An experimental study of mild flameless combustion of methane/hydrogen mixtures

This paper presents an experimental study of mild flameless combustion regime applied to methane/hydrogen mixtures in a laboratory-scale pilot furnace with or without air preheating. Results show that mild flameless combustion regime is achieved from pure methane to pure hydrogen whatever the CH₄/H₂ proportion. The main reaction zone remains lifted from the burner exit, in the mixing layer of fuel and air jets ensuring a large dilution correlated to low NOx emissions whereas CO₂ concentrations obviously decrease with hydrogen proportion. A decrease of NOx emissions is measured for larger quantity of hydrogen due mainly to the decrease of prompt NO formation. Without air preheating, a slight increase of the excess air ratio is required to control CO emissions. For pure hydrogen fuel without air preheating, mild flameless combustion regime leads to operating conditions close to a "zero emission furnace", with ultra-low NOx emissions and without any carbonated species emissions.     

Modelling and simulation of lean premixed turbulent methane/hydrogen/air flames with an effective Lewis number approach

Recent work on reaction modelling of turbulent lean premixed combustion has shown a significant influence of the Lewis number even at high turbulence intensities, if different fuels and varied pressure is regarded. This was unexpected, as the Lewis number is based on molecular transport quantities (ratio of molecular thermal diffusivity to mass diffusivity), while highly turbulent flames are thought to be dominated from turbulent mixing and not from molecular transport. A simple physical picture allows an explanation, assuming that essentially the leading part of the wrinkled flame front determines the flame propagation and the average reaction rate, while the rear part of the flame is of reduced importance here (determining possibly the burnout process and the flame brush thickness but not the flame propagation). Following this argumentation, mostly positively curved flame elements determine the flame propagation and the average reaction rate, where the influence of the preferential molecular diffusion and the Lewis number can easily seen to be important. Additionally, an extension of this picture allows a simple derivation of an effective Lewis number relation for lean hydrogen/methane mixtures. The applicability and the limit of this concept is investigated for two sets of flames: turbulent pressurized Bunsen flames, where hydrogen content and pressure is varied (from CNRS Orléans), and highly turbulent pressurized dump combustor flames where the hydrogen content is varied (from PSI Baden). For RANS simulations, comparison of flame length data between experiment and an effective Lewis number model shows a very good agreement for all these flames with hydrogen content of the fuel up to 20 vol.%, and even rather good agreement for 30% and 40% hydrogen.     

Reactive structures and NOx emissions of methane/hydrogen mixtures in flameless combustion

Methane/hydrogen combustion represents a concrete solution for the energy scenario to come. Indeed, the addition of hydrogen into the natural gas pipeline is one of the solutions foreseen to reduce CO₂ emissions. Nevertheless, the replacement of methane by hydrogen will enhance the reactivity of the system, increasing NOx emissions. To overcome this issue, non-conventional combustion technologies, such as flameless combustion represent an attractive solution. This study aims to improve our understanding of the behaviour of methane/hydrogen blends under flameless conditions by means of experiments and simulations. Several experimental campaigns were conducted to test fuel flexibility for different methane/hydrogen blends, varying the injector geometries, equivalence ratio and dilution degree. It was found that a progressive addition of hydrogen in methane enhanced the combustion features, reducing the ignition delay time and loosing progressively the flameless behaviour of the furnace. Reducing the air injector diameter or increasing the fuel lance length were found to be efficient techniques to reduce the maximum temperature of the system and NOx emissions in the exhausts, reaching values below 30 ppm for pure hydrogen. MILD conditions were achieved up to 75%H₂ in molar fraction, with no visible flame structures. Additionally, RANS-based simulations were also conducted to shed further light on the effect of adding hydrogen into the fuel blend. A sensitivity study was conducted for three different fuel blends: pure methane, an equimolar blend and pure hydrogen. The effect of chemistry detail, mixing models, radiation modeling and turbulence models on in-flame temperatures and NOx emissions was also studied. In particular, it was found that the usage of detailed chemistry for NOx, coupled with an adjustment of the PaSR model, filled the gap between experiments and predictions. Finally, a brute-force sensitivity revealed that NNH is the most important route for NOx production.     

