Rice husk and rice straw torrefaction: Properties and pyrolysis kinetics of raw and torrefied biomass

Abstract

Torrefaction is an effective pretreatment process of biomass to enhance energy density and reduce moisture and oxygen contents. The kinetics analysis is important to estimate the torrefaction efficiency and understand the thermal degradation characteristics of torrefied biomass. In this study, the pyrolysis kinetics (Flynn–Wall–Ozawa method, Kissinger–Akahira–Sunose​ method, and Starink’s method) of rice husk (RH), rice straw (RS), and their torrefied solid products were investigated under different heating rates. The results demonstrated that the increase of torrefaction temperature decreased the content of hemicellulose and cellulose. TG-FTIR and TG/DTG curves of RH and RS showed a right shift trend after torrefaction. The average activation energy calculated was 149.79 and 161.76 kJ mol −1 for RH and RS pyrolysis. The activation energy of torrefied RH and RS showed an increasing trend with torrefaction temperature. According to the change of activation energy, the pyrolysis model of RH and RS was predicted by Z ( α ) master plots and compensation effect, respectively. Torrefaction could convert the First-order reaction model (F1) to nucleation and growth model (An) gradually for RH, and raise the reaction order of the pyrolysis reaction for RS. • Rice straw was more sensitive to torrefaction temperature than rice husk. • The average E a were 150 and 162 kJ mol -1 for rice husk and rice straw pyrolysis. • The activation energy of torrefied biomass was higher than that of raw biomass. • Torrefaction raised the pyrolysis reaction order of rice straw. • Pyrolysis of torrefied rice husk was dominated by nucleation and growth model.