Experimental investigation of thermoacoustic coupling using blended hydrogen–methane fuels in a low swirl burner

In this study, experimental testing and analysis were performed to examine the combustion instability characteristics of hydrogen–methane blended fuels for a low-swirl lean premixed burner. The aim of this study is to determine the effect of hydrogen addition on combustion instability, and this is assessed by examining the flame response to a range of constant amplitude, single frequency chamber acoustic modes. Three different blends of hydrogen and methane (93% CH₄–7% H₂, 80% CH₄–20% H₂ and 70% CH₄–30% H₂ by volume) were employed as fuel at an equivalence ratio of 0.5, and with four different acoustic excitation frequencies (85, 125, 222 and 399 Hz). Planar laser induced fluorescence of the hydroxyl radical (OH-PLIF) was employed to measure the OH concentration at different phases of acoustic excitation and a Rayleigh Index was then calculated to determine the degree of thermoacoustic coupling. It was found, as has been previously reported, that the combustion characteristics are very sensitive to the fraction of hydrogen in the fuel mixture. The flame shows significant increases in flame base coupling and flame compaction with increasing hydrogen concentration for all conditions. While this effect enhances the flame response at non-resonant frequencies, it induces only minimal compaction and appears to decreases the coupling intensity at the resonant frequency.     

An assessment of fuel variability effect on biogas-hydrogen combustion using uncertainty quantification

Effects of fuel variability involving small fluctuations in fuel composition on physicochemical properties of premixed biogas-hydrogen combustion are quantified using global sensitivity analysis approach. Different proportions of hydrogen addition, and different CH₄:CO₂ ratios in biogas-hydrogen fuel with uncertainties are investigated from a statistical point of view. Analyses show that small fluctuation of biogas-hydrogen fuel composition does not lead to significant fluctuations in physicochemical properties of combustion such as laminar flame speed and adiabatic flame temperature. A fast growth of laminar flame speed fluctuation from lean to rich combustion of biogas-hydrogen fuel is observed implying a less stable flame at rich condition, and lower dimensional studies show that hydrogen uncertainty takes predominant responsibility for the rapid increase of flame speed. Apart from the uncertainty of hydrogen, it is found that carbon dioxide concentration fluctuation has a larger negative effect on stabilising biogas-hydrogen flame compared to that of methane. It is also found that for biogas-hydrogen fuel with high hydrogen content, contribution of carbon dioxide variability to flame speed fluctuation decreases while methane contribution increases.     

Development of 1D model for the analysis of heat transport in cylindrical micro combustors

A 1D flame model was developed to analyze the heat transport occurring in the cylindrical micro combustors. The one-step global reaction mechanisms were employed for three fuel–air mixtures (H₂–air, CH₄–air and C₃H₈–air) to account for the difference of fuel property in terms of the kinetics. The effects of various parameters such as the combustor size, fuel property, fuel–air equivalence ratio and unburned mixture temperature on the heat loss ratio (defined as Q_l/Q_in) and the heat recirculation ratio (defined as Q_recir/Q_l) were investigated. The results indicated that these parameters have significant effects on the two ratios, and therefore should be carefully managed in order to achieve efficient and stable combustion. After comparing the results of different fuel–air mixtures, it is concluded that hydrogen is superior to methane and propane as the fuel for micro combustion engines owing to its higher flame temperature and thinner flame thickness, which favors the reduction of heat loss from the flame zone.     

