Studies on properties of laminar premixed hydrogen-added ammonia/air flames for hydrogen production

In order to evaluate the potential of burning and reforming ammonia as a carbon-free fuel in production of hydrogen, fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) for laminar premixed hydrogen-added ammonia/air flames were studied both experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of hydrogen substitution. Results show the substantial increase of laminar burning velocities with hydrogen substitution, particularly under fuel-rich conditions. Also, predicted flame structures show that the hydrogen substitution enhances nitrogen oxide (NO_x) and nitrous oxide (N₂O) formation. At fuel-rich conditions, however, the amount of NO_x and N₂O emissions and the extent of the increase with the hydrogen substitution are much lower than those under fuel-lean conditions. These observations support the potential of hydrogen as an additive for improving the burning performance with low NO_x and N₂O emissions in fuel-rich ammonia/air flames and hence the potential of using ammonia as a clean fuel. Increasing the amount of added hydrogen tends to enhance flame sensitivity to stretch.     

Effects of ammonia substitution on hydrogen/air flame propagation and emissions

In order to evaluate the potential of partial ammonia substitution to improve the safety of hydrogen use and the effects on the performance of internal combustion engines, the propagation, development of surface cellular instability and nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions of spark-ignited spherical laminar premixed ammonia/hydrogen/air flames were studied experimentally and computationally. With ammonia being the substituent, the fundamental unstretched laminar burning velocities and Markstein numbers, the propensity of cell formation and the associated flame structure were determined. Results show substantial reduction of laminar burning velocities with ammonia substitution in hydrogen/air flames, similar to hydrocarbon (e.g., methane with a similar molecular weight to ammonia) substitution. In all cases, ammonia substitution enhances the NO_x and N₂O formation. At fuel-rich conditions, however, the amount of NO_x emissions increases and then decreases with ammonia substitution and the increased amount of NO_x and N₂O emissions with ammonia substitution is much lower than that under fuel-lean conditions. These observations support the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use with low NO_x and N₂O emissions in fuel-rich hydrogen/air flames. The potential of ammonia as a suppressant of both preferential-diffusional and hydrodynamic cellular instabilities in hydrogen/air flames was also found particularly for fuel-lean conditions, different from methane substitution. However, it should be noted that the use of ammonia also imposes considerable technological challenges and public concerns, particularly those associated with toxicity and the specific properties such as high reactivity with container materials and water, which should be completely resolved.     

An updated short chemical-kinetic nitrogen mechanism for carbon-free combustion applications

Chemical-kinetic combustion mechanisms for hydrogen-oxygen-nitrogen systems, motivated originally by concerns about NO_x emissions during hydrogen burning, have recently acquired renewed interest as a result of the possibility of employing ammonia-hydrogen mixtures in gas turbines and reciprocating engines as drop-in fuel to replace the use of natural gas. Specifically, this is of relevance to the implementation of engineering approaches for economical power generation with carbon sequestration or to large-scale energy-storage schemes, based on hydrogen or efficient hydrogen carriers such as ammonia. Because computational investigations are facilitated by short mechanisms (since the use of large mechanisms is often prohibitively expensive in reactive flow simulations), in response to the original concerns, a short nitrogen mechanism was developed in San Diego in the 1990s (not updated since 2004), without consideration of ammonia combustion. In view of the renewed interest in this topic, that mechanism has now been expanded to encompass 60 elementary steps among 19 reactive chemical species, including ammonia burning and NO_x production, as reported herein, greatly improving predictions. With particular attention to high reactant temperatures and high-pressure conditions, relevant to industrial applications, it is shown that the present short mechanism retains satisfactory accuracy, exhibiting deviations that in most cases are within acceptable bounds (±20%). The revisions maintain the shortness of the original mechanism, adding only one more reactive species and six more elementary steps (while updating values of rate parameters of nine other steps, on the basis of newly available information). In addition, the short mechanism is applied herein to the analysis of fundamental combustion properties of ammonia/hydrogen/nitrogen-air laminar premixed flames, at unstrained and strained conditions, for comparison with methane-air flames as a reference gas-turbine fuel. It is found by comparing carbon-free and hydrocarbon laminar flames that these reactive mixtures, even if characterized by nearly identical adiabatic flame temperature and laminar flame speeds, nevertheless exhibit substantially different resistance to strain, with the ammonia/hydrogen flames exceeding the strain limit of methane flames by a factor of 5.     

