Evaluation of the effect of high sulfur subbituminous coal on the devolatilization of biomass residue by using model free, model fitting and combined kinetic methods

Abstract

In order to sustainably utilize waste biomass and low-grade coal for energy generation, a comprehensive thermo-kinetic investigation was done to understand the effect of addition of a high sulfur subbituminous coal on the devolatilization behaviour of a biomass residue. At first, kinetics, mechanisms and thermodynamic parameters were evaluated by model-free and model-fitting methods. Combine kinetic model (CK-model) was then used to corroborate the results obtained from these methods. Results reveal that the addition of coal improves biomass devolatilization behaviour and best performance is observed when 50% biomass is replaced by coal. Mean activation energies (Eo) and Arrhenius constants (Ao) of biomass determined by KAS/DAE method are 197.6 kJ.mol−1 and 5.5 × 1014 s−1, respectively. Similarly, Eo and Ao values are respectively 161.7 kJ mol−1 and 3.01 × 106 s−1 for 50% coal containing blend and 169.0 kJ mol−1 and 2.0 × 107 s−1 for 75% blend. A positive synergistic effect of coal addition on biomass pyrolysis is observed from the kinetic behaviour, which is also dependent on the biomass to coal ratios. Pyrolysis of biomass with or without coal initially follow order-based model which gradually changes to diffusion model at later stage of conversion. Single-step CK-model based pyrolysis mechanisms of plain, 50 and 75% coal blended biomass are f(α) = (1 − α)1.109.α−2.9037.[−ln(1 − α)]1.1035, f(α) = (1 − α)1.0725.α−2.4681.[−ln(1-α)]0.9967, and f(α) = (1 − α)1.1256.α−2.9990.[−ln(1 − α)]1.2453, respectively. Activation energies and reaction orders predicted by CK-model are similar to the values predicted by isoconversional- and CR-methods, respectively. Results also show that the kinetic parameters obtained from CK-model satisfactorily explains the devolatilization kinetics of biomass-coal blends. Therefore, the results obtained from this comprehensive investigation are helpful in the designing of pyrolysis reactor systems for co-conversion of biomass-coal mixture to liquid and gaseous products.