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Energy & Fuels 2000, 14, 1296-1303
Performance of Mn and Cu Mixed Oxides as Regenerable Sorbents for Hot Coal Gas Desulfurization E. Garcı´a,† J. M. Palacios,*,‡ L. Alonso,‡ and R. Moliner† Instituto de Carboquı´mica, CSIC, Poeta Luciano Gracia 5, 50015-Zaragoza, Spain, and Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Campus Universidad Auto´ noma, Cantoblanco, 28049-Madrid, Spain Received May 31, 2000. Revised Manuscript Received July 25, 2000
Mixed oxides, prepared by calcination at 950 °C for 6 h of powder mixtures of manganese and copper oxides, have been studied as regenerable sorbents for hot coal gas desulfurization. For the stabilization of copper in the oxidation states 2+ or 1+ under strong reducing conditions of coal gas, different concentrations of component oxides have been used enhancing the formation of different mixed oxides. Copper was not stabilized by manganese oxides, but its presence in the fresh sorbent was completely necessary because it increases the sorbent reactivity and keeps the H2S concentration in the outlet gas from a reactor below 50 ppmv. Thermogravimetric experiments and performance tests in a fixed bed reactor have allowed the optimization of the operating conditions for a sorbent MC (1:0.1) showing a good performance in multicycle tests without apparent decay.
Introduction Many systems for clean and efficient electric power generation from coal gas, such as IGCC or MCFC, are demanding desulfurization processes taking place at high temperatures (>500 °C) achieving sub-ppm H2S levels. Some mixed oxides based on Zn, Fe, and/or Ti, have shown good short-term performances in different types of reactors. However, their use is only economically feasible if, at least, they are able to withstand 100 successive sulfidation-regeneration cycles without apparent performance decay. Unfortunately, in long-term tests these sorbents show a progressive efficiency loss at breakthrough and high degradation of their mechanical properties as the number of cycles increases. Recent studies have been drawn to the study of other more efficient inorganic oxides. As compared with ZnO, manganese oxides exhibit not so favorable sulfidation thermodynamics; consequently, the expected equilibrium H2S concentration in the outlet gas from reactor is comparatively higher.1 However, some studies have shown that the rate of sulfidation of manganese-based sorbents was substantially higher than that exhibited by these more conventional ZnObased sorbents. Because of this high reactivity, a high efficiency at breakthrough could be expected although the use of these manganese-based sorbents should, probably, be restricted to applications in which high levels of gas desulfurization were not required (100200 ppmv). * To whom all correspondence should be addressed: Tel: 34 91 585 47 87. Fax: 34 91 585 47 60. E-mail:
[email protected]. † Instituto de Carboquı´mica, CSIC. ‡ Instituto de Cata ´ lisis y Petroleoquı´mica, CSIC. (1) Ben-Slimane, B.; Hepworth, M. T. Energy Fuels 1994, 8, 11751183.
Temperature is a limiting factor for the use of zincbased sorbents, during both sulfidation and regeneration. During sulfidation, above 650 °C, zinc oxide is reduced to metallic zinc and, subsequently, partially lost by evaporation. Regeneration, due to the stability of zinc sulfate, has to be undertaken at temperatures higher than 720 °C that, presumably, leads to excessive thermal sintering. Using zinc-based sorbents, sulfidation and regeneration have to be carried out at different temperatures that bring some additional technological problems and loss of thermal efficiency of the overall process. In the case of manganese-based sorbents, MnO is the stable phase in sulfidation in all range of temperatures and manganese sulfate is stable below 800 °C during the regeneration process. Using this type of sorbents, sulfidation and regeneration must be carried out at very high temperatures. At these operating conditions, some thermogravimetric studies have shown that the sulfidation reactivity is high enough in the temperature range of 800-900 °C. Thermodynamic calculations reveal that coal gas desulfurization levels down to 150 ppmv of H2S could be achieved,1 being below the levels set up by environmental legislations. Additionally, because the sulfidation reaction is not so right-hand shifted as in the zinc oxide case, a study has shown the possibility of sorbent regeneration by steam in the absence of oxygen. At these conditions, the formation of manganese sulfate and the occurrence of thermal spots, derived of the great exothermiticity of the oxidative regeneration, were prevented. Unfortunately, steam regeneration cannot replace completely the oxidative process because it is too slow and usually incomplete.2 (2) Ataku¨l, H.; Wakker, J. P.; Garritsen, A. W.; van den Berg, P. J. Fuel 1996, 75, 373-378.
