In the past several years, however, a variety of factors have combined to limit the viability of biomass power. Electric utilities have gone from a condition of undercapacity to adequate capacity. Coal prices are relatively low (about $1.00 to $1.50 per million Btu) and the utility avoided cost for coal-fired electricity can be as low as 2 to 3 cents per kilowatthour, whereas the breakeven range for biomass power may be in the range of 4 to 7 cents per kilowatthour. Many utilities are not renewing expired purchase contracts with small producers. Section 29 of the Internal Revenue Service code (which provides for a 1.5-cent-per- kilowatthour tax credit for biomass-based electricity generation) is due to expire December 31, 1995, and extension is currently being debated in Congress. In addition to these factors, the problems of small power producers in the Western United States have been compounded by a limited supply of waste wood for fuel use, due to forest management constraints on logging.
On the other hand, the pulp and paper industry, which is by far the largest consumer of biomass among independent power producers, may not be as strongly affected by the above developments as are other power producers. Pulp and paper facilities are large rather than small in operating scale, and the wood and wood byproducts they burn for power and steam are largely waste materials that would otherwise represent a disposal problem. Also, much of the power generated by pulp and paper mills is consumed by the mills themselves.
Biomass fuels, primarily wood, fired in these systems are supplied by the forest and agricultural sector. A significant portion of the biomass power industry is comprised of cogenerators in the pulp and paper industry. These cogenerators use black liquor, bark and wood residues as fuel. Most wood fuels from the forestry and agricultural sectors have a high moisture content (up to 50 percent) and low heating value (4,000 to 5,000 Btu per pound). Urban wood wastes are generally drier (between 5 and 15 percent moisture content) and have a higher heating value (6,000 to 8,000 Btu per pound), with the exception of fresh wood trimmings, which are similar to forest residues.
Most biomass power plants operating today are characterized by low boiler efficiencies (65 to 75 percent) and low net plant efficiencies (20 to 25 percent). Beyond the fuel characteristics, the small size of most facilities contributes to the low efficiencies. Resource limitations and capacity caps promulgated under the Public Utility Regulatory Policies Act of 1978 (PURPA) limited biomass-fired plants to 50 megawatts; thus, the designs have not harnessed economies of scale, such as reheat steam loops and multistage feedwater heating. Some gasification technologies, which gasify biomass and burn the gases, have been used successfully in small-scale commercial applications, but they have not yet been integrated into large power plant designs. The Vermont Gasification Project at the McNeil Power Plant in Burlington may be the first large-scale utility demonstration of the new, higher efficiency, gasification technology.
Coal-fired power plants have also co-fired biomass with coal for many years. Recent data from controlled test burns promise significant reductions in sulfur dioxide emissions, and perhaps nitrogen oxide emissions, although the latter results are still experimental. The benefits of sulfur dioxide reductions, coupled with carbon dioxide recycling, have increased the interest in co-firing within the utility industry. Co-firing biomass with fossil fuels, however, may require boiler modifications,(23) electrostatic precipitator improvements, and changes in fuel handling systems. Nonetheless, this option may be an attractive and cost-effective emission reduction strategy, particularly because of the reduced sulfur dioxide emissions. In the future, even natural gas could be co-fired with gasified biomass, enhancing fuel substitution strategies.
One issue that continues to create technical difficulties for direct-fired systems is alkali fouling. Alkaline compounds, such as potassium and sodium, contained in the biomass melt at low temperatures (for boilers). When the molten or partially molten ash particles come in contact with the boiler walls or the heat exchanger tubes, they cool and form glass-like coatings that reduce the boiler efficiency over time. Wood fuels generally minimize this phenomenon, but other biomass fuels, such as straw and agricultural products, still present a technological challenge to the industry.
Gasification involves the transformation of solid biomass into a gaseous state, followed by burning in advanced gas turbines, such as combined-cycle turbines, which have overall efficiencies of 40 percent or higher. Specific areas of research and development include improving hot gas cleanup to remove alkaline compounds, identifying the source of turbine blade deposits, and identifying methods to remove particulates through temperature control using mechanical systems and feedstock additives. Demonstration units have been tested, and the first commercial-scale gasifier is planned for 1996 in Burlington, Vermont. These systems have the potential to reduce biomass power costs to levels competitive with natural gas.
Fast pyrolysis produces "biocrude," a liquid similar to crude oil, by subjecting the biomass to extreme pressure and temperatures. Research agendas include determining the combustion characteristics of various types of biocrude made from different biomass resources, understanding the interactions between fast pyrolysis conditions and the resulting characteristics of the oils, validating combustion tests, removing ash, and developing acid-resistant components.
Several types of fuel cells are under development (phosphoric acid, molten carbonate, and solid oxide fuel cells), and several are in commercial use, fueled by natural gas, methanol, or ethanol feedstocks.(28),(29) Fuel cells produce electricity through chemical reactions, as opposed to combustion, and can approach efficiencies of 60 percent, making them very attractive options. Gas or fuel quality is a particular concern for fuel cell manufacturers and operators, since small amounts of contaminants create significant problems. Much of the research in hot gas cleanup for gasification will be applicable to fuel cells. Also, programs to produce ethanol or methanol from biomass will significantly affect the economics of fuel cell operations. Expanded availability and lower cost of feedstocks will expand fuel cell opportunities.
U.S. forest ecosystems contain nearly 58 billion tons of carbon and represent an important environmental resource for reducing atmospheric carbon dioxide.(31) In the United States, live trees are currently accumulating carbon from the atmosphere at an average rate of 1,252 pounds per acre per year, representing a 2.7-percent yearly increase in sequestered carbon. Society realizes an annual ■bonus■ of 117 million tons of carbon sequestered additionally■the estimated net annual increase stored by forest systems. This is the amount of carbon left stored after the total quantity accumulated by live and dead trees (508 million tons) minus the carbon removed by timber harvest, land clearing, and fuelwood production. However, 117 million tons of carbon is equivalent to only 9 percent of total annual U.S. atmospheric carbon emissions.(32)
Biomass combustion does produce ash, but it results in less ash than coal combustion does, reducing ash disposal costs and landfill space requirements. The biomass ash can also be used as a soil additive on farmland.