The decreasing effect of ammonia enrichment on the combustion emission of hydrogen,methane, and propane fuels

In the present study, non-premixed combustion and NOx emission of H₂, NH₃, C₃H₈, and CH₄ fuels have been studied in a combustion test unit under lean mixture conditions (λ = 4) at 8.6 kW thermal capacity. Furthermore, the combustion and NOx emission of the H₂, C₃H₈, and CH₄ fuels have been investigated for various NH₃ enrichment ratios (5, 10, 20, and 50%) and excess air coefficients (λ = 1.1, 2, 3, and 4) at the same thermal capacity. The obtained results have been compared for each fuel. Numerical simulation results show that H₂ emits intense energy through the reaction zone despite the lowest fuel consumption in mass, among others, due to its high calorific value. Therefore, it has a higher flame temperature than others. At the same time, C₃H₈ has the lowest flame temperature. Besides, NH₃ has the shortest flame length among others, while C₃H₈ has the most extended flame form. The highest level of NOx is released from the NH₃ flame in the combustion chamber, while the lowest NOx is released from the CH₄. However, the lowest NOx emission at the combustion chamber exit is obtained in NH₃ combustion, while the highest NOx emission is obtained with H₂ combustion. It results from the shortest flame length of NH₃, short residence time, and backward NOx reduction to N₂ for NH₃. As for H₂, high flame temperature and relatively long flame, and high residence time of the products trigger NOx formation and keep the NOx level high. On the other hand, excess air coefficient from 1.1 to 2 increases NOx for H₂, CH₄, and NH₃ due to their large flame diameters, unlike propane. Then, NOx emission levels decrease sharply as the excess air coefficient increases to 4 for each fuel. NH₃ fuel also emits minimum NOx in other excess air coefficients at the exit, while H₂ emits too much emission. With NH₃ enrichment, the NOx emissions of H₂, CH₄, and C₃H₈ fuels at the combustion chamber exit decrease gradually almost every excess air coefficient apart from λ = 1.1. As a general conclusion, like renewable fuels, H₂ appears to be a source of pollution in terms of NOx emissions in combustion applications. In contrast, NH₃ appears to be a relatively modest fuel with a low NOx level. In addition, the high amount of NOx emission released from H₂ and other fuels during the combustion can be remarkably reduced by NH₃ enrichment with an excess air combustion.     

Rich methane premixed laminar flames doped by light unsaturated hydrocarbons: III. Cyclopentene

In line with the studies presented in Parts I (methane flame seeded with allene and propyne) and II (methane flame seeded with 1,3-butadiene) of this paper, the structure of a laminar rich premixed methane flame doped with cyclopentene has been investigated. The gases of this flame contain 15.3% (molar) of methane, 26.7% of oxygen, and 2.4% cyclopentene, corresponding to an overall equivalence ratio of 1.79 and a C₅H₈/CH₄ ratio of 15.7%. The flame has been stabilized on a burner at a pressure of 6.7 kPa using argon as dilutant, with a gas velocity at the burner of 36 cm/s at 333 K. The measured temperature ranged from 627 K close to the burner up to 2027 K. Species quantified by gas chromatography included the usual methane C₀–C₂ combustion products, but also propyne, allene, propene, propane, 1-butene, 1,3-butadiene, 1,2-butadiene, vinylacetylene, diacetylene, cyclopentadiene, 1,3-pentadiene, benzene, and toluene. A new mechanism for the oxidation of cyclopentene has been developed and added to the former model for the oxidation of small unsaturated hydrocarbons, benzene, and toluene described in Parts I and II. The whole mechanism involved 175 species in 1134 reactions. The main reaction pathways of consumption of cyclopentene and of formation of benzene and toluene are presented and discussed from flow rate analyses.     

Experimental investigation of the flame characteristics of a fuel mixture with high hydrogen content enriched with oxygen under the externally acoustic enforcement conditions

