N2 Dilution Research Articles

Experimental investigations on laminar burning velocity variation of CH4 + air mixtures at elevated temperatures with CO2 and N2 dilution

Experimental investigations on laminar burning velocity variation of CH4 + air mixtures at elevated temperatures with CO2 and N2 dilution

Experimental study on the instability characteristics of CO2-diluted n-butane/air cellular flames on McKenna burner

Present experimental study reports on the effects of CO2 dilution on intrinsic instabilities of non-adiabatic n-butane/air cellular flames on McKenna burner at initial 300 K temperature and 0.1 MPa pressure. The Planar Laser Induced Fluorescence (OH-PLIF and CH2O-PLIF) optical diagnostic technology was utilized to probe the structure of cellular flames. The results show that the cellular flames can be obtained under the conditions with lean, stoichiometric and rich CO2 diluted n-butane/air mixtures. At lean conditions, hydrodynamic instability was dominant and the cellular flames were formed by connected wrinkles. However, at rich conditions, diffusive-thermal instability was dominant and the cellular flames were formed by separated cells. It confirmed that the CO2 dilution can enhance flame instability significantly, as well as the enhanced effect was positive correlation with CO2 dilution ratio. Furthermore, the enhanced effects of flame instability was stronger by CO2 dilution than by N2 dilution. The chemical and thermal effects by CO2 dilution were attributed to enhance flame instability.

Investigations on Emission Characteristics of Methane in Small Scale a Swirl Flameless Combustor: Using Preheating Air Diluted CO2 and N2 Gas at Various Temperatures

In order to reduce the damage to the environmental, it is desirable to have low emission combustion and high-efficiency system of operation. In achieving this, the multi-stages and exhaust gas recirculation (EGR) were adopted to reduce NOx formation during combustion. In this paper, a numerical study of the impact of diluted preheated oxidizer variables on NOx emissions from methane combustion was performed. This study utilizes Computational Fluid Dynamics (CFD) analysis to determine the influence of preheating fuel and air on the combustion efficiency when asymmetric vortex flameless was applied on different supply tangential air location at different oxygen concentrations. The design parameters utilized are diluted N2 and CO2, oxygen concentrations of 10%, 7% and 5%, and air temperatures that were set at 300 K, 500 K, 700 K and 900 K, respectively. The exhaust gas recirculation (EGR) was simulated using CO2 gas and N2 gas. The results show that by oxygen concentration has direct relationship with NOx emission for all equivalence ratios. Apart from that, CO2 dilution air offers better results than N2 dilution in reducing NOx emissions. In terms of preheating air, higher NOx emission was observed with the increase of air temperature. However, the effect of preheating air on the NOx emission is greater than the effect of preheating of fuel. Based on these results, numerous ports were considered important to achieve a good mix of air and fuel. The analysis outcome shows that more inlet tangential air has a significant influence on combustion temperature and NOx emission.

Thermodynamic simulation of the co-gasification of biomass and plastic waste for hydrogen-rich syngas production

Co-gasification of biomass and plastics is a waste-to-energy conversion that reduces waste volume and enhances product quality, thus improving overall process efficiency. Thermodynamic equilibrium simulation of biomass co-gasification with plastic is studied using HSC Chemistry Software. Properties of effluent products are evaluated as product gas yields, higher heating value and carbon conversion efficiency to gas. Among different plastics tested, polypropylene is the most beneficial. Increasing plastic-to-biomass ratio, up to 5, improves hydrogen and CO yields and increases the HHV of the gas from 21 to 25 MJ/kg. Using CO2 as gasifying agent lowers H2 quantity due to reverse water gas shift reaction and reduces HHV. Air gasification decreases the HHV, compared to oxygen, due to N2 dilution effect. Steam is the highly efficient gasifying agent, and steam-to-carbon ratio of unity is a good compromise for high gas yield and heating value. Finally, thermodynamic data are validated with published experimental work.

