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.     

Laminar burning velocities and flame characteristics of CO–H2–CO2–O2 mixtures

Laminar burning velocities of CO–H₂–CO₂–O₂ flames were measured by using the outwardly spherical propagating flame method. The effect of large fraction of hydrogen and CO₂ on flame radiation, chemical reaction, and intrinsic flame instability were investigated. Results show that the laminar burning velocities of CO–H₂–CO₂–O₂ mixtures increase with the increase of hydrogen fraction and decrease with the increase of CO₂ fraction. The effect of hydrogen fraction on laminar burning velocity is weakened with the increase of CO₂ fraction. The Davis et al. syngas mechanism can be used to calculate the syngas oxyfuel combustion at low hydrogen and CO₂ fraction but needs to be revised and validated by additional experimental data for the high hydrogen and CO₂ fraction. The radiation of syngas oxyfuel flame is much stronger than that of syngas–air and hydrocarbons–air flame due to the existence of large amount of CO₂ in the flame. The CO₂ acts as an inhibitor in the reaction process of syngas oxyfuel combustion due to the competition of the reactions of H + O₂ = O + OH, CO + OH = CO₂ + H and H + O₂(+M) = HO₂(+M) on H radical. Flame cellular structure is promoted with the increase of hydrogen fraction and is suppressed with the increase of CO₂ fraction due to the combination effect of hydrodynamic and thermal-diffusive instability.     

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 H₂/CO synthetic mixture flames were discussed. Results indicate the flame structures and flame extinction are impacted by the compositions of syngas mixture significantly. From H₂-enriched syngas to CO-enriched syngas fuels, the dominant chain reactions are shifting from OH + H₂→H + H₂O for H₂O production to OH + CO→H + CO₂ for CO₂ production through the key chain-branching reaction of H + O₂→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 CO₂, N₂, and H₂O, showed the maximum flame temperature is decreased the most with CO₂ as the dilution gas, while CO-enriched syngas flames with H₂O 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 syngas fuel compositions on the occurrence of instability of laminar diffusion flame

The paper presents a numerical investigation of the critical roles played by the chemical compositions of syngas on laminar diffusion flame instabilities. Three different flame phenomena – stable, flickering and tip-cutting – are formulated by varying the syngas fuel rate from 0.2 to 1.4 SLPM. Following the satisfactory validation of numerical results with Darabkhani et al. [1], the study explored the consequence of each species (H₂, CO, CH₄, CO₂, N₂) in the syngas composition. It is found that low H₂:CO has a higher level of instability, which however does not rise any further when the ratio is less than 1. Interestingly, CO encourages the heat generation with less fluctuation while H₂ plays another significant role in the increase of flame temperature and its fluctuation. Diluting CH₄ into syngas further increases the instability level as well as the fluctuation of heat generation significantly. However, an opposite effect is found from the same action with either CO₂ or N₂. Finally, considering the heat generation and flame stability, the highest performance is obtained from 25%H₂+75%CO (81 W), followed by EQ+20%CO₂, and EQ+20%N₂ (78 W).     

The synergistic effect of equivalence ratio and initial temperature on laminar flame speed of syngas

The laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas in a wide equivalence ratio range (0.6–5) and initial temperature (298–423 K) was studied by Bunsen burner. The results show that the laminar flame speed first increases and then decreases as the equivalence ratio increasing, which is a maximum laminar flame speed at n = 2. The laminar flame speed increases exponentially with the increase of initial temperature. For different equivalent ratios, the initial temperature effects on the laminar flame speed is different. The initial temperature effects for n = 2 (the most violent point of the reaction) is lower than others. It is found that H, O and OH are affected more and more when the equivalence ratio increase. When the equivalence ratio is far from 2, the reaction path changes, and the influence of initial temperature on syngas combustion also changes. The laminar flame speed of syngas is more severely affected by H + O₂ = O + OH and CO + OH = CO₂ + H than others, which sensitivity coefficient is larger and change more greatly than others when the initial temperature and equivalence ratio change. Therefore, the laminar flame speed of syngas/air premixed gas is affected by the initial temperature and equivalence ratio. A new correlation is proposed to predict the laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas under the synergistic effect of equivalence ratio and initial temperature (for equivalence ratios of 0.6–5, the initial temperature is 298–423 K).     

