Numerical study on NO formation in CH4–O2–N2 diffusion flame diluted with CO2

Numerical study with momentum‐balanced boundary conditions has been conducted to grasp chemical effects of added CO₂, to either fuel‐ or oxidizer‐side on flame structure and NO emission behaviour in CH₄–O₂–N₂ diffusion flames. Cautious investigation is made for the comparison among the behaviours of principal chain branching and important H‐removal key reactions. This describes successfully the reason why flame temperatures for fuel‐side dilution are higher than those for oxidizer‐side dilution. The role of the principal chain branching reaction is also recognized to be important even in the change of major flame structure caused by chemical effects. The importantly contributing reaction steps to NO production are examined. The reduced production rates of thermal NO and prompt NO due to chemical effects are much more remarkable for fuel‐side dilution. It is also found that the reaction step, H+NO+M=HNO+M plays a decisive role of the formation and destruction of prompt NO. Copyright © 2005 John Wiley & Sons, Ltd.     

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.     

Effects of CO addition on laminar flame characteristics and chemical reactions of H2 and CH4 in oxy-fuel (O2/CO2) atmosphere

An experimental study is conducted to investigate the effect of CO addition on the laminar flame characteristics of H₂ and CH₄ flames in a constant-volume combustion system. In addition, one-dimensional laminar premixed flame propagation processes at the same conditions are simulated with the update mechanisms. Results show that all mechanisms could well predict the laminar flame speeds of CH₄/CO/O₂/CO₂ mixtures, when Z_CO is large. For mixtures with lower CO, the experimental laminar flame speeds are always smaller than the calculated ones with Han mechanism. For mixtures with larger or smaller Z_CO2, GRI 3.0, San diego and USC 2.0 mechanisms all overvalue or undervalue the laminar flame speeds. When CO ratio in the CH₄/CO blended fuels increases, laminar flame speed firstly increases and then decreases for the CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, San diego, Davis and Li mechanisms all undervalue the laminar flame speeds of H₂/CO/CO₂/CO₂ mixtures. Existing models could not well predict the nonlinear trend of the laminar flame speeds, due to complex chemical effects of CO on CH₄/CO or H₂/CO flames. Then, the detailed thermal, kinetic and diffusive effects of CO addition on the laminar flame speeds are discussed. Kinetic sensitivity coefficient is far larger than thermal and diffusive ones and this indicates CO addition influences laminar flame speeds mainly by the kinetic effect. Based on this, radical pool and sensitivity analysis are conducted for CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures. For CH₄/CO/O₂/CO₂ mixtures, elementary reaction R38H + O₂ ↔ O + OH and R99 OH + CO ↔ H + CO₂ are the most important branching reactions with positive sensitivity coefficients when CO ratio is relative low. As CO content increases in the CH₄/CO blended fuel, the oxidation of CO plays a more and more important role. When CO ratio is larger than 0.9, the importance of R99 OH + CO ↔ H + CO₂ is far larger than that of R38H + O₂ ↔ O + OH. The oxidation of CO dominates the combustion process of CH₄/CO/O₂/CO₂ mixtures. For H₂/CO/O₂/CO₂ mixtures, the most important elementary reaction with positive and negative sensitivity coefficients are R29 CO + OH ↔ CO₂ + H and R13H + O₂(+M) ↔ HO₂(+M) respectively. The sensitivity coefficient of R29 CO + OH ↔ CO₂ + H is increasing and then decreasing with the addition of CO in the mixture. Chemical kinetic analysis shows that the chemical effect of CO on the laminar flame propagation of CH₄/CO/O₂/CO₂ and H₂/CO/O₂/CO₂ mixtures could be divided into two stages and the critical CO mole fraction is 0.9.     

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.     

Numerical analysis of the effect of CO2 on combustion characteristics of laminar premixed methane/air flames

The freely-propagating laminar premixed flames of (CH₄+CO₂)/air mixtures were calculated with the PREMIX code at various CO₂ contents (0–0.35) and equivalence ratios (0.7–1.3). The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of CO₂ addition. The chemical effects of CO₂ and the changes of the mole fraction on the important active radicals CH₃, OH, H and O and the sensitivity of the main reactions contributing to their information were analyzed. The results show that with the increase of X_CO2, the laminar burning velocity and the adiabatic flame temperature are decreased. Moreover, the amount of NO_x produced and the mole concentrations and the net rates of main reactions of CH₃, OH, H and O also decrease as CO₂ is added. The dominant reactions responsible for the four free radicals are R38 H + O₂ = OH + H, followed by R52 H + CH₃ (+ M) = CH₄ (+ M) and R35 H + O₂ + H₂O = H₂O + HO₂.     

