Energy & Fuels 2008, 22, 1021–1026
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Screening of Zinc-Based Sorbents for Hot-Gas Desulfurization Joong B. Lee,† Chong K. Ryu,*,† Chang K. Yi,‡ Sung H. Jo,‡ and Sung H. Kim*,§ Global EnVironment Group, Korea Electric Power Research Institute, 103-16 Munjidong, Yusungku, Taejeon 305-389, Korea, Korea Institute of Energy Research, 71-2 Jangdong, Yusungku, Taejeon 305-343, Korea, and Department of Chemical and Biological Engineering, Korea UniVersity, 1-5 Ka Anam-dong, Seoungbuk-ku, Seoul 136-701, Korea ReceiVed July 15, 2007. ReVised Manuscript ReceiVed October 29, 2007
Highly reactive and attrition-resistant ZnO-based sorbents that are suitable for bubbling fluidized-bed reactors can be produced using the spray-drying method. Most of the ZnO-based sorbents prepared here (ZAC-X, X ) 18N-25N) satisfy the physical and chemical criteria for bubbling fluidized-bed application [spherical shape, average particle size, 90-110 µm; size distribution, 40-230 µm; bulk density, 0.9-1.0 g/mL; attrition index (AI), 40-80%; sulfur sorption capacity, 14-17 wt %; sorbent use, 70-80%]. The performance test of the ZAC-C sorbent at Korea Institute of Energy Research (KIER) with a bubbling fluidized-bed for 70 h also demonstrated that it had good sulfidation and regeneration performance (11 wt % sorption capacity and 52% sorbent use) as well as reasonable attrition resistance (1.1% attrition loss for 70 h).
Introduction Recently, a number of environmental issues have been raised worldwide. With regard to CO2 emissions, which can lead to climate change, and SOx and NOx emissions, which cause acid rain, the focus in most industrially developed countries has been on the use of clean fossil energy. Coal is the most commonly used fossil fuel in electricity-generating units. The limitations of coal-fired power plants has been the main driving force behind technological innovations aimed at improving both the efficiency and environmental performance of power generation from coal. Emerging technologies, such as the integrated gasification combined cycle (IGCC), is based on coal gasification rather than combustion, and they require that sulfur-bearing species be removed at high temperatures to conserve the sensible heat in the gasifier fuel gas, which would improve the efficiency of this process. In addition, it is economically and environmentally desirable to use regenerable sorbents for the removal of sulfur species. Hot-gas desulfurization (HGD) was selected as the key technology, which has benefits, including higher plant efficiency (>45%), lower capital cost, and less environmental impact.1 HGD in Korea has focused on process and sorbent development. The desulfurization process using fluidized-bed/transport reactors has several advantages over fixed-bed and moving-bed processes. These include excellent gas contact with smaller sorbent particles, ease of sorbent refill and removal, ease of controlling the exothermic regeneration reaction temperature, and a continu* To whom correspondence should be addressed. Telephone: +82-42865-5770. Fax: +82-42-865-5725. E-mail:
[email protected] (C.K.R.); Telephone:+82-2-3290-3297.Fax:+82-2-926-6102.E-mail:
[email protected] (S.H.K.). † Korea Electric Power Research Institute. ‡ Korea Institute of Energy Research. § Korea University. (1) U.S. Department of Energy (DOE). Clean Coal Technology Demonstration Program. Project Fact Sheets, Report to DOE/METC. Report number DOE/FE-0339. Morgantown Energy Technology Center, U.S. Department of Energy, WV, July, 1995.
