Flow reversal reactor for the catalytic combustion of lean methane mixtures

This paper describes an experimental investigation of a pilot scale reverse flow reactor for the catalytic destruction of lean mixtures of methane in air. It was found that using reverse flow it was possible maintain elevated reactor temperatures which were capable of achieving high methane conversion of methane in air streams at methane concentrations as low as 0.19% by volume. The space velocity, cycle time and feed concentration are all important parameters that govern the operation of the reactor. Control of these parameters is important to prevent the trapping of the thermal energy within the catalyst bed, which can limit the amount of energy that can be usefully extracted from the reactor.     

Optimization of a flow reversal reactor for the catalytic combustion of lean methane mixtures

This paper describes a parametric study of a catalytic flow reversal reactor used for the combustion of lean methane in air mixtures. The effects of cycle time, velocity, reactor diameter, insulation thickness, thermal mass and thermal conductivity of the inert sections are studied using a computer model of the system. The effects on the transient behaviour of the reactor are shown. Emphasis is placed on the effects of geometry from a scale-up perspective. The most stable system is obtained when the thermal mass of the inert sections is highest, while thermal conductivity has only a minor effect on reactor temperature. For a given operation, the stationary state depends on the combination of velocity and switch time. Provided that complete conversion is achieved, highest reactor temperature is achieved with the highest switch time. The role of the insulation is not only to prevent heat loss to the environment, but also to provide additional thermal mass. During operation heat is transfer to and from the insulation. The insulation effect leads to higher reactor temperature up to a maximum thickness. The insulation effect diminishes as the reactor diameter increases, and results in higher temperatures at the centreline.     

Catalytic combustion assisted methane steam reforming in a catalytic plate reactor

A theoretical study of methane steam reforming coupled with methane catalytic combustion in a catalytic plate reactor (CPR) based on a two-dimensional model is presented. Plates with coated catalyst layers of order of micrometers at distances of order of millimetres offer a high degree of compactness and minimise heat and mass transport resistances. Choosing similar operating conditions in terms of inlet composition and temperature as in industrial reformer allows a direct comparison of CPRs with the latter. It is shown that short distance between heat source and heat sink increases the efficiency of heat exchange. Transverse temperature gradients do not exceed across the wall and across the gas-phase, in contrast to difference in temperature of outside wall and mean gas phase temperature inside the tube usually observed in conventional reformers. The effectiveness factors for the reforming chemical reactions are about one order of magnitude higher than in conventional processes. Minimisation of heat and mass transfer resistances results in reduction of reactor volume and catalyst weight by two orders of magnitude as compared to industrial reformer. Alteration of distance between plates in the range 1- does not result in significant difference in reactor performance, if made at constant inlet flowrates. However, if such modifications are made at constant inlet velocities, conversion and temperature profiles are considerably affected. Similar effects are observed when catalyst layer thicknesses are increased.     

Catalytic combustion of methane in non-permselective membrane reactors with separate reactant feeds

Catalytic combustion of methane over Pd and Pt/SiO₂/-Al₂O₃ membranes was studied in the temperature range 300–650 °C. Fuel and oxygen were fed at opposite membrane sides. In order to improve reactor controllability the -Al₂O₃ membranes were impregnated with SiO₂ sol resulting to smaller pore size. Methane conversions up to 100% for the palladium membrane and up to 42% for the platinum membrane were achieved. The results indicated a transition from kinetic to mass transfer control within the temperature range investigated. This was accompanied by reduction of methane slip from tube to shell side with increasing temperature. CO and H₂ were detected in the product gases of the palladium membrane. Their concentration could be reduced by applying a trans-membrane pressure difference. Low concentrations of CO were observed for the Pt/SiO₂/-Al₂O₃ membrane, while no CO or H₂ were detected for a Pd/-Al₂O₃ membrane operating in dead-end configuration.     

