Effect of oxygen on thermal behaviors and kinetic characteristics of biomass during slow and flash pyrolysis processes

Abstract

Pyrolysis is usually considered as the initial stage of solid fuel combustion and gasification, and the pyrolysis characteristics in oxidizing atmosphere greatly affect the ignition behavior, flame stability and product distribution. However, the effect of oxygen on thermal behaviors and kinetic characteristics of biomass pyrolysis was poorly understood and has been studied only based on thermogravimetric analysis (TGA). TGA has heat transfer limitations, resulting in a significant difference between the calculated kinetic results and actual ones. In this work, a bubbling fluidized bed with an on-line analysis system was developed to investigate the real-time gas releasing behaviors of biomass flash pyrolysis in inert and oxidizing atmospheres. The slow pyrolysis characteristics were investigated in TGA for comparison. The effects of oxygen on the kinetics of flash and slow pyrolysis were investigated by combining the distributed activation energy model (DAEM) isothermal model and non-isothermal models including Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS) and Starink. Under slow heating conditions, the presence of oxygen led to a higher maximum pyrolysis rate in a smaller temperature interval, and the maximum pyrolysis rate shifted to the lower temperature. In oxidizing atmosphere, the pore volume and specific surface of char were about 4–5 times higher than that of the char prepared in inert atmosphere. The values of apparent activation energy (Ea) at the initial stage of pyrolysis reaction in oxidizing atmosphere were much higher than those in inert atmosphere. As the conversion rate reached 0.4–0.5, the presence of oxygen led to char gasification and combustion, as well as a considerable change in Ea values. In addition, the Ea values of biomass flash pyrolysis obtained by the fluidized bed were about 4–5 times smaller than those of slow pyrolysis obtained by TGA.