Torrefaction and hydrothermal carbonization of waste from the paper industry: Effects of atmosphere choice and pretreatment with natural acidic reagent on fuel properties

[Display omitted] •Paper mill and biological sludge have biofuel potential after thermal treatment.•Determination of torrefaction operation time in N2 atmosphere using gas analyses.•Acidification of raw samples or acidic media in HTC increases C content and HHV.•Higher torrefaction temperatures and longer operation time in HTC increase HHV.•Acid pretreatment impacts the combustion characteristics of chars. The hydrothermal carbonization (HTC) and torrefaction of paper sludges and biological sludges from the paper industry were investigated and the effect of pretreatment of the samples with alcoholic vinegar was evaluated. The HTC was performed at two different reaction times, while the torrefaction took place at two different temperatures and two atmospheres. The gas analysis showed that the optimal torrefaction time for the N2 atmosphere can be determined by observing the CO2 content in the gas phase, which is not possible for the CO2 atmosphere. The TGA analysis showed that the choice of atmosphere influences further combustion, with samples torrefied at lower temperatures being more sensitive to it. The carbon (C) content in the thermally treated samples increases from 39.85 to 48.06 wt% for biological sludge and from 40.02 to 60.21 wt% for paper sludge. The H/C and O/C ratios decrease and approach the values of conventional fuels. The fuel ratio is higher for the torrefied samples. The higher heating value (HHV) increased from 17.4 to 21.4 MJ/kg for biological sludge and from 16.2 to 22.4 MJ/kg for paper sludge. Higher temperature and longer treatment time led to products with a higher HHV and a higher energy density. The highest energy yields were achieved during torrefaction at 250 °C, regardless of the atmosphere, with temperature and type of material having the greatest influence. Acidification of the samples or acidic media in the HTC affects the properties of the products by increasing the C content and HHV and affecting combustion.