Influence of fuel composition on chemiluminescence emission in premixed flames of CH4/CO2/H2/CO blends

The growing concern about pollutant emissions and depletion of fossil fuels has been a strong motivator for the development of cleaner and more efficient combustion strategies, such as the gasification of coal, biomass or waste, which have increased the interest in using a new type of fuels, mainly composed of CH₄, H₂, CO and CO₂.These new fuels, commonly called syngas, display a wide range of compositions, which affects their combustion characteristics and, in some cases, are more prone to instabilities or flashback. Since flame properties have been demonstrated to be strongly related to equivalence ratio, a precise measurement of the flame stoichiometry is a key pre-requisite for combustion optimization and prevention of unstable regimes. In particular, chemiluminescence emission from flames has been largely tested for stoichiometry monitoring for methane flames, but its use in syngas flames has been far less studied. Consequently, the main goal of this work is analyzing the effect of fuel composition on the chemiluminescence vs. equivalence ratio curves for different fuel blends, as a first approach for a wide range of syngas compositions. The experimental results revealed that the ratio OH*/CH*, which had been widely demonstrated to be the best option for methane, may not be suitable for monitoring with certain fuels, such as those with a high percent of hydrogen. Alternatively, other signals, in particular the ratio OH*/CO₂*, appear as viable stoichiometry indicators in those cases.The analysis was also completed by numerical predictions with CHEMKIN. The comparisons of calculations with different flame models and experimental data reveals differences in the chemiluminescence vs. equivalence ratio curves for the different combustion regimes, depending on the range of the equivalence ratio ranges and fuel compositions. This finding, which confirms previous observations for a much narrower range of fuels, could have important practical consequences for the application of the technique in real combustors.     

A numerical study on combustion and emission characteristics of premixed hydrogen air flames

Main challenges for micro power generators that utilize combustion process for energy production are inadequate residence time, destructive radical wall interactions and intensified heat loss which are mainly rooted from size limitation of such devices. To achieve high and uniform energy output, and bring in a solution to these challenges in an environment friendly manner without any kind of fundamental modification, effect of equivalence ratio on combustion and emission behavior of premixed hydrogen/air flames is numerically investigated in this study. For this purpose, an experimentally tested micro cylindrical combustor model is constructed and premixed hydrogen/air combustion in this model is simulated by varying equivalence ratio between 0.5 and 1.2 to find an optimal equivalence ratio with respect to drawbacks of micro power generators. Combustion and turbulence models implemented in this study are Eddy Dissipation Concept and Standard k-ε models, respectively. A detailed hydrogen/air reaction mechanism which consists of 9 species and 19 steps is employed to accurately gain insight into combustion process. Simulation results show that as the equivalence ratio decreases; centerline temperature distribution gets a lower value and the place where chemical reactions take place moves downstream. The most uniform temperature distribution is achieved between 0.8 and 1.0 equivalence ratios. The highest NO_x formation is at 0.9 equivalence ratio and its mass fraction decreases sharply when the equivalence ratio reduces from 0.9 to 0.5.     

Combustion behavior and stability map of hydrogen-enriched oxy-methane premixed flames in a model gas turbine combustor

The article describes an experimental study and comparison of the combustion behavior and determines the stability map of turbulent premixed H₂-enriched oxy-methane flames in a model gas turbine combustor. Static stability limits, in terms of flashback and blow-out limits, are recorded over a range of hydrogen fraction (HF) at a fixed oxygen fraction (OF) of 30% and a particular inlet bulk velocity, and the results are compared with the non-enriched case (HF = 0%). The static stability limits are also recorded for different inlet bulk velocity (4.4, 5.2, and 6 m/s) and the results are compared to explore the effect of flow dynamics on operability limits of H₂-enriched flames. The stability maps are presented as a function of equivalence ratio (0.3–1.0) and HF (0%–75%) plotted on the contours of adiabatic flame temperature (AFT), power density (PD), inlet Reynolds number (Re) and reacting mixture mass flow rate ($\dot{m}$) to understand the physics behind flashback and blow-out phenomena. The results indicated that both the flashback and blow-out limits tend to move towards the leaner side with increasing HF due to the improved chemical kinetics. The stability limits are observed to follow the Reynolds number indicating its key role in controlling flame static stability limits. The results showed that H₂ enrichment is effective in the zone from HF = 20% up to HF = 50%, and O₂ enrichment is also effective in a similar zone from OF = 20% up to 50%, with wider stability boundaries for H₂ enrichment. Axial and radial temperature profiles are presented to explore the effect of HF on the progress of chemical reactions within the combustor and to serve as the basis for validation of numerical models. Flame shapes are recorded using a high-speed camera and compared for different inlet velocities to explore the effects of H₂-enrichment and equivalence ratio on flame stability. The equivalence ratio at which a transition of flame stabilization from the inner shear layer (ISL) to the outer recirculation zone (ORZ) occurs is determined for different inlet bulk velocities. The value of the transition equivalence ratio is found to decrease while increasing the inlet bulk velocity. Flame shapes near flashback limit, as well as near blow-out limit, are compared to explore the mechanisms of flame extinctions. Flame shapes are compared at fixed adiabatic flame temperature, fixed inlet velocity and fixed flow swirl to isolate their effects and investigate the effect of kinetic rates on flame stability. The results showed that the adiabatic flame temperature does not govern the flame static stability limits.     

