Reaction Mechanism For Combustion Research Articles

Quantum Chemistry Calculation Study on Chain Reaction Mechanisms and Thermodynamic Characteristics of Coal Spontaneous Combustion at Low Temperatures.

The coal spontaneous combustion phenomenon seriously affects the safety production of coal mines. Aiming at the problem of complex coal molecular structure and incomplete reaction sequences at present, the mechanisms and thermodynamic parameters of coal spontaneous combustion chain reactions were explored by combining experimental detections and molecular simulations. First, the active groups on the surface of coal were obtained by Fourier transform infrared spectroscopy (FTIR), mainly including methyl (−CH3), methylene (−CH2), methyne (−CH), phenolic hydroxyl (−ArOH), alcohol hydroxyl (−ROH), carboxyl (−COOH), aldehyde (−CHO), and ether (−O−), and the coal molecular models containing functional groups and radicals were established. According to the charge density, electrostatic potential, and frontier orbital theories, the active sites and active bonds were obtained, and a series of reactions were given. The thermodynamic and structural parameters of each reaction were explored. In the chain initiation reaction stage, O2 chemisorption and the self-reaction of radicals play a leading role. In this stage, heat gradually accumulates and various radicals begin to generate, where the intramolecular hydrogen transfer reaction of a peroxide radical (−C–O–O·) can produce the key hydroxyl radical (−O·). In the chain propagation reaction stage, O2 and −O· continuously consume active sites to accelerate the reaction sequences and increase the temperature of coal, and index gases such as CO and CO2 generate, causing the chain cycle reactions to gradually form. The chain termination reaction stage is the formation of stable compounds such as ethers, esters, and quinones, which can inhibit the development of chain reactions. The results can further explain the reaction mechanism of coal spontaneous combustion and provide references for the development and utilization of chemical inhibitors.

Investigation on the flammability limit and limiting oxygen concentration of N2-diluted H2/CO/air mixtures at high temperature and pressure

H2/CO binary mixtures are common, and their combustion characteristics and reaction mechanism are important. Currently, there are few studies on the combustion characteristics of H2/CO mixtures at high temperature and pressure conditions, and they are mainly limited by the test conditions. In this work, a flammability limit test device was used to determine the flammability limit and limiting oxygen concentration of a combustible mixture with a composition of H2/CO = 1:1 at high initial temperature (∼80 °C) and high initial pressure (∼10 MPa) conditions. The effects of temperature and pressure on the flammability limit and limiting oxygen concentration were obtained, and the results showed that the initial temperature and initial pressure expanded the range and reduced the limiting oxygen concentration, which significantly increased the risk of combustion. As the pressure increased, the effect of pressure on the flammability limit and limiting oxygen concentration decreased. In addition, an obvious coupling effect of initial temperature and pressure on the upper flammability limit and the limiting oxygen concentration (<5 MPa) was found. Moreover, kinetic simulations of the CO/H2 mixture were carried out to verify the synergistic oxidation effect, which was explained using chain reaction theory. We found that the chain initiation reactions caused by H2 generated OH products and the heat required for CO oxidation. Lastly, we also observed oxygen competition between CO and H2, which decreased the CO conversion rate as H2 increased.

Reduced Chemical Kinetic Reaction Mechanism for Dimethyl Ether-Air Combustion

Development and validation of a new reduced dimethyl ether-air (DME) reaction mechanism is presented. The mechanism was developed using a modular approach that has previously been applied to several alkane and alkene fuels, and the present work pioneers the use of the modular methodology, with its underlying H/C1/O base mechanism, on an oxygenated fuel. The development methodology uses a well-characterized H/C1/O base mechanism coupled to a reduced set of fuel and intermediate product submechanisms. The mechanism for DME presented in this work includes 30 species and 69 irreversible reactions. When used in combustion simulation the mechanism accurately reproduced key combustion characteristics and the small size enables use in computationally demanding Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS). It has been developed to accurately predict, among other parameters, laminar burning velocity and ignition delay times, including the negative temperature regime. The evaluation of the mechanism and comparison to experimental data and several detailed and reduced mechanisms covers a wide range of conditions with respect to temperature, pressure and fuel-to-air ratio. There is good agreement with experimental data and the detailed reference mechanisms at all investigated conditions. The mechanism uses fewer reactions than any previously presented DME-air mechanism, without losing in predictability.

