Energy Conversion from Biomass

The various process used for conversion of biomass into energy or bio fuels can be classified as follows:

1. Direct combustion
2. Thermo chemical conversion
3. Biochemical conversion

1. The direct combustion of biomass in presence of oxygen/air to produce heat and by products is called direct combustion. The complete combustion of biomass into ash is called incineration.
2. This heat energy in the product gases or in the form of steam can be used for various applications like space heating or cooling, power generation, process heating in industries or any other application.
3. Various designs of boilers and furnaces are available to burn biomass like wood, dung. dried vegetable wastes from food industry, pulp, bagasse from sugar industry and municipal wastes etc.
4. The moisture content in biomass and their wide range of composition tends to produce low calorific value of fuel. However, if biomass energy by combustion is used as co-generation with conventional fuels, the utilisation of biomass energy makes it an attractive proposition.

The thermo-chemical reactions can convert the organic biomass into more valuable and convenient form of products as gaseous and liquid fuels, residues and by-products etc. at different pressures and temperatures. These processes can be carried out in following ways :

• Gasification : Heating of biomass in presence of limited oxygen and air (deficient O/air) is called gasification. It produces gaseous fuels like H₂, CO, CH₄, N₂ of low calorific value.
• Pyrolysis : It is the heating of biomass in a closed vessel at temperatures in the range of 500°C-900°C in absence of O/air or with steam. It produces solid, liquids and gases. The pyrolysis process can use all type of organic materials including plastic and rubbers. The gases produced by this process include the mixture of CO, CH₄, N₂, H₂, CO₂ and other hydrocarbons. The liquids produced are oil like materials (acetic acid, methanol, tar) and solids produced are similar to pure carbon charcoal.

3. Blochemical Conversion : The process of biochemical conversion of biomass makes use of metabolic action of microbial organism on biomass to produce liquid and gaseous fuels. These processes are of two types.

• Fermentation of biomass
• Anaerobic digestion of biomass
• 1. Fermentation : Fermentation is a process of decomposition of complex molecules of organic compound under the influence of micro-organism (ferment) such as yeast, bacteria, enzymes etc. The example of fermentation process is the conversion of grains and sugar crops into ethanol and CO₂ in presence of yeast. 10% of ethanol so produced can be blended with petrol to produce gasohol. The fermentation process of sugar is carried out at about 30°C in acidic conditions of ph value 4 to 5 and the completion of fermentation process takes about 50 hours.
• Anaerobic digestion of biomass : The anaerobic digestion or anaerobic fermentation process involves the conversion of decaying wet biomass and animal wastes into biogas through decomposition process by the action of anaerobic bacteria. The anaerobic bacteria is a micro-organism that can live and grow in absence of O₂ or air. The biogas production in anaerobic digestion depends on the type of biomass used, temperatures, ph value of mixture etc. For example the production of biogas from human and animal wastes takes about 10 days at optimum temperature of 35°C while the decomposition of biomass like sewage sludge, green plants etc. takes longer time. The gas produced is mixture of methane (55% to 70%) and remainder is CO₂ with other impurities. The most useful biomass for production of biogas are animal and human waste, algae, hyacinth, plant residue and other organic waste materials with high moisture content.