NOx Model Research Articles

Numerical modelling of MILD combustion for coal

Emissions of nitrogen oxides from coal combustion are a major environmental problem because they have been shown to contribute to the formation of acid rain and photochemical smog. Moderate and Intensive Low oxygen Dilution (MILD) combustion is a promising technology for decreasing pollutant emissions and improving combustion efficiency. A combination of air preheating and fuel dilution with combustion products of low oxygen concentration are the main features of this technique. In the MILD combustion mode, preheated air and fuel are gradually mixed with large amounts of recirculated exhaust gas. The objective of the present work is to investigate the capability of present fuel NO mechanisms for pulverised coal combustion to predict the observed nitrogen oxide levels in MILD combustion mode. For this purpose, knowledge of the fate of coal nitrogen during the combustion process is vital. The interaction between turbulence and chemistry is modelled by an advanced Eddy Dissipation Concept (EDC). The NOx model is used to predict NO profiles that are compared to measurements obtained from semi-industrial scale experiments.

A Novel Approach to Predicting NOx Emissions From Dry Low Emissions Gas Turbines

An empirical modeling concept for the prediction of NOx emissions from dry low emissions (DLE) gas turbines is presented. The approach is more suited to low emissions operation than are traditional approaches (Lefebrre, A. H., 1998, Taylor and Francis, New York). The latter, though addressing key operating parameters, such as temperature and pressure drop, do not address issues such as variation in fuel/air distribution through the use of multifuel stream systems, which are commonly applied in DLE combustors to enable flame stability over the full operating range. Additionally, the pressure drop dependence of NOx in such systems is complex, and the exponent of a simple pressure drop term can vary substantially. The present approach derives the NOx model from the equations that govern the NOx chemistry, the fuel/air distribution and the dependence of the main reaction zone on its controlling parameters. The approach is evaluated through comparing its characteristics to data obtained from high-pressure testing of a DLE combustor fueled with natural gas. The data were acquired at a constant pressure and preheat temperature (14 Bara and 400°C) and a range of flame temperatures and flow rates. Though the model is configured to address both relatively fast and slow NOx formation routes, the present validation is conducted under conditions where the latter is negligible. The model is seen to reproduce key features apparent in the data, in particular, the variable pressure drop dependence without any ad hoc manipulation of a pressure drop exponent.

Short-Time Forecasting of Atmospheric NOx Concentration by Neural Networks

The possibilities of modeling of NO and NO2 concentrations at air monitoring stations were analyzed in this paper. Artificial neural networks (ANN) were used in time series analysis to predict the levels of concentrations. Investigations were carried out based on two data sets collected at air monitoring stations in South Poland: Zabrze (1994–1997) and Ke˛dzierzyn-Koźle (1994–1999). The set of hourly values of NOx concentrations were used in the analysis. The main purpose of the studies was to estimate prediction accuracies for both chosen pollutant concentrations. The effect of changes of lookahead, the ANN time series parameter, was analyzed. The accuracies of different obtained models were compared. The main result was that the accuracy of prognosis quickly decreases when lookahead rises. Time series models of NOx concentration are not accurate, except for short lookaheads. The accuracy of NO2 concentration modeling is apparently better than NO concentration modeling. The nonlinear models are not distinctly more precise than their linear equivalents. The linear models seem to be more advantageous because of their simplicity and very short time of calculation. The precision of NOx modeling at the more polluted station (Zabrze) is higher than at the less polluted one ( Kędzierzyn-Koźle). Other methods of modeling could be more accurate in prediction of NOx levels, so possibilities of practical implementation of ANN time series analysis are limited.

Sensitivity Analysis of NOx Formation Kinetics in Pilot-Ignited Natural Gas Engines

A sensitivity analysis of NOx formation in pilot-ignited natural gas dual fuel engines is performed based on a phenomenological combustion model. The NOx formation mechanism employed in this study incorporates a super-extended Zel’dovich mechanism (up to 43 reactions). The sensitivity analysis compares the contribution of each major reaction to NOx formation, and identifies the rate-controlling NOx formation reactions in different high-temperature regions—the burning pilot spray, the premixed flame associated with the gaseous fuel-air mixture, and the burned combustion products. The formation rates for reactions involving NOx are also investigated to reveal the primary NOx formation paths. Results show two main NOx formation paths both in the pilot spray (also called the packets zone) and the burned zone. The rate-limiting reactions for the packets zone are O+N2=NO+N and N2+HO2=NO+HNO. Rate-limiting reactions for the burned zone are N2O+M=N2+O+M and N2+HO2=NO+HNO. Since the aforementioned reactions significantly influence the net NOx prediction, it is important that the corresponding reaction rates be determined accurately. Finally, because the quasi-steady-state assumption is commonly used for certain species in NOx modeling, a transient relative error is estimated to evaluate the validity of the assumption. The relative error in NOx prediction with and without the use of the steady-state assumption is small, of the order of 2%. This work also confirms that sensitivity analysis can provide valuable insight into the possible NOx formation pathways in engines and improve the ability of current prediction tools to obtain more reliable predictions.

