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Ind. Eng. Chem. Res. 2009, 48, 10498–10503
Glass-Based Processing of Mixed-Oxide Desulfurization Sorbents Michael D. Dolan,* Alex Y. Ilyushechkin, Keith G. McLennan, Ty Nguyen, and Sunil D. Sharma CSIRO Energy Technology, Post Office Box 883, Kenmore 4069, Queensland, Australia
The favorable thermodynamics of the reaction between H2S and oxides of iron, nickel, and zinc make these materials suitable for the sorption of sulfur from coal-derived synthesis gas. As an alternative to traditional solid-state processing, the suitability for coal gas desulfurization of glass-ceramics based on transition metal oxides has been evaluated. Glass-ceramics based on NiAl2O4 and ZnFe2O4 have similar sorption properties and a high resistance to agglomeration, but the inexpensive raw materials and moderate processing temperature of the ZnFe2O4-based glass makes it more suitable for a commercial application. The overall performance is highly dependent on the particle size of the sorbent, but these materials can be formed with relatively high surface areas by using fiber-based processing. This material is a potential alternative to mixed oxide sorbents prepared by solid-state processing, as it offers comparable performance combined with the ability to process the glass by existing glass-processing techniques. 1. Introduction Sulfur compounds (mainly as H2S and COS) are the most abundant impurity in coal-derived synthesis gas (syngas). Due to their detrimental effect on the environment and downstream processes, these species must be removed from the syngas prior to its utilization. Studies of the thermodynamics of the reaction between H2S and transition metal oxides1,2 have shown that, up to 600 °C, the equilibrium conversion of ZnO, CuO, and NiO to their respective sulfides is around 100%. Above this temperature, due to the exothermic nature of the reaction and volatilization of the transition metal oxide, the H2S conversion decreases sharply. Pelletized, porous ZnO-based sorbents have been commercially available for some time, but they are generally nonregenerable due to the detrimental effect of successive sulfidation/oxidation cycles on the physical structure of the sorbent. In recent years considerable effort has been devoted to the processing of mixed oxide desulfurization sorbents for coal-derived synthesis gas, culminating in regenerable ZnFe2O4,3-6 zinc titanate,7-9 and copper chromite10-12 sorbents. Such sorbents feature an active oxide (e.g., CuO or ZnO) that is stabilized against reduction, volatilization, and pore closure by a second oxide (e.g., TiO2 or Cr2O3). In the case of ZnFe2O4, the Fe2O3 component also contributes to sulfur sorption. After granulation, mixed-oxide sorbents are sintered at high temperature in order to introduce the strength and attrition resistance necessary for repeated use in a hightemperature sulfur-removal process. Sorbent activity, however, is inversely proportional to strength.11 While higher sintering temperatures provide greater strength and resistance to attrition, this comes at the expense of sorption capacity, which is decreased by a reduction in surface area and by conversion of the active oxide to a less-reactive mixed oxide. It is therefore a major challenge to overcome this inverse relationship between strength and capacity. As an alternative to solid-state processing, sorbents for sulfur removal can potentially be produced from oxide glasses, and more specifically, glass-ceramics, so-called as they are formed from the controlled devitrification of a glass. The use of glassceramics allows an active component (for example, a mixed oxide of nickel, copper, or zinc) to be crystallized from a dense, * Author to whom correspondence should be addressed. E-mail:
[email protected]; tel: +61733274126.
inert silicate support. Glass-based processing offers several potential advantages over solid-state processing, particularly in terms of strength and the ability to form the sorbent into complex geometries from the molten state. Reducible oxides such as CuO and NiO13,14 are effective H2S sorbents, but they have received less attention than ZnO-based sorbents due to their tendency to reduce and agglomerate. The problem can be overcome if the metal forms as discrete deposits on an otherwise inert surface. NiO-based glass-ceramics form microstructures that appear to be favorable for the potential application. Crystallized NiO-Al2O3-SiO2 glasses form dendritic (branched, treelike) NiAl2O4 spinel growths within a SiO2 matrix.15 NiO-MgO-Al2O3-SiO2 glass-ceramics may also be suitable, as they can crystallize an active nickel cordierite along with an inert magnesium cordierite phase.16,17 In these glasses, reduction of the spinel to Ni and Al2O3 may result in a highsurface area porous network through which the metallic Ni is dispersed. ZnO-based glass-ceramics have also been reported, including compositions crystallizing Zn2SiO4 silicate18 and ZnFe2O4.19,20 Zinc-ferrite formulations are of particular interest due to the ability of the Fe2O3 component to sorb H2S, thus potentially providing greater sorption capacity than other ZnO-based mixed oxide sorbents. Furthermore, ZnFe2O4 glass-ceramics can be formed from metallurgical wastes,19 which can significantly reduce the cost of raw materials. This paper will describe the processing and sorption characteristics of glass-ceramics based on NiAl2O4 and ZnFe2O4, with the aim of identifying suitable glass-ceramics best suited for high-temperature syngas desulfurization. The H2S sorption capacity of these materials will be compared against a commercially available ZnO-based sorbent, and a recommendation of the viability of this approach will be made. 2. Methods 2.1. Sorbent Preparation and Characterization. Glassceramics 1 and 2, crystallizing NiAl2O4 and ZnFe2O4, respectively, were prepared according to the compositions and conditions shown in Table 1. Glass batches were blended from high-purity oxides, homogenized in a high-speed rotary mill and melted in air by use of platinum-alloy crucibles. The melts were quenched in air or on a steel plate and then crushed and
