Routes for energy and bio-silica production from rice husk: A comprehensive review and emerging prospect

Abstract The high energy and silica contents of rice husks make them attractive for use in renewable energy and green chemical strategies. This review comprehensively discussed the potential route of rice husk utilization to energy and bio-silica simultaneously which is still lacking to investigate. The ash content in the dry rice husk could be up to 25 wt% and the high silica content in the rice husk ash could reach 99.77 d b. wt%. About 1 tonne/h of rice husk combustion could produce about 600–700 kWh of averaged net electricity so that it can be employed to support a small and simple power plant. Apart from that, applying acid leaching as a pretreatment before rice husk ash extraction with NaOH could achieve bio-silica with 98% recovery, 99.7% purity, and 400.69 m2/g surface area. Rice husk combustion in a suspension combustor seems to be appropriate to maintain the low combustion temperature at 700 °C in order to prevent the crystalline silica formation in the rice husk ash. The combustion efficiency is very high with 0.8% of unburnt carbon content in the ash as well as CO and NOx emissions are below 250 ppm. This review reveals that the industrial-scale for energy and amorphous bio-silica production from rice husk through a suspension combustor with optimized operating parameters emerges as a future prospect. It can lead to sustainability and self-reliance on chemicals and energy sectors, of which there are many research and development challenges to overcome.