Recent progress in Japan on hot gas cleanup of hydrogen chloride, hydrogen sulfide and ammonia in coal-derived fuel gas

The present review paper highlights on the recent progress in Japan on the hot gas cleanup of HCl, H₂S and NH₃ in raw fuel gas for coal-based, combined cycle power generation technologies. It has been shown that NaAlO₂, prepared by mixing Na₂CO₃ solution with Al₂O₃ sol, can reduce HCl in an air-blown gasification gas from the initial 200 ppm to < 1 ppm at 400 °C, and it is tolerable for 200 ppm H₂S. With regard to the removal of H₂S, studies on the stability and durability of ZnFe₂O₄ sorbent in a simulated fuel gas have indicated the presence of an optimal operation temperature from the viewpoint of the suppression of both vaporization of metallic Zn and carbon formation from CO. High-performance TiO₂-supported ZnFe₂O₄, which can decrease 1000 ppm H₂S to < 1 ppm at 450 °C and 1 MPa, has been developed by the homogeneous precipitation method using a mixture of SiO₂ sol and an aqueous solution of Zn and Fe nitrates, followed by mixing with TiO₂. Although this sorbent is regenerable and durable, the sorption ability should be improved in a syngas-rich fuel gas from an O₂-blown gasifier. A novel method to prepare carbon-supported ZnFe₂O₄ and CaFe₂O₄ by impregnating the corresponding nitrate solution with brown coal has been proposed, and the large desulfurization capacity of almost 100% has been achieved in the removal of 4000 ppm H₂S around 450 °C. The present authors have demonstrated that an Australian limonite rich in α-FeOOH is practically feasible as the catalyst material for the decomposition of 2000 ppm NH₃ in a syngas-rich gas of 25 vol.% H₂/50 vol.% CO at 750 °C, because small amounts of H₂O and CO₂ added to the gas can work efficiently for inhibiting carbon deposition from the CO.     

Study of the ammonia decomposition over iron catalysts

The decomposition of ammonia is a reaction associated with the process of the nitriding of metals. The kinetics of the ammonia decomposition on iron catalysts has been studied using a differential reactor with internal mixing. The balance between the inlet and outlet ammonia quantity has been used to determine the degree of conversion. The rate of ammonia decomposition could be described by the following expression: r = k₀ exp (Ea/RT)pNH₃. The activation energy of the ammonia decomposition process has been found for samples with potassium as E _a= 96 kJ/mol, for samples without potassium as E _a= 87 kJ/mol. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbon-supported cobalt–iron catalysts for ammonia synthesis

The cobalt, iron and Co–Fe catalysts deposited on carbon were prepared, characterised (XRD, H₂ TPD) and studied in ammonia synthesis at 90 bar (H₂:N₂ = 3:1). Partly graphitised carbon material obtained via high temperature treatment (1900 °C) of commercial activated carbon was used as a support for the active metals (10 wt.%) and barium or potassium were used as promoters. XRD studies of unpromoted materials have shown that cobalt (5–20% in Co + Fe) dissolves in the iron phase (alloy formation); the average sizes of crystallites (20–30 nm) are roughly independent of the metal kind (Co, Fe, Co–Fe). The effect of Ba and that of K on the catalyst performance proved to be strongly dependent on the choice of an active phase (Co or Fe or Co–Fe). In the case of Co/C, the promotional effect of barium was extremely large. Furthermore, the Ba–Co/C system was found to be less inhibited by the ammonia product than Ba–Fe/C. At low temperature (400 °C) and at high conversion (8% NH₃ in the gas), the surface-based reaction rate (TOF) for Ba–Co/C is about six times higher than that for Ba–Fe/C.     

