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Energy & Fuels 1999, 13, 146-153
Factors Affecting the H2S Reaction with Noncalcined Limestones and Half-Calcined Dolomites L. F. de Diego, F. Garcı´a-Labiano, and J. Ada´nez* Instituto de Carboquı´mica (CSIC), P.O. Box 589, 50080 Zaragoza, Spain
J. M. Palacios Instituto de Cata´ lisis y Petroleoquı´mica (CSIC), Campus Universidad Auto´ noma, Cantoblanco, 28049 Madrid, Spain Received June 25, 1998. Revised Manuscript Received October 14, 1998
Sulfidation experiments at atmospheric pressure were performed in a thermogravimetric analyzer with three uncalcined limestones and a half-calcined dolomite at temperatures between 600 and 850 °C and with sorbent particle sizes between 0.4 and 1.6 mm. The effects of reaction temperature, sorbent particle size, gas-phase composition, and H2S concentration were analyzed. For the dolomite, the sulfidation rate increased when the reaction temperature increased and when the particle size decreased. For the limestones, the sulfidation rate did not depend on particle size and the initial reaction rates increased with increasing reaction temperature until 750 °C. When the conversion increased, the reaction rate dropped sharply at the highest temperatures, giving a maximum conversion at 650 °C for Sastago and Alborge limestones and at 700 °C for Omyacarb limestone. Morphological studies with SEM-EDX equipment, BET surface areas measurements, and special tests in the TGA were performed to analyze this behavior. In addition, the effect of the gas composition on the sulfidation reaction was also analyzed with the uncalcined limestone and the half-calcined dolomite and different behaviors were detected. For the dolomite the gas composition (CO2, CO, H2, and H2O) had no effect using the same H2S + COS partial pressure; however, for the limestones, the sulfidation rate was affected by the COS concentration.
Introduction One of the most attractive of the clean coal technologies being developed for electric power generation is the integrated gasification combined cycle (IGCC). In these processes, coal is gasified to produce a synthesis gas that is fired in a gas turbine for power generation. The sulfur contained in coal reacts in the reducing atmosphere of a gasifier forming H2S and COS. This gaseous contaminant must be removed prior to combustion of the coal gas, either inside the furnace or in the gas cleanup system, to prevent damage to turbine equipment and infringement of emissions legislation. H2S can be removed by traditional gas absorption processes at low temperature, but Squires et al.1 showed that removing the sulfur and other contaminants from coal gas at high temperature and pressure offers numerous advantages over other gas cleaning methods, mainly higher thermal efficiency and reduced volume of processing vessels. Many sorbents have been developed to be applied for high-temperature coal gas desulfurization, both synthetic regenerable sorbents, which are relatively expensive but have a long lifetime, and natural sorbents, which are cheap but need to be stabilized before deposition. Among the natural sor* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Squires, A. M.; Graff, R. A.; Pell, M. Chem. Eng. Prog. Symp. Ser. 1971, 67 (115), 23-34.
bents, those most often used in industrial coal gasification processes are limestone and dolomite. Limestone is primarily CaCO3, and dolomite is CaCO3-MgCO3. Upon heating, the MgCO3 decomposes into MgO and CO2 and the CaCO3 can decomposes into CaO and CO2 or remain as CaCO3 depending on the partial pressure of CO2 in the system. The partial pressure of CO2 in the coal gas varies with the gasification process employed, and both uncalcined and calcined conditions are possible. The CaO or CaCO3 reacts with H2S to form calcium sulfide (CaS), but MgO does not. Thus, for calcined or uncalcined conditions, the reactions taking place are
CaO(s) + H2S(g) a CaS(s) + H2O(g) CaO‚MgO(s) + H2S(g) a CaS‚MgO(s) + H2O(g) (1) or
CaCO3(s) + H2S(g) a CaS(s) + CO2(g) + H2O(g) CaCO3‚MgO(s) + H2S(g) a CaS‚MgO(s) + CO2(g) + H2O(g) (2) The sulfidation of calcined limestone and fully calcined dolomite (reaction 1) have been widely studied.2 (2) Ada´nez, J.; de Diego, L. F.; Garcı´a-Labiano, F.; Abad, A. Energy Fuels 1998, 12 (3), 617-625.