10.1021/ef0001179 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/14/2000
Sorbents for Hot Coal Gas Desulfurization
In oxidative regeneration, thermobalance studies show that manganese sulfate becomes unstable above 800 °C. Despite that, surprisingly, sorbent reactivity increased in 5-cycles tests as the number of cycles increased, a feature that authors correlated with the development of a crack network that could facilitate reactant diffusion not affecting the mechanical strength of the sorbent.3 Tests for 15-cycles carried out in a fixed bed reactor at 900 °C, using for sulfidation a gas simulating a Tampella-U fuel gas and air for regeneration, showed prebreakthrough H2S levels of 200 ppmv with no decay of sulfur capacity and mechanical strength of the sorbent. Consequently, manganese-based sorbents can be considered high-temperature regenerable sorbents allowing relatively low levels of gas desulfurization. On the other side, copper-based sorbents have the most favorable thermodynamics allowing gas desulfurization down to sub-ppm levels. However, under the strong reducing power of coal gas they are reduced to metallic copper whose sulfidation thermodynamics is not so favorable.4 These sorbents are easily regenerated in oxidant atmosphere because copper sulfate becomes unstable at the usual high temperatures. Many efforts have been drawn to achieve copper stabilization in oxidation states 2+ or 1+, through the formation of different mixed oxides with Fe, Al, Ti, Cr, or Ce.5-8 Taking advantage of the good reactivity and stability of manganese oxide and the favorable thermodynamics exhibited by copper oxide, mixed oxides could be prepared by calcination at high temperature showing, presumably, improved performance as regenerable sorbents. In fact, a first study9 showed that these sorbents exhibited high reactivity and complete regeneration, depending on the concentration of the component oxides and the operating temperature. In general terms, copper-rich sorbents showed the best performance at low operating temperature while manganese-rich sorbents show the best one at a higher operating temperature. Unfortunately, lacking the necessary characterization studies of sorbents no clear understanding of the exhibited behavior could be achieved. Now, a detailed study of characterization of the fresh, reduced, sulfided, and regenerated sorbents by using different physical techniques has been undertaken.10 Taking into account these characterization results, the aim of the paper is the study of the reactivity and performance in multicycle sulfidation-regeneration tests, of manganese-based sorbents prepared by using different concentrations of copper oxides, to optimize the performance in a fixed (3) Ben-Slimane, R.; Hepworth, M. T. Energy Fuels 1994, 8, 11841191. (4) Tamhankar, S. S.; Bagajewicz, M.; Gavalas, G. R.; Sharma, P. K.; Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 429-437. (5) Ibarra, J. V.; Cilleruelo, C.; Garcı´a, E.; Pineda M.; Palacios, J. M. Vib. Spectrosc. 1998, 16, 1-10. (6) Siriwardane, R. V.; Poston, J. A. Appl. Surf. Sci. 1993, 68, 6580. (7) Pineda, M.; Fierro, J. L. G.; Palacios, J. M.; Cilleruelo, C.; Garcı´a, E.; Ibarra, J. V. Appl. Surf. Sci. 1997, 119, 1-10. (8) Li, Z.; Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Res. 1997, 36, 187-196. (9) Desai, M.; Brown, F.; Cahmberland B.; Jalan, V. Prepr. Paps Am. Chem. Soc., Div. Fuel Chem. 1990, 35, 87-94. (10) Alonso, L.; Palacios, J. M.; Garcı´a, E.; Moliner, R. Fuel Proc. Technol. 2000, 62, 31-44.
Energy & Fuels, Vol. 14, No. 6, 2000 1297 Table 1. Characterization of the Fresh Sorbents sorbent MC(0.13:1) MC(1.04:1) MC(1.6:1) MC(1:0.1)
Vp crush strength (cm3/g) (N/mm) 0.06 0.14 0.16 0.23
26 4 1 1
XRD identification CuO Cu(1+x)Mn(2-x)O4, x ) 0.47 Cu(1+x)Mn(2-x)O4, x ) 0.15 Mn2O3
bed reactor as regenerable sorbents for coal gas desulfurization. Experimental Section Fresh sorbents were prepared from powdered pure oxides of particle size
