VALLEJOS, F.; DÍAZ, N.; SILVA, Á.; JIMÉNEZ, R.; GARCÍA, X.; RADOVIC, L.:
Carbon, 109, 253-263 (2016).
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.