Decarbonisation potential of dimethyl ether/hydrogen mixtures in a flameless furnace: Reactive structures and pollutant emissions

This article sheds light on the combustion characteristics of dimethyl ether and its mixtures with methane/hydrogen under flameless conditions at different equivalence ratios. It was found that combustion of 100% dimethyl ether in flameless conditions minimises the NO formation, keeping it less than 10 ppm with no CO or unburned hydrocarbons. Progressive addition of methane was found to reduce the NO, reaching up to zero value at 50% methane in molar fraction along with a marginal CO₂ reduction. However, large amounts of CO were found for higher methane levels, greater than 60% CH₄ in molar fraction. Reactive structures based on OH1 chemiluminescence revealed that adding methane results in increased ignition delay times and, consequently, a more distributed reaction zone characterised by reduced temperature gradients. No visible flame was observed for pure dimethyl ether as well as dimethyl ether/methane mixtures. Furthermore, a more intense and narrower reaction zone, characterised by the presence of a visible flame, was formed upon hydrogen addition. Adding hydrogen by 50% in molar fraction did not cause a noticeable rise in NO levels; however, CO₂ was lowered by about 18%. Further addition of hydrogen resulted in increased peak temperatures of about 1700 K and higher NO emissions of about 50 ppm. Additionally, a skeletal Chemical Reactor Network was built and simulated with the commercial software CHEMKIN Pro to investigate the effect of the different mixtures and operating conditions on NO formation from a chemical point of view. N₂O pathway was observed to be the root source of NO emissions for pure DME and DME/CH₄ mixtures, however; the thermal pathway became gradually more important as hydrogen concentration was increased in the mixture.     

Design and numerical analysis of a 3 kWe flameless microturbine combustor for hydrogen fuel

In this work, a new 3 kWe flameless combustor for hydrogen fuel is designed and analyzed using CFD simulation. The strategy of the design is to provide a large volumetric combustion for hydrogen fuel without significant rise of the temperature. The combustor initial dimensions and specification were obtained from practical design procedures, and then optimized using CFD simulations. A three-dimensional model for the designed combustor is constructed to further analysis of flameless hydrogen combustion and consideration that leads to disappearance of flame-front and flameless combustion. The key design parameters including aerodynamic, temperature at walls and flame, NO_X, pressure drop, combustion efficiency for the hydrogen flame is analyzed in the designed combustor. To well demonstrate the combustor, the NO_X and entropy destruction and finally energy conversion efficiency, and overall operability in the microturbine cycle of hydrogen flameless combustor is compared with a 3 kWe design counterpart for natural gas. The findings demonstrate that hydrogen flameless combustion is superior to derive the microturbines with significantly lower NO_X, and improvements in energy efficiency, and cycle overall efficiency with low wall temperatures guaranteeing the long-term operation of combustor and microturbine parts.     

First and second thermodynamic-law analyses of hydrogen-air counter-flow diffusion combustion in various combustion modes

In this paper, from the viewpoints of both the first and the second law of thermodynamics, we conduct a comprehensive study on hydrogen-air counter-flow diffusion combustion in various modes. The effects of air inlet temperature (T_oxi) and effective equivalence ratio of fuel (φ) on the reaction zone structure and entropy generation of combustion are revealed over a wide range of T_oxi and φ. Through the present work, five interesting features of combustion of this kind, which are quite different from that reported in the literature, are presented. Especially, for the first time we divide various combustion modes in the φ − T_oxi map instead of the popular way used in previous studies. Such innovation can help judge the final combustion regime more straightforwardly for any given operative condition.     

Effects of multi-component diffusion and heat release on laminar diffusion flame liftoff

Numerical simulations were conducted of the liftoff and stabilization phenomena of laminar jet diffusion flames of inert-diluted C₃H₈ and CH₄ fuels. Both non-reacting and reacting jets were investigated, including multi-component diffusivities and heat release effects (buoyancy and gas expansion). The role of Schmidt number for non-reacting jets was investigated, with no conclusive Schmidt number criterion for liftoff previously arrived at in similarity solutions. The cold-flow simulation for He-diluted CH₄ fuel does not predict flame liftoff; however, adding heat release reaction lead to the prediction of liftoff, which is consistent with experimental observations. Including reaction was also found to improve liftoff height prediction for C₃H₈ flames, with the flame base location differing from that in the similarity solution - the intersection of the stoichiometric and iso-velocity (equal to 1-D flame speed) is not necessary for flame stabilization (and thus liftoff). Possible mechanisms other than that proposed for similarity solution may better help to explain the stabilization and liftoff phenomena.     

