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%.     

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.     

Rates of reaction of SO2 with metal oxides

Rates of sorption of SO₂ from synthetic flue gas by 14 metal oxides were determined. These oxides had been selected for their potential applicability to processes for removing SO₂ from flue gas using thermal regeneration of sorbent. Measurements were performed using thermogravimetric analysis, and rates were fitted to semi-empirical expressions; CeO₂, Co₃O₄, Cr₂O₃, CuO, Fe₂O₃, and NiO displayed measurable rates in the range 200° to 500°C, and were converted to sulphates. Rates were immeasurably small at 25° to 800°C for Al₂O₃, Sb₂O₅, SnO₂, TiO₂, V₂O₅, WO₃, ZnO, and ZrO₂, CuO and CeO₂ showed the highest rates     

Kinetic studies on the reactions involved in the hot gas desulfurization using a regenerable iron oxide sorbent—II: Reactions of iron sulfide with oxygen and sulfur dioxide

In the hot gas desulfurization process using iron oxide sorbent, the regeneration of the sulfided iron oxide sorbent consists of two reactions: the oxidation of iron sulfide with air, and its reaction with the sulfur dioxide formed during the air oxidation. This part describes the kinetic studies on the reactions of iron sulfide (formed by the reactions of Fe₂O₃ with H₂CO mixture and subsequendy with H₂S) with oxygen and sulfur dioxide. The experimental and analysis procedures used are similar to those outlined in Part I of this paper.The activation energies for the oxygen and the sulfur dioxide reactions are found to be 15.63 and 17.5 kcal/mol, respectively. Notably, the product oxides formed in the two cases are different. With air, the reaction is fast and the final product is Fe₂O₃, whereas with SO₂, the major product is Fe₃O₄, which slowly oxidizes to Fe₂O₃ in a secondary step. Also, in the latter reaction elemental sulfur is formed.     

Regenerable Sorbents for High-Temperature Desulfurization of Syngas from Biomass Gasification

Single-metal high-temperature solid sorbents for syngas cleaning using Mn, Ca, Fe, Cu, or Mo supported on γ-Al₂O₃ were synthesized, characterized, and tested in a fixed-bed reactor. H₂S and SO₂ concentrations in the gas after treatment at T = 400 to 700 °C were compared with thermodynamic calculations. The Mn-based sorbent showed the best ability to achieve a low sulfur residual in the gas, especially at temperatures above 600 °C. Sorbents with Fe, Cu, and Mo gave SO₂ formation in the initial phase, but this could be avoided by a pre-reduction treatment of the sorbent material.     

Lean NOxCatalysis over Sn/γ-Al2O3Catalysts

Sn/γ-Al₂O₃were effective and highly stable catalysts for NO reduction with propene under high partial pressures of oxygen and water. The activity depended on the Sn loading. For a 10 wt% Sn/Al₂O₃at 475–500°C, 58% conversion of 1000 ppm NO to N₂was obtained in the presence of 10% water and 15% O₂, at a space velocity of 30,000 h⁻¹, and 77% conversion at 15,000 h⁻¹. The NO conversion increased with O₂partial pressure but was suppressed by water below 500°C. The activity was also suppressed by SO₂but could be restored slowly after the removal of SO₂.     

SO2 abatement over nanocrystalline MgAl2O4 spinel-supported catalysts

Thermally stable magnesium-rich MgAl₂O₄ spinel with mesoporous nanostructures and high surface area has been prepared by co-precipitation and post hydrothermal treatment, using glucose as organic template. Physical and chemical properties were characterized by XRD, N₂ sorption, TG, FTIR, SEM, and TEM. The synthesized MgAl₂O₄ showed a surface area of 324 m² g^?1 and centralized mesopore distribution (ca. 3.3 nm pore width) after calcination at 700 °C for 3 h. The prepared MgAl₂O₄ were impregnated with metal oxides as sulfur transfer catalysts for high-temperature SO₂ adsorption reaction. The results showed that ferric doped MgAl₂O₄ had the highest SO₂ pick-up capacity up to 58 % and best regeneration up to 81 %. These results showed that thermally stable nanostructured MgAl₂O₄ are a promising candidate as catalyst for desulfurization in fluid catalytic cracking process.     

