Precious metal catalysts in the clean-up of biomass gasification gas Part 1: Monometallic catalysts and their impact on gasification gas composition

The performance of Rh, Ru, Pt, and Pd on modified commercial zirconia support (m-ZrO₂) was compared to a benchmark Ni/m-ZrO₂ catalyst in the presence of H₂S in the clean-up of gasification gas from tar, methane, and ammonia. The aim was to produce ultra clean gas applicable for liquid biofuel production. In general, the activity towards the decomposition decreased in the order of aromatic hydrocarbons, ethylene > methane > ammonia. Hydrocarbon decomposition on m-ZrO₂ supported Rh, Ni, and Ru catalysts mainly occurred at 800-900 °C through reforming and/or dealkylation reactions. Aromatic hydrocarbon decomposition reactions proceeded on Pt/m-ZrO₂ and Pd/m-ZrO₂ via oxidation reactions at temperatures of 600-800 °C, while at 900 °C, the reforming and/or dealkylation reactions were dominating also on Pt/m-ZrO₂ and Pd/m-ZrO₂ catalysts. During longer test runs of ten hours at 800 °C, the activity of the Rh/m-ZrO₂ catalyst declined in the presence of 100 ppm H₂S due to the sulfur poisoning effects, coke formation, and the particle size growth. Although the performance of Rh/m-ZrO₂ declined, it still remained better than Ni/m-ZrO₂ both towards naphthalene and total aromatic hydrocarbon, while only Ni/m-ZrO₂ and Ru/m-ZrO₂ decomposed ammonia in the presence of sulfur. Nevertheless, the most promising catalyst for clean gas production was Rh/m-ZrO₂.     

Thermal plasma-sprayed nickel catalysts in the clean-up of biomass gasification gas

Plasma spraying is a potential catalyst preparation method for hot gas clean-up which requires very durable catalytic coatings. In this work, the multi-layer composite coating powders consisted of a gibbsite or boehmite core together with a hydrotalcite coating. We used the coating powders in the preparation of the powder form Ni catalysts, which were characterized by XRD, BET, XPS, and XRF. The coating powders were also used in the preparation of plasma sprayed metal substrate supported Ni catalysts. Selected plasma-sprayed nickel containing catalysts were characterized by XRD and SEM. In addition, for comparative purposes we prepared and used a ceramic monolithic Ni catalyst on modified ZrO₂. The performance of all catalysts were studied in the clean-up of synthetic gasification gas, i.e. the decomposition of tar, ammonia, and methane, at 700 and 900 °C, without and with the addition of H₂S. Catalysts with a gibbsite-core showed higher activity and better sulfur resistance than those with a boehmite-core. The high activity of the gibbsite-core Ni catalyst with a sulfur-containing gas together with the promising mechanical and thermal strength makes the plasma spraying method an interesting alternative for gas clean-up catalysts in biomass gasification processes.     

Catalytic clean-up of gasification gas with precious metal catalysts - A novel catalytic reformer development

Several precious metal catalysts were prepared on modified zirconia and tested for the selective catalytic clean-up of the gasification gas. The activity of the precious metal catalysts were compared to that of the modified zirconia supported nickel catalyst and to the support. The activities of the catalysts were tested in a monolithic form in a quartz laboratory reactor at temperatures of 600-900 °C under atmospheric pressure using synthetic sulfur containing gas mixture. In addition, the stability of the Ni and Rh catalysts was examined by measuring the activities at 800 °C for 10 h using sulfur containing gas. The simulated gas contained CO, CO₂, CH₄, C₂H₄, H₂, N₂, H₂O, H₂S, NH₃ and a tar model compound, i.e. a mixture of naphthalene and toluene. The addition of metal on the support promoted the activity in tar model compound decomposition only at the temperature range of 850-900 °C. The order of activity was Rh ≈ Ni > Pd > Ir > Ru > Pt. Almost complete tar model compound conversion was achieved with Rh, as well as with Ni, at 900 °C. At lower temperatures, the support showed higher activity in tar model compound decomposition compared to the metal/support catalysts tested. Only Ni and Ru showed moderate activity in ammonia decomposition. In regard to sulfur tolerance at 800 °C, Rh was activated during the 10 h experiment while the activity of Ni decreased. The performance of both was restored after the overnight N₂ flush and the conversion of the tar model compound was higher for Rh (64%) than for Ni (46%).     

