A decoupled approach for NO_x-N_2O 3-D CFD modeling in CFB plants

摘要

In this study, a 3D CFD model for the formation of NO_x and N_2O in a lignite fired 1.2 MWth CFB pilot plant is developed. The decoupled approach (decoupled from combustion simulation) is tested for the minimization of computational cost. As combustion simulation is prerequired, this was achieved through a simplified 3-D CFD combustion model. The developed model is then applied to the pilot-scale 1.2 MWth, CFB plant and validated against experimental data. As concerns the NO_x-N_2O model, an extensive literature review is also carried out for the incorporation of the appropriate reactions network and respective reaction rates expressions. Results show that homogenous reactions are favoured on the lower section of the bed, due to the abundance of fuel devolatilization products. On the other hand, on the upper section, heterogeneous reactions govern nitric oxide formation/reduction. It is found that for the lignite examined in this work, HCN is released in negligible amounts during char combustion. The proposed and validated CFD model for NO_x and N_2O, is capable of examining the effect of different operational parameters and coal properties on the overall nitric oxides emissions from a CFB combustor, with low computational cost and without the additional expenses for pilot-scale experiments.