COMBUSTION OF SOLID BIOFUEL

Solid Bio-fuels - Characteristics - Properties - Preparation

of biomass as feedstock for combustion reactor-

Combustion Process and equipment - Combined Cycle

Operation

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What is biomass and biofuel? Biomass is plant matter such as trees, grasses,

agricultural crops (residues), tree borne oil seeds or

other biological material. It may be divided into

woody, non-woody and aqueous waste with high

BOD content. Fuels derived from biomass are called

biofuels compared with petroleum based fuels.

It can be used as a solid fuel, or converted into liquid

or gaseous forms for the production of electric power,

heat, chemicals, or fuels.

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Cooking energy constitutes about 85 percent of Indian

rural energy demand and has traditionally been met

by biomass fuels such as firewood, agricultural

residues and animal wastes. Under the National

Programme on Improved Cook stoves, about 30

million cook stoves have so far been installed, which

are helping to cut back and conserve fuelwood use.

RURAL DOMESTIC USE: FUEL FOR COOKING

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What inputs are needed to develop biofuels?

Biofuel as energy source of rural poor people

requires social and technical inputs to make

it a modern sustainable system.

Wasteland may be utilized.

From Forests areas, wastes _ forest tree twigs,

shrubs may be available

Marginal Croplands

Overcome disadvantage of low bulk energy

density of particulate biomass by preparatory

process like briquetting.

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Forestry in the New Millennium

Tropical India, with its adequate sunlight,

rainfall, land and labour, is ideally suitable for

tree plantations.

With the enhanced plan outlay for forestry

sector and financial support from donor

agencies, the country is able to march ahead

towards the target of 33 percent forest cover.

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Can solid biofuels be used in Power

Plants?_ Yes, in small scale power plants

Plants fired solely by biomass are small compared with

conventional coal, gas, oil or nuclear stations. This size

depends on biomass growing capacity and delivery.

Transport of biomass, a material with a low bulk energy

density is expensive when conveyed over the long

distances to larger plants.

Specific capital cost, efficiency and specific operational

cost advantages of larger power plants are balanced

against fuel transport issues.

Give examples of solid bio-fuels

WOOD [Forestry / agro-forestry based]

BAGASSE [Agro - industry residue]

RICE HUSK [Crop residue]

NEEM ETC.- [Multi-purpose trees]

PROSOPIS ETC.- [Energy crops]

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Common sources of solid bio-fuels

APPLCATIONS OF

BIOMASS COMBUSTION

• RURAL DOMESTIC: COOKING

• HEAT & STEAM FOR SMALL SCALE

PROCESS

• COGENERATION / COMBINED CYCLE FOR

ELECTRIC POWER GENERATION

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What and when does solid bio-fuels have

advantage over coal for combustion?

• For decentralized and small / medium scale

operation where coal mines are remote but forest

resource / woodlands are near.

• Restricts the emission of green house gases and

air pollutants (like SO2).

•Lessens our dependence on imported

hydrocarbon fuels, creates rural employment.

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For combustion, mention disadvantages

of solid bio-fuel over coal?

• Availability is location specific and limited.

• Lower calorific value; variable particle size.

• Preparation like drying, briquetting etc. needed for wood /agro-residue.

• Price depends on production system like energy plantation, social forestry, saw mills, paddy processing etc.

• India‟s Forest resource is highly stressed

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Important Properties

and

Suitability as fuel

MOISTURE CONTENT

ELEMENTAL ANALYSIS

CHEMICAL COMPOSITION

PARTICLE SIZE & DISTRIBUTION

SUITABLE WHERE LOCALLY AVAILABLE

THROGHOUT THE YEAR AND

FOR SMALL & MEDIUM SCALE OPERATION

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Proximate and Ultimate Analysis,

and HHV

Proximate Analysis: (1) Moisture,

(2) Volatile,

(3) Fixed carbon and

(4) Ash. [Wt %]

Ultimate Analysis : C, H, O, N, S. [Wt %]

Higher Heating Value, MJ/Kg

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Providing the Draft

P 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.

Natural: by means of a chimney

Artificial: The fan is used to suck the gases away from the furnace [induced draft], or to force the air required [forced draft] through the grate.

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COMBUSTION PROCESS

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Thermal decomposition steps for a biofuel:

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COMBUSTION REACTIONS

C6n[H2O]5n = 6nC + 5nH2O

Biomass

C + O2 + 3.79N2 = 3.79N2 +CO2

EXOTHERMIC, 395400 KJ / KG ATOM

OF CARBON

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COMBUSTION PROCESS

• Combustion: Oxidation of reduced forms of carbon and hydrogen by free radical processes.

• Chemical properties determine the higher heating value & the pathways of combustion.

• Bio-fuel: A wet (50% moist), dirty, light in weight, heterogeneous in particle size, and quite reactive condition.

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COMBUSTION PROCESS continued

Biofuel is

highly reactive,

volatile,

oxygenated

fuel of moderate heating value.

Moisture content lowers the combustion efficiency

and affects the economics of the fuel utilization. 22

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• Drying,

• Pyrolysis,

• Release of volatiles and

• Formation of char

are followed by

• pre-combustion gas phase reactions

• char oxidation reactions.

