FREE ALKALI-INDEX FOR OPTIMIZING THE FUEL MIXTURE IN BIOMASS CO-FIRING

Co-firing biomass with fossil fuels provides a cheap and efficient reduction of the CO2 emissions in electricity generation. One of the main challenges in co-firing is the increased fouling and corrosion rates of the heat transfer surfaces when biomass is added to the fuel mixture. Optimization of the fuel mixture from the fouling and corrosion point of view would be an easier task, if some fuel properties would provide correlation with the fouling and corrosion rates. Currently there is no clear understanding of the most useful fuel indices. The fouling indices used in coal firing have proved to be less useful in biomass cofiring. Prediction of the fouling rate from the the (S/Cl) molar ratio or the (S/available alkali) molar ratio have shown some success. However, generally accepted threshold values for these are not available. The newly developed fouling indices in biomass co-firing are basically trying to predict the formation of alkali chlorides on the heat transfer surfaces. Alkali chlorides, especially KCl are believed to be the substances responsible for the high fouling and corrosion rates in biomass combustion and cofiring, whereas the formation of alkali sulfate seems to have a beneficial effect in the minimization of the fouling and corrosion rates. However, the mechanism of the formation of the alkali chlorides and sulfates on the heat transfer surfaces is not known in detail so the threshold values of the indices are based on empirical correlations found to fit the data of a particular experiment. In this paper, a new fouling index, (x·(Na+K)-2·S)/LHV, is presented. Theoretical background of the index is described and examples are given how to use the index in optimization of the fuel mixture in biomass co-firing. Finally, correlation of the index with available literature data is presented. Now the validation of the usefulness of the index requires further experimental work