Modeling NH 3 and HCN emissions from biomass circulating fluidized bed gasifiers ☆

Abstract

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 NH 3, 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 NH 3 emissions generally agree with the literature data. A sensitivity analysis of some reaction rates included in the model on NH 3 emissions has also been carried out. It has been revealed that the catalytic activity of bed materials towards the oxidation of NH 3 has the greatest influence on the predicted NH 3 emissions. In addition, the predicted NH 3 emissions are also affected by the catalytic activity of bed materials towards the decomposition of NH 3 and the homogenous reaction rates of NH 3 decomposition and the reduction of NO by NH 3 in the presence of oxygen.