Energy & Fuels 1992,6, 21-27
of secondary gas-phase reaction on this method. The coal was preheated at 200 "C for 1h while traveling through a screw feeder. The total volatile matter and the tar yield increased at d temperatures between 650 and 850 "C. At 850 "C the yield of valuable chemicals such as naphthalene as well as the yield of total tar significantly increased. Thus it was clarified that the flash pyrolysis of coal pre-
21
heated at low temperature is a simple and effective method to increase the total volatile matter and the tar yield.
Acknowledgment. This work was financially supported by the Ministry of Education, Culture and Science of Japan through the Grant-in-Aid on Priority-Area Research (Grant No. 63603014, 01603012, and 02203112).
Development of Zinc Ferrite Sorbents for Desulfurization of Hot Coal Gas in a Fluid-Bed Reactor R. Gupta* and S. K. Gangwal Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709
S. C . Jain Morgantown Energy Technology Center, U.S. Department of Energy, Morgantown, West Virginia 26507 Received July 15, 1991. Revised Manuscript Received September 30, 1991
Advanced integrated gasification combined cycle (IGCC) power generation systems require the development of high-temperature, regenerable, desulfurization sorbents capable of removing hydrogen sulfide from coal gasifier gas to very low levels. Fluidized-bed desulfurization reactors offer significant potential advantages in IGCC systems compared to fixed- and moving-bed reactors because of their ability to control the highly exothermic regeneration involved. However, a durable, attrition-resistant sorbent in the 100- to 300-pm-size range is needed. Thus, the objective of this investigation was to identify and demonstrate methods for enhancing the long-term chemical reactivity and mechanical strength of zinc ferrite, a leading regenerable sorbent, for fluidized-bed applications. A bench-scale, high-temperature, high-pressure, fluidized-bed reactor (3-in. i.d.) system capable of operating up to 350 psig and 850 "C was designed and built. A total of 175 sulfidation-regeneration cycles were carried out using KRW-type coal gas with various zinc ferrite formulations. A number of sorbent manufacturing techniques, including spray drying, impregnation, crushing and screening, and granulation, were investigated. While fluidizable sorbents prepared by crushing the durable pellets and screening had acceptable sulfur capacity, they underwent excessive attrition during multicycle testing. The sorbent formulations prepared by a proprietary granulation technique were found to have excellent attrition resistance and chemical reactivity during multicycle testing. However, application of zinc ferrite was limited to a maximum temperature of about 550 "C, above which excessive sorbent weakening was observed possibly due to chemical transformations.
Introduction An integrated gasification combined cycle (IGCC) power system employing hot-gas desulfurization of the coal-derived gas is the most promising advanced technology for producing electric power from coal. Compared to conventional pulverized coal combustion power plants,' the IGCC system has the potential of greater reductions in pollutant emissions and improvements in electrical generation efficiencies. Recent developments in hot-gas desulfurization technologies have focused on regenerable, solid mixed metal oxide sorbents, such as zinc ferrite, to remove reduced sulfur species (H2S,COS,etc.) from coal gas.2 The sulfided sorbents are reused after regeneration with air. These sflidation-regeneration steps form a cycle. A commercially viable sorbent should withstand numerous (1) Williams, M. C.; Bedick, R. C. 'Gas Stream Cleanup"; Technology Status Report, DOE/METC-89/0263, 1988. ( 2 ) Jalan, V. High-Temperature Desulfurization of Coal-Gases. Acid and Sour Gas Treating Processes; Gulf Publishing Co.: Houston, TX, 1985; pp 466-491.
(at least 100) cycles before deactivating to an unacceptable level of reactivity, e.g., sorbent sulfur absorption capacity dropping below 10-20% of its theoretical value. Zinc ferrite, ZnFe204,originally developed at the U.S. Department of Energy/Morgantown Energy Technology Center (DOE/METC) by Grindley and Steinfield,3 contains an equimolar mixture of zinc oxide and iron oxide calcined at 800-850 "C with a suitable binder. It combines the high desulfurization efficiency of zinc oxide and the high capacity and reactivity of iron oxide. It can be easily regenerated using air/diluent mixtures and has consistremovals to less ently demonstrated hydrogen sulfide (H3) than 10 parts per million (ppm) by volume over multiple cycle~.~3~ The chemistry of H2Sremoval by zinc ferrite and regeneration of sulfided sorbent is discussed e l s e ~ h e r e . ~ , ~ The theoretical maximum sulfur absorption capacity of zinc ferrite is close to 40 g of sulfur per 100 g of sorbent (3) Grindley, T.; Steinfeld, G. "Development and Testing of Regenerable Hot Coal-Gas Desulfurization Sorbents", DOE/MC/16545-1125, 1981.
