Biomass can be routed through following different processes for energy conversion:
Thermal decomposition of organic matter is carried out in the presence of excess air, liberating heat and leaving behind incombustible ash.
Fuel + Air ? Heat + Ash + Inert Gases
In the combustion mode, the Biomass and air are combined under efficient and controllable conditions to provide energy for utilization. The direct combustion of Biomass can be done in two ways:
• Fixed Bed Combustion
The combustion of Biomass is usually carried out in fixed bed combustors. The main drawback of fixed bed combustion is a low combustion efficiency of 70%. It is associated with problems of ash removal and inability to use effectively high moisture content Biomass. Further, fixed combustion involves high inventory of fuel over the grate resulting in time consuming start-up and shut down procedures.
• Fluidised bed combustion
In a fluidized bed system, a bed of fine particles is fluidized by a gas stream passing upwards through it at a controlled velocity. The bed is continuously subjected to high rate of mixing and agitation resulting in high heat and mass transfer rates. Fluidised bed combustion is best suited for burning fuels like rice husk etc.
The advantages of fluid bed combustion over fixed bed combustion are high combustion efficiency (95%), multi-fuel combustion facility, and consistent rate of combustion and ability to burn low grade and high moisture content fuels.
Gasification is the thermo-chemical process of obtaining energy from solid matter in a gaseous form. In principle, the process is a thermal decomposition of organic matter in the presence of limited supply of air or oxygen to produce combustible gases thus converting calorific value of organic material into a gaseous energy carrier. The amount of electricity that can be produced from Biomass power systems can be increased by 50% by replacing steam based generation units with Biomass gasifiers close-coupled to gas turbines.
In contrast to complete combustion of solid carbonaceous material, the process of pyrolysis refers to combustion in a deficient supply of air / oxygen. The process gives out carbon-mono-oxide and methane, which are condensed to form tar and aqueous liquor. The latter is then distilled to give methanol and other organic substances. This process produces three useful fuels - charcoal, oil and gas. About 70% of the energy in Biomass can be converted to higher forms of energy - charcoal or oil, which are storable and transportable.