Sulfidation-regeneration Cycles Research Articles

Functionalized Zn-based desulfurizer with ordered mesoporous structure based on insights into sulfur-release mechanism during hot coal gas desulfurization

Sulfur-release effect impedes the engineering application of hot coal gas desulfurization. However, the related mechanism is not clear. This study aims to clarify the aforementioned effects and directionally design/fabricate ordered mesoporous Zn-based sorbents with function of inhibiting sulfur-release behavior. The results reveal that COS is mainly formed via R-1 (reaction between CO and H2S) before the reaction gas contacts Zn-based sorbents. R-2 (reaction between CO2 and H2S) catalyzed by ZnS is more kinetically favorable than its non-catalytic process. Furthermore, R-1 and R-2 contribute almost equally to the overall amount of released COS during desulfurization process. Then, considering the composition of coal gas, the strategy of promoter (with function of catalytic hydrogenolysis of COS) modification was proposed to inhibit sulfur-release behavior. A series of functionalized sorbents with four Zn/Me (Me: Mo, Ni) molar ratios (88:12–97:3) were constructed. The sorbents exhibit ordered pore-arrangement structure. Compared to unmodified sorbents, Mo- and Ni-doping both result in significant decline (by 98 %−99 %) in the amount of released COS before H2S-breakthrough. Moreover, the doping of Mo/Ni into ZnO generally produces positive effects on H2S-uptake activity. However, regeneration leads to notable deterioration of performance for Mo-doped sorbents. It is ascribed to the inadequate conversion of ZnS and structure destruction of sorbents. On the contrast, Ni-doped sorbents with Zn/Ni molar ratio of 90:10 could not only greatly suppress the sulfur-release effect and but also maintain 92 %−95 % of initial sulfur capacity during five sulfidation-regeneration cycles.

Insights into H2S-absorption and oxidation-regeneration behavior of Ni-doped ZnO-based sorbents supported on SBA-15 for desulfurization of hot coal gas

Sulfur removal from hot coal gas is one of important technologies for highly efficient coal-based energy system. H2S-absorption and oxidation-regeneration behavior of Ni-doped ZnO-based sorbents supported on SBA-15 (ZnNi/SBA-15) was analyzed in this work. The results indicate that ZnNi/SBA-15 shows highest H2S-uptake capacity (16 g S/100 g sorbent) and lowest amount of produced COS (0.0004 g COS/100 g sorbent) when used for desulfurization at 500 °C. Removal of H2S at 700 °C leads to lowest breakthrough sulfur capacity of sorbents. It is ascribed to the destruction of typical microscopic morphology of SBA-15. Boltzmann function is suitable for modeling the H2S-absorption behavior of ZnNi/SBA-15. The evolution rate of H2S for sorbents varies from 0.096 to 0.228 min−1 at 400–700 °C. H2S-CO and H2S-CO2 reactions both contribute to the formation of COS at the stage of H2S-absorption saraturation. Increase of temperature from 550 to 600 °C results in the disappearance of ZnS in regenerated ZnNi/SBA-15. Moreover, most of plugged pore structure caused by sulfidation reaction could restore after regeneration at 600 °C. Further increase of regeneration temperature causes the decline of regeneration efficiency and less porous sorbent. Additionally, the sorbents have stable desulfurization ability and low COS emission during multiple sulfidation-regeneration cycles.