Autoignited laminar lifted flames of methane, ethylene, ethane, and n-butane jets in coflow air with elevated temperature

The autoignition characteristics of laminar lifted flames of methane, ethylene, ethane, and n-butane fuels have been investigated experimentally in coflow air with elevated temperature over 800 K. The lifted flames were categorized into three regimes depending on the initial temperature and fuel mole fraction: (1) non-autoignited lifted flame, (2) autoignited lifted flame with tribrachial (or triple) edge, and (3) autoignited lifted flame with mild combustion.For the non-autoignited lifted flames at relatively low temperature, the existence of lifted flame depended on the Schmidt number of fuel, such that only the fuels with Sc > 1 exhibited stationary lifted flames. The balance mechanism between the propagation speed of tribrachial flame and local flow velocity stabilized the lifted flames. At relatively high initial temperatures, either autoignited lifted flames having tribrachial edge or autoignited lifted flames with mild combustion existed regardless of the Schmidt number of fuel. The adiabatic ignition delay time played a crucial role for the stabilization of autoignited flames. Especially, heat loss during the ignition process should be accounted for, such that the characteristic convection time, defined by the autoignition height divided by jet velocity was correlated well with the square of the adiabatic ignition delay time for the critical autoignition conditions. The liftoff height was also correlated well with the square of the adiabatic ignition delay time.     

Analysis of high-pressure hydrogen, methane, and heptane laminar diffusion flames: Thermal diffusion factor modeling

Direct numerical simulations are conducted for one-dimensional laminar diffusion flames over a large range of pressures (1?P₀?200 atm) employing a detailed multicomponent transport model applicable to dense fluids. Reaction kinetics mechanisms including pressure dependencies and prior validations at both low and high pressures were selected and include a detailed 24-step, 12-species hydrogen mechanism (H₂/O₂ and H₂/air), and reduced mechanisms for methane (CH₄/air: 11 steps, 15 species) and heptane (C₇H₁₆/air: 13 steps, 17 species), all including thermal NO_x chemistry. The governing equations are the fully compressible Navier-Stokes equations, coupled with the Peng-Robinson real fluid equation of state. A generalized multicomponent diffusion model derived from nonequilibrium thermodynamics and fluctuation theory is employed and includes both heat and mass transport in the presence of concentration, temperature, and pressure gradients (i.e., Dufour and Soret diffusion). Previously tested high-pressure mixture property models are employed for the viscosity, heat capacity, thermal conductivity, and mass diffusivities. Five models for high-pressure thermal diffusion coefficients related to Soret and Dufour cross-diffusion are first compared with experimental data over a wide range of pressures. Laminar flame simulations are then conducted for each of the four flames over a large range of pressures for all thermal diffusion coefficient models and results are compared with purely Fickian and Fourier diffusion simulations. The results reveal a considerable range in the influence of cross-diffusion predicted by the various models; however, the most plausible models show significant cross-diffusion effects, including reductions in the peak flame temperatures and minor species concentrations for all flames. These effects increase with pressure for both H₂ flames and for the C₇H₁₆ flames indicating the elevated importance of proper cross-diffusion modeling at large pressures. Cross-diffusion effects, while not negligible, were observed to be less significant in the CH₄ flames and to decrease with pressure. Deficiencies in the existing thermal diffusion coefficient models are discussed and future research directions suggested.     

Effect of hydrogen and helium addition to fuel on soot formation in an axisymmetric coflow laminar methane/air diffusion flame

A detailed numerical study was conducted to investigate the effects of hydrogen and helium addition to fuel on soot formation in atmospheric axisymmetric coflow laminar methane/air diffusion flame. Detailed gas-phase chemistry and thermal and transport properties were employed in the numerical calculations. Soot was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Numerical results were compared with available experimental data. Both experimental and numerical results show that helium addition is more effective than hydrogen addition in reducing soot loading in the methane/air diffusion flame. These results are different from the previous investigations in ethylene/air diffusion flames. Hydrogen chemically enhances soot formation when added to methane. The different chemical effects of hydrogen addition to ethylene and methane on soot formation are explained in terms of the different effects of hydrogen addition on propargyl, benzene, and pyrene formation low in the flames.     

Effect of hydrogen percentage and air jet Reynolds number on fuel lean flame stability of LPG-fired inverse diffusion flame with hydrogen enrichment

The flame type studied in this paper is a circumferential-fuel – jet inverse diffusion flame, and the fuel is liquefied petroleum gas enriched with hydrogen gas. Fuel lean flame stability limit regarding to the volumetric percentage of hydrogen and the air jet Reynolds number was investigated. There were three flame stable-related limits examined: local extinction limit, restore limit, and complete extinction limit. Global Energy Consumption Rate of fuel, fuel jet velocity, and overall equivalence ratio of the air/fuel mixture at the three stable-related limits were presented. Experimental results indicate that with hydrogen addition, the inverse diffusion flame can sustain burning with a lower global energy than without it. The most significant stabilization effect was obtained with 30% hydrogen addition for complete extinction limit and 30%–90% for local extinction limit. The corresponding fuel jet velocity at complete extinction limit also decreases with hydrogen addition. However, fuel jet velocities at local extinction limit and restore limit increase significantly, when hydrogen percentage is larger than 70%. Air jet Reynolds number does not show notable influence on Global Energy Consumption Rate or fuel jet velocity at the three stability limits. In addition, overall equivalence ratio, which is an important parameter of inverse diffusion flame combustion dropping dramatically with air jet Reynolds number when it is less than 2000.     

