Effects of CO2/H2O on the characteristics of chars prepared in CO2/H2O/N2 atmospheres

Abstract

CO2 and H2O play vital roles in the evolution of char structures and reactivities, particularly in wet oxy-fuel combustion. This work aims to investigate the effects of CO2/H2O gasification on the characteristics of char prepared in CO2/H2O/N2 atmospheres. Lignite samples from Zhundong coalfield in China were pyrolyzed and gasified in different atmospheres at a slow heating rate of 20 °C/min to generate char. Raman spectroscopy was used to identify the microstructure of coal chars, which was correlated with the char combustion reactivity measured with thermogravimetric analyzer in O2/H2O/CO2 atmosphere. The combustion reactivity index Tindex was correlated with Raman structural parameters to demonstrate the applicability of Raman spectroscopy in evaluating char-combustion reactivity. Results indicated that the gasification of CO2 and H2O with coal significantly contributed to the weight loss of char. The Raman band ratio I(Gr+Vl+Vr)/ID decreased and the total Raman peak area increased upon adding CO2 and H2O. Thus, the char became more condensed and contained additional O-containing functional groups. Char-combustion reactivity in oxy-fuel combustion was decreased which originated from the changes in char structures induced by CO2 and H2O gasification. Moreover, Tindex had positive and negative linear correlations with the Raman parameters I(Gr+Vl+Vr)/ID and IAll, respectively. For char prepared in CO2-containing atmosphere, the correlation among Tindex, I(Gr+Vl+Vr)/ID, and IAll can be expressed by the function Tindex = 13.2299I(Gr+Vl+Vr)/ID − 1.7255E-4IAll + 421.260 with R = 0.94.