Ignition Delay Times Of Methane Research Articles

Autoignition enhancement of methane by admixture of low fraction of acetaldehyde: Simulations and RCM experiments in stoichiometric and rich mixtures

AbstractThe effect of small fractions of acetaldehyde (CH3CHO) on the ignition delay time of methane (CH4) was examined at high pressure. Measurements are reported for the ignition delay time obtained in a rapid compression machine (RCM) at a compression pressure (Pc) of ∼60 bar and temperatures after compression (Tc) in the range 750–900 K for fuel‐air equivalence ratios ϕ in the range 1–4. The results show that mixtures of 2%–5% CH3CHO in CH4 ignite under conditions at which pure methane does not ignite experimentally. The efficiency of acetaldehyde as a promoter seems to be comparable to that of other oxygenated fuels like alcohols and ethers. For comparison with the experimental results, ignition delay times are computed using an updated reaction mechanism and two mechanisms from the literature for CH3CHO oxidation. For most conditions, the simulations using the current mechanism agree with the measurements to within a factor of two. The ignition profile shows a pre‐ignition temperature rise and two‐stage ignition similar to that previously observed in low fractions of dimethyl ether in ammonia; both phenomena are captured by the simulations. Analysis of simulations at constant volume indicates that CH3CHO is oxidized much more rapidly than CH4, producing reactive species that initiate the oxidation of CH4 and generates heat that accelerates oxidation toward ignition. The low‐temperature chain‐branching reactions of CH3CHO are important in the early oxidation of the fuel mixture. Additional simulations were performed for equivalence ratios of ϕ = 1 and 4, at a compression pressure (Pc) of 100 bar and Tc = 750–1000 K. The simulations indicate that CH3CHO has a strong ignition‐enhancing effect on CH4, with small fractions reducing the ignition delay time by up to a factor of 100, depending on the temperature, as compared to pure CH4.

Autoignition of Methane–Hydrogen Mixtures below 1000 K

In the range of 800–1200 K, both experiments and kinetic modeling demonstrate a significant difference in the dependence of the ignition delay time of methane and hydrogen on pressure and temperature, with the complex influence of these parameters on the autoignition delay time of methane–hydrogen–air mixtures. In connection with the prospects for the widespread use of methane–hydrogen mixtures in energy production and transport, a detailed analysis of their ignition at temperatures below 1000 K, the most important region from the point of view of their practical application, is carried out. It is shown that such a complex behavior is associated with the transition in this temperature range from low-temperature mechanisms of oxidation of both methane and hydrogen, in which peroxide radicals and molecules play a decisive role, to high-temperature mechanisms of their oxidation, in which simpler radicals dominate. A kinetic interpretation of the processes occurring in this case is proposed.

Identification of homogeneous chemical kinetic regimes of methane-air ignition

Sensitivity analysis results for ignition delay time (IDT) may be very different depending on the initial temperature, pressure and equivalence ratio φ, but similar in some regions of these variables. This phenomenon was investigated systematically by carrying out ignition simulations and local sensitivity calculations of methane−air mixtures using the Aramco-II-2016 mechanism at 14,417 combinations of initial temperature (changed between 500 and 3000 K), initial pressure (0.05−500 atm) and φ (0.05−8.0) values. The cluster analysis of the sensitivity vectors identified five large kinetically homogeneous regions. Each region has well defined borders in the (T, p φ) space and can be characterized by different sets of important reactions. The related kinetic scheme is very different in each region. Regions 1 and 2 are dominated by catalytic cycles based on species CH3O2/CH3O2H and HO2/H2O2/CH3O, respectively. In regions 3, 4, and 5 the H atoms are converted to CH3 in an identical chain branching sequence, but the back conversion is via three different routes. Literature experimental data on the IDTs of methane−air mixtures were sorted according to these five regions. Regions 1 to 5 contain 214, 328, 3, 0, and 237 experimental data points, respectively. In regions 1, 2 and 5 the data points are well reproduced by the Aramco-II-2016 mechanism, but little or no experimental information is available about kinetic regions 3 and 4. Further experimental exploration of the ignition of methane−air mixtures may aim the study of these regions. A similar approach can be used for the characterization of other combustion systems and sorting the related experimental data.

