(729c) Recycling of Used Railroad Ties Via a Thermochemical Process Using a Semi-Pilot Scale Auger Reactor System
AIChE Annual Meeting
2016 AIChE Annual Meeting
Sustainable Engineering Forum
Conversion of Biomass Based Renewable Resources to Synthesis Gases and Pyrolysis Oils II
Thursday, November 17, 2016 - 4:05pm to 4:30pm
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.