Critical review on production, characterization and applications of microalgal hydrochar: Insights on circular bioeconomy through hydrothermal carbonization

Exploitation of microalgal biomass as a valuable resource is hindered by the challenges associated with high downstream processing costs, including biomass harvesting, drying, and product extraction. Direct utilization of microalgae as a solid fuel source, soil conditioner, capacitor or adsorbent material raises environmental concerns. Hydrothermal carbonization (HTC) is a highly efficient and promising technology for microalgal biomass conversion. This comprehensive review provides an in-depth understanding of the HTC reaction mechanisms involved in microalgal hydrochar production, shedding light on the underlying processes and factors affecting the quality of hydrochar. HTC has the potential to improve fixed carbon content, thermal stability and nutrient availability in the resulting hydrochar. Furthermore, this review explores the integration of HTC with anaerobic digestion (AD) to establish a circular bioeconomy, thereby promoting sustainability in energy generation. The synergistic combination offers a promising approach for the efficient utilization of microalgal biomass, where hydrochar can serve as a renewable energy source while the aqueous fraction can be utilized as a nutrient-rich feedstock for biogas production. By highlighting the potential benefits and futuristic directives associated with microalgal biomass valorisation through HTC, this review aims to contribute to the development of sustainable waste management strategies for recovery of value-added compounds from microalgae. Ultimately, this review strives to foster the transition towards a more environmentally friendly and resource-efficient bioeconomy.