Effect of CO2-O2 oxidizing atmospheres on the combustion characteristics of metallurgical coke and anthracite

As a novel method for blast furnace ironmaking, CO2 is utilized in the hot blast and the pulverized coal carrier gas. In order to clarify the function mechanism of CO2/O2 oxidizing atmospheres on the metallurgical reaction of metallurgical coke and anthracite, the combustion characteristics of anthracite and coke in CO2/O2 atmosphere were obtained by non-isothermal thermogravimetry. The physical and chemical structural characteristics of anthracite and coke were measured systematically by using X-ray diffraction(XRD), X-ray fluorescence spectrometer (XRF), laser particle size analysis, scanning electron microscopy(SEM) and Raman spectroscopy analysis. The results show that when the proportion of CO2 in the reaction gas increases, the ignition performance and comprehensive combustion performance of anthracite and coke decrease, and higher heating rate is not conducive to the reaction. The ignition performance and the comprehensive combustion performance of the samples rank in the order anthracite > coke. The volume model (VM) and unreacted core model(URCM) were used to fit the experimental data, and the reaction kinetic parameters and combustion reaction mechanism were obtained. The performance of VM model is slightly better than URCM model.

Combustion kinetics of alternative jet fuels, Part-II: Reaction model for fuel surrogate

Conventional transportation fuels used in aviation (jet fuel) or in ground transportation (gasoline, diesel) contain multitude of hydrocarbon components and are difficult to be modeled, if one has to consider each of the component present. A typical approach is the definition of a fuel surrogate with a limited number of fuel components. In this context, a single semi-detailed high temperature reaction kinetic mechanism is presented in this work, which contains all the important molecular classes required for the detailed surrogate modeling of a hydrocarbon fuel. The appeal of the mechanism is the suitability for a broad range of technical fuels covering gasoline, diesel and jet fuels. The reaction mechanism for hydrocarbon combustion is consisted of 238 species and 1814 reactions and is rigorously validated for 70 neat hydrocarbon components over a wide range of experimental conditions including combustion setups such as shock-tubes, laminar flames, jet-stirred and flow reactors.The purpose of this study is to provide a single reaction model that (1) includes variety of hydrocarbon molecules of varying degree of complexity and carbon numbers, (2) has capability to model a spectrum of different fuels, initially aviation fuels, and (3) is compact to apply both in simple (fundamental kinetic investigations) and complex geometries (CFD studies) of combustion system enabled through customized mechanism reductions. The ultimate goal is to resolve the fuel differences using the model predictions obtained from the reaction mechanism that will supply parameters for fuel design and optimization of fuels. Extensive supporting information is available in this work.

Competition between reduction reaction and carbon deposition for CaSO4 oxygen carrier in chemical-looping combustion: A first-principle study and reaction equilibrium analysis

To understand the reaction mechanism of chemical looping combustion with calcium-based oxygen carrier, the competition mechanisms between reduction reaction and carbon deposition for CaSO4 oxygen carrier were calculated by density functional theory and simulated by chemical reaction equilibrium analysis. In this paper, the reaction paths of reduction reaction and carbon deposition at different CaSO4(010) surface outer oxygen content (100%, 75%, 50%, 25%) were studied, and the reversibility of the reaction was fully considered. Furthermore, based on the kinetic analysis of the transition state theory, the kinetic parameters of the elementary reaction were obtained. Moreover, the reaction equilibrium simulation revealed the competition of reactions under different conditions. The results show that the energy barriers of forward and reverse reactions decrease with the oxygen content decreasing on the CaSO4 (010) outer layer surface in the reduction reaction. Besides, the forward energy barriers at each stage are lower than the reverse energy. In the carbon deposition reaction, the energy barriers of the forward reaction are higher than the reverse reaction, and the energy barriers of the reverse reaction gradually increase with oxygen content decreasing. The kinetic analysis results show the rate-limiting steps of forward and reverse reaction in reduction reaction are 75% and 25% oxygen content stage. According to the reaction equilibrium, the conversion rate of CO oxidation to CO2 decreases with temperature increasing, and the conversion rate of carbon deposition increases. The rate of CO oxidation to CO2 increase and the occurrence of carbon deposition reaction decrease with oxygen carrier ratio increasing. Thus, when the oxygen carrier is insufficient or most of CaSO4 is reduced, the carbon deposition reaction occurs easily.

