Enhancement of biogas/air combustion by hydrogen addition at elevated temperatures

In this study, combustion characteristics of various biogas/air mixtures with hydrogen addition at elevated temperatures were experimentally investigated using bunsen burner method. Methane, CH₄, was diluted with different concentrations of carbon dioxide, CO₂, 30 to 40% by volume, to prepare the biogas for testing. It is followed by the hydrogen, H₂, enrichment within the range of 0 to 40% by volume and the temperature elevation of unburned gas till 440 K. Blowoff velocities were measured by lowering the jet velocity until a premixed flame could be stabilized at the nozzle exit, while laminar burning velocities were calculated by analyzing the shape of the directly captured premixed bunsen flames. The results showed that hydrogen had a positive effect on the blowoff velocity for all three fuel samples. Nonlinear growth of the blowoff velocity with hydrogen addition was associated to the dominance of methane-inhibited hydrogen combustion process. It was also observed that the increase in the initial temperature of the unburned mixture led to a linear increase of the blowoff velocity. Moreover, specific changes in flame structure such as flame height, standoff distance, and the existence of tip opening were attributed to the change in the blowoff velocity. The effect of CO₂ content in the mixture was examined with regards to laminar burning velocity for all compositions. The outcome of the experiment showed that the biogas mixture with higher content of CO₂ possessed lower values of laminar burning velocity over the wide range of equivalence ratios. A reduced GRI-Mech 3.0 was used to simulate the combustion of biogas/air mixtures with different compositions using ANSYS Fluent. The numerically simulated stable conical flames were compared with the experimental flames, in terms of flame structure, showing that the reduced GRI-Mech 3.0 was suitable for modeling the combustion of biogas/air mixtures.     

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

Investigation of turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

Methane and hydrogen-enriched (25 vol% and 50 vol% H₂-enriched CH₄) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H₂ concentrations. The effects of equivalence ratio (Φ), Re, and H₂ enrichment on flame structure were examined employing OH-PLIF measurement. For CH₄/air cases, the flame was stabilized with an M shape; while for H₂-enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H₂ enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH₄/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH₄ flames with M shape, whereas the BLF occurs for all H₂-enriched flames with П shape.     

Stabilization regimes and pollutant emissions from a dual fuel CH4/H2 and dual swirl low NOx burner

Burning hydrogen in gas turbines is a relevant technological solution to decarbonize power production and propulsion systems. However, ensuring low NOx emission and preventing flashback can be challenging with hydrogen. Stabilization regimes and pollutant emissions from partially premixed CH₄/H₂/air flames above a coaxial Dual Fuel Dual Swirl injector are investigated in a laboratory-scale combustor at atmospheric conditions for increasing hydrogen contents. The injector consists of an external annular swirler providing premixed methane/air and a central channel fed with pure hydrogen. This burner virtually removes the risk of flashback due to the late injection of hydrogen. Flame stabilization regimes, CO and NOx emissions are analyzed for different configurations of the injector and operating points. The effect of swirling the hydrogen stream is investigated together with the influence of the hydrogen injector recess, i.e. its nozzle position with respect to the backplane of the combustion chamber. It is shown that swirling the central hydrogen stream favors aerodynamically stabilized flames resulting in a low thermal stress on the injector and limited NOx emissions. The study also highlights that a small recess of the central hydrogen injector widely extends the operability range of the burner with aerodynamically stabilized flames. With a sufficient inner swirl and a small recess, flames detach from the injector rim when the hydrogen bulk velocity is large enough. In this configuration, it is found that NOx emissions remain low even for operation with pure hydrogen. Moreover, NOx emissions decrease when increasing the thermal power for a fixed equivalence ratio.     

