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.     

Effects of CO and H2 on the formation of N2O via catalytic NO reduction

The formation of N₂O in a mixture of NO, CO, H₂, O₂ and N₂ was investigated experimentally in a tubular-flow reactor containing a catalyst.It was found that the reduction of NO is enhanced by the presences of H₂, and to a lesser extent CO, and that N₂O is formed as a by-product of NH₃ decomposition and NO reduction in the presence of H₂, and through NO reduction in the presence of CO. The main product of NH₃ oxidation is N₂ in addition to the products of NO and N₂O, and the rate of conversion for NH₃ to N₂O is about 10%. The conversion of NO to N₂O is higher in the presence of H₂ than in the presence of CO at lower temperature, and there is a range of temperature in which the formation of N₂O is enhanced in the presence of either H₂ or CO, whereas CO enhances N₂O production from catalytic NO reduction more than H₂ in a CO/H₂/NO/O₂/N₂ gas system.     

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.     

Formation of NO in high temperature hydrogenair and ethyleneair detonative combustion

The formation of NO in overdriven H₂air and C₂H₄air detonations was experimentally and numerically investigated under various temperature and pressure conditions. The NO concentrations were measured by (0,1)NO-γ resonance absorption with additional OH and CO + O emission measurements. It was found that the NO formation rates near the detonation front are greater by a factor of 1≤ψ≤4 than predicted by the Zeldovich three-reaction mechanism assuming equilibrium O and OH concentrations. By computer simulation of a one-dimensional detonation wave together with hydrogen oxidation and NO formation kinetics, a nearly complete fit of the NO profiles measured under different conditions was possible. In the same way the simulation of C₂H₄air detonative combustion was successful using a scheme of 52 elementary reactions to describe the C₂H₄ oxidation and of 9 reactions for NO formation. It was possible to fit all measured NO profiles by use of an adjusted rate coefficient for the reaction CH + N₂, which is part of the NO formation scheme.     

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.     

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.     

A proposed reaction mechanism for the selective oxidation of methane with nitrous oxide over Co-ZSM-5 catalyst forming synthesis gas (CO + H2)

Probable reaction intermediates and mechanistic steps involved for the selective oxidation of methane (CH₄) with nitrous oxide (N₂O) forming synthesis gas (CO + H₂) over Co-ZSM-5 was examined. To assess the reaction kinetics reactions were conducted within a temperature range of 300 °C–550 °C and at atmospheric pressure, and the composition of reactant feed (N₂O/CH₄) was varied between three fixed ratios (5, 3, and 1). TPD, XRD, TEM and FT-IR techniques were used for catalyst characterization and mechanistic studies. It was found, that H₂ selectivity is higher for feeds with lower N₂O concentration. Furthermore, CO formation commences at lower temperatures in comparison to H₂ gas which is a result of the formation of methanol (CH₃OH) and carbon oxides (CO and CO₂) at low CH₄ conversions.Based on the reaction conditions, the literature proposes direct and an indirect mechanistic route for synthesis gas formation. Our catalytic study suggests a direct route for the synthesis gas formation; i.e. H₂ gas is formed by the decomposition of intermediate species (methoxy and CH₂O). This is supported by spectroscopic investigation. Under favourable reaction conditions and over the Co-ZSM-5 catalyst, N₂O first oxidizes CH₄ to CH₃OH, an active reaction intermediate, which then subsequently is converted into (formaldehyde) CH₂O and H₂. CH₂O is highly reactive and decomposes into CO and H₂.     

Numerical investigation on the effect of CO2 and steam for the H2 intermediate formation and NOX emission in laminar premixed methane/air flames