Enhancing ϕ-sensitivity of ignition delay times through dilution of fuel-air mixture

The high ϕ-sensitivity (η) of ignition delay time (τIDT) is one of the desirable fuel properties for the high-load extension of advanced compression ignition engine. Recent studies revealed the effectiveness of a high dilution rate (xD) for enhancing η at the engine-relevant conditions. This study aims to quantify the effect of dilution on the η of isooctane using a combined experiment-simulation approach. The ignition delay of the isooctane/air mixture was measured with an Advanced Fuel Ignition Delay Analyzer (AFIDA) over the temperature range of 623 – 923 K at 10 bar pressure with global ϕ = 0.3 – 0.6, with and without 28.6% of additional N2 dilution. For precise evaluation of experimental η, the facility effect of the AFIDA experiment was characterized with three-dimensional computational fluid dynamics (3-D CFD) simulation. The temperature in the combustion chamber from 3-D CFD indicated a substantial temporal dependency, varying up to ∼52 K by charge-cooling of fuel injection and heat transfer from the wall. We introduced the dimensionless number θ(t) for characterizing the temporal profile of chamber temperature. Consideration of the facility effect using θ(t) resulted in better agreement between the experimental η and zero-dimensional (0-D) kinetics simulation. The refined η were then further utilized to quantify the effectiveness of dilution to η. The extent of η enhancement with dilution strategy was maximized at the low-temperature chemistry regime, increasing η by 77% with a 28.6% dilution rate. Further analysis on the dilution effect was carried out using 0-D kinetics simulation, revealing the critical dimensionless numbers relevant to the effectiveness of dilution to η enhancement. This study is the first experiment-simulation combined research to quantify the effect of dilution on η, facilitating the kinetics model refinement for better reproduction of ϕ-sensitivity.

Experimental and numerical study of the laminar burning velocity of syngas in oxyfuel conditions

ABSTRACT This study evaluates the laminar burning velocity of oxyfuel syngas mixtures. Experimental measurements were performed in standard conditions (298 K and 1 atm), varying the oxyfuel syngas composition (H2/CO/CO2/O2) and equivalence ratio for lean mixtures. The experimental setup uses the heat flux method, validated with CH4/air and H2/CO/air measurements. The results were compared with six chemical kinetic mechanisms developed for syngas, aiming at evaluating their predictive capabilities. The mixtures presented a tendency to the appearance of cellular instability in the flame front that was prevented to a certain extent. The CO2 effects on the flames were explored through numerical analysis, isolating the thermodynamic properties, transport properties, thermal radiation, and chemical effects. The CO2 or N2 effect as bath gas in the oxidizer is also evaluated in terms of the laminar burning velocity and mixture’s effective Lewis number. Limitations due to cellular instability were studied, analyzing the influence of fuel dilution and composition on the onset of cellular instability at the flames. The results showed a good prediction of the experimental data by the evaluated kinetic mechanisms. The expected reduction in the laminar burning velocity with CO2 dilution is primarily due to the effect of carbon dioxide’s thermodynamic properties, followed by its transport properties (when compared to N2 dilution), chemical effects, and heat losses by radiation. Cellular instability is attenuated with high dilution levels due to the reduction of hydrodynamic instability. At the same time, the increase of the H2 concentration in the mixture leads to more unstable flames due to both hydrodynamic and thermal-diffusive instability. A laminar burning velocity threshold above which the measurements were not possible due to the effects of cellular instabilities was identified and analyzed in detail.

Thermodynamic limitations to direct CO2 utilisation within a small-scale integrated biomass power cycle

Partially recycling CO2-rich exhaust gases from a syngas fuelled internal combustion engine to a biomass gasifier has the capability to realise a new method for direct carbon dioxide utilisation (CDU) within a bioenergy system. Simulation of an integrated, air-blown biomass gasification power cycle was used to study thermodynamic aspects of this emerging CDU technology. Analysis of the system model at varying gasifier air ratios and exhaust recycling ratios revealed the potential for modest system improvements under limited recycling ratios. Compared to a representative base thermodynamic case with overall system efficiency of 28.14 %, employing exhaust gas recycling (EGR) enhanced gasification system efficiency to 29.24 % and reduced the specific emissions by 46.2 g-CO2/kWh. Further investigation of the EGR-enhanced gasification system revealed the important coupling between gasification equilibrium temperature and exhaust gas temperature through the syngas lower heating value (LHV). Major limitations to the thermodynamic conditions of EGR-enhanced gasification as a CDU strategy result from the increased dilution of the syngas fuel by N2 and CO2 at high recycling ratios, restricting equilibrium temperatures and reducing gasification efficiency. N2 dilution in the system reduces the efficiency by up to 2.5 % depending on the gasifier air ratio, causing a corresponding increase to specific CO2 emissions. Thermodynamic modelling indicates pre-combustion N2 removal from an EGR-gasification system could decrease specific CO2 emissions by 9.73 %, emitting 118.5 g/kWh less CO2 than the basic system.

