A Numerical Investigation of Stabilized Methane/Air Combustion and Pollutants Formation in Porous Burners (original) (raw)

Two-dimensional Numerical Simulation of Combustion and Heat Transfer in Porous Burners

Engineering Letters, 2007

In this study, combustion in a 5kW porous burner is simulated. The two dimensional Navier-Stokes, the energy and the chemical species transport equations are solved and a multistep kinetics mechanism (5 reactions and 7 species) is employed. Finite volume method is used for simulation. Thermal nonequilibrium is accounted for gas and solid temperature and radiation heat transfer is considered for solid phase. Gas and solid temperature profiles and species mole fractions on the burner centerline are presented. Calculated CO and NO emissions are compared to experimental data for several excess air ratios. The effects of excess air ratio and solid phase radiation are investigated. The predicted temperature profile and pollutants formation are in good agreement with the available experimental data.

Mathematical modeling of porous combustion under various working conditions

Chemical Engineering Communications , 2023

Porous burner offers attractive advantages including high combustion performance and power ranges, and approximately zero pollutant emission. In this paper, the thermal behavior of premixed flame of a CH4/air mixture in a porous burner was examined using an unsteady In-house CFD code with an appropriate single reaction mechanism. In this regard, to solve the transient term, an implicit scheme was used, and displacement terms were solved using the upwind methodology. Additionally, the discretized central difference was employed to solve diffusion terms. At first, the results were compared with available published data, and then the influences of various working parameters, including solid matrix conductivity and volumetric heat transfer coefficient, were investigated on the flame structure. Thermal performance of the burner is studied by varying relevant parameters within a practical range. The predicted gas temperature profile near the reaction zone is much broader than that in an adiabatic premixed flame. The effect of volumetric heat transfer coefficient on the temperature profile is investigated. The local gas temperature decreases in the reaction zone and the solid temperature increases in the preheat zone with an increase in convective heat transfer. Also, the corresponding local convective heat transfer rates for different values of volumetric heat transfer coefficient are calculated and compared. Simulation results also indicate that increasing the effective thermal conductivity of the solid will decrease the solid phase temperature downstream of the flame location. Due to high temperature gradients in the solid matrix, its effective thermal conductivity has a significant impact on the conductive heat transfer rates.

Numerical Modelling of Porous Radiant Burners Using Full and Reduced Kinetics Mechanisms

Iran. J. Chem. Chem. Eng. …, 2008

The present paper compares full kinetics mechanisms in numerical modelling of porous radiant burners (PRB), with their reduced forms. The two most frequently used mechanisms of methane combustion (GRI3.0 and Miller) were selected and their effects were examined on temperature, species concentration, burning speed, and pollutant emission. While the findings of numerical simulation of PRB show fine concurrence between each full mechanism and its related reduced mechanism, no significant temperature differences are observed in the results of full mechanisms. However, CO concentration along burner axis shows a small difference between two full mechanisms, which is related to HCO and HO 2 concentrations. The inconsistency is more pronounced for NO concentration along porous axis, which is due to prompt NO evaluation. The present research finds deviation also between burning speeds, calculated by numerical simulation and experimental results. This difference is much more significant in rich mixtures. GRI3.0 mechanism estimated the burning velocities as closer to the experimental values than those predicted using Miller mechanism.

Numerical simulation of turbulent combustion in porous materials

International Communications in Heat and Mass Transfer, 2009

This paper presents one-dimensional simulations of combustion of an air/methane mixture in porous materials using a model that 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. Four different thermomechanical models are compared, namely Laminar, Laminar with Radiation Transport, Turbulent, Turbulent with Radiation Transport. Combustion is modeled via a unique simple closure. Preliminary testing results indicate that a substantially different temperature distribution is obtained depending on the model used. In addition, for high excess air peak gas temperature is reduced and the flame front moves towards the exit of the burner. Also, increasing the inlet flow rate for stoichiometric mixture pushes the flame out of the porous material.

Trends in modeling of porous media combustion

Progress in Energy and Combustion Science, 2010

Porous media combustion (PMC) has interesting advantages compared with free flame combustion due to higher burning rates, increased power dynamic range, extension of the lean flammability limits, and low emissions of pollutants. Extensive experimental and numerical works were carried out and are still underway, to explore the feasibility of this interesting technology for practical applications. For this purpose, numerical modeling plays a crucial role in the design and development of promising PMC systems. This article provides an exhaustive review of the fundamental aspects and emerging trends in numerical modeling of gas combustion in porous media. The modeling works published to date are reviewed, classified according to their objectives and presented with general conclusions. Numerical modeling of liquid fuel combustion in porous media is excluded.

