Autoignition Of Methane Research Articles

Autoignition of methane and methane/hydrogen blends in CO2 bath gas

Combustion in large dilutions of carbon dioxide (CO2) has the potential to enable fossil fuel combustion with 100 % inherent carbon capture by simplifying the post-combustion carbon capture process into the facile separation of CO2 and water. Emerging technologies that employ CO2 as a bath gas, exemplified by the Allam-Fetvedt cycle, are gaining widespread international attention, evidenced by ongoing plant developments in both the United Kingdom and the United States. This study presents new ignition delay time datasets (IDTs) for the combustion of methane and methane/hydrogen blends at 20 and 40 bar. The chemical kinetics governing these distinct conditions are investigated by utilizing two distinct chemical kinetic mechanisms, namely UoS sCO2 2.0 and NUIGMech1.1. Ignition delay datasets of methane/hydrogen blends are presented alongside a detailed discussion of the effect CO2, in contrast to N2, on the combustion mechanism. The differences in kinetics of pure methane and pure hydrogen are also discussed with regards to key reactions and the radical pool. This study aims to elucidate the intricate interplay of factors shaping ignition dynamics in CO2– enriched environments.

A shock-tube experimental and kinetic simulation study on the autoignition of methane at ultra-lean and lean conditions

Coalbed methane represents an important kind of natural gas resource in many countries. However, the low-concentration property of coalbed methane limits its applications. To gain insight into the combustion kinetics of coalbed methane and facilitate its combustion utilization, this work reports an experimental and kinetic simulation study on the autoignition properties of methane at ultra-lean and lean conditions. A shock-tube (ST) facility is used for ignition delay time (IDT) measurements with equivalence ratios at 0.5, 0.1, and 0.05 with pressure at 2 and 10 bar under the temperature ranging from 1320 to 1850 K. The measured IDTs can be correlated into a general Arrhenius expression, and the equivalence ratio effect on IDTs is then analyzed. Seven detailed chemical kinetic mechanisms are employed to predict the IDTs and statistical error indicators are used to evaluate their performance. Detailed kinetic analysis via sensitivity and reaction path analysis is performed to uncover the kinetic differences among the seven mechanisms. It is shown that some of the reaction paths only exist in the NUIGMech1.3 mechanism, while the other detailed mechanisms do not consider them. Reaction path analysis indicates that the reactions related to O2, OH and O species become more important compared to the reactions involving CH3 and H radicals as the equivalence ratio decreases from lean to ultra-lean conditions. Detailed chemical kinetics analysis is also conducted to demonstrate the uncertainty of key reactions. The present work should be valuable to gain insight into the methane ignition characteristics and to facilitate kinetic mechanism optimization of methane combustion.

Understanding the Nonlinear Reactivity Promoting Effect of n-Heptane Addition on the Binary Mixture From Low to Intermediate Temperature: A Case of Methane/n-Heptane Mixtures

Abstract To understand the effect of n-heptane (NC7H16) addition on the auto-ignition of methane (CH4) at low to intermediate temperatures, the ignition delay times (IDTs) of stoichiometric CH4/NC7H16 blends with varying NC7H16 concentrations were measured at temperatures from 600 to 1000 K, pressures of 20 and 40 bar. Detailed chemical kinetic mechanisms were validated against the newly measured IDTs. Adding NC7H16 in the binary mixture shows a nonlinear promoting effect on the IDTs: micro-addition of NC7H16 can significantly reduce the IDTs of the binary mixture when the NC7H16 is lower than 20%. However, the decrease of the IDTs becomes much slower when further increasing the NC7H16 addition. Affected by the negative temperature coefficient (NTC) behavior of NC7H16, this nonlinear effect is particularly notable at around 795 K, the low boundary of the NTC region. To reveal the nonlinear reactivity-promoting effect of NC7H16 addition on the binary mixture, reaction flux, ignition sensitivity, rate of production of the key radicals along heat production analyses were conducted. Apart from contributing more O˙H production through the low-temperature chain-branching reaction pathways of NC7H16, adding NC7H16 also promotes the pre-ignition heat release of the binary mixture. The heat release raises the system temperature and further promotes the mixture ignition, enhancing the nonlinear effect at low temperatures.