A Critical Evaluation of NOx Modeling in a Model Combustor

Reliable NOx modeling depends on the accurate prediction of both velocity and temperature fields. The velocity and temperature fields of a propane diffusion flame combustor, with interior and exterior conjugate heat transfers, were first numerically studied. The results from three combustion models, together with the renormalization group (RNG) k-ε turbulence model and the discrete ordinates radiation model are discussed, and compared with comprehensive experimental measurements. The flow patterns and the recirculation zone length in the combustion chamber are excellently predicted, and the mean axial velocities are in fairly good agreement with the experimental data for all three combustion models. The mean temperature profiles are fairly well captured by the probability density function (PDF) and eddy dissipation (EDS) combustion models. However, the EDS-finite-rate combustion model fails to provide an acceptable temperature field. Based on the acceptable velocity and temperature fields, a number of NO modeling approaches were evaluated in a postprocessing mode. The partial-equilibrium approach of O and OH radical concentrations shows a significant effect on the thermal NO formation rate. In contrast, the prompt NO, the NO reburn mechanism and the third reaction of the extended Zeldovich mechanism have negligible effects on the overall NO formation in the present study. This study indicates that the semiempirical, postprocessing NO model can provide valuable NO simulations as long as the velocity and temperature fields are adequately predicted.

A HYBRID GA FOR NOX EMISSION MODELLING IN POWER GENERATION PLANTS

This paper reviews a grey-box (GB) modelling method that uses genetic algorithms (GA). The GA-GB modelling framework is used for finding NOx emission from coal-fired power generation boilers using operator controlled variables. The main contribution of this paper is the inclusion of a distributed elitist scheme within the GA. This enables a local gradient-based optimisation routine to be incorporated within the GA. The new hybrid GA-GB modelling procedure is shown to be able to produce NOx models having similar performance to those of the original method but requires less computational effort.

00/02211 A comparison of electron staining agents for the transmission electron microscopy examination of coal

Computational flow modeling has been done to study NOx formation under laminar flow conditions in drop tube furnaces (DTF) burning pulverized coal. The generation of NOx from nitrogen in the coal’s volatile matter was simulated using de Soete’s reactions involving HCN, NO and N2 as the basic homogeneous gas-phase system. The effects of ammonia release and hydrocarbon reburn were then included. Three approaches to the oxidation of char-nitrogen were investigated, one simply using experimental conversion data. The simulations for a number of coals and chars were compared to three experimental studies in the literature. Even though laminar flow combustion conditions were modeled, it was possible to achieve carbon burnouts representative of those obtained experimentally. Consideration of disturbances to the flow from jets of volatiles improved the predicted carbon burnout for fuel-lean conditions (φ < 1) in one furnace. The NOx models gave good results with bituminous and sub-bituminous coal under fuel-lean conditions at 1750 K, and reasonable results in fuel-rich conditions. At 1250 K the models failed and it is concluded that all the reaction mechanisms are inadequate at that low temperature. With higher rank coals at boiler temperatures, the full NOx chemistry involving ammonia and hydrocarbon reburn gave the best results. With lignite coals, the simple gas-phase kinetics of de Soete gave good predictions, and the inclusion of ammonia and reburning did not improve the result. For the high-temperature (1750 K) combustion of char or petroleum coke, two of the models fitted equally well to the experiments on the oxidation of char-bound nitrogen. One model involved an effectiveness factor, where char-N was converted to NO and then reduced to N2 while diffusing into the particle. The second assumed that all char-N was converted to HCN, which was then subject to gas-phase reactions.

