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Energy & Fuels 2009, 23, 762–765
Study on the Stability of Sorbents Removing H2S from Hot Coal Gas Li-Ping Chang,† Zong-You Zhang,†,‡ Xiu-Rong Ren,† Fan Li,*,† and Ke-Chang Xie† Key Laboratory of Coal Science and Technology, Taiyuan UniVersity of Technology, Ministry of Education and Shanxi ProVince, Taiyuan, Shanxi 030024, People’s Republic of China, and Shandong Haihua Coal Chemical Company Ltd., Zaozhuang, Shandong 277000, People’s Republic of China ReceiVed July 30, 2008. ReVised Manuscript ReceiVed October 27, 2008
Mixed metal oxide containing iron with the high-sulfur capacity and reactivity is considered as one of the most favorable sorbents for desulfurization in hot gas. The stability and life of iron-based sorbents are the main challenges for the hot gas cleanup techniques. Not only the effect of gas atmosphere but also the effect of ZnO and MgO on the stability of iron-based sorbent was studied in this work. The mechanism and factors influencing sorbent stability are discussed. The results showed that the coexistence of CO and H2 result in the instability of the zinc-iron-based sorbents. The reaction of carbon deposit is the crucial step affecting the stability of sorbent for hot gas desulfurization. ZnO in the sorbent is adverse to the physical stability of the iron-based sorbents. MgO in the sorbent hardly affects the physical stability of the iron-based sorbents but improves the capacity of removing the hydrogen sulfide from hot coal gas at 773 K.
Introduction Hydrogen sulfide in coal-derived gas must be removed to comply with the increasingly stringent future environmental standards and to protect the equipment in continuous processes. Several commercial techniques are available for the removal of hydrogen sulfide, including wet absorption. However, these commercial techniques for the purification of coal gas share one disadvantage. That is, the hot coal gas must be cooled to ambient temperature and then preheated to a high temperature in the gas turbine. To avoid heat loss and save energy, hightemperature desulfurization technology has been widely developed and appears to be the main technique for removal of hydrogen sulfide from hot coal gas.1-3 It is well-known that the integrated gasification combined cycle (IGCC) is an advanced power generation technology with high efficiency power generation, good environmental performance, and simpler plant configuration.4 One of the potential processes for removal of hydrogen sulfide in the IGCC power plant is the use of metal oxide sorbents. They are investigated for various selections of materials and reactor designs. Desired properties for the sorbent material are mainly performance and durability. For actual operation of the hydrogen sulfide removal system, sorbent life is an important feature, especially when the fixed-bed type reactor is chosen. Iron-based sorbent is more attractive in terms of both sulfidation and economic considerations.5,6 Zinc ferrite is one of the potential materials that is able to reduce sulfur * To whom correspondence should be addressed. Telephone: +86-3516010482. Fax: +86-351-6010588. E-mail:
[email protected]. † Taiyuan University of Technology. ‡ Shandong Haihua Coal Chemical Company Ltd. (1) Na-oki, I.; Yousuku, O.; Hiroaki, M.; Toshimitsu, S. Fuel 2004, 83 (6), 661–669. (2) Pineda, M.; Palacios, J. M.; Alonso, L.; Garcia, E.; Moliner, R. Fuel 2000, 79 (8), 885–895. (3) Zeng, Y.; Kaytakoglu, S.; Harrison, D. P. Chem. Eng. Sci. 2000, 55 (21), 4893–4900. (4) Atimtay, A. T. Clean Prod. Proc. 2001, 2, 197–208. (5) Shen, F.; Li, C. H.; Shang Guan, J.; Fan, H. L.; Liang, S. Z. Coal ConVers. 2004, 27, 54–56.
