Lean Methane Mixtures Research Articles

Methane Combustion Kinetics over Palladium-Based Catalysts: Review and Modelling Guidelines

Fugitive methane emissions account for a significant proportion of greenhouse gas emissions, and their elimination by catalytic combustion is a relatively easy way to reduce global warming. New and novel reactor designs are being considered for this purpose, but their correct and efficient design requires kinetic rate expressions. This paper provides a comprehensive review of the current state of the art regarding kinetic models for precious metal catalysts used for the catalytic combustion of lean methane mixtures. The primary emphasis is on relatively low-temperature operation at atmospheric pressure, conditions that are prevalent in the catalytic destruction of low concentrations of methane in emission streams. In addition to a comprehensive literature search, we illustrate a detailed example of the methodology required to determine an appropriate kinetic model and the constants therein. From the wide body of literature, it is seen that the development of a kinetic model is not necessarily a trivial matter, and it is difficult to generalize. The model, especially the dependence on the water concentration, is a function of not only the active ingredients but also the nature of the support. Kinetic modelling is performed for six catalysts, one commercial and five that were manufactured in our laboratory, for illustration purposes.

Ageing Studies of Pt- and Pd-Based Catalysts for the Combustion of Lean Methane Mixtures

This paper presents results obtained for the thermal and hydrothermal ageing of seven commercial precious metals-based catalysts for the combustion of methane. Experiments are performed in a large excess of oxygen representing lean conditions. Temperatures used are those typically found in lean burn compression ignition engines. The precious metals used were platinum, palladium and rhodium, present either singly or in combination. The most active catalyst contains a platinum and palladium mixture, with palladium being dominant. This catalyst was also the least affected by both thermal and hydrothermal ageing. The second most active catalyst contained only palladium, but this catalyst also demonstrated more susceptibility to ageing. The least active catalyst contained only platinum, although this catalyst was also the least affected by hydrothermal ageing. The addition of rhodium to either palladium or platinum–palladium catalysts caused a more rapid loss in activity at higher temperatures, although the loss in activity at lower temperatures was similar in magnitude to those catalysts without rhodium. In some cases, cycling the reactor temperature between high and low restored some activity to the catalyst. In all cases, the catalyst activity was observed to be lower in the presence of water, after both thermal and hydrothermal ageing.

Diagnostics of plasma formed during the oxidation of hydrocarbons in shock waves by electric probes with a conductive and dielectric surface

Experiments were carried out behind shock waves with recoding the electric current flowing through cylindrical probes with a conductive and dielectric surface under a negative (-9V) potential relative to the walls of the shock tube, with the displacement and total currents measured, respectively. Simultaneously, the signals of chemiluminescent emission from electronically excited OH* (λ = 308 nm) and CH* (λ = 430 nm) radicals were recorded. Experiments were carried out with various lean mixtures of methane and acetylene with oxygen diluted in argon. Preliminary calculations were performed using a theoretical model of an electric probe with a dielectric and conducting surface. Simulations showed that the displacement current on a probe under a negative potential with a dielectric surface is controlled by the chemical ionization rate, the surface area of the probe, and the electric potential. A close correlation was observed between the times of reaching the maximum of the probe current and the maximum of the chemiluminescent emission signals from the electronically excited OH* and CH* radicals.

Effect of carbon black nanoparticles on methane/air explosions: Influence at low initial turbulence

Nanoparticles are widely used in industrial applications as additives to modify materials properties such as resistance, surface, rheology or UV-radiation. As a consequence, the quantification and characterization of nanoparticles have become almost compulsory, including the understanding of the risks associated to their use. Since a few years ago, several studies of dust explosion properties involving nano-sized powder have been published. During the production and industrial use of nanoparticles, simultaneous presence of gas / vapor / solvents and dispersed nanoparticles mixtures might be obtained, increasing the risk of a hybrid mixture explosion. The aim of this work is to study the severity of the explosion of carbon black nanoparticles/methane mixtures and understand the influence of adding nanopowders on the behavior of the gas explosions. These results are also useful to understand the influence of soot on the efficiency of the gas combustion. Two grades of carbon black nanoparticles (ranging from 20 to 300 nm average diameter) have been mixed with methane. Tests have been performed on these mixtures in a standard 20 L explosion sphere. Regarding the scale precision, the lowest concentration of carbon black nanoparticles was set at 0.5 g.m-3. Tests were also performed at 2.5 g.m-3, which is still far below 60 g.m-3, the minimum explosive concentration of such powders previously determined in our laboratory. The influence of carbon black particles on the severity of the explosions has been compared to that of pure gas. It appears that the use of carbon black nanoparticles increases the explosion overpressure for lean methane mixtures at low initial turbulences by c. 10%. Similar results were obtained for high initial turbulent systems. Therefore, it seems that carbon black nanoparticles have an impact on the severity of the explosion even for quiescent systems, as opposed to systems involving micro-sized powders that require dispersion at high turbulence levels. Concerning the increment in the maximum rate of pressure rise, the addition of carbon black nanoparticles increased it by a factor of 1.15 in the case of lean fuel mixture. However, this behavior is only observed at high initial turbulence levels. The increment on the maximum rate of pressure rise is higher for powders with lower elementary particle diameter, which is notably due to the fragmentation phenomena that promotes the heat exchange.