Experimental and modeling study of laminar burning velocity of biomass derived gases/air mixtures

Laminar burning velocities of four biomass derived gases have been measured at atmospheric pressure over a range of equivalence ratios and hydrogen contents, using the heat flux method on a perforated flat flame burner. The studied gas mixtures include an air-blown gasification gas from an industrial gasification plant, a model gasification gas studied in the literature, and an upgraded landfill gas (bio-methane). In addition, co-firing of the industrial gasification gas (80% on volume basis) with methane (20% on volume basis) is studied. Model simulations using GRI mechanisms and detailed transport properties are carried out to compare with the measured laminar burning velocities. The results of the bio-methane flame are generally in good agreement with data in the literature and the prediction using GRI-Mech 3.0. The measured laminar burning velocity of the industrial gasification gas is generally higher than the predictions from GRI-Mech 3.0 mechanism but agree rather well with the predictions from GRI-Mech 2.11 for lean and moderate rich mixtures. For rich mixtures, the GRI mechanisms under-predict the laminar burning velocities. For the model gasification gas, the measured laminar burning velocity is higher than the data reported in the literature. The peak burning velocities of the gasification gases/air and the co-firing gases/air mixtures are in richer mixtures than the bio-methane/air mixtures due to the presence of hydrogen and CO in the gasification gases. The GRI mechanisms could well predict the rich shift of the peak burning velocity for the gasification gases but yield large discrepancy for the very rich gasification gas mixtures. The laminar burning velocities for the bio-methane/air mixtures at elevated initial temperatures are measured and compared with the literature data.     

Study of ozone-enhanced combustion in H2/CO/N2/air premixed flames by laminar burning velocity measurements and kinetic modeling

The enhancement effect of ozone addition for H₂/CO/N₂/Air premixed flames at ambient condition is investigated both experimentally and computationally. Adiabatic laminar velocities under different amount of O₃ addition were directly measured using the Heat Flux Method. The ozone concentration in the oxidizer is monitored online to ensure the precise control and stability of ozone injection. Experimental data shows significant enhancement of the burning velocities due to O₃ addition. With 8500 ppm ozone seeded, maximum 18.74% of burning velocity enhancement is observed at equivalence ratio Φ = 0.7. Kinetic modeling works were conducted by integrating ozone sub-mechanism with three kinetic mechanisms: GRI-Mech 3.0, Davis mechanism and USC Mech II. The modeling results were compared with experimental data. GRI-Mech 3.0 + Ozone mechanism demonstrated the ability to reproduce the experimental data. Extra OH radicals promoted by ozone was found in the pre-heat regime which initiates the chain-branching reaction and results in the combustion enhancement.     

A consistent chemical mechanism for oxidation of substituted aromatic species

Computational studies of combustion in engines are typically performed by modeling the real fuel as a surrogate mixture of various hydrocarbons. Aromatic species are crucial components in these surrogate mixtures. In this work, a consistent chemical mechanism to predict the high temperature combustion characteristics of toluene, styrene, ethylbenzene, 1,3-dimethylbenzene (m-xylene), and 1-methylnaphthalene is presented. The present work builds on a detailed chemical mechanism for high temperature oxidation of smaller hydrocarbons developed by Blanquart et al. [Combust. Flame 156 (2009) 588-607]. The base mechanism has been validated extensively in the previous work and is now extended to include reactions of various substituted aromatic compounds. The reactions representing oxidation of the aromatic species are taken from the literature or are derived from those of the lower aromatics or the corresponding alkane species. The chemical mechanism is validated against plug flow reactor data, ignition delay times, species profiles measured in shock tube experiments, and laminar burning velocities. The combustion characteristics predicted by the chemical model compare well with those available from experiments for the different aromatic species under consideration.     

