A Study of the Sulfidation and Regeneration Reaction Cycles of Zn-Ti-Based Sorbents with Different Crystal Structures

Korean J. Chem. Eng., 27(5), 1428-1434 (2010) DOI: 10.1007/s11814-010-0237-0 INVITED REVIEW PAPER A study of the sulfidation and regeneration reaction cycles of Zn-Ti-based sorbents with different crystal structures Suk Yong Jung**, Jung Je Park*, Soo Jea Lee***, Hee Kwon Jun***, Soo Chool Lee*, and Jae Chang Kim*,† *Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, Korea **Department of Chemical Engineering, Pohang University of Science and Technology, Hyoja-dong, Nam-gu, Pohang, Gyeongbuk 790-784, Korea ***GS Fuel Cell Co., 453-2, Seongnae-dong, Gangdong-gu, Seoul 134-848, Korea (Received 20 October 2009 • accepted 24 December 2009) Abstract−The reaction cycles of the sulfidation and regeneration of Zn-Ti-based sorbents prepared by a physical mixing method (ZT-700 and ZT-1000) or co-precipitation method (ZT-cp) were tested in a fixed bed micro-reactor at o o middle temperature (Sulfidation; 480 C, regeneration; 580 C). The ZnS produced during sulfidation from the Zn2TiO4 with a spinel structure (ZT-1000, ZT-cp) was easily regenerated even at 550 oC, while the ZnS produced from the ZnO with a hexagonal structure (ZT-700) needed a temperature higher than 610 oC. After regeneration, each sorbent was restored to its original crystal structure. The differences in the regeneration properties and the reaction cycles of the sorbents were related to the original crystal structures rather than to the physical properties such as pore volume and surface area. To study these differences further, FT-IR, FT-Raman, XRD and TPR were used, and their results including the reaction cycles of the sulfidation and regeneration on the Zn-Ti based sorbents with different crystal structures were discussed. Key words: Desulfurization, Regeneration, Sorbent, Hot Coal Gas, Crystal Structure, Reaction Cycles INTRODUCTION tion of a gasifier, must be removed from the hot coal gas because these species entering the gas turbine are converted to SOx, which Coal is extremely abundant and more widely distributed in com- are known as precursors of acid rain and whose emission into the parison with other fossil fuels like natural gas and oil. In addition, atmosphere is limited by strict government regulation. In addition, coal is one of the most abundant fossil fuel sources on earth with the H2S gases cause corrosion of the turbine and deactivation of reserves available for several hundred years. For these reasons, coal the catalyst used in other processes such as NH3 decomposition pro- is expected to maintain a significant share of this demand for the cess, water gas shift reaction and CO2 absorption process et al. [1-4]. near future. So far, coal has been mostly used for electricity genera- Typically, several commercial techniques are available for the tion, which has produced various pollutants such as SOx and NOx. removal of H2S and COS using a Selexol and similar processes [2]. Recently, for the environmental and efficient use of coal, several However, the disadvantage of these commercial techniques for the thermal cycles with coal as fuel have been proposed, such as the purification of coal gas is that the hot coal gas must be cooled to an pressurized fluidized bed combustion (PFBC) and integrated gas- ambient temperature and then preheated to a high temperature before ification combined cycle (IGCC) [1]. In particular, IGCC is consid- it can be used in a gas turbine. To avoid heat loss and save energy, ered to be among the most efficient and environmentally acceptable high (higher than 550 oC) or middle (350-550 oC) temperature de- technologies for the generation of power from coal. IGCC systems sulfurization technology has been widely developed and appears to are capable of significantly lower discharge rates of gaseous, liquid, be the major technique for the removal of hydrogen sulfide from and solid wastes relative to conventional coal-based systems [2]. hot coal gas [2,3]. An IGCC system with hot-gas clean up is a pro- IGCC technology is an environmentally benign power generation cess that shows promise for higher thermal efficiency and superior technology advanced by many developed countries with funding environmental performance than the conventional process. Although of several billions of dollars until now. The most critical cost and thermal efficiencies of 30-35% are typical for conventional pulver- the risks involved with a construction cost for the plant. But the con- ized coal plants, efficiencies of 43-46% may be easily attainable by struction cost of a coal IGCC plant after 2010 is expected to drop IGCC with hot gas cleanup [5]. to less than US$ 1100 kW. With higher efficiency and less genera- Recent studies have indicated that desulfurization system com- tion of pollutants, it would be cheaper than the pulverized coal power ponents become prohibitively expensive with increasing operating