In search of more sustainable options for plastic waste disposal, thermochemical conversion to produce alternative fuels or chemicals appears as an outstanding alternative due to its environmental benefits such as having a lower carbon footprint than fossil fuel extraction.1
One method of thermochemical conversion is fast pyrolysis, the thermal degradation of plastic waste in the absence of oxygen. Fast pyrolysis at high temperatures, with reaction times shorter than 10 s, has been used to produce hydrocarbon products from plastic waste that can be converted into fuel or chemical intermediates. This study analyzes the degradation products of high density polyethylene (HDPE), the most abundant plastic waste, with respect to temperature and vapor residence time. Through the use of a novel, high-temperature reactor extension to an existing micro-pyrolysis unit (CDS Analytical 5200 HP Pyroprobe), the residence time at each temperature was varied by adjusting the helium carrier gas flowrate. Residence times were varied from ~0.0 to 5.6 s (25mL/min He to 100mL/min He). The temperature of the micropyrolysis unit and the reactor extension was established at three different temperatures (650°C, 675°C, and 700°C) for each set of residence times. The pyrolysis products were analyzed in a ThermoFisher K8880181 Trace Gas Chromatograph coupled to a DSQ II mass spectrometer. The observed chemical species were alkanes, alkenes, alkadienes and aromatics and the carbon content of the pyrolysis products ranged from 2 to 32 atoms per molecule. At constant reactor temperature of 700°C, the slight variation in residence time from ~0.0 to 1.3 s greatly shifted the product distribution towards lower molecular weight compounds (LMWC, C15 and below) as opposed to higher molecular weight compounds (HMWC, C16 and above). Without extended reaction, 25% of the products were LMWC, while with extended reaction, 96% were LMWC. When the pyrolysis temperature was lowered to 675 °C the amount of LMWC decreased 6% to 90%. These results illustrated the trend that as temperature and residence time increased, there was a greater abundance of lower molecular weight compounds in the product distribution, mostly alkenes with some alkanes but almost lacking dienes. Changes in chemical species distribution were also observed. An increase in the composition of aromatic compounds was exhibited at longer residence times and higher temperatures whilst the opposite trend occurred for high molecular weight alkenes and alkadienes. These results show a promising pathway for HDPE waste as a potential source for hydrocarbon fuel fractions and chemical intermediates.
1. Gracida-Alvarez, U.R.; Keenan, L.M.; Sacramento-Rivero, J.C. & Shonnard, D.R. Resource and Greenhouse Gas Assessments of the Thermochemical Conversion of Municipal Solid Waste in Mexico. ACS Sustainable Chemistry and Engineering 4, 5972-5978 (2016).