Laminar flame speed of H2/CH4/air mixtures with CO2 and N2 dilution

The laminar flame speeds of H2/CH4/air mixtures with CO2 and N2 dilution were systematic investigated experimentally and numerically over a wide range of H2 blending ratios (0–75 vol%) with CO2 (0–67 vol%) and N2 (0–67 vol%) dilution in the fuels. The experimental measurements were conducted via the Bunsen flame method incorporating the Schlieren technique under the condition of equivalence ratios from 0.8 to 2.0. To gain an insightful understanding of the experimental observations, detailed numerical simulation was carried out using Chemkin-Pro with GRI3.0-Mech. The experimental measurements were also used to validate the corresponding performance of a semiempirical correlation derived through asymptotic analysis method coupled with the reduced chemistry mechanism. The results showed that at lower H2 fraction (xH2 ≤ 0.5), the laminar flame speeds of H2/CH4/air mixtures displayed great linearly increase with the growth of H2 fractions. The combustion of mixtures with low H2 contents was dominated by CH4 conversion which was mainly controlled by the increasing OH radicals produced from the key oxidation reactions of H + O2 = O + OH. With the further increase of H2 fractions, the methane-dominated combustion gradually transformed into the methane-inhibited hydrogen combustion, resulting to the growth of laminar flame speeds was dramatical and non-linear. Due to the larger heat capacity and chemical kinetic effect, CO2 presented a stronger dilution effect on reducing the laminar flame speeds than N2. With the addition of CO2, the increasing stronger competition for H radical through CO + OH = CO2 + H with H + O2 = O + OH due to the significant reduction of H mole fractions, leading to the larger decrease of laminar flame speeds of mixtures. Besides, although the contribution of thermal effect of CO2 decreased near the equivalence ratio, the thermal effect of CO2 still preformed the dominated contribution to the total dilution effect. A comparison between the experimental data and estimated results using the semiempirical correlation showed that, the correlation using new modified coefficients provided the satisfactorily accuracy predictions on the laminar flame speeds of diluted H2/CH4/air mixtures at lower xH2 (xH2 ≤ 0.5) and lower xdilution (xdilution = 0.25). The estimated results were generally located within a deviation range of ±20% errors except for two unsatisfactory eases occurred at conditions of xH2 = 0.75 and xCO2 = 0.67. The considerably poor predictions were attributed to the significantly variation of the chemical kinetics under high H2 content and large CO2 dilution conditions.

Detonation behaviors of stoichiometric H2-O2 mixture diluted with He, N2, CO2 at different initial pressures

In this study, the detonation parameters for stoichiometric H2-O2 mixtures diluted with varying amount of He, N2, CO2 are measured and analyzed in a small bore tube at different initial pressure P0. Signals of flame front and cell patterns are obtained by photodiodes and smoked foil technique, respectively. Experimental results indicate that as dilution mole fraction increases, the average detonation velocity deviates more from the theoretical value, and the detonation velocity deficit increases. The detonation velocity deficit of the H2-O2-N2 mixtures is relatively smaller than that of the H2-O2-He mixtures. The maximum detonation velocity occurs at 65% He dilution fraction. The cell sizes of the H2-O2 mixture diluted with CO2 are most affected by P0. The dilution mole fraction less than 10% has little effect on the cell size at higher initial pressure. The H2-O2-He mixture can make the induction zone length shorter than that of H2-O2 mixture as P0 increases. The detonation instability increases as N2 or CO2 dilution fraction increases, and the cellular instability decreases as He dilution fraction increases. Detonation parameters such as reaction zone thickness are calculated to introduce a cell size predicting formula based on the Ng model. Linear relationships are determined by: λ = 42.457 Δi (H2-O2-N2 mixture), λ = 48.945 Δi (H2-O2-He mixture).

