Heterogeneous Combustion Research Articles

La-Ce-hexaaluminate doped by multivalent metal ion as the oxygen carrier for the optimization of hydrogen production

The efficiency of porous media combustion was greatly improved by combining the high-temperature catalyst of hexaaluminate. In this paper, a double-layer porous media burner filled with cylinders was built with the loading of La0.8Ce0.2MAl11O19 (M = Fe, Co, Cu). The effects of hexaaluminate and cylinders parameters on the products concentration and temperature distribution were studied at different working conditions. Results indicated that the La0.8Ce0.2CuAl11O19 obtained the excellent catalytic performance with the most adsorbed oxygen and oxygen channels, so that the maximum hydrogen mole fraction was achieved. With the increasing of the 13 mm cylinders volume ratio from 0 to 1, the hydrogen and carbon monoxide yield increased by 53.3 % and 50.9 %, respectively. Meanwhile, the increased heat releasing of the higher velocity contributed to improving the combustion temperature and syngas production. These results provided theoretical guidance for the catalytic porous media reactor in energy utilization and environmental protection.

Transition to unstable oscillatory flames in porous media combustion

The flames inside porous burners with low filtration velocity are prone to oscillatory behavior similar to unstable combustion in narrow channels but accompanied by complicated dynamics due to the irregularity of porous structure, flow field, and thermal gradient formed as a result of heat recuperation. This work is a numerical study within the framework of the pore-scale simulation approach devoted to investigating the flame oscillation mechanisms and characteristics in a two-dimensional pseudo-irregular packed bed of particles generated using the Voronoi tessellation algorithm with constant pore size. The results demonstrate that unstable flame oscillations like flames with repetitive extinction and ignition (FREI) lead to the velocity pulsation in adjacent pore channels, where the stable flame fragments are exposed to the hydrodynamically-driven fluctuations. In the case of the flow rate close to the stability threshold, the flow pulses can reach a magnitude enough to shift the stable flame in the upstream quenching zone, promoting its oscillations with repetitive extinction and ignition. Results of parametric simulation with a variation of the particle diameter and flow velocity were summarized in the regime diagram revealing the transition between stable normal flame, FREI, and stable weak flame with the velocity reduction. The pulsating transitional regime was detected between the stable normal flame and the FREI. The amplitude and intensity of such pulsations increase monotonically with the flow rate reduction in a narrow range. Oscillations of the unstable flame and hydrodynamically-driven flame pulsations are similar in dynamics but different in their physical mechanisms. However, in irregularly packed beds of particles, both these phenomena unavoidably coexist and interfere significantly with each other due to the connectivity of the pore channels.

MvK mechanism dominated methane combustion over Ni-CeO2 derived from MOF by flame pyrolysis

Methane catalytic combustion is a significant technique in the fields of enhanced combustion, methane removal, and methane detection with the development of the natural gas industry and the demand for low-carbon combustion. Here, using a facile flame pyrolysis technique, two different types of Ni-CeO2 catalysts for methane combustion were synthesized from metal-organic framework (MOF) and metal salt (MS). Moreover, the Mars–van Krevelen (MvK) mechanism was investigated by combining kinetics with density functional theory (DFT) calculation. The performance shows that Ni-CeO2(BTC) catalysts give high activity owing to the large surface area, abundant Ni cations, and adsorbed oxygen. The high intrinsic activity is also related to a large number of oxygen vacancies which are essential for the MvK mechanism. In terms of kinetic analysis, the methane concentration determines the catalysis, and a redox MvK mechanism shows a good description of the experimental results. DFT calculations further clarify that, according to the MvK mechanism, the reduction of the active site is via dissociative adsorption of CH of CH4 on the catalytic site NiOCe, resulting in the formation of an oxygen vacancy, followed by dissociation of gas-phase oxygen on the oxygen vacancy to reoxidize the active site. Summarily, the MvK mechanism dominates the methane heterogeneous combustion over Ni-CeO2 and provides insight into the catalyst design for strengthening combustion.