One-dimensional premixed freely-propagating flames for (CH₄+CO₂/H₂O)/air(79%N₂+21%O₂) mixtures were modeled using ChemkinⅡ/Premix Code with the detailed mechanism GRI-Mech 3.0. The investigation of the effects of CO₂ and steam addition on the H₂ intermediate formation and NO emission was conducted at the initial conditions of 1 atm and 398 K. Both physical and chemical effects of CO₂, H₂O on laminar burning velocities and adiabatic flame temperatures were also analyzed. The calculations show that with the increase of α_CO2 and α_H2O, both physical and chemical effects of CO₂ and H₂O result in the reduction of laminar burning velocities (LBVs) and adiabatic flame temperatures (AFTs) in which the chemical effects of CO₂ addition are more significantly than H₂O. Especially, the chemical effects of steam promote the increase of AFTs and the influence in rich BG65 flames are larger than in methane. With a proper amount of H₂O addition, the chemical effects of H₂O on the peak concentration of H₂ are more significantly than physical at Φ = 1.2. Moreover, CO₂, steam and their mixture addition have significant reduction on the NO emission. The most sensitive reaction for the formation of H₂ and NO emission were determined. The responsible reactions for H₂ formation and NO emission are R84 OH + H₂ <=> H + H₂O and R240 CH + N₂ <=> HCN + N (a prompt routine), 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.     

Homogeneous formation of NO and N2O from the oxidation of HCN and NH3 at 600–1000°C

The oxidation of HCN and NH₃ with CO, CH₄, or H₂ addition has been studied in the temperature range between 600 to 1000°C. In most of the tests 10% oxygen was used. The experiments were carried out under well-defined conditions in a flow tube reactor made of quartz glass. The effects of NO addition and oxygen level have been tested. To study the importance of O/H radicals in the reaction mechanism and to confirm previous studies, iodine was added in some tests. A detailed chemical kinetic model was used to analyze the experimental data. In general, the model and experimental results are in good agreement. The results show that under the conditions tested CO significantly promotes NO and N₂O formation during HCN oxidation. During NH₃ oxidation carbon-containing gaseous species such as CO and CH₄ are important to promote homogeneous NO formation. In the system with CH₄ addition, the conversion of HCN to N₂O is lower compared to the other systems. In the HCN/NO/CO/O₂ system NO reduction starts at 700°C and the maximum reduction of approx. 40% is obtained at 800°C. For the NH₃/NO/CO/O₂ system the reduction starts at 750°C and the maximum reduction is 50% at 800°C. Iodine addition shifts the oxidation of HCN, NO, and N₂O formation as well as NO reduction to higher temperatures. Under the conditions tested, it was found that iodine mainly enhances the recombination of the O-radicals. No effect on NO formation was found in the HCN/CH₄/O₂ system when oxygen was increased from 6% to 10%, but when oxygen was increased from 2% to 6% NO formation decreased. The role of hydrocarbon radicals in the destruction of NO is likely to become important at low oxygen concentrations (2%) and at high temperatures (1000°C).     

Fuelnitrogen transformations in one-dimensional coal-dust flames

Experiments on the distribution of fuel nitrogen in pulverized-coal/O₂/Ar flames stabilized on a flat-flame burner are reported. The gas-phase concentration of HCN, NO, NH₃, and N₂, and the solid-phase nitrogen content were measured as a function of reaction time. A complete inventory of the major nitrogen-containing species is attained for two fuel-rich stoichiometries. Coal-nitrogen release is found to be more complete in the leaner flame. HCN and NO account for most of the gas-phase nitrogen early in the flames, while N₂ predominates later. Particle and gas temperature profiles were obtained with an infrared pyrometer. Early in the flame the particle and gas temperatures do not differ much, but the gas may eventually be as much as several hundred Kelvin hotter than the particles. The experimental results are used to determine overall rate constants for coal-nitrogen devolatilization and for N₂ formation.     

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.     

Ammonia chemistry in oxy-fuel combustion of methane

The oxidation of NH₃ during oxy-fuel combustion of methane, i.e., at high [CO₂], has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high CO₂ level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and CO₂, are responsible for the effect of a high CO₂ concentration on ammonia conversion. When CO₂ is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NH₂+CO. However, reactions NH₂+O to form HNO and NH₂+H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NH₂ with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NH₂ availability to form NO by other pathways like via HNO or NH and increase the probability of forming N₂ instead of NO.     

