Thermochemical liquefaction of rice husk for bio-oil production with sub- and supercritical ethanol as solvent

•The liquefaction characteristics of rice husk in ethanol were systematically studied.•Too high temperature and solvent filling ratio resulted in lower bio-oil yields.•The bio-oil yields declined continuously with the increment of solid–liquid ratio.•NaOH was certified to be an ideal catalyst for rice husk liquefaction.•The bio-oils mainly consisted of phenolic compounds, long-chain alkanes and esters.

Rice husk was converted into bio-oil via thermochemical liquefaction with ethanol as solvent in an autoclave (500 mL). The influences of reaction parameters on the yields of liquefaction products were investigated. Liquefaction experiments were performed at various reaction temperatures (T, 513–633 K), solid–liquid ratios (R1, 5–15%), and solvent filling ratios (R2, 10–30%) with or without catalyst. Two types of catalysts were involved, including iron-based catalysts (FeSO4 and FeS) and alkali metal compounds (NaCO3 and NaOH). The dosage of catalyst (R3) was also optimized. Without catalyst, the bio-oil yields ranged from 11.8% to 24.2%, depending on T, R1 and R2. And the bio-oil yields increased firstly and then decreased with increasing T and R2, while the bio-oil yields continuously declined with increasing R1. NaOH was certified to be an ideal catalyst for rice husk liquefaction and the optimal dosage was approximately 10%. The obtained bio-oils had much higher caloric values of 20.9–24.8 MJ/kg compared to 14.9 MJ/kg for the crude rice husk sample. Without catalyst, the main components of bio-oil were phenolic compounds. In the case of NaOH as catalyst, long-chain alkanes were the major compositions of bio-oil.