Investigating the characterisation, kinetic mechanism, and thermodynamic behaviour of coal-biomass blends in co-pyrolysis process

The integration of biomass into existing thermochemical conversion processes for bioenergy production is expected to play a key role in the energy transition to reduce the reliance on depleting fossil fuels and mitigate fossil carbon emissions. In this study coal-biomass blends containing hemp and sawdust were prepared with various blending ratios for co-pyrolysis. The coal-biomass blends were characterised using ultimate analysis (CHN), gross calorific value (GCV), FTIR, and TGA. Co-pyrolysis was performed in TGA that was applied to study the thermokinetic behaviour of the respective blends. The deviation between the experimental and calculated values of TGA mass loss (ML), the residue left (RL), and maximum mass loss rate (DTG(max)) were calculated to observe the synergistic effect. The positive deviation in the ML and DTG(max) values indicated the presence of a synergistic effect during co-pyrolysis. Kinetic parameters were analysed by employing the Coats-Redfern method with thirteen integral functions. The activation energy (E-a) for individual coal was 39 kJ/mol through a one and a half chemical reaction (F3/2), while individual sawdust and hemp showed 60 kJ/mol through a deceleratory reaction mechanism for contracting sphere (R3) and 44 kJ/mol through the second-order chemical reaction (F2), respectively. Thermodynamic parameters such as the change in enthalpy (Delta H) and change in Gibbs free energy (Delta G) showed positive values that indicate the reaction was non-spontaneous. Additionally, the change in entropy (Delta S) was negative that suggested a more ordered state. The coal-sawdust blends were found to be suitable for the production of bio-oil as the individual sawdust contained a higher number of volatiles, whereas the coal-hemp blends were better suited for the production of biochar since the individual hemp produced more residue after co-pyrolysis. Hemp biochar was further characterised by FTIR, TGA, GCV, and SEM-EDX analysis to investigate its potential in environmental and energy applications.