Temperature-resolved non-additivity in coal–biomass co-pyrolysis: governing roles of coal rank and biomass type in temperature-opposed synergy

Coal–biomass co-pyrolysis offers a flexible pathway for valorizing solid carbon resources, yet contradictory conclusions regarding synergistic effects remain common because conventional evaluations rely primarily on endpoint residue deviations. To address this limitation, a temperature-resolved evaluation framework integrating the endpoint deviation (ΔWf) with the temperature-dependent deviation function (ΔW(T)) was established and validated using thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy (TGA–FTIR) and apparent kinetic analysis for blends of three coal ranks with two representative biomass types. The results demonstrate that non-additive behavior evolves continuously during co-pyrolysis and may reverse direction over different temperature intervals, a characteristic interaction pattern defined here as temperature-opposed synergy. Bituminous coal–poplar sawdust blends exhibit relatively stable apparent promotion, whereas lignite- and anthracite-based systems show pronounced ratio-dependent sign-switching, while spirulina platensis blends display stronger temperature dependence owing to the combined influence of biomass biochemical composition and inorganic constituents. Apparent kinetic fitting and FTIR measurements provide complementary evidence supporting the proposed temperature-resolved interaction patterns. Overall, the proposed framework offers a unified basis for interpreting non-additive behavior and reconciling previously contradictory observations in coal–biomass co-pyrolysis.