Methane-air Combustion Research Articles

Effect of hydrogen addition on the methane-air combustion under normal and reversed gravity

Effect of hydrogen addition on the methane-air combustion under normal and reversed gravity

Insight into ventilation air methane combustion of ultralow sub-nanometer palladium catalyst within the MFI zeolite

Insight into ventilation air methane combustion of ultralow sub-nanometer palladium catalyst within the MFI zeolite

Development of a reduced mechanism for methane combustion in OpenFOAM: A computational approach for efficient and accurate simulations

The primary aim of this study is to develop a more efficient reaction mechanism for accurately simulating methane combustion, with a specific focus on ignition delay, laminar flame speed, and 2-D simulated flames, while also reducing computational time. Ten reduced reaction mechanisms for methane combustion were evaluated, with only one, "SK30," meeting the required accuracy standards. However, SK30 proved to be computationally intensive when simulating a 2-D premixed flame at a microscale. To address this challenge, a two-step reduction process was implemented. Firstly, an automated algorithm utilized direct relation graphs and sensitivity analysis with ignition delays as a reference to streamline the mechanism while maintaining accuracy. Subsequently, the second step involved identifying key reactions that had a more significant impact on flame speed than ignition delay through sensitivity analysis. Any missing reactions were then added judiciously, prioritizing the retrieval of the missing but important reactions to overall accuracy while minimizing computational cost. This process resulted in a novel mechanism comprising 25 species and 132 reactions for methane-air combustion. The validity of this mechanism was confirmed through comparison with a benchmark model, demonstrating satisfactory agreement in 1-D flame speed and 2-D premixed flame modeling. Most notably, the new mechanism substantially reduced processing time, achieving a 50 % speedup compared to SK30.

A numerical study of catalytic combustion of methane-air in excess oxygen and deficient oxygen environments with increasing initial pressure: A molecular dynamic approach

In the low-temperature range, catalytic combustion results in few emissions of nitrogen oxide and happens without a visible flame. This work investigates the effect of initial pressure (IP) on catalytic methane-air combustion (CMAC) in a microchannel using the molecular dynamics (MD) method. Palladium particles with an atomic ratio of 4 % were used as a catalyst. The study also investigates the CMAC in two environments: excess oxygen (EO) and deficient oxygen (DO). The research investigates the changes in density (Den), velocity (Velo), temperature (Temp) profiles, heat flux (HF), thermal conductivity (TC), and combustion efficiency (CE). The results of the MD simulation indicate that the maximum values of Den and velocity decrease as the IP increases to 10 bar. This reduction was more pronounced in the EO medium than in the DO medium. The maximum values of density and velocity decrease to 0.105 atom/Å and 0.17 Å/ps, respectively, in the EO medium. These values decrease to 0.080 atom/Å and 0.20 Å/ps, respectively, in the DO medium. Additionally, the HF and TC values decrease in both mediums, with the EO medium showing values of 1839 W/m2 and 1.01 W/m.K, and the DO medium showing values of 1869 W/m2 and 1.04 W/m.K. If there was a DO medium, the atoms and particles of the system had a greater ability to heat transfer to different parts. Therefore, TC and HF are more in this case.

Exploring combustion characteristics of a novel micro combustor with a fan swirler for thermophotovoltaic system

