Experimental and numerical study on the laminar burning velocity of hydrogen enriched biogas mixture

The laminar burning velocities of biogas-hydrogen-air mixture at different fuel compositions and equivalence ratios were determined and studied using the spherical flame method. The combined effects of H₂ and CO₂ on the laminar burning velocity were investigated quantitatively based on the kinetic effects and the thermal effects. The results show that the laminar burning velocities of the BG40, BG50 and BG60 are increased almost linearly with the H₂ addition owing to the improved fuel kinetics and the increased adiabatic flame temperature. The dropping trend of laminar burning velocity from the BG60-hydrogen to the BG40-hydrogen is primarily attributed to the decreased adiabatic flame temperature (thermal effects). The GRI 3.0 mechanism can predict the laminar burning velocity of biogas-hydrogen mixture better than the San Diego mechanism in this study. Whereas, the GRI mechanism still needs to be modified properly for the hydrogen-enriched biogas as the CO₂ proportion exceeds 50% in the biogas at the fuel-rich condition. The increased CO₂ exerts the stronger suppression on the net reaction rate of H + O₂=OH + O than that of H + CH₃(+M) = CH₄(+M), which contributes to that the rich-shift of peak laminar burning velocity of biogas-hydrogen mixture requires higher H₂ addition as the CO₂ content is enhanced. For the biogas-hydrogen fuel, the H₂ addition decreases the flame stability of biogas fuel effectively due to the increased diffusive-thermal instability and hydrodynamic instability. The improved flame stability of biogas-hydrogen fuel with the increased CO₂ content is resulted from the combined effects of diffusive-thermal instability and hydrodynamic instability.     

Experimental and numerical investigation on the effect of hydrogen addition and N2/CO2 dilution on laminar burning velocity of methane/oxygen mixtures

An experimental and numerical study on the combined effect of N₂/CO₂ dilution and hydrogen addition on the laminar burning velocity (LBV) of methane-oxygen mixtures was conducted. The experiments were performed at atmospheric conditions using the heat flux method for effective equivalence ratios (ϕ_F) varying from 0.7 to 1.3. The results show that the hydrogen addition causes an increase in LBV for all the mixture conditions. The variation in LBV based on hydrogen addition parameter (R_H) for all N₂ dilution conditions were following a linearly increasing trend. The strong effect of hydrogen addition on LBV is observed at lean and rich mixtures compared to that at near stoichiometric mixture conditions. The experimental results show that the percentage variation in LBV with R_H at rich mixture is more substantial at 75% N₂ dilution compared to that at 65% N₂ dilution.     

Experimental and numerical study of the laminar burning velocity of NH3/H2/air premixed flames at elevated pressure and temperature

Ammonia (NH₃) is a carbon-free fuel that shows great research prospects due to its ideal production and storage systems. The experimental data of the laminar burning velocity of NH₃/H₂/air flame at different hydrogen ratios (X_H2 = 0.1–0.5), equivalent ratios (φ = 0.8–1.3), initial pressures (P = 0.1–0.7 MPa), and initial temperatures (T = 298–493 K) were measured. The laminar burning velocity of the NH₃/H₂/air flame increased upon increasing the hydrogen ratios and temperature, but it decreased upon increasing the pressure. The equivalent ratio of the maximum laminar burning velocity was only affected by the proportion of reactants. The equivalence ratio value of the maximum laminar burning velocity was between 1.1 and 1.2 when X_H2 = 0.3. The chemical reaction kinetics of NH₃/H₂/air flame under four different initial conditions was analyzed. The less NO maximum mole fraction was produced during rich combustion (φ > 1). The results provide a new reference for ammonia as an alternative fuel for internal combustion engines.     

Experimental investigations on laminar burning velocity variation of CH4+H2+air mixtures at elevated temperatures

The present work reports experimental investigations on laminar burning velocity variation of CH₄+H₂+air mixtures at elevated temperatures (300–650 K) using an externally heated diverging-channel method. The effect of mixture equivalence ratio (? = 0.7–1.3) and H₂ fraction (0–50% by volume) on burning velocity have been reported at elevated temperatures. The experimental measurements are compared with numerical simulations using GRI Mech 3.0 and FFCM-1 kinetic models. The obtained results exhibit an increase in the laminar burning velocity with H₂ fraction due to the formation of H-atom as an intermediate. The temperature dependency is established through a power-law correlation. The temperature-exponent shows a parabolic variation with a minimum value at ? = 1.1. Reaction pathway diagram interprets the major oxidation paths followed by reactants for higher carbon-consumption with varying H₂ fraction. The P2 pathway involving ethane breakdown plays a major role in enhancing the burning velocity at rich mixture conditions.     

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.     

