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.     

Numerical simulation of turbulent combustion in porous materials

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 thermo-mechanical 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.     

Calculation of premixed combustion within inert porous media with model parametric uncertainty quantification

The present study focuses on uncertainties existing in porous media parameters and in the inlet reactant mixture conditions of solid oxide fuel cell off-gas combustion. Propagation of uncertainty from the model input parameters to the output stochastic variables is quantified using a non-intrusive spectral projection method based on polynomial chaos expansion. The non-intrusive nature of this method allows the solution of the stochastic problem to be obtained directly from the deterministic model without requiring modification of the governing equations. Quantification of uncertainty is investigated in a one-dimensional model for premixed combustion within inert porous media. The model includes detailed chemistry and solves the gas- and solid-phase energy balances coupled by convective heat exchange, including radiative heat transfer in the solid-phase. The results denote that the uncertainties in the porous media heat transfer parameters are relevant and originate a relatively high error bar on the CO emission and burning velocity. When the inlet reactant mixture uncertain conditions is also accounted for, it overcomes the influence of the other uncertain parameters on the gas- and solid-phase temperatures error bar. Both types of parametric uncertainty sources (inlet conditions and porous media parameters) are important in order to establish the error bar on the CO emission and burning velocity predictions.     

Turbulent flow with combustion in a moving bed

This paper presents a mathematical model for treating turbulent combusting flows in a moving porous bed, which might be useful to design and analysis of modern and advanced biomass gasification systems. Here, one explicitly considers the intra-pore levels of turbulent kinetic energy and the movement of the rigid solid matrix is considered to occur at a steady speed. 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. Results indicate that fixing the gas speed and increasing the speed of the solid matrix pushes the flame front towards the end of the reactor. Also, since the rate of production of turbulence is dependent on the relative velocity between phases, as the solid velocity approaches that of the gas stream, the level of turbulence in the flow is reduced.     

A review of investigations on liquid fuel combustion in porous inert media

Utilization of a porous medium for combustion of liquid fuels is proved to be a promising approach for future applications. The porous medium burner for liquid fuels is more advantageous than the conventional open spray flame burner for several reasons; these include enhanced evaporation of droplet spray owing to regenerative combustion characteristics, low emission of pollutants, high combustion intensity with moderate turn-down ratio and compactness. This article provides a comprehensive picture of the global scenario of research and developments in combustion of liquid fuels within a porous medium that enable a researcher to determine the direction of further investigation. Accordingly, a glossary of the important terminology, the modeling approach, advances in numerical and experimental works and applications are included. The papers published in standard journals are reviewed and summarized with relevant comments and suggestions for future work.     

A numerical evaluation of combustion in porous media by EGM (Entropy Generation Minimization)

Combustion in Porous Media provides interesting advantages compared with the free flame combustion due to the higher burning rates, increased power dynamic range, the extension of lean flammability limits, and the low emissions of pollutants. A numerical code is developed in order to evaluate the effects of different parameters of combustion in porous media. The governing equations including Navier–Stokes, the solid and gas energy and the chemical species transport equations are solved using a multi-step reduced kinetic mechanism. Flame stabilization and the burner optimization are studied by EGM (Entropy Generation Minimization) method considering the effects of chemical affinities and reaction. It is found that the flames occurring at the upstream half of the porous layer are more stable and more efficient, producing less emissions than those occur at the downstream half of porous layer. Also at a specified equivalence ratio both the heat recirculation efficiency and the Merit number have similar trend by changing the flame location. For a FFL (Fixed Flame Location), there is an optimum value of equivalence ratio at which the burner efficiency is a maximum.     

