(729c) Recycling of Used Railroad Ties Via a Thermochemical Process Using a Semi-Pilot Scale Auger Reactor System

A thermochemical process, an initial thermal treatment followed by an intermediate pyrolysis step, was investigated to recover wood preservatives and produce bio-oil and biochar containing low amount of polycyclic aromatic hydrocarbons (PAHs) from used creosote-treated railroad ties. The material was first treated using a semi-pilot scale auger pyrolysis reactor at 250, 280, and 300 °C for 15min. The yield of the liquid fraction was 28, 35, and 38 wt% while the solid fraction yield was 65, 57 and 54 wt%, respectively. The liquid fraction had two phases: aqueous phase in the upper and organic phase (enriched in creosote) in the bottom. As thermal desorption temperature increased, the aqueous phase exhibited a decrease in water content from 95 to 88% and pH from 3.2 to 2.6 and an increase in total acid number (TAN) from 20 to 63 mgKOH/g. The yield of the creosote phase increased from 2.0 to 4.5 wt% of total creosote content of 7 wt% found in the wood tie. As thermal desorption temperature increased, in addition to creosote, the organic phase also contained larger amount of carbohydrates- and lignin-derived compounds, decreasing the purity of the recovered creosote. The thermally treated samples were subsequently pyrolyzed using the same semi-pilot scale auger pyrolysis system at 500 oC for 72s residence time. The yield of bio-oil produced from the untreated, 250, 280, and 300 °C-treated materials was 61, 42, 35, and 30 % while the yield of biochar was 21, 25, 32, and 33 %, respectively. The characterization of the bio-oil produced from the thermally treated ties showed that as the desorption temperature increased, water content decreased from 36 to 32 %,TAN value decreased from 127 to 113 mg KOH/g, and density increased from1.1 to 1.2 g/mL. PAHs traces found in biochar from the thermally treated wood tie were dramatically lower (less than 0.2 wt%) than that (0.8 wt%) in biochar from the untreated ties. This result demonstrates that a thermal preservative-removal step can recover valuable creosote for re-use as preservatives and subsequently supply a feedstock without significant levels of contaminant hazardous pollutants for pyrolysis to produce bio-oil and biochar.