Investigation on Desulfurization Performance and Pore Structure of

Energy & Fuels 1997, 11, 887-896

887

Investigation on Desulfurization Performance and Pore Structure of Sorbents Containing Zinc Ferrite Makoto Kobayashi,* Hiromi Shirai, and Makoto Nunokawa Yokosuka Research Laboratory, Central Research Institute of Electric Power Industry, Nagasaka 2-6-1, Yokosuka 240-01, Japan Received December 26, 1996X

Regenerable and durable sorbents for high-temperature sulfur removal are desired to achieve advanced power generation using coal gas fuel. Different precipitation methods using ammonia and urea were studied in this work to prepare zinc ferrite-silica composite particles. Two sorbents containing those powders were extruded and were evaluated in desulfurization performance and durability during cycles of repeated desulfurization at pressurized simulated coal gas conditions. Both sorbents could reduced sulfur compounds to less than 1 ppm during consecutive desulfurization cycles operated at 723 K and 0.98 MPa. The sorbent prepared by ammonia precipitation exhibited higher performance at initial sulfidation but declined in sulfidation kinetics after 20 cycles of tests. The urea-precipitated sorbent maintained its performance during the same desulfurization cycles. Although the residual sulfur was gradually increased for both sorbents, the sorbents maintained over 75% of their initial sulfur capacity at the end of the test. Pore distribution obtained by mercury porosimetry showed significant loss of mesopores of the ammonia-precipitated sample during the cycles, while the urea-precipitated sorbent maintained its mesopore structure. The primary particles of zinc ferrite in the fresh and spent sorbents were observed using FE-TEM. Primary particles of zinc ferrite in the ammonia-precipitated powder were much smaller (10-20 nm) than those of the urea-precipitated powder (100 nm diameter). Rapid decline in the ammonia-precipitated sorbent was due to the sintering of the fine primary particles. The latter kept their size after cyclic desulfurization. The durability of the sorbent was strongly affected by the primary particle size as determined by the preparation method and sorbent preparation is an important factor to enhance durability of the sorbent.

Introduction A high-performance cleanup system is essential for the purification of coal gas for use in molten carbonate fuel cells.1 The system can be simplified by introducing an advanced desulfurization process that requires a high-performance desulfurization sorbent.2 Many attempts have been made to develop high-temperature desulfurization sorbents, using iron oxide, zinc ferrite,3-5 zinc titanate,6 and other metal oxides.7 We propose that low-temperature operation of sorbent containing zinc ferrite as the oxide has favorable thermodynamic equiAbstract published in Advance ACS Abstracts, June 1, 1997. (1) Kinoshita, K.; McLarnon, F. R.; Cairns E. J. U.S. DOE Report DOE/METC-88/6096, 1988. Pigeaud, A. U.S. DOE Report DOE/MC/ 25009-2838, 1989, pp 1-25. Kobayashi, M.; Nakayama, T.; Shirai, H.; Matsuda, H.; Tanaka, T. Denryoku Chuou Kenkyuuzyo Houkoku 1990, W90014. (2) Kobayashi, M.; Nakayama, T.; Matsuda, H.; Ito, S.; Shirai, H.; Tanaka T. Denryoku Chuou Kenkyuuzyo Houkoku 1991, W91016. (3) Underkoffler, V. S. U.S. DOE Report DOE/MC/21098-2247, 1986, Vol. 1, pp 1-182. (4) Gangwal, S. K.; Harkins, S. M.; Woods, M. C.; Jain, S. C.; Bossart; S. J. Environ. Prog. 1989, 8, 265-269. (5) Ayala, R. E.; Gal, E.; Gangwal, S. K.; Jain, S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel. Chem. 1990, 35, 120-127. (6) Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Res. 1989, 28, 535-541. Lew, S.; Sarofim, A. F.; FlytzaniStephanopoulos, M. AIChE J. 1992, 38, 1161-1169. Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Chem. Eng. Sci. 1992, 47, 14211431. Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Res. 1992, 31, 1890-1899. (7) Anderson, G. L.; Berry, F. O.; Hill, A. H.; Ong, E.; Laurence, R. M.; Shah, R.; Feldkirchner, H. L. U.S. DOE Report DOE/MC/221442722, 1988, pp 1-166. X

