Combustion In Inert Porous Media Research Articles

Modelling challenges of volume-averaged combustion in inert porous media

In porous burners, the flame thickness can be smaller than the pore size, resulting in sharp and locally-anchored flame fronts. The presence of such steep gradients at the pore level is a major hurdle for the derivation of volume-averaged models, particularly for the highly non-linear reaction rates. With the intent to address the difficulties associated with volume-averaging for porous media combustion, this work makes use of 3D Pore-Level Direct Numerical Simulations including conjugate heat transfer and complex chemistry in burners of finite length. These detailed 3D simulations are compared to their 1D volume-averaged counterpart, with effective properties estimated directly on the computational domains and of identical thermo-chemical scheme. Discrepancies in terms of burning rate, profiles, and a priori analysis on the reaction rates are discussed. Various pore sizes and geometries are considered. At the pore level, it is shown that preheating, wrinkling and wall quenching are the three main factors driving the global burning rate. Importantly, hydrodynamic dispersion is shown to have an indirect role on combustion processes. From the observations of combustion at pore scale, a new closure for reaction rates based on a flamelet assumption is proposed. It accounts for flame wrinkling and eliminates the unwanted effect of hydrodynamic dispersion on burning rate.

Three-dimensional pore-scale numerical simulation of methane-air combustion in inert porous media under the conditions of upstream and downstream combustion wave propagation through the media

Abstracts Porous media combustion of lean and ultra-rich methane-air mixtures was numerically studied using the approach when the process is simulated at the pore scale with explicit consideration of the three-dimensional porous structure and interstitial flow with thermal interaction between fluid and solid phases including the detailed chemical kinetics model and solid-to-solid radiation. The geometrical model of the packed bed was generated synthetically by simulation of particles falling under gravity force. A novel workflow for a randomly packed bed meshing at the pore scale was used to avoid the point contact problem. The results demonstrate that the model predicts a thermal nonequilibrium and heat recuperation mechanism in the combustion wave in the low-velocity regime correctly. The inhomogeneous structure of the porous media leads to large spatial variation of the process parameters such as velocity and temperature. It was shown that radiation makes a significant contribution to heat transfer in the region of a large temperature gradient. Radiative heat flux is transferred through the bed via radiation layer-by-layer due to restricted mutual visibility of the opaque particles. The mixture composition has a significant impact on the heat release rate and wave velocity and direction as well as thermal flame thickness and exhaust gas composition. At the macro scale, the mode of combustion wave propagation is quasi-steady whereas at the pore scale with little time steps the unsteady effects can be observed. Different subregions of the flame front propagate with not equal velocity, stand or accelerate depending on the local flow and porous matrix inhomogeneity. For the lean mixture the oscillatory regime of propagation was detected and analyzed.

Hydrocarbons conversion to syngas in inert porous media combustion

Experimental combustion in inert porous media of Liquefied Petroleum Gas (LPG), Butane, Propane, Diesel Fuel and Heavy Fuel Oil (HFO) is presented to assess the syngas production within different stages of refinement of fuels, considering light gases up to a residual oil. The porous media is composed by a fixed bed of alumina spheres and temperatures and gas composition were acquired while stable combustion waves under rich fuel conditions were produced. The results of this study showed that it is possible to partially oxidize HFO producing up to a 10% of H2 and CO in the products for the equivalence ratio of ϕ = 1.3, with a superior performance than any other fuel tested in this work. The behavior of the recorded temperature results showed good agreement with the available literature. Upstream regimes of propagation of the combustion wave are observed for the experiments with liquid fuels, while gaseous fuels presented downstream regimes for ϕ > 2 up to ϕ = 4. Results suggest that in order to efficiently transform the fuels into syngas, it is convenient to operate the reactor under an upstream regime considering the chemical energy at inlet and outlet. Furthermore, liquid fuels, regardless of its refining level, have the potential to produce syngas in inert porous media in promising volumes.

