A model of limestone attrition and SO2 capture in a large-scale pressurised fluidised bed combustor

This study presents a model of attrition and sulphation of uncalcined limestone. It was applied to analyses of data from the literature of a 71 MWe pressurised fluidised bed combustor (71 MWe PFBC). First, fragmentation of limestone by thermal shock was experimentally evaluated using an atmospheric fluidised bed reactor. The same limestone as that used in the 71 MWe PFBC was employed. Experimental results showed that fragmentation by thermal shock was not important. Therefore, a model to estimate the particle size distribution (PSD) of the bed material without assuming fragmentation is necessary. In the attrition model, different dependency of the rate of change in particle size (dD/dt) on particle size (D) was assumed as: dD/dt=-knDn. Different values of the exponent were examined: n=0,1, and 2. The best fit between calculated PSD and the experimental value was with n=2. Sulfation models with different n were developed based on the shrinking core model. The continuous attrition model and intermittent attrition model were used for the sulphation-attrition model. With increasing n, the value of the interval of attrition that gave good agreement with the experimental results decreased for test series without fly-ash recirculation. For test series with fly-ash recirculation during coal combustion, the continuous attrition model under steady-state sulphation-attrition conditions yielded values that resembled the experimental results. During high-sulphur fuel combustion under fly-ash recirculation conditions, the present model results agreed with experimental values when SO2 capture by the fresh limestone surface was taken into consideration.