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Desulfurization Behavior of Fe-Mn-Based Regenerable Sorbents for High-Temperature H2S Removal Bing Zeng, Hairong Yue, Changjun Liu, Tao Huang, Jing Li, Bin Zhao, Ming Zhang, and Bin Liang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502092v • Publication Date (Web): 04 Feb 2015 Downloaded from http://pubs.acs.org on February 10, 2015
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Desulfurization Behavior of Fe-Mn-Based Regenerable Sorbents for High-Temperature H2S Removal *
Bing Zeng, Hairong Yue, Changjun Liu, Tao Huang, Jing Li, Bin Zhao, Ming Zhang, Bin Liang Multi-phases Mass Transfer and Reaction Engineering Laboratory, College of Chemical Engineering, Sichuan University，Chengdu 610065, China
*Corresponding author:
[email protected] ACS Paragon Plus Environment
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ABSTRACT: A series of Fe-Mn-based sorbents with different Fe/Mn mole ratios were prepared via co-precipitation for the high-temperature removal of H2S. Performance tests were carried out at 1123 K in a fixed-bed reactor，indicating that metallic Fe and MnO were the active components of the Fe-Mn-based sorbents in the hot gas. Single Fe-based sorbents exhibited a low desulfurization efficiency and effective sulfur capacity. The addition of manganese (Fe/Mn mole ratios less than 8:2) considerably improved the desulfurization efficiency and effective sulfur capacity of the Fe-based sorbents. In the first sulfidation test, effective sulfur capacities of 20.71, 20.72, and 20.14 g S/(100 g sorbent) were obtained for Mn7Fe3, Mn5Fe5, and Mn3Fe7, respectively. During five sulfidation-regeneration cycles, Mn7Fe3, Mn5Fe5, and Mn3Fe7 were stable, maintaining high activities and sulfur capacities, and reduced the amount of H2S to a few ppmv. After sulfidation, the sulfided sorbents could easily be regenerated with 2% O2 in N2 at 1123 K to obtain SO2 and S2. The elemental sulfur recovery rate increased with the decrease of manganese content. Characterization with XRD, SEM and BET showed that Fe-Mn-based sorbents kept stable structures during successive sulfidation−regeneration cycles. KEYWORDS:
High
temperature
H2S
removal;
Desulfurization; Sulfur recovery
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Regenerable
sorbents;
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1. INTRODUCTION Integrated gasification combined cycle (IGCC) is a promising process for advanced electric power regeneration, which is environmental friendly and high efficiency.1-3 In the IGCC process, coal is gasified and hot coal gas is used as the fuel gas for turbines to generate electricity. In order to protect the turbine from corrosion and generate flue gas with a low sulfur emission, H2S contained in the coal gas should be removed prior to its entry into the turbine. A coal gasifier typically operates at temperatures greater than 1273 K, and the temperature of the hot coal gas is typically between 1123 and 1173 K.4 For high efficiency, the desulfurization temperature should be around 1123 K. In sponge iron production, high-temperature H2S removal from hot coal gas is also required. The shaft furnace for sponge iron production operates at 1123 K, where the ferrous ore is directly reduced with the hot coal gas. However, the hot coal gas must be desulfurized to below 15 ppmv of H2S in order to avoid the formation of sulfides.5
Because of the high temperature operation, traditional commercial desulfurization processes are not suitable. Solid sulfur acceptors are the only choice for the high temperature desulfurization process. In order to develop a high temperature regenerative sorbent a number of systems such as iron-based, 6 zinc-based, 7-18 and copper-based sorbents18-21 have been evaluated. These sorbents showed high desulfurization efficiencies and activities at the temperatures lower than 873 K. However, when these sorbents are used in reducing atmosphere at temperatures greater than 1023 K, the metal oxides are reduced either into their metallic states, or
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form corresponding metal carbides. This influences the mechanical strength, sulfur capacity and desulfurization efficiency of the sorbent. In order to overcome the drawbacks of these sorbents used alone, number of mixed metal oxides was investigated in the literature to achieve excellent desulfurization performance in the removal of H2S, such as Fe-Mn-based sorbents. Compared with other sorbents, Fe-based sorbents have the advantages of high sulfur capacity, high activity and low price and have been widely investigated in high temperature coal gas desulfurization.