Experimental and numerical investigation of stability and emissions of hydrogen-assisted oxy-methane flames in a multi-hole model gas-turbine burner

This paper reports experimental and numerical study of stability and combustion characteristics of premixed oxy-methane flames with hydrogen-enrichment (CH₄–H₂/O₂–CO₂ flames) in a model multi-hole burner for clean energy production in gas turbines. The combustor lean blow-out (LBO) limit was presented on an equivalence ratio (Ø) - hydrogen fraction (HF: volumetric fraction of H₂ in a mixture of H₂+CH₄) map spanning over Ø-values of 0.1–1 and HF-values of 0–70% at fixed hole jet velocity and oxygen fraction (OF: volumetric fraction of O₂ in a mixture of O₂+CO₂) of 5.2 m/s and 30%, respectively. The condition of the combustion chamber is assumed to be depicted by the corrugated premixed flame regime. The premixed turbulent flame was modeled using the reaction progress variable flame front topology approach with the Large Eddy Simulation (LES) technique. The recorded combustor stability maps showed great resistance of the micromixer burner technology to flashback, recommending its use for stable gas turbine operation. The results show that H₂-enrichment widens the combustor operability limits (higher turndown ratio) by extending the LBO from Ø = 0.45 at HF = 0% down to Ø = 0.15 at HF = 70% with a slight reduction in the heat release factor by 0.1. The high reactivity and higher flame speed of H₂ ensures the sustenance of flame at lower equivalence ratios. At high equivalence ratios, H₂ addition enhances the reaction rates and makes both the primary and secondary reaction zones shorter and more intense. Increasing HF leads to increase in the Damköhler number (Da) and decrease in both the Karlovitz number (Ka) and flame thickness. The CO emission at the combustor outlet reduced significantly from 241 ppm at HF = 0% to 33.1 ppm at HF = 10%, then it increased back to 364 ppm at HF = 50%.     

Adiabatic burning velocity of H2–O2 mixtures diluted with CO2/N2/Ar

Global warming due to CO₂ emissions has led to the projection of hydrogen as an important fuel for future. A lot of research has been going on to design combustion appliances for hydrogen as fuel. This has necessitated fundamental research on combustion characteristics of hydrogen fuel. In this work, a combination of experiments and computational simulations was employed to study the effects of diluents (CO₂, N₂, and Ar) on the laminar burning velocity of premixed hydrogen/oxygen flames using the heat flux method. The experiments were conducted to measure laminar burning velocity for a range of equivalence ratios at atmospheric pressure and temperature (300 K) with reactant mixtures containing varying concentrations of CO₂, N₂, and Ar as diluents. Measured burning velocities were compared with computed results obtained from one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics and good agreement was obtained. The effectiveness of diluents in reduction of laminar burning velocity for a given diluent concentration is in the increasing order of argon, nitrogen, carbon dioxide. This may be due to increased capabilities either to quench the reaction zone by increased specific heat or due to reduced transport rates. The lean and stoichiometric H₂/O₂/CO₂ flames with 65% CO₂ dilution exhibited cellular flame structures. Detailed three-dimensional simulation was performed to understand lean H₂/O₂/CO₂ cellular flame structure and cell count from computed flame matched well with the experimental cellular flame.     

Laminar burning velocities and flame instabilities of diluted H2/CO/air mixtures under different hydrogen fractions

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H₂/CO/air mixtures. The laminar burning velocity of H₂/CO/air mixtures diluted with CO₂ and N₂ was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H₂/CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H₂ fraction in H₂/CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H₂/30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H₂/70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed.     

Burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane–air flames

The burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane–air flames were experimentally studied by laser tomography visualization method using a V-shaped flame configuration. Turbulent burning velocity was measured and the variation of flame surface characteristics due to hydrogen addition was analyzed. The results show that hydrogen addition causes an increase in turbulent burning velocity for lean premixed CH₄–air mixtures when turbulent level in unburned mixture is not changed. Moreover, the increase of turbulent burning velocity is faster than that of the corresponding laminar burning velocity at constant equivalence ratio, suggesting that the kinetics effect is not the sole factor that results in the increase in turbulent burning velocity when hydrogen is added. The further analysis of flame surface characteristics and brush thickness indicates that hydrogen addition slightly decreases local flame surface density, but increases total flame surface area because of the increased flame brush thickness. The increase in flame brush thickness that results in the increase in total surface area may contribute to the faster increase in turbulent burning velocity, when hydrogen is added. Besides, the stretched local laminar burning velocity may be enhanced with the addition of hydrogen, which may also contribute to the faster increase rate of turbulent burning velocity. Both the variation in flame brush thickness and the enhancement in stretched local laminar burning velocity are due to the decreased fuel Lewis number when hydrogen is added. Therefore, the effects of fuel Lewis number and stretch should be taken into account in correlating burning velocity of turbulent premixed flames.     

