1

COMBUSTION OF BIOMASS

―chemical reaction between fuel and oxidizer involving significant release of energy as

heat‖.

2

Biomass Power Potential (MW)-2000

Source Potential (MW)

From surplus Biomass 16000

From bagasse based

co-generation in the

existing sugar mills

3500

Total 19500

Ref: MNES Annual Report, 1999

Source Approximate

Potential

Status

(as on 31 MARCH

1998)

Biogas plants 12 million 2.71 million

Improved wood

Stoves

120 million 28.49 million

Biomass power

and gasifiers

1700 MW 29.5 MW

Biomass based cogeneration 3500 MW 84 MW

Whether the use of biomass is to be made by direct combustion or by including other

conversion processes is decided by considering the fuel moisture content, density,

thermal value and the physical form of the material particularly as related to its

mechanical handling.

PRINCIPLES OF COMBUSTION: [FUEL – OXYGEN – TEMPERATURE – PRODUCT REMOVAL- HEAT TRANSFER]

COMPARISON OF COAL AND WOOD AS FUEL FOR COMBUSTION:

Coal Wood

1. Solid fuel, high ash content,

used for Raising HP steam,

power production with Rankine

cycle

Solid fuel, less ash, more volatile content, reactive,

can be used for Raising HP steam, power

production with Rankine cycle

2. Gas Turbine cycles,

Brayton cycle

More difficult

3. Can be used for producing

process steam for direct

heating

Can be used for producing

process steam for direct heating

4. Large scale availability

near mines and ports assured

Assured availability is only on small scale--

Variable

5. Technology for handling,

storage and Processing well

established

Large scale processing. storage and energy

conversion technology not established in India

6. Sulfur content and ash content

are problems

Moisture content, low bulk density,

Location specific availability are

problems

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Combustion is a chemical process involving oxidation of reduced forms of carbon

and hydrogen by free radical processes. Chemical properties of the biomass fuels

determine the higher heating value of the fuel and the pathways of combustion.

Biomass fuel enters a combustor in a wet (50% moist), dirty, light in weight,

heterogeneous in particle size, and quite reactive condition. Moisture content lowers the

combustion efficiency and affects the economics of the fuel utilization. Biomass fuels are

highly reactive, volatile, oxygenated fuels of moderate heating value. (See Table 1.)

Changes during the process of combustion are due to the effect of heating and

decomposition as the exothermic oxidation proceeds.

Drying, pyrolysis of solid particle, release of volatiles and formation of char are followed

by pre-combustion gas phase reactions and char oxidation reactions.

PROXIMATE & ULTIMATE ANALYSIS AND HHV:

For expressing the complete composition of any solid fuel, besides the organic

composition, proximate analysis and ultimate or elemental analysis are used.

Typical values of chemical composition of some biomass are shown in Table 1.

Table 2. shows average composition, ultimate analysis and bulk density of hardwood.

Table 3. and 4.are data of typical compositions of solid fuels.

To determine the quantity of air required for complete combustion of a fuel, the ultimate

analysis is useful.

C + O2 = CO2 +97644 cal /mole [15o C]

H2 +O2 = H2O + 69000 cal / mole [15o C]

Calorific value of a fuel is the total heat produced when a unit mass of a fuel is

completely burnt with pure oxygen. It is also called heating value of the fuel. When the

c.v. is determined, water formed is considered as in vapour state, net c. v. is got.

Gross calorific value or higher heating value of a fuel containing C, H and O is given by

the expression:

Cg =[C x 8137 + (H--O/8) x 34500]/100 where C, H and O are in % and Cg is in calories.

Net calorific value is the difference between GCV and latent heat of condensation of

water vapor present in the products

AIR REQUIRED FOR BIOMASS COMBUSTION:

Excess air % = (40*MCg)/(1- MCg) where MCg is moisture content on total wt

basis (green). For typical biomass fuels at 50 % moisture content, for grate firing system

about 40% excess air may be required.

For suspension fired and fluidized bed combustion, air required may be 100 % excess.

They are high because the air must keep the particles in suspension or fluidize the bed

medium.

Distribution of air and whether it is pre-heated is also important.