Iron catalyst for ammonia synthesis doped with lithium oxide

Iron catalysts doped with Al₂O₃, CaO were obtained by melting iron oxide with 2.2 wt.% of Al₂O₃, 2.1 wt.% of CaO. The reduced catalyst was impregnated with lithium hydroxide water solution. Activity measurements were carried out in the laboratory installation in the temperature range 623–773 K under the pressure of 10 MPa. The activity of catalyst containing 0.79 wt.% of Li₂O and reduced at 773 K was similar to the activity of an industrial iron catalyst doped with potassium oxide. After reduction at 923 K the catalyst containing 0.48 wt.% of Li₂O was about 15% more active than the industrial catalyst. Increasing Li₂O concentration results in the decrease of the surface area of a catalyst reduced at 923 K. The most active catalysts doped with lithium oxide were more active than the industrial catalyst when their activity was calculated and scaled down to surface area units.     

Effects of pre-treatment in air microwave plasma on the structure of CNTs and the activity of Ru/CNTs catalysts for ammonia decomposition

Carbon nanotubes (CNTs) were treated in air microwave plasma at three different microwave powers 80, 100 and 150 W. The effects of the treatment on the structure of CNTs and the dispersion of Ru particles supported on CNTs were evaluated using XRD, N₂ physisorption, FTIR, TPD, XPS, TEM and CO chemisorptions. The results show that the bulk structure and the texture of the CNTs were unchanged, but phenolic hydroxyls and carboxyls appeared on the surface. The total concentration of these groups increases with the microwave power to 100 W, then decreases. The phenolic hydroxyls and carboxyls on the CNTs interact with Ru ions during the preparation of Ru/CNTs resulting in highly dispersed Ru particles. The activity of Ru/CNTs for ammonia decomposition depends on the dispersion of Ru particles, but is negatively affected by any remaining oxygen-containing groups on the surface.     

A study on the reactivity of Ce-based Claus catalysts and the mechanism of its catalysis for removal of H2S contained in coal gas

In this study, Claus reaction was applied for the selective removal of H₂S contained in the gasified coal gas, and the characteristics of Claus reaction over the Ce-based catalysts were investigated to propose the reaction mechanism. The Ce-based catalysts showed a high activity on Claus reaction. Specially, Ce_0.8Zr_0.2O₂ catalyst had a higher activity than CeO. On the basis of our experimental results, it was proposed that the selective oxidation of H₂S was carried out by the lattice oxygen in the Ce-based catalysts and that the reduction of SO₂ was performed by the lattice oxygen vacancy in the reduced catalyst. Since the mobility of the lattice oxygen in Ce_0.8Zr_0.2O₂ composite catalyst was better than the one in CeO₂, Ce_0.8Zr_0.2O₂ provided more lattice oxygen for the selective oxidation of H₂S. It was presumed that the reaction mechanism to convert H₂S and SO₂ into elemental sulphur over our prepared catalysts was different from the mechanism over the solid-acid catalysts. It is believed that Claus reaction over the Ce-based catalysts was carried out by the redox mechanism. Since the moisture was contained in the major components, CO and H, of the gasified fuel gas, the effects of CO and H₂O on the catalytic reaction were investigated over a Ce-based catalyst. The conversion of H₂S and SO₂ was decreased in Claus reaction over the Ce-based catalysts as the concentration of either H₂O or CO in the gasified coal gas was increased. Under the circumstances of the coexistence of both moisture and CO, however, the conversion was increased as the concentration of CO was increased. The reactivity of Claus reaction was varied in terms of the concentration ratio of CO to H₂O. The maximum conversion of H₂S and SO₂ was achieved in the condition of that the concentration of CO contained in the reacting gas was higher than the one of H₂O. The conversions of H₂S and SO₂ did not match to the stoichiometric ratios of Claus reaction. The higher conversion of H₂S was obtained in the higher concentration of H₂O, while the higher conversion of SO₂ was achieved in the higher concentration of CO. It was another evidence to indicate that the Claus reaction over the Ce-based catalysts was carried out by the redox mechanism.     

