ARTICLE pubs.acs.org/EF
High-Temperature Desulfurization of Gasifier Effluents with Rare Earth and Rare Earth/Transition Metal Oxides Kerry M. Dooley,* Vikram Kalakota, and Sumana Adusumilli Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States
bS Supporting Information ABSTRACT: We have improved the application of mixed rare-earth oxides (REOs) as hot gas desulfurization adsorbents by impregnating them on stable high surface area supports and by the inclusion of certain transition metal oxides. We report comparative desulfurization experiments at high temperature (900 K) using a synthetic biomass gasifier effluent containing 0.1 vol % H2S, along with H2, CO2, and water. More complex REO sorbents outperform the simpler CeO2/La2O3 mixtures, in some cases significantly. Supporting REOs on Al2O3 (∼20 wt % REO) or ZrO2 actually increased the sulfur capacities found after several cycles on a total weight basis. Another major increase in sulfur capacity took place when MnOx or FeOx is incorporated. Apparently most of the Mn or Fe is dispersed on or near the surface of the mixed REOs because the capacities with REOs greatly exceeded those of Al2O3-supported MnOx or FeOx alone at these conditions. In contrast, incorporating Cu has little effect on sulfur adsorption capacities. Both the REO and transition metal/REO adsorbents could be regenerated completely using air for at least five repetitive cycles.
1. INTRODUCTION For both coal and biomass gasification, removal of H2S at high temperatures is a critical step toward better process economics and heat integration.1 It is part of the larger problem of syngas cleanup and conditioning, which represents roughly 70% of the conversion and processing costs of, e.g., ethanol from biomass.2 Aside from the problems posed by sulfur to downstream catalysts, there are environmental limits on sulfur discharges; current DOE targets are for less than 50 ppm, with ∼1 can be maintained as a single phase to temperatures greater than 1000 K.15 However, while they can be initially effective as adsorbents for H2S and other sulfur compounds, they still rapidly lose sulfur capacity (>80% after three redox cycles at 923 K).16 This is because the mixtures, while generally less crystallizable than CeO2 itself, still undergo appreciable crystal ripening at high-temperature hydrothermal conditions.9,11,15 r 2011 American Chemical Society
A third oxide (e.g., ZrO2, Tb2O3, Gd2O3, Al2O3) can be added to further stabilize the adsorbent, as these oxides are known to retard CeO2 or La2O3 crystal ripening by limiting cation diffusion.17-27 These oxides can in some instances increase the number of surface oxygen vacancies and so also the oxygen mobility, leading to a more reactive (often more easily reduced) surface-phenomena observed for Tb,21,23,26,28-30 Gd,20,31,32 Zr,25,33-35 and Al.36,37 This third oxide could be present either as a support for the binary REO (ZrO2, Al2O3) or as a dopant in small quantity (Tb2O3, Gd2O3). For such ternary oxides (or binary oxides interacting with a support), the optimal Ce/La ratios may differ significantly from those of the simpler Ce/La oxides. Another popular class of high-temperature sulfur adsorbents, with large total sulfur capacities, is based on iron, copper, or manganese oxides. However, the pure or mixed (sometimes with ZnO) oxides of this type also exhibit poor long-term hydrothermal stability.38,39 It may be possible to combine one of these transition metal oxides with the REOs in order to improve the stability of the former and the total capacity of the latter. The goals of this work were therefore to examine the potential of both ternary and transition metal-doped REOs for cleanup of biomass or coal/biomass gasifier effluents at high-temperature conditions where the sulfidation will take place in the presence of appreciable amounts of CO2 and water. Both CO2 and water can compete with sulfur adsorption, as seen when comparable materials are examined for H2S adsorption/reaction on a water-free or CO2-free basis.8,16,40-42 We also examined whether these materials could be successfully regenerated using air or an inert gas instead of the more common dilute O2 mixtures. Received: November 3, 2010 Revised: January 18, 2011 Published: February 11, 2011 1213
