Comprehensive evaluation of municipal solid wastes and mixed feedstocks for commercial hydrothermal liquefaction in bio-refineries

Hydrothermal liquefaction (HTL) is a promising thermochemical conversion technology that converts waste biomass into valuable chemicals and energy. The process induces physical and chemical transformations of organic molecules under sub-critical or supercritical water. During HTL, the biomass molecules are hydrolyzed and degraded into low molecular weight, reactive species that may undergo recombination reactions to form condensed fragments. A substantial amount of oxygen from the biomass is removed via dehydration and decarboxylation during HTL. A number of existing studies in the literature have focused on hydrothermal conversion of feedstocks like wood, algae, plastics and the effect of different operating conditions on their hydrothermal conversion. However, literature on HTL of a heterogeneous feedstock like municipal solid wastes (MSW) is very limited. An understanding of the transformation of the different fractions present in MSW can be obtained from HTL of individual components. Available literature on HTL of bio-degradable wastes, non-bio-degradable polymers and MSW is reviewed in this paper. Consequently, catalytic upgradation to improve the yield and quality of the bio-crude, and continuous hydrothermal processing are also discussed. More importantly, a detailed discussion on the challenges involved in scaling up of the HTL process with MSW as the feedstock is presented using the available protocols for fuel readiness and data from commercial HTL plants. Due focus is given on the possibilities of deriving value-added products from the HTL of MSW on a large scale, while simultaneously tackling the waste management problem associated with MSW.