Effect of multistage combustion on NOx emissions in methane–air flames (original) (raw)
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A numerical study on NOx formation in laminar counterflow CH4/air triple flames
Combustion and Flame, 2005
Formation of NO x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO x , compared to the corresponding premixed flames. A triple flame produces more NO and NO 2 than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N 2 O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO 2 formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N 2 O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified. Crown
NO formation analysis of turbulent non-premixed coaxial methane/air diffusion flame
International Journal of Environmental Science and Technology, 2015
Natural gas combustion is one of the primary sources of harvesting energy for various processes and has gained a wide attention during the past decade. One of the most recent applications of natural gas combustion can be found in non-premixed combustion of methane in a coflow burner system. One of the main environmental concerns that arises from the natural gas combustion is the formation of NO produced by thermal NO and prompt NO mechanisms. Current paper is devoted on an examination of a 2D numerical simulation of turbulent non-premixed coaxial methane combustion in air enclosed by an axisymmetric cylindrical chamber to study the effects of species concentrations of reactants on NO formation, their individual contributions, and the chamber outlet temperature. A finitevolume staggered grid method is utilized to solve conservation equations of mass, energy, momentum, and species concentrations. In order to handle radiation heat transfer, discrete transfer method is used to solve radiation equation. Utilizing weighted-sum-of-gray-gases model, based on the newly obtained high-temperature molecular spectroscopic data, local variations of species absorption coefficients are taken into account. To calculate NO concentration, a single-or joint-variable probability density function in terms of a normalized temperature, mass fractions of species, or a combination of both is employed. Plus, published relevant experimental data are used to validate temperature and species concentration fields. It is shown that a decrease in N 2 concentration contributes to reducing NO. More importantly for higher O 2 mass fraction, thermal NO formation becomes the dominant mechanism responsible for NO emission.
Ammonia conversion and NOx formation in laminar coflowing nonpremixed methane-air flames
Combustion and Flame, 2002
This paper reports on a combined experimental and modeling investigation of NO x formation in nitrogen-diluted laminar methane diffusion flames seeded with ammonia. The methane-ammonia mixture is a surrogate for biomass fuels which contain significant fuel-bound nitrogen. The experiments use flue-gas sampling to measure the concentration of stable species in the exhaust gas, including NO, O 2 , CO, and CO 2. The computations evolve a two-dimensional low Mach number model using a solution-adaptive projection algorithm to capture fine-scale features of the flame. The model includes detailed thermodynamics and chemical kinetics, differential diffusion, buoyancy, and radiative losses. The models shows good agreement with the measurements over the full range of experimental NH 3 seeding amounts. As more NH 3 is added, a greater percentage is converted to N 2 rather than to NO. The simulation results are further analyzed to trace the changes in NO formation mechanisms with increasing amounts of ammonia in the fuel.
A numerical simulation of none-premixed methane-air combustion is performed. The purpose of this paper is to provide information concerning the effect of inlet-air velocity (dry air) on the exhaust gas emissions of oxides of nitrogen (NO), for a simple type of combustor. Effects of increased inlet-air velocity on NOx formation are examined. Numerical results show that NO formation mechanisms, decrease, with increasing inlet-air velocity. The simulation has been performed using the Computational Fluid Dynamics (CFD) commercial code ANSYS CFX release 15, including laminar flamelet model, for simulating the methane combustion mixing with air (none-premixed combustion) and predicting concentration of (CH-CH2-CH2O-CH3-CH4-CHO-CO-CO2-O-O2-H-H2-H2O-HO2-N2-H2O2-OH).k-ε model was also investigated to predict the turbulent combustion reaction, which indicated the simulation results of velocities, temperatures and concentrations of combustion productions. A thermal and prompt Nox formation is performed for predicting NO emission characteristics. A comparison between the various inlet-air velocity, and their effects on NO emission characteristics and temperature fields are presented.
A numerical investigation of NOx formation in counterflow CH4/H2/air diffusion flames
Proceedings of …, 2006
A detailed numerical study was carried out for the effect of hydrogen enrichment on flame structure and NO X formation in counterflow CH 4 /air diffusion flames. Detailed chemistry and complex thermal and transport properties were employed. The enrichment fraction was changed from 0 (pure CH 4) to 1.0 (pure H 2). The result indicates that for flames with low to moderate stretch rates, with the increase of the enrichment fraction from 0 to 0.5~0.6, NO emission index keeps almost constant or only slightly increases. When the enrichment fraction is increased from 0.5~0.6 to about 0.9, NO emission index quickly increases, and finally NO formation decreases again when pure hydrogen flame condition is approached. However, for flames with higher stretch rates, with the increase of hydrogen enrichment fraction from 0 to 1.0, the formation of NO first quickly increases, then slightly decreases and finally increases again. Detailed analysis suggests that the variation of the characteristics in NO formation in stretched CH 4 /air diffusion flames is caused by the change of flame structure and NO formation mechanism, when the enrichment fraction and stretch rate are changed.
