Syngas Flames Research Articles

Two-dimensional temperature characteristics of syngas flames with nitrogen, carbon dioxide, and steam diluents by planar laser-induced fluorescence thermometry

ABSTRACTIn this research, the effects of nitrogen, carbon dioxide, and steam diluents on the two-dimensional temperature structures and characteristics of syngas flames were investigated by two-line planar laser-induced fluorescence thermometry. The spatial resolution and the total uncertainty of the measured results were 41.7 µm and ±8.45%, respectively. The experimental results demonstrated that diluting with carbon dioxide in the syngas flame resulted in the lowest flame temperature, followed by steam diluent. The measured temperature fields for these three diluents exhibited a symmetrical but nonuniform structure. It was found that the high-temperature region exists in the downstream of flame front, and the temperature decreases gradually inside-out in the burned zone of syngas flame. The findings in this work will improve our understanding of the temperature characteristics for the syngas flames with different diluents.

Effects of nitrogen dilution on the NOx and CO emission of H2/CO/CH4 syngases in a partially-premixed gas turbine model combustor

In this study, the nitrogen dilution effect in H2/CO/CH4 syngas flames on the characteristics of NOx and CO emissions were investigated in a partially premixed gas turbine model combustor. Nitrogen was diluted into the fuel side and combustor liner temperature, and the flame structures were observed in order to find information that was critical for analyzing the results of the NOx and CO emissions. The addition of nonflammable N2 was effective in decreasing the combustion temperature, which was attributed to the reduction in the reaction rate of syngas oxidation as well as the increase in the overall heat capacity of the air-fuel mixture at a constant heat input. NOx suppression was successfully achieved up to the one digit ppmv level for all heat input and all fuel composition by the 60%-fuel side dilution of nitrogen. However, increases in nitrogen dilution increased the possibility of CO emission; therefore, extremely high CO was emitted in a 100%-CO flame at a low load with 60%-nitrogen dilution reaching 10,000 ppmv. The dilution of nitrogen in syngas considerably changed the flame structure, such as flame length, thickness, angle, location, and intensity. These flame structures provided valuable clues for the analysis of the NOx and CO emission results in relation to the flame temperature results. Based on the integration of the test results, a cause-and-effect diagram of NOx and CO emissions was plotted, and the root causes of NOx suppression were determined to be the decreases in both temperature and residence time in the hot combustion zone, and the critical path of these root causes was clarified. Although the nitrogen dilution technique has been investigated in detail, and the effects are fully understood, the careful monitoring of CO and flame detachment from the fuel nozzle is recommended when applying the nitrogen dilution technique to reduce NOx emission, especially in cases of low load operation. A practical dataset was collected from the combustion test at gas turbine relevant conditions for a wide range of fuel compositions. These results and the knowledge gained from this study could be usefully applied for the environmentally friendly and safe operation of integrated gasification combined cycle and synthetic natural gas firing gas turbine power plants.

Effects of inert dilution on the lean blowout characteristics of syngas flames

The paper presents an experimental study on the lean blowout (LBO) characteristics of inert-diluted syngas fuels. Real syngas fuels often contain inert diluents such as N2 and CO2. The diluents have negative impact on the syngas flame stability, which needs to be understood for the design and operation of gas turbine and industrial burners. In this work, effects of diluents on the LBO of premixed syngas flames were systematically studied under atmospheric pressure using a swirling flow model combustor. LBO limits, i.e., the equivalence ratio at which the flame blows out, were measured for syngas mixtures with various dilution ratios. Results show that the LBO limits generally increase with dilution ratio. In particular, inert dilution dominates the LBO behavior of syngas which has low H2 content. Detailed analyses based on a well stirred reactor model reveal that the experimental data can be well explained by the thermal and chemical kinetics effects of diluents. A physics-based correlation, using Damköhler number and normalized flame temperature, is proposed for the prediction of LBO limits of inert-diluted syngas flames.