Pollutant emissions are one of the major problems for the World. In this regard, researchers focus on the studies on emission reduction. Hydrogen is an alternative solution for this problem. Hydrogen produces only water as a result of combustion with oxygen. Therefore, this study examines the combustion stability and emissions of a high hydrogen content fuel mixture. The fuel mixture containing 45% H₂ by volume was supported with 5% CH₄ in order to provide stable combustion. In addition, in order to reduce the instabilities caused by the high laminar burning rate of hydrogen, it was diluted with 50% CO₂ which equal volume with the fuel mixture. After the fuel mixture containing 45% H₂ - 5% CH₄ - 50% CO₂ was burned with air containing 21% O₂, enrichment was applied at the rates of 24% and 27% O₂. The flame that contains different oxygen ratios was acoustically forced through the speakers around the combustion chamber. The stability data, dynamic pressure, and light intensity fluctuation of the flame were recorded under different acoustic resonance frequencies (110 Hz, 190 Hz, 260 Hz). In this way, the oxygen enrichment performance and flame characteristics of hydrogen in a premixed burner, which is promising in zero-emission studies, were investigated. As a result, when the combustion condition of 21% O₂ and 24% O₂ ratios are compared, the instability increased slightly from 801 Pa to 887 Pa, respectively. However, at 27% O₂, the flame could not perform a stable combustion under acoustic enforcement. The flame flashbacked with a dynamic pressure fluctuation of 1577 Pa under an acoustic frequency of 110 Hz. In addition, it was observed that CO emissions have decreased with the increase in oxygen enrichment rate. CO emission measured at 1080 ppm at 21% O₂ decreased to 542 ppm and 276 ppm respectively at 24% and 27% oxygen enrichment levels. While NOx emission was measured at 10 ppm in the case of combustion with air, it was observed that decreased to 4 ppm at the rate of 27% O₂.     

Effects of fuel type and equivalence ratios on the flickering of triple flames

An experimental study has been conducted in axisymmetric, co-flowing triple flames with different equivalence ratios of the inner and outer reactant streams (2<?in<3 and 0??out<0.7). Different fuel combinations, like propane/propane, propane/methane or methane/methane in the inner and outer streams respectively, have been used in the experiments. The structures of the triple flames have been compared for the different fuel combinations and equivalence ratios. The conditions under which triple flames exhibit oscillation have been identified. During the oscillation, the non-premixed flame and the outer lean premixed flame flicker strongly, while the inner rich premixed flame remains more or less stable. The flickering frequency has been evaluated through image processing and fast Fourier transform (FFT) of the average pixel intensity of the image frames. It is observed that, for all the fuel combinations, the frequency decreases with the increase in the outer equivalence ratio, while it is relatively invariant with the change in the inner equivalence ratio. However, an increase in the inner equivalence ratio affects the structure of the flame by increasing the heights of the inner premixed flame and non-premixed flame and also enlarges the yellow soot-laden zone at the tip of the inner flame. A scaling analysis of the oscillating flames has been performed based on the measured parameters, which show a variation of Strouhal number (St) with Richardson number (Ri) as St ∝ Ri^0.5. The fuel type is found to have no influence on this correlation.     

A lean methane premixed laminar flame doped with components of diesel fuel: I. n-Butylbenzene

To better understand the chemistry involved in the combustion of components of diesel fuel, the structure of a laminar lean premixed methane flame doped with n-butylbenzene has been investigated. The inlet gases contained 7.1% (molar) methane, 36.8% oxygen, and 0.96% n-butylbenzene corresponding to an equivalence ratio of 0.74 and a ratio C₁₀H₁₄/CH₄ of 13.5%. The flame has been stabilized on a burner at a pressure of 6.7 kPa using argon as diluent, with a gas velocity at the burner of 49.2 cm/s at 333 K. Quantified species included the usual methane C₀-C₂ combustion products, but also 16 C₃-C₅ hydrocarbons, and 7 C₁-C₃ oxygenated compounds, as well as 20 aromatic products. A new mechanism for the oxidation of n-butylbenzene is proposed whose predictions are in satisfactory agreement with measured species profiles in flames and flow reactor experiments. The main reaction pathways of consumption of n-butylbenzene have been derived from flow rate analyses.     