Detailed Multi-dimensional Study of Pollutant Formation in a Methane Diffusion Flame
Energy & Fuels, 2012
This paper describes a method to produce chemical reactor networks (CRNs) consisting of large numbers of perfectly stirred reactors (PSRs) from computational fluid dynamics (CFD) simulations to predict pollutant emissions from combustion systems accurately, flexibly, and efficiently using detailed kinetic schemes and the kinetic post-processor (KPP) developed at Politecnico di Milano. Benefits of the method described here include its applicability to a wide range of combustion systems, its ability to predict emissions of a variety of pollutant species, and its speed. CFD and CFD−CRN simulation results of the Sandia D piloted methane−air diffusion round-jet flame are successfully validated against experimental data for axial velocity, mixture fraction, temperature, and speciation, including CO and NO mass fractions. A CRN consisting of a large number of PSRs is found to be required to simulate the system accurately, while ensuring independence of the solution from CRN size. The results of CFD−CRN analysis for a 1114 PSR network are used to study the pathways (thermal, prompt, N 2 O, and NO 2) by which NO and NO 2 , the constituents of NO x , are formed in the flame. Results of CFD−CRN analysis show that NO is produced in the high-temperature (T > 1850 K) flame brush by a combination of the prompt, N 2 O, and thermal pathways and in the intermediate-temperature (1000 < T < 1600 K) post-flame region by a reversal of the NO 2 pathway. NO is consumed in the fuelrich (mixture fraction, f > 0.43) region, where a low O atom concentration encourages a reversal of the prompt pathway (i.e., NO reburning), and in low-temperature (T < 1000 K) regions by the NO 2 pathway, which oxidizes NO to NO 2. Rate of production analysis, performed using CHEMKIN PRO at specified locations throughout the flame, shows that the trends of NO production and consumption observed in these simulations agree with expected and published results. Finally, the study predicts that, of the total NO x produced by the Sandia D flame, 47% is due to the prompt pathway, 32% is due to the N 2 O pathway, and 21% is due to the thermal pathway. As future steps in this work, the CFD−CRN method will be adapted and used to predict and study emissions from a range of more complex combustion systems.
Thermal Science and Engineering Progress, 2018
The present work reports on the numerical investigation of NOx in three turbulent piloted diffusion flames of different levels of extinction. The study involves two-dimensional axisymmetric modeling of combustion in these flames with fairly detailed chemistry, i.e. GRI 3.0 mechanism. The main focus of the study is to analyze the effects of the two different combustion model approaches, such as infinitely fast chemistry based unsteady flamelet and finite rate chemistry based EDC, in predicting the NOx formation in three piloted methane jet flames (Sandia D, E, and F). The EDC approach is able to predict the passive scalar quantities but shows over-prediction in the reactive scalar quantities and NO prediction, while the unsteady flamelet modeling is found to be essential in predicting the accurate formation of slow kinetic species like NOx. The inability of flamelet and EDC approach in capturing localized flame extinction is observed, which lead to an over-prediction of NOx at larger downstream locations. Further, the dominance of NOx formation pathways is investigated in all three flames.
The structure and extinction of nonpremixed methane/nitrous oxide and ethane/nitrous oxide flames
Proceedings of the Combustion Institute, 2013
Knowledge of combustion of hydrocarbon fuels with nitrogen-containing oxidizers is a first step in understanding key aspects of combustion of hypergolic and gun propellants. Here an experimental and kinetic-modeling study is carried out to elucidate aspects of nonpremixed combustion of methane (CH 4) and nitrous oxide (N 2 O), and ethane (C 2 H 6) and N 2 O. Experiments are conducted, at a pressure of 1 atm, on flames stabilized between two opposing streams. One stream is a mixture of oxygen (O 2), nitrogen (N 2), and N 2 O, and the other a mixture of CH 4 and N 2 or C 2 H 6 and N 2. Critical conditions for extinction are measured. Kinetic-modeling studies are performed with the San Diego Mechanism. Experimental data and results of kinetic-modeling show that N 2 O inhibits the flame by promoting extinction. Analysis of the flame structure shows that H radicals are produced in the overall chain-branching step 3H 2 + O 2 2H 2 O + 2H, in which molecular hydrogen is consumed. Hydrogen is also consumed in the overall step N 2 O + H 2 N 2 + H 2 O where stable products are formed. Inhibition of the flames by N 2 O is attributed to competition between these two overall steps.