Hydrogen-enriched Methane Research Articles

Emissions and Flame Stability Assessment of Hydrogen Addition and Air Dilution in a Micro Gas Turbine Combustor Using 0D/1D Modeling by Chemical Reactor Network

Abstract Hydrogen emerges as a promising fuel for clean and sustain- able electricity production when utilized in micro gas turbines (mGTs). However, some challenges linked to hydrogen usage must be overcome as its high reactivity can lead to increased NOx emissions and flame instabilities. Literature suggests that combustion air humidification and/or exhaust gas recirculation (EGR) may be methods to reduce this reactivity. However, these measures are limited due to the small operating range of the mGT combustor. In this context, a 20 kWth mGT combustor is investigated un- der various inlet conditions to assess the effect of EGR towards increased flame stability and emission control when operating under hydrogen-enriched methane firing, using a Chemical Re- action Network (CRN) model. The CRN modeling is a fast and low computational complexity tool to model combustion perfor- mance and emissions by representing complex reactive flow fields through idealized reactor models, leading to reduced computa- tional costs. This study examines partial load conditions, fuel compositions, and the effect of combustion air dilution through EGR. The CRN model of the combustion process is designed based on combustion zones extracted from the main flow fields with similar thermo-chemical states from CFD, while emission predictions are experimentally validated for different operating points. Predicted NOx and CO emissions by the proposed CRN model show an acceptable agreement with the experimental data at high power loads. However, its accuracy diminishes for loads below 70% due to the variability in model tuning parameters across different loads, whereas the

Effects of combustion progress variable and Karlovitz number on the scalar dissipation rate of turbulent premixed hydrogen-enriched methane–air flames: An experimental study

The scalar dissipation rate plays a key role in the modeling of turbulent premixed flames. Despite the above, our knowledge concerning the effects of several parameters, such as the combustion progress variable, Karlovitz number, and fuel composition on the scalar dissipation rate requires to be developed. This is carried out experimentally here. Two mixtures, methane–air (Lewis number of unity) and hydrogen-enriched methane–air (Lewis number of 0.8), are examined. Three passive (perforated plates) and one active turbulence generators are used, leading to a wide range of turbulence intensities with the corresponding Karlovitz number changing from 0.1 to 34.5. The hot-wire anemometry and the Rayleigh scattering techniques are used separately to study the background non-reacting turbulent flow and the reacting scalar dissipation rate, respectively. Three models that are available in the literature for estimating the Favre-averaged scalar dissipation rate are experimentally assessed. Using experimental results obtained in this study, it is shown that moving from fresh reactants towards combustion products increases the above modeling parameters by 2–3 times, depending on the tested Lewis number. For each tested Lewis number, empirical formulations that include the effects of both the Karlovitz number and combustion progress variable on the modeling parameters are proposed. Care must be taken in extending the proposed models of the present study to test conditions that are not examined here.

Exergy Analysis in Highly Hydrogen-Enriched Methane Fueled Spark-Ignition Engine at Diverse Equivalence Ratios via Two-Zone Quasi-Dimensional Modeling

