Source: Unnasch 1994.
Lumbermill waste and forest thinnings were considered for plants operating on forest material. Removing biomass from forests that have a high risk of fire is a source of feedstock for electric power generation and planned ethanol production. The benefits of harvesting forest thinnings also include increasing water available for larger trees, reduced fire fighting costs, and potentially reducing insect damage.
Providing a steady amount of forest material year round is not always possible as environmental constraints limit timber harvesting to non-rainy months. The amount of forest material that could be available in proximity to a potential methanol plant limits the plant size to about 40 million gallons per year. This plant size is relatively small for a capital-intensive methanol facility. A mixture of urban waste and agricultural waste was assumed necessary to provide a plant size over 100 million gallons per year. Urban wood waste and tree trimmings could provide additional biomass feedstocks for methanol production. Sewage sludge has also been considered a feedstock for gasification.
Wood waste, tree trimmings, and yard waste are separated in many areas. Competing uses for the highest quality of urban wood waste would require blending with lower value feedstocks, such as tree prunings, to reduce feedstock costs. Most urban wood waste that is currently burned in biomass power plants consists of larger branches from tree pruning and removal with very little clean wood residue from furniture and lumber operations. Urban wood waste is a limited resource for existing biomass power plants and if used as a methanol feedstock the price and transportation distance would increase. Chipped tree branches and yard waste are other potential feedstocks. These materials are either composted or used for landfill cover and are not suitable as fuels for biomass power plants. Sorting and quality control steps may need to be taken with branches and yard waste as these can quickly rot, may contain unexpected contaminants, and can have a high ash content.
Energy crops could provide additional feedstocks for methanol production. Eucalyptus was assumed as a potential energy crop since it has low water requirements and could be grown in many parts of California. It also could be used in areas where ground water contamination may be mitigated by planting trees. Table 2-6 shows the composition of some biomass materials.
2.3 Electric and Steam Energy
Electrical energy in this study is reported in electric kWh. Thermal energy used in generating electricity and other fuels is reported in Btu. With this approach, electrical energy and thermal energy should not be confused. Converting electrical energy