The combustion and thermal characteristics of non-premixed methane (CH4)–air flames in a swirl micro combustor are numerically investigated for potential use in a thermophotovoltaic (TPV) system. Swirling flows associated with methane–air combustion are simulated using a Reynolds stress model and detailed chemistry of GRI-Mech 3.0. The optimal condition for improved emission and combustion performance is established by taking into account six blade angles and three fuel-lean equivalence ratios, to consider among other geometric features of the fan swirler and operating conditions.The reactive swirling flows in a micro combustor with a fan swirler are modified by altering the blade angle (θb). For θb = 5°, 10°, and 15°, the swirl micro combustor with a two-vortex structure exhibits a significant enhancement of the fuel–air mixing and thermal performance compared to no-swirl combustor. As θb increases, inner vortices close to the fuel stream and outer vortices near the combustor wall behind the fan blades are developed according to the interaction of swirling flows and recirculating flows due to the blockage effect of the fan blades. For θb = 15°, the strong interaction between two vortex regions maximizes combustion efficiency. The combustion efficiency is highest at θb = 15° and lowest at θb = 75°. For three fuel-lean conditions of θb = 15°, the flame zone is enlarged towards the combustion chamber wall, resulting in a high and uniform wall temperature. The resulting radiant emitter efficiencies for a TPV system are increased by 28.3 %–35.1 % compared to that of the no-swirling micro combustor. The results indicate that the fan swirler has recirculation flows for the best emitter and combustion efficiency unlike vane-type swirlers.

Thermal and structural performance of additively manufactured ceramic porous media burners

This study examines the effect of porosity, topology and material composition on the performance of additively manufactured ceramic porous media burners. Thermal-structural simulations of triply periodic minimal surfaces (TPMS) were performed to investigate thermal-stress magnitude and distribution. Alumina and mullite TPMS structures were 3D-printed and tested in a methane-air combustion experiment, where the mullite structures showed superior durability compared to alumina. X-ray imaging revealed notable correlation between predicted high tensile stress regions and experimental crack formation. At equivalent porosity and cell size, TPMS structures with higher specific surface area and tortuosity were found to have lower thermal strain and propensity for structural failure. Structures with these features, namely diamond and I-WP, demonstrated favorable thermal and structural behavior under identical thermal conditions. These findings can guide the design and manufacturing of high-temperature applications with embedded porous structures, such as heat exchangers, catalytic converters, and thermal management systems.

Comparison of the flame stabilities during oxy-methane and air-methane combustion in a two-layer porous burner

The development of the oxy-fuel porous media combustion technique not only takes advantage of the characteristics of porous media combustion, such as a relatively low lean-burn limit and small volume, but also dramatically facilitates carbon capture and storage. In this paper, a two-layer porous burner model is established, in which a two-temperature equation model is adopted for calculation purposes. Differences in combustion behaviour between oxy-fuel and air-fuel conditions are compared. The results show that the difference in physical properties between CO2 and air is the main reason for the significant variation of combustion behaviours under the conditions of oxy-fuel compared to air-fuel. The specific performance is lower combustion temperature and flame propagation speed. The temperature under oxy-fuel combustion conditions is 300 K lower than that under air-fuel combustion conditions at an equivalence ratio of 0.6, and the stable range of oxy-fuel combustion is only approximately 50% of air-fuel combustion. The impact of porous media material parameters on combustion behavior has been investigated. These variables include the equivalence ratio, material thermal conductivity, volume heat transfer coefficient, and extinction coefficient, while the ratio of O2/CO2 remains fixed at 0.21/0.79. The influence of these variables on the stable velocity range and temperature field is consistent, but a large difference in values occurs. Reducing the thermal resistance of the burner by adjusting the properties of the porous matrix can increase the velocity limit of stable combustion. However, even if the thermal conductivity and extinction coefficient are increased by 5 times, there will be no significant impact on the stable range. Reducing the pore size of the combustion zone by 50% will increase the stable range by over 100%.

Investigating the Ignition and Stability Limits of Premixed Methane/Air Combustion in Micro-Channels

Understanding and improving the performance of miniature devices powered by micro-combustion have been the focus of continued attention of researchers recently. The goal of the present work is to investigate the behavior of premixed methane–air combustion in a quartz microreactor with an externally controlled wall temperature. Specifically, the impacts of the flow inlet velocity, the equivalence ratio, and the microreactor channel size are examined. This study is conducted by means of computational simulations, and the results are validated against prior experimental data, as well as by other similar studies in the literature. Utilizing simulation results with detailed chemistry, the present work provides more in-depth insight into a variety of phenomena, such as ignition, flame propagation, flames with repetitive extinctions and ignitions (FREI), and flame stabilization. In particular, the ignition, the flame span, and the FREI-related characteristics are scrutinized to understand the underlying physics of the flame stability/instability modes. It is shown that the flames appear stable at higher inlet velocities, while the FREI mode is detected at a lower inlet velocity, depending on the equivalence ratio and the channel size. The findings also explain how different operating conditions impact the flame characteristics in both stability modes.