In the endeavor to accomplish a fully de-carbonized globe, sparkling interest is growing towards using natural gas (NG) having as vastly major component methane (CH4). This has the lowest carbon/hydrogen atom ratio compared to other conventional fossil fuels used in engines and power-plants hence mitigating carbon dioxide (CO2) emissions. Given that using neat hydrogen (H2) containing nil carbon still possesses several issues, blending CH4 with H2 constitutes a stepping-stone towards the ultimate goal of zero producing CO2. In this context, the current work investigates the exergy terms development in high-speed spark-ignition engine (SI) fueled with various hydrogen/methane blends from neat CH4 to 50% vol. fraction H2, at equivalence ratios (EQR) from stoichiometric into the lean region. Experimental data available for that engine were used for validation from the first-law (energy) perspective plus emissions and cycle-by-cycle variations (CCV), using in-house, comprehensive, two-zone (unburned and burned), quasi-dimensional turbulent combustion model tracking tightly the flame-front pathway, developed and reported recently by authors. The latter is expanded to comprise exergy terms accompanying the energy outcomes, affording extra valuable information on judicious energy usage. The development in each zone, over the engine cycle, of various exergy terms accounting too for the reactive and diffusion components making up the chemical exergy is calculated and assessed. The correct calculation of species and temperature histories inside the burned zone subsequent to entrainment of fresh mixture from the unburned zone contributes to more exact computation, especially considering the H2 percentage in the fuel blend modifying temperature-levels, which is key factor when the irreversibility is calculated from a balance comprising all rest exergy terms. Illustrative diagrams of the exergy terms in every zone and whole charge reveal the influence of H2 and EQR values on exergy terms, furnishing thorough information. Concerning the joint content of both zones normalized exergy values over the engine cycle, the heat loss transfer exergy curves acquire higher values the higher the H2 or EQR, the work transfer exergy curves acquire slightly higher values the higher the H2 and slightly higher values the lower the EQR, and the irreversibility curves acquire lower values the higher the H2 or EQR. This exergy approach can offer new reflection for the prospective research to advancing engines performance along judicious use of fully friendly ecological fuel as H2. This extended and in-depth exergy analysis on the use of hydrogen in engines has not appeared in the literature. It can lead to undertaking corrective actions for the irreversibility, exergy losses, and chemical exergy, eventually increasing the knowledge of the SI engines science and technology for building smarter control devices when fueling the IC engines with H2 fuel, which can prove to be game changer to attaining a clean energy environment transition.

Comprehensive pollutant emission prediction models from hydrogen-enriched methane combustion in a gas-fired boiler based on box-behnken design method

Hydrogen-enriched methane (HEM) combustion is an effective method for reducing the consumption of carbonaceous fuels. However, high adiabatic flame temperature of H2 can cause large NO emissions. To reduce the NO and CO2 emissions from HEM combustion, the Box-Behnken design (BBD) method combining response surface experimental design, analysis of variance, and multi-nonlinear regression models was adopted. The effects of the H2 doping ratio (XH2), preheated air temperature (Tair), excess air coefficient of burners arranged at the bottom (Ex,b), and their interactions on the NO, N2O, and CO2 emissions of a gas-fired boiler were investigated. According to the BBD method, the NO, N2O, and CO2 emissions prediction models with R2 exceeding 0.98 were proposed, respectively. The results showed that the increase of Tair had a positive effect on reducing NO emissions, as the NO exhausted from HEM combustion is mainly thermal NO. Compared with Tair and Ex,b, XH2 was the most significant factor affecting N2O and CO2 emissions, indicating that an increase in XH2 can significantly reduce greenhouse gas emissions. Additionally, the formation and consumption of N2O emissions presented a competitive mechanism at XH2 increased from 0 to 0.8, and CO2 emissions reduced with the increase of XH2, Tair, and Ex,b. Furthermore, a comprehensive emission prediction model with NO, N2O, and CO2 emissions was proposed. With the target of minimizing total NO, N2O and CO2 emissions, three groups of combinations for XH2, Tair, and Ex,b were obtained from the equal weight, entropy weight method, and criteria importance through the inter-criteria correlation method as 1:1:1, 24:9:17, and 21:11:18. The results showed that for minimize emissions, there is not much difference in the results calculated based on the optimal boundary conditions under different weights. For various reduction targets for nitrogen, carbon, and greenhouse gases, three groups of optimal combinations of XH2, Tair, and Ex,b were obtained and their accuracies were verified based on numerical calculations with relative errors below 6%. The proposed comprehensive pollutant emission prediction models are conducive to calculating the pollutant emissions of HEM combustion and guiding the investigation of prediction models considering other operating conditions.