S0887-0624(96)00231-9 CCC: $14.00

librium for sulfidation below 723 K.8 Sulfidation performance is usually diminished by limited kinetics at around this temperature. Although kinetics of zinc ferrite limit the sulfidation performance, the restriction can be significantly reduced by preparing it with homogeneous precipitation using urea. As we reported previously, homogeneous precipitation activates zinc ferrite so that the sorbent exhibits a higher sulfidation rate even at temperatures as low as 723 K.8 The other problem in using zinc ferrite was carbon deposition potentially expected to occur in a coal gas environment. This reaction is likely to initiated by metal carbide formed from reduced metals;9 iron will act as an initiator in the case of zinc ferrite. Additives such as silicon dioxide or sodium carbonate are effective at inhibiting the soot formation. We applied silicon dioxide additive to a iron oxide sorbent and zinc ferrite sorbent,10 which results in preventing soot formation at 723 K and 9.8 × 105 Pa. Supporting material has important rolls to maintain structural strength and to provide pores through which reactants and products are transferred between gas phase and reactive metal oxides. Titanium dioxide was a sufficient material as support for iron (8) Kobayashi, M.; Nunokawa, M.; Shirai, H.; Watanabe, M. CRIEPI Report EW93005, 1994, pp 1-44. (9) Sasaoka, E.; Iwamoto, Y.; Hirano, S.; Uddin, M. A.; Sakata, Y. Energy Fuels 1995, 9, 344-353. (10) Kobayashi, M.; Nunokawa, M.; Shirai, H. High Temperature Gas Cleaning. Proceedings of the Third International Symposium and Exhibition on Gas Cleaning at High Temperatures; Institut fu¨r Mechanische Verfahrenstechnik und Mechanik Universita¨t Karlsruhe (TH): Karlsruhe, Germany, 1996; pp 618-629.

© 1997 American Chemical Society

888 Energy & Fuels, Vol. 11, No. 4, 1997

oxide sorbent.11 Thus, we prepared high-performance desulfurization sorbents by supporting zinc iron double oxides and silicon dioxide on titanium dioxide. Test results in a desulfurization cycle that included reduction, sulfidation, and oxidation showed their ability to reduce sulfur compounds below 1 ppm and regenerability.12 Although the initial performance is acceptable to our target performance, the performance of the sorbent in repeated use of the sorbent in a high-temperature, high-pressure coal gas environment is gradually decreased.10 Residual sulfur and sintering are major possible causes of the performance loss, while the mechanism of degradation is still not clear. The degradation mechanism should be verified to enhance durability of the sorbent for reliable hot gas cleaning technology. The primary object of this study is to determine the degradation mechanism during multiple desulfurization cycles in a pressurized simulated coal gas environment by comparing the durability of sorbents prepared with different precipitation methods.

Kobayashi et al. Table 1. Specifications of Desulfurization Sorbents A and B precipitation method (precipitant) calcination temp of powder (K) Zn:Fe:Si molar ratio structure of Zn-Fe oxide content of Zn and Fe (mmol/g of sorbent) weight ratio of double oxide and support material (TiO2) calcination temp of pellet (K)

(11) Nakayama, T.; et al. CRIEPI Report EW89015, 1989. (12) Shirai, H.; Kobayashi, M.; Nunokawa, M.; Watanabe, M. Denryoku Chuou Kenkyuuzyo Houkoku 1995, W94006.

B coprecipitation (ammonia)

1.0:2.0:2.0 ZnFe2O4-SiO2 2.5

1.0:2.0:2.0 ZnFe2O4-SiO2 2.5

3.0:7.0

3.0:7.0

973

1073

1073

Table 2. Conditions for Pressurized Desulfurization Cycle Tests temp, K pressure, MPa gas comp, vol %

Experimental Section Preparation of Desulfurization Sorbent Pellets. The desulfurization sorbents consisted of oxide particles supported on titania. We prepared composite powders of zinc iron double oxide and silicon dioxide. The powder was prepared by precipitation from aqueous solutions of salts. The raw materials were zinc nitrate hexahydrate (Wako Pure Chemical Industries, Inc., S grade), iron(III) nitrate nonahydrate (Wako Pure Chemical Industries, Inc., S grade), and silica sol (Catalysts & Chemicals Industries Co., Ltd., Cataloid-SN). Solutions of these salts and the sol were mixed so that the molar ratio of Zn:Fe:Si was 1.0:2.0:2.0. The concentrations of zinc ion and iron ion in the mixture were 0.40 and 0.79 mol/L, respectively. As the sol was stabilized in acidic solution, gelation did not occur at this point. The mixture was then subjected to precipitation reactions. The precipitant was ammonia or urea solution according to the precipitation method, coprecipitation or homogeneous precipitation. Coprecipitation was carried out at room temperature by pouring the stoichiometric amount of ammonia solution into the mixture liquid with continuous stirring. The obtained precipitate then stood in the mother liquor for 12 h. In the case of homogeneous precipitation, urea solution was added to the mixture in a flask with a condenser. The molar ratio of urea:NO32- was 3.0:2.0. The flask containing the mixed solution was continuously stirred and was heated in an ethylene glycol bath at 378 K so that the solution temperature was stabilized between 369 and 371 K. This procedure was continued while bubbling of CO2 lasted, typically for 7 h. Then the precipitate stood for 8 h for aging. The precipitate was filtered from solution with a Kiriyama funnel and cleaned with distilled water until the pH of the filtrate was below 8. The precipitate was air-dried at 333 K for 8 h and crushed before calcination. It was then placed in a crucible made of alumina of high purity (>99.98%) and calcined in air for 6 h by heating in an electric furnace. The product was crushed and sieved through standard sieves. Particles of diameter less than 500 µm were used for the pellet preparation. Table 1 summarizes the preparation conditions and resulting properties of the pellet sorbents containing zinc-iron double oxide powder. Titanium(IV) oxide (anatase) powder, whose specific surface area is 25 m2/g, was used as a support-

