Energy & Fuels, Vol.34, No.2, 2553-2565, 2020
Behavior and Kinetics of Drying, Pyrolysis, Gasification, and Combustion Tested by a Microfluidized Bed Reaction Analyzer for the Staged-Gasification Process
In this research, a systematic analysis and comparison between the behavior and the kinetics of coal drying in Ar, coal pyrolysis in Ar, char gasification in CO2, and char combustion in air were conducted on a microfluidized bed reaction analyzer according to the isothermal method. To avoid the thermal decomposition of the coal particle at high temperature, the wet char sample with a moisture content of 20% was adopted to simulate the process of coal drying. For the tested experimental temperature in the range of 1123-1223 K, the process of coal pyrolysis in Ar can be finished in 10 s while the processes of char gasification in CO2, char combustion in air, and wet char drying in Ar were much longer, on a minute scale. The drying process spent most of the time in char gasification and combustion, all of which happened nearly simultaneously rather than subsequently in the actual gasifier. At 1123 K, the ratios of the maximum reaction rate between drying and gasification, combustion and gasification, and pyrolysis and combustion were 2.2, 34.0, and 225.0, respectively. For char combustion, the delayed phenomenon extended the whole reaction time and lowered the reaction rate significantly at a conversion above 0.6. Thus, the main limiting steps in the gasifier were char gasification and the delayed combustion. The calculated activation energies (E-a) for pyrolysis in Ar, drying in Ar, normal combustion in air, delayed combustion in air, and gasification in CO2 were 30.5, 67.0, 56.4, 143.5, and 176.4 kJ/mol, respectively. Finally, from the viewpoint of the reaction behavior and kinetics, some suggestions were provided for the design and operation of the fluidized bed two-stage gasification process.