Disposition of Solids from Hydrothermal Liquefaction of Biomass: Current Understanding, Research Gaps, and Opportunities

The characteristics of solid residuals generated during the hydrothermal liquefaction (HTL) of biomass are reported to establish disposition options for these residuals to reduce the liability or barriers to the commercialization of HTL. HTL solids from the pilot scale are reported and compared to the existing HTL solid literature for a variety of biomass feedstocks and HTL configurations, noting differences among the literature HTL solids due to solid collection methods. Reported analyses include CHNOS, moisture, ash, inorganics by inductively coupled plasma (ICP), and a high heating value (HHV). HTL solids are determined to be substantially compatible with the U.S. Environmental Protection Agency (EPA) disposal regulations within the existing data. Disposition and valorization routes are identified, yet combustion as valorization is determined to be not economically beneficial. Research gaps are identified, which include the need for future research to be performed on HTL solids that are representative of a continuous, commercializable flowsheet; verification that specific HTL solids meet EPA disposal requirements; exploration of valorization options; and the need for the invention of continuous HTL solid separation/management.