Reaction Mechanisms for Methane Ignition

2000 ◽  
Author(s):  
S. C. Li ◽  
F. A. Williams
Keyword(s):  
Temperature Sensitivity ◽  
Reaction Mechanisms ◽  
Ignition Delay ◽  
Low Temperatures ◽  
Gas Turbines ◽  
Elementary Reactions ◽  
Detailed Mechanism ◽  
Ignition Delay Times ◽  
Delay Times ◽  
Flame Chemistry

To help understand how methane ignition occurs in gas turbines, dual-fuel diesel engines and other combustion devices, the present study addresses reaction mechanisms with the objective of predicting autoignition times for temperatures between 1000 K and 2000 K, pressures between 1 bar and 150 bar and equivalence ratio between 0.4 and 3. It extends our previous methane flame chemistry and refines earlier methane ignition work. In addition to a detailed mechanism, short mechanisms are presented that retain essential features of the detailed mechanism. The detailed mechanism consists of 127 elementary reactions among 31 species and results in 9 intermediate species being most important in autoignition, namely, CH3, OH, HO2, H2O2, CH2O, CHO, CH3O, H, O. Below 1300 K the last 3 of these are unimportant, but above 1400 K all are significant. To further simplify the computation, systematically reduced chemistry is developed, and an analytical solution for ignition delay times is obtained in the low-temperature range. For most fuels, a single Arrhenius fit for the ignition delay is adequate, but for hydrogen the temperature sensitivity becomes stronger at low temperatures. The present study predicts that, contrary to hydrogen, for methane the temperature sensitivity of the autoignition delay becomes stronger at high temperatures, above 1400 K, and weaker at low temperatures, below 1300 K. Predictions are in good agreement with shock-tube experiments. The results may be employed to estimate ignition delay times in practical combustors.

Reaction Mechanisms for Methane Ignition

2002 ◽  
Vol 124 (3) ◽  
pp. 471-480 ◽  
Author(s):  
S. C. Li ◽  
F. A. Williams
Keyword(s):  
Temperature Sensitivity ◽  
Reaction Mechanisms ◽  
Ignition Delay ◽  
Low Temperatures ◽  
Gas Turbines ◽  
Elementary Reactions ◽  
Detailed Mechanism ◽  
Ignition Delay Times ◽  
Delay Times ◽  
Flame Chemistry

To help understand how methane ignition occurs in gas turbines, dual-fuel diesel engines, and other combustion devices, the present study addresses reaction mechanisms with the objective of predicting autoignition times for temperatures between 1000 K and 2000 K, pressures between 1 bar and 150 bar, and equivalence ratio between 0.4 and 3. It extends our previous methane flame chemistry and refines earlier methane ignition work. In addition to a detailed mechanism, short mechanisms are presented that retain essential features of the detailed mechanism. The detailed mechanism consists of 127 elementary reactions among 31 species and results in nine intermediate species being most important in autoignition, namely, CH3, OH, HO2,H2O2,CH2O,CHO, CH3O, H, O. Below 1300 K the last three of these are unimportant, but above 1400 K all are significant. To further simplify the computation, systematically reduced chemistry is developed, and an analytical solution for ignition delay times is obtained in the low-temperature range. For most fuels, a single Arrhenius fit for the ignition delay is adequate, but for hydrogen the temperature sensitivity becomes stronger at low temperatures. The present study predicts that, contrary to hydrogen, for methane the temperature sensitivity of the autoignition delay becomes stronger at high temperatures, above 1400 K, and weaker at low temperatures, below 1300 K. Predictions are in good agreement with shock-tube experiments. The results may be employed to estimate ignition delay times in practical combustors.

Comparison Between RCCE and Shock Tube Ignition Delay Times at Low Temperatures

2015 ◽  
Vol 137 (6) ◽  
Author(s):  
Ghassan Nicolas ◽  
Hameed Metghalchi
Keyword(s):  
Free Energy ◽  
Shock Tube ◽  
Ignition Delay ◽  
Low Temperatures ◽  
Reduction Technique ◽  
Experimental Measurements ◽  
Ignition Delay Times ◽  
Delay Times ◽  
Constrained Equilibrium ◽  
Rate Controlled

The rate-controlled constrained-equilibrium (RCCE) method is a reduction technique based on local maximization of entropy or minimization of a relevant free energy at any time during the nonequilibrium evolution of the system subject to a set of kinetic constraints. In this paper, RCCE has been used to predict ignition delay times of low temperatures methane/air mixtures in shock tube. A new thermodynamic model along with RCCE kinetics has been developed to model thermodynamic states of the mixture in the shock tube. Results are in excellent agreement with experimental measurements.

