Methane-air System Research Articles

Operating strategies for thermally coupled combustion-decomposition catalytic microreactors for hydrogen production

Aiming towards COx-free hydrogen production for fuel cell applications, the catalytic decomposition of ammonia over ruthenium coupled with the steady catalytic combustion of fuel-lean propane-air mixtures in multifunctional microreactors consisting of alternating endothermic and exothermic reaction channels was investigated numerically. Comparisons with methane-air systems were also made. A two-dimensional CFD (computational fluid dynamics) model was developed, and numerical studies were performed to evaluate the effect of main design parameters and to provide guidelines for optimal design. Operating strategies were provided, and operating diagrams were constructed by different operating lines. Furthermore, the attainable operating region delimited from the breakthrough limit, material stability limit, self-sustained operation limit, and maximum power output limit was mapped out. It is shown that ammonia decomposition at millisecond contact times is feasible. High-conductivity materials are preferable due to their rather wide operating region, whereas low-conductivity materials result in slightly increased ammonia conversion at the expense of hot spot (temperature peak) formation. Sufficiently high ammonia flow velocities serve a dual purpose by improving hydrogen yields and reducing reactor temperatures. Higher hydrocarbons not only can extend the flow velocity operating region but also can improve the hydrogen yield; however, at high ammonia conversions, the choice of hydrocarbon fuel is immaterial when using suitable compositions to maintain the same energy input. The co-current flow configuration should be preferred due to its rather wide operating region and the improved thermal coupling, although co- and counter-current systems are essentially equivalent for highly conductive materials. Finally, channel gap sizes should be small enough to minimize mass-transfer effects.

Influence of oxygen on the resonant photoacoustic signal from methane excited at the ν 3 mode

This paper shows that, when samples of methane in dry air are irradiated with an OPO tuned at the ν 3 mode at 3019 cm−1, the photoacoustic signal is lower than in mixtures in pure nitrogen. In order to explain this change, we developed a rate-equation-based model which describes the methane excitation and relaxation to heat by collisions with the buffer. In particular, we reduced the problem to a system of six energy levels taking into account the fast energy exchange between methane and oxygen resonant states and the slow relaxation rate of oxygen. Thus, the photoacoustic signal is calculated for methane–nitrogen and methane–air systems and a very good agreement is found with the experimental results.

Simulations of flame acceleration and deflagration-to-detonation transitions in methane–air systems

Flame acceleration and deflagration-to-detonation transitions (DDT) in large obstructed channels filled with a stoichiometric methane–air mixture are simulated using a single-step reaction mechanism. The reaction parameters are calibrated using known velocities and length scales of laminar flames and detonations. Calculations of the flame dynamics and DDT in channels with obstacles are compared to previously reported experimental data. The results obtained using the simple reaction model qualitatively, and in many cases, quantitatively match the experiments and are found to be largely insensitive to small variations in model parameters.

Systematically reduced rate mechanisms and presumed PDF models for premixed turbulent combustion

The use of reduced kinetic mechanisms in models for turbulent premixed flames, at large but finite Damköhler numbers, is described. Taking as examples the two cases of hydrogen–air and methane–air systems for which there exist reduced kinetic schemes characterized by two independent scalar variables only, it is shown that the mean chemical production rate can be in a general way expressed as the product of a mixing factor, depending on the segregation of species due to turbulence, with a reaction rate factor. In this latter factor the probability density function (PDF) of the two scalars appears only as a constant integral quantity. The extension of this analysis to systems involving more than two independent scalars is envisaged. The general result is applied to the two specific cases of interior PDF’s modelled by (i) a single Dirac delta function and (ii) a laminar flamelet.

Uncertainty quantification in chemical systems

AbstractWe demonstrate the use of multiwavelet spectral polynomial chaos techniques for uncertainty quantification in non‐isothermal ignition of a methane–air system. We employ Bayesian inference for identifying the probabilistic representation of the uncertain parameters and propagate this uncertainty through the ignition process. We analyze the time evolution of moments and probability density functions of the solution. We also examine the role and significance of dependence among the uncertain parameters. We finish with a discussion of the role of non‐linearity and the performance of the algorithm. Copyright © 2009 John Wiley & Sons, Ltd.