Explosions of methane/air/nanoparticles mixtures: Comparison between carbon black and inert particles

Abstract During incomplete combustions or nano-size carbon blacks generation, atmospheres of carbonaceous nanopowders and combustible gases are encountered. These hybrid mixtures exhibit specific explosive behaviors, which can notably be caused by the modification of the initial turbulence level or by changes in oxidation reactions. In order to either support or reject such assertions, various nanoparticles/methane mixtures were tested, some with carbonaceous nanopowders, some with inert nanopowders (alumina). The aim of this work is then to compare the influences of alumina and carbon black nanoparticles insertion on the explosion severity and on the flame velocity of methane. Tests were performed in a 20 L explosion sphere and in a 1 m vertical flame propagation tube. An estimation of the unstretched flame velocity is obtained assuming a linear relationship between the burning velocity and Karlovitz stretch factor. It appears that the use of carbon black nanoparticles increases the explosion overpressure for lean methane mixtures by approximately 10%. Similar behaviors have been observed for hybrid mixtures involving alumina particles for fuel lean conditions. For alumina, non-significant changes are observed for fuel rich mixtures. Moreover, a considerable diminution of the explosion severity was noted for fuel rich mixtures when carbon black nanoparticles are dispersed into the reaction vessel. Regarding the flame propagation test for stoichiometric methane concentration, higher unstretched burning velocities were obtained for carbon black hybrid mixtures compared to alumina mixtures. These results suggest soot or carbonaceous nanopowders not only impact the oxidation kinetics, but also the flame stretching and heat transfer.

An efficient computational scheme for building operating maps for a flow reversal reactor

This paper describes the use of a two dimensional heterogeneous computational model for the catalytic reverse flow reactor for the combustion of lean methane mixtures. A uniform design method is used to generate a table of 41 operating points. The results from these points were used to generate a simple empirical model for the reactor at a single specified value of the superficial gas velocity. Stability maps that show the envelope for operation that satisfy a minimum level of conversion and a maximum operating temperature are generated using the empirical model. These maps delineate all possible combinations of methane concentration, switch time and mass extraction rate that are capable of giving stable operating conditions that meet these two criteria. The model is potentially useful in model predictive control of the reactor.

The stability of Co3O4, Fe2O3, Au/Co3O4 and Au/Fe2O3 catalysts in the catalytic combustion of lean methane mixtures in the presence of water

Nano-sized Co3O4, Fe2O3, Au/Co3O4 and Au/Fe2O3 catalysts were prepared and evaluated for catalytic combustion of lean methane-air mixtures. Characteristics and catalytic activities under dry and wet feed conditions were investigated at gas hourly space velocities up to 100000h−1 mimicking the typical flow and conversion requirements of a catalytic system designed to treat a ventilation air methane stream. In order to gain a better understanding of the interaction between H2O and the catalyst surface, temperature-programmed desorption of water over fresh and used samples were studied, and supported by other catalyst characterization techniques such as N2-adsorption desorption, XRD, TEM, SEM and XPS analyses. The activity measurements of the catalysts studied identify Co3O4 as the most active material. Co-precipitating gold particles with cobalt oxide or iron oxide do not enhance the activity of the catalyst, which is most likely due to blocking the active site of support by the gold particle. The presence of strong hydroxyl bonds on the catalyst surface is substantiated by TPD and XPS analyses, and is suggested to be responsible for the rapid deactivation of Fe2O3 and Au/Fe2O3 catalysts.

