Syngas Flames Research Articles

Numerical investigation of the effects of fuel variability on the dynamics of syngas impinging jet flames

Numerical simulations using the Large-eddy simulation technique is presented to study the effects of fuel variability on the dynamics of hydrogen and syngas impinging flames. The compositions of CO and H2 are varied in a syngas mixture, including a pure H2 case as the baseline Case 1, 20% CO with 80% H2 for Case 2, 40% CO with 60% H2 for Case 3, and 20% CO with 20% CO2 and 60% H2 for Case 4. The impinging flame configuration has a distance to nozzle diameter ratio of H/d=20 and the inlet velocity of the fuel is 27m/s. The fuel is issued from a circular nozzle and mixes with air in a non-premixed configuration. The results show that the flames develop vortical structures in the primary jet associated with the buoyancy and shear layer instability, and the wall jet progresses parallel to the impinging plate forming large-scale vortex rings at different locations and strengths as a consequence of the fuel compositions. A comprehensive analysis of vortical structures in the primary and secondary jet streams, along with a description of their effects on the near-wall heat transfer and instabilities of syngas flames is presented here. Pollutant emissions and species formations are also investigated in order to gain further insight into the syngas burning characteristics for future cleaner combustion systems.

Experimental Investigation of Double-Swirled Non-premixed Syngas Flames by Planar Laser-Induced Fluorescence

Experimental measurements are carried out on a non-premixed model combustor, equipped with a double-swirled syngas burner. Mixtures of H2, CO, and CO2 with appropriate contents are used to simulate...

Computed NOx emission characteristics of opposed-jet syngas diffusion flames

This paper reported a numerical study on the NOx emission characteristics of opposed-jet syngas diffusion flames. A narrowband radiation model was coupled to the OPPDIF program, which used detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames with flame radiation. The effects of syngas composition, pressure and dilution gases on the NOx emission of H2/CO synthetic mixture flames were examined. The analyses of detailed flame structures, chemical kinetics, and nitrogen reaction pathways indicate NOx are formed through Zeldovich (or thermal), NNH and N2O routes both in the hydrogen-lean and hydrogen-rich syngas flames at normal pressure. Zeldovich route is the main NO formation route. Therefore, the hydrogen-rich syngas flames produce more NO due to higher flame temperatures compared to that for hydrogen-lean syngas flames. Although NNH and N2O routes also are the primary NO formation paths, a large amount of N2 will be reformed from NNH and N2O species. For hydrogen-rich syngas flames, the NO formation from NNH and N2O routes are lesser, where NO can be dissipated through the reactions of NH+NO→N2+OH and NH+NO→N2O+H more actively. At a rather low pressure (0.01atm), NNH-intermediate route is the only formation path of NO. Increasing pressure then enhances NO formation reactions, especially through Zeldovich mechanisms. However, at higher pressures (5–10atm), NO is then converted back to N2 through reversed N2O route for hydrogen-lean syngas flames, and through NNH as well for hydrogen-rich syngas flames. In addition, the dilution effects from CO2, H2O, and N2 on NO emissions for H2/CO syngas flames were studied. The hydrogen-lean syngas flames with H2O dilution have the lowest NO production rate among them, due to a reduced reaction rate of NNH+O→NH+NO. But for hydrogen-rich syngas flames with CO2 dilution, the flame temperatures decrease significantly, which leads to a reduction of NO formation from Zeldovich route.

Syngas combustion radiation profiling through spectrometry and DFCD processing techniques

Experimental investigations of H2 and H2-enriched syngas flame radiation properties have been conducted through spectroscopic and DFCD (Digital Flame Colour Discrimination) techniques. A spectrograph was employed to quantify the emission profile of H2-based flames in the UV–visible spectral domain. The OH* emission was found to be the strongest in reactants with highest amount of H2. Further addition of CO and/or CO2 resulted in the reduction of OH* intensity with the addition of CO2 causing greater radical intensity-loss than that of CO. The decrease in OH* intensity is accompanied by a corresponding increase in the CO–O* broadband continuum in the short-wavelength domain of the visible spectrum. Such reduction of OH* along with increase in CO–O* intensity can be related to the endothermic reaction mechanism of CO + OH => CO2 + H, which describes the role of CO/CO2 addition in H2-enriched syngas flames. Comparison with direct imaging results provided additional credence to the effect of temperature reduction as flames with CO and/or CO2 additions resembled colourations closer to typical bluish premixed hydrocarbon flame. The employment of DFCD processing effectively characterised different syngas combustion conditions by combining aspects of digital flame colour intensities with spatial combustion distributions. This colour signal quantification method was shown to yield useful characterisation of H2-based flames, similar to the use of OH* chemiluminescence intensity variation from spectrometry. Also, DCFD analysis was able to depict the variances between the burning of different syngas gaseous constituents. Thus, useful image-based parameters related to the H2-based combustion can be derived and potentially applied as a practical monitoring and characterisation mean for syngas combustion.