Experimental investigation of combustion in porous heating burners

The combustion characteristics of liquefied petroleum gas inside porous heating burners have been investigated experimentally under steady-state and transient conditions. Cooling tubes were embedded in the postflame region of the packed bed of a porous heating burner. The flame speed, temperature profile, and [NO_x] and [CO] in the product gases were monitored during an experiment. Due to the heat removal by the cooling tubes, a phenomenon termed metastable combustion was observed; this is that only one flame speed exists at a particular equivalence ratio for maintaining stable combustion within the porous bed of the porous heating burner. This behavior is quite different from that of porous burners without cooling tubes, in which an extended range of flame speeds usually is found for maintaining stable combustion. After metastable combustion has been established in a porous heating burner, a change in the equivalence ratio will stop the metastable combustion and drive the flame out of the packed bed. From the steady-state results, the porous heating burner was shown to maintain stable combustion under fuel-lean conditions with an equivalence ratio lower than the flammability limit of a normal free-burning system. The flame speed in a porous heating burner was found to decrease with an increase in the length of the porous bed. Combustion within a porous heating burner has the features of low flame temperature, extended reaction zone, high preheating temperature and low emissions of NO_x and CO. The flame temperature ranged from 1050 to 1250 °C, which is ∼200 °C lower than the adiabatic flame temperature at the corresponding equivalence ratio. The length of the reaction zone could be more than 70 mm and the preheating temperature ranged from 950 to 1000 °C. Both [NO_x] and [CO] were low, typically below 10 ppm.     

Maximum superadiabatic temperature for stabilized flames within porous inert media

This work analyzes the superadiabatic temperature for laminar stationary lean premixed flames within porous inert media. The analysis is based on the excess enthalpy function applied to the one-dimensional volume-averaged equations. This formulation, with results obtained in a previous work, allows for the construction of an analytical solution valid over a large range of equivalence ratios. The model reveals the existence of a maximum non-dimensional superadiabatic temperature at a precisely determined equivalence ratio and connects previous works for near-stoichiometric and ultra-lean mixtures.     

Future progress in turbulent combustion research

Turbulent combustion research is projected to be an important area of research well into the twenty-first century. Issues of current interest in turbulent flame structure and computational prediction are outlined and forecasts are made for approaches that are likely to lead to significant advances. There is a mounting body of evidence that concepts and models derived from the laminar flamelet hypothesis are not valid over many of the conditions of practical interest for both premixed and non-premixed systems. Approaches such as Conditional Moment Closure and Monte–Carlo simulation of the transport equation for the probability density function are considered to have the most promise for pollutant prediction in non-premixed systems. Large Eddy Simulation may be necessary for non-stationary premixed problems and for bluff-body and swirling flows.     

The dynamics of in-situ combustion fronts in porous media

The sustained propagation of combustion fronts in porous media is a necessary condition for the success of in situ combustion for oil recovery. Compared to other recovery methods, in situ combustion involves the complexity of exothermic reactions and temperature-dependent chemical kinetics. In the presence of heat losses, the possibility of ignition and extinction also exists. In this paper, we address some of these issues by studying the properties of forward combustion fronts propagating at a constant velocity in the presence of heat losses. We extend the analytical method used in smoldering combustion [7], to derive expressions for temperature and concentration profiles and the velocity of the combustion front, under both adiabatic and non-adiabatic conditions. Heat losses are assumed to be relatively weak and they are expressed using two modes: 1) a convective type, using an overall heat transfer coefficient; and, 2) a conductive type, for heat transfer by transverse conduction to infinitely large surrounding formations. In their presence we derive multiple steady-state solutions with stable low and high temperature branches, and an unstable intermediate branch. Conditions for self-sustaining front propagation are investigated as a function of injection and reservoir properties. The extinction threshold is expressed in terms of the system properties. An explicit expression is also obtained for the effective heat transfer coefficient in terms of the reservoir thickness and the front propagation speed. This coefficient is not only dependent on the thermal properties of the porous medium but also on the front dynamics.     

