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.     

Laminar flame speed correlations for pure-hydrogen and high-hydrogen content syngas blends with various diluents

Hydrogen is an attractive fuel for large-scale combustion systems due to its high flame speeds and clean burning characteristics. This paper presents a new set of correlations for the laminar flame speeds of hydrogen-oxygen mixtures with nitrogen (air) and helium as diluents, using a recently updated chemical kinetics mechanism. A wide excursion of equivalence ratios (φ = 0.5–5.0), pressures (1–30 atm) and temperatures (270–620 K) was performed. Flame speed correlations were developed at five pressures, namely 1, 5, 10, 20, and 30 atm for the pure-hydrogen case. The disparities between the kinetic model predictions and the correlation estimates, commonly associated with existing correlations, were significantly reduced, and the correlation estimates are within ±13 cm/s of the model predictions. Also, a correlation for lean and high-hydrogen content (HHC) syngas blends of H₂ + CO + H₂O was developed from the pure-hydrogen correlations. A wide range of pressures (1–30 atm), initial temperatures (323–550 K), steam contaminant levels (5–15%), and hydrogen content in the fuel blend (15–100%) were simulated. A design of experiments approach was adopted to determine the critical mixtures necessary to develop the correlation. The developed HHC correlation agrees within ±12% of the model predictions.     

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.     

Experimental studies on the three-dimensional effects of opposed-flow flame spread over thin solid materials

The three-dimensional effects of flame spread over thin solid materials were experimentally studied using a natural-convection-suppressing horizontal narrow-channel. In a sufficiently wide narrow-channel, the variation of flame spread against the width of the material sample showed different trends for different gas flow speeds and oxygen concentrations. The extent of three-dimensional effects was inversely proportional to the gas flow speed or its square. Near quenching extinction limits, the effects were significant because weak combustion is sensitive to a slight variation of heat loss and oxygen concentration. The effects may be due to different factors such as side heat loss, side oxygen diffusion, or both. Far away from quenching extinction limits, the effects were weak because vigorous combustion is insensitive to a small variation of oxygen concentration and heat loss. In all tests, the effects were limited to the samples of width less than 10 times of the diffusion length. Moreover, a higher oxygen concentration suppressed the effects at a lower gas flow speed. For sufficiently wide samples, in the most range of gas flow speeds, the channel width had almost no effect on flame spread. However, near extinction limits, the flame spread rate decreased with the increasing channel width.     

An experimental and kinetic modeling study of n-octane and 2-methylheptane in an opposed-flow diffusion flame

Fischer–Tropsch (FT) fuels derived from biomass syngas are renewable fuels that can replace conventional petroleum fuels in jet engine and diesel engine applications. FT fuels typically contain a high concentration of lightly methylated iso-alkanes, whereas petroleum derived jet and diesel fuels contain large fractions of n-alkanes, cycloalkanes, and aromatics plus some lightly methylated iso-alkanes. In order to better understand the combustion characteristics of FT and petroleum fuels, this study presents new experimental data for 2-methylheptane and n-octane in an opposed-flow diffusion flame. The high temperature oxidation of 2-methylheptane and n-octane has been modeled using an extended transport database and a reaction mechanism consisting of 3401 reactions involving 714 species. The proposed model shows good qualitative and quantitative agreement with the experimental data. The measured and predicted concentrations of 1-alkenes and ethylene are higher in the n-octane flame, while the concentrations of iso-alkenes (especially iso-butene) and propene are higher in the 2-methylheptane flame. The proposed chemical kinetic model is used to delineate the reactions pathways leading to these observed differences in product species concentrations. An uncertainty analysis was conducted to assess experimental and modeling uncertainties. The results indicate that the simulations are sensitive to the transport parameters used to calculate fuel diffusion.     

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.     

