Bio-oils from microwave assisted pyrolysis of kraft lignin operating at reduced residual pressure

Every year approximately 50 million tonnes of lignin are obtained worldwide as by-products of the paper industry and represent a potential renewable aromatic feedstock for a sustainable future carbon economy. In spite of this the availability of lignin remains largely unused so long the most part of it (ca. 98%) is still burned. For this reason pyrolysis-based technologies, such as fast pyrolysis and gasification, are considered promising methods for converting lignin into biochemicals, biomaterials, and biofuels. In this work the pyrolysis of kraft lignin were studied at reduced pressure under Microwave Assisted Pyrolysis (MAP). Experiments were carried out at different pressure (1 bar, 0.13 bar, 0.013 bar) also using a fractionating system. A multimode MW (Microwave) batch reactor was employed as oven using carbon as MW absorber. The most relevant achievements were gained at residual pressure of 0.013 kPa obtaining a 37 wt% of bio-oil in 9 min. Compositions of bio-oils were evaluated through 1H NMR, FT-IR ATR and a quantitative GC–MS method. Analysis showed high concentration of multisubstituted aromatic ring and light linear/cyclic compounds (C2-C5) from advanced thermal degradation of side chains of the lignin structure.