Integrated high temperature gas cleaning: Tar removal in biomass gasification with a catalytic filter

A nickel-based catalytic filter material for the use in integrated high temperature removal of tars and particles from biomass gasification gas was tested in a broad range of parameters allowing the identification of the operational region of such a filter. Small-scale porous alumina filter discs, loaded with approximately 2.5 wt% Al₂O₃, 1.0 wt% Ni and 0.5 wt% MgO were tested with a particle free synthetic gasification gas with 50 vol% N₂, 12 vol% CO, 10 vol% H₂, 11 vol% CO₂, 12 vol% H₂O, 5 vol% CH₄ and 0–200 ppm H₂S, and the selected model tar compounds: naphthalene and benzene. At a typical face velocity of 2.5 cm/s, in the presence of H₂S and at 900 °C, the conversion of naphthalene is almost complete and a 1000-fold reduction in tar content is obtained. Technically, it would be better to run the filter close to the exit temperature of the gasifier around 800–850 °C. At 850 °C, conversions of 99.0% could be achieved in typical conditions, but as expected, only 77% reduction in tars was achieved at 800 °C.

Conversion data can be reasonably well described with first order kinetics and a dominant adsorption inhibition of the Ni sites by H₂S. The apparent activation energies obtained are similar to those reported by other investigators: 177 kJ/mol for benzene and 92 kJ/mol for naphthalene. The estimated heat of adsorption of H₂S is 71 kJ/mol in the benzene experiments and 182 kJ/mol in the naphthalene experiments, which points at very strong adsorption of H₂S. Good operation of the present material can hence only be guaranteed at temperatures above 830 °C mainly due to the strong deactivation by H₂S at lower temperatures.     

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.     

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.     

贵金属含量对催化剂抗硫和抗水热老化性能的影响

将车用催化剂活性涂层涂于金属载体,负载不同含量的铂、钯贵金属,制得车用催化剂样品,采用XRD对催化剂样品进行表征,并通过傅里叶红外评价装置评价催化剂水热老化及硫老化前后的催化性能。结果表明,催化剂抗硫老化及抗水热老化性能随贵金属含量的增加呈先增后减的趋势。贵金属负载质量分数为1.0%时,具有较好的抗硫老化及抗水热老化性能。硫老化后,对HC的劣化率为1.83%,对CO的劣化率为14.93%。水热老化后,对HC的劣化率为2.19%,对CO的劣化率为5.24%。     

Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 2: Mathematical modelling and model validation

In gasification of biomass, coal and blended biomass and coal, there are two steps including an initial pyrolysis process followed by gasification of solid char. The latter process is a slow process and thus dominates the whole gasification. In our previous paper (Xu et al., in press), the differences between steam gasification of biomass chars and that of coal chars have experimentally been investigated and the results show that these differences are mainly due to the difference in microstructures of these two fuels. In this work, a mathematical model of char gasification is developed based on reaction kinetics and gas transportation of both the producer gas and the gasification agent (steam). The model also includes mass conservation equations for each of the gas components and solid carbon involved in the gasification process. This has resulted in a set of highly nonlinear differential equations which have been solved using a numerical technique to predict gas production rate, gas compositions and carbon consumption rate during the gasification.The developed mathematical model is validated using experimental results reported in previous paper (Xu et al., in press), and close agreement between the simulation results and the experimental values have been observed. From the modelling, it has been confirmed that the char gasification is mainly determined by the characteristics of char matrix including the exposed surface area and the micro-pore size. The former determines intrinsic reaction rate and the latter influences the intra-particle mass transportation. Biomass char has more amorphous structure, thus the intrinsic reaction rate is enhanced. For coal char, the larger pore size enables the high transport rate of the gasification agent (water vapour) into the char particles but the resultant gases have higher resistance to transfer through compact clusters. For simulation of the blended biomass and coal, the blend properties were determined based on the blend proportion of each fuel. The close agreement between the simulation results and experimental data suggests that the approach in this work can adequately quantify the gasification kinetics and the gas composition.     