Experimental study on the effect of diluent gas on H2/CO/air mixture turbulent premixed flame

To study the effects of different diluents on the propagation characteristics of H₂/CO/air mixture turbulent premixed flames, a series of experiments were carried out in a turbulent premixed flame experimental system. The effects of turbulence intensity (0.49–1.31 m/s), dilution gas content (10%, 20%, and 30%), hydrogen fraction (50%, 70%, and 90%), and equivalence ratio (0.6, 0.8, and 1.0) on the turbulent premixed flame were studied. The results show that with the increase in hydrogen fraction or turbulence intensity or equivalence ratio, the S_T and u_t increase at the same radius. Compared with N₂ dilution, CO₂ dilution showed a more obvious inhibition effect on S_T. With the increase of Ka, S_T;35mm/u’ gradually decreased, and the extent of S_T;35mm/u’ decrease gradually became smaller. As the intensity of turbulence increases or the hydrogen fraction increases, the slope of S_T,35mm/u’ with Da/Le gradually decreases. In the turbulence intensity range of this experiment, the u_t,35mm/μ_l under nitrogen dilution condition has a larger floating range. The growth rate of u_t,35mm/μ_l at a low equivalence ratio is significantly higher than that at a high equivalence ratio.     

Experimental investigation on performance of hydrogen additions in natural gas combustion combined with CO2

Natural gas with H₂ is widely used for lean-burn combustion, which leads to NO_x emission as the main problem for it. For decreasing NO_x emission and increasing thermal efficiency, the investigation on seeking the influence of H₂ fractions on the mixture of CH₄ and CO₂ was conducted. Firstly, the ignition timing was decided through thermal efficiency and brake mean effective pressure (BMEP) for CH₄ only. Then, combustion characteristics of CH₄, CH₄+CO₂ and CH₄+CO₂+H₂ were compared with volume percentage of H₂ changing from 5% to 30%. Finally, the H₂ injection strategy was checked between closed and open valve injections. Among these discussions, thermal efficiency, power output, BMEP and fuel consumption were evaluated. Results show that CO₂ addition decreases power output and BMEP, leading to much more fuel consumption and lower thermal efficiency. When H₂ is added, at the rich mixture conditions (λ＜1.0), power output and thermal efficiency decrease sharply as the mixture is enriched. However, at the lean-burn conditions (λ＞1.0), the decrease in flow rate of lower heating value (LHV) and increase in power output finally result in the higher efficiency with H₂ addition. Moreover, when λ＞1.0, both low fuel consumption and high efficiency can be obtained with H₂ addition to achieve the high BMEP. Furthermore, the open valve injection could obtain higher thermal efficiency, power output and BMEP with lower fuel consumption, suggesting that the H₂ injection strategy should be well controlled with the ignition timing.     

An experimental study of combustion and emissions in a spark-ignition engine fueled with coal-bed gas

Coal-bed gas has been considered an attractive alternative fuel for internal combustion engines due to its abundant source and low emissions. In the present study, a combustion system with a swirl chamber has been developed for a spark-ignition engine using coal-bed gas. Detailed experiments have been carried out to investigate the combustion and emission characteristics of the engine operating with three different grades of coal-bed gas. The results have shown that this combustion system allows satisfactory operation of the engine with a wide range of methane content in the supplied coal-bed gas. For all tested conditions, the CO emission has a maximum value of 0.062%, and the HC emission is less than 380 ppm. The NO emission increases with the engine load but is less than 1800 ppm, demonstrating a great advantage of coal-bed gas as a relatively clean engine fuel.     

