Kinetic analysis and modeling of coal pyrolysis with model-free methods

Knowledge about the rate of coal pyrolysis is of great importance because it exerts remarkable effect on its thermal conversions such as combustion, gasification and liquefaction. Different approaches can be used to obtain the kinetics of coal pyrolysis, the simplest are empirical and global kinetics, where Arrhenius expression is used to correlate the mass loss with temperature. This work conducted pyrolysis of a lignite and a bituminite at various heating rates with a thermo-gravimetric analyzer (TGA). Thermogravimetry coupled with mass spectroscopy (TG-MS), solid state C-13 Cross Polarization/Magic Angle Spinning nuclear magnetic resonance (C-13 CP/MAS NMR) and Raman analyses were also conducted to correlate the structural characteristics and pyrolysis behavior of the coals. Kinetic analysis was performed by using three model-free methods including Distributed Activation Energy model (DAEM), Ozawa-Flynn-Wall (OFW) and Friedman method. Kinetic modeling was also performed based on the DAEM model with multiple Gaussian sub-distributions of activation energy. The results show that DAEM, OFW and Friedman methods are almost equally effective for analyzing activation energy, which gradually increases with the extent of pyrolysis. Pyrolysis of both the coals can be divided into three stages corresponding to the reactions of various covalent bonds. Three-Gaussian distributed activation energy model (DAEM-G3) is applicable to describing pyrolysis of both coals. The well-matched modeling results with the experimental data over the entire temperature range indicate the validity of DAEM-G3 model in studying and understanding the reaction mechanism of coal pyrolysis.