An experimental and kinetic study of syngas/air combustion at elevated temperatures and the effect of water addition

Laminar flame speeds of premixed syngas/air mixtures were measured at various fuel equivalence ratios (0.6–3.0), H2 content of the fuel, and preheat temperatures (298–500K) using a spherically expanding flame configuration. The measured laminar flame speeds were compared with simulations using with three existing chemical kinetic models – GRI Mech 3.0, H2/CO Davis Mechanism and San Diego mechanism. Reasonable agreement between computations and measurements was achieved at room temperature that validated the new experimental configuration. However, at higher preheat temperatures discrepancies between computed and measured values were large, especially for fuel rich mixtures. Addition of H2O to two fuels (H2/CO=5/95 and 50/50) up to 40% in the fuel–air mixture was studied to understand the effect of moisture in coal derived syngas. For the H2/CO=5/95 fuel, flame speed was observed to increase with up to 20% H2O addition and then to decrease with any further water addition. However, the higher H2 content fuel (H2/CO=50/50) only showed a decrease in flame speed with an increasing water concentration in the fuel–air mixture. The different trends have been explained as a result of the competing chemical and physical (dilution and thermal) effects of H2O addition on the syngas flames using sensitivity analyses and by analyzing reaction rates and radical concentrations.

A computational study of combustion and extinction of opposed-jet syngas diffusion flames

This paper reports a numerical study on the combustion and extinction characteristics of opposed-jet syngas diffusion flames. A model of one-dimensional counterflow syngas diffusion flames was constructed with constant strain rate formulations, which used detailed chemical kinetics and thermal and transport properties with flame radiation calculated by statistic narrowband radiation model. Detailed flame structures, species production rates and net reaction rates of key chemical reaction steps were analyzed. The effects of syngas compositions, dilution gases and pressures on the flame structures and extinction limits of H 2/CO synthetic mixture flames were discussed. Results indicate the flame structures and flame extinction are impacted by the compositions of syngas mixture significantly. From H 2-enriched syngas to CO-enriched syngas fuels, the dominant chain reactions are shifting from OH + H 2→H + H 2O for H 2O production to OH + CO→H + CO 2 for CO 2 production through the key chain-branching reaction of H + O 2→O + OH. Flame temperature increases with increasing hydrogen content and pressure, but the flame thickness is decreased with pressure. Besides, the study of the dilution effects from CO 2, N 2, and H 2O, showed the maximum flame temperature is decreased the most with CO 2 as the dilution gas, while CO-enriched syngas flames with H 2O dilution has highest maximum flame temperature when extinction occurs due to the competitions of chemical effect and radiation effect. Finally, extinction limits were obtained with minimum hydrogen percentage as the index at different pressures, which provides a fundamental understanding of syngas combustion and applications.

Effect of flame stretch in downstream Interaction between premixed syngas-air flames

The effect of strain rate in downstream interactions between lean (rich) and lean (rich) premixed syngas flames with the fuel composition of 50% H 2 and 50% CO is numerically investigated by varying the strain rate in the range of 5∼500 s −1. The flame stability maps for several strain rates are presented and main concerns are focused on the downstream interactions on the lean and rich extinction boundaries. The fuel composition of 50% H 2 and 50% CO with effective Lewis numbers larger than unity for both lean and rich extinction boundaries is chosen for grasping the important role of hydrogen with the deficient reactant Lewis numbers much smaller than unity. The results show that the lean extinction boundaries have the slanted shape, thereby leading to strong interactions; meanwhile the rich extinction boundaries at appropriately low strain rates are of square, indicating weak interactions. However, at highly strained interacting rich flames, the rich extinction boundaries show a slanted shape, thereby leading to strong interactions even for Lewis numbers much larger than unity. In such situations, thermal and chemical interactions are explained in detail. It is found that, in interacting flames, the excessive heat loss of the stronger flame partly to the weaker flame and mostly to the ambience is the mechanism of flame extinction.

