Lean methane catalytic combustion over the mesoporous MnOx-Ni/MgAl2O4 catalysts: Effects of Mn loading

In the current investigation, the textural features and catalytic efficiency of the Mn-promoted Ni/MgAl₂O₄ catalysts were evaluated in the catalytic combustion of lean methane. The results demonstrated that adding manganese oxide up to 5 wt.% to the catalyst improved the structural features and light-off temperatures due to the increase of the reduction degree and oxygen vacancies. The role of the T_calcination and processing factors on the methane conversion of the 5%Mn-20%Ni/MgAl₂O₄ catalyst was also studied. The results indicated that the CH₄ conversion decreased with the increment of O₂/CH₄ molar ratio and GHSV value. The stability test revealed that the prepared sample exhibited high stability during 10 h time on stream. Furthermore, the obtained results indicated that the BET area, reduction degree, oxygen mobility, and catalytic performance increased with decreasing the calcination temperature from 700 to 500 °C.     

CeO2-promoted BaO-MnOx catalyst for lean methane catalytic combustion at low temperatures: Improved catalytic efficiency and light-off temperature

In this research, the effect of various promoters (Zr, La, Ce, and Y) on the physicochemical features and catalytic activities of the BaO(10)-MnO_x prepared by the mechanochemical preparation route was investigated in the catalytic combustion of lean methane. Incomplete methane combustion as a result of the burning of natural gas in industries is the major reason to use a catalytic system to reduce the pollutants formation (NO_x, CO_x, etc.) and improve the combustion efficiency and proceeds towards complete combustion. In our previous study, we performed the methane combustion reaction on the mechanochemical synthesized BaO(x)-MnO_x catalysts with various contents of BaO (5–20 wt%), and the results indicated that the addition of 10 wt% of BaO to MnO_x could remarkably improve the catalytic efficiency due to the enhance the oxygen mobility and reduction features. In the present study, the structural properties of the synthesized catalysts were specified by XRD, BET, H₂-TPR, O₂-TPD, and SEM techniques. It is observed from the O₂-TPD technique that the addition of promoters into the BaO-MnO_x catalyst resulted in the increase in oxygen mobility, which could rise the catalytic activity. The addition of CeO₂ has a more positive effect on the catalytic activity due to the higher surface area and oxygen storage capacity. The obtained results revealed that the CeO₂(3)-BaO(10)-MnO_x catalyst possessed the superior performance in the methane combustion. The 90% of CH₄ conversion was obtained at about 350 °C over this catalyst. The effect of calcination temperature, feed ratio, GHSV, hysteresis curve, pretreatment atmospheres, and the presence of CO₂ and moisture was evaluated on the catalytic efficiency. After 50 h of continuous reaction at 450 °C, the selected catalyst showed high stability and the catalyst morphology was not significantly altered. Furthermore, CO oxidation was performed over the CeO₂(3)-BaO(10)-MnO_x catalyst, and the results indicated that the CO conversion was reached 100% at 250 °C.     

Ammonia chemistry in oxy-fuel combustion of methane

The oxidation of NH₃ during oxy-fuel combustion of methane, i.e., at high [CO₂], has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high CO₂ level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and CO₂, are responsible for the effect of a high CO₂ concentration on ammonia conversion. When CO₂ is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NH₂+CO. However, reactions NH₂+O to form HNO and NH₂+H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NH₂ with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NH₂ availability to form NO by other pathways like via HNO or NH and increase the probability of forming N₂ instead of NO.     