Experimental and numerical study of the effects of N2 dilution and CH4 addition on the laminar burning characteristics of syngas

The study of combustion characteristics of complex biomass syngas under changing conditions is helpful for its practical utilization. In this paper, the effects of N2 dilution (dilution rates from 0% to 60%) and CH4 addition (mixing ratios from 0% to 20%) on the laminar flame properties of H2/CO/N2 syngas were researched. The laminar burning velocities (LBVs) were obtained by constant volume combustion chamber system. Five common mechanisms were used for numerical simulation using the CHEMKIN software package and the results were compared with the measurement data. The Li mechanism that was well matched with the experimental data was selected for kinetic analysis. The results indicate that the LBVs of H2/CO/N2/air mixtures decrease due to the dilution of N2. The addition of CH4 decreases the LBVs of H2/CO/N2/CH4/air mixtures and the LBVs of fuel-rich mixtures decrease faster. Sensitivity analysis illustrates that R29 is the most important branching reaction affecting LBV in H2/CO/N2/air premixed flame, the net reaction rate of all branched reactions and the mole fraction of H and OH in the mixture decrease due to the increase of N2 content. After adding CH4, the dominant branching reaction becomes R1. Due to the increase of CH4 addition ratio, the concentrations of H and O decrease, but the concentrations of OH and CH3 increase slightly. The chemical effect of adding CH4 increases LBV, but the physical effect decreases LBV. The reduction in the proportion of chemical effect is the reason for the faster reduction of LBV in the fuel-rich region. Finally, the formation of NO was analyzed in detail using the GRI 3.0 mechanism. The results show that N2 dilution reduces the mole fraction of NO, but CH4 addition increases the mole fraction of NO. Pathway analysis shows that N2 dilution and CH4 addition decrease and increase the effect of thermal mechanism on NO generation, respectively.

The effects of N2 and steam dilution on NO emission for a H2/Air micromix flame

The different effects of N2 and water steam dilution on the NO emission from a micromix H2/air flame were experimentally and numerically studied. NO emission was measured using Fourier transform infrared spectroscopy gas analyzer (FTIR). Numerical simulation of the H2/air flame was carried out with realizable k-ε model and the Eddy Dissipation Concept (EDC) model. Two pseudo species were introduced in the simulations to numerically isolate the thermal-dilution, direct chemical and third-body effect of the steam. The experimental and numerical results show that NO emission with steam dilution is about 20%∼50% of the emission with N2 dilution at the same equivalence ratio (φ) and dilution ratio (D). The present simulation results show that the thermal-dilution and direct chemical effects of steam account for 50.8%∼92.1% and 5.8%∼15.8% of the total NO emission, respectively, while the third-body effect of steam shows less impact on NO emission. Specifically, the third-body effect slightly promotes the formation of H and NO at lower φ (0.6–0.8) and suppresses the formation of H and NO at higher φ (0.9–1.0) through inhibiting the third-body reaction of H + O2 + M ⇔ HO2 + M. In addition, the direct chemical and third-body effects of steam always decrease the formation of NO from the NNH route.

Preliminary Results of Biomass Gasification Obtained at Pilot Scale with an Innovative 100 kWth Dual Bubbling Fluidized Bed Gasifier

Biomass gasification is a favourable process to produce a H2-rich fuel gas from biogenic waste materials. In particular, the dual bubbling fluidized bed (DBFB) technology consists of the separation of the combustion chamber, fed with air, from the gasification chamber, fed with steam, allowing to obtain a concentrated syngas stream without N2 dilution. In a previous work, an innovative design of a DBFB reactor was developed and its hydrodynamics tested in a cold model; in this work, the novel gasifier was realized at pilot scale (100 kWth) and operated for preliminary biomass gasification tests. The results showed a high-quality syngas, composed of H2 = 35%, CO = 23%, CO2 = 20%, and CH4 = 11%, as a confirmation of the design efficacy in the separation of the reaction chambers. The dry gas yield obtained was 1.33 Nm3/kg of biomass feedstock and the carbon conversion was 73%. Tars were sampled and measured both in the raw syngas, giving a content of 12 g/Nm3, and downstream from a traditional conditioning system composed of a cyclone and a water scrubber, showing a residual tar content of 3 g/Nm3, mainly toluene. The preliminary tests showed promising results; further gasification tests are foreseen to optimize the main process parameters.