Investigation of the Syngas Flame Characteristics at Elevated Pressures Using Optical and Laser Diagnostic Methods

The effect of pressure on the characteristics of syngas flames is investigated under gas turbine relevant conditions using planar laser induced fluorescence of OH radicals and OH* chemiluminescence imaging. An optically accessible combustor fitted with a swirl burner was operated with two different syngas mixtures, preheated air at 700 K, and pressures ranging from 5 to 20 bars. The thermal load varied from 15 to 25 kW/bar at an equivalence ratios 0.5. The OH-PLIF measurements show that the flames under all conditions exhibited two reaction fronts, one at the shear layer between the inner recirculation zone and the fuel inlet, and one between the fuel inlet and the air nozzle. The more or less continuous reaction front at low pressure turned into a highly corrugated flame front at higher pressures, with isolated regions of ignition and extinction. The probability density distribution of the flame curvature for the mixtures studied showed that the inner and outer flame responded differently to the pressure increase, with the mean curvature magnitude also depending on the mixture composition and thermal load. The measurements clearly shows the limitations associated with the use of OH* chemiluminescence images as a marker for the heat release rate especially in case of syngas mixtures.

Investigation of nitrogen-diluted syngas non-premixed flames measured by planar laser-induced fluorescence of hydroxyl and particle image velocimetry

In order to investigate the effects of nitrogen dilution on combustion behaviour of syngas flames, a model combustor with optical access for swirl non-premixed flames was developed. Experimental results from planar laser-induced fluorescence (PLIF) of OH and particle image velocimetry (PIV) are presented. The syngas consists of hydrogen and carbon monoxide of volume fraction ratio kept at 0.78. Up to 60 per cent (by volume) of nitrogen was added into syngas, as well as reference fuels including methane, hydrogen, and carbon monoxide, for dilution. Flow fields obtained by PIV reveal that the averaged typical swirling flow structure is not influenced by dilution content, which has more effect on turbulence intensities in recirculation zones and shear layers. Additionally, analysis of reaction zones and regions of burnt gas from OH-PLIF measurement shows that although syngas flame burns closer to fuel spray exit than methane, the latter shows more combustion stability, probably because of the different stabilization mechanisms for these two flames. With less support from hot burned gases in recirculation zones, the content of hydrogen plays a crucial role in syngas flame stabilization. Experimental results also imply that the increase of dilution content in fuel leads to less flame opening angle and thinner flame base.

Validation of flow simulation and gas combustion sub-models for the CFD-based prediction of NOx formation in biomass grate furnaces

While reasonably accurate in simulating gas phase combustion in biomass grate furnaces, CFD tools based on simple turbulence–chemistry interaction models and global reaction mechanisms have been shown to lack in reliability regarding the prediction of NOx formation. Coupling detailed NOx reaction kinetics with advanced turbulence–chemistry interaction models is a promising alternative, yet computationally inefficient for engineering purposes. In the present work, a model is proposed to overcome these difficulties. The model is based on the Realizable k–ϵ model for turbulence, Eddy Dissipation Concept for turbulence–chemistry interaction and the HK97 reaction mechanism. The assessment of the sub-models in terms of accuracy and computational effort was carried out on three laboratory-scale turbulent jet flames in comparison with the experimental data. Without taking NOx formation into account, the accuracy of turbulence modelling and turbulence–chemistry interaction modelling was systematically examined on Sandia Flame D and Sandia CO/H2/N2 Flame B to support the choice of the associated models. As revealed by the Large Eddy Simulations of the former flame, the shortcomings of turbulence modelling by the Reynolds averaged Navier–Stokes (RANS) approach considerably influence the prediction of the mixing-dominated combustion process. This reduced the sensitivity of the RANS results to the variations of turbulence–chemistry interaction models and combustion kinetics. Issues related to the NOx formation with a focus on fuel bound nitrogen sources were investigated on a NH3-doped syngas flame. The experimentally observed trend in NOx yield from NH3 was correctly reproduced by HK97, whereas the replacement of its combustion subset by that of a detailed reaction scheme led to a more accurate agreement, but at increased computational costs. Moreover, based on results of simulations with HK97, the main features of the local course of the NOx formation processes were identified by a detailed analysis of the interactions between the nitrogen chemistry and the underlying flow field.