Accelerating Chemical Kinetics Calculations With Physics Informed Neural Networks

Abstract Detailed chemical kinetics calculations can be very computationally expensive, and so various approaches have been used to speed up combustion calculations. Deep neural networks (DNNs) are one promising approach that has seen significant development recently. Standard DNNs, however, do not necessarily follow physical constraints such as conservation of mass. Physics Informed Neural Networks (PINNs) are a class of neural networks that have physical laws embedded within the training process to create networks that follow those physical laws. A new PINN-based DNN approach to chemical kinetics modeling has been developed to make sure mass fraction predictions adhere to the conservation of atomic species. The approach also utilizes a mixture-of-experts (MOE) architecture where the data is distributed on multiple subnetworks followed by a softmax selective layer. The MOE architecture allows the different subnetworks to specialize in different thermochemical regimes, such as early stage ignition reactions or post-flame equilibrium chemistry, then the softmax layer smoothly transitions between the subnetwork predictions. This modeling approach was applied to the prediction of methane-air combustion using the GRI-Mech 3.0 as the reference mechanism. The training database was composed of data from 0D ignition delay simulations under initial conditions of 0.2–50 bar pressure, 500–2000 K temperature, an equivalence ratio between 0 and 2, and an N2-dilution percentage of up to 50%. A wide variety of network sizes and architectures of between 3 and 20 subnetworks and 6,600 to 77,000 neurons were tested. The resulting networks were able to predict 0D combustion simulations with similar accuracy and atomic mass conservation as standard kinetics solvers while having a 10-50× speedup in online evaluation time using CPUs, and on average over 200× when using a GPU.

Numerical study of the impact of hydrogen addition, swirl intensity and equivalence ratio on methane-air combustion

Abstract Hydrogen-blended fuel is a promising resource for future generations of gas turbine engines, due to its capability of reducing carbon-based emissions. This paper presents a numerical study to assess hydrogen-enriched combustion in a laboratory-scale burner operating at a high turbulence level and under lean and stoichiometric burning conditions. Moreover, a wide range of H2 (up to 90 %) is used for enriching CH4-air combustion in combination with two different swirl levels. The results show that a high swirl intensity results in shorter flames, due to the increased turbulent intensity, which reduces the flame surface area and uniformness the reacting zone. Besides, increasing swirl intensity further increase flame temperature for a given H2-blended fuel. Overall, the results suggest that high swirl intensity in combination to lean mixtures is favorable when using H2-blended fuel with high H2 concentrations. The simulation results also demonstrate that considering radiation heat loss is influential, as it yields a reduction of the outlet temperature by not less than 100 K, bringing down NO x emissions by half.

Reduced chemical kinetic model for CH[formula omitted]-air non-premixed flames including excited and charged species

Electric fields can impact small laminar flames by changing their shape and overall behavior by acting on charged species produced in combustion. However, no reduced chemical kinetic model has been developed considering both major species and minor species related to flame characterization and flame behavior in the presence of an electric field. This study presents a reduced chemical kinetic model for methane-air combustion which includes minor excited species (CH* and OH*) and charged species (H3O+, HCO+, C2H3O+, CH5O+, O2−, OH−, e−, CO3−, CHO2−, O−, CHO3−). The results employing the reduced chemistry model have been validated for a two-dimensional flame geometry by comparison with (i) detailed chemistry simulation results for species location and peak values, and (ii) experimental CH* chemiluminescence location, considering the self-repulsion of charges yet without externally applied electric field to the flame.This reduced chemical kinetic model, with 45 species and 216 reactions, shows a computational demand one-third that of employing its equivalent detailed chemistry (83 species and 394 reactions). The reduction is modest but significant considering that high fidelity is needed to capture the behavior of the chemi-ion and chemiluminescent species. Future works will involve the use of this model for simulations predicting flame behavior with applied electric field (i.e., field strength ≠0 kV/cm).

