Ind. Eng. Chem. Res. 1998, 37, 4675-4681
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Sol-Gel-Derived Alumina-Supported Copper Oxide Sorbent for Flue Gas Desulfurization Zhong-Min Wang and Y. S. Lin* Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171
Nanostructed mesoporous CuO/γ-Al2O3 granular sorbents were prepared by the sol-gel method. Performance of the sol-gel-derived CuO/γ-Al2O3 sorbents for SO2 removal was studied in a fixedbed adsorption system. SO2 breakthrough curves with a feed stream of air containing 2000 ppm SO2 were measured at different temperatures (300-500 °C) and flow rates (interstitial velocity of 0.25-6.96 cm/s). The optimum sulfation and regeneration temperature on the solgel-derived sorbent was found to be 400 °C. The properties of the sol-gel-derived sorbents are compared with a similar sorbent from a commercial source used in the pilot-scale copper oxide flue gas desulfurization process. At high temperatures (>400 °C) the sol-gel-derived CuO sorbents exhibit catalytic properties for converting SO2 to SO3. The crush strength of the solgel-derived sorbents is about 5 times that of the commercial sample, while the attrition rate of the former is about 4-10 times smaller than the latter. The SO2 sorption capacity of the solgel-derived sorbent is about 3 times that of the commercial sorbent with a similar amount of CuO loading. The better mechanical properties and higher sulfation capacity of the sol-gelderived alumina-supported copper oxide sorbents are due to their unique microstructure and the method used for coating CuO. The sulfation and regeneration study shows good regenerability and stability of the sol-gel-derived CuO/γ-Al2O3 sorbents. Introduction Dry regenerative processes for flue gas desulfurization and NOx removal offer a number of advantages over the established throwaway processes.1 These include (1) no solid or liquid waste is generated, (2) salable sulfur byproduct is produced, (3) reheating of flue gas after the SO2/NOx process is not required, and (4) wet materials are not handled and water input requirements are minimal. The dry sorbents are usually made by coating an oxide of transition, alkali, or alkalineearth metal, most notably, CuO, on the surface of γ-alumina or silica.2-4 The sorption process using supported CuO sorbents (referred to as the copper oxide process) is one of the most promising dry, regenerative processes for combined removal of SO2 and NOx from flue gas. For flue gas desulfurization the copper oxide process is based on the sulfation reaction with CuO coated on the surface of porous support as
CuO(solid) + SO2(gas) + 1/2O2 w CuSO4(solid) The sulfated sorbent can be regenerated by using a reducing gas, such as methane, as
CuSO4(solid) + 1/2CH4 w Cu(solid) + SO2 + 1/2CO2 + H2O which is followed by oxidation of Cu to CuO using air to complete the sulfation and regeneration cycle. On the process aspect, UOP in the U.S. licensed the fixed-bed copper oxide process developed by Shell in the * To whom correspondence should be addressed. Telephone: (513)556-2769. Fax: (513)556-3473. E-mail: JLIN@ ALPHA.CHE.UC.EDU.
1970s for flue gas desulfurization.5,6 Concurrently, Pittsburgh Energy Technology Center independently developed a fluidized-bed copper oxide process for flue gas treatment.2,7 In the 1980s, Rocketdyne Division of Rockwell International developed a more effective moving-bed copper oxide reactor system. They have demonstrated that the copper oxide process could remove over 95-98% of SO2 and NOx from flue gas.1 Currently, a team of Southern Illinois University in Carbondale, IL, Tecogen Inc. in Waltham, MA, and Sargent and Lundy in Chicago, IL, is developing a testing-scale copper oxide bed regenerable application (COBRA) scrubber based on the moving-bed copper oxide process. This COBRA scrubber is the gas cleanup portion of a project aimed to demonstrate a 70-MW plant with a lowemission boiler system. In the typical coal-based electric power plant, warm flue gas from the economizer is fed into the moving-bed copper oxide process for SO2 and NOx removal. The ash particulate and attrition sorbent fines, together with the flue gas being treated, pass through the moving-bed adsorber and are collected by a screen at the outlet of the adsorber or captured by a downstream baghouse. The adsorber incorporates a vertically moving bed of sorbent retained between a louvered partition or a screen on the gas inlet side and a sheet of filter materials on the gas exit side. The flue gas flows in the direction perpendicular to the movement of the sorbent. The sulfated sorbent is regenerated in another moving-bed regeneration reactor with methane or syngas as the reducing agent. The granular γ-Al2O3 supports of the commercial sorbents (e.g., Alcoa and UOP) used in the copper oxide process are prepared from alumina powder by a traditional granulation method such as tumble growth.8 The γ-Al2O3-supported CuO sorbents are then produced by impregnating the porous γ-Al2O3 support with a solution
10.1021/ie980343u CCC: $15.00 © 1998 American Chemical Society Published on Web 11/04/1998
4676 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 1. Flowchart of the sol-gel process for preparation of γ-Al2O3-supported CuO granular sorbent.