Modulation of Heat Transfer in a Porous Burner Based on Triply Periodic Minimal Surface

Abstract The list of reacting flows in porous media applications is quite long, including porous media combustion, syngas production, and fuel cells. Porous media combustion is recognized as a cutting-edge combustion technique for increasing flammability. In this process, heat is transferred from the exothermic reaction zone to the incoming reactants through porous media. This role of porous media distinguishes reacting flows in porous media from free combustion processes. Local heat transfer, such as solid conduction, solid–solid radiation, and solid–gas convection, as well as the response behavior, are affected by the topology of the porous material. Theoretical studies indicate that continuously graded porous materials can significantly enhance the performance benefits of heat transfer. However, topology design is challenging for smooth graded porous media, and thus investigations of combustion within graded porous media are still required. In this study, we constructed a porous structure of type W/P/D/G (porosity ε = 0.3–0.5, hydraulic diameter dh = 1.33–3.86 mm) using a triply periodic minimal surface (TPMS), and a computational model of the combustion reaction in porous media was established to compare the range of flame stability within different pore types. In addition, topology gradation was achieved via TPMS to modulate the heat transfer to ensure the dependable functioning of premixed flames and improved heat recirculation. Heat transfer in the graded TPMS-based porous structure was analyzed numerically. The conclusions obtained from this study can effectively address the aforementioned challenges related to porous media burner design.

Conjugate heat and mass transfer at heterogeneous burning surface of AP/HTPB/Al composite propellant at 2–10 MPa

Conjugate heat and mass transfer at heterogeneous burning surface has significant effect on combustion characteristics of AP/HTPB/Al composite propellant. In the present study, the conjugate heat and mass transfer characteristics of AP/HTPB/Al composite propellant are studied experimentally and numerically based on formulated combustion model and experimental facilities. With the increase of pressure and Al content, flame temperature increases significantly and leads to stronger heat transfer to the burning surface. With the content of Al varying from 18 wt% to 9 wt%, the total heat transfer coefficient decreases for all the sampled positions of the burning surface with a maximum decrease of 12% at the AP/HTPB interface. It is found that heat convection accounts for more than 97% of the total heat transfer coefficient. However, with pressure increasing from 2 MPa to 10 MPa, the total heat transfer coefficient and the respective contribution of heat convection and radiation vary slightly. Owing to the heterogeneous sandwich-like structure of the propellant, the temperature distribution at the burning surface of propellant is highly non-uniform. Therefore, heat transfer characteristics vary significantly at different positions of the burning surface. The heterogeneity of the burning surface is exacerbated by the non-uniform distribution of heat transfer coefficients at different positions of the burning surface. The burning rate of the propellant varies from 0.5 mm/s to 9.5 mm/s for different positions of the burning surface at 10 MPa. The results of micro-scale flame structure show that the mass transfer of fine Al particles in the combustion of AP/HTPB/Al composite propellant can be basically divided into four stages, i.e., migration, melting, agglomeration and oxidation/combustion. Coral reef-like aggregate structure is formed by some of the fine Al particles before oxidation and combustion. Increasing pressure is conducive to avoiding formation of large Al agglomerates and enhancing mass transfer at the heterogeneous burning surface.

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.

Second stage porous media burner for syngas enrichment

The syngas production from hydrocarbons by porous media combustion (or conventional gasification) processes has been intensively and extensively studied due its calorific value and its applications in the energy sector. However, the syngas produced in a first stage gasifier can still have concentrations of light hydrocarbons (e.g., methane) which can be post-processing for further enrichment of hydrogen and carbon monoxide. The present work numerically investigates the performance of a second stage porous media burner to enrich the syngas content, mainly hydrogen and carbon monoxide. A one-dimensional model based on a two-temperature approximation is implemented, based on the PREMIX code, and supported by CHEMKIN and GRI-MECH 3.0 database and routines. From hybrid porous bed reactor experiments, five types of mixtures in the equivalence ratio range between 0.4 and 2.4 are tested: pure methane as baseline, pure Eucalyptus nitens syngas, pure Pinus radiata syngas, and two mixtures of methane with each of the biomass syngas in equal volume quantities, as transition points. The results obtained show that in the enriched syngas, in comparison to the first content after the first stage reactor, the concentrations of hydrogen and carbon monoxide increase, due to the partial oxidation of methane, however part of the hydrogen is consumed in the process. The intermediate species present in methane processing for pure syngas mixtures have broader reaction zones and in lower concentrations compared to the use of pure methane. For equivalence ratios greater than 1.9, pure syngas mixtures show higher conversion efficiencies compared to the baseline. At the equivalence ratio of 2.4, the pure syngas from Eucalyptus nitens and Pinus radiata has an energy return over energy invested (EROI) of 58.27% and 53.95%, respectively, and a maximum hydrogen and carbon monoxide yields of 31.66% and 48.40%, respectively. In the case of the Pinus radiata, the outlet syngas concentrations of the hydrogen and carbon monoxide on dry basis expose an increment about two times in comparison to the initial concentrations.