Furthermore,
many
reports
have
indicated
the
excellent
sulfidation-regeneration performance, high sulfur capacity, high desulfurization efficiency, and high stability of alumina-supported MnO at temperatures up to 1123 K.22-26 Therefore, Fe-Mn-based sorbents were investigated widely to reserve the advantages possessed by iron and manganese and avoid their disadvantages. In the studies of Ren 27, it was shown that the addition of manganese oxide can improve the desulfurization capacity of iron-based sorbent at 773 K, Fe3O4 and MnO are the active components of the Fe-Mn-based sorbents, sorbents with the Fe/Mn mole ratios of 7:3 and 3:7 have bigger sulfur capacities. The desulfurization performance of Fe-Mn-based was also investigated by Zhang28, the studies showed that the sorbent’s reactivity and regenerability were improved at 923 K when Mn and Fe were supported upon a suitable support; the maximum breakthrough sulfur capacity of 4.2g S/100g sorbent was obtained at the mole ratio of 0.5 of Fe/Mn; the breakthrough sulfur capacity of regenerated sorbent was slightly decreased compared with the fresh sorbents when the number of sulfidation-regeneration cycles exceeded seven. Sena
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Yasyerli29 made an activity comparison of Ce-Mn V-Mn, Zn-Mn and Fe-Mn sorbents, found that the sulfur retention capacity of the Fe–Mn mixed oxide sorbent was quite high at 873 K. These studies have shown that the addition of Mn can improve the desulfurization performance of Fe-based sorbent. Reviewing the current researches about Fe-Mn-based sorbents, the desulfurization temperature is below 873 K, under this temperature, the reduction product of iron oxide is mainly Fe3O4 or FeO, while the reduction product of manganese oxide is MnO, and these reduced oxides are the active components in the removal of H2S. However, in order to improve the efficiency of IGCC and meet the requirements of sponge iron production, the most appropriate desulfurization temperature is 1123 K. At this high temperature of 1123 K, iron oxide may be likely to be deeply reduced to the metallic state, resulting in different reduction and sulfidation processes compared with
lower
temperatures.
According
to
thermodynamic
calculation,
the
desulfurization efficiency of Fe is low in the removal of H2S, this leads to the decrease of the effective sulfur capacity of Fe-based sorbents, while MnO has excellent desulfurization performance at up to 1123 K. Therefore, we designed a Fe-Mn-based sorbent for the high-temperature desulfurization of hot coal gas at 1123 K and expected the Fe-Mn-based sorbents have the potential of increasing desulfurization efficiency and sulfur capacity. If it is achievable to reserve the specific advantages possessed by iron and manganese and avoid their disadvantages, the optimal sorbents to remove H2S from hot coal gas at 1123 K should be obtained. Based on this consideration, in this study, a series of sorbents with different Fe/Mn
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mole
ratios
were
prepared
via
co-precipitation,
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and
the
reducibility,
sulfidation-regeneration performance and stability of the sorbents were investigated at 1123 K。 2. EXPERIMENTAL SECTION 2.1 Preparation of sorbents. A series of Fe-Mn-based sorbents with different Fe-Mn mole ratios (the quality fraction of alumina was fixed about 60wt % in different sorbents) were prepared via co-precipitation. 1 mole/L solutions of Mn (NO3)2, Al (NO3)3 and Fe (NO3)3 were separately prepared from manganous nitrate (AR), aluminum nitrate (AR) and ferric nitrate (AR). The solutions were mixed with different ratios and then neutralized with10% NH3•H2O. During the precipitation, the nitrate solution and the NH3•H2O were simultaneously pumped into a reactor filled with a small amount of bottom water, and the pH value was kept between 11–11.4 by regulating the flow rates of the base and salt solutions. The temperature was kept at 323 K during neutralization. The precipitation slurry was aged for 12 h at 323 K and filtered. The filtration cake was washed with distilled water and dried overnight in air at 473 K. The dried cake was crushed and sieved to the size of 100–120 mesh, and was then calcined at 1173 K in air for 12 h. For comparison, single Fe-based and Mn-based sorbents were also prepared using the same method. 2.2 Analysis and characterization of sorbents. The manganese content of the sorbents was analyzed by the ammonium iron || sulfate titrimetric method (Chinese analysis standard method, GB1506-2002-T); the iron content was analyzed by the
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Titanium trichloride || potassium dichromate titrimetric method (Chinese analysis standard method, GB/T8654.1-2007). The crystalline structures of the sorbents were determined by X-ray diffraction (XRD). A DX-2700 CSC goniometer was used to scan the sorbents. The scan angle (2θ) was varied from 20°to 80°. An S-4800 scanning electron microscope (SEM) was used to characterize the surface morphology of selected samples. The pore volume and the specific surface area of the sorbents were analyzed with N2 adsorption on an ASAP 2020 BET system. 2.3
Sulfidation-regeneration
tests.
