BURN SOLID BIOMASS FOR ENERGY

Small Scale Heat release process

Solid bio-fuel combustion OK for small power plant:

• Suitable for decentralized and small / medium scale operation as assured availability is small - location specific. • Restricts the emission of green house gases and air pollutants (like SO2). •Creates rural employment, lessens dependence on imported fuels

Unfavourable characteristics of solid biomass as a fuel:

• Location specific and limited availability.• Lower calorific value; variable particle size• Preparation like drying, briquetting etc.

needed for wood /agro-residue• Price depends on production system like

energy plantations, social forestry, saw mills, paddy processing etc.

• India’s Forest resource is highly stressed

SOLID BIOFUELS: examples

WOOD [Forestry]

BAGASSE [Agro-industry residue]

RICE HUSK[Crop residue]

PROSOPIS ETC.- [Energy crops]

NEEM ETC.- [Multi-purpose trees]

BIOMASS COMBUSTION

• RURAL DOMESTIC: COOKING

• HEAT & STEAM: SMALL SCALE

• ELECTRIC POWER GENERATION:

• COGENERATION/ COMBINED CYCLE

Important Properties

MOISTURE CONTENTELEMENTAL ANALYSISCHEMICAL COMPOSITIONPARTICLE SIZE & DISTRIBUTIONSUITABLE WHERE LOCALLY AVAILABLE

THROGHOUT THE YEAR AND FORSMALL & MEDIUM SCALE OPERATION

Prox.anal., Ult. Anal., & 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

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 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.

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.

COMBUSTION PROCESS continued

• Biomass fuels are highly reactive, volatile, oxygenated fuels of moderate heating value.

• Moisture content lowers the combustion efficiency and affects the economics of the fuel utilization.

• Drying, pyrolysis of solid particle, release of volatiles and formation of char are followed by pre-combustion gas phase reactions and char oxidation reactions.

Requirements of efficient combustion:

Sufficient air to provide oxygen needed for complete burning; higher than theoretical air.

Distribution of air supply: mix with fuelSecondary air to burn the volatile Volume of furnace; Flow path for flue gasMinimize heat losses

Factors influencing thermal efficiency in wood combustion

Enthalpy of the fuelMoisture content of the fuelLevel of excess air employedFinal stack temperatureNote: Theoretical flame temperature

depends on moisture content, % excess air and preheating of air

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.

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.

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.

Screw Press for briquettes

Ram press for briquettes

FURNACE FOR BIOFUEL COMBUSTION

Horizontal grate furnaceInclined step grate furnaceSpreader-stoker systemSuspension burning systemCyclone firing systemFluidized bed combustion system

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.

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

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.

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.

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.

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

COMBINED HEAT & POWER

• STEAM INJECTED GAS TURBINE

• INTERCOOLED STEAM INJECTED GAS

TURBINE

• COMBINED CYCLE

Cogeneration with biomass fuel

• SUITABLE FOR SMALL SCALE (<10 MW) GENERATION

• PRODUCES LESS AIR POLLUTANTS AND SOLID WASTES

• AUGMENTS POWER SUPPLY TO INDUSTRY

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 TEN YEARS.

Reference Books1. A. Chakraverthy, “Biotechnology and Alternative Technologies for Utilizations of Biomass / Agricultural Wastes”, Oxford & IBH publishing Co., N. Delhi, 1989.2. 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

Reference Books/journals

3. Wood Combustion, Tillman, Ch. 5 ‘Heat production & release from wood combustion’,4. Progress in biomass Conversion, vol 3, Edited by K V Sarkanen, D A Tillman and. E C Jahn, Academic Press, 1982