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.     

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.     

Characteristics of the removal of mercury vapor in coal derived fuel gas over iron oxide sorbents

The characteristics of a novel method for Hg removal using H₂S and sorbents containing iron oxide were studied. Previously, we have suggested that this method is based on the reaction of Hg and H₂S over the sorbents to form HgS. However, the reaction mechanism is not well understood. In this work, the characteristics of the Hg removal were studied to clarify the reaction mechanism. In laboratory made sorbents containing iron oxide were used as the sorbent to remove mercury vapor from simulated coal derived fuel gases having a composition of Hg (4.8 ppb), H₂S (400 ppm), CO (30%), H₂ (20%), H₂O (8%), and N₂ (balance gas). The following results were obtained: (1) The presence of H₂S was indispensable for the removal of Hg from coal derived fuel gas; (2) Hg was removed effectively by the sorbents containing iron oxide in the temperature range of 60-100 °C; (3) The presence of H₂O suppressed the Hg removal activity; (4) The presence of oxygen may play very important role in the Hg removal and; (5) Formation of elemental sulfur was observed upon heating of the used sample.     

Kinetic behaviour of iron oxide sorbent in hot gas desulfurization

Although a number of reports on sorbents containing ZnO for H₂S removal from coal-derived gases can be found in the literature, it is shown in our study that a special sorbent containing Fe₂O₃·FeO (SFO) with minor promoters (Al₂O₃, K₂O, and CaO) as the main active species is more attractive for both sulfidation and regeneration stages, also under economic considerations. This paper presents the kinetic behaviour of SFO in a hot gas desulfurization process using a thermogravimetric analysis under isothermal condition in the operating range between 500 and 800 °C. The gas stream was N₂ with a 2% wt of H₂S. Experiences carried out on sorbent sulfidation with SFO (particle sizes in the range of 0.042-0.12 mm) indicate that the sorbent sulfidation capacity sharply increases with temperature in the range of 500-600 °C. It is also shown that the sample weight reaches its maximum absorption capacity, near saturation, at 600 °C so that it makes no sense to increase the sulfidation temperature from this point. To make a comparison between SFO and a zinc titanate based sorbent, a set of sulfidation tests was carried out at 600 °C during 7200 s using the same sieve range for both sorbents between 42 and 90 μm. Results show that the sulfidation capacity of SFO is 1.9 times higher than that of zinc titanate.     

Desulfurization matching with coal poly-generation system based on dual gas resources

The coal poly-generation system for the production of alcohol and ether fuels as well as power is one of advanced coal utilization techniques. The team leaded by Professor Xie Kechang is carrying out the research on the poly-generation system to produce the syngas from the combination of gasified and pyrolyzed coal gas (dual gas resources) for the alcohol ether synthesis. Gas desulfurization is one of the key technologies for this system. The desulfurization matching with dual gas resources based poly-generation system for the production of alcohol and ether fuels as well as power is presented according to gas components, sulfur content, sulfur species and desulfurization accuracy in this technology. This matching desulfurization is classified into hot gas desulfurization, normal gas desulfurization, warm gas desulfurization and organic sulfur catalytic conversion. The preparation of H₂S removal sorbents, organic sulfur hydrolysis catalyst and the evaluation of their activities involved in the system were investigated. The H₂S removal efficiencies of the crude and fine desulfurization sorbents prepared for hot gas desulfurization are 90% and 99% at 500 °C in simulating coal gas, and their sulfur capacities are 21.85 wt.% and 24.91 wt.%, respectively. The organic sulfur catalyst shows the high hydrolysis activity, and the hydrolysis conversion of COS is more than that of CS₂ on the same catalyst. The research will provide necessary information for the matching desulfurization technology in the demonstration project on dual gas resources coal poly-generation system.     

