Thermal pretreatment of a high lignin SSF digester residue to increase its softening point

Abstract Residues high in lignin and ash generated from the simultaneous saccharification and fermentation of corn stover were thermally pretreated in an inert (N 2 ) atmosphere to study the effect of time and temperature on their softening points. These residues are difficult to feed into gasifiers due to premature thermal degradation and formation of reactive liquids in the feed lines, leading to plugging. The untreated and treated residues were characterized by proximate and ultimate analysis, and then analyzed via TGA, DSC, 13 C NMR, Py-GC–MS, CHNO/S, and TMA. Interpretation of the compositional analysis indicates that the weight loss observed during pretreatment is mainly due to the thermal decomposition and volatilization of the hemicelluloses and amorphous cellulose fractions. Fixed carbon increases in the pretreated material, mostly due to a concentration effect rather than the formation of new extra poly-aromatic material. The optimal processing time and temperature to minimize the production of carbonyl groups in the pretreated samples was 300 °C at a time of 30 min. Results showed that the softening point of the material could be increased from 187 °C to 250 °C, and that under the experimental conditions studied, pretreatment temperature plays a more important role than time. The increase in softening point was mainly due to the formation of covalent bonds in the lignin structures and the removal of low molecular weight volatile intermediates.