Accelerating wet torrefaction rate and ash removal by carbon dioxide addition

• CO2 was found to improve the reactivity of forest residues during wet torrefaction, compared with N2.
• Wet torrefaction in CO2 produced hydrochar with better physical properties, compared with wet torrefaction in N2.
• CO2-enriched wet torrefaction removed up to 60–69% of the ash content from raw forest residues, compared to 14–26% in N2.

In this study, CO2 was employed as an environment friendly catalyst for wet torrefaction of fresh Norwegian forest residues. The effects of CO2 addition on the yield and the properties of hydrochar were experimentally studied and compared with the base case, wet torrefaction in N2. The results showed that wet torrefaction in CO2 and N2 has similar effects on the improvement in the fuel properties of biomass. However, CO2 was found to improve the reactivity of forest residues during wet torrefaction, producing the same hydrochar yield at less severe conditions compared with N2. The hydrochars produced in CO2 had lower heating value and energy yield, but better grindability and hydrophobicity, compared with those in N2. More importantly, CO2-enriched wet torrefaction can remove up to 60–69% of the ash content in the forest residues, compared to only 14–26% for wet torrefaction in N2.