Inert Porous Media Research Articles

Quantifying oxygen diffusion during thermal degradation of combustible porous media

Oxygen diffusion controlled combustion occurs when local oxygen transport is slower than the chemistry, commonly found in porous combustible material or combustible material embedded within an inert porous medium. This mode of combustion, such as smouldering, can pose dangerous fire risks and also be harnessed in environmentally beneficial applications. However, the oxygen diffusion limitation is poorly understood in all contexts and persists as a key knowledge gap. Quantitative analysis of oxygen diffusion effects is therefore crucial for understanding the combustion behavior of combustible porous media and developing precise smouldering simulation models. In this paper, a reactive transport model incorporating both oxygen diffusion and chemical consumption was developed. Using coal as the model fuel, the impacts of key parameters on global mass loss during the one-dimensional diffusion combustion of coal samples were simulated and compared with TGA experiments conducted within a range of oxygen concentrations between 3–21%. Using this method, key kinetic and oxygen diffusion parameters were obtained within reasonable ranges by using a genetic algorithm optimization method. With these optimized parameters, the local oxygen distribution profiles in the samples at different inlet oxygen concentrations were simulated. The results indicate that oxygen diffusion can lead to large oxygen concentration differences within the coal samples, exceeding 63% of the inlet oxygen concentration. These oxygen differences can impact the local chemistry throughout the sample, and lead to fundamental errors in analyzing global kinetic analyses, if the transport effects are not considered. Altogether, this study delivers new insights into a potentially rate-limiting phenomenon that is relevant in progressing knowledge on many fire problems and engineering applications.

Исследование инициирования волны фильтрационного горения газа открытым пламенем

One of the processes of filtration combustion, namely the process of filtration combustion of gas, is considered. Its importance and practical significance are emphasized. The current state of research is shown and the need to study the non-stationary processes of filtration combustion of gas is revealed. The article examines one of the non-stationary processes of filtration combustion of gas, which is of great importance: the process of formation of a wave of filtration combustion of gas, which occurs when a burning gas mixture moves through an inert porous medium towards the flow of a combustible mixture. This process occurs when devices using filtration combustion of gas are operating: devices for afterburning poor gas mixtures, radiation burners, etc. A comprehensive study of this process has been carried out, including mathematical modeling and experimental research. The necessity of using a comprehensive study is shown. A method for processing experimental data is proposed. The factors influencing the process of formation of the filtration combustion wave of gas when it is initiated by an open flame are established. The influence of the gas mixture feed rate, heat loss from the porous medium to the external environment, and the grain size of the porous medium on the ignition process and the movement of the filtration combustion wave of gas was revealed.

FORMULAS FOR CALCULATING THE STATIONARY WAVE VELOCITY AND IGNITION TEMPERATURE IN FILTRATION COMBUSTION OF HYDROGEN-AIR MIXTURE

We present formulas for calculating the steady-state wave velocity and ignition temperature during filtration combustion of a hydrogen-air mixture in an inert porous medium. According to the obtained formula, the ignition temperature rises smoothly with the increase of the mixture blowing velocity, and decreases with the increase of the porous medium particle diameter. The calculated wave velocities increase with the increase of the mixture inlet velocity and pass into other velocity regimes.

Direct pore-level simulation of hydrogen flame anchoring mechanisms in an inert porous media

This work addresses the stabilization of hydrogen flames inside porous media. Hydrogen is a peculiar fuel with a large molecular diffusivity, triggering preferential diffusion and other effects related to non-unity Lewis numbers. Consequently, the macro-scale behavior of hydrogen flames is highly influenced by the flame-wall interactions at the pore-scale. In this study, direct pore-level simulations are used for the investigation of anchoring mechanisms and burning-rate enhancement for lean hydrogen flames embedded in inert porous media. It is observed that preferential diffusion greatly enhances flame anchoring, in the wake of the struts, leading to a high increase in flame surface. Also, the local heat release rate is enhanced not only by the higher gas temperature resulting from the heat recirculation but also by the higher local equivalence ratio resulting from preferential diffusion. Furthermore, most of the heat produced by a hydrogen flame is released by the hydroperoxyl reactions, effective at low temperatures but necessitating radicals, mostly H. These radicals are produced by the shuffle reactions, effective at higher gas temperatures. The coupling between those two reaction groups is very dependent on the transport of radicals, which can be impacted by the pore scale velocity gradient, leading to hydrodynamic dispersion and flame wrinkling. Consequently, it is shown that the burning-rate enhancement observed within porous burners fueled with hydrogen is not only due to heat recirculation but also to increased transports of radicals and preferential diffusion effects, which should be accounted for in macroscopic models.