Mesoporous Zn-Fe-based binary metal oxide sorbent with sheet-shaped morphology: Synthesis and application for highly efficient desulfurization of hot coal gas

Mesoporous Zn-Fe-based binary metal oxide sorbent for hot coal gas desulfurization was derived from the Zn-Fe-based layer double hydroxide (ZnFe-LDH). Considering the great disparity of the pH required for the precipitation of Zn2+ and Fe3+, a two-step approach, namely the co-precipitation (Zn2+ and Fe2+) plus subsequent H2O2 oxidation, was proposed for the synthesis of the ZnFe-LDH, the precursor of the sorbent. The preliminary synthesis mechanism of the ZnFe-LDH was discussed. The results reveal that the formation of the layered structure occurs during the oxidation step process rather than the precipitation stage. A series of LDH-derived double metal oxides (DMOs) with the Zn/Fe molar ratios ranging from 1:1 to 5:1 were obtained and contain both ZnO and ZnFe2O4. Furthermore, sheet-shaped morphology is observed for most of the DMOs. The desulfurization performance of the DMOs was evaluated over a fixed-bed reactor. The results indicate that the DMO with the Zn/Fe molar ratio of 5:1 performs best and has the highest sulfur capacity (25 g of sulfur per 100 g of sorbent) at 550 °C. The reaction kinetics was analyzed via the deactivation model. The sorbent has lower deactivation rate constants (0.0185–0.0282 min−1) compared to other Zn-based sorbents, beneficial for the sulfidation reaction. In comparison to ZnO, the DMO sorbent shows lower regeneration temperature. Furthermore, regeneration could effectively recover the plugged pore structure caused by the sulfidation reaction. In addition, the sorbents could maintain high sulfur capacity during four sulfidation-regeneration cycles, showing great potential for industrial desulfurization of hot coal gas.

High-temperature H2S removal performance over ordered mesoporous La-Mn-supported Al2O3-CaO sorbents

A series of La-Mn supported on ordered mesoporous Al2O3-CaO (OMA-2Ca) sorbents with various Mn/La molar ratio were first designed and synthesized for removal of H2S at 650–850°C. 40% 98Mn2La/OMA-2Ca exhibited satisfactory desulfurization performance at 800°C with a breakthrough sulfur capacity (BSC) of 16.15gS/100g sorbent. Elemental sulfur was formed during desulfurization as a result of synergistic catalysis of γ-Al2O3 and lanthanum oxysulfide at high temperature. The existence of high content steam in hot coal gas obviously reduced the sulfur capacity because of the formation of MnAl2O4. After five successive sulfidation-regeneration cycles, high BSC (about 90% of the initial BSC) was maintained. In addition, the characterization results indicated that the mesostructure of OMA-2Ca remained intact during sulfidation and regeneration except that the amorphous wall was converted to γ-Al2O3 phase, and active species were still well dispersed on support, which indicated the stability of Mn-La/OMA-2Ca sorbent.

High H2O-resistance CaO-MnOx/MSU-H sorbents for hot coal gas desulfurization

A series of xMnyCa/MSU-H sorbents with various Mn/Ca molar ratio were first designed and synthesized with a sol-gel method. The desulfurization performance of the new sorbent was investigated at 600–800°C in hot coal gas. 90Mn10Ca/MSU-H exhibited better desulfurization performance at 750°C with a breakthrough sulfur capacity (BSC) of 18.69g S/100g sorbent compared to other supported Mn-based sorbents (13.2g S/100g sorbent) in similar desulfurization condition, and strong durability in multiple sulfidation-regeneration cycles using oxidation/reduction regeneration method which resolved the scientific issue of that CaSO4 is hardly decomposed to CaO. The introduction of Ca species effectively promoted the dispersion of active constituents, which improved the desulfurization activity. More importantly, 90Mn10Ca/MSU-H showed excellent H2O-resistance ability due to the fact that CaO enhanced the sorption of H2O. Moreover, the utilization of MSU-H with large pore size and excellent thermal stability effectively assured fast mass-transfer and confined the migration of active particles, which led to long lifetime stability of sorbents.