Exergy destruction behavior of chemical reactions during the auto-ignition of methane doped with hydrogen

Chemical reaction is the major source of combustion irreversibility in premixed conditions, and revealing the details of exergy destruction can provide a more essential perspective on exploring high-efficiency combustion strategies. The present study focuses on the homogeneous combustion process of CH4/H2/air mixtures, aiming to reveal the exergy destruction behavior of chemical reactions from the viewpoint of different combustion stages and oxidation paths. Results indicate that exergy destruction is mainly accumulated in the ignition delay stage, and the exergy destruction caused by different elementary reactions depends on the temperature at which the reaction takes place and the resulting change of free energy. By dividing the chemical reactions into four groups, the main reaction channels that contribute to the change in exergy destruction with combustion boundaries have been identified. Global reaction flux analysis reveals the effect mechanism of different oxidation paths on exergy destruction. Furthermore, the staged combustion concept with endothermic reactions reacting firstly at lower temperatures followed by dilute combustion in the high-temperature heat release stage is discussed from the point of the exergy destruction evolution, which offers the potential to simultaneously reduce exergy destruction and suppress NOx formation.

Skeletal reaction models for methane combustion

A local-sensitivity-analysis technique is employed to generate new skeletal reaction models for methane combustion from the foundational fuel chemistry model (FFCM-1). The sensitivities of the thermo-chemical variables with respect to the reaction rates are computed via the forced-optimally time dependent (f-OTD) methodology. In this methodology, the large sensitivity matrix containing all local sensitivities is modeled as a product of two low-rank time-dependent matrices. The evolution equations of these matrices are derived from the governing equations of the system. The modeled sensitivities are computed for the auto-ignition of methane at atmospheric and high pressures with different sets of initial temperatures, and equivalence ratios. These sensitivities are then analyzed to rank the most important (sensitive) species. A series of skeletal models with different number of species and levels of accuracy in reproducing the FFCM-1 results are suggested. The performances of the generated models are compared against FFCM-1 in predicting the ignition delay, the laminar flame speed, and the flame extinction. The results of this comparative assessment suggest the skeletal models with 24 and more species generate the FFCM-1 results with an excellent accuracy.

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.

An experimental and kinetic modeling study of NOx sensitization on methane autoignition and oxidation

An experimental and kinetic modeling study of the influence of NOx (i.e. NO2, NO and N2O) addition on the ignition behavior of methane/‘air’ mixtures is performed. Ignition delay time measurements are taken in a rapid compression machine (RCM) and in a shock tube (ST) at temperatures and pressures ranging from 900–1500 K and 1.5–3.0 MPa, respectively for equivalence ratios of 0.5–2.0 in ‘air’. The conditions chosen are relevant to spark ignition and homogeneous charge compression ignition engine operating conditions where exhaust gas recirculation can potentially add NOx to the premixed charge. The RCM measurements show that the addition of 200 ppm NO2 to the stoichiometric CH4/oxidizer mixture results in a factor of three increase in reactivity compared to the baseline case without NOx for temperatures in the range 600–1000 K. However, adding up to 1000 ppm N2O does not show any appreciable effect on the measurements. The promoting effect of NO2 was found to increase with temperature in the range 950–1150 K, while the sensitization effect decreases at higher pressures. The experimental results measured are simulated using NUIGMech1.2 comprising an updated NOx sub-chemistry in this work. A kinetic analysis indicates that the competition between the reactions ĊH3 + NO2 ↔ CH3Ȯ + NO and ĊH3 + NO2 (+M) ↔ CH3NO2 (+M), the former being a propagation reaction and the latter being a termination reaction governs NOx sensitization on CH4 ignition. Recent calculations by Matsugi and Shiina (A. Matsugi, H. Shiina, J. Phys. Chem. A. 121 (2017) 4218–4224) for the nitromethane formation reaction CH3 + NO2 (+M) ↔ CH3NO2 (+M), together with the recently calculated rate constants for HONO/HNO2 reactions significantly improve ignition delay time predictions in the temperature range 600–1000 K. Furthermore, the experiments with NO addition reveal a non-monotonous sensitization impact on CH4 ignition at lower temperatures with NO initially acting as an inhibitor at low NO concentrations and then as a promoter as NO concentrations increase in the mixture. This non-monotonous trend is attributed to the role of the chain-termination reaction ĊH3 + NO2 (+M) ↔ CH3NO2 (+M) and the impact of NO on the transition to the chain-branching steps CH2O + HȮ2 ↔ HĊO + H2O2, H2O2 (+M) ↔ ȮH + ȮH (+M), HĊO ↔ CO + Ḣ followed by CO + O2 ↔ CO2 + Ö and Ḣ + O2 ↔ Ö + ȮH. NUIGMech1.2 is systematically validated against the new ignition delay measurements taken here together with species measurements and high temperature ignition delay time data available in the literature for CH4/oxidizer mixtures diluted with NO2/N2O/NO and is observed to accurately capture the sensitization trends.

