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Bioenergy refers to liquid or solid fuels derived from biological sources and used for heat, electricity generation, or transportation. Electricity generation using biomass is much the same as that from fossil fuels; it generally involves a steam turbine cycle. The key difference is that typical output for a wood-based biomass power plant is about 50 MW, while conventional coal-fired plants generally produce anywhere from 100 to 1,500 MW (NRC, 2009a).

In the United States, interest in biomass for energy production is usually in the form of liquid transportation fuels. Such biofuels currently take several forms, including biodiesel, the sugarcane-based ethanol systems used widely in Brazil, and the corn-based ethanol system that has been encouraged through subsidies in the United States. While the sugarcane system has an energy output that is more than five times greater than the energy input, corn ethanol has an energy output that on average is slightly greater than its input, and thus does not significantly reduce GHG emissions (Arunachalam and Fleischer, 2008; Farrel et al., 2006). Ongoing research into cellulosic feedstocks, algae-based fuels, and other next-generation biofuel sources could lead to more favorable bioenergy effects and economics. Other areas of research include improving the productivity of current bioenergy crops through genetic engineering (Carroll and Sommerville, 2009), reducing the environmental impact of bioenergy crops by growing native species on marginal lands (McLaughlin et al., 2002; Schmer et al., 2008), and developing biofuels that can be used within the current, petroleum-based fuel infrastructure (NRC, 2009b).

Many different disciplines are contributing to the development of new bioenergy strategies, including biochemistry, bioenergetics, genomics, and biomimetics research. For example, research in plant biology, metabolism, and enzymatic properties will support the development of new forms of biofuel crops that could potentially have high yields, drought resistance, improved nutrient use efficiency, and tissue chemistry that enhances fuel production and carbon sequestration potential. Significant research is also being directed toward strategies for cellulose treatment, sugar transport, and the use of microbes to break down different types of complex biomass, as well as on advanced biorefineries that can produce biofuels, biopower, and commercial chemical products. Many developments in biofuels have been recently summarized (see DOE, 2009c; NRC, 2008a, 2009b).

Widescale development of bioenergy crops could have significant unintended negative consequences if not managed carefully. Conversion of solar energy to chemical energy by ecosystems is typically less than 0.5 percent efficient, yielding less than 1 W/m2, so relatively large land areas would be required for biomass to be a major source of energy (Larson, 2007; Miyamoto, 1997; NRC, 1980a). If the land required to grow bioenergy crops comes from deforesting or converting natural lands, there could be a net increase in GHG emissions as well as losses of biodiversity and ecosystem services. If grown on marginal lands, increased emissions of N2O, a potent GHG, may result as a side effect of nitrogen fertilizer use (Wise et al., 2009b). If bioenergy crops are grown on existing agricultural areas, food prices and food security could be compromised (Crutzen et al., 2008; Searchinger et al., 2008). Production of bioenergy crops also has the potential to negatively impact water quality and availability for other uses (NRC, 2008i), and methods are needed to more fully assess their potential impacts on ecosystem services (Daily and Matson, 2008). The recent report Liquid Transportation

Fuels from Coal and Biomass (NRC, 2009b) contains a more detailed discussion of the potential environmental and ecosystem impacts and provides recommendations for sustainable methods for increased bioenergy use. Focused interdisciplinary research efforts are needed to develop such methods and more fully assess the full spectrum of possible benefits and side effects associated with different bioenergy production strategies.

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