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“Hot Coal Gas” Desulfurization by Perovskite-type Sorbents Vijayanand Rajagopalan and Michael D. Amiridis* Department of Chemical Engineering, The University of South Carolina, Columbia, South Carolina 29208
We have evaluated the performance of perovskite-based sorbents for hot coal gas desulfurization. These sorbents (LaMnO3, LaCoO3, LaFeO3, and La2CuO4) were tested in a fixed-bed reactor and characterized by X-ray diffraction studies and BET surface area measurements. The results indicate a high initial sulfur removal capacity for the perovskites (0.15-0.34 g of H2S/g of sorbent) but poor regenerability. The analysis of spent and regenerated sorbents has revealed complete segregation of the A-site and B-site elements following sulfidation in the case of LaMnO3 and LaCoO3. No such segregation was observed in the case of LaFeO3, while an intermediate behavior was exhibited by La2CuO4. The BET measurements indicate a substantial loss of surface area following the first cycle of sulfidation and regeneration, which is in part responsible for the poor performance of the sorbents in successive cycles. Introduction The integrated gasification combined cycle (IGCC) process is an advanced technology for generating “clean” electricity (i.e., with minimal generation of atmospheric pollutants) from coal. The process uses the “hot coal” gas produced by the gasification of coal (gas-solid reaction of coal with steam and either air or oxygen at high temperature and pressure) in a combined cycle of gas and steam turbines. IGCC power plants generate electricity with higher efficiency than conventional coal fired power plants.1 The coal gas exits the gasifier at temperatures between 500 and 800 °C. Under the highly reducing conditions of the gasifier, the sulfur present in the coal takes the form of hydrogen sulfide (H2S). H2S needs to be removed from the hot coal gas to protect the equipment in the later stages of the process from its corrosive effects and to meet the strict government regulations for sulfur emissions. Low-temperature scrubbing processes are not appropriate for the desulfurization of hot coal gas, since the required cooling results in a huge loss in thermodynamic efficiency. Alternatively, regenerable sorbents operating at high temperatures represent the preferred mode of desulfurization. Many transition metal oxide-based sorbents have been developed and extensively studied in the past for this application. Zinc,2-5 copper,6-8 iron,9-11 and manganese12-14 containing sorbents are the ones that have attracted most attention. The performance of several of these oxides is limited by thermal instability at the higher end of the temperature range of interest, attributed to either chemical or mechanical degradation.5 Among them, the most promising are copper oxides because of their high sulfidation equilibrium constants8 and manganese oxides because of their reactivity at high temperatures with H2S.14 This study focuses on the potential utilization of perovskite-based sorbents for hot coal gas desulfurization. Perovskites are mixed metal oxides with the general formula ABO3, where A is a rare-earth, alkalineearth, alkali, or other large cation that can occupy the dodecahedral sites of the framework and B is a 3d, 4d, * To whom correspondence should be addressed. Phone: 803-777-7294. Fax: 803-777-8265. E-mail: amiridis@ engr.sc.edu.
or 5d transition metal cation that can occupy the octahedral sites of the framework. Most of the metallic elements in the periodic table can be accommodated in the perovskite structure. Perovskites can also tolerate partial substitution and nonstoichiometry while still maintaining their structure. At the same time they are known to exhibit oxygen mobility. All of these properties were expected to have a favorable influence on the reactivity of these oxides with H2S. Finally, since they are synthesized at high temperatures, perovskites were also expected to be structurally stable at the conditions required for hot coal gas desulfurization. In this paper we report on the desulfurization capacity and regeneration characteristics of four different types of perovskite-based sorbents (i.e. LaMnO3, LaCoO3, LaFeO3, and La2CuO4). Desulfurization and regeneration studies were conducted in a fixed-bed reactor, and BET measurements and X-ray diffraction (XRD) studies were used for the analysis of fresh, spent, and regenerated sorbents. Experimental Approach Materials. The sorbents used in this work were synthesized via the amorphous citrate precursor method.15 In brief, appropriate amounts of aqueous solutions of the nitrates of the constituent metals of the perovskites to be synthesized were mixed with an aqueous solution of citric acid, so that the molar ratio of citric acid to total metal cations was equal to one. The resulting solution was stirred and heated to 80 °C for about 2 h to obtain a sol, which was dried overnight in a vacuum oven at 80 °C. Following calcination of the amorphous precursors thus obtained at 650 °C, XRD patterns revealed the presence of a typical perovskite phase in the cases of LaCoO3, LaFeO3, and LaMnO3 and a K2NiF4-type structure (a structure consisting of alternating layers of LaCuO3 perovskite slabs and LaO rock-salt structures) in the case of La2CuO4. This material also contained an amount of XRD-amorphous CuO (on the order of 15 wt %), since our original intention was to synthesize LaCuO3, and hence, an excess of Cu was used. A “benchmark” sorbent, 10% CuO/Al2O3, was also prepared by the same method.