Laminar burning characteristics of ammonia/hydrogen/air mixtures with laser ignition

Ammonia, as a zero-carbon fuel, is drawing more and more attention. The major challenge of using ammonia as a fuel for the combustion engines lies in its low chemical reactivity, and therefore more fundamental researches on the combustion characteristics of ammonia are required to explore effective ways to burn ammonia in engines. In this study, the laminar burning characteristics of the premixed ammonia/hydrogen/air mixtures are investigated. In the experiment, the laser ignition was used to achieve stable ignition of the ammonia/air mixtures with an equivalence ratio range from 0.7 to 1.4. The propagating flame was recorded with the high-speed shadowgraphy. Three different processing methods were introduced to calculate the laminar burning velocity with a consideration of the flame structure characteristics induced by the laser ignition. The effects of initial pressure (0.1 MPa–0.5 MPa), equivalence ratio (0.7–1.4), hydrogen fraction (0–20%) on the laminar burning velocity were investigated under the initial ambient temperature of 360 K. The state-of-the-art kinetic models were used to calculate the laminar burning velocities in the CHEMKIN-pro software. Both the simulation and experimental results show that the laminar burning velocity of the ammonia mixtures increases at first, reaches the peak around ϕ of 1.1, and then decreases with the equivalence ratio increasing from 0.7 to 1.4. The peak laminar burning velocities of the ammonia mixture are lower than 9 cm/s and are remarkably lower than those of hydrocarbon fuels. The laminar burning velocity of the ammonia mixture decreases with the increase of the initial ambient pressure, and it can be drastically speeded up with the addition of hydrogen. While the models except for those by Miller and Bian can give reasonable predictions compared to the experimental results for the equivalence ratio from 0.7 to 1.1 in the ammonia (80%)/hydrogen (20%)/air mixtures, all the kinetic models overpredict the experiments for the richer mixtures, indicating further work necessary in this respect.     

Laminar burning velocities and flame stability analysis of H2/CO/air mixtures with dilution of N2 and CO2

Experimental measurement of the laminar burning velocities of H₂/CO/air mixtures and equimolar H₂/CO mixtures diluted with N₂ and CO₂ up to 60% and 20% by volume, respectively, were conducted at different equivalence ratios and conditions near to the sea level, 0.95 atm and 303 ± 2 K. Flames were generated using contoured slot-type nozzle burners and Schlieren images were used to determine the laminar burning velocity with the angle method. Numerical calculations were also conducted using the most recent detailed reaction mechanisms for comparison with the present experimental results. Additionally, a study was conducted to analyze the flame stability phenomenology that was found in the present experiments. The increase in the N₂ and CO₂ dilution fractions considerably reduced the laminar burning velocity due to the decrease in heat release and increase in heat capacity. At the same dilution fractions this effect was higher for the case of CO₂ due to its higher heat capacity and dissociation effects during combustion. Flame instabilities were observed at lean conditions. While the presence of CO in the fuel mixture tends to stabilize the flame, H₂ has a destabilizing effect which is the most dominant. A higher N₂ and CO₂ dilution fraction increased the range of equivalence ratios where unstable flames were obtained due to the increase in the thermal-diffusive instabilities.     

Impact of dissociated Methanol addition on premixed Toluene reference fuel-air mixtures in a constant-volume chamber

The effects of dissociated methanol addition on a premixed toluene reference fuel was studied in a constant-volume chamber at initial conditions of 0.1 MPa and 398 K. Hydrogen was also added in separate proportions under the same conditions in order to assess its effects. Dissociated methanol fractions of 30%, 50%, 70%, 90%, hydrogen fractions of 50% and 90%, and equivalence ratios from 0.6 to 1.4 were analysed. The laminar burning velocities of the toluene reference fuel-dissociated methanol-air mixtures and the toluene reference fuel-hydrogen-air mixtures were measured by the spherically expanding flame method with a schlieren photography system. The effects of dissociated methanol addition on the laminar burning velocities was evaluated, and the relationships between the laminar burning velocity and the equivalence ratio or dissociated methanol fraction at 398 K and 0.1 MPa were fitted with a third-order polynomial function. Furthermore, the flame instabilities of toluene reference fuel-dissociated methanol-air mixtures and toluene reference fuel-hydrogen-air mixtures were also discussed.     