Effect of methane reforming before combustion on emission and calorimetric characteristics of its combustion process

In this study, combustion and emission characteristics of methane mixed with steam (CH₄/H₂O) and the products of methane reforming with steam (CO/H₂/H₂O) were compared. Four fuel compositions were analysed: CH₄+H₂O, CH₄+2H₂O, and products of complete methane reforming in these mixtures, respectively. A comparison was carried out through the numerical model created via Ansys Fluent 2019 R2. A combustion process was simulated using a non-premixed combustion model, standard k-ϵ turbulence model and P-1 radiation model. The combustor heat capacity for interrelated fuel compositions was kept constant due to air preheating before combustion. The inlet air temperature was varied to gain a better insight into the combustion behaviour at elevated temperatures. The effect of steam addition on the emission characteristics and flame temperatures was also evaluated. NO_x formation was assessed on the outlet of the combustion zone. The obtained results indicate that syngas has a higher combustion temperature than methane (in the same combustor heat capacity) and therefore emitted 27% more NO_x comparing to methane combustion. With the air inlet temperature increment, the pollutant concentration difference between the two cases decreased. Steam addition to fuel inlet resulted in lesser emissions both for methane and syngas by 57% and 28%, respectively. In summary, syngas combustion occurred at higher temperature and produced more NO_x emissions in all cases considered.     

Effect of steam addition and distance between inlet nozzles on non-catalytic POX process under MILD combustion condition

Moderate or intensive low-oxygen dilution (MILD) combustion is a novel combustion technology with high efficiency and low emissions. Few studies have been performed on the application of this technology for partial oxidation processes. In this research, a Computational Fluid Dynamics study for the effect of different parameters on the natural gas partial oxidation under MILD combustion conditions has been carried out. The combustion chamber was in the form of cylinder with a diameter of 300 mm and a length of 1500 mm. The effect of parameters such as different kinetic mechanisms, adding ethane and propane to methane (shale gas feed), adding steam to the feed and distance (interval) between methane and oxygen nozzles were investigated. Results showed that addition of ethane and propane to methane increased the mole fraction of CO and C₂H₂ so that, in the case of mixed methane with ethane and propane compared to the case of pure methane, an increase of 18.75% and 12.93% was observed for CO and C₂H₂, respectively. In addition, with increasing the percentage of steam in the inlet feed at a constant flow rate, methane conversion increased so that it in the case of 30% inlet steam was 77.17%, which showed 11% promotion compared to the pure methane case. Also, increasing the distance between the fuel and oxidizer nozzles led to an increase in the maximum temperature in the combustion chamber.     

A lean methane premixed laminar flame doped with components of diesel fuel part III: Indane and comparison between n-butylbenzene, n-propylcyclohexane and indane

To better understand the chemistry of the combustion of components of diesel fuel, the structure of a laminar lean premixed methane flame doped with indane has been investigated. The inlet gases contained 7.1% (molar) of methane, 36.8% of oxygen and 0.9% of indane corresponding to an equivalence ratio of 0.67 and a ratio C₁₀H₁₄/CH₄ of 12.8%. The flame has been stabilized on a burner at a pressure of 6.7 kPa (50 Torr) using argon as diluent, with a gas velocity at the burner of 49.1 cm s⁻¹ at 333 K. Quantified species included the usual methane C₀-C₂ combustion products, but also 16 C₃-C₅ non-aromatic hydrocarbons, 6 C₁-C₃ non-aromatic oxygenated compounds, as well as 22 aromatic products, namely benzene, toluene, xylenes, phenylacetylene, ethylbenzene, styrene, propenylbenzene, allylbenzene, n-propylbenzene, methylstyrenes, ethyltoluenes, trimethylbenzenes, n-butylbenzene, dimethylethylbenzene, indene, methylindenes, methylindane, benzocyclobutene, naphthalene, phenol, benzaldehyde, and benzofuran. A new mechanism for the oxidation of indane was proposed whose predictions were in satisfactory agreement with measured species profiles in both flames and jet-stirred reactor experiments. The main reaction pathways of consumption of indane have been derived from flow rate analyses in the two types of reactors. A comparison of the effect of the addition of three components of diesel fuel, namely indane, n-butylbenzene and n-propylcyclohexane (parts I and II of this series of paper), on the structure of a laminar lean premixed methane flame is also presented.     