A detailed numerical study of NOx kinetics in low calorific value H2/CO syngas flames

The present study provides an extensive and detailed numerical analysis of NO_x chemical kinetics in low calorific value H₂/CO syngas flames utilizing predictions by five chemical kinetic mechanisms available out of which four deal with H₂/CO while the fifth mechanism (GRI 3.0) additionally accounts for hydrocarbon chemistry. Comparison of predicted axial NO profiles in premixed flat flames with measurements at 1 bar, 3.05 bar and 9.15 bar shows considerably large quantitative differences among the various mechanisms. However, at each pressure, the quantitative reaction path diagrams show similar NO formation pathways for most of the mechanisms. Interestingly, in counterflow diffusion flames, the quantitative reaction path diagrams and sensitivity analyses using the various mechanisms reveal major differences in the NO formation pathways and reaction rates of important reactions. The NNH and N₂O intermediate pathways are found to be the major contributors for NO formation in all the reaction mechanisms except GRI 3.0 in syngas diffusion flames. The GRI 3.0 mechanism is observed to predict prompt NO pathway as the major contributing pathway to NO formation. This is attributed to prediction of a large concentration of CH radical by the GRI 3.0 as opposed to a relatively negligible value predicted by all other mechanisms. Also, the back-conversion of NNH into N₂O at lower pressures (2–4 bar) was uniquely observed for one of the five mechanisms. The net reaction rates and peak flame temperatures are used to correlate and explain the differences observed in the peak [NO] at different pressures. This study identifies key reactions needing assessment and also highlights the need for experimental data in syngas diffusion flames in order to assess and optimize H₂/CO and nitrogen chemistry.     

Experimental study of premixed syngas/air flame deflagration in a closed duct

The propagation behaviour of a deflagration premixed syngas/air flame over a wide range of equivalence ratios is investigated experimentally in a closed rectangular duct using a high-speed camera and pressure transducer. The syngas hydrogen volume fraction, φ, ranges from 0.1 to 0.9. The flame propagation parameters such as flame structure, propagation time, velocity and overpressure are obtained from the experiment. The effects of the equivalence ratio and hydrogen fraction on flame propagation behaviour are examined. The results indicate that the hydrogen fraction in a syngas mixture greatly influences the flame propagation behaviour. When φ, the hydrogen fraction, is ≥0.5, the prominently distorted tulip flame can be formed in all equivalence ratios, and the minimum propagation time can be obtained at an equivalence ratio of 2.0. When φ < 0.5, the tulip flame distortion only occurs in a hydrogen fraction of φ = 0.3 with an equivalence ratio of 1.5 and above. The minimum flame propagation time can be acquired at an equivalence ratio of 1.5. The distortion occurs when the maximum flame propagation velocity is larger than 31.27 m s^?1. The observable oscillation and stepped rise in the overpressure trajectory indicate that the pressure wave plays an important role in the syngas/air deflagration. The initial tulip distortion time and the plane flame formation time share the same tendency in all equivalence ratios, and the time interval between them is nearly constant, 4.03 ms. This parameter is important for exploring the quantitative theory or models of distorted tulip flames.     

Study on laminar flame speed and flame structure of syngas with varied compositions using OH-PLIF and spectrograph

Various Bunsen flame information of premixed syngas/air mixtures was systematically collected. A CCD camera was used to capture the flame images. The OH-PLIF technique was applied to obtain the flame OH distribution and overall flame radiation spectra were measured with a spectrograph. Experiments were conducted on a temperature un-controlled burner and syngas over a wide range of H₂/CO ratios (from 0.25 to 4) and equivalence ratios (from 0.5 to 1.2). Results show that increasing hydrogen fraction (XH₂$X_{{\text{H}}_2}$) extends the blow-off limit significantly. The measured laminar flame speed using cone-angle method based on CCD flame imaging and OH-PLIF images increases remarkably with the increase of XH₂$X_{{\text{H}}_2}$, and these measurements agrees well with kinetic modeling predictions through Li's mechanism when the temperature for computation is corrected. Kinetic study shows that as XH₂$X_{{\text{H}}_2}$ increases, the production of H and OH radicals is accelerated. Additionally, the main H radical production reaction (or OH radical consumption reactions) changes from R29 (CO + OH = CO₂ + H) to R3 (H₂ + OH = H₂O + H) as XH₂$X_{{\text{H}}_2}$ increases. Sensitivity analysis was conducted to access the dominant reactions when XH₂$X_{{\text{H}}_2}$ increases. The difference on flame color for different XH₂$X_{{\text{H}}_2}$ mixtures is due to their difference in radiation spectrum of the intermediate radicals produced in combustion.     

