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.     

Effects of CO2 and N2 dilution on the characteristics and NOX emission of H2/CH4/CO/air partially premixed flame

For the combustion of the mixture of blast furnace gas, natural gas, and coke oven gas in industrial burners, how to improve combustion efficiency and reduce pollutant emission are of significance. To accomplish this, an industrial partially premixed burner with a combustion diagnostic system is used to experimentally reveal the characteristics and NO_X emission of H₂/CH₄/CO/air flame under CO₂, N₂, and CO₂/N₂ (replacing half of N₂ with CO₂) dilution. NO_X emission and flame length, temperature profile, along with CO, CH₄, and CO₂ concentration profiles are analyzed with the three diluents in the fuel stream under different dilution rates (0–32% by volume). Experimental results show that for lean H₂/CH₄/CO combustion, greater proportions of CO₂ in the diluent affect flame characteristics in various ways. These effects include longer flame length, lower highest flame temperature, the highest flame temperature being located farther away from the nozzle, and the highest CO₂ concentration being located nearer the nozzle. Furthermore, results of CO, CH₄, and CO₂ concentrations indicate that chemical reactions in the flame are significantly affected by CO₂ owing to the series reaction CH₄?CH₃→CO?CO₂. Finally, increasing diluents or the ratio of CO₂ in diluents has the benefit of reducing NO_X emission.     

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.     

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.     

Effect of methane reforming before combustion on emission and calorimetric characteristics of its combustion process

In this study, combustion and emission characteristics of methane mixed with steam (CH₄/H₂O) and the products of methane reforming with steam (CO/H₂/H₂O) were compared. Four fuel compositions were analysed: CH₄+H₂O, CH₄+2H₂O, and products of complete methane reforming in these mixtures, respectively. A comparison was carried out through the numerical model created via Ansys Fluent 2019 R2. A combustion process was simulated using a non-premixed combustion model, standard k-ϵ turbulence model and P-1 radiation model. The combustor heat capacity for interrelated fuel compositions was kept constant due to air preheating before combustion. The inlet air temperature was varied to gain a better insight into the combustion behaviour at elevated temperatures. The effect of steam addition on the emission characteristics and flame temperatures was also evaluated. NO_x formation was assessed on the outlet of the combustion zone. The obtained results indicate that syngas has a higher combustion temperature than methane (in the same combustor heat capacity) and therefore emitted 27% more NO_x comparing to methane combustion. With the air inlet temperature increment, the pollutant concentration difference between the two cases decreased. Steam addition to fuel inlet resulted in lesser emissions both for methane and syngas by 57% and 28%, respectively. In summary, syngas combustion occurred at higher temperature and produced more NO_x emissions in all cases considered.     

Effect of CO2/N2/CH4 dilution on NO formation in laminar coflow syngas diffusion flames

The effect of CO₂/N_2/CH₄ dilution on NO formation in laminar coflow H₂/CO syngas diffusion flames was experimentally and numerically investigated. The results reveal that the NO emission index increases with H₂/CO mole ratio. In all cases, CO₂/N₂/CH₄ dilution can reduce the peak temperature of syngas flame and have the ability to reduce peak flame temperature is decreased in the following order: CO₂>N₂>CH₄. CO₂/N₂ dilution reduces the NO formation in syngas flame while CH₄ dilution promotes the NO formation. Besides, the dilution of CO₂/N₂/CH₄ can reduce the peak mole fraction of OH and its variations with H₂/CO mole ratio and dilution ratio show the same trend as the peak flame temperature variations. The height of the flame with CO₂ and N₂ dilution increases with dilution ratio. The flame with CH₄ dilution becomes higher and wider with the increase of dilution ratio.     