High temperature ignition and combustion enhancement by dimethyl ether addition to methane–air mixtures

The effects of dimethyl ether (DME) addition on the high temperature ignition and burning properties of methane–air mixtures were studied experimentally and numerically. The results showed that for a homogeneous system, a small amount of DME addition to methane resulted in a significant reduction in the high temperature ignition delay. The ignition enhancement effect by DME addition was found to exceed that possible with equivalent amounts of hydrogen addition, and it was investigated by using radical pool growth and computational singular perturbation analysis. For a non-premixed methane–air system, it was found that two different ignition enhancement regimes exist: a kinetic limited regime and a transport limited regime. In contrast to the dramatic ignition enhancement in the kinetic limited regime, the ignition enhancement in the transport limited regime was significantly less effective. Furthermore, laminar flame speeds as well as Markstein lengths were experimentally measured for methane–air flames with DME addition. The results showed that the flame speed increases almost linearly with DME addition. However, the Markstein length and the Lewis number of the binary fuel change dramatically at small DME concentrations. Moreover, comparison between experiments and numerical simulations showed that only the most recent DME mechanism well reproduced the flame speeds of both DME–air and CH 4–air flames.

EVALUATION OF REDUCED MECHANISM FOR MODELING COMBUSTION OF PYROLYSIS GAS IN WILDLAND FIRE

ABSTRACT Fuel supply to the largely gaseous flames in an intense wildland fire is primarily a result of pyrolysis of cellulose present in live wood. Gaseous fuel released from high temperature pyrolysis of ground fuel is a complex and highly variable mixture, which includes four main gases (CO, H2, CH4, and CO2) and other aromatics. Although a detailed reaction mechanism (over a hundred reactions and around 50 species) is possible for an accurate description of the chemistry, it is currently impractical for predicting fire behavior under adverse conditions. Therefore, the idea of reducing the detailed mechanism to a few steps has always been attractive. We have developed a reduced four-step mechanism for combustion of gaseous fuel released from high temperature pyrolysis of wood using steady state assumptions for minor species. Three test environments composed of a perfectly stirred reactor, premixed flame, and opposed diffusion flame are used to validate this mechanism by comparing calculations performed with full, skeletal, and reduced chemistry over a wide range of pyrolysis fuel gas composition, fuel/air equivalence ratio, and strain rate. The reduced mechanism designed for the methane-air system is also used to model the combustion of pyrolysis fuel gas. The reduced model developed in this paper generally provides a good description of the oxidation process of pyrolysis fuel gas. The calculated temperature distribution, species profiles, and flame speed from a reduced model, and opposed diffusion flame structure agree well with the results obtained by full mechanism.

A simplified approach to the numerical description of methane-air diffusion flames

Starting with a three-step reduced chemistry description that employes H2 and CO as the only intermediates not in steady state, a simplified formulation aimed at facilitating numerical computations of non-premixed methane-air systems is developed. The analysis retains finite rates for radical recombination and CO oxidation but assumes infinitely fast fuel consumption taking place in a diffusion-controlled manner in an infinitely thin reaction sheet. To remove stiffness associated with the fast fuel consumption, the conservation equations for the major species and the temperature are written in terms of generalized coupling functions that for predictive accuracy permit species diffusivities that differ from the thermal diffusivity. The resulting formulation, which automatically determines the position of the fuel-consumption laver without necessity of front tracking or further interface approximations, can be used for analytical, computational, and modeling studies of both laminar and turbulent flows, removing stiffness difficulties associated with highly disparate chemical time scales. Comparisons of results of the simplified formulation in the counterflow mixing layer with those obtained with detailed chemistry and transport descriptions indicate that the proposed formulation applies with good accuracy to strain conditions ranging from weakly strained, robust flames to near-extinction flames.

