Samples of calcined limestone particles having a diameter of about 1 mm were exposed to simulated coal gases containing between 500 and 18 000 ppm H2S for temperatures ranging from 560 to 1100 °C in a differential tube reactor. The formation of CaS was followed quantitatively as well as qualitatively to elucidate the reaction mechanism. Contrary to the limited conversion of CaCOs to CaS, it was found that the limestone particles could be completely converted to CaS by 1% H2S in about 1 h if the particles are precalcined or if the rate of calcination is higher than the rate of sulfidation. The reaction then takes place between CaO and H2S and follows a shrinking-core mechanism. The reaction kinetics is controlled by the diffusion of H2S through the pores of the CaS product layer formed around the lime particle (effective diffusivity between 2.8 x 10-6 and 5.1 x 10-6 m2/s). The kinetics of the sorption of H2S by CaO is relatively insensitive to the reaction temperature, and the reaction rate does not decrease significantly when the CaO is severely sintered for several hours at 1050 °C prior to sulfidation.

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H2S sorption by 18-35 mesh particles (average mass radius of 0.40 mm) of three different calcium-based sorbents (limestone, CaC03; dolomitic limestone, dolomite, MgC03-CaC03) was tested under simulated coal gas in a differential tube reactor.Two fundamentally different behaviors were observed. Above the calcination temperature of CaC03, complete conversion of CaC03 to C a s can be achieved with all three sorbents; the reaction rate increases as the magnesium-to-calcium ratio increases in the sorbent and the reaction rate is controlled by the diffusion of H2S through the Cas product layer and by the kinetics of the calcination of CaC03 to CaO. However, below the calcination temperature of CaC03 (about 900 "C under 1 bar of C o d , less than 20% of the CaC03 in limestone can be converted to C a s compared t o 100% in dolomite. For the dolomitic limestone, all the calcium atoms associated with the dolomite regions can be converted to C a s whereas only 20% of those associated with the limestone regions can be converted. Above 710 "C, the sulfidation rate of dolomite and dolomitic limestone is controlled by the diffusion of H2S through the product layer. Below 710 "C, the kinetics of calcination of MgC03 as well as the rate of the chemical reaction between CaC03 and H2S become the limiting steps in the overall reaction kinetics.

The calcium carbonate contained in limestone becomes thermodynamically capable of sorbing hydrogen sulfide from high-pressure coal gas at temperatures above 600 °C, typically well below the calcination temperature. Limestone can be used more effectively as a sorbent for hydrogen sulfide in high-temperature gas-cleaning applications if it is prevented from undergoing calcination since calcium oxide may sinter rapidly. For large (millimeter) sized particles typically used in gas cleaning, sintering of uncalcined limestone was found to be insignificant in the temperature range 750-900 °C. Poor conversion of the solid upon reaction with H2S is caused by sintering of the CaS product layer, which can be seen from scanning electron microscopy photographs. Sintering of CaS is rapid in an atmosphere that contains C02, but is slow under N2 or H2. The kinetics of CaS sintering under C02 was determined for the temperature range 750-900 °C.

Concentration profiles in several configurations of sorption systems (moving, packed, and fluidized beds and entrained-flow) were determined using the grain and the unreacted shrinking core model for the description of the gas−solid reaction kinetics. General equations for the design and analysis of all these reactor configurations are given, and simple analytical solutions are proposed. The behavior of these reactors can be expressed as a function of only four parameters:  the size of the sorbent pellet and the Péclet, Sherwood (or Biot), and Damköhler numbers. These general equations were then applied to the lime(stone)/H2S systems. Using limestone particles with diameters on the order of 100 μm for entrained-flow and with diameters on the order of 1 mm for the other configurations, it was found that H2S could be removed from the hot coal gas to near its equilibrium value with more than 75% sorbent utilization and with reasonable bed depth (about 1 m, or 1-s contact time for entrained-flow) when the reactor temperature is maintained 25−50 °C above the calcination temperature of the calcium carbonate.