Suppressive effects of potassium salt modified dry water material on hydrogen/methane mixture explosion

As a new type of energy, hydrogen-enriched methane mixed fuel has a high risk of deflagration in the application process. In order to better understand the deflagration and inhibition characteristics of hydrogen-enriched methane fuel, the effect of potassium salt modified dry water material (DW) on the deflagration characteristics of hydrogen-enriched methane fuel was studied by 20 L spherical explosive apparatus, combined with pyrolysis characteristics and kinetic model. The results show that when the addition of different modifier concentrations, it can be divided into three types: small curvature, medium curvature and large curvature according to the change of pressure rising rate. With the increase of the proportion of potassium salt modifiers, the number of areas with small curvature is gradually increasing, the number of large curvature is gradually decreasing, and the number of medium curvature is almost unchanged. When the proportion of potassium salt modifier is 15% and ϕ ≤ 0.7, all enter the area below the medium curvature. When the addition of different modifiers, 15%KCl-DW has the best inhibition effect on Pmax and (dP/dt) max, while CH3COOK-DW and K2CO3-DW have better inhibition effect on Δt2 and Δt2. In general, when the hydrogen addition ratio is less than 70%, the addition of inhibitors can significantly reduce the explosion intensity. In terms of inhibition mechanism, we established the decomposition model of potassium salt modified dry water and found that potassium salt modified dry water will produce a large amount of gaseous K and KOH in the explosion system. And gaseous K and KOH will catalyze each other and combine with H and OH to form stable H2O, which significantly reduces the mass fraction of H and OH radicals in the chain reaction. The results of this paper provide a theoretical basis for the safe use of hydrogen-enriched methane fuel and the research and development of explosion emergency prevention and control technology.

Effect of hydrogen-blending ratio and wall temperature on establishment, NO formation, and heat transfer of hydrogen-enriched methane MILD combustion

Hydrogen-enriched methane moderate or intense low-oxygen dilution (MILD) combustion is a promising technology that offers an attractive pathway to achieve low NOx and CO2 emissions in furnaces and boilers. This paper numerically investigates the influence of hydrogen-blending ratios (XH2 = 0–100 %) and wall temperature (Twall = 1500–300 K) on establishment, NO formation/reduction mechanisms, and heat transfer behaviors of hydrogen-enriched methane MILD combustion with non-preheated air in a laboratory-scale furnace. Results show that hydrogen addition increases the peak Damköhler number from 7.5 to 20.3, which could cause departure from MILD combustion at high wall temperatures. However, lowering Twall can slow down the MILD reaction rate and thus reduce Damköhler number closer to unity, which is beneficial for achieving hydrogen-enriched methane MILD combustion. Interestingly, as XH2 is increased from 0 to 100 %, the threshold wall temperature for achieving MILD combustion is extended from 1170 to 690 K. In other words, MILD combustion could be sustained/stabilized under low Twall conditions (encountered in boilers) when burning pure hydrogen or fuels with a high percentage of hydrogen. Moreover, as Twall is lowered, the major NO formation pathway is changed from thermal-NO to NNH and N2O-intermediate in pure hydrogen MILD combustion; also, H reburning via the pathway of NO→+HHNO→+HNH→+NON2 becomes more important in reducing NO at a lower Twall level. As XH2 is increased from 0 to 100 %, the radiative heat transfer is enhanced by 29.4 % while the convective heat transfer is reduced by 16.8 %, ultimately resulting in an increase of 15.0 % in the total heat transfer (Twall = 1300 K). Along the furnace sidewall, the distribution of heat flux has a low value near the burner exit, peaks in the downstream furnace, and declines further downstream close to the furnace exit in MILD combustion with non-preheated air.

Transient Combustion Characteristics of Methane-Hydrogen Mixtures in Porous Media Burner.

The combustion of conventional methane-hydrogen mixtures is associated with challenges such as combustion instability and excessive pollutant emissions. This study explores the advantages of porous media, which include a wide operating range, enhanced combustion stability, high combustion efficiency, and reduced pollutant emissions. We conducted numerical transient simulations to investigate methane-hydrogen combustion within a porous media, focusing on a cylindrical double-layer porous burner geometry. The research analyzes the temperature, combustion rate, and diffusion characteristics of the methane-hydrogen-precipitated gas flame within the porous media. Additionally, it examines variations in the position and width of the high-temperature region along with changes in carbon and nitrogen emissions. The computations were carried out for different hydrogen blending ratios over the time interval of 0-0.4 s. The results unveil the transient combustion characteristics of hydrogen-enriched methane within a porous media, offering valuable insights for the subsequent optimization of porous media burners (PMB). This study provides a theoretical foundation for enhancing the efficiency and environmental performance of combustion processes involving methane-hydrogen mixtures.

