Valorisation of woody biomass by combining enzymatic saccharification and pyrolysis
The emerging industrialization of cellulosic bioethanol will create large amounts of lignin-rich residues, so called hydrolysis-lignin, which may be used as a raw material for further fuel production. In this study, fermentable sugars were removed from hardwood and softwood substrates by combining steam explosion and enzymatic saccharification with a cellulose hydrolysis yield of 90.1% and 41.5%, respectively. The remaining lignin-residues were further processed using intermediate pyrolysis to produce bio-oils, gas and char. The pyrolysis of hydrolysis-residues was compared with the pyrolysis of the original hardwood and softwood substrates. Overall, the lignin-enriched residues yielded up to 30 wt% of bio-oils. By applying low cost ZSM-5 vapour cracking catalysts, produced by template-free methods, the content of monophenolics in this fraction could be increased to more than 80 wt%. Thus, this study shows that by using enzyme technology, lignin and carbohydrates can be separated and a liquid sugar stream generated. This wood-based sugar stream can then be converted by fermentation or chemical methods to fuels and chemicals. The remaining lignin-rich residue is a valuable source for bio-based aromatics. This process of separate conversion of sugars and lignin further results in an improved overall product yield compared to the direct pyrolysis of the original wood biomass. Additionally, the pyrolysis of lignin-residues produces a simpler oil of higher quality than the pyrolysis of wood. The process of thermochemically converting lignin-rich hydrolysis residues is a simple and viable option for existing bioethanol plants to improve their process economics and diversify the product portfolio.