NO_x Formation in Natural Gas CombustionEvaluation of Simplified Reaction Schemes for CFD Calculations

Abstract

There is a demand for further reducing NO x emissions from natural gas burners in heat and power production. For this issue, computational fluid dynamics (CFD) is a powerful tool, gaining in importance. However, because of computational efforts, there is the necessity for simplified models in order to simulate the combustion reactions and the NO x formation, respectively. In this work, different simplified models for the NO x formation, i.e., thermal NO and prompt NO formation, are validated against the predictions of a detailed chemical kinetic mechanism. NO x formation from CH 4 combustion becomes relevant above ∼1200 °C. For temperatures >1600 °C, thermal NO formation is dominant. At lower temperatures, the N 2 O/NO and NNH routes have a significant contribution. For the conditions investigated, prompt NO formation is not relevant except for fuel-rich combustion. The simplified models capture thermal NO formation quite well. The superequilibrium radical concentration in the flame zone can be modeled by the "partial equilibrium approach" for the O radical, assuming its concentration in the relevant temperature range is ∼30% higher than that for the equilibrium approach. However, this model overestimates NO formation under postflame conditions. For the prompt NO model, CH 4 and O 2 concentrations are necessary. These are obtained from the combustion model, which can be a local thermodynamic equilibrium (mixing rate limiting), a global reaction (reaction and/or mixing rate limiting), or a simplified reaction mechanism (flamelet model 1 ). The prompt NO model based on these combustion calculations is able to reasonably capture the NO emissions predicted by the detailed mechanism. However, the performance of the prompt NO model is rather dependent on the combustion model. Thus, this one must be chosen carefully. Critically, it has to be stated that, although the simplified prompt NO model captures the observed trends acceptably, it lacks a physical basis. It assumes the prompt NO formation to be proportional to the fuel concentration. The detailed reaction mechanism shows that the NO formation is more strongly related to the fuel oxidation rate, i.e., the radical concentration. Moreover, the detailed mechanism shows that prompt NO formation is of secondary relevance. Low-temperature NO formation is more related to the N 2 O/NO and the NNH routes.

Emission Factors of NOx, SO2, PM and VOCs in Pharmaceuticals, Brick and Food Industries in Shanxi, China

“…The EF I values ranged from 1.91 to 11.2 kg t -1 (mean: 5.86 ± 2.95), and they were 5.01-231 kg MY -1 (mean: 130 ± 88.3) for EF II (Figs. 1 and (Löffler et al, 2005). EF II values (reported in kg MY -1 ) of NO x varied from 5.63 of P8 to 93.8 of P3 with the mean value as 56.6 (Fig.…”

“…All the 9 enterprises were not equipped with NO x RFs, so the NO x formation processes became the determining factors for NO x emissions. Unlike SO 2 forming by combustion of sulfur in coal, NO x mainly originated from oxidation of air N 2 at high temperature regardless of that small proportion of NO x formed by burning of fuel-nitrogen (Löffler et al, 2005), so SO 2 emissions were more depended on the coal compositions than NO x due to the different formation mechanisms between them. Therefore, NO x EF I values experienced minor fluctuation compared to SO 2 EF I , which ranged from 2.58 to 4.72 kg t -1 with the mean value as 3.09 kg t -1 .…”

Numerical Study on Heat Transfer and Combustion Characteristics of Gas-fired Boiler under Different Loads

“…In this study, SIMPLEC algorithm was carried out to solve the time averaged conservation equation for mass, momentum and energy [9] . The realizable 𝑘 − 𝜀 model was chosen to describe the turbulent flow process [10] .…”