Methane combustion over palladium catalyst within the confined space of MFI zeolite

Isolated cationic Pd species encapsulated in MFI zeolite, i.e., Pd@MFI, have been successfully prepared via in situ hydrothermal route followed by oxidative treatment. The as-prepared Pd@MFI samples are investigated as promising catalysts in the reaction of methane combustion. Typically, Pd@H-ZSM-5 shows remarkable activity in methane catalytic combustion with a low apparent activation energy value of 70.7 kJ/mol as well as good catalytic stability even in excess water vapor. Detailed characterization results demonstrate the strong interaction between Pd sites and zeolite framework in Pd@ZSM-5 and the efficient stabilization of isolated Pd sites by zeolite thereof. Spectroscopy analyses reveal that the presence of Brønsted acid sites is beneficial to methane adsorption and its subsequent activation on adjacent Pd sites, constructing cooperation between Brønsted acid sites and Pd sites within the confined space of MFI zeolite toward high-efficiency methane catalytic combustion. The reaction mechanism of methane combustion catalyzed by Pd@H-ZSM-5 model catalyst is finally discussed.

The variations of free radical and index gas CO in spontaneous combustion of coal gangue under different oxygen concentrations

SummaryIn order to study the effect of oxygen concentrations on the change of active groups in the oxidation process of coal gangue, the airflows of different oxygen concentrations were prepared to simulate the oxidation and heating process of coal gangue under oxygen concentrations of 0%, 5%, 10%, 15%, and 21%. During the oxidation and heating process, the free radical parameters were measured at different temperatures in real time. The results show that in the early stage of coal gangue oxidation, the g factor remains at a lower level and do not start to increase until certain temperatures are reached. Besides, the higher the oxygen concentration is, the lower the temperature at which the g factor begins to increase. When the oxygen concentrations are 0%, 5%, 10%, 15%, and 21%, the corresponding temperatures are 225, 200, 150, 150, and 125°C, respectively. Furthermore, with the increase of oxidation temperature, the free radical concentrations experience four stages, namely, the slow increase stage, the rapid increase stage, the stagnant increase stage, and the final increase stage. And the higher the oxygen concentration is, the shorter the free radical stagnant increase stage is. During the oxidation of coal gangue, the line widths decrease slowly before 150°C, and then start to plunge and finally stabilize near the minimum at 350°C. Moreover, the higher the oxygen concentration is, the smaller the minimum values of line widths are. In the oxidation process of coal gangue, the gas product CO amounts are first at a lower level, and then begin to increase rapidly after 150°C. At the same temperature, the increased rates of CO production in coal gangue under the oxygen concentrations of 10%, 15%, and 21% are significantly higher than those under the oxygen concentrations of 0% and 5%. Based on the variation law of free radical parameters in coal gangue under different oxygen concentrations, the actual spontaneous combustion of gangue hills was analyzed and the free radical reaction mechanism of spontaneous combustion of gangue hills was discussed. The research results are of guiding significance for the treatment of spontaneous combustion of coal gangue.