Effect of operating conditions on gas components in the partial coal gasification with air/steam

With increasing environmental considerations and stricter regulations, coal gasification, especially partial coal gasification, is considered to be a more attractive technology than conventional combustion. Partial coal gasification was conducted in detail under various experimental conditions in a lab-scale fluidized bed to study the factors that affected gas components and heating value, including fluidized air flow rate, coal feed rate, and steam feed rate, gasification temperature, static bed height, coal type and catalyst type. The experiment results indicate that gasification temperature is the key factor that affects components and the heating value of gas is in direct proportion to gasification temperature. There exists a suitable range of fluidized air flow rate, coal feed rate, steam feed rate and static bed height, which show more complex effect on gas components. High rank bitumite coal is much more suitable for gasification than low rank bitumite coal. The concentrations of H₂, CO and CH₄ of bitumite coal are more than those of anthracite coal. Compounds of alkali/alkaline-earth metals, such as Ca, Na, K etc., enhance the gasification rate considerably. The catalytical effects of Na₂CO₃ and K₂CO₃ are more efficient than that of CaCO₃. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

镍基芳烃加氢金属催化剂抗硫性的研究进展

从助剂、载体、还原度、非晶态合金和制备方法五个方面介绍了在油品的芳烃加氢过程中提高镍金属催化剂抗硫性研究的新进展。镍催化剂中添加碱金属、碱土金属和稀土助剂后,对调节镍金属的还原度和改善催化剂的制备方法等均有较好的效果。     

大型煤制天然气项目气化技术方案的主要特点和技术优势的探讨

介绍了粉煤气化和碎煤气化两种工艺技术特点,并探讨了两种工艺组合在大型煤制合成天然气项目中应用前景。     

氧化铈和铈锆固溶体对Pd催化剂抗硫性能影响的研究

采用等体积浸渍法制备了质量分数为1%Pd/γ－Al₂O₃、1%Pd/10%CeO₂/γ-Al₂O₃以及1% Pd/10%Ce_0.6Zr_0.4O₂ /γ－Al₂O₃催化剂,研究了CeO₂和Ce_0.6Zr_0.4O₂ 固溶体对Pd催化剂抗硫性能的影响。利用XRD和XPS技术表征了催化剂的固相结构、表面元素组成和反应后催化剂中硫物种（或表面硫酸盐）的存在形态。利用SO₂-TPD研究了SO₂在催化剂上的吸附和脱附行为。实验结果表明,在催化剂载体中引入CeO₂和Ce_0.6Zr_0.4O₂ 降低了催化剂起燃活性;SO₂ 在Pd/Ce_0.6Zr_0.4O₂ /γ－Al₂O₃催化剂表面上发生化学反应生成Zr(SO₄)₂,其热稳定性大于在Pd/γ－Al₂O₃和Pd/CeO₂/γ－Al₂O₃催化剂表面上的硫酸盐或硫酸盐物种。     

Influence of the co-feeding of CO, H2, CO2 or H2O in the partial oxidation of methane over Ni and Rh supported catalysts

The influence of the addition of 5 vol.% of carbon monoxide, hydrogen, carbon dioxide or water to the feed of partial oxidation of methane was investigated over Ni/γ-Al₂O₃ and Rh/γ-Al₂O₃ catalysts. In addition to catalytic tests, thermodynamic calculations were performed to predict the effect of these gas co-feeds. Compared to the thermodynamic trends, differences in the influence of the co-feeding on catalytic performances were observed between both catalysts. Co-feeding of CO, H₂, CO₂ or H₂O can modify the oxidation state and dispersion of the metal component of the catalysts during reaction, and as a consequence, their performances. Changes in catalysts can be due to dynamic processes occurring during reaction. It is suggested to take these processes into account in a more complex kinetic equation for the reactions involved.     

The preparation of palladium metal catalysts supported on carbon part II: Deposition of palladium and metal area measurements

Four techniques for Pd deposition were compared and various carbons from Part I were used. Increasing burnoff produced higher area of the support and higher available metal area. However, excessive removal of carbon resulted in a decrease in Pd area for a given loading. Although HNO₃ treatment did not produce an increase in carbon area, there were significant increases in catalyst areas for some deposition techniques. Carbon deposition appeared to block micropores to some extent, but was disappointing in the areas of Pd found on these samples. A favorable technique would seem to be to use 15 to 20% burnoff followed by HNO₃ acid washing and the impregnation adsorption deposition method.     

生物质裂解气中焦油转化催化剂的研究现状

生物质裂解气中焦油的存在严重影响了燃气的热值和后续流程，因此焦油转化对生物质裂解制取合成气有着重要的意义，介绍了用于生物质裂解气中的焦油转化的催化剂体系，以及国内外在此领域的研究成果，展望了焦油转化催化剂的发展方向以及急需解决的问题。     

助剂对PdO/Al2O3催化剂的抗硫中毒性能影响

浸渍法制备了不同助剂掺杂的PdO/Al<,2>O<,3>催化剂,以H<,2>S为硫源考察了催化剂的抗硫中毒性能以及催化剂对甲烷的催化燃烧性能,并用库仑滴定法测定了中毒及再生后催化剂表面的含硫量,以此分析催化剂中毒模式.结果表明,助剂MgO、ZnO<,2>、K<,2>O的添加均不同程度的提高了催化剂的抗硫中毒性能.再生后...     