Experimental and modelling studies of the catalytic combustion of methane

Ceramic honeycomb monoliths with a noble metal-alumina based washcoat were used as burners for the combustion of very lean methane-air mixtures below the conventional lower flammability limit without the emission of CO, NO_x, or unburned fuel gas. Measurements and modelling in the steady state proved that the near zero emissions could have been equally due to gas phase combustion than to catalytic combustion for the long monoliths. However, only catalytic oxidation reactions could account for the complete and clean combustion observed for the shortest burners, indicating that even in the longest monoliths, the combustion had been catalytic. Thus the onset of gas phase combustion was inhibited by catalytic combustion. This phenomenon was investigated using numerical modelling and experimental studies on a catalytic stagnation point flow reactor, with a polycrystalline Pt foil as the catalyst. These studies showed the extent of the phenomenon of inhibition of gas phase ignition and how catalytic combustion is an extremely stable and clean process.     

Catalytic combustion of methane over mixed oxides derived from Co-Mg/Al ternary hydrotalcites

Co_xMg_3 − x /Al composite oxides (xCoMAO-800) were prepared by calcination of Co_xMg_3 − x/Al hydrotalcites (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, respectively) at 800 °C. The materials were characterized using XRD, TG-DSC, N₂ adsorption-desorption and TPR. The methane catalytic combustion over the xCoMAO-800 was assessed in a fixed bed micro-reactor. The results revealed that cobalt can be homogenously dispersed into the matrices of the hydrotalcites and determines the structure, specific surface areas and porosity of the derived xCoMAO-800 oxide catalysts. The thermal stability and homogeneity of the hydrotalcites markedly depends on the cobalt concentration in the hydrotalcites. The Co-based hydrotalcite-derived oxides exhibit good activity in the catalytic combustion of methane. The catalytic activity over the xCoMAO-800 oxides enhances with increasing x up to 1.5, but subsequently decreases dramatically as cobalt loadings are further increased. The 1.5CoMAO-800 catalyst shows the best methane combustion activity, igniting methane at 450 °C and completing methane combustion around 600 °C. The catalytic combustion activity over the xCoMAO-800 oxides are closely related to the strong Co-Mg/Al interaction within the mixed oxides according to the TG-DSC, TPR and activity characteristics.     

Catalytic combustion of methane over bimetallic catalysts a comparison between a novel annular reactor and a high-pressure reactor

The effects of adding a co-metal, Pt or Rh, to Pd/γ-Al₂O₃ catalysts were studied with respect to the catalytic activity for methane combustion and compared to a Pd/γ-Al₂O₃ catalyst, using both a pressurized pilot-scale and a lab-scale annular reactor. Temperature programmed oxidation (TPO) experiments were also carried out to investigate the oxygen release/uptake of the catalyst materials. Palladium showed an unstable behavior both in the pilot and lab-scale experiments at temperatures well below the PdO to Pd transformation. An addition of Pt to Pd stabilized, and in some cases increased, the catalytic activity for methane combustion.

The TPO experiments showed that the oxygen release peak was shifted to lower temperatures even for low additions of Pt, i.e. Pd:Pt=2:1. For additions of rhodium only small beneficial effects were seen. The steady-state behavior of the lab-scale annular reactor correspond well to the pressurized pilot-scale tests.     

Investigation of oxy-fuel combustion for methane and acid gas in a diffusion flame

Co-combustion of methane(CH4)and acid gas(AG)is required to sustain the temperature in Claus reac-tion furnace.In this study,oxy-fuel combustion of methane and acid gas has been experimentally studied in a diffusion flame.Three equivalence ratios(ER=1.0,1.5,2.0)and CH4-addition ratios(CH4/AG=0.3,0.5,0.7)were examined and the flame was interpreted by analyzing the distributions of the temperature and species concentration along central axial.CH4-AG diffusion flame could be classified into three sec-tions namely initial reaction,oxidation and complex reaction sections.Competitive oxidation of CH4 and H2S was noted in the first section wherein H2S was preferred and both were mainly proceeding decom-position and partial oxidation.SO2 was formed at oxidation section together with obvious presence of H2 and CO.However,H2 and CO were inclined to be sustained under fuel rich condition in the complex reac-tion section.Reducing ER and increasing CH4/AG contributed to higher temperature,H2S and CH4 oxida-tion and CO2 reactivity.Hence a growing trend for CH4 and AG to convert into H2,CO and SO2 could be witnessed.And this factor enhanced the generation of CS2 and COS in the flame inner core by interactions of CH4 and CO2 with sulfur species.COS was formed through the interactions of CO and CO2 with sulfur species.The CS2 production directly relied on reaction of CH4 with sulfur species.The concentration of COS was greater than CS2 since CS2 was probably inhibited due to the presence of H2.COS and CS2 could be consumed by further oxidation or other complex reactions.     