A numerical study of laminar flame speed of stratified syngas/air flames

Numerical simulations are performed to study the flame propagation of laminar stratified syngas/air flames with the San Diego mechanism. Effects of fuel stratification, CO/H₂ mole ratio and temperature stratification on flame propagation are investigated through comparing the distribution of flame temperature, heat release rate and radical concentration of stratified flame with corresponding homogeneous flame. For stratified flames with fuel rich-to-lean and temperature high-to-low, the flame speeds are faster than homogeneous flames due to more light H radical in stratified flames burned gas. The flame speed is higher for case with larger stratification gradient. Contrary to positive gradient cases, the flame speeds of stratified flames with fuel lean-to-rich as well as with temperature low-to-high are slower than homogeneous flames. The flame propagation accelerates with increasing hydrogen mole ratio due to higher H radical concentration, which indicates that chemical effect is more significant than thermal effect. Additionally, flame displacement speed does not match laminar flame speed due to the fluid continuity. Laminar flame speed is the superposition of flame displacement speed and flow velocity.     

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).     

Measurement of the instantaneous flame front structure of syngas turbulent premixed flames at high pressure

Instantaneous flame front structure of syngas turbulent premixed flames including the local radius of curvature, the characteristic radius of curvature, the fractal inner cutoff scale and the local flame angle were derived from the experimental OH-PLIF images. The CO/H₂/CO₂/air flames as a model of syngas/air combustion were investigated at pressure of 0.5 MPa and compared to that of CH₄/air flames. The convex and concave structures of the flame front were detected and statistical analysis including the PDF and ADF of the local radius of curvature and local flame angle were conducted. Results show that the flame front of turbulent premixed flames at high pressure is a wrinkled flame front with small scale convex and concave structures superimposed with large scale flame branches. The convex structures are much more frequent than the concave ones on flame front which reflects a general characteristic of the turbulent premixed flames at high pressure. The syngas flames possess much wrinkled flame front with much smaller fine cusps structure compared to that of CH₄/air flames and the main difference is on the convex structure. The effect of turbulence on the general wrinkled scale of flame front is much weaker than that of the smallest wrinkled scale. The general wrinkled scale is mainly dominated by the turbulence vortex scale, while, the smallest wrinkled scale is strongly affected by the flame intrinsic instability. The effect of flame intrinsic instability on flame front of turbulent premixed flame is mainly on the formation of a large number of convex structure propagating to the unburned reactants and enlarge the effective contact surface between flame front and unburned reactants.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

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.     

Experimental and numerical study of the effect of initial temperature on the combustion characteristics of premixed syngas/air flame

The effects of different initial temperatures (T = 300–500 K) and different hydrogen volume fractions (5%–20%) on the combustion characteristics of premixed syngas/air flames in rectangular tubes were investigated experimentally. A high-speed camera and pressure sensor were used to obtain flame propagation images and overpressure dynamics. The CHEMKIN-PRO model and GRI Mech 3.0 mechanism were used for simulation. The results show that the flame propagation speed increases with the initial temperature before the flame touches the wall, while the opposite is true after the flame touches the wall. The increase in initial temperature leads to the increase in overpressure rise rate in the early flame propagation process, but the peak overpressure is reduced. The laminar burning velocity (LBV) and adiabatic flame temperature (AFT) increase with increasing initial temperature. The increase in initial temperature makes the peaks of H, O, and OH radicals increase.     

