Rice husk valorization: production of bio-oil and silica-rich solid phase using direct thermal liquefaction

Rice husk is a major byproduct of rice milling, containing an approximate composition of 72–85 wt.% lignocellulose and 15–28 wt.% silica. The lignocellulosic part of the rice husk can be utilized to produce value-added products such as fuel, chemicals, etc., while silica can be utilized to produce silicon-based materials. This work reports a process route that utilizes the complete rice husk. Herein, a direct thermal liquefaction technique is used that converts lignocellulosic fraction into bio-oil with an exceptionally high yield of ~ 90 wt.% and leaves behind a silica-rich solid residue with very low organics (~ 7 wt.%) in it. Investigation of process parameters, such as temperature, reaction time, water concentration, and biomass loading, reveals that a liquid yield of ~ 90 wt.% can be achieved at 280–300 ⁰C for 20–30 min of reaction time. Most of the minerals in the rice husk landed in the solid residue. The resulting solid residue was found to contain mainly silica in it with a concentration of ~ 98 wt.% (excluding organics). The silica can be extracted from the solid residue without the need for an incineration step. The bio-oil can be further used to produce biofuels or chemicals.