A homogeneous precipitation (urea) 973

gas flow rate, L/min (NTP) space velocity, h-1 sorbent particle size, mm bed volume, mL sorbent weight, g

sulfidation

regeneration

reduction

723 0.98 CO, 20.0 CO2, 5.0 H2, 8.0 H2O, 5.0 H2S, 1.0 N2, balance 3.3

723 0.98 O2, 1.6 N2, balance

723 0.98 CO, 20.0 CO2, 5.0 H2, 8.0 H2O, 5.0 N2, balance

3.7

1.1

3200 1.0-1.4

3700 1.0-1.4

1100 1.0-1.4

61 60

61 60

61 60

ing material. The composite powder was mixed with the titanium dioxide to the ratio shown in Table 1 in presence of 29 wt % moisture. The mixture was kneaded at 353 K until the water content was reduced to 24% in weight. The mixture was extruded to a cylindrical shape of 3 mm diameter. The pellets were dried and were calcined at the temperatures of Table 1. The weight ratio of zinc to iron was adjusted to the stoichiometric value to form zinc ferrite. The preparation of sorbent A was virtually the same as for sorbent B, but the precipitant was urea and the calcination temperature was 973 K. The reason for the selected calcination temperature was that the urea-precipitated zinc ferrite had a higher initial sulfidation rate compared to the powder calcined at 1073 K, at which desulfurization of the product oxide was activated.13 Apparatus and Procedures. Fixed-Bed Reactor Tests. Sulfidation cycle tests were carried out in our apparatus with a fixed-bed reactor (37.5 mm i.d.) under pressurized conditions. A continuous sulfur concentration analyzer (Yanaco Ohgi, TSA-1001) equipped with a FPD detector to measure concentrations of sulfur compounds up to 2 vol % in desulfurization was used to analyze the exit gas stream. The data were used to calculate the sulfur balance of the sorbents. An oxygen analyzer (Servomex, Ltd., Oxygen Analyzer 540A) was used to determine termination of oxidation reactions. Reaction conditions are summarized in Table 2. The pellet sorbents were crushed and sieved to obtain pellets of 1.0-1.4 mm in diameter, which were packed into the fixed-bed reactor and heated to a predetermined temperature in a stream of nitrogen gas. Sulfidation was performed by introducing a gas mixture that simulates coal gas produced in a dry coal fed, air-blown coal gasifier. Sulfidation was continued until complete breakthrough. Desulfurization characteristics of the sorbent were determined on the basis of the concentration of sulfur com(13) Kobayashi, M.; Nakayama, T.; Shirai, H. Denryoku Chuou Kenkyuuzyo Houkoku 1992, W92026.

Sorbents Containing Zinc Ferrite

Energy & Fuels, Vol. 11, No. 4, 1997 889

Figure 1. Apparatus for evaluation of desulfurization sorbent for coal gas. pounds at the inlet and outlet of the sorbent bed during sulfidation. Regeneration was carried out in oxygen mixed with nitrogen as shown in Table 2, until breakthrough of oxygen. A reduction step followed regeneration to complete the removal of sulfur. Simulated coal gas without sulfur compounds was also used in reduction (Table 2). Sulfur elution from the reactor outlet during regeneration and reduction was analyzed to evaluate regenerability of the sorbent. The spent and fresh sorbents were tested in another fixedbed reactor that is specifically designed to determine the sulfur removal efficiency of sorbents. Figure 1 shows a schematic flow diagram of the apparatus for sulfidation reactions of desulfurization sorbent pellets and determination of sulfur compounds whose concentrations are less than 10 ppm under high-temperature and high-pressure conditions. The sorbents were settled in a 30 mm i.d. quartz tube inside a stainless steel pressure vessel of the reactor. The reaction gas was supplied as a mixture of several pure gases and steam so as to simulate coal gas including hydrogen sulfide. All pure gases were supplied from a gas cylinder, and the flow rate was controlled with mass flow controllers at 9.8 × 105 Pa. Steam was generated in an evaporator with a high-pressure dual cylinder pump and supplied through a heated tube. The part of the heated tube in contact with the reaction gas was coated with glass or Teflon to prevent corrosion or adsorption of contaminants. The pressure of the effluent gas from the reactor outlet was reduced to 2.5 × 105 Pa and introduced to sulfur analyzer units. The rest of the gas was cooled and mist eliminated for further analysis of gas composition. The samples were fresh sorbent or spent sorbent reduced after the final desulfurization cycle. The sorbents were reacted during reduction and sulfidation under essentially the same conditions as in the desulfurization cycle test. As indicated by italic type in Table 3, the altered conditions were flow rate, sulfur concentration, and amount of sorbent. In these tests, the gas flow rate was always 3.0 L/min and hydrogen sulfide was mixed at 0.1 vol % during sulfidation. The amount of sorbent packed in the reactor was 30 g, this being one-half the sorbent used in the desulfurization cycle tests. Sulfidation in all cases was preceded by 1 h of reduction with simulated coal gas without sulfur compounds. Desulfurization was conducted until the outlet concentration of total sulfur exceeded 10 ppm. Sulfur concentration at the outlet of the sorbent was measured with two instruments equipped with FPD detectors. Values less than 10 ppm were measured by gas chromatography (Yanaco Ohgi, AG-1/CTHFPD) that is capable to determine the concentrations of hydrogen sulfide and carbonyl sulfide up to 10 ppm. Sulfur concentration was expressed as the sum of those concentrations. Values exceeding 10 ppm were determined with a low-sulfur continuous monitor (Best Instruments Co., Ltd., BEX-70UN) whose measuring range is up to 60 ppm.