Complete oxidation of methane at low temperature over Pt and Pd catalysts for the abatement of lean-burn natural gas fuelled vehicles emissions: influence of water and sulphur containing compounds

The catalytic activity of fresh Pd and Pt catalysts supported on γ-alumina in the complete oxidation of CH₄ traces under lean-burn conditions was studied in the presence or the absence of water or H₂S. Steam-aged catalysts were also studied in order to simulate long-term ageing in real lean-burn natural gas fuelled vehicles (NGVs) exhaust conditions. Without water or H₂S added to the feed, Pd catalysts exhibit a superior catalytic activity in methane oxidation compared to Pt ones, whatever the catalysts were fresh or aged. The addition of 10 vol.% water vapour to the feed strongly affects the activity of the fresh Pd catalyst, thus being only slightly more efficient than the fresh Pt one. H₂S has a strong poisoning effect on the catalytic activity of Pd catalysts, while Pt catalysts are more resistant. The fresh H₂S-poisoned Pd/Al₂O₃ catalyst was studied by TPD in O₂/He. Poisoning species decompose above 873 K as SO₂ and O₂ in relative concentrations consistent with the decomposition of surface sulphate species. However, a treatment in O₂/He at temperatures as high as 923 K does not allow the complete regeneration of the catalytic activity of H₂S-poisoned Pd/Al₂O₃. A mechanism involving the poisoning of PdO by sulphate species is proposed. Different diffusion processes by which these sulphate species can migrate back and forth between PdO and the support, depending on the experimental conditions, are suggested.     

Cu/γ-Al2O3 catalyst for the combustion of methane in a fluidized bed reactor

The applicability of a catalyst based on copper dispersed on γ-Al₂O₃ spheres (1 mm diameter) for fluidized bed catalytic combustion of methane has been assessed. Catalyst properties have been determined by physico-chemical characterization techniques and fixed bed activity tests revealing the presence of a surface CuAl₂O₄ spinel phase, still active and stable in methane combustion after repeated thermal ageing treatments at 800 °C. Methane catalytic combustion experiments have been performed in a 100 mm premixed fluidized bed reactor under lean conditions (0.15–3% inlet methane concentration), showing that complete CH₄ conversion can be attained below 700 °C in a fluidized bed of 1 mm solids with a gas superficial velocity about twice the incipient fluidization velocity.     

Auto-cyclic reactor: Design and evaluation for the removal of unburned methane from emissions of natural gas engines

A non-adiabatic fixed bed auto-cyclic reactor (ACR) consisting of two counter-current concentric compartments was designed and built for removing low concentrations of methane from exhaust gases from natural gas engines. The length was based on simulations by a simple heterogeneous one dimensional model using literature parameters and kinetic data, while the diameter was selected to assure a linear fluid velocity between 0.5 and 2 m/s. Its innovative design consists of a judicious combination of 14 longitudinal fins welded to the outlet part of inner reactor compartment to maximize the heat transfer to the inlet section and highly active pellet type catalyst filling the space between fins to lower the ignition temperature.The experimental ACR pilot unit was loaded by a combination of highly active laboratory prepared catalysts: palladium/alumina pellets and palladium/alumina coated cordierite monoliths. The efficiency of methane removal from air and from synthetic exhaust gas containing 7 vol% CO₂ and 14 vol% H₂O was evaluated under a wide range of operating conditions: temperature from 290 to 500 °C, methane concentration between 500 and 3800 ppm. The reactor performance was monitored in terms of axial temperature profiles and methane conversion both in transient and steady state conditions.Reproducible performance of the ACR was observed even after 1200 h of cumulative operation and complete methane removal was obtained at relatively low temperatures.To simulate the obtained experimental data, a heterogeneous one-dimensional model was developed to suit the final reactor configuration using actual laboratory determined kinetic data. The model described adequately the experimental temperature profiles and methane conversion when heat transfer between the reactor compartments and heat loss were taken into account.     