NO formation of opposed-jet syngas diffusion flames: Strain rate and dilution effects

The NO formation characteristics and reaction pathways of opposed-jet H₂/CO syngas diffusion flames were analyzed with a revised OPPDIF program which coupled a narrowband radiation model with detailed chemical kinetics in this work. The effects of strain rates ranging from 0.1 to 1000 s^?1 and diluents including CO₂, H₂O and N₂ on NO production rates were investigated for three typical syngas compositions. The numerical results demonstrated that NO is produced primary through NNH-intermediate route and thermal route at high strain rates, where the reaction of NH + O = NO + H (R51) also become more active. Near the strain rate of 10 s^?1, the flame temperature is the highest and thermal route is the dominant NO formation route, but NO would be consumed by reburn route where NO is converted to NH through HNO, especially for H₂-rich syngas. At low strain rates, radiative heat loss results in a lower flame temperature and further reduce NO formation, while the reaction of N + CO₂ = NO + CO (R140) become more important, especially for CO-rich syngas. With the diluents, NO production rates decreased with increasing dilution percentages. When the flame temperature is very high as the thermal route is dominant near strain rate of 10 s^?1, CO₂ dilution makes flame temperature and NO production rate the lowest. Toward both lower and higher strain rates, adding H₂O is more effective in reducing NO because R140 and NNH-intermediate route are suppressed the most by H₂O dilution respectively.     

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.     

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.     

Thermal and chemical contributions of added H2O and CO2 to major flame structures and NO emission characteristics in H2/N2 laminar diffusion flame

Numerical simulation with detailed chemistry has been carried out to clearly discriminate the thermal and chemical contributions of added diluents (H₂O and CO₂) to major flame structures and NO emission characteristics in H₂/N₂ counterflow diffusion flame. The pertinence of GRI, Miller–Bowman, and their recent modified mechanisms are estimated for the combined fuel of H₂, CO₂, and N₂. A virtual species X, which displaces the individual CO₂ and H₂O in the fuel sides, is introduced to separate chemical effects from thermal effects. In the case of H₂O addition the chain branching reaction, H + O₂ → O + OH is considerably augmented in comparison with that in the case of CO₂ addition. It is also seen that there exists a chemically super‐adiabatic effect in flame temperature due to the breakdown of H₂O. The reaction path of CH₂O→CH₂OH→CH₃ and the C1‐branch reactions become predominant due to the breakdown of CO₂. In NO emission behaviour super‐equilibrium effects caused by the surplus chain carrier radicals due to the breakdown of added H₂O are more superior to the enhanced effects of prompt NO with the breakdown of added CO₂. Especially, it is noted that thermal NO emission is directly influenced by the chemical super‐equilibrium effects of chain carrier radicals in the case of H₂O addition. As a result the overall NO emission in the case of the addition of H₂O is higher than that in the case of CO₂ addition. Copyright © 2002 John Wiley & Sons, Ltd.     

Influence of H2 on the response of lean premixed CH4 flames to high strained flows

A combined experimental and numerical investigation on the effects of H₂ addition to lean-premixed CH₄ flames in highly strained counterflow fields (with strain rates up to 8000 s⁻¹) using preheated flows indicate significant enhancement of lean flammability limits and extinction strain rates for relatively small amounts of H₂ addition. Numerical modeling of the counterflow opposed jet configuration used in this study indicated extinction strain rates which were within 5% of experimentally measured values for equivalence ratios ranging from 0.75 to less than 0.4. Both experimental and numerical results indicate that increasing H₂ in the fuel significantly increases flame speeds and thus extinction strain rates. Furthermore, increasing H₂ decreases the dependency of extinction equivalence ratio on the strain rate of the flow. For all of the mixtures investigated, extinction temperatures depend primarily on equivalence ratio and not fuel composition for the range of H₂ content studied, which suggests that extinction can be correlated to flame temperature and O₂ concentration. Nonetheless, H₂ addition greatly increases the maximum allowable strain rate before extinction temperatures are reached. Inspection of the model-predicted species profiles suggest that the enhancement of CH₄ burning rates with H₂ addition is driven by early H₂ breakdown increasing radical production rates early in the flame zone to enhance CH₄ ignition under conditions where otherwise CH₄ combustion might be prone to undergo extinction.     

Chemical effect of diluents on flame structure and NO emission characteristic in methane‐air counterflow diffusion flame

The dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically simulated with detailed chemistry in CH₄/air counterflow diffusion flame. The volume percentage of diluents (H₂O, CO₂, and N₂) in air stream is systematically changed from 0 to 10. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour especially at low strain rates. The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO₂>H₂O>N₂, even if heat capacity of H₂O is the highest. It is also found that the addition of CO₂ shows the tendency towards the reduction of flame temperature in both the thermal and chemical sides, while the addition of H₂O enhances the reaction chemically and restrains it thermally due to a super‐equilibrium effect of the chain carrier radicals caused by the breakdown of H₂O in high‐temperature region. The comparison of the nitrogen chemical reaction pathway between the cases of the addition of CO₂ and H₂O clearly displays that the addition of CO₂ is much more effective to reduce NO emission. Copyright © 2002 John Wiley & Sons, Ltd.     