Experimental and kinetic study of laminar flame characteristics of H2/O2/diluent flame under elevated pressure

Laminar burning velocity, Markstein length, and critical flame radius of an H₂/O₂ flame with different diluents, He, Ar, N₂ and CO₂, were measured under elevated pressure with different diluent concentrations. The effects of pressures, diluents, and dilution and equivalence ratios were studied by comparing calculated and experimental results. The laminar burning velocity showed non-monotonic behavior with pressure when the dilution ratio was low. The reason is the radical pool reduced with increasing pressure and leads to the decrease of overall reaction order from larger than 2 to smaller than 2, and further leads to this non-monotonic phenomenon. A modified empirical equation was presented to capture the relationship between active radicals and laminar burning velocity. Critical radii and Markstein lengths both decrease with initial pressure and increase with equivalence ratio and dilution ratio. The calculated critical radii indicate that the Peclet number and flame thickness control the change of R_cr. It can be found that Le_eff has a significant influence on Peclet number and leads to the decrease of critical flame radii of Ar, N_2, and CO₂ diluted mixture. Interestingly, the CO₂ diluted mixture has the lowest Markstein length under stoichiometric conditions and a high value under fuel-rich conditions, consistent as the flame instability observed on the flame images. The reason is that the Le_eff of CO₂ diluted mixture increased rapidly with the equivalence ratio.     

Numerical and experimental study on laminar burning velocity of syngas produced from biomass gasification in sub-atmospheric pressures

The laminar burning velocity of syngas mixtures has been studied by various researches. However, most of these studies have been conducted in atmospheric conditions at sea level. In the present study, the effect of sub atmospheric pressure was evaluated on the laminar burning velocity for a mixture of H₂, CO and N₂ (20:20:60 vol%) in real sub atmospheric condition. The measurements was conducted in an altitude of 2130 m.a.s.l (0.766 atm) and 21 m.a.s.l (0.994 atm) to evaluate the effect of pressure, the temperature and relative humidity were controlled using an air conditioning unit and was maintained in 295 ± 1 K and 62.6 ± 2.7% respectively. The Flames were generated using contoured slot-type nozzle burner, and an ICCD camera was used to capture chemiluminescence emitted by OH∗-CH∗ radicals present in the flame and thus obtain the flame front and determinate the laminar burning velocity using the angle method. The experimental results were compared with numerical calculations, conducted using the detailed mechanisms of Li et al. and the GRI-Mech 3.0. It was found that the laminar burning velocity increases at lower pressure, for an equivalence ratio of 1.1, the laminar burning velocity increases by almost 23% respect to the sea level conditions.     

Experimental and numerical evaluation of low-temperature combustion of bio-syngas

Environmental regulations have strongly incentivized the development of alternative technologies and renewable sources for the energy supply, including bio-syngas and low-temperature combustion. However, accurate estimation methods for low-temperature chemistry of these mixtures are still missing. Hence, experimental data with the limited impact of fluid dynamics aspects are strongly required. To this aim, the heat flux burner has been adopted in this work for the measurements of the laminar burning velocity. Data were compared to evaluate the accuracy of the kinetic mechanism developed at the University of Bologna (KIBO) and exiting mixing rules. Hirasawa's correlation has been found as the most reliable empirical correlation. Furthermore, the KIBO model has been validated and compared with existing models for the investigated conditions. These results have allowed for the evaluation of the effects of fuel composition on the preferable reaction paths and on the NO_x production rate.     

A comprehensive study of light hydrocarbon mechanisms performance in predicting methane/hydrogen/air laminar burning velocities

In order to obtain the precise predicted values of methane/hydrogen/air burning velocities from simulations, the performances of GRI mech 3.0, Aramco mech 1.3, USC mech 2.0 and San diego mech mechanisms were systematically studied under various conditions by PREMIX code and compared with experimental data from literature. The conditions where each mechanism gave their good performance are obtained and concluded. The flowrate sensitivity and rate constants of key elementary reactions were analyzed to insight the different behavior of each mechanism. The results showed that all these widely used small hydrocarbon mechanisms could gave reasonable predictions for pure methane and methane hydrogen blends. Nevertheless, they lack sensitivity for rich hydrogen at elevated pressures due to their complex reactions competitions controlled by hydrogen sub model. USC mech 2.0 was found more suitable for being used at low hydrogen contents while San diego mech gained better results at high hydrogen contents. GRI 3.0 gave good predictions for methane hydrogen blends except for high initial pressures. Generally, Aramco mech 1.3 showed the best performance for all testing conditions. Moreover, there was relatively large deviation from the predicted results and experimental data in the transition regime where the hydrogen fractions were between 60% and 80%, it may could be optimized by tuning the rate constants of reactions.     

