Elemental sulfur production through regeneration of a SO2-adsorbed V2O5-CoO/AC in H2

H₂ regeneration of an activated carbon supported vanadium and cobalt oxides (V₂O₅-CoO/AC) catalyst–sorbent used for flue gas SO₂ removal is studied in this paper. Elemental sulfur is produced during the H₂-regeneration when effluent gas of the regeneration is recycled back to the reactor. The regeneration conditions affect the regeneration efficiency and the elemental sulfur yield. The regeneration efficiency is the highest at 330 °C, with SO₂ as the product. The production of elemental sulfur occurs at 350 °C and higher with the highest elemental sulfur yield of 9.8 mg-S/g-Cat. at 380 °C. A lower effluent gas recycle rate is beneficial to elemental sulfur production. Intermittent H₂ feeding strategy can be used to control H₂S concentration in the gas phase and increase the elemental sulfur yield. Two types of reactions occur in the regeneration, reduction of sulfuric acid to SO₂ by AC and reduction of SO₂ to elemental sulfur through Claus reaction. H₂S is an intermediate, which is important for elemental sulfur formation and for conversion of CoO to CoS that catalyzes the Claus reaction. The catalyst–sorbent exhibits good stability in SO₂ removal capacity and in elemental sulfur yield.     

Regeneration of Fe2O3-based high-temperature coal gas desulfurization sorbent in atmosphere with sulfur dioxide

Regeneration of a high-temperature coal gas desulfurization sorbent is a key technology in its industrial applications. A Fe₂O₃-based high-temperature coal gas desulfurizer was prepared using red mud from steel factory. The influences of regeneration temperature, space velocity and regeneration gas concentration in SO₂ atmosphere on regeneration performances of the desulfurization sorbent were tested in a fixed bed reactor. The changes of phase and the composition of the Fe₂O₃-based high-temperature coal gas desulfurization sorbent before and after regeneration were examined by X-ray diffraction (XRD) and X-ray Photoelectron spectroscopy(XPS), and the changes of pore structure were characterized by the mercury intrusion method. The results show that the major products are Fe₃O₄ and elemental sulfur; the influences of regeneration temperature, space velocity and SO₂ concentration in inlet on regeneration performances and the changes of pore structure of the desulfurization sorbent before and after regeneration are visible. The desulfurization sorbent cannot be regenerated at 500°C in SO₂ atmosphere. Within the range of 600°C–800°C, the time of regeneration becomes shorter, and the regeneration conversion increases as the temperature rises. The time of regeneration also becomes shorter, and the elemental sulfur content of tail gas increases as the SO₂ concentration in inlet is increased. The increase in space velocity enhances the reactive course; the best VSP is 6000 h⁻¹ for regeneration conversion. At 800°C, 20 vol-% SO₂ and 6000 h⁻¹, the regeneration conversion can reach nearly to 90%.     

Regeneration of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-based high-temperature coal gas desulfurization sorbent in atmosphere with sulfur dioxide

Regeneration of a high-temperature coal gas desulfurization sorbent is a key technology in its industrial applications. A Fe₂O₃-based high-temperature coal gas desulfurizer was prepared using red mud from steel factory. The influences of regeneration temperature, space velocity and regeneration gas concentration in SO₂ atmosphere on regeneration performances of the desulfurization sorbent were tested in a fixed bed reactor. The changes of phase and the composition of the Fe₂O₃-based high-temperature coal gas desulfurization sorbent before and after regeneration were examined by X-ray diffraction (XRD) and X-ray Photoelectron spectroscopy(XPS), and the changes of pore structure were characterized by the mercury intrusion method. The results show that the major products are Fe₃O₄ and elemental sulfur; the influences of regeneration temperature, space velocity and SO₂ concentration in inlet on regeneration performances and the changes of pore structure of the desulfurization sorbent before and after regeneration are visible. The desulfurization sorbent cannot be regenerated at 500°C in SO₂ atmosphere. Within the range of 600°C–800°C, the time of regeneration becomes shorter, and the regeneration conversion increases as the temperature rises. The time of regeneration also becomes shorter, and the elemental sulfur content of tail gas increases as the SO₂ concentration in inlet is increased. The increase in space velocity enhances the reactive course; the best VSP is 6000 h^?1 for regeneration conversion. At 800°C, 20 vol-% SO₂ and 6000 h^?1, the regeneration conversion can reach nearly to 90%.     