Preparation and thermal shock resistance investigation of ZTA-La2O3 composite ceramics for porous medium combustion materials

Zirconia-alumina composite ceramics are the preferred materials for high temperature and thermal shock applications because of their excellent mechanical properties. In this paper, La2O3 doped zirconia-alumina composite ceramics were prepared by using fused zirconia corundum and ZrO2 as starting materials. The effects of La2O3 on the phase composition, microstructure and properties of the composite ceramics were investigated by XRD and SEM. The results show that La2O3 can reduce the sintering temperature and improve the bending strength of the samples. The bending strength and fracture toughness of the composite ceramics with 1.5 wt% La2O3 sintered at 1480 °C are 299.17 MPa and 5.61 MPa m1/2, respectively. The strength loss rate of the sample after 30 thermal shocks (room temperature ∼ 1000 °C) is 2.67%, SEM shows that the grain sizes of alumina and zirconia in the sintered sample were about 5 μm and 2 μm, respectively. After 30 thermal shocks, the phase composition and microstructure of the sample have no significant change, indicating good thermal shock resistance of the composite ceramics. The excellent thermal shock resistance ensures porous medium combustion materials has a long service life.

Development in comprehensive CFD simulation of fire and explosion

In this study, an outline of developments is presented in comprehensive computational fluid dynamics (CFD) simulations of fire and explosion with respect to safety science. Fires can be divided into two types: conventional and spontaneous. Conventional fires typically address unwanted combustion in air, whereas spontaneous ignition fires typically address unwanted combustion in porous media. Given that the porous media has a dominant effect on the flow, the behavior of a spontaneous ignition fire completely differs from that of a conventional fire. Although a fire mainly comprises a diffusion flame, where the fuel and oxidant are initially separated, and low-speed flow that can be considered incompressible, explosion usually occurs with premixed combustion, where the fuel and oxidant are initially well-mixed, and high-speed flow where the compressible effect must be included. Owing to the complexity of fires and explosions, a comprehensive CFD simulation should carefully consider turbulence, turbulent combustion, two-phase flow (for cases where liquid droplets and/or solid particles are involved), conjugate heat transfer between gas and solid (including thermal radiation, convective heat transfer, and heat conduction inside solids), and pyrolysis of combustible solids. These interactive processes are also discussed. Furthermore, some developments by the author are presented along with illustrative simulations performed using Simtec software [1], which is used to implement the developments.

Experimental analysis and performance of refinery fuel gas on lean and rich fuel–air premixed combustion

Refinery fuel gas (RFG) is a blend of gas streams produced as by-products in various refinery processes. Large amounts of RFG are burning around the clock in the air with thermal energy waste and adverse effects on the environment. The present work investigates experimentally the performance of different compositions of RFG from lean to rich fuel–air premixed combustion, with focus on an efficient and low-emission combustion for its heat recovery. Radiant and porous media burners are the combustion technologies used. For the radiant burner, fuel compositions are propane and ethane with an average ethane ratio of 5%, 10%, and 15% (E05, E10, E15 respectively), which are contrasted with pure propane and liquefied petroleum gas (LPG). The pure gases fluxes are fixed as 1 and 2 m3/h for low and high flame configurations, respectively. The lean fuel–air flames investigated are stable for all ethane ratios with low carbon monoxide (CO) and nitrate oxides (NOx) emissions. Maximum temperature is given by pure propane flame (besides LPG), followed by E05, E10 and E15, for both flame configurations. Then, the radiation heat flux of ethane mixtures exhibits a reduction in comparison with LPG. For the porous media burner, lean and rich fuel–air mixture composed by natural gas (NG, 97% methane) and representative flue samples of 55% (C1) and 70% (C2) methane concentration are tested. The C1 and C2 flue samples present high combustion temperatures, showing a positive swap from NG. The NG rich-fuel combustion exposes high hydrogen conversion (27%) at high equivalence ratio (ϕ=1.8). Finally, for both burners lean fuel–air premixed combustion shows high combustion efficiencies, 74.8% with E05 and 80.7% with C1 flue sample.

