3.2.3 Biomass Based Power Generation

Biomass materials such as firewood and agro-residues essentially contain carbon, hydrogen, and oxygen along with some moisture and ash. Direct combustion of biomass is generally inefficient and smoky that cannot easily be controlled. About 200 million tonnes of firewood and equivalent amount of agricultural residues are burnt annually in India with end-use efficiency of approximately 10%. Under controlled conditions characterized by low oxygen supply and high temperatures, most of the biomass materials can be converted into gaseous fuel known as “producer gas”, which comprises carbon monoxide, hydrogen, carbon dioxide, methane and nitrogen. This gas has a lower calorific value than natural gas or liquefied petroleum gas, but can be burnt with high efficiency and good degree of control without emitting smoke. The conversion efficiency of the gasification process is in the range of 60%–70%. Usage of gasifiers in place of conventional direct burning devices leads to savings of a minimum of 50% fuel.

For thermal applications, the technology has been well proven and gasifier systems are already working in the field. The capacity installed so far covers a wide range of applications at different capacities. It has range from 30 KW(t) to 500 KW(t).

Gasifier is essentially a chemical reactor where several thermo-chemical processes such as pyrolysis, combustion and reduction take place. Depending on the movement of gases relative to the fuel bed, various gasifier designs can be classified: updraft, downdraft and cross-draft gasifiers. Traditional downdraft gasifiers have throats or choke plates in order to reduce the tar content of the gases, but throat-less designs are limited. Advanced designs such as fluidized bed systems, high-pressure gasifiers, or designs with tar recycling are yet to be perfected. Gasifier designs usually depend on type of fuel, moisture content, ash content, fuel pellet size, etc. Certain biomass fuels such as rice husk have the tendency to form slag at high temperatures, and hence may require different designs.

Advantages of Gasification

Conversion of solid biomass into combustible gas offers all the advantages associated with using gaseous and liquid fuels.
It is attributed to clean combustion, compact burning, high thermal efficiency and good degree of control. In locations where biomass is already available at reasonable low prices (e.g. rice mills, coffee/corn processing units, sugar mills, etc.) or in applications utilizing fuel-wood (e.g. institutional cooking, silk reeling units, etc), gasifier systems offer definite economic advantages.

Advantages of using producer gas

• Reduces firewood consumption by at least 50% in large stoves
• Saves LPG in large-scale cooking
• Saves LDO (light diesel oil) and furnace oil in boiler applications
• Replaces up to 80% of diesel oil in diesel generator sets operated in dual fuel mode.

Application of the technology: Steam generation

Institutional/large-scale cooking

• Fuel-wood is used in large quantities for cooking in hostels, hospitals, hotels, marriage parties, and sweet shops
• Thermal efficiency of large stoves using firewood is low (approx. 10%) and requires large quantities of firewood
• Use of gasifier reduces fuel-wood consumption by about 50%
• Power delivered can vary, thereby causing the cooking process faster
• Depending on availability of biomass and space LPG users can shift to firewood/biomass briquettes.

Silk reeling industry of South India