Qualitative characterization of solid residue from hydrothermal liquefaction of biomass using thermochemolysis and stepwise pyrolysis-gas chromatography-mass spectrometry

Hydrothermal liquefaction (HTL) produces solid residue (SR) as a side-product with an organic fraction of char highly dependent on the feedstock. In this work the char from batch HTL of poplar, Spirulina, and their 1 : 1 mixture was characterized for the first time using stepwise thermal desorption and pyrolysis-gas-chromatography-mass spectrometry (py-GC-MS) along with thermochemolysis. Three distinct compound fractions were identified in the form of trapped or strongly adsorbed compounds, residual lignin, and repolymerized phenolics. The trapped or adsorbed fraction resembled the compounds in the bio-crude and aqueous phase from both poplar and Spirulina. Residual lignin was only found from poplar while repolymerized phenolics were predominantly observed from poplar through ortho and para-directed polymerization. Multiple aliphatic hydrocarbons and some alkylated pyrroles were observed from Spirulina. Co-liquefaction of biomasses led to a markedly different SR from the individual biomasses with multiple alkylated pyrroles and indoles, both volatile and non-volatile, while repolymerized phenolics diminished due to imine formation. This work demonstrates that potential bio-crude is present in the SR from both poplar and Spirulina while co-liquefaction hinders repolymerization of phenolics but also produces a vast number of volatile and non-volatile pyrroles. The work shows that additional information on the reaction pathways of HTL may be found by the characterization of the SR and provides researchers investigating biomass conversion with a method to evaluate the effects on SR formation.