Stabilization of methane–hydrogen flames inside a divergent porous media reactor

The energy transition process triggered by the threat of climate change has created the need for cleaner heat generating systems. Using blends of hydrocarbons with increasing presence of green fuels, such as green hydrogen, has been proposed as an initial step of this process. The present study aims to investigate methane–hydrogen flames stabilized inside a divergent inert porous media burner. Experimentally, four stable operating points were found for the considered burner with a hydrogen presence between 0% and 30% in the fuel mixture at a constant thermal output of 2 kW. Temperatures are presented for each condition, noticing a decreasing trend as hydrogen presence was increased, since the equivalence ratio was gradually reduced to achieve stabilization. CO and NOx emissions were below 15 ppm for all studied cases. A 2-dimensional numerical model incorporating the GRI-Mech 3.0 mechanism was developed and validated against experimental data.The model revealed that the stabilized flame front is observed as a straight line that runs parallel to the radial coordinate. This line exhibits a slight curvature close to the burner wall, which is caused by heat losses and the influence of the divergent geometry. Solid surface temperature was not constant due to the reduced convective heat exchange with flue gases in the periphery of the burner. To reduce this issue a lower expansion angle is proposed as an alternative. In addition, changes in the chemical kinetics due to H2 addition were also evaluated. As results, a general trend towards reaction shifting was found with a 56% of the reactions being strongly promoted by H2 presence, which is associated with an increase in radical concentration in the flame front due to H2 decomposition.

Experimental Investigation of a Self-Sustained Liquid Fuel Burner Using Inert Porous Media

A self-sustained porous burner without a sprayed atomizer was built for diesel oil. It consisted of metal fiber felt as an evaporator upstream and ceramic foam as an emitter downstream. The liquid fuel underwent film boiling in the porous evaporator and was rapidly evaporated by the heat recirculated from the porous emitter to the porous evaporator through intense irradiative heat flux. The effect of the porous structure and its installation location on the performance of the porous burner was investigated. The results indicated that the evaporation and combustion of liquid fuel could be prompted by the radiation of porous media. The position of the flame moved downstream, and the flame temperature decreased when the distance between the metal fiber felt and the ceramic foam was increased. The lowest NOx concentration was obtained when the distance between the foam and the metal fiber felt was 90 mm. When the diameter of the central hole of the ceramic foam was increased, the position of the flame moved towards the burner outlet, and the flame temperature and NOx emission declined. The flame temperature of the divergent configuration as emitter was higher than that of the convergent configuration, and the flame temperature of the C–D configuration was higher than that of the D–C configuration. Different ceramic foam structures had a significant effect on the temperature and emission in the combustion chamber, which showed that the evaporation and radiation performance of inert porous media burners with different structures is quite different.

Modelling oxygen-limited and self-sustained smoldering propagation: Thermochemical treatment of food waste in an inert porous medium

Smoldering treatment is emerging as a valuable engineering tool for many processes, including food waste treatment. However, smoldering systems are currently not well-understood nor optimized. Therefore, numerical models provide invaluable insight into the process dynamics, which improves our understanding and supports the development of novel systems. These smoldering models couple heat, mass, and momentum transfer with pyrolysis and oxidation chemical reactions within porous media. While recent models have untangled many aspects of these systems, local oxygen transport rates from bulk flow to the fuel surface are still not well-resolved. In this work, local oxygen-transport equation was approximated by an analytic derivation based on the gas–solid oxygen non-equilibrium hypothesis. With the improved oxygen-transport equation, a 2D model with five-step reaction scheme for smoldering propagation of food waste in sand was developed. Kinetic parameters obtained from TG experiments were incorporated into the bed-scale smoldering propagation model. The developed model was validated with experimental data that stretched from robust to weak smoldering propagation. It was demonstrated that the developed model matches well with experiments. Furthermore, this model revealed: (i) the emergence of non-uniform gas flow in the reactor, (ii) the evolution of the kinetic- and oxygen-transport-limiting regimes, and (iii) valuable insight into the fundamental changes with smoldering robustness.

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.

