Karin Laursen

Sulfation and reactivation characteristics of nine limestones

The sulfation and steam hydration reactivation characteristics of nine limestones were evaluated based on laboratory sulfation and hydration tests and scanning electron microscope analyses. The calcium utilization and the sulfation pattern of the limestones were found to depend on the morphology and microstructure of the calcined limestones. Three sulfation patterns were observed in the limestones: unreacted core, uniformly sulfated and network. The steam hydration behavior of the sulfated limestones was dependent on the sulfation pattern. Unreacted core particles appeared to be easier to reactivate, thereby giving higher overall calcium utilization, compared to network-sulfated particles. Uniformly sulfated particles did not react upon hydration and could not be reactivated with steam at 250 or 450°C.

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.

Crystallization and fracture: formation of product layers in sulfation of calcined limestone

Abstract Sulfation experiments were carried out and product layers were characterized to investigate the reaction kinetics and to test a Crystallization and Fracture (CF) model describing the mechanism of formation of product layers. Sintered samples of calcined limestone were sulfated in a quartz reactor for short and extended times in order to obtain initial and fully developed product layers. The product layers were characterized by means of Scanning Electron Microscopy (SEM) visualization and measurements of BET surface area and pore size distribution. At lower temperatures, the sulfation reaction ceased after a relatively short time, whereas at higher temperatures, the reaction continued at an approximately constant rate for as long as 30 h. The reaction product was formed by crystallization. The product “layer” formed in the early stages of the reaction was not a true layer, but isolated nuclei and crystals. The “continuous” product layer formed in the later stages of the reaction was found to be a monolayer of individual crystals with pores of size 20–30 A along the boundaries. The product layer was more porous when developed from larger stable nuclei formed during the initial reaction at higher temperatures and lower SO 2 concentrations. These observations support the mechanism described by the CF model.