Non premixed operation strategies for a low emission syngas fuelled reverse flow combustor

In the present study, we investigate the combustion of low-calorific value syngas in a reverse-flow combustor at a fuel thermal input of 3.3 kW. A reverse-flow combustor is known to improve stability while reducing pollutant emissions due to internal recirculation. The study is significant for renewable energy applications of syngas obtained from biomass gasification. Equivalence ratio and O2% in the oxidizer have been varied to establish different modes of operation at atmospheric pressure (P = 1.013 bar). Temperature, heat release, CO, and NOx emissions have been measured to characterize the operation modes. The heat release varies significantly with O2% and influences the CO emissions. The NOx emission is insensitive to the parameters due to the low-calorific value of syngas which limits the maximum temperature to around 1700 K. The lowest CO emission of 32 ppm and NOx < 1 ppm has been achieved in the present study. The combustion efficiency of the combustor is greater than 99%. The variations in pollutant formation were explained using reactor network models. The instantaneous images of heat release revealed key features of the operation modes. As the N2 dilution level is increased, the chemical timescale increases, leading to a volumetric heat release. Suppression of localized heat release reduces the maximum temperature and consequently the NOx emissions.

Influence of gasoline fuel formulation on lean autoignition in a mixed-mode-combustion (deflagration/autoignition) engine

Stoichiometric spark-ignition engines suffer efficiency penalties due to throttling losses at low loads, a low specific-heat ratio of the stoichiometric working fluid, and limits on compression ratio due to end-gas autoignition leading to undesirable knocking. Mixed-Mode Combustion (MMC) mitigates these shortcomings by using a lean working fluid where a spark-initiated pilot-stabilized deflagrative flame front is followed by controlled end-gas autoignition. This MMC study investigates the effects of initial conditions (intake air temperature, intake pressure, equivalence ratio, and intake oxygen fraction) on autoignition tendency of four gasoline-range fuels with varying properties and composition. The use of fuels with varying octane sensitivity (S) allowed exploring the importance of low-temperature heat release in triggering autoignition. Fuels with high S were less reactive for conditions that promote low-temperature chemistry (operation at high intake air pressure or without N2 dilution). Conversely, an Alkylate fuel with low S showed a greater autoignition resistance at operating conditions that were unfavorable for low-temperature chemistry. Next, the effect of residual gas composition on autoignition tendency of fuels was examined with a chemical-kinetics model. Among the various molecules in the residual gas, nitric oxide (NO) enhanced the low-temperature chemistry and increased the autoignition tendency most significantly. The fuels’ autoignition response to increasing NO amount corroborates the experimental observations. Next, the sequential autoignition of the end-gas was assessed to be less impacted by thermal stratification because of lean mixtures showing relatively less low-temperature chemistry, when compared to stoichiometric mixtures. Next, the effect of changing equivalence ratio on the autoignition was found to be similar for all fuels, regardless of their S. With changing intake air temperature, the response of fuels’ autoignition tendency depended on the dilution level used. At high dilution (i.e. low intake [O2]), fuels’ reactivity increased with increasing intake air temperature. In contrast, for operation without dilution, the autoignition tendency of the low-S Alkylate fuel decreased with increasing intake air temperature, while that of high-S High Cycloalkane fuel still increased with increasing intake air temperature. In conclusion, conventional octane metrics (RON and MON) have utility in assessing the autoignition tendency under lean MMC operation. Moreover, the fuel requirements for MMC align with that of stoichiometric operation: i.e., high RON and high S fuels are desirable for stable non-knocking operation.