The Effects of Combustion Parameters on Pollutant Emissions in a Porous Burner

This paper reports a two-dimensional numerical prediction of premixed methane/air combustion in inert porous media. The two dimensional Navier-stokes equations, the two separate energy equations for solid and gas and conservation equations for chemical species are solved using finite volume method based on SIMPLE algorithm. The burner under study is a rectangular one with two different regions. First region is a preheating zone (low porosity matrix) that followed by the actual combustion region (high porosity matrix). For simulating the chemical reactions, skeletal mechanism (26 species and 77 reactions) is used. For studying the pollutant emissions in this porous burner, the effects of porous matrix properties, excess air ratio and inlet velocity are studied. The predicted gas temperature contour and pollutant formations are in good agreement with the available experimental data. The results indicate that the downstream of the burner should be constructed from materials with high conductivity, high convective heat transfer coefficient and high porosity in order to decrease the CO and NO emissions. Also, with increasing the inlet velocity of gas mixture and the excess air ratio, the pollutant emissions are decreased.

Hydrogen and syngas production from propane and polyethylene

Hybrid filtration combustion of propane in a porous medium composed of aleatory polyethylene pellets and alumina spheres is studied to examine the suitability of the concept for hydrogen and syngas production. Temperature, velocity, and chemical products of the combustion waves were recorded experimentally in the range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 1.65. The model predictions for combustion in inert porous media using GRI 3.0 reaction mechanism are in good qualitative agreement with experimental data. Hydrogen, carbon monoxide, methane and carbon dioxide are dominant combustion products for upstream sub-adiabatic temperatures recorder in all the range of equivalence ratios studied. The maximum hydrogen and carbon monoxide yields are close to 48% and 89%, respectively.

Laminar flow with combustion in inert porous media

This work presents one-dimensional numerical results for combustion of an air/methane mixture in inert porous media using laminar and radiation models. Comparisons with experimental data are reported. The burner is composed by a preheating section followed by a combustion region. Macroscopic equations for mass, momentum and energy are obtained based on the volume average concept. Distinct energy equations are considered for the porous burner and the flowing gas. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to relax the entire equation set. Inlet velocity, excess air, porosity and solid-to-fluid thermal conductivity ratio were varied in order to investigate their effect on temperature profiles. Results indicate that higher inlet velocities result in higher gas temperatures, following a similar trend observed in the experimental data used for comparisons. Burning of mixtures close to the stoichiometric conditions also increased temperatures, as expected. Increasing the thermal conductivity of the preheating section reduced peak temperature in the combustion region. The use of porous material with very high thermal conductivity on the combustion region did not affect significantly temperature levels in the combustion section.

Analysis of turbulent combustion in inert porous media

The objective of this paper is to present an extension of a simplified reaction kinetics model that, combined with a thermo-mechanical closure, entails a full-generalized turbulent combustion model for flow in porous media. In this model, one explicitly considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. These mass fractions are double decomposed in time and space and, after applying simultaneous time-and-volume integration operations to them, distinct terms arise, which are here associated with the mechanisms of dispersion and turbulence. Modeling of these extra terms remains an open question and the derivations herein might motivate further development of models for turbulent combustion in porous media.

Analytical modelling of filtration combustion in inert porous media

A study is made of the self-sustaining combustion waves during the filtration of lean methane–air mixtures in inert porous media using the two-temperature approximation. Such processes are characterized by an intense thermal interaction between the gas and the porous material. Due to interfacial heat transfer, the solid phase is able to redistribute heat absorbed from reaction products to the unburned mixture. The analytical solution is built in three different regions: the pre-heating region, the reaction region and the region occupied by the combustion products. Analytic expressions predicting the temperature and methane mass fraction profiles in the wave, as well as the combustion wave velocity and the longitudinal extension of the reaction region are derived. The results are confirmed by numerical calculations using the finite difference method and a full set of basic equations.

Transient Numerical Simulation of Combustion in Inert Porous Media

AbstractThis paper presents methods for the time‐dependent numerical solution of a two‐dimensional model governing combustion an inert porous media. The spatial discretization is based on a mixed finite element method for the flow problem and a cell‐centered finite volume scheme for the transport equations. Some results of numerical simulations show the influence of a jump in the porosity of the solid matrix on the localization of the combustion zone.