Effect of sulfidation/oxidative regeneration cycle on the solid structural changes of sorbent for high-temperature removal of H2S

In order to investigate the effects of sulfidation/oxidative regeneration cycle on the change of structural properties and removal capacity of sorbent, sulfidation/regeneration cycle was carried out up to 15 times in a fixed-bed reactor. The effluent gases from the fixed-bed reactor were analyzed by gas chromatography, and XRD, SEM, and liquid nitrogen physisorption method were used to characterize the reacted sorbents. The sorbent treated first sulfidation/regeneration cycle exhibited maximum specific surface area and the highest H₂S removal capacity. Hysteresis of adsorption isotherm of the regenerated sorbent reflected the growth of pores of fresh sorbent and pore size distribution confirmed this fact. Furthermore constant H₂S removal capacity was maintained up to 15 times of sulfidation/regeneration cycle.     

SO2 retention at low temperatures by Ca(OH)2-derived CaO: a model for CaO regeneration

The retention of SO₂ by Ca(OH)₂-derived CaO has been studied at 573 K. The thermal treatment of Ca(OH)₂ forms CaO sorbents with high activity to retain SO₂. At the temperature of 573 K, it has been proved that SO₂ interacts with CaO to form a surface CaSO₃ specie.The regeneration process by thermal decomposition of CaSO₃, giving SO₂ and CaO, has been examined using techniques such as in situ FTIR and in situ XRD. In addition, CO₂ chemisorption and SO₂ sorption and sorbent regeneration was evaluated by thermogravimetry.The thermal regeneration process used to decompose CaSO₃ causes an important loss in the SO₂ retention capacity of the sorbent. Using several experimental procedures, a series of reactions have been analysed (Ca(OH)₂ decomposition, formation of superficial CaSO₃, change from surface to bulk CaSO₃ and CaSO₃ decomposition) and an insight into the regeneration process has been obtained.The decrease in SO₂ retention, following the thermal regeneration step, is due to both an increase in the particle size and the conversion of CaSO₃ into CaSO₄ and CaS (CaSO₃ disproportionation) upon heat treatment. The increase in the particle size is responsible for about 88% of the loss of activity, whereas CaSO₃ disproportionation explains a 12% of loss.     

A study on regeneration of zinc titanate sorbents for H2S removal

In this study the regeneration of used zinc titanate sorbents for high temperature desulfurization of fuel gases was investigated. Zinc titanate sorbents with Zn/Ti molar ratio of 1.5 were prepared and the regeneration was carried out in quartz fixed-bed reactor of 1 cm diameter. Regeneration of zinc titanate sorbents at high temperature is exothermic reaction that brings about deterioration of sorbents. So far experimental parameters such as reaction temperature, concentration of oxygen, flow rate and steam content have been considered to obtain suitable regeneration conditions. H₂S and SO₂ breakthrough curves were obtained during desulfurization-regeneration. Also, properties of the sorbents before and after regeneration were analyzed using SEM, XRD, Hg-porpsimetry and BET method. From such results, we obtained the most suitable regeneration conditions including 650 ‡ C of regenerating temperature, 5 % of oxygen content and 10 % of steam in the gas stream.     

High temperature desulfurization over nano-scale high surface area ceria for application in SOFC

In the present work, suitable absorbent material for high temperature desulfurization was investigated in order to apply internally in solid oxide fuel cells (SOFC). It was found that nano-scale high surface area CeO₂ has useful desulfurization activity and enables efficient removal of H2S from feed gas between 500 to 850°C. In this range of temperature, compared to the conventional low surface area CeO₂, 80–85% of H2S was removed by nano-scale high surface area CeO₂, whereas only 30–32% of H2S was removed by conventional low surface area CeO₂. According to the XRD studies, the product formed after desulfurization over nano-scale high surface area CeO₂ was Ce₂O₂S. EDS mapping also suggested the uniform distribution of sulfur on the surface of CeO₂. Regeneration experiments were then conducted by temperature programmed oxidation (TPO) experiment. Ce₂O₂S can be recovered to CeO₂ after exposure in the oxidation condition at temperature above 600°C. It should be noted that SO₂ is the product from this regeneration process. According to the SEM/EDS and XRD measurements, all Ce₂O₂S forming is converted to CeO₂ after oxidative regeneration. As the final step, a deactivation model considering the concentration and temperature dependencies on the desulfurization activity of CeO₂ was applied and the experimental results were fitted in this model for later application in the SOFC model.     