of cupric salt which is decomposable to Cu ion or CuO. The major problems associated with the use of these CuO/γ-Al2O3 sorbents in the moving-bed copper oxide process are high attrition rate and lower chemical reactivity of the sorbents used.2,7,9 The weak mechanical attrition resistance of the sorbents is related to poor mechanical strength of the γ-Al2O3 support prepared by the conventional method. The poor chemical attrition resistance of the conventional sorbents reflects the weak binding of CuO on the support surface. Furthermore, these commercial alumina supports have a smaller surface area (98.5%), and the commercial copper oxide sorbent (UOP SOX-3) were also measured. The experimental conditions were the same as those for the sol-gel-derived sorbents (0.5 g of sorbent, air containing 2000 ppm SO2 as the feed, feed flow rate of 8.7 mL/min). Figures 11 compares the breakthrough curve of the sol-gel-derived sorbent with lime sorbent (typically operated in 25-80 °C). For the sol-gelderived sorbent the concentration of sulfur dioxide in the effluent stream is zero in the first 6 h. After 3 h, the concentration of SO2 in the effluent stream begins to increase, and it takes more than 20 h to reach the input concentration. The sol-gel-derived sorbent has a breakthrough time similar to that of the lime sorbent. The total sorption of the sol-gel-derived sorbent is much larger than that of the lime sorbent. It should be pointed out that the lime sorbent cannot be regenerated at temperatures lower than 1200 °C.21 However, the CuO-coated sorbent can be regenerated in 400-500 °C with methane or hydrogen.
Figure 12. Comparison of SO2 breakthrough curves from a fixed bed packed with sol-gel-derived γ-Al2O3-supported CuO sorbent and commercial γ-Al2O3-supported CuO sorbent.
Figure 12 compares the breakthrough curves of the sol-gel-derived and commercial copper oxide sorbents. At 100% SO2 removal efficiency the sol-gel-derived sorbent could treat about 1800 mL of flue gas (corresponding to a 200 min breakthrough point), about 3 times that of the commercial sorbent (80 min). Considering the total area bound by the breakthrough curve, the C/C0 axis, and the horizontal straight line above the breakthrough curve, the total sorption capacity of the sol-gel-derived sorbent is also about 3 times that of the commercial sorbent. Obviously, the sol-gelderived sorbent has a higher sulfation capacity than that of the commercial sorbent. The slightly higher amount of CuO coated on the sol-gel-derived sorbent alone would not contribute to the significantly improved SO2 removal capacity of the sol-gel-derived sorbent. The excellent sulfation reactivity of the sol-gel-derived sorbents is a result of more uniform dispersion of CuO on the support surface. Conclusions The sol-gel-derived γ-Al2O3-supported CuO granular sorbents have large surface area and pore volume and uniform pore size. These sorbent particles consist of nanoscale γ-Al2O3 crystallites with CuO uniformly dispersed on their grain surface. The sol-gel-derived sorbents exhibit crush strength and attrition resistance 3-5 times and SO2 removal capacity 2-3 times those of the similar commercial sorbent. At 100% SO2 removal efficiency in the fixed-bed operation, the sol-gel-derived sorbent (with 9 wt % CuO) could treat the flue gas with about 3 times that of the commercial sorbent with similar CuO loading. The total sorption capacity of sol-gel-derived sorbent is also about 3 times that of the commercial sorbent. The excellent sulfation reactivity of the sol-gel-derived sorbents used in this work is a result of more uniform dispersion of CuO on the support surface and the specific nanoscale γ-Al2O3 crystallites. This is another promising feature of the sol-gel-derived sorbents. Sulfation and regeneration experiments show good regenerability and stability of the sol-gel-derived sorbent. Acknowledgment The project was supported by the National Science Foundation (CTS-9502437) and Ohio Coal Development Office through Grant D-95-1R.
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Received for review June 1, 1998 Revised manuscript received September 25, 1998 Accepted October 4, 1998 IE980343U