Effects of Bio-Syngas Blends on Combustion Characteristics of Coal: Kinetic Analysis

The coupled combustion of biomass and coal can utilize large amounts of renewable biomass and reduce the emission of pollutants during power generation. In this study, the coal combustion characteristics were analyzed at different heating value-based biomass blending ratios and temperatures in the coupled combustion of bio-syngas and coal. The kinetic reaction mechanism of coal combustion was investigated by Micro-fluidized Bed Reaction Analyzer (MFBRA). The results indicated that the reaction time decreased with increasing the heating value-based biomass blending ratio and temperature in the coupled combustion process of bio-syngas and coal. The major conversion process of coal combustion usually took 15 s at 1273 K and the maximum reaction rate was usually below 0.14 at 873 K to 1273 K and decreased with increasing reaction temperature. The nucleation and growth model offered a most reasonable description of the coal combustion process. The activation energy was about 121.04 kJ/mol and the pre-exponential factor was of the order of 5.01 × 104 s−1 in the combustion of coal. These data are important to the understanding of the coupled combustion mechanism of bio-syngas and coal, which is beneficial for improving combustion efficiency as well as operating a combustion furnace.

A detailed reaction mechanism for elemental sulphur combustion in the furnace of sulphuric acid plants

AbstractElemental sulphur combustion is traditionally used to generate SO2 to produce sulphuric acid that is consumed in chemical industries, but is now also being considered as an energy vector for power generation. Despite the fact that sulphur combustion has been practiced for decades, there are limited reaction mechanisms for its combustion chemistry, which is critical for the optimization of the sulphuric acid production process to maximize efficiency and reduce cost. In this paper, a detailed reaction mechanism is developed and validated with different sets of experimental data from lab‐scale and industrial plant studies. The reaction mechanism is used to conduct sulphur furnace simulations, where the effects of feed air/sulphur ratio and oxygen enrichment of air stream on furnace temperature and the concentrations of SO2, SO3, and O2 are investigated. The dominant reaction pathways in sulphur combustion, particularly for the production of SO2 and SO3, are identified. It was found that the feed air/sulphur ratio monitors the furnace temperature and can be used to obtain the desired O2/SO2 ratio at the furnace exit for the optimal operation of catalytic converter (for SO2 oxidation to SO3) that follows the furnace in the sulphuric acid plant. Moreover, high oxygen enrichment above 35% (while maintaining the desired O2/SO2 ratio at the furnace exit) significantly increased the furnace capacity through reduced total gas flow (thus decreasing blower energy requirement and equipment size). The developed reaction mechanism provides a method to obtain optimized furnace parameters to achieve high efficiency and reduced costs in sulphuric acid plants.

Unraveling the low-temperature oxidation mechanism between methyl crotonate radicals and O2

In this paper, chemical reaction kinetics at low temperatures on three different methyl crotonate (MC, C5H8O2) radicals with O2 were conducted via quantum chemical methods. The potential energy surfaces (PESs) for these reactions were investigated by M062x/6-311++G(d,p) and CBS-QB3 methods. The related rate coefficients also have been solved by master equations based on Rice-Ramsperger-Kassel-Marcus theory, predicting the competitive relationships over 300 to 1500 K and 0.001 to 100 atm. The calculated results indicated that the rate constants of O2 addition reaction at ester methylic site were higher than those at allylic sites, which showed that the conjugation effect caused by the C=C double bond has a crucial effect on the reaction process. Formation of initial adducts and intramolecular H-transfer reactions play a great role in the low temperature oxidation of MC. Furthermore, the mechanism of O2 addition to MC radicals was verified by the previous combustion model. The updated model did a good job to replicate the previous experimental results. This work not only provides the necessary rate constants for the reaction mechanism of MC combustion but also serves as a solid starting point for the further understanding of combustion kinetics of large molecule unsaturated biodiesels.

Automated Calibration of a Poly(oxymethylene) Dimethyl Ether Oxidation Mechanism Using the Knowledge Graph Technology.

