Staged thermal fractionation for segregation of lignin and cellulose pyrolysis products: An experimental study of residence time and temperature effects

- Multiple-stage pyrolysis produces streams with reduced chemical complexity.
- Optimal stage 2 and stage 3 residence time and temperature were found.
- High selectivity to levoglucosan from cellulose in stage 3 is demonstrated.

Thermal conversion technologies may be the most efficient means of production of transportation fuels from lignocellulosic biomass. In order to increase the viability and improve the carbon emissions profile of pyrolysis biofuels, improvements must be made to the required catalytic upgrading to increase both hydrogen utilization efficiency and final liquid carbon yields. However, no current single catalytic valorization strategy can be optimized to convert the complex mixture of compounds produced upon fast pyrolysis of biomass. Staged thermal fractionation, which entails a series of sequentially increasing temperature steps to decompose biomass, has been proposed as a simple means to create vapor product streams of enhanced purity as compared to fast pyrolysis. In this work, we use analytical pyrolysis to investigate the effects of time and temperature on a thermal step designed to segregate the lignin and cellulose pyrolysis products of a biomass which has been pre-torrefied to remove hemicellulose. At process conditions of 380 °C and 180 s isothermal hold time, a stream containing less than 20% phenolics (carbon basis) was produced, and upon subsequent fast pyrolysis of the residual solid a stream of 81.5% levoglucosan (carbon basis) was produced. The thermal segregation comes at the expense of vapor product carbon yield, but the improvement in catalytic performance may offset these losses.