Characterization of syngas laminar flames using the Bunsen burner configuration

Laminar flame speeds of syngas mixtures (H 2/CO/Air) have been studied using the Bunsen flame configuration with both straight and nozzle burners. The flame surface area and flame cone angle methodologies, respectively based on the OH* chemiluminescence and Schlieren imaging techniques, have been performed to extract flame speeds for a wide range of equivalence ratios (0.3 < φ < 1.2) and mixture compositions (1% < %H 2 < 100%). As a result, a flame speed correlation established for lean syngas flames with 0.6 < φ < 1.0 and 10% < %H 2 < 70% is proposed. A particular attention has been devoted to the development and validation of the OH* chemiluminescence methodology with the identification of important parameters governing the measurement accuracy.

Experimental studies of the fundamental flame speeds of syngas (H2/CO)/air mixtures

Syngas laminar flame speed measurements have been carried out at atmospheric pressure and ambient temperature using spherically expanding flames. Mixture compositions ranging from 5/95% to 50/50% H2/CO and equivalence ratios from 0.4 to 5.0 have been investigated. Both state-of-the-art linear and non-linear extrapolation methodologies have been tested and extracted laminar flame speeds have been compared to datasets available in the literature as well as computations using two leading kinetic mechanisms for syngas combustion. Syngas flame sensitivities to stretch have been characterized by extracting the corresponding Markstein lengths. In order to explain important disparities found for rich syngas flames among datasets of the literature, computations have been performed to quantify the possible extent of velocity reduction corresponding to small contents of iron pentacarbonyl.

A Comparative Study of Eight Finite-Rate Chemistry Kinetics for Co/H2 Combustion

We compare the performance and computational cost of 8 kinetic models (3 global and 5 elementary) that describe the finite-rate chemistry of syngas combustion. We apply them in simulating a turbulent jet flame with syngas diluted by 30% nitrogen. We model the turbulence by a modified k-epsilon model and the turbulence-chemistry interaction by the partially stirred reactor approach. To integrate the chemistry equations, we nominally use explicit fifth-order embedded Runge-Kutta ODE solver. But semi-implicit Bulirsch-Stoer and implicit Euler were also used. The computational time depends on the number of reaction steps and the ODE solver. Five models overpredict the maximum flame temperature (by 200 K-320 K). Two models underpredict it by 240 K and 580 K. The global model that is based on the Westbrook-Dryer (1981) model for hydrocarbon fuels gives the best agreement with measurements, and also has low computational demand. Therefore, it is recommended for modeling turbulent syngas flames.

Large eddy simulation of extinction and reignition with artificial neural networks based chemical kinetics

Large eddy simulation (LES) of a non-premixed, temporally evolving, syngas/air flame is performed with special emphasis on speeding-up the sub-grid chemistry computations using an artificial neural networks (ANN) approach. The numerical setup for the LES is identical to a previous direct numerical simulation (DNS) study [1], which reported considerable local extinction and reignition physics, and hence, offers a challenging test case. The chemical kinetics modeling with ANN is based on a recent approach [2], and replaces the stiff ODE solver (DI) to predict the species reaction rates in the subgrid linear eddy mixing (LEM) model based LES (LEMLES). In order to provide a comprehensive evaluation of the current approach, additional information on conditional statistics of some of the key species and temperature are extracted from the previous DNS study and are compared with the LEMLES using ANN (ANN-LEMLES, hereafter). The results show that the current approach can detect the correct extinction and reignition physics with reasonable accuracy compared to the DNS. The syngas flame structure and the scalar dissipation rate statistics obtained by the current ANN-LEMLES are provided to further probe the flame physics. It is observed that, in contrast to H 2 , CO exhibits a smooth variation within the region enclosed by the stoichiometric mixture fraction. The probability density functions (PDFs) of the scalar dissipation rates calculated based on the mixture fraction and CO demonstrate that the mean value of the PDF is insensitive to extinction and reignition. However, this is not the case for the scalar dissipation rate calculated by the OH mass fraction. Overall, ANN provides considerable computational speed-up and memory saving compared to DI, and can be used to investigate turbulent flames in a computationally affordable manner.