Experimental and modeling study on ignition delays of lean mixtures of methane, hydrogen, oxygen, and argon at elevated pressures

Ignition delays of lean mixtures of methane–hydrogen with various hydrogen volumetric contents were experimentally studied in a shock tube together with modeling analysis. Results show that the ignition behavior of the methane–hydrogen mixture depending on pressure resembles that of methane for hydrogen fraction less than 40%, with the ignition delays decreasing with increasing pressure. For the hydrogen fraction equal 60%, a negligible promoted effect of pressure on the ignition of the methane–hydrogen mixture is exhibited. For hydrogen fractions equal or greater than 80%, however, the ignition response resembles that of hydrogen in that the ignition delay exhibits a complex dependence on pressure and two-step transition in the global activation energy. Compared with calculated values using four available mechanisms, the NUI Galway mechanism yielded the closest agreement, and was adopted in the sensitivity analysis of the ignition kinetics. The sensitivity analysis well explained the experimental results which the ignition delay decreases with increasing temperature regardless of whether methane (typical fuel 80%CH 4/20%H 2) or hydrogen (typical fuel 20%CH 4/80%H 2) dominates the ignition process. Rate of production analysis shows that the promoted effect of the hydrogen on the oxidation of the methane is mainly due to the concentrations of the free radicals such as H, O and OH increase with increasing hydrogen fraction, and lead to the total reaction rate is enhanced. Consumption of methane is mainly through these reactions in which the active free radicals participate.

Modeling catalytic oxidation of lean mixtures of methane–air in a packed-bed reactor

Abstract A mathematical model developed for simulation of combustion of preheated lean homogeneous methane–air mixtures in an adiabatic catalytic packed bed is described. The unsteady one-dimensional model includes inter-phase heat and mass transfers, longitudinal mass dispersion in the gas phase and longitudinal conduction and radiation heat transfer modes. The solid and gas phases are assumed not to be in local thermal equilibrium. Multi-step reaction mechanisms are employed for both the surface and gas phase chemical reactions. The surface reaction mechanism for the oxidation of methane on Pt includes 36 elementary surface reactions and 11 surface adsorbed species. The gas phase reaction mechanism includes 28 elementary reactions and 21 species. The governing equations are the unsteady equations of conservation of mass, chemical species and separate energy equations for solid and gas phases. These equations were solved using the commercial CFD code ‘Fluent’ and a number of modifying subroutines especially are developed to include the equations of surface reactions in the computations. The model has been used to investigate the effects of operational conditions such as the mixture inlet temperature (600–1250 K), approach velocity (0.5–15 m/s) and equivalence ratio (0.15–0.5) on the oxidation of methane within both the catalytic and non-catalytic packed-bed reactors under adiabatic conditions. The calculated values of methane conversion showed good agreement with the corresponding available experimental data. Moreover the amount of heat release at different inlet velocities was calculated and the optimum value discussed.

Studies on combustion catalytic activity of some pure and doped lanthanum cobaltites

The aim of this work is to compare the catalytic activity of some pure and doped lanthanum cobaltites, prepared by solution combustion technique, with a Pt/Al2O3 commercial catalyst. As test reaction we used the combustion of lean methane mixtures. The experimental data evidenced a significantly better catalytic activity of a cerium-doped lanthanum cobaltite, prepared by using α-alanine as fuel. The combustion data, for the lean methane mixtures we tested, are well fitted by a first-order kinetic expression. The calculated activation energy values are in good agreement with the published data.

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.

MODEL PREDICTIVE CONTROL OF A CATALYTIC FLOW REVERSAL REACTOR WITH HEAT EXTRACTION

This paper presents the formulation of a controller for a Catalytic Flow Reversal Reactor (CFRR) with heat extraction. The controller is based on the Model Predictive Control (MPC) concept. The MPC scheme uses a model that assumes plug flow and neglects radial gradients in the reactor but accounts for the two phases within the reactor. The prediction of the future output behavior from the model is obtained by using the Method of Characteristics as proposed by Shang et al. (2004) for convection dominated distributed parameter systems. The formulated controller is applied to a CFRR unit for the catalytic oxidation of fugitive lean methane mixtures. The objective of the control algorithm is to maintain stable reactor operation, while extracting the maximum amount of useful energy by hot gas removal from the mid-section of the reactor. Simulations are used to show the performance of the designed controller.

