Validation of flow simulation and gas combustion sub-models for the CFD-based prediction of NO_x formation in biomass grate furnaces

摘要

While reasonably accurate in simulating gas phase combustion in biomass grate furnaces, CFD tools based on simple turbulence-chemistry interaction models and global reaction mechanisms have been shown to lack in reliability regarding the prediction of NO_x formation. Coupling detailed NO_x reaction kinetics with advanced turbulence-chemistry interaction models is a promising alternative, yet computationally inefficient for engineering purposes. In the present work, a model is proposed to overcome these difficulties. The model is based on the Realizable k-ε model for turbulence, Eddy Dissipation Concept for turbulence-chemistry interaction and the HK97 reactionmechanism. The assessment of the sub-models in terms of accuracy and computational effort was carried out on three laboratory-scale turbulent jet flames in comparison with the experimental data. Without taking NO_x formation into account, the accuracy of turbulence modelling and turbulence-chemistry interaction modelling was systematically examined on Sandia Flame D and Sandia CO/H2/N2 Flame B to support the choice of the associated models. As revealed by the Large Eddy Simulations of the former flame, the shortcomings of turbulence modelling by the Reynolds averaged Navier-Stokes (RANS) approach considerably influence the prediction of the mixing-dominated combustion process. This reduced the sensitivity of the RANS results to the variations of turbulence-chemistry interaction models and combustion kinetics. Issues related to the NO_x formation with a focus on fuel bound nitrogen sources were investigated on a NH3-doped syngas flame. The experimentally observed trend in NO_x yield from NH3 was correctly reproduced by HK97, whereas the replacement of its combustion subset by that of a detailed reaction scheme led to a more accurate agreement, but at increased computational costs. Moreover, based on results of simulations with HK97, the main features of the local course of the NO_x formation processes were identified by a detailed analysis of the interactions between the nitrogen chemistry and the underlying flow field.