CH4 O2 Research Articles

Upgrading low-concentration oxygen-bearing coal bed methane by dual-reflux vacuum swing adsorption

Fugitive methane (CH4) is a typical by-product of mining processes, which is commonly known as coal bed methane (CBM) or coal mine gas (CMG). The capture of these CH4 gases can simultaneously avoid greenhouse gas emissions and provide extra energy benefits. However, the explosion risk of low-concentration CBM (CH4 molar fraction ≤ 30%) requires strictly safe operating protocols to conduct the capture process. Dual reflux vacuum swing adsorption (DR-VSA) is a promising candidate with a vacuum operating condition which can lower the explosion risk and simultaneously reach CH4 enrichment and O2 removal targets in product and effluent streams. Herein, a low-concentration oxygen-bearing CBM (20% CH4, 16% O2 and 64% N2) can be upgraded to 69.7 mol% in the product gas while ensuring an effluent concentration of 2.5 mol% by the DR-VSA cycle using ionic liquidic zeolites (ILZ) as the adsorbents. A rigorous safety analysis has been conducted to investigate the explosion risk in the adsorption column and product tank, suggesting that the DR-VSA process is a safe technology for upgrading low-concentration oxygen-bearing methane.

Analyzing the combustion characteristics of premixed methane-oxygen with different hydrogen addition ratios in a catalytic micro-combustor

Experimental and numerical study of premixed methane-oxygen combustion characteristics in a catalytic micro-combustor with different hydrogen addition ratios has been presented in this paper. The objective is to examine the effects of platinum catalysts and hydrogen additions on methane and oxygen combustion. The simulation of a three-dimensional micro rectangular model was carried out with a gas phase reaction mechanism combined with a surface reaction mechanism, and the model was validated with experimental results with a maximum deviation of just 1.56 percent. In the experimental phase, the effects of flammability limit, equivalence ratio, and inlet velocity were studied, while in the numerical phase, the effect of hydrogenation on combustion characteristics was investigated. Experimental results show that adding 10 % hydrogen to a premixed methane-oxygen mixture significantly increases its flammability limit. The maximum external wall temperature along the centerline was observed at an equivalent ratio of 1. Additionally, an increase in the inlet velocity leads to a corresponding increase in the external wall centerline temperature. Catalytic reaction wall temperatures show a gradual decline, with the highest temperatures located at the inlet. An observed trend revealed a decrease in the external wall centerline temperature as the hydrogen addition increased from 0 % to 40 % due to the reduced total heating value of the blended fuel. Due to its higher reactivity and ability to promote complete combustion, hydrogen also plays an important role in CH4 and O2 conversion. In addition, CO and CO2 emissions decreased with increased hydrogen percentage in fuel mixtures. Furthermore, hydrogen addition can reduce external wall heat losses as well. After adding 0–40 % hydrogen to a premixture of (CH4–O2) fuel, the total heat loss of the external wall was reduced from 19.48 W to 17.11 W.

Metal-Free Photocatalytic CO2 Reduction to CH4 and H2 O2 under Non-sacrificial Ambient Conditions.

Photocatalytic CO2 reduction to CH4 requires photosensitizers and sacrificial agents to provide sufficient electrons and protons through metal-based photocatalysts, and the separation of CH4 from by-product O2 has poor applications. Herein, we successfully synthesize a metal-free photocatalyst of a novel electron-acceptor 4,5,9,10-pyrenetetrone (PT), to our best knowledge, this is the first time that metal-free catalyst achieves non-sacrificial photocatalytic CO2 to CH4 and easily separable H2 O2 . This photocatalyst offers CH4 product of 10.6 μmol ⋅ g-1 ⋅ h-1 under non-sacrificial ambient conditions (room temperature, and only water), which is two orders of magnitude higher than that of the reported metal-free photocatalysts. Comprehensive in situ characterizations and calculations reveal a multi-step reaction mechanism, in which the long-lived oxygen-centered radical in the excited PT provides as a site for CO2 activation, resulting in a stabilized cyclic carbonate intermediate with a lower formation energy. This key intermediate is thermodynamically crucial for the subsequent reduction to CH4 product with the electronic selectivity of up to 90 %. The work provides fresh insights on the economic viability of photocatalytic CO2 reduction to easily separable CH4 in non-sacrificial and metal-free conditions.

