Fluidizable catalyst for methane reforming

Fluidizable catalysts are developed in this study for advancing an integral approach towards a new methane reforming process. With this end, catalysts constituted by nickel supported on -alumina, NaY, and USY were developed using the incipient wetness technique producing bulk nickel loadings in the 0–20 wt.% range. These catalysts were also tested under relevant conditions for industrial operation in a novel Riser Simulator. It was found that, for the case of ‘dry’ reforming of methane, nickel deposited in zeolites is a promising catalyst given that it allows for close control of metal dispersion–redispersion process. In fact, when this catalyst was exposed to repeated oxidation and reduction cycles, nickel dispersions remained stable at 25% for NaY zeolite and at 15% for USY zeolite. This catalyst offers, however, limited application for steam reforming of methane given the potential collapse of the zeolite structure under steam atmosphere. As an alternative and for cases where steam reforming of methane is preferred, nickel on -alumina catalyst was considered. In these cases, optimum catalytic activity was achieved with 2.5 wt.% of nickel on -alumina with 3–6% nickel dispersion.     

A kinetic study on the conversion of methane to higher hydrocarbons in a radio-frequency discharge

This study investigated methane conversion with direct current discharge at low pressure in a radio frequency. The main gaseous products of the reaction were ethane, ethylene, acetylene and propane. This study was concentrated on the influence of discharge conditions on the conversion of methane to higher hydrocarbons. Reaction temperature, electron density and mean residence time were calculated from experimental data and mathematical relations. The maximum conversion of the methane was about 45% with the pure methane as a reactant. Ethane was the main product when the reaction occurred in the glow discharge. Ethane selectivity decreased with the increase of the gas temperature. The kinetics of reactions was also analyzed from possible reaction equations and various rate constant data. Consequently, the dissociation constant and the density of radicals could be obtained at any experimental conditions.     

Upgrading of diesel fuels and mixtures of hydrocarbons by means of oxygen low pressure plasmas: a comparative study

The oxidation of n-heptane, 1-octene, toluene, cis-decahydronaphthalene, mixtures of them, 4-phenyl-1-butene, 1,2,3,4-tetrahydronaphthalene, and three commercial diesel fuels, all in the liquid phase, by means of low pressure high-voltage oxygen plasmas was studied. Oxygen pressure was 0.2 mbar, applied power was 35 watts and reaction times ranged from 1 min to 23 h. Both individually and forming part of mixtures, olefins were the most reactive with ground-state atomic oxygen, O(³P). Olefinic double bonds reacted ca. 150 times faster than C-H bonds. Products were: epoxides and aldehydes for olefins; alcohols and ketones for alkanes; phenols for aromatics. Addition of 4.7-7.8% wt of oxygen was achieved for the diesels, depending on the particular composition, those with higher content of olefins being favoured, followed by those with higher content of alkanes.     

Co/Al2O3 catalysts promoted with noble metals for production of hydrogen by methane steam reforming

The effect of noble metal addition on the catalytic properties of Co/Al₂O₃ was evaluated for the steam reforming of methane. Co/Al₂O₃ catalysts were prepared with addition of different noble metals (Pt, Pd, Ru and Ir 0.3 wt.%) by a wetness impregnation method and characterized by UV–vis spectroscopy, temperature programmed reduction (TPR) and temperature programmed oxidation (TPO) of the reduced catalysts. The UV–vis spectra of the samples indicate that, most likely, large amounts of the supported cobalt form Co species in which cobalt is in octahedral and tetrahedral symmetries. No peaks assigned to cobalt species from aluminate were found for the promoted and unpromoted cobalt catalysts. TPO analyses showed that the addition of the noble metals on the Co/Al₂O₃ catalyst leads to a more stable metallic state and less susceptible to the deactivation process during the reforming reaction. The Co/Al₂O₃ promoted with Pt showed higher stability and selectivity for H₂production during the methane steam reforming.     