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.     

Computed NOx emission characteristics of opposed-jet syngas diffusion flames

This paper reported a numerical study on the NO_x 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 NO_x emission of H₂/CO synthetic mixture flames were examined. The analyses of detailed flame structures, chemical kinetics, and nitrogen reaction pathways indicate NO_x 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 (0.01 atm), 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 (5–10 atm), 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.     

Numerical study on flame structure in H2–O2/CO2 laminar flames

Numerical study, aimed at the understanding of the flame structure in O₂/CO₂ recycling combustion system, has been conducted with detailed chemistry. Special concern is focused on addition effect of carbon dioxide on flame structure in H₂–O₂ counterflow diffusion flame as a simulating configuration. To clarify chemical and thermal effects on flame structure, the comparison between predicted results with a virtual species X to displace the real carbon dioxide and with added carbon dioxide in oxidizer stream is made according to strain rate and the concentration of added CO₂. From the systematical comparison of a dominant radical producing reaction with a chain termination reaction the effects of strain rate and composition control of oxidizer stream on flame structure are estimated. It is found that the behaviours of C₁‐ and C₂‐branch species are a direct outcome of that of produced CO due to the breakdown of added CO₂. There exists a temperature dependency in the behaviour of produced CO and this competes for the behaviour of the produced CO with chemical effects due to the backward reaction of CO+OH=CO₂+H. Copyright © 2003 John Wiley & Sons, Ltd.     

Role of CO2 in the CH4 oxidation and H2 formation during fuel-rich combustion in O2/CO2 environments

The effect of CO₂ reactivity on CH₄ oxidation and H₂ formation in fuel-rich O₂/CO₂ combustion where the concentrations of reactants were high was studied by a CH₄ flat flame experiment, detailed chemical analysis, and a pulverized coal combustion experiment. In the CH₄ flat flame experiment, the residual CH₄ and formed H₂ in fuel-rich O₂/CO₂ combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O₂/CO₂ combustion was noticeably higher than that in air. In addition to this experiment, calculations were performed using CHEMKIN-PRO. They generally agreed with the experimental results and showed that CO₂ reactivity, mainly expressed by the reaction CO₂ + H → CO + OH (R1), caused the differences between air and O₂/CO₂ combustion under fuel-rich condition. R1 was able to advance without oxygen. And, OH radicals were more active than H radicals in the hydrocarbon oxidation in the specific temperature range. It was shown that the role of CO₂ was to advance CH₄ oxidation during fuel-rich O₂/CO₂ combustion. Under fuel-rich combustion, H₂ was mainly produced when the hydrocarbon reacted with H radicals. However, the hydrocarbon also reacted with the OH radicals, leading to H₂O production. In fact, these hydrocarbon reactions were competitive. With increasing H/OH ratio, H₂ formed more easily; however, CO₂ reactivity reduced the H/OH ratio by converting H to OH. Moreover, the OH radicals reacted with H₂, whereas the H radicals did not reduce H₂. It was shown that OH radicals formed by CO₂ reactivity were not suitable for H₂ formation. As for pulverized coal combustion, the tendencies of CH₄, CO, and H₂ formation in pulverized coal combustion were almost the same as those in the CH₄ flat flame.     

Numerical study on flame structure and NO formation in CH4–O2–N2 counterflow diffusion flame diluted with H2O

Numerical study on flame structure and NO emission behaviour has been conducted to grasp chemical effects of added H₂O on either fuel‐ or oxidizer‐side in CH₄–O₂–N₂ counterflow diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties of added H₂O, is introduced to feasibly isolate the chemical effects. Special concern is focused on the important role of remarkably produced OH radicals due to chemical effects of added H₂O on flame structure and NO emission. The reason why the difference of behaviours between the principal chain branching reaction rate and flame temperature appear is attributed to the drastic change of reaction step (R120) from the production to the consumption of OH. It is also, however, seen that the most important contribution of produced OH due to chemical effects of added H₂O is through reaction step (R127). The importantly contributing reaction steps to NO production are also examined. The production rates of thermal NO and prompt NO are suppressed by chemical effects of added H₂O. The contribution of the reaction steps related to HNO intermediate species to the production of prompt NO is also stressed. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

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.     