Numerical study on effects of CH4–CO2 internal reforming on electrochemical performance and carbon deposition of solid oxide fuel cell

CH4–CO2 fueled solid oxide fuel cells (SOFCs) are the high-efficiency and low-carbon power generation technology. However, carbon deposition severely hinders its application. Here, the numerical model considering the complex anode CH4 internal reforming reactions, H2 electrochemical reaction, gas diffusion and charge transfer of SOFC using CH4–CO2 fuel has been established. The CH4–CO2 internal reforming and electrochemical reactions of SOFC under different CO2/CH4 ratios and temperatures are studied to reveal the optimal operating conditions. The distributions of different reaction rates, gas components and carbon deposition activity at the anode are analyzed. The results show that the increase of CO2 concentration can decrease the carbon activity but also cause the decrease of electrochemical performance. It is found that the good comprehensive performances including power density, carbon activity and methane conversion rate can be achieved under the CO2/CH4 molar ratio of 1. The thermodynamic calculation of three different methane reforming modes shows that CH4–CO2 reforming leads to the greatest carbon deposition risk compared with CH4–H2O and CH4–O2 reforming, implying the importance of anti-carbon for CH4–CO2 fueled SOFC.

Simulating the Effect of Electric Bias Voltages on the Electrical Characteristics of Oxyfuel Preheat Flame Using Reduced Combustion Mechanism

Abstract A three-dimensional computational model is presented in this paper that illustrates the detailed electrical characteristics, and the current–voltage (i–v) relationship throughout the preheating process of premixed methane-oxygen oxyfuel cutting flame subject to electric bias voltages. As such, the equations describing combustion, electrochemical transport for charged species, and potential are solved through a commercially available finite volume computational fluid dynamics (CFD) code. The reactions of the methane-oxygen (CH4–O2) flame were combined with a reduced mechanism, and additional ionization reactions that generate three chemi-ions, H3O+, HCO+, and e−, to describe the chemistry of ions in flames. The electrical characteristics such as ion migrations and ion distributions are investigated for a range of electric potential, V ∈ [−5 V, +5 V]. Since the physical flame is comprised of twelve Bunsen-like conical flames, inclusion of the third dimension imparts the resolution of fluid mechanics and the interaction among the individual cones. It was concluded that charged “sheaths” are formed at both torch and workpiece surfaces, subsequently forming three distinct regimes in the i–v relationship. The i–v characteristics obtained from this study have been compared to the previous experimental and two-dimensional computational model for premixed flame. In this way, the overall model generates a better understanding of the physical behavior of the oxyfuel-cutting flames, along with more validated i–v characteristics. Such understanding might provide critical information toward achieving an autonomous oxyfuel-cutting process.

Investigation of the Active-Site Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Using In Situ UV–Vis Spectroscopy

