This study, as part II of two companion papers, investigated the environmental performance of liquid hydrocarbon biofuel production via fast pyrolysis of pine through two pathways: a one-step pathway via fast pyrolysis only, and a two-step pathway that includes a torrefaction step prior to fast pyrolysis. Fast pyrolysis in all cases took place at a temperature of 530 °C whereas for the two-step pathways, torrefaction was investigated at temperatures of 290, 310, and 330 °C. Bio-oil produced was then catalytically upgraded to hydrocarbon biofuel. Different scenarios for providing the required process heat either by using fossil energy or renewable energy, as well as the effect of heat integration, were also investigated. Our life cycle analysis indicated that using the energy allocation approach, a two-step heat integrated pathway with torrefaction taking place at 330 °C had the lowest global warming potential among all scenarios of about 29.0 g CO 2 equiv/MJ biofuel. Using the system expansion approach, significantly higher reductions in GHG emissions of about 56 to 265% relative to conventional gasoline were observed for the heat integrated processes. More modest percentage reduction in emissions of about 34 to 67% was observed across all scenarios using the energy allocation approach.

Polyethylene plastics are a major source of industrial and household wastes, the majority of which end up in the environment or in landfills. These wastes pose challenges for microbial biodegradation due to their polymeric structure. There is a critical need for a process that aids in the breakdown and reuse of plastic compounds. Pyrolysis of high-density polyethylene (HDPE) has previously been used to induce chemical changes in plastic compounds, resulting in more structurally simplistic compounds. Here, we demonstrate the ability of pyrolysis to produce microbially biodegradable intermediate compounds. Biodegradation of pyrolysis-treated plastics has not previously been demonstrated. We found that enrichment cultures derived from six different environmental inocula were able to achieve extensive biodegradation of polyethylene pyrolysis products over the course of 5 days. We verified the biodegradation by quantifying residual compound concentrations of alkenes using gas chromatography/mass spectrometry (GC/MS). 16S rRNA gene amplicon sequencing results demonstrated that the most dominant taxa in the microbial community belonged to the phylum Proteobacteria. Many organisms in this phylum have previously been shown to metabolize hydrocarbons. Our results indicate that the coupling of chemical and biological processes can speed up the breakdown and conversion of polyethylene to bacterial biomass by microbial consortia.

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The rapid thermal degradation of olefin plastics is a promising chemical recycling technology to create useful products from waste plastics. In this study, pyrolysis vapors from polyethylene (HDPE and LDPE) and polypropylene were subjected to secondary degradation using a new two-stage micropyrolysis reactor (TSMR) accessory to a commercial micropyrolysis unit. Variations in reactor temperature (550–600 °C) and vapor residence time (VRT) (1.4–5.6 s) showed a strong effect on the product distribution, which was comprised of mostly alkene hydrocarbons over a broad carbon number range, with minor production of alkanes and alkadienes. On the basis of the generated micropyrolysis data, a very practical lumped kinetic model comprised of 10 reactions and 6 lumped “species” was created to describe the plastic pyrolysis and to understand how temperature and VRT turn the product distribution into different product classes of compounds (plastic, wax, heavy oil, light oil, gas, and aromatics). The kinetic parameters, such as the activation energy and frequency factor, were solved for using the method of least squares. The presented kinetic model shows good agreement with the data and with known degradation mechanisms.