Regeneration performance and characteristic of iron oxide/arenaceous sorbents in the atmosphere of O2/N2

The performance and the change characteristic of iron oxide/arenaceous sorbent in the regeneration reaction are systematically investigated by studying the effects of temperature, space velocity and O2 concentration on the sorbent in a fixed-bed quartz reactor. Sulfidation-regeneration cycles were also conducted to evaluate the reusability of the sorbents. The surface and bulk structural features of the sorbents, both before and after regeneration, were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope, energy dispersive spectroscopy, Fourier transform infrared spectroscopy and N2 adsorption-desorption. The results show that the sorbents exhibit the highest regeneration rate at the conditions of 6vol.% O2, 650°C, and space velocity of 3000h−1. After five sulfidation-regeneration cycles, the breakthrough curves of regenerated sorbents changed slightly and the regeneration rates of the regenerated sorbents still exceeded 95%. The characterization data indicates that the outer layer electron density of surface atoms decreased in the regeneration process, which adversely affects the adsorption rate of acidic H2S and the reaction rate of H2S with Fe2O3 on the surface of sorbents. It can be seen from XRD analysis that the particle size of the sorbent gradually increased as the number of sulfidation-regeneration cycles increased, leading to reduced surface area and blocked pore structure. Furthermore, the internal structure deteriorated due to sintering, which resulted in the lowered sulfidation performance of the regenerated sorbent.

High capacity Mn-Fe-Mo/FSM-16 sorbents in hot coal gas desulfurization and mechanism of elemental sulfur formation

A series of newly developed Mo incorporated Mn-Fe supported on cost-effective mesoporous silica for the reactive adsorption desulfurization process is reported. The performance of the new sorbents was evaluated at 550–750°C over a fixed-bed reactor. 50% 4Mn1Fe-3%Mo/FSM-16 exhibited better desulfurization performance at 600°C with a breakthrough sulfur capacity (BSC) of 18.15g S/100g sorbent compared to 50% 4Mn1Fe/FSM-16 sorbent. High sulfur capacity (90% of the initial sulfur capacity) of sorbent was maintained after multiple sulfidation-regeneration cycles, which revealed 50% 4Mn1Fe-3%Mo/FSM-16 possessed strong durability. A large amount of elemental sulfur was produced instead of sulfur dioxide (SO2) by a simple regeneration. Moreover, the mechanism of sulfidation-regeneration was confirmed in situ using highly sensitive time-of-flight mass-spectrometry (TOF-MS) for the first time.

Effects of microwave irradiation on H2S removal activity from hot coal gas by modified semicoke-supported ZnO sorbents

To improve the desulfurization performance of sorbents at a low cost, modified semicoke-supported zinc oxide sorbents were prepared for hot coal gas by calcining the precursors using microwave and conventional heating. The desulfurization performances of these sorbents were evaluated over a fixed-bed reactor. The effects of microwave-assisted calcination temperature and time on the properties of sorbents were also investigated. These sorbents were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and nitrogen adsorption. The results show that the sorbents calcined by microwave exhibited a much higher sorbent utilization and desulfurization efficiency than those heated by conventional method. Compared to conventional heating, even and rapid heating of microwave led to smaller crystal size and better dispersion of active species. The Brunauer–Emmett–Teller results confirm that microwave irradiation modified the microstructure improved the pore structure (an increase in the specific surface area and pore volume), thus facilitating the gas–solid interactions during the desulfurization processes. A higher concentration of elements Zn and O was observed on the surface of sorbents by XPS analysis, facilitating the reaction of active species with H2S. The performance of the sorbents heated by microwave irradiation appeared to be stable over multiple sulfidation–regeneration cycles.