The effects of humidity on combustion characteristics of a nonpremixed syngas flame

With the rapid development of economy, environmental protection and energy shortage are becoming more and more acute in China. The Integrated Gasification humid air (IGHAT) cycle is highlighted in providing efficient and clean options for meeting future coal-utilizing power generation needs. Syngas combustion in humid air is a bottleneck problem in IGHAT cycle for power generation. This paper carried out experiments on a non-premixed model combustor, equipped with a double-swirled syngas burner. Planar laser induced fluorescence (PLIF) of OH radical measurement is adopted to identify main reaction zones and burnt gas regions as well. With the temperature and emission measurements in the exhaust section, some important characteristics of the syngas flame are investigated overall. And the effects of the CO/H2 molar ratio consisting of syngas fuel are investigated under different humidity. With the increase of CO/H2 ratios, the concentration field of OH radicals is gradually away from the nozzle exit, and the nozzle exit almost no existence of OH radicals, forming a typical lifted flame. In addition, fluorescent signal strength of OH radicals pronounced weakening, the flame gradually appeared W type distribution and more and more obvious with the increased of humidification amount. At the same time there are almost no changes of the average exhaust temperature of combustor or CO and NOx emissions. The study can provide a reliable database for high moisture gas turbine combustor design and combustion numerical simulation.

Numerical study of the distribution of oxygen atoms in the combustion products of CO/H2/air flames

The distribution of air oxygen in the oxidation products of H 2 and CO in syngas flames has been studied by numerical simulation using the tracer method. The reaction pathways and the role of the oxygen atom of the CO molecule in heat release in these flames have been proposed.

Effect of H2O Addition on the Flame Front Evolution of Syngas Spherical Propagation Flames

ABSTRACTAn experimental study on the cellular instability of spherical propagation syngas flames with H2O addition was performed in a constant volume combustion bomb over a wide range of CO/H2 ratios at elevated pressures. Schlieren images were recorded to observe and analyze the flame front evolution of spherical propagation flames. Results show that the cellular instability of syngas flames is promoted at atmospheric pressure and suppressed at elevated pressure with H2O addition for syngas mixtures with CO/H2 ratio of 80/20. At elevated pressure, critical radius, beyond which flame front starts to accelerate, increases with H2O addition for syngas mixtures with higher CO/H2 ratios of 65/35 and 50/50, while decreases for syngas mixtures with lower CO/H2 ratios of 20/80 and 0/100. Critical Peclet number decreases with H2O addition for all of the mixtures. The acceleration exponent increases with the flame radius and reaches a constant value at some points. This implies that both self-acceleration and self-similarity regions exist in spherical syngas flames. The acceleration exponent in a self-similar region changes little with H2O addition due to the minor change of intrinsic flame instability.

A numerical investigation of structure and NO emissions of turbulent syngas diffusion flame in counter-flow configuration

This paper reports a numerical simulation of turbulent non-premixed counter-flow syngas flames structure and NO emissions at a high strain rate with a special focus on mixing. The analysis is conducted over a wide range of hydrogen percentage (H2/CO ratio between 0.4 and 2.0) and operating pressure (from 1 to 10 atm). Numerical model is based on RANS (Reynolds Averaged Navier–Stokes) technique including k-ε turbulence model. SLFM (Steady laminar flamelet model) is used for flame structure calculations and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Mixing is described by mixture fraction and its variance. Radiation effects are also considered.Computational results show an improvement of mixing with hydrogen enrichment and ambient pressure rise. Maximum flame temperature decreases with H2 addition and increases with pressure. NO levels decrease towards hydrogen-rich syngas flames and increase with pressure. Zeldovich route is found to be the main NO formation path in the operating conditions considered.

Radiative heat release from premixed oxy-syngas and oxy-methane flames

The abundant supply of coal globally and its energy content per unit mass make the use of this fuel an attractive option for power generation. However, with increasingly strict environmental regulations imposed on greenhouse gas emission technologies the future use of coal in its current form is questionable. One solution to reduce pollutant emissions is oxy-combustion with coal-derived syngas, which can provide a method of using coal efficiently with carbon capture. This paper presents a study of the radiative heat release properties from oxy-syngas and oxy-methane flames. This is relevant since the radiative properties of prospective oxy-combustion systems are not fully known. In many oxy-fuel systems the flue gas stream is recycled and premixed with the fuel and oxidizer to reduce flame temperatures. It is estimated that the radiative heat transfer from these recirculated gases will significantly increase due to the increased emission of radiation from CO2 and H2O in the 1.2μm to 5μm wavelength range, which can impact future combustor design models. Motivated by this, this work aims to provide a study of the global and spectral radiation properties of oxy-syngas combustion and how relevant variables affect radiative emissions. Theses variables include the effect of CO2 acting as a diluent, percentage of H2 in the fuel, firing input, and the effects of equivalence ratio on the flame’s radiative heat release and spectral radiation of CO2 and H2O. The current study reveals that the radiative heat release factor of syngas flames decreases at higher firing inputs. It was also observed that the radiative heat factor decreased at high hydrogen concentrations. An increase in heat release factor is also measured at higher recirculation ratios of CO2.

