Environmental Problems, Their Causes, and Sustainability 1

Biomass consists of plant materials and animal wastesthat can be burned directly as a solid fuel or convertedinto gaseous or liquid biofuels (Figure 18-25). Mostbiomass is burned directly for heating, cooking, and industrialprocesses or indirectly to drive turbines andproduce electricity. Burning wood and manure forheating and cooking supplies about 10% of the world’senergy and about 30% of the energy used in developingcountries (90% in the poorest countries such asBangladesh, Ethiopia, Burundi, and Bhutan).In 2002, about 350 biomass power plants suppliedabout 3% of the commercial energy and 2% of the electricityused in the United States. The U.S. governmenthas a goal of increasing the use of biomass energy to9% of the country’s total commercial energy by 2010.One way to produce biomass fuel is to plant, harvest,and burn large numbers of fast-growing trees (especiallycottonwoods, poplars, sycamores, willows,and leucaenas), shrubs, perennial grasses (such asswitchgrass), and water hyacinths in biomass plantations.In agricultural areas, crop residues (from sugarcane,rice, cotton, and coconuts) and animal manure can becollected and burned or converted into biofuels. Insome developing countries the poor gather animal manureor dung by hand, dry it, and burn it for heat andcooking. On the surface, this appears to be a free andlogical use of wasted biomass.But some ecologists argue that it makes moresense to use animal manure as a fertilizer and cropresidues to feed livestock, retard soil erosion, and fertilizethe soil. Not allowing these animal and cropwastes to return to the soil as natural fertilizer can reducefood production and food supplies in poor countries.Also burning dried dung in open fires wastesabout 90% of its heat content.Figure 18-26 lists the general advantages and disadvantagesof burning solid biomass as a fuel. Oneproblem is that burning biomass produces CO 2 . However,if the rate of use of biomass does not exceed therate at which it is replenished by new plant growth(which takes up CO 2 ), there is no net increase in CO 2emissions. But repeated cycles of growing and harvestingbiomass plantations can deplete the soil of keynutrients.How Can Gaseous Fuels Be Produced fromBiomass? Bacteria and Chemistry to theRescueSome forms of biomass can be converted intogaseous and liquid biofuels.Bacteria and various chemical processes can convertsome forms of biomass into gaseous biofuels (Figure18-25). One of them is biogas—a mixture of 60%methane and 40% CO 2 .In rural China, anaerobic bacteria in more than500,000 biogas digesters on farms and in homes convertSolid Biomass FuelsWood logs and pelletsCharcoalAgricultural waste(stalks and other plant debris)Timbering wastes(branches, treetops, and wood chips)Animal wastes (dung)Aquatic plants (kelp and water hyacinths)Urban wastes (paper, cardboard,and other combustible materials)Direct burningGaseous BiofuelsSynthetic natural gas(biogas)Wood gasConversion to gaseousand liquid biofuelsFigure 18-25 Principal types of biomass fuel.Liquid BiofuelsEthanolMethanolGasoholplant and animal wastes into methane gas that is usedfor heating and cooking. After the biogas has been removed,the almost odorless solid residue is used asfertilizer on food crops or, if it is contaminated, ontrees. When they work, biogas digesters are very efficientand burning natural gas produced from dungproduces much more heat than burning the dung itself.But they are slow and unpredictable, a problemthat could be corrected by developing more reliablemodels. They also add CO 2 to the atmosphere.In some places in the United States, bacteria convertlivestock wastes from cattle, hogs, and chickensto biogas. One way to do this is to put the wastes in along, lined, insulated pit. A flexible liner stretchingacross the digester pit inflates like a balloon as itcollects the biogas, which can then be burned to heatthe digester or nearby farm buildings or to produceelectricity.398 CHAPTER 18 Energy Efficiency and Renewable Energy