Char-nitrogen oxidation under fluidised bed combustion conditions: single particle studies

Abstract

Oxidation of bituminous coal char-nitrogen to NO, N 2O, and N 2 has been studied by single particle modelling. The single particle model forms an essential part of the CFB NO x /N 2O emission tendency predictor under development. In a CFB a significant part of nitrogen oxide emissions originate from oxidation of char-nitrogen, and the char inventory in the reactor affects indirectly the fate of nitrogen components by numerous heterogeneous or heterogeneously catalysed reactions. In an earlier work a parametric study with the single particle char oxidation model was carried out, and showed a need for improvement of the model for more accurate prediction of nitrogen oxides in a CFB. In this work a re-estimation of the kinetic rate constants in the model has been carried out based on available experimental data under a wide condition range (750–950 °C, 0–21% O 2, 0–2% CO, 0–200 ppm NO, 0–100 ppm N 2O) and including measured data on CO/CO 2 ratio close to the particle surface. A new reaction has also been added in the mechanism: the char catalysed N 2O reduction by CO, which may lead to enhanced N 2 formation at FBC conditions. Further, an energy balance has been incorporated, and the description of external mass transfer has been improved in order to better account for its different rates in different parts in a CFB reactor. The new set of rate constants differs mostly from the original constants by a higher rate constant values for NO reduction by CO catalysed by char, and for N 2O reduction on the char. The temperature dependence for the ratio of the rate constants k 2/ k 1 describing the split of oxidised surface intermediate (–CNO) to N 2O and NO, respectively, is opposite. The new model and the original model lead to different predictions for nitrogen oxides. The conversion factors of char-N to NO and N 2O are lower than predicted by the original model. The predictions with the new model seem to be in better agreement with available experimental quantitative data. The new model gives also better fit for the burning times and the temperature effect on char reactivity. The most severe limitations in the model are assumed to be the omission of the HCN formation in the mechanism and the boundary layer reactions. The latter will lead to the fact that the model experiences more oxidising conditions than if the film layer reactions were included.