Preparation and selection of Fe-Cu sorbent for COS removal in syngas

A series of iron-based sorbents prepared with iron trioxide hydrate, cupric oxide by a novel method was studied in a fixed-bed reactor for COS removal from syngas at moderate temperature. In addition, the sorbents mixed with various additives in different ratios were tested. The effects of additive type and ratio on the breakthrough capacity and desulfurization performance, as well as the influence of operating conditions on sulfidation behavior of the sorbent, were investigated. The simulate gas contained 1% COS, 5% CO₂, 20%–30% CO and 60%–70% H₂. The outlet gases from the fixed-bed reactor were automatically analyzed by on-line mass spectrometry, and the COS concentration before breakthrough can be kept steady at 1 ppmv. The result shows that the breakthrough sulfur capacity of the sorbent is as high as 25 g-S/100 g. At 700 K and space velocity of 1000 h⁻¹, the efficiency of sulfur removal and breakthrough sulfur capacity of the sorbent increase with the increase of copper oxide with an optimum value. The result shows that the species and content of additives also affect desulfurization performance of the sorbent.     

Correlation of H<Subscript>2</Subscript>S and COS in the hot coal gas stream and its importance for high temperature desulfurization

Thermodynamic analysis of the correlation of H₂S and COS has been carried out at the temperature range of 400–650 °C at which high temperature desulfurization of coal gas is usually performed. The correlation of the two sulfur species is mainly through the reaction H₂S+CO→COS+H₂. Simulated coal gas with the following composition CO 32.69%, H₂ 39.58%, CO₂ 18.27%, N₂ 8.92% and H₂S 0.47% was used in this study, and the equilibrium concentrations of the two species at different temperatures were calculated. The results of Fe-based sorbents during sulfidation were compared with calculations. It is concluded that the above reaction may reach equilibrium concentration in the presence of the Fe-based sorbents, which means the Fe-based sorbents may effectively catalyze the reaction between H₂S and CO. Because of the correlation of the two sulfur species, both can be effectively removed at high temperatures simultaneously, offering high temperature desulfurization some advantages over low temperature desulfurization processes.     

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.     

粉煤灰作为高温煤气脱硫剂载体的可行性研究

基于XRD、SEM、激光粒度分析仪和成分分析等不同测试方法,比较了不同大小的粉煤灰颗粒的物化性质和微观形貌特征。以不同颗粒大小的粉煤灰为载体,制备了一系列铁铈氧化物高温煤气脱硫剂,以此研究粉煤灰作为高温煤气脱硫剂载体的可行性。在600℃的硫化实验表明,粉煤灰作为铁基高温煤气脱硫剂的载体,不仅能有效提高脱硫剂的脱硫精度和其硫化再生后的机械强度,且粉煤灰自身也有一定的脱硫作用。     

Development of iron-based sorbents for Hg0 removal from coal derived fuel gas: Effect of hydrogen chloride

Laboratory studies were conducted to develop an elemental mercury (Hg⁰) removal process based on the reaction of H₂S and Hg⁰ using iron-based sorbents for coal derived fuel gas. It is well known that hydrogen chloride (HCl) is present in fuel gases derived from some types of coal, but the effect of HCl on the Hg⁰ removal performance of iron-based sorbents in coal derived fuel gas is not yet well understood. In this study, the effects of HCl on the removal of Hg⁰ from coal derived fuel gases over iron-based sorbents such as iron oxide (Fe₂O₃), supported iron oxides on TiO₂, iron oxide–Ca(OH)₂, and iron sulfides were investigated. The Hg⁰ removal experiments were carried out in a laboratory-scale fixed-bed reactor at 80 °C using simulated fuel gas. In the case of iron oxide (Fe₂O₃), the presence of HCl suppressed the Hg⁰ removal rate. In the case of Fe₂O₃ (2 or 5 wt%)/TiO₂, the presence of HCl did not suppress the Hg⁰ removal rate and the activity was stable. The Hg⁰ removal performance of reagent FeS₂ was higher than that of the iron oxide, and not affected by the presence of HCl. The Hg⁰ removal rate of iron oxide–Ca(OH)₂ was not effected by the presence of HCl, because HCl was captured by Ca(OH)₂. The reagent FeS₂ showed higher Hg⁰ removal activity than that of FeS₂ ore. However, the Hg⁰ removal performance of ground and kneaded FeS₂ ore was comparable to that of reagent FeS₂ probably due to the increase in porosity of the FeS₂ ore by grinding and kneading.     

