CaSO4 and CaS are the only sulphur compounds found in solid residues from the sulphation process in FBC boilers. Early results indicated very high levels of CaS in ashes from small-scale FBC pilot plant combustors. However. these results were due to the presence of high levels of char carbon in standpipes and cyclones. In commercial-scale FBC boilers, CaS concentrations are normally less than 0.1% although levels of up to 0.6% are seen in some ashes. Sometimes elevated CaS concentrations are found in deposits in CFBC boilers firing high levels of petroleum coke (although this is unusual), and in ashes from stripper coolers. In both cases. this can be minimized by proper design. CaS can also be deliberately introduced into FBC boilers with the char from a topping cycle gasifier. Test work using hydration in a similar manner to a reactivation strategy has been shown to be ineffective for sulphide destruction when used directly with char from a topping cycle gasifier. Instead it is recommended that reactivation be used on ashes from the CFBC boiler receiving the sulphided char residues. This guarantees overall high sorbent utilization and the effective elimination of sulphides in the ashes finally discharged from the CFBC boiler. (c) 2006 Elsevier Ltd. All rights reserved.