Hydrothermal liquefaction of biomass for the generation of value-added products

The growing concerns on climate change, energy demand and depleting fossil fuel reserves have diverted the focus of the 21st century toward renewables. A wide range of technologies such as transesterification, gasification, pyrolysis, aqueous phase catalytic reforming and hydrothermal liquefaction (HTL) are used for producing biofuels and chemicals from a wide range of feedstocks. Due to its immense flexibility in handling a wide variety of organic substrates, ranging from dry to wet residual biomass, algae, sewage sludge, plastics and municipal solid wastes, HTL is regarded as a “feedstock agnostic technique.” The bio-crude obtained from HTL is a dark viscous liquid, which is immiscible with water and has a better calorific value with lower oxygen content than the bio-oil obtained from pyrolysis. Various organic compounds such as aromatics, aldehydes, ketones, alcohols, carboxylic acids, esters, and straight and cyclic hydrocarbons make up a major portion of the bio-crude, with its composition strongly dependent on the type of feedstock. The products from HTL can be considered as a renewable source of biofuels, bio-based specialty chemicals, and bio-products in a circular bioeconomy. This critical review will assess the possibilities of deriving chemicals, fuel molecules and bio-products from a range of feedstocks via HTL technology with special emphasis given to (a) characteristics and applications of different products from HTL, (b) possibilities of tailoring the selectivity to specific chemicals by using catalysts, (c) challenges and opportunities in integrating the HTL products in the existing refinery infrastructure and (d) industrial potential and economics of the process.