Alkaline wet oxidation of biorefinery lignin from wheat straw

Abstract: Climate change and depletion of fossil resources have generated a need for energy production based on renewable resources such as lignocellulosic biomass. Agricultural residues, like wheat straw, is lignocellulosic biomass that could contribute to the energy supply if it was used for biogas production. One issue with biogas production from lignocellulosic materials is, however, the high content of recalcitrant lignin. The inhibiting lignin fraction can, nevertheless, become more easily degraded if it undergoes pretreatment. A well-known industrial process to degrade lignin is alkaline wet oxidation, which is examined in this study with biorefinery lignin from wheat straw as raw material. The effect of temperature, dry matter (DM) content, reaction time and NaOH amount is examined, with the objective to contribute to the development of lignin pretreatment methods for subsequent biogas production. The optimal reaction conditions using 10 bar O2 were found at 180˚C, 6.5% DM content, 23 minutes reaction time and 9.8 wt% NaOH. The total yield of the identified phenolic compounds, including vanillin, vanillic acid and guaiacol, was 0.197%. The approximate yield of all organic acids was 35.5%, including formic acid, acetic acid and seven other unidentified compounds. pH stability is a critical factor in the outcome of the oxidation and is very influenced by the relation of DM content and NaOH loading. In this study, it has been shown that alkaline wet oxidation can be applied to degrade wheat straw lignin, although many improvements should be made in the experimental procedure. In future studies, alternative wet oxidation processes with more benefits for subsequent biogas production could, however, be examined.