Comprehensive numerical study of molecular diffusion effects and Eddy Dissipation Concept model in MILD combustion

Moderate or Intense Low-oxygen Dilution (MILD) combustion is a clean combustion technology with high thermal efficiency and low levels of emissions. In this paper, by employing Adelaide Jet-in-Hot-Co-flow (AJHC), several approaches are examined to increase the numerical solution accuracy. First, molecular diffusion effects are investigated in MILD combustion. Second, adjusting the Eddy Dissipation Concept (EDC) coefficients is comprehensively discussed, and finally, the reaction fraction coefficient and EDC formulation are investigated. The results show that the effect of enthalpy transport caused by molecular diffusion on the energy equation must be considered in the low oxygen concentration regions. Also, the maximum temperature in the MILD region can be kept constant by adjusting EDC coefficients. Furthermore, it is shown that applying the reaction fraction factor increases the accuracy of the numerical solution in the MILD region.     

Characterization and challenge of development of producer gas fuel combustor: A review

Producer gas, which derived from a biomass gasification process, is considered as one of the alternative fuels, which is suitable for the heating process and power generation. Due to low heating density and impurities, combustion in an external combustion chamber constitutes an obvious option for the utilization of producer gas via the combustion process. This paper reviews the technical challenges and the development of the producer gas combustor. Various combustion techniques are reviewed. A stable flame combustion with low emissions (both CO and NO_x) constitutes a main requirement of the producer gas combustion. Flame stabilization techniques such as swirl-vane coupled with bluff-body, swirl flow configuration, and staging combustion were successfully employed to enhance the stability and performance of the producer gas combustion. As shown in the results of the studies, the combustion process can operate in a wide range of equivalence ratios with the exhaust gas temperature >600 °C. This temperature is sufficiently hot for the power generation and heating applications. Overall, NOx and CO emissions were below 700 ppm and 1.3%, respectively. In the flameless combustion mode, ultra-low emission for both CO and NOx were recorded. However, higher emission can be found when operated at a higher thermal load combustor. Homogeneity of the thermal field and low polluting emissions make flameless combustion a promising lean and clean combustion technology. Integration of the benefits of flameless combustion and producer gas fuel is an outstanding contribution in reducing emissions and enhancing the efficiency of the combustion systems.     

Aluminium clusters for molecular hydrogen storage and the corresponding alanes as fuel alternatives: A structural and energetic analysis

Bare and hydrogenated aluminium clusters Al_n (n = 5–7) are studied using density functional theory in order to evaluate the ability to store molecular hydrogen and to estimate the energy release upon combustion with the aim to understand these species as alternative fuel resources. Six sequential molecular hydrogenations are considered and are shown to occur without or very low activation barriers. The H₂ molecule uses its occupied σ orbital to react with an appropriate electron-deficient site of the cluster; this is generally followed by H migration leading to the favoured geometry. The combustion process produces alumina (Al₂O₃), water and a significant quantity of energy. The exothermicity seems to be largely independent of cluster size but it depends instead on the stored hydrogen quantity.     

Effects of fuel compositions on the heat generation and emission of syngas/producer gas laminar diffusion flame

Demand for the clean and sustainable energy encourages the research and development in the efficient production and utilisation of syngas for low-carbon power and heating/cooling applications. However, diversity in the chemical composition of syngas, resulting due to its flexible production process and feedstock, often poses a significant challenge for the design and operation of an effective combustion system. To address this, the research presented in this paper is particularly focused on an in-depth understanding of the heat generation and emission formation of syngas/producer gas flames with an effect of the fuel compositions. The heat generated by flame not only depends on the flame temperature but also on the chemistry heat release of fuel and flame dimension. The study reports that the syngas/producer gas with a low H_2:CO maximises the heat generation, nevertheless the higher emission rate of CO₂ is inevitable. The generated heat flux at H_2:CO = 3:1, 1:1, and 1:3 is found to be 222, 432 and 538 W m^-2 respectively. At the same amount of heat generated, H₂ concentration in fuel dominates the emission of NO_x. The addition of CH₄ into the syngas/producer gas with H_2:CO = 1:1 also increases the heat generation significantly (e.g. 614 W m^-2 at 20%) while decreases the emission formation. In contrast, adding 20% CO₂ and N₂ to the syngas/producer gas composition reduces the heat generation from 432 W m^-2 to 364 and 290 W m^-2, respectively. The role of CO₂ on this aspect, which is weaker than N₂, thus suggests CO₂ is preferable than N₂. Along with the study, the significant role of CO₂ on the radiation of heat and the reduction of emission are examined.     