Investigation of Heterogeneous Detonation Wave Interaction with Porous Medium

ABSTRACT Mathematical model of aluminum heterogeneous detonation interaction with inert porous media was developed. Main regimes of interaction of heterogeneous detonation wave of stoichiometric mixture of aluminum particles in oxygen with porous inserts were studied. One- and two-dimensional calculations for micron and submicron particles and comparison of these calculations were made. Main regimes of detonation interaction with porous zone and critical conditions of detonation attenuation in porous zones were obtained. Calculations show that for micron particles reducing the diameter of burning particles lead to increasing the volume concentration of inert phase in the porous body that needs for detonation attenuation. Main difference of the propagation regimes for micron and submicron aluminum particles was found.

Applied smouldering for co-waste management: Benefits and trade-offs

Smouldering combustion is emerging as a valuable application for many environmentally beneficial purposes, including waste-to-energy. While many applied smouldering systems rely on small fractions of fuel mixed within inert porous media (IPM) like coarse grain silica sand, there is also an opportunity to pursue co-waste management with much higher fuel loadings. This study explores these co-waste systems by blending non-smoulderable wastes (e.g., sewage sludge) with smoulderable wastes (e.g., construction waste woodchips) to form porous solid fuels (PSFs). Key differences between these IPM and PSF systems were identified by contrasting the temperature profiles in space and time, specific mass loss rates, and emissions profiles across experiments in multiple reactors (with 0.054, 0.080, and 0.300 m radii). For example, the PSF systems exhibited higher throughputs, a more straightforward path towards continuous operation, improved scalability, and higher production of potentially useful by-products (e.g., CH4, H2) than the IPM systems. However, the PSF systems were more sensitive to extinction and exhibited lower fuel moisture content limitations than the IPM systems. Altogether, this study illuminates the benefits and trade-offs of co-waste smouldering.

Effect of quenching region characteristics on the performance of two-region porous inert medium burners at low-power operation

Although the effects of quenching and combustion regions on the performance of two-region porous inert medium (PIM) burners are different, only the combustion regions have received the utmost consideration. This study focuses on quenching regions by exploring the effect of varying their characteristics on the performance of PIM burners operated at low powers (1–5 kW). Therefore, the axial real-time temperature distributions and the steady-state CO– and NOx-emissions due to the combustion of wide ranges of well-premixed LPG-air mixtures within a well-designed PIM burner were measured. To vary the quenching region characteristics, two different materials of alumina and cordierite and three different thicknesses of the alumina material were used. The combustion stability limits were extracted from the measured stable temperature distributions and presented as a function of the applied power. Both the lower and higher stability limits decayed toward lower excess air ratios. However, the higher limit decayed at a faster rate, thereby causing the modulation ability of the excess air ratio to shrink continuously, which limited the burner’s adaptability and capacity. Meanwhile, using the alumina material instead of the cordierite improved the entire lower stability limit by at least 0.1. Conversely, using the cordierite material improved the higher stability limit by 0.1, but only for the two lower powers of 1 and 2 kW. In addition, increasing the alumina material’s thickness from 13 to 20 mm enhanced the whole modulation ability by 0.2, where each limit was entirely improved by 0.1. Therefore, these findings emphasize the effects of quenching region characteristics on the performance of two-region PIM burners, revealing the importance of optimizing this region during their design for actual practical applications. Finally, except for the CO-emission at 1 and 2 kW, outstanding insignificant CO– and NOx-emissions were observed.

Numerical study of flame stability within inert porous media with variable void area

This numerical study brings a comparison of a cylindrical porous burner with graded porosity and a conical porous burner with uniform porosity. Both burners have the same void area, creating a similar hydrodynamic stabilization mechanism. The study was performed for a laminar premixed CH4/air flame for different equivalence ratios with the FGM reduction technique employed to model the chemical kinetics. The results demonstrate that the cylinder with graded porosity presents a wider range of stable flames compared with the conical porous burner because the graded porosity leads to a larger heat recirculation as the flame approaches the outlet surface, which is mainly due to an increase of the solid-phase radiative heat transfer. The two-dimensional structure of the flame front was investigated in detail showing that the non-alignment of flow streamlines and solid-phase heat flux lines creates regions of sub-adiabatic and super-adiabatic temperatures along the flame front. Flame area increases with the inlet flow rate with the graded porosity burner presenting a smaller area and larger local consumption velocity than conical burner.