A simplified fuel-NOx model based on regression analysis

The mechanism of detailed chemical reactions in combustion is very complicated, especially when the formation of a pollutant such as NOx is considered. Based on regression analysis, a simplified fuel–NOx model is developed for premixed flame. The new model is available for a wider range of temeperature and fuel–air ratios compared with De Soete's (1975) fuel NO model. The reduction CHi species on NO is considered so that the model can be used in both fuel-lean and fuel-rich combustion systems. After a great simulation of a one-dimensional premixed flame system using Miller and Bowman's (1989, Prog. Energy Combust. Sci., 15, 287) detailed elementary reaction model, the calculation of all the reaction rates is presented based on regression analysis. Copyright © 1999 John Wiley & Sons, Ltd.

Air pollution exposure monitoring and estimation. Part II. Model evaluation and population exposure.

The air pollution dispersion model EPISODE has been developed at the Norwegian Institute for Air Research (NILU) over the past several years in order to meet the needs of modern air quality management work in urban areas. The model has recently been used as a basis for exposure calculations of NOx and NO2 in order to assess the effects of different traffic diversion measures on health and well being for the residents in the Vålerenga-Ekeberg-Gamlebyen area in Oslo. Here we describe some results from the most recent evaluations of the model for NOx and NO2 at station Nordahl Brunsgate in Oslo for the period 1 October 1996-19 November 1996. In addition examples of population exposure calculations for Oslo performed during the winter period of 1995-96, are also presented.

State-Level Air Emissions Trading: The Michigan and Illinois Models

ABSTRACT Since passage of the 1990 Amendments to the U.S. Clean Air Act, there has been growing interest in the use of economic incentives for air pollution control. This trend is epitomized by the federal Acid Rain Program and the RECLAIM program for smog control in the Los Angeles basin. The adoption of these programs for attainment of the ozone standard is problematic, because of vexing issues of geography, atmospheric chemistry, source coverage, and monitoring and enforcement. These issues are especially salient in the use of emission reduction credit (ERC) trading systems. Cap and trade programs circumvent some of these difficulties by limiting total emissions and increasing source coverage but may still face monitoring challenges. Finally, the U.S. Environmental Protection Agency (EPA) is currently proposing that states use another incentive program, known as Open Market Trading. Michigan and Illinois have both developed new market incentive programs for ozone compliance. Michigan adopted an ERC model for NOx and VOCs, while Illinois opted for a cap and trade program to reduce VOCs in the Chicago area. Though these programs are fairly young, their strengths and weaknesses can be identified. Problems with the Michigan program are so serious that it has been initially disapproved by the EPA out of concern that down-state areas could backslide into non-attainment status. Among the concerns are pre-enactment ERCs and counting ERC generation from facility shutdowns and curtailments. Although the Illinois program is more promising, it has problems of its own, such as low emission source coverage. Nonetheless, it is only through experimentation with market programs that their ultimate utility for ozone compliance can be determined.

98/03131 Application of CFD modelling for the pre-optimization of boiler layout in the frame of the Vado Ligure #4 coal over Coal Reburn Demonstration Project

Nitric oxide (NO) concentrations in a swirling pulverized coal flame were predicted from a mathematical model and compared with experimental measurements. The model consisted of a two-dimensional, axi-symmetric parent CFD code combined with a NOx post-processor. Both thermal and fuel NO are included in the NOx chemistry. The parent code is used to calculate the combustion aerodynamics within the flame, while the NOx (post-processor) model predicts the local concentrations of HCN, NH3 and NO. The volatile nitrogen is released either as HCN or NH3, depending on its functional form in the coal. Several models studying the conversion of char nitrogen to NO are tested, as well as the effect of a hydrocarbon reburn mechanism. The experimental data were obtained from a swirling pulverized coal flame (aerodynamically air staged burner) operated at the International Flame Research Foundation. A sensitivity analysis on the primary components of the NOx chemistry is included. When the only reactions considered were those involving HCN and NO, the NO concentrations were underpredicted. The full NOx chemistry model was able to achieve good NO predictions by including ammonia as a NOx precursor and incorporating a hydrocarbon mechanism to recycle NO to HCN.

Parametric Modeling of Exhaust Gas Emission From Natural Gas Fired Gas Turbines

Increased focus on air pollution from gas turbines in the Norwegian sector of the North Sea has resulted in taxes on CO2. Statements made by the Norwegian authorities imply regulations and/or taxes on NOx emissions in the near future. The existing CO2 tax of NOK 0.82/Sm3 (US Dollars 0.12/Sm3) and possible future tax on NOx are analyzed mainly with respect to operating and maintenance costs for the gas turbine. Depending on actual tax levels, the machine should be operated on full load/optimum thermal efficiency or part load to reduce specific exhaust emissions. Based on field measurements, exhaust emissions (CO2, CO, NOx, N20, UHC, etc.) are established with respect to load and gas turbine performance, including performance degradation. Different NOx emission correlations are analyzed based on test results, and a proposed prediction model presented. The impact of machinery performance degradation on emission levels is particularly analyzed. Good agreement is achieved between measured and predicted NOx emissions from the proposed correlation. To achieve continuous exhaust emission control, the proposed NOx model is implemented to the on-line condition monitoring system on the Sleipner A platform, rather than introducing sensitive emission sensors in the exhaust gas stack. The on-line condition monitoring system forms an important tool in detecting machinery condition/degradation and air pollution, and achieving optimum energy conservation.