Structures and burning velocity of biomass derived gas flames

Biomass derived gases produced via gasification, pyrolysis, and fermentation are carbon neutral alternative fuels that can be used in gas turbines, furnaces, and piston engines. To make use of these environmentally friendly but energy density low fuels the combustion characteristics of these fuels have to be fully understood. In this study the structure and laminar burning velocity of biomass derived gas flames are investigated using detailed chemical kinetic simulations. The studied gaseous fuels are the air-blown gasification gas, co-firing of gasification gas with methane, pyrolysis gases, landfill gases, and syngas, a mixture of carbon monoxide and hydrogen. The simulated burning velocities of reference fuel mixtures using two widely used chemical kinetic mechanisms, GRI Mech 3.0 and the San Diego mechanism, are compared with the experimental data to explore the uncertainties and scattering of the simulation data. The different chemical kinetic mechanisms are shown to give a reasonable agreement with each other and with experimental data, with a discrepancy within 7% over most of the conditions. The results show that the structures of typical landfill gas flames and co-firing of methane/gasification gas flames share essential similarity with methane flames. The reaction zones of these flames consist of a thin inner layer and a relatively thick CO/H-2 oxidation layer. In the inner layer hydrocarbon fuel (methane) is converted through chain reactions to intermediates such as CH3, CH2O, CO, H-2, etc. The structures of gasification gas flames, pyrolysis gas flames, syngas flames share similarity with the oxidization layer of the methane/air flames. Overall, the chemical reactions of all biomass derived gas flames occur in thin zones of the order of less than 1 mm. The thickness of all BDG gas flames is inversely proportional to their respective laminar burning velocity. The laminar burning velocities of landfill gases are found to increase linearly with the mole fraction of methane in the mixtures, whereas for gasification gas, syngas and pyrolysis gas where hydrogen is present, the laminar burning velocities scale linearly with the mole fraction of hydrogen. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. (Less)

Laboratory Investigations of Low-Swirl Injectors Operating With Syngases

The low-swirl injector (LSI) is a lean premixed combustion technology that has the potential for adaptation to fuel-flexible gas turbines operating on a variety of fuels. The objective of this study is to gain a fundamental understanding of the effect of syngas on the LSI flame behavior, the emissions, and the flowfield characteristics for adaptation to the combustion turbines in integrated gasification combined cycle clean coal power plants. The experiments were conducted in two facilities. Open atmospheric laboratory flames generated by a full size (6.35 cm) LSI were used to investigate the lean blow-off limits, emissions, and the flowfield characteristics. Verification of syngas operation at elevated temperatures and pressures were performed with a reduced scale (2.54 cm) LSI in a small pressurized combustion channel. The results show that the basic LSI design is amenable to burning syngases with up to 60% H2. Syngases with high H2 concentration have lower lean blow-off limits. From particle image velocimetry measurements, the flowfield similarity behavior and the turbulent flame speeds of syngases flames are consistent with those observed in hydrocarbon and pure or diluted hydrogen flames. The NOx emissions from syngas flames show log-linear dependency on the adiabatic flame temperature and are comparable to those reported for the gaseous fuels reported previously. Successful firing of the reduced-scale LSI at 450 K&lt;T&lt;505 K and 8 atm verified the operability of this concept at gas turbine conditions.

Chemical effects of added CO 2 on the extinction characteristics of H 2/CO/CO 2 syngas diffusion flames

Chemical effects of added CO 2 on flame extinction characteristics are numerically studied in H 2/CO syngas diffusion flames diluted with CO 2. The two representative syngas flames of 80% H 2 + 20% CO and 20% H 2 + 80% CO are inspected according to the composition of fuel mixture diluted with CO 2 and global strain rate. Particular concerns are focused on impact of chemical effects of added CO 2 on flame extinction characteristics through the comparison of the flame characteristics between well-burning flames far from extinction limit and flames at extinction. It is seen that chemical effects of added CO 2 reduce critical CO 2 mole fraction at flame extinction and thus extinguish the flame at higher flame temperature irrespective of global strain rate. This is attributed by the suppression of the reaction rate of the principal chain branching reaction through the augmented consumption of H-atom from the reaction CO 2 + H→CO + OH. As a result the overall reaction rate decreases. These chemical effects of added CO 2 are similar in both well-burning flames far from extinction limit and flames at extinction. There is a mismatching in the behaviors between critical CO 2 mole fraction and maximum flame temperature at extinction. This anomalous phenomenon is also discussed in detail.