ous steady operation.2 Because of its origin, sorbent development was aimed at fluidized beds and transport-reactor applications in Korea. Gupta and co-workers2,3 suggested that the sorbents used in transport-reactor applications must have a high attrition resistance, good sorption capacity with fast chemical reactivity within a short contact time of 1-2 s, low-temperature initiation of regeneration, good regenerability, and good flowability. Therefore, the key characteristic for fluidized applications is the ability of the sorbents to retain their reactivity and physical integrity during repeated sulfidation and regeneration cycles in fast sorbent circulating processes under harsh conditions. Early studies on the IGCC system integration reported that the optimum desulfurization temperature appears to be in the range of 350-600 °C.4 A recent study on the impact of lower temperature hot-gas cleanup on the performance of the baseline plant designs (400 MWe KRW IGCC) supports the previous results with the range of 370-604 °C.5 A current U.S. VISION 21 program also revealed two ultra-low gas stream purification process programs: one being developed at the Research Triangle Institute (RTI), which is in the range of 150-450 °C,6 the other (2) Gupta, R. P.; Gangwal, S. K.; Ciecero, D. C.; Henningsen, G. B.; Katta, S. Hot Coal Gas Desulfurization in Fluidized-Bed Reactors Using Zinc Titanate Sorbents. In High Temperature Gas Cleaning; Schmidt, E., et al., Eds.; University of Karlsruhe: Karlsruhe, Germany, 1996; pp 543– 556. (3) (a) Gupta, P. K.; Turk, B. S.; Vierheilig, A. A. Desulfurization Sorbents for Transport-Bed Applications. Proceedings of the Advanced CoalBased Power and Environmental Systems 1997 Conference, Pittsburgh, PA, July 22–24, 1997. (b) Gupta, P. K.; Turk, B. S.; Vierheilig, A. A. Desulfurization Sorbents for Transport-Bed Application. Proceedings of the Advanced Coal-Based Power and Environmental Systems 1998 Conference, Morgantown, WV, July 21–23, 1998 (2A.2 on CD-ROM). (4) NOVEM. System Study High Temperature Gas Cleaning at IGCC System. Netherlands Agency for Energy and the Environment, 1991. (5) Rutkowski, M., et al. IGCC Database and Hot Gas Cleanup Systems Studies. Proceedings of the Coal-Fired Power Systems 93—Advances in IGCC and PFBC Review Meeting, DOE/METC-93/6131, 1993; p 359. (6) Gupta, R. P.; Turk, B. S.; Merkel, T. C.; Krishnan, G. W.; Ciecero, D. C. Gaseous Contaminant Control for IGCC Application. Proceedings of the 17th Annual International Pittsburgh Coal Conference, 2000 (available on CD-ROM).
10.1021/ef7004089 CCC: $40.75 2008 American Chemical Society Published on Web 01/12/2008
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Figure 1. Schematic diagram of the thermogravimetric analyzer. Table 1. TGA Test Conditions and Gas Compositions items gas composition (vol %) temperature (°C) pressure sample load (mg) total gas flow rate (mL/min)
sulfidation 3% H2S, 9.7% H2, 0.4% CH4, 17.5% CO, 6.1% CO2, N2 balance 500 ambient 10 50
regeneration
Table 2. Test Conditions for the BSU (Batch) Run at KIER items
air
gas composition (vol %)
650 ambient 10 50
temperature (°C) pressure (atm) sorbent inventory (g) superficial gas velocity (m/s) total gas flow rate (slpm) bed height (m)
at the Gas Technology Institute (GTI), which is in the range of 350-550 °C.7 In the lower temperature HGD process, the thermodynamic equilibria of many metal oxides improve as the temperature decreases, making many metal-oxide sorbents suitable for the HGD process in the range of 350-550 °C.7 Although most early HGD processes used ZnO-based sorbents, which suffer from sulfate formation and Zn volatilization, this lower temperature process would promote the better use of regenerable ZnO-based sorbents because of their H2S sorption capacity and fast kinetics. Zinc titanate and zinc oxide are some of the leading sorbents currently used. The early types of such sorbents were prepared using pellet, extrusion, and granulation techniques, which were reviewed by Mitchell.8 With the adaptation of fluidized-bed/transport reactor technology to HGD, the spray-drying technique provides sorbent particles with a highly uniform size and 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.9,10 With the adaptation of the fluidized/transport desulfurization to remove H2S from hot-gasified fuel gas streams, this study (7) Abbasian, J.; Slimane, R. B.; Williams, B.; Akpolat, O.; Lau, F.; Newby, R. A.; Jain, S. Novel Gas Cleaning/Conditioning for Integrated Gasification Combined Cycle. Proceedings of the 17th Annual International Pittsburgh Coal Conference, 2000 (available on CD-ROM). (8) Mitchell, S. C. Hot Gas Cleanup of Sulphur, Nitrogen, Minor and Trace Elements. IEA Coal Research, 1998 (CCC/12, IBBN ). (9) Ryu, C. K.; Lee, J. B.; Ahn, D. H.; Kim, J. J. Hot Gas Desulfurization Sorbents for Fluidized-Bed Applications. Proceedings of the 18th Annual International Pittsburgh Coal Conference, 2001 (available on CD-ROM).