Among Cu-containing zeolites, Cu-CHA exhibits relatively high catalytic performance in direct oxidation of CH4 to CH3OH in a continuous-flow reaction of a CH4–O2–H2O gas mixture. The catalytic activity of Cu-CHA varies with its composition, e.g., Cu loading and Si/Al ratio, suggesting formation of different Cu active-site structures depending on the composition. In the present study, to identify catalytically active structures of Cu species, 18 different Cu-CHA catalysts having three Si/Al ratios and six Cu loadings at each Si/Al ratio were prepared, and their catalytic performance was evaluated. Based on the catalytic performance, the Cu species in Cu-CHA catalysts were categorized into three groups: those having high selectivity but low catalytic activity were formed at a low Cu/Al ratio (group A); those having high selectivity and high catalytic activity for CH3OH production were formed at a medium Cu/Al ratio (group B); and those having high activity for the complete oxidation of CH4 to CO2 were formed at a high Cu/Al ratio (group C). Thus, three representative catalysts corresponding to the three groups were selected at each Si/Al ratio and analyzed using in situ UV–vis spectroscopy, from which the spectral features of Cu species involved in the catalytic reaction were extracted from the difference between the spectra under reduction conditions and those under oxidation conditions. Based on the comparison of the spectral features with the simulated spectra of various Cu active-site structures in the literature, the main species of the three groups were suggested: Cu2+ species in the six-membered ring of SSZ-13 zeolites and water-coordinated CuOH+ in the eight-membered ring of SSZ-13 zeolite for group A; mono-(μ-oxo) di-Cu species for group B; and μ-η2:η2 peroxide di-Cu, trans-μ-1,2-peroxo di-Cu and/or mono-/bis-(μ-hydroxo) di-Cu species for group C. Therefore, the mono-(μ-oxo) di-Cu species in Cu-CHA is proposed as the active species for the catalytic direct oxidation of CH4 to CH3OH.

Dietary supplementation with xylooligosaccharides and exogenous enzyme improves milk production, energy utilization efficiency and reduces enteric methane emissions of Jersey cows

BackgroundSustainable strategies for enteric methane (CH4) mitigation of dairy cows have been extensively explored to improve production performance and alleviate environmental pressure. The present study aimed to investigate the effects of dietary xylooligosaccharides (XOS) and exogenous enzyme (EXE) supplementation on milk production, nutrient digestibility, enteric CH4 emissions, energy utilization efficiency of lactating Jersey dairy cows. Forty-eight lactating cows were randomly assigned to one of 4 treatments: (1) control diet (CON), (2) CON with 25 g/d XOS (XOS), (3) CON with 15 g/d EXE (EXE), and (4) CON with 25 g/d XOS and 15 g/d EXE (XOS + EXE). The 60-d experimental period consisted of a 14-d adaptation period and a 46-d sampling period. The enteric CO2 and CH4 emissions and O2 consumption were measured using two GreenFeed units, which were further used to determine the energy utilization efficiency of cows.ResultsCompared with CON, cows fed XOS, EXE or XOS + EXE significantly (P < 0.05) increased milk yield, true protein and fat concentration, and energy-corrected milk yield (ECM)/DM intake, which could be reflected by the significant improvement (P < 0.05) of dietary NDF and ADF digestibility. The results showed that dietary supplementation of XOS, EXE or XOS + EXE significantly (P < 0.05) reduced CH4 emission, CH4/milk yield, and CH4/ECM. Furthermore, cows fed XOS demonstrated highest (P < 0.05) metabolizable energy intake, milk energy output but lowest (P < 0.05) of CH4 energy output and CH4 energy output as a proportion of gross energy intake compared with the remaining treatments.ConclusionsDietary supplementary of XOS, EXE or combination of XOS and EXE contributed to the improvement of lactation performance, nutrient digestibility, and energy utilization efficiency, as well as reduction of enteric CH4 emissions of lactating Jersey cows. This promising mitigation method may need further research to validate its long-term effect and mode of action for dairy cows.

Highly Active and Durable Rh–Mo-Based Catalyst for the NO–CO–C3H6–O2 Reaction Prepared by Using Hybrid Clustering

We developed a method for preparing catalysts by using hybrid clustering to form a high density of metal/oxide interfacial active sites. A Rh–Mo hybrid clustering catalyst was prepared by using a hybrid cluster, [(RhCp*)4Mo4O16] (Cp* = η5-C5Me5), as the precursor. The activities of the Rh–Mo catalysts toward the NO–CO–C3H6–O2 reaction depended on the mixing method (hybrid clustering > coimpregnation ≈ pristine Rh). The hybrid clustering catalyst also exhibited high durability against thermal aging at 1273 K in air. The activity and durability were attributed to the formation of a high-density of Rh/MoOx interfacial sites. The NO reduction mechanism on the hybrid clustering catalyst was different from that on typical Rh catalysts, where the key step is the N–O cleavage of adsorbed NO. The reducibility of the Rh/MoOx interfacial sites contributed to the partial oxidation of C3H6 to form acetate species, which reacted with NO+O2 to form N2 via the adsorbed NCO species. The formation of reduced Rh on Rh4Mo4/Al2O3 was not as essential as that on typical Rh catalysts; this explained the improvement in durability.