Experimental investigation of the effect of the electromagnetic field on the stability of the hydrogen enriched methane flame under acoustic enforcement

ABSTRACT The current research has investigated the effects of magnetic fields on molecular structures, especially on flame formation, flame behavior and emission gases. This study particularly examines the phenomenon of combustion instability and emission characteristics of CH4 and CH4/H2 fuels under lean combustion conditions and under external acoustic forcing. Conducted within a combustor utilizing premixing and swirl techniques, experiments strategically generated magnetic fields in specific regions: the burner input, pre-mixer, and fuel supply lines. Trials maintained a constant magnetic field intensity of 3500 gauss, with a thermal output of 3 kW, swirl number of 1, and equivalency ratio of 0.7. Evaluation of acoustic resonance was performed at 95 Hz and 175 Hz frequencies within the combustion chamber. Findings suggest that applying a magnetic field positively impacts the combustion process, reducing instabilities in fuels like CH4 and CH4/H2, especially at a 95 Hz frequency. During pure methane combustion, CO emissions initially measured 4785 ppm but decreased to 4143 ppm under the magnetic field’s influence. Introducing the magnetic field to the pre-mixer increased CO emissions to 4233 ppm, while its application to the fuel line reduced emissions to 4104 ppm. For the CH4/H2 fuel mix, CO emissions decreased from 2638 ppm to 1961 ppm with the magnetic field, indicating improved combustion and reduced pollutant gases, including CO and NOx. This study highlights the potential of magnetic fields to improve combustion efficiency and address environmental concerns.

Numerical Study of Ignition and Combustion of Hydrogen-Enriched Methane in a Sequential Combustor

Ignition and combustion behavior in the second stage of a sequential combustor are investigated numerically at atmospheric pressure for pure CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document} fueling and for two CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document}-H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} fuel blends in 24:1 and 49:1 mass ratios , respectively, using Large Eddy Simulation (LES). Pure CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document} fueling results in a turbulent propagating flame anchored by the hot gas recirculation zones developed near the inlet of the sequential combustion chamber. As the H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} content increases, the combustion process changes drastically, with multiple auto-ignition kernels produced upstream of the main flame brush. Analysis of the explosive modes indicates that, for the highest H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} amount investigated, flame stabilization in the combustion chamber is strongly supported by auto-ignition chemistry. The analysis of fuel decomposition pathways highlights that radicals advected from the first stage flame, in particular OH, induce a rapid fuel decomposition and cause the reactivity enhancement that leads to auto-ignition upstream of the sequential flame. This behavior is promoted by the relatively large mass fraction of OH radicals found in the flow reaching the second stage, which is approximately one order of magnitude greater than it would be at chemical equilibrium. The importance of the out-of-equilibrium vitiated air on the ignition behavior is proven via an additional LES that features weak auto-ignition kernel formation when equilibrium is artificially imposed. It is therefore concluded that parameters affecting the relaxation towards chemical equilibrium of the vitiated flow can have an important influence on the operability of sequential combustors fueled with varying fractions of H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} blending.

