Understanding the stability of pyrolysis tars from biomass in a view point of free radicals

Fast pyrolysis of biomass has attracted increasing attention worldwide to produce bio-tars that can be upgraded into liquid fuels and chemicals. However, the bio-tars are usually poor in quality and stability and are difficult to be upgraded. To better understand the nature of the bio-tars, this work reveals radical concentration of tars derived from pyrolysis of two kinds of biomass. The tars were obtained by condensing the pyrolysis volatiles in 3 s. It shows that the tars contain large amounts of radicals, at a level of 1016 spins/g, and are able to generate more radicals at temperatures of 573 K or higher, reaching a level of 1019 spins/g at 673 K in less than 30 min. The radical generation in the tar samples is attributed to the formation of THF insoluble matters (coke), which also contain radicals. The radical concentrations of the aqueous liquids obtained in pyrolysis are also studied.