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.     

The oxidation of Fe(II) with Cu(II) in acidic sulfate solutions with air at elevated pressures

The oxidation of ferrous ions in acidic sulfate solutions in the presence of cupric ions at elevated air pressures was investigated in a high-intensity gas–liquid contactor. The study was required for the design of the regeneration steps of the novel Vitrisol^® desulphurization process. The effects of the Fe²⁺ concentration, Cu²⁺ concentration, Fe³⁺ concentration, initial H₂SO₄ concentration, and partial oxygen pressure on the reaction rate were determined at three different temperatures, i.e., T?=?50?°C, 70?°C, and 90?°C. Most of the experiments were determined to be affected by the mass transfer of oxygen, and therefore true intrinsic kinetics could not be fully determined. An increase in Fe²⁺ and Cu²⁺ concentrations, as well as the partial pressure of oxygen and temperature, increased the Fe²⁺ oxidation rate. H₂SO₄ did not influence the Fe²⁺ oxidation rate. An increase in Fe³⁺ concentration decreased the Fe²⁺ oxidation rate. Although determined from experiments partially affected by mass transfer, a first order of reaction in Fe²⁺ was observed, fractional orders in both Cu²⁺ and O₂ were measured, a zero order in H₂SO₄ was determined, and a negative, fractional order in Fe³⁺ was obtained. The activation energy was estimated to be 31.3?kJ/mol.     

Sulfidation and regeneration of iron-based sorbents supported on activated-chars prepared by pressurized impregnation for coke oven gas desulfurization

The sulfidation and regeneration properties of lignite char-supported iron-based sorbent for coke oven gas (COG) desulfurization prepared by mechanical stirring (MS), ultrasonic assisted impregnation (UAI), and high pressure impregnation (HPI) were investigated in a fixed-bed reactor. During desulfurization, the effects of process parameters on sulfidation properties were studied systematically. The physical and chemical properties of the sorbents were analyzed by X-ray diffraction (XRD), scanning electron microscope coupled with energy dispersive spectroscopy (SEM-EDS), Fourier transform infrared (FTIR) and BET surface area analysis. The results of desulfurization experiments showed that high pressure impregnation (HPI) enhanced the sulfidation properties of the sorbents at the breakthrough time for char-supported iron sorbents. HPI method also increased the surface area and pore volume of sorbents. Sulfur capacity of sorbents was enhanced with increasing sulfidation temperatures and reached its maximum value at 400 °C. It was observed that the presence of steam in coke oven gas can inhibit the desulfurization performance of sorbent. SO₂ regeneration of sorbent resulted in formation of elemental sulfur. HPIF10 sorbent showed good stability during sulfide-regeneration cycles without changing its performance significantly.     

Removal of nitric oxide and sulfur dioxide from flue gases using a FeII-ethylenediamineteraacetate solution

The combined absorption of NO and SO₂ into the Fe(II)-ethylenediamineteraacetate(EDTA) solution has been realized. Activated carbon is used to catalyze the reduction of Fe^III-EDTA to Fe^II-EDTA to maintain the ability to remove NO with the Fe-EDTA solution. The reductant is the sulfite/bisulfite ions produced by SO₂ dissolved into the aqueous solution. Experiments have been performed to determine the effects of activated carbon of coconut shell, pH value, temperature of absorption and regeneration, O₂ partial pressure, sulfite/bisulfite and chloride concentration on the combined elimination of NO and SO₂ with Fe^II-EDTA solution coupled with the Fe^II-EDTA regeneration catalyzed by activated carbon. The experimental results indicate that NO removal efficiency increases with activated carbon mass. There is an optimum pH of 7.5 for this process. The NO removal efficiency increases with the liquid flow rate but it is not necessary to increase the liquid flow rate beyond 25 ml min^?1. The NO removal efficiency decreases with the absorption temperature as the temperature is over 35 °C. The Fe²⁺ regeneration rate may be speeded up with temperature. The NO removal efficiency decreases with O₂ partial pressure in the gas streams. The NO removal efficiency is enhanced with the sulfite/bisulfite concentration. Chloride does not affect the NO removal. Ca(OH)₂ and MgO slurries have little influence on NO removal. High NO and SO₂ removal efficiencies can be maintained at a high level for a long period of time with this heterogeneous catalytic process.     

