Environmental Engineering Reference
Figure 7.3 A gasifier at a power plant in Burlington, Vermont, processes about 200 tons per day of wood
chips and waste, generating a gaseous fuel that provides 8 MW of electric power from a steam plant. (By
permission of DOE/NREL-PIX.)
to this use of an otherwise valueless byproduct, electric power plants fueled by wood harvested
for fuel only purposes have been constructed. The harvesting and transportation costs of the wood
fuel for these plants, added to the market value of the wood component as pulp feed or lumber (in
contrast to the residue component), generates a costly fuel and makes these plants uneconomical
compared with fossil fuel power plants. 8
Figure 7.3 shows a gasifier that converts wood waste to gaseous fuel for use in an electric
power plant. By converting from a solid to a gaseous form, the fuel can be used in a combined
cycle power plant, doubling the thermal efficiency of about 20% obtained in conventional wood
burning power plants.
The use of crops or their residues to supply fuel to replace fossil fuel creates environmental
impacts similar to those associated with agriculture and silviculture: consumption of manufactured
fertilizers, spreading of pesticides and herbicides, soil erosion, consumption of irrigation water,
and interference with natural ecosystems. Air emissions from the combustion of biomass fuels are
not always less than those from the fossil fuels they replace.
8 Expressed in 1999 dollars, the wood-fueled plants produce electric power at a cost of 8-12 cents per kWh
compared to 4-6 cents per kWh for current fossil fuel plants.