Numerical Analysis of the Catalytic Combustion of Premixed Methane/Air Mixtures in Microtubes

The combustion characteristics and extinction limits for the catalytic combustion of a methane/air mixture in a microtube are investigated computationally using the commercial CFD code FLUENT coupled to an external subroutine DETCHEM. The effects of the microtube dimensions, conductivities of wall materials, external heat losses and flow velocity on the combustion stability, are also studied. The numerical model is set as either adiabatic or non‐adiabatic with a fixed exterior heat transfer coefficient. Numerical results indicate that thermal conductivity and wall thickness are vital to preheat the methane/air mixture through the conducting wall. Two types of extinction occur, i.e., thermal quenching and blow out. These extinction limits are characterized by wall surface temperature in the microtube and the ratio of Pt(s)/O(s).     

Combustion characteristics and flame stability at the microscale: a CFD study of premixed methane/air mixtures

A two-dimensional elliptic, computational fluid dynamics (CFD) model of a microburner is solved to study the effects of microburner dimensions, conductivity and thickness of wall materials, external heat losses, and operating conditions on combustion characteristics and flame stability. We have found that the wall conductivity and thickness are very important as they determine the upstream heat transfer, which is necessary for flame ignition and stability, and the material's integrity by controlling the existence of hot spots. Two modes of flame extinction occur: a spatially global type for large wall thermal conductivities and/or low flow velocities and blowout. It is shown that there exists a narrow range of flow velocities that permit sustained combustion within a microburner. Large transverse and axial gradients are observed even at these small scales under certain conditions. Periodic oscillations are observed near extinction in cases of high heat loss. Engineering maps that delineate flame stability, extinction, and blowout are constructed. Design recommendations are finally made.     

Transport limited pattern formation in catalytic monoliths

We consider the problem of flow in a tube with an exothermic surface reaction and show that the azimuthally symmetric steady-state can lose stability giving rise to patterned states with nonuniform concentration and temperature profiles. The primary cause for this transport limited pattern formation is the slow relative rate of heat and mass diffusion compared to surface reaction. Patterned states in which the temperature and concentration profiles vary in the azimuthal direction, can exist (or coexist with symmetric states) for all values of the fluid Lewis number (Le_f) though the patterned states are more pronounced and exist in a wider range of parameter space when Le_f<1. We analyze a three-dimensional model of catalytic monolith and develop analytical criteria for identifying the parameter regions in which patterned states exist. These criteria indicate that patterned states are formed whenever the local balance equations have multiple solutions and the characteristic reaction time is much smaller compared to the heat/mass diffusion time. Examination of the numerical values of the various parameters shows that most catalytic monoliths and combustors may operate in the region in which a large number of patterned states may exist. It is also found that when nonuniform and three-dimensional temperature and concentration fields exist, there can be hot spots in which the temperature exceeds the adiabatic temperature even when Le_f=1.     

Hot Spots in Energetic Materials

Porosity in high explosives increases the sensitivity against shock initiation and fast heating. An idealized energetic material is studied 3-dimensionally assuming that some mechanism like adiabatic heating of the pores by compression waves or the percolation of hot gases propagate in one direction like a plane wave with a constant or varying velocity. It induces successively hot spots distributed randomly or in regular structures. A conversion mechanism is included by an exothermal zero order reaction of Arrhenius type. The spreading of the heat input and the subsequent reactions depend on the energy input to the hot spots, the distance of the hot spots and the propagation velocity.     