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.

Auto Ignition Study of Methane and Bio Alcohol Fuel Blends

Abstract Methane (CH4) and bio alcohols have different ignition properties. These have been extensively studied and the resulting experimental data have been used to validate chemical kinetic models. Methane is the main component of natural gas, which is of interest because of its relative availability and lower emissions compared to other hydrocarbon fuels. Given growing interest in fuel-flexible systems, there can be situations in which the combustion properties of natural gas need to be modified by adding biofuels such as bio alcohols. This can occur in dual-fuel internal combustion engines or gas turbines with dual-fuel capabilities. The combustion behavior of such blends can be understood by studying the auto ignition properties in fundamental combustion experiments. Studies of the ignition of such blends are very limited in the literature. In this work, the auto ignition of methane and bio alcohol fuel blends is investigated using a shock tube facility. The chosen bio alcohols are ethanol (C2H5OH) and n-propanol (NC3H7OH). Experiments are carried out at 3 atm and 10 atm for stoichiometric and lean mixtures of fuel, oxygen, and argon. The ignition delay times of the pure fuels are first established at conditions of constant oxygen concentration and comparable pressures. The ignition delay times of blends with 50% methane are then measured. The pyrolysis kinetics of the blends is further explored by measuring CO formation during pyrolysis of the alcohol and methane–alcohol blends. The resulting experimental data are compared with the predictions of selected chemical kinetic models to establish the ability of these models to predict the disproportionate enhancement of methane ignition by the added alcohol.

Effect of the CH3 + O2 Reaction on the Kinetics of Autoignition of Hydrocarbons at High Temperatures

The effect of the reaction between methyl radical and molecular oxygen on autoignition of various carbohydrates at temperatures above 1300 K is studied using numerical simulation. Calculations using the detailed kinetic mechanism showed that the interaction between CH3 and O2 is essential for autoignition of methane and acetone, while having no significant effect for other hydrocarbons. The sensitivity to this channel of interaction between CH3 and O2 is found to weaken with temperature. Model calculations have shown that an increase in the number of components and reactions in the detailed kinetic mechanisms may weaken the effect of interaction between CH3 and O2.

Measurement of methane autoignition delays in carbon dioxide and argon diluents at high pressure conditions

The directly fired supercritical carbon dioxide (sCO2) power cycle has high efficiency while allowing nearly complete carbon dioxide (CO2) capture. The operating conditions of sCO2 power cycle (100–300 bar) combustors are dramatically different from conventional gas turbine combustors. However, combustion properties such as autoignition delay are not well understood at these conditions. This study reports methane autoignition delay measurements for diluted carbon dioxide environments at 100 and 200 bar and at temperatures within the range of 1139–1433 K using a high pressure shock tube. To study the effect of CO2 on ignition, similar experiments are conducted at 100 and 200 bar by replacing carbon dioxide with argon. The experimental data is then compared with calculations using different chemical kinetics models. For the conditions of this study, predictions of the Aramco Mech 2.0 show the overall best agreement with experimental measurements, while predictions of the GRI 3.0 kinetic model have the largest (by a factor of 3) deviation with experiments. Sensitivity and reaction pathway analyses reveal that methyl (CH3) recombination to form ethane (C2H6) and oxidation of CH3 to form methoxide (CH3O) are the most important reactions controlling the ignition behavior at temperatures greater than approximately 1250 K. However, at temperatures below approximately 1250 K, an additional reaction pathway for methyl radicals is found through CH3+O2+M = CH3O2+M which leads to formation of methyldioxidanyl (CH3O2). This reaction pathway plays a distinct role in dictating the ignition trends at lower temperature conditions.

Promoting effect of halogen- and phosphorus-containing flame retardants on the autoignition of a methane–oxygen mixture

This paper presents a numerical and experimental study of the effect of flame-retardant additives on the autoignition of methane behind shock waves. It is shown that at a temperature of 1300–1900 K, the compounds CCl4, CF3H, and (CH3O)3PO not only do not suppress ignition but significantly reduce the induction time of methane–oxygen mixtures. A kinetic mechanism is proposed which relates the promoting effect to the reactivity of the pyrolysis products of the additives.