Calcium-based regenerable sorbents for high temperature H 2 S removal

Abstract Ca–Al–O regenerable sorbents with various calcium contents have been prepared by co-precipitation method for H 2 S removal at 850 °C. For the sorbent with a calcium content of 52.5 wt%, its sulfur capacity reached 36 g S/100 g sorbent. The saturated S/Ca molar ratios of the sulfided sorbents changed from 0.82 to 0.88, depending on the Ca content of sorbent. They are much higher than the case of pure CaO. Regeneration by diluted air, a cyclic oxidation/reduction process, or water vapor was applied for the regeneration of sulfided sorbents. The sulfided sorbent cannot be regenerated completely in the diluted air regeneration operation. It was completely regenerated in the oxidation/reduction process, in which the oxidation used 3% O 2 and the reduction used 10% H 2 . Repeating the oxidation/reduction 4 times, CaS was completely converted to CaO. Under such regeneration conditions, the sorbent performance was stable during 5 sulfidation–regeneration cycles. Sintering was observed in high O 2 concentration regeneration due to the strong exothermic reaction, which resulted in activity drop. During 6 cycles of steam regeneration with 81% H 2 O at 850 °C, the sulfur capacities of the regenerated sorbents with an initial calcium content of 61.2 wt% remained stable at 36 g S/100 g sorbent.

Desulfurization Behavior of Fe–Mn-Based Regenerable Sorbents for High-Temperature H2S Removal

A series of Fe–Mn-based sorbents with different Fe/Mn mole ratios was prepared via coprecipitation 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 ...

Regeneration characteristics and kinetics of Fe2O3/lignite semi-coke hot gas desulfurizer at O2/N2 atmosphere

The Fe2O3/lignite semi-coke sorbent was prepared by co-precipitation method with assistance of ultrasonic irradiation and underwent 4 sulfidation–regeneration cycles with O2/N2 as regeneration gases. The fresh, sulfided, and regenerated sorbents were characterized using XRD, SEM, XPS, and BET technologies in this paper. The regeneration mechanism was also discussed. It was found that in the oxygen atmosphere, FeS in the sulfided sorbent reacted with oxygen to produce Fe2O3 and SO2, the sulfate formed in regeneration process is easy to decompose at higher temperature. Regeneration kinetic studies were also performed at regeneration temperatures of 625–700 °C. It was found that the reaction order of regeneration with respect to O2 is first order. The equivalent grain model can be effectively used to correlate with the experimental data. In the early stage of reaction (x 70 %). According to the model, the apparent activation energy and the corresponding frequency factor for two different stages are 14.73 kJ mol−1 and 4.43 × 10−2 m s−1, 31.32 kJ mol−1 and 5.77 × 10−4 m2 s−1, respectively.

Manganese-based regenerable sorbents for high temperature H2S removal

A series of sorbents with different manganese contents have been prepared by co-precipitation method for 850°C regenerative H2S removal. The sulfur capacity increased linearly with the increase of manganese content. The performance of sorbents prepared by co-precipitation of Mn nitrate and Al nitrate appears to be stable over five sulfidation–regeneration cycles. Al2O3 plays an important role in the performance of sorbents and the recovery of elemental sulfur in O2 regeneration. With increasing Mn content in the sorbent, increasing O2 concentration in the regeneration gas or increasing total regeneration gas flow rate, the recovery of elemental sulfur decreased. Elemental sulfur is the only product with SO2 regeneration. The sorbents cannot be completely regenerated with SO2 under the conditions used. The sorbents, however, can be completely regenerated by steam. The main product for steam regeneration is H2S. Steam regeneration results in less sintering than O2 regeneration.

Combustion synthesis of porous ZnO monoliths for sulfur removal

Highly porous ZnO monoliths are prepared with potential applications as regenerable adsorbents for sulfur removal. ZnS/NaCl composite materials are first obtained by combustion synthesis from mixtures of zinc, sulfur and sodium chloride powders. After NaCl removal by lixiviation with water, they are converted into ZnO by thermal treatment under air. The crystalline structure, the microstructure and the porosity of the resulting monoliths are investigated by X-ray diffraction, scanning electron microscopy and Hg porosimetry. Their evolution after sulfidation–regeneration cycles is also considered.

