Modeling NH3 and HCN emissions from biomass circulating fluidized bed gasifiers

A circulating fluidized bed biomass gasification model is developed in the present study. The model consists of sub-models for devolatilization, tar cracking and a chemical reaction network of main gasification reactions and nitrogen chemistry. A total of forty global chemical reactions are included in the model, of which twenty-eight reactions belong to fuel-nitrogen reaction network. Individual reaction rates are selected from the literature, wherever possible, based on studies of woody biomass fuels. Volatile nitrogen is assumed to consist of NH3, HCN and N-2 with the distribution between three species as input parameters to the model. Modeling of the hydrodynamics of the riser is simplified by using solids concentration profile along the riser as an input to the model. Both gaseous phase and solids phase are assumed to be in plug flow. Modeling results are compared with the experimental results published in the literature. Predicted effects of bed temperature, equivalence ratio and fuel moisture content on main gaseous composition, tar and NH3 emissions generally agree with the literature data. A sensitivity analysis of some reaction rates included in the model on NH3 emissions has also been carried out. It has been revealed that the catalytic activity of bed materials towards the oxidation of NH3 has the greatest influence on the predicted NH3 emissions. In addition, the predicted NH3 emissions are also affected by the catalytic activity of bed materials towards the decomposition of NH3 and the homogenous reaction rates of NH3 decomposition and the reduction of NO by NH3 in the presence of oxygen. (C) 2003 Elsevier Science Ltd. All rights reserved.