Lean Methane Research Articles

Experimental and numerical investigation of hetero-/homogeneous combustion-based HCCI of methane–air mixtures in free-piston micro-engines

The hetero-/homogenous combustion-based HCCI (homogeneous charge compression ignition) of fuel–lean methane–air mixtures over alumina-supported platinum catalysts was investigated experimentally and numerically in free-piston micro-engines without ignition sources. Single-shot experiments were carried out in the purely homogeneous and coupled hetero-/homogeneous combustion modes, involved temperature measurements, capturing the visible combustion image sequences, exhaust gas analysis, and the physicochemical characterization of catalysts. Simulations were performed with a two-dimensional transient model that includes detailed hetero-/homogeneous chemistry and transport, leakage, and free-piston motion to gain physical insight and to explore the hetero-/homogeneous combustion characteristics. The micro-engine performance concerning combustion efficiency, mass loss, energy density, and free-piston dynamics was investigated. The results reveal that both purely homogeneous and coupled hetero-/homogeneous combustion of methane–air mixtures in a narrow cylinder with a diameter of 3mm and a height of approximately 0.3mm are possible. The coupled hetero-/homogeneous mode can not only significantly improve the combustion efficiency, in-cylinder temperature and pressure, output power and energy density, but also reduce the mass loss because of its lower compression ratio and less time spent around TDC (top dead center) and during the expansion stroke, indicating that this coupled mode is a promising combustion scheme for micro-engine. Heat losses result in higher mass losses. Heterogeneous reactions cause earlier ignition, which is very favorable for the micro-device.

Highly Active and Thermally Stable Supported Pd@SiO2 Core–Shell Catalyst for Catalytic Methane Combustion

AbstractA series of Pd‐based core–shell catalysts (Pd@SiO2, Pd@CeO2, and Pd@ZrO2) supported on Si‐modified Al2O3 were prepared for the catalytic combustion of lean methane. Pd@SiO2 exhibited slightly lower catalytic activity than Pd@CeO2 and higher catalytic activity than Pd@ZrO2 in dry conditions; however, Pd@SiO2 had the highest catalytic activity among the three catalysts in the presence of high concentrations of water vapor. Furthermore, the Pd@SiO2 catalyst showed good thermal stability and resistivity to water poisoning. CO chemisorption and TEM results demonstrated that the core–shell structure could help in the stabilization of the active phase compared to the uncoated Pd/Si‐Al2O3 catalyst. XRD and X‐ray photoelectron spectroscopy results indicated that a balanced mixed phase of PdOx/Pd0 is the main active phase of these Pd‐based catalysts.

Three-dimensional direct numerical simulation of turbulent lean premixed methane combustion with detailed kinetics

The interaction of maintained homogeneous isotropic turbulence with lean premixed methane flames is investigated using direct numerical simulation with detailed chemistry. The conditions are chosen to be close to those found in atmospheric laboratory experiments. As the Karlovitz number is increased from 1 to 36, the preheat zone becomes thickened, while the reaction zone remains largely unaffected. A negative correlation of fuel consumption with mean flame surface curvature is observed. With increasing turbulence intensity, the chemical composition in the preheat zone tends towards that of an idealised unity Lewis number flame, which we argue is the onset of the transition to distributed burning, and the response of the various chemical species is shown to fall into broad classes. Smaller-scale simulations are used to isolate the specific role of species diffusion at high turbulent intensities. Diffusion of atomic hydrogen is shown to be related to the observed curvature correlations, but does not have significant consequential impact on the thickening of the preheat zone. It is also shown that susceptibility of the preheat zone to thickening by turbulence is related to the ‘global’ Lewis number (the Lewis number of the deficient reactant); higher global Lewis number flames tend to be more prone to thickening.

Combustion of lean methane–air mixtures in a slot burner with adiabatic outer walls

The problem of the combustion of a methane–air mixture in a slot burner with adiabatic outer walls is solved. The problem is investigated numerically in a one-dimensional formulation and dimensional variables. The range of existence of stable high-temperature combustion of the methane–air mixture is determined as a function of gas flow rate and methane content in the mixture. The mechanism of failure of stable combustion is determined. The effect of heat transfer of the gas insert with an inner insert on combustion stability is shown.

