Raw, Thermally Treated, and Densified Biomass for Solid-Fuel Combustion and Coal Co-Firing: A Critical Review

This review presents a comparative assessment of raw agricultural and wood-processing residues, thermally treated biomass, biochar, and densified fuels used alone or in coal blends. Drawing on 50 primary experimental and/or numerical studies and informed by previous reviews, it provides a comparative systematization of the advantages and disadvantages of biomass utilization routes in terms of fuel properties, combustion behavior, emissions, and practical application. Feedstocks included straw, husks, stalks, cobs, bagasse, sawdust, bark, leaves, and needles. The reported lower heating values ranged from 9 to 23 MJ kg−1 for raw biomass and from 23 to 32 MJ kg−1 for coal. The optimal biomass proportions were system-specific, depending on feedstock combinations and optimization criteria. In the studies reviewed, thermal treatment increased the heating values to as high as 31 MJ kg−1. Densification was shown to improve fuel density, mechanical strength, storage, and handling. The effect of biomass addition on emissions was not uniform: reductions were reported for some fuel systems, whereas CO, CO2, or other emissions increased in others. These outcomes depended on feedstock composition, blend ratio, pretreatment, and combustion conditions. Study heterogeneity and predominantly laboratory-scale evidence limit the transferability of these findings to pilot and industrial applications.