COMPOSITION PARAMETERS AFFECTING COMBUSTION

Net energy density available in combustion of biomass varies from about 10

MJ/kg (green wood) to about 40 MJ/kg (Oils/fats). Water requires 2.3 MJ/(kg of water) to

evaporate. Moisture content (MC) influences efficiency more than any variable. A system

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which gives a thermal efficiency of about 80% while firing a fuel of MC 15%, gives

reduced efficiencies of 65% when the fuel MC is 50 % or more.

Cellulose embedded in a matrix of hemi-cellulose and lignin is the main

constituent of woody biomass. Compared to coal, biomass has less mineral content and

wood gives less ash than agro-residue.

Table: 1. Chemical composition of some biomass material

Species Total ash

%

Solvent

Soluble %

Water

Soluble %

Lignin

%

Hemi-cellulose

%

Cellulose

Soft wood 0.5 2.0 - 27.9 24.0 40.8

Hard wood 0.3 3.1 - 19.5 35.0 39

Wheat Straw 6.0 3.1 7.1 16.0 28.1 39.7

Rice Straw 16.1 4.6 13.1 11.9 24.1 30.2

Bagasse 2.2 8.3 10.0 18.4 28.0 33.1

.

Table2.Av. Comp. & properties of hardwood:

GROUPWISE COMPOSITION, percent air dried

Cellulose 45-55

Lignin 25-35

Hemi-cellulose 4-6

Fats, waxes, and resins 0.5-2

Water 10-15

Ultimate Analysis and other properties, dry basis Carbon % 50

Hydrogen % 6

Oxygen, % 43.5

Ash, % 0.5

Density,kg/m2 650

Calorific value, kcal/kg 4,600

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TABLE 3.TYPICAL COMPOSITIONS OF SOLID FUELS

Types Proximate Analysis Ultimate Analysis Heating

Value, dry

basis,

kcal/kg

Moisture Volatile Fixed

carbon

Ash C H O N S

Wood

Oak(dry)

------ 85.6 13.0 1.4 58.2 6.0 43.3 0.1 - 4622

Pine(dry) ------ 87.0 12.8 0.7 52.2 7.0 40.2 0.2 - 5338

Peat 56.8 26.0 11.2 6.0 23.1 9.6 59.6 1.3 0.4 4625

Lignite 34.8 28.2 30.8 6.2 42.4 6.7 43.3 0.7 0.7 6110

Coal (Range

Of property) 3-20 16-40 40-

80

3.0-

40

60-

50

3.0--

6

3.0-

6

1-

1.5

0.3-

4.3

4000

to

8000

Bagasse ----- 80.5 17.0 2.5 48 6.0 43.2 0.3 0.1 4430

Coke 0.8 1.4 87.1 10.7 85 0.8 1.2 1.3 1.0 7105

Charcoal 12.0 1.9 83.1 3.0 84 2.3 10.7 -- -- 7130

Table 4. Douglas

Fir

wood

Western

Hemlock

Bagasse Rice

Husk

Pittsburgh

Bitum.

coal

All on oven dry basis

Proximate-Wt %

Volatiles

85.8

83.8

83.8

64.5

33.9

Fixed Carbon 13.4 14.0 12.7 12.9 55.8

Ash 0.8 2.22 3.5 22.6 10.3

Ultimate-Wt%

Hydrogen

6.3

5.8

5.8

4.4

5.0

Carbon 52.3 50.4 48.8 38.3 75.5

Oxygen 40.5 41.4 41.7 33.9 4.9

Nitrogen 0.1 0.1 0.2 0.8 1.2

Sulphur -- 0.1 -- -- 3.1

Ash 0.8 2.22 3.5 22.6 10.3

Higher Heating Value,

MJ/kg

21.05 20.05 19.32 13.81 31.74

Conditions for efficient Combustion:

1. Sufficient air to provide oxygen needed for complete burning of the fuel. Higher

than stoichiometric amount of air is supplied.

2. Free and intimate contact between fuel and oxygen by distribution of air supply.

3. Secondary air to burn the volatile mass leaving the fuel bed completely before it

leaves the combustion zone.

4. Volatile matter leaving the fuel bed should not cool below combustion

temperature by dilution with the flue gas. Flow path should assure this.

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5. Volume of the furnace should be arranged so as to provide for expansion of gases

at high temperature and complete burning of volatile matter before flowing away.

DRAFT: The pressure difference required to make the air flow through the fuel bed and

to the flue gas discharge height is called draft of air in a furnace and is expressed in

millimeters of water.