sulfidation
regeneration air
H2 (18), CO (38), CO2 (17), H2O (10), H2S (1), N2 balance 500 5 1500 0.05
500 5 1500 0.05
39
39
0.23
0.23
prepared a series of spray-dried regenerable ZnO-based sorbents that are suitable for sulfidation and regeneration reactions in the range of 450–550 °C., Experimental Section 1. Sorbent Preparation. A single- and multibinder matrix system and direct incorporation of the regeneration promoter was adopted. Sorbent forming was performed using a spraydrying technique that is easily scalable to commercial levels. The spray-drying process of the sorbent includes the selection and comminution of raw materials, colloidal slurry preparation, spray drying, and calcination steps. Among these steps, the colloidal slurry preparation is the most important step for yielding sorbent particles suitable for fluidized-bed applications. The (10) Ryu, C. K.; Lee, J. B.; Ahn, D. H.; Kim, J. J., Yi, C. K. Highly Attrition Resistant Zinc Oxide-Based Sorbents for H2S Removal by Spray Drying Technique. Proceedings of the 5th International Symposium on Gas Cleaning at High Temperature, Morgantown, WV, September 17–20, 2002 (available on CD-ROM). (11) Yi, C. K.; Jo, S. H.; Han, M. H.; Son, J. E.; Jin, G. T. Continuous Operation for 100 h in a Bench-Scale Fluidized Hot Gas Desulfurization Process. Proceedings of the 18th Annual International Pittsburgh Coal Conference, 2001 (available on CD-ROM). (12) Yi, C. K.; Jo, S. H.; Jin, G. T.; Ahn, Y. S.; Han, M. H.; Son, J. E.; Ryu, C. K. Continuous Operation of Spray-Dried Zinc Based Sorbent in a Hot Gas Desulfurization Process Consisting of a Transport Desulfurization and a Fluidized Regenerator. Proceedings of the 5th International Symposium on Gas Cleaning at High Temperature, Morgantown, WV, September 17–20, 2002 (available on CD-ROM).
Hot-Gas Desulfurization of Zinc-Based Sorbents
Energy & Fuels, Vol. 22, No. 2, 2008 1023 AI ) [total fine 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 a total fine generated over 5 h and from a total amount of sample used initially CAI ) [(total fine collected for 5 h - fine collected for first 1 h)/ (amount of initial sample - fine collected for first 1 h)] × 100
Figure 2. Schematic diagram of the KIER fluidized HGD processes. Table 3. Slurry Properties of the ZAC Series Sorbents group A sorbent binder matrix scale (kg) pHa viscosity (cP)b solid content (wt %)
group B sorbent
group C sorbent
18N/19N
20N
21N
22N
23N
24N
single 2/2 7.22 605/1400 40
single 4 7.41 568 38.7
binary 4 7.49 1210 35
binary 8 7.43 490 43.1
ternary 16 7.76 778 39.4
ternary 16 7.44 585 39.7
a pH was measured without any dilution of slurry. b Viscosity was measured with spindle number 4 at 60 rpm (Brooks Instrument Model number RV DV I+).
