Mild hydrothermal conditioning prior to torrefaction and slow pyrolysis of low-value biomass

•Mild hydrothermal conditioning reduces the ash content in the biomass.•Hydrothermal conditioning has a big impact on the yield of thermochemical conversions.•Heating values similar to those of high-volatile bituminous coals can be produced.

The aim of this research was to establish whether hydrothermal conditioning and subsequent thermochemical processing via batch torrefaction or slow pyrolysis may improve the fuel quality of grass residues. A comparison in terms of fuel quality was made of the direct thermochemical processing of the feedstock versus hydrothermal conditioning as a pretreatment prior to thermochemical processing. Hydrothermal conditioning reduced ash content, and particularly nitrogen, potassium and chlorine contents in the biomass. The removal of volatile organic matter associated with thermochemical processes can increase the HHV to levels of volatile bituminous coal. However, slow pyrolysis only increased the HHV of biomass provided a low ash content (<6%) feedstock was used. In conclusion, hydrothermal conditioning can have a highly positive influence on the efficiency of thermochemical processes for upgrading low-value (high-ash) biomass to a higher quality fuel.

Graphical abstractOverview of the experimental set-up: thermochemical conversion of mild hydrothermally treated biomass and not treated biomass.Figure optionsDownload full-size imageDownload as PowerPoint slide