Investigation of dilution effects on partially premixed swirling syngas flames using a LES-LEM approach

The dilution effects of CO₂ and H₂O on partially premixed swirling syngas flames are investigated with the large eddy simulation (LES) method. The linear-eddy model (LEM) is employed to directly resolve the unclosed molecular diffusion, scalar mixing and chemical reaction processes occurring at subgrid scale level using their specific length and time scales instead of modelling, which makes the LES-LEM approach quite attractive for hydrogen fuel combustion as the obviously different diffusion and reaction characteristics of H₂ and H compared to other species in the syngas mixture. Firstly, adding CO₂ into the fuel stream can significantly decrease the flame temperature during the partially premixed combustion. The concentration of H and OH radicals decreases upon CO₂ dilution and thus the chemical reaction processes are modified. Compared with CO₂, H₂O is less effective in changing the temperature field because of the chemical effects of H₂O. The simultaneous addition of H₂O and CO₂ as dilution gases with volume ratio 1:1 into the fuel stream is also conducted to identify the effects of H₂O and CO₂ on partially premixed combustion dynamics by comparing with single H₂O and CO₂ cases. The obtained results are expected to provide helpful information for the design and operation of gas turbine combustion systems with syngas fuels.     

Influences of different diluents on NO emission characteristics of syngas opposed-flow flame

This paper used the opposed-flow flame model and GRI 3.0 mechanism to investigate NO emission characteristics of H₂-rich and H₂-lean syngas under diffusion and premixed conditions, respectively, and analyzed influences of adding H₂O, CO₂ and N₂ on NO formation from the standpoint of thermodynamics and reaction kinetics. For diffusion flames, thermal route is the dominant pathway to produce NO, and adding N₂, H₂O and CO₂ shows a decreasing manner in lowering NO emission. The phenomenon above is more obvious for H₂-rich syngas because it has higher flame temperature. For premixed flames, adding CO₂ causes higher NO concentration than adding H₂O, because adding CO₂ produces more O radical, which promotes formation of NO through NNH + O = NH + NO, NH + O = NO + H and reversed N + NO = N₂ + O. And in burnout gas, thermal route is the dominant way for NO formation. Under this paper's conditions, adding N₂ increases the formation source of NO as well as decreases the flame temperature, and it reduces the NO formation as a whole. In addition, for H₂-lean syngas and H₂-rich syngas with CO₂ as the diluent, N + CO₂ = NO + CO plays as an important role in thermal route of NO formation.     

Turbulent flame topology and the wrinkled structure characteristics of high pressure syngas flames up to 1.0 MPa

The turbulent flame topology characteristics of the model syngas with two different hydrogen ratios were statistically investigated, namely CO/H₂ ratio at 65/35 and 80/20, at equivalence ratio of 0.7. The combustion pressure was kept at 0.5 MPa and 1.0 MPa, to simulate the engine-like condition. The model syngas was diluted with CO₂ with a mole fraction of 0.3 which mimics the flue gas recycle in the turbulent combustion. CH₄/air flame with equivalence ratio of 1.0 was also tested for comparison. The flame was anchored on a premixed type Bunsen burner, which can generate a controllable turbulent flow. Flame front, which is represented by the sharp increased interface of the OH radical distribution, was measured with OH-PLIF technique. Flame front parameters were obtained through image processing to interpret the flame topology characteristics. Results showed that the turbulent flames possess a wrinkled character with smaller scale concave/convex structure superimposed on a larger scale convex structure under high pressure. The wrinkled structure of syngas flame is much finer and more corrugated than hydrocarbon fuel flames. The main reason is that scale of wrinkled structure is smaller for syngas flame, resulting from the unstable physics. Hydrogen in syngas can increase the intensity of the finer structure. Moreover, the model syngas flames have larger flame surface density than CH₄/air flame, and hydrogen ratio in syngas can increase flame surface density. This would be mainly attributed to the fact that the syngas flames have smaller flame intrinsic instability scale l_i than CH₄/air flame. S_T/S_L of the model syngas tested in this study is higher than CH₄/air flames for both pressures, due to the high diffusivity and fast burning property of H₂. This is mainly due to smaller L_M and l_i. V_f of the two model syngas is much smaller than CH₄/air flames, which suggests that syngas flame would lead to a larger possibility to occur combustion oscillation.     

