All of the more important technical options for the direct co-firing of biomass materials in large coal fired power plants have been implemented in Britain over the past 3-4 years.Here will brief descriptions of all of the available direct co-firing options, and a discussion of the preferred options for future projects, are presented. There is also a discussion of the potential impacts of co-firing on coal fired boiler plant performance and integrity, and on the means available for the assessment and minimization of these impacts for both retrofit and new build projects.
Biomass Cofiring in Coal-Fired Boilers Using this time-tested fuel-switching technique in existing federal boilers helps to reduce operating costs, increase the use of renewable energy, and enhance our energy security Executive Summary To help the nation use more domestic fuels and renewable energy technologies—and increase our energy security In biomass cofiring, biomass can substitute for up to 20% of the coal used in the boiler. The biomass and coal are combusted simultaneously. When it is used as a supplemental fuel in an existing coal boiler, biomass can provide the following benefits: lower fuel costs, avoidance of landfills and their associated costs, and reductions in sulfur oxide, nitrogen oxide, and greenhouse-gas emissions. Other benefits, such as decreases in flue gas opacity, have also been documented. Biomass cofiring is one of many energy- and cost-saving technologies to emerge as feasible for federal facilities in the past 20 years.
Cofiring is a proven technology; it is also proving to be life-cycle cost-effective in terms of installation cost and net present value at several federal sites. Energy-Saving Mechanism Biomass cofiring projects do not reduce a boiler’s total energy input requirement. In fact, in a properly implemented cofiring application, the efficiency of the boiler will be the same as it was in the coal-only operation. However, cofiring projects do replace a portion of the nonrenewable fuel—coal—with a renewable fuel—biomass. Cost-Saving Mechanisms Overall production cost savings can be achieved by replacing coal with inexpensive biomass fuel sources—e.g., clean wood waste and waste paper. Typically, biomass fuel supplies should cost at least 20% less, on a thermal basis, than coal supplies before a cofiring project can be economically attractive.
Cofiring is a near term, low-cost option for efficiently and cleanly converting biomass to electricity by adding biomass as a partial substitute fuel in high-efficiency coal fired water tube boilers. It has been demonstrated, tested, and proved in all boiler types commonly used by electric utilities. There is little or no loss in total boiler efficiency after adjusting combustion output for the new fuel mixture. This implies that biomass combustion efficiency to electricity would be close to 33%-37% when cofired with coal. Extensive demonstrations and tests also confirmed that biomass energy can provide as much as 15% of the total energy input with only feed intake system and burner modifications. The opportunities for biomass cofiring are great because large scale coal-powered boilers represent 310 gigawatts of generating capacity. Cofiring biomass with coal offers several environmental benefits. Cofiring reduces emissions of carbon dioxide, a greenhouse gas that can contribute to the global warming effect (see picture on the reverse side). Also, biomass contains significantly less sulfur than most coal. This means that cofiring will reduce emissions of sulfurous gases such as sulfur dioxide that will then reduce acid rain. Early test results with woody biomass cofiring showed a reduction potential as great as 30% in oxides of nitrogen, which can cause smog and ozone pollution.
Co-firing coal with biomass is a potentially valuable tool to help decrease greenhouse gas and other emissions in coal-fueled fired boilers. Use of biomass at low to moderate biomass to coal ratios appears to produce the best performance enhancements and can result in overall life-cycle energy consumption reductions, as well as reduced solid waste generation. However, there are many potential obstacles to biomass use that can decrease efficiency, as well as increase costs, maintenance (corrosion, slagging, etc.), and boiler down time if biomass use is not managed very carefully.