The draft is produced either naturally by means of a chimney or mechanically by

a fan. Mechanical draft can be either induced draft or a forced draft depending on

whether the fan is used to suck the gases away from the furnace or to force the air

required for combustion through the grate.

COMBUSTION PROCESS

Combustion of solid biomass like wood involves heating and drying, pyrolysis of solid

particle, forming volatiles and char; Pre-combustion gas phase reactions and char

oxidation reactions.

The pyrolysis and combustion of the biomass fuel takes place as the temperature rises

when the particle is in the hot fuel bed. At lower temperature, pyrolysis produces a gas

mixture of carbon dioxide, water vapor and carbonaceous char. The released volatile

mass and secondary air mixes and undergoes flaming combustion raising temperature

further. Oxidation of the active char results in glowing or smoldering combustion at a

lower rate.

Intensity of combustion can be expressed by

I c = H25 dw/dt -----(1)

Where I c is intensity of combustion, H25 is heat of combustion at 25 o C,

For the reaction: fuel +oxygen CO2 +H2O, and dw /dt is rate of loss of mass

The rate of burning depends upon the composition and the size of the fuel, air to fuel

ratio, and the heat and mass transfer of the whole system. At the lower temperature, the

chemical kinetics of pyrolysis and combustion control the burning rate, whereas at higher

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temperatures when these reactions take place at a very high speed, the heat and mass

transfer become the controlling factors.

Bioenergy options and their carbon reduction potential:

Bioenergy technologies draw energy from the biomass derived from plants and have the

advantage of restricting the emission of air pollutants. Their development and use not

only decreases CO2 emission but also lessens our dependence on fossil fuels, improves air

quality and creates rural employment.

Table 5. Examples of appropriate fuel delivery and furnace technologies according to

the form and particle size of the fuel:

Form Maximum

Particle size

(mm)

Appropriate delivery system Appropriate

Furnace technology

Bulk

material

<5 Direct injection,

Pneumatic conveyors

Direct-fired furnaces,

Muffle furnaces, cyclone

burners, CFB

Bulk

material

<50 Screw conveyers Underfeed stokers, grate

firings, BFB, CFB

Bulk

material

<100 Vibro-conveyers, Troughed chain

conveyers

Grate firing, BFB

Bulk

material

<500 Sliding bar conveyer, Sliding bar

conveyer

Grate firing, BFB

Standard or

cut bales

<50 Cutters/shredders followed by

Pneumatic conveyors or screw

conveyers

Direct-fired furnaces,

Grate firing, BFB, CFB

Pellets <30 Screw conveyers Under-feed stokers, BFB,

CFB

Briquettes <120 Sliding bar conveyer, Sliding bar

conveyer

Grate firing, BFB

Availability of annual crop/agro-residue in

India (1995-96) MT = Million tons

Agro-residue India, MT T.Nadu, MT

Wheat Straw 83.3 9.2

Rice Husk 39.8 3.3

Sugar Cane Bagasse 93.4 9.2

Coconut shell 3.4 0.4

Coconut pith 3.4

Groundnut shells 2.6 0.6

Cotton Stalks 27.3 0.8

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Rice husk based power plant:

(Nandini Chemical Journal, 6(8): 54, 1999)

A power plant that can generate 6 MW of power has been inaugurated in Raipur district

of M.P. It uses 7 tonnes of rice husk an hour to produce high pressure steam (at 480 o

C)

that is used to produce electricity. To burn the husk, the plant uses fluidized bed

combustion type boiler supplied by Thermax. The plant is owned by Indo-Lahari Power

Limited. The estimated capital cost for a megawatt of power produced is 35 million

rupees as against 40 million rupees for a coal based power plant. In Raipur area one tonne

of rice husk costs about rupees 550 per tonne as compared to rupees 1400 per tonne of

coal.

Combustion equipment for solid biomass (wood):

Inclined step grate furnace: In the inclined grate system, fuel is fed to the top of the

grate.In this system, heating and drying can occur very near to the fuel feed shoot. Solid

phase pyrolysis can occur as the fuel is sliding down the grate. Char oxidation can occur

at the base of the grate and on the dumping grate. Gas phase reactions can be controlled

by over-fire air distribution and separated completely from solid phase reactions.