colloidal slurry must be flowable, homogeneous, dispersed, and stable. These properties are controlled by the concentration, viscosity, and pH through the addition of organic additives, such as dispersants. With the direct incorporation of the regeneration promoter into the slurry, a number of formulations with multibinder matrices prepared by spray drying satisfy the criteria for fluidized-bed applications. These are designated as ZAC-X sorbents (X ) 18N-25N). Several zinc-based sorbents were prepared in this study. First-, second-, and third-group sorbents were prepared with different binder matrices but with the same weight percentage of the total binder. For each group, the sorbents consisted of 50 wt % of the active component and 50 wt % matrices containing the binder including a Ni-based promoter by 7.5 wt %. 2. Sorbent Characterization. (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 the fluidizedbed application was measured with a modified three-hole air-jet attrition tester based on the American Society for Testing and Materials (ASTM) D 5757-95. The attrition was determined at 10 standard L/min (slpm) over 5 h as described in the ASTM method and also at 7 slpm for comparitive purposes. The attrition index (AI) is the percent fines generated over 5 h. The fines were collected after 1 and 5 h from the start
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 below 30% for the transport reactor or even below 60% for the bubbling fluidized-bed reactor. A lower value of AI or CAI indicates better attrition resistance of bulk particles. However, the attrition test presented here is merely the physical attrition properties of sorbent particles because the sorbent undergoes not only the physical attrition but also chemical attrition caused by continuous volume changes during repeated sulfidation and regeneration reactions in practice. (b) Particle Size and Particle Size Distribution (PSD). The PSDs of the samples, both as fresh (in the calcined state) and after air-jet attrition testing, were measured using a MEINZER II sieve shaker. The results of several measurements for each sample were averaged to minimize the error. ASTM E-11 shows the detailed sampling and measuring procedures. Both distributions were combined according to the weight percentage of each portion to calculate the average particle size. (c) Tapping (or Bulk) Density. The tapping density of each sorbent was determined using the Autotap instrument (Quanta chrome) proposed in ASTM D 4164-88, which has been demonstrated to mechanically measure the tapped density that is characteristic of the formed catalyst or catalyst carrier. The tapping density was obtained by dividing the known particle mass by its tapped volume. (d) Brunauer–Emmett–Teller (BET) Surface Area and Pore Size Distribution. The BET surface areas and pore size distributions of the sorbents were determined by N2 physisorption using a Quantachrome Autosorb-2000 automated system. These parameters were determined only for fresh calcined sorbents. Each sample was degassed in the system at 100 °C for 1 h and then at 200 °C for 2 h, before each measurement. (e) Particle Morphology (Shape). The particle morphology for each sorbent was obtained using a Jeol JSM 6400 scanning electron microscope (SEM). The samples were coated with gold before the measurements. (f) ThermograVimetric Analyzer. The chemical reactivity of each sorbent was assessed using a thermogravimetric analyzer (STA 1500, Reometircs). Figure 1 shows a schematic diagram of the thermogravimetric analyzer. The powder was sieved to a fraction with sizes ranging from 38 to 212 µm in diameter and used in the thermogravimetric analysis (TGA) tests. The flow rate of the reaction gas was 50 mL/min at NPT. The reaction conditions and gas compositions were the same as those shown in Table 1. The sulfur (H2S) sorption capacity was determined from the increase in weight during sulfidation, and the regenerability was also estimated from the decrease in weight during oxidative regeneration. The data were used to guide