Flaming combustion <> Glowing combustion

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Requirements of efficient

combustion:

• Sufficient air to provide oxygen needed for

complete burning; higher than theoretical

air.

• Distribution of air supply: mix with fuel

• Secondary air to burn the volatile

• Volume of furnace; Flow path for flue gas

• Minimize heat losses

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Factors influencing thermal efficiency

in wood combustion

• Enthalpy of the fuel

• Moisture content of the fuel

• Level of excess air employed

• Final stack temperature

Note: Theoretical flame temperature

depends on moisture content, % excess

air and preheating of air

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Briquetting in India

• Indian briquettes made from: groundnut

shell, cotton stalk, saw dust, coffee husk,

bagasse, mustard stalk and press mud.

Whi

• Southern region: groundnut shell and saw

dust

• Western and Northern regions: bagasse,

groundnut shell, cotton stalk, mustard

stalk and press mud briquettes.

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Briquetting continued

A recent addition: Municipal solid waste

densified for use as fuel in process

industries (tea, tobacco, textile,

chemical, paper, starch, tyre re-

treading, tiles, etc.) for thermal

applications.

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Screw and Ram Press

• Both the machines give briquettes with a

density of 1-1.2 gm /cc, suitable as industrial

solid fuels.

• The screw type machines: briquettes with a

concentric hole-- better combustibility- a

preferred fuel.

• These briquettes can also be more

conveniently deployed in small furnaces and

even cook-stoves than solid briquettes

generated by a ram press.

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Screw Press for briquettes

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Ram press for briquettes

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FURNACE FOR BIOFUEL

COMBUSTION

Horizontal grate furnace

Chain grate furnace

Inclined step grate furnace

Spreader-stoker system

Suspension burning system

Cyclone firing system

Fluidized bed combustion system

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Inclined step grate furnace:

• Fuel is fed to the top of the grate

• 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.

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Inclined step grate furnace:

….continued

• 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

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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

• Solid phase pyrolysis and char oxidation

occur on the grate.

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Spreader stoker …continued

• 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.

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Suspension burning system:

horizontal cyclone furnace

• A horizontal or slightly inclined cylinder lined

with firebricks into which air is ejected

tangentially at a velocity of 6000- 7000 m/min.

• 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.

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Horizontal cyclone furnace

• 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

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Circulating Fluidized bed combustion

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Circulating Fluidized bed combustion

COMBINED HEAT & POWER

• STEAM INJECTED GAS TURBINE

• INTERCOOLED STEAM INJECTED GAS

TURBINE

• COMBINED CYCLE

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Biofuel use in

cogeneration cycle

• SUITABLE FOR SMALL SCALE (<10 MW)

GENERATION

• PRODUCES LESS AIR POLLUTANTS AND

SOLID WASTES

• AUGMENTS POWER SUPPLY TO

INDUSTRY

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BIOMASS INTEGRATED GASIFIER /GAS

TURBINE (BIG/ GT) TECHNOLOGY

HIGH THERMODYNAMIC CYCLE EFFICIENCY

GAS TURBINES TECHNOLOGY IS MADE AVAILABLE NOW AT REASONABLE COSTS

LOW UNIT CAPITAL COST AT MODEST SCALES FEASIBLE

IT IS EXPECTED THAT THIS TECHNOLOGY WILL BE COMMERCIALLY SUCCESSFUL IN THE NEXT FIVE YEARS.

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Biomass-coal co-combustion represents a near-term,

low-risk, low-cost, sustainable, renewable energy option

that promises reduction in effective CO2 emissions,

reduction in SOx and often NOx emissions, and several

societal benefits. Technical issues with cofiring: fuel

supply, potential increases in corrosion, decreases in

overall efficiency, ash deposition issues, pollutant

emissions, and overall economics.

Biomass - Coal co-combustion

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Operating costs are higher for biomass than for coal.

A sensitive factor is the cost of fuel, as energy crops suffer

large economic disadvantages relative to residues. Fuel

transportation, preparation, and on-site handling typically

increase its effective cost per unit energy such that it

sometimes exceeds that of coal. Biomass cofiring is slightly

more expensive than dedicated coal system. Compared to

alternative renewable energy sources, biomass cofiring

generally turns out to be significantly cheaper.

Biomass - Coal co-combustion

Reference Books

• A. Chakraverthy, “Biotechnology and Alternative Technologies for Utilisation of Biomass / Agricultural Wastes”, Oxford & IBH publishing Co., N. Delhi, 1989.

• Samir Sarkar, Fuels and Combustion, 2nd Edition, Orient Longman, 1990

• Chapters on Combustion process Stoichiometry and Thermodynamics, Combustion Kinetics and Combustion Appliances. pages 217 to 326

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Reference Books / journals

Journal—„Biomass and Bio-energy‟,

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

b) 1998, 14(1): 33-56 „De-centralized 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

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6. Solid Fuels Combustion and Gasification,

Marcio L.de Souza-Santos, MARCEL DEKKER,

2005

7. Wood Energy News, October 1999, Vol. 14,

No 9, The Regional Wood Energy Development

Programme in Asia (RWEDP), E-mail:

rwedp@fao.org, „Wood energy in India‟

Reference Books / journals