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Biopower

This biomass gasifier in Burlington, Vermont operates on wood chips. Photo credit: Warren Gretz and DOE/NREL

Biopower is the process of using biomass (plant and organic matter) to generate electricity. Biomass has been used for lighting, cooking, and heating ever since humans first discovered fire. Today, modern biomass generating plants can produce hundreds of megawatt-hours of electricity in a manner similar to generating electricity using fossil fuels. In essence, the fossil fuels are simply replaced with plant matter as a fuel source, creating a cleaner, renewable energy alternative.

There are three different types of biofuel systems: direct-fire, co-firing, and gasification. Direct-firing involves burning biomass directly to produce steam. This steam is then captured and directed to spin a turbine that produces electricity. This system is very similar to conventional power production that uses coal or oil to generate electricity. The important benefit with biopower is that it relies on a renewable resource with significantly less harmful emissions.

Co-firing is similar to direct-firing except for the fact that biomass materials are burned in combination with a fossil fuel, most often coal, in a high-efficiency boiler. Utility companies use this approach to reduce overall air pollution emissions, most notably sulfur.

Gasification systems are different than the other two methods, in that high temperatures are used in an oxygen-starved environment to convert biomass into a gas (a mixture of hydrogen, carbon monoxide, and methane). This gas can then be used to fuel a combined-cycle gas turbine, which is much like a jet engine only it turns an electric generator instead of propelling a jet. This is a benefit over direct- and co-firing, given that combined gas-turbines are the most efficient of all power conversion technologies. Although gasification is not yet widely used in large power plants, it has the potential to be highly efficient and low polluting. This process is being tested by the United States Department of Energy at a facility in Burlington, Vermont.

Bioenergy Sources

Biomass for power production comes from four different sources: agriculture waste, forestry waste, municipal and industrial solid waste, and energy crops. Each year, agriculture production in the United States produces millions of tons of waste that could be utilized for energy production. Similarly, the forest products industry produces millions of tons of waste that could be used for the same purpose. Forestry waste includes residue left over from logging, saw mills, and paper production. In addition, a significant amount of biomass residue is generated when non-commercial tree species are thinned from the forest to allow more valuable tree species to grow to maturity. Much of the waste generated by municipalities and industry consists of plant and organic waste that could be removed from the solid waste stream and used as a source of fuel in a biomass power plant. Finally, energy crops can be grown specifically for the purpose of producing electricity. In the quest to identify suitable energy crops, researchers are experimenting with fast-growing trees, shrubs, and grasses, such as hybrid poplars, willows, and switchgrass. Each year millions of acres of agricultural land are idle, making them available for growing energy crops.

Economics and Future Prospects

Biomass offers great potential to replace existing fossil fuel power plants. The United States currently relies on bioenergy for about 1.4 percent of the nation's electricity. Bioenergy is generally cost competitive with fossil-fuel-generated electricity when biomass residue prices are very low or negative. Given that transportation costs are key to the price, biomass power plants must be sited near an ongoing, reliable biomass source. Given the prevalence of the forest product industry in the Northeast, we have several biomass power plants located throughout northern New England. There remains a significant untapped potential in the Northeast, given the fact that only a small portion of the plant and organic waste generated in the region is currently used for power production.

Most analysts believe that the economics of bioenergy will improve as larger plants are constructed with higher efficiencies. Increasing efficiency is the key to lowering the overall costs of bioenergy. The gasification process discussed above offers the potential to significantly enhance the economics of bioenergy, given that significantly higher efficiencies are achieved using modern turbine technology.

Environmental Issues

Although direct-fired biopower systems produce air pollution emissions, they are cleaner than coal-fired power plants because they do not release sulfur dioxide, a key pollutant contributing to acid rain. Furthermore, bioenergy systems have nowhere near the global warming impacts of fossil-fuel plants. They can be referred to as carbon dioxide neutral, given that the plant material absorbs as much carbon dioxide during its life as released when burned to produce electricity. The main concern with regard to emissions from biomass power production is the release of unburned particles that contribute to human health concerns. Pollution control technologies are available for removing these particulates from the smokestack.

The production of energy crops for bioenergy production can also be a cause for concern. If these crops are grown in the same unsustainable manner as most food crops are today, bioenergy does not offer a sustainable alternative to current fossil-fuel-fired power production. However, if energy crops are produced with minimal use of chemicals, they could potentially enhance agricultural lands and improve wildlife habitat. Many of the crops that are being considered for energy production could be grown in a sustainable, low-impact manner.

For Further Information

Visit the following web sites:

• The Energy Efficiency and Renewable Energy Network of the US Department of Energy (www.eren.doe.gov)
• The National Renewable Energy Laboratory (www.nrel.gov)