Influences of flame-vortex interactions on formation of oxides of nitrogen in curved methane-air diffusion flamelets

Previous work has identified a parabolic flamelet in a uniform flow as a useful model for studying flame-vortex interactions and has presented an asymptotic analysis of this flamelet structure for two-step reduced chemistry of the methane-air system. The present paper addresses production rates of oxides of nitrogen in this flamelet by one-step reduced-chemistry descriptions of both thermal and prompt mechanisms, for both two-step and three-step methane-air reduced chemistry, and also reports some results of calculations of production rates with a full-chemistry description of planar counterflow flames, for purposes of comparison. The comparisons suggest that the asymptotic approximations significantly overestimate production rates and fail as extinction is approached but give qualitatively correct trends away from extinction. These trends show that increasing the tip curvature of the flamelet increases the prompt contribution while decreasing the thermal contribution. It is concluded that more research is needed on both elementary rates and asymptotic descriptions, especially for the prompt mechanism.

Combustion By-Products Destruction by Arc Plasma

The parameters affecting complete thermal decomposition of methane, and 1,1–dichloroethane in a dc argon plasma were investigated. The 1,1–dichloroethane was chosen as a representative reactant from chlorinated–compounds and chemical warfare agents. Theoretical studies were conducted using STANJAN thermochemical equilibrium solver–code on stoichiometric mixtures of methane–air and dichloroethane–air systems. In addition, the effect of 10% excess oxygen in the stoichiometric methane–air system was studied in order to understand the role of oxygen at a plasma condition. The results from these studies give valuable information in predicting the species concentrations and evaluating the quenching process. To conduct experimental investigations, a 10 kW laboratory scale dc plasma generator was built and operated to produce a stable argon plasma. The representative organic species: methane, and dichloroethane were included in the argon stream for decomposition in the plasma. The decomposition of the species in the plasma was monitored by optical emission spectroscopy. The extent of thermal decomposition was measured by the emission spectrum obtained from the data acquisition system. The gas–flow rate of the reactants and the power dissipated in the plasma were found to be important parameters for achieving complete decomposition of a particular chemical species. The argon–methane plasma was used as a reference, because methane is the simplest saturated hydrocarbon and we could use it to simulate the effluent from an incinerator. The plasma destruction of hazardous wastes was thus found to be very promising.

95/06345 Nonpremixed flames in stagnating turbulence part I - the k-e theory with equilibrium chemistry for the methane-air system

95/06345 Nonpremixed flames in stagnating turbulence part I - the k-e theory with equilibrium chemistry for the methane-air system

Nonpremixed flames in stagnating turbulence part I— The [formula omitted] theory with equilibrium chemistry for the methane-air system

Non premixed turbulent flames in opposed air and methane streams are analyzed on the basis of the k ̃ - ε ̃ theory under conditions of chemical equilibrium. The methane-air system is chosen to permit comparison with existing experimental data. It is found that the thickness of the experimentally observed flames is so large as to prelude application of the current thin flame analyses. Accordingly, the usual thin flame approach is avoided and solutions for the entire flow from the exit plane of one jet to that of the other are obtained. Difficulties are encountered in comparing theory and experiment as a consequence of the flow conditions and the instrumentation used to collect the data. Nevertheless it is considered worthwhile to attempt this comparison; indeed some aerothermochemical quantities are satisfactorily predicted. We examine the influence of several aerothermochemical quantities on the character of these flames.