The Effects of Fuel Mixtures in Nonpremixed Combustion for a Bluff-Body Flame

A numerical investigation is presented assessing the effects of hydrogen compositions and nonflammable diluent mixtures on the combustion and NO emission characteristics of syngas nonpremixed flames for a bluff-body burner. An assessment of turbulent nonpremixed modeling techniques is presented and is compared with the experiments of Correa and Gulati (1992, “Measurements and Modeling of a Bluff Body Stabilized Flame,” Combust. Flame, 89(2), pp. 195–213). The realizable k–ε and the Reynolds stress (RSM) turbulence models were found to perform the best. As a result, increased hydrogen content caused the radial velocity and strain rate to decrease, which was important for mixing whereby NO production decreased. Also, the effectiveness of nonflammable diluent mixtures of N2, CO2, and H2O was characterized in terms of the ability to reduce NO emission in syngas nonpremixed flames. Results indicated that CO2 was the most effective diluent to reduce NO emission and H2O was more effective than N2. CO2 produced low levels of OH radical, which made CO2 the most effective diluent. Although H2O increased OH radicals, it was still effective to reduce thermal NO because of its high specific heat. It will be numerically shown that hydrogen concentration in the H2/CO/N2 flame does not significantly affect temperature but dramatically decreases NO emission, which is important for industrial applications that can use hydrogen in syngas flames.

Nitric oxide pollutant formation in high hydrogen content (HHC) syngas flames

Three-dimensional direct numerical simulations (DNS) of high hydrogen content (HHC) syngas nonpremixed jet flames with a Reynolds number of Re = 6000 have been carried out to study the nitric oxide (NO) pollutant formation. The detailed chemistry employed is the GRI 3.0 updated with the influence of the NCN radical chemistry using flamelet generated manifolds (FGM). Preferential diffusion effects have been considered via FGM tabulation and the reaction progress variable transport equation.The DNS based quantitative results indicate a strong correlation between the flame temperature and NO concentration for the pure hydrogen flame, in which NO formation is mainly characterised by the Zeldovich mechanism. The results also indicate a rapid decrease of maximum NO values in H2/CO syngas mixtures due to lower temperatures associated with the CO-dilution into H2. Results on NO formation routes in H2/CO syngas flames show that while the Zeldovich mechanism dominates the NO formation at low strain rates, the high NO formation rate at high strain rates is entirely caused by the NNH mechanism. We also found that the Fenimore mechanism has a least contribution on NO formation in H2/CO syngas flames due to absence of CH radicals in the oxidation of CO. It is found that, due to preferential diffusion, NO concentration exhibits higher values near the flame base depending on the hydrogen content in H2/CO syngas fuel mixture.

합성가스(H<sub>2</sub>/CO) 예혼합 충돌 제트화염에서 신장률에 따른 부상된 화염 구조에 관한 연구

A study has been conducted numerically to investigate the lifted flat syngas flame structure of impinging jet flame configuration with the global strain rates in 10% hydrogen content. In this study, the effects of strain rate were major parameters on chemistry kinetics and flame structure at stagnation point. The numerical results were calculated by SPIN application of the CHEMKIN package. The strain rates were adjusted with Reynolds numbers of premixed syngas-air mixture. Different flame shapes were observed with different strain rates. As strain rate has increased, the flame temperature and axial velocity have been decreased due to the flame heat loss increment, and the OH radical reaction zones become narrower but each mole fractions are still constant. Also, the reversion of H2O product near stagnation point has been found out when strain rate has increased. This phenomenon is attributed to the rapid production of oxidizing radical reaction such as the R12 (H+O2(+M) = HO2(+M)), which makes the R18 (HO2 + OH = O2 + H2O) reaction increment.