Experimental and numerical investigation of flame stabilization and pollutant formation in matrix stabilized ammonia-hydrogen combustion

Ammonia (NH3) is a carbon-free fuel that offers an attractive alternative for reducing greenhouse gas emissions. However, the slow flame speed, low heating value, and emissions of nitrogen-containing pollutants present significant issues for practical combustion applications. To address these issues, we investigate the use of matrix stabilized combustion. In this type of burner, combustion is performed within an inert porous ceramic foam, heat is recirculated by solid conduction and radiation, which enhances flame speed and combustion stabilization, thereby permitting combustion over a wide range of equivalence ratio conditions. We present a new porous media burner (PMB) capable of stabilizing NH3/air flames at ambient conditions. An extensive experimental characterization of the stability of this burner is conducted with up to 30% by volume of hydrogen (H2) in the fuel stream. A 15:1 turndown ratio is demonstrated, with a high thermal power density of 62MWm−3. Concentrations of NO, unburnt NH3, and H2 in the exhaust stream are measured. Two regimes are identified for low NO operation: rich and very lean. For rich conditions, NO emissions decrease with increasing equivalence ratio and decreasing H2 blending. Unburnt NH3 emissions follow opposite trends. These measurements are complemented by simulations in which the burner is represented by a coupled solid-gas reactor network. This model captures the burner’s pollutant emissions to good accuracy and is used to analyze the mechanisms of pollutant formation.

Numerical evaluation of heat irreversiblity in porous medium combustion of third-grade fluid subjected to Newtonian cooling

Understanding combustion efficiency and reduction of environmental pollution are one of the main challenges encountered in porous medium combustion in many industrial and chemical engineering applications in recent times. In this regard, we present a numerical investigation of the thermal stability of the process and inherent irreversibility analysis for combustible third-grade fluid through a non-Darcian porous medium. The heated fluid flow is assumed to flow steadily through parallel Riga plates with asymmetrical convective cooling. The equations governing the flow are formulated and converted to a set of nonlinear Boundary-value problems by dimensional analysis. The dimensionless equations are solved numerically by spectral Chebyshev collocation method and validated with Shooting-Runge-Kutta method. Graphical and tabular illustrations are provided with sufficient explanations for the velocity, temperature, entropy generation, and irreversibility ratio profiles.

Numerical simulation of ethylene combustion characteristics under a new double-layer porous media burner

The efficient use of energy is an important strategic issue concerning our energy security and double carbon goals. Many industrial waste gases, such as oil refineries, contain a lot of ethylene, which is a gas that is harmful to people and the environment. The use of high-efficiency burners to reduce ethylene emissions is a proven and convenient way, so the improvement and optimization of high-efficiency burners has become a hot research issue nowadays. In this paper, a new porous media burner is designed, and a numerical model of the structure is established based on finite element software. The porous media burner is used to treat low calorific value ethylene and effectively reduce the direct emission of low calorific value ethylene in wastewater treatment and chemical industries. Based on the model, the combustion characteristics and pollutant emission characteristics of premixed gas at different equivalence ratios and different flow rates were investigated. The results show that the maximum combustion temperature can reach 1757K with a constant inlet flow rate of premixed gas and a full combustion at an equivalent ratio of 0.85; the maximum temperature increases rapidly with the increase of flow rate at an inlet flow rate of 0.65-0.85 m/s with a constant equivalent ratio, and the temperature rise effect increases slowly when the flow rate is higher than 0.85 m/s; the new porous media The maximum reduction of pollutant emissions of the new porous media burner reaches 37.9%. This study has a guiding effect on the design and engineering application of porous media combustion devices.

Effects of infrared radiant coating on ceramic porous media combustion using simulated natural gas and air mixture

Effects of infrared radiant coating on ceramic porous media combustion using simulated natural gas and air mixture

Numerical Simulation of Methane Combustion in Two-Layer Porous Media Under Oxy-Fuel Condition

Numerical Simulation of Methane Combustion in Two-Layer Porous Media Under Oxy-Fuel Condition

Efficient Combustion of Low Calorific Industrial Gases: Opportunities and Challenges

It is becoming increasingly important to develop effective combustion technologies for low calorific industrial gases (LCIG) because of the rising energy demand and environmental issues caused by the extensive use of fossil fuels. In this review, the prospect of these opportunity fuels in China is discussed. Then, the recent fundamental and engineering studies of LCIG combustion are summarized. Specifically, the differences between LCIG and traditional fuels in the composition and fundamental combustion characteristics are described. The state-of-the-art combustion strategies for burning LCIG are reviewed, including porous media combustion, flameless combustion, oxy-fuel combustion, and dual-fuel combustion. The technical challenges and further development needs for efficient LCIG combustion are also discussed.