Combustion of methane lean mixtures in reverse flow reactors: Comparison between packed and structured catalyst beds

The scope of this work is to compare systematically the performance of particle beds and monolithic beds in catalytic reverse flow reactors used for combustion of lean methane/air mixtures, using alumina-supported palladium as catalyst. Different values of gas surface velocity (0.1–0.3 m/s), particle diameter (3–6 mm, for particle bed), cell density (200–400 cpsi, for structured bed) and catalyst/inert ratio (0.4–1) were used for the simulation of the combustion of 3500 ppm methane in both kinds of reverse flow reactor. An unsteady one-dimensional heterogeneous model has been developed and solved using a MATLAB code. The model, physical parameters and transport properties used had been experimentally validated in a previous work, operating with a particle bed reverse flow reactor. Results obtained indicate that the reverse flow reactor is more stable when the catalyst particle beds are use, although the difference with the monolith bed decreases as surface velocity increases. In contrast, pressure drops in the bed are higher for the particle bed.     

Transient behaviour of perovskite-based monolithic reactors in the catalytic combustion of methane

The transient behaviour of perovskite-based catalysts prepared via active phase dispersion on La/γ-Al₂O₃ washcoated cordierite monoliths has been investigated in the autothermal combustion of lean methane mixtures. During start-up and shut-down operations, the reaction front moves from the outlet towards the inlet (ignition) or vice versa (extinction), with a time scale significantly higher than space time. The CH₄/O₂/N₂ feed mixture is completely converted to CO₂ and H₂O provided its inlet temperature is about 500°C, a value not affected by catalyst length and gas flow rate, the phenomenon being kinetically controlled. Gas flow rate significantly affects solid steady-state temperature, as at higher flow rates the thermal power produced by combustion is higher in comparison with heat losses by radiation and conduction and temperature rise is closer to the adiabatic value. The fresh catalysts weakly deactivate during the first 60 h of operation under reaction conditions, but after 120 h the activity is still very high and not significantly affected by further ageing. The transient behaviour of the system has been simulated by a mathematical model, characterised by an increased solid thermal conductivity to take into account the relevant contribution of internal radiation between channel surfaces.     

Al-MCM-41 supported palladium catalyst for methane combustion: Effect of the preparation methodologies

This work aimed at elucidating the beneficial effect of plasma treatment on the catalytic performance of palladium (Pd) catalysts in methane combustion with the ordered mesoporous molecular sieve Al-MCM-41 as the model support. The plasma treated Pd/Al-MCM-41 catalyst exhibited a higher initial activity and a better stability in comparison with the untreated counterpart catalyst. To clarify the plasma effect, the catalysts were characterized by N₂ sorption analysis, X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH₃-TPD), pyridine adsorption-infrared spectroscopy (Py-IR), high resolution-transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (CH₄-TPR) experiments. The results obtained confirmed that palladium oxide (PdO) was the active phase. Plasma treatment enhanced the acidity of catalyst and improved the dispersion of PdO particles, which lead to a higher initial activity. The better stability for plasma treated Pd-based catalyst was proved to be closely related to the stronger interaction between palladium oxide and the molecular sieve support. In addition, the sintering of PdO particles over the plasma treated catalyst was not significant during the stability test. These findings may provide useful guidelines for further catalyst design for methane combustion.     

Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century

The very large reserves of methane, which often are found in remote regions, could serve as a feedstock for the production of chemicals and as a source of energy well into the 21st century. Although methane currently is being used in such important applications as the heating of homes and the generation of hydrogen for ammonia synthesis, its potential for the production of ethylene or liquid hydrocarbon fuels has not been fully realized. A number of strategies are being explored at levels that range from fundamental science to engineering technology. These include: (a) stream and carbon dioxide reforming or partial oxidation of methane to form carbon monoxide and hydrogen, followed by Fischer–Tropsch chemistry, (b) the direct oxidation of methane to methanol and formaldehyde, (c) oxidative coupling of methane to ethylene, and (d) direct conversion to aromatics and hydrogen in the absence of oxygen. Each alternative has its own set of limitations; however, economical separation is common to all with the most important issues being the separation of oxygen from air and the separation of hydrogen or hydrocarbons from dilute product streams. Extensive utilization of methane for the production of fuels and chemicals appears to be near, but current economic uncertainties limit the amount of research activity and the implementation of emerging technologies.     

