Elucidation of Biomass Pyrolysis Products Using a Laminar Entrained Flow Reactor and Char Particle Imaging

We have mapped the formation of tars during white oak thermochemical conversion using a bench scale laminar entrained flow reactor (LEFR). White oak particles (80 mesh, <180 μm) were pyrolyzed under conditions not limited by heat transfer. Measurements were made with residence times of 0.2, 0.4, 0.6, and 0.8 s, between 500 and 900 at 100 °C increments, and with residence times of 1 s at temperature from 450 to 950 °C at 25 °C increments. Products were monitored with a molecular beam mass spectrometer (MBMS), and the mass spectra were analyzed using model-free multivariate analysis (multivariate curve resolution). Six groups of correlated masses were identified that suggest the mechanisms of pyrolysis and gasification. The first group of masses (lowest temperature) is associated with primary species from lignin and hemicellulose, followed by cellulose products. The next two groups (increasing temperature) are composed of secondary products resulting from the cracking of carbohydrate vapors and the cracking of lignin in the gas or solid phase. Molecular weight growth products are seen in the next two groups including substituted aromatic compounds in the fifth group and polycyclic aromatic hydrocarbons (PAHs) in the sixth group. The results of this study show that as the temperature of pyrolysis is increased, the molecular weight of the tars decreases up to 750 °C, because the pyrolysis vapors are cracked. As the temperature increases beyond 750 °C, molecular weight growth is seen with increasing temperature. The analysis also shows that as the temperature increases from 450 to 950 °C, oxygen is lost from the tars and converted into CO and CO₂. The char samples were collected and analyzed with light and electron microscopy. This analysis revealed that micropores develop in the cell wall around 550 °C and increase in size and coalesce into a cenosphere morphology with increasing temperature. Above 850 °C, these cenospheres appear to rupture, releasing their contents into the gas phase. This rupture event correlates with increased MBMS signals from PAH-associated masses.