Modeling Of NOx Formation Research Articles

Numerical Investigation on the Effects of Inlet Air Temperature on Spray Combustion in a Wall Jet Can Combustor Using the k − ϵ Turbulence Model

A three-dimensional numerical study was performed to assess the effects of inlet temperature and equivalence ratio on the spray combustion and subsequent NOx emission in a wall jet can combustor (WJCC) installed with twin-fluid air-assisted fuel atomizers. The RNG k − ϵ turbulence model, eddy breakup (EBU) combustion model, and the Zeldovich model of NOx formation were utilized in the numerical study. The WJCC was implemented with a swirling air jet at the fuel nozzle exit and two other air jets, known as primary and dilute jets, at downstream locations. The inlet air temperature and overall equivalence ratio were varied from 373 to 1000 K and from 0.3 to 0.6, respectively. Our computational study showed that the inlet air of high temperature induced flow acceleration and sufficient jet penetration, which were desirable for achieving uniform temperature distribution at the combustor outlet but unfavorably yielded increased NOx emission. While the inlet air temperature had no prominent influence on the evaporation rate of the fuel drops in the upstream primary zone, its influence appeared to be prominent further downstream.

Computational modelling of NO<SUB align=right>x emissions from biodiesel combustion

A detailed numerical spray atomisation, ignition, combustion and NOx formation model was developed for direct injection diesel engines using KIVA-3V code that could be applied to biodiesel fuels and this model was used to investigate the NOx emissions mechanisms of biodiesel compared with diesel fuel. In addition, computational modelling was applied to evaluate strategies for reducing NOx emissions from biodiesel combustion. The physical and thermodynamic properties of biodiesel used in the model were based on fatty acid composition. The model was verified with experimental data from an engine fuelled with diesel fuel, soyabean methyl ester, Yellow Grease Methyl Ester (YGME) and genetically modified soyabean methyl ester. Strategies for reducing NOx emissions from biodiesel combustion were evaluated with the aid of the model. Increasing spray cone angle, retarding start of injection, applying Exhaust Gas Recirculation (EGR) and charge air cooling were all effective approaches to reducing NOx emissions from biodiesel fuel.

The shock tube pyrolysis of pyridine

Fossil fuels such as coal and heavy fuel oils contain up to about 2 per cent by weight of fuel nitrogen, most of this being present in pyridine or pyrolic aromatic structures. Under pyrolytic conditions these ring structures decompose to give HCN and CH3CN. For present day computer modelling of NOx formation in flames it is necessary to know the mechanism and rates of reaction. A number of previous studies of pyridine pyrolysis have been undertaken using shock tubes or flow reactors. In this present study a shock tube was used to obtain the kinetics of pyridine decomposition in the range 1590–2335 K and with pressures between 2.2 and 3.4 atm. Two independent sets of data were obtained. One set of results was found to be represented by an Arrhenius rate constant k=109.7±0.5 exp (−220.0 kJ mol−1 RT−1)s−1. For the other work k= 109.8±0.5 exp (−228.191±18.42 kJ mol−1 RT−1)s−1. In addition, the pyrolysis mixtures of pyridine plus toluene have also been studied to understand the synergistic effects. The results indicated the strong involvement initiated by fission of the pyridine ring system. Copyright © 2000 John Wiley & Sons, Ltd.

Predictions of NOx Formation Under Combined Droplet and Partially Premixed Reaction of Diffusion Flame Combustors

A methodology is presented in this paper on the modeling of NOx formation in diffusion flame combustors where both droplet burning and partially premixed reaction proceed simultaneously. The model simulates various combustion zones with an arrangement of reactors that are coupled with a detailed chemical reaction scheme. In this model, the primary zone of the combustor comprises a reactor representing contribution from droplet burning under stoichiometric conditions and a mixing reactor that provides additional air or fuel to the primary zone. The additional flow allows forming a fuel vapor/air mixture distribution that reflects the unmixedness nature of the fuel injection process. Expressions to estimate the extent of deviation in fuel/air ratios from the mean value, and the duration of droplet burning under stoichiometric conditions were derived. The derivation of the expressions utilized a data base obtained in a parametric study performed using a conventional gas turbine combustor where the primary zone equivalence ratio varied over a wide range of operation. The application of the developed model to a production combustor indicated that most of the NOx produced under the engine takeoff mode occurred in the primary as well as the intermediate regions. The delay in NOx formation is attributed to the operation of the primary zone under fuel rich conditions resulting in a less favorable condition for NOx formation. The residence time for droplet burning increased with a decrease in engine power. The lower primary zone gas temperature that limits the spray evaporation process coupled with the leaner primary zone mixtures under idle and low power modes increases the NOx contribution from liquid droplet combustion in diffusion flames. Good agreement was achieved between the measured and calculated NOx emissions for the production combustor. This indicates that the simulation of the diffusion flame by a combined droplet burning and fuel vapor/air mixture distribution offers a promising approach for estimating NOx emissions in combustors, in particular for those with significant deviation from traditional stoichiometry in the primary combustion zone.