Solar syngas production from CO2 and H2O in a two-step thermochemical cycle via Zn/ZnO redox reactions: Thermodynamic cycle analysis

Solar syngas production from CO₂ and H₂O is considered in a two-step thermochemical cycle via Zn/ZnO redox reactions, encompassing: 1) the ZnO thermolysis to Zn and O₂ using concentrated solar radiation as the source of process heat, and 2) Zn reacting with mixtures of H₂O and CO₂ yielding high-quality syngas (mainly H₂ and CO) and ZnO; the ZnO is recycled to the first, solar step, resulting in net reaction βCO₂ + (1 − β)H₂O → βCO + (1 − β)H₂. Syngas is further processed to liquid hydrocarbon fuels via Fischer-Tropsch or other catalytic processes. Second-law thermodynamic analysis is applied to determine the cycle efficiencies attainable with and without heat recuperation for varying molar fractions of CO₂:H₂O and solar reactor temperatures in the range 1900-2300 K. Considered is the energy penalty of using Ar dilution in the solar step below 2235 K for shifting the equilibrium to favor Zn production.     

Numerical investigation of diluent influence on flame extinction limits and emission characteristic of lean-premixed H2-CO (syngas) flames

In recent years, research efforts have been channeled to explore the use of environmentally-friendly clean fuel in lean-premixed combustion so that it is vital to understand fundamental knowledge of combustion and emissions characteristics for an advanced gas turbine combustor design. The current study investigates the extinction limits and emission formations of dry syngas (50% H₂-50% CO), moist syngas (40% H₂-40% CO-20% H₂O), and impure syngas containing 5% CH₄. A counterflow flame configuration was numerically investigated to understand extinction and emission characteristics at the lean-premixed combustion condition by varying dilution levels (N₂, CO₂ and H₂O) at different pressures and syngas compositions. By increasing dilution and varying syngas composition and maintaining a constant strain rate in the flame, numerical simulation showed among diluents considered: CO₂ diluted flame has the same extinction limit in moist syngas as in dry syngas but a higher extinction temperature; H₂O presence in the fuel mixture decreases the extinction limit of N₂ diluted flame but still increases the flame extinction temperature; impure syngas with CH₄ extends the flame extinction limit but has no effect on flame temperature in CO₂ diluted flame; for diluted moist syngas, extinction limit is increased at higher pressure with the larger extinction temperature; for different compositions of syngas, higher CO concentration leads to higher NO emission. This study enables to provide insight into reaction mechanisms involved in flame extinction and emission through the addition of diluents at ambient and high pressure.     

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.     

Extinction studies of near-limit lean premixed syngas/air flames

Extinction studies of weakly-stretched near-limit lean premixed syngas/air flames were conducted in a twin-flame counterflow configuration. Experiments showed that buoyancy-induced natural convection at normal gravity strongly disturbed these flames. In order to validate the simulation, accurate extinction data was obtained at micro-gravity. Experimental data obtained from the 3.6 s micro-gravity drop tower showed that the extinction equivalence ratio increased with the increasing global stretch rate and decreased with the increasing H₂ mole fraction in the fuel. Numerical simulation was conducted with CHEMKIN software using GRI 3.0 and USC-Mech II mechanisms. The predicted extinction limit trend was in agreement with the micro-gravity experimental data. Sensitivity analyses showed that the competition between the main branching reaction H + O₂ ⇔ O + OH and the main termination reaction H + O₂ + M ⇔ HO₂ + M in the H₂/O₂ chemistry determined the extinction limits of the flames. The dominant species for syngas/air flame extinction was the H radical. The key exothermal reaction changed from OH + CO ⇔ H + CO₂ to OH + H₂ ⇔ H + H₂O with the increasing H₂ mole fraction in the fuel. Also, the mass diffusion played a more important role than chemical kinetics in the flame extinction. When the H₂ mass diffusion was suppressed, the reaction zone was pushed to the stagnation plane and the flame became weaker; while H mass diffusion is suppressed, the reaction zone slightly shifted towards the upstream and the flame was slightly strengthened.     

Computed flammability limits of opposed-jet H2/CO syngas diffusion flames

Extensive computations were made to determine the flammability limits of opposed-jet H₂/CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate the minimum hydrogen concentrations for H₂/CO syngas to be combustible are larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s⁻¹ where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7% for the equal-molar syngas fuels (H₂/CO = 1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and pressures or dilution gases percentages as the coordinates. By adding dilution gases such as CO₂, H₂O, and N₂ to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H₂O contributes to higher flame temperature, while the radiation effect of H₂O and CO₂ plays an important role in the flame extinction at low strain rates.     

