Impact of salt composition and temperature on low-temperature torrefaction of pine in molten nitrate salts

Abstract

An experimental investigation of torrefaction of pine in molten, nitrate salt blends was conducted. Five blends, consisting of varying amounts of LiNO3 (20–40 wt%), NaNO3 (0–20 wt%), and KNO3 (50–70 wt%), and torrefaction temperatures from 210 to 240 °C were investigated and compared to torrefaction in nitrogen. The effect of temperature and salt blend composition on the appearance, higher heating value (HHV), proximate analysis (volatiles, fixed carbon, and ash), and mass yields are investigated. The results show that the molten salts effectively catalyze the reaction. At low temperatures, there is significant increase in the fixed carbon content and HHV compared to the raw pine. For example, at 225 °C and depending on the salt composition, the fixed carbon content increases by 51–166% and the HHV by 6–20% relative to raw. Compared to nitrogen torrefaction at equivalent torrefaction temperatures, salt-torrefied pine is more severely torrefied and is more sensitive to temperature. LiNO3 has the strongest the catalytic effect. Blends with higher LiNO3 content result in more severely torrefied biomass at a given temperature than other blends regardless of changes in sodium and potassium content. NaNO3 content is the next most influential component salt followed by KNO3.