The rate limiting step in the sulfation of natural limestones during fluidized bed coal combustion

Abstract

A study was performed of the sulfation of naturally occurring limestones under conditions simulating the bubbling fluidized bed combustion of coal. Limestones with impurity contents ranging from 2–18% (by weight) were evaluated with respect to reactivity towards SO 2 at temperatures ranging from 750 to 935°C. It was found that the calcination rates of the sorbents were sufficiently slow so as to influence the ultimate sulfation capacities. For the conditions investigated, the calcination rate dominated the overall reaction rate initially, followed by the sulfation rate once a significant amount of CaO had been generated. Sorbent utilization was improved by increasing the operating temperature and/or lowering the CO 2 concentration in the reaction atmosphere. Higher sorbent utilization was observed for (relatively) impure limestones. The presence of impurities enhanced the rate of calcination and lowered the decomposition temperature of the sorbent, in addition to affecting (both catalytically and non-catalytically) the sulfation reaction. The results of this investigation suggest that the calcination rates of sorbents, as derived from laboratory experiments, can be used to determine suitability for use in fluidized bed combustion applications. The higher the calcination rate, the better is the performance of the limestone.