Laminar flame speed of H2/CH4/air mixtures with CO2 and N2 dilution

The laminar flame speeds of H₂/CH₄/air mixtures with CO₂ and N₂ dilution were systematic investigated experimentally and numerically over a wide range of H₂ blending ratios (0–75 vol%) with CO₂ (0–67 vol%) and N₂ (0–67 vol%) dilution in the fuels. The experimental measurements were conducted via the Bunsen flame method incorporating the Schlieren technique under the condition of equivalence ratios from 0.8 to 2.0. To gain an insightful understanding of the experimental observations, detailed numerical simulation was carried out using Chemkin-Pro with GRI3.0-Mech. The experimental measurements were also used to validate the corresponding performance of a semiempirical correlation derived through asymptotic analysis method coupled with the reduced chemistry mechanism. The results showed that at lower H₂ fraction (x_H2 ≤ 0.5), the laminar flame speeds of H₂/CH₄/air mixtures displayed great linearly increase with the growth of H₂ fractions. The combustion of mixtures with low H₂ contents was dominated by CH₄ conversion which was mainly controlled by the increasing OH radicals produced from the key oxidation reactions of H + O₂ = O + OH. With the further increase of H₂ fractions, the methane-dominated combustion gradually transformed into the methane-inhibited hydrogen combustion, resulting to the growth of laminar flame speeds was dramatical and non-linear. Due to the larger heat capacity and chemical kinetic effect, CO₂ presented a stronger dilution effect on reducing the laminar flame speeds than N₂. With the addition of CO₂, the increasing stronger competition for H radical through CO + OH = CO₂ + H with H + O₂ = O + OH due to the significant reduction of H mole fractions, leading to the larger decrease of laminar flame speeds of mixtures. Besides, although the contribution of thermal effect of CO₂ decreased near the equivalence ratio, the thermal effect of CO₂ still preformed the dominated contribution to the total dilution effect. A comparison between the experimental data and estimated results using the semiempirical correlation showed that, the correlation using new modified coefficients provided the satisfactorily accuracy predictions on the laminar flame speeds of diluted H₂/CH₄/air mixtures at lower x_H2 (x_H2 ≤ 0.5) and lower x_dilution (x_dilution = 0.25). The estimated results were generally located within a deviation range of ±20% errors except for two unsatisfactory eases occurred at conditions of x_H2 = 0.75 and x_CO2 = 0.67. The considerably poor predictions were attributed to the significantly variation of the chemical kinetics under high H₂ content and large CO₂ dilution conditions.     

Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

A study on the effect of CO₂ and H₂O dilution on the laminar burning characteristics of CO/H₂/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO₂ and H₂O diluted CO/H₂/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO₂ or H₂O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO₂ and H₂O diluted CO/H₂/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO₂ and H₂O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H₂ ratio of 20/80 changes from reaction R3 (OH + H₂ = H + H₂O) to R16 (HO₂+H = OH + OH) when CO₂ and H₂O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO₂ and H₂O are added. Laminar burning velocities and kinetics analysis indicate that CO₂ has a stronger chemical effect than H₂O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO₂ and H₂O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.     

The role of N2O and NNH in the formation of NO via HCN in hydrocarbon flames

A new way of forming HCN in flames via N₂O and NNH reacting with CH_i radicals is proposed and tested for rich and lean gaseous premixed flames of CH₄ and air and also of CH₄, N₂O and Ar. This new route is thermodynamically more probable than Fenimore’s direct reaction of N₂ with CH_i radicals. In fact, it is shown that the new mechanism is more important than Fenimore’s reaction in both rich and lean flames. Rate constants of the new reactions forming NO have been suggested on the basis of numerical modeling. It has been shown that the formation of NO through HCN is most effective as the result of reactions initiated by N₂O + CH₃ → CH₂NH + NO, followed by CH₂NH + H → H₂CN + H₂ and CH₂NH + O → H₂CN + OH. In flames of CH₄ and air, a substantial source of N₂O comes from the reverse of the reaction N₂O + CH₃ → CH₃O + N₂ in the reaction zone. A formula based on the steady state assumption and partial equilibrium limits the number of nitrogen conversion reactions to only 12; this was tested using a premixed flame of CH₄ and air.     

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.     

Flame structure and kinetic studies of carbon dioxide-diluted dimethyl ether flames at reduced and elevated pressures