Catalytic decomposition of N2O over supported Rh catalysts: effects of supports and Rh dispersion

The study of catalytic decomposition of nitrous oxide to nitrogen and oxygen over Rh catalysts supported on various supports (USY, NaY, Al₂O₃, ZrO₂, FSM-16, CeO₂, La₂O₃) showed that the activities of Rh/Al₂O₃ and Rh/USY (ultrastable Y zeolite) catalysts were comparable to or higher than the other catalysts reported in the literatures. The catalytic activity of N₂O decomposition was sensitive not only to the Rh dispersion but also to the preparation variables such as the Rh precursors and the supports used. A pulsed N₂O experiment over a Rh/USY catalyst suggested that the catalytic N₂O decomposition occurs on oxygen-covered surface and that O₂ may be freed on collision of N₂O molecules with the adsorbed oxygen atoms.     

Development of Ni catalysts for gas production from biomass gasification. Reactivity in steam- and dry-reforming

Biomass gasification can be optimised in a fluidised bed by the use of metallic nickel as active phase grafted on olivine. Natural olivine ((Mg, Fe)₂SiO₄) has been chosen as catalyst support because of its activity in biomass steam gasification and tar cracking, its high attrition resistance.

After impregnation of nickel oxide on olivine and calcination at 900, 1100 or 1400°C, different interactions between the precursor and the support have been revealed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy coupled to energy dispersive X-ray spectroscopy. Temperature programmed reduction has completed this study and permitted to control the reducibility of the catalysts. The most promising catalyst determined after these different characterisation studies contained 2.8 wt.% of Ni and was calcined at 1100°C. It exhibited strong nickel–olivine interaction but the grafted nickel oxide particles stayed reducible under catalytic test conditions.

Already at 750°C, this catalyst presented a high activity in dry-reforming (95% methane conversion) and steam-reforming (88% methane conversion) and yield in syngas (80% and 75% CO yield, respectively). An excess of water content in steam-reforming inhibited the catalytic activation which could be retrieved by addition of a reducer like H₂.

No sintering of nickel particles and very little carbon deposition has been observed on this catalytic system by characterisation studies after catalytic tests. This can explain its very good ageing behaviour (at least 260 h at 800°C) and justifies its use in a fluidised bed pilot plant.     

Performance of alumina-supported noble metal catalysts for the combustion of trichloroethene at dry and wet conditions

The performance of four different alumina-supported noble metal catalysts (0.5% of Pd, Pt, Rh and Ru, respectively) for the deep oxidation of trichloroethene (1000–2500 ppmV, WHSV = 55 h⁻¹) in air was studied in this work. Experiments were carried out at both dry and wet (20,000 ppm of H₂O) conditions. Catalysts were compared in terms of activity, selectivity for the different reaction products (CO₂, HCl, Cl₂, C₂Cl₄, CCl₄ and CHCl₃), and stability at reaction conditions.

As general trend, the activity of the catalysts decreases in the order Ru  Pd > Rh > Pt. Concerning to the effect of the water addition, no important effect on the catalyst activity was observed, except in the case of Pt, for which an increase of the catalytic activity was observed. Reaction mechanism (and hence product distribution) is very similar for Rh, Pd and Pt, being in these cases C₂Cl₄ the only organochlorinated by-product detected. In the case of Ru, the reaction mechanism seems to be quite different, CCl₄ and CHCl₃ being the main organic by-products.

Simple power-law kinetic expressions (first order on trichloroethene concentration for Pd, Rh and Ru, and zeroth order for Pt) provide fairly good fits for catalytic performance of the studied catalysts.

Finally, deactivation studies show that both formation of active metal chlorides (especially in the case of Rh) and fouling (especially for Pd and Pt) are the main deactivation causes.     

甲醇合成气中羰基金属化合物对催化剂的影响及对策

羰基铁和羰基镍是铜基合成甲醇催化剂的毒物。论述了羰基铁和羰基镍的形成机理及对甲醇催化剂的影响,指出使用脱羰基铁、羰基镍吸附剂是延长催化剂使用寿命、增加甲醇产量和提高甲醇产品质量的有效措施。