Effect of hydrogen enrichment on swirl/bluff-body lean premixed flame stabilization

This paper reports the mechanism of hydrogen enrichment in stabilizing swirl/bluff-body CH₄/air lean premixed flame. Large Eddy Simulation (LES) coupled with Thickened Flame (TF) model was performed to resolve the turbulent reacting flow. A detailed chemistry was used to describe the oxidization of CH₄/H₂/air mixtures. Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence of OH (OH-PLIF) simultaneous measurements were conducted to obtain the velocity fields and flame structures respectively. The numerical methods were validated by experimental data and showing good agreements. Both the experimental and numerical results show that, the flame brush attachment tends to leave the inner shear layer with increasing hydrogen addition, which will reduce the risk of flame lift-off. The chemical analyses prove that the attachment of CH₄/air flame is inherently weak. On the one hand, the CH₄/air flame is stabilized by the hot products inside the recirculation. On the other hand, the burnt gas suppresses the oxidation of H₂ and CO through H₂ + OH = H + H₂O and CO + OH = CO₂ + H, respectively. Although the proportion of CH₄ decomposition through CH₄ + OH = CH₃ + H₂O will be reduced by hydrogen addition, the path of CH₄ + H = CH₃ + H₂ will be enhanced significantly. Hydrogen addition will not only increase the overall reaction rate, but also change the combustion intensity at the nozzle exit from relatively weak to strong, which is also important for flame stabilization. The robust flame attachment obtained by hydrogen addition can attributed to the enhanced reactions of H₂ + OH = H + H₂O and CH₄ + H = CH₃ + H₂.     

Effects of CO addition on laminar flame characteristics and chemical reactions of H2 and CH4 in oxy-fuel (O2/CO2) atmosphere

An experimental study is conducted to investigate the effect of CO addition on the laminar flame characteristics of H₂ and CH₄ flames in a constant-volume combustion system. In addition, one-dimensional laminar premixed flame propagation processes at the same conditions are simulated with the update mechanisms. Results show that all mechanisms could well predict the laminar flame speeds of CH₄/CO/O₂/CO₂ mixtures, when Z_CO is large. For mixtures with lower CO, the experimental laminar flame speeds are always smaller than the calculated ones with Han mechanism. For mixtures with larger or smaller Z_CO2, GRI 3.0, San diego and USC 2.0 mechanisms all overvalue or undervalue the laminar flame speeds. When CO ratio in the CH₄/CO blended fuels increases, laminar flame speed firstly increases and then decreases for the CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, San diego, Davis and Li mechanisms all undervalue the laminar flame speeds of H₂/CO/CO₂/CO₂ mixtures. Existing models could not well predict the nonlinear trend of the laminar flame speeds, due to complex chemical effects of CO on CH₄/CO or H₂/CO flames. Then, the detailed thermal, kinetic and diffusive effects of CO addition on the laminar flame speeds are discussed. Kinetic sensitivity coefficient is far larger than thermal and diffusive ones and this indicates CO addition influences laminar flame speeds mainly by the kinetic effect. Based on this, radical pool and sensitivity analysis are conducted for CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures. For CH₄/CO/O₂/CO₂ mixtures, elementary reaction R38H + O₂ ↔ O + OH and R99 OH + CO ↔ H + CO₂ are the most important branching reactions with positive sensitivity coefficients when CO ratio is relative low. As CO content increases in the CH₄/CO blended fuel, the oxidation of CO plays a more and more important role. When CO ratio is larger than 0.9, the importance of R99 OH + CO ↔ H + CO₂ is far larger than that of R38H + O₂ ↔ O + OH. The oxidation of CO dominates the combustion process of CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, the most important elementary reaction with positive and negative sensitivity coefficients are R29 CO + OH ↔ CO₂ + H and R13H + O₂(+M) ↔ HO₂(+M) respectively. The sensitivity coefficient of R29 CO + OH ↔ CO₂ + H is increasing and then decreasing with the addition of CO in the mixture. Chemical kinetic analysis shows that the chemical effect of CO on the laminar flame propagation of CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures could be divided into two stages and the critical CO mole fraction is 0.9.     

