Catalytic combustion of lean methane–air mixtures

Catalytic combustion of lean methane–air mixtures was studied on supported iron oxide and platinum monolith catalysts. Flameless catalytic combustion was investigated in the temperature range 600–900°C, GHSV up to 10 000 h⁻¹, and methane concentration in initial gas mixture 1–9 vol%. It was shown that under certain process conditions complete combustion of methane at 4.5 vol% inlet methane concentration occurs.     

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.     

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.     

Effective approach to cyclic steady state in the catalytic reverse-flow combustion of methane

Computer-based simulations of a reverse-flow reactor should be carried out till the attainment of the so-called cyclic steady state. Usually this state is achieved by the method of direct dynamic simulations. In the paper of Unger et al. (Comput. Chem. Eng. 21 (1997) 5167.) special approaches, making use of various minimization algorithms based most often on Newton algorithms, are proposed. In the present paper one deals with a comparison and an appraisal of these methods, applied to the reverse-flow catalytic combustion of methane that occurs in coal-mine ventilation air.     

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.     

Deactivation of palladium catalyst in catalytic combustion of methane

Catalytic combustion of natural gas, for applications such as gas turbines, can reduce NO_x emissions. Palladium-on-stabilised alumina has been found to be the most efficient catalyst for the complete oxidation of methane to carbon dioxide and water. However, its poor durability is considered to be an obstruction for the development of catalytic combustion. This work was aimed at identifying the origin of this deactivation: metal sintering, support sintering, transformation or coking.

Catalytic combustion of methane was studied in a 15 mm i.d. and 50 mm length lab reactor and in a 25 mm i.d. pilot test rig on monolithic honeycomb substrates. Experiments were performed at GHSV of 50 000 h⁻¹ in lab test and 500 000 h⁻¹ in pilot test. The catalysts used were palladium on different supports on cordierite substrate. The catalysts were characterised by XRD, STEM, ATG and XPS.

In steady-state conditions, deactivation has been found to be dependent on the air/methane ratio, the palladium content on the washcoat and the amount of washcoat on the substrate. An oscillating behaviour of the methane conversion was even observed under specific conditions, due to the reducibility of palladium oxide PdO to Pd. The influence of the nature of the support on the catalyst deactivation was also investigated. It has been shown that some supports can surprisingly eliminate this oscillating behaviour. However, in pilot test, deactivation was found to be very rapid, even with stabilised alumina supports. Furthermore, successive tests performed on the same catalyst revealed that the activity (light-off temperature, conversion) falls strongly from one test to another.

Then, the stabilised alumina support was calcined at 1230°C for 16 h prior to its impregnation by palladium, in order to rule out its sintering. Experiments carried out on precalcined catalysts point out that deactivation is mostly correlated to the metal transformation under reaction conditions: activity decreases gradually as PdO sinters, but it dropped much more steeply in relation to appearance of metallic palladium.     

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.     

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.     

Modelling catalytic combustion in monolith reactors – challenges faced

When searching for a design concept in which a catalytic combustor is utilised, or looking for areas where improvements can be made to an existing design, then mathematical modelling is an important tool. However, models are only as good as the way in which the physico-chemical processes are modelled and the quality of the physical and chemical parameters (e.g. kinetic expressions, physical properties) acquired for use in the models. When selecting a basis for a model, there are many questions that need to be asked and answered by the developer of the chemical reaction engineering model of the catalytic combustor. Many challenges arise from having to make decisions on compromises that need to be made, and in recognising the consequences of such action. Examples of such challenges are outlined and, for some, clues are offered as to where the answers may lie. The examples include challenges in: the selection of appropriate kinetic expressions, recognition of the role that intraphase diffusion may play, the choice of pressure for catalytic kinetic and pilot scale studies, the selection of heat and mass transfer correlations, and the modelling of transients.     

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.     

Effect of the mode of heat withdrawal on the asymmetry of temperature profiles in reverse-flow reactors. Catalytic combustion of methane as a test case

Results of simulations are presented concerning a reverse-flow reactor for the catalytic combustion of methane that occurs in coal-mine ventilation air. Two variants of heat withdrawal are analysed. The simulations show that a relation exists between the method for heat withdrawal and the asymmetry in the profiles of the catalyst temperature over half-cycles of flow reversal. Too strong asymmetry impairs the efficient utilization of the heat produced. The results reveal that the recovery of heat by hot gas withdrawal from the central part of the reactor, and then, the introduction of the gas into a boiler wherein it is cooled to about 333 K (60 °C) guarantee more favourable symmetry of the half-cycle profiles and much better utilization conditions than the direct withdrawal of heat from the mid-section of the reactor (central cooling).     

Analysis of design sensitivity of flow-reversal reactors: Simulations, approximations and oxidation experiments

The analysis of a flow-reversal reactor, constructed from a catalytic bed imbedded within two inert beds and catalysing an instantaneous reaction, shows that the main parameters that determine the maximal reactor temperature are the thermodynamic parameters, heat loss through the walls and the conductivity and length of the inert zones. We show how these parameters can be easily extracted from the experimental data of very fast reactions, and demonstrate it for ethylene and for propane oxidation. We also derive an approximation for the maximal temperature for fast or slow reactions. These approximations are compared with experimental results obtained during propane (with low feed concentration) or methane oxidation.     

