Biomass cofiring: the technology, the experience, the combustion consequences

Cofiring, the practice of supplementing a base fuel with a dissimilar fuel, is an extension of fuel blending practices common to the solid fuels community. Recently, there has been considerable emphasis on cofiring biomass opportunity fuels with coal in pulverized coal (PC) and cyclone boilers owned and operated by electricity generating utilities in order to address such issues as potential portfolio standards, voluntary actions to reduce fossil CO2 emissions, customer service, and the generation of green power within the context of deregulation. Biomass fuels considered for cofiring include wood waste, short rotation woody crops, short rotation herbaceous crops (e.g., switchgrass), alfalfa stems, various types of manure, landfill gas and wastewater treatment gas. Of these, the solid biofuels such as sawdust, urban wood waste and switchgrass have received the most attention. The Electric Power Research Institute (EPRI), along with the Tennessee Valley Authority (TVA), GPU Genco, Northern Indiana Public Service Company (NIPSCO), Central and South West Utilities (C&SW), Southern Company, Madison Gas & Electric (MG&E), New York State Electric and Gas (NYSEG) and others developed a concerted effort to commercialize direct combustion cofiring of biomass with coal. The USDOE joined the program with a cooperative agreement between the Federal Energy Technology Center (FETC) - now the National Energy Technology Laboratory (NETL) and EPRI; this agreement was extensively supported by the energy efficiency and renewable energy (EERE) element of USDOE. European cofiring programs also have been extensive and include gasification-based cofiring in Lahti, Finland and straw cofiring in Denmark. Three general techniques comprise the cofiring technology family: blending the biomass and coal in the fuel handling system and feeding that blend to the boiler; preparing the biomass fuel separately from coal, and injecting it into the boiler without impacting the conventional coal delivery system; and gasifying the biomass with subsequent combustion of the producer gas in either a boiler or a combined cycle combustion turbine (CCCT) generating plant. Commercialization has proceeded on the direct combustion approaches to cofiring, beginning with engineering and economic studies, parametric testing and the construction of demonstration projects. The direct combustion cofiring techniques are now ready for commercial deployment. This paper reviews the key projects, and details some of the influences of cofiring on the combustion process. (C) 2000 Elsevier Science Ltd. All rights reserved.