Experimental study of N2 and CO2 dilution in CH4 fuel stream with buoyancy-induced low-stretch diffusion flames

This study experimentally investigated the structures, dynamics and extinction behaviors of buoyancy-induced ultra-low-stretch diffusion flames with N2/CO2 dilution in a CH4 stream, using porous spherical burners with large radii. In quasi-steady states associated with low dilution and comparatively small fuel mixture injection speeds (uF), as stretch rate dropped and uF enlarged, the flame standoff distance increased, while the burner surface temperature decreased due to surface heat losses. The flame temperature, Tf, dropped at lower uF and smaller stretch rates, primarily because of reduced heat generation and increased flame radiative loss, respectively. The dilution effect of N2/CO2 reduced Tf. Moreover, the chemical and thermal effects of CO2 further decreased Tf and soot formation. With increased dilution, two instability patterns occurred before extinction. One was a stripes/holes pattern, evolving from waves/bumps and cellular flames, at higher uF. The other was a periodic holes pattern appearing at lower uF. These patterns were associated with Rayleigh-Taylor instability, thermal-diffusive instability and instability related to heat loss. The dilution extinction limits at varied stretch rates increased with increases in uF until they reached constant levels, corresponding to the intrinsic dilution limits of N2 and CO2 for CH4, and the suppression effect of CO2 was greater than that of N2. At the high dilution extinction limit, extinction was independent of uF and stretch rate, but dominated by lower heat release compared with robust flames, and also by substantial flame heat losses. At the low dilution extinction limit, both gas-phase and burner surface heat losses contributed to extinction, furthermore, lower stretched flames had obvious higher dilution extinction limits, primarily due to smaller gas-phase heat losses requiring less heat generation to balance. This work enriches the knowledge of ultra-low-stretch gaseous diffusion flames, especially the flame structure, instability and extinction, and also provides information regarding the dilution gas effects.

Effects of CO2 and N2 dilution on the characteristics and NOX emission of H2/CH4/CO/air partially premixed flame

For the combustion of the mixture of blast furnace gas, natural gas, and coke oven gas in industrial burners, how to improve combustion efficiency and reduce pollutant emission are of significance. To accomplish this, an industrial partially premixed burner with a combustion diagnostic system is used to experimentally reveal the characteristics and NOX emission of H2/CH4/CO/air flame under CO2, N2, and CO2/N2 (replacing half of N2 with CO2) dilution. NOX emission and flame length, temperature profile, along with CO, CH4, and CO2 concentration profiles are analyzed with the three diluents in the fuel stream under different dilution rates (0–32% by volume). Experimental results show that for lean H2/CH4/CO combustion, greater proportions of CO2 in the diluent affect flame characteristics in various ways. These effects include longer flame length, lower highest flame temperature, the highest flame temperature being located farther away from the nozzle, and the highest CO2 concentration being located nearer the nozzle. Furthermore, results of CO, CH4, and CO2 concentrations indicate that chemical reactions in the flame are significantly affected by CO2 owing to the series reaction CH4⇌CH3→CO⇌CO2. Finally, increasing diluents or the ratio of CO2 in diluents has the benefit of reducing NOX emission.

Experimental and kinetic studies of laminar burning velocities of ammonia with high Lewis number at elevated pressures

The present study aims to expand the available database of laminar burning velocities to a higher initial pressure for ammonia kinetic model validation and to investigate the ammonia flame chemistry especially for the effect of N2 addition on N2 and NOx production. In the present study, a high-pressure constant-volume combustion vessel with the pressure release tank was developed and validated. The variations of Lewis number with diluent (He/N2, He/Ar) ratio and the relationships of Lewis number and adiabatic flame temperature with initial pressures were investigated to prevent the flame instability effects and create safe experiments at high pressures. The laminar burning velocities of NH3/O2/He/Ar mixtures with Lewis number around 1.5 and the fixed adiabatic flame temperature of 2250 K at elevated pressures (up to 1.5 MPa) were measured and the overall reaction order of about 1.6 was determined, besides, the effects of pressure on laminar burning velocities and NOx formations were investigated. Moreover, to provide more useful validation data at high pressure, the laminar burning velocities of NH3/O2/He/Ar mixtures at different adiabatic flame temperatures and initial pressure of 1.0 MPa were presented. Additionally, the predictions of four kinetic models on the experimental data were compared and the discrepancies between the calculated and measured results were found. Since ammonia contains nitrogen element, the effects of N2 addition on the laminar burning velocities and the production of N2 and NOx in ammonia combustion were investigated and the physical and chemical effects of N2 addition were separated. Results showed that the laminar burning velocities and NOx concentration for NH3/O2/N2 flames decreased with the increase of N2 dilution and a slight non-linear tendency was observed. Both physical and chemical effects inhibit the N2 production in the NH3/O2/N2 mixtures. The chemical effects play a dominant role in the inhibition of N2 production at the N2 dilution rates below 0.3. The main kinetic effect of N2 addition on NO production is that the addition of N2 shifts the reaction N + NO <=> N2 + O to the left-hand side.