Carbon dioxide reforming of methane with a free energy minimization approach

Carbon dioxide reforming of methane to syngas is one of the primary technologies of the new poly-generation energy system on the basis of gasification gas and coke oven gas. A free energy minimization is applied to study the influence of operating parameters (temperature, pressure and methane-to-carbon dioxide ratio) on methane conversion, products distribution, and energy coupling between methane oxidation and carbon dioxide reforming methane. The results show that the methane conversion increases with temperature and decreases with pressure. When the methane-to-carbon dioxide ratio increases, the methane conversion drops but the H₂/CO ratio increases. By the introduction of oxygen, an energy balance in the process of the carbon dioxide reforming methane and oxidation can be realized, and the CO/H₂ ratio can be adjusted as well without water-gas shift reaction for Fischer-Tropsch or methanol synthesis. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Study on the active centers of methane coupling catalysts by means of fluorescence emission of Eu

Eu³⁺ ion was adopted as a probe to detect the probability of entrance of alkali elements into the crystal lattice of MgO, CaO and La₂O₃ by means of its characteristic emission. Based on the experimental data it is concluded that Li⁺ and Na⁺ ions can substitute Mg²⁺ and Ca²⁺ ions and only a small amount of K⁺ ion can enter into the lattice of CaO. Whilst Li⁺ ion can not enter into the lattice of lanthana. The conclusion of this investigation is in good agreement with that obtained by Lunsford by ESR studies.     

Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation

A thermodynamic equilibrium analysis on the multi-reaction system for carbon dioxide reforming of methane in view of carbon formation was performed with Aspen plus based on direct minimization of Gibbs free energy method. The effects of CO₂/CH₄ ratio (0.5-3), reaction temperature (573-1473 K) and pressure (1-25 atm) on equilibrium conversions, product compositions and solid carbon were studied. Numerical analysis revealed that the optimal working conditions for syngas production in Fischer-Tropsch synthesis were at temperatures higher than 1173 K for CO₂/CH₄ ratio being 1 at which about 4 mol of syngas (H₂/CO = 1) could be produced from 2 mol of reactants with negligible amount of carbon formation. Although temperatures above 973 K had suppressed the carbon formation, the moles of water formed increased especially at higher CO₂/CH₄ ratios (being 2 and 3). The increment could be attributed to RWGS reaction attested by the enhanced number of CO moles, declined H₂ moles and gradual increment of CO₂ conversion. The simulated reactant conversions and product distribution were compared with experimental results in the literatures to study the differences between the real behavior and thermodynamic equilibrium profile of CO₂ reforming of methane. The potential of producing decent yields of ethylene, ethane, methanol and dimethyl ether seemed to depend on active and selective catalysts. Higher pressures suppressed the effect of temperature on reactant conversion, augmented carbon deposition and decreased CO and H₂ production due to methane decomposition and CO disproportionation reactions. Analysis of oxidative CO₂ reforming of methane with equal amount of CH₄ and CO₂ revealed reactant conversions and syngas yields above 90% corresponded to the optimal operating temperature and feed ratio of 1073 K and CO₂:CH₄:O₂ = 1:1:0.1, respectively. The H₂/CO ratio was maintained at unity while water formation was minimized and solid carbon eliminated.     

Decomposition of polyethylene in radio-frequency nitrogen and water steam plasmas under reduced pressures

The possibility of radio-frequency (RF) nitrogen and water steam plasmas under reduced pressures for gasification of plastic waste as a thermal recycling method has been investigated in order to develop an innovative method for directly recycling plastic waste to hydrogen, synthesis gases or fuels. The products of pyrolysis were analyzed and classified into gaseous fraction and solid soot; and analytical interest was focused on the gaseous product composition. It was found that the electrode geometry, input power, reactor pressure and plasma working gas were the key parameters affecting the plasma characteristics and pyrolysis product. Experiments with different plasma media indicated that when polyethylene (PE) powder was injected into nitrogen plasma, the PE was decomposed and hydrogen formed as a main product by reaction with the plasma; when water steam plasma was used for conversion of PE, the carbon conversion to gas was dramatically enhanced in the presence of water steam, and the main gas products were carbon monoxide and hydrogen. Preliminary solid products analysis and pyrolysis mechanisms for the different plasmas processes were also discussed.     

