Petrological analysis of fast pyrolysis bio-oil distillation residues

Authors: HACKLEY, PAUL - Us Geological Survey (USGS); Elkasabi, Yaseen; JUBB, AARON - Us Geological Survey (USGS)

Submitted to: International Journal of Coal Geology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2026
Publication Date: 3/16/2026
Citation: Hackley, P., Elkasabi, Y.M., Jubb, A.M. 2026. Petrological analysis of fast pyrolysis bio-oil distillation residues. International Journal of Coal Geology. https://doi.org/10.1016/j.coal.2026.104979.
DOI: https://doi.org/10.1016/j.coal.2026.104979

Interpretive Summary: Non-edible agricultural crops and edible crop residues can be converted into crude oils ('bio-oil') in a process called pyrolysis, which is the heating in the absence of oxygen. These oils, while they greatly differ from petroleum chemically can be refined into products including fuel and chemicals. As with petroleum, distillation is used to fractionate bio-oils and the heaviest of these are solid carbon materials. The goal of this work was to compare these biobased carbon solids to fossil solids like coal and heavy petroleum using analyses typically used for characterizing fossil resources with the goal of better understanding how to make the best use of biobased carbons. This information will be useful to petroleum refiners looking to make use of biobased resources, biorefiners looking for new uses and will benefit farmers and producers by finding new uses and new markets for products derived from agricultural residues.

Technical Abstract: Distillation of fast pyrolysis bio-oils leaves solid residues analogous to those from distillation of fossil fuels or from natural subsurface petroleum formation (i.e., solid bitumen) or other natural carbon concentration processes (e.g., coal formation). Much like petroleum-based residues, bio-oil residues vary in terms of quality, yield, and versatility for downstream applications. The goal of this work was to elucidate parallels and contrasts between bio-oil distillation residues and fossil-derived solid fuels using petrological analysis techniques (reflectance and fluorescence microscopy, Raman and infrared spectroscopy) supported by geochemical analysis (elemental and programmed temperature pyrolysis analysis). Continuous dual-bed fast pyrolysis was used to convert hardwood and switchgrass biomass into oils with varying oxygen content. Compositions and petrographic presentation of the biomass distillation residue solids were contrasted to natural solid hydrocarbon products and lignite-subbituminous coal. Microscopic evaluation shows distillation residues are similar to low to moderate thermal maturity solid hydrocarbons in reflectance, fluorescence, isotropism, and the absence of biological structure, confirming that traditional petrographic approaches can be used to assess progress of bio-oil distillation via recovery of volatiles. Distillation residues contain significantly more structural oxygen than solid hydrocarbons and in that way are more similar to coal derived from fossil higher plant materials. Distillation residues contain significant content of weakly bound oxygen-containing terpenoid compounds relative to the comparison solid hydrocarbons, indicating their volatility and environmental reactivity, and potentially suggesting unrealized valorisation for bio-oil distillation residues as byproduct energy fuels to be recycled into the industrial process.