Fast pyrolysis of lignins with different molecular weight: Experiments and modelling

Lignins with number average molecular weights between 350 Da and 1900 Da were characterised and subsequently pyrolysed in a screen-heater at pressures of 500 Pa and 10⁵ Pa between 425 °C and 793 °C. Upwards of 530 °C, the temperature turned out to have only a minor influence on the yields and composition of the oils produced. Clear trends were observed as a function of the molecular weight and pressure – (1) at increasing molecular weight of the lignin, the oil yield decreases while yields of char and gas increase, (2) the molecular weight of the oil is lower for oils produced at 10⁵ Pa as compared to the ones obtained at 500 Pa, (3) above a certain molecular weight of the lignins, ∼400 Da for 10⁵ Pa and ∼800 Da for 500 Pa, the molecular weight of the oil becomes independent of the molecular weight of the lignin. A mathematical model has been developed, which includes three concurrently occurring processes, viz. cracking and polymerisation reactions and removal, hence mass transport, of unconverted lignin and reaction products from the reaction zone. This model can describe all the trends observed experimentally and provides, after parametrisation, reasonable qualitative predictions of the yield and molecular weight of the oils. Knowledge of the role of the interplay between mass transport and chemistry in the pyrolysis process is further accumulating, and from this the development of lignin valorisation can avail. For instance, it has become clear that in the pyrolysis process the molecular weight of lignin oil, which is an important characteristic for the upgrading of the oil to chemicals and/or fuels, can be steered with the pressure.