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 radiant burner for stationary and mobile applications

In present discussions on energy conversion processes aimed at producing both thermal and process heat, catalytic burners provide an alternative approach for future applications. Catalytic burners are advantageous in that they cause only low pollutant emissions during the process of converting chemical energy into heat. In addition, novel engineering concepts require the complete combustion of a variety of fuels and fuel mixtures. Against this background, a novel catalytic radiant burner was developed at the Research Centre Jülich. Under near-stoichiometric conditions, this catalytic burner burns both natural gas with hydrogen admixture in a heat recovery boiler for stationary heat production and methanol with hydrogen admixture in a reformer producing process heat to be used in a fuel cell drive system. The emission data of the catalytic heater were recorded at a nominal power of 11.5 kW, a nominal air/fuel ratio of 1.15 and different hydrogen ratios between 0% and 50% and were 7–3 mg/kW h for carbon monoxide and 3.3–3.9 mg/kW h for nitrogen oxides. The test runs for a catalytic burner to be used for heating a compact reformer in a fuel cell vehicle were carried out at a power density of 15–60 kW/m², a nominal air/fuel ratio of 1.1 and different hydrogen, carbon dioxide and water ratios. For nitrogen oxides emissions of less than 0.4 mg/kW h, the measured carbon monoxide amount ranges between 0 and 13 mg/kW h.     

Simulation of a Reverse Flow Reactor for the Catalytic Combustion of Lean Methane Emissions

This work is focused on the performance prediction of pilot scale catalytic reverse flow reactors used for combustion of lean methane-air mixtures. An unsteady one-dimensional heterogeneous model for t...     

End wall effects on thermal stratification and heat transfer in a vertical enclosure with offset partitions

End wall effects on thermal stratification and heat transfer in a vertical enclosure with offset partitions has been studied by numerically solving the governing differential equations. Two limiting end wall conditions are investigated — adiabatic end walls and perfectly conducting end walls. Two offset partition positions and three partition heights are considered. It is observed that with adiabatic end walls, the heat transfer from the vertical hot and cold walls is always greater. The effect of end wall conditions is most significant when the top partition is offset toward the cold wall and the bottom partition toward the hot wall. In this position strong thermal stratification in the core is observed. When the direction of offset is reversed, i.e., top partition is moved closer to the hot wall and the bottom partition closer to the cold wall, strong stratification effects are noted in the partition-near side wall region. Adiabatic end wall conditions promote these stratification effects.     

Stability and performance of catalytic microreactors: Simulations of propane catalytic combustion on Pt

A pseudo-two-dimensional (2D) model is developed to analyze the operation of platinum-catalyzed microburners for lean propane-air combustion. Comparison with computational fluid dynamics (CFD) simulations reveals that the transverse heat and mass transfer is reasonably captured using constant values of Nusselt and Sherwood numbers in the pseudo-2D model. The model also reasonably captures the axial variations in temperatures observed experimentally in a microburner with a gap size. It is found that the transverse heat and mass transport strongly depend on the inlet flow rate and the thermal conductivity of the burner solid structure. The microburner is surface reaction limited at very low velocities and mass transfer limited at high velocities. At intermediate range of velocities (preferred range of reactor operation), mass transfer affects the microburner performance strongly at low wall conductivities, whereas transverse heat transfer affects stability under most conditions and has a greater influence at high wall conductivities. At sufficiently low flow rates, complete fuel conversion occurs and reactor size has a slight effect on operation (conversion and temperature). For fast flows, propane conversion strongly depends on residence time; for a reactor with gap size of , a residence time higher than 6 ms is required to prevent propane breakthrough. The effect of reactor size on stability depends on whether the residence time or flow rate is kept constant as the size varies. Comparisons to homogeneous burners are also presented.     

High vs. low temperature reforming for hydrogen production via microtechnology

Hydrogen production from a multifunctional microdevice consisting of thermally coupled catalytic plate combustion and reforming microreactors is simulated for methane and methanol reforming as representatives of a high and a low temperature process, respectively. Both reforming processes are feasible at microscales with high conversions over a wide combustible and reforming stream inlet velocity range, and can be tuned to provide variable power output. Interestingly, in low temperature reforming, the fraction of heat release that is wasted, as “excess enthalpy” in the products, is not significantly lower than in high temperature reforming at the breakthrough limit. Along the breakthrough line, more than 30% higher power efficiency to hydrogen is predicted for the methanol system due to the high CO₂:CO ratio. Finally, matching the reaction zones in the two channels via proper choice of catalyst loadings and channel gap sizes can alleviate hot spots and axial temperature gradients promoted by low conductivity materials.     