Double-diffusive turbulent natural convection in a porous square cavity with opposing temperature and concentration gradients

This paper presents results for coupled heat and mass transport under laminar and turbulent flow regimes in porous cavities. Two driving mechanisms are considered to contribute to the overall momentum transport, namely temperature driven and concentration driven mass fluxes. Aiding and opposing flows are considered, where temperature and concentration gradients are either in the same direction or of different sign, respectively. Modeled equations are presented based on the double-decomposition concept, which considers both time fluctuations and spatial deviations about mean values. Turbulent transport is accounted for via a macroscopic version of the k–ε model. Variation of the cavity Nusselt and Sherwood numbers due to changes on N, where N is the ratio of solute to thermal Grashof numbers, is presented. Results indicate that for adding cases, mass and heat transfer across the cavity are enhanced faster than for cases with opposing temperature and concentration gradients. For the conditions here investigated, the use a turbulence model gave results for Nu and Sh that were nearly double when compared with laminar results for the same conditions.     

Syngas production from polyethylene and biogas in porous media combustion

The production of syngas from biogas (surrogate of CH₄/CO₂: 55/45 v/v) and polyethylene in a porous media combustion reactor is experimentally studied. The employed setup is novel and has not been studied before. A semi-continuous feed of solid fuel and a constant filtration velocity of the gaseous reactants of 17 cm/s were considered. Temperature, velocity of propagation, and composition of the syngas produced in the combustion waves were registered in a tubular reactor packed with a ceramic foam porous medium and two solid fuel inlets. In the first part of the study, a baseline determined by the filtration combustion of a biogas/air mixture through the ceramic foam at the equivalence ratio ($\mathrm{?}$) range $0.7\le \mathrm{?}\le 1.6$, having transient (upstream and downstream) and stationary combustion wave propagation regimes, is established. In the second part of this work, portions of the ceramic foam in two different separated zones are extracted, leaving space for the semi-continuous supply of polyethylene. In this second part the biogas-air combustion was performed only for $\mathrm{?}=0.8$ and $\mathrm{?}=1.6$. Although the combustion temperature decreased by the presence of polyethylene, it was found that the syngas (both H₂ and CO) yield was larger than for the baseline. The highest degrees of conversion to hydrogen and carbon monoxide was reached under the presence of polyethylene, having 45% and 67% for $\mathrm{?}=0.8$, and 45% and 60% for $\mathrm{?}=1.6$, respectively. These results are very promising and they demonstrate the capabilities of the presented methodology and experimental setup, which should encourage future attempts of applications of the technology.     

The use of inert porous media based reactors for hydrogen production

The present review paper examines the production of hydrogen in inert porous media based reformer by thermal partial oxidation. Here we consider, specifically, the rich combustion of hydrocarbon fuels and the conversion of H₂S to hydrogen. The different technologies to produce hydrogen beside the experimental and numerical work done in this field are presented. The effect of different operating conditions, such as the equivalence ratio, the mass flow rate and the reactant feed temperature are explained. Additionally, design parameters, including the reactor geometry and porous material specifications, are discussed.     

Hydrogen production in ultrarich combustion of hydrocarbon fuels in porous media

Rich and ultrarich combustion of methane, ethane, and propane inside inert porous media is studied experimentally and numerically to examine the suitability of the concept for hydrogen production. Temperature, velocities, and chemical products of the combustion waves were recorded experimentally at a range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 2.5, for a filtration velocity of 12 cm/s. Two-temperature numerical model based on comprehensive heat transfer and chemical mechanisms is found to be in a good qualitative agreement with experimental data. Partial oxidation products of methane, ethane, and propane (H₂, CO, and C₂ hydrocarbons) are dominant for ultrarich superadiabatic combustion. The maximum hydrogen yield is close to 50% for all fuels, and carbon monoxide yield is close to 80%.     

