CO2 Gasification Kinetics of Biomass Char Derived from High-Temperature Rapid Pyrolysis

The char of three typical biomasses, rice straw char (RS char), chinar leaves char (CL char), and pine sawdust char (PS char), was prepared in a high-frequency furnace, which could efficiently reduce secondary reactions under rapid pyrolysis conditions at 800-1200 degrees C. The rapid pyrolysis char produced was isothermally gasified in a thermogravimetric analyzer (TGA) under a CO2 atmosphere. Effects of biomass type and pyrolysis temperature on intrinsic carbon structures and morphologic structures of char and further on gasification characteristics of char were investigated using a Raman spectrum analyzer, scanning electron microscopy (SEM), and a surface area and pore size distribution analyzer. Gasification kinetic models were also contrastively discussed under different conditions. Results show that gasification rates decrease with the increasing pyrolysis temperature. Under the morphologic characteristic reserved conditions, morphologic structures present obvious effects on gasification rates. Gasification reactivity of the three biomass chars is in the order of CL char > RS char > PS char. Melting and shrinkage happen during rapid pyrolysis of PS, and the disappearance of the pore and decrease of the specific surface area of PS char lead to the low specific surface area and gasification rates of PS char. Unobvious melting happens to RS char and CL char, and the initial physical structures can be almost reserved, while CL char presents larger porosity and specific surface area, which make its gasification rates higher than those of RS char. In most conditions, the random pore model (RPM) performs well to describe gasification rates of biomass char studied in this work. However, for gasification of PS char at high temperatures, during which high gasification rates can be maintained in high conversion ranges, the modified random pore model (M-RPM) performs better. For gasification of RS char and CL char at low temperatures, during which gasification rates present a sharp decrease and trailing in medium high conversion ranges, the shifted M-RPM performs better.