Valorization of biomass waste to engineered activated biochar by microwave pyrolysis: Progress, challenges, and future directions

Biomass waste represents the promising surrogate of fossil fuels for energy recovery and valorization into value-added products. Among thermochemical conversion techniques of biomass, pyrolysis appears to be most alluring owing to its low pollutant emission and diverse products formation. The current pyrolysis applications for valorization of biomass waste is reviewed, covering the key concepts, pyrolysis mode, operating parameters and products. To date, existing types of pyrolysis include conventional pyrolysis (poor heat transfer due to non- selective heating), vacuum pyrolysis (lower process temperature because of vacuum), solar pyrolysis (entirely "green" with solar-powered), and a newly touted microwave pyrolysis. In microwave pyrolysis of biomass, the heat transfer is more efficient as the heat is generated within the core of material by the interaction of microwave with biomass. The plausible mechanisms of microwave heating are dipole polarization, ionic conduction and interfacial polarization. The lack of top-tier reactor design is identified as the main obstacle that impedes the commercialization of microwave pyrolysis in biomass recycling. Based on the existing works, it is surmised that microwave pyrolysis of biomass produces solid biochar as a main product. To confront the great market demand of activated biochar, it is proposed that the solid char could be upgraded into engineered activated biochar with desirable properties for wide application in pollution control, catalysis and energy storage. Hence, the production of engineered activated biochar from microwave pyrolysis process and its applications are reviewed and explicitly discussed to fill the research gap, and the key implications for future development are highlighted.