The
sorbents
were
subjected
to
sulfidation–regeneration tests in a vertically oriented quartz tube reactor, with an inner diameter of 6 mm. In each case, 0.5 g of the sorbent was packed to a height of rough 2 cm. The reactor was heated with a pipe-type electrical furnace and the packed sorbent was placed within the constant temperature zone. A typical experiment consisted of several repeated cycles, and each cycle included sulfidation, flushing, regeneration, and re-flushing. A gas mixture containing H2S/H2 and inert gas was used as the simulation gas during sulfidation. The H2S content was 1% (10000 ppmv) and the flow rate was controlled at 50.9 mL/min (STP) by a mass flow controller. The sulfidation temperature was 1123 K. The breakthrough curve was expressed as a plot of the outlet concentration of H2S versus time. In addition, the amount of sulfur captured by the sorbent at the breakthrough onset (lower than 50 ppmv sulfur in the outlet) was denoted as the breakthrough sulfur capacity or effective sulfur capacity, calculated using equation
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1:27 SC ( gS /100 g sorbent ) tb M =WHSV × S × ∫ (Cin − Cout）dt × 10 −4 0 Vm
(1)
where SC is the effective sulfur capacity of the sorbent, WHSV is the weight hourly space velocity (L h-1 g-1), Ms is the mole weight of sulfur (32.06 g mol-1), Vm is the mole volume of H2S at 1 atm and 298 K (24.5 L mol-1), and tb is the breakthrough time of the sorbent (h); Cin and Cout are the inlet and outlet concentrations of H2S (ppmv), respectively. During regeneration, diluted air was used to regenerate the sulfided sorbents. The regeneration products included SO2 and elemental sulfur. The regeneration was discontinued when the measured concentration of SO2 was close to the detection limit. The regenerated sorbent was used directly in the sulfidation test in the subsequent cycle. The concentrations of H2S and SO2 were measured by a SC-2000 gas chromatograph (GC), equipped with a thermal conductivity detector (TCD) and a flame photometric detector (FPD); the lowest detectable concentrations of H2S and SO2 were less than 1 and 50 ppmv, respectively 3. RESULTS AND DISCUSSION 3.1 Preparation of sorbents by co-precipitation method. By varying the Fe/Mn mole ratio in the precipitation solution, seven sorbents with different Mn/Fe mole ratios were obtained (Table 1). The sorbents were characterized by XRD and the results are shown in Figure 1. In
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Mn10Fe0, high manganese oxides are not detected, but peaks of mixed metal oxide Mn2AlO4 are observed, indicating that manganese is well dispersed and a spinel structure forms during high-temperature calcination. Upon addition of Fe, Mn2AlO4 and iron oxides are observed. In Mn7Fe3, iron primarily exists in the form of Fe3O4, and the peaks observed in Mn7Fe3 are broader than those observed in pure Fe3O4. In Mn5Fe5, Mn3Fe7, and Mn2Fe8, Fe2O3 is not detected, but Fe3O4 is observed. Fe2O3 and little Fe3O4 are observed in Mn1Fe9 and Mn0Fe10. 3.2 Reduction stability of sorbents. The oxides and metallic stats of manganese and iron with desulfurization activity include Mn, MnO, Mn3O4, Mn2O3, Fe, FeO, Fe3O4, and Fe2O3. All of the aforementioned compounds can absorb H2S from coal gas at high-temperatures. The corresponding reactions are as follows: Manganese oxides and Mn Mn2O3+2H2S+H2=2MnS+3H2O
(2)
Mn3O4+3H2S+H2=3MnS+4H2O
(3)
MnO+H2S =MnS+H2O
(4)
Mn+H2S=MnS+H2
(5)
Iron oxides and Fe Fe2O3+2H2S+H2=2 FeS+3H2O
(6)
Fe3O4+3H2S+H2=3FeS+4H2O
(7)
FeO+H2S = FeS+H2O
(8)
Fe+H2S=FeS+H2
(9)
The existing form of the metal compound depends on the reducing atmosphere and
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operation temperature. Owing to the presence of H2 in hot coal gas, Mn3O4 and Mn2O3 are readily reduced to MnO under slightly reducing atmospheres at lower temperatures. However, MnO is not reduced to elemental manganese. On the contrary, the reduction of iron oxides strongly depends on the reducing atmosphere and operation temperature. In order to investigate the stability of the sorbents under reducing conditions, 10% H2 without H2S was passed through the reactor, using identical operating conditions as those used during sulfidation, the results are shown in Figure 2. During the first 5 min, H2 concentration of the off-gas increases rapidly to the concentration of feeding gas. It means that the reduction in the initial period is rapid and the reduction completes within 10 min. In sulfidation tests, a simulation gas containing 99% H2 is used as sulfidation gas; therefore, the sorbents should be completely reduced in far less than 10 min. The XRD measurements of the reduced sorbents taken from the reactor at the end of the experiments indicate that all iron oxides are reduced to metallic Fe and all manganese oxides are reduced to MnO (see Figure 3). In order to make a comparison of reduction rate and sulfidation rate of the sorbents, the changing phases of Mn5Fe5 sample under sulfidation conditions were detected by XRD. Figure 4 illustrates the XRD patterns of sorbent Mn5Fe5 undertaking sulfidation in different times, similar results are obtained with the other sorbents. In the sulfidation of the fresh sorbent, XRD results show that Mn2AlO4 and Fe3O4 phase completely disappear within 10 min, but bivalent manganese (referring to the peaks at
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34.9°，40.6°，58.7° and 70.2° in the XRD pattern) and metallic Fe (at 44.6° and 65.1°) are still observed. The complete sulfidation time (about 130 min) of sorbents is much longer than the complete reduction time (