Effects of CO content on laminar burning velocity of typical syngas by heat flux method and kinetic modeling

Variations in syngas composition could bring a challenge for its combustion with both high efficiency and low emission. In this study, the effect of CO content on the laminar burning velocity of typical syngas was determined by the heat flux method and by kinetic simulations. For the 0% H₂ in syngas, the laminar burning velocity increased monotonically with CO content until its maximum value and then dropped rapidly with further increase of CO content, while for the 25% H₂ case, the laminar burning velocity increased almost linearly with CO content. Based on the kinetic simulations, consumption rate changes of CO and OH and the discrepancy of the heat release rate in the preheat zone contribute to these trends. At sufficient OH, the increase in the reaction rate between OH and CO corresponds to a faster heat release in the preheat zone, whereas at insufficient OH, oxidation of CO by OH is inhibited and the heat release process is delayed, decelerating the release rate and decreasing the laminar burning velocity.     

Effects of water vapor addition on the laminar burning velocity of oxygen-enriched methane flames

The effects of steam addition on the laminar burning velocity of premixed oxygen-enriched methane flames are investigated at atmospheric pressure. Experiments are carried out with an axisymmetric burner on which laminar conical flames are stabilized. A newly devised steam production system is used to dilute the reactants with water vapor. The oxygen-enrichment ratio in the oxidizer, defined as O₂/(O₂ + N₂) (mol.), is varied from 0.21 (air) to 1.0 (pure oxygen). The equivalence ratio ranges from 0.5 to 1.5 and the steam molar fraction in the reactive mixture is varied from 0 to 0.50. For all compositions examined, the reactive mixture is preheated to a temperature T_u = 373 K. Laminar flame speeds are determined with the flame area method using a Schlieren apparatus. The deviations induced by stretch effects due to aerodynamic strain and flame curvature are assessed using Particle Imaging Velocimetry measurements and flame images, and these data are used to estimate the uncertainty of the flame speed measurements. The experiments are completed by numerical simulations conducted with the PREMIX code using different detailed kinetic mechanisms. It is shown that the laminar flame speed of CH₄/O₂/N₂/H₂O_(v) mixtures features a quasi-linear decrease with increasing steam molar fraction, even at high steam dilution rates. Numerical predictions are in good agreement with experimental data for all compositions explored, except for low dilution rates X_H2O<0.10 in methane–oxygen mixtures, where the flame speed is slightly underestimated by the calculations. It is also shown that steam addition has a non-negligible chemical impact on the flame speed for methane–air flames, mainly due to water vapor high chaperon efficiency in third-body reactions. This effect is however strongly attenuated when the oxygen concentration is increased in the reactive mixture. For highly oxygen-enriched flames, steam can be considered as an inert diluent.     

Sensitivity to change in laminar burning velocity and Markstein length resulting from variable hydrogen fraction in blast furnace gas for changing ambient conditions

The sensitivity to changes in fuel characteristics has been investigated for combustion of Blast Furnace Gas resulting from small volumetric increases in H₂ concentration. A nonlinear methodology has been employed to quantify unstretched flame speeds and the effect of flame stretch from outwardly propagating spherical flames. Following benchmarking work with CH₄, results were obtained under ambient conditions of 303 K and 0.1 MPa, with small absolute change in hydrogen concentration shown to at least triple the laminar burning velocity for all tested mixtures. Fuel composition and equivalence ratio were shown to independently influence mixture diffusivity and Lewis number, quantified by change in the obtained values of Markstein length. Temperature and pressure were increased to respective values of 393 K and 0.2 MPa to investigate influence of ambient conditions, with a power law correlation presented. Finally the performance of several published chemical reaction mechanisms has been evaluated through comparison of 1-D flame models.     

Experimental and numerical studies of laminar flame characteristics of ethyl acetate with or without hydrogen addition

Hydrogen (H₂) is an effective additive to improve the issue of low laminar burning velocity of some biofuels. In order to better understand the laminar burning characteristics of ethyl acetate (EA) with or without H₂ addition, experimental investigations of laminar burning characteristics were carried out by using the high-speed Schlieren photography technique in a constant volume combustion chamber. Tests were conducted under various equivalence ratios ranging from 0.5 to 1.4 with an initial temperature of 358 K, an initial pressure of 0.1 MPa and a H₂/air proportion of 0%, 4%, 8% and 12% by volume. Laminar burning velocities, together with other parameters such as laminar burning flux, flame thickness, Markstein length and Markstein number, were calculated and discussed. In addition, the experimental data were compared with numerical simulations based on the Dayma model. Results showed that the laminar burning velocity of EA was enhanced with the increase of H₂ addition, and the maximum value reached 95.09 cm/s at φ = 0.6 with 12% H₂, a value more than twice as fast as that of pure EA (39.3 cm/s). Moreover, H₂ was found to extend the lower flammability limit of EA. The laminar burning velocities simulated with the Dayma model agreed well with the experimental results of EA at various H₂ additions.     