In this paper, we develop a knowledge graph-based framework for the automated calibration of combustion reaction mechanisms and demonstrate its effectiveness on a case study of poly(oxymethylene)dimethyl ether (PODEn, where n = 3) oxidation. We develop an ontological representation for combustion experiments, OntoChemExp, that allows for the semantic enrichment of experiments within the J-Park simulator (JPS, theworldavatar.com), an existing cross-domain knowledge graph. OntoChemExp is fully capable of supporting experimental results in the Process Informatics Model (PrIMe) database. Following this, a set of software agents are developed to perform experimental result retrieval, sensitivity analysis, and calibration tasks. The sensitivity analysis agent is used for both generic sensitivity analyses and reaction selection for subsequent calibration. The calibration process is performed as a sampling task, followed by an optimization task. The agents are designed for use with generic models but are demonstrated with ignition delay time and laminar flame speed simulations. We find that calibration times are reduced, while accuracy is increased compared to manual calibration, achieving a 79% decrease in the objective function value, as defined in this study. Further, we demonstrate how this workflow is implemented as an extension of the JPS.

Studies on the kinetics of catalytic combustion of deuterium using 0.5% platinum loaded on Dixon ring catalyst

When hydrogen isotopes are present in stoichiometric ratio of 2:1 with oxygen in inert gas such as helium, catalytic combustion is the most promising option to reduce the concentration of hydrogen isotopes and to minimize the associated safety hazards. This is the case, when deuterium and oxygen is formed during radiolysis of heavy water in moderator circuit of Pressurized Heavy Water Reactors (PHWR) and mixed with helium in the cover gas system. In order to design the catalytic reactor for recombination of deuterium and oxygen, the data on catalytic combustion kinetics under the similar process conditions are essential. However, the catalytic combustion data is generally reported for combustion of hydrogen in air, where oxygen is available in excess. The studies on the kinetics of catalytic combustion of deuterium where oxygen is present in stoichiometric ratio of 2:1 in an inert gas medium such as helium, is very scarce. In the present study, the multi-step reaction mechanism of catalytic combustion of hydrogen in presence of platinum catalyst is analyzed for the present process conditions. Based on the analysis, a simple rate expression is proposed for the present process conditions of catalytic combustion of deuterium. Further, 0.5% platinum catalyst is prepared on a stainless steel Dixon ring support with an objective to achieve better heat transfer characteristics and lesser reduction in the catalytic activity due to water adsorption. The kinetic data is generated using a differential packed bed reactor operating in a closed loop. The experiments were conducted at different temperatures using a stoichiometric mixture of deuterium and oxygen in helium. The rate constant for the above proposed model is estimated based on the experimental data at different temperatures. Further, the activation energy and frequency factor are determined and the activation energy for the present catalyst is found to be on the lower side in comparison to literature data.

Experimental investigation of temperature distribution and spontaneous combustion tendency of coal gangue stockpiles in storage.

In this study, an experimental model about the coal gangue stockpiles in semi-open storage was developed. According to the model's requirements, the corresponding coal gangues were piled up in the factory building, the heat source and collection points were arranged, and the four operating temperatures were selected from 70 to 350 °C for heating. A series of fire experiments concerning the temperature distributions of the coal gangue piles were conducted systematically. The spontaneous combustion tendency of coal gangue samples under kinds of the four heat sources was analyzed using scanning electron microscope (SEM), thermal gravity analysis, and differential thermal gravity (TG-DTG). Under the action of thermal damage, the surface of micropores in coal gangue becomes rough. Heat accumulation is, in nature, most likely to occur near 0.1~0.4 m away from the heat source of coal pile. Simultaneously, on each of the measured flat layers, the greater the horizontal distance from the heat source is, the lower the heated temperature of gangues is, and the lower the temperature change rate is, indicating that the horizontal heat conduction is also gradually weakened. The experimental model provides an empirical basis for studying the distribution of temperature field in the depth of gangue pile and kinetics reaction mechanism of spontaneous combustion.

Construction of a Reduced Diesel/Polyoxymethylene Dimethyl Ether 3 (PODE3) Reaction Mechanism for Combustion and Emission Analysis

Polyoxymethylene dimethyl ether 3 has been regarded as a promising additive for reducing soot emissions in diesel engines. To better reflect the physical and chemical properties and combustion, emi...