04/02271 Flow reversal reactor for the catalytic combustionof lean methane mixtures: Salomons, S. et al. Catalysis Today, 2003, 83, (1–4), 59–69

04/02271 Flow reversal reactor for the catalytic combustionof lean methane mixtures: Salomons, S. et al. Catalysis Today, 2003, 83, (1–4), 59–69

Modelling a reverse flow reactor for the catalytic combustion of fugitive methane emissions

This paper describes the development and validation of a computer simulator for the modelling of a reverse flow catalytic reactor for the combustion of lean mixtures of methane in air. The simulator uses a heterogeneous two dimensional model for the reactor. The reactor uses a packed bed for catalytic sections and ceramic monoliths for inert sections, although the simulator is written in a general fashion so that any combination of packing can be used, in any desired variety. Validation is performed using a 200 mm internal diameter reactor over various flowrates and methane concentrations. The reverse flow reactor is observed to yield stable auto-thermal operation even for low methane concentrations. Higher methane concentrations are observed to give dual temperature peaks in the reactor. The transfer of energy is observed to be a significant factor in the reactor operation, which is shown by comparison of the heterogeneous model to a pseudo-homogeneous reactor model. The simulator can model the pilot reactor in real time for typical operating conditions.

Modelling a reverse flow reactor for the catalytic combustion of fugitive methane emissions

This paper describes the development and validation of a computer simulator for the modelling of a reverse flow catalytic reactor for the combustion of lean mixtures of methane in air. The simulator uses a heterogeneous two dimensional model for the reactor. The reactor uses a packed bed for catalytic sections and ceramic monoliths for inert sections, although the simulator is written in a general fashion so that any combination of packing can be used, in any desired variety. Validation is performed using a 200 mm internal diameter reactor over various flowrates and methane concentrations. The reverse flow reactor is observed to yield stable auto-thermal operation even for low methane concentrations. Higher methane concentrations are observed to give dual temperature peaks in the reactor. The transfer of energy is observed to be a significant factor in the reactor operation, which is shown by comparison of the heterogeneous model to a pseudo-homogeneous reactor model. The simulator can model the pilot reactor in real time for typical operating conditions.

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.

Transient behaviour and start-up of periodic flow reversal reactors for catalytic decontamination of waste gases

A full dynamic one-dimensional model for the simulation of the catalytic reverse-flow reactor and a code for its solution have been developed. A wide set of simulations has been carried out for the combustion of lean mixtures of methane. The influence of changes in the design parameters (bed length, cycle period and fraction of inert material) and in the operating parameters (inlet concentration and flow rate) on the transient starting has been investigated in detail.

Numerical studies of wall effects with laminar methane flames

Wall effects in the combustion of lean methane mixtures have been studied numerically using the CHEMKIN software. To gain a deeper understanding of the flame-wall interaction in lean burn combustion, and in particular the kinetic and thermal effects, we have simulated lean and steady methane/air flames in a boundary layer flow. The gas-phase chemistry is modeled with the GRI mechanism version 1.2. Boundary conditions include an inert wall, a recombination wall and catalytic combustion of methane. Different pressures, wall temperatures and fuel-air ratios are used to address questions such as which part of the wall effects is most important at a given set of conditions. As the results are analyzed it can be seen that the thermal wall effects are more significant at the lower wall temperature (600 K) and the wall can essentially be modeled as chemical inert for the lean mixtures used. At the higher wall temperature (1,200 K), the chemical wall effects become more significant and at the higher pressure (10 atm) the catalytic surface retards homogeneous combustion of methane more than the recombination wall because of product inhibition. This may explain the increased emissions of unburned fuel observed in engine studies, when using catalytic coatings on the cylinder walls. The overall wall effects were more pronounced for the leaner combustion case (φ = 0.2). When the position of the reaction zone obtained from the boundary layer calculations is compared with the results from a one-dimensional premixed flame model, there is a small but significant difference except at the richer combustion case (φ = 0.4) at atmospheric pressure, where the boundary layer model may not predict the flame position for the given initial conditions.

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 60h of operation under reaction conditions, but after 120h 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.

Simulation of the ignition of lean methane mixtures using CFD modelling and a reduced chemistry mechanism

This paper applies a recently developed coupled chemistry/hydrodynamics model to the study of combustion ignition. The problem studied is that of determination of the location of the ignition region for a lean methane mixture (1 vol%) at a low temperature (1000–1200 K). This is achieved by solving a skeletal chemical mechanism, involving 16 species and 28 reactions, together with hydrodynamic equations using the CFX4 model. Once validated, the model is used to demonstrate how such modelling can be used in the design of combustors to utilize low concentration methane mixture from coal mines or waste dumps.