Study of intrinsic sulfidation behavior of Fe2O3 for high temperature H2S removal

Iron-based sorbent was preferable for desulfurization from coal-derived gas due to economic consideration and favorable dynamic property. The intrinsic behavior of Fe-based sorbent should be primarily understood in the sulfidation process for improving its performance. A series of tests were carried out with Fe₂O₃, Fe and other compounds containing-Fe (FO) made from the same precursor FeC₂O₄·2H₂O in H₂S-N₂ mixture in this study. The formation of H₂ was observed with Fe and FO as sorbents. While SO₂ was detected with FO and Fe₂O₃ as sorbents, its concentration in outlet was gradually decreased. The crystal phase and surface chemical state of fresh and sulfided Fe₂O₃ with different reaction times were characterized by XRD and XPS measurements. The result suggested that the intrinsic H₂S removal by Fe₂O₃ would produce multi-phase of sulfides. The possible mechanism of sulfidation reaction was discussed.     

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.     

Sulfidation/regeneration multicycle testing of nickel-modified ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> 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.     

The effect of HCl and H<Subscript>2</Subscript>O on the H<Subscript>2</Subscript>S removing capacities of Zn-Ti-based desulfurization sorbents promoted by cobalt and nickel oxide

The sulfur removing capacities of various Zn-Ti-based sorbents were investigated in the presence of H₂O and HCl at high-(sulfidation, 650 °C; regeneration, 800 °C) and medium-(sulfidation, 480 °C; regeneration, 580 °C) temperature conditions. The H₂O effect of all sorbents was not observed at high-temperature conditions. At mediumtemperature conditions, the reaction rate of ZT (Zn/Ti : 1.5) sorbent decreased with the level of H₂O concentration, while modified (ZTC, ZTN) sorbents were not affected by the water vapor. HCl vapor resulted in the deactivation of ZT sorbent with a cycle number at high-temperature due to the production of ZnCl₂ while the sulfur removing capacities of ZTC and ZTN sorbents were maintained during 4–5 cyclic tests. In the case of medium-temperature conditions, ZT sorbent was poisoned by HCl vapor while cobalt and nickel added to ZT sorbent played an important catalytic role to prevent from being poisoned by HCl due to providing heat, emitted when these additives quickly react with H₂S even at medium-temperature conditions, to the sorbents     

Effect of rice husk ash addition on CO2 capture behavior of calcium-based sorbent during calcium looping cycle

Rice husk ash/CaO was proposed as a CO₂ sorbent which was prepared by rice husk ash and CaO hydration together. The CO₂ capture behavior of rice husk ash/CaO sorbent was investigated in a twin fixed bed reactor system, and its apparent morphology, pore structure characteristics and phase variation during cyclic carbonation/calcination reactions were examined by SEM-EDX, N₂ adsorption and XRD, respectively. The optimum preparation conditions for rice husk ash/CaO sorbent are hydration temperature of 75 °C, hydration time of 8 h, and mole ratio of SiO₂ in rice husk ash to CaO of 1.0. The cyclic carbonation performances of rice husk ash/CaO at these preparation conditions were compared with those of hydrated CaO and original CaO. The temperature at 660 °C–710 °C is beneficial to CO₂ absorption of rice husk ash/CaO, and it exhibits higher carbonation conversions than hydrated CaO and original CaO during multiple cycles at the same reaction conditions. Rice husk ash/CaO possesses better anti-sintering behavior than the other sorbents. Rice husk ash exhibits better effect on improving cyclic carbonation conversion of CaO than pure SiO₂ and diatomite. Rice husk ash/CaO maintains higher surface area and more abundant pores after calcination during the multiple cycles; however, the other sorbents show a sharp decay at the same reaction conditions. Ca₂SiO₄ found by XRD detection after calcination of rice husk ash/CaO is possibly a key factor in determining the cyclic CO₂ capture behavior of rice husk ash/CaO.     

