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

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 H₂ exhibited a 20 to 30-fold increase at >550 °C. The CO/H₂ ratio of pyrolysis of the waste in CO₂ is ∼1100% of that of pyrolysis of the waste in N₂ at 720 °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 (i.e., 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.