Table 3. Conditions for Sulfur Removal Efficiency Tests reduction

sulfidation

temp, K pressure, MPa gas comp, vol %

723 0.98 CO, 20.0 CO2, 5.0 H2, 8.0 H2O, 5.0 N2, balance

gas flow rate, L/min (NTP) space velocity, h-1 sorbent particle size, mm bed volume, mL sorbent weight, g

3.0 5800 1.0-1.4 31 30

723 0.98 CO, 20.0 CO2, 5.0 H2, 8.0 H2O, 5.0 H2S, 0.1 N2, balance 3.0 5800 1.0-1.4 31 30

Thermobalance Reactor Tests. A thermobalance reactor (ULVAC Shinku-Riko, Inc., TGD-5500RH S) was used to estimate products during reduction and the sulfur absorbing capacity of metal oxides. A schematic diagram of the thermobalance is shown in Figure 2. This apparatus was operated at 0.10 MPa. Samples were crushed in a mortar made of agate and sieved through standard sieves. The powder was sieved to a fraction of 53-125 µm in diameter and used in the thermobalance tests. The flow rate of the reaction gas was 380 mL/min at NTP. Reaction temperatures and gas compositions were the same as in Table 2. Sulfur capacity was determined from the weight increase during sulfidation. Reduction products were estimated from the weight change during reduction. The data were used to guide the desulfurization cycle tests in the fixed-bed reactor. Sorbent Characterization and Chemical Analysis. Specific surface areas of the prepared sorbents and spent sorbents were determined by the N2-BET method using the adsorption isotherm measured with an automatic BET instrument (BEL Japan Inc., Belsorp 36). Pore distribution and pore volume were determined with a mercury porosimeter (Micromeritics, Autopore II 9220). The crystal structure was identified from diffraction patterns between 2θ ) 20° and 80° obtained with an X-ray diffractometer (JEOL, JDX-8030). The microstructure of the zinc ferrite particle was observed by a transmission electron microscope equipped with a field emission type electron gun (HITACHI, HF-3000). The sample was crushed and dispersed in ethanol and dropped on a collodion membrane. The acceleration voltage of the microscope was 300 kV. Magnification of TEM images were between 8 × 104 and 1.6 × 106. The selected area image and selected area diffraction were obtained to discuss the crystalline size of the primary particle. Sulfur contents of sorbents are measured by an instrument (Horiba, Ltd., EMIA-1120) which integrates the amount of sulfur dioxide evolved from a sample incinerated in oxygen. Zinc content and iron content of fresh and spent sorbents are determined by chemical analyses. Zinc and iron contained in


Kobayashi et al.

Figure 2. Schematic diagram of thermogravimetric analyzer apparatus. the sorbents were dissolved in an aqueous solution. The zinc concentration was determined with an atomic adsorption spectrometer (Hitachi, Z-8000) at a wavelength of 213.8 nm. The iron concentration was measured by an inductively coupled plasma atomic emission spectrometer (Shimadzu, ICPS-2000) at a wavelength of 259.9 nm.