Numerical study on the effects of CO2/H2O dilution on the ignition delay time of methane

AbstractIn this study, the effects of CO2/H2O dilution on the ignition delay time (IDT) of methane under different conditions were studied by the method of sensitivity analysis, mole fraction analysis and reaction path analysis. The predictions of six published kinetic models were first compared with experimental data from the literature; the kinetics of the model judged best were then analyzed in greater detail to investigate coupling effects of different ratios of CO2/H2O dilution, temperature, pressure, and equivalence ratios on the IDT of methane. Through this research, it was found that under high temperature conditions (1700–2000 K), the IDT increases with CO2/H2O ratios increasing. At the equivalence ratio of 2.0 and low to medium temperatures (1000–1700 K), the diluent gas at the ratio of CO2/H2O = 0.4/0.6 has the maximum suppression effect on methane ignition, and the inhibitory effect of diluted gas in the IDT can be enhanced by 4.9% compared with other cases. Through the path analysis, it was found that the reaction path under the condition of CO2/H2O = 0.4/0.6 is changed into the O radicals generation reactions and CH3 consuming Reactions (R155, R106, and R158) comparing with the condition of CO2/H2O = 0.8/0.2. By providing insight into the factors affecting the IDT of methane under a wide variety of conditions, the present study extends our understanding of methane combustion and can be used to guide the development of a combined application of EGR and in‐cylinder water injection.

Measurement of the Ignition Delay Times of Methane in Air in a Shock Tube Installation at Low Initial Pressures

Measurement of the Ignition Delay Times of Methane in Air in a Shock Tube Installation at Low Initial Pressures

Reduced reaction mechanism for natural gas combustion in novel power cycles

Abstract We study the auto-ignition behavior of several natural gas surrogates; in particular, the influence of ethane and propane on the ignition delay time of methane is studied. A rapid compression machine was used to obtain experimental measurements of ignition delay times at two different compression pressures (10 and 15 bar) and a wide range of compression temperatures (904–1151 K), for both stoichiometric and fuel-rich ( ϕ = 2 ) mixtures. The experimental results are compared to homogeneous reactor model simulations (HOMREA). A first set of simulations treats chemical reaction in detail, using the AramcoMech 3.0 reaction mechanism. It was observed that the mechanism predicted the observed ignition delay times well. Experimental results indicate that both ethane and propane have an ignition enhancing effect on the mixture, shortening the ignition delay time. Propane, in particular, appears to have a higher influence compared to ethane at low temperatures. In general, fuel rich mixtures show shorter ignition delay times. These trends are well captured by the AramcoMech 3.0 mechanism showing great agreement with experimental data. To make the chemistry underlying the AramcoMech 3.0 available in a concise form, e.g., for CFD applications or similar, a reduced chemistry scheme was developed (reaction mechanism with 49 species and 332 reactions). Simulations using the reduced model showed a similarly good agreement to which was developed based on the detailed AramcoMech 3.0 reaction mechanism. The experimental results are predicted well by the reduced model, in a similar fashion like the detailed scheme. Furthermore, the reduced reaction mechanism is validated against experimental data found in literature, covering a wide range of conditions. This establishes the reduced model as a useful, computationally efficient substitute for describing the effect of ethane and propane onto the auto-ignition of methane.