Trends in modeling of porous media combustion

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 modelling of a three-zone combustion for heavy fuel oil in inert porous media reactor

Numerical model for heavy fuel oil and air mixtures combustion is presented to simulate the behavior of the fuel in an inert porous medium reactor for hydrogen production. Three-zone combustion of oil and petroleum cokes separated by temperature ranges starting from ambient temperature to 560 K, from 560 K to 673 K, and above 673 K, is presented. Hydrogen production is achieved using water gas shift equilibrium reaction on the combustion products at different temperatures. Results show a high enthalpy contribution due to coke combustion formed in the low temperature oxidation reaction, being the most important reaction in relation to its zone size. Simulations increasing filtration velocity (from 0.05 to 0.9 m/s) has a favorable effect on the maximum temperature and the combustion front velocity. The effect of the simplified combustion model lowers computational time, with acceptable results for temperature as well as hydrogen production in contrast to laboratory tests and other software simulation such as COMSOL Multiphysics.     

Experimental study of hydrogen production and soot particulate matter emissions from methane rich-combustion in inert porous media

A detailed experimental study of stationary Thermal Partial Oxidation (TPOX) within inert porous media has been conducted. The reaction zone of the tested TPOX reformer is designed so as to enable stationary conversion of fuel/air mixtures for a wide range of operational conditions. Operating characteristics of the process have been examined for two different porous matrices, with different thermal and transport properties, namely SiSiC open foam structure and a packed bed of pure Al₂O₃ packing material in the form of cylindrical rings. The influence of reactants preheating was also examined since the reformer is meant for integration within high temperature fuel cell systems. The operating regime was scanned for reactants' inlet temperature of 400 °C and 550 °C, varying the thermal load in a range from 350 kW/m² up to 2600 kW/m² and the equivalence ratio from 1.9 up to 2.9. Temperature profiles within the reaction region of the reformer were recorded for all tested conditions while gas samples were on-line analyzed for the major species H₂, CO, CO₂, and minor species CH₄, C₂H₂. At reactants' inlet temperatures of 400 °C and 600 °C, for a fixed thermal load of 1540 kW/m² and for selected equivalence ratios around the sooting limit of the process (φ = 2.2–2.6), soot particle size distributions were measured in the exhaust gas with a Scanning Mobility Particle Sizer (SMPS). The results show that the better thermal properties and the higher porosity in the case of the SiSiC matrix enables longer residence times for slow reforming reactions to evolve towards equilibrium and yields syngas with significantly less soot in terms of particle numbers and mass concentration.     

Computation of turbulent heat transfer in a moving porous bed using a macroscopic two-energy equation model

Heat transfer between phases in a moving porous bed is analyzed. This work proposes a set of transport equations for solving problems involving turbulent flow and heat transfer in a moving bed equipment. The device is modeled as a saturated porous matrix in which the solid phase moves with a steady imposed velocity. Additional drag terms appearing the momentum equation, as well as interfacial heat transfer between phases, are assumed to be a function of the relative velocity between the fluid and solid phases. Turbulence transport equations are here also dependent on the speed of the solid material. Results indicate that, as the phases attain velocities of equal order, turbulence in damped and heat transfer between solid and fluid occurs mainly by conduction mechanism.     

A fractal permeability model for bi-dispersed porous media

In this paper a fractal permeability model for bi-dispersed porous media is developed based on the fractal characteristics of pores in the media. The fractal permeability model is found to be a function of the tortuosity fractal dimension, pore area fractal dimension, sizes of particles and clusters, micro-porosity inside clusters, and the effective porosity of a medium. An analytical expression for the pore area fractal dimension is presented by approximating the unit cell by the Sierpinski-type gasket. The pore area fractal dimension and the tortuosity fractal dimension of the porous samples are determined by the box counting method. This fractal model for permeability does not contain any empirical constants. To verify the validity of the model, the predicted permeability data based on the present fractal model are compared with those of measurements. A good agreement between the fractal model prediction of permeability and experimental data is found. This verifies the validity of the present fractal permeability model for bi-dispersed porous media.