Reduction ZnOFe2O3 + 1/3H2 f ZnO + 2/3Fe3O4 + 1/3H2O (1) Sulfidation ZnO + 2/3Fe3O4 + 3H2S + 2/3H2 f ZnS + 2FeS +

Results and Discussion Chemical Reactions in a Desulfurization Cycle. Reaction products related to zinc ferrite in a desulfurization cycle are extensively studied and authorized.14 We conducted desulfurization cycle tests in the thermobalance instrument to confirm applicability of those reaction schemes to developed sorbent in the conditions applied in this study. Reduction tests of zinc ferrite powder conducted in the thermobalance apparatus at 723 K and 0.1 MPa showed that the samples lost about 2.3% of the initial weight. This value is very close to the theoretical weight loss, 2.2%, which assumes reduction products ZnO and Fe3O4. The weight loss was apart from the theoretical values presuming elemental zinc formation and metal iron formation of 6.6% and 20.0%, respectively. The reduction products of zinc ferrite are thus zinc oxide and magnetite as shown in eq 1. We assumed that the sulfidation reactions of the products were stoichiometric. Then we obtain the sulfidation reaction scheme as in eq 2, and it can be expressed in equivalent equations as eqs 3 and 4. Sulfidation test results indicated the conversion of zinc ferrite to exceed 0.9 assuming eq 2. This is consistent with the stoichiometry of the sulfidation reaction. The regeneration of sulfides is believed to be as in eq 5. Sulfate may also be produced from reactions of corresponding sulfides with oxygen and evolved sulfur dioxide as in eqs 6-8. During reduction of regenerated sorbent in simulated coal gas without sulfur compounds using a fixed-bed reactor, sulfur evolution was detected at the outlet of the sorbent. Thus the decomposition of sulfate formed in regeneration may occur. The reaction schemes of sulfate decomposition in eqs 9 and 10 appear applicable. (14) Focht, G. D.; Ranade, P. V.; Harrison, D. P. Chem. Eng. Sci. 1988, 43, 3005-3013. Focht, G. D.; Ranade, P. V. ; Harrison, D. P. Chem. Eng. Sci. 1989, 44, 2919-2926.

2

11 H O (2) 3 2

ZnO + H2S f ZnS + H2O

(3)

/3Fe3O4 + 2H2S + 2/3H2 f 2FeS + 8/3H2O

(4)

Regeneration and Sulfate Formation ZnS + 2FeS + 5O2 f ZnOFe2O3 + 3SO2

(5)

ZnS + 2O2 f ZnSO4

(6)

ZnO + 1/2O2 + SO2 f ZnSO4

(7)

2FeS + 5O2 + SO2 fFe2(SO4)3

(8)

Reduction of Residual Sulfate ZnSO4 + 4H2 f ZnS + 4H2O Fe2(SO4)3 + 10H2 f 2FeS + SO2 + 10H2O

(9) (10)

We examined data from chemical analysis of residual sulfur and metal content in the spent sorbent and other test data in the following sections on the basis of the reactions described above. Sulfur Removal Efficiency. Results obtained in the desulfurization tests in the fixed-bed apparatus are shown in Figure 3 and Figure 4 for sorbents A and B, respectively. The breakthrough curves in Figure 3 indicate sulfur removal characteristics in the first three cycles and in the 21st cycle. In the first cycle, the period for maintaining the outlet concentration below 1 ppm was slightly shorter than that of the following cycles; sorbent A reduced the outlet sulfur concentration below 1 ppm for about 100 min through the second to 21st cycle. During this 100 min, the sorbent absorbed 13.4 mmol of hydrogen sulfide; this corresponds to 18% of the sulfur capacity in the second cycle. When outlet sulfur exceeded 10 ppm, sorbent A absorbed 50 mmol



Figure 3. Characteristics of sulfur removal by sorbent A.

Figure 6. Breakthrough curves of sulfur compounds for sorbent B.

Figure 4. Characteristics of sulfur removal by sorbent B.

Figure 7. Variation in sulfur capacity of desulfurization sorbents during 20 desulfurization cycles.

Figure 5. Breakthrough curves of sulfur compounds for sorbent A.

of sulfur in this cycle. Though sorbent B decreased the sulfur level below 1 ppm in the first cycle, the outlet sulfur concentration exceeded 1 ppm at only 60 min in the 21st cycle. The outlet sulfur concentration at the beginning of sulfidation in the 21st cycle was higher than that of the first cycle, indicating that the sulfur removal efficiency of this sorbent was reduced with cycling. Sulfur Absorbing Capacity. The sulfur absorbing capacity was determined from the breakthrough curves in the cyclic desulfurization tests in the fixed bed reactor. Subsets of typical breakthrough curves up to 20 cycles are shown in Figure 5 and Figure 6 for sorbents A and B, respectively. Sulfur uptake was integrated until the outlet sulfur concentration exceeded 90% of the inlet concentration. Figure 7 shows the variation in sulfur capacity for the two sorbents for the first 20 desulfurization cycles. The ratio of this capacity in the final cycle to the first cycle was around 75% for

Figure 8. Sulfur capacity and residual sulfur of desulfurization sorbents during desulfurization cycles.