10.1021/ie901170f CCC: $40.75 2009 American Chemical Society Published on Web 10/01/2009
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Table 1. Preparation Conditions for the Commercial ZnO-Based Sorbent and Two Mixed-Oxide Sorbents Based on Transition-Metal Silicate Glasses commercial ZnO-based sorbent
glass-ceramic1
glass-ceramic 2
composition (wt %) active phase chemical formula of sulfide phasea melting conditions
90% ZnO + 10% Al2O3 ZnO ZnS
22.8% NiO + 31.2% Al2O3 + 46.0% SiO2 13.8% ZnO + 27.1% Fe2O3 + 28.5% CaO + 30.6% SiO2 NiAl2O4 ZnFe2O4 Ni3S2 (+ Al2O3) ZnFe2S4
N/A
crystallization conditions activation conditions
N/A N/A
1550 °C for 1 h in Pt/Rh crucible in air, air-quenched 1200 °C for 3 h in air 600 °C for 12 h under 5% H2 in N2
a
1300 °C for 1 h in Pt/Au crucible in air, quenched on steel plate in air 950 °C for 24 h in N2 N/A
Predicted by thermodynamic simulation software.23
Table 2. Measured BET Surface Areas of Various Sorbents sample
size
treatment
BET surface area, m2 g-1
commercial ZnO-based sorbent commercial ZnO-based sorbent glass-ceramic 2 glass-ceramic 2 glass-ceramic 2 glass-ceramic 2 commercial borosilicate glass fiber
425-600 µm 425-600 µm 425-600 µm 425-600 µm 106-180 µm 106-180 µm ∼10 µm diameter
as received postsulfidation as melted postsulfidation/regeneration as melted postsulfidation/regeneration as received
44.0753 ( 0.2214 16.6857 ( 0.3330 0.1032 ( 0.0005 0.1663 ( 0.0024 0.1377 ( 0.0022 0.1886 ( 0.0017 0.4399 ( 0.0089
sized by use of ASTM sieves. Sized fractions were exposed to a controlled heat treatment to induce crystallization. The microstructures of the crystallized sorbents were examined by X-ray diffractometry (Bragg-Brentano geometry with Cu KR radiation at 1.542 Å) and scanning electron microscopy in backscattered electron (BSE) mode to show compositional variation. The temperature dependence of crystalline phase formation in glass-ceramic 2 was examined by in situ high-temperature X-ray diffractometry. Glass was crushed to 20% (w/
Figure 11. Heat-treated fiber formed from glass-ceramic 2.
w) of sulfur. This value is approximately equal to the theoretical maximum of 22% (w/w). Given that theoretical sorption capacities are rarely achieved, this raises the possibility that CaO is contributing to sorption in a significant way and/or that both chemisorption and physisorption are occurring at the sorbent surface. Given the temperature involved, this high capacity is most likely due to the contribution of CaO to sorption; however, as CaS is not regenerable, it would be expected that sorption capacity should decrease with successive sulfidation/regeneration cycles. It is likely, therefore, that the exposure of fresh sorbent with successive cycles outweighs the nonregenerability of the CaO component. The surface area of glass-ceramic 2 increased as a consequence of repeated sulfidation/regeneration cycles (see Table 2), although it should be noted that the surface area of this dense material is several orders of magnitude less than the highly porous ZnO-based material. Furthermore, as with glass-ceramic 1, the sorbent suffered from no agglomeration. Figure 10 shows that glass-ceramic 2 does not exhibit plugflow behavior. The increase in effluent [H2S] is gradual, rather than a sharp breakthrough, and could be indicative of several phenomena. Channeling or bypass could be responsible, but this is unlikely given the size and uniformity of the sorbent and the absence of adhesion between particles or densification of the sorbent bed during sorption. Another possible explanation is that the rate of the sorption reaction decreases with time. Given that this sorbent is dense, reaction between H2S and fresh sorbent is limited by solid-state diffusion, and the rate of sorption will decrease as this reaction layer becomes thicker. 3.6. Suitability for Fiber Drawing. The performance of the glass-ceramic sorbents has been shown to be related to the particle size of the sorbent. The formation of fibers is one means by which the surface area, and thus the performance, of a glassy material can be maximized. This is illustrated by the respective surface areas of 425-600 µm glass-ceramic 2 (0.10 m2 g-1) and a commercial borosilicate glass fiber (0.44 m2 g-1). Given that fiber-based processing is an anticipated application of these materials, the fiberforming ability of glass-ceramic 2 was examined. Glass-ceramic 2 can be readily drawn into fibers at 20% (w/w)], is formed from inexpensive raw materials, and possesses suitable glass-phase processing properties (including the ability to draw fibers at a moderate temperature). ZnFe2O4-based sorbents prepared via glass-ceramic processing are therefore a promising alternative to mixed oxide sorbents prepared by solid-state processing. The glass-ceramics investigated were a representative selection but are by no means the only suitable compositions. Many further compositions, particularly those based on ferrites, can potentially be applied for this application. Acknowledgment We acknowledge the financial support provided by the Centre for Low Emission Technology and CSIRO’s Energy Trans-
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ReceiVed for reView July 23, 2009 ReVised manuscript receiVed September 7, 2009 Accepted September 15, 2009 IE901170F