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion was carried out in this study to investigate the influences of gas flow rate, oxidative atmosphere, and inert porous medium involvement on the properties of products. The experiments were carried out at 300–600 °C under atmospheres of N2, 5% O2/95% N2, 10% O2/90% N2, and 15% O2/85% N2, with Darcy flow rates of 1.0 and 3.5 cm/s, respectively, with dried sewage sludge loaded individually or as a mixture with sand. As a result, both the increment of gas flow rate and involvement of sand leaded to lower yields of char and higher yields of bio-oil and gas under N2 at temperature of ≤500 °C, due to the enhanced efficiency of pyrolysis reaction and gas transportation. However, when temperature increased to 600 °C, the influencing trends on product distributions changed due to the mechanisms of secondary cracking reaction and volatile-char interaction. The involvement of oxygen in fraction of ≤15 vol% at temperatures of 400–500 °C would lead to the intense decreasing yields of char and bio-oil, and increasing yield of the gaseous (dominated by CO2 and CO), due to the involved oxidation reaction during pyrolysis. Both increment of temperature and oxygen fraction would lead to the delay of ignition and the increase of activation energy of the produced char, except for that of char produced at 400 °C under 5% O2/95% N2, whose calculated activation energy was lower and volatile content was higher compared to that of char produced from pyrolysis at 400 °C under N2. The bio-oil from pyrolysis under N2 was dominated by aliphatic acids, phenols, steroids, amides, and indoles, etc., and the involvement of partial oxidation would lead to the weakened formation of aromatics, phenols, and S/Cl/F-containing compounds in bio-oil.

Green hydrogen production based on the co-combustion of wood biomass and porous media

The production of green hydrogen by utilizing wood pellets is an effective way to solve the problem of energy shortage and environmental pollution. In order to improve the hydrogen production efficiency of biomass, a co-combustion system of inert porous media and wood pellets was built, and the combustion characteristic of wood pellets was studied with different porous media structures. The results demonstrated that the increasing of air velocity was beneficial to increase the combustion temperature, and hydrogen production reached the maximum value at v = 4 cm/s. Meanwhile, the hydrogen production increased with the increasing of inert bed length due to the intensification of heat circulation. When the length ratio of the hybrid to inert bed was 6: 3, the lower heating value of syngas was 3.69 MJ/Nm3 and the molar fraction of hydrogen increased by 69 % compared to the single wood pellets combustion. Moreover, the decreasing of pellets diameter was conducive to increase syngas production. The co-combustion of inert porous media and wood pellets realized the efficient utilization of biomass, and maximum growth rate of hydrogen reached 142 %.

Fast moving smoldering fronts in thin liquid fuel films dispersed on inert porous media

An experimental study of smoldering in thin layers of pyrolysis oil on an inert substrate of calcium silicate boards has been conducted. The smoldering fronts propagated with velocities up to 0.4mm/s, which is significantly faster than other types of smoldering without forced air flow. The smoldering propagation velocity increased both with thinner layer of pyrolysis oil and higher initial fuel temperature. Our experimental observations are in close correspondence with a theoretical model based on energy balance, where the availability of oxygen at the substrate surface is the main limiting mechanism. Both the front peak temperature and the front width are correlated to the initial fuel temperature. We also report observations of instability phenomena, including double fronts propagating in close succession. The temperature dynamics were observed by high resolution infrared thermography. The findings contribute to the understanding of the governing mechanisms of smoldering combustion of liquids on inert media, and in particular the fast smoldering fronts that have been observed along the surface of porous insulating materials in experiments with smoldering cotton.

Experimental Study on Effect of Infiltration Depth on Burning Characteristic and Fuel Residual Feature of Inert Porous Media Bed Soaked by Combustible Liquid

Experimental Study on Effect of Infiltration Depth on Burning Characteristic and Fuel Residual Feature of Inert Porous Media Bed Soaked by Combustible Liquid

An experimental study on burning characteristics and temperature distribution of inert porous sand bed soaked by leaked combustible liquid