A Novel Technology for Natural Gas Conversion by Means of Integrated Oxidative Coupling and Dry Reforming of Methane

A novel process concept for the oxidative coupling of methane followed by the oligomerization to liquids has been developed within the frame of the EU integrated project OCMOL. This technology is based on process intensification principles via cutting‐edge structured microreactor technology. It is also a fully integrated industrial process through the re‐use and the recycling of by‐products, in particular CO₂, at every process stage. The focus of this contribution is on the reaction engineering aspects of the core steps, i.e., catalysts, kinetics and reactor design for the methane coupling and reforming.     

Ni-Ce-ZrO2催化剂在二氧化碳甲烷重整制合成气中的研究

采用XRD、氢化学吸附作用、TPR和XPS等技术研究了共沉淀方法制备N i-Ce-ZrO2催化剂对二氧化碳甲烷重整的性能。N i的载入量和CeO2与ZrO2的比率系统地被改变将使N i-Ce-ZrO2催化剂最优化。发现15w t%N i与Ce0.8Zr0.2O2共沉淀有体相的状态,在800℃用CH4制合成气超过97%,并且经过100h的反应,活性被维持没有重大的损失。     

Cobalt–magnesia catalyst by oxalate co-precipitation method for dry reforming of methane under pressure

Catalytic performance of cobalt–magnesia catalyst prepared by oxalate co-precipitation method (Co–MgO) was investigated for dry reforming of methane at 1 MPa, 1023 K. Co–MgO (7 mol% Co) showed stable activity at such high space velocity as 400,000 cm³ h⁻¹ g⁻¹ whereas reactor was plugged during the reforming reaction with 10 mol% Co–MgO, and the activity of Co–MgO with Co content less than 6 mol% gradually decreased by the oxidation of cobalt species. Well-balanced cobalt content is essential for high and stable activity.     

Partial oxidation (POX) reforming of gasoline for fuel-cell powered vehicles applications

As a part of the development of a gasoline processor for integration with a proton-exchanged membrane (PEM) fuel cell, we carried out the POX reforming reaction ofiso-octane, toluene and gasoline over a commercial methane reforming catalyst, and investigated the reaction conditions required to prevent the formation of carbon and the effect of fuel constituents and sulfur impurities in gasoline. The H₂ and CO compositions increased with increasing reaction temperature, while those of CO₂ and CH₄ decreased. It is desirable to maintain an O/C molar ratio of more than 0.6 and an H₂O/C molar ratio of 1.5 to 2.0 for vehicle applications. It has been found that carbon formation in the POX reforming ofiso-octane occurs below 620 °C, whereas in the case of toluene it occurs below 640 °C. POX reforming of gasoline constituents led to the conclusion that hydrogen production is directly related to the constituents of fuels and the operating conditions. It was also found that the coke formation on the surface of catalysts is promoted by sulfur impurities in fuels. For the integration of a fuel processor with PEM fuel cell, studies are needed on the development of new high-performance transition metal-based catalysts with sulfur and coke-resistance and the desulfurization of fuels before applying the POX reformer based on gasoline feed.     

Strontium carbonate supported cobalt catalyst for dry reforming of methane under pressure

Catalytic performance of Co–SrO catalyst for dry reforming of methane was investigated at 1 MPa, 1023 K. The catalyst prepared by oxalate co-precipitation method or citric acid method showed a steady activity for dry reforming of methane under pressure. The importance and stability of cobalt metal with strontium carbonate were suggested for the Co–SrO catalyst, and thus it should be denoted as Co–SrCO₃. In addition, cobalt supported on strontium carbonate prepared by impregnation method (Co/SrCO₃) showed the comparable activity with high tolerance to oxidative atmosphere under reaction conditions.     

Carbon dioxide reforming of methane over Ni/Al2O3 treated with glow discharge plasma

Ni/Al₂O₃ catalyst was first treated by argon glow discharge plasma followed by calcination in air. The catalyst prepared this way exhibits an improved low-temperature activity for carbon dioxide reforming of methane, compared to the catalyst prepared without plasma treatment. The catalyst characterization using XRD, chemisorption and TEM analyses show that the plasma treatment followed by calcination thermally induces a generation of specific nickel species on the support. This kind of “plasma” metal species is highly dispersed on the support and can remain stable during reforming reactions. The average size of the “plasma” metal particles is ca. 5 nm. The plasma treatment can also enhance the anti-carbon deposition performance of the catalyst. The formation of carbon species that is responsible for catalyst deactivation can be inhibited. The catalyst stability is therefore improved.     

