This study investigated the slow pyrolysis behavior of raw rice husk (RRH) and fermented rice husk (FRH) in a fixed-bed reactor at temperatures in the range of 200–600 °C. The effects of pyrolysis temperature on the biochar yield, composition, and physiochemical properties were examined to evaluate the energy potential of biochars produced from RRH and FRH. The FRH-derived biochar produced at 600 °C was found to be more suitable than the RRH-derived biochar because of its higher carbon content (68.9% vs 42.1%), GCV (31.6 vs 24.1 MJ kg-1), and true density (1.94 vs 1.54 g cm-3). The slow pyrolysis in the high-temperature regime facilitated the formation of lignin-rich and aromatically condensed biochar, making it particularly useful for producing carbon-rich materials. Thus, slow pyrolysis can be a technically viable approach for producing high-energy-density solid fuels that can replace medium-ranking coals in co-firing.