Char Structural Evolution during Pyrolysis and Its Influence on Combustion Reactivity in Air and Oxy-Fuel Conditions

Three kinds of Chinese coal and a biomass were pyrolyzed by N-2 and CO2 in a bench scale fluidized bed reactor. Fourier transform (FT)-Raman/infrared (IR) spectroscopy was used to identify microstructure and to evaluate the structural evolution of chars generated in N-2 and CO2 environments, which are the main diluting gases of air and oxy-fuel environments. The Raman spectra were fitted with five Lorentzian bands. The reactivities of the char were measured by a thermogravimetric analyzer from room temperature to 1373 K in air and oxy-fuel conditions with O-2 concentration of 21%. The derived activation energy for different samples was correlated with the Raman structural parameters. Results showed that more disordered char was formed with the pyrolysis in CO2 than that in N-2, and new O-containing functional structures would be introduced into the char structure in CO2 atmosphere. The char structures became less ordered as the sample rank decreased. The reactivity of CO2 char was higher than that of N-2 char, while the combustion atmospheres rarely affected the char reactivity, indicating CO2 played a more important role on the devolatization process for coal than for char combustion. The activation energy had a good linear correlation for N-2 char with Raman characterizations, while the data points for CO2 char were perfectly fitted with exponential functions.