A review on research and development of iron-based sorbents for removal of hydrogen sulfide from hot coal gases

In poly-generation and integrated gasification-combined cycle (IGCC) systems for clean energy conversion, it is essential to remove impurities such as sulfur species from hot coal gases prior to entering the subsequent units. This paper provides a comprehensive review on previous studies on high temperature removal of hydrogen sulfide from high temperature coal gases using iron-based sorbents. A two-step desulphurization process for hot coal gas cleanup is highlighted, which is integrated with direct production of elemental sulfur during regeneration of iron-based sorbents in the primary desulphurization step. Different kinetic modeling approaches for sulfidation and regeneration were compared. Limited research on activated carbon supported sorbents was also briefly summarized.     

Effects of reduction of metal oxide sorbents on reactivity and physical properties during hot gas desulphurization in IGCC

In this study, the changes of physical properties and reactivity of the metal oxide sorbents were investigated under the reducing conditions of coal gas. Metal oxide sorbents are converted into metal sulphides as a result of reaction with H₂S in synthesis gas. This could cause the reduced reactivity of sorbents if the metal oxides were converted into metallic elements due to the reduction by either hydrogen or carbon monoxide. In this experiment, the changes of physical properties and reactivity of the metal oxides were investigated over the temperature range 480-700 °C. It is confirmed that the reactivity of sulphidation and the reduction of metal oxide increased with increasing temperature. Even though the sulphur capacity of the sorbents in the early stage was high, it reduced rapidly due to the progressive reduction of metal oxides as the sulphidation/regeneration process was repeated. The reduction of metal oxide and the extent of reduction were verified by measuring the amount of oxygen consumed and the amount of SO₂ produced during the regeneration of sulphidated sorbents with the aids of a gas analyser. It was concluded that the reactivity of the metal oxide sorbents was influenced by reduction with coal gas at high temperature.     

Reactivity of Metal Oxide Sorbents for Removal of Sulfur Compounds from Coal Gases at High Temperature and Pressure

Abstract

Hot-gas desulfurization for the integrated gasification combined cycle (IGCC) process has been investigated to effectively remove hydrogen sulfide with various metal oxide sorbents at high temperatures and pressures. Metal oxide sorbents such as zinc titanate oxide, zinc ferrite oxide, copper oxide, manganese oxide, and calcium oxide were found to be promising sorbents in comparison with other removal methods such as membrane separation and reactive membrane separation. The removal reaction of H₂S from coal gas mixtures with zinc titanate oxide sorbents was conducted in a batch reactor. The main objectives of this research are to formulate promising metal oxide sorbents for removal of hydrogen sulfide from coal gas mixtures, to compare reactivity of a formulated sorbent with a sorbent supplied by the Research Triangle Institute at high temperatures and pressures, and to determine effects of concentrations of moisture contained in coal gas mixtures, and to determine effects of concentrations of moisture contained in coal gas mixtures on equilibrium absorption of H₂S into metal oxide sorbents. Promising durable metal oxide sorbents with high-sulfur-absorbing capacity were formulated by mixing active metal oxide powders with inert metal oxide powders and calcining these powder mixtures.     

