Pyrolysis kinetics of hulless barley straw using the distributed activation energy model (DAEM) by the Tg/DTA technique and SEM/XRD characterizations for hulless barley straw derived biochar

The pyrolysis kinetics of hulless barley straw at different heating rates of 5, 10, 15, 20 and 30 deg C/min were investigated via thermogravimetry, and the activation energy distribution E and pre-exponential factor k₀ were calculated using the Distributed Activation Energy Model (DAEM) from thermogravimetric analysis (TGA) curves, and characterizations of pyrolysis product of biochar were analyzed by techniques of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The pyrolysis process consisted mainly of the dehydration stage (50-150 deg C), the active pyrolysis stage (200-400 deg C) and the passive pyrolysis stage (400-800 deg C). The E ranged from 73.45 to 214.11 kJ/mol within the conversion rate range of 0.10-0.55, and changed from 214.11 to 141.55 kJ/mol within the conversion rate range of 0.55-0.90, and the average value of E was 172.23 kJ/mol. The values of k₀ changed greatly with E values at different mass conversion. The wide E and k₀ distributions obtained from the kinetic analysis are attributed to the complex chemical reactions of pyrolysis. The structure of biochar was degraded or ruptured due to the increase in temperature. The XRD analysis confirmed that the biochar was amorphous, dominated by disordered graphitic crystallites. (author)