Global mechanism of methane autoignition: Approach and algorithm

An approach to constructing universal global mechanisms of methane autoignition is proposed. It is based on the requirement of obligatory reproduction of all the kinetic stages of the autoignition process and the types of their kinetics. It is shown that satisfying these requirements enables to construct global mechanisms of methane autoignition that are applicable in a wide range of initial conditions and adaptable to new problems. A global autoignition mechanism (10 species and reactions 9) is developed, as well as an extended global mechanism for describing plasma-induced methane autoignition (10 species, 10 reactions).

Kinetic nature of blue flames in the autoignition of methane

Kinetic calculations within the framework of detailed reaction mechanisms showed that the blue flames observed during autoignition of methane in an internal combustion engine are combined cool-blue flames. In the course of methane oxidation, methyl hydroperoxide CH3O2H and hydrogen peroxide H2O2 appear and accumulate almost simultaneously. The partial decomposition of these species yields a hydroxyl OH concentration peak, thereby giving rise to a local acceleration of the oxidation reaction, accompanied by luminescence. Kinetic curves for the main products were obtained and the boundaries of cool-blue flames were determined, which turned out to be in satisfactory agreement with the experimental data.

Modeling Negative Temperature Coefficient region in methane oxidation

Standard kinetic models are essential tools for predicting and interpreting the evolution of oxidation processes and obtain useful information for designing and dimensioning practical combustion facilities. Quite often a large part of the development work consists in the determination of the most suited chemical kinetics scheme to use in numerical simulations. This step is even more critical in the case of innovative technologies. In fact, in this case, models are required to work in extrapolative conditions, i.e. in range of parameters outside the ones for which they have been optimized. This is the case of prediction methane autoignition at atmospheric pressure, in diluted conditions, corresponding to MILD combustion conditions, where no experimental data are available. The aim of the present work is to compare the efficacies in predicting the existence of Negative Temperature Coefficient (NTC) behavior of ignition time of methane at atmospheric pressure of several kinetic models available in the literature. Such phenomenology is extensively described in the literature for high molecular weight paraffin but few experimental evidences are reported about its occurrence in methane oxidation. Methane autoignition time in dependence of temperature, reaction pathways with rate of production, sensitivity and flow diagram analysis have been exploited in order to highlight the kinetic controlling steps of methane autoignition at different temperature ranges. It has been shown that the prevalence of either the oxidation or the recombination results in a speeding or a slowing down of the reactive process. In this reactive network, a key role is covered by the active oxidation pathway. At the same time, in dependence of working temperature, the branching routes of H2/O2 reaction mechanism supply a great part of radicals needed for ignition. Thus, numerical results presented in the paper clearly show that the Negative Temperature Coefficient region in the Arrhenius plot of methane ignition delay marks the shift from one principal reaction route to the others.

Auto-ignition of methane–air mixtures flowing along an array of thin catalytic plates

In this paper, the heterogeneous ignition of a methane–air mixture flowing along an infinite array of catalytic parallel plates has been studied by inclusion of gas expansion effects and the finite heat conduction on the plates. The system of equations considers the full compressible Navier–Stokes equations coupled with the energy equations of the plates. The gas expansion effects which arise from temperature changes have been considered. The heterogeneous kinetics considers the adsorption and desorption reactions for both reactants. The limits of large and small longitudinal thermal conductance of the plate material are analyzed and the critical conditions for ignition are obtained in closed form. The governing equations are solved numerically using finite differences. The results show that ignition is more easily produced as the longitudinal wall thermal conductance increases, and the effects of the gas expansion on the catalytic ignition process are rather small due to the large value of the activation energy of the desorption reaction of adsorbed oxygen atoms.

