The combustion process comprises four basic phases: heating and drying, distil­lation of volatile gases, combustion of these volatile gases, and combustion of the residual fixed carbon. Prior to the actual combustion process, the biomass is first subjected to pelletizing and/or briquetting in order to increase the density of the biomass and simultaneously reduce the moisture content. This also increases the calorific value of the biomass and increases the easy handling of the biomass during transportation and processing. The following steps are involved in pellet­izing of biomass [9]:

1. Drying. The biomass is dried to a moisture content of about 8-12% (weight basis) before pelletizing.

2. Milling. Size reduction of the biomass is done in hammer mills.

3. Conditioning. Conditioning of the biomass is done by addition of steam, whereby the particles are covered with a thin liquid layer to improve adhesion.

4. Pelletizing. Flat die or Ring die pelletizers are used to convert the above material into compact pellets.

5. Cooling. The temperature of the pellets increases during the densification process. Therefore, careful cooling of the pellets is required before the pellets leave the press, to ensure high durability of the pellets.

Pelletization is expensive compared to briquetting where the biomass is com­pressed and extruded in heavy duty extruders into solid cylinders. This pelletized or briquetted biomass is subjected to heat, which breaks down the plant cells. The volatile matter is driven off from the compacted biomass and instead of being released directly into the atmosphere, it is made to pass through a high temperature zone (above 630°C) in presence of secondary air. Here, the gases are combusted and release more heat. A carbonaceous residue called char, containing the mineral components is left behind.

After briquetting or pelletizing, the biomass is fed into the combustion furnace after which combustion proceeds in four phases [7]:

Phase 1: Heating and drying

Moisture in the biomass varies from 10 to 50% of the total weight (wet basis). This moisture reduces the dry heat value of the biomass and slows down the heating and drying process. It is therefore essential to remove this moisture in order to increase the efficiency of the combustion process. The size of the feed particles is also important because most biomass is woody in nature and wood is a poor conductor of heat. The larger the particle size, the lower the rate of heat transmission through the feed bed. The biomass is hence reduced in size so that the maximum distance from the center of the particle of the feed to the surface does not exceed 20-30 mm. Thus, wood chips, sawdust, shredded straw, and pulverized biomass fuels such as bagasse are preferred.

Phase 2: Distillation of volatiles

After the evaporation of moisture is complete, the heat supplied gets used in volatilization of the liquid constituents present in the biomass. This occurs between 180 and 530°C. Distillation occurs during this phase. The gases released comprise complex saturated and unsaturated organic compounds such as paraffin, phenols, esters, and fatty acids. These distill at different distillation temperatures thus making the concept of ‘‘biorefinery’’ possible.

Phase 3: Combustion of volatiles

Ignition of the volatilized components takes place at temperatures between 630 and 730°C. This involves an exothermic reaction between the volatilized gases and oxygen, as a result of which, heat is produced and CO2 and water vapor are released. The flame temperature in this phase depends on the amount of excess air present and the amount of moisture initially present in the biomass (because this evaporated moisture is present as water vapor in this gas phase). Here, supply of excess oxygen in the form of secondary air supply is essential because this will maintain high temperatures during this phase. In absence of this, incomplete combustion will result in lower process efficiency . The unburnt carbonaceous part is called soot. This soot absorbs volatile components which condense in the cooler parts of the furnace and form an oily product called tar.

Phase 4: Combustion of residual fixed carbon

After the moisture and volatiles have been removed, the fixed carbon component of the biomass remains as char. This char begins to burn as oxygen is now available, and carbon monoxide is released which, in the presence of oxygen gets converted into CO2. This CO2 is finally emitted from the furnace.