Modeling of nitric oxide formation in spark ignition engines with a multizone burned gas

A review of the literature on nitrogen oxide concludes that the thermal nitric oxide (NO) mechanism is the most relevant to the nitrogen oxide emissions from spark ignition engines. The review has also shown that the use of multiple burned gas zones is likely to be important for the accurate prediction of NO emissions. The multizone formulation developed here results in a particularly efficient formulation, as burning can occur within a zone. Previous multizone spark ignition engine simulations all appear to have created a new zone for each crank-angle increment during combustion. The present studies indicate that 5 or 10 burned zones are likely to be sufficient for most purposes. In reality there will be some mixing between the zones, and it could well be that with discrete burned zones, the most accurate simulation will be with a finite number of zones. The investigations with the multizone model showed lower predictions of NO than the single burned zone model; a result consistent with that of earlier workers. The outputs from the simulation have been compared with measurements from a gas engine, that encompass particularly wide variations in ignition timing (2–30 deg btde) and nondimensional air fuel ratio (0.8 < λ < 1.8). The earlier multizone NOx models had been compared with a limited range of mixtures close to stoichiometric, and in most cases the Minimum ignition advance for the Best Torque (MBT ignition timing). The opportunity was also taken to compare the NOx predictions, resulting from using different recommendations for the reaction rates in the extended Zeldovich mechanism. Finally, mean cycle modeling and cycle-by-cycle modeling were compared. There was close agreement in NO predictions only in the region of the MBT ignition timing. The significance of cycle-by-cycle modeling should be investigated further, with carefully calculated burn rate data.

NOx model for lean combustion concept

In an effort to reduce the emissions produced by gas turbine combustors to meet the expected trends of more stringent regulations, a model to calculate NOx produced by lean combustion concepts is proposed. The model simulates the combustor by a number of reactors representing the main combustion as well as pilot zones. A detailed reaction rate scheme was used to provide a fundamental basis for the derivation of reaction temperature and NOx formation expressions. The model validation involved the utilization of the data obtained for a combustor that comprised a prechamber in which fuel was vaporized and mixed with air, and a variable geometry feature to control air split between various zones. In addition to the satisfactory agreement with the measurements obtained under a wide range of operations, the model provided insight into the roles of the primary zone and pilot in forming NOx. The present investigation also shows that multidimensional calculations can provide important assistance for emissions model development. Methods of NOx reduction may be implemented in light of such information.

A pseudolinearized NOx model

Using the outcomes of nonlinear chemical reactions, describing the formation of photooxidants and of nitric acid/nitrate from hydrocarbons, oxides of nitrogen and carbon monoxide in a Lagrangian trajectory model, ‘linear’ reaction rates are derived for the reaction NO x( = NO + NO 2) → N(5) (= HNO 3 + nitrates), which are valid for different conditions. The reaction rate depends on the O 3 concentration and the photon flux during the day essentially. During the night the main factors are the O 3 concentration, the reaction rate between the NO 3- radical as well as the N 2O 5 molecule, and wet aerosols, and the local source strength of NO. The ‘linear’ rate constants during the day and during the night are given by regression equations between the constants and the specified variables. Some examples, describing real situations, are given.

Measurements of stratospheric NO2 from the Solar Mesosphere Explorer satellite: 1. An overview of the results

The visible light spectrometer on board the Solar Mesosphere Explorer spacecraft measures stratospheric NO2 in the 20–40 km altitude region and provides accurate daytime NO2 density profiles with nearly complete latitudinal coverage over an extended period of time. The instrument and data analysis are discussed in detail, and NO2 results for winter/spring 1982 are presented and compared to current theoretical models. Agreement with other measurements is good, and comparison with NOx models indicates that although the overall agreement is acceptable, improvements in the models are required before good agreement is reached at all latitudes. The data indicate that NO2 has a strong memory of the physical conditions present in the stratosphere over a time period of several days.