Sulfur emission control in fossil fuel gasification plants implies the removal of H2S from the product gas either inside the furnace or in the gas clean-up system. In a fluidized-bed gasifier, in-bed sulfur capture can be accomplished by adding a calcium-based sorbent such as limestone or dolomite to the bed and removing the sulfur from the system with the bottom ash in the form of CaS. This work describes the H2S uptake by a set of physically and chemically different limestones and dolomites under pressurized conditions, typically for those in a pressurized fluidized-bed gasifier (2 MPa, 950 °C). The tests were done with a pressurized thermobalance at two p CO 2 levels. Thus, the sulfidation of both calcined and uncalcined sorbents could be analyzed. The effect of p H 2 S was also investigated for uncalcined limestones and half-calcined dolomites. The results are presented as conversion of CaCO3 or CaO to CaS vs time plots. The results are also compared with the sulfur capture performance of the same sorbents under pressurized combustion conditions.

The physical structure of a limestone or dolomite to be used in in-bed sulfur capture in fluidized bed gasifiers has a great impact on the efficiency of sulfur capture and sorbent use. In this study an unreacted shrinking core model with variable effective diffusivity is applied to sulfidation test data from a pressurized thermogravimetric apparatus (P-TGA) for a set of physically and chemically different limestone and dolomite samples. The particle size was 250−300 μm for all sorbents, which were characterized by chemical composition analysis, particle density measurement, mercury porosimetry, and BET internal surface measurement. Tests were done under typical conditions for a pressurized fluidized-bed gasifier, i.e., 20% CO2, 950 °C, 20 bar. At these conditions the limestone remains uncalcined, while the dolomite is half-calcined. Additional tests were done at low CO2 partial pressures, yielding calcined limestone and fully calcined dolomite. The generalized model allows for determination of values for the initial reaction rate and product layer diffusivity.

Regenerable mixed metal oxide sorbents are the prime candidates for the removal of hydrogen sulfide (the main pollutant) from the hot coal gas in the simplified integrated gasification combined cycle (IGCC) processes. As part of the sulfur removal process development, Carbona Corp. is developing fluidized bed reactor models for scale-up. It is essential for this work to apply a reliable and simple correlation for the conversion rate of zinc titanate or hydrogen sulfide in the sulfidation reaction. Two different models, the unreacted shrinking core (USC) model and overlapping grain (OG) model, are applied to this purpose. The parameter values obtained from ambient pressure tests are compared with those reported earlier for zinc titanates. Potential reasons for the differences are discussed. In the modeling of high-pressure sulfidation data, reaction rate constants from the literature are used, thus leaving the product layer diffusion coefficient as a fitting parameter. The values obtained indicate a linear dependence on process pressure.

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High-temperature high-pressure sulfur removal is considered to be one of the key steps in the hot gas cleanup train of an integrated gasification combined cycle (IGCC) process. Regenerable mixed metal oxide sorbents such as zinc titanates are prime candidates for this purpose. As part of sulfur removal process development, Carbona Corp. is developing fluidized bed reactor models for scale-up. In the first part of this work, the parameters for the unreacted shrinking core and overlapping grain models were determined to describe the rate of zinc titanate in H2S capture. A method using these sorbent particle conversion models for fluidized bed application is presented in this paper. Both models show a reasonably good fit to the high-pressure fluidized bed sulfidation test data, with the same order of magnitude parameters as obtained in the previously reported solid conversion rate tests.