Experimental investigation on flame stability and emissions of lean premixed methane–air combustion in a developed divergent porous burner

Flame stability and pollutant emissions are issues of the greatest concern for low-calorific-value gas combustion. Although the conventional straight porous burner is conducive to improving the flame stability, it still cannot meet the industrial demand. To improve the flame stability and reduce the pollutant emissions during fuel-lean combustion, a divergent double-layer porous burner was creatively proposed in this study. The influences of the inlet velocity and equivalence ratio on the flame stability, temperature distributions, the emissions were experimentally investigated in detail. The results showed that the flame front gradually moved downstream, the preheating zone length increased as the inlet velocity increased. Compared with the traditional straight porous burner, the divergent porous burner extended the blowout limit by 300%. Since the burner was operated under fuel-lean conditions (the equivalence ratio range was 0.45–0.6), the CO emissions were relatively high when the inlet velocity was less than 0.4 m/s. When the inlet speed was equal to or greater than 0.4 m/s, the maximum CO emissions concentration was 132 ppm. The burner showed superior performances in terms of NO emissions, with emissions concentrations of less than 20 ppm under all test conditions. In summary, the great advantage of the divergent porous burner lies in extending the flame stability limit. For clean combustion it is recommended that the burner be operated at a higher velocity (greater than or equal to 0.4 m/s) to obtain lower CO and NO emission levels.

From Fossil‐Fueled to Hydrogen‐Fueled Annealing Furnaces: Effects on the Oxidation of Stainless Steels

The possibility of replacing methane with hydrogen as the fuel gas in an annealing furnace is studied regarding the oxidation of ferritic AISI 441 and austenitic AISI 304 stainless steels during simulated short‐term annealing. Oxide‐scale formation using the simulated atmospheres based on methane–air, methane–oxygen, and hydrogen–oxygen combustions with different oxidant gas excesses is compared at steel grades’ furnace temperature ranges in an annealing process. Thermogravimetry is used to determine variations of weight gain during oxidation, and field‐emission scanning electron microscopy and glow discharge optical emission spectroscopy are used to characterize oxide scales. The transition to H2 as the fuel gas is more suitable for the ferritic grade than the austenitic grade at the studied temperatures. High water vapor content in hydrogen– and methane–oxygen combustion atmospheres promotes breakaway oxidation for both steel grades. Nodule formation is significantly observed under the most demanding annealing conditions for the ferritic grade, while only the lightest conditions remain without the signs of breakaway oxidation for the austenitic grade. Changes in the oxide‐scale structures produced in both hydrogen–oxygen and methane–oxygen atmospheres are in significant, which can facilitate the exchanges between these atmospheres in relation to methane–air combustion.

Quantitative analyses of NO formations from different reaction pathways in methane-air combustion at various pressures and temperatures

Numerical calculations of the laminar counterflow diffusion flames of methane-air are conducted to quantitatively study NO formations from different reaction pathways (mechanisms) at various pressures and/or initial air temperatures. A modified method, which deactivates the initiating reactions in a specific reaction pathway only for NO production while maintaining interactions with other pathways, is developed to calculate NO formation in each mechanism. Results indicate that in the thermal, prompt, N2O, and NNH reaction mechanisms, the NO production rate largely increases as the pressure or the initial air temperature increases, and in particular, as these two parameters are increased together as in a practical combustion system. In the thermal pathway, as the pressure is increased to 15 atm, NO is produced almost entirely on the fuel-lean side. In the prompt pathway, NO production generally occurs in regions with the progress variable ranging from 0.8 to 0.95; the maximum NO production rate moves from the fuel-rich towards the stoichiometric region as the pressure is increased. NO formation from the N2O or NNH mechanism occurs mainly at high pressures, but the corresponding production rate is an order of magnitude smaller than that from the thermal or prompt mechanism, respectively. In all the reaction mechanisms, NO consumption occurs through the reburn mechanism on the fuel-rich side, and the consumption rate significantly increases as the pressure and initial air temperature increase. Results herein provide a quantitative understanding of the effects of pressure and temperature on NO formations from different reaction mechanisms.