concentration down to parts per million (ppm) level in coalderived gas conditions. The previous study mainly described the physical and chemical degradations by controlling pore structure of the sorbent or varying additives,7 but the factors influencing the physical and chemical degradations are little dealt with in essence. This paper mainly studied the reasons resulting from the physical degradation in different atmosphere gas and addition quantity of zinc oxide or magnesium oxide. Experimental Section Sorbents Preparation. Iron-based sorbents were prepared by mixing uniformly red mud, zinc oxide (AR), magnesia oxide (AR), starch, inorganic binder, etc. and then molding. All of the sorbents were formed about inner diameter (i.d.) 5 × 5 mm cylinder, then oven-dried, and calcined for 4 h in 1073 K based on our previous work experience.8 The composition of red mud and binder in sorbent is respectively shown in Tables 1 and 2. Table 1. Composition of Red Mud Used in the Experiments material
Fe
FeO
Fe3O4 Fe2O3 Al2O3 CaO MgO SiO2
red mud (wt %) 6.50 23.11 20.73 19.13
0.53
14.72 0.62
6.35
Table 2. Composition of Binder Used in the Experiments binder
SiO2 Al2O3 Fe2O3 TiO2 CaO MgO K2O Na2O LOI
brick 54.24 10.87 clay (wt %)
4.41
0.80 12.13 1.59 1.92
1.28 11.16
Apparatus and Operative Procedure. The sulfidation experiments were run in a fixed-bed reactor heated externally by an electric furnace at atmospheric pressure with gas flow controllers. The reactor consists of a quartz tube, 2.0 cm i.d., 2.2 cm outer diameter (o.d.), and 66 cm length. The quartz fibers were set in the (6) Kobayashi, M.; Shirai, H.; Nunokawa, M. Energy Fuels 1997, 11 (4), 887–896. (7) Tamhankar, S. S.; Hasatani, M.; Wen, C. Y. Chem. Eng. Sci. 1981, 36 (7), 1181–1191. (8) Liang, M. S. Ph.D. Thesis, Taiyuan University of Technology, People’s Republic of China, 2005.
10.1021/ef8005838 CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
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Energy & Fuels, Vol. 23, 2009 763
reactor to support the sorbents. In these tests, 20 mL of sorbent was used, and the gas space velocity was about 2000 h-1. Analysis of the outlet and inlet gas from the reactor was carried out by gas chromatography using a thermal conductivity detector (TCD) and a flame photometric detector (FPD) for low concentrations of sulfur compounds with sensitivity higher than 0.1 ppmv. The identification of crystalline phases associated with the sorbent was performed with a Rigaku D/ max2500 X-ray diffraction (XRD) instrument with nickel-filtered Cu KR radiation, a power setting of 40 kV and 10 mA, scanning range of 10-80°, and scanning rate of 4°/min. H-temperature-programmed reduction (TPR) was performed with a 40% H2-N2 gas mixture in a quartz reactor (10 mm i.d.). A total of 100 mg of sorbents was packed in this reactor, and the gas flow rate was maintained at 200 mL/min and controlled by mass flow controllers. The outlet gases from the reactor were automatically analyzed by a TCD. The temperature was varied from 200 to 800 °C at the heating rate of 10 °C/min.
Figure 2. XRD patters of sorbent AZF before (X) and after (Y) calcinated and after (Z) sulfidation.
Results and Discussion Effect of Sorbent Composition on Stability. The composition of mixed gas for sulfidation is about 39.6% H2, 32.7% CO, 19.5% CO2, H2S of 1318 ppmv, and balance gas of N2. The ratios of materials, symbol of sorbent, and results after sulfidation for 7 h in 773 K were showed in Table 3. It is seen that the extent of sorbent pulverized is obviously different: the physical degradation of sorbent AF was negligible, while AZF was pulverized entirely. Sorbent AZF could remove H2S to the ppmv level in coal gas, and AF only did it to about 20 ppmv. It was identified that the main active species are Fe2O3 in fresh sorbent AF and zinc ferrite in sorbent AZF from XRD results in Figures 1 and 2. It is known that zinc ferrite may be decomposed into ZnO and Fe2O3 in sulfidation.9,10 To investigate if the volatilization of Zn in reductive atmosphere affects the experimental result, sorbent AZ was run in above operation conditions, and the sulfidation result is shown in Table 3. Although white elemental zinc is observed in the outlet of the reactor, it can be seen that no physical degradation occurred. This indicates that the volatilization of Zn in reductive atmosphere is not the main reason for the physical degradation of the AZF sorbent. The results in Figures 1-3 show the difference of sorbent AZF with AF and AZ after sulfidation. It is obvious that there are
Figure 1. XRD patters of sorbent AF before (X) and after (Y) calcinated and after (Z) sulfidation. Table 3. Ratios of Materials, Symbol of Sorbents, and Sulfidation Results in Simulated Coal Gas ratios of materials (wt %) sorbents
red mud (F)
brick clay
zinc oxide (Z)
AF AZF AZ
70 54 0
30 30 30
0 16 70
physical degradation no yes no
H2S in outlet (ppmv) 20 20
Figure 3. XRD patters of sorbent AZ before (X) and after (Y) calcinated and after (Z) sulfidation.
