Reactivity of calcium sulfate from FBC systems

A relative estimate of the reactivity of calcium sulfate in a number of coal combustion ash samples was obtained, using the rate of solution in water as a parameter. Measurements were also performed on standard samples of calcium sulfate prepared in different ways, for comparison. The temperature of previous treatment appeared as the most important factor determining the reactivity of CaSO4; the grain size distribution was less important, and the duration of heating (even to 105 days) had very little influence. No correlation between specific surface of ash samples and their reactivity was apparent. Calcium sulfate in FBC ash samples was much more reactive than that contained in high-temperature ashes, and than calcium sulfate heated, alone or with various additions, at 850 degrees C for 2 days. Of the six FBC samples tested, five showed similar behaviour, including a sample from a pressurized system; only a deposit from 96 days operation of an industrial CFBC boiler burning petroleum coke showed considerably less reactivity. Surprisingly, CaSO4 from two FBC samples placed in an oven for 60 days under sulfating conditions showed a very similar rate of solution to that of the other FBC samples, while a third sample kept in the oven for 105 days also showed no decrease in reactivity. Only when one of these samples had agglomerated (which occurred between 60 and 105 days) did it show decreased reactivity, suggesting that the agglomeration process rather than duration is significant in promoting sintering and reducing the sulfate reactivity. (C) 1997 Elsevier Science Ltd.