Catalytic combustion of sulphur-containing methane lean emissions in a reverse-flow reactor with integrated adsorption

Lean methane emissions (0.15–0.5% CH4) often contain low concentrations of sulphur compounds (e.g. H2S). Regenerative catalytic oxidation is an efficient process for methane removal and upgrading of these streams, but the presence of sulphur compounds deactivates typical combustion catalysts. We propose in this work a new strategy for overcoming this problem, by using a reverse flow reactor provided with integrated adsorption beds. Therefore, it is possible to operate autothermically with low methane concentrations, as well as effectively separate the sulphur compounds before reaching catalytic bed. The working principle of this device has been experimentally demonstrated in a bench-scale reactor working at conditions typical for industrial emissions (GHSV=1146h−1, 4300ppm CH4, 100–500ppm H2S). It has been found that molecular sieve 5A is a suitable adsorbent for this device. The influence of the main reverse flow reactor variables, switching time (200–400s) and methane feed concentration (4000–4500ppm), on the performance of the process has been studied. The integrated adsorption performs better at switching time 400s, while methane concentration has negligible influence, provided that the reactor is maintained ignited.

Kinetic effects of hydrogen addition on the catalytic self-ignition of methane over platinum in micro-channels

The hetero-/homogeneous chemistry coupling of hydrogen and methane over platinum in catalytic micro-channels was investigated numerically for hydrogen–methane–air mixtures with lean methane–air equivalence ratios φ=0.7, 0.8 and 0.9, containing 0–5% hydrogen (on a molar basis). Simulations were carried out with a 2-D computation that included detailed hetero-/homogeneous chemistry and transport, with emphasis on the kinetic effects of hydrogen addition on the catalytic self-ignition of methane. Simulations indicated that hydrogen can successfully cause catalytic self-ignition of methane–air mixtures in micro-channels, eliminating the need for startup devices. While, methane kinetically inhibits catalytic combustion of hydrogen at low hydrogen compositions, and a two-stage catalytic ignition can be observed under certain conditions. A minimum hydrogen composition for catalytic self-ignition of methane–air mixtures is found to be relatively constant, which is at the level of approximately 3.2% molar fraction, irrespective of the methane–air composition. Furthermore, kinetic analysis suggested that this constant is exactly the critical point from thermal effect (preheating region) to kinetic effect (self-ignition region) with the increase of hydrogen composition, resulting in a sharp reduction in ignition temperature, and providing a good startup strategy and an opportunity to self-ignite hydrocarbons. However, once the hydrogen concentration in the feed is sufficiently low, i.e., below the above critical point, external heating is necessary to ignite methane–air mixtures. Finally, surface coverage simulations in catalytic micro-channels revealed that hydrogen promotes methane catalytic combustion because of the increase in free Pt(s) sites, and richer hydrogen composition or lower temperature enhances this kinetic effect.

On the correlation of inverted flame blow-off limits with the boundary velocity gradient at the flame holder surface

Conductive heat losses from the base of a lean methane–air inverted flame stabilized behind the trailing edge of a thin rod have been experimentally evaluated. The results favor the view that the heat losses to the flame holder play a crucial role in the inverted flame stabilization and blow-off. Simple estimations have been performed, which indicate that the well-established correlation between the mixture composition and the boundary velocity gradient at the flame holder, usually considered as a proof of the flame stretch theory of blow-off, can be explained without involving the flame stretch concept. The suggested explanation of this correlation is based on the assumption that the heat loss to the flame holder is the main factor that determines the inverted flame blow-off behavior and on the similarity between the mechanisms of energy and momentum diffusion in gases (Pr≈ 1).

Comparison of simulated results for CO fields at the flame front by the RANS and LES methods

Turbulent combustion of a lean methane–air mixture in a model combustion chamber behind a step is analyzed by numerical methods. Comparison of the RANS and LES methods shows that the neglect of pulsations in the RANS method may result in significant errors in the calculation of CO fields.