Analysis of spontaneous ignition of hydrogen-enriched methane in a rectangular tube

This study investigates the spontaneous ignition of high-pressure hydrogen-enriched methane in air within a rectangular tube. A computationally efficient approach has been adopted, utilizing a reduced reaction mechanism and ignition delay model within a 3D Large Eddy Simulation (LES) framework. This approach overcomes the limitations of traditional 1D and 2D simulations with detailed chemistry models, which are unable to accurately reproduce the complex 3D shock wave structures within the tube. The simulated shock wave behavior during 9 MPa hydrogen leakage (case 1) and 11 MPa 90 vol% hydrogen/10 vol% methane mixture leakage (case 2) are found to agree well with experimental observations. In case 2, the hot spots generated by reflected shock waves and Mach reflections ignite the hydrogen/methane-air mixture, resulting in three sequential spontaneous ignitions. The flame is observed to primarily propagate along the tube corners and wall centers, with the central ignition spreading across the entire cross-section. For the 25 MPa 24 vol% hydrogen/76 vol% methane mixture leakage (case 6), the shock intensity is significantly reduced due to the increased methane proportion, leading to spontaneous ignition only at the tube corners when the hemispherical shock wave reflects from the wall. The flame predominantly forms downstream along the tube corner, gradually spreading along the tube wall. It is indicated that while the probability of spontaneous ignition decreases with increasing methane content, the risk remains significant under sufficiently high pressures. To the best our knowledge, this study represents the first 3D large eddy simulation of spontaneous ignition for high-pressure hydrogen-enriched methane leakage into air, providing valuable insights into the underlying physical phenomena.

Quantitative studies of NO emissions from various reaction pathways in swirling combustion of hydrogen-enriched methane

Hydrogen-enriched methane is a promising alternative fuel for carbon dioxide reduction. Large eddy simulations of turbulent combustion and NO emission are carried out using a new numerical method, which incorporates an improved NO prediction model and a recently-developed NO reaction pathway separation approach. Quantitative analyses of NO formations from all reaction pathways are conducted in an open swirl burner (combustion in the open environment) with hydrogen-enriched methane at a constant thermal power of 10.85 kW at the atmospheric pressure. Results indicate that different NO reaction pathways response very differently with hydrogen addition. In the open burner with hydrogen enrichment up to 60 %, the prompt NO makes the largest (around 50 %) contribution to the total NO emission, but its relative contribution decreases with hydrogen addition. The thermal NO strongly increases with hydrogen addition, but it accounts for only around 25 % of the total NO emission in hydrogen-enriched cases, because of the reduced high-temperature zone by co-flow air dilution. NO formation from the NNH pathway largely increases and contributes around 20 % to the total NO emission with hydrogen enrichment. Results would provide quantitative understanding of NO emission in the bluff-body stabilized swirling combustion of hydrogen-enriched methane in an open burner.

Assessing the cyclic variability of combustion and NO emissions in hydrogen-methane fueled HSSI engine via quasi-dimensional modeling under the influence of flame-kernel turbulence and equivalence ratio variation mechanisms

This paper assesses the cyclic variability (cycle-by-cycle variations (CCV)) of combustion, performance and nitric oxide (NO) emissions in high-speed (HS), spark-ignition (SI) engine fueled with hydrogen-enriched methane (CH4), for which experimental data are available. Thus, in-house, two-zone, quasi-dimensional turbulent combustion model is used that closely trails the flame-front movement, which was validated before regarding performance, emissions, exergy, and CCV for gasoline- or methane-run HSSI engines. It is enhanced herein to study CCV of performance, combustion, and NO emissions, with engine fueled by various hydrogen-methane blends (up to 50 % by vol. hydrogen) and equivalence ratios (EQR). Hence, the influence of flame-kernel turbulence and EQR variation mechanisms on CCV attributes is considered and assessed. The numerical results are compared with measured data of cylinder pressure, indicated mean effective pressure (IMEP), and NO emissions at ‘steady’ conditions, and further for validation against available experimental CCV results for IMEP at all operating conditions. That CCV analysis is extended to peak pressure, IMEP, and NO emissions using coefficients of variation (COV), frequency distributions, and pertinent diagrams illuminating their cycle-by-cycle variations. The CCV variations decrease with hydrogen content in the blend whereas they increase with the mixture leanness delineating the relative merits of the two mechanisms.