Dry gas cleaning process by adsorption of H2S into activated cokes in gasification of carbon resources

Gasification of carbon resources including biomass and coal is one of promising energy production technologies. The R&D on effective and convenient gas cleaning processes for removal of contaminants as well as high efficient reliable gasifiers is essential for industrial application in broad fields. In this study, a dry process of synthesis gas cleaning by adsorption of H₂S into activated cokes was proposed as a candidate of desulfurization technologies in gasification. The H₂S adsorption performance of activated coke produced from coal, which are used industrially for de-SO_x and de-NO_x, was evaluated by the thermogravimetric analyses and the adsorption examination in a fixed bed under the atmospheric and high pressures. Activated coke was not only the most active at about 423 K for the H₂S adsorption rate but also regenerative over 573 K by H₂S desorption with a sufficient rate under an inert gas flow of nitrogen. The H₂S adsorption performance of the activated coke was not inhibited by the co-existence of CO₂ or COS but enhanced rather by the co-existence. The adsorbent was promisingly active for both H₂S and COS adsorption as well. These behaviors suggest that the activated coke are available for simultaneous desulfurization of H₂S and COS. The H₂S breakthrough examination in the fixed bed revealed that it was possible to remove H₂S to lower level than 1 ppm for a long time depending on the residence time of gas flow in the bed. When the adsorption operation was carried out under high pressures up to 0.6 MPa, the regeneration of activated coke by H₂S desorption took place under the pressure reduced to the atmosphere. As the results, it was implied that the present activated coke could be applicable to the desulfurization process in coal gasification.     

Development of a pilot-scale acid gas removal system for coal syngas

The Korean pilot-scale gasification facility consists of a coal gasifier, hot gas filtering system, and acid gas removal (AGR) system. The syngas stream from the coal gasification at the rate of 100–120 Nm³/hr included pollutants such as fly ash, H₂S, COS, etc. The acid gas, such as H₂S and COS, is removed in the AGR system before generating electricity by gas engine and producing chemicals like Di-methyl Ether (DME) in the catalytic reactor. A hydrolysis system was installed to hydrolyze COS into H₂S. The designed operation temperature and pressure of the COS hydrolysis system are 150 °C and 8 kg/cm². After the hydrolysis system, COS was reduced below 1 ppm at the normal operating condition. The normal designed operation temperature and pressure of the AGR system are below 40 °C and 8 kg/cm². Fe-chelate was used as an absorbent. H₂S was removed below 0.5 ppm in the AGR system when the maximum concentration of H₂S was 900 ppm. A small scale COS adsorber was also installed and tested to remove COS below 0.5 ppm. COS was removed below 0.1 ppm after the COS adsorbents such as the activated carbon and ion exchange resin. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Research progress of hot gas filtration,desulphurization and HCl removal in coal-derived fuel gas: A review

The present review paper highlighted on the recent progress of hot gas filtration, desulphurization and HCl removal in coal-derived fuel gas for combined cycle power generation (IGCC) or molten carbonate fuel cells (MCFC) technologies. As a critical process in the gasification system, hot gas filtration in the particulate control device (PCD) was introduced with enhanced understanding of equipment and operation, filter element and failsafe material properties, and gasification ash characteristics. The issues associated with the commercialization of hot gas filtration were also addressed, and some novel systems and methods were also discussed. The hot gas desulphurization in coal-derived fuel gas has concentrated on developing regenerable sorbents including the single and composite oxides of Zn, Fe, Cu, Mn and other species, and the reduction of metal oxides in the highly reducing atmosphere followed by vaporization of elements can be a problem for reactivity and regeneration. With regard to the removal of HCl, the studies have indicated sorbents prepared by pelletizing the powders of naturally available alkali metal and alkali earth metal substances can rapidly react with HCl vapor and reduce the HCl vapor concentration to less than 1 ppmv, and some sorbents lab-made have very high chlorine capacity. The sorbents based hot gas cleaning also has some challenges. Kinetics studies showed that unreacted shrinking core (USC) can be applied to the modeling of H₂S and HCl removal by sorbents at high temperature, and the surface chemical reaction and reactant diffusion by product layers between solid sorbents and gases were very important mechanisms. The paper also proposed and discussed a rational concept for the simultaneous removal of multiple contaminants including ash, H₂S and HCl, which will offer a possible cost reduction by two or more processes in a single vessel for hot gas cleaning.     