Hollow spheres as inert packed bed from lean to rich combustion in porous media

• Hollow spheres were used in filtration combustion processes. • Lean and rich fuel-air filtration combustion was investigated. • Numerical and experimental comparison of solid and hollow spheres. • Hydrogen and syngas were produced at high temperatures. Hollow spheres as inert packed bed is an innovative technology with high potential in filtration combustion. Their lower specific density, geometry, and the radiation inside the sphere enhance the overall heat transfer of the sphere, and therefore, increase the effective conductivity of the packed bed, while the other thermo-physical properties remain unchanged. The present work investigates numerically and experimentally the effects of the packed bed made of hollow alumina spheres filled with air, from lean to rich premixed methane-air combustion within this porous media, with focus on the production of hydrogen and carbon monoxide species on rich conditions. The experimental part considers the equivalence ratio from 0.8 to 1.3. The numerical approach contemplates a one-dimensional model based on two-temperature approximation (gas and solid) with a four-step chemical reaction mechanism for homogeneous reactions at equivalence ratios between 0.4 and 1.6, for different spheres geometries. The results of experimental and numerical investigations are in good agreement. Both indicate that the use of hollow spheres packed bed (HS), in contrast to the packed bed made of the solid spheres (SS), shows higher temperatures and flame front velocities in all the equivalence ratio ranges. At the stoichiometric equivalence ratio, the numerical investigation in the HS case results in higher temperatures of 168 K and 21 K, in the solid and gas phases, respectively, in comparison with the SS case. The combustion products for different spheres geometries and equivalence ratios present the same trend and values for both HS and SS cases. The yields of carbon monoxide and hydrogen goes up to 37% and 42%, respectively.

Investigation on effects of hydrogen addition to the thermal performance of a traditional counter-flow combustor

The hydrogen applicability of current gas turbines designed for traditional fuels needs to be improved. The influence of hydrogen addition to the performance of swirling methane-air combustion has been investigated for a counter-flow combustor. The combustor was simulated under certain swirl intensity for a 100 kW micro gas turbine fuelled by methane–hydrogen mixture. The predicted outlet temperature and the measured value based on practical combustors were compared. The effects of air distribution scheme and hydrogen content in fuel mixture on thermal characteristics were analyzed from the perspectives of multi fields and synergy. The results show that the scheme with primary air of 50%, secondary air of 20% and dilution air of 30% leads to an ideal temperature distribution at the exit. With the increase of hydrogen addition ratio, the outlet temperature decreases and the emission of pollutant NOX is controlled within an ultra-low range. When the hydrogen volume content exceeds 20%, the hydrogen addition has a great impact on the field synergy in the combustor, which is beneficial to the uniform temperature distribution at the exit and the relatively low flow resistance. These findings may supply guidance for the redesign and operation of the gas turbine using hydrogen-doped fuels.