Predictions for Observable Atmospheres of Trappist-1 Planets from a Fully Coupled Atmosphere–Interior Evolution Model

The Trappist-1 planets provide a unique opportunity to test the current understanding of rocky planet evolution. The James Webb Space Telescope is expected to characterize the atmospheres of these planets, potentially detecting CO2, CO, H2O, CH4, or abiotic O2 from water photodissociation and subsequent hydrogen escape. Here, we apply a coupled atmosphere–interior evolution model to the Trappist-1 planets to anticipate their modern atmospheres. This model, which has previously been validated for Earth and Venus, connects magma ocean crystallization to temperate geochemical cycling. Mantle convection, magmatic outgassing, atmospheric escape, crustal oxidation, a radiative-convective climate model, and deep volatile cycling are explicitly coupled to anticipate bulk atmospheres and planetary redox evolution over 8 Gyr. By adopting a Monte Carlo approach that samples a broad range of initial conditions and unknown parameters, we make some tentative predictions about current Trappist-1 atmospheres. We find that anoxic atmospheres are probable, but not guaranteed, for the outer planets; oxygen produced via hydrogen loss during the pre-main sequence is typically consumed by crustal sinks. In contrast, oxygen accumulation on the inner planets occurs in around half of all models runs. Complete atmospheric erosion is possible but not assured for the inner planets (occurs in 20%–50% of model runs), whereas the outer planets retain significant surface volatiles in virtually all model simulations. For all planets that retain substantial atmospheres, CO2-dominated or CO2–O2 atmospheres are expected; water vapor is unlikely to be a detectable atmospheric constituent in most cases. There are necessarily many caveats to these predictions, but the ways in which they misalign with upcoming observations will highlight gaps in terrestrial planet knowledge.

Exploring the connection of physical habitat health of the wetland with its gas regulating services

Many scholars assessed habitat health and measured gas regulation service of wetlands separately. However, no such targeted work was done to answer how habitat health can control gas regulation? Therefore, the present study investigated the role of habitat health on the gas regulation service of wetlands. Multi-parametric Machine Learning (ML) based modelling approach was applied to assess habitat health considering physical wetland hydrological factors and habitat degrading factors. Three primary atmospheric gas regulation functions of wetland, namely Methane (CH4) emission, Carbon (C) sequestration, and Oxygen (O2) release, were observed associating with habitat health. Very poor habitat health prevailed over 227 km2 of wetland, whereas very good habitat health covered 299 km2 of wetland. But out of the very good habitat health zone, 69% belonged to the perennial river of this region. On the other hand, it was estimated that the amount of annual CH4 emission, C sequestration, and O2 release from the wetlands of this region were respectively 8.01 × 1000 tons, 908.98 × 1000 tons and 664.51 × 1000 tons. The association of habitat health was positive with CH4 emission and negative with C sequestration and O2 release. Following the association, habitat health zone-wise variation of emission, sequestration, and release was also very prominent. The very good zone of habitat health was observed to have 60% higher CH4 average emission and 56% lesser C sequestration than very poor habitat health.

Assessment of (La1-xSrx)MnO3±δ perovskites as oxygen- carrier materials in chemical-looping processes

