Carbonization of rice husk in an oxygen-limited counter-current combustion and KOH activation of the biochar

The biochar prepared from biomass by carbonization has been widely used due to the developed pore structure and chemically active surface. In this study, one carbonization method of rice husk via self-sustaining oxygen-limited counter-current combustion was proposed. Such a combustion process has a dynamic propagating combustion front with a temperature of 632–1023 °C, allowing the effective carbonization of rice husk with a removal ratio of volatile matter inside rice husk as high as 98.3 %. For this special combustion process, increasing the oxygen concentration achieves a milder but faster carbonization process, characterized with shorter residence time, lower combustion front temperature and higher biochar yield. Compared to the pyrolyzed biochar prepared by the external heating method, the biochar prepared via this combustion process is of a more developed amorphous structure and more oxygen-containing function groups, making the biochar more easily chemically etched and inspiring more activation reactions. Such biochar is suitable to prepare the mesopore-dominated activated carbon, especially at low mass ratios of KOH to char. At the ratio of 4, an optimal activation effect with a specific surface area (S_BET) of 2863 m² g⁻¹ and total pore volume (V_TOT) of 1.854 cm³ g⁻¹ is achieved.