Structures and burning velocity of biomass derived gas flames

Biomass derived gases produced via gasification, pyrolysis, and fermentation are carbon neutral alternative fuels that can be used in gas turbines, furnaces, and piston engines. To make use of these environmentally friendly but energy density low fuels the combustion characteristics of these fuels have to be fully understood. In this study the structure and laminar burning velocity of biomass derived gas flames are investigated using detailed chemical kinetic simulations. The studied gaseous fuels are the air-blown gasification gas, co-firing of gasification gas with methane, pyrolysis gases, landfill gases, and syngas, a mixture of carbon monoxide and hydrogen. The simulated burning velocities of reference fuel mixtures using two widely used chemical kinetic mechanisms, GRI Mech 3.0 and the San Diego mechanism, are compared with the experimental data to explore the uncertainties and scattering of the simulation data. The different chemical kinetic mechanisms are shown to give a reasonable agreement with each other and with experimental data, with a discrepancy within 7% over most of the conditions. The results show that the structures of typical landfill gas flames and co-firing of methane/gasification gas flames share essential similarity with methane flames. The reaction zones of these flames consist of a thin inner layer and a relatively thick CO/H₂ oxidation layer. In the inner layer hydrocarbon fuel (methane) is converted through chain reactions to intermediates such as CH₃, CH₂O, CO, H₂, etc. The structures of gasification gas flames, pyrolysis gas flames, syngas flames share similarity with the oxidization layer of the methane/air flames. Overall, the chemical reactions of all biomass derived gas flames occur in thin zones of the order of less than 1 mm. The thickness of all BDG gas flames is inversely proportional to their respective laminar burning velocity. The laminar burning velocities of landfill gases are found to increase linearly with the mole fraction of methane in the mixtures, whereas for gasification gas, syngas and pyrolysis gas where hydrogen is present, the laminar burning velocities scale linearly with the mole fraction of hydrogen.     

Theoretical investigation of the combustion performance of ammonia/hydrogen mixtures on a marine diesel engine

Ammonia is a good hydrogen carrier and can be well combined with hydrogen for combustion. The combustion performance of the mixtures of ammonia and hydrogen in a medium-speed marine diesel engine was investigated theoretically. The HCCI combustion mode was selected for reducing thermal-NO_x production. The start fire characteristic of the NH₃–H₂ mixtures was studied under different equivalence ratio, hydrogen-doped ratio, and intake air temperature and pressure. Then, the combustion performance of the NH₃–H₂ mixtures (doping 30% hydrogen) was analyzed at a typical operation condition of engine. The addition hydrogen improved the laminar flame velocity of ammonia, and affected the NO_x emission. For the medium-speed marine engine fueled with NH₃–H₂, reducing combustion temperature, introducing EGR and combining with post-treatment technology would be a feasible scheme to reduce NO_x emission.     

Effects of syngas addition on flame propagation and stability in outwardly propagating spherical dimethyl ether-air premixed flames

Experiments in outwardly propagating spherical flame were carried out to investigate unstretched laminar burning velocity and flame instability by adding 25%, 50%, and 75% syngas to DME-air mixtures at room temperature and elevated pressures up to 0.3 MPa. The measured unstretched laminar burning velocities were compared to numerical predictions using PREMIX code with Zhao reaction mechanism and good agreement was found between them. Flame instability was also investigated through evaluating Markstein length and cellular instability. Behavior of the Markstein lengths was described well by the deficient reactant Lewis number and highly affected by the amount of syngas addition to the DME-air mixtures. Effects of syngas addition and increased initial pressure on cell formation on the flame surface were also examined through evaluating the Lewis number, flame thickness, and thermal expansion ratio. Regardless of syngas addition, the cellular instability was enhanced mainly by the hydrodynamic instability due to decreased flame thickness while diffusional-thermal instability was minor.     

Measurements of laminar burning velocities and flame stability analysis for dissociated methanol–air–diluent mixtures at elevated temperatures and pressures

The laminar burning velocities and Markstein lengths for the dissociated methanol–air–diluent mixtures were measured at different equivalence ratios, initial temperatures and pressures, diluents (N₂ and CO₂) and dilution ratios by using the spherically outward expanding flame. The influences of these parameters on the laminar burning velocity and Markstein length were analyzed. The results show that the laminar burning velocity of dissociated methanol–air mixture increases with an increase in initial temperature and decreases with an increase in initial pressure. The peak laminar burning velocity occurs at equivalence ratio of 1.8. The Markstein length decreases with an increase in initial temperature and initial pressure. Cellular flame structures are presented at early flame propagation stage with the decrease of equivalence ratio or dilution ratio. The transition positions can be observed in the curve of flame propagation speed to stretch rate, indicating the occurrence of cellular structure at flame fronts. Mixture diluents (N₂ and CO₂) will decrease the laminar burning velocities of mixtures and increase the sensitivity of flame front to flame stretch rate. Markstein length increases with an increase in dilution ratio except for very lean mixture (equivalence ratio less than 0.8). CO₂ dilution has a greater impact on laminar flame speed and flame front stability compared to N₂. It is also demonstrated that the normalized unstretched laminar burning velocity is only related to dilution ratio and is not influenced by equivalence ratio.     

