VOLUME 10, NUMBER 2
MARCH/APRIL 1996
© Copyright 1996 American Chemical Society
Symposium Symposium on Biomass Fuels: An Introduction David C. Dayton* and Helena Li Chum Industrial Technologies Division, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401-3393 Received January 15, 1996 On August 21 and 22, 1995, at the 210th American Chemical Society National Meeting in Chicago, Illinois, the ACS Division of Fuel Chemistry sponsored a symposium on Biomass Fuels. Twenty-three papers devoted to the thermochemical conversion of biomass to fuels or electric power were presented by a diverse, international group of researchers. Revised and updated versions of several papers appear in the following special section of this issue. Recent concerns about global climate change contributed to by the use of fossil fuels, among others, has made consideration of alternative sources important. Biomass is an attractive alternative energy resource. It is domestic and renewable. With the appropriate considerations in its use to make it sustainable, the increased utilization of biomass fuels could improve significantly both the environment and the economy of the United States. Current biomass resources comprise primarily industrial waste materials such as sawdust or pulp process wastes, hog fuel, forest residues, clean wood waste from landfills, and agricultural prunings and residues. The increased use of biomass fuels would diversify the nation’s fuel supply while reducing net CO2 production (because CO2 is withdrawn from the atmosphere during plant growth) and reduce the amount of waste material that eventually ends up in landfills. It is important that biomass uses have a high process efficiency to increase the overall resource productivity from past commercial applications. Thermochemical conversion of biomass typically is achieved by using one of three processes: combustion, gasification, or pyrolysis. Direct biomass combustion systems can be used to generate process heat and electricity. Biomass can also be cocombusted with coal to take advantage of the high efficiency of coal-fired power plants. Integrated gasification combined cycles 0887-0624/96/2510-0267$12.00/0
(IGCC) can be used to generate synthesis gas, or syngas, a mixture of primarily hydrogen and carbon monoxide with traces of hydrocarbons and other impurities. Clean syngas can be burned in high-efficiency gas turbines to produce electricity or converted to methanol for fuel use. The future use of syngas in high-efficiency fuel cells also shows promise for economically competitive electricity production. The product of biomass pyrolysis is an oil called biocrude that can be stored and transported more easily than the foundation biomass material. This biocrude can be combusted in a boiler today, or, in the future, used in higher efficiency applications such as a diesel engine, or in a gas turbine to produce electricity. Alternatively, it could be upgraded chemically or catalytically to produce fuels and other higher value chemicals. Next-generation biomass thermochemical conversion facilities are being designed in conjunction with dedicated feedstock supply systems that are projected to provide low-cost energy crops for power generation, fuels, and chemical production. These multiple developing uses of biomass ensure that a type of biomass refinery could be achieved where, in addition to conventional feed, food, and fiber applications, biomass can contribute to fuels, chemicals, and electricity production in a sustainable way. In the United States, the increased use of biomass fuels faces difficult challenges because of the low cost of fossil fuels and petroleum-derived products. However, the increased use of biomass fuels around the globe is proving to be an economical and socially acceptable alternative to fossil fuel consumption for electricity production and district heating. In the European Community, large excesses of straw (wheat straw and rye straw for example) are available as byproducts from the agricultural industry. Biomass fuels such as straw are exempt from the CO2 tax imposed on facilities consum© 1996 American Chemical Society
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ing fossil fuels. As a result, considerable efforts are being made to improve the technologies for utilizing biomass for electricity production and district heating to meet environmental challenges. Biomass fuels are also being considered in developing countries where the fossil fuels infrastructure is not well established. In addition to providing an energy source, biomass utilization also helps increase regional employment. The participants of the symposium addressed many of the technical issues and challenges associated with biomass thermochemical conversion described above; some of these are conveyed in the following articles. Two of the articles assume the challenge of modeling different types of biomass combustors using simplified models to describe the chemistry and heat transfer process occurring. Bryden et al. model a deep, fixedbed combustor based on the Whole-Tree Energy concept. The advantages of using whole trees as opposed to doing substantial feed reduction and preparation are discussed. Gas compositions, surface temperatures, and heat-release rates are presented. Bech et al. describe their modeling efforts involving the combustion of straw bales in Danish cigar burners. Straw is an abundant agricultural byproduct in the European Community and there is considerable interest in using straw to generate electricity and heat. The advantages and challenges associated with burning whole straw bales as opposed to further feedstock preparation are discussed. One of the more serious technical challenges facing the increased use of biomass fuels involves the alkali metals that are present in biomass. Alkali metals are essential for plant growth; however, when biomass is thermochemically processed, these alkali metals can volatilize and cause severe fouling and slagging problems. Fouling and slagging in biomass combustors reduce efficiencies for electricity production and, in the worst case, lead to unscheduled downtime. The article by Dayton and Frederick confronts the issue of alkalimetal vapor release during biomass combustion. Black liquor, a residual product of the papermaking process, is the biomass fuel used in the forest product industry to generate process electricity and steam for the papermaking process. The high sodium content of black liquor causes fouling and plugging in recovery boilers. The fate of alkali metals during biomass pyrolysis is discussed by Agblevor and Besler. For pyrolysis oil to be a suitable gas-turbine fuel, the alkali metal content
Dayton and Chum
of the oil must be very low to minimize erosion and corrosion of the turbine blades. Effective hot gas filtration, prior to condensation, appears to be a promising technique for reducing the alkali metal content of biomass-derived pyrolysis oils to suitably low levels. Pressurized pyrolysis and combustion of wheat straw is discussed in the article by Fjellerup et al. [This paper will be published in the next issue.] Bench-scale combustion and pyrolysis experiments reveal the reactivity of wheat straws under various conditions. Organic emissions from biomass combustion are the focus of the article by Hoerning et al. The use of wood residues (such as plywood and particleboard) in combustion facilities is a near-term method for increasing the use of biomass for producing electricity. Unlike the straws, wood and wood wastes can have a very low alkali-metal content, which makes them attractive fuels for use in combustion applications including cofiring with coal in power plants. As the use of wood wastes increases, the emissions from the combustion of such processed wood will need to be addressed. We thank all of the participants who contributed to the Biomass Fuels symposium and the editor, John Larsen, for making this special section possible. The articles in this special section represent a fourth of those presented during the symposium. The issues involving biomass gasification have not been addressed, in particular hot gas conditioning of syngas and integrated gasification combined cycle systems. Some more applied aspects of these areas were presented at the Second Biomass Conference of the Americas: Energy, Environment, Agriculture and Industry (Portland, Oregon, August, 1995, Proceedings NREL/CP-200-8093; DE 9500 9230). We also encourage all of the readers who are interested in the increased sustainable utilization of biomass fuels to contact us (David C. Dayton: (303) 384-6216, FAX (303) 384-6103; e-mail daytond@ tcplink.nrel.gov. Helena Li Chum: (303) 275-2949, FAX (303) 275-2905; e-mail
[email protected]) or any of the authors who contributed to the special section. An open dialogue about the issues and challenges of increasing the use of biomass fuels will encourage global participation and optimize the use of valuable research resources. EF960008N