An insight into the reaction kinetics of CH3 + O2(a1Δg) and its enhancement effect on methane ignition

The reaction between CH3 and O2(a1∆g) is crucial to understand the effects of electronically excited oxygen in plasma-assisted combustion of methane and other hydrocarbons. In the present work, multireference quantum chemical methods were used to investigate the potential energy surface of CH3 + O2(a1∆g). The RRKM/master equation simulation was employed to compute the rate coefficients of various pathways to this reaction over the temperature range of 300–2000 K and a pressure range of 0.1–100 atm. Special attention has been paid to the nonadiabatic transition between the excited state and ground state, which directly leads to a quenching channel from CH3 + O2(a1∆g) to CH3 + O2(X3∑g−). This quenching reaction has been overlooked by previous theoretical and kinetic modeling studies. We also conducted kinetic modeling to examine the effect of this reaction on the ignition enhancement of methane oxidation. Although the channel of CH3 + O2(a1∆g) quenching to CH3 + O2(X3∑g−) has nonnegligible rate constants comparing with other reaction channels, modeling result with the inclusion of 5% O2(a1∆g) in molecular oxygen shows that the titled reactions shorten the ignition delay time of methane by more than twenty times at 900 K, 1 atm. The ignition enhancement is mainly from the chain branching channels to CH2O + OH and CH3O + O which has been greatly promoted by excess energy from O2(a1∆g). The present study uncovers the kinetic mechanism of this nonadiabatic reaction and provides reasonable rate coefficients for further kinetic modeling of plasma-assisted combustion of methane and other hydrocarbons.

Revisiting the chemical kinetics of CH3 + O2 and its impact on methane ignition

The reaction between the methyl radical and molecular oxygen is one of the most important combustion reactions—it plays a controlling role in methane ignition. Although the reaction kinetics of CH3 + O2 has been extensively studied (at least experimentally) in the literature, high-level theoretical studies are still desired to understand the chemistry of this “small but complicated” reaction system and provide rate constants over wide range of temperature and pressure. In the present study, the rate coefficients of three major reaction channels of CH3 + O2, i.e., CH3OO stabilization (R1), CH3O + O channel (R2), and CH2O + OH channel (R3), were computed with temperature ranging from 300 to 2500 K and pressure from 0.01 to 100 atm by combining high-level quantum chemical calculations (CCSD(T)-F12/cc-pVQZ-F12//QCISD/6-311++G(2df,2p)) and RRKM/master equation simulations (with variable reaction coordinate transition state theory simulation for the barrierless channels). The computational results were compared with previously reported rate constants based on a thorough literature review. They show satisfactory agreement with those most recent measurements over the limited experimental temperature ranges—the discrepancies are within a factor of two for the three dominant reactions. The rate constants of R2 and R3 have very high sensitivity coefficients to the ignition delay time of methane. The impact of this reaction kinetics on methane ignition was evaluated by putting our currently computed rate coefficients into two recently reported kinetic models for methane combustion. There was a significant difference on the simulated ignition delay for an RCM experiment by replacing the rate constants of R2 and R3 with the newly computed results. It indicates the risk of estimating rate constants by simple extrapolation from experimental measurements over limited temperature ranges. The modified Arrhenius representations of the reaction channels of both CH3 + O2 and CH3OO dissociation were provided for utility in combustion modeling.

Experimental and Numerical Study of the Effect of CO2 on the Ignition Delay Times of Methane under Different Pressures and Temperatures

Pressurized oxy-fuel combustion is regarded as a new generation of oxy-fuel technology. The ignition delay times of methane in an O2/N2 atmosphere (0.21O2 + 0.79N2) and an O2/CO2 atmosphere (0.21O2 + 0.79CO2) were measured in a shock tube at a pressure of 0.8 atm, an equivalence ratio of 0.5, and within a temperature range of 1501–1847 K. The present experimental data and the experimental data of Hargis and Peterson at 1.75 and 10 atm were adopted to evaluate five representative chemical kinetic models. This paper studied the chemical effects (chaperon effects of CO2 and the effects of reactions containing CO2) and physical effects of CO2 on the ignition of methane at different pressures and temperatures in detail using a modified model. Artificial materials X and Y were employed to analyze the chemical and physical effects. The analysis showed that the physical effects of CO2 inhibit the ignition of methane and are not sensitive to the temperature. The chemical effects of CO2 vary greatly with the pressu...