Two-dimensional simulation of premixed laminar flame at microscale

Microcombustion is one of the most essential topics in the development of micropower generators for portable devices, microaerial vehicles and sensors. Although one-dimensional numerical modelling of combustion has been developed, maturing a lot during the last forty years, two-dimensional numerical modelling of combustion, particularly at very small scales has only started in the last decade. In this particular study, two-dimensional numerical simulations of premixed, laminar, lean methane–air flames at atmospheric pressure formed in a 2mm diameter microcombustor are performed. A skeletal mechanism consisting of 16 species and 41 reactions is employed. This study shows the importance of applying low target residuals for convergence in CFD, which plays a critical role in obtaining an accurate solution in the numerical modelling of microcombustion. The main focus of this work is to investigate the effect of preheating the reactants on the flame structure and stability with adiabatic and non-adiabatic combustor walls. The flame in the 2mm diameter circular microcombustor under adiabatic wall conditions shows a laminar Bunsen type flame behaviour. On the other hand, under the non-adiabatic wall conditions, a cone shaped high temperature flame zone is observed. Preheating the reactants widens the flame stability at high incoming flow velocities under both conditions, however, under non-adiabatic wall conditions, at low incoming flow velocities and/or low mass flow rates, preheating the reactants contributes neither to flame stability nor combustion characteristics. On the contrary, it results in low combustor exit temperature. Finally, the effects of convective heat transfer coefficients and heat loss, with and without radiation, on the microcombustion are analysed.

An efficient computational scheme for building operating maps for a flow reversal reactor

This paper describes the use of a two dimensional heterogeneous computational model for the catalytic reverse flow reactor for the combustion of lean methane mixtures. A uniform design method is used to generate a table of 41 operating points. The results from these points were used to generate a simple empirical model for the reactor at a single specified value of the superficial gas velocity. Stability maps that show the envelope for operation that satisfy a minimum level of conversion and a maximum operating temperature are generated using the empirical model. These maps delineate all possible combinations of methane concentration, switch time and mass extraction rate that are capable of giving stable operating conditions that meet these two criteria. The model is potentially useful in model predictive control of the reactor.

Solvothermal synthesis and characterization of mixed oxides with perovskite-like structure

Solvothermal synthesis was successfully used to prepare LaMeO3 (Me=Mn, Co, Fe) and La1−xSrxMnO3 (x=0.1, 0.3, 0.5), mixed oxides with perovskite-like structure. The prepared materials were examined by X-ray diffraction (XRD), nitrogen adsorption at low temperature (BET), transmission electron microscopy (TEM), decomposition of cyclohexanol (CHOL), temperature programmed reduction with hydrogen (TPRH2), temperature programmed desorption of oxygen (TPDO2) and X-ray photoelectron spectroscopy (XPS). Furthermore, catalytic activity for combustion of 0.6% vol. methane in air (GHSV=40000h−1) was determined.All prepared materials are nanocrystalline and well-crystallized, and they exhibit relatively well-developed specific surface area (9.2–39.2m2/g). The trigonal structure of the Mn-containing samples corresponds to the structure of the high-temperature bulk phase, which cannot be recovered at ambient conditions. Generally, the Mn-containing materials exhibit higher SSA, more basic character of the surface, lower temperature of maximum H2 consumption and higher catalytic activity in lean methane combustion than the corresponding Co- and Fe-based nanocrystals. The highest catalytic activity for lean methane combustion (100% of CH4 conversion at 451°C) is observed for LaMnO3. A partial substitution of La by Sr lowers content of Mn4+ on the surface of the corresponding materials, resulting in lower catalytic activity in lean methane combustion.

Effects of Electric Fields on the Combustion Characteristics of Lean Burn Methane-Air Mixtures

In this work, the effects of the electric fields on the flame propagation and combustion characteristics of lean premixed methane–air mixtures were experimentally investigated in a constant volume chamber. Results show that the flame front is remarkably stretched by the applied electric field, the stretched flame propagation velocity and the average flame propagation velocity are all accelerated significantly as the input voltage increases. This indicates that the applied electric field can augment the stretch in flame, and the result is more obvious for leaner mixture. According to the analyses of the combustion pressure variation and the heat release rate, the peak combustion pressure Pmax increases and its appearance time tp is advanced with the increase of the input voltage. For the mixture of λ = 1.6 at the input voltage of −12 kV, Pmax increases by almost 12.3%, and tp is advanced by almost 31.4%, compared to the case of without electric fields. In addition, the normalized mass burning rate and the accumulated mass fraction burned are all enhanced substantially, and the flame development duration and the rapid burning duration are remarkably reduced with the increase of the input voltage, and again, the influence of electric field is more profound for leaner mixtures. The results can be explained by the electric field-induced stretch effects on lean burn methane-air mixture.