Lean methane premixed laminar flames doped by components of diesel fuel II: n-Propylcyclohexane

For a better understanding of the chemistry involved during the combustion of components of diesel fuel, the structure of a laminar lean premixed methane flame doped with n-propylcyclohexane has been investigated. The inlet gases contained 7.1% (molar) methane, 36.8% oxygen, and 0.81% n-propylcyclohexane (C₉H₁₈), corresponding to an equivalence ratio of 0.68 and a C₉H₁₈/CH₄ ratio of 11.4%. The flame has been stabilized on a burner at a pressure of 6.7 kPa (50 Torr) using argon as diluent, with a gas velocity at the burner of 49.2 cm/s at 333 K. Quantified species included the usual methane C₀–C₂ combustion products, but also 17 C₃–C₅ hydrocarbons, seven C₁–C₃ oxygenated compounds, and only four cyclic C₆₊ compounds, namely benzene, 1,3-cyclohexadiene, cyclohexene, and methylenecyclohexane. A new mechanism for the oxidation of n-propylcyclohexane has been proposed. It allows the proper simulation of profiles of most of the products measured in flames, as well as the satisfactory reproduction of experimental results obtained in a jet-stirred reactor. The main reaction pathways of consumption of n-propylcyclohexane have been derived from rate-of-production analysis.     

Study on combustion and ignition characteristics of natural gas components in a micro flow reactor with a controlled temperature profile

Combustion and ignition characteristics of natural gas components such as methane, ethane, propane and n-butane were investigated experimentally and computationally using a micro flow reactor with a controlled temperature profile. Special attention was paid to weak flames which were observed in a low flow velocity region. The observed weak flame responses for the above fuels were successfully simulated by one-dimensional computations with a detailed kinetic model for natural gas. Since the position of the weak flame indicates the ignition characteristics as well as the reactivity of each fuel, the experimental and computational results were compared with research octane number (RON) which is a general index for ignition characteristics of ordinary fuels. At 1 atm, ethane showed the highest reactivity among these fuels, although RON of ethane (115) is between those of methane (120) and propane (112). Since the pressure conditions are different between the present experiment and the general RON test, weak flame responses to the pressure were investigated computationally for these fuels. The order of the fuel reactivity by the reactor agreed with that by RON test when the pressure was higher than 4 atm. Reaction path analysis was carried out to clarify the reasons of the highest reactivity of ethane at 1 atm among the employed fuels in this study. The analysis revealed that C₂H₅ + O₂ ⇔ C₂H₄ + HO₂ is a key reaction and promotes ethane oxidation at 1 atm. The effect of the pressure on the fuel oxidation process in the present reactor was also clarified by the analysis. In addition, weak flame responses to various mixing ratios of methane/n-butane blends were investigated experimentally and computationally. The results indicated a significant effect of n-butane addition in the blends on combustion and ignition characteristics of the blended fuels.     

An experimental study of syngas combustion in a bidirectional swirling flow

The paper reports on the results of an experimental study of methane and syngas combustion as well as their co-firing in a bidirectional swirling flow. The results confirmed that the bidirectional flow structure provides a significant decrease in the lean blow-off equivalence ratio as well as that of emissions of main pollutants. The combustion intensification becomes more evident when using syngas is as fuel. The composition of the used syngas is as follows (by volume): H₂ - 29.42%; CO - 14.32%; CH₄ - 3.8%; N₂ - 49.11%; H₂O - 3.35%. In this case, the lean blow-off is achieved at ? < 0.1, NOx emission is halved, while CxHy and CO emissions become 20 times less compared to pure methane combustion. However, according to experimental results, the co-combustion of syngas (volume fraction V_syn = 15%) and methane is the most appropriate fuel utilization mode. It provides blow-off and emission properties similar to those for combustion of pure syngas, whereas energy consumption for its production is much lower. Moreover, unlike hydrocarbon fuel combustion, that of syngas in a bidirectional swirling flow is characterized by the presence of density stratification. This is accompanied by the flame formation at significantly different locations in the combustion chamber at lean and “ultra-lean” modes of operation. Hydrogen combustion most likely to occur in the core region at near-blow-off modes ? < 0.1, whereas normal ‘operating modes in the range 0.2 = ? ≤ 0.4 result in the formation of a conical flame surface where CH₄ and CO combustion occurs. These new results with respect to the flame structure as well as blow-off and emission properties make it possible to consider bidirectional vortex combustors for application in modern gas turbine power plants in order to meet the strict environmental and energy requirements.     