10.1021/ie9902986 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/03/1999
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Figure 1. Breakthrough curves for the fresh sorbents: (b) LaCoO3; (O) LaMnO3; (4) La2CuO4; (0) LaFeO3; (9) 10% CuO/ Al2O3.
Figure 2. Comparison of breakthrough curves after regeneration at 700 °C: (b) LaCoO3; (O) LaMnO3; (4) La2CuO4; (0) LaFeO3; (9) 10% CuO/Al2O3.
Table 1. Sulfur Removal Capacity per Gram of Sorbent for the Different Perovskites in Cycle 1
Table 2. Crystalline Phases Present in the Sulfided and Regenerated Samples of LaCoO3
sorbent
sulfur removal capacity (g H2S/g of sorbent)
10% CuO/Al2O3 LaCoO3 LaMnO3 LaFeO3 La2CuO4
0.03 0.33 0.34 0.15 0.20
Reactor Studies. Desulfurization and regeneration studies were carried out in a fixed bed quartz reactor placed inside an electric furnace. The sorbent bed was supported by a fritted quartz disk at the bottom and a layer of quartz wool at the top. Glass-lined stainless steel tubing was used to construct the inlet and outlet lines of the reactor to avoid reaction of sulfur compounds with stainless steel. A filter was placed near the exit of the reactor to prevent plugging of the downstream line due to condensation of elemental sulfur. Blank breakthrough runs were conducted at the reaction conditions and confirmed that no adsorption/reaction of H2S was taking place anywhere along the lines. The temperature was monitored by a K-type thermocouple clamped to the reactor so that its tip was exactly at the top of the sorbent bed and controlled by an Omega CN 76000 temperature controller. A six-port sampling valve with a 5-mL sampling loop (VALCO) was used to sample the inlet and outlet gases for analysis purposes. Analysis was conducted on line with a gas chromatograph (Buck scientific Model 910; packed silica gel column at 100 °C) equipped with a thermal conductivity detector. Peaksimple software (SRI Inc.) was used to analyze the detector signal and control the conditions in the gas chromatographer. The amounts of the sorbents examined in this study were calculated using 1 g of 10% CuO-Al2O3 as the basis and maintaining the moles of the transition metal constant in all cases. Prior to their use, sorbents were pressed into a pellet and then crushed and sieved to obtain a particle size of 100-120 mesh. Before the first desulfurization run, the sorbent bed was treated for 45 min at 500 °C with a 10% O2 in N2 mixture, to ensure that the sample was fully oxidized and remove any adsorbed impurities from its surfaces. A 3000 ppmv H2S in N2 mixture (250 cm3/min), prepared by dilution of an 1% H2S in N2 certified gas mixture (HOLOX Inc.), was then introduced to the sample. At this point periodic sampling of the outlet gas (every 6 min) was initiated. The desulfurization run was considered complete when
temp (°C) sulfidation regeneration 500
not done
500
500
500
700
500
800
state of sample
crystalline phases
sulfided (1 cycle)
La2O2S (major phase) La2O2SO4 CoO regenerated La2O2SO4 (major phase) (1 cycle) La2(SO4)3 La2O2S Co3O4 regenerated La2O2SO4 (major phase) (1 cycle) La2O2S Co3O4 regenerated La2O2SO4 (major phase) (1 cycle) Co3O4
a constant value of H2S concentration, approximately equal to the inlet concentration, was obtained over five successive samplings. The reactor was then flushed with nitrogen for approximately 15 min and heated to the appropriate regeneration temperature. Regeneration was conducted for 2 h with a 79% N2/21% O2 mixture (250 cm3/min). The same procedure was repeated several times to obtain breakthrough curves over a few cycles. Characterization. BET surface area measurements were performed with a Micromeritics Pulse Chemisorb 2700 analyzer. X-ray diffraction patterns were obtained with a Rigaku D\Max-2200 powder diffractometer using a Bragg-Brentano geometry, with Cu KR radiation (λ ) 1.54 Å). The patterns were analyzed with the JADE (for windows) XRD pattern-processing software system. Results and Discussion Fixed-Bed Reactor Studies. The breakthrough curves for the fresh sorbents examined in this work are shown in Figure 1. The breakthrough times (defined as the point in time when a substantial amount of H2S is first observed in the outlet stream) for LaCoO3, LaMnO3, La2CuO4, LaFeO3, and CuO/Al2O3 were 74, 68, 38, 8, and 8 min, respectively. The sulfur removal capacities of the different sorbents were calculated from the results of Figure 1 by numerically integrating the area above the breakthrough curve. These capacities normalized per gram of sorbent are shown in Table 1. On the basis of several repeat experiments, we have established that the obtained
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Figure 3. Overall hydrogen sulfide removal capacity per gram of the sorbent over several cycles: (b) LaCoO3; (O) LaMnO3; (4) La2CuO4; (0) LaFeO3; (9) 10% CuO/Al2O3.
sulfur removal capacities are reproducible within (0.02 g of H2S per gram of sorbent for the perovskites. A higher accuracy is associated with the calculations for the CuO-Al2O3 sorbent (