Combustion of lean methane–air mixtures in monolith beds: Kinetic studies in low and high temperatures

The paper summarizes results of Gosiewski et al. (2009), Pawlaczyk and Gosiewski (2013) and Pawlaczyk (2013) kinetic studies of non-catalytic (thermal) combustion of lean methane–air mixtures, carried out in a low and high temperature, in monolith bed and compared with similar results for a free space. The study reveals an influence of size, type of monolith’s surface and temperature in combustion zone on the reaction mechanism and its kinetics. It was formulated a hypothesis that the share of combustion type: heterogeneous with surface effect (on the monolith’s wall) and homogeneous (in the free space) depends on the temperature in the combustion zone. The appropriate kinetic equations were estimated. Moreover the paper presents corrected values of kinetic parameters which in contrast to Gosiewski et al. (2009) and Pawlaczyk and Gosiewski (2013) were estimated using concentrations of components related to actual gas volume in the combustion zone.

The stability of Co3O4, Fe2O3, Au/Co3O4 and Au/Fe2O3 catalysts in the catalytic combustion of lean methane mixtures in the presence of water

Nano-sized Co3O4, Fe2O3, Au/Co3O4 and Au/Fe2O3 catalysts were prepared and evaluated for catalytic combustion of lean methane-air mixtures. Characteristics and catalytic activities under dry and wet feed conditions were investigated at gas hourly space velocities up to 100000h−1 mimicking the typical flow and conversion requirements of a catalytic system designed to treat a ventilation air methane stream. In order to gain a better understanding of the interaction between H2O and the catalyst surface, temperature-programmed desorption of water over fresh and used samples were studied, and supported by other catalyst characterization techniques such as N2-adsorption desorption, XRD, TEM, SEM and XPS analyses. The activity measurements of the catalysts studied identify Co3O4 as the most active material. Co-precipitating gold particles with cobalt oxide or iron oxide do not enhance the activity of the catalyst, which is most likely due to blocking the active site of support by the gold particle. The presence of strong hydroxyl bonds on the catalyst surface is substantiated by TPD and XPS analyses, and is suggested to be responsible for the rapid deactivation of Fe2O3 and Au/Fe2O3 catalysts.

Simulation of the Flame Propagation in a Methane–Air Mixture in the Presence of Water Aerosol

We have formulated a physicomathematical model of the flame propagation in a combustible gas containing water aerosol based on the thermal-diffusion model of the laminar flame propagation in a gas and taking into account the processes of heat and mass transfer between the phase and liquid drops. Computational-theoretical studies of the influence of water aerosol characteristics on the flame velocity in a lean methane–air mixture have been made. Comparison of the results of calculations with experimental data has shown that there is good agreement between them. Comparison of the efficiency of using water aerosol and inert gas to stop the spread of fire has shown that there exists a limiting size of the dispersed phase above which the efficiency of using water aerosol and inert powders to stop the spread of fire becomes equal.

Structure of hydrogen-rich transverse jets in a vitiated turbulent flow

This paper reports the results of a joint experimental and numerical study of the flow characteristics and flame structure of a hydrogen rich jet injected normal to a turbulent, vitiated crossflow of lean methane combustion products. Simultaneous high-speed stereoscopic PIV and OH PLIF measurements were obtained and analyzed alongside three-dimensional direct numerical simulations of inert and reacting JICF with detailed H2/CO chemistry. Both the experiment and the simulation reveal that, contrary to most previous studies of reacting JICF stabilized in low-to-moderate temperature air crossflow, the present conditions lead to a burner-attached flame that initiates uniformly around the burner edge. Significant asymmetry is observed, however, between the reaction zones located on the windward and leeward sides of the jet, due to the substantially different scalar dissipation rates. The windward reaction zone is much thinner in the near field, while also exhibiting significantly higher local and global heat release than the much broader reaction zone found on the leeward side of the jet. The unsteady dynamics of the windward shear layer, which largely control the important jet/crossflow mixing processes in that region, are explored in order to elucidate the important flow stability implications arising in the inert and reacting JICF. The paper concludes with an analysis of the ignition, flame characteristics, and global structure of the burner-attached flame. Chemical explosive mode analysis (CEMA) shows that the entire windward shear layer, and a large region on the leeward side of the jet, are highly explosive prior to ignition and are dominated by non-premixed flame structures after ignition. The predominantly mixing limited nature of the flow after ignition is examined by computing the Takeno flame index, which shows that ∼70% of the heat release occurs in non-premixed regions.