Development of iron oxide sorbents for Hg removal from coal derived fuel gas: Sulfidation characteristics of iron oxide sorbents and activity for COS formation during Hg removal

The aim of this study is to develop a process for the removal of Hg⁰ using H₂S over iron oxides sorbents, which will be located just before the wet desulfurization unit and catalytic COS converter of a coal gasification system. It is necessary to understand the reactions between the iron oxide sorbent and other components of the fuel gas such as H₂S, CO, H₂, H₂O, etc. In this study, the sulfidation behavior and activity for COS formation during Hg⁰ removal from coal derived fuel gas over iron oxides prepared by precipitation and supported iron oxide (1 wt% Fe₂O₃/TiO₂) prepared by conventional impregnation were investigated. The iron oxide samples were dried at 110 °C (designated as Fe₂O₃-110) and calcined at 300 and 550 °C (Fe₂O₃-300 and Fe₂O₃-550). The sulfidation behavior of iron oxide sorbents in coal derived fuel gas was investigated by thermo-gravimetric analysis (TGA). COS formation during Hg⁰ removal over iron oxide sorbents was also investigated using a laboratory-scale fixed-bed reactor. It was seen that the Hg⁰ removal activity of the sorbents increased with the decrease of calcinations temperature of iron oxide and extent of sulfidation of the sorbents also increased with the decrease of calcination temperature. The presence of CO suppressed the weight gain of iron oxide due to sulfidation. COS was formed during the Hg⁰ removal experiments over Fe₂O₃-110. However, in the cases of calcined iron oxides (Fe₂O₃-300, Fe₂O₃-550) and 1 wt% Fe₂O₃/TiO₂, formation of COS was not observed but the Hg⁰ removal activity of 1 wt% Fe₂O₃/TiO₂ was high. Both FeS and FeS₂ were active for Hg⁰ removal in coal derived fuel gas without forming any COS.     

Oxidative regeneration of sulfided sorbent by H2S without emission of SO2

Much SO₂, another perilous air pollutant, was emitted during the oxidative regeneration of sulfided sorbent by H₂S. In order to prevent emission of SO₂, we carried out oxidative regeneration with the physical mixture of CaO and sulfided sorbent and investigated the effect of regeneration temperature and oxygen concentration on the reactivity of CaO with S0₂. The effluent gases were analyzed by G.C. and the properties of sorbent were characterized by XRD. SEM, TG/DTA and EPMA. Deterioration of reactivity of CaO with S0₂ resulted in increment of emission of SO₁₂ due to the structural changes of CaO above 750°C and that at 850°C was more severe. Furthermore EPMA and XRD analysis revealed that product layer diffusion through the solid product, CaSO₄, was the rate limiting step for CaO sulfidation. The reaction of CaO w:.th SO₂ was first order approximately and that was accelerated by high O₂ concentration.     

Sorbent development for continuous regenerative H2S removal in a rotating monolith reactor

A high capacity and regenerable manganese based sorbent for desulfurization of hot dry fuel gas from coal gasification has been developed. Pure γ-Al₂O₃ and washcoated cordierite monoliths impregnated with manganese acetate and calcined at 973 K resulted in highly dispersed Mn₃O₄ on γ-Al₂O₃. MnS was formed during sulfidation and MnAl₂O₄ during subsequent regeneration with steam. The optimal operation temperature was found to be between 1123 and 1223 K. The maximum capacity of the acceptor was 17 mass% sulfur which was obtained for a 32 mass% manganese loading. A deactivation test of 65 subsequent sulfidation and regeneration cycles showed minor deactivation during the first cycles followed by a stable performance. This sorbent will be used in a rotating monolith reactor in which absorption and regeneration takes place simultaneously in separate sections, which enables a continuous operation.     

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.     

