Microwave applied to the thermochemical conversion of biomass: A review

Renewable fuels and valuable chemicals derived from biomass offer a promising solution to address the global energy shortage and carbon emission challenges. However, the commercial application of traditional biomass thermochemical conversion technologies remains hindered, due to low heat transfer and processing efficiencies. In recent years, microwave heating, as a novel method, has garnered significant attention for its advantages in dielectric heating, superior heat transfer efficiency, and potential for carbon emission reduction. Nevertheless, the precise role of microwaves in various applications remains unclear. Therefore, this research provides an overview of microwave applications in different types of biomass thermochemical conversion processes, examines the role of microwave, and suggests research directions to enhance the benefits. This research summarized common knowledge points across areas including the principles of microwave heating, the microwaves-biomass interactions, microwave absorbents/catalysts, and microwave effects. The material transformation of biomass during thermochemical conversion and their interactions with microwave was analyzed. Based on whether microwave primarily provide rapid thermal energy or enhance activity during the process, the technologies were categorized into microwave-driven technologies (drying, pyrolysis, torrefaction, hydrothermal technology) and microwave-assisted technologies (gasification, tar reforming, combustion). Additionally, the technical characteristics and advancements in each technology was conducted. Furthermore, an in-depth analysis of the challenges and opportunities from the viewpoints of theoretical investigation, equipment research, and industrial application was concluded. This review aims to provide in-depth understanding and inspiration for research and development in the thermochemical conversion biomass with microwave.