Temperature and emissions characteristics of a micro-mixing injection hydrogen-rich syngas flame diluted with N2

Micro-mixing injection combustion is an effective way to reduce the NOx emissions of gas turbine, but it can also cause high temperature near the burner's nozzle. In this paper, micro-mixing injection combustion and N2 dilution were combined to resolve this problem. The results indicate that this method not only reduces NOx emissions effectively but also protects the nozzles by lowering the temperature of the flame and that near the nozzle. The larger the dilution amount, the lower the temperature and NOx emissions could be. Under various experimental operating conditions, NOx emissions could be reduced to less than 6 ppm (@15% O2), which is lower than the emissions of most conventional gas turbine burners. According to the characteristics of temperature and NOx emissions, it seems the Zeldovich route is not the control route for the flame. With most of the flames, the CO emission hardly changed and remained at a relatively low level within almost 6 ppm (@15% O2).

Evaluation of the Accuracy of Selected Syngas Chemical Mechanisms

The implementation of reduced syngas combustion mechanisms in numerical combustion studies has become inevitable in order to reduce the computational cost without compromising the predictions' accuracy. In this regard, the present study evaluates the predictive capabilities of selected detailed, reduced, and global syngas chemical mechanisms by comparing the numerical results with experimental laminar flame speed (LFS) values of lean premixed (LPM) syngas flames. The comparisons are carried out at varying equivalence ratios, syngas compositions, operating pressures, and preheat temperatures to represent a range of operating conditions of modern fuel flexible combustion systems. NOx emissions predicted by the detailed mechanism, GRI-Mech. 3.0, are also used to study the accuracy of the selected mechanisms under these operating conditions. Moreover, the selected mechanisms' accuracy in predicting the laminar flame thickness (LFT), species concentrations of the reactants, and OH profiles at different equivalence ratios and syngas compositions are investigated as well. The LFS is generally observed to increase with increasing equivalence ratio, hydrogen content in the syngas, and preheat temperature, while it is decreased with increasing operating pressure. This trend is followed by all mechanisms understudy. The global mechanisms of Watanabe–Otaka and Jones–Lindstedt for syngas are consistently observed to over-predict and under-predict the LFS up to an average of 60% and 80%, respectively. The reduced mechanism of Slavinskaya has an average error of less than 20%, which is comparable to the average error of the GRI-Mech. 3.0. It however over-predicts the flame thickness by up to 30% when compared to GRI-Mech. 3.0. The NO prediction by Li mechanism and the reduced mechanisms are observed to be within 10% prediction range of the GRI-Mech. 3.0 at intermediate equivalence ratio (φ=0.74) up to stoichiometry. Moving toward more lean conditions, there is significant difference between the GRI-Mech. 3.0 NO prediction and those of the reduced mechanisms due to relative importance of the prompt NOx at lower temperature compared to thermal NOx that is only accounted for by the GRI-Mech. 3.0.

Effect of H2/CO Composition on Extinction Strain Rates of Counterflow Syngas Flames

This work presents a detailed experimental and numerical investigation of the effect of H-2/CO composition on extinction characteristics of premixed and nonpremixed syngas flames. Experimental measurements of local and global extinction strain rates in counterflow diffusion flames have been reported at atmospheric pressure for six different compositions of syngas fuel. The concentration of H-2 was varied from 5 to 20% with a 3% increment, and correspondingly, CO was decreased from 35 to 20% in steps of 3%. Particle imaging velocimetry has been used to determine the local extinction strain rates. Local extinction strain rates increased with an increase in the H-2/CO ratio in both nonpremixed and premixed flames. The predicted extinction strain rates for both nonpremixed and premixed counterflow flames using five different mechanisms available in the literature were compared with measurements. The Davis H-2/CO and Ranzi H-2/CO mechanisms predicted extinction strain rates within 10% of experimental values irrespective of the H-2/CO ratio. In the nonpremixed case, the Cl mechanism by Li et al., GRI 3.0, and the Ranzi H-2/CO mechanism predicted extinction strain rates well for low H-2/CO ratios (from 5:35 to 14:26) but deviated from experiments for higher H-2/CO values (17:23 and 20:20). In addition to kinetics, preferential diffusion effects were found to affect the reaction zone significantly and create distinct localized reaction zone structures in nonpremixed flames, which could contribute to discrepancies in extinction predictions.