(La1-xSrx)MnO3±δ (x = 0, 0.3, 0.5, 0.7, 1) perovskites prepared by co-precipitation of nitrate salts have been evaluated for a chemical looping process with fuel CH4 and CO2 or O2 as oxidants. The CH4 conversion is initially, at small deficiencies, high and is directed to total oxidation products. Subsequently, and as the available oxygen is being reduced, the CH4 conversion decreases and the main reaction products become CO and H2. The higher the Sr content the higher the CH4 conversion and the concentration of created oxygen vacancies. When the CO2 splitting reaction is used for the oxidation of the reduced solid, only a part of the created deficiency is used up, which decreases as Sr content increases. SrMnO3 oxidates CH4 almost exclusively to CO2 and, even when strongly oxygen deficient, does not react with CO2. The oxygen transport capacity with CO2 as oxidant decrease as Sr content increases and attain average values of approximately 1 wt% for LaMnO3 and 0 wt% for SrMnO3. Instead, the oxygen transport capacity with O2 as oxidant increase as Sr content increase and attain average values of approximately 1 wt% for LaMnO3 and 6.5 wt% for SrMnO3. The experimental results have been explained on basis of proposed defect models for (La1-xSrx)MnO3±δ according to which the Mn4+ → Mn3+ reduction creates oxygen vacancies the concentration of which increases with Sr content and do not participate in the CO2 splitting reaction. On the other hand, the subsequent Mn3+ → Mn2+ reduction creates oxygen vacancies the concentration of which increases as Sr content decreases and their elimination, upon oxidation, participates in the CO2 splitting reaction.

Transient Potassium Peroxide Species in Highly Selective Oxidative Coupling of Methane over an Unmolten K2WO4/SiO2 Catalyst Revealed by In Situ Characterization

The oxidative coupling of methane (OCM) over a K2WO4/SiO2 catalyst was investigated using a CH4–O2 flow system. The addition of H2O to the reactant stream caused an increase in the CH4 conversion rate and C2 selectivity, consistent with the kinetics associated with the equilibrated OH radical generation, followed by H-abstraction from CH4 with OH radicals. The C2+ (a sum of C2 and higher hydrocarbons) yield reaches a maximum of 24.7% (C2H4 yield 17.0%) at a CH4 conversion of 47.1%, comparable to those over the previously reported Na2WO4/SiO2 catalyst. In situ near ambient pressure X-ray photoelectron spectroscopy uncovered K2O2 and KO2 species at and above 560 °C in the presence of gas-phase O2. These K2O2/KO2 species are considered to catalyze H2O activation for OH radical generation. By in situ high-temperature X-ray diffraction, K2WO4 maintained a crystalline phase in the bulk during the OCM reaction at 850 °C in contrast to the molten state of Na2WO4 previously reported. This indicates that the melting of the supported component and the covering of support are not essential to achieve a high C2 yield in supported alkali metal tungstate catalysts as long as inert support materials are used toward OCM.

Theoretical study of the catalytic effect of TM-CmHm (TM = Cr, Mn, Sc, Ti, V, and m = 4, 5) on the activation of oxygen at the cathode and methane at the anode in the fuel cell reaction “CH4–O2”: a DFT study

Theoretical study of the catalytic effect of TM-CmHm (TM = Cr, Mn, Sc, Ti, V, and m = 4, 5) on the activation of oxygen at the cathode and methane at the anode in the fuel cell reaction “CH4–O2”: a DFT study

Enhanced Catalytic NO Reduction in NO–CO–C3H6–O2 Reaction Using Pseudo-Spinel (NiCu)Al2O4 Supported on γ-Al2O3

Enhanced Catalytic NO Reduction in NO–CO–C₃H₆–O₂ Reaction Using Pseudo-Spinel (NiCu)Al₂O₄ Supported on γ-Al₂O₃