Flame front evolution and laminar flame parameter evaluation of buoyancy-affected ammonia/air flames

For flames with very low burning speed, the flame propagation is affected by buoyancy. Flame front evolution and laminar flame parameter evaluation methods of buoyancy-affected flame have been proposed. The evolution and propagation process of a center ignited expanding ammonia/air flame has been analyzed by using the methods. The laminar flame parameters of ammonia/air mixture under different equivalence ratio (ER) and initial pressure have been studied. At barometric pressure, with the increase of ER, the laminar burning velocity (LBV) of ammonia/air mixture undergoes a first increase and then decrease process and reaches its maximum value of 7.17 cm/s at the ER of 1.1, while the Markstein length increases monotonously. For ammonia/air flames with ER less than unity, the flame velocity shows a decreasing trend with stretch rate, resulting in the propensity to flame instability, but no cellular structure was observed in the process of flame propagation. As the initial pressure increases, the LBV decreases monotonously as well as the Markstein length. The flame thicknesses of ammonia/air mixtures decrease with initial pressure and are much thicker than those of hydrogen flames, which makes a stronger stabilizing effect of curvature on the flame front. The most enhancement of LBV is contributed by the dehydrogenation reaction of NH₃ with OH. The NO concentration decreases significantly with the increase of ER.     

Effects of O2 enrichment on NH3/air flame propagation and emissions

The potential utilization of ammonia as a carbon-free fuel under oxygen (O₂)-enriched condition is demonstrated, suggesting its practically appropriate burning conditions by measuring and predicting the combustion characteristics of outwardly-propagating spherical O₂-enriched NH₃/air premixed flames at normal temperature and pressure. Measured and computed laminar burning velocities and predicted flame structure exhibit that the O₂-enriched ammonia/air flames become thinner and propagate faster with O₂ enrichment. Observed flame morphologies and measured and computed Markstein numbers reveal that all the present O₂-enriched flames are stable in terms of the flamefront cellular instability due to preferential diffusion and the effects of O₂ enrichment on the instability are negligible. Volume-based 35–40% O₂ in the nonfuel mixtures demonstrates the proper burning intensity for practical applications, comparable to the typical hydrocarbon/air flames. In the present flame configuration, however, local nitrogen oxides emissions are found to be high, which should be substantially reduced in the practical systems.     

Determination of laminar burning velocities for lean low calorific H2/N2 and H2/CO/N2 gas mixtures

Combustion of lean and ultra-lean synthetic H₂/CO mixtures that are highly diluted in inert gases is of great importance in several fields of technology, particularly in the field of post combustion for combined heat and power (CHP) systems based on fuel cell technology. In this case H₂/CO mixtures occur via hydrocarbon reforming and their complete conversion requires efficient, compact and low emission combustion systems. In order to design such systems, knowledge of global flame properties like the laminar burning velocity, is essential. Using the heat flux burner method, laminar burning velocities were experimentally determined for highly N₂ diluted synthetic H₂ and H₂/CO mixtures with low calorific value, burning with air, at ambient temperature and atmospheric pressure. Furthermore, numerical 1-D simulations were performed, using a series of different chemical reaction mechanisms. These numerical predictions are analysed and compared with the experimental data.     

不同初始压力下掺氢天然气的燃烧特性

在定容燃烧弹内研究了不同初始压力下天然气-氢气-空气混合气的火焰传播规律,得到了不同掺氢比例和初始压力下,不同燃空当量比时混合气的层流燃烧速率,并分析了火焰的稳定性及其影响因素.研究结果表明,随着天然气中掺氢比例的增加,混合气的燃烧速率增加,且增长速率逐渐加快,而马克斯坦长度值则随着掺氢比例的增加而减小,即火焰的稳定性下降.不同初始压力下,随着燃空当量比的增加,马克斯坦长度值在不同掺氢比例下均增加,显示火焰的稳定性增加.无拉伸层流燃烧速率随着初始压力的增加略有减小,且在化学当量比附近,变化的初始压力和掺氢比对无拉伸层流燃烧速率的影响最为明显.     