An experimental and modeling study of the influence of flue gases recirculated on ethylene conversion

The influence of those gaseous compounds that can be typically present in combustion processes with flue gas recirculation (FGR) techniques: CO₂, H₂O, CO, NO, NO₂, N₂O and SO₂, on ethylene conversion was analyzed through an experimental and modeling study. Ethylene oxidation experiments in the presence of the different gaseous compounds were carried out in the 700–1400 K temperature range, at atmospheric pressure, from fuel-lean to fuel-rich conditions, using N₂ as bath gas. These experiments were modeled by means of a detailed gas-phase chemical kinetic mechanism, which was used to identify the implications of the different gaseous compounds recirculated for the ethylene oxidation scheme, as well as for their own conversion. Overall, good agreement was obtained between the experimental data and the modeling, and thus the proposed mechanism can be successfully used to model the ethylene oxidation in the presence of flue gases recirculated (CO₂, H₂O, CO, NO, NO₂, N₂O and SO₂) in a wide range of operating conditions.     

On the influence of the char gasification reactions on NO formation in flameless coal combustion

Flameless combustion is a well known measure to reduce NO_x emissions in gas combustion but has not yet been fully adapted to pulverised coal combustion. Numerical predictions can provide detailed information on the combustion process thus playing a significant role in understanding the basic mechanisms for pollutant formation. In simulations of conventional pulverised coal combustion the gasification by CO₂ or H₂O is usually omitted since its overall contribution to char oxidation is negligible compared to the oxidation with O₂. In flameless combustion, however, due to the strong recirculation of hot combustion products, primarily CO₂ and H₂O, and the thereby reduced concentration of O₂ in the reaction zone the local partial pressures of CO₂ and H₂O become significantly higher than that for O₂. Therefore, the char reaction with CO₂ and H₂O is being reconsidered. This paper presents a numerical study on the importance of these reactions on pollutant formation in flameless combustion. The numerical models used have been validated against experimental data. By varying the wall temperature and the burner excess air ratio, different cases have been investigated and the impact of considering gasification on the prediction of NO formation has been assessed. It was found that within the investigated ranges of these parameters the fraction of char being gasified increases up to 35%. This leads to changes in the local gas composition, primarily CO distribution, which in turn influences NO formation predictions. Considering gasification the prediction of NO emission is up to 40% lower than the predicted emissions without gasification reactions being taken into account.     

Research on reaction conditions and mechanism for simultaneous removal of NO and Hg0 over CuFe modified activated carbon at low temperature

In this study, modified microwave coconut shell activated carbon (CuFe/MCSAC) was used to simultaneously remove NO and Hg⁰ in the industrial tail gas. The influences of reaction condition were investigated, such as NH₃ concentration, O₂ content, gas hourly space velocity (GHSV), resistance of SO₂ and H₂O. Under the optimal reaction conditions (600 ppm NH₃, 3% O₂ and 30000 h⁻¹ GHSV), CuFe/MCSAC showed high NO conversion rate (86%), high Hg⁰ conversion rate (61%) and low N₂O concentration (81 ppm) at 225 °C. NH₃ was conducive to the removal of NO, but it had a competitive adsorption effect for Hg⁰. O₂ promoted oxidation of NO and Hg⁰, and led to the formation of N₂O. Low GHSV had the external diffusion effect, which affected the reaction rate. High GHSV could not provide enough reaction time. H₂O and SO₂ were not conducive to the selectivity of SCR reaction and oxidation of Hg⁰.     

A wide range modeling study of formation and nitrogen chemistry in hydrogen combustion

The chemistry of nitrogen species and the formation of NO_x in hydrogen combustion are analyzed here on the basis of a large set of experimental measurements.The detailed kinetic scheme of H₂/O₂ combustion was updated and upgraded using new kinetic and thermodynamic measurements, and was validated over a wide range of temperatures, pressures and equivalence ratios. The mechanism's performance at high pressures was greatly improved in particular by adopting higher rate parameters for the H+OH+M=H₂O reaction.The NO_x sub-mechanism was further validated and updated. The kinetic parameters of the NO₂+H₂=HONO+H and N₂H₂+NO=N₂O+NH₂ reactions were updated in order to improve model predictions in specific conditions.Sensitivity analyses were carried out to determine which reactions dominate the H₂/O₂ and H₂/NO_x systems at particular operating conditions.Good overall agreement was observed between the model and the wide range of experiments simulated.     

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.