Introduction To Biomass Gasification and Types Of Gasifier
• One biomass energy based system, which has been proven reliable and had been extensively used for transportation and on farm systems during World War II is wood or biomass gasification.
• Biomass gasification means incomplete combustion of biomass resulting in production of combustible gases consisting of Carbon monoxide (CO), Hydrogen (H2) and traces of Methane (CH4). This mixture is called producer gas.
Producer gas can be used to run internal combustion engines (both compression and spark ignition), can be used as substitute for furnace oil in direct heat applications and can be used to produce, in an economically viable way.
Wood wastes can be used in fuel boiler but the equipment is expensive and energy recovery is low. As a result it is often advantageous to convert this waste into more readily usable fuel from like producer gas. The price of petroleum fuels is high or where supplies are unreliable the biomass gasification can provide an economically viable system – provided the suitable biomass feedstock is easily available. During both world wars and particularly World War II, shortage in petroleum supplies led to widespread introduction of gasification.
The production of generator gas (producer gas) called gasification, is partial combustion of solid fuel (biomass) and takes place at temperatures of about 10000C. The reactor is called a gasifier. The combustion products from complete combustion of biomass generally contain nitrogen, water vapor, carbon dioxide and surplus of oxygen. However in gasification where there is a surplus of solid fuel (incomplete combustion) the products of combustion are combustible gases like Carbon monoxide (CO), Hydrogen (H2) and traces of Methane and non useful products like tar and dust. The key to gasifier design is to create conditions such that a) biomass is reduced to charcoal and, b) charcoal is converted at suitable temperature to produce CO and H2.
Types of gasifier
Since there is an interaction of air or oxygen and biomass in the gasifier, they are classified according to the way air or oxygen is introduced in it.
There are three types of gasifiers Downdraft, Updraft and Crossdraft.
The choice of one type of gasifier over other is dictated by the fuel, its final available form, its size, moisture content and ash content. therefore, the advantages and disadvantages generally found for various classes of gasifiers.
Comparison Of Various Types of Gasifires:
1) Updraft Gasifiers:
- Advantages: Small pressure drop, Good thermal efficienty, Little tendency towards slag formation.
- Disadvantages: Great sensitivity to tar and moisture and moisture content of fuel, Relatively long time required for start up of IC engine, Poor reaction capability with heavy gas load.
2) Downdraft Gasifiers:
- Advantages: Flexible adaptation of gas production to load, Low sensitivity to charcoal dust and tar content of fuel.
- Disadvantages: Design tends to be tall, Not feasible for very small particle size of fuel.
3) CrossDraft Gasifiers:
- Advantages: Short design height, Very fast responce time to load, Flexible gas production.
- Disadvantages: Very high sensitivity to slag formation, High Pressure Drop.
Applications of producer gas:
The combustible gases from the gasifier can be used
a) in internal combustion engines,
b) for direct heat applications and
c) as feedstock for production of chemicals like methanol.
However in order for the gas to be used for any of the above applications it should be cleaned of tar and dust and be cooled.
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