Article pubs.acs.org/EF
CO2 and H2S Adsorption on γ‑Al2O3‑Supported Lanthanum Oxide Benjamin J. Feist and Josephine M. Hill*
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 31, 2015 | doi: 10.1021/acs.energyfuels.5b01294
University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada ABSTRACT: The use of high temperatures (>873 K) for desulfurization of syngas has the potential to significantly increase the efficiency of gasification processes. Rare earth oxides are effective adsorbents at high temperatures but expensive. In this work, lanthanum oxide at low weight loadings (2.5−7.5 wt %) is supported on γ-Al2O3 to more efficiently use lanthanum oxide for hot gas desulfurization. The adsorbents are characterized with CO2 adsorption and tested in a fixed-bed reactor for H2S capture at 873 K over multiple cycles. The interaction between lanthanum oxide and the support varied with loading, and regeneration with humidified argon is better than with dilute oxygen. Although the capacity per gram of lanthanum is higher for supported lanthanum oxide, these adsorbents are not selective for the adsorption of H2S in the presence of CO2, CO, and CH4 likely as a result of site competition with CO2 and H2O. regenerated with gas streams free of H2S.14 Further work in this group demonstrated that lanthana doped with praseodymium sorbents have sulfur capacities exceeding 50 mg of S gsorbent−1 at 1 ppm breakthrough when exposed to a simulated reformate gas stream containing 250 ppm of H2S, 25% CO, 5% CO2, and 5% H2O, with the balance being He. The performance of these sorbents was not affected by intermittent exposure to a simulated combustor exhaust stream containing 90% air, 5% CO2, and 5% H2O.17 Another alternative may be to support the active phase on a less expensive material to improve dispersion, resulting in smaller particles that can be regenerated quickly. Interaction with the support is beneficial if the stability of the active phase is improved but may be detrimental if the adsorptive or catalytic properties of the active phase are hindered. Supported rare earth oxide systems have not been extensively studied in terms of adsorption capacity. La2O3 has been studied on supports, including alumina,18−31 silica,30,32 and zeolites,33 but these studies have focused on the role of La2O3 to prevent sintering of the γ-Al2O3 support,18,19,21,22 to change the acidity of the support,24 or to increase NO reduction.25 A few studies have specifically investigated the change in adsorption properties with La loading. Shen et al. have shown that low-weight loadings (1.4 wt %) of La2O3 on γ-Al2O3 have similar CO2 adsorption characteristics to γ-Al2O3, while high-weight loadings (42 wt %) produced a different broader adsorption energy distribution.24 Bettman et al. found a linear increase in CO2 adsorption capacity with increasing La (i.e., slope) of 0.225 atom of CO2/atom of La at lanthanum loadings of up to 10.5 wt % and 0.04 atom of CO2/atom of La at lanthanum loadings from 10.5 to 37 wt %.20 The authors indicate the formation of a LaAlO3 phase between 10.5 and 37 wt % and have no explanation for the low CO2/La stoichiometry observed below 10.5 wt %.20 Huang et al. found that the adsorption of NO on 10% La2O3/γ-Al2O3 was similar to γAl2O3, while 40% La2O3/γ-Al2O3 was similar to bulk La2O3.28
1. INTRODUCTION Chemical processes convert energy from one form to another. In industries using fossil fuels, solids or viscous liquids are converted into less viscous liquids or gases, which can be used directly or as building blocks for the synthesis of other chemicals. One of the challenges with these conversions is dealing with contaminants in the original feed. For example, in coal gasification, coal can be gasified with steam to yield syngas,1,2 but contaminants, including particulates, hydrogen sulfide, ammonia, chloride, alkali, and heavy metal vapors (i.e., K, As, Se, Sb, Pb, and Hg), must be removed from the syngas prior to its ultimate use.3−5 Hydrogen sulfide, in particular, must be removed to prevent the deactivation of catalysts downstream. Currently, coal gas is cooled below 423 K for liquid-phase scrubbing to remove H2S, reducing the thermal efficiency of the power generation.6 Typical efficiencies of pulverized coal plants are 30−35%. The efficiency can be increased to 43% with integrated gasification combined cycle (IGCC) power generation and up to 47% with the use of hot gas desulfurization.6,7 This increase in efficiency results in lower fuel requirements for the same power output, thus decreasing the emissions of CO2 while maintaining the required level of purity. Many studies have focused on the development of sulfur-tolerant catalysts; however, these catalysts are generally not as active as the conventional catalysts.8−12 The acceptable level of sulfurcontaining compounds varies with the downstream application but is generally below 100 ppm and possibly less than 1 ppm of H2S.13 Rare earth oxides have been studied for hot gas cleanup.14−17 In particular, unsupported lanthanum oxide (La2O3) has been shown to reduce gas-phase hydrogen sulfide concentrations to sub-parts per million (ppm) levels at temperatures of 1073 K.14,17 Bulk sulfidation of the lanthanum oxide particles can occur during adsorption, making it difficult to regenerate lanthanum oxide. As a solution, Flytzani-Stephanopoulos et al. developed unsupported lanthanum oxide sorbents that were converted to oxysulfides, which had capacity for H2S adsorption. High space velocities were used for quick sulfidation/regeneration cycles. The sorbents could be © XXXX American Chemical Society
Received: June 10, 2015 Revised: August 26, 2015
A
DOI: 10.1021/acs.energyfuels.5b01294 Energy Fuels XXXX, XXX, XXX−XXX
Article
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 31, 2015 | doi: 10.1021/acs.energyfuels.5b01294
Energy & Fuels
adsorbed species. The samples were then cooled to the desired temperature and exposed to CO2 (40 sccm) for 1 h, followed by a N2 purge (40 sccm) for 1 h to remove weakly adsorbed CO2. N2 adsorption at 77 K was performed on a Micromeritics Tristar 3000 instrument, and the adsorption data were analyzed using the Brunauer−Emmett−Teller (BET) equation to determine the surface area. The samples were degassed for 3 h at 573 K under vacuum prior to the adsorption. Each measurement was performed 3 times to obtain an estimate of the measurement error. Volumetric chemisorption was performed with a Quantachrome Autosorb 1-C instrument at temperatures between 473 and 673 K, at 25 K intervals, using CO2. Each sample was pretreated by heating to 1023 K under He at 30 sccm for 1 h and then held under a vacuum of
