Accelerating wet torrefaction rate and ash removal by carbon dioxide addition

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. Copyright © 2015 Elsevier B.V. All rights reserved