Pretreatment of Korean pine (Pinus koraiensis) via wet torrefaction in inert and oxidative atmospheres

This study investigates the possibility of utilizing waste heat sources such as hot flue gas or hot air for wet torrefaction (WT) processes. Although waste heat sources are cheaper alternatives than pure nitrogen used as purging and pressurizing gas for WT, they contain some fractions of non-inert gases and potentially affect the yield and fuel properties of the solid product (hydrochar). To assess these possible influences, Korean pine (Pinus koraiensis) was subjected to WT in different atmospheres, including N₂, CO₂, O₂, air and synthetic flue gas (SFG), and the produced hydrochars were characterized. The results show that WT in different gas atmospheres at 200 °C and 20 bar yields 72.24–73.82% of hydrochar. In general, the fuel properties of the hydrochars are found to be superior to those of the untreated pine: the O/C ratio decreases from 0.703 (raw pine) to 0.582–0.588 (hydrochars), and the HHV increases from 19.22 MJ/kg (raw pine) to 20.80–20.99 MJ/kg (hydrochars). Furthermore, analysis of variance (ANOVA) and pairwise t-test were performed, and the results reveal that the effects of different atmospheres in WT are not statistically meaningful (p-values >0.05), except for the influence of CO₂ on ash content. The results also indicate that the presence of oxidative gases such as O₂ and CO₂ in WT process at appropriate pressures and temperatures has almost insignificant effects on the yields and fuel properties of the hydrochar products. The findings from this study could encourage the utilization of waste heat sources for WT to reduce operating costs.