0887-062419212506-0021$03.00/ 0 0 1992 American Chemical Society
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Gupta et al.
22 Energy & Fuels, Vol. 6, No. 1, 1992 He
Regeneration
'
Seals
co2 co
lntemal Cyclone
HZ 10% HzS
Feed Gases
Mass Flow
Comrollers
and Vem
Sobent Cage (7.62-cm or 5 08-cm dia) Sobent
Water
Melenng Punp
Figure 1. Bench-scale fluid-bed sorbent test facility.
Thermowell
based on the stoichiometry of the sulfidation reaction. Typically, about 40-50% of the theoretical capacity is acnieved because sulfur loading varies across the bed before concentration of H2S in the effluent rises to an unacceptable level, e.g., above 50 ppmv. At this point the bed must be regenerated. Early testing of zinc ferrite was conducted in a fixed-bed reactor system using cylindrical e~trudates."~However, the fixed-bed reactor system was of limited practical importance because of severe restrictions imposed by (1)the need for values operating at high-temperature, highpressure (HTHP) conditions; (2) the greater difficulty of the fixed-bed reactor to handle the heat released by the highly exothermic regeneration reaction; and (3) the nonuniform composition of regeneration off-gas. Fixed-bed development efforts were, therefore, not continued. To overcome some of the limitations of a fixed-bed system, moving- and fluidized-bed reactor systems are currently being developed with the sponsorship of DOE/METC. The General Electric (GE) moving-bed system is currently being demonstrated in a pilot plant.9910 Fluidized-bed reactor systems using zinc ferrite sorbents are being developed at the Research Triangle Institute (RTI) and at DOE/METC. A major obstacle to commercialization of fluidized-bed hot-gas cleanup systems is poor sorbent durability, i.e., excessive sorbent loss from the reactor in the form of fines due to attrition. Sorbents for fluidized-bed reactors must withstand stresses induced by rapid temperature swings, chemical transformations, (4) Grindley, T.; Goldsmith, H. 'Development of Zinc Ferrite Desulfurization Sorbents for Large Scale Testing". Presented at the AIChE Annual Meeting, Session 114d, New York, November 15-20, 1987. (5) Gangwal, S. K.; Harkins, S. M.; Stogner, J. M.; Bossart, S. J. Testing of Novel Sorbents for H2S Removal from Coal Gas. Enuiron. Prog. 1989, BO), 26. (6)Haldipur, G. B.; Schmidt, D. K.; Smith, K. J.; Lucas, J. L. KRW Process Development Coal Gasification/Hot Gas Cleanup. In Proceedings of the S h e n t h Annual Gasification and Gas Stream Cleanup Systems Contractors Review Meeting; DOE/METC-87/6079, Vol. 2, NTIS/DE87006496, 1987; pp 668-677. (7) Wu, J. C.; Robin, A. M.; Kassman, J. S. Integration and Testing of Hot Gas Desulfurization and Entrained Flow Gasification for Power Generation Systems. In Proceedings of the Ninth Annual Gasification and Gas Stream Cleanup Systems Contractors Review Meeting; Vol. 1, DOE/METC-89/6107, Vol. I (DE89011706),June 1989. (8) Gangwal, S. K.; Harkins, S. M.; Woods, M. C.; Jain, S. C.; Bossart, S. J. Bench Scale Testing of High-Temperature Desulfurization Sorbents. Enuiron. Prog. 1989, 8(4), 265. (9) Gal, E. M.; Cook, S. C.; Furman, A. H.;Smith, D. P. Design Studies for Hot Gas Cleanup System in a Load Following Mode. Proceedings of the VIIth Annual Gasification Gas Stream Cleanup Systems Contractors Review Meeting; DOE/METC-87/6079, Vol. 2 (DE87006496),1987; pp 668-677. (IO) Cook, C. S.; Parekh, B.; Gal, E.; Furman, A. H. Integrated Operation of a Pressurized Fixed-Bed Gasifier and Hot Gas Desulfurization Systems. In Proceedings of the Ninth Annual Gasification and Gas Stream Cleanup Systems Contractors Review Meeting; DOE/METC89/6107, Vol. I, NTIS/DE89011706, June 1989; pp 37-46.