LR spectroscopic study of chemisorbed dinitrogen species on ammonia synthesis iron catalysts

The results of the LRS study of N-containing chemisorbed species on the promoted iron catalyst for ammonia synthesis have further substantiated the existence of two species of N₂(a) as the dominant N-containing chemisorbed species under the functioning catalyst conditions. A model of active site, as 3-Fe cluster on (111) or (211) surface of -Fe, and two modes of multinuclear coordination activation for the observed two species of N₂(a) were proposed. It was further illustrated from reaction energetics that the mechanism of the dominant reaction pathway for ammonia synthesis/decomposition may be associative, rather than dissociative.The work was supported by a grant from the National Natural Science Foundation of China; parts of the work were presented at 24th ICCC (Athens, 1986).     

Catalytic decomposition of ammonia over fly ash supported Ru catalysts

Fly ash (FA), an industrial waste material, has been treated by physical and chemical methods. These materials were then employed as supports for preparation of Ru-based catalysts for H₂ generation from ammonia decomposition. The physicochemical properties of the supports and Ru-based catalysts were characterised using several techniques. The results revealed that the surface area of FA could be enhanced and thus improved the dispersion of Ru particles, resulting in higher catalytic activity. Ru/FA-800 exhibits the highest conversion due to higher surface loading of Ru, stronger NH₃ adsorption and least acid sites.     

影响Ru/MO_x氨合成催化剂活性关键因素

氨合成催化剂是多相催化领域中许多基础研究的起点,低温高活性的钌系催化剂为继熔铁催化剂之后的第二代氨合成催化剂。从影响氧化物负载的Ru基催化剂性能的关键因素出发,对氧化物载体的改性、助剂的作用并将其深入到反应条件下的化学状态等方面进行了分析和探讨,为新型高活性Ru基氨合成催化剂及其他多相催化剂的设计研发提供借鉴。     

圆柱状成形Ru/AC氨合成催化剂制备及性能

将水洗处理的椰壳活性炭研磨至小于0.08 mm,加入粘合剂挤压成长度3～10 mm、3～5 mm的圆柱形,颗粒经120 ℃烘干、1 900 ℃高温处理和400 ℃经氧、氮等混合气处理后,作为Ru/AC催化剂载体。其侧压强度从无定形的40 N·cm^－1提高到成形后的91 N·cm^－1,磨耗率从无定形的5%左右降低到成形后的0.05%。以成形椰壳炭为载体的Ru/AC催化剂的活性与无定形活性炭为载体时相当,具有在低温、低压、低氢氮比和原料气高氨含量下高活性的特点,且具有很好的耐热性。与无定形椰壳炭相比,以成形椰壳炭为载体制备的Ru/AC氨合成催化剂更适合工业氨合成装置使用。     

Methanol decomposition to synthesis gas over supported Pd catalysts prepared from synthetic anionic clays

Supported Pd or Rh catalysts were prepared by the solid-phase crystallization method starting from hydrotalcite anionic clay minerals based on [Mg₆Al₂(OH)₁₆CO ₂ ²⁻ ]·4H₂O as the precursors. The precursors were prepared by a coprecipitation method from the raw materials containing Pd²⁺ and various trivalent metal ions which can replace each site of Mg²⁺ and Al³⁺ in the hydrotalcite. Rh³⁺ was also used for preparing the catalyst as comparison. The precursors were then thermally decomposed and reduced to form supported Pd or Rh catalysts and used for the methanol decomposition to synthesis gas. Among the precursors tested, use of Mg–Cr hydrotalcite containing Pd²⁺ resulted in the formation of efficient Pd supported catalysts for the production of synthesis gas by selective decomposition of methanol at low temperature. Although Pd²⁺ cannot well replace the Mg²⁺ site in the hydrotalcite, the Pd supported catalyst (Pd/Mg–Cr) prepared by the solid-phase crystallization method formed highly dispersed Pd metal particles and showed much higher activity than that prepared by the conventional impregnation method. When the precursor was prepared under mild conditions, more fine particles of Pd metal were formed over the catalyst, resulting in high activity. It is likely that the high activity may be due to the highly dispersed and stable Pd metal particles assisted by the role of Cr as the co-catalyst. This revised version was published online in November 2006 with corrections to the Cover Date.     