Homogeneous vs. catalytic combustion of lean methane—air mixtures in reverse-flow reactors

To carry out a comparative assessment of a recently proposed idea of using thermal flow-reversal reactors (TFRR) for mine ventilation air, the results for the catalytic flow-reversal reactor (CFRR) investigated within the European Project (2003) are briefly presented. Next, experimental investigations of thermal combustion are presented in this paper. These consisted of the kinetic study of homogeneous combustion in the pelletized bed and in the monolith. Kinetic equations for the two cases are derived and discussed. Experimental autothermal reverse-flow operation in a laboratory setup was performed. Due to the high heat capacity of the wall and insulation of the pelletized bed reactor, with considerable heat losses to the surroundings, autothermal operation was successful only in the monolithic reactor. It is finally concluded that the thermal combustion can be competitive compared with the catalytic oxidation.     

Microemulsion synthesis of MgO-supported LaMnO3 for catalytic combustion of methane

Catalysts with 20% LaMnO₃ supported on MgO have been prepared via CTAB-1-butanol-iso-octane-nitrate salt microemulsion. The preparation method was successfully varied in order to obtain different degrees of interaction between LaMnO₃ and MgO as shown by TPR and activity tests after calcination at 900 °C. Activity was tested on structured catalysts with 1.5% CH₄ in air as test gas giving a GHSV of 100,000 h⁻¹. The activity was greatly enhanced by supporting LaMnO₃ on MgO compared with the bulk LaMnO₃. After calcination at 1100 °C both the surface area and TPR profiles were similar, indicating that the preparation method is of little importance at this high temperature due to interaction between the phases. Pure LaMnO₃ and MgO were prepared using the same microemulsion method for comparison purposes. Pure MgO showed an impressive thermal stability with a BET surface area exceeding 30 m²/g after calcination at 1300 °C. The method used to prepare pure LaMnO₃ appeared not to be suitable since the surface area dropped to 1.1 m²/g already after calcination in 900 °C.     

Al18B4O33 aluminium borate: A new efficient support for palladium in the high temperature catalytic combustion of methane

Well crystallised aluminium borate Al₁₈B₄O₃₃ has been synthesised from alumina and boric acid with a BET area of 18 m²/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al₁₈B₄O₃₃ was prepared by conventional impregnation of Pd(NO₃)₂ aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al₁₈B₄O₃₃ in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al₂O₃. Pd/Al₁₈B₄O₃₃ exhibited a much lower activity than Pd/Al₂O₃ when treated in hydrogen at 500 °C or aged in O₂/H₂O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al₁₈B₄O₃₃ which became as active as Pd/Al₂O₃. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al₂O₃ and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al₁₈B₄O₃₃. The existence of two distinct types of PdO species formed on Al₁₈B₄O₃₃ and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O₂.     

High temperature stable magnesium oxide catalyst for catalytic combustion of methane: A comparison with manganesesubstituted barium hexaaluminate

The activity of magnesium oxide for catalytic combustion of methane was examined and the results were compared with experimental results for manganese-substituted barium hexaaluminate. The catalysts were calcined at temperatures up to 1 500°C and the effects of temperature, space velocity and calcination temperature were examined. The catalysts were also characterized with BET and XRD. For magnesium oxide calcined at 1 100°C the ignition temperature T_10% was decreased by 270°C compared to the non-catalyzed reaction. For the same catalyst T_50% was measured to be 795°C. The corresponding temperature for the hexaaluminate was 640°C. The difference between the two catalysts decreased after calcination at 1 500°C. For the magnesium oxide the influence of catalytically initiated homogeneous gas phase reactions was studied by varying the post catalytic volume of the reactor (and hence the residence time in the heated zone after the catalyst). It was shown that these catalytically initiated homogeneous gas phase reactions are significant for the methane conversion.