Reactor
From Rengeneration Preheater From Sulfidation .-L Preheater
1 Ceramic Packing
Figure 2. Fluid-bed desulfurization reactor.
and fluidization and transport. This paper reports the development and testing of zinc ferrite sorbents for fluidized-bed applications. It briefly describes RTI's fluidized-bed, bench-scale reador facility, sorbent characterization techniques, and testing of zinc ferrite sorbent formulations prepared using a variety of methods,
Bench-Scale Reactor System The cyclic (sulfidation/regeneration) desulfurization tests for sorbent development were carried out in a highpressure, semibatch, bench-scale reactor designed for operation at pressures up to 350 psig and temperatures up to 850 "C. Figure 1 is a schematic diagram of the test facility. The reactor is constructed using a 4-in., schedule-160,316 stainless steel pipe. Most of the other system components are constructed with either 316 or 304 stainleas steel. All hot H,S-wetted parts are Alon-processed (a high-temperature aluminum vapor treatment) to prevent corrosion of stainless steel by sulfur gases in the presence of steam. The main components of the reactor facility are (1)gas delivery system, (2) reactor assembly, (3) data acquisition and process control, (4) gas analysis system, and (5) reador off-gas venting system. Each is briefly described below; further details are presented e1sewhere.l' A battery of seven mass flow controllers capable of operation at pressures up to 100 atm controls the flow rate and composition of simulated coal gas using bottled gases for CO, H2, C02, N2, H2S, 02,and air. A positive displacement pump feeds deionized water to a boiler and superheater to generate steam. The delivery system can generate simulated coal gasifier gases representative of all types of gasifiers. The fluid-bed desulfurization reactor is shown in Figure 2. The unique feature of this reactor is a removable cage for easy loading and unloading of the sorbent. The reactor can accommodate both 7.62 cm (3 in.) and 5.1 cm (2 in.) diameter sorbent cages. A removable a-alumina distributor plate is positioned at the bottom of each cage to introduce hot coal gas into the reactor. The reactor is housed inside a three-zone furnace equipped with separate temperature controllers for each zone and can heat the (11) Gupta, R.; Gangwal, S. K. 'Enhanced Durability of Desulfurization Sorbents for Fluidized-bed Applications"; Topical Report to DOE/METC, no. DOE/MC/25006-3011,NTIS/DE91002090,June 1991.
Zinc Ferrite Sorbents
reactor up to 850 "C. Ceramic thimble filters downstream of the reactor capture particulates from the sflidation and regeneration exit lines prior to condensers. The reactor exit gas, after passing through the thimble filters, is cooled using heat exchangers. The reactor temperature is monitored at the bed inlet below the distributor, halfway in the bed, and at the bed outlet in the freeboard using Type-K thermocouples. The thermocouples are equipped with a digital display and are connected to a data acquisition system. Pressure is controlled precisely by two back-pressure regulators in series. An electronic differential pressure monitor across the reactor provides an indication of fluidization behavior of the hot sorbent in the reactor and detects an onset of reactor plugging. The reactor plugging can possibly be caused by reduction of zinc oxide present in the sorbent by Hz and CO in the coal gas, followed by zinc vaporization and its subsequent condensation in the gas exit lines resulting in a solid mass. A small slipstream of steam-free gas from the reactor is diverted to an on-line gas analysis system, which consists of two gas chromatographs (GCs): a Carle series 400 AGC with a thermal conductivity detector (TCD), and a Varian 3300 with a flame photometric detector (FPD). Both FPD and TCD are connected to Spectra Physics SP4270 integrators to intermittently measure the gas composition. Multiple GC sampling valves and dual loops in the Varian FPD measure H$ and COS from 1500 ppmv to