both sorbents (the capacity of sorbent A is larger than that of sorbent B during these 20 cycles). The capacities of both sorbents also appear to converge to about same final value. Degradation of sulfur absorbing capacity was attributed to potential residual sulfur after reduction, metal vaporization, or sintering. Figure 8 shows the sulfur balance observed in cyclic desulfurization tests of sorbents A and B. Absorbing capacity was determined from the breakthrough curve as in Figure 7. The residual sulfur was determined from analysis of sulfur in the spent sorbent after the reduction step, i.e. just


Figure 9. Metal content in the sorbents during desulfurization cycles.

prior to the next sulfidation. The figure indicates that the amount of residual sulfur is equal to the decrease in sulfur absorbing capacity observed in the first few cycles. However, at the 20th cycle, both sorbents showed an additional decrease in absorbing capacity. Metal vaporization and sintering were then examined as possible causes of the decrease in sulfur absorbing capacity. Metal vaporization can occur as the sorbents contain zinc oxide which can be reduced by coal gas and has considerable vapor pressure at high temperature.15 The metal content was analyzed for the two sorbents and is summarized in Figure 9. The content was converted to the weight of metal per unit weight of fresh sorbent. This figure shows that vaporization of zinc apparently did not occur during the 20 desulfurization cycles. However, it is known from the literature16 that migration of zinc to the surface of the particle can take place, leading to sintering and physical structure degradation. Additionally, thermal sintering without zinc migration is also possible. The changes of the physical properties of the sorbents during desulfurization cycles were next studied. Effect of Desulfurization Cycles on the Physical Properties of the Pellets. The specific surface areas of the reduced and regenerated sorbents are shown in Figure 10. The specific surface area of the sorbent B considerably decreased with the number of desulfurization cycles, while that of sorbent A only slightly decreased during these cycles. The large decrease in specific surface area of sorbent B well corresponds to the larger additional decrease in sulfur absorbing capacity after the 20th cycle (Figure 8). As the decrease was not apparent at earlier cycles, however, while the decrease in surface area was continuous, this explanation is not adequate. We have to consider the combined effect of sulfate formation and sintering on pore structure because both of them may cause reduction of the surface area. Effects of Desulfurization Cycles on Rate of Desulfurization. We evaluated the sulfidation reaction rate of the sorbents from the profile of the breakthrough curve obtained in the series of desulfurization tests. Figures 5 and 6 show variation in the break(15) Sasaoka, E.; Hirano, S.; Kasaoka, S.; Sakata, Y. Energy Fuels 1994, 8, 763-769. (16) Lew, S. High-Temperature Sulfidation and Reduction of Zinc Titanate and Zinc Oxide Sorbents. Doctoral Thesis, MIT, 1990. Flytzani-Stephanopoulos, M.; Yu, T. U.; Lew, S. MIT Topical Report to Texaco, Inc., under subcontract DOE cooperative agreement, No. DE-FC21-87MC23277, 1988.

Kobayashi et al.

Figure 10. Decrease in the specific surface area of sorbents with the number of desulfurization cycles.

Figure 11. Pore distribution of fresh sorbent B and its constituent oxides.

through curves of sulfur compounds for the sorbents. The slope of the increasing part of the sulfur concentration profile is related to the sulfidation rate of the sorbents. The slope of sorbent B decreased with cycling, indicating a drop in the desulfurization rate. On the other hand, the sulfidation rate of sorbent A maintained during cycling as evidenced by the parallel breakthrough profiles of Figure 5. Pore Size Distribution of Zinc Ferrite Sorbents. The pore size distribution of unreacted sorbent B was measured by mercury porosimetry. As shown in Figure 11, the fresh sorbent had both mesopores and macropores. The sorbent is a mixed product of zinc ferritesilica composite powder and titanium dioxide powder. The both powders were prepared separately and calcined at 1073 K, at which the sorbent was also calcined. The pore distributions of the constituents were measured to obtain information about the origin of the pore structure and are also shown in Figure 11. The titanium dioxide applied to the sorbent has a macropore distribution whose peak diameter is almost the same as the macropore size obtained for the sorbent. The mesopore distribution obtained for the zinc ferrite powder was in good agreement with the mesopore distribution of the sorbent. By comparing these three distributions, the mesopores were attributed to the composite powder, while TiO2 contributed to the macropores. The geometrical arrangement of zinc ferrite and silica in the composite powder was not clear from the pore distribution. We considered the existence of micropores of the sorbents and the validity of the mesopore distribution obtained by the mercury porosimetry. The distribution of mesopores has a peak diameter at 7 nm corresponding


Figure 12. Adsorption isotherm of nitrogen gas at 77 K for fresh sorbent B.


Figure 14. Pore distribution of fresh and spent sorbent A.