dx.doi.org/10.1021/ef101487v | Energy Fuels 2011, 25, 1213–1220
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2. EXPERIMENTAL SECTION 2.1. Materials. The CeO2/La2O3 mixed REOs or ternary (with Gd2O3 or Tb2O3) REOs were synthesized as high surface area mesoporous materials at >2.0 nm average pore diameter by adapting a surfactant-templated method.43,44 Measured amounts of ceric(IV) ammonium nitrate (NH4)2Ce(NO3)6 (Aldrich 99.9%; FW = 548.25) and lanthanum precursor La(NO3)3 3 6 H2O (Alfa Aesar 99.9%; FW = 433.1) were added to water (∼0.5% salts) and 1% tetramethylammonimum hydroxide (Acros, 25% in methanol) surfactant with stirring. The salts dissolved immediately to a clear solution, to which was slowly added NH4(OH) (Alfa Aesar, 28-30% NH3), until precipitation occurred (pH ∼10.3-10.5). The temperature was raised to 363 K and the gel stirred for 4 days, adjusting the pH by further ammonia addition as necessary. The centrifuged precipitate was washed with acetone and deionized water, dried overnight at 373 K, then calcined in flowing air at 773 K with a ramp of 2 K/min and a final hold of 6 h. When Gd or Tb were added, the Gd or Tb precursors were the chloride hexahydrates (Sigma, 99% and Aldrich, 99.9%, respectively). Mn, Fe, or Cu were supported on the dried and calcined CeO2/La2O3 mixed oxides or on Al2O3 by incipient wetness impregnation from Mn(NO3)2, FeCl2 3 4H2O, or Cu(NO3)2 3 2.5 H2O followed by drying and calcining as before. The supported REO mixed oxides were prepared by incipient wetness impregnation from concentrated solutions of the nitrate salts on either Al2O3 (BASF Al-3945E, 1/12 in. extrudate) or ZrO2/Al2O3 (BASF Zr-0510, 87% ZrO2, 1/8 in. extrudate). The total amounts of REO used were calculated to give roughly a monolayer if they were fully dispersed on the support surface. The resulting materials were characterized by high-temperature steaming (sintering) tests with before and after surface area measurements and N2 porosimetry (Quantachrome AS-1, 40 points, BarrettJoyner-Halenda algorithm), X-ray diffraction (XRD), and differential scanning calorimetry-thermogravimetric analysis (DSC-TGA). XRDs of powdered samples were obtained using a Rigaku Miniflex 2005C103 diffractometer using Cu-KR radiation. Samples were scanned from 5 to 60° 2θ at 1°/min with a 0.05° step. DSC-TGAs were measured using a TA SDT 2969 instrument in N2 flow from 313 to 673 K at 10 K/min, then 673-1373 K at 5 K/min. 2.2. Desulfurization Experiments. To explore these materials’ utility in gasifier effluent purification, we employed adsorption of H2S from a simulated gasifier effluent, followed by temperature programmed desorption and then final regeneration. The adsorption tests were at 900 K, and after saturation the gas was switched to helium and temperatureprogrammed from 900 to 1073 K. The simulated effluent gas for the adsorption/reaction step contained 0.1% H2S, 24% H2, 32% CO2, 3.3% H2O, balance N2. This gas approximates a bio- or coal gasifier effluent but without CO and with slightly less water. The sulfur content is higher than is characteristic of effluents from biogasifiers but lower than from coal gasifiers.38,45-48 These are short-bed, nonequilibrium data with a 1 g bed at GHSV = 15 500 and a prebreakthrough concentration of roughly 10 ppmv. Final regeneration was at 873 K in air for 30 min.