Spreader Stoker: In the spreader stoker, fuel particles are fed into the firebox and flung,

mechanically or pneumatically across the grate. Some heating and drying and possibly

some pyrolysis occurs while the particle is in suspension. For the most part however,

solid phase pyrolysis and char oxidation occur on the grate. Pre-combustion gas phase

reactions occur between the grate and the zone where secondary air is introduced. Gas

phase oxidation occurs either throughout the firebox or in the vicinity of the zone where

secondary air is introduced if the under-grate air is limited to sub-stoichiometric

quantities.

Combustion equipment for solid biomass (particulates--wood and agro-residue):

CYCLONIC, SUSPENSION FIRED COMBUSTION SYSTEM

Horizontal Cyclone Furnace: A horizontal cyclone furnace consists of a horizontal or

slightly inclined cylinder lined with firebricks into which air is ejected tangentially at a

velocity of 6000- 7000 m/min so that the flame in the furnace revolves at a rpm of 1200

to 1800 .The fuel introduced at the cyclone tip is entrained by the revolving mass and is

thrown against the cyclone walls where it burns. The flue gases that escape at high

velocities through the aperture at the other end of the cyclone are substantially free from

fly ash. The heat release rate of (2-5) X 106 kcal/m2-hr can be achieved for pulverized

coal in a cyclone furnace.

The rotary motion imparted to the flame results in an intensive mixing of the flame mass

and the fuel particles are subjected to the action of centrifugal force. This increases the

residence time of the fuel in the furnace and combustion is complete.

FLUIDISED BED COMBUSTION SYSTEM

In fluidized bed combustion, bio-fuel is dispersed and burned in a fluidized bed of inert

particles. Temperature of the bed is maintained in the range of 750 to 1000 o

C so that

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combustion of the fuel is completed but particle sintering is prevented. The gaseous

products leave the bed at its operating temperature, removing about 50% of the heat

generated. The remainder of the heat is available for direct transmission to heat transfer

surfaces immersed within the bed; in boiler applications these comprise a set of steam

raising tubes. The heat transfer to immersed surfaces is uniformly high in comparison

with the variation of radiation heat transfer through a conventional combustion chamber.

Consequently less heat transfer surface is required for a given output and a boiler system

occupies a smaller volume.

Principles of furnace design calculations:

Thermal load of furnace grate area:

It is the amount of heat generated in kilo-calories by the complete combustion of a solid

fuel on one square meter of grate area per hour.

Thermal load of furnace grate area , QA = W.Cn / A kcal/m2.hr

QA = Thermal load of fire grate area, kcal/m2.hr

W = Fuel burned kg / hr,

Cn = Net calorific value of fuel, kcal / kg

A = furnace grate area, m2

Thermal load of volume of furnace:

It is the amount of heat generated in kilo-calories by the complete combustion of a solid

fuel, in one cubic meter of furnace volume per hour.

Thermal load of volume of furnace, QV =. W Cn / V kcal/m3.hr

QV = Thermal load of volume of furnace, kcal/m3.hr

V = volume of furnace space, m3

Thermal efficiency of furnace:

Thermal efficiency of furnace is the ratio of actual heat delivered by furnace to the

available heat in the fuel

Thermal efficiency of furnace, ηF =

(Heat generated – Heat losses) / (Net calorific value of fuel)

= (M.h) / (W Cn)

H = enthalpy of flue gas kilocalories/ m3

M = Flow rate of fluegas, m3/hr

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Example1. Combustion of Municipal Solid Waste (MSW):

The ultimate analysis of MSW is given below.

C- 30% H- 4% O- 22% H2O – 24% and ash-- metal, etc-20%;Compute the actual air

required and the flue gases produced per kg. of MSW if 50% excess air is supplied for

complete combustion.

Solution:

Basis: 100 kg MSW

Constituent Constituent

kg

Constituent

kg-mole

Oxygen required

kg-mole

Products,

kgmole

C 30 2.5 2.5 2.5

H 4 2.0 1.0 2.0

O 22 0.7 -0.7 --

H2O 24 1.3 -- 1.3

Total 2.8 5.8

Theoretical O2 required = 2.8 kg-moles

Actual O2 supplied = 2.8x1.5 = 4.2 kg-moles

Excess O2 present in the flue gas = 1.4 kg-moles

Actual air supplied = [100/21] x 4.2 x 29 = 600 kg

Weight of air supplied per weight of unit weight of refuse

= [600 / 100] = 6 kg / kg refuse

Quantity of N2 present in air supplied = [79 / 100] x 20 =15.8 kg-moles

Total amount of flue produced =5.8 + 1.4 +15.8

=23 kg-moles

= 23 x 29=667 kg

(assuming M.W. of flue gas is 29)