Simultaneous experiments of sulfidation and regeneration in two pressurized fluidized-bed reactors for hot gas desulfurization of IGCC

Hot Gas Desulfurizarion for IGCC is a new method to efficiently remove H2S in fuel gas with regenerable sorbents at high temperature and high-pressure conditions. The Korea Institute of Energy Research did operation of sulfidation in a desulfurizer and regeneration in a regenerator simultaneously at high pressure and high temperature conditions. The H₂S concentration at exit was maintained continuously below 50ppmv at 11,000 ppmv of inlet H₂S concentration. The sorbent had little effect on the reducing power in the inlet gas in the range from 11% to 33% of H₂. As inlet H₂S concentration was increased, H₂S concentration in the product gas was also increased linearly. The sorbent was maintained at low sulfur level by the continuous regeneration and the continuous solid circulation at the rate of 1.58× 10⁻³ kg/s with little mean particle size change.     

Sulfur transfers from pyrolysis and gasification of direct liquefaction residue of Shenhua coal

The sulfur transformation during pyrolysis and gasification of Shenhua direct liquefaction residue was studied and the release of H₂S and COS during the process was examined. For comparison, the sulfur transfer of Shenhua coal during pyrolysis and that of pyrolyzed char during gasification were also studied. The residue was pyrolyzed at 10 °C /min to 950 °C. During pyrolysis about 33.6% of sulfur was removed from the residue, among which 32.1% was formed H₂S in gas and 1.5% was transferred into tar, 66.4% of the sulfur was remained in residue char. Compared with coal, the residue has generated more H₂S due to presence of Fe_1−xS which was enriched in residue during liquefaction process. There is a few COS produced at 400-500 °C during pyrolysis of coal, but it was not detected form pyrolysis of the residue. During CO₂ gasification, compared with pyrolysis and steam gasification, there are more COS and less H₂S formation, because CO could react with sulfide to form COS. During steam gasification only H₂S was produced and no COS detected, because H₂ has stronger reducibility to form H₂S than CO. After steam gasification no sulfur was detected in the gasification residue. The XRD patterns show after steam gasification, only Fe₃O₄ is remained in the gasification residue. This indicates that the catalyst added during the liquefaction of coal completely reacted with steam, resulting in the formation of H₂ and Fe₃O₄.     

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.     

Preparation of mesoporous MCM-41 supported zinc sorbents by microwave in-situ oxidation for H2S removal in coal gas

Zn-based MCM-41 supporter sorbents were prepared using the microwave in-situ oxidation method. Other sorbents were heated using a conventional heating method to contrast the performance for H₂S removal. The sorbents were tested at 500°C in fixed reactor and dried simulated Texaco coal gas was employed for the sulphurized atmosphere. The results show that sorbents prepared by microwave oxidation had a better toleration for the adsorption of H₂S. A 13.2% improvement occurred in the sulphur capacity of the sorbents heated by the microwave method. XRD, SEM with EDS-element mapping, TEM, N₂ adsorption, and XPS were used to characterize the properties of the sorbents. Due to the selective heating of the microwave and the superiority of the in-situ oxidation method, the sorbents heated by microwave exhibited more appropriate structures for sulphurization. Meanwhile, the even heating environment supplied by the microwave resulted in a more uniform distribution of the active component. The microwave also had an effect on the chemical bond and reduced the binding energy of the active component, which enhanced the reactivity between the H₂S and the sorbents. The preferable features generated by microwave in-situ oxidation accelerate the replacement of S to O, and therefore the Zn-based MCM-41 sorbents prepared by the microwave method have an increased capability for H₂S removal in high temperature coal gas.