Reduction of kinetic mechanisms for fuel oxidation through genetic algorithms

New automotive engine combustion concepts, such as the Homogeneous Charge Compression Ignition (HCCI) or the Controlled Autoignition (CAI), have been developed in the last years in order to fulfil the very stringent European regulations limiting pollutant emissions (mainly nitrogen oxides and particulate matter). These combustion modes are not controlled by the physical phenomena prior to combustion, as usual in traditional Compression Ignition (CI) and Spark Ignition (SI) engines, but by the fuel chemical kinetics leading to autoignition, which is described by the corresponding reaction mechanism. Since the detailed reaction mechanism of surrogate fuels (which are used instead of the original fuels in order to simulate their oxidation process) consists of hundreds of species and thousands of reactions, it cannot be used for engine simulation purposes due to the very excessive computational time requirements. Thus, reduced reaction mechanisms, which simulate properly some characteristics of the detailed one (autoignition time, combustion temperature, etc.), are used instead. Among the typical reduction techniques (Quasi-Steady-State Assumption (QSSA), Reaction Analysis, etc.), Genetic Algorithms (GA) allow for both an efficient reduction of small/medium size reaction mechanisms and, when combined with other techniques, a more effective reduction of large reaction mechanisms. In this work, a general and innovative GA methodology for the reduction of kinetic mechanisms describing the hydrocarbon oxidation processes has been proposed, and its application to the reduction of two different mechanism sizes (a medium one describing the methane autoignition and a larger one derived from the oxidation of a diesel fuel surrogate) has provided better results when compared to other GA methodologies or reduction techniques. The reduced mechanisms provide very similar autoignition time and combustion temperature values to those obtained from the detailed mechanism under different engine operating conditions (intake pressure and temperature, equivalence fuel/air ratio and exhaust gas recirculation rate).

Transport budgets in turbulent lifted flames of methane autoigniting in a vitiated co-flow

Autoignition of hydrocarbon fuels is an outstanding research problem of significant practical relevance in engines and gas turbine applications. This paper presents a numerical study of the autoignition of methane, the simplest in the hydrocarbon family. The model burner used here produces a simple, yet representative lifted jet flame issuing in a vitiated surrounding. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD package, FLUENT. The in situ adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. An analysis of species concentrations and transport budgets of convection, turbulent diffusion, and chemical reaction terms is performed with respect to selected species at the base of the lifted turbulent flames. This analysis provides a clearer understanding of the mechanism and the dominant species that control autoignition. Calculations are also performed for test cases that clearly distinguish autoignition from premixed flame propagation, as these are the two most plausible mechanisms for flame stabilization for the turbulent lifted flames under investigation. It is revealed that a radical pool of precursors containing minor species such as CH 3, CH 2O, C 2H 2, C 2H 4, C 2H 6, HO 2, and H 2O 2 builds up prior to autoignition. The transport budgets show a clear convective–reactive balance when autoignition occurs. This is in contrast to the reactive–diffusive balance that occurs in the reaction zone of premixed flames. The buildup of a pool of radical species and the convective–reactive balance of their transport budgets are deemed to be good indicators of the occurrence of autoignition.

Species reconstruction using pre-image curves

All chemistry dimension-reduction methodologies, such as quasi-steady-state approximation (QSSA), rate-controlled constrained equilibrium (RCCE), and intrinsic low-dimensional manifolds (ILDM), either implicitly or explicitly, provide a means of species reconstruction. Given a reduced description of the chemical composition (e.g., in terms of some “major” species), species reconstruction is a procedure to estimate the full composition. In this paper, we develop and demonstrate a new species reconstruction methodology, which is based on a pre-image curve of the reaction mapping. By definition, in the composition space, if a reaction trajectory is followed from any point on the pre-image curve, then at some time the reduced composition on the trajectory intersects the given reduced composition. The reconstructed composition is taken to be the full composition on the reaction trajectory at this intersection. The method is tested for a case of methane autoignition in which the detailed mechanism has 27 degrees of freedom, and two reduced descriptions are considered with 4 and 6 degrees of freedom. It is shown that the normalized reconstruction errors are very small; they are two orders of magnitude smaller than those in RCCE; and with 6 degrees of freedom they are smaller than those in QSSA with 12 degrees of freedom. The reconstructed compositions (for all reduced compositions) form the reconstructed manifold. It is shown that, to an excellent approximation, this manifold is inertial (i.e., it is composed of reaction trajectories). In comparison to other dimension-reduction methodologies, this is the first approach which determines, locally, points on a low-dimensional inertial manifold. The combination of in situ adaptive tabulation and species reconstruction can be used to reduce substantially the computational cost of applying detailed chemistry to turbulent combustion calculations.

Global Kinetic Parameters for Determining the Autoignition Limits and Induction Periods for Mixtures of Methane, Ammonia, Oxygen, and Nitrogen

Formally kinetic constants for the autoignition of methane–ammonia–oxygen–nitrogen mixtures were determined. The results obtained allow one to estimate the critical conditions and autoignition delays of the examined mixtures to assess the explosion hazard of reactors for the synthesis of hydrocyanic acid.