Comparative study on flame instability and combustion characteristics of CH4/O2/CO2 and CH4/O2/N2 mixtures

To investigate flame instabilities and combustion characteristics of oxygen-enriched methane in a closed chamber, a series of combustion experiments on the CH4/O2/CO2 and CH4/O2/N2 mixtures were conducted at the initial temperature and pressure of 301 ± 2 K and 1.0 atm. The results show that the distorted tulip flames and corrugated flames, which can be differentiated by a 2D plane of firstly wall-touching pressure (Pwall/P0) versus flame position (Ztipwall/L), are for the first time observed in the oxygen-enriched methane mixtures. Particularly, the corrugated flames can emerge by increasing the reactivity of CH4/O2/N2 mixtures. There are distinct flame instabilities between the CO2/N2 diluents. Specifically, the CH4/O2/CO2 mixtures always show stronger Thermal-Diffusive instability whereas the CH4/O2/N2 mixtures always exert stronger Darrieus-Landau instability. There is a linearly positive correlation between the Darrieus-Landau instability and oxygen enrichment ratio for both mixtures. In contrast, the oxygen enrichment ratio plays positive (N2 dilution) and negative (CO2 dilution) role for the Thermal-Diffusive instability. A linear prediction between the maximum combustion pressures and Sl·E/lfg is proposed by considering the Darrieus-Landau instability. However, the Rayleigh-Taylor instability, as an additional mechanism for flame distortion, makes the linearity less accurate. As the oxygen enrichment ratio is further increased, the Richtmyer-Meshkov instability instead of the Rayleigh-Taylor instability plays a role in converting distorted tulip flames to corrugated flames, and could revalidate this linear prediction.

Effect of diluent gas on propagation and explosion characteristics of hydrogen-rich syngas laminar premixed flame

Syngas, a promising renewable alternative fuel, shows different combustion characteristics depending on its exact composition. In this study, the effect of different diluents on the laminar premixed flame of hydrogen-rich syngas was experimentally investigated using a 90% H2/10% CO/air mixture in a constant-volume combustion bomb. In particular, this investigation revealed the effects of varying the equivalence ratio (0.6, 0.8, and 1.0) and dilution content (10%, 20%, and 30%) on the flame propagation and explosion characteristics of syngas/air mixture. The results showed that for a given dilution gas content, the flame propagation speed under CO2 dilution is less than that under N2 dilution; however, the flame acceleration effect of CO2 dilution is significantly higher than that of N2. As the dilution increases, the rate of pressure rise and the maximum combustion explosion pressure gradually decrease. For a given equivalence ratio, the maximum rate of pressure rise and the deflagration index of the flame mixed with N2 were higher than those for CO2.

Oxidative Coupling of Methane over Pt/Al2O3 at High Temperature: Multiscale Modeling of the Catalytic Monolith

At high temperatures, the oxidative coupling of methane (OCM) is an attractive approach for catalytic conversion of methane into value-added chemicals. Experiments with a Pt/Al2O3-coated catalytic honeycomb monolith were conducted with varying CH4/O2 ratios, N2 dilution at atmospheric pressure, and very short contact times. The reactor was modeled by a multiscale approach using a parabolic two-dimensional flow field description in the monolithic channels coupled with a heat balance of the monolithic structure, and multistep surface reaction mechanisms as well as elementary-step, gas phase reaction mechanisms. The contribution of heterogeneous and homogeneous reactions, both of which are important for the optimization of C2 products, is investigated using a combination of experimental and computational methods. The oxidation of methane, which takes place over the platinum catalyst, causes the adiabatic temperature increase required for the generation of CH3 radicals in the gas phase, which are essential for the formation of C2 species. Lower CH4/O2 ratios result in higher C2 selectivity. However, the presence of OH radicals at high temperatures facilitates subsequent conversion of C2H2 at a CH4/O2 ratio of 1.4. Thereby, C2 species selectivity of 7% was achieved at CH4/O2 ratio of 1.6, with 35% N2 dilution.