The world's first biomass-based power plant promises a 'cleaner' alternative to coal-fired plants
biomass has moved a key step towards competitiveness with fossil fuels. The McNeil generating station, a biomass power plant in Burlington, us, when completed, will house the world's first gasifier capable of burning biomass for electric power generation. Biomass gasification has the potential to marry the environmental advantages of biomass fuels with power generation efficiency levels matching those of coal and oil power plants.
Biomass fuel can be viewed as solar energy stored by photosynthesis in living plants. It includes any harvested plant matter and can be used directly as fuel or converted into fluid fuels for power generation.
Common feedstocks to a biomass plant are residues from paper mills, sawmills, the wood products manufacturing industry, urban wood surplus (such as tree trimmings) and agri-cultural residues. The capacity of a power plant is limited by the availability of feedstock within a distance of 80-120 km. As a result, plants are typically of capacities less than 25 mega watt.
At present, nearly all biomass plants use steam turbines. Boiler systems include manually-fed ovens, in which biomass burns in a pile; automated, stocker-fed systems in which biomass burns on a stationary or moving grate; or fluidised-bed units in which biomass burns as it is fluidised from below by a continuous jet of combustion air. Fluidised-bed units offer advantages of low nitrogen oxide and carbon monoxide emissions and high carbon burnout. Overall, biomass steam-turbine system efficiency is limited to about 25 per cent due to the feedstock's inherently high moisture content (about 50 per cent) and the constraints of availability of feedstocks to the power plants.
In biomass gasification, heat applied to biomass transforms it into a gas, fuelling the turbine for electricity generation. For higher efficiency, the gas-turbine cycle can be combined with a steam cycle in an integrated gasifier combined cycle system, or used with a steam-injected gas turbine.
In conventional systems, heat is directly applied to the biomass in the presence of a gasifying agent such as air or pure oxygen. The gasifier used at the McNeil station uses two separate circulating fluidised-bed reactors -- a gasification reactor in which the biomass is converted into gas and residual char and a combustion reactor that burns the residual char to provide heat for gasification. The heat is applied to the biomass indirectly by a 'carrier' -- in this case, sand -- that circulates between the two reactors. The combustor heats the sand to about 1,800-1,900f (1,000c approximately). The sand is then fed to the gasification reactor, where it literally surrounds every biomass particle, quickly turning it into gas before returning to the combustor to be reheated.
Since the hot sand method does not directly use air or oxygen, it avoids the generation of by-products such as carbon dioxide and nitrogen. It is a quicker method, thus enabling the new design to reduce capital costs compared to conventional systems. In a plant performance projection, the system's estimated net efficiency is approximately 36 per cent.
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