Catalytic influence of mineral compounds on the reactivity of cellulose-derived char in O₂-, CO₂-, and H₂O-containing atmospheres

The catalytic effects of mineral compounds on the conversion of a biomass-derived char in air- and oxyfuel-related atmospheres were investigated by thermogravimetric analysis at atmospheric pressure. The applied char originated from the hydrothermal carbonization (HTC) of cellulose followed by pyrolysis at 1073 K and subsequent mixing with 20 wt% of minerals by grinding to achieve tight contact. The reactivities of the mineral-loaded HTC chars were evaluated based on isothermal experiments in O₂-, CO₂-, and H₂O-containing atmospheres as a function of their composition applying a magnetic suspension balance. The reactivity sequence K₂CO₃ > Na₂CO₃ ≫ Fe₂O₃ > CaO > MgO ≥ mineral-free was derived consistently for char oxidation in O₂/inert as well as for char gasification in diluted H₂O and CO₂ mixtures. In addition to this qualitative assessment, the kinetic experiments were first modelled based on a simple global n^th-order power-law rate expression. Then, the more complex Carbon Burnout Kinetics (CBK/G) model and the PoliMi model were applied. All three modeling approaches enabled a systematic quantification of the catalytic effects and led to a comparable lowering in the apparent activation energy. In combination with the kinetic parameters determined for the mineral-free char, the lowered apparent activation energies specific for the applied mineral and atmosphere facilitate the implementation of catalytic effects on the conversion of biomass-derived char into combustion models.