Study on activity, stability limit and reaction mechanism of CO self-sustained combustion over the LaMnO3, La0.9Ce0.1MnO3 and La0.9Sr0.1MnO3 perovskite catalysts using sugar agent

The LaMnO3, La0.9Ce0.1MnO3 and La0.9Sr0.1MnO3 catalysts are synthesized using sugar agent, and the CO self-sustained combustion is investigated, where the catalytic performance is decided by temperature with CO conversions of 10% (T10), 50% (T50), and 90% (T90). The results show that self-sustaining combustion is successfully realized on the catalyst, and the order of activity decrease is as follows: La0.9Ce0.1MnO3 (with sugar) > La0.9Sr0.1MnO3 (with sugar) > LaMnO3 (with sugar) > LaMnO3 (without sugar) > La0.9Sr0.1MnO3 (without sugar) > La0.9Ce0.1MnO3 (without sugar). Combined with the results of XPS, H2-TPR, O2-TPD and CO-TPD techniques, the excellent activity of La0.9Ce0.1MnO3 (with sugar) can be attributed to the high content of Mn4+ ions and reactive oxygen vacancies enriched on the catalyst surface, sound low-temperature reduction, and uniform dispersion. Besides, in situ IR spectroscopy results indicate that the catalytic combustion of CO over manganese-based perovskite catalysts follows the L-H mechanism: the chemisorption of CO and O2 takes place to produce monodentate carbonates and bicarbonate species, which then decompose to yield CO2 release. The high-temperature stability test provides evidence that the La0.9Ce0.1MnO3 (with sugar) gives 100% CO conversion and that the activities remain almost unchanged after reaction for 12 h, where the temperature of catalyst bed reaches about 717 °C. The results obtained are helpful to accept this technology on efficient and clean energy utilization in iron and steel industry.

Thermal behaviors, combustion mechanisms, evolved gasses, and ash analysis of spent potlining for a hazardous waste management

An unavoidable but reusable waste so as to enhance a more circular waste utilization has been spent potlining (SPL) generated by the aluminum industry. The combustion mechanisms, evolved gasses, and ash properties of SPL were characterized dynamically in response to the elevated temperature and heating rates. Differential scanning calorimetric (DSC) results indicated an exothermic reaction behavior probably able to meet the energy needs of various industrial applications. The reaction mechanisms for the SPL combustion were best described using the 1.5-, 3- and 2.5-order reaction models. Fluoride volatilization rate of the flue gas was estimated at 2.24%. The SPL combustion emitted CO2, HNCO, NO, and NO2 but SOx. The joint optimization of remaining mass, derivative thermogravimetry, and derivative DSC was achieved with the optimal temperature and heating rate combination of 783.5 °C, and 5 °C/min, respectively. Interaction between temperature and heating rate exerted the strongest and weakest impact on DSC and remaining mass, respectively. The fluorine mainly as the formation of substantial NaF and CaF2 in the residual ash. Besides, the composition and effect of environment of residual solid were evaluated. The ash slagging tendency and its mineral deposition mechanisms were elucidated in terms of turning SPL waste into a benign input to a circular waste utilization.

The effects of steam addition on the unstable behavior of hydrogen-air lean premixed flames under the adiabatic and non-adiabatic conditions