An experimental study was conducted to investigate the burning characteristics and temperature distribution of inert porous sand bed soaked by leaked combustible liquid. 99.9% pure ethanol was selected as fuel and sand with different mean sizes were used as porous bed. Typical burning parameter, mass loss rate, flame height, plume temperature, sand bed temperature, and fuel residual percentage, etc., were measured and identified. Results shown that the overall variation trend of mass loss rate of liquid fuel-soaked inert porous sand bed exhibits a rapid increase at first and then a gradual decay. Specifically, the burning process can be divided into four stages, among which stage II and stage III account for a large proportion of burning duration. In Stage II, mass loss rate decreases with the increase of sand size, but that of stage III is not sensitive to sand size. Within current experimental range, by correlating dimensionless flame height and heat release rate, an empirical relationship is proposed for flame height in stage II . Axial temperature distribution of flame and plume did not vary significantly with sand size. The axial temperature profile could be described by classic plume relationship, but temperature decay rate is faster. In range of 76.1–77.1 °C, an obvious retardation appears in temperature growth of sand bed. The fuel residual percentage increases with the increase of sand size, however, due to the strong thermal conductivity of sand bed, low flash point and low viscosity of ethanol liquid, the maximum fuel residual percentage in current experiment is only 5.4%. This work can provide some references for fire safety and decontamination treatment in such combustible liquid-soaked inert porous media environment (soil, sand, etc).

Asymptotic study of premixed flames in inert porous media layers of finite width: Parametric analysis of heat recirculation phenomena

In this paper, we present an investigation on super-adiabatic premixed flames in an inert porous medium layer of finite length. The combustion process is modeled by a one-step Arrhenius kinetics in which density variations are taken into account. The asymptotic case of large ratio of thermal conductivities of solid to gas phases and the high activation energy limit are explored. The results obtained for these limiting cases are compared to those for finite values of these parameters. The use of the flame sheet model allows us to obtain steady-state solutions in an analytical form thus facilitating the parametric analysis.The investigation focuses on the phenomenon of multiplicity of steady-state solutions. It is shown that two or three (nontrivial) steady-state solutions are possible, depending on the flow rate intensity. The critical values of the parameters for the existence of multiple solutions are also determined. The stability of the obtained solutions is finally investigated by means of time-dependent simulations.

Combustion regimes in inert porous media: From decoupled to hyperdiffusive flames

Based on the classical volume-averaged equations, this work proposes a classification of porous media combustion in three distinct regimes for increasing interphase heat transfer, namely: decoupled, intermediate and hyperdiffusive. In the decoupled regime, flames behave as preheated free-flames. In the intermediate regime, large superadiabaticities are found. In the hyperdiffusive regime, flames are governed solely by an increase in thermal conductivity. The transition between these regimes is shown to be driven by two dimensionless parameters. The extent of the intermediate regime and the maximal superadiabaticity are proven to be related to the ratio between the diffusive and reactive length scales of the reference free-flame. Eventually, it is emphasized how the heat-recirculating system acts locally as a Lewis-changing device, whose effect harmonizes in the hyperdiffusive regime.

Dynamics of Thermoacoustic Oscillations in Swirl Stabilized Combustor without and with Porous Inert Media

Lean premixed (LPM) combustion processes are of increased interest to the gas turbine industry due to their reduction in harmful emissions. These processes are susceptible to thermoacoustic instabilities, which are produced when energy added by an in-phase relationship between unsteady heat release and acoustic pressure is greater than energy dissipated by loss mechanisms. To better study these instabilities, quantitative experimental resolution of heat release is necessary, but it presents a significant challenge. Most combustion systems are partially premixed and therefore will have spatially varying equivalence ratios, resulting in spatially variant heat release rates. For laminar premixed flames, optical diagnostics, such as OH chemiluminescence, are proportionally related to heat release. This is not true for turbulent and partially premixed flames, which are common in commercial combustors. Turbulent eddies effect the strain on flame sheets which alter light emission, such that there is no longer a proportional relationship. In this study, phased, averaged, and spatially varying heat release measurements are performed during a self-excited thermoacoustic instability without and with porous inert media (PIM). Previous studies have shown that PIM can passively mitigate thermoacoustic instabilities, and to the best of the authors’ knowledge, this is the first-time that heat release rates have been quantified for investigating the mechanisms responsible for mitigating instabilities using PIM. Heat release is determined from high-speed PIV and Abel inverted chemiluminescence emission. OH ∗ chemiluminescence is used with a correction factor, computed from a chemical kinetics solver, to calculate heat release. The results and discussion show that along with significant acoustic damping, PIM eliminates the direct path in which heat release regions can be influenced by incoming perturbations, through disruption of the higher energy containing flow structures and improved mixing.