A reduced mechanism for ethylene/methane mixtures with excessive NO enrichment

A detailed mechanism for methane–ethylene mixtures enriched with excessive amount of NO was systematically reduced for efficient numerical simulations of flames in arc-heated co-flowing air. Methane and ethylene were selected as the surrogate fuel in the present study due to their drastically different features of ignition and extinction properties and flame propagation speeds, such that the mixtures of them may be utilized to mimic practical hydrocarbon fuels with various kinetic properties in experiments. The recently released USC Mech-II for C1–C4 was grafted with the NO_x sub-mechanism in GRI-Mech 3.0 with updated reaction parameters for prompt NO formation. The resulting detailed mechanism with 129 species and 900 reactions was first validated against experiments involving NO_x enrichment and reasonably good agreements were observed. The detailed mechanism was then employed as the starting mechanism for the reduction. A skeletal mechanism with 44 species and 269 reactions was derived using the methods of directed relation graph (DRG) and DRG-aided sensitivity analysis (DRGASA); a 39-species reduced mechanism with 35 semi-global reaction steps was further obtained using the linearized quasi steady state approximations (LQSSA). Five species related to prompt NO were retained in the reduced mechanism because of their significant impacts on the fuel oxidation. The reduced mechanism closely agrees with the detailed mechanism for ignition and extinction of homogenous mixtures, as well as selected 1-D flames over a wide range of parameters with NO concentrations between 0% and 3%. The observed worst-case relative error of the reduction is approximately 20%. The reduced mechanism was further validated with experiments involving excessive NO_x enrichment.     

Hydrogen peroxide for improving premixed methane–air combustion

In this study, the effects of hydrogen peroxide on laminar, premixed, methane–air flames at atmospheric pressure and temperature were investigated using CHEMKIN III and GRI 3.5 mechanism. The range of fuel/air equivalence ratio (φ) was varied from 0.6 to 1.2, and the amount of hydrogen peroxide was altered from 0% to 20% volumetric fraction of the methane–hydrogen peroxide (air excluded) mixture. The burning velocity was found to increase with increasing hydrogen peroxide addition, with a relatively larger increase for the fuel-richer mixtures (ΔS_u up to 15 cm/s for φ≈1.2). The adiabatic flame temperature rose with hydrogen peroxide addition, and the temperature rise per unit hydrogen peroxide addition was more significantly (ΔT up to 100 K) for the leaner mixtures. For the same mixture stoichiometry, adding hydrogen peroxide also increased CO concentration and NO_x emissions somewhat. Accordingly, the benefits of adding hydrogen peroxide to the combustion conditions considered here can be best realized by burning leaner mixtures.     

Transported PDF modeling of high-Reynolds-number premixed turbulent flames

The transported PDF approach, closed at the joint composition-enthalpy level, is applied to model premixed turbulent flames at a wide range of Reynolds numbers. The initial aim of the study is to establish the impact of closure approximations for the scalar dissipation rate upon the relationship between turbulence fluctuations and predicted turbulent burning velocities. The cases considered feature stoichiometric methane-air flames with the chemical source term extracted from a detailed chemistry simulation of the corresponding unstrained laminar flame. The transported PDF approach is subsequently combined with a systematically reduced C/H/O mechanism featuring 142 reactions and 14 solved and 15 steady-state species and applied to piloted premixed stoichiometric methane-air flames investigated experimentally by Chen et al. [Combust. Flame 107 (1996) 223-226]. The cases considered here feature Re=24,200 (flame F3) and 52,500 (flame F1) and Damköhler numbers approaching unity. The effects of variations in the time-scale ratio (2?C??8) and heat losses to the burner were investigated, along with the impact of an extended algebraic relationship for the scalar dissipation rate that accounts for small-scale properties. Comparisons with experimental data show that the modified Curl's model and the extended scalar dissipation-rate closure produce turbulent burning velocities in close agreement with measurements. The study further indicates that a closure at the joint scalar level combined with comprehensive chemistry has the potential to reproduce the detailed chemical structure of premixed turbulent flames. The importance of boundary conditions and comprehensive scalar statistics, including the scalar dissipation rate, is also emphasized by the study.