plants installed with desulfurization and de-NOx facilities [3]. To temperature and that the overall IGCC process efficiency gains of use this technology, it is necessary to remove the pollutants from carrying out desulfurization above 550 oC may not be sufficient to the coal-derived fuel gas. Among the pollutant gases, sulfur, which justify operation at such high temperature. Due to equipment limita- exists in the form of H2S or COS under the highly reducing conditions and other variables, the optimum temperature for fuel gas desulfurization in this process is estimated to be between 350 and 550 oC †To whom correspondence should be addressed. (middle temperature range), where technical viability and process E-mail: [email protected] efficiency result in a lower overall process cost [6]. 1428 A study of the sulfidation and regeneration reaction cycles of Zn-Ti-based sorbents with different crystal structures 1429 Several metal oxides, such as Ca-based sorbent [7], Zn-based dissolved in water. Precipitation was carried out by adding a raw sorbent [8-10], Cu-based sorbent [11-13], Mn-Fe-Zn containing salt solution to the NaOH (1.5 mol solution) under vigorous mix- sorbent [14,15] et al., have been studied for the development of re- ing at room temperature. The product of the precipitation was aged generable sorbents to remove hydrogen sulfide from coal-derived for 12 hrs, and then washed and separated by filtration. An extruder gas in high or middle temperature ranges under severe reduction was used to form pellets from the slurry with an outer diameter of conditions. 1 mm. The product was dried at 150 oC, calcined at 700 oC, and then The affinity of many metal oxides for reduced sulfur species is ground to a particle size ranging between 250-300 µm in diameter. In improved as the temperature is decreased, making many solids po- addition, in order to identify crystalline phases in the mixed oxides, an tentially suitable for hot gas clean up applications in the middle tem- X-ray diffraction (XRD) study was performed with a Philips XPERT perature range. Especially, the Zn-Ti based sorbents are known to instrument using a CuKα radiation source at the Korea Basic Sci- be among the best metal oxide sorbents, having the most favorable ence Institute. thermodynamics for H2S removal and regeneration properties in 2. Apparatus and Procedure the middle temperature range. In this temperature range, the con- Multiple cycles of sulfidation/regeneration were performed in a ventional Zn-Ti based sorbents prepared by the physical mixing fixed-bed quartz reactor with a diameter of 1 cm in an electric fur- method have shown some inferior properties such as low initial sulfur nace. One gram of sorbent was packed into the reactor and the space capacity, regeneration properties and deactivation with an increase velocity (SV) was maintained at 5,000 h−1 to minimize a severe pres- in cycle number [16-19]. To solve these problems, various promot- sure drop and channeling phenomena. All of the volumetric gas ers, such as cobalt [20-23], nickel [21-23] and iron [24] et al., were flows were measured under standard temperature and pressure (STP) added to the Zn-Ti based sorbents. The sorbents with an excellent conditions. The temperature of the inlet and outlet lines of the reactor sulfur removing capacity could be prepared throughout the co-pre- was maintained above 120 oC to prevent the condensation of water cipitation of these promoters. Jung et al. [17] also reported that the vapor in the sulfidation process. The outlet gases from the reactor sulfur-removing capacity of the Zn-Ti based sorbent depended on were automatically analyzed every 8 min by a thermal conductivity the physical properties, such as pore volume, pore size and surface detector (TCD) equipped with an auto-sampler (Valco). The col- area et al. But, the details of the study on the different crystal struc- umn used in the analysis was a 1/8-in. A teflon tube was packed tures formed by various preparation methods and pretreatment con- with Porapak T. The sulfidation and regeneration conditions and ditions, which are expected to severely affect their sulfidation and the composition of mixed gases are shown in Table 1. When the regeneration behaviors, have not yet been understood. H2S concentration of the outlet gases reached 15,000 ppmv, the con- In this work, to investigate these behaviors and the sulfidation/ centration of H2S at the inlet stream of mixed gases, an inert nitrogen regeneration cycles with different crystal structures, Zn-Ti based gas without H2S, was introduced to purge the system until it reached sorbents were prepared by various methods such as the co-precipi- the regeneration temperature. Finally, nitrogen gas mixed with 3% tation and physical mixing method and tested in a fixed-bed reac- oxygen was introduced to regenerate the sulfurized sorbents until tor at a middle temperature range.

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