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In 2020, energy-related CO2 emissions reached 31.5 Gt, leading to an unprecedented atmospheric CO2 level of 412.5 ppm. Hydrogen blending in natural gas (NG) is a solution for maximizing clean energy utilization and enabling long-distance H2 transport through pipelines. However, insufficient comprehension concerning the combustion characteristics of NG, specifically when blended with a high proportion of hydrogen up to 80%, particularly with minority species, persists. Utilizing the heat flux method at room temperature and 1 atm, this experiment investigated the laminar burning velocities of CH4/NG/H2/air/He flames incorporating minority species, specifically C2H6 and C3H8, within NG. The results point out the regularity of SL enhancement, reaching its maximum at an equivalence ratio of 1.4. Furthermore, the propensity for the enhancement of laminar burning velocity aligned with the observed thermoacoustic oscillation instability during fuel-rich regimes. The experimental findings were contrasted with kinetic simulations, utilizing the GRI 3.0 and San Diego mechanisms to facilitate analysis. The inclusion of H2 augments the chemical reactions within the preheating zone, while the thermal effect from temperature is negligible. Both experimental and simulated results revealed that CH4 and NG with a large proportion of H2 had no difference, no matter whether from a laminar burning velocity or a kinetic analysis aspect. [ABSTRACT FROM AUTHOR]

• A novelty micro-mixing multi-injector burner was constructed for burning high-hydrogen-content CH 4 /H 2 fuels. • Under all experimental conditions, the micro-mixing burner can achieve ultra-low NOx emissions (less than 8 ppm@15 %O 2). • The impact of H 2 content on the flame morphology, flame stability and emission characteristics were investigated. • Distinct mechanisms for flame stabilization between pure methane and high-hydrogen-content CH 4 /H 2 fuels were observed. Micro-mixing combustion technology premixes air and fuel in a microscale channel, enabling efficient mixing within a short distance can preventing flame flashback suitable for H 2 -enriched fuels. This approach effectively mitigates the formation of localized high-temperature zones in the flame and shortens the residence time of fuel in the high-temperature zone, thereby significantly minimizing NOx emissions. In this study, a self-designed micro-mixing multi-injector burner was constructed for burning high-hydrogen-content CH 4 /H 2 fuels. The H 2 / CH 4 volume ratios range from 0:10 to 8:2. The effects of fuel composition and equivalence ratio on flame structure, flame stability, and pollutant emission of the CH 4 /H 2 micro-mixing combustion were investigated experimentally. The results demonstrated that even at an adiabatic flame temperature of 2100 K, the micro-mixing burner can achieve low NOx emissions (less than 8 ppm@15 %O 2). Furthermore, compared to pure methane, highly hydrogen-enriched fuel can exhibit lower NO x emissions under stable combustion conditions. Based on the measurement of the distribution of OH radicals using Planar Laser-Induced Fluorescence (PLIF), and the captured OH* chemiluminescence signals, the impact of H 2 concentration, equivalence ratio and Reynolds number on the flame morphology were investigated. The measurements suggest that hydrogen plays a pivotal role in the formation of OH radicals. For fuels with higher H 2 concentration, the flame height and lift-off height are smaller, and the heat release area is more concentrated. The flame with higher H 2 concentration can be stabilized at a lower equivalence ratio. As the H 2 concentration decreases, the distance between flame jets shortens, leading to gradual coupling and eventual reunion of the jet flames. When the combustion state approaches the lean blowout (LBO) limit, the flame structure of high hydrogen concentration fuel exhibits notable disparities compared to that of pure methane fuel, indicating distinct mechanisms for flame stabilization between these two fuels. [ABSTRACT FROM AUTHOR]

