Effects of microstructural evolutions of pyrolysis char and pulverized coal on kinetic parameters during combustion

Abstract

Pyrolysis chars have potential as fuels for pulverized coal injection (PCI); however, their proper and efficient utilization requires evaluation of char combustion kinetics. The combustion characteristics of two chars (F-char and M-char) and two pulverized coals (H-PCI and P-PCI) were analyzed herein using thermogravimetric analysis–mass spectrometry. The apparent activation energy (Ea) of the sample under non-isothermal combustion conditions was obtained using the Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose methods, and the reaction mechanism for the fuels was established using the Malek method. Additionally, changes in the microscopic pore structure and carbon chemical structure of the fuels at different stages of combustion were characterized using N2 adsorption and X-ray diffraction to analyze the relationship between microstructural evolution and Ea. The results suggested that Ea of the sample first rapidly decreased and then became stabilized during combustion. Compared with pulverized coals, the two chars presented more developed microscopic pore structure, less-ordered carbon chemical structure and lower Ea during reaction. During combustion, the stacking height of the aromatic layer first decreased and then increased, whereas the specific surface area first increased and then decreased. The volatile content significantly influenced Ea only during the initial stage of combustion. During the middle stage, Ea was controlled more by the microscopic pore structure and the carbon chemical structure, and those influences disappeared in the later stage. The transition point of the structures affecting Ea occurred at a combustion rate between 52.9% and 72.0%. In general, the microscopic pore structure and the carbon chemical structure influenced kinetic parameters more than the volatile content.