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.     

Application of catalytic combustion to a 1.5 MW industrial gas turbine

A catalytic combustor is described for a 1.5 MW gas turbine engine. The catalyst temperature is limited and the high combustor outlet temperatures required by the turbine are generated downstream of the catalyst. The combustor design places a low NO_x preburner upstream of the catalyst and uses this preburner to achieve optimum catalyst operation by providing the desired catalyst inlet temperature. The combustor system employs the catalyst during engine acceleration and loading. The catalyst design has been tested on a sub-scale rig under full pressure and flow conditions simulating turbine operation over the entire operating range including acceleration and loading. The design should achieve emissions at full load operation of <3 ppm NO_x and <10 ppm CO and UHC. Low emissions operation is expected over the 75–100% load range. In addition, long-term sub-scale rig test results are reported at simulated full load operating conditions including cyclic operation and full load trips.     

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.     

Encapsulated nano-heat-sinks for thermal management of heterogeneous chemical reactions

This paper describes a new way to control temperatures of heterogeneous exothermic reactions such as heterogeneous catalytic reaction and polymerization by using encapsulated nanoparticles of phase change materials as thermally functional additives. Silica-encapsulated indium nanoparticles and silica encapsulated paraffin nanoparticles are used to absorb heat released in catalytic reaction and to mitigate gel effect of polymerization, respectively. The local hot spots that are induced by non-homogenous catalyst packing, reactant concentration fluctuation, and abrupt change of polymerization rate lead to solid to liquid phase change of nanoparticle cores so as to avoid thermal runaway by converting energies from exothermic reactions to latent heat of fusion. By quenching local hot spots at initial stage, reaction rates do not rise significantly because the thermal energy produced in reaction is isothermally removed. Nanoparticles of phase change materials will open a new dimension for thermal management of exothermic reactions to quench local hot spots, prevent thermal runaway of reaction, and change product distribution.     

The catalytic heat exchanger using catalytic fin tubes

The characteristics of a catalytic heat exchanger which integrates heat generation and heat exchange into one equipment have been investigated by the experiment and numerical simulation. The surface of the fin tubes was catalyzed by the formation of the oxide layer and the subsequent washcoating of ZrO₂, followed by the impregnation of Pd catalyst. The experimental results showed that the performance of catalytic combustion in the catalytic heat exchanger was more significantly affected by the inlet velocity of the mixture than by its inlet temperature and equivalence ratio. It was also found that the catalytic surface area was a critical parameter to obtain the complete conversion of the mixture. Numerical simulation has been performed with a commercial software FLUENT. The calculated results indicated that the performance of the catalytic combustion was influenced by the catalytic fin configuration as well as the flow pattern of the mixture over the catalytic fins. The results recommend that the number and thickness of catalytic fins should be designed above 6 pieces/inch and less than to achieve the best performance in the catalytic heat exchanger.     

A catalytic diesel particulate filter with a heat recovery function

Based on a folded sheet design, we made and tested a miniature diesel particulate filter (DPF) that can transfer the heat generated by catalytic oxidation in the DPF to its upstream, thus promoting substantial temperature rise at the position where pieces of SiC felt working as PM filters are situated. When 0.6% of H₂, corresponding to 50 K in adiabatic temperature rise, was added to a 43 L/min of exhaust gas, the observed maximum temperature rise at the filter material exceeded 350 K, from which the heat recovery rate was estimated to be more than 86%. The PM filtration rates were 80–90%.     