complicated FexC (x ) 0-3) and zinc ferrite in sorbent AZF. It is suggested that the physical degradation may have some relation to the carbonaceous gases in the experimental gas and the addition of Zn in the sorbent. Effect of Environmental Gases on Sorbent Stability. AZF sorbent has the efficiency of deep removal H2S from reductive hot gases, but the physical degradation is serious. To study the main reason influencing the instability of AZF, the effect of the experimental gases on physical degradation of sorbent AZF was studied during sulfidation. The compositions of the experimental gases (H2S in feeding gas is 1318 ppmv) and the results after sulfidation 7 h at 773 K were shown in Table 4. For all experiments, except for AZF1, the concentration of H2S and COS in the outlet could all be removed down to 1 ppmv, although the extent of the sorbent pulverized was different. In comparison to the composition in the outlet of AZF1-AZF3, it was detected that SO2 and element sulfur existed in the outlet only for AZF1. These indicated that the following reactions may have existed: MO + (1 - 2x)H2S f MS1-3x + xSO2 + (1 - 2x)H2O (1) where MO means metal oxide 2H2S + SO2 f 3S + 2H2O
(2)
When reductive gas of H2 or CO existed in experimental gases, the following reactions maybe take place: (9) Barin, I.; Knacke, O. Thermochemical Properties of Inorganic Substance; Springer Verlag: New York, 1973. (10) Barin, I.; Knacke, O.; Kubaschewski, O. Thermochemical Properites of Inorganic Substance, Supplement; Springer Verlag: New York, 1976.
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SO2 + 3H2 f H2S + 2H2O
(3)
or, SO2 + 3CO f COS + 2CO2
(4)
COS + MO f CO2 + MS
(5)
In comparison to AZF2 and AZF3, the difference could be shown as follow: MexOy(s) + zH2(g) f MexO(y-z)(s) + zH2O(g) + ∆rH11 (6) MexOy(s) + zCO(g) f MexO(y-z)(s) + zCO2(g) + ∆rH12 (7) Predigested as H2(g) + O* f H2O(g) ∆H773 ) -245.9 kJ mol-1
(8)
Figure 5. H-TPR curves of sorbents AF, AZF, and AZ.
CO(g) + O* f CO2(g) ∆H773 ) -270.4 kJ mol-1
(9)
the deoxidization reaction of the sorbent at 873 than 773 K (in Figure 5). All of the above results indicate that the heat of the deoxidizing reaction from both H2 and CO with the sorbent has not caused AZF physical degradation. XRD patterns of sorbent AZF after sulfidation in different feeding gas are shown in Figure 6. The results show that Fe exists in the sorbent from the AZF2 (H2-N2 and H2S gases) experiment. It is reported that the element Fe is the catalyzer of the carbon deposit reaction: 2CO f C + CO2, ∆H773 ) -159.9 KJ mol-1.11 This reaction is also an exothermic reaction. However, for the AZF7 run, when the gas atmosphere in the reactor was the mixed gases of H2 and N2 first and then CO and N2, the sorbent was also not pulverized. This result indicates that the carbon deposit reaction is still not the main reason causing sorbent physical degradation after the sorbent deoxidized by H2 alone.
The above reactions indicate that the deoxidization of sorbent is an exothermic reaction. However, there was no physical degradation for AZF2 and AZF3. This indicates that the deoxidizing reaction of sorbent in single H2 or CO is not the main reason of AZF sorbent physical degradation. To explain whether the heat concussion from the deoxidizing reaction of H2 and CO for sorbent results in the physical degradation of AZF sorbent, the AZF4 and AZF5 experiments were run. Both of them were pulverized entirely, and there was no different of the physical degradation between AZF4 and AZF5. Thus, the heat concussion from the deoxidizing reaction of sorbent in both H2 and CO may not also be the main reason of AZF sorbent physical degradation. There is little change for the content of H2 and CO in the inlet and outlet of the reactor when the AZF4 experiment was run (shown in Figure 4). H-TPR of AZF in Figure 5 also indicates that there is a much lower peak intensity of H2 consumed at 773 than 873 K. These results indicate that the heat from the deoxidizing reaction is limited at 773 K. Moreover, in terms of kinetics, the reductive activity of CO is not as high as that of H2 at 773 K. Even the reductive activity of 10% H2 is twice as high as 37% CO.8 However, when the AZF6 experiment was run at 873 K, the sorbent was also not pulverized. From the H-TPR, there should be more heat from
Figure 6. XRD patterns of sorbent AZF after sulfidation in different feeding gas (X, N2 and H2S; Y, H2-N2 and H2S; Z, H2-CO-N2 and H2S). Figure 4. H2 and CO concentration in the inlet and outlet of the reactor for the AZF4 experiment.
