Evaluation of pyrolysis behavior and products properties of rice husk after combined pretreatment of washing and torrefaction

Highlights: • Effect of combined pretreatment on pyrolysis mechanism was evaluated. • KAS method and model-fitting method was combined to obtain kinetic parameter. • Char with high SiO₂ content had the potential to prepare silica products. • Pretreatment combined the advantages of acid washing and torrefaction. -- Abstract: Effect of combined pretreatment on the mechanism of pyrolysis behavior and pyrolysis products (bio-oil, non-condensable gas and char) of rice husk was investigated using thermogravimetric analyzer (TGA) and laboratory-scale fixed-bed reactor. A coupling method combining iso-conversional method and model-fitting method was used to obtain the pyrolysis kinetic parameter. Pyrolysis kinetics results indicated that activation energy (E) gradually increased with the increase of conversion rate (α), which was due to the differences in thermal stability of biomass components. Acid washing pretreatment slightly increased the average activation energy (E_a), and then subsequent torrefaction process further increased it. The entire pyrolysis reaction process of all the rice husk samples can be described by the reaction-order model. In addition, a laboratory-scale fixed-bed reactor was used to investigate the yields and detailed characteristics of pyrolysis products. The results suggested that phenols and sugars contents in bio-oil increased, while that of small-molecule components with high thermal instability decreased, which was favor for the storage and subsequent utilization of bio-oil. Rice husk char obtained from pyrolysis also has the potential for preparation of silica products. We concluded that combined pretreatment of washing and torrefaction significantly improved the thermochemical utilization potential of rice husk for fuels and chemicals by pyrolysis.