The flame structure and kinetics of dimethyl ether (DME) flames with and without CO₂ dilution at reduced and elevated pressures were studied experimentally and computationally. The species distributions of DME oxidation in low-pressure premixed flat flames were measured by using electron-ionization molecular-beam mass spectrometry (EI-MBMS) at an equivalence ratio of 1.63 and 50 mbar. High-pressure flame speeds of lean and rich DME flames with and without CO₂ dilution were measured in a nearly-constant-pressure vessel between about 1 and 20 bar. The experimental results were compared with predictions from four kinetic models: the first was published by Zhao et al. (2008) [9], the second developed by the Lawrence Livermore National Laboratory (LLNL) (Kaiser et al., 2000) [13], and the third has been made available to us as the Aramco mechanism (Metcalfe et al., 2013) [14]; as the fourth, we have used an updated model developed in this study. Good agreement was found between measurements and predictions from all four models for all major and most typical intermediate species with and without CO₂ addition in low-pressure flat flame experiments. However, none of the models was able to reliably predict high-pressure flame speeds. Although the updated model improved the prediction of flame speeds for lean mixtures, errors remained for rich conditions at elevated pressure, likely due to uncertainty in the rates of CH₃ + H(+M) = CH₄(+M) and the branching and termination reaction pair of CH₃ + HO₂ = CH₃O + OH and CH₃ + HO₂ = CH₄ + O₂. CO₂ addition considerably decreased the flame speed. Kinetic comparisons between inert and chemically active CO₂ in DME flames showed that CO₂ addition affects rich and lean DME flame kinetics differently. For lean flames, both the inert third-body effect and the kinetic effect of CO₂ reduce H-atom production. However, for rich flames, the inert third-body effect increases H-atom production via HCO(+M) = H + CO(+M) and suppression of the kinetic effect of CO₂ by shifting the equilibrium of CO + OH = CO₂ + H.     

Numerical investigation of the effects of H2/CO/syngas additions on laminar premixed combustion characteristics of NH3/air flame

Co-firing NH₃ with H₂/CO/syngas (SYN) is a promising method to overcome the low reactivity of NH₃/air flame. Hence, this study aims to systematically investigate the laminar premixed combustion characteristics of NH₃/air flame with various H₂/CO/SYN addition loadings (0–40%) using chemical kinetics simulation. The numerical results were obtained based on the Han mechanism which can provide accurate predictions of laminar burning velocities. Results showed that H₂ has the greatest effects on increasing laminar burning velocities and net heat release rates of NH₃/air flame, followed by SYN and CO. CO has the most significant effects on improving NH₃/air adiabatic flame temperatures. The H₂/CO/SYN additions can accelerate NH₃ decomposition rates and promote the generation of H and NH₂ radicals. Furthermore, there is an evident positive linear correlation between the laminar burning velocities and the peak mole fraction of H + NH₂ radicals. The reaction NH₂ + NH <=> N₂H₂ + H and NH₂ + NO <=> NNH + OH have remarkable positive effects on NH₃ combustion. The mole fraction of OH × NH₂ radicals positively affects the net heat release rates. Finally, it was discovered that H radicals play an important role in the generation of NO. The H₂/CO/SYN additions can reduce the hydrodynamic and diffusional-thermal instabilities of NH₃/air flame. The NH₃ reaction pathways for NH₃–H₂/CO/SYN-air flames can be categorized mainly into NH₃–NH₂–NH–N–N₂, NH₃–NH₂–HNO–NO(?N₂O)–N₂ and NH₃–NH₂(?N₂H₂)–NNH–N₂. CO has the greatest influence on the proportions of three NH₃ reaction routes.     

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.     

Water enhanced methanol decomposition for simultaneous heat recovery and ready-to-use synthesis gas production over CO2 capture enhanced Ni/zeolite 4A catalyst

Methanol decomposition is considered as a “one stone two birds” approach for simultaneously recovering waste heat and affording synthesis gas. However, this approach requires efficient catalysts with high CO selectivity and low selectivity to byproducts. Herein, a rational design of CO₂ capture enhanced Ni/zeolite 4 A catalyst for synthesis gas production by water enhanced methanol decomposition is reported. 5%-Ni/NaA-500 catalyst achieves the Y_H2 of 80.6%, Y_CO of 76.2%, H₂/CO molar ratio of 2.11, high stability, low selectivity to CO₂ and CH₄, and no coke at 325 °C. Ni atoms highly disperse on the surface and microporous of zeolite 4 A, and the strong interaction between Ni atoms and zeolite 4 A inhibits the reduction of Ni atoms. Consequently, Ni³⁺, Ni²⁺ and Ni⁰ coexist in 5%-Ni/NaA-500, and the redox couples of Ni³⁺↔Ni²⁺, Ni²⁺↔Ni⁰, and Ni³⁺↔Ni⁰ will enhance the redox processes during methanol decomposition. CO₂ capture capacity of x%-Ni/NaA-Y below 350 °C promotes the reverse water gas shift reaction by concentrating CO₂ molecules, which hence increases CO selectivity and declines the selectivity to byproducts. The reaction path follows CH₃OH→CH₃O→CH₂O→CHO→CO. This work will pave the way to industrial applications that combine ready-to-use synthesis gas production and heat recovery.