A review on the stepwise processes of hydrothermal liquefaction (HTL): Recovery of nitrogen sources and upgrading of biocrude

Abstract

With the rapid industrialization over the past decades, proper disposal of organic wastes has attracted scientific interest in pursuit of sustainable development and valorization. Among them possible disposal methods, hydrothermal liquefaction (HTL) is an ideal technique to treat high-moisture nitrogen (N)-containing materials. A stepwise process based on direct HTL was recently developed that exhibit a higher potential for simultaneously producing both clean biofuels and valuable chemicals by switching the role of N from “harmful” to “helpful” species. Unfortunately, although there are relevant efforts devoted to either the direct HTL of organic waste or the reuse of N sources in processed water, few studies have investigated this coupled process for comprehensive utilization. Consequently, the present review provides a systematic comparison of direct and stepwise HTL from technical and economic perspectives, where the former process serves as a foundation for the evaluation of the latter process. Regarding direct HTL, studies have focused on the transformation of various substances and their N species in aqueous systems. More attentions has been subsequently given to exploring approaches for separating the hydrolysis of protein-N in the aqueous phase and its further condensation in the oil phase through stepwise HTL, with the purposes of recycling nitrogenous chemicals and upgrading biofuels at the same time. A techno-economic analysis and the actual examples of large-scale application are also discussed to provide an objective understanding of the profitability and feasibility of the direct and stepwise HTL techniques. Finally, innovative proposals and future directions for technical improvements throughout the processing chain of stepwise HTL are presented, thereby serving as a guideline for future studies and industrial tests on the upgrading of biofuel and recovery of nutrients from N-rich organic wastes.