10.1021/ef980145f CCC: $18.00 © 1999 American Chemical Society Published on Web 11/20/1998
Factors Affecting the H2S Reaction
However, there are only a few studies for CaCO3-H2S reaction in high CO2 partial pressure (reaction 2). Reaction 2 was studied by Ruth et al.3 with calcium carbonate in the form of half-calcined dolomite, CaCO3‚ MgO. They reported that small particles of half-calcined dolomite had a much greater reactivity than fully calcined dolomite (CaO‚MgO) at temperatures up to 800 °C and a higher activation energy, suggesting that the reaction is at least partially controlled by the chemical reaction between CaCO3 and H2S. Borgwardt and Roache4 studied the sulfidation of limestone for particles of different sizes between 1.6 and 100 µm. They found the rate of reaction to be inversely proportional to particle size and to be high initially but to fall off rapidly above 11% conversion for large limestone particles (Dp >15 µm). No completely satisfactory explanation for such poor reactivity has been suggested. Borgwardt and Roache4 proposed that a loss of the initial porosity of CaCO3 due to limestone sintering caused the poor conversion. However, similar experiments made by Fenouil et al.5 showed no significant surface area loss with limestone pellets, eliminating CaCO3 sintering as the cause of poor reactivity of H2S with millimeter-size particles of limestone. They attributed the poor reactivity of the limestone to sintering of the CaS layer around the limestone grains so that the CaS coats the grain with a nonporous, quasi-impermeable layer. There is no agreement about the effect of coal gas composition on the CaCO3-H2S reaction. Both Ruth et al.3 and Borgwardt and Roache4 reported that CO2 and H2O enhanced the sulfidation reaction. However, Ruth et al.3 found that adding hydrogen had no effect, but Borgwardt and Roache4 found it to act as an inhibitor. Illerup et al.6 observed no significant influence of the CO2 and CO partial pressures on the rate of sulfidation and over the sulfur capacity of the sorbent in the range from 2 to 20 vol %. They observed also a slight increase in the final sulfur capacity when H2 content was lowered from 20 to 2 vol %. Lin et al.7 found that both the initial reaction rate and final conversion decreased sharply with increasing H2O partial pressure. Finally, Fenouil et al.5 showed that CaS did not sinter under a N2 or H2 atmosphere, whereas it loses about one-half of its initial surface area at 850 °C in 40 min when CO2 is present in the gas phase. Unfortunately, as indicated by Fenouil et al.,5 most of these research efforts did not take proper account of interations between the gas-phase species and between the gases and solid reagents and products. Coal gas is a multicomponent gas mixture, and the composition of the gas is set by the equilibrium of a number of reactions. At the temperatures at which sorption would be carried out, the most important of these is the watergas-shift reaction (3) Ruth, L. A.; Squires, A. M.; Graff, R. A. Environ. Sci. Technol. 1972, 12 (6), 1009-1014. (4) Borgwardt, R. H.; Roache, N. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 742-748. (5) Fenouil, L. A.; Towler, G. P.; Lynn, S. Ind. Eng. Chem. Res. 1994, 33, 265-272. (6) Illerup, J. B.; Dam-Johansen, K.; Johnsson, J. E. In Gas Cleaning at High Temperatures, 1st ed.; Blackie Academic & Professional: London, 1993; pp 492-509. (7) Lin, S. Y.; Suzuki, T.; Aida, C.; Horio, M. Proceedings of the 13th International Conference on Fluidized Bed Combustion, Orlando, FL, 1995; ASME: Fairfield, NJ, 1995; pp 1043-1048.
Energy & Fuels, Vol. 13, No. 1, 1999 147 Table 1. Chemical Analysis and Physical Characteristics of Sorbents composition (wt %)
Omyacarb limestone
Sa´stago limestone
Alborge limestone
CaCO3 MgCO3 Na2O K2O SiO2 Al2O3 Fe2O3 BET specific surface areaa (m2 g-1) porositya(%)
97.1 0.2 1.1