the desulfurization cycle tests in the 4 in. fluidized-bed reactor. In the TGA test, the weight of a powder sample hanging from a microbalance was monitored as a function of time. The weight gain is proportional to the extent of the reaction in the sample. The scope and baseline tests were carried out in the temperature range of 400-650 °C and at 500 °C, respectively, using simulated coal gas (KRW gasifier), except for 3 vol % of H2S and 10 vol % of H2O. (g) Fluidized-Bed Reactor Test. The sulfidation cycle tests were performed using the apparatus at Korea Institute of Energy Research (KIER) with a bubbling fluidized-bed reactor (75.5 mm I.D.) under pressurized conditions. An online gas chromatography (HP 5890
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Figure 3. SEM images of ZAC-9 (left, 100×) and ZAC-9N (right, 250×). Table 4. Physical and Chemical Properties of the ZAC-4N and ZAC-9N Sorbents ZAC-9N (HTC)
ZAC-9N (AC)
properties
ZAC-4N
average particle size (µm) size distribution (µm) surface area (BET) (m2/g) bulk density (g/cc) attrition resistance (%) AI (CAI) at 7 slpm AI (CAI) at 10 slpm fresh TGA sulfur capacity (wt %)
109 56–178 20 1.1
102 48–171 17–19 1.1
102 48–171 20 1.1
4.6 (2.0) 33 (22) 21
7.2 (2.0) 41 (17–22) 20.5
75 (40) 17
series II with a flame photometric detector) and two continuous H2S gas analyzers (Radas 2, Hartmann and Braun Co.), a paramagnetic oxygen analyzer (775R, Rosemount Analytical, Inc.), an IR-type SO2 analyzer (URAS 4, Hartmann and Braun Co.), and a TCD-type H2 analyzer (CALDOS5G, Hartmann and Braun Co.) were used to analyze the exit gas stream. Table 2 gives a summary of the reaction conditions. Sulfidation was performed by introducing a simulated gas mixture, which simulated the coal gas produced in the dry-coal fed, air-blown KRW coal gasifier. When the H2S concentration of the exit gases for sulfidation reached breakthrough, which is specified as 200 ppmv, the inlet stream of the simulated coal gases was stopped and inert nitrogen gas was introduced to purge the system until no reactant could be detected. The sulfided sorbents were then regenerated by introducing dry air in the same manner as in the sulfidation process. Figure 2 gives a schematic diagram of the experimental setup used in this study.
Results and Discussion A series of zinc-based sorbents were prepared to screen the chemical reactivity and attrition resistance. Table 3 shows the slurry properties of the sorbent formulations that were suitable to form spherical solid particles by the spray drier. 1. Early Development of ZAC-4N and ZAC-9N Sorbents by Spray Drying. During the earlier stages of the sorbent screening process using the spray-drying technique, KEPRI prepared 11 initial ZnO-based sorbent formulations, which are designated as the ZAC-X series (X ) 1-11). These formulations used a single ceramic binder on a 2 kg solid scale and were prepared by varying the organic additives and the operation conditions of the spray drier. Only four formulations yielded spherical sorbents, with a mean particle size of 110 µm and a size distribution of 40-180 µm. After the spray-dried green particles were predried and calcined, the two formulations were further incorporated with the regeneration promoter using a dryimpregnation method and are designated as ZAC-4N and ZAC9N. The SEM photographs of ZAC-9 and ZAC-9N (AC) in Figure 3 show that the particles of the impregnated promoter are mainly coated at the outer surface of the sorbent particles. Table 4 shows a comparison of the physical and chemical properties of the ZAC-4N and ZAC-9N sorbents. ZAC-9N
(HTC) and ZAC-9N (AC) represent hydrothermal and air calcinations, respectively. The bulk density and BET surface area of ZAC-4N and ZAC-9N sorbents are similar and 1.1 g/cc and about 20 m2/g, respectively. The attrition indices of both sorbents were measured initially at 7 slpm at 273.17 K and 1 bar for 5 h using a modified attrition tester based on ASTM D 5757-95 (three-hole air jet). The results at 7 slpm show that the AI and CAI were acceptable (
