System Application Benefits and Impacts

The current fleet of low-cost, coal-fired, base load electricity generators are producing over 50% of the nation's power supply [5]. With the 1990 Clean Air Act Amendments (CAAA) requiring reductions in emissions of acid rai n precursors such as sulfur dioxide (SO2) and nitrogen oxides (NOx) from utility power plants, co-firing biomass a t existing coal-fired power plants is viewed as one of many possible compliance options. In addition, co-firing usin g biomass fuels from su stainably grown, dedicated energy crops is viewed as a possible option for reducing net emissions of carbon dioxide (CO2), a greenhouse gas that contributes to global warming. Coupled with the need of the industrial sector to dispose of biomass residues (generally clean wood byproducts or remnants), biomass co-firing offers th e potential for solving multiple problems at potentially modest investment costs. These opportunities have caught th e interest of power companies in recent years.

Unlike coal, most forms of biomass contain very small amounts of sulfur. Hence, substitution of biomass for coal can result in significant reductions in sulfur dioxide (SO 2) emissions. The amount of SO2 reduction depends on the percent of heat obtained from biomass and the sulfur content of the coal. Co-firing biomass with coal can allow powe r producers to earn SO2 emission allowances under Section 404(f) of the CAAA [6]. An allowance is earned for eac h ton of SO2 emissions reduced (1 allowance = 1 ton = 0.91 tonnes; 1 tonne = 1 metric ton). This section of the CAAA includes provisions for e arning credits from SO2 emissions avoided through energy conservation measures (i.e., demand side management or DSM) and renewable energy. In addition to any allowances which the producer earned by no t emitting SO2, two allowances can be given to the utility from an allowance reserve for every gigawatt-hour (106 kWh) produced by biomass in a co-fired boiler. These allowances may then be sold or traded to others who need them t o remain in compli ance with the CAAA. The value of an SO 2 allowance has ranged from $135 in 1993 to a current value of about $80.

As with fossil fuels, a result of burning biomass is the emission of CO 2. However, biomass absorbs about the same amount of carbon dioxide during its growing cycle as is emitted from a boiler when it is burned. Hence, when biomass production is undertaken on a sustainable or "closed-loop" basis by raising energy crops or by using the standard practice in the U.S. of growing at least as much forest as is being harvested, net CO2 emissions on a complete fuel cycle basis (from growth to combustion) are considered to be nearly zero [7]. Therefore, biomass co-firing may be one o f the most practical strategic options for complying with restrictions on generation of greenhouse gases. Fossil CO 2 reductions are currently being pursued voluntarily by utilities in the U.S. through the federal government's Climat e Challenge program. These utilities may be able to receive early credit for their fossil CO 2 emission reductions fo r future use in the event that legislation is passed which creates market value for CO 2 reductions. Total estimated emissions of both SO2 and CO2 from power plants operating in coal-only modes and when co-firing with biomass ar e shown in Table 3 (Section 4.2).

In addition to these emissions reductions and being a base load renewable power option, biomass co-firing has othe r possible benefits. The use of biomass to produce electricity in a dedicated feedstock supply system, where biomass i s grown specifically for the purpose of providing a fuel feedstock, will provide new revenue sources to the U.S . agriculture industry by providing a new market for farm production. These benefits will result in substantial positiv e economic effects on rural America. Using urban wood residues as a fuel reduces landfill material and subsequentl y extends landfill life. For industries served by the utilities, rising costs of tipping fees, restrictions on landfill use, an d potential liabilities associated with landfill use represent opportunities for power companies to assist industria l customers while obtaining low-cost biomass residues for use as alternative fuels. These residues can be mixed wit h more expensive biomass from energy crops to reduce the overall cost of biomass feedstocks. Finally, firing biomas s in boilers with pollution control can reduce burning of wood residues in uncontrolled furnaces or in open fields, an d hence provides another means of reducing air emissions.

Potential negative impacts associated with co-firing biomass fuels include: (1) the possibility for increased slaggin g and fouling on boiler surfaces when firing high-alkali herbaceous biomass fuels such as switchgrass, and (2) th e potential for reduced fly ash marketability due to concerns that commingled biomass and coal ash will not meet existing ASTM fly ash standards for concrete admixtures, a valuable fly ash market. These two issues are the subject o f continued research and investigation. Two factors indicate that biomass co-firing (using sources of biomass such a s energy crops or residues from untreated wood) will have a negligible effect on the physical properties of coal fly ash . First, the mass of biomass relative to coal is small for co-firing applications, since biomass provides 15% or less of the heat input to the boiler. Second, combustion of most forms of biomass results in only half as much ash when compared to coal. Despite these factors, significant efforts will be required to ensure that commingled biomass and coal ash will meet ASTM standards for concrete admixture applications. In the immediate future (three to five years), the AST M standards that preclude the use of non-coal ash will probably remain unchanged. Estimated ash effluents are show n in Table 3 (Section 4.2) for power plants operating in the coal-only mode and when co-firing with biomass.

Guide to Alternative Fuels

Guide to Alternative Fuels

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