Unit 10: Energy Challenges // Section 7: Biomass Energy and Feedstocks
Biomass energy sources are all around us. They include many types of plants and plant-derived material, such as agricultural crops and wastes; trees, forestry wastes, and mill residues; animal wastes; and municipal solid waste. About 11 percent of world primary energy today is produced from biomass, but this comes mostly from low-technology applications in developing countries, such as burning crop waste, wood, or dung for home heating and cooking.
While these fuels might seem like low-impact energy sources, biomass burning in primitive stoves is very inefficient and generates large quantities of pollutants such as fine particulates, sulfur and nitrogen oxides, and carbon monoxide, which are especially harmful when released in poorly ventilated houses. Indoor air pollution is a serious health risk and causes many respiratory illnesses in developing countries (for more details, see Unit 6, "Risk, Exposure, and Health"). The inefficient combustion of biomass fuels in primitive stoves also means that much of their energy content is wasted. Programs to help users switch fuels or install more efficient stoves are among the cost-effective options to reduce health impacts from using traditional biofuels (Fig. 13).
Figure 13. A ceramic cook stove saves fuel in Myanmar
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Source: © G. Bizzarri, Food and Agriculture Organization of the United Nations.
Advanced biomass technologies that use organic material cleanly and efficiently offer much greater opportunities. One of the fastest-growing biomass applications today is production of transportation fuels from plant sources. Notably:
- Ethanol, also known as ethyl alcohol or grain alcohol, can be fermented from sugars found in corn and other crops and added to conventional gasoline. As an additive, ethanol lowers reliance on conventional oil and increases the combustion efficiency of gasoline, reducing pollutant emissions. In Brazil, which has a sizeable ethanol industry based on sugar cane, all gasoline sold contains 25 percent alcohol, and over 70 percent of the cars sold each year can run on either ethanol or gasoline.
- Biodiesel, which is essentially vegetable oil, can also be derived from a wide range of plant sources, including rapeseed, sunflowers, and soybeans, and can be used in most conventional diesel engines. Because it burns more cleanly than its petroleum-based counterpart, biodiesel can reduce pollution from heavy-duty vehicles such as trucks and buses.
Both ethanol and biodiesel are viable sources of renewable energy that can reduce our dependence on conventional fossil fuels and reduce harmful emissions. However, growing biofuel crops—especially corn for ethanol—requires major quantities of fossil fuel to manufacture fertilizer, run farm machines, and ship the fuel to market, so these biofuels do not always offer significant net energy savings over gasoline and diesel fuel. Growing corn is also water-intensive and removes significant levels of nutrients from soil (hence its high need for fertilizer). In addition, relying too heavily on these sources could mean diverting crops from the food supply at some point to produce energy.
Analysts generally agree that, at best, corn ethanol offers modest energy savings over gasoline, and that the real promise lies in making ethanol from cellulosic (woody) plants such as switchgrass, willows, and poplars (footnote 3). Some of these plants, notably switchgrass, also sequester large amounts of carbon, restore nutrients to soil, and can be used to stabilize land threatened by erosion (Fig. 14). Cellulosic plant tissue is tough and must be broken down before it can be fermented, but contains substantially more energy per unit than carbohydrates such as corn. Current research focuses on developing quick and efficient methods of breaking down cellulosic plant tissue for fermentation into fuel (footnote 4). Many experts believe that cellulosic ethanol will become a significant energy source in the United States in the next one to two decades.
Figure 14. Switchgrass
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Source: © Oak Ridge National Laboratory.
Biomass can also be used to generate electricity. A number of U.S. power plants either run completely on biomass fuels or co-fire them with coal to reduce emissions. In most cases, biomass fuel is burned directly to boil water and turn steam turbines, but some advanced plants convert biomass fuels to gas by heating them in a low- or zero-oxygen environment. The resulting gas burns more efficiently than solid wood waste or plant material, thus extracting more energy from the fuel with fewer pollutants (footnote 5). Many industrial facilities, especially in the pulp and paper industry, produce significant quantities of electric power using residual biomass fuels such as wood pulp that are generated from their own production facilities.
Human and animal wastes can also produce electricity. On farms, devices called anaerobic digesters use microbes to break down manure into organic solids and biogas, which typically contains about 60 percent methane and 40 percent CO2. The methane can be burned to generate electricity and reduce greenhouse gas emissions. Similarly, many large landfills collect the biogas that is generated by decomposition of buried organic waste and burn it to generate electricity.
One of the most important environmental benefits of using biofuels is that biomass energy is carbon-neutral—that is, using it does not increase long-term greenhouse gas levels in the atmosphere. Biomass fuels such as timber release carbon when they are burned, but this carbon was sequestered from the atmosphere when the original trees grew and would have been released when they died and decayed, so using biofuels simply completes the natural carbon cycle. In contrast, burning coal or oil releases carbon that was previously sequestered underground for millions of years and would have stayed there if it were not mined for energy, so it represents a net transfer of carbon from terrestrial sinks to the atmosphere. (For more on the carbon cycle, see Unit 2, "Atmosphere.") Biomass energy thus does not contribute to global climate change unless it is harvested more quickly than it regenerates—for example, when large forest areas are clear-cut.