Moreover, as shown in Table 4 and Figure 6, about 0.3% of CH4 and complicated FexC (x ) 0-3) in the reactor outlet were
Table 4. Composition of Experimental Gas and the Results of Sulfidation Tests with AZF Sorbent inlet gas composition (vol %) (db) number AZF1 AZF2 AZF3 AZF4 AZF5 AZF6 AZF7 AZF8 AZF9 a
N2 100.0 60.4 67.3 27.7 63.9 60.4 60.4/67.3 7.9 7.9
H2
CO
CO2
H2Oa
temperature (K)
19.8 19.8
0 0 0 0 0 0 0 0 10
773 773 773 773 773 873 773 773 773
39.6 39.6 19.8 39.6 39.6/0 39.6 39.6
32.7 32.7 16.3 0/32.7 32.7 32.7
Calculated by total gas. b Except inlet gas.
composition of outlet gasb S-SO2 CO2-COS CO2-COS-CH4 CO2-COS-CH4 COS-CH4 COS-CH4
physical degradation no no no yes yes no no yes yes
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detected for all sulfidation runs of the serious physical degradation. Thus, there may be the carbon deposit reaction: CO + H2 f C + H2O ∆H773 ) -112.6 kJ mol-1
(10)
C + 2H2 f CH4 ∆H773 ) -63.9 kJ mol-1
(11)
Sum up as CO + 3H2 f CH4 + H2O ∆H773 ) -176.5 kJ mol-1 (12) That is to say, the reaction above may be the main reason of the physical degradation for the AZF sorbent. The process of the carbon-depositing reaction could be demonstrated as Figure 7. With the Zn-Fe-O sorbent, the CO was translated into C and H2O in the H2 atmosphere, then part of C reacts with H2 into CH4, and the rest of C reacts with Fe into FexC (x ) 0-3). The heat from the above reactions and the formation of FexC contribute to the physical instability of AZF. To avoid the physical degradation, it is necessary to decrease the quantity of CH4 and FexC (x ) 0-3) formed during sulfidation. Therefore, the first step reaction: CO + H2 f C + H2O should be restrained. Thus, additives added in sorbent to restrain this reaction should be further researched.
of the iron-based sorbents. That is to say, ZnO could not restrain the carbon-deposited reaction. Instead, it accelerates this reaction. On the basis of the tests above, the sorbents M0F, M1F, M2F, and M3F containing MgO additive 0, 1, 3, and 5% were also run respectively in the feeding gas of mixed gas at 773 K. The results are shown in Figure 8. The physical degradation did not appear, and CH4 was also not detected in the outlet of reactor. It is known that MgO could restrain the carbon-deposited reaction.12 In other words, the addition of MgO is helpful for the stability of iron-based sorbent by restraining the carbondeposited reaction, while it is also helpful for the deep removal of H2S.
Figure 8. H2S concentration (a) in outlet and sulfidation efficiency (b) during sulfidation of sorbents with different MgO content.
Conclusions Figure 7. Schematic process of the carbon deposit reaction.
Effect of ZnO and MgO on Sorbent Stability. The physical degradation of sorbent AF and ZAF is obviously different. The addition of zinc oxide may have a relation to the instability of iron-based sorbent. Table 5 is the experimental results of sorbents with different percentages of zinc oxide (ZF1, ZF2, and ZF3) in the feeding gas of simulated coal gas for about 7 h at 773 K. The results show that the physical degradation yield of the sorbent ZF3 without zinc oxide was zero, but the deep removal of H2S is not well-performed. The more ZnO added in sorbents, the more efficient H2S is removed. However, this also results in much more physical degradation. This indicates that the addition of zinc oxide is one of the reasons for the instability Table 5. Results of Sorbent Physical Degradation after ZF1, ZF2, and ZF3 Sulfidation at 773 K sorbents
ZF1
ZF2
ZF3
Zn (wt %) yield of physical degradation (wt %) H2S concentration in outlet (ppm)
8 100.0
