Waste biomass valorization for the production of biofuels and value-added products: A comprehensive review of thermochemical, biological and integrated processes

Waste biomass can be converted to green fuels and value-added products via thermochemical and biological conversion processes. The thermochemical processes endure limitations such as high processing costs due to high-temperature requirements. In contrast, the challenges of biological processes include low product yield and long processing time. Integrating different technologies, especially thermochemical and biological conversion processes, helps to enhance resource utilization and promote a circular economy. The combination of different technologies would help alleviate their limitations. In this respect, the integration of biological processes (e.g., syngas fermentation and anaerobic digestion) and thermochemical processes (e.g., pyrolysis, gasification, hydrothermal carbonization etc.) was the focus of this review. Integrated conversion processes often reduce the environmental impact compared to a standalone process. Hybrid pyrolysis-anaerobic digestion processes are promising from economics and ecological perspectives. However, more studies are required to understand how to effectively recycle and utilize the residue from pyrolysis and other thermochemical processes. Hydrothermal liquefaction (HTL) and pyrolysis are promising bio-oil production. However, HTL-derived oils are characterized by higher heating values and lower oxygen contents. The maturity level of most biological processes for waste biomass valorization is in the range of technology readiness level (TRL) 4–5. In contrast, thermochemical processes are expected to reach a TRL of 9 in the next two decades through detailed research and development. Moreover, the TRL of integrated processes described in the present study should also be assessed to evaluate their maturity and commercialization potential. The study will help researchers and policymakers to identify the knowledge gaps in integrating thermochemical and biological conversion processes. The present review is the first of its endeavor to present the advances, progress, and prospects of combining thermochemical and biological conversion processes for biofuels and value-added chemicals production. Most of the studies available in the literature focus on the advances in thermochemical or biological processes as a standalone conversion pathway despite many possible integration scenarios available. Therein lies the motivation for the present study. The current review is helpful for new researchers and policymakers to acquire the fundamental knowledge and possible pathways to unlock full biomass conversion pathways. Therefore, the authors made a significant effort to explain each conversion pathway in the first few sections before outlining their advantages and limitation. The final section was entirely on the different possible integration scenarios followed by process integration challenges. This study aims to open up the possibility of future research and help researchers identify the knowledge gaps in integrating thermochemical and biological conversion processes.