Circulating Fluidised Bed Gasification of Biomass: Modelling of Fuel-Bound Nitrogen Evolution

A model for a CFB biomass gasifier is developed aiming at predicting the pollutant emissions. This model considers a total of eighteen chemical reactions and 8 species (C, H₂, CO, H₂O, CO₂, CH₄, NH₃ and NO), of which nine reactions are related to fuel-bound nitrogen (FBN). The model implies that the FBN is transformed into gas, crude tar and char in certain proportions during fast pyrolysis, depending on the kind of biomass, operating temperature and heating rate, and shows that, of Tar-N and Char-N, less reactive-N is delaying in participating in the further reaction compared with reactive-N despite the proportion of less reactive-N to reactive-N and the time lag varies with the type of biomass and operating temperature. NH₃ is the sole nitrogenous pollutant considered in the model due to the concentration of NO being much lower compared with that of NH₃. The trend of NH₃ concentration predicted by our model is in good accordance with previous results reported in literature. The model predicts the significant decrease of NH₃ emission with temperature increase, and indicates the importance of O₂ action and its contact with NH₃ for the formation and destruction of NH₃.