On the structural and reactivity differences between biomass- and coal-derived chars

Rapidly growing research on biomass reactivity has not benefited enough from the very abundant knowledge acquired on coal-derived materials. Literature comparisons using similar methods are scant and inconclusive. To clarify differences (or similarities) between coal- and biomass-derived chars, we performed a systematic study using three different carbon-based solids at different stages of coalification. Two biomass materials and a low-rank coal were demineralized and heat-treated between 550 and 1450 °C. Even after heat treatment, biomass-derived chars remained less structurally ordered, more microporous, and more reactive towards oxygen than their coal-derived counterparts. Through computational chemistry, we compared the thermodynamic driving forces and kinetic barriers for biomass-like and coal-like representative graphene clusters. The oxidation pathway to CO was the same for both clusters, but different for CO₂ formation. We predict that the energy barrier of CO desorption is higher for coal-derived chars, whereas that of CO₂ desorption is higher for biomass-derived chars; this is consistent with the dominant formation of CO₂ at the relatively low reaction temperatures used in our study. We demonstrate also that in a kinetically controlled reaction regime, in the absence of catalytically active minerals, the reactivity differences can be explained on the basis of differences in char surface chemistry.