Comparison of four combustion models for simulating the premixed combustion in inert porous media

This work reports one-dimensional predictions of methane/air fuel combustion in inert porous media using four combustion models: full mechanism (FM, 49 species and 227 elemental reactions), skeletal mechanism (SM, 26 species and 77 elemental reactions), 4-step reduced mechanism (4RM, 9 species) and 1-step global mechanism (1GM). The effects of these models on temperature, species, burning speeds and pollutant emissions are examined. The calculations are compared with available experimental data. It is concluded that the already known limitation of the 1-step global mechanism can be partially eliminated by the present 4-step reduced mechanism. This 4RM model compares very satisfactorily with the full mechanism in the simulation of combustion in porous media. This conclusion is encouraging for the simulation of practical porous media burners because the 4RM model improves the stability of the calculation process and can be used with reduced computational resources and cost. © 1998 John Wiley & Sons, Ltd.

Comparison of four combustion models for simulating the premixed combustion in inert porous media

This work reports one-dimensional predictions of methane/air fuel combustion in inert porous media using four combustion models: full mechanism (FM, 49 species and 227 elemental reactions), skeletal mechanism (SM, 26 species and 77 elemental reactions), 4-step reduced mechanism (4RM, 9 species) and 1-step global mechanism (1GM). The effects of these models on temperature, species, burning speeds and pollutant emissions are examined. The calculations are compared with available experimental data. It is concluded that the already known limitation of the 1-step global mechanism can be partially eliminated by the present 4-step reduced mechanism. This 4RM model compares very satisfactorily with the full mechanism in the simulation of combustion in porous media. This conclusion is encouraging for the simulation of practical porous media burners because the 4RM model improves the stability of the calculation process and can be used with reduced computational resources and cost.

Characteristics of stationary spherical waves of gas combustion in inert porous media

Stationary spherical waves of gas combustion in porous media were studied theoretically and experimentally. It is shown that in spherically convergent and divergent gas flows, the waves differ in the type of stability: they are stable in a divergent flow and unstable in a convergent flow. A mathematical model for standing spherical waves is proposed that establishes the dependences of the coordinate of a standing wave on the gas-flow rate, mixture composition, and parameters of the porous medium and the gas and describes the experiment adequately. It is shown that allowance for heat losses leads to the appearance of limits of existence for standing spherical waves.

Numerical Study of Combustion and Pollutants Formation in Inert Nonhomogeneous Porous Media

Combustion in inert porous media offers an interesting and promissing route towards high power density, high power dynamic range and very low emission of pollutants such as NOx and CO. This work assumes one dimensional combustion in a porous burner but considers the detailed reaction mechanism of methane/air, 27 species and 73 reactions, and also the conductive heat transfer of solid matrix and radiative heat transfer in the porous media. In this work, we present a numerical study which show the effects of excess air ratio, thermal power, solid conductivity and radiative heat transfer on the combustion and pollutants formation in inert nonhomigeneous porous media. It was concluded that NO and CO emission depend mainly on the excess air ratio and thermal power. The peak flame temperature is reduced with the reduction of th solid conductivity resulting in a decrease of NO emission. This important conclusion means that NO emission can be decreased through the utilization of porous medium with small solid conductivity. Radiative heat transfer is also important for the combustion and heat transfer in porous media. The flame can be stabilized easily at the interface between two different porosity ceramic blocks due to the radiative heat feedback. In addition the predicted gas and solid temperatures were compared and discussed with reported measurements of centre line temperature in a cylindrical porous burner. Special emphasis was given to the predicted pollutants formation and their comparison with experiments. The good agreement with experimental observations suggests that the numerical model is an excellent tool to investigate combustion and pollutants formation in porous media as well as to achieve optimised porous combustion designs.

Combustion in a Porous Medium-Advances and Applications

Abstract In this paper research and development work of the authors is described, which led to the design of a new combustor-heat exchanger system based on the combustion in porous media. Combustion in inert porous media is possible if the Peclet-number is high enough ( > 65), so that quenching of the flame inside the pores is prohibited. The heat transfer from the combustion zone, to the porous medium itself is very effective because of the very large surface between them. The combustion temperature can be controlled through the porous medium temperature. Prompt and thermal NOx formation, which is temperature dependent, can be controlled by appropriate cooling of the combustion zone. The heat transfer mechanisms in the combustor are discussed and new designs of porous materials are proposed, which allow a high power density and a better control of the temperature level in the combustion zone. The possible application field and the expected benefits of this combustion technique are discussed.

Fundamental laws governing subsonic and detonating gas combustion in inert porous media

Fundamental laws governing subsonic and detonating gas combustion in inert porous media