Figure 15. Pore distribution of fresh and spent sorbent B. Figure 13. Pore distribution of sorbent B obtained by the Dollimore-Heal method.

to the injection pressure of about 400 MPa. As the pore structure of zinc ferrite may be affected by the application of high pressure, the mesopore distribution was compared with the one observed in an alternative method. Figure 12 shows adsorption isotherms of nitrogen obtained for fresh sorbent B at 77 K, and Figure 13 is a pore distribution derived from analysis of the isotherms. The isotherm possesses an adsorption hysteresis loop at relative pressures over 0.6. In the pressure region lower than 0.6, the shape of the curve is similar to that of a type II isotherm. These characteristics are typically shown as type IV isotherms.17 The type of isotherm agrees with the existence of mesopores in the sample. The BET plot was linear for the relative pressure region between 0.05 and 0.28. The “c parameter” which indicates the magnitude of interaction between adsorptive and adsorbent was 87. Thus, existence of micropores was not suspected; the amount adsorbed at relative pressures around zero, less than 10 mL/g, is assigned to the monolayer adsorption capacity. The micropore volume had negligible contribution to the pore volume measured by the adsorption isotherm. We applied the Dollimore-Heal method18 to obtain the pore distribution. We selected the De Boer model as a standard curve of adsorption on the sorbent because the sorbent is a mixture of metal oxides. The distribution of the mesopore obtained from the desorption branch of the isotherm was in good agreement in its position and width with the results of the mercury porosimeter. The pore volume calculated between 3 and 30 nm was 0.07 mL/g. This value also agreed with the value obtained from mercury porosimetry, 0.06 mL/g. Thus, we confirmed the validity of the measurement with the porosimeter. (17) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982; pp 111-193. (18) Dollimore, D.; Heal, G. R. J. Appl. Chem. 1964, 14, 109-114.

Pore Size Distribution of Sorbents A and B. Pore size distributions of fresh and spent sorbents were compared to obtain structural information about sintering during repeated use. Observed pore size distributions of sorbents A and B are shown in Figures 14 and 15, respectively. In the measurable range of pore distribution, between 3 and 1.5 × 105 nm, pores of diameter larger than 1000 nm were not observed for both sorbents. Thus we show the distribution between 3 and 1000 nm in both figures. A drastic change was observed after cyclic use of sorbent B. The mesopores of sorbent B almost disappeared after the cycling, while the mesopores of sorbent A were maintained after the same numbers of cycles in use. The deterioration in mesopore structure of sorbent B corresponds well to the large decrease of surface area. Primary Particle Structure of Zinc Ferrite Observed by Transmission Electron Microscope. The difference in the variation of mesopores during cyclic use of the sorbents can be attributed to the sintering characteristics of the zinc ferrite powders. Consequently, we examined the primary particle structure of the zinc ferrite powder by transmission electron microscopy. As the composite powder or sorbent has a porous structure, it is difficult to obtain thin samples without damaging the pore structure. Thus, we selected a field emission type electron gun as its electron beam is bright enough to transmit through thick samples. This enabled us to observe the powders without special pretreatment. The TEM images are shown in Figures 1619. Each figure is displayed in terms of a relatively low magnification image and a high-resolution image. Particle geometry can be captured from the former images. The high magnification image enabled us to distinguish the crystalline particle from the amorphous particle by the stripe patterns, which are evidence of an interference between the electron beam and crystal lattice. Figure 16 shows the images of the composite powder prepared by the homogeneous precipitation method. The


Figure 16. TEM image of primary particle of zinc ferrite of fresh sorbent A.

low magnification image indicates coexistence of two phases: fine particles with diameters between 10 and 20 nm and relatively larger particles whose diameters are around 100 nm. The interference fringes on the high-resolution image show that the larger particles are crystalline phase, while the fine particles are amorphous. Therefore we conclude that the former is zinc ferrite and the latter is primary particles of silica. This was confirmed by qualitative analysis using the EDX of the TEM instrument. The fringes also indicate that the larger particles are agglomerates of primary particles of zinc ferrite as marked with arrows in the highresolution image. It was revealed that the ureaprecipitated composite powder has larger zinc ferrite primary particles surrounded by fine silica primary particles. The primary particle images of the powder in the spent sorbent are shown in Figure 17. Smoothedged, round particles shown in the low magnification image are TiO2 particles in the sorbent. The primary particles of zinc ferrite were found to be same in size as the fresh one, while the silica primary particles were slightly larger. The TEM images of ammonia-precipitated, fresh composite powder that are calcined at 1073 K and spent sorbent B are shown in Figures 18 and 19, respectively. Figure 18 shows that the ammonia-precipitated zinc ferrite has much smaller primary particles with diameters around 10 nm, which are comparable to the silica primary particles. These primary particles of zinc ferrite were aggregated together with the silica primary particles. The fine primary particles, however, could not maintain their size during repeated use of the

Kobayashi et al.