Stationary and traveling hot spots in the catalytic combustion of hydrogen in monoliths

In the course of catalytic combustion of hydrogen (1-5% H₂ in air) in monolith reactors, strongly localized stationary and traveling hot spots arise in response to a sudden and persistent rise of gas flow velocity. Such hot spots may occur, e.g. in a catalytic converter following the acceleration of a car or in a catalytic combustor as a result of a load increase. This phenomenon is illustrated by simulations using a two-phase reactor model. The temperature overshoot of the adiabatic limit is typically of the order of the adiabatic temperature rise itself.The following mechanism underlies this behavior. Light fuel is supplied to the catalytic wall by fast diffusion (in the direction perpendicular to flow), while the heat released by reaction is removed from the wall by the slower, mixture-averaged heat conduction. This leads to accumulation of heat at the catalytic surface that eventually saturates at high temperatures. The hot spots may exhibit intricate dynamics, propagating downstream or upstream, or they may remain stationary. The direction of propagation depends on the relative strength of convective downstream and conductive upstream contributions to the overall displacement of reaction fronts. Generally, the hot spot tends to drift downstream at low flow velocities, remain stationary at intermediate flow velocities, and drift upstream at high flow velocities.     

Simulation and interpretation of catalytic combustion experimental data

The catalytic combustion of CH₄/air in monoliths has been simulated through a commercial numerical fluid dynamic code. The program has been suitably modified in order to describe the heterogeneous reaction at the channel walls. Different flow arrangements have been studied in an attempt to closely match an experimental investigation reported in the literature. Single step overall rate equation has been used and identification of suitable kinetic constants performed through the use of optimization techniques. A framework for kinetic investigation accounting for complex flow structure and interaction with the chemistry is suggested. The relevant and sometimes overwhelming role of transport phenomena is discussed.     

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.     

Structured reactors for kinetic measurements under severe conditions in catalytic combustion over palladium supported systems

The collection of chemical kinetics data in catalytic combustion over very active palladium catalysts under conditions relevant to practical applications (e.g. gas turbine combustors) is extremely difficult, mainly due to strong exothermicity and very fast rate of combustion reactions. Within this purpose in this paper two types of laboratory structured reactors, which closely resemble industrial monolith catalysts, are investigated: (a) the annular reactor, consisting of a catalyst coated ceramic tube, co-axially placed in a quartz tube; (b) the metallic plate-type reactor, consisting of an assembled packet of metallic slabs coated with a ceramic catalytic layer.

The design of the annular reactor configurations for kinetic investigations is first addressed by mathematical modeling. The resulting advantages, including: (i) negligible pressure drops; (ii) minimal impact of diffusional limitations in high temperature–high GHSV experiments; (iii) effective dissipation of reaction heat are then experimentally demonstrated for the case of CH₄ combustion over a PdO/γ-Al₂O₃ catalyst with high noble metal loading (10% (w/w) of Pd).

The feasibility of a near-isothermal operation with the metallic plate-type reactor by an extremely effective dissipation of reaction heat through proper selection of highly conductive support material and of the geometry of the metallic slabs is finally discussed and experimentally demonstrated for the case of combustion of CO at high concentrations over a PdO/γ-Al₂O₃ (3% (w/w) of Pd) catalyst.     

Catalytic combustion over platinum group catalysts: fuel-lean versus fuel-rich operation

Performance data are presented for methane oxidation on alumina-supported Pd, Pt, and Rh catalysts under both fuel-rich and fuel-lean conditions. Catalyst activity was measured in a micro-scale isothermal reactor at temperatures between 300 and 800 °C. Non-isothermal (near adiabatic) temperature and reaction data were obtained in a full-length (non-differential) sub-scale reactor operating at high pressure (0.9 MPa) and constant inlet temperature, simulating actual reactor operation in catalytic combustion applications.

Under fuel-lean conditions, Pd catalyst was the most active, although deactivation occurred above 650 °C, with reactivation upon cooling. Rh catalyst also deactivated above 750 °C, but did not reactivate. Pt catalyst was active above 600 °C. Fuel-lean reaction products were CO₂ and H₂O for all three catalysts.

The same catalysts tested under fuel-rich conditions demonstrated much higher activity. In addition, a ‘lightoff’ temperature was found (between 450 and 600 °C), where a stepwise increase in reaction rate was observed. Following ‘lightoff’ partial oxidation products (CO, H₂) appeared in the mixture, and their concentration increased with increasing temperature. All three catalysts exhibited this behavior.

High-pressure (0.9 MPa) sub-scale reactor and combustor data are shown, demonstrating the benefits of fuel-rich operation over the catalyst for ultra-low emissions combustion.     

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.