Interactions between coal and biomass when cofiring

Cofiring biomass with coal is one of the most promising short-term options for reducing the net CO2 emissions from coal-fired power plants. This paper examines the effects of cofiring biomass and coal on ash deposition, corrosion, and NOx emissions under conditions representative of those found in pulverize-coal (pc) boilers. Experiments were conducted with blends of six different fuels-two types of bituminous coal, two types of straw, switchgrass, and wood. For each fuel, reference tests of unblended fuel establish a baseline to examine the results from the cofiring tests. The deposition rates for the blends of fuels considered by this investigation lav between the measured deposition rates of the unblended fuels. Therefore, cofiring can mitigate some df the fouling difficulties associated with high-fouling coals and biofuels. By accounting for factors other than fuel composition, we can improve our predictions of the effects of cofiring on ash deposition rates. For the fuel blends examined here, available alkali is a relatively good predictor for the ash deposition rate. Sulfation of alkali chlorides is the major interaction that occurs within deposits formed while cofiring. Sulfation reduces the chlorine content of the deposits, potentially reducing the corrosion potential of the deposits. Cofiring biomass can decrease NOx emissions relative to firing unblended coal under typical pc-combustion conditions when the biomass fuel contains little nitrogen. There does not appear to be any fundamental synergistic interaction between the coal and biomass that significantly reduces NOx emissions. These measurements demonstrate that cofiring biomass with coal can result in decreased pollutant production, decreased ash deposition, and decreased effective CO2, but judicious choices of fuels and operating conditions are required to accomplish these objectives.