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BIOMASS ENERGY: THE TECHNOLOGY
Biopower is the use of biomass to generate electricity. There are three major types of biopower systems. They are:
Direct-firing plants that burns 100% biomass fuel,
Co-firing power plants that uses biomass as an adjunct to coal, and
Gasification-based power plants that convert biomass to a low- or medium-heating value gaseous fuel, usually for combustion in a gas turbines/engine.
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Most of the biopower plants in the world are direct-fired systems. They burn biomass feedstock directly to produce steam. The steam, as in any other boiler plant, is captured by a turbine and converted into electrical energy by a generator. |
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For utilities that generate electricity by means of coal-fired plants, co-firing with biomass represents one of the least-cost renewable energy options. Co-firing involves replacing a part of the coal with biomass using an existing power plant boiler. This can be done by either mixing biomass with coal before fuel is introduced into the boiler, or by using separate fuel feeds for coal and biomass. Depending on the boiler design and fuel feed system; biomass can replace up to 15% of coal in a co-firing operation. Boiler technologies where co-firing has been practised, tested, or evaluated, include wall fired and tangentially designed pulverised coal boilers, coal-fired cyclone boilers, fluidised-bed boilers and spreader stokers. Preparing biomass for co-firing involves well-known and commercial technologies. After ‘tuning’ the boiler’s combustion output, there is little or no loss in total efficiency. |
Gasification
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Biomass gasifiers operate by heating the biomass in an
oxygen-starved environment until it breaks into its constituent chemical components. The
process requires the input of heat energy for the endothermic chemical reaction to proceed
and split the molecules apart. In order to produce combustible gas, the wood should first be heated. It is common to heat it by burning a small portion of the wood. The heating dries the fuel, and not until then will the temperature be increased. At a temperature of approximately 200°C, the pyrolysis begins where the volatile constituents of the wood are given off. They consist of a mixture of gases and tars. When the pyrolysis is completed, the wood has been converted to volatile constituents and a solid carbon residual (char). |
In the second step a fluidising agent (steam, air or oxygen) is added to convert the char into gas. The combustible constituents in the gas are primarily carbon monoxide, hydrogen and a little methane. The gas also contains incombustible constituents such as nitrogen and carbon dioxide. Small fractions of tar and other carbohydrates may also be present. In order to use the gas in an engine, the tar and distillates must be destroyed in the hearth.
Many types of gasifiers have been developed. They are normally classified according to how fuel and air are fed in relation to one another. The most common is the updraft and downdraft gasifiers. There are also other gasification principles e.g. fluidised bed gasification, which has its stronghold in large systems.
The gas is then used in a combustion turbine or engine to produce electricity.