Effects of fuel compositions on the heat generation and emission of syngas/producer gas laminar diffusion flame

Demand for the clean and sustainable energy encourages the research and development in the efficient production and utilisation of syngas for low-carbon power and heating/cooling applications. However, diversity in the chemical composition of syngas, resulting due to its flexible production process and feedstock, often poses a significant challenge for the design and operation of an effective combustion system. To address this, the research presented in this paper is particularly focused on an in-depth understanding of the heat generation and emission formation of syngas/producer gas flames with an effect of the fuel compositions. The heat generated by flame not only depends on the flame temperature but also on the chemistry heat release of fuel and flame dimension. The study reports that the syngas/producer gas with a low H_2:CO maximises the heat generation, nevertheless the higher emission rate of CO₂ is inevitable. The generated heat flux at H_2:CO = 3:1, 1:1, and 1:3 is found to be 222, 432 and 538 W m^-2 respectively. At the same amount of heat generated, H₂ concentration in fuel dominates the emission of NO_x. The addition of CH₄ into the syngas/producer gas with H_2:CO = 1:1 also increases the heat generation significantly (e.g. 614 W m^-2 at 20%) while decreases the emission formation. In contrast, adding 20% CO₂ and N₂ to the syngas/producer gas composition reduces the heat generation from 432 W m^-2 to 364 and 290 W m^-2, respectively. The role of CO₂ on this aspect, which is weaker than N₂, thus suggests CO₂ is preferable than N₂. Along with the study, the significant role of CO₂ on the radiation of heat and the reduction of emission are examined.     

Influence of reactive species on the lean blowout limit of an industrial DLE gas turbine burner

In order to achieve ultra-low emissions of both NO_X and CO it is imperative to use a homogeneous premixed combustor. To lower the emissions further, the equivalence ratio can be lowered. By doing so, combustion is moved towards the lean blowout (LBO) limit. To improve the blowout characteristics of a burner, heat and radicals can be supplied to the flame zone. This can be achieved using a pre-chamber combustor. In this study, a central body burner, called the RPL (rich-pilot-lean) section, was used as a pre-chamber combustor to supply heat and radicals to a downscaled industrial burner. The flue gas from the RPL is mixed with the surrounding fresh mixture and form a second flame zone. This zone acts as a stabilizer for the investigated burner. The LBO limit was modeled using two perfectly stirred reactors (PSRs) in series, which allows the chemical influence on the LBO limit to be isolated. The resulting trends for the modeled LBO limit were in agreement with measured data. Increasing the equivalence ratio in the RPL section, thus increasing the energy supplied by the fuel, is a major contributor to combustion stability up to a limit where the temperature decrease is too large support combustion. For lean RPL combustion, the reactive species O, H and OH in combination affect the stability to a greater extent than the temperature alone. At rich equivalence ratios, the conversion of methane to hydrogen and carbon monoxide in the RPL section is a factor influencing the LBO limit. The results are compared with emission probe measurements that were used to investigate the LBO limit for methane and a generic syngas (10% CH₄, 67.5% H₂, and 22.5% CO). The syngas was also investigated after being diluted with nitrogen to a Wobbe index of 15 MJ/m³.     