A comparative study of the effect of clay binders on iron oxide sorbent in the high-temperature removal of hydrogen sulfide

The purpose of this study is to investigate the effect of clay binder, an important additive, on the performance of iron oxide sorbent in high temperature coal gas desulfurization. The four clay binders chosen for the study were kaolinite, diatomite, bentonite and brick clay. The sulfidation–regeneration cycles were conducted in a fixed-bed reactor. XRD, DTA and FTIR, together with texture characterizing techniques, such as mercury porosimetry and nitrogen adsorption, were adopted to characterize the sorbents and raw materials. The results obtained show that sorbents prepared from various clay binders exhibit different breakthrough behaviors. In addition, a correlation between pore volume and sulfur capacity reveals that sorbents with a greater number of pores larger than 200nm (diameter), exhibit higher sulfur capacity. The reason for this is that a greater number of large pores can improve diffusion and provide a larger space for relieving heat impact. However, too many large pores may result in weak strength and very low bulk density, thus a balance between large pores and the density must be achieved. This study also reveals that clay binder can contribute to the modification of a sorbent's texture as gas is released when the mineral structure changes during calcination. In addition, a clay mineral with an active interlayer has been shown to be beneficial in improving the dispersion of active components in the sorbent, because of the existence of an interaction between the mineral and red mud.

Sulfidation/regeneration multicycle testing of nickel-modified ZnFe2O4 desulphurization sorbent

A commercial metal oxide sorbent for the desulphurization of coal-derived gas requires high desulphurization reactivity, mechanical strength, ability to regenerate, and stability to endure many sulfidation-regeneration cycles. In this paper, the sulfur capacity and multiple cycles of a nickel-modified ZnFe2O4 sorbent prepared by the sol-gel auto-combustion method were measured in a fixed-bed reactor at middle temperature of 300°C (sulfidation temperature) and 500°C (regeneration temperature). Also, the BET surface area, pore volume, average pore diameter and X-ray diffraction (XRD) patterns of the sorbent through multicycles were studied. Multicycle runs indicate that the sulfidation reactivity decreases slightly during the second cycle and keeps steady in the following cycles. The results indicate that the nickel-modified ZnFe2O4 keeps high reactivity and structural stability in the multicycle testing of sulfidation/regeneration.

Desulfurization performance of iron-manganese-based sorbent for hot coal gas

A series of iron-manganese-based sorbents were prepared by co-precipitation and physical mixing method, and used for H2S removal from hot coal gas. The sulfidation tests were carried out in a fixed-bed reactor with space velocity of 2000 h−1(STP). The results show that the suitable addition of manganese oxide in iron-based sorbent can decrease H2S and COS concentration in exit before breakthrough due to its simultaneous reaction capability with H2S and COS. Fe3O4 and MnO are the initial active components in iron-manganese-based sorbent, and FeO and Fe are active components formed by reduction during sulfidation. The crystal phases of iron affect obviously their desulfurization capacity. The reducibility of sorbent changes with the content of MnO in sorbent. S7F3M and S3F7M have bigger sulfur capacities (32.68 and 32.30 gS/100 g total active component), while S5F5M has smaller sulfur capacity (21.92 gS/100 g total active component). S7F3M sorbent has stable sulfidation performance in three sulfidation-regeneration cycles and no apparent structure degradation. The sulfidation performance of iron-manganese-based sorbent is also related with its specific surface area and pore volume.

Effect of binder on the properties of iron oxide sorbent for hot gas desulfurization

The most difficult problem in hot gas desulfurization in Integrated Coal Gasification Combined Cycle (IGCC) is the pulverization of sulfur removal sorbents. Appropriate binders for hot gas sulfur removal sorbents can solve the pulverization problem. In this paper, six sorbents with binders of different argillaceous minerals were prepared by mechanical mixing method. Desulfurization behavior for hot gas desulfurization sorbents was investigated in a fixed-bed reactor. Result showed that sorbent NTKW2 with binder of clay had a better sulfidation performance. NTKW2 had a more stable performance than other sorbents in the continuous sulfidation-regeneration cycles. Sulfur capacity of sorbent remained the same in each cycle. The desulfurization efficiency and mechanical strength of NTKW2 were the best among the tested sorbents. The behavior of NTKW2 at different temperatures showed different performances, and the best reaction temperature was 550 °C. Higher heat stability, sulfur capacity and desulfurization efficiency were found on NTKW2 in six continuous sulfidation-regeneration cycles.

