to that of coal

(, 2 80%).

3. The fractional heat contribution by VM in biomass is of

the order of, 70% compared to, 36% for coal.

4. Biomass char has more oxygen compared to coal.

5. The heating value of volatiles is lower for biomassfuels compared to those from coal fuel.

6. Pyrolysis of biomass chars mostly releases CO, CO2

and H2O.

7. Biomass fuels have ash that is more alkaline in nature,

which may aggravate the foulin problems.

8. Conclusion

Biomass has a significantly lower heating value than

most coal. This is in part due to the generally higher

moisture content and in part due to the high oxygen content.

It was observed that the investigated biomass materialsshowed different combustion characteristics. The structural,

proximate and ultimate analyses results of biomass and

wastes differ considerably.

Cofiring biomass with coal, in comparison with single

coal firing, helps reduce the total emissions per unit energy

produced. To reduce greenhouse gas emissions, the pressure

is on conventional coal-fired utilities to burn renewable

fuels such as waste product or energy crop-derived biomass

fuels as a lowest-cost option for reducing greenhouse gas

emissions. Coal and biomass fuels are quite different in

composition. Cofiring biomass with coal has the capability

to reduce both NOx and SOx levels from existing pulverized-

coal fired power plants. Cofiring may also reduce fuel costs,

minimize waste and reduce soil and water pollutiondepending upon the chemical composition of the biomass

Table 14

Inorganic properties of typical fuel samples

Si2O Al2O2 TiO2 Fe2O3 CaO MgO Na2O K 2O SO3 P2O5 Cl

Coal 42.0 20.0 1.2 17.0 5.5 2.1 1.4 5.8 5.0 – –

Red oak wood 49.0 9.5 – 8.5 17.5 1.1 0.5 9.5 2.6 1.8 0.8

Wheat straw 48.0 3.5 – 0.5 3.7 1.8 14.5 20.0 1.9 3.5 3.6

Source: Ref. [56].

Table 15

Proximate analyses of typical fuel samples

Moisture (% of fuel) Ash (% of dry fuel) Volatile matter (% of dry fuel) Fixed carbon (% of dry fuel)

Coal 4.8 ^ 2.6 8.3 ^ 1.5 2.4 ^ 5.9 43.6 ^ 3.8

Oak wood 6.5 ^ 0.8 0.5 ^ 0.1 78.6 ^ 3.8 21.5 ^ 2.1

Wheat straw 7.3 ^ 1. 12.7 ^ 3.6 64.0 ^ 5.1 23.4 ^ 2.5

Source: Ref. [56].


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used. Cofiring must be implemented with other approaches

to provide a meaningful, long-term solution.

Pyrolysis produces energy fuels with high fuel-to-feed

ratios, making it the most efficient process for biomass

conversion, and the method most capable of competing and

eventually replacing non-renewable fossil fuel resources.

The conversion of biomass to crude oil can be have an

efficiency of up to 70% for flash pyrolysis processes. The so-

called biocrude can be used in engines and turbines. Its use

as feedstock for refineries is also being considered. Some

interesting trends have been obtained, especially with

respect to the effect of net heating rate and temperature on

the pyrolysis time. The reported literature results indicate adecrease in Final pyrolysis time as the net heating rate or

temperature is increased [58].

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