Laminar burning velocity and interchangeability analysis of biogas/C3H8/H2 with normal and oxygen-enriched air

Numerical and experimental measurements of the laminar burning velocities of biogas (66% CH₄ – 34% CO₂) and a biogas/propane/hydrogen mixture (50% biogas – 40% C₃H₈ – 10% H₂) were made with normal and oxygen-enriched air while varying the air/fuel ratio. GRI-Mech 3.0 and C₁–C₃ reaction mechanisms were used to perform numerical simulations. Schlieren images of laminar premixed flames were used to determine laminar burning velocities at 25 °C and 849 mbar. The mixture's laminar burning velocity was found to be higher to that of pure biogas due to the addition of propane and hydrogen. An increase in the laminar burning velocities of both fuels is reported by enriching air with oxygen, a phenomenon that is explained by the increased reactivity of the mixture. Additionally, an analysis of interchangeability based on both the Wobbe Index and the laminar burning velocity between methane and a biogas/propane/hydrogen mixture is presented in order to consider this mixture as a substitute for natural gas. It was found that the variations of these properties between the fuels did not exceed 10%, enabling interchangeability.     

Numerical study of laminar flame properties of diluted methane-hydrogen-air flames at high pressure and temperature using detailed chemistry

Technical limits of high pressure and temperature measurements as well as hydrodynamic and thermo-diffusive instabilities appearing in such conditions prevent the acquisition of reliable results in term of burning velocities, restraining the domain of validity of current laminar flame speed correlations to few bars and hundreds of Kelvin. These limits are even more important when the reactivity of the considered fuel is high. For example, the high-explosive nature of pure hydrogen makes measurements even more tricky and explains why only few correlations are available to describe the laminar flame velocity of high hydrogen blended fuels as CH₄-H₂ mixtures. The motivation of this study is thereby to complement experimental measurements, by extracting laminar flame speeds and thicknesses from complex chemistry one-dimensional simulations of premixed laminar flames. A wide number of conditions are investigated to cover the whole operating range of common practical combustion systems such as piston engines, gas turbines, industrial burners, etc. Equivalence ratio is then varied from 0.6 to 1.3, hydrogen content in the fuel from 0 to 100%, residual burned gas mass ratio from 0 to 30%, temperature of the fresh mixtures from 300 to 950 K, and pressure from 0.1 to 11.0 MPa. Many chemical kinetics mechanisms are available to describe premixed combustion of CH₄-H₂ blends and several of them are tested in this work against an extended database of laminar flame speed measurements from the literature. The GRI 3.0 scheme is finally chosen. New laminar flame speed and thickness correlations are proposed in order to extend the domain of validity of experimental correlations to high proportions of hydrogen in the fuel, high residual burned gas mass ratios as well as high pressures and temperatures. A study of the H₂ addition effect on combustion is also achieved to evaluate the main chemical processes governing the production of H atoms, a key contributor to the dumping of the laminar flame velocity.     

Laminar burning velocities and flame characteristics of CO–H2–CO2–O2 mixtures

Laminar burning velocities of CO–H₂–CO₂–O₂ flames were measured by using the outwardly spherical propagating flame method. The effect of large fraction of hydrogen and CO₂ on flame radiation, chemical reaction, and intrinsic flame instability were investigated. Results show that the laminar burning velocities of CO–H₂–CO₂–O₂ mixtures increase with the increase of hydrogen fraction and decrease with the increase of CO₂ fraction. The effect of hydrogen fraction on laminar burning velocity is weakened with the increase of CO₂ fraction. The Davis et al. syngas mechanism can be used to calculate the syngas oxyfuel combustion at low hydrogen and CO₂ fraction but needs to be revised and validated by additional experimental data for the high hydrogen and CO₂ fraction. The radiation of syngas oxyfuel flame is much stronger than that of syngas–air and hydrocarbons–air flame due to the existence of large amount of CO₂ in the flame. The CO₂ acts as an inhibitor in the reaction process of syngas oxyfuel combustion due to the competition of the reactions of H + O₂ = O + OH, CO + OH = CO₂ + H and H + O₂(+M) = HO₂(+M) on H radical. Flame cellular structure is promoted with the increase of hydrogen fraction and is suppressed with the increase of CO₂ fraction due to the combination effect of hydrodynamic and thermal-diffusive instability.     

