Challenges and progresses in the chemical investigation of high molecular weight species in condensed pyrolysis products of coal and biomass

Abstract

The condensed products of pyrolysis embrace a wide range of compounds, ranging from relatively low molecular weight (MW) hydrocarbons ("light tar") to progressively heavier species, including large PAHs ("heavy tar"), ending up with fine carbonaceous particles (soot) of nanometric and, in some cases, even micrometric size. While most part of "light tar” from either coal or biomass pyrolysis can be effectively detected by conventional gas chromatography/mass spectrometry (GC/MS), a detailed analysis of “heavy tar” and soot turns out to be rather cumbersome and more susceptible of errors, especially when the techniques, originally developed for fossil fuels-derived products, are applied to the oxygen rich biomass products. Size exclusion chromatography (SEC) coupled with UV-Visible (UV-Vis) absorption detector is applied to both coal and biomass “heavy tar”, providing evidence of high mass components reaching MWs of thousands of u, similar to incipient soot. Carbon particles reach the micrometric scale in some coal pyrolysis products, so that "ad hoc" solvent-based separation procedures have been developed in order to distinguish soot from char fines. In biomass pyrolysis products, carbon rich particulate remains confined to the nano-size range. The latter exhibits fluorescence emission in the green wavelength range, with a relatively high quantum efficiency, and can therefore be regarded as carbon dots. Recent progress shows that the molecular mass and size of the heaviest species in biomass heavy pyrolysis products can be assessed by SEC with a fluorescence detector. However, the technique still requires refinement due to the lack of SEC calibration data valid for all tar species. Laser desorption ionization time of flight mass spectrometry (LDI TOFMS) turns out to be a valid support to evaluate the molecular range of such ill-defined products.