A numerical study on the effects of H2 addition in non-premixed turbulent combustion of C3H8–H2–N2 mixture using a steady flamelet approach

This study has been implemented in two sections. At first, the turbulent jet flame of DLR-B is simulated by combining the k–ε turbulence model and a steady flamelet approach. The DLR-B flame under consideration has been experimentally investigated by Meier et al. who obtained velocity and scalar statistics. The fuel jet composition is 33.2% H₂, 22.1% CH₄ and 44.7% N₂ by volume. The jet exit velocity is 63.2 m/s resulting in a Reynolds number of 22,800. Our focus in the first part is to validate the developed numerical code. Comparison with experiments showed good agreement for temperature and species distribution. At the second part, we exchanged methane with propane in the fuel composition whilst maintaining all other operating conditions unchanged. We investigated the effect of hydrogen concentration on C₃H₈–H₂–N₂ mixtures so that propane mole fraction extent is fixed. The hydrogen volume concentration rose from 33.2% up to 73.2%. The achieved consequences revealed that hydrogen addition produces elongated flame with increased levels of radiative heat flux and CO pollutant emission. The latter behavior might be due to quenching of CO oxidation process in the light of excessive cold air downstream of reaction zone.     

Investigation of the effect of air turbulence intensity on NOx emission in non-premixed hydrogen and hydrogen-hydrocarbon composite fuel combustion

In this paper, the effect of air turbulence intensity on NO formation in the combustion of mixed hydrogen-hydrocarbon fuel is numerically studied. The fuels used in this study are 100% H₂, 70% H₂ + 30% CH₄, 10% H₂ + 90% CH₄ and 100% CH₄. Finite volume method is utilized to solve the governing equations. The obtained results using realizable k-ε and β-PDF models show good agreement with other numerical and experimental results. The results show that increasing air turbulence intensity decreases NO concentration in the flame zone and at the combustor outlet. With increasing air turbulence intensity, maximum decreasing of NO at the combustor outlet is for the case of pure hydrogen fuel. It is also found that adding hydrogen to methane rises the peak temperature of the flame.     

Impact of co-flow air on buoyant diffusion flames flicker

This paper describes experimental investigation of co-flow air velocity effects on the flickering behaviour of laminar non-lifted methane diffusion flames. Chemiluminescence, high-speed photography, schlieren and Particle Imaging Velocimetry (PIV), have been used to study the changes in the flame/vortex interactions as well as the flame flickering frequency and magnitude by the co-flow air. Four cases of methane flow rates at different co-flow air velocities are investigated. It has been observed that the flame dynamics and stability of co-flow diffusion flames are strongly affected by the co-flow air velocity. When the co-flow velocity has reached a certain value the buoyancy driven flame oscillation was completely suppressed. The schlieren and PIV imaging have revealed that the co-flow of air is able to push the initiation point of the outer toroidal vortices beyond the visible flame to create a very steady laminar flow region in the reaction zone. Then the buoyancy driven instability is only effective in the plume of hot gases above the visible flame. It is observed that a higher co-flow rate is needed in order to suppress the flame flickering at a higher fuel flow rate. Therefore the ratio of the air velocity to the fuel velocity, γ, is a stability controlling parameter. The velocity ratio, γ, was found to be 0.72 for the range of tested flow rates. The dominant flickering frequency was observed to increase linearly with the co-flow rate (a) as; f = 0.33a + 11. The frequency amplitudes, however, were observed to continuously decrease as the co-flow air was increasing.     

