Thermal degradation behaviors and pyrolysis kinetics of model compounds of bio-oil heavy fractions

Eugenol, vanillic aldehyde and levoglucosan were selected as the model compounds of bio-oil heavy fractions for phenols, aldehydes and saccharides, respectively; their thermal degradation behaviors and pyrolysis kinetics were investigated by thermogravimetric and Fourier transform infrared spectrometer analyses (TG-FT-IR) under non-isothermal conditions. The results indicated that the hydroxylbenzenes exhibits the greatest tendency to decompose, followed by the aldehydes and carbohydrate. One stage thermal degradation is identified for eugenol and vanillic aldehyde, with the formation of water, CO₂, CO, alkanes, alkenes, as well as small molecules of phenols and aromatic aldehydes. Levoglucosan pyrolysis proceeds slowly in two stages at medium-high temperature (180~370℃); the products are mainly composed of CO₂, aldehydes, ketones, cyclic ethers, and a small amount of water and CO. As for the mixture of the model compounds, there are three stages in the pyrolysis process; the pyrolysis products evolved share the same compositions of three model samples besides the formation of some low molecular acetal polymers. Compared with the single model compounds, the interaction between the carbonyl groups and hydroxyl groups in the mixture of model compounds may produce polycondensates at high temperature (≥ 300℃), which makes a complete pyrolysis of the mixture more difficult. The saccharide should be the key substance that dominates the pyrolysis rate of heavy fractions. By fitting the dynamic profiles of each stage, kinetic parameters of thermal degradation were determined. For the pyrolysis of levoglucosan, the apparent activation energy and reaction order are 115.80kJ/mol and 0.5 (first stage) and 141.19kJ/mol 2/3 (second stage), respectively; for eugenol, the apparent activation energy is 42.29kJ/mol, with the reaction order of 0.7; for vanillic aldehyde, the apparent activation energy is 36.53kJ/mol, with the reaction order of 0.95; for the mixture of model compounds, the apparent activation energy and reaction order are 54.46kJ/mol and 1 (first stage) and 50.67kJ/mol 2/5 (second stage), respectively.