Quality Enhancement of a Sewage Sludge Derived Biocrude Under Hydrothermal Conditions: an Explorative Study Using Zero-valent Metals as Hydrogen Generators

Hydrothermal liquefaction (HTL) is one of the most suitable processes for the thermochemical conversion of moist biomass to produce liquid biofuel precursors. Despite its potential, this technology is still not mature enough to be extensively applied, mainly due to the inadequate quality of the obtained biocrude. Its high viscosity and the excessive presence of heteroatoms as nitrogen and oxygen strongly hinders the application of HTL biocrude as drop in fuel, leading to the necessity of a downstream upgrading process. Conventional upgrading methods often require harsh conditions and significant consume of hydrogen, which reduce the overall sustainability of the process. This study aims to explore an alternative upgrading approach based on the redox potential of zero-valent metals (ZVMs) to generate hydrogen in situ under hydrothermal conditions, without external hydrogen supplies. The upgrading experiments were conducted on biocrude obtained from the HTL of digested sewage sludge, using Fe and Zn as hydrogen generators. While hydrothermal conditions alone appeared to notably reduce nitrogen and oxygen content, by 20 % and 35 % respectively, ZVMs appeared to enhance the upgraded biocrude recovery after the process. GC-MS analysis also revealed interesting results when using ZVMs, which produced a significant decrease of fatty acids concentration, increasing their corresponding reduction products, as aldehydes, alcohol and aliphatic hydrocarbons. These results indicate a potential exploitation of ZVMs conditions to improve hydrothermal upgrading processes.