3. RESULTS 3.1. Characterization of Materials. We found that our modified surfactant-templated method reliably produced high surface area (sometimes >200 m2/g) mesoporous REOs, regardless of the precursor salts used. Initial surface areas, postcalcination, are shown in Table 1. Nitrogen porosimetry showed that the materials consisted of relatively uniform pores of average diameter 3.5-4 nm. An example pore size distribution is shown in Figure 1. We also modified a reverse microemulsion method for ZrO2 films49,50 in order to prepare larger mesoporous (>6 nm) CeO2/La2O3 composites, using block
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Table 1. Composition and Properties of Sample Materials calcined designation
composition (molar ratio)
used, g3
surface cycles area (m2/g) (m2/g)
Ce0.9/La
Ce/La = 0.9
110
21
Ce3/La
Ce/La = 3
240
94
Ce6/La
Ce/La = 6
200
24
Mn0.4/Ce3/La
Mn/Ce/La = 0.4/3/1
120
Mn0.2/Ce0.9/La Mn/Ce/La = 0.2/0.9/1
20 14
Mn0.6/Ce0.9/La Mn/Ce/La = 0.6/0.9/1 Fe0.6/Ce0.9/La Fe/Ce/La = 0.6/0.9/1
62 130
10 21
Cu0.2/Ce0.9/La Cu/Ce/La = 0.2/0.9/1
93
22
La0.35/Zr
La/Zr = 0.35
65
43
Ce3/La/Al
Ce/La/Al = 3/1/27
170
120
Ce0.9/La/Al
Ce/La/Al = 0.9/1/13
160
150
Gd/Ce/La/Al
Gd/Ce/La/Al = 0.2/0.9/1/14
160
120
Tb/Ce/La/Al
Tb/Ce/La/Al = 0.2/0.9/1/14
170
93
Mn/Al Fe/Al
Mn/Al = 0.78 (25 wt % MnO2) Fe/Al = 0.95 (25 wt % FeO)
150 170
Figure 1. Pore size distribution for Ce3/La computed by the BJH algorithm. The pore volume V is in centimeter3/gram.
EO-PO copolymer (Pluronic F127 and P123) templates, but early runs with these materials showed that they performed no better in the desulfurization experiments than the sorbents synthesized by the simpler surfactant-templated method, and as their synthetic yields were as low as 70% we concentrated on the simpler method. Using DSC, we measured the heat evolution from the crystallization exotherm of as-calcined materials, in N2. This exotherm was smaller for intermediate Ce/La atomic ratios (Table 2). This suggests that the intermediate oxides are less crystallizable and therefore might better retain surface area. Sulfidation tests (minimum three cycles) with before-and-after porosimetry confirmed this, in that an intermediate composition (Ce3/La) retained the highest surface area upon use (Table 1); however, it did not show the absolute lowest heat of crystallization, so sulfidation must also depend upon factors other than (absence of) crystal ripening. The as-calcined mixed oxides showed broad, weak diffraction peaks that can be attributed to the cubic fluorite phase of CeO2 mixed with La2O3 (Figure 2a). Upon use in several cycles of sulfidation/oxidative regeneration, these peaks are sharpened but little, and their breadth and the large bowl at low 2θ values 1214
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Table 2. DSC of Calcined Materials Ce/La (molar)
-ΔH (J/g)
0.65
2100
0.9
2900
3
2000
6 Inf.
670 5980
Figure 3. XRD patterns of supported Ce/La mixed oxides after use in sulfur adsorption tests at 900 K and air regeneration, for a minimum of three cycles after the first cycle: b, peaks characteristic of La(OH)(CO3); 2, La0.1Zr0.9O1.95; , La2O2(CO3).
Figure 4. Sulfur adsorption capacity and amount desorbed using synthetic gasifier effluent for adsorption at 900 K, with TPD at 9001073 K in N2. Average of three cycles, not using the first cycle, with final air regeneration at 873 K. Figure 2. (a) XRD patterns of Ce/La mixed oxides, calcined at 873 K but before use in sulfur adsorption. (b) XRD patterns of Ce/La mixed oxides after use in sulfur adsorption tests at 900 K and air regeneration, for a minimum of three cycles after the first cycle. b, peaks characteristic of La(OH)(CO3). 4, hexagonal La2O3.