Synergistic tuning of the phase structure of alumina and ceria-zirconia in supported palladium catalysts for enhanced methane combustion

CeO₂–ZrO₂–Al₂O₃ composite oxides supported palladium catalysts (Pd/CZA) are promising candidates for catalytic oxidation reactions. However, the efficient and stable oxidation of methane over Pd-based catalysts remains a longstanding challenge. Herein, we present a facile strategy to boost the catalytic performance of Pd/CZA through elaborately tuning the phase structure of supports. Calcining supports at relatively high temperatures (1200, 1300 °C) induced the phase transition of alumina (from γ-to α-) and the development of CeO₂–ZrO₂ solid solution (CZ). The weak interaction between α-Al₂O₃ and PdO resulted in an improved reducibility of catalysts. Meanwhile, the higher oxygen mobility originated from well-crystallized CZ phase contributed to the reoxidation of Pd to PdO, giving rise to abundant surface active Pd²⁺ species. Coupled with the hydrophobicity of α-Al₂O₃, the catalyst prepared with CZA supports calcined at 1300 °C demonstrated an excellent low-temperature activity, astounding stability and greatly enhanced water resistance towards methane combustion.     

Perovskite catalysts for methane combustion: applications,design, effects for reactivity and partial oxidation

This review underlines the importance of the developments in perovskite catalysts for methane combustion from the past up to the present. In this review, after a general and brief introduction to perovskites, the mechanisms of catalytic combustion of methane have been included. Moreover, current studies on perovskites have been summarized including the effects of substitutions, doped perovskites, perovskite preparation methods, and the effect of sulfur presence on perovskite catalysts. Besides, recent studies on perovskite oxides and phenomenon of oxygen (O₂) deficiency, porous perovskite oxides, and nanostructured perovskites have been conducted. In addition, partial oxidation of methane (POM) has been reviewed. The loss of active component during the POM reaction can take place in the nickel catalyst, in particular. Since nickel has a lower melting point than noble metals and other active components, such as Co and Fe, in general, to deactivate nickel is easier. Compared with conventional structure, the porous structure with the unique morphology significantly enhances the catalytic activity through a much larger surface area (SA) and greater reactivity of the active sites. Furthermore, the monolithic nanoarrayed perovskite presents very good results in well‐defined faceted catalysts and takes part in porous channel hydrocarbon combustion. This review study is prepared as a guide to cover the profound knowledge of perovskite oxides catalysts, considering the methane combustion reaction mechanisms, and addresses prospective studies in this field for researchers.     

Thermal management and catalytic combustion stability characteristics of premixed methane/air in heat recirculation meso‐combustors

In order to illuminate heat recirculation effect on catalytic combustion stability and further improve energy conversion efficiency in meso‐combustor, the catalytic combustion characteristics of the combustor with/without preheating channels are numerically studied at steady conditions. It is found that methane conversion rate and combustion efficiency increases by 2% to 3% and approximately 9% in the heat recirculation meso‐combustor, indicating that heat recirculation effect facilitates more complete combustion of methane and medium components. Preheating channels show positive effects on improving combustion stability in the heat recirculation meso‐combustor. On one hand, preheating channels facilitate heat recirculation effect, and heat recirculation rate exceeds 10% for all cases and reaches 31.8% with an inlet velocity of 0.5 m/s, leading to significant increment of methane‐specific enthalpy at the preheating channel outlet. On the other hand, Rh(s)/O(s) ratios of catalytic surface and catalytic surface temperature in main reaction zone are enlarged by the preheating channels, facilitating methane adsorption at catalytic surface. Specially, most of fuels are consumed in a shorter distance with higher methane conversion speed, which brings benefits to promote combustion efficiency and may be helpful to inhibit the combustion instability in heat recirculation meso‐combustors.     

Bifurcation behaviors of catalytic combustion in a micro-channel

Bifurcation analysis of ignition and extinction of catalytic combustion in a short micro-channel is carded out with the laminar flow model incorporated as the flow model. The square of transverse Thiele modulus and the residence time are used as bifurcation parameters. The influences of different parameters on ignition and extinction behavior are investigated. It is shown that all these parameters have great effects on the bifurcation behaviors of ignition and extinction in the short micro-channel. The effects of flow models on bifurcation behaviors of combustion are also analyzed. The results show that in comparison with the flat velocity profile model, for the case of the laminar flow model, the temperatures of ignition and extinction of combustion are higher and the unsteady multiple solution region is larger.     

