Contributions of Thermal Ejection and Evaporation to the Formation of Condensable Volatiles during Cellulose Pyrolysis

Thermal ejection may play a key role in the formation of primary volatiles from biomass fast pyrolysis. As the first in the field, this study proposes a new method based on the tracers that are only present in the ejected aerosols or the evaporated vapors, to determine the true yields of the ejected aerosols and the evaporated vapors in the condensable volatiles (so-called tar) produced from cellulose pyrolysis in a wire mesh reactor at 600 °C and 1–1000 K/s. The results show that some non-volatile compounds (i.e., cellotriosan, cellotetraosan, and cellopentaosan) are only present in the ejected aerosols with similar collection efficiencies, whereas volatile compounds such as 5-hydroxymethylfurfural are only present in the evaporated vapors with high collection efficiencies (>90%). Consequently, the true yields of the ejected aerosols and the evaporated vapors can be determined using those key compounds as “tracers”. The results show that the true yield of the ejected aerosols from cellulose pyrolysis continuously increases from 29% at 1 K/s to 57% at 1000 K/s, accompanied by the continuous decrease in the true yield of the evaporated vapors from 43% at 1 K/s to 28% at 1000 K/s. The total primary tar yield of cellulose pyrolysis still increases with the heating rate (from 72% at 1 K/s to 86% at 1000 K/s), with the contribution of the ejected aerosols increasing from 40% at 1 K/s to 67% at 1000 K/s. The data clearly demonstrate the key role of thermal ejection in the formation of condensable volatiles during cellulose pyrolysis, greatly influencing the yield and composition of tar generated from cellulose pyrolysis. Overall, the results of this study provide significant insights into the formation mechanisms of primary volatiles during cellulose/biomass pyrolysis.