The most familiar forms of renewable energy are the wind and sun. But biomass – plant and animal waste – supplies almost 15 times more energy in the US than wind and solar combined. Types of biomass resources include tree and grass crops and forests, agricultural products, and urban wastes.
The old way of converting biomass to energy is simply to burn it to produce heat. The heat can be used directly for heating, cooking, and industrial processes or indirectly, to produce electricity. The problems with burning biomass are that much of the energy is wasted and that it can cause some pollution if it is not carefully controlled.
Mixed prairie grasses have emerged as a leader in the quest to produce biofuels.
Photo credit: Cedar Creek LTER site.
One approach that could increase the use of biomass energy in the short term is to burn it mixed with coal in power plants – called co-firing. Biomass feedstock can substitute up to 20% of the coal used in a boiler. The benefits associated with biomass co-firing include lower operating costs, reductions of harmful emissions, and greater energy security. Co-firing is also one of the more economically viable ways to increase biomass power generation today.
The Chariton Valley Biomass Project, a joint effort including Alliant Energy, the US DOE, and local biomass groups, began testing switchgrass co-firing with coal at Alliant’s Ottumwa Generating Station in Iowa. The project has proved so successful that in 2005, five years after the project started, Alliant received permission to build a permanent biomass processing facility at the plant.
Biomass conversion can be accomplished in three ways:
Thermochemical: When plant matter is heated but not burned, it breaks down into various gases, liquids, and solids. These products can be further processed and refined into useful fuels such as methane and alcohol. Biomass gasifiers capture methane released from the plants and burn it in a gas turbine to produce electricity.
Biochemical: Bacteria, yeasts, and enzymes also break down carbohydrates. Fermentation changes biomass liquids into alcohol. A similar process is used to convert corn into grain alcohol or ethanol, which is mixed with gasoline to make gasohol. Also, when bacteria break down biomass, methane and carbon dioxide are produced. This methane can be captured in sewage treatment plants and landfills and burned for heat and power.
Chemical: Biomass oils such as soybean and canola oil can be chemically converted into a liquid fuel similar to diesel fuel and into gasoline additives. Cooking oil from restaurants, for example, has been used as a source to make biodiesel for trucks.
In the US, we generate 45 billion kW-hr of electricity from biomass – about 1.2% of the nation’s total electric sales. We also generate nearly 4 billion gal. of ethanol, approximately 2% of the liquid fuel used in cars and trucks. With better conversion technology and more attention paid to energy crops, we could produce more.
Union of Concerned Scientists
Filed Under: Energy management + harvesting, Green engineering