催化燃烧技术的应用进展

介绍催化燃烧的实质和特点,论述催化燃烧技术在汽车尾气处理、提高锅炉燃煤效率、提高铁矿烧结质量等领域中的应用,以及在炉管烧焦过程中应用的前景。催化燃烧技术有利于资源的合理利用,是环境友好的过程,加快和扩展催化燃烧研究的领域,对生产进步有促进作用。     

Numerical studies of catalytic combustion in a catalytically stabilized combustor

This work aims to investigate numerically the catalytic combustion of a catalytically stabilized combustor. The numerical model treated a catalytic channel deposited with Pt and used a plug model of laminar, one‐dimensional, and steady‐state flow. The predicted conversions of mixture and ignition temperatures of surface reaction agreed well with the measured data when a multi‐step mechanism was used for the CH₄ surface reaction over Pt. The flame speed of a mixture supported by catalytic surface reaction was found to increase compared with a mixture without a catalytic combustion. CO mole fractions were analysed for three cases—gas reaction, surface reaction, and gas reaction coupled with surface reaction. The case of solely gas reaction produced the most CO emission and the case of solely surface reaction generated the least CO emission. The position where flame ignites was also evaluated numerically. There was only a small difference between the measured and predicted results on the starting points of flame in the catalytic channel. As a result, the plug model was shown to model surface ignition very well, however, it did not predict well the position of flame ignition. Copyright © 2000 John Wiley & Sons, Ltd.     

Combustion of methane in catalytic honeycomb monolith burners

The heat transfer efficiency, stability, and pollutant emissions characteristics of ultra‐lean methane–air combustion in some precious metal‐based honeycomb monoliths were investigated. The interpretation of the experimental results was assisted using numerical modelling of the gas‐phase combustion process. The thermal radiation output of the monoliths varied between 27 and 30 per cent of the thermal input, and this compared favourably with equivalent porous inert media burners. The minimum fuel concentrations for very‐low emission stable combustion were found to be significantly lower than for conventional gas‐phase combustion and were shown to vary with the nature and loading of the catalyst, as well as with flow rates. The palladium catalyst was found to have a larger window of mixture strengths and flow rates for stable operation than the platinum one. During all the runs under stable combustion conditions, only extremely small amounts of CO, NO_x and unburnt hydrocarbons were detected. Thus, the operating conditions verified ‘near‐zero’ pollutant emissions that only a catalytic combustion process can achieve at present. Temperature profiles inside the monoliths channels proved that the catalyst's role was not only to enable the ignition of fuel mixtures below flammability limits, but also to ensure the complete oxidation of the fuel to CO₂ via surface reactions in the steady state. The reaction zone inside the catalysts was found to end at about 10 mm from the monolith's entrance. The effect of monolith length was investigated and a reduction of 70 per cent in the original length was found possible. Copyright © 2000 John Wiley & Sons, Ltd.     

Solution combustion synthesized lithium cobalt oxide as a catalytic precursor for the hydrolysis of sodium borohydride

Solution combustion synthesis (SCS) has recently been explored as one method to synthesize metal oxides (e.g. Co₃O₄) that can serve as catalytic precursors for the hydrolysis of sodium borohydride (NaBH₄). In this work, SCS is used to produce the mixed metal oxide lithium cobalt oxide (LiCoO₂) from a solution of cobalt nitrate, lithium acetate, and glycine. Its subsequent use as an effective catalyst precursor for NaBH₄ hydrolysis is characterized and compared to commercially available LiCoO_2. To remove residual impurities from the SCS material the materials were heated at a rate of 10 °C min^?1 and held for 2 h at temperatures ranging from 500 to 800 °C and subsequently characterized. It was found that the layered phase of LiCoO₂ results at heat treat temperatures above 700 °C. Using a 0.6 wt.% aqueous solution of NaBH₄ at 25 °C and a 1 wt.% catalyst precursor loading, an optimized HGR of 2.09 L min^?1 g_cat^?1 was achieved for the solution combustion synthesized LiCoO₂. In contrast, at the same conditions, a HGR of 0.29 L min^?1 g_cat^?1 was obtained for commercial materials even though the specific surface area was much higher.     

