Bio-coal and bio-coke production from agro residues

Biomass-derived bio-coal has shown excellent potential as a partial replacement for coal to achieve a closed carbon cycle and mitigate CO2 emissions by at least 18-40% in steel industries. The bio-coal higher heating value (30-32 MJ/kg) is in the range of the heating value of coal (30-32 MJ/kg) used in the steel industries. Therefore, different bio-coal production methods, such as pelletization, torrefaction, pyrolysis, hydrothermal carbonization, and steam explosion to produce bio-coal, have been critically reviewed. Moreover, the effect of different agro residues such as bagasse, rice husk, and wheat straw and their composition on the properties of the bio-coal are discussed. However, agro residues yield a maximum of 35 wt% bio-coal; thus, blending bio-coal with conventional coal to produce bio-coke is also discussed. Furthermore, bio-coke production from only agro residues is also reviewed. Eventually, the review concludes with an outlook and future challenges.