Zirconia-supported tungsten oxides for cyclic production of syngas and hydrogen by methane reforming and water splitting

ZrO₂-supported tungsten oxides were used for cyclic production of syngas and hydrogen by methane reforming (reduction) and water splitting (re-oxidation). The reduction characteristics of WO₃ to WO₂ and WO₂ to W were examined at various temperatures (1073–1273 K) and reaction times. Significant portions of the tungsten oxides were also reduced by the produced H₂ and CO. The extent of reduction by H₂ varied greatly depending on temperature and WO₃ content and also on the reduction of either WO₃ or WO₂, while that by CO was consistently low. When the overall degree of reduction became sufficiently high, methane decomposition started to proceed rapidly, resulting in considerable carbon deposition and H₂ production. Consequently, the H₂/(CO + CO₂) ratio varied from around 1 to higher than 2. During the repeated cyclic operations with a proper reduction time at a given temperature, the syngas and hydrogen yields decreased gradually while the H₂/(CO + CO₂) ratio remained nearly constant and the carbon deposition was negligible.     

Performance improvement of the proton-conducting solid oxide electrolysis cell coupled with dry methane reforming

The proton-conducting solid oxide electrolysis cell (H-SOEC) is a clean technology for syngas production from H₂O and CO₂ through electrochemical and chemical reactions. However, it provides a low CO₂ conversion and produces a syngas product with a high H₂O content. To improve the H-SOEC for syngas production, H-SOEC coupled with a dry methane reforming process (H-SOEC/DMR) was proposed in this work. The process flowsheet of the H-SOEC/DMR was developed and further used to evaluate the performance of the H-SOEC/DMR process. From the performance analysis of the H-SOEC/DMR process, it was found that the CO₂ and CH₄ conversions were higher than 90% and 80%, respectively, when the process was operated in the temperature range of 1073–1273 K. In addition, the result showed that a syngas product with a low H₂O content could be obtained. Energy efficiency was then considered and the results indicated that the highest energy efficiency of 72.80% could be achieved when the H-SOEC/DMR process was operated at a temperature of 1123 K, pressure of 1 atm, and current density of 2500 A m^?2. Based on a pinch analysis, a heat exchanger network was applied to the H-SOEC/DMR process that improved the energy efficiency to 81.46%. Finally, an exergy analysis was performed and showed that the H-SOEC/DMR unit had the lowest exergy efficiency as a high-temperature exhaust gas was released.     

Efficient recovery of syngas from dry methane reforming product by a dual pressure swing adsorption process

In recent years, there has been growing interest in the dry reforming of methane (using CO₂ instead of H₂O) to obtain syngas, due to its economic and environmental advantages. In this reaction, to achieve conversions close to 100%, it is necessary to work at temperatures higher than 1000 °C. However, to attenuate the catalyst deactivation by sintering is convenient to work at lower temperatures, so that normally the syngas (mixture CO + H₂) is obtained mixed with unreacted CO₂ and CH₄. In this work a process has been simulated to recover the syngas from the product of a dry reforming reaction of methane at 700 °C by means of a Dual PSA (Dual Pressure Swing Adsorption) process with heavy reflux using BPL activated carbon as an adsorbent, operating at 25 °C. The process can recover syngas with purity and recovery higher than 99%. Unreacted CO₂ and CH₄ can be recycled to the reactor, leading to effective CO₂ and CH₄ conversions close to 100%. The process specific energy input (SEI) is 4.7 thermal kJ per L (STP) of syngas. The process can also be used to recover the syngas contained in the tail gas of a H₂ purification PSA from SMR-off gas.     

Highly selective high-silica SSZ-13 zeolite membranes for H2 production from syngas

A highly CO₂-selective high-silica SSZ-13 zeolite membrane was used for H₂ production by separating CO₂ from syngas (CO₂/H₂ mixture). High-silica SSZ-13 zeolite membranes were fabricated using outside asymmetric alumina tubes by secondary growth of ball-milled SSZ-13 seeds. The composition of membrane gel and synthesis time were modified. The Si/Al ratio in framework of the membrane was as high as 42 when SiO₂/Al₂O₃ ratio of the gel increased to 140. The effects of test parameters such as pressure drop, temperature, feed flow rate and concentration on membrane performance were investigated. The test pressure drop was up to 2 MPa. The ultra-high CO₂/H₂ selectivity of 161 with excellent CO₂ permeances of ~6.3 × 10⁻⁷ mol/(m² s Pa) (=3760 GPU) were observed for the best membrane at 243 K and pressure drop of 0.2 MPa. Carbon dioxide permeance through high-silica SSZ-13 zeolite membrane was 4.2 × 10⁻⁷ mol/(m² s Pa) (=2500 GPU) at 298 K and pressure drop of 2.0 MPa, and the CO₂/H₂ selectivity was 17.4. The current high-silica SSZ-13 zeolite membranes exceeded the upper bound of polymeric membranes and other inorganic membranes in CO₂/H₂ plots and owned great potentials for H₂ production from syngas.     