Figure 17. TEM image of primary particle of zinc ferrite in spent sorbent A.

sorbent. The micrographs in Figure 19 clearly show the growth of zinc ferrite, which is an evidence of sintering of zinc ferrite crystal during repeated desulfurization cycles. Sorbent B exhibited higher performance initially due to the fine primary particles of zinc ferrite. However, the fine particles grew rapidly during cyclic desulfurization, resulting in retardation of the sulfidation rate and loss of sulfur capacity of the sorbent B. We can assume that the zinc ferrite crystals in the agglomerates did not grow across the boundary between the aggregates as they are separated by the titanium dioxide particles. Thus, the maximum size of the grown zinc ferrite of the spent sorbent B is that of the aggregates at most. The pore distribution of the sorbent B shown in Figure 15 also indicates that the size of the grown particles does not exceed the size of titanium dioxide particles. The observation of primary particles of the sorbent B showed that the final sizes of the zinc ferrite primary particles are comparable to those of spent zinc ferrite in the sorbent A. This indicates the existence of the other effect on the inferior sulfidation rate of spent sorbent B. The most probable reason for the degradation in the spent sorbent B is effect of silica. Silica and iron or zinc can make silicates that have poor performance as desulfurization sorbents.12 The fine primary particles of zinc ferrite and silica in the sorbent B are nearly the same size and are mixed with each other in the aggregates. This geometrical peculiarity might promote silicate formation, resulting in a lower sulfidation rate of the sorbent B.


Figure 18. TEM image of primary particle of zinc ferrite of fresh sorbent B.

Evaluation of Pore Structures of Developed Desulfurization Sorbents. We combined the information on the pore structure of sorbent A obtained with mercury porosimetry and TEM observation and displayed it in Figure 20. The zinc ferrite-silica composite powder contains macropores due to the larger primary particles of zinc ferrite, which can be distinguished from the mesopores formed by the primary particles of silica. Thus, the mesopores observed in the pore distribution of sorbent A are mainly attributed to silicon dioxide primary particles. The larger zinc ferrite particles form macropores whose distribution is overlapped with that of titanium dioxide. Although the pore distribution of sorbent A is similar to that of sorbent B in terms of having both mesopores and macropores, their origin is quite different. Mesopores in sorbent B are caused from coagulated primary particles of both zinc ferrite and silicon dioxide, and macropores are due to titanium dioxide only. The fine structure of zinc ferrite was subjected to rapid sintering resulting in the low durability of sorbent B. We concluded that sorbent A has superior pore structure in terms of sulfur absorbing capacity, sulfidation rate, and durability. Conclusions Different precipitation methods using urea or ammonia were applied to prepare the powder of zinc ferrite combined with silicon dioxide. These zinc ferrite-silica composite powders were mixed with titanium dioxide powder and extruded to desulfurization sorbents A and


Figure 19. TEM image of primary particle of zinc ferrite in spent sorbent B.

Figure 20. Pore distribution of fresh sorbent A and its assignment to primary particles.

B containing the composite powder precipitated by urea and ammonia, respectively. These sorbents were tested in cyclic desulfurization tests. Up to 20 cycles of sulfidation, oxidation, and reduction were conducted using a fixed bed, plug flow reactor at 723 K and 9.8 × 105 Pa. The following results were obtained: (1) The sorbents decreased sulfur compounds to less than 1 ppm after 20 repetitions of desulfurization cycles. (2) The sulfur capacity of each sorbent decreased by 22-25% with respect to the initial capacity. Slopes of breakthrough curves showed that the sulfidation rate was affected by repetitive use of sorbent B but not of A. (3) Metal composition remained constant throughout desulfurization cycles, and thus, residual sulfur and sintering would be the main contribution to sulfur


capacity reduction. For the first few cycles, a small decrease in sulfur capacity is due to residual sulfur alone. At 20 cycles, both sintering and residual sulfur affect the sulfur capacity. (4) The primary particle size of zinc ferrite is affected by the precipitation methods applied to zinc ferrite preparation. The homogeneous precipitation method produces relatively larger primary particles that are more resistant to sintering during the desulfurization cycles. The particles are considered to have minimal interaction with silica, resulting in conservation of reactivity of zinc ferrite. These properties of zinc ferrite are suitable for the production of durable zinc ferrite

Kobayashi et al.

sorbent applicable to long cyclic use in a practical coal gas desulfurization scheme. Sorbent A is superior to the other sorbent in sulfur absorbing capacity, sulfidation rate, and ability to reduce sulfur concentration. The durability of the sorbent was higher than that of sorbent B caused by larger primary particle formation in the homogeneous precipitation method. Acknowledgment. We acknowledge many helpful discussions with and suggestions in regard to primary particle observation by Dr. Haruo Kishida of CRIEPI. EF960231K

Investigation on Desulfurization Performance and Pore Structure of

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