The synergistic effect of equivalence ratio and initial temperature on laminar flame speed of syngas

The laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas in a wide equivalence ratio range (0.6–5) and initial temperature (298–423 K) was studied by Bunsen burner. The results show that the laminar flame speed first increases and then decreases as the equivalence ratio increasing, which is a maximum laminar flame speed at n = 2. The laminar flame speed increases exponentially with the increase of initial temperature. For different equivalent ratios, the initial temperature effects on the laminar flame speed is different. The initial temperature effects for n = 2 (the most violent point of the reaction) is lower than others. It is found that H, O and OH are affected more and more when the equivalence ratio increase. When the equivalence ratio is far from 2, the reaction path changes, and the influence of initial temperature on syngas combustion also changes. The laminar flame speed of syngas is more severely affected by H + O₂ = O + OH and CO + OH = CO₂ + H than others, which sensitivity coefficient is larger and change more greatly than others when the initial temperature and equivalence ratio change. Therefore, the laminar flame speed of syngas/air premixed gas is affected by the initial temperature and equivalence ratio. A new correlation is proposed to predict the laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas under the synergistic effect of equivalence ratio and initial temperature (for equivalence ratios of 0.6–5, the initial temperature is 298–423 K).     

Direct numerical simulation of lean premixed CH4/air and H2/air flames at high Karlovitz numbers

Three-dimensional direct numerical simulation with detailed chemical kinetics of lean premixed CH₄/air and H₂/air flames at high Karlovitz numbers (Ka ∼ 1800) is carried out. It is found that the high intensity turbulence along with differential diffusion result in a much more rapid transport of H radicals from the reaction zone to the low temperature unburned mixtures (∼500 K) than that in laminar flamelets. The enhanced concentration of H radicals in the low temperature zone drastically increases the reaction rates of exothermic chain terminating reactions (e.g., H + O₂+M = HO₂ + M in lean H₂/air flames), which results in a significantly enhanced heat release rate at low temperatures. This effect is observed in both CH₄/air and H₂/air flames and locally, the heat release rate in the low temperature zone can exceed the peak heat release rate of a laminar flamelet. The effects of chemical kinetics and transport properties on the H₂/air flame are investigated, from which it is concluded that the enhanced heat release rate in the low temperature zone is a convection–diffusion-reaction phenomenon, and to obtain it, detailed chemistry is essential and detailed transport is important.     

Effects of CO addition on the characteristics of laminar premixed CH4/air opposed-jet flames

The effects of CO addition on the characteristics of premixed CH₄/air opposed-jet flames are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position, temperature, and velocity are performed in stoichiometric CH₄/CO/air opposed-jet flames with various CO contents in the fuel. Thermocouple is used for the determination of flame temperature, velocity measurement is made using particle image velocimetry (PIV), and the flame front position is measured by direct photograph as well as with laser-induced predissociative fluorescence (LIPF) of OH imaging techniques. The laminar burning velocity is calculated using the PREMIX code of Chemkin collection 3.5. The flame structures of the premixed stoichiometric CH₄/CO/air opposed-jet flames are simulated using the OPPDIF package with GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position, temperature, and velocity of the stoichiometric CH₄/CO/air flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that as the CO content in the fuel is increased from 0% to 80%, CO oxidation (R99) increases significantly and contributes to a significant level of heat-release rate. It is also shown that the laminar burning velocity reaches a maximum value (57.5 cm/s) at the condition of 80% of CO in the fuel. Based on the results of sensitivity analysis, the chemistry of CO consumption shifts to the dry oxidation kinetics when CO content is further increased over 80%. Comparison between the results of computed laminar burning velocity, flame temperature, CO consumption rate, and sensitivity analysis reveals that the effect of CO addition on the laminar burning velocity of the stoichiometric CH₄/CO/air flames is due mostly to the transition of the dominant chemical kinetic steps.     

