Conventional and microwave-assisted pyrolysis of gumwood: A comparison study using thermodynamic evaluation and hydrogen production

The application of microwave energy in biomass pyrolysis has gained increasing interest due to its fast, volumetric, selective and efficient heating. However, the dominance may be levelled off if the energy and exergy efficiencies of overall system are taken into consideration. Hence, based on the lab-scale experimental data (at 600, 700 and 800 °C), energy and exergetic assessment of pyrolysis-derived gas, char and oil from gumwood under conventional and microwave heating were investigated in this work. The results showed that at each temperature, the corresponding energy and exergy rates of gas under microwave heating were found to be 23% and 26%, respectively, higher than those of conventional one. Meanwhile, the values for char were around 46%. This was mostly because microwave-derived pyrolysis product contained more gaseous and char products compared to the conventional process. In contrast, opposite trend was noticed for bio-oil due to the reduction of oil yield in pyrolysis product. It was demonstrated that the total energy and exergy value of individual pyrolysis products were significantly influenced by the energy and exergy content of individual component. For example, the increment rate of energy and exergy values of H₂ and CH₄ were sufficiently higher than the decrement rate of energy and exergy values of CO and CO₂ with the increase of temperature; the total values were consequently increased at the elevated temperature. For the case of char and oil, the energy and exergy rate were decreased with temperature due to the reduction of individual product yield. Pyrolysis system efficiency (PSE), the overall performance indicator used in this study, of the microwave-assisted process was 13.5% higher than that of the conventional process. Moreover, the experimental results were further analysed with the conduct of a hydrogen plant simulation using Aspen Plus™ whose results confirmed that an improved performance of microwave heated system was achieved by producing 120 gH₂/kg gumwood, 15% higher compared to that of conventional system.