Aspects of laminar premixed flame extinction limits

Numerical methods have been used to examine (1) the combined effects of stretch and upstream heatloss on the properties of a stoichiometric, laminar, pre-mixed methane-air flame, and (2) the combined effects of stretch and an extended, radiative heat loss on the properties of a fuel-lean methane-air flame, equivalence ratio Σ=0.57. Appropriate opposed flow configurations were investigated in both cases. In the first case, fresh gas was discharged towards the stagnation plane from identical, coaxial, plug flow nozzles maintained at constant temperature and separated by a fixed distance. Continuation methods were used in tandem with Newton solutions to study the flame behaviour as the stretch rate (or nozzle inlet velocity) was varied. High and low stretch extinction limits were observed as positions of vertical tangency on the steady-state solution curves of flame property vs stretch rate. The complete series of solutions between these positions formed a limit cycle with both stable and unstable branches. The chemical behaviour at the limits and along the unstable branch is discussed. The effect of radiative losses on the fuel-lean flame was studied in the unburnt-to-unburnt (UTU)configuration as above, but also in the unburnt-to-burnt (UTB) configuration where the ignited unburnt stream is opposed by a stream of its own equilibrium combustion products. If the flame is adiabatic, the product stream is input at the adiabatic equilibrium flame temperature. Such a system shows no abrupt extinction limits. On the other hand, if the radiative “sink” temperature is below some critical value, then, irrespective of the magnitude of the heat loss rate on a volumetric basis, the unburnt-to-burnt system will show abrupt extinctions if the stretch rate is increased sufficiently. Because of extremely slow numerical convergence, the numerical investigation of these limits for the methane-air system is not yet complete. However, such an investigation is in progress, and this combination of stretch and radiative loss effects is clearly of importance in relation to behaviour at composition limits of flammability.

Turbulent non-premixed combustion in partially premixed diffusion flamelets with detailed chemistry

The formulation of an extended laminar flamelet model of non-premixed turbulent combustion is developed. The microscopic elements in the turbulent ensemble are taken as laminar diffusion flamelets that are distinguished by the extent of stretching to which they are exposed and, additionally, by the degree to which their reactants are premixed. Extensive laminar flamelet computations are performed using a detailed mechanism for the methane-air system consisting of more than 250 elementary reactions of 39 chemical species. The degree both of flow field stretching and of partial premixing of reactants is varied systematically. The agreement of the numerical results with experimental data available from the literature is found to be excellent. First numerical simulations of turbulent flames on the basis of the extended flamelet model are performed. The results show that partial premixing of laminar diffusion flamelets is essential for the prediction of turbulent flame structures.

A Diffusion Combustor and Methane-Air Flame Propagation in Concentration Gradient Fields

Abstract An experimental apparatus for the study of the fundamentals of flame propagation phenomena in the gas mixtures with concentration gradient has been developed. The combustor provides one (or two) horizontal combustible layer(s) with severe concentration gradients formed only by interdiffusion of two (or three) initially homogeneous and isolated gases. The preliminary experiments were conducted for methane-air systems in case of a single combustible layer. A new schlieren photographic technique has been employed to make records of the flame propagation processes instead of the conventional cinephotographic technique, by which simultaneous measurements of the flame characteristics in space and time are possible. The results show that the flame propagating perpendicular to concentration gradient decelerates and may even lead to extinction for certain conditions. It is found that ignition energy, concentration gradient, and local equivalence ratio play an important role in such flame quenching phenomena.

The mechanisms of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube

The mechanisms by which lean limit upward and downward propagating flames extinguish in a standard flammability tube (51mm ID, 1.8m long) were deduced using schlieren and direct light photography and temperature measurements and by observing the behavior of the flame under transient loading. The lean methane-air system was studied. The upward propagating flame takes the shape of a stable hemispherical cap attached to a trailing skirt. This flame always extinguishes first at the holding region on the axis of the tube and a failure wave subsequently washes down the flame until extinguishment is complete. The extinguishment occurs because the flame is stretched at the tip and heat loss to the walls is not important to this extinguishment. On the other hand the extinguishment of a downward propagating flame is a multistep process. First heat loss to the walls causes the flame to extinguish near the walls and a smaller residual flame is left in the center of the tube. Once this happens, differential buoyancy of the hotter central gases and the cooler surrounding gases forces the cooler gases to move ahead of the flame. Finally, the downward propagating flame is observed to rise slightly. At this point the flame extinguishes completely.

A Gasdynamic Analysis of a Counter-Flow Diffusion Flame

Abstract A theoretical analysis of the gasdynamic structure of a counter-flow diffusion flame established in the forward stagnation region of a porous cylinder is made on the basis of the flame surface and the boundary layer approximations. The numerical calculation is performed to obtain the gasdynamic flame structure for the methane-air system and the obtained profiles of temperature, density and velocity are found to correlate satisfactory with those of experiment.