Numerical investigation of the combustion performance in inert porous media

The aim of the present work is to develop a numerical scheme to investigate the combustion performance of porous burner. The finite volume method for three-dimensional Navier–Stokes equations, energy equations, and chemical species transport equations are solved and heat release described by a multi-step kinetics mechanism. The simulated results for temperature profiles are compared with published experimental data and a good agreement is observed. Predicted 3D temperature fields are also presented and the effects of material’s grain size and flow characteristics including excess air ratio on flame characteristics are investigated. It is observed that temperature profiles, gas velocity, and convective heat transfer between solid and gas phases are affected by the perturbations associated with these parameters.

Design and Evaluation of a Porous Burner for the Mitigation of Anthropogenic Methane Emissions

Environmental Science & Technology, 2009

Methane constitutes 15% of total global anthropogenic greenhouse gas emissions. The mitigation of these emissions could have a significant near-term effect on slowing global warming, and recovering and burning the methane would allow a wasted energy resource to be exploited. The typically low and fluctuating energy content of the emission streams makes combustion difficult; however porous burnerssan advanced combustion technology capable of burning low-calorific value fuels below the conventional flammability limitsare one possible mitigation solution. Here we discuss a pilot-scale porous burner designed for this purpose. The burner comprises a cylindrical combustion chamber filled with a porous bed of alumina saddles, combined with an arrangement of heat exchanger tubes for preheating the incoming emission stream. A computational fluid dynamics model was developed to aid in the design process. Results illustrating the burner's stable operating range and behavior are presented: stable ultralean combustion is demonstrated at natural gas concentrations as low as 2.3 vol%, with transient combustion at concentrations down to 1.1 vol%; the system is comparatively stable to perturbations in the operating conditions, and emissions of both carbon monoxide and unburned hydrocarbons are negligible. Based on this pilotscale demonstration, porous burners show potential as a methane mitigation technology.

Numerical Computation of Reacting Flow in Porous Burners With an Extended CH

Volume 2, Parts A and B, 2004

The present paper presents, numerical computations for flow, heat transfer and chemical reactions in an axisymmetric inert porous burner. The porous media re-radiate the heat absorbed from the gaseous combustion products by convection and conduction. In the present work, the porous burner species mass fraction source terms are computed from an 'extended' reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e. the porosity is not uniform over the entire domain. Therefore, it has to be included inside the partial derivatives of the transport governing equations. Finite-difference equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to insure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved, iteratively, for U, V, p'(pressure correction), enthalpy and species mass fractions, utilizing a grid of (60X40) nodes. The sixty grid nodes in the axial direction are needed to resolve the detailed structure of the thin reaction zone inside the porous media. The porous burner uses a premixed CH 4-air mixture, while its radiating characteristics are computed numerically, using a four-flux radiation model. Sixteen species are included, namely CH 4 , CH 3 , CH 2 , CH, CH 2 O, CHO, CO, CO 2 , O 2 , O, OH, H 2 , H, H 2 O, HO 2 , H 2 O 2 , involving 49 chemical reaction equations. It was found that 900 iterations are sufficient for complete conversion of the computed results with errors less than 0.1%. The computed temperature profiles of the gas and the solid show that, heat is conducted from downstream to the upstream of the reaction zone. Most stable species, such as H 2 O, CO 2 , H 2 , keep increasing inside the reaction zone staying appreciable in the combustion products. However, unstable products, such as HO 2 , H 2 O 2 and CH 3 , first increase in the preheating region of the reaction zone, they are then consumed fast in the postreaction zone of the porous burner. Therefore, it appears that their important function is only to help the chemical reactions continue to their inevitable completion of the more stable combustion products..

Study on Oxy-Methane Flame Stability in a Cylindrical Porous Medium Burner

Processes

Combustion in a porous medium can be beneficial for enhancing reaction rate and temperature uniformity. Therefore, considering the combination with oxy-fuel combustion can address some shortcomings in oxy-fuel burners, a cylindrical two-layer porous burner model is established based on OpenFOAM in this paper. A two-temperature equation model is adopted for the simulation of the heat transfer process. The CH4 skeletal kinetic mechanism is adopted for complex chemistry integration based on OpenSMOKE++. Corresponding experimental methods were used for complementary studies. The walls of the burner are wrapped with three types of thermal insulation materials to present different levels of heat loss. The results show that considering the convection and radiative heat loss of the burner wall, the temperature near the wall is reduced by more than 300 K compared to the adiabatic condition. As a result, the flame propagation speed and CO oxidation rate slowed down. The stable range will be d...