Combined organic acid leaching and torrefaction as pine wood pretreatment before fast pyrolysis

Abstract

Reducing pine wood particle size is beneficial for acid leaching (shorter leaching time, smaller equipment size) and also for pyrolysis (low pyrolysis time, high oil and sugar yields). Torrefaction helps to improve the energy efficiency of grinding. However, it is not well established whether alkali and alkaline earth metals can still be removed effectively, after torrefaction, by leaching with an acetic acid solution, while keeping high oil and sugar yields. To investigate this, an experimental study was carried out combining torrefaction (290?C, 20 minutes) and subsequently acid leaching of pine wood as pretreatment step before fast pyrolysis of the feedstock at 530?C. The oil, char, gas, water, pyrolytic lignin, light oxygenates and levoglucosan yields were compared with the results obtained from fast pyrolysis of untreated pine wood and acid leached pine wood at 530?C. In addition, pyrolysis vapors were condensed step-wise into two distinguished fractions (condensation T = 80?C) to increase the levoglucosan concentration up to 45 wt.% with or without applying torrefaction as pretreatment. Intra-particle reactions during torrefaction are more profound in the presence of alkali and alkaline earth metals. Cellulose and lignin derived products (e.g. levolucosan (~0.2 kg/kg pine wood) and pyrolytic lignin (~0.11 kg/kg pine wood) were not affected by the torrefaction pre-treatment of acid leached pine wood. In general, torrefaction of acid leached pine wood followed by fast pyrolysis showed similar total organics, char, gas, light oxygenates and produced water yields compared to fast pyrolysis of acid leached pine wood. Contrary, when acid leaching is applied after torrefaction the organics (dry bio-oil) and light oxygenates yields are quite comparable to the results of untreated pine wood fast pyrolysis.