Comparative Investigation of N-Heptane Droplet Ignition in High Temperature Convective Environments

2007 ◽  
Author(s):  
J. Chen ◽  
X. F. Peng ◽  
Y. G. Ju ◽  
B. X. Wang
Keyword(s):  
High Temperature ◽  
Ignition Delay ◽  
Comparative Investigation ◽  
Elementary Reactions ◽  
Fuel Droplets ◽  
Ignition Delay Times ◽  
Delay Times ◽  
One Step ◽  
Flame Behavior ◽  
Skeletal Mechanism

A 2-D numerical model was proposed to investigate the ignition of liquid fuel droplets in convective environments at high temperature. This model employed a skeletal mechanism consisting of 34 reactive species and 56 elementary reactions, rather than one-step overall reaction as in normal 2-D droplet ignition models, because the skeletal mechanism for n-heptane reproduces ignition delay times at various temperatures and pressures reasonably well. In present investigation an emphasis was addressed on the comparative analysis of suitability of the model, particularly numerical simulations were compared with experiments available in the literature, or for N-heptane droplets ignition in the convective air at temperature in a range of 1100K∼1400K and velocity of 2m/s. The ignition delay time and ignition position were obtained using an ignition criterion based on OH radical mass fraction. The flame behavior after ignition was also studied comparatively. The agreement between numerical simulation and experiments is reasonably good.

DME as a Potential Alternative Fuel for Gas Turbines: A Numerical Approach to Combustion and Oxidation Kinetics

2011 ◽  
Author(s):  
Pierre A. Glaude ◽  
Rene´ Fournet ◽  
Roda Bounaceur ◽  
Michel Molie`re
Keyword(s):  
Natural Gas ◽  
Gas Turbine ◽  
Heat Input ◽  
Ignition Delay ◽  
Gas Turbines ◽  
Ignition Temperature ◽  
Alternative Fuels ◽  
Ignition Delay Times ◽  
Auto Ignition ◽  
Delay Times

Many investigations are currently carried out in order to reduce CO2 emissions in power generation. Among alternative fuels to natural gas and gasoil in gas turbine applications, dimethyl ether (DME; formula: CH3-O-CH3) represents a possible candidate in the next years. This chemical compound can be produced from natural gas or coal/biomass gasification. DME is a good substitute for gasoil in diesel engine. Its Lower Heating Value is close to that of ethanol but it offers some advantages compared to alcohols in terms of stability and miscibility with hydrocarbons. While numerous studies have been devoted to the combustion of DME in diesel engines, results are scarce as far as boilers and gas turbines are concerned. Some safety aspects must be addressed before feeding a combustion device with DME because of its low flash point (as low as −83°C), its low auto-ignition temperature and large domain of explosivity in air. As far as emissions are concerned, the existing literature shows that in non premixed flames, DME produces less NOx than ethane taken as parent molecular structure, based on an equivalent heat input to the burner. During a field test performed in a gas turbine, a change-over from methane to DME led to a higher fuel nozzle temperature but to a lower exhaust gas temperature. NOx emissions decreased over the whole range of heat input studied but a dramatic increase of CO emissions was observed. This work aims to study the combustion behavior of DME in gas turbine conditions with the help of a detailed kinetic modeling. Several important combustion parameters, such as the auto-ignition temperature (AIT), ignition delay times, laminar burning velocities of premixed flames, adiabatic flame temperatures, and the formation of pollutants like CO and NOx have been investigated. These data have been compared with those calculated in the case of methane combustion. The model was built starting from a well validated mechanism taken from the literature and already used to predict the behavior of other alternative fuels. In flame conditions, DME forms formaldehyde as the major intermediate, the consumption of which leads in few steps to CO then CO2. The lower amount of CH2 radicals in comparison with methane flames seems to decrease the possibility of prompt-NO formation. This paper covers the low temperature oxidation chemistry of DME which is necessary to properly predict ignition temperatures and auto-ignition delay times that are important parameters for safety.