Some issues in modelling methane catalytic decomposition in fluidized bed reactors

This paper reports a model of fluidized bed thermo-catalytic decomposition (TCD) of methane. The novelty of the model consists of taking into account the occurrence of different competitive phenomena: methane catalytic decomposition, catalyst deactivation due to carbon deposition on the catalyst particles and their reactivation by means of carbon attrition. Comparison between theoretical and experimental data shows the capability of the present model to predict methane conversion and deactivation time during the process. The model demonstrates to be also a useful tool to investigate the role played by operative parameters such as fluidizing gas velocity, temperature, size and type of the catalyst. In particular, the model results have been finalized to characterize the attrition phenomena as a novel strategy in catalyst regeneration.     

Experimental investigation of premixed combustion limits of hydrogen and methane additives in ammonia

In this study, a specially designed premixed combustion chamber system for ammonia-hydrogen and methane-air laminar premixed flames is introduced and the combustion limits of ammonia-hydrogen and methane-air flames are explored. The measurements obtained the blow-out limits (mixed methane: 400–700 mL/min, mixed hydrogen: 200–700 mL/min), mixing gas lean limit characteristics (mixed methane: 0–82%, mixed hydrogen: 0–37%) and lean/rich combustion characteristics (mixed methane: ? = 0.6–1.9, mixed hydrogen: ? = 0.9–3.2) of the flames. The results show that the ammonia-hydrogen-air flame has a smaller lower blow-out limit, mixing gas ratio, lean combustion limit and higher rich combustion limit, thereby proving the advantages of hydrogen as an effective additive in the combustion performance of ammonia fuel. In addition, the experiments show that increasing the initial temperature of the premixed gas can expand the lean/rich combustion limits of both the ammonia-hydrogen and ammonia-methane flames.     

Effects of hydrogen addition on methane catalytic combustion in a microtube

The objective of this paper is to study hydrogen-assisted catalytic combustion of hydrocarbon on a microscale experimentally. In the experiment, neither methane nor ethane can be ignited by itself, but hydrogen can be ignited and burn steadily in this tube. It is found that there is no significant difference between hydrogen added to the hydrocarbon and hydrogen alone as fuel without the platinum thermocouple, but the temperature will increase and the efficiency of methane combustion will increase considerably when the platinum thermocouple was put into the microtube. Methane can burn steadily without adding hydrogen after ignited by hydrogen. It can be concluded that the addition of hydrogen to hydrocarbon is favorable to ignition and the platinum thermocouple catalyzes the hydrocarbon combustion. The experiment result showed that the added hydrogen acts as an assistant for ignition and expands the range for methane steady burn. After igniting, methane can burn steadily alone at catalytic condition. This is useful for optimization microcombustion fuel.     

Numerical study of rich catalytic combustion of syngas

The rich catalytic combustion of syngas/air mixtures over platinum has been investigated numerically in a two-dimensional circular channel using steady simulations and detailed hetero-homogeneous chemistry. The channel dimensions are representative of a catalytic monolith. Simulations have been conducted in the pressure range of 1–10 bar and Φ = 3–5 with varying inlet velocities, residence time, H₂/CO ratio and CH₄ percentage. Detailed kinetic studies including the reaction path diagram (RPD) in a plug flow reactor have also been conducted to understand the kinetic interactions between H₂, CO, and CH₄. It has been observed that the homogeneous reaction rates are significant at higher pressures and cannot be neglected, although they were highly localized. The channel temperature significantly affected the relative conversion of H₂ and CO. The kinetic coupling between H₂ and CO oxidation was studied and the reason for the differential consumption of O₂ by the reactants was addressed by analyzing the reaction pathways. The residence time in the channel affected the species oxidation and four operation regimes were identified. Both the water-gas shift reaction and the reverse water-gas shift reaction were observed under varying conditions of pressure and equivalence ratio. The effect of H₂/CO ratio has also been investigated. The present study shows that rich catalytic combustion of syngas is fundamentally different from lean combustion.     