Sustainability of hydrogen-enriched methane MILD combustion over a wide range of hydrogen-blending ratios in a strongly heat-extracted well-stirred reactor

Moderate or intense low-oxygen dilution (MILD) combustion of methane blended with hydrogen offers a potential solution for gas-fired industrial furnaces and even boilers to realize rapid decarbonization while maintaining ultra-low NO emissions. In this work, the combustion mode map for identifying MILD combustion when burning CH4/H2 mixtures from pure methane to pure hydrogen is proposed, and then the sustainability of hydrogen-enriched methane MILD combustion over a wide range of hydrogen-blending ratios from 0 to 100% under strongly heat-extracted conditions is revealed. Results show that, there exists a threshold oxygen mole fraction (XO2*) below which hydrogen-enriched methane MILD combustion can be achieved as long as the inlet temperature (Tin) is above the self-ignition temperature of reactants. As the hydrogen-blending ratio increases from 0 to 100%, XO2* is reduced from 9.3 to 6.5% and thus the unconditional MILD combustion region is concentrated, implying that hydrogen addition requires a larger oxygen dilution level to sustain MILD combustion. Interestingly, the sustainable operating range of oxygen concentration to sustain hydrogen-enriched methane MILD combustion can be extended by increasing the heat extraction ratio (HER) properly, e.g., XO2* up to 13.2% at HER = 30% for pure hydrogen fuel. However, methane MILD combustion would become unstable and even extinguished once HER exceeds about 49%, whereas MILD combustion of methane blended with a high percentage of hydrogen could be sustained even under highly heat-extracted conditions, e.g., a maximum allowable HER of about 91% for pure hydrogen fuel. In pure hydrogen MILD combustion, heat extraction can further reduce NO emissions, mainly due to less NO production via the N2O-intermediate, NNH, and thermal pathways, and NO reduction by H radicals tends to be weakened with heat extraction. In conclusion, high-percentage hydrogen co-firing makes it possible to achieve stable MILD operation with low NO emissions under strongly heat-extracted conditions encountered in boilers.

A simplified mechanism of hydrogen addition to methane combustion for the pollutant emission characteristics of a gas-fired boiler

The reaction mechanism of hydrogen-enriched methane (HEM) combustion is significant in the combustion process. In this study, four widely used reaction mechanisms for methane combustion are simplified to investigate the pollutant emission characteristics of a gas-fired boiler used for HEM combustion. The laminar burn velocity (LBV) and ignition delay time (IDT) of the four detailed and four simplified mechanisms are compared, and the simplified San Diego mechanism is identified as the optimal mechanism using relative error and population standard deviation analysis. The temperatures and key components of the simplified San Diego mechanism are also compared with those of the detailed San Diego mechanism under various XH2. Based on the simplified San Diego mechanism, a chemical reactor network model for gas-fired boilers is further established to investigate the pollutant emissions of HEM combustion, in which the hydrogen doping ratio XH2 ranged from 0 to 80% and the inlet air and fuel temperature Tin ranged from 300 K to 600 K. Results showed that at Tin = 300 K with XH2 ranging from 0 to 80%, the mole fraction of NO increase by 1.94 times because of the increase in peak flame temperature; moreover, the mole fractions of N2O, CO, and CO2 decrease by approximately 25%, 50%, and 52%, respectively. With XH2 ranging from 0 to 40% and Tin ≥ 500 K, the mole fraction of NO decreases because the reduction reaction of NO + H = N + OH is promoted, whereas when XH2 is further increased to 80%, the increased H promotes the increase in NO again. CO emissions decreased as XH2 increased from 0 to 80%, whereas they increased by approximately 1.3 times as Tin increased from 300 K to 600 K because of the CO2 decomposition. N2O emissions decreased by more than 25% as XH2 increased from 0 to 80%. Additionally, CO2 emissions decrease by 52% with an increase in XH2 and by only 5% with an increase in Tin, indicating that XH2 has a larger impact on CO2 emissions. The proposed optimal HEM combustion mechanism is conducive to improving the efficiency of numerical calculations.