indicate the CeO2/La2O3 mixtures are still made up of small, semicrystalline nanostructures (Figure 2b). The absence of a dominant peak at 2θ = 28-28.5° suggests that the CeO2/La2O3 mixture is poorly crystalline at best or that the dominant structure is not a simple mixed oxide.9,51-53 The peaks in Figure 2b are actually more characteristic of the two polymorphs of hydrated lanthanum oxycarbonates usually denoted La(OH)(CO3).52,54,55 However, the relative intensities (especially those of Ce3/La) suggest varying degrees of hydration/carbonation for the different samples. For example, the peak near 39° in Ce3/La is also a major peak in La(OH)3,54 suggesting more hydration and less carbonation for this used sample. The XRD patterns of the used, supported CeO2/La2O3 mixed oxides (on Al2O3) and of La2O3 (on ZrO2) are shown in Figure 3. The patterns of the Al2O3-supported materials are relatively featureless with broad peaks, indicating a nanocrystalline material. The absence of intensity from 2θ = 15-21° suggests less water incorporation and more CO2, as the other peaks are characteristic of the hexagonal form of La2O2(CO3)2, in roughly
the correct ratios of intensities.54,56 There is no evidence of either CeAlO3 or LaAlO3. The pattern of La2O3/ZrO2 also indicates a nanocrystalline material. There is little evidence of a separate crystalline La2O3 phase (indicated by relatively weak intensity at 30 and 46°), but once again the peaks characteristic of lanthanum hydroxycarbonates are present. The intensity at 2θ = 50° is too great to result from these polymorphs alone, and in combination with other peaks present (see Figure 3) suggests that some mixed phase of nominal composition Zr0.9La0.1O1.95 (pdf 00-017-0450) is present. The used adsorbents containing Mn gave patterns similar to those shown in Figure 2b, especially similar to the one for Ce/La = 0.9. In particular, there were no peaks indicating a separate MnO2, Mn3O4, or MnO phase. 3.2. Sulfur Adsorption/Desorption. The sulfur adsorptive capacity in the presence of the competing adsorbates H2O and CO2 was determined. We used temperature-programmed desorption (TPD) in He following the adsorption to saturation only to estimate how much sulfur was more weakly bound to the sorbents. This TPD was always followed by air regeneration at 873 K. The best simple CeO2/La2O3 sorbents were in the Ce/La = 3-6 range (Figure 4), as might be expected from the results in Tables 1 and 2. About half this amount was bound weakly enough that it could be removed with inert gas. 1215
dx.doi.org/10.1021/ef101487v |Energy Fuels 2011, 25, 1213–1220
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Table 3. Sulfur Capacities of REO/Transition Metal Oxide Mixturesa amount H2S designation Fe/Al Mn/Al
a
Figure 5. Sample breakthrough curves for Ce3/La/Al. Nmax corresponds to ∼1000 ppmv of H2S in the feed.
The stable capacity (after the first cycle) for repeat runs was ∼60 μmol of H2S/g for Ce/La = 3 (Figure 4). The stable capacity for Ce/La = 0.9 was only ∼20 μmol of H2S/g and for pure CeO2 less than this. For such binary REOs, total capacities from 25 to 1000 μmol/g have been reported previously, with the higher values in the absence of CO2 and/or H2O.5,8,12,16,40 Amounts near 1 mmol/g (2 mmol/g without CO2 or water) are required to match the lower-temperature capacities of Znbased sorbents,48,57 so further evolutionary development of REO-based materials is called for. However, the Zn-based sorbents are not thermally stable at >870 K.58,59 Supporting the REOs on either Al2O3 or ZrO2 increased the sulfur capacities; the increases were especially notable on a per rare-earth atom basis. Averaged sulfur capacity results for the better materials are shown in Figure 4; a commercial lower temperature zeolite/Al2O3 sulfur adsorbent (BASF CDX) is shown for comparison. The data show that going to a less crystallizable mixed REO in order to enhance hydrothermal stability under reducing conditions (see surface areas in Table 1) greatly increases the desulfurization capacity, especially on an atomic (REO atom) basis. Note that the Al2O3 itself is almost inert under these conditions (