The effect of hydrogen containing fuel blends upon flashback in swirl burners

Lean premixed swirl combustion is widely used in gas turbines and many other combustion Processes due to the benefits of good flame stability and blow off limits coupled with low NO_x emissions. Although flashback is not generally a problem with natural gas combustion, there are some reports of flashback damage with existing gas turbines, whilst hydrogen enriched fuel blends, especially those derived from gasification of coal and/or biomass/industrial processes such as steel making, cause concerns in this area. Thus, this paper describes a practical experimental approach to study and reduce the effect of flashback in a compact design of generic swirl burner representative of many systems. A range of different fuel blends are investigated for flashback and blow off limits; these fuel mixes include methane, methane/hydrogen blends, pure hydrogen and coke oven gas. Swirl number effects are investigated by varying the number of inlets or the configuration of the inlets. The well known Lewis and von Elbe critical boundary velocity gradient expression is used to characterise flashback and enable comparison to be made with other available data.Two flashback phenomena are encountered here. The first one at lower swirl numbers involves flashback through the outer wall boundary layer where the crucial parameter is the critical boundary velocity gradient, G_f. Values of G_f are of similar magnitude to those reported by Lewis and von Elbe for laminar flow conditions, and it is recognised that under the turbulent flow conditions pertaining here actual gradients in the thin swirl flow boundary layer are much higher than occur under laminar flow conditions. At higher swirl numbers the central recirculation zone (CRZ) becomes enlarged and extends backwards over the fuel injector to the burner baseplate and causes flashback to occur earlier at higher velocities. This extension of the CRZ is complex, being governed by swirl number, equivalence ratio and Reynolds Number. Under these conditions flashback occurs when the cylindrical flame front surrounding the CRZ rapidly accelerates outwards to the tangential inlets and beyond, especially with hydrogen containing fuel mixes. Conversely at lower swirl numbers with a modified exhaust geometry, hence restricted CRZ, flashback occurs through the outer thin boundary layer at much lower flow rates when the hydrogen content of the fuel mix does not exceed 30%. The work demonstrates that it is possible to run premixed swirl burners with a wide range of hydrogen fuel blends so as to substantially minimise flashback behaviour, thus permitting wider used of the technology to reduce NOx emissions.     

Burning velocity and flame structure of CH4/NH3/air turbulent premixed flames at high pressure

Ammonia is one of the most promising alternative fuels. In particular, ammonia combustion for gas turbine combustors for power generation is expected. To shift the fuel for a gas turbine combustor to ammonia step-by-step, the partial replacement of natural gas by ammonia is considered. To reveal the turbulent combustion characteristics, CH₄/NH₃/air turbulent premixed flame at 0.5 MPa was experimentally investigated. The ammonia ratio based on the mole fraction and lower heating value was varied from 0 to 0.2. The results showed that the ratio of the turbulent burning velocity and unstretched laminar burning velocity decreased with an increase in the ammonia ratio. The reason for this variation is that the flame area decreased with an increase in the ammonia ratio as the flame surface density decreased and the fractal inner cutoff increased. The volume fractions in the turbulent flame region were almost the same with ammonia addition, indicating that combustion oscillation can be handled in a manner similar to that for the case of natural gas for CH₄/NH₃/air flames.     

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.     

Effect of hydrogen ratio on turbulent flame structure of oxyfuel syngas at high pressure up to 1.0 MPa

The CO/H₂/CO₂/O₂, CO/H₂/CO₂/air turbulent premixed flames as the model of syngas oxyfuel and syngas/air combustion were studied experimentally and compared to that of CH₄/air mixtures at high pressures up to 1.0 MPa. Hydrogen ratio in syngas was set to be 35%, 50% and 65% in volumetric fraction. Four perforated plates are used to generate wide range of turbulence intensity and scales. The instantaneous flame structure was measured with OH-PLIF technique and then statistic flame structure parameters and turbulent burning velocity were derived to interpret the multi scale turbulence-flame interaction. Results show that the flame structure of syngas is wrinkled and convex cusps to the unburned mixtures are sharper and deeper comparing to that of CH₄ flames. Pressure has a dominating effect on flame wrinkling other than mixtures composition at high pressure of 1.0 MPa. The flame surface density, Σ of syngas is larger than that of CH₄. The Σ of syngas flames is almost independent on pressure and hydrogen ratio especially when hydrogen ratio is over 50% which is a significant feature of syngas combustion. Larger flame surface density for syngas flames mainly comes from the finer structure with smaller wrinkles which is the result of more intensive flame intrinsic instability. The S_T/S_L of syngas is larger than CH₄ and it slightly increases with the pressure rise. The S_T/S_L of syngas oxyfuel is similar to that of syngas/air flames in the present study. The S_T/S_L increases with the increase of hydrogen ratio and keeps almost constant when hydrogen ratio is over 50%.