Co-Firing in FBC: A Challenge for Fuel Characterization and Modeling

Co-combustion of coal with biomass or firing biomass alone is used more and more in a first step in meeting the Finnish commitments under the Kyoto protocol. A frequently used technique for firing mixtures of fuels is fluidized bed combustion (FBC). Firing coal, co-combustion with biomass or firing biomass alone may, however, lead to unwanted ash-related problems. Prediction of ash formation behavior can help to avoid these problems before taking new fuels into use. Standard fuel analyses have shown to provide insufficient information for proper prediction especially when considering fuel mixtures. In an attempt to minimize the number of lab scale and pilot scale combustion experiments an extensive database is under development. This database contains data used as input for prediction models such as standard fuel analyses, results from stepwise leaching experiments, SEM/EDS analyses of original and partly burned-out fuels and thermodynamic estimations of the melting behavior of the fuels. Today the database contains 51 fuels, i.e. 8 bark fuels, 10 wood fuels, 3 annual biomasses, 8 peats, 6 coals and 16 miscellaneous fuels, such as RDF, sludge, hulls and husks, bagasse and other residues. Standardized fuel analysis is available for all fuels; melting calculations have been carried out for some 33 fuels. SEM/EDS analysis has been carried out for 20 fuels. The extended utilization of these data with computational fluid dynamic modeling (CFD) has proven to be a useful tool in prediction of deposits in FBC boilers. An example of the prediction tool shows the ability of deposit formation prediction.