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The process of biomass gasification is distinctly different form that of biogas production. Gasification is the process by which solid biomass materials are broken down using heat to produce a combustible gas, commonly known as producer gas. Common feedstocks for combustion include wood, charcoal, rice husks and coconut shells.
The biomass gasification plant consists of a reactor, similar to a simple stove, into which the solid biomass fuel is fed. The supply of air to the fuel is, however, carefully controlled to allow only partial combustion of the fuel to take place. During this process gases are given off which are captured and can be used as a fuel gas. Several combustible gases are given off - hydrogen, carbon monoxide and methane - as well as carbon dioxide and nitrogen.
Two reactor types exist; the fluidised bed reactor, which is used with large-scale gasification system, and the fixed bed reactor, which is employed for small-scale producer gas systems. There are three varieties of fixed bed reactor; updraft, downdraft and crossdraft. Each reactor type produces a different ratio of gases at different temperatures and with a differing level of cleanliness.
The gas has several applications. It can be used directly in a burner to provide process heat or it can be used in internal combustion engines, but requires cleaning and cooling for this application. Plant ratings for small-scale power output can range from several kilowatts up to several hundred kilowatts and for heat production the output can be several megawatts. System efficiencies vary considerably depending on fuel, reactor type and application. Producer gas is commonly used for commercial cooking applications.
The small-scale gasifier technology is reasonably simple and cheap and can be manufactured locally, although care should be taken to ensure safety standards are maintained as carbon monoxide, which is produced during combustion, is a toxic gas. In China, a downdraft reactor design has been in production since the 1960's which uses rice husks as feedstock and hundreds of these systems are in use. They have also been installed in Mali, Surinam and India (Stassen 1995). During the Second World War, when fuel supplies were in short supply, millions of vehicles in Europe were adapted to run on producer gas, and today in countries such as Brazil and the Philippines gasifiers are commercially available for a variety of applications.
In India, there has been widespread development and dissemination of gasification technology to meet a variety of rural energy needs - for example, irrigation pumping and village electrification.
References and resources
- Stassen, H.E., Small-scale biomass gasifiers for heat and power: a global review. World Bank technical paper no. 296, Energy Series 1995.
- Quaak, P., Knoef, H. and Stassen, H.E., Energy from biomass: a review of combustion and gasification technologies. World Bank technical paper no. 422, Energy Series 1999.
|This article or section includes content from Original:Biogas and liquid biofuels by Practical Action. Used with permission.|
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