Comparative study on the effects of heating rate on char gasification behaviors by thermogravimetric analyzer and high-temperature stage microscope under non-isothermal condition

Abstract

Understanding the intrinsic gasification reactivity and kinetics of coal char is vital in the process optimization, reactor design, and scaling of industrial gasifiers. Thermogravimetric analyzer (TGA) and visual high-temperature stage microscope (HTSM) were developed to investigate the mass loss and morphological evolution of coal char under non-isothermal conditions at different heating rates of 2.5, 5, and 10 °C/min, respectively. The gasification reactivity and kinetics obtained from different systems were comprehensively compared with the consideration of heating rate. Results showed that the consumption of carbon matrix and sintering could be responsible for the appearance of two reaction rate peaks in the rapid reaction stage in the HTSM experiments. As the heating rate increased from 2.5 to 10 °C/min, the limitation of heat and mass transfer and shortened residence time were conducive to the increase of observed reactivity and characteristic temperatures. Also, more serious diffusion and limitation could be responsible for the lower reactivity monitored by TGA and the difference in increasing amplitude of characteristic temperatures. Kissinger-Akahira-Sunosen (KAS) iso-conversional method was efficiently proposed to predict the gasification process. The activation energy varied with the carbon conversion, and then the average activation energies determined by TGA and HTSM were 191.51 and 199.01 kJ/mol, respectively. The evidence proved powerfully that the results close to the intrinsic behaviors could be provided by HTSM due to lower diffusion and limitation from heat and mass transfer.