Selective catalytic oxidation of H2S to elemental sulfur over V2O5/Zr-pillared montmorillonite clay

The performance of vanadia-doped zirconia-pillared clay (V/Zr-PILC) for the selective catalytic oxidation of H2S was investigated in this study. A series of vanadia-doped zirconia pillared clays (V/Zr-PILCs) with various amounts of vanadia were prepared and characterized using X-ray diffraction (XRD), surface area-pore volume measurements, chemical analysis, infrared (FTIR) spectroscopy, UV–vis-diffuse reflectance spectroscopy (UV–vis-DRS), X-ray photoelectron spectroscopy (XPS), and thermogravimetric (TG) analysis. V/Zr-PILCs showed better catalytic performance than Zr-PILC at temperatures ranging from 220–300 °C without any considerable SO2 emission. The H2S conversion over V/Zr-PILCs increased with increasing vanadia content up to 6 wt%. However, it decreased at higher vanadia loadings due to the decrease of surface area and to the formation of the crystalline V2O5 phase. The results of a regeneration experiment using 6 wt% V/Zr-PILC catalyst showed good reproducibility in its catalytic activity, even through five successive sulfidation-regeneration cycles at 280 °C.

Investigation of nano-CuO/mesoporous SiO 2 materials as hot gas desulphurization sorbents

Gases from gasification processes contain H 2S which is highly harmful for almost all catalyst materials and metal constructions such as in IGCC plants. Metal oxides and/or their admixtures are widely used for the removal of H 2S from gases at high temperatures. These sorbents, however, suffer from several problems which adversely affect their performances. In this study, a new method was explored to prepare sorbents from nano-size (sub-micron) CuO supported on SiO 2 materials. Three sorbents with 38.5 wt.%, 41.5 wt.% and 49.6 wt.% Cu contents were prepared and characterized. Results indicated that, the produced materials are mesoporous composite type materials of nano-size CuO and amorphous SiO 2. The H 2S adsorption characteristics of the prepared sorbents were studied in a laboratory scale fixed-bed reactor through sulfidation–regeneration cycles in the temperature range of 788–838 K at atmospheric pressure. The average sulfur uptake capacities of the sorbents, based on the breakthrough point H 2S concentration of 20 ppm, were determined to be in the range of 2.38–2.91 g S/100 g sorbent after 3 cycles. Although the process at high temperatures appeared to disturb the structure of the sorbents to some extent, the drop in their sulfur adsorption capacities remained limited.

Investigation of CuO/mesoporous SBA-15 sorbents for hot gas desulfurization

Metal oxides, are widely used as sorbents to remove H 2S from hot gases produced by gasification processes. These oxides, however, suffer from several problems such as low capacity, sintering, evaporation, low duration and mechanical strength. Pure metal oxides or their admixtures loaded on inert materials have been used to overcome these problems. In this study, Cu/SBA-15 type sorbents were prepared by the wet impregnation method, in which copper and SBA-15 are active and support material, respectively. The prepared sorbent samples contained 21.97% and 40.2% Cu by weight. The H 2S removal characteristics of the samples were investigated by using a laboratory scale fixed bed reactor through sulfidation–regeneration cycles in the temperature range of 788–838 K. Characterization results showed that the preparation method does not have a serious destructive effect on the structure of the support material. The average breakthrough point sulfur uptake capacities of the sorbents with 21.97% Cu and 40.22% Cu were determined to be 0.63 and 1.57 g S/100 g adsorbent, respectively, after three cycles. The XRD and nitrogen adsorption analysis indicated that the structures of the sorbents were destructed to some extent during the sulfidation–regeneration cycles. Changes in the sulfur up take, however, were not comparable and remained limited.