Effects of hydrogen addition on structure and NO formation of highly CO-Rich syngas counterflow nonpremixed flames under MILD combustion regime

The present study has numerically investigated the Moderate or Intense Low oxygen Dilution (MILD) combustion regime, combustion processes and NO formation characteristics of the highly CO-rich syngas counterflow nonpremixed flames. To realistically predict the flame properties of the highly CO-rich syngas, the chemistry is represented by the modified GRI 3.0 mechanism. Computations are performed to precisely analyze the flame structure, NO formation rate, and EINO of each NO sub-mechanism. Numerical results reveal that the hydrogen enrichment and oxygen augmentation substantially influence the NO emission characteristics and the dominant NO production route in the CO-rich syngas nonpremixed flames under MILD and high temperature combustion regimes. It is found that the most dominant NO production routes are the NNH path for the lowest oxygen level (3%) and the thermal mechanism for the highest O₂ condition (21%). For the intermediate oxygen level (9%), the most dominant NO production routes are the NNH route for the hydrogen fraction up to 5%, the CO₂ path for the hydrogen fraction range from 5% to 10% and the thermal mechanism for the hydrogen fraction higher than 10%, respectively. To evaluate the contribution of the specific reaction on EINO the sensitivity coefficients are precisely analyzed for NO formation processes with the dominance of NNH/CO₂/Thermal mechanism under the highly CO-rich syngas flames.     

Numerical and experimental study of the influence of CO2 dilution on burning characteristics of syngas/air flame

In this study, effect of carbon dioxide dilution on explosive behavior of syngas/air mixture was investigated numerically and experimentally. Explosion in a 3-D cylindrical geometry model with dimensions identical to the chamber used in the experiment was simulated using ANSYS Fluent. The simulated results showed that after ignition, the flame front propagated outward spherically until it touched the wall, like the propagating flame observed in the experiment. Both experimental and simulated results presented a same trend of decreasing the maximum explosion pressure and prolonging the explosion time with CO₂ dilution. The results showed that for CO₂ additions, the maximum explosion pressure decreased linearly and the explosion time increased linearly, while the maximum rate of pressure rise decreased nonlinearly, which can be correlated to an exponential equation. In addition, both results showed a good agreement for syngas/air flame with CO₂ addition up to 20% in volume. However, larger discrepancies were observed for higher levels of CO₂ dilutions. Of the three diluents tested, carbon dioxide displayed the strongest effect in reducing explosion hazard of syngas/air flame compared to helium and nitrogen. Chemical kinetic analysis results showed that maximum concentration of major radicals and net reaction rates of important reactions drastically decreased with CO₂ addition, causing a reduction of laminar flame speed.     

Laminar flame speeds of moist syngas mixtures

This work experimentally investigates the effect of the presence of water vapor on the laminar flame speeds of moist syngas/air mixtures using the counterflow twin-flame configuration. The experimental results presented here are for fuel lean syngas mixtures with molar percentage of hydrogen in the hydrogen and carbon monoxide mixture varying from 5% to 100%, for an unburned mixture temperature of 323 K, and under atmospheric pressure. At a given equivalence ratio, the effect of varying amount of water vapor addition on the measured laminar flame speed is demonstrated. The experimental laminar flame speeds are also compared with computed values using chemical kinetic mechanisms reported in the literature. It is found that laminar flame speed varies non-monotonically with addition of water for the carbon monoxide rich mixtures. It first increases with increasing amount of water addition, reaches a maximum value, and then decreases. An integrated reaction path analysis is further conducted to understand the controlling mechanism responsible for the non-monotonic variation in laminar flame speed due to water addition. On the other hand, for higher values of H₂/CO ratio the laminar flame speed monotonically decreases with increasing water addition. It is shown that the competition between the chemical and thermal effects of water addition leads to the observed response. Furthermore, reaction rate sensitivity analysis as well as binary diffusion coefficient sensitivity analysis are conducted to identify the possible sources of discrepancy between the experimental and predicted values. The sensitivity results indicate that the reaction rate constant of H₂ + OH = H₂O + H is worth revisiting and refinement of binary diffusion coefficient data of N₂–H₂O, N₂–H₂, and H₂–H₂O pairs can be considered.