Flame front structure of turbulent premixed flames of syngas oxyfuel mixtures

In order to investigate oxyfuel combustion characteristics of typical composition of coal gasification syngas connected to CCS systems. Instantaneous flame front structure of turbulent premixed flames of CO/H₂/O₂/CO₂ mixtures which represent syngas oxyfuel combustion was quantitatively studied comparing with CH₄/air and syngas/air flames by using a nozzle-type Bunsen burner. Hot-wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Image processing and statistical analyzing were performed using the Matlab Software. Flame surface density, mean progress variable, local curvature radius, mean flame volume, and flame thickness, were obtained. Results show that turbulent premixed flames of syngas possess wrinkled flame front structure which is a general feature of turbulent premixed flames. Flame surface density for the CO/H₂/O₂/CO₂ flame is much larger than that of CO/H₂/O₂/air and CH₄/air flames. This is mainly caused by the smaller flame intrinsic instability scale, which would lead to smaller scales and less flame passivity response to turbulence presented by Markstain length, which reduce the local flame stretch against turbulence vortex. Peak value of Possibility Density Function (PDF) distribution of local curvature radius, R, for CO/H₂/O₂/CO₂ flames is larger than those of CO/H₂/O₂/air and CH₄/air flames at both positive and negative side and the corresponding R of absolute peak PDF is the smallest. This demonstrates that the most frequent scale is the smallest for CO/H₂/O₂/CO₂ flames. Mean flame volume of CO/H₂/O₂/CO₂ flame is smaller than that of CH₄/air flame even smaller than that of CO/H₂/O₂/air flame. This would be due to the lower flame height and smaller flame wrinkles.     

Kinetic effects of non-equilibrium plasma-assisted methane oxidation on diffusion flame extinction limits

The kinetic effects of low temperature non-equilibrium plasma assisted CH₄ oxidation on the extinction of partially premixed methane flames was studied at 60 Torr by blending 2% CH₄ by volume into the oxidizer stream of a counterflow system. The experiments showed that non-equilibrium plasma can dramatically accelerate the CH₄ oxidation at low temperature. The rapid CH₄ oxidation via plasma assisted combustion resulted in fast chemical heat release and extended the extinction limits significantly. Furthermore, experimental results showed that partial fuel mixing in the oxidizer stream led to a dramatic decrease of O concentration due to its rapid consumption by CH₄ oxidation at low temperature. The products of plasma assisted CH₄ oxidation were measured using the Two-photon Absorption Laser-Induced Fluorescence (TALIF) method (for atomic oxygen, O), Fourier Transform Infrared (FTIR) spectroscopy, and Gas Chromatography (GC). The product concentrations were used to validate the plasma assisted combustion kinetic model. The comparisons showed the kinetic model over-predicted the CO, H₂O and H₂ concentrations and under-predicted CO₂ concentration. A path flux analysis showed that O generated by the plasma was the critical species for extinction enhancement. In addition, the results showed that O was produced mainly by direct electron impact dissociation reactions and the collisional dissociation reactions of electronically excited molecules with O₂. Moreover, these reactions involving electron impact and excited species collisional dissociation of CH₄ contributed approximately a mole fraction of 0.1 of total radical production. The present experiments produced quantitative species and extinction data of low temperature plasma assisted combustion to constrain the uncertainties in plasma/flame kinetic models.     

Effect of hydrogen ratio on turbulent flame structure of oxyfuel syngas at high pressure up to 1.0 MPa

The CO/H₂/CO₂/O₂, CO/H₂/CO₂/air turbulent premixed flames as the model of syngas oxyfuel and syngas/air combustion were studied experimentally and compared to that of CH₄/air mixtures at high pressures up to 1.0 MPa. Hydrogen ratio in syngas was set to be 35%, 50% and 65% in volumetric fraction. Four perforated plates are used to generate wide range of turbulence intensity and scales. The instantaneous flame structure was measured with OH-PLIF technique and then statistic flame structure parameters and turbulent burning velocity were derived to interpret the multi scale turbulence-flame interaction. Results show that the flame structure of syngas is wrinkled and convex cusps to the unburned mixtures are sharper and deeper comparing to that of CH₄ flames. Pressure has a dominating effect on flame wrinkling other than mixtures composition at high pressure of 1.0 MPa. The flame surface density, Σ of syngas is larger than that of CH₄. The Σ of syngas flames is almost independent on pressure and hydrogen ratio especially when hydrogen ratio is over 50% which is a significant feature of syngas combustion. Larger flame surface density for syngas flames mainly comes from the finer structure with smaller wrinkles which is the result of more intensive flame intrinsic instability. The S_T/S_L of syngas is larger than CH₄ and it slightly increases with the pressure rise. The S_T/S_L of syngas oxyfuel is similar to that of syngas/air flames in the present study. The S_T/S_L increases with the increase of hydrogen ratio and keeps almost constant when hydrogen ratio is over 50%.