Effects of inert dilution on the propagation and extinction of lean premixed syngas/air flames

The dilution effects of inert components N2 and CO2 on the propagation and extinction of lean premixed H2/CO/air syngas flames were experimentally and numerically investigated. Extinction stretch rates were measured using the counterflow technique while laminar flame speed data were obtained from literatures. Numerical simulations were conducted at 1-D freely propagating configuration and opposed-jet configuration with detailed chemistry and molecular transport description. The numerical results well predicted the experimental measurements. Both results revealed that CO2 dilution had more profound effect on flame propagation and extinction than N2 dilution. In addition, numerical simulation assessed the preferential importance in a rather quantitative manner among the three effects, i.e., the thermal effect, the diffusivity change effect and the chemical effect with artificial manipulation of mass diffusivities and chemical reactions of CO2 and N2. The results showed that the thermal effect dominated the reduction of laminar flame speed and extinction strain rate. The chemical effect caused by CO2 dilution was slightly stronger to the reduction of extinction limit than to that of laminar flame speed. The diffusivity change effect is negligible for both CO2 and N2 dilutions. N2 only acts as a thermal inert in the propagation and extinction of the H2/CO/air flames.

A numerical investigation of structure and emissions of oxygen-enriched syngas flame in counter-flow configuration

The aim of the present study is to investigate the enhancement of syngas combustion using the promising oxygen-enrichment technology, with a particular attention on optimal operating conditions in regard to NOx emissions. For this purpose, a numerical study is conducted on a syngas counter-flow diffusion flame, using air enriched with oxygen as the oxidizer. The oxygen concentration ranges from 21% to 30% by volume. Two syngas compositions are considered with H2/CO rates equal to 0.25 and 4, respectively. Flame structure is characterized by solving flamelet equations with the consideration of radiation. The chemical reaction mechanism used is GRI 3.0. Computational results showed that oxygen addition increases the flame temperature and intensifies the radiative heat transfer. It also considerably extends flammability limits allowing stable flames at high values of the scalar dissipation rates and for lean syngas composition. NOx formation is substantially increased with oxygen increment, and Zeldovich mechanism is found to be the main route of NOx formation. H2-lean syngas flames produce less NOx at low scalar dissipation while H2-rich syngas flames NOx emissions are low at high scalar dissipation rates.

ICOPE-15-C133 DNS Investigation of the Interaction between Premixed Turbulent Syngas Flame Front and Vortex at Different Reynolds Numbers

ICOPE-15-C133 DNS Investigation of the Interaction between Premixed Turbulent Syngas Flame Front and Vortex at Different Reynolds Numbers

The Scalar Structure of Turbulent Oxy-fuel Non-premixed Flames

This study addresses the scalar structure of turbulent oxy-fuel syngas non-premixed jet flame in the context of direct numerical simulation (DNS) and detailed chemistry. The main objective is to identify the influence of the Reynolds number on oxy-fuel flame structure, and to clarify the differences of scalar structures between oxy-fuel syngas and syngas-air flames at a similar higher Reynolds number. Two oxy-syngas flames at Reynolds numbers of 3000 and 6000 and one syngas-air flame at Reynolds number of 6000 were simulated. These studies show that the existence of CO2 in the oxidant has a profound effect on scalar structure of oxy-fuel syngas flame compared to syngas-air flame.

Operational issues in premixed combustion of hydrogen-enriched and syngas fuels

The growing interest in the use of syngas, as a means to reduce fossil fuel consumption and CO2 emissions, has promoted many works focused on evaluating their combustion characteristics before their use in existing combustors, generally optimized for conventional fuels. Although the effect of fuel composition on the stability range (flashback and blowout) has been analyzed in a wide number of previous studies, much less attention has been devoted to other issues, such as the relation between flashback and combustion instabilities. The present work is, therefore, focused on analyzing the main concerns in syngas combustion: stability range, combustion instabilities and pollutant emissions, also specifically addressing the relation between pressure fluctuations and flashback appearance.The results obtained revealed that the fuel composition has an important effect on the stability range, especially on flashback appearance. However, different trends were observed not only as a function of composition but also as a function of burner configuration. As a consequence, although some approximations, such as the definition of a critical Damköhler number, seemed to give quite accurate predictions of the stability range for a certain range of fuels or configurations, a thorough analysis should be made for each fuel composition before using it in a given burner configuration. In addition, in fuels with high hydrogen content, under certain circumstances, evidence of the existence of a triggering mechanism between pulsating flashback episodes and thermo-acoustic instabilities was found. This result has a high practical importance since, although thermo-acoustic instabilities had been analyzed in methane flames throughout many previous works, their appearance in syngas flames, where they can be also triggered by flashback events, was clearly less tested. Finally, the effect of fuel composition on pollutant emissions has been also analyzed.

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.