The interplay between chemistry and transport phenomena during the fast pyrolysis of cellulose, lignin and biomass

Abstract

Depleting fossil energy resources and the changes in climatic conditions have propelled humanity to find alternative sustainable resources to satisfy the increasing energy demand. Lignocellulosic biomass is one of the carbon-based renewable resources which can be used to produce fuels and chemicals. Fast pyrolysis is a promising technologies in which lignocellulosic biomass is decomposed at ~500 °C and in the absence of oxygen, to produce oil (~75%), char (~12%) and gas (~13%). Pyrolysis oil can be upgraded to fuel or platform chemicals via downstream processing. The research described in this thesis aims at advancing the current understanding of the three simultaneously occurring processes, viz. chemical reactions (decomposition, cracking, polymerisation), heat transfer and mass transfer (evaporation, sublimation, random ejection), and their interplay during the fast pyrolysis of cellulose, lignin and lignocellulosic biomass. For that, a dedicated screen-heater reactor was used, which was designed to minimise non-isothermality, to control the reaction time inside the reacting particle by the escape rate of compounds from the reaction zone by variation of the pressure, and to minimise reactions outside the reacting particle by minimising hot vapour residence time followed by fast quenching of products. Experiments were also carried out in a bench-scale 1 kg h-1 fluidised bed unit equipped with a fractional condensation system to investigate the effect of hot vapour residence time. From this work it can be concluded that besides the heating rate of sample, hot vapour residence time of products and the temperature during the pyrolysis, the system pressure is the key parameter, which alters the residence time of products in/on the hot reacting particle, thereby, providing a means to steer the yields and composition of the products of pyrolysis.