Rice Husk and Its Pretreatments for Bio-oil Production via Fast Pyrolysis: a Review

Abstract

Rice husk is a prospective bio-oil feedstock due to its plentiful supply, but its unfavorable characteristics like high moisture content, high ash content, and low energy density tend to jeopardize both the yield and quality of bio-oil produced by fast pyrolysis. Lately, various pretreatments, namely, washing, torrefaction (dry and wet), and their combined pretreatments, have been researched on rice husk with the aim of improving its unfavorable characteristics for bio-oil production. However, the influences of different pretreatments on pretreated rice husk and the subsequent bio-oil produced have not been compared. Hence, this review paper presents an overview of rice husk as a bio-oil feedstock and its pretreatment methods for bio-oil production via fast pyrolysis. Particular emphasis is placed on the rice husk characteristics and their impacts on the bio-oil production via fast pyrolysis, as well as the different types of rice husk pretreatment and their influences on the characteristics of pretreated rice husk, product yields of fast pyrolysis, and the composition and physical properties of the bio-oil produced. A comparison of the physicochemical properties of rice husk- and other biomass-based bio-oil alongside those of petroleum fuel oil is also outlined. Major challenges and future prospects towards the utilization of rice husk as a bio-oil feedstock and the integration of rice husk pretreatment with fast pyrolysis for large-scale applications are also discussed.