How is nox formed in combustion

Conversion of fuel-bound nitrogen to NOx is strongly dependent on the fuel-air stoichiometry but is relatively independent of variations in combustion zone temperature. Therefore, this conversion can be controlled by reducing oxygen availability during the initial stages of combustion. Techniques such as controlled fuel-air mixing and staged combustion provide a significant reduction in NOx emissions by controlling stoichiometry in the initial devolatilization zone.

There are a variety of ways to control NOx in a boiler. Combustion technology -based NOx control is typically the lowest cost solution.

NOx formation is promoted by rapid fuel-air mixing. This produces high peak flame temperatures and excess available oxygen which, in turn, promotes NOx emissions.

Combustion system developments responsible for reducing NOx formation include low NOx burners, staged burning techniques overfire air , and flue gas recirculation FGR. The specific NOx reduction mechanisms include controlling the rate of fuel-air mixing, reducing oxygen availability in the initial combustion zone, and reducing peak flame temperatures.

Additional NOx control techniques can be applied downstream of the combustion zone to achieve further reductions. In either technology, NOx is reduced to nitrogen N2 and water H2O through a series of reactions with a reagent or reagents injected into the flue gas.

SNCR typically is limited to lower NOx reduction levels but may be the more economical choice depending on the required NOx reduction or the unique project requirements. Both technologies have been successfully applied for NOx reduction on multiple fuels and boiler types. A combination of combustion and post-combustion NOx control is frequently the most economical approach for existing installations and is required for installations that cannot achieve necessary emissions reduction by combustion methods alone.

Please fill out the form and a representative will contact you shortly. We look forward to hearing from you. Renewable Overview. Thermal Overview. Company Overview. The chemical source term in the post-processing transport equation for NO was obtained from the reduced mechanisms, and the predictions of NO showed adequate agreement with the experimental data of the Adelaide JHC flame Dally et al. Hence, the reduced mechanisms performed reasonably well for the conditions on which they were tailored.

However, errors may be introduced when extrapolating to different conditions than those used for fitting: as for the simplest post-processing tool mentioned above, the mechanism reduction relies on the assumptions of partial equilibrium and quasi-steady state that can fail under certain conditions and affect the predictivity of the reduced chemical schemes.

A CRN approach that employs a detailed mechanism with relatively low computational costs was presented by Frassoldati et al. Frassoldati et al. The numerical algorithm of the CRN approach was refined Cuoci et al. The formation of NO x was quite successfully modeled in laboratory flames Bergmann et al.

The joint CFD-CRN method was also employed to predict NO x emissions in a interturbine flameless combustor for gas turbines: the flamelet generate manifolds FGM method was used to model the combustion process in the CFD simulation, whereas a detailed mechanism was considered in the resolution of the network of PSRs Perpignan et al. Although the CRN approach can be used to evaluate the variation of NO x emissions under different operating conditions in a cost-effective way, the reliability of the CRN predictions is strongly dependent on the accuracy of the underlying CFD simulation.

Moreover, network parameters such as number and type of reactors, mass flow rates and residence times, have to be tuned on the combustion conditions and a general effective method for the optimization of these parameters has not been validated yet. Detailed kinetic mechanisms that include both hydrocarbons oxidation and NO x chemistry are necessary to improve predictivity and comprehensiveness of CFD simulations of turbulent MILD combustion systems.

They include mutual NO-hydrocarbons interactions that are incorporated in the NO x chemistry, especially in prompt and reburning routes. The effects of turbulence-related fluctuations on the mean reaction rates become more prominent when detailed kinetic mechanisms are considered for NO x formation.

Turbulent combustion models have been developed over the years following two main assumptions: infinitely fast chemistry compared to the scale of turbulence and finite-rate chemistry. Hence, models that account for the effects of turbulence interactions on finite-rate chemical reactions should be considered Minamoto and Swaminathan, The Eddy Dissipation Concept EDC Granm and Magnussen, is a finite-rate chemistry combustion model and has found wide application for the simulation of MILD combustion systems thanks to affordable, although not negligible, computational costs when compared to more sophisticated models such as the Transported Probability Density Function TPDF methods.

Several numerical investigations Mardani and Tabejamaat, ; Gao et al. PaSR and EDC are conceptually similar since they both model combustion as a sequence of reaction and mixing steps in locally uniform regions. However, estimating the formation of NO x may require the resolution of chemical source terms with different characteristic time scales, since the reactions forming NO occur in a wider range of time scales compared to those of the main combustion process. Figure 2 provides a comparison of the capabilities of some of the CFD modeling approaches mentioned above to replicate the emission of NO x measured in the Adelaide JHC burner.

The burner has been the target of numerous numerical studies in the context of NO x formation modeling. It consists of an insulated and cooled central fuel jet supplying an equimolar mixture of CH 4 and H 2.

The fuel jet is surrounded coaxially by an annulus containing the hot co-flow flue gases produced by a secondary burner mounted upstream. The hot combustion products are further mixed with air and nitrogen to control the oxygen levels.

The burner is placed inside a tunnel. Details of the geometry can be found in Dally et al. Measurements of NO were taken via the single-point Raman-Rayleigh-laser-induced fluorescence technique. The selected numerical results are provided by RANS studies that differs from one another in terms of employed chemical mechanism [either KEE, Bilger et al.

In contrast, model M2 considers the same global scheme for prompt formation as model M1 and multi-step mechanisms for the other routes. Figure 2. The experimental profiles include both the mean values and the error bar with The relevant settings of the numerical simulations are reported in the explanatory table at the bottom. The latter is due to the shift of the predicted profiles of relevant quantities for NO x formation, such as temperature and OH concentration, reported in Iavarone et al.