Hydrocarbon flame inhibition by C3H2F3Br (2-BTP)

A kinetic mechanism for hydrocarbon flame inhibition by the potential halon replacement 2-BTP (2-Bromo-3,3,3-trifluoropropene) has been assembled, and is used to study its effects on premixed methane–air flames. Simulations with varying CH4–air stoichiometry and agent loading have been used to understand its flame inhibition mechanism. In particular, the response of lean methane–air flames is examined with addition of 2-BTP, CF3Br, C2HF5, and N2 to illustrate the effect of agent heat release on these flames. The results predict that addition of 2-BTP or C2HF5 can increase the burning velocity of very lean flames, and 2-BTP is less effective for lean flames than for rich. The flame inhibition mechanism of 2-BTP involves the same bromine-species gas-phase catalytic cycle as CF3Br, which drives the flame radicals to equilibrium levels, which can be raised, however, by higher temperatures with added agent (for initially lean flames). Simulations for pure 2-BTP–O2–N2 mixtures predict burning velocities on the order of 1cm/s at 300K initial temperature.

Dynamic Behavior of Reverse Flow Reactor for Lean Methane Combustion

The stability of reactor operation for catalytic oxidation of lean CH4 has been investigated through modeling and simulation, particularly the influence of switching time and heat extraction on reverse flow reactor (RFR) performance. A mathematical model of the RFR was developed, based on one-dimensional pseudo-homogeneous model for mass and heat balances, incorporating heat loss through the reactor wall. The configuration of the RFR consisted of inert-catalyst-inert, with or without heat extraction that makes it possible to store the energy released by the exothermic reaction of CH4 oxidation. The objective of this study was to investigate the dynamic behavior of the RFR for lean methane oxidation and to find the optimum condition by exploring a stability analysis of the simple reactor. The optimum criteria were defined in terms of CH4 conversion, CH4 slip, and heat accumulation in the RFR. At a switching time of 100 s, the CH4 conversion reached the maximum value, while the CH4 slip attained its minimum value. The RFR could operate autothermally with positive heat accumulation, i.e. 0.02 J/s. The stability of the RFR in terms of heat accumulation was achieved at a switching time of 100 s.

Prediction of NOx in premixed high-pressure lean methane flames with a MMC-partially stirred reactor

The multiple mapping conditioning (MMC) method is used to extend the stochastic PDF approach to the flamelet regimes of premixed combustion. MMC permits for a desired degree of flamelet-like conditions while reflecting the fluctuating nature of turbulent flames and preserving the integrity and universality of the chosen mixing model. The model is implemented in the context of the partially stirred reactor (PaSR), which is therefore generalised to have a wider range of applicability. A stochastic formulation of original MMC is deployed, where mixing of particle scalar values is conditioned on a Markovian reference variable which emulates an implied particle position relative to a flame. The model interactions with the reference variable are controlled through the flamelet localness parameter, Λ, which is also related to the ratio of diffusive to convective time scales. The model is implemented in a Monte Carlo numerical scheme using detailed GRI3.0 chemical kinetics without adjustments of kinetic coefficients. Predictions of NOx emissions are validated against experimental data for a lean premixed high-pressure combustor in which reactions fall between the flamelet and distributed regimes. There is good agreement between model predictions and experimental data.

Local quenching phenomena of a lean premixed flat flame impinging with a pulsating air jet

Local quenching phenomena of a lean methane air premixed flat flame formed horizontally in a wall stagnating flow impinging with a pulsating air jet has been investigated experimentally. The burner system consists of 40mm inverted nozzle burner and a solid wall with 8mm diameter air jet placed in line vertically. The pulsating frequencies set up to 100Hz while the jet intensities generate up to 6 m/s by a loud speaker. Approximately '00mm disk shape flame front is curved by the pulsating air jet and the air jet impacting point is locally quenched. The fuel concentration of quenching start condition increases with increasing the intensity of air jet, because the increased jet intensity linked with the flame strain rate gain. For weak jet intensity range, the quenching hole becomes directly to develop the whole flame extinction. On the other hand, for moderate or strong jet condition, the flame can recover from the local quenching phenomena. In this condition, once the quenching hole creates, but the hole may close by the flame propagation or reigniting process. Then, the whole flame extinction limits are lower than no jet impacting condition depending on the circumstances.