Ignition delay times of methane and hydrogen highly diluted in carbon dioxide at high pressures up to 300 atm

The need for more efficient power cycles has attracted interest in super-critical CO2 (sCO2) cycles. However, the effects of high CO2 dilution on auto-ignition at extremely high pressures has not been studied in depth. As part of the effort to understand oxy-fuel combustion with massive CO2 dilution, we have measured shock tube ignition delay times (IDT) for methane/O2/CO2 mixtures and hydrogen/O2/CO2 mixtures using sidewall pressure and OH* emission near 306 nm. Ignition delay time was measured in two different facilities behind reflected shock waves over a range of temperatures, 1045–1578 K, in different pressures and mixture regimes, i.e., CH4/O2/CO2 mixtures at 27–286 atm and H2/O2/CO2 mixtures at 37–311 atm. The measured data were compared with the predictions of two recent kinetics models. Fair agreement was found between model and experiment over most of the operating conditions studied. For those conditions where kinetic models fail, the current ignition delay time measurements provide useful target data for development and validation of the mechanisms.

Theoretical and Experimental Study of Chemical Transformations of a Methane–Hydrogen–Coal Particles Mixture in a Rapid-Compression Machine

Results of an experimental and numerical study of the ignition of a stoichiometric methane–air mixture in the presence of coal particles of diameters 20–52 μm in the range of temperatures 850–1150 K and pressures 1.5–2.0 MPa are presented. It has been found that the particles begin to burn at a temperature of the oxidizing medium above 850 K. At a temperature above 1000 K, burning particles reduce the time and limiting temperature of ignition of the methane–air mixture. A comparison has been made of the calculated data on ignition-delay times of coal in an air–coal mixture and on ignition-delay times of methane and coal in a methane–air–coal mixture with the experimental data. A satisfactory agreement is shown between the data on ignition-delay times of coal and ignition-delay times of methane in all the mixtures in question.

Physical and mathematical modeling of ignition of methane-air mixture in the presence of coal microparticles

The results of numerical investigation of the ignition of a stoichiometric methane-air mixture in the presence of carbon particles with diameters of 20-52 μm in the temperature range 950-1150 K and pressures of 1.5-2.0 MPa are presented. The calculated data of the ignition delay times of coal particles in the coal particles/air mixture and of the ignition delay times of methane and coal particles in the methane/coal particles /air mixture are compared with the experimental ones. A satisfactory agreement of the data on the coal particles ignition delay times and methane ignition delay times in all the mixtures considered is shown.

Kinetic modeling of pressure and equivalence ratio effects on methane oxidation

The kinetics of methane oxidation in a jet-stirred reactor was modeled using a comprehensive kinetic reaction mechanism including the most recent findings concerning the kinetics of the reactions involved in the oxidation of C 1 -C 4 hydrocarbons. The computed results are discussed in terms of pressure and equivalence ratio (o) effects on methane oxidation. The previously validated mechanism is able to reproduce experimental data obtained in our high-pressure jet stirred reactor (concentration profiles for CH 4 , CO, CO 2 , H 2 , C 2 H 4 , C 2 H 6 , et C 2 H 2 ; 900 ≤ T/K ≤ 1300 ; 1 ≤ P/atm ≤ 10 ; 0.1 ≤ o ≤ 2) and methane ignition delay times measured in shock tube (800 ≤ T/K ≤ 2000 ; 1 ≤ P/atm ≤ 13 ; 0.1 ≤ o ≤ 2). It is also able to reproduce H and O atoms concentrations measured in shock tube at ≈ 2 atm. Burning velocities of methane in air between 1 and 3 atm and methane-air flame structures were also modeled. The same detailed kinetic mechanism can also be used to model the oxidation of ethane, ethylene, propene, and propane in similar conditions.