Recycling of used railroad ties via two-staged pyrolysis for fractionation of wood preservatives and bio-oil: pyrolytic characterization by TGA and Py-GC/MS
Pyoungchung Kim, J Lloyd, Jae-Woo Kim, N Labbe
Creosote and copper naphthenate (CuNap) (in an oil carrier) treated railroad tie materials (crossties or sleepers) were initially heat-treated at 200 – 300 oC and subsequently pyrolyzed via thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) to recover wood preservatives and produce a higher quality bio-oil. Preservative-desorptive temperatures at 200 and 250 oC by TGA removed water and creosote (6.6 – 9.3 wt%) from the creosote-treated crosstie materials, and water and CuNap components (21.2 – 23.3 wt%) from the CuNap-treated crossties. Temperature at 300 oC removed a shoulder DTG peak at 305 - 325 oC and weight loss accounted for 25.5 wt% in the creosote-treated tie and 30.6 wt% in the CuNap-treated tie. Temperature at 200 – 300 oC by Py-GC/MS desorbed creosote-derived chemicals such as naphthalene, acenaphthene, fluorine, anthracene, fluoranthene, pyrene and other polycyclic aromatic hydrocarbon (PAHs) derived from the creosote-treated tie; and mineral oil (alkane hydrocarbons such as tetradecane, nonadecane, hexadecane and octadecane, and heptacosane) from the CuNap-treated ties. Pyrolysis of the wood tie with elevated temperature produced a high amount of carbohydrate- and lignin-derived compounds from wood ties. Fast pyrolysis of the 200 oC-treated crossties by Py-GC/MS produced a high fraction of creosote- and CuNap compounds most of which were then not subsequently recovered through fast pyrolysis of the 250 and 300 oC-treated samples. Fast pyrolysis of the thermally preservatives-removed tie samples produced high anhydrosugars such as levoglucosan and low acetic acid, furfural and ketones as well as high pyrolytic lignin-derived compounds, which shows good potential for phenolic-based chemical production. The results demonstrate that a thermal preservative-removal step (similar to a torrefaction step) can successfully remove valuable creosote and CuNap components for re-use as preservatives and subsequently supply a clean wood without significant levels of contaminant hazardous air pollutants for use as boiler fuel, more efficient pyrolysis to produce higher quality bio-oil, gasification or other uses.