Conditional Moment Closure Modeling for a Three-Dimensional Turbulent Non-premixed Syngas Flame with a Cooling Wall

The fully coupled conditional moment closure (CMC) modeling with an unstructured-grid finite volume method has been developed to realistically simulate the structure of complex, turbulent non-premixed syngas-air flames, where flame structures could be considerably influenced by turbulence, transport history, and heat transfer. In this study, conservative CMC formulation is used to improve robustness and accuracy in the boundary treatment of CMC-related fluxes. To correctly account for the transport effect, the CMC transport equations fully coupled with equations for the flow and mixing fields are numerically solved. Moreover, to reduce the excessive computational burden required for three-dimensional computations of the fully coupled CMC approach with detailed chemistry, parallel strategies are implemented into the general CMC formulation. The present fully coupled CMC formulation together with a parallel processing procedure successfully demonstrates the capability to realistically predict detailed struc...

Flame Extinction Limits of H2‐CO Fuel Blends

The flame extinction limits of syngas (H2‐CO) flames were measured using a twin-flame counterflow burner. Plots of extinction limits (%f: volumetric percent of fuel in air) versus global stretch rates were generated at different fuel blend compositions and were extrapolated to determine the flame extinction limit corresponding to an experimentally unattainable zero-stretch condition. The zero-stretch extinction limit of H2‐CO mixtures decreases with the increase in H2 concentration in the mixture. The average difference between the measured flame extinction limit and the Le Chatelier’s calculation is around 7% of the mean value. The measured OH chemiluminescence data indicates that regardless of blend composition the OH radical concentration reduces to a critical value prior to the flame extinction. The measured laminar flame velocity close to the extinction indicates that regardless of fuel composition, the premixed flame of hydrogen fuel blends extinguishes when the mixture laminar flame velocity falls below a critical value.

A numerical and experimental study of counterflow syngas flames at different pressures

Synthesis gas or “Syngas” is being recognized as a viable energy source worldwide, particularly for stationary power generation due to its wide availability as a product of bio and fossil fuel gasification. There are, however, gaps in the fundamental understanding of syngas combustion and emissions characteristics, especially at elevated pressures that are relevant to practical combustors. This paper presents a numerical and experimental investigation of the combustion and NO x characteristics of syngas fuel with varying composition, pressure and strain rate. Experiments were performed at atmospheric conditions, while the simulations considered different pressures. Both experiments and simulations indicate that stable non-premixed and partially premixed counterflow flames (PPFs) can be established for a wide range of syngas compositions and strain rates. Three chemical kinetic models, GRI 3.0, Davis et al., and Mueller et al. are examined. The Davis et al. mechanism is found to agree best with the experimental data, and hence used to simulate the PPF structure at different pressure and fuel composition. For the pressure range investigated, results indicate a typical double flame structure with a rich premixed reaction zone (RPZ) on the fuel side and a non-premixed reaction zone (NPZ) on the oxidizer side, with RPZ characterized by H 2 oxidation, and NPZ by both H 2 and CO oxidation. While thermal NO is found to be the dominant route for NO production, a reburn route, which consumes NO through NO + O + M→ NO 2 + M and H + NO + M → HNO + M reactions, becomes increasingly important at high pressures. The amount of NO formed in syngas PPFs first increases rapidly with pressure, but then levels off at higher pressures. At a given pressure, the peak NO mole fraction exhibits a non-monotonic variation with syngas composition, first decreasing to a minimum value, and then increasing as the amount of CO in syngas is increased. This implies the existence of an optimum syngas composition that yields the lowest amount of NO production in syngas PPFs, and can be attributed to the combined effects of thermal and reburn mechanisms.