Studies on nickel-based catalysts for carbon dioxide reforming of methane

The catalytic activity and coke resistance of La₂O₃ promoted nickel-based catalysts are investigated in a fixed-bed flow reactor. The contents of carbon deposited on catalysts were measured by a carbon combustion method. Catalysts were characterized by CO–TPD, CO₂–TPD, TPR, XPS and XRD techniques, and the results were correlated with the coke resistance of the catalysts. It is found that the catalytic activity, resistance to carbon deposition and the stability of the catalysts can be greatly improved with the addition of a rare earth oxide. It is found that BaTiO₃ is an ideal support. Thus 5.0 wt.% Ni–0.75 wt.% La–BaTiO₃ catalyst shows great resistance to coke formation and higher thermal stability as well as higher catalytic activity, than the catalysts 5.0 wt.% Ni/La–BaTiO₃ (Ba/La = 1/0.002) and 5.0 wt.% Ni–1.5 wt.% La/BaTiO₃.     

Temperature profiles of the monolith catalyst in CO<Subscript>2</Subscript> reforming of methane with<Emphasis Type="Italic">in-situ</Emphasis> combustion of methane and ethane

The temperature profiles in a monolith reactor were measured in CO₂ reforming of CH₄ within-situ combustion of methane and ethane in order to find out in what sequence the reactions are occurring. The reaction gas flowed both upward and downward. A hot spot was observed at low furnace temperatures, and it tended to disappear as the furnace temperature increased. This is due to natural extinguishment of the flame caused by the endothermic reforming reactions occurring. The hot spot disappeared at a lower temperature with the up-flow when compared with the down-flow. When the hot spot appears, H₂O and CO₂ are produced by complete oxidation and accordingly the steam reforming and the CO₂ reforming occur competitively in the rear part of the monolith. If the hot spot does not appear, it is considered that the partial oxidation of methane occurs predominantly over the complete oxidation, resulting in more efficient CO₂ removal.     

Pure hydrogen production by methane steam reforming with hydrogen-permeable membrane reactor

Low temperature steam reforming of methane mainly to hydrogen and carbon dioxide (CH₄ + 2H₂O → 4H₂ + CO₂) has been performed at 773 and 823 K over a commercial nickel catalyst in an equilibrium-shift reactor with an 11-μm thick palladium membrane (Mem-L) on a stainless steel porous metal filter. The methane conversion with the reactor is significantly higher than its equilibrium value without membrane due to the equilibrium-shift combined with separation of pure hydrogen through the membrane. The methane conversion in a reactor with an 8-μm membrane (Mem-H) is similar to that with Mem-L, although the hydrogen permeance through Mem-H is almost double of that through Mem-L. The amount of hydrogen separated in the reaction with Mem-H is significantly large, showing that the hydrogen separation overwhelms the hydrogen production because of the insufficient catalytic activity.     

Effects of ceria in CO2 reforming of methane over Ni/calcium hydroxyapatite

In CO₂ reforming of methane over a calcium hydroxyapatite-supported nickel catalyst, the carbon deposition occurred more severely with increase of the methane partial pressure and at temperatures below about 1,000 K. The effects of ceria that was added as a promoter to the nickel catalyst were investigated. It was observed that the ceria not only enhanced the catalyst stability but also increased the activity, and this is considered owing to the oxygen storage capacity of ceria. The TGA analysis demonstrated that the ceria promoted the removal of the deposited carbon. The optimum Ce/Ni mole ratio was ca. 0.3/2.5. The deposited carbon could easily be removed by oxygen treatment at 1,023 K and the catalytic activity could be restored.     

CH4与CO2催化重整制合成气研究进展

本文综述了CH4与CO2重整制合成气的研究进展,对重整催化剂、重整反应机理及重整动力学进行了评述。