钡离子对镍基钙钛矿分解氨的影响

镍基钙钛矿分解氨制取氢气虽有稳定性好、价格低廉的优点,但也有完全分解温度偏高的缺点。为降低完全分解氨制取氢气的温度,实验采用柠檬酸络合法,通过改变A位掺杂离子与掺杂量对LaNiO₃改性,并改变载体及负载量,制备一系列的催化剂。采用XRD、SEM、TEM表征技术进行表征,考察了Ba 含量对催化剂结构与性能的影响。实验表明在催化剂装填量为1mL、空速为10000h^-1、原料气为纯氨的条件下,氨分解的最佳电子助剂离子为Ba、最优催化剂为w(NiO)=20%的La_0.9Ba_0.1NiO₃/MCM-41,氨气完全分解温度由650℃降为575℃。随着Ba掺杂量的增加,催化剂活性先增加,在x=0.1达到最大值,之后减小,掺杂Ba过多催化剂的结构由钙钛矿变为非钙钛矿,非钙钛矿结构的催化剂活性不如钙钛矿。     

Development and reactivity tests of Ce-Zr-based Claus catalysts for coal gas cleanup

Claus reaction (2H₂S + SO₂ ↔ 3/nS_n + 2H₂O) was used to clean the gasified coal gas and the reactivity of several metal oxide-based catalysts on Claus reaction was investigated at various operating conditions. In order to convert H₂S contained in the gasified coal gas to elemental sulfur during Claus reaction, the catalysts having the high activity under the highly reducing condition with the moisture should be developed. CeO₂, ZrO₂, and Ce_1−xZr_xO₂ catalysts were prepared for Claus reaction and their reactivity changes due to the existence of the reducing gases and H₂O in the fuel gas was investigated in this study. The Ce-based catalysts shows that their activity was deteriorated by the reduction of the catalyst due to the reducing gases at higher than 220 °C. Meanwhile, the effect of the reducing gases on the catalytic activity was not considerable at low temperature. The activities of all three catalysts were degraded on the condition that the moisture existed in the test gas. Specifically, the Ce-based catalysts were remarkably deactivated by their sulfation. The Ce-Zr-based catalyst had a high catalytic activity when the reducing gases and the moisture co-existed in the simulated fuel gas. The deactivation of the Ce-Zr-based catalyst was not observed in this study. The lattice oxygen of the Ce-based catalyst was used for the oxidation of H₂S and the lattice oxygen vacancy on the catalyst was contributed to the reduction of SO₂. ZrO₂ added to the Ce-Zr-based catalyst improved the redox properties of the catalyst in Claus reaction by increasing the mobility of the lattice oxygen of CeO₂.     

On the influence of oxygen on iron catalysts during ammonia synthesis and catalyst characterization

Once again attention is brought to the sensitivity of iron catalysts to poisoning by sub ppm levels of oxygen and a procedure is suggested to evaluate the purity of the gas supply to a steady state flow reactor setup. It is demonstrated that extremely high gas purities are essential, in particular for studies of alkali promoted samples.     

Sulphur poisoning of nickel-based hot gas cleaning catalysts in synthetic gasification gas II. Chemisorption of hydrogen sulphide