Heat and mass transfer correlations and bifurcation analysis of catalytic monoliths with developing flows

We review and compare the literature correlations for estimating the heat and mass transfer coefficients as well as pressure drop in catalytic monoliths with simultaneously developing velocity, concentration and temperature profiles. We present accurate correlations for estimating the local Nusselt and Sherwood numbers for developing flows with constant flux (slow reaction) and constant wall concentration or temperature (fast reaction) cases for a channel of arbitrary shape. These new correlations need only a single parameter, namely, the asymptotic value, which depends on the channel geometric shape. We establish the accuracy of the proposed correlations by comparing the predicted values with the exact numerical values available for a few cases. We use the new correlations to analyze the effect of flow conditions near the inlet of the channel on the ignition and extinction behavior of catalytic monoliths used in combustion and after-treatment applications as well as laboratory experiments. It is shown that the bifurcation behavior, such as the number and location of the ignition/extinction points, the number of stable steady-states and the hysteresis locus is sensitive to the flow conditions in the entry region, and hence the heat and mass transfer correlations used, especially for large values of the transverse Peclet number (high space velocities or very short monoliths) or adiabatic temperature rise or when the axial catalyst loading is not uniform.     

Particle Motion in Simple Shear Flow with Gravity

The motion of aerosol particles in simple shear flow, subject to gravity, is analyzed. The combination of gravity and shear-induced lift is shown to give rise to particle drift. It is shown that in shear flow near a wall, when gravity points in the direction of flow, particles drift towards the wall, while for gravity pointing against the flow the drift is away from the wall. These results are also demonstrated experimentally, with fair qualitative agreement between analysis and experiments.     

Are 3‐D models necessary to simulate packed bed reactors? analysis and 3‐D simulations of adiabatic and cooled reactors

Simulations and analysis of transversal patterns in a homogeneous three‐dimensional (3‐D) model of adiabatic or cooled packed bed reactors (PBRs) catalyzing a first‐order exothermic reaction were presented. In the adiabatic case the simulation verify previous criteria, claiming the emergence of such patterns when (ΔT_ad/ΔT_m)/(Pe_C/Pe_T) surpasses a critical value larger than unity, where ΔT_ad and ΔT_m are adiabatic and maximal temperature rise, respectively. The reactor radius required for such patterns should be larger than a bifurcation value, calculated here from the linear analysis. With increasing radius new patterned branches, corresponding to eigenfunction of the problem emerge, whereas other branches become unstable. The maximal temperature of the 3‐D simulations may exceed the 1‐D prediction, which may affect design procedures. Cooled reactor may exhibit patterns, usually axisymmetric ones that can be characterized by two anomalies: the peak temperature may exceed the corresponding value of an adiabatic reactor and may increase with wall heat‐transfer coefficient, and the peak temperature in a sufficiently wide reactor need not lie at the center but rather on a ring away from it. In conclusions, we argue that transversal patterns are highly unlikely to emerge in practical adiabatic PBRs with a single exothermic reaction, as in practice Pe_C/Pe_T > 1. That eliminates patterns in stationary and downstream‐moving fronts, whereas patterns may emerge in upstream‐moving fronts, as shown here. This conclusion may not hold for microkinetic models, for which stationary modes may be established over a domain of parameters. This suggests that a 1‐D model may be sufficient to analyze a single reaction in an adiabatic reactor and a 2‐D axisymmetric model is sufficient for a cooled reactor. The predictions of a 2‐D cylindrical thin reactor with those of a 3‐D reactor were compared, to show many similarities but some notable differences. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Unsteady state treatment of very lean waste gases in a network of catalytic burners

Catalytic reactors in forced non-stationary operation enable autothermal VOC oxidation even at extremely low adiabatic temperature rise. A network of three reactors in series with large inert sections is presented as an alternative to the reverse-flow reactor.

A one-dimensional model for simulation of VOC oxidation in forced non-steady state packed bed catalytic reactors has been developed and implemented into a program that uses standard mathematical software. Numerical simulation reveals that the network of three catalytic reactors in series with large inert sections is a suitable design for VOC oxidation at low concentration. The system can be controlled by a simple set of two switches acting according to temperature setpoints.

Maximum temperature is very sensitive to heat transfer, which cannot be considered as infinitely fast despite small temperature gradients between gas and solid. These dependencies are somewhat less pronounced at higher load.