Study of ozone-enhanced combustion in H2/CO/N2/air premixed flames by laminar burning velocity measurements and kinetic modeling

The enhancement effect of ozone addition for H₂/CO/N₂/Air premixed flames at ambient condition is investigated both experimentally and computationally. Adiabatic laminar velocities under different amount of O₃ addition were directly measured using the Heat Flux Method. The ozone concentration in the oxidizer is monitored online to ensure the precise control and stability of ozone injection. Experimental data shows significant enhancement of the burning velocities due to O₃ addition. With 8500 ppm ozone seeded, maximum 18.74% of burning velocity enhancement is observed at equivalence ratio Φ = 0.7. Kinetic modeling works were conducted by integrating ozone sub-mechanism with three kinetic mechanisms: GRI-Mech 3.0, Davis mechanism and USC Mech II. The modeling results were compared with experimental data. GRI-Mech 3.0 + Ozone mechanism demonstrated the ability to reproduce the experimental data. Extra OH radicals promoted by ozone was found in the pre-heat regime which initiates the chain-branching reaction and results in the combustion enhancement.     

Numerical investigation on mixing performance and diffusion combustion characteristics of H2 and air in planar micro-combustor

In the present work, the effects of inlet velocity and channel height (H₀ = 0.6 mm, 1.0 mm and 1.4 mm) on the mixing performance, flame stability limit and combustion efficiency of H₂ and air in a 2D planar micro-combustor with a separating plate were studied numerically. The results demonstrate that improved mixing can be achieved with a decrease in inlet velocity and channel height. Moreover, the flame blow-off limit is the largest for a micro-combustor with H₀ = 0.6 mm; the flame becomes inclined at a high velocity and the direction varies with the inlet velocity. Furthermore, a micro-combustor with a medium height (H₀ = 1.0 mm) can achieve the largest blowout limit among the three cases. Finally, for identical inlet velocities, the combustion efficiency increases with decreasing combustor height. In summary, these findings can provide a guideline for the optimal design of such micro-combustors.     

Numerical investigation of the effects of H2/CO/syngas additions on laminar premixed combustion characteristics of NH3/air flame

Co-firing NH₃ with H₂/CO/syngas (SYN) is a promising method to overcome the low reactivity of NH₃/air flame. Hence, this study aims to systematically investigate the laminar premixed combustion characteristics of NH₃/air flame with various H₂/CO/SYN addition loadings (0–40%) using chemical kinetics simulation. The numerical results were obtained based on the Han mechanism which can provide accurate predictions of laminar burning velocities. Results showed that H₂ has the greatest effects on increasing laminar burning velocities and net heat release rates of NH₃/air flame, followed by SYN and CO. CO has the most significant effects on improving NH₃/air adiabatic flame temperatures. The H₂/CO/SYN additions can accelerate NH₃ decomposition rates and promote the generation of H and NH₂ radicals. Furthermore, there is an evident positive linear correlation between the laminar burning velocities and the peak mole fraction of H + NH₂ radicals. The reaction NH₂ + NH <=> N₂H₂ + H and NH₂ + NO <=> NNH + OH have remarkable positive effects on NH₃ combustion. The mole fraction of OH × NH₂ radicals positively affects the net heat release rates. Finally, it was discovered that H radicals play an important role in the generation of NO. The H₂/CO/SYN additions can reduce the hydrodynamic and diffusional-thermal instabilities of NH₃/air flame. The NH₃ reaction pathways for NH₃–H₂/CO/SYN-air flames can be categorized mainly into NH₃–NH₂–NH–N–N₂, NH₃–NH₂–HNO–NO(?N₂O)–N₂ and NH₃–NH₂(?N₂H₂)–NNH–N₂. CO has the greatest influence on the proportions of three NH₃ reaction routes.     

Effect of different turbulence models on combustion and emission characteristics of hydrogen/air flames

This paper aims to present modeling results of hydrogen/air combustion in a micro-cylindrical combustor. Modeling studies were carried out with different turbulence models to evaluate performance of these models in micro combustion simulations by using a commercially available computational fluid dynamics code. Turbulence models implemented in this study are Standard k-ε, Renormalization Group k-ε, Realizable k-ε, and Reynolds Stress Transport. A three-dimensional micro combustor model was built to investigate impact of various turbulence models on combustion and emission behavior of studied hydrogen/air flames. Performance evaluation of these models was executed by examining combustor outer wall temperature distribution; combustor centerline temperature, velocity, pressure, species and NO_x profiles. Combustion reaction scheme with 9 species and 19 steps was modeled using Eddy Dissipation Concept model. Results obtained from this study were validated with published experimental data. Numerical results showed that two equation turbulence models give consistent simulation results with published experimental data by means of trend and value. Renormalization Group k-ε model was found to give consistent simulation results with experimental data, whereas Reynolds Stress Model was failed to predict detailed features of combustion process.     