Conversion of Methane in Plasma of Pulsed Nanosecond Discharges

Investigations of methane conversion in mixtures CH4–CO2 and CH4–O2 of various composition, pressure, and temperature processed by pulsed nanosecond discharges of various types and durations were carried out. The main products of carbon dioxide conversion (dry reforming) of methane were carbon oxide (CO), hydrogen (H2), ethane (C2H6), and carbon (C). Gas mixture treatment by discharges with pulse duration 1, 5, and 15 ns demonstrated that the energy efficiency of carbon dioxide conversion of methane depends on the power of the discharge pulse. Dependencies of CH4 conversion on gas mixture pressure for spark, diffuse, and corona discharge showed that the most efficient dry reforming of methane occurred in spark discharge with pulse duration of 15 ns at a pressure of 1 atm. with energy cost of conversion of one CH4 molecule being 15 electron-volts per molecule. The main products of oxidative conversion of methane were ethane and ethylene C2H4. It was shown that the catalyst NaOH/CaO was considerably more effective than nickel catalyst, and the use of it allowed to increase methane oxidative conversion degree by more than an order of magnitude. It was found that CH4 oxidative conversion and C2H6 yield were growing up with an increase in temperature, whereas C2H4 yield began to decrease at a temperature more than 420 K. The most efficient oxidative conversion of methane was realized in spark discharge in the presence of NaOH/CaO catalyst with energy cost of conversion of one CH4 molecule being 50 electron-volts per molecule.

Investigation of intra-day variability of gaseous measurements in sheep using portable accumulation chambers.

Portable accumulation chambers (PAC) enable short-term spot measurements of gaseous emissions including methane (CH4), carbon dioxide (CO2), and oxygen (O2) consumption from small ruminants. To date the differences in morning and evening gaseous measurements in the PAC have not been investigated. The objectives of this study were to investigate: 1) the optimal measurement time in the PAC, 2) the appropriate method of accounting for the animal’s size when calculating the animal’s gaseous output, and 3) the intra-day variability of gaseous measurements. A total of 12 ewe lambs (c. 10 to 11 months of age) were randomly selected each day from a cohort of 48 animals over nine consecutive days. Methane emissions from the 12 lambs were measured in 12 PAC during two measurement runs daily, AM (8 to 10 h) and PM (14 to 16 h). Animals were removed from Perennial ryegrass silage for at least 1 h prior to measurements in the PAC and animals were assigned randomly to each of the 12 chambers. Methane (ppm) concentration, O2 and CO2 percentage were measured at 5 time points (T1 = 0.0 min, T2 = 12.5 min, T3 = 25.0 min, T4 = 37.5 min, and T5 = 50.0 min from entry of the first animal into the first chamber) using an Eagle 2 monitor. The correlation between time points T5-T1 (i.e., 50 min minus 0 min after entry of the animal to the chamber) and T4-T1 was 0.95, 0.92, and 0.77 for CH4, O2, and CO2, respectively (P < 0.01). The correlation between CH4 and CO2 output and O2 consumption, calculated with live-weight and with body volume was 0.99 (P < 0.001). The correlation between the PAC measurement recorded on the same animal in the AM and PM measurement runs was 0.73. Factors associated with CH4 production included: day and time of measurement, the live-weight of the animal and the hourly relative humidity. Results from this study suggest that the optimal time for measuring an animal’s gaseous output in the PAC is 50 min, that live-weight should be used in the calculation of gaseous output from an animal and that the measurement of an animal’s gaseous emissions in either the AM or PM does not impact on the ranking of animals when gaseous emissions are measured using the feeding and measurement protocol outlined in the present study.

Catalytic oxidation of methane to methanol over Cu-CHA with molecular oxygen

Catalytic production of CH3OH in a CH4–O2–H2O flow reaction is improved using Cu-CHA having improved redox property involved in the C–H activation of CH4.

Chemiluminescence of electronically excited species during the self-ignition of acetylene behind reflected shock waves

Acetylene is an effective additive to various rocket fuels. Acetam, an equilibrium highly concentrated solution of acetylene in liquid ammonia, is a promising fuel for use in liquid−propellant jet engines. To gain insights into the processes occurring in such systems and provide means for predicting their characteristics, the self-ignition of model acetylene−oxygen−argon mixtures is experimentally and theoretically studied. The experiments are performed behind reflected shock waves in a temperature range from 1100 to 1870 K at a total gas concentration of [M] ≈ 1.0 × 10−5 mol/cm3, with chemiluminescence signals from electronically excited C2* (λ ≅ 516 nm), CH* (429 nm), CO2*(363 nm), and OH* (308 nm) recorded. Numerical simulations within the framework of various detailed kinetic mechanisms closely reproduce the measured temperature dependences of the ignition delay time for the tested mixtures, but in some cases fail to satisfactorily describe the time profiles of the emission signals from C2*, CH*, CO2*, and OH*. The performed numerical simulations make it possible to explain the observed characteristic features of the emission profiles of electronically excited C2*, CH*, CO2*, and OH* during the self-ignition of C2H2–O2−Ar mixtures. Sensitivity analysis demonstrates that the existence and characteristics of the preignition stage in the CO2* and OH* emission signals are largely determined by the kinetic behavior of intermediate species, such as the ketene molecule and CH2 radical. Based on the results obtained, a new detailed kinetic mechanism has been proposed for describing processes in the combustion products of acetylene-containing propellants, in particular chemiluminescent glow, which are of considerable interest for the development of new technologies for astronautics and aeronautics.

