Biomass Explained
There are two types of Biomass systems, closed loop and open loop. In a closed loop biomass system, the material to be burned in the plant has been planted and harvested specifically for that purpose. For example, if switchgrass is planted, cultivated, harvested, and then taken directly to the power plant to be burned then this would be a closed loop system.
In contrast, an open loop biomass system uses material that was not originally intended for use as a fuel source. This is most likely to be the waste products from another process, such as wood chips and sawdust from a lumber mill, animal waste, farm waste, and paper mill waste.
There are also two major ways to use biomass in a power plant to produce electricity, co-firing and direct firing. A co-firing power plant burns both biomass material and another type of fuel source. If coal is used, this results in less coal being used in the process of providing electricity and making the power plant production process greener. Other sources, qualifying under the tax code, can be burned to produce even less carbon emissions in the process. In contrast, direct firing is where only biomass material is burned in the plant to produce electricity.
Recent standards proposed by the EPA would greatly impact this energy source in a negative way. The standards relate to the amount of carbon emissions released in the burning process, and would set the standards at an extremely high level that currently even the most efficient plants cannot meet. These standards were contested by members of congress on the basis that they will be impossible to meet and would decimate this growing industry. On July 1, 2011, the EPA formally announced that it would delay the enforcement of these standards for three years to assess biomass’ impact on air quality.
Biomass Electricity Production
The production capabilities of a biomass plant make it attractive to college campuses, prisons, and small communities. By definition, a 25 MW power plant produces 25 megawatts of electricity per hour at peak performance. One megawatt equals 1,000 kilowatts, so a 25 MW plant would produce 25,000 kW of electricity. The energy content of fuel is measured in Btu’s, or British Thermal Units. A pound of bone dry (zero percent moisture) wood fuel contains roughly 7,600-9,600 Btu’s of combustible energy. The moisture content affects the Btu level of the fuel source. The higher the moisture content is, the lower the Btu level, and the less efficient the system. One kW is equivalent to 3,413 Btu’s, so 25 MW would require 85,325,000 Btu’s of energy. Dividing by the low end of 7,600, to produce 25 kW would require about 11,200 pounds of fuel input. In summary, if you were to burn 11,000 pounds of ideal fuel source for an hour in a peak efficient system under ideal conditions you would produce 25 MW of electricity. A full year’s worth of production would require over 90,000,000 pounds of fuel.
According to the Department of Energy website, the average annual electricity consumption for a US residential utility customer was 11,040 kWh in 2008. So, if we were to calculate, using 8,000 hours a year and assuming that the plant could operate at peak efficiency for all 8,000 hours, a 25 MW plant could produce 200,000,000 kWh of electricity in a year. Dividing by the average annual consumption, the plant could provide approximately 18,000 homes with electricity for a year.