Enhancement of Methane-Air Combustion with Increasing Oxygen Ratio

Increasing combustion efficiency and minimal emissions of pollutants are among the fundamental issues in combustion research. In this regard, the phenomenon of combustion and its influencing factors, such as the fuel / air ratio as one of the most important factors in flame formation, have attracted the attention of many researchers. In this study, the effect of increasing the oxygen ratio over its proportion in the stoichiometric reaction for methane – air combustion was studied. The equations for the conservation of mass, momentum, and energy for a methane-air mixture have been solved by using CFD. A FORTRAN-90 program based on GENMIX will be developed. In order to verify the validity of the present program, the present numerical results were compared with the previous experimental results, and the comparison showed a good agreement. From the results, it was found that increasing the proportion of oxygen leads to an improvement in the temperature distribution inside the combustion cylinder, and there is no effect was observed on the nature of the velocity profiles inside the combustion space. As for the combustion of fuel, there was an improvement in the fuel combustion rate with the increasing in oxygen ratio.

Application of large eddy simulation in methane-air explosion prediction using thickening flame approach

In this work, large eddy simulation (LES) is carried out to investigate the stoichiometric methane-air explosion through using thickening flame model. The power-law model is used to evaluate the efficiency factor and a reduced two-step combustion mechanism is considered for the methane-air combustion. The adaptive grid refinement is adopted and different thickening factors are simulated. The numerical model is validated by comparing the predicted results with experimental data. It is seen that the numerical model can reproduce the experiment data quantitatively and qualitatively. All the stages of tulip shaped flame are well repeated by LES in both 2D and 3D forms. For the flame propagating in smooth duct with tulip shaped flame formation, the effect of efficiency factor can be ignored due to its small value. The thickening factor should be estimated through using thermal thickness of premixed flame and it needs to ensure that the thickened flame can cover at least 5 grid cells. The ignition radius needs to be larger than the thickness of thickened flame to ensure the premixed flame to grow steady after the ignition.

Investigation of the partially premixed turbulent combustion through the PRECCINSTA burner by large eddy simulation

The turbulent combustion processes within full annular PRECCINSTA combustor were investigated numerically in the present study. Dynamic thickened flame model was implemented to simulate the partially premixed turbulent methane-air combustion processes. The governing equations considering wall heat loss were solved by using the large eddy simulation method. In order to validate the implemented model, the temperature and main species distribution profiles within the burner were compared to the experimental data and good agreement was observed. The vortex motion characteristics of the cold flow field, the characteristics of the flame surface area, heat release rate and velocity vector changes with time during the circumferential cross-fire process were studied. Results showed that considering the wall heat loss significantly improved the prediction accuracy of the temperature and intermediate species distributions causing flame structure change from “M-shaped” to “V-shaped”. Numerical analysis proved that the circumferential velocity in different direction induced by the swirling flow near the inner and outer walls of the annular combustor are coupled with the combustion heat expansion, causing difference gain effects to the flame propagation on two sides of the combustor.

Optimal swirl number and equivalence ratio for minimizing the soot and NO emissions in turbulent methane combustion

The simultaneous effect of the swirl number (SN) and equivalence ratio on NO and soot emissions in turbulent methane–air combustion was numerically investigated. The realizable k-ε model has been applied for modeling turbulence and eddy dissipation (ED) and probability density function (PDF) models were adopted for chemical reaction. The general gradient diffusion hypothesis (GGDH), high order algebraic model (HOGGDH), and simple eddy diffusivity model have been applied for modeling the turbulent heat flux (THF) vector. The Brooke–Moss model has been employed to predict the soot particle quantities. Comparing the results of the different numerical models (PDF and EDM with algebraic THF) with available experimental data indicated that employing the second-order models significantly leads to the modification of predicting temperature distribution in EDM. However, due to the effect of the PDF model on soot modeling, the PDF model provides more accurate results. The numerical simulations have been performed for various SNs (SN = 0–1.33) and equivalence ratios (0.125–0.75). In all of equivalence ratios the flame temperature and pollutants emission (NO and soot) were strongly affected by the SN. Also, the SN has different effects on NO and soot emission. Finally, for each of NO and soot emission, a correlation relative to effective combustion factors such as SN and equivalence ratio was presented. It is found that by increasing the equivalence ratio, the NO emission increases with a power of 2.3, and soot emission decreases with a power of 8.7.