Effect of the variation of oxygen concentration on the laminar burning velocities of hydrogen-enriched methane flames

The combustion properties of hydrogen-containing fuel mixtures and the effect of the variation of the oxygen content of the oxidizer are at the center of recent research interest. Laminar burning velocity measurements with varied oxygen content can help in the validation of reaction mechanisms for better simulations of combustion systems using exhaust gas recirculation (EGR) or oxygen-enriched atmosphere. Such measurements were carried out in hydrogen-enriched methane−air flames using the heat flux method with higher accuracy and a wider range of initial oxygen and hydrogen concentrations compared to the similar studies in the literature. The mole fractions of the hydrogen and oxygen contents of the initial fuel and oxidizer mixtures were varied between xH2 = 0 and 0.20, and xO2 = 0.14 and 0.23, respectively. The initial gas temperature and pressure were 298 K and 1 bar, respectively. It is demonstrated that the increase of combustion rate by the hydrogen enrichment can be compensated with the decrease of the oxygen content. This compensating effect was investigated in detail in a wide range of equivalence ratio (φ). The experimental data were simulated with 11 widely used methane combustion reaction mechanisms. The prediction accuracies of the mechanisms at lean and rich equivalence ratios were significantly different and the important reaction steps were identified using sensitivity analysis for three mechanisms. Mechanisms POLIMI-2014 and Caltech-2015 gave the best overall predictions.

Stabilization and soot/NOx emission of hydrogen-enriched methane flames in a turbulent jet with coaxial air under elevated pressures

Adding hydrogen to methane has gained interest as a means of reducing CO2 emissions from combustion systems. However, this process can lead to several issues, such as flashback and NOx emissions. Therefore, it is essential to determine the optimal conditions for flame stabilization and emission to design effective hydrogen burners. This study chose a turbulent fuel jet with coaxial air as the basic configuration, and its combustion characteristics were investigated under elevated pressures. Additionally, the depth of the fuel tube in the coaxial air tube was actively controlled to regulate flame stabilization, and its effects were examined for various pressures, velocities, and hydrogen concentrations. Three flame stabilization modes were observed clearly, i.e., inner-attached, outer-attached, and inner-lifted modes. Furthermore, the parameters that influenced the flame modes were identified, and the characteristics of soot and NOx emissions were studied. The inner-lifted mode demonstrated the best combustion performance, and the corresponding conditions were predicted to be extended through hydrogen enrichment and pressure elevation. Therefore, the inner-lifted flame is recommended as the conceptual design goal for hydrogen-enriched high-pressure burners.

Performance enhancement of swirl-assisted distributed combustion with hydrogen-enriched methane

Effect of hydrogen addition to methane fuel on the performance of swirl-assisted distributed combustion was studied at 5.72 MW/m3-atm thermal intensity using different hydrogen-enrichment (at 0, 10, 20, and 40%) to methane fuel and CO2 and N2 as the flowfield diluents. High-speed chemiluminescence imaging performed (without spectral filtering) exhibited a gradual increase of chemiluminescence intensity along with gradually narrowing of the flame shape for both swirl and distributed combustion with increase in hydrogen-enrichment from increased reactivity in flame brush. Fluctuations of pressure (p′) and heat release (q′) were signals showed the existence of a peak in swirl combustion indicating the possibility of thermo-acoustic coupling. This peak mostly diminished in distributed combustion. Investigation of fluctuation with the proper orthogonal decomposition exhibited no prominent structures in distributed combustion. Measurement of lean blowoff equivalence ratios (ϕLBO) at different combustion conditions showed extended ϕLBO in distributed combustion indicating wider operational limits in distributed combustion. The NO emission increased in conventional swirl combustion while it consistently decreased in distributed combustion even at increased H2 enrichment to methane fuel. The exhaust CO2 gradually decreased with increase in H2 enrichment for both swirl and distributed reaction zones at constant thermal intensity. Reduced pollutants emission with hydrogen enrichment was also observed with air preheats in the range of 373–573 K to support distributed combustion for gas turbine conditions.