Effects of dimethyl methylphosphonate on premixed methane flames

The impact of dimethyl methylphosphonate (DMMP) was studied in a premixed methane/oxygen/N₂-Ar flame in a flat flame burner slightly under atmospheric pressure at two different equivalence ratios: rich and slightly lean. CH₄, CO, CO₂, CH₂O, CH₃OH, C₂H₆, C₂H₄, and C₂H₂ profiles were obtained with a Fourier Transform Infrared (FTIR) spectrometer. Gas samples, analyzed in the FTIR, were extracted from the reaction zone using a quartz microprobe with choked flow at its orifice. Temperature profiles were obtained by measuring the probe flow rate through the choked orifice. Flame calculations were performed with two existing detailed chemical kinetic mechanisms for organophosphorus combustion. DMMP addition caused all profiles except that of CH₃OH to move further away from the burner surface, which can be interpreted as a consequence of a reduction in the adiabatic flame speed. Experimentally, the magnitude of the shift was 50% greater for the near-stoichiometric flame than for the rich flame. Experimental CH₃OH profiles were four to seven times higher in the doped flames than in the undoped ones. The magnitude of this effect is not predicted in the calculations, suggesting a need for further mechanism development. Otherwise, the two mechanisms are reasonably successful in predicting the effects of DMMP on the flame.     

Numerical investigation on NO formation in laminar counterflow methane/n-heptane dual fuel flames

To understand the fundamental mechanisms of NO formation in natural gas-diesel dual fuel combustion, a numerical study on NO formation in laminar counterflow methane (CH₄)/n-heptane (n-C₇H₁₆) dual fuel flames is conducted. The results reveal that the flame structure and NO formation vary with the fuel equivalence ratio. For a given n-C₇H₁₆/air mixture, the NO emission index decreases with increasing the equivalence ratio of the CH₄/air mixture (φ(CH₄/air)). The NO formation route analysis suggests that the prompt and thermal routes dominate the NO formation. The increase in φ(CH₄/air) causes the decrease in the contribution of the prompt route to overall NO formation. NO formation by prompt route is mainly caused by rich n-C₇H₁₆ combustion. As φ(CH₄/air) increases, the mole fractions of the radicals (OH, O and H) related to CH formation in the reaction zone of rich n-C₇H₁₆/air flame branch are decreased, which reduces the formation of NO by prompt route.     

The features of ignition and combustion of composite propane-hydrogen fuel: Modeling study

The extensive numerical analysis of the features of self ignition and formation of NO and CO during combustion of blended fuel, consisting of propane and hydrogen, with air is considered on the basis of extended detailed kinetic model involving both high and low temperature submechanisms of propane oxidation. It has been shown that for the blended C₃H₈–H₂ fuel there exists the temperature region, where the ignition of the C₃H₈–H₂–air mixture occurs faster compared to pure propane. However, this region is not broad enough and has low and high temperature boundaries (T_b and T_h, respectively). At the initial temperature of fuel–air mixture T₀ < T_b, the induction time of blended C₃H₈–H₂ fuel is greater than that of pure propane and, at T₀ > T_h, the admixture of a small amount of propane (1 ∼ 5% per volume) to hydrogen accelerates the ignition. The values of T_b and T_h depend on the composition of blended fuel and initial pressure. It has been revealed that the addition of hydrogen to propane increases the flame speed and extends the flammability thresholds both in fuel-lean and in fuel-rich regions, but doesn't result in the substantial change of the concentrations of main pollutants NO and CO in the combustion exhaust. However, the addition of hydrogen to fuel-lean propane–air mixture allows one to provide the stable combustion of leaner fuel–air mixture and, thus, to reduce notably the emission of NO and CO compared to that for the combustion of pure propane–air mixture.