Weight of flue gas produced = 667 /100 =6.67 kg / kg refuse

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Example 2: Combustion of rice husk:

The ultimate analysis of rice husk is as follows:

C- 39 %, H- 5 %, O- 32.7 % , S- 0.1 %, N-.0 %, H2O- 3.6%, and ash =17.6 %

Assuming M.W. of air and flue gas as 29 compute the actual air required and flue gas

produced per kg of rice husk, if 20% excess air is supplied for complete combustion of

rice husk.

Solution:

Stoichiometric air required and flue gas produced for combustion of 100 kg rice husk.

Basis: 100kg rice husk

Rice husk

constituents

Constituent,

kg

Constituent,

kg-mole

O2 required,

kg-mole

Products,

kg-mole

C 39.0 3.250 3.250 3.25

H 5.0 2.500 1.250 2.250

O 32.7 1.022 --1.022 --

S 0.1 0.003 0.003 0.003

N 2.0 0.071 -- 0.071

H2O 3.6 0.200 -- 0.200

Total 3.481 6.024

Theoretical air required for combustion is as follows:

Theoretical air required per kg husk = [100 x 3.481 x 29] / [21 x 100] =4.8 kg.

Actual air supplied and actual flue gas produced are as follows:

Actual O2 supplied = 3.481x1.2 =4.18 kg-moles

Actual air supplied = [100/21]x 4.18 =19.9 kg-moles

Actual air supplied per kg husks = [100x4.18x29]/[21x100] =5.77 kg

N2 present in the actual air = [79/100]x19.9 = 15.72 kg-moles

Therefore total flue gas produced

Stoichiometric Chemical products + Inert Nitrogen + Excess O2 in air

= 6.024 + 15.72 + 0.79

= 22.434 kg-moles

Therefore flue gas produced per kg husk = [22.43x 29] / [100] = 6.5 kg.

Example 3: Design of a cyclone Furnace:

Design a cyclone furnace to supply hot flue gas-air mixture required for drying 2 tonnes

parboiled paddy per hour assuming the following data:

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Initial and final moisture contents of

parboiled paddy =

30% and 15% (w. b.)

respectively

Latent heat of vaporization

of paddy moisture =

580 kcal./kg

Average net calorific value

of rice husk =

3000 kcal / kg

Excess air supplied = 40%

Flame Temperature = 1100 o C

Ambient air temperature = 25 o C

Average inlet and outlet drying

gas-air mixture temperatures =

100 and 60 o C

respectively

Average wet & dried parboiled

Paddy temperatures

35 and 60 o C

respectively

Mean sp. Heats of paddy, flue gas[600],

Flue gas [80] =

0.4; 0.28; and 0.26

kcal./ kg o C

respectively

Average M.W.s of air& flue gas = 29

Efficiency of cyclone furnace = 78%

Length/ diameter = 1.5

Thermal load on furnace volume for rice husk = 2x105 kcal/m

3-hr

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Reference books for Combustion:

1. A. Chakraverthy, ―Biotechnology and Alternative Technologies for Utilisation of

Biomass/Agricultural Wastes‖, Oxford & IBH publishing Co., N.Delhi, 1989.

2. Fuels and Combustion, 2nd

Edition, Samir Sarkar, Orient Longman, 1990

Chapters on Combustion process Stoichiometry and Thermodynamics,

Combustion Kinetics and Combustion Appliances. pages 217 to 326

3. Journal—‗Biomass and Bio-energy‘,

a) 1996, 11(4): 271-281 ‗Biomass Combustion for power generation‘

b) 1998, 14(1): 33-56 ‗Decentralized biomass combustion: state of the art and future

development‘

4. Wood Combustion, Tillman, Ch. 5 ‗Heat production & release from wood

combustion‘,

5. Progress in biomass Conversion, vol 3, Edited by K V Sarkanen, D A Tillman

and. E C Jahn, Academic Press, 1982

6. Sharma S.P. and Chandramohan, ―Fuels and combustion‖ Tata McGraw Hill

(1987).