Hydrogen production by catalytic methane decomposition over yttria doped nickel based catalysts

Catalytic methane decomposition can become a green process for hydrogen production. In the present study, yttria doped nickel based catalysts were investigated for catalytic thermal decomposition of methane. All catalysts were prepared by sol-gel citrate method and structurally characterized with X-ray powder diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and Brunauer, Emmet and Teller (BET) surface analysis techniques. Activity tests of synthesized catalysts were performed in a tubular reactor at 500 ml/min total flow rate and in a temperature range between 390 °C and 845 °C. In the non-catalytic reaction, decomposition of methane did not start until 880 °C was reached. In the presence of the catalyst with higher nickel content, methane conversion of 14% was achieved at the temperature of 500 °C. Increasing the reaction temperature led to higher coke formation. Lower nickel content in the catalyst reduced the carbon formation. Consequently, with this type of catalyst methane conversion of 50% has been realized at the temperature of 800 °C.     

Oxidation of lean methane in a two-chamber preheat catalytic reactor

This paper presents the results of lean methane oxidation in a two-chamber preheat catalytic reactor. A preheat catalytic reactor was built, and the effects of the space velocity (3800 h^?1 to 8100 h^?1), the inlet methane concentration (0.6 vol.% to 0.8 vol.%) and the inlet temperature of catalytic oxidation bed (420 °C–540 °C) were experimentally investigated. The results showed that when the space velocity is low, the methane conversion rate maintains a high value. But when the space velocity is higher than 7100 h^?1, the methane conversion rate decreases dramatically. With the increases of the inlet methane concentration and the inlet temperature, the overall temperature of the oxidation bed increases rapidly, the temperature increment of the first catalytic ceramic layer increases, the methane conversion rate increases.     

Synthesis, characterization and catalytic performances of Ce-SBA-15 supported nickel catalysts for methane dry reforming to hydrogen and syngas

A series of Ce-incorporated SBA-15 mesoporous materials were synthesized through direct hydrothermal synthesis method and further impregnated with 12 wt.% Ni. The samples were characterized by ICP-AES, XRD, N₂ physisorption, XPS, TPR, H₂ chemisorption, TGA, temperature-programmed hydrogenation (TPH) and TEM measurements. The low-angle XRD and N₂ physisorption results showed the Ce successfully incorporated into the framework of SBA-15. The catalytic properties of these catalysts were investigated in methane reforming with CO₂. The Ce/Si molar ratio had a significant influence on the catalytic performance. The highest catalytic activity and long-term stability were obtained over the Ni/Ce-SBA-15 (Ce/Si = 0.04) sample. The improved catalytic behavior could be attributed to the cerium impact in the framework of SBA-15, where cerium promoted the dispersion of nano-sized Ni species and inhibited the carbon formation. In comparison with the effect of CeO₂ crystallites in SBA-15, cerium in the framework of SBA-15 promoted the formation of the nickel metallic particles with smaller size. The XRD and TGA results exhibited that carbon deposition was responsible for activity loss of Ni/SBA-15 and Ni/Ce-SBA-15 (Ce/Si = 0.06) catalysts. TEM results showed that the hexagonal mesopores of SBA-15 were still kept intact after reaction and the pore walls of SBA-15 prevented the aggregation of nickel.     

氢气预混合微尺度催化燃烧的数值模拟

通过耦合计算流体力学软件FLUENT和化学反应动力学软件CHEMKIN并采用空间气相和表面催化详细化学反应机理,对氢气和空气的预混合气体在微型管道内的催化燃烧过程进行了数值模拟,讨论了不同反应模型的燃烧特性以及预混合气体入口速度、当量比Φ和管径对催化燃烧反应的影响。计算结果表明：表面催化反应对空间气相反应有抑制作用;随着入口速度的增大,燃烧过程同时存在着表面催化反应和空间气相反应两种控制因素;当量比Φ和管径对氢气的催化燃烧过程有重要的影响。