Pretreating biomass via demineralisation and torrefaction to improve the quality of crude pyrolysis oil

Pretreating biomass prior to pyrolysis was investigated. Three undesirable catalysts naturally present in biomass were identified: inorganics, acids, and water. A pretreatment sequence incorporating acid leaching and torrefaction was developed to reduce/remove these catalysts. Acid leaching targeted reducing the biomass's inorganic content. The acidic liquor produced during torrefaction was rich in acetic and formic acid; this solution was recycled as the acid leaching reagent. The optimal leaching conditions were at 30 °C with 1% acetic acid for 4 h, which decreased the inorganic content from 0.41 wt% to 0.16 wt% for leached biomass. Torrefaction targeted reducing the biomass's moisture and acetyl content and was optimal at 270 °C for 20 min. Bio-oil from pyrolysis of demineralisation and torrefied biomass was depleted in organic acids, pyrolytic lignin, and water but was rich in levoglucosan and aromatics. Decreasing the biomass's acetyl and inorganic content reduced organic acid formation. The water content in the bio-oil was lower because less water entered the system, and water plays an auto-catalytic role during pyrolysis, promoting the production of pyrolytic water. The high levoglucosan yield confirmed that secondary reactions were limited to a much higher degree when both pretreatments were implemented compared to demineralisation or torrefaction alone.