A computational study on the combustion of hydrogen/methane blended fuels for a micro gas turbines

To understand the combustion performance of using hydrogen/methane blended fuels for a micro gas turbine that was originally designed as a natural gas fueled engine, the combustion characteristics of a can combustor has been modeled and the effects of hydrogen addition were investigated. The simulations were performed with three-dimensional compressible k-ε turbulent flow model and presumed probability density function for chemical reaction. The combustion and emission characteristics with a variable volumetric fraction of hydrogen from 0% to 90% were studied. As hydrogen is substituted for methane at a fixed fuel injection velocity, the flame temperatures become higher, but lower fuel flow rate and heat input at higher hydrogen substitution percentages cause a power shortage. To apply the blended fuels at a constant fuel flow rate, the flame temperatures are increased with increasing hydrogen percentages. This will benefit the performance of gas turbine, but the cooling and the NO_x emissions are the primary concerns. While fixing a certain heat input to the engine with blended fuels, wider but shorter flames at higher hydrogen percentages are found, but the substantial increase of CO emission indicates a decrease in combustion efficiency. Further modifications including fuel injection and cooling strategies are needed for the micro gas turbine engine with hydrogen/methane blended fuel as an alternative.     

Effects of Lewis number and preferential diffusion on flame characteristics in 80%H2/20%CO syngas counterflow diffusion flames diluted with He and Ar

Numerical study is conducted to grasp flame characteristics in H₂/CO syngas counterflow diffusion flames diluted with He and Ar. An effective fuel Lewis number, applicable to premixed burning regime and even to moderately stretched diffusion flames, is suggested through the comparison among fuel Lewis number, effective Lewis number, and effective fuel Lewis number. Flame characteristics with and without the suppression of the diffusivities of H, H₂, and He are compared in order to clarify the important role of preferential diffusion effects through them. It is found that the scarcity of H and He in reaction zone increases flame temperature whereas that of H₂ deteriorates flame temperature. Impact of preferential diffusion of H, H₂, and He in flame characteristics is also addressed to reaction pathways for the purpose of displaying chemical effects.     

Hysteresis loops of methane catalytic partial oxidation for hydrogen production under the effects of varied Reynolds number and Damköhler number

Hysteresis loops of catalytic partial oxidation of methane (CPOM) for hydrogen production under the effects of varied Reynolds number and Damköhler number are investigated numerically in this study. The physical phenomena are predicted using the indirect mechanism, which consists of the total oxidation (or combustion), steam reforming and CO₂ reforming of methane in a catalyst bed. Numerical results reveal that, when the Damköhler number is relatively low, a hysteresis loop of CPOM from varying Reynolds number develops. Increasing the Damköhler number leads to the loop shifting toward the regime of high Reynolds number. However, once the Damköhler number is large to a certain extent, the chemical reactions are always exhibited for the Reynolds number less than 2000. A closed loop is thus not observed. Alternatively, for a given Reynolds number, an ignited Damköhler number and an extinguished Damköhler number can be obtained. Accordingly, three different regions in the plot of Damköhler number versus Reynolds number are identified. Physically, when the role played by Damköhler number on CPOM is much more important than by the Reynolds number (Region I), the thermal effect governs the chemical reactions. In contrast, if the Reynolds number plays a key role in determining the CPOM (Region III), the chemically frozen flow prevails over the catalyst bed. When the residence times of the total oxidation and convection in the catalyst bed are in an equivalent state (Region II), CPOM is characterized by a dual-solution, rendering the hysteresis loops. From the distributions of ignited and extinguished Damköhler numbers, the catalytic reactor and operation of partial oxidation of methane and other fuels can be designed accordingly.     

直喷式柴油机两段喷射计算模拟

以KIVA-3为计算平台，计算模拟4气门直喷式柴油机不同预喷油提前角的两段喷射燃烧历程。其结论是：150CA BTDC以后的预喷提前角，其预喷与主喷柴油都以扩散燃烧为主；预喷柴油燃烧与缸内流动的共同作用，使缸内温度、质量分数分布更加复杂。氧质量分数分布差异是不同预喷油提前角对主喷燃烧放热率影响的主要因素。