Improving the thermophotovoltaic performance of a hydrogen-fueled planar combustor with four-corner inlets via partially inserting porous medium

Micro thermophotovoltaic (TPV) generator is a potentially efficient approach to achieve microscale fuel-to-electricity conversion with high-energy–density, high-efficiency, and small-scale portability, where micro combustor provides the high-temperature continuous combustion process as well as a heat source of thermal radiation. In this study, a hydrogen-fueled planar combustor with four-corner inlets by partially inserting porous medium is proposed and investigated by a three-dimensional numerical model of porous medium combustion with detailed reaction mechanism and local thermal non-equilibrium. The combustor performance with and without partially inserting porous medium under various inlet velocities Vin and equivalence ratios ϕ are compared and analyzed to reveal the inherent strengthening mechanism. Moreover, the effects of various porous parameters on the energy conversion performance are examined: the porosity ɛ, thermal conductivity ks, and porous medium filled volume. The results demonstrate that the inserted porous medium can significantly enhance combustion efficiency, combustor wall temperature and its uniformity, and the largest increase in mean wall temperature reaches 79.7 K at Vin = 9 m/s, resulting in a 35.8 % increase in micro-TPV system efficiency. As ɛ and ks decrease, the combustor energy conversion performance increases while the temperature uniformity decreases. A moderate channel width L = 2.0 mm is good for further improving temperature uniformity and energy conversion efficiency. This study paves the way for the improvement of efficient, practical, and portable microscale generators.

Study of flash pyrolysis and combustion of biomass powders using the Godbert-Greenwald furnace: An essential step to better understand organic dust explosions

An organic dust explosion is a heterogeneous system on a space and time scale. Predicting the parameters characteristic of its severity needs experimental and theoretical approaches to find the optimal compromise between consistency with reality and modelling time. A hybrid method is proposed to study flash pyrolysis and combustion of several organic powders (cellulose, wheat starch, oak wood, Douglas fir and olive pomace). A Godbert-Greenwald furnace was employed to perform the experiments to mimic the fundamental characteristics of a dust explosion: high particle heating rate, high reaction temperature and short residence times. At 973 K, the residence time is a critical parameter: the large particles of cellulosic compounds (wood, cellulose) do not reach their pyrolysis temperature and only fibres smaller than 20 or 30 µm are fully converted. As the particle size distribution of starch is smaller, heat transfer is not directly the limiting phenomenon but rather the strong tendency for powders to agglomerate during pyrolysis. At higher temperatures, secondary reactions of primary tars are evidenced, stressing the influence of the pyrolysis stage and leading to heterogeneous combustion. The composition of the pyrolysis gases as a function of the nature of the powder and the temperature was also determined. A lumped-kinetic model adapted to dust explosion was developed and validated for cellulose. The kinetics constants corresponding to levoglucosan to permanent gases and cellulose to char and water reactions are significantly different from those proposed by the literature, demonstrating that dust explosion kinetic parameters must be obtained under conditions consistent with such phenomenon.

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.

Pore-scale study of coke formation and combustion in porous media using lattice Boltzmann method

In-situ combustion (ISC) has long been recognized as a promising technique for heavy oil recovery. However, ISC includes multiple physicochemical processes, which are still poorly understood and difficult to predict and control. This study establishes a lattice Boltzmann (LB) model to simulate the two important aspects of ISC at the pore scale: coke formation and combustion. The LB model includes thermal expansion effects and solves the reactive air-coke interface without iterations. Moreover, this model improves upon previous models by considering both coke formation and two-step coke combustion, as well as the growth of solid geometry. Results show that the LB model correctly captures coke combustion properties. Meanwhile, the newly introduced coke formation and two-step combustion yield important findings. As heat released from combustion transfers downstream, oil cracking and coke formation ahead of the combustion front are successfully tracked. The generated coke fuels the upstream combustion, making the system self-sustained. During coke formation and combustion, four coke transition states are identified. In addition, a parametric study demonstrates that the large inlet oxygen content and driving force are desirable, while too high a driving force should be avoided as it causes high burning temperature. Furthermore, it suggests that the inlet air temperature should be set appropriately. On one hand, a high temperature may promote coke formation and retard the front propagation. On the other hand, a low temperature may slow down the combustion of coke 2, even though it is high enough to ensure the ignition of coke 1. The decelerated coke 2 combustion may further cause the insufficient heat release and the failed coke formation, thus inducing the early termination of combustion. Such effects of the inlet temperature indicate the necessity of considering coke formation and two-step coke combustion. These results help to improve the understanding and facilitate the development of ISC.