Low nitrogen and high value hydrochar preparation through co-hydrothermal carbonization of sludge and saw dust with acid/alcohol assistance

High hydrothermal temperature and solid-N enrichment were the key issues during co-hydrothermal carbonization (co-HTC) of sludge and lignocellulose biomass. The technology of acid-alcohol assisted co-HTC was proposed and the relevant reaction mechanisms were investigated in this study. The results showed that acetic acid and ethanol promoted the Maillard and Mannich reaction respectively at 453 K through catalytic degradation of different organics. Above 483 K, acetic acid contributed to deamination and reduced solid-N by 9.18%, but there was no decrease in yield because acetic acid participated in hydrochar formation by repolymerization. Ethanol preferred to increase production by 5.82% without extra solid-N enrichment, promoting NH4+-N transformation into quaternary-N with more organic groups substitution. Solvent mixture had best performance on dehydration, carbonization and denitrification reaction to upgrade hydrochar fuel quality. The hydrochar obtained was more approach to lignite with high fixed carbon and heating value as well as low nitrogen content. Thus the fuel property and substance composition could be selected by adjusting the combination of hydrothermal temperature and organic solvent.