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Common biomass fuels—such as trees, plants, cow manure, apricot seeds and paper—contain chemical energy. To produce electricity, the energy is converted from one form to another. Some biomass processes are akin to what happens when we eat and our bodies digest the food, converting the calories into energy.
Chemical energy is stored in the bonds that hold matter together. Biomass is a special class of chemical that contains hydrogen and carbon, also known as a hydrocarbon (HC), or organic matter.
Like a relay race, energy can be passed from one form to another. In most biomass processes, the fuel is first burned to convert it to heat. In other cases, bacteria is used to convert complex chemicals into methane, which is easier to burn. Heat can be further converted either to steam, the motion of an engine piston, or the spinning of turbine blades. The resulting motion spins a generator that produces electricity. Fuel cells, like photovoltaic solar cells, bypass the intermediate step involving motion and produce electricity directly.
In general, biomass generating facilities use materials that would otherwise be wasted, or that are more valuable for their energy than for other purposes. For example, wood chips are useful in landscaping but may be more valuable when converted to electric energy.
Putting biomass to work
Last year, SCE purchased 1,436 million kilowatt hours (kWh) of electricity from 21 biomass energy producers, enough to power approximately 217,000 homes. The electricity came from different types of biomass fuel. Some basic ways those fuels are converted to electricity include:
- Landfills: In a landfill, buried trash decomposes with the help of bacteria already in the organic material. This conversion of complex organic material to simpler chemicals, primarily methane (the principal component of natural gas) is called digestion. The methane is collected in pipes and burned in a boiler to make steam used to drive a turbine, which turns a generator. Alternatively, the methane can be burned in a piston or turbine engine, which turns a generator directly.
- Sewage treatment & digester plants: These facilities use digestion, like landfills, but the digestion process goes on in manmade containers under more controlled conditions. The most effective bacteria is used and the temperature is held steady. Some digester plants process materials such as tree trimmings, agricultural waste such as rice straw, or manure from dairy farms. Digestion processes are becoming more attractive for electrical generation because they produce fewer emissions than direct combustion.
- Direct combustion: In direct burning, the fuel is not converted first. Instead, it is burned in a large furnace or boiler to heat water hot enough to become steam. The steam then turns turbine blades, spinning the generator which makes electricity. For example, animal manure can be burned either directly or first digested to make methane gas.
For more information
Biomass Standard Contracts
Contact: George Wiltsee
E-mail: BiomassStandardContracts@sce.com
Phone: (626) 302-4945 |