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.     

Numerical study on laminar burning velocity of ammonia flame with methanol addition

The combustion characteristics of ammonia/methanol mixtures were investigated numerically in this study. Methanol has a dramatic promotive effect on the laminar burning velocity (LBV) of ammonia. Three mechanisms from literature and another four self-developed mechanisms constructed in this study were evaluated using the measured laminar burning velocities of ammonia/methanol mixtures from Wang et al. (Combust.Flame. 2021). Generally, none of the selected mechanisms can precisely predict the measured laminar burning velocities at all conditions. Aiming to develop a simplified and reliable mechanism for ammonia/methanol mixtures, the constructed mechanism utilized NUI Galway mechanism (Combust.Flame. 2016) as methanol sub-mechanism and the Otomo mechanism (Int. J. Hydrogen. Energy. 2018) as ammonia sub-mechanism was optimized and reduced. The reduced mechanism entitled ‘DNO-NH3’, can accurately reproduce the measured laminar burning velocities of ammonia/methanol mixtures under all conditions. A reaction path analysis of the ammonia/methanol mixtures based on the DNO-NH3 mechanism shows that methanol is not directly involved in ammonia oxidation, instead, the produced methyl radicals from methanol oxidization contribute to the dehydrogenation of ammonia. Besides, NO_x emission analysis demonstrates that 60% methanol addition results in the highest NO_x emissions. The most important reactions dominating the NO_x consumption and production are identified in this study.     

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.     

Chemical structure and laminar burning velocity of atmospheric pressure premixed ammonia/hydrogen flames

This paper presents experimental data on the flame structure of laminar premixed ammonia and ammonia/hydrogen flames at different equivalence ratios (φ = 0.8, 1.0 and 1.2) and the laminar flame speed of ammonia/hydrogen flames (φ = 0.7–1.5) at 1 atm. Experimental data were compared with modeling results obtained using four detailed chemical-kinetic mechanisms of ammonia oxidation. In general, all models adequately predict the flame structure. However, for the laminar burning velocity, this is not so. The main nitrogen-containing species present in the post-flame zone in significant concentrations are N₂ and NO. Experimental data and numerical simulations show that the transition to slightly rich conditions enables to reduce NO concentration. Numerical simulation indicate that increasing the pressure rise also results into reduction of NO formation. However, when using ammonia as a fuel, additional technologies should be employed to reduce NO formation.     

A model for the laminar flame speed of binary fuel blends and its application to methane/hydrogen mixtures

Recent studies have demonstrated promising performance of adding hydrogen to methane in internal combustion engines and substantial attention has been devoted to binary fuel blends. Due to the strong nonlinearity of chemical reaction process, the laminar flame speed of binary fuel blends cannot be obtained from linear combination of the laminar flame speed of each individual fuel constituent. In this study, theoretical analysis is conducted for a planar premixed flame of binary fuel blends and a model for the laminar flame speed is developed. The model shows that the laminar flame speed of binary fuel blends depends on the square of the laminar flame speed of each individual fuel component. This model can predict the laminar flame speed of binary fuel blends when three laminar flame speeds are available: two for each individual fuel component and the third one for the fuel blends at one selected blending ratio. The performance of this model as well as models reported in the literature is assessed for methane/hydrogen mixtures. It is demonstrated that good agreements with calculations or measurements can be achieved by the present model prediction. Moreover, it is found that the present model also works for other binary fuel blends besides methane/hydrogen.     

Laminar burning velocities and flame stability analysis of hydrogen/air premixed flames at low pressure

An experimental and numerical study on laminar burning velocities of hydrogen/air flames was performed at low pressure, room temperature, and different equivalence ratios. Flames were generated using a small contoured slot-type nozzle burner (5 mm × 13.8  mm). Measurements of laminar burning velocity were conducted using the angle method combined with Schlieren photography. Numerical calculations were also conducted using existing detailed reaction mechanisms and transport properties. Additionally, an analysis of the intrinsic flame instabilities of hydrogen/air flames at low pressure was performed. Results show that the behavior of the laminar burning velocity is not regular when decreasing pressure and that it depends on the equivalence ratio range. The behavior of the laminar burning velocity with decreasing pressure can be reasonably predicted using existing reaction mechanisms; however changes in the magnitude of the laminar burning velocity are underestimated. Finally, it has been found experimentally and proved analytically that the intrinsic flame instabilities are reduced when decreasing the pressure at sub-atmospheric conditions.