Effect of natural gas enrichment with hydrogen on combustion process and emission characteristic of a dual fuel diesel engine

The paper presents results of experimental research on a dual-fuel engine powered by diesel fuel and natural gas enriched with hydrogen. The authors attempted to replace CNG with hydrogen fuel as much as possible with a constant dose of diesel fuel of 10% of energy fraction. The tests were carried out for constant engine load of IMEP = 0.7 MPa and a rotational speed of n = 1500 rpm. The effect of hydrogen on combustion, heat release, combustion stability and exhaust emissions was analyzed. In the test engine, the limit of hydrogen energy fraction was 19%. The increase in the fraction caused an increase in the cycle-by-cycle variation and the occurrence of engine knocking. It was shown that the enrichment of CNG with hydrogen allows for the improvement in the combustion process compared to the co-combustion of diesel fuel with non-enriched CNG, where the reduction in the duration of combustion by 30% and shortening the time of achieving 50% of MFB by 50% were obtained. The evaluation of the spread of the end of combustion is also presented. For H₂ energetic share over 20%, the spread of end of combustion was 48° of crank angle. Measurement of exhaust emissions during the tests revealed an increase in THC and NO_x emissions.     

Experimental investigation of premixed combustion limits of hydrogen and methane additives in ammonia

In this study, a specially designed premixed combustion chamber system for ammonia-hydrogen and methane-air laminar premixed flames is introduced and the combustion limits of ammonia-hydrogen and methane-air flames are explored. The measurements obtained the blow-out limits (mixed methane: 400–700 mL/min, mixed hydrogen: 200–700 mL/min), mixing gas lean limit characteristics (mixed methane: 0–82%, mixed hydrogen: 0–37%) and lean/rich combustion characteristics (mixed methane: ? = 0.6–1.9, mixed hydrogen: ? = 0.9–3.2) of the flames. The results show that the ammonia-hydrogen-air flame has a smaller lower blow-out limit, mixing gas ratio, lean combustion limit and higher rich combustion limit, thereby proving the advantages of hydrogen as an effective additive in the combustion performance of ammonia fuel. In addition, the experiments show that increasing the initial temperature of the premixed gas can expand the lean/rich combustion limits of both the ammonia-hydrogen and ammonia-methane flames.     

Effect of radiation emission and reabsorption on flame temperature and NO formation in H2/CO/air counterflow diffusion flames

The radiation effect on flame temperature and NO emission of H₂-lean (0.2H₂ + 0.8CO) and H₂-rich (0.8H₂ + 0.2CO) syngas/air counterflow diffusion flames was numerically investigated using OPPDIF code incorporated with the optical thin model, statistical narrow band model and adiabatic condition. Firstly, the coupled effect of strain rate and radiation was studied. Disparate tendencies of NO emission with an increasing strain rate between H₂-lean and H₂-rich syngas flames were found at very small strain rate, and the effect of radiation reabsorption on NO formation can be neglected when the strain rate was greater than 100 s^?1 for both H₂-lean and H₂-rich syngas flames. Because the radiation effect is vital to flames with small strain rate, its impact on flame temperature and NO emission was investigated in detail at a strain rate of 10 s^?1. The results indicated that NO formation is more sensitive to radiation reabsorption than flame temperature, especially for the H₂-rich syngas flame. The underlying mechanism was discovered by using reaction pathway analysis. Furthermore, the radiation effect under CO₂ dilution of the syngas fuel was examined. It was demonstrated that the radiation effect on flame temperature became more prominent with the increase of CO₂ concentration for both H₂-lean and H₂-rich syngas. The radiation effect on NO emission increased first and then decreased with an increasing CO₂ content for H₂-lean syngas, whereas for H₂-rich syngas the radiation effect is monotonic.     