• First-time measurements were taken for CH 4 + H 2 S + N 2 + O 2 S L with varied oxygen ratio. • The S L tendencies against ϕ , x O 2 , x H 2 S were systematically compared and analysed. • The Mulvihill mechanism reproduces well all experimental data, without optimization. • Distinct kinetic features in fuel-rich CH 4 + H 2 S + N 2 + O 2 flames were identified. • Enhanced S formation in rich low- x O 2 flame simulations may help in applications. Sour gas is a special type of natural gas that contains H 2 S and takes a large percentage of present natural gas reserves. Facing the increasing world energy demand, direct combustion of sour gas has been proposed as a potential choice, which requires systematic investigations on the sour gas combustion characteristics. In this work, the laminar burning velocities S L of 85% CH 4 + 15% H 2 S + N 2 + O 2 and CH 4 + N 2 + O 2 flames were measured using the heat flux method at 1 atm and 298 K, where the oxygen ratio x O 2 and equivalence ratio ϕ ranges covered 0.17–0.24 and 0.7–1.4, respectively. Part of the conditions have never been reported before in the literature, and the S L tendencies against all the three dimensions of ϕ , x O 2 , x H 2 S were systematically compared. Simulations using the Mulvihill mechanism were carried out, showing good agreement with all the experimental data measured. By the kinetic analyses including maximum flame temperatures, reaction sensitivities, reaction paths, and maximum mole fractions of key radicals, distinct kinetic features of the fuel-rich CH 4 + H 2 S + N 2 + O 2 flames were found, which are different from the lean and stoichiometric flames. S-containing products in the CH 4 + H 2 S + N 2 + O 2 flames were also discussed using the simulation results. Besides the most dominant SO 2 among all the conditions investigated in the present study, it's noticed that the formation of the element sulfur, especially S 2 , is enhanced at fuel-rich conditions with small x O 2 . This phenomenon can be helpful for the design of real applications as the S 2 particles can be easily condensed and reduce the load of desulfurization. [ABSTRACT FROM AUTHOR]

• Laminar burning velocities of NH 3 /CH 4 /H 2 S/air flames were measured with a wide range of conditions that have not yet been investigated before. • Adding H 2 S into the fuel mixture has an inhibitory effect on laminar burning velocities of NH 3 /CH 4 /H 2 S/air flames, and the inhibition degree varies with equivalence ratio. • Reactions involving sulfur-containing species become important in laminar flame speed determination as H 2 S fraction in the fuel mixture increases. • The addition of H 2 S changes burning velocities of NH 3 /CH 4 /H 2 S/air flames by affecting the free radical pool in the flames and changing the reaction path. Recently, ammonia has been favored by scientists and engineers as a new type of carbon-free fuel for future carbon–neutral society. Before large-scale application, some existing drawbacks including extremely low reactivity need to be overcomed by enhancing combustion technology. Blending ammonia with sour gas, a type of natural gas resource containing amounts of H 2 S, can be a solution to enhance the combustion of ammonia as well as efficiently utilize natural gas resource. In this study, the laminar burning velocities of NH 3 /CH 4 /H 2 S/air flames were investigated using the Heat Flux method over a wide range of equivalence ratios and and mixing ratios. A kinetic model for NH 3 /CH 4 /H 2 S/air flames simulation was developed by integrating ammonia sub-mechanism with the mechanism of Han et al. and validated based on the experimental data. The experimental results show that the H 2 S addition has a negative effect on the flame speed, which reaches 13.36 % decrease at ϕ = 1.25 when the H 2 S mole fraction is 12 %. Kinetic analysis was performed for exploring the effect of H 2 S addition on the laminar flame propagation of NH 3 /CH 4 /H 2 S mixtures. The results show that the elevated H 2 S content significantly impacts the radicals pool in flame, where the mole fraction of H, OH radicals decreases while the S-containing species content increases obviously. Sensitivity analysis shows that the reactions involving S-containing species become important for laminar flame speed determination at high H 2 S content. Besides, the increament of NH 3 ratio in the fuel significantly improves the concentration level of NH 2 radical in the flame, leading to the increased importance of NH 2 chemistry. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) is a promising carbon-free alternative fuel, and blending NH 3 with other more active fuels can enhance the low burning rate of pure NH 3 flames, which makes it more compatible with existing combustors. There are laminar burning velocity data of NH 3 +H 2 , NH 3 +CO/syngas, and NH 3 +CH 4 flames in literature, however, blending NH 3 with the oxygenated alternative fuel alcohols have not yet been checked. In the present work, the laminar burning velocities of NH 3 +CH 3 OH/air and NH 3 +C 2 H 5 OH/air flames were measured using the heat flux method at 1 atm with varied equivalence ratios and mixing ratios. Measurements were carried out at 298 K and elevated temperatures until 448 K unburnt temperatures, in order to check the data consistency via the temperature coefficient α in S L S L 0 = (T u T u 0) α. A new updated kinetic mechanism, based on our previous mechanism named CEU-NH3, was proposed and validated based on the present and published experimental data for laminar burning velocity, ignition delay times and species profiles. This mechanism contains 91 species and 444 reactions, which is small in size and thus advantageous to be used in the modeling of complex combustion fields. By numerical analyses using the CEU-NH3 and the detailed Konnov mechanisms, the interactions between C- and N- containing species are found to be insignificant for the laminar burning velocities of the various NH 3 blending mixtures. [ABSTRACT FROM AUTHOR]