Development of a regenerable calcium-based sorbent for hot gas cleanup
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Experiments were conducted to develop a regenerable calcium-based sorbent for hot coal gas desulfurization. Spherical sorbent pellets were fabricated in a novel core-in-shell configuration. All pellets were prepared using a bench-scale pelletizer. In accordance with the core-in-shell concept, a reactive calcium compound (CaO) was surrounded by a strong shell consisting of a composite material. Limestone (CaCO3) and hemihydrate (CaSO4·0.5 H2O) were used as starting materials that were eventually decomposed to produce CaO. Different types of cements and aluminas were investigated as strength enhancing shell materials.;The sorbent pellets were characterized by measuring their compressive strength, surface area, pore volume and porosity. Sectioned pellets were also viewed with an electron microscope. A thermal gravimetric analyzer was used to determine the absorption rate, absorption capacity, optimum service temperature, concentration-initial rate relationship, and the regenerability of several sorbent formulations. Sulfidation runs were performed with 0.55--3.0% H2S in nitrogen at temperatures of 840--1000°C. Regeneration of the spent sorbent was conducted at 1050°C using a cyclic oxidation/reduction scheme.;For all core-in-shell pellets tested, it was found that thicker shells provided greater strength, but reduced the absorption capacity. While Portland cement shells provided enormous strength after curing, most of this strength was lost at high temperature. Of the cement formulations, those containing high calcium aluminate concentrations provided the greatest high temperature strength. However, all cement formulations required a minimum curing time to obtain adequate calcined strength. On the other hand, a sintered alumina-based shell produced pellets that met the strength requirements without the need for the extra curing step.;For most formulations, the initial reaction rate was directly proportional to the H2S concentration, and the optimum service temperature was found to be in the neighborhood of 920°C. Unlike the limestone-based formulations, the hemihydrate-based sorbents showed no loss in sorbent capacity when subjected to repeated sulfidation and regeneration cycles. The kinetics of sulfidation were represented well by a shrinking core reaction model that accounted for the effect of chemical reaction on the surface of the unreacted core.