For over 20 years, it has been known that there is a sulfur capture maximum around 850°C for limestone sorbents in FBC systems, albeit that this maximum appears to depend both on the characteristics of the FBC unit and the sorbent itself. Numerous explanations have been given for the temperature maximum at higher temperatures, including reducing reaction of CaSO4 with CO, sintering of sorbent particles which results in lower porosity and surface area. Other explanations include equilibrium between SO2 and SO3 with higher temperatures reducing the availability of SO3 for reaction with CaO, blocked sorbent pores at higher temperatures, and depletion of oxygen in the dense phase of the bed at higher temperatures. The most plausible explanation is that the temperature maximum results from a competition between sulfation and reduction of the sorbent, with reduction becoming more important at higher temperatures. Clear elucidation of the factors that affect the temperature dependence and an explicit relationship for the temperature maximum ought to permit improvements in sulfur capture efficiency to be achieved. Currently, no explicit relation appears to exist, and hence we provide an analysis for the temperature maximum based on the competition between sulfation and reduction, and derive an expression for the sulfur capture efficiency as a function of gas composition, sorbent residence time and apparent reaction rate coefficients, all of which are dependent on temperature. The expression relates operation conditions and sorbent activity to the temperature maximum, and may serve as a stepping-stone for future studies in this area.

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