Hydrothermal liquefaction: A promising pathway towards renewable jet fuel

Abstract

Conversion of wet biomass and waste products via hydrothermal liquefaction (HTL) has been evolving as an alternative thermochemical technology for the production of liquid biofuels. Processing of biomass slurries with approximately 20 % solids content under high temperature and pressure mimics the natural formation of fossil crude on earth. With reaction times of around 10 to 30 minutes, temperatures of 350 °C and pressures of around 200 bar, HTL converts any biomass feedstock to a liquid bio-crude. This raw product roughly resembles petroleum, but exhibits higher oxygen contents (~10 %) and has a higher viscosity. Therefore, development of the hydrothermal liquefaction technology has concentrated on the upgrading of bio-crude via hydrotreatment to reduce its heteroatom content, viscosity, boiling point and density. Upgraded bio-crude can then be further refined via distillation or other established processes into renewable gasoline, diesel and jet fuel. The upgraded fuel's chemical composition, with a high concentration of aliphatic hydrocarbons showing carbon numbers in the range of C8 to C18, appears promising for application as renewable jet fuel. The specific composition of the refined fuel products (as well as of the bio-crude) is, however, affected to a significant extent by the type of feedstock applied. For example, using lignocellulosic feedstock results in increased concentrations of aromatic hydrocarbons in the final product. The versatility of the HTL technology in terms of feedstocks and products represents a major advantage over other thermochemical conversion processes. Future developments should address tailoring the process to meet specific fuel requirements, e.g. those of renewable aviation fuels. Recent HTL reactor developments have led to proven continuous operation on a variety of feedstocks, but current reactor capacities of about ~1 bbl/d of bio-crude are still limited. Initial environmental and economic assessments of the hydrothermal liquefaction technology are promising, but in-depth studies covering a representative range of feedstock have not yet been published, rendering estimations of minimum fuel selling prices and greenhouse gas (GHG) balances of HTL derived liquid fuels difficult. To advance the technological maturity of hydrothermal liquefaction towards industrial implementation, development efforts should focus on process integration along the entire production chain encompassing pre-treatment, HTL processing, hydrotreatment, distillation and utilization of process water.