96/05493 3-D modelling of NOx formation in a 275 MW utility boiler

96/05493 3-D modelling of NOx formation in a 275 MW utility boiler

Mathematical model of NOx formation in a fluidized-bed combustor

Mathematical model of NOx formation in a fluidized-bed combustor

96/01836 Errors due to correlations in evaluating mean density from Favre-averaged enthalphy and composition in turbulent reactive flow

The applicability of the laminar flamelet concept for the formation and destruction of nitric oxides in laminar and turbulent diffusion flames has been studied. In a first step, temperatures and species concentrations in an axisymmetric laminar diffusion flame have been calculated (i) by solving the detailed conservation equations and (ii) by applying the laminar flamelet concept. The main purpose of this step was the identification of differences between results from both approaches. It turned out that for highly temperature sensitive or relatively slow chemical processes, the inclusion of the full range of the prevailing scalar dissipation rates plays a major role for the calculated species concentrations. This behavior is obvious from the concept of the laminar flamelet model, where the scalar dissipation rate can be discussed in terms of the reciprocal of a residence time for attaining chemical equilibrium. In a second step, flamelet modeling of NOx formation was extended to a turbulent hydrogen diffusion flame. In both the steps, the flow fields of the flames were calculated by solving the Navier–Stokes equations in axisymmetric formulation using the SIMPLER algorithm. For the turbulent flow, Favre-averaged equations have been used and turbulence was modeled with the standard k–ϵ model including a correction term for axisymmetric systems. The averaging of the species concentrations was accomplished with presumed shape probability density functions (pdfs). The pdf of the mixture fraction was described with a β-function whereas that of the scalar dissipation rate was assumed to be log-normal. Buoyancy effects have been taken into account. The calculated temperatures and concentrations were compared with data from different experiments.

A Computational Method for Determining Global Fuel-NO Rate Expressions. Part 1

Global chemical reaction rates used in the modeling of NOx formation in comprehensive combustion codes have traditionally been obtained through correlation of experimental data. In this paper, a computational approach for obtaining global rates is presented. Several premixed flames were simulated, and sensitivity analysis of species concentration profiles was used to suggest global pathways in fuel-nitrogen conversion to NO. Based on these analyses, the global reaction rates were formulated. The predicted species concentration profiles and their derivatives were then used in the determination of the global rate constants. The correlation of rate constants for the two fuel-NO global rates (HCN + NO → N2 + ... and HCN + O2 → NO + ...) are discussed. Comparisons of the computed global rate constants with those deduced from experimental data show good agreement. The global rates provide practical kinetics for simulating nitrogen pollutant chemistry in complex flames.

Relation between functional forms of coal nitrogen and NOx emissions from pulverized coal combustion

NOx emissions during pulverized coal combustion, the thermal decomposition behaviour of fuel-bound nitrogen during rapid pyrolysis and the functional forms of coal nitrogen were investigated to develop the general index estimating NOx levels for coals covering a wide range of rank. NOx levels under excess air and two-stage combustion strongly depended on coal type. The effect of nitrogen content in the parent coal on NOx levels was not a continuous relationship, seemingly because of the yields of volatile nitrogen species which evolve during the early stage of combustion. An improved model of NOx formation was proposed to explain the influence of coal type. The dominant factors for NOx reduction were derived on the basis of the improved model. The volatile nitrogen yield and the NH 3 HCN ratio strongly affect NOx formation. An NOx index to predict NOx levels was proposed based on the relation between the functional forms of coal nitrogen and the yields of nitrogen-containing species. The index involves the proportion of quaternary, pyrrole- and pyridine-type nitrogen.

噴霧燃焼シミュレーションによるガスタービン燃焼器燃焼特性の予測

In this study, we investigated the combustion characteristics of a gas turbine combustor using the spray combustion simulation method which had been developed by our group.As a result of the simulation, it was clear that the temperature distribution in the combustor was closely related to the combustion gas flow and the fuel droplets behaviors. We measured, moreover, the averaged temperature at the outlet and the mean temperature profile along the centerline of the combustor, and compared the measured data with the calculated results. As for the averaged temperature at the outlet, the simulation agreed with the measurements, but it could not predict well the temperature profile along the centerline, especially in the lower fuel feed rate. It was guessed that the discrepancy was caused by the measurement error of temperature due to the difficulty of measurement in the closed system and the calculation error in the complex flow field.We also tried to predict the NOx formation characteristic in the combustor and compared with the measured data of the NOx concentration in the exhaust gas. As a result, it was necessary to simulate both of thermal NOx and prompt NOx formation. Thus, we developed the prompt NOx formation model in which its complex formation mechanism was modeled simply.