Pyrolysis of wastes generated through saccharification of oak tree by using CO2 as reaction medium

Abstract

In this study, the production of bioethanol was evaluated through a series of saccharification and fermentation of lignocellulosic biomass (e.g., oak tree) pre-treated with H2SO4, NH3, or NaOH using a yeast (Pichia stipitis). In addition, it was investigated the effects of CO2 on pyrolysis of the biomass wastes remaining after saccharification of the three pre-treated oak tree (BWs: BW-H2SO4, BW-NH3, and BW-NaOH). Thus, this work emphasizes the mechanistic understanding of CO2 in pyrolysis of BWs. The effect of CO2 was most noticeable in syngas, as the ratio of CO and H-2 exhibited a 20 to 30-fold increase at >550 degrees C. The CO/H-2 ratio of pyrolysis of the waste in CO2 is similar to 1100% of that of pyrolysis of the waste in N-2 at 720 degrees C. Such proliferation of syngas led to the subsequent reduction of tar since the substantial amount of tar was consumed as a precursor of syngas: CO2 not only expedited the thermal cracking of volatile organic compounds (VOCs), but also reacted with those VOCs. The morphologic modification of biochars also occurred in the presence of CO2 via heterogeneous reaction between CO2 and surface of BWs. In summary, this study shows a utilization of an oak tree waste generated from saccharification for bioethanol production as a pyrolysis feedstock to recover energy (Le., syngas production). The use of CO2 as pyrolysis medium not only enhanced syngas production from oak tree waste but also reduced tar formation by thermal decomposition of VOCs and reaction between VOCs and CO2. The process shown in this study can be used as a potential high energy recovery from a biomass waste by utilizing potent greenhouse gas such as CO2.