Effect of feedstocks and free-fall pyrolysis on bio-oil and biochar attributes

This study evaluates the potential of a mixed fast and slow pyrolysis process for the conversion of four feedstocks (i.e., hybrid poplar, maple, pine, and sugarcane bagasse) into bio-oil and biochar. The novelty of this study lies within the integration of a free-fall reactor with a batch reactor to take advantage of both fast and slow pyrolysis, and adapt conversion parameters (e.g., carrier gas pressure, feedstock particle size, bulk density, reactor height, and path tortuosity) to maximize the desired product quality and yields. Thermogravimetric analysis was used to provide insight into the thermal degradation of the specified feedstock. Biomass samples were pyrolyzed at 500–550 °C, and the conversion products were evaluated. Biochar properties were characterized using proximate and ultimate analyses and surface area. Bio-oil characterization was obtained by gas chromatography-mass spectrometry (GC-MS), electrospray ionization-mass spectrometry (ESI-MS), Fourier transform infrared (FTIR) spectroscopy, and thermal stability testing. GC-MS identified dominant bio-oil compounds, such as levoglucosan, furfural, and acetic acid. ESI detected molecular weights that are higher than average, necessitating further in-situ cracking (e.g., thermal or catalytic). FTIR showed similar peak patterns and functional groups across the feedstock. The results show that maple produced the greatest biochar yield at 56%, while hybrid poplar had the greatest bio-oil yield at 38%. Among the feedstocks, bio-oil produced from hybrid poplar presents a promising oil with the lowest moisture content (<20%), high phenol content, and the most thermally stable properties, while bio-oil from pine had the next best thermal stability, but with the highest content of levoglucosan (a favorable platform molecule). Analytical results are compared to prior studies to identify the advantages and disadvantages of the free-fall fast pyrolysis reactor. Shortcomings of the current reactor configuration are identified, along with the direction for future studies.