Shock tube study on ignition delay of multi-component syngas mixtures – Effect of equivalence ratio

Ignition delays were measured in a shock tube for syngas mixtures with argon as diluent at equivalence ratios of 0.3, 1.0 and 1.5, pressures of 0.2, 1.0 and 2.0 MPa and temperatures from 870 to 1350 K. Results show that the influences of equivalence ratio on the ignition of syngas mixtures exhibit different tendency at different temperatures and pressures. At low pressure, the ignition delay increases with an increase in equivalence ratio at tested temperature. At high pressures, however, an opposite behavior is presented, that is, increasing equivalence ratio inhibits the ignition at high temperature and vice versa at intermediate temperature. The affecting degree of equivalence ratio on ignition delay is different for each mixture at given condition, especially for the syngas with high CO concentration. Sensitivity analyses demonstrate that reaction H + O₂ = O + OH (R1) dominates the syngas oxidation under all conditions. With the increase of CO mole fraction, reactions CO + OH = CO₂ + H (R27) and CO + HO₂ = CO₂ + OH (R29) become more important in the syngas ignition kinetics. With the increase of pressure, the reactions related to HO₂ and H₂O₂ play the dominate role. The opposite influence of equivalence ratio on ignition delay at high- and intermediate-temperatures is chemically interpreted through kinetic analyses.     

Biomass-derived CO2 rich syngas conversion to higher hydrocarbon via Fischer-Tropsch process over Fe–Co bimetallic catalyst

Biomass-derived syngas (CO₂ + CO + H₂) has emerged as a potential non-fossil fuel source to yield transportation fuel via Fischer Tropsch Synthesis (FTS) reaction. Thus, the present study demonstrates the conversion of CO₂ containing syngas into fuel range hydrocarbon via Fischer Tropsch Synthesis over Fe–Co bimetallic catalyst. The experimental tests were carried out in a fixed bed continuous reactor to investigate the effect of CO₂ on CO/CO₂ conversion. Accordingly, obtained data were validated by FTS kinetic model for a plug flow reactor. It was found that the unique combination of Fe and Co bimetallic catalyst facilitates both FTS and water gas shift (WGS) reaction simultaneously that helps to convert CO₂ along with CO. It was also observed that the presence of iron in the catalyst helps in conversion of CO₂ into hydrocarbons, only when a particular concentration of CO₂ in syngas is reached, i.e., critical ratio R_C (CO₂/CO + CO₂) due to the occurrence of reverse water gas reaction (RWGS) which varies with the temperature and the feed gas composition (H₂/CO/CO₂ molar ratio). At 240 °C and hydrogen deficient condition, the critical ratio was measured to be 0.74 whereas for hydrogen balanced condition, it was measured 0.6. The kinetic model developed in the present study predicted trends for % CO conversion, % carbon conversion, and % CO₂ conversion which is applicable for a wide range of critical ratio R_C (CO₂/(CO + CO₂) = 0 to 1). The model also predicted that a positive conversion of CO₂ could be achieved at lower CO₂ concentration by increasing the reaction temperature. At 260 °C and 280 °C, the value of Rc were 0.31 and 0.18 were measured.     