The cause may be attributed not only to the kinetic mechanism and the combustion model employed, but also to the choice of the turbulence model, which must be able the replicate the spreading of the jet flame. This overestimation is likely due to the absence of the reburning route and the assumption of a global scheme for the prompt route, which was found to be the main contributor to NO formation in the corresponding study Galletti et al.

For models M5-M6, Iavarone et al. Same results have been obtained for simulation M4, showing that the contribution rank is not affected by using a different combustion model. These trends can be related to the structure of the JHC flame.

Three subregions can be identified: a fuel-rich region located inside the jet, a reaction region inside the shear layer, and a fuel-lean region. The thermal route is suppressed under the low temperature conditions, and the N 2 O mechanism is not important. This is in agreement with the results of Gao et al. Finally, it can also be noticed from Figure 2 that biases in the combustion closure can be as important as the level of the accuracy of the chemical scheme employed.

MILD combustion represents a promising technology to comply with restrictive policies on pollutant emissions prompted by growing environmental issues. MILD combustion can provide low emissions of NO x thanks to the alteration of the corresponding chemistry. This alteration is caused by the peculiar features of MILD regime, namely homogeneous reaction zones, reduced temperature peaks, and changes in the composition of reactants and subsequent radical species pool. As a consequence, prompt, N 2 O-intermediate, NNH-intermediate, and reburning routes, which may be typically neglected in conventional combustion systems, end up playing a crucial role in the formation and destruction of NO x in MILD regime.

Although several studies have analyzed the relative impact of the NO x chemical pathways on the emissions of NO x in MILD combustion, a dominant source has not been identified. The lack of consensus is given by several factors: a variability of the operating conditions at which the MILD regime is established; b considerable uncertainty still present in the kinetics of NNH and prompt routes; c strong interconnection between the hydrocarbon chemistry and the NO x chemistry, via prompt and reburning routes in particular; d ambiguity in the methods of isolating the contribution of a single route with respect to the others, given their interdependence; e role of closure sub-models, such as hydrocarbon oxidation kinetics and turbulence-chemistry interactions.

A summary of NO x formation modeling approaches employed in CFD simulations has been presented in this review. The approaches span from the estimation of the NO x emissions through CFD post-processing with simple kinetic mechanisms to the modeling of NO x formation with detailed kinetic schemes incorporated in turbulence-chemistry interaction models.

Even though fairly good predictions of NO x emissions may be obtained by post-processing techniques, their use can be negatively affected by: a lack of necessary detail when simple kinetic mechanisms are used; b lack of generality, with risks when extrapolating, since important parameters must be adjusted on the combustion conditions and are thus case-dependent; c strong dependence on the accuracy of the underlying CFD simulation.

On the other hand, the use of detailed mechanisms with reactor-based combustion models, desirable in terms of improved predictivity at still affordable computational costs, can be affected by the challenge of accounting for the gap of chemical time scales between nitrogen chemistry and fuel-oxidizer reactions. Therefore, models that bridge these different scales are necessary, since it has been shown that biases in the combustion closure are as important as the level of the accuracy of the chemical scheme employed.

To reduce the uncertainty related to the use of combustion models, one could leverage Direct Numerical Simulations DNS , which solve the entire range of spatial and temporal scales of the turbulence avoiding the need for a combustion closure. DNS has been recently used to provide useful insights on the physics of MILD combustion and validate combustion models under this regime Swaminathan, Indeed, a DNS study confirmed the flaws of the flamelet approaches in MILD combustion due to the presence of microscopic flame interactions that result into distributed reaction zones at the macroscopic level Minamoto and Swaminathan, A-priori assessments of combustion models can be beneficial also for the prediction of NO x emission with detailed kinetic mechanisms incorporated in finite-rate chemistry combustion models.

So far, affordable DNS solvers can accommodate reduced kinetic schemes. However, in the next few years, the increasing computational resources can facilitate the inclusion of more detailed schemes, with the possibility to explore the range of time scales involved in the NO x chemistry and shed light on the importance of specific NO x pathways in MILD combustion. More validation studies of newly available kinetic mechanisms and combustion models are to be accomplished using data coming from either experiments or DNS of turbulent MILD burners to reach clear conclusions about the role and the modeling of NO x formation routes, fuel oxidation chemistry, and turbulence-chemistry interactions in the predictions of NO x emissions in MILD combustion.

SI and AP have contributed to the research concept and design. They equally contributed to the manuscript through literature review and useful discussions. The work contains parts of their joint research activities. SI has assembled the data and written the first release of the manuscript. AP has critically revised the paper up to the final version. This work has been financially supported by the Wiener Anspach Foundation and has received funding from the European Research Council ERC under the European Union's Horizon research and innovation programme under grant agreement No.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Energy Fuels 27, — Dally, B. Structure of turbulent non-premixed jet flames in a diluted hot coflow. This method is normally confined to larger, utility-size equipment.

Pre-combustion method prevents NOx from forming in the first place. Pre-combustion NOx reduction is accomplished by either staging the combustion process or recirculating flue gases into the combustion process FGR. FGR is accomplished by forcing the flue gases with a separate fan back into the combustion zone forced FGR , or by drawing the flue gases through the combustion air fan induced FGR.

Both methods reduce the bulk flame temperature in the furnace to inhibit the chemical reaction between the nitrogen and oxygen. FGR systems reduce NOx emissions without reducing efficiency.