Performance optimization of an electromembrane reactor for recycling and resource recovery of desulfurization residuals

An environmentally friendly method for electrochemically regenerating alkali‐sorbent (NaOH) and recovering sulfur in the flue gas as H₂SO₄, while producing H₂ as a clean energy source from flue gas desulfurization (FGD) residuals in an electromembrane reactor, was proposed in this article. To optimize and improve the performance, the optimal operating conditions were deduced from the numerical simulation and validated using experimental data. Under the optimized conditions, the current efficiencies of alkali‐sorbent regeneration and H₂SO₄ reached 84 and 87%, respectively, which is comparable to those obtained in the chlor‐alkali industry. Therefore, this method has the potential to be scaled up. If this technology is integrated into an existing FGD facility, the money‐consuming chemical process could be transferred into a renewable resource and clean energy conversion process. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2613–2624, 2014     

Experimental and theoretical analysis of element mercury adsorption on Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/Ag composites

A novel magnetic nano-sorbent Fe₃O₄/Ag was synthesized and applied to capture the elemental mercury from the simulated flue gas. The morphology, components and crystal phase of the sorbents were characterized by transmission electron microscope (TEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD), respectively. The mercury removal performance of the sorbents was investigated through the fixed-bed tests. The results indicated that silver was successfully loaded on the surface of Fe₃O₄ particles, which could significantly enhance the Hg⁰ removal performance of the sorbents. Flue gas components, including CO₂, SO₂, and NO, have little impact on the Hg⁰ removal performance of Fe₃O₄/Ag sorbents, while O₂ has a slightly positive effect. The Hg⁰ removal efficiency decreased with the increasing of temperature, Hg⁰ inlet concentration and gas hourly space velocity. Only one broad mercury desorption peak at approximately 210 °C could be observed during the temperature-programmed desorption (TPD) process, which indicated that mercury species existing on the surface of Fe₃O₄/Ag sorbents might be elemental mercury instead of oxidized mercury. Furthermore, the reusability tests showed that the Fe₃O₄/Ag sorbents could be efficiently regenerated and reused. Finally, the theoretical analysis based on the DFT method showed that a weak chemisorption of Hg⁰ on Fe₃O₄ sorbents changed to a strong chemisorption when silver was loaded. The results of theoretical analysis conformed to the experiments results well.     

Removal of SO2 from flue gas using a new semidry flue gas desulfurization process with a powder-particle spouted bed

A spouted bed of binary particle mixture was applied to a low temperature desulfurization process in order to develop a new type of semidry flue gas desulfurization (FGD) technology. We investigated the effects of operating parameters, such as type of SO₂ sorbent, diameter of SO₂ sorbent particles, apparent residence time of gas in the bed, approach to saturation temperature and Ca/S molar ratio, on SO₂ removal in a bench-scale powder-particle spouted bed. We also investigated the utilization rate of SO₂ sorbent and ways to enhance the efficiency of SO₂ removal and SO₂ sorbent utilization. The experimental results showed that SO₂ removal is significantly affected by the approach to saturation temperature and Ca/S molar ratio, and that a high SO₂ removal efficiency and effective sorbent utilization can be obtained under appropriate operating conditions. Thus, this new simple process of flue gas desulfurization is highly efficient and has little impact on the environment.     

Sulfation diffusion model for SO2 capture on the T-T sorbent at moderate temperatures

A sulfation model was developed for dry flue gas desulfurization (FGD) at moderate temperatures to describe the reaction characteristics of the T-T sorbent clusters and the fine CaO particles that fall off the sorbent grains in a circulating fluidized bed (CFB) reactor. The cluster model describes the calcium conversion and reaction rate for various size sorbent clusters. The sulfation reaction is first order with respect to the SO₂ concentration above 973 K. The calcium conversion and reaction rate for the CaO particles were obtained by extrapolation. In the model for CaO particle, the reaction rate is linearly related to the calcium conversion and the SO₂ concentration in the rapid reaction stage and linearly related only with the calcium conversion after the product layer forms. The sulfation model accurately describes the sulfation of the T-T sorbent flowing through a CFB reactor. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

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.