Optical and Thermodynamic Investigations of a Methane- and Hydrogen-Blend-Fueled Large-Bore Engine Using a Fisheye Optical System

The following paper presents thermodynamic and optical investigations of hydrogen-enriched methane combustion, showing the potential of a hydrogen admixture as a means to decarbonize stationary power generation. The optical investigations are carried out through a fisheye optical system directly mounted into the combustion chamber, replacing one exhaust valve. All of the tests were carried out with constant fuel energy producing 16 bar indicated mean effective pressure. The engine under investigation is a port-fueled 4.8 L single-cylinder large-bore research engine. The test series compared the differences between a conventional spark plug and an unscavenged pre-chamber spark plug as an ignition system. The fuel blends under investigation are 5 and 10%V hydrogen mixed with methane and pure natural gas acting as a reference fuel. The thermodynamic results show a beneficial influence of the hydrogen admixture on both ignition systems and for all variations concerning the lean running limit, combustion stability and indicated efficiency, with the most significant influence being visible for the tests using conventional spark plugs. With the unscavenged pre-chamber spark plug and the combustion of the 10%V hydrogen admixture, an increase in the indicated efficiency of 0.8% compared to NG is achievable. The natural chemiluminescence intensity traces were observed to be predominantly influenced by the air–fuel equivalence ratio. This results in a 20% higher intensity for the unscavenged pre-chamber spark plug for the combustion of 10%V hydrogen compared to the conventional spark plug. This is also visible in the evaluations of the flame color derived from the dewarped combustion image series. The investigation of the torch flames also shows a difference in the air–fuel equivalence ratio but not between the different fuels. The results encourage the development of hydrogen-based fuels and the potential to store surplus sustainable energy in the form of hydrogen in existing gas grids.

Methanol, isobutanol, kerosene, dimethylfuran, ethanol, and isopropanol additives effects on soot concentration at hydrogen-enriched methane flames

One exciting research topic is biofuels’ effects on soot formation. For this purpose, the combustion behavior of different biofuel pairs, with the main fuel being hydrogen-enriched methane, was investigated in a burner, using laser-induced incandescence (LII). The soot concentrations in several flames were observed for methanol, isobutanol, kerosene, dimethylfuran, ethanol, and isopropanol fuels coupled with hydrogen-enriched methane. Soot concentration was determined for the additive fuels and the effects of soot particles in the flame and Reynolds number on the flame oscillation and soot interaction were examined. The highest soot formation was observed in the h-methane + kerosene and led to large soot fractal aggregates. The isobutanol has 8.8% higher Re sensitivity for oscillation than isopropanol for soot concentration. Also, isobutanol causes a higher amount of soot production as Re increases. The flame area/soot area ratio decreases 1.7 times faster with dimethylfuran usage compared to kerosene. The dimethylfuran created a higher soot field in the flame by 23.9% compared to methanol and 25.9% compared to ethanol. At the tests, the lowest soot concentration was observed in the addition of isopropanol in hydrogen-enriched methane flame.

Impact of Methane and Hydrogen-Enriched Methane Pilot Injection on the Surface Temperature of a Scaled-Down Burner Nozzle Measured Using Phosphor Thermometry

The surface temperature of a burner nozzle using three different pilot hardware configurations was measured using lifetime phosphor thermometry with the ZnS:Ag phosphor in a gas turbine model combustor designed to mimic the Siemens DLE (Dry Low Emission) burner. The three pilot hardware configurations included a non-premixed pilot injection setup and two partially premixed pilot injections where one had a relatively higher degree of premixing. For each pilot hardware configuration, the combustor was operated with either methane or hydrogen-enriched methane (H2/CH4: 50/50 in volume %). The local heating from pilot flames was much more significant for hydrogen-enriched methane compared with pure methane due to the pilot flames being in general more closely attached to the pilot nozzles with hydrogen-enriched methane. For the methane fuel, the average surface temperature of the burner nozzle was approximately 40 K higher for the partially premixed pilot injection configuration with a lower degree of mixing as compared to the non-premixed pilot injection configuration. In contrast, with the hydrogen-enriched methane fuel, the differences in surface temperature between the different pilot injection hardware configurations were much smaller due to the close-to-nozzle frame structure.