Syngas production through CH4-assisted co-electrolysis of H2O and CO2 in La0.8Sr0.2Cr0.5Fe0.5O3-δ-Zr0.84Y0.16O2-δ electrode-supported solid oxide electrolysis cells

High temperature co-electrolysis of H₂O/CO₂ allows for clean production of syngas using renewable energy, and the novel fuel-assisted electrolysis can effectively reduce consumption of electricity. Here, we report on symmetric cells YSZ-LSCrF | YSZ | YSZ-LSCrF, impregnated with Ni-SDC catalysts, for CH₄-assisted co-electrolysis of H₂O/CO₂. The required voltages to achieve an electrolysis current density of ?400 mA·cm^?2 at 850 °C are 1.0 V for the conventional co-electrolysis and 0.3 V for the CH₄-assisted co-electrolysis, indicative of a 70% reduction in the electricity consumption. For an inlet of H₂O/CO₂ (50/50 vol), syngas with a H₂:CO ratio of ≈2 can be always produced from the cathode under different current densities. In contrast, the anode effluent strongly depends upon the electrolysis current density and the operating temperature, with syngas favorably produced under moderate current densities at higher temperatures. It is demonstrated that syngas with a H₂:CO ratio of ≈2 can be produced from the anode at a formation rate of 6.5·mL min^?1·cm^?2 when operated at 850 °C with an electrolysis current density of ?450 mA·cm^?2.     

Hydrogen-enriched non-premixed jet flames: Compositional structures with near-wall effects

The species concentrations of non-premixed hydrogen and syngas flames were examined using results obtained from direct numerical simulation technique with flamelet generated manifold chemistry. Flames with pure H₂ and H₂/CO mixtures are discussed for an impinging jet flame configuration. Single-point data analyses are presented illustrating the effects of fuel composition on species concentrations. In general, scatterplots of all species show the effects of fuel variability on the flame compositional structures. The behaviours of major combustion products and key radicals species indicate the effects of CO concentration on the 2/CO syngas combustion. In particular, high concentration of CO tends to induce local extinction in the 2/CO flames in which critical chemical reactions of the fuel mixture such as CO + OH become important. The unsteady fluctuations of species profiles in the wall jet region characterise the complexity of the distributions of compositional structures in the near-wall region with respect to the effects of CO concentration on the combustion of hydrogen-enriched fuels.     

NO mechanisms of syngas MILD combustion diluted with N2, CO2, and H2O

The NO mechanism under the moderate or intense low-oxygen dilution (MILD) combustion of syngas has not been systematically examined. This paper investigates the NO mechanism in the syngas MILD regime under the dilution of N₂, CO₂, and H₂O through counterflow combustion simulation. The syngas reaction mechanism and the counterflow combustion simulation are comprehensively validated under different CO/H₂ ratios and strain rates. The effects of oxygen volume fraction, CO/H₂ ratio, pressure, strain rate, and dilution atmosphere are systematically investigated. For all the MILD cases, the contribution of the prompt and NO-reburning routes to the overall NO emission is less than 0.1% due to the lack of CH₄ in fuel. At atmospheric pressure, the thermal route only accounts for less than 20% of the total NO emission because of the low reaction temperature. Moreover, at atmospheric pressure, the contribution of the NNH route to NO emission is always larger than 55% in the N₂ atmosphere. The N₂O-intermediate route is enhanced in CO₂ and H₂O atmospheres due to the increased third-body effects of CO₂ and H₂O through the reaction N₂ + O (+M) ? N₂O (+M). Especially in the H₂O atmosphere, the N₂O-intermediate route contributes to 60% NO at most. NO production is reduced with increasing CO/H₂ ratio or pressure, mainly due to decreased NO formation from the NNH route. Importantly, a high reaction temperature and low NO emission are simultaneously achieved at high pressure. To minimize NO emission, the reactions should be operated at high values of CO/H₂ ratios (i.e., >4) and pressures (e.g., P > 10 atm), low oxygen volume fractions (e.g., X_O2 < 15%), and using H₂O as a diluent. This study provides a new fundamental understanding of the NO mechanism of syngas MILD combustion in N₂, CO₂, and H₂O atmospheres.     

Chemical effects of added CO2 on the extinction characteristics of H2/CO/CO2 syngas diffusion flames

Chemical effects of added CO₂ on flame extinction characteristics are numerically studied in H₂/CO syngas diffusion flames diluted with CO₂. The two representative syngas flames of 80% H₂ + 20% CO and 20% H₂ + 80% CO are inspected according to the composition of fuel mixture diluted with CO₂ and global strain rate. Particular concerns are focused on impact of chemical effects of added CO₂ 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 CO₂ reduce critical CO₂ 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 CO₂ + H→CO + OH. As a result the overall reaction rate decreases. These chemical effects of added CO₂ are similar in both well-burning flames far from extinction limit and flames at extinction. There is a mismatching in the behaviors between critical CO₂ mole fraction and maximum flame temperature at extinction. This anomalous phenomenon is also discussed in detail.