Sorption-enhanced water gas shift reaction by sodium-promoted calcium oxides

The water gas shift reaction was evaluated in the presence of novel carbon dioxide (CO₂) capture sorbents, both alone and with catalyst, at moderate reaction conditions (i.e., 300-600 °C and 1-11.2 atm). Experimental results showed significant improvements to carbon monoxide (CO) conversions and production of hydrogen (H₂) when CO₂ sorbents are incorporated into the water gas shift reaction. Results suggested that the performance of the sorbent is linked to the presence of a Ca(OH)₂ phase within the sorbent. Promoting calcium oxide (CaO) sorbents with sodium hydroxide (NaOH) as well as pre-treating the CaO sorbent with steam appeared to lead to formation of Ca(OH)₂, which improved CO₂ sorption capacity and WGS performance. Results suggest that an optimum amount of NaOH exists as too much leads to a lower capture capacity of the resultant sorbent. During capture, the NaOH-promoted sorbents displayed a high capture efficiency (nearly 100%) at temperatures of 300-600 °C. Results also suggest that the CaO sorbents possess catalytic properties which may augment the WGS reactivity even post-breakthrough. Furthermore, promotion of CaO by NaOH significantly reduces the regeneration temperature of the former.     

SO2 retention on CaO/activated carbon sorbents. Part III. Study of the retention and regeneration conditions

The retention of SO₂ on CaO/activated carbon sorbents is studied. The effect of several variables such as the reaction temperature, partial pressure of SO₂ for different calcium loads, and O₂ presence are analysed. Additionally, the regeneration and reutilization of spent sorbents is investigated. In all cases presence of well-dispersed CaO in the sorbents improves SO₂ retention in comparison with the activated carbon. In absence of O₂ in the gas mixture, the amount of SO₂ retained does not depend on the SO₂ partial pressure in the range of partial pressures studied and, as expected, SO₂ physisorption on the activated carbon support occurs at room temperature. SO₂ retention occurs in surface CaO between 100 °C and 250 °C, and in bulk CaO above 300 °C. The total calcium conversion is reached at 500 °C. Above 550 °C calcium-catalysed carbon gasification by SO₂ occurs. In presence of O₂ in the gas mixture, the studied sorbents are very effective for SO₂ removal. However, the SO₂ retention process in presence of oxygen must be carried out at temperatures lower than 300 °C to avoid carbon gasification by O₂. The thermal regeneration of the spent sorbents can be done under inert atmosphere (880 °C) with only 20% activity loss after the first regeneration cycle due to sintering and formation of CaS. No additional activity loss is detected in the subsequent cycles.     

Improving the SO<Subscript>2</Subscript> absorption rate of CeFeMg-based sorbent promoted with titanium promoter

To improve the poor SO₂ absorption rate of CeFeMgTi sorbent with high sulfur removal capacity and fast regeneration, a new sorbent, CeFeMgTi-sol was prepared by the modified co-precipitation method and tested in a packed bed reactor at RFCC conditions (sulfation of MgO to MgSO₄ in the presence of low concentration of SO₂ at 973 K, regeneration of MgSO₄ to MgO and H₂S in the presence of H₂ at 803 K). The CeFeMgTi-sol sorbent showed excellent SO₂ absorption and sulfur removal capacity (46.2 sulfur g/g absorbent×100). It was found that the SO₂ absorption rates were related to the structure of the Mg and Ti and the textural properties such as surface area and pore volume. In the case of the fresh state of CeFeMgTi sorbent, CeO₂, MgO and MgTiO₃ structures were observed. But the new CeFeMgTisol sorbent before SO₂ absorption, showed a separated MgO and TiO₂ peak only. These differences in the sorption rates were discussed by the difference in the XRD pattern, surface area and pore volume.     

Kinetic study of domestic limestone (Daesung)

Sulfidation and sulfation reactions of Daesung limestone, which is a calcium-based sorbent chosen out of domestic limestone for the removal of H₂S and SO₂, were investigated by using TGA (thermal gravimetric analyzer) Effects of H and H₂S on the sulfidation were also investigated The conversion rate of CaS to CaSO₄ in oxidation was low since the concentration of SO₂ used for this study was low and CaO was not completely converted into CaS It was observed that the effects of H₂ concentration on the sulfidation were relatively small and the maximum conversion rate and reaction rate increased with increase of H₂S concentration In the sulfation reaction, conversion rate could be raised with the injection of air at a sulfation reaction temperature above 800 °C However, the conversion rate decreased without the injection of air due to the blockage of sorbent pores.     