The effect of different components of gasification gas on sulphur poisoning of nickel catalysts were studied. In addition, the sulphur distribution and content of nickel catalyst beds were analysed to account the poisoning effect of sulphur on the activity of catalysts to decompose tar, ammonia and methane. The desorption behaviour of chemisorbed sulphur from the bed materials was monitored by temperature programmed hydrogenation (TPH). It was established that bulk nickel sulphide was active in decomposing ammonia in high-temperature gasification gas-cleaning conditions. The decomposing activity of methane was not affected by bulk nickel sulphide formation, but that of toluene was decreased. The activity of the catalyst regained rapidly when H₂S was removed from the gas. However, the conversion of ammonia was not regained at as high a level as before sulphur addition, most probably due to irreversible sulphur adsorption on the catalyst. The temperature increase could also be used to regenerate the catalyst performance especially in respect to methane and toluene. Sulphur adsorbed on nickel catalysts in different chemical states depends on the process conditions applied. At >900°C the sulphur adsorbed on the catalyst formed an irreversible monolayer on the catalyst surfaces, while at <900°C the adsorbed sulphur, probably composed of polysulphides (multilayer sulphur), was desorbed from the catalyst in sulphur-free hydrogen containing atmosphere. However, a monolayer of sulphur still remained on the catalyst after desorption. The enhanced effect of high total pressure on sulphur-poisoning of nickel catalysts could be accounted for the increased amount of sulphur, probably as a mode of polysulphides, adsorbed on the catalyst.     

燃煤锅炉烟气氨法脱硫技术概况

介绍了氨法脱硫的原理、主要的脱硫工艺以及实际应用情况。实践表明,大型锅炉采用氨法脱硫效率高,投资省,用电量少,可生产硫酸或硫酸铵。烟气即使不降温,直接进入脱硫塔,也可达到较高的脱硫效率。通过对多种工艺的比较,为燃煤烟气脱硫技术的选择提供参考。     

Sulphur poisoning of nickel-based hot gas cleaning catalysts in synthetic gasification gas: I. Effect of different process parameters

The effect of different process parameters on sulphur poisoning of nickel catalysts in tar (toluene), ammonia and methane decomposition was studied. Tests were carried out in a fixed-bed tube reactor at 800–1000°C at 5 and under 20 bar total pressure using a synthetic gasification gas mixture. In the same conditions, sulphur affected less the toluene and methane decomposing activity than the ammonia decomposing activity. Ammonia conversion was affected by the catalyst type but not by the nickel content of catalyst. When temperature was increased the effect of sulphur poisoning was decreased. At 20 bar pressure the poisoning effect of sulphur was stronger than at 5 bar pressure. To prevent sulphur poisoning of nickel catalysts in the tar and ammonia decomposition process at high pressure (20–30 bar), the catalyst process should operate at > 900°C. In addition, by decreasing the space velocity of the process the sulphur poisoning effect could be compensated in the test conditions.     

Low temperature ruthenium catalyst for ammonia synthesis supported on BaCeO3 nanocrystals

The nanocrystalline barium cerate (BaCeO₃) was synthesized and examined as a support promoting ammonia synthesis catalyzed by particulate Ru. It was quite worthy noting that Ru/BaCeO₃ catalyst exhibited superior activity to the conventional ruthenium catalysts supported on γ–Al₂O₃, MgO and CeO₂ at low reaction temperature. The characterization including XRD, HRTEM, XPS and TPR results demonstrated that electronic interaction between Ce (IV) and Ru metals combined with strong metal-support interaction was responsible for such high catalytic activity at low reaction temperature.     

ZSM-5 supported iron catalysts for Fischer-Tropsch production of light olefin

Fischer-Tropsch Synthesis (FTS) for olefin production from syngas was studied on Fe-Cu-K catalysts supported on ZSM-5 with three different Si/Al ratios. The catalysts were prepared by slurry-impregnation method of metallic components, and were characterized by BET surface area, XRD, hydrogen TPR and ammonia TPD. Fe-Cu-K/ZSM-5 catalyst with a low Si/Al ratio (25) is found to be superior to the other catalysts in terms of better C₂-C₄ selectivity in the FTS products and higher olefin/(olefin + paraffin) ratio in C₂-C₄ because of the facile formation of iron carbide during FTS reaction and also due to a larger number of weak acidic sites that are present in these catalysts.