Characterization of steam reactivation mechanisms in limestones and spent calcium sorbents

Three limestones and three spent calcium sorbents with differing sulfation patterns were subjected to sulfation and steam hydration at 250 and 450°C in an effort to identify the optimum conditions for increasing their ability to capture sulfur. Visual and chemical investigations of the hydrated and resulfated samples using a scanning electron microscope indicate that the sulfation pattern controls whether or not a limestone can be reactivated significantly using steam. The increasing calcium utilization with decreasing hydration temperature results from fracturing during both hydration and de-hydration. Spent sorbents were harder to reactivate than pure limestones. Only low temperature (250°C) steam hydration led to an increase in utilization for two of the spent sorbents. Problems reactivating the spent sorbent may be related to the higher temperatures encountered in the boilers in which they were produced compared to the limestones prepared in the laboratory. The sulfation ‘history’ of the spent sorbents, as well as reactions between the limestone and the coal (both the organic and inorganic fractions), may also contribute in reducing the response to hydration of spent sorbents.