Effect of CO2 on oxy-fuel combustion of coal-char particles in a fluidized bed: Modeling and comparison with the conventional mode of combustion

A char combustion model is developed to study the effect of CO2 on the combustion of coarse char particles under oxy-fuel conditions in a fluidized bed (FB). It is a transient one-dimensional model, taking into account the heat and mass transfer from the bed to the particle and the heterogeneous combustion and gasification of char. The model shows good ability to predict the char temperature history measured in our previous work for different combinations of O-2/CO2 and O-2/N-2 with various coal types. Simulations are carried out to establish the role of CO2 in oxy-fuel conversion at different O-2 levels, particle sizes, and bed temperatures. The model is used to analyze the relative contribution of carbon in the char consumed by CO2 (gasification) and O-2 (combustion), as well as the differences of the peak temperatures and the burnout times in O-2/CO2 and O-2/N-2 for char particles in a commercial FB combustor. The results indicate that the conversion of coarse (mm size) char particles in an oxy-FB is controlled by the diffusion of O-2 both in the O-2/CO2 and O-2/N-2 case. The burn-out time decreases with the bed temperature also in both cases. The lower O-2 diffusion rate in CO2 compared to N-2, is the main reason for the longer burnout time and lower peak temperature found using O-2/CO2 at bed temperatures of 1073-1173 K. In that temperature window, the contribution of the CO2-char gasification is limited, being notable only at high bed temperature in O-2/CO2, e.g. 1223 K. In such high temperature conditions (rarely expected to be found in commercial coal FBC) the predicted burnout time of a lignite char-particle becomes shorter in O-2/CO2 than in O-2/N-2. (C) 2016 Elsevier Ltd. All rights reserved.