Shockless Explosion Combustion: Experimental Investigation of a New Approximate Constant Volume Combustion Process

2016 ◽  
Author(s):  
Thoralf G. Reichel ◽  
Bernhard C. Bobusch ◽  
Christian Oliver Paschereit ◽  
Jan-Simon Schäpel ◽  
Rudibert King ◽  
...  
Keyword(s):  
Shock Waves ◽  
Ignition Delay ◽  
Gas Turbines ◽  
Control Algorithm ◽  
Ambient Pressure ◽  
Pressure Sensors ◽  
Constant Volume ◽  
Ignition Delay Times ◽  
Delay Times ◽  
Constant Volume Combustion

Approximate constant volume combustion (aCVC) is a promising way to achieve a step change in the efficiency of gas turbines. This work investigates a recently proposed approach to implement aCVC in a gas turbine combustion system: shockless explosion combustion (SEC). The new concept overcomes several disadvantages such as sharp pressure transitions, entropy generation due to shock waves, and exergy losses due to kinetic energy which are associated with other aCVC approaches like, e.g., pulsed detonation combustion. The combustion is controlled via the the fuel/air mixture distribution which is adjusted such that the entire fuel/air volume undergoes a spatially quasi-homogeneous autoignition. Accordingly, no shock waves occur and the losses associated with a detonation wave are not present in the proposed system. Instead, a smooth pressure rise is created due to the heat release of the homogeneous combustion. An atmospheric combustion test rig is designed to investigate the autoignition behavior of relevant fuels under intermittent operation, currently up to a frequency of 2Hz. Application of OH*- and dynamic pressure sensors allows for a spatially- and time-resolved detection of ignition delay times and locations. Dimethyl ether (DME) is used as fuel since it exhibits reliable autoignition already at 920K mixture temperature and ambient pressure. First, a model-based control algorithm is used to demonstrate that the fuel valve can produce arbitrary fuel profiles in the combustion tube. Next, the control algorithm is used to achieve the desired fuel stratification, resulting in a significant reduction in spatial variance of the auto-ignition delay times. This proves that the control approach is a useful tool for increasing the homogeneity of the autoignition.

Assessment of detailed reaction mechanisms for reproduction of ignition delay times of C2–C6 alkenes and acetylene

2019 ◽  
Vol 206 ◽  
pp. 37-50 ◽  
Author(s):  
Agnieszka Jach ◽  
Wojciech Rudy ◽  
Andrzej Pękalski ◽  
Andrzej Teodorczyk
Keyword(s):  
Reaction Mechanisms ◽  
Ignition Delay ◽  
Ignition Delay Times ◽  
Delay Times

Development of a Global Mechanism for Oxy-Methane Combustion in a CO2 Environment

2018 ◽  
Author(s):  
Owen M. Pryor ◽  
Subith Vasu ◽  
Xijia Lu ◽  
David Freed ◽  
Brock Forrest
Keyword(s):  
Time Scales ◽  
Ignition Delay ◽  
Batch Reactor ◽  
Methane Combustion ◽  
Combustion Mechanism ◽  
Detailed Mechanism ◽  
Main Challenge ◽  
Ignition Delay Times ◽  
Delay Times ◽  
Time Required

There has been some recent work on the global kinetic modeling of flames in oxy-fuel combustion for methane. The main challenge is that none of the mechanisms were developed to understand the time-scales of ignition. Here, a 3-step mechanism was developed for methane combustion in oxy-fuel environment. The mechanisms were simulated using a closed batch homogeneous batch reactor with constant pressure and compared to baseline simulations performed using a detailed mechanism. All simulations were performed for methane used a mixture of XCH4 = 0.05, XO2 = 0.10 and XCO2 = 0.85. Mechanisms were altered using the global mechanism equilibrium approach to ensure that the steady-state values matched the reference values and were further altered using an optimization scheme to match experimental data that was taken in a shock tube. Simulation results of methane, CO time-histories, and temperature profiles from the global mechanism were compared to those from the detailed mechanism. Ignition delay times were used to represent the time-scales of combustion. This was defined for current simulations as the time required for methane concentration to reach 5% of its initial value during combustion. Using this approach, the 3-step methane combustion mechanism showed excellent improvement in the ignition timing over a range of pressures (1 to 10 bar) and initial temperatures (1500 to 2000 K) for both lean and stoichiometric mixtures but fails to predict ignition delay times at 30 bar or the ignition delay times of fuel rich mixtures. Ongoing effort focuses on extending this new global mechanism to higher pressures and to syngas mixtures.

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