India produces a huge quantity of biomass material in its agricultural, agro-industrial, and forestry operations. According to some estimates, over 500 million tonnes of agricultural and agro-industrial residue alone is generated every year. This quantity, in terms of heat content, is equivalent to about 175 million tonnes of oil. A portion of these materials is used for fodder and fuel in the rural economy. However, studies have indicated that at least 150–200 million tonnes of this biomass material does not find much productive use, and can be made available for alternative uses at an economical cost. These materials include a variety of husks and straws. This quantity of biomass is sufficient to generate 15 000–25 000 MW of electrical power. In addition, electricity can also be generated from biomass grown on wastelands, road and rail track side plantations, etc. The quantum of electricity that can be produced from such biomass has been estimated to be in excess of 70 000 MW. Thus, the total electricity generation potential from biomass could reach a figure of about 100 000 MW.
Technology used for biomass based power generation
The technology for generation of electricity from these biomass materials is similar to the conventional coal-based thermal power generation. The biomass is burnt in boilers to generate steam, which drives a turbo alternator for generation of electricity.
Typical capital costs for biomass power projects range from Rs 3 crores/ MW to Rs 4 crores/MW. Costs of generation depend upon the cost of biomass, the plant load factor, and the efficiencies of conversion.
What is biomass gasification?
Biomass gasification is thermo-chemical conversion of solid biomass into a combustible gas mixture (producer gas) through a partial combustion route with air supply restricted to less than that theoretically required for full combustion. Typical composition of producer gas is as follows.
Why gasify biomass?
What type of biomass can be gasified?
Most commonly available gasifiers use wood/woody biomass; some can use rice husk as well. Many other non-woody biomass materials can also be gasified, although gasifiers have to be specially designed to suit these materials and the biomass may have to be compacted in many cases.
How do gasifiers work?
Gasifiers can be of ‘updraft’ or ‘downdraft’ types. In the downdraft type of gasifier, fuel and air move in a co-current manner. In updraft gasifiers, fuel and air move in counter-current manner. However, the basic reaction zones remain the same.
Fuel is loaded into the reactor from the top. As the fuel moves down, it is subjected to drying and pyrolysis. Air is injected into the reactor in the oxidation zone, and through the partial combustion of pyrolysis products and solid biomass, the temperature rises to 1100 °C. This helps in breaking down heavier hydrocarbons and tars. As these products move downwards, they enter the reduction zone where producer gas is formed by the action of carbon dioxide and water vapour on red-hot charcoal. The hot and dirty gas is passed through a system of coolers, cleaners, and filters before it is sent to engines.
What can be done with the producer gas?
The clean producer gas can be used for electrical power generation, either through dual-fuel IC engines (where diesel oil is replaced to an extent of 60%–80%), or through 100% gas-fired spark ignition engines. The producer gas can also be used for heating to replace conventional forms of energy in many applications like small boilers, furnaces, hot air generators, dryers, etc.
Biomass gasifier-based systems are being made in capacities ranging from a few kilowatts to a megawatt of electricity equivalent. For heating applications, the current upper limit on the unit size is equivalent to 200–300 kg/h of oil consumption.
The typical costs of biomass gasifier-based electricity generation systems range from Rs 4 crores/MWe to Rs 4.5 crores/MWe. The cost of power generation depends on cost of biomass, plant load factor, etc., and is estimated to be between Rs 2.50/kWh and Rs 3.50/kWh. For thermal applications, the capital costs are estimated to be about Rs 0.5–0.7 crores for each 1 million kcal capacity.
In simple terms, cogeneration is the process of using a single fuel to produce more than one form of energy in sequence. Cogeneration of steam and electricity can significantly increase the overall efficiencies of fuel utilization in process industries. A minimum condition for cogeneration is the simultaneous requirement of heat and electricity in a favourable ratio, which is well fulfilled in the sugar industry. During electricity production, there happens a rejection of a large quantity of heat to a lower temperature sink. In normal electricity generation plants, this heat rejection takes place in condensers where up to 70% of heat in steam is rejected to the atmosphere. In cogeneration mode, however, this heat is not wasted and is instead used to meet process heating requirement. The potential for cogeneration projects is estimated at 3500 MW of additional power generation from the country’s existing functional sugar mills.
The main equipment required for these projects comprise high-pressure bagasse-fired boilers, steam turbines, and grid-inter-phasing system. All these equipment are manufactured indigenously.
The overall efficiency of fuel utilization can thus be increased to 60 % or even higher in some cases. Capacity of cogeneration projects can range from a few kilowatts to several megawatts of electricity generation along with simultaneous production of heat ranging from less than a hundred kWth (kilowatts thermal) to many MWth (megawatts thermal).
The capital costs of installed bagasse-based cogeneration projects in sugar mills are known to range between Rs 3 crores/MW and Rs 4 crores/MW. It has been seen that for a typical sugar mill having an average crushing season of 160 days, investments for additional power generation through cogeneration will turn out to be beneficial in the long term.
Source : Biomass booklet by Ministry of New and Renewable Energy, GoI