Sulphur dioxide removal using South African limestone/siliceous materials

This study presents an investigation into the desulfurization effect of sorbent derived from South African calcined limestone conditioned with fly ash. The main aim was to examine the effect of chemical composition and structural properties of the sorbent with regard to SO₂ removal in dry-type flue gas desulfurization (FGD) process. South African fly ash and CaO obtained from calcination of limestone in a laboratory kiln at a temperature of 900 °C were used to synthesize CaO/ash sorbent by atmospheric hydration process. The sorbent was prepared under different hydration conditions: CaO/fly ash weight ratio, hydration temperature (55-75 °C) and hydration period (4-10 h). Desulfurization experiments were done in the fixed bed reactor at 87 °C and relative humidity of 50%. The chemical composition of both the fly ash and calcined limestone had relatively high Fe₂O₃ and oxides of other transitional elements which provided catalytic ability during the sorbent sorption process. Generally the sorbents had higher SO₂ absorption capacity in terms of mol of SO₂ per mol of sorbent (0.1403-0.3336) compared to hydrated lime alone (maximum 0.1823). The sorbents were also found to consist of mesoporous structure with larger pore volume and BET specific surface area than both CaO and fly ash. X-ray diffraction (XRD) analysis showed the presence of complex compounds containing calcium silicate hydrate in the sorbents.     

Kinetics of the reaction of iron blast furance slag/hydrated lime sorbents with SO2 at low temperatures: effects of sorbent preparation conditions

Sorbents highly reactive towards SO₂ have been prepared from iron blast furnace slag and hydrated lime under different hydration conditions. The reaction of the dry sorbents with SO₂ has been studied under the conditions similar to those in the bag filters in the spray-drying flue gas desulfurization system. The reaction was well described by a modified surface coverage model which assumes the reaction rate being controlled by chemical reaction on sorbent grain surface and takes into account the effect of sorbent Ca molar content and the surface coverage by product. The effects of sorbent preparation conditions on sorbent reactivity were entirely represented by the effects of the initial specific surface area (S_g0) and the Ca molar content (M⁻¹) of sorbent. The initial conversion rate of sorbent increased linearly with increasing S_g0, and the ultimate conversion increased linearly with increasing S_g0M⁻¹. The initial conversion rate and ultimate conversion of sorbent increased significantly with increasing relative humidity of the gas. Temperature and SO₂ concentration had mild effects on the initial conversion rate and negligible effects on the ultimate conversion.     

Role of water vapor in oxidative decomposition of calcium sulfide

CaS formed from the CaO sorbent during desulfurization in coal gasifiers has to be converted to CaSO₄ before disposal. CaS is mainly decomposed to CaO and SO₂ by O₂ and then CaO is converted to CaSO₄ by SO₂ and O₂. The role of H₂O in the oxidative decomposition of CaS with O₂ was studied using reagent grade CaS and H₂¹⁸O. The following results were obtained: (1) there is a synergistic effect of H₂O and O₂ on the oxidative decomposition of CaS to CaO and SO₂; (2) H₂O reacts with CaS to form CaO, SO₂ and H₂ in the absence of O₂; (3) the oxidative decomposition of CaS to CaO and SO₂ occurs stepwise; (4) H₂O directly reacts with CaS in the presence of O₂; (5) H₂O plays an important role in the oxidative decomposition of CaS even if the O₂ concentration is high.     

应用CaSO4载氧体燃烧技术的CaO再生过程

引言近年来CaO作为CO2吸收剂在近零排放煤直接制氢、生物质气化、吸收增强型天然气重整制氢、焦炉煤气重整制氢、燃煤电站CO2捕集等过程的应用受到国内外的持续关注和大量研究。在这些过程中CO2以CaCO3的形式固化下来,为循环利用CaO吸收剂并收集CO2,需要将产物CaCO3煅烧分解,这一过程称为CaO再生过程。CaO再生是强吸热过程,为该过程提供热量的一