Numerical analysis of the effect of CO2 on combustion characteristics of laminar premixed methane/air flames

The freely-propagating laminar premixed flames of (CH₄+CO₂)/air mixtures were calculated with the PREMIX code at various CO₂ contents (0–0.35) and equivalence ratios (0.7–1.3). The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of CO₂ addition. The chemical effects of CO₂ and the changes of the mole fraction on the important active radicals CH₃, OH, H and O and the sensitivity of the main reactions contributing to their information were analyzed. The results show that with the increase of X_CO2, the laminar burning velocity and the adiabatic flame temperature are decreased. Moreover, the amount of NO_x produced and the mole concentrations and the net rates of main reactions of CH₃, OH, H and O also decrease as CO₂ is added. The dominant reactions responsible for the four free radicals are R38 H + O₂ = OH + H, followed by R52 H + CH₃ (+ M) = CH₄ (+ M) and R35 H + O₂ + H₂O = H₂O + HO₂.     

Nozzle effect on heat transfer and CO emission of impinging premixed flames

Experiments were carried out to investigate the heat transfer and CO emission characteristics of a premixed LPG/air circular flame jet impinging upwards normal to a flat plate. The effects of nozzle diameter and nozzle arrangement on the heat transfer and CO emission under different fuel/air mixture flow rates (Q_mix), equivalence ratios (Ф) and normalized nozzle-to-plate distances (H/d) were examined. For the effect of nozzle diameter, burners of nozzle diameters of d = 7.9, 9 and 10 mm were used, and for the effect of nozzle arrangement, a twin-nozzle burner and a triple-nozzle burner, each with a cross-sectional area equal to that of the 9 mm diameter burner, were investigated under different normalized jet-to-jet spacing, S/d, of 3, 5 and 7. The heat transfer rate and CO emission index (EICO) are enhanced significantly with the decrease in the nozzle diameter for the single-nozzle flames. For the twin- and triple-nozzle flames, when the other operational conditions including Q_mix, Ф and H/d are invariant, the moderate S/d of 5 gives the highest heat transfer rate, whereas the EICO increases with increasing S/d. Comparison of the flames from all the burners shows that the highest heat transfer rate and EICO are obtained on the single-nozzle burner with the smallest nozzle diameter while the lowest heat transfer rate and EICO are obtained on the triple-nozzle burner with the smallest S/d.     

Ultra-lean combustion through the backflow of burned gas in rotating counterflow twin premixed flames

Rotating counterflow twin premixed flames of methane–air were numerically simulated with detailed chemistry based on a similarity solution to explore the leanest extinction limit without preheating and to elucidate the mechanism of “ultra-lean” combustion. We focused on high rotation rate cases in which unrealistic backflow from infinity is allowed to occur since ultra-lean combustion was found to be realized in such a situation. It was found that the reaction zone is in the backflow zone, where the flame’s apparent burning velocity is negative, and that the flame zone width is much smaller than that of a 1-D planar premixed flame due to an inversion of the convexity directions of the profiles of temperature and main species concentrations. The decrease of the flame width seems to make the flame less extinguishable. The equivalence ratio of the leanest flame obtained neglecting radiative heat loss is 0.32, while that obtained with an optically thin radiation model is 0.42, which is still much leaner than the ratio of 0.49 for a 1-D planar premixed flame generated using the same radiation model.     

Numerical study on CH4 laminar premixed flames for combustion characteristics in the oxidant atmospheres of N2/CO2/H2O/Ar-O2

The freely-propagating laminar premixed flames of CH₄–N₂/CO₂/H₂O/Ar-O₂ mixtures were conducted with the PREMIX code. The effects of the equivalence ratio and various oxidant atmospheres on the basic combustion characteristics were analyzed with the initial pressure and temperature of 1 atm and 398 K, respectively, O₂ content in the oxidant of 21%. The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of the oxidant atmospheres of N₂/O₂, CO₂/O₂, H₂O/O₂, and Ar/O₂ on the adiabatic flame temperature, laminar burning velocity, flame structure, free radicals, intermediate species, net heat release rate and specific heat of the fuel/oxidant mixtures. The numerical results show that the maximum adiabatic flame temperatures and laminar burning velocities are at Ar/O₂ atmosphere. The mole fractions of CO and H₂ increased fastest at CO₂/O₂ atmosphere and H₂O/O₂, respectively. The mole fractions of CH₃ and H follow the order Ar/O₂> N₂/O₂>H₂O/O₂>CO₂/O₂. In addition, for 4 oxidant atmospheres, the peak mole fraction of C₂H₂ is following the order H₂O/O₂>Ar/O₂>N₂/O₂>CO₂/O₂ and the net heat release rate is following the order Ar/O₂>N₂/O₂>H₂O/O₂>CO₂/O₂ for all equivalence ratios.