The effects of steam addition on the unstable behavior of hydrogen-air lean premixed flames under the adiabatic and non-adiabatic conditions were investigated using numerical calculations. We adopted the detailed chemical reaction mechanism for hydrogen-oxygen combustion, modeled with seventeen reversible reactions of eight reactive species and a diluent. Two-dimensional unsteady reactive flow was treated, based on the compressible Navier-Stokes equations. The burning velocity of a planar flame decreased as the steam addition and heat loss increased. A sufficiently small disturbance was superimposed on a planar flame to study the characteristics of intrinsic instability. We obtained the relation between the growth rate and wave number, i.e. the dispersion relation, and the linearly most unstable wavelength, i.e. the critical wavelength. As the steam addition increased, the unstable range became narrower, and the critical wavelength became longer. Taking account of heat loss, we obtained smaller growth rates and narrower unstable range. The superimposed disturbance developed owing to intrinsic instability, and then the cellular shape of flame fronts appeared. In cellular flames, compared with planar flames, high (low) concentration of active chemical species was found in downstream of convex (concave) fronts. This was caused by high (low) temperature at convex (concave) fronts due to the diffusive-thermal effects. The concentration of active chemical species became lower with increasing the steam addition and heat loss, which was because of the reduction of flame temperature. Moreover, the burning velocity of a cellular flame increased monotonically with an increase in the scale of premixed flames. The burning velocity of a cellular flame normalized by that of a planar flame increased as the steam addition and heat loss increased. The numerical results denoted that the steam addition and heat loss had a great influence on the unstable behavior of hydrogen-air lean premixed flames.

Facile synthesis of birnessite-type K2Mn4O8 and cryptomelane-type K2-xMn8O16 catalysts and their excellent catalytic performance for soot combustion with high resistance to H2O and SO2

Soot particles emitted from the diesel exhausts have caused serious air pollution and catalytic soot combustion is promising for soot abatement. In this work, two kinds of soot oxidation catalysts, including phase-pure birnessite-type K2Mn4O8 (K-OL-1) and cryptomelane-type K2-xMn8O16 (K-OMS-2), were successfully prepared by a very simple method. In preparation of these catalysts, KMnO4 and glucose were used in various proportions and different calcination temperatures employed. The as-prepared catalysts exhibit excellent catalytic performance in the loose contact mode with the presence of water (10 %). Specifically, most of them completely remove soot particles below 450 °C. More importantly, the birnessite-type K2Mn4O8 catalysts exhibit good tolerance for sulfur and water and the presence of water in the interlayer space is the reason for this property. The possible reaction mechanisms for soot combustion on K2Mn4O8 catalysts are related to reaction of active oxygen species and NO2-assisted oxidation. This work shed a light on the important role of the interlayer gallery species (K+, H2O) in the catalytic activity of the birnessite materials in soot combustion, which so far has not been recognized. Meanwhile, the birnessite-type K2Mn4O8 catalysts are low-cost, the excellent catalytic performance and simple synthesis method give them a great potential for industrial applications.

Improvement of H2/O2 chemical kinetic mechanism for high pressure combustion

An updated H2/O2 kinetic mechanism was proposed by incorporating carefully selected reaction rate coefficient and great progress in radical chain mechanisms, in which the uncertainties of rate coefficient were discussed. The performance of the current mechanism was compared to other H2 mechanism and validated against a wide range kinetic targets, including oxidation, decomposition in shock waves, ignition, flame speed and flame structure. Results show that the current mechanism obtains an overall improvement of performance, especially for the flame speed. By using the updated binary diffusion coefficient from ab initio calculations and the chemically termolecular reactions, the current mechanism presents better agreement with the new experimental flame speed at atmospheric pressure and obtains the improved performance with respect to the negative pressure dependence of high-pressure H2 flame. Furthermore, the flame speed predictions are strongly sensitive to the H2O third body efficiency in the H2 mechanism, affecting the water-contained H2 flame. The modeling results of rapid compression machine ignition show that present mechanism can more accurately predicts the ignition delay under engine-like conditions. However, all three mechanisms cannot accurately reproduce the negative pressure dependence behavior of mass burning rate in high-pressure H2 flame, which may be attributed to the fact that the important reaction O + OH(+M) = HO2(+M) that significantly affects lean high-pressure H2 flame is not included in current mechanism. Consequently, continuous works should be emphasized on the reactions that are important but neglected in H2 mechanism. All these not only develop an improved H2 reaction mechanism for high-pressure combustion, but also point out the direction for refining the H2 mechanism.