Computational investigation of real fluid effects in cryogenic laminar premixed CH4–O2 flames

This study investigates the effects of real fluid thermodynamic and transport properties on the structure and propagation of canonical planar, unstretched, premixed methane-oxygen flames at transcritical conditions, which is relevant to the ignition phase in a liquid rocket engine. The ideal gas law is inapplicable at such extreme conditions, and real fluid thermodynamic and transport properties are required to accurately model the combustion physics at supercritical conditions.The computational framework used in this study integrates real fluid property estimation into the steady-state, freely-propagating flame solver available in the Cantera combustion suite [Goodwin et al., 2017]. The Peng-Robinson equation of state provides closure for the thermodynamic terms in the governing equations. High-pressure thermal conductivity and mass diffusivity coefficients are obtained using the Chung and Takahashi correlations, respectively. A mixture-averaged mass diffusion model is assumed, while neglecting the Soret and Dufour effects. The reaction rate terms are modeled with the standard Arrhenius ideal gas kinetics model using the CH4–O2 mechanism from Lindstedt. The effects on laminar flame structure are presented. It is found that the choice of chemical mechanism has a much greater influence on mass burning rates than the real fluid effects. Real fluid properties produce lower mass burning rates by just ∼10% near the critical region, while the laminar flame speeds are reduced by a factor of ∼5. It is shown that using dilute gas transport properties in combination with a real fluid equation of state and differential diffusion effects may cause a qualitatively different, non-monotonic density profile in the flame.

Operability of a premixed combustor holding hydrogen‐enriched oxy‐methane flames: An experimental and numerical study

This work presents a detailed investigation on the stability, flame macrostructure and combustion characteristics of oxy-methane/hydrogen (H2CH4O2CO2) flames in a premixed swirl-stabilized model gas turbine combustor. The effects of equivalence ratio, inlet Reynolds number and inlet swirl number on stability and combustion characteristics were examined experimentally and numerically. A base operation case was selected as a reference to study the effects of the different parameters with equivalence ratio of 1.0, bulk mean throat velocity of 5.2 m/s, and swirl number of 0.98. Fixed compositions of the oxidizer (30%O2/70%CO2) and fuel (50%H2/50%CH4) streams were considered throughout the study. Large eddy simulation (LES) technique was adopted to compute the premixed turbulent flame regime. Experimentally captured flame images were compared with the flame contours obtained from the numerical simulations, and good conformations were observed. The results showed that near-stoichiometry flames exhibit strong inner recirculation zone (IRZ) and are stabilized between two shear layers. At lower equivalence ratios, the flames are corner-stabilized, and the flow is dominated by a single primary eddy in the IRZ. The calculated strain rates predict flame quenching at equivalence ratio of 0.5, which was also confirmed experimentally. It was observed that the effect of Reynolds number on the flame shape is not significant; however, the flow is dominated by larger primary eddy with strong IRZ as the Reynolds number increases. The IRZ also gets stronger with the increase in the flow swirl number. At higher swirl numbers, the flame stabilization occurs at the inner shear layer and the outer recirculation zone (ORZ) diminishes. It was found that the chemical time scale for all cases under investigation is shorter than the Kolmogorov time scale, signifying that the chemical reactions are dominated by the laminar flame speed.