Ind. Eng. Chem. Res. 2008, 47, 4455–4464
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Highly Attrition-Resistant Zinc Oxide-Based Sorbents for H2S Removal by Spray-Drying Technique Joong B. Lee,† Jeom-In Baek,† Chong K. Ryu,*,† Chang K. Yi,‡ Sung H. Jo,‡ and Sung H. Kim*,§ Global EnVironment Group, Korea Electric Power Research Institute, 103-16 Munji-dong, Yuseong-gu, Daejon 305-380, Korea, Korea Institute of Energy Research, 71-2 Jang-dong, Yusung-ku, Daejon 305-343, Korea, Department of Chemical and Biological Engineering, Korea UniVersity, 1-5 Ka Anam-dong, Sungbuk-ku, Seoul 136-701, Korea
A ZnO-based sorbent, ZAC 32N, applicable to transport reactors was successfully prepared by the spraydrying technique. Another sorbent, ZAC 32SU, was prepared by scale-up preparation of ZAC 32N sorbent. The physical properties of the sorbents such as attrition resistance, specific surface area, pore volume, and particle size were extensively characterized and exhibited a good potential for use in transport applications. The chemical reactivity tested in the thermogravimetric analyzer and microreactor exhibited desirable characteristics for effective desulfurization of syngas streams in the range of 450-550 °C. Bench-scale tests for the sorbent ZAC 32SU were performed for a continuous 160 h with a steady solid circulation of 54.6 kg/h. The results showed 99.5%+ desulfurization at 500-550 °C and reasonable regenerability at 550-620 °C. Test results on the physical properties and chemical reactivity indicated that the performance of developed sorbents proved to be outstanding. 1. Introduction In many nations, the advanced high-efficiency integrated gasification combined-cycle power systems are being developed to produce power from coal. In these systems, coal is gasified to produce a gas under high-temperature and high-pressure (HTHP) conditions. The hot gas is cleaned and burned in a gascombustion turbine. Integrated gasification combined-cycle (IGCC) systems have lower emissions than conventional pulverized-coal-fired power plants. Hot gas cleanup offers the potential for higher plant thermal efficiencies and lower costs because of the elimination of syngas cooling and associated heat exchangers.1 Research on hot gas desulfurization has been focused on the fluidized-bed hot gas desulfurization process and sorbent development. A preferred method for removal of hydrogen sulfide would be to contact the gas stream with a sorbent capable of undergoing many cycles of sulfidation and regeneration by means of oxidation. The desulfurization process using a fluidized-bed/transport reactor has several potential advantages over fixed-bed and moving-bed processes. These advantages include excellent gas contact with smaller sorbent particles, ease in sorbent refill and removal, ease in control of exothermic regeneration-reaction temperature, and continuous steady operation.2 Gupta and co-workers2,3 suggested that the sorbents for transport reactor applications must be highly attrition resistant, have a good sorption capacity with fast chemical reactivity within a short contact time (1-2 s), require lowtemperature initiation of regeneration with good regenerability, and have good flowability.2,3 The early types of such sorbents were prepared using pellet, extrusion, and granulation techniques that were reviewed by Mitchell.4 With the adaptation of fluidized-bed/transport reactor * Corresponding author. Tel.: +82-42-865-5230 (C.K.R.), +82-23290-3297 (S.H.K.). Fax: +82-42-865-5708 (C.K.R.), +82-2-926-6102 (S.H.K.). E-mail:
[email protected] (C.K.R.),
[email protected] (S.H.K.). † Korea Electric Power Research Institute. ‡ Korea Institute of Energy Research. § Korea University.
technology to hot gas desulfurization, the spray-drying technique provides sorbent particles with a highly uniform size, spherical shape, and texture and is also readily accessible for the addition of sorbent ingredients during the slurry-preparation steps and a scale-up to industrial production.5–7 The primary issues in the fluidized-bed/transport reactor process are the attrition resistance of the sorbent, the sorption capacity and regenerability, the durability, and the cost. The overall objective of this project is the development of a superior attrition-resistant zinc oxide-based sorbent for hot gas cleanup in the integrated coal gasification combined cycle (IGCC). Sorbents applicable to the fluidized-bed hot gas desulfurization process must have a high attrition resistance to withstand the fast solid circulation between desulfurizer and regenerator, fast kinetic reactions, and high sulfur-sorption capacity.8–10 The oxidative regeneration of zinc oxide-based sorbents is usually initiated at temperatures greater than 600 °C and has a highly exothermic nature that causes the deactivation of the sorbent as well as complicates the sulfidation process with side reactions. By focusing on sorbent attrition and the regenerability of zinc oxide-based sorbents, we have adopted multibinder matrixes and the direct incorporation of the regeneration promoter. In this study, we have investigated the physical properties and chemical reactivity of two spray-dried regenerable zinc oxide-based sorbents that are suitable for the fluidized-bed/transport desulfurization process.3 This work aimed to develop and verify the performance of sorbents with high sulfur-sorption capacity and good physical properties to remove sulfur compounds from coal-derived synthesis gas at high temperatures. Comprehensive data, such as attrition index, surface area, particle size, packing density, reactivity, etc., required to evaluate the developed sorbents, ZAC 32N and ZAC 32SU, were presented. 2. Experimental Section 2.1. Sorbent Preparation. Zinc oxide-based sorbent was prepared for this study. We adopted a multibinder matrix system and the direct incorporation of the regeneration promoter to improve the attrition and regenerability of the sorbent. Sorbent
10.1021/ie070962f CCC: $40.75 2008 American Chemical Society Published on Web 06/07/2008
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Figure 1. Schematic diagram of the thermogravimetric analyzer.
forming was performed using a spray-drying technique that is easily scalable to commercial levels. Spray-drying technique was employed to prepare free-flowing solid-sorbent particles having suitable shape, particle size, and particle-size distribution for a fluidized-bed or transport process and reacting efficiently with hydrogen sulfide (H2S). Sorbent preparation procedure is as follows. Solid raw materials and organic additives were wellmixed in the pure water to obtain uniform slurry. The mixed slurry was comminuted with high energy ball mill to control particle size and make homogeneous slurry. The homogenized slurry was spray-dried to form sorbent particles (i.e., green body). The green body was calcined after predrying. Among these steps, the colloidal slurry preparation is the most important step for yielding sorbent particles suitable for fluidized-bed applications. The colloidal slurry must be flowable, homogeneous, dispersed, and stable; these properties are controlled by the concentration, viscosity, and pH by the addition of organic additives such as dispersants. Both sorbents consisted of 50 wt % of ZnO and 50 wt % matrixes containing the binder including a Ni-based promoter by 7.5 wt %. Each sorbent was designated as ZAC 32N and ZAC 32SU. ZAC 32SU was prepared by scale-up preparation of ZAC 32N. 2.2. Sorbent Characterization. 2.2.a. Attrition Resistance. The calcined sorbents were sieved using the standard sieves of 38 and 212 µm before attrition testing. The sieving was performed until no particles passed through. The attrition resistance of the calcined sorbents for fluidized-bed application was measured with a modified three-hole air-jet attrition tester based on the American Society for Testing and Materials (ASTM) D 575795. The attrition was determined at 10 standard L/min (slpm) over 5 h as described in the ASTM method. The attrition index (AI) is the percent fines generated over 5 h. The fines were collected after 1 and 5 h from the start: AI ) [total fines collected for 5 h/amount of initial sample (50 g)] × 100. The corrected attrition index (CAI) is the percent fines generated only over 4 h; that is, the fines generated over the first 1 h were subtracted from the total fines generated over 5 h and from a total amount of sample used initially: CAI ) [(total fines collected for 5 h - fines collected for first 1 h)/(amount of initial sample - fines collected for first 1 h)] × 100.
The AI and CAI of fresh Akzo and Davison fluid catalytic cracking (FCC) catalysts as references are 22.5% (18%) and 18.4% (13.1%) at the same conditions (10 slpm), respectively. It would be acceptable in a fluidized-bed desulfurization (H2S removal) process that materials have an AI of
