Fast and ex situ catalytic copyrolysis of switchgrass and waste polyethylene

Authors: Mullen, Charles; Strahan, Gary; Elkasabi, Yaseen; Ellison, Candice

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2024
Publication Date: 10/8/2024
Citation: Mullen, C.A., Strahan, G.D., Elkasabi, Y.M., Ellison, C.R. 2024. Fast and ex situ catalytic copyrolysis of switchgrass and waste polyethylene. Energy and Fuels. 38(20):19639-19647. https://www.doi.org/10.1021/acs.energyfuels.4c03510.
DOI: https://doi.org/10.1021/acs.energyfuels.4c03510

Interpretive Summary: Crop residues, forest thinnings, and grasses are examples of lignocellulosic biomass, which is the most abundant resource for the replacement of fuels, chemicals and materials that are currently sourced from petroleum. A process called pyrolysis can convert this biomass to a crude bio-oil which can be refined into gasoline, diesel and sustainable aviation fuels and a variety of chemical products. However, to fully refine this pyrolysis oil, substantial hydrogen addition, usually from fossil sources is required. A way to decrease this need for hydrogen is to increase the hydrogen content of the bio-crude by adding inexpensive hydrogen rich additives such as waste plastics. This both adds to the quality of the pyrolysis crude oil and offers a route for disposal of plastics. In this study the copyrolysis of switchgrass and waste polyethylene was studied. It was found that addition of a catalytic step after the pyrolysis resulted in a high quality pyrolysis oil that is suitable for further refining and that the quaility and yield was higher with waste plastic added compared with switchgrass alone. This information will be useful to those considering developing thermochemical conversion based biorefineries for agricultural residues.

Technical Abstract: Continuous fast- and ex situ catalytic- pyrolysis of blends of switchgrass with 15wt% polyethylene were studied using a fluidized bed pyrolysis system. Higher than typical temperatures for biomass pyrolysis were utilized to overcome the higher thermal stability of polyethylene, at 630 °C. For fast pyrolysis, the high pyrolysis temperature led to a lower yield of oil and a higher yield of gas from the switchgrass. When polyethylene was blended in a small increase in the yield of oil was noted, and the oil had a slightly lower oxygen content and high hydrogen content. GC/MS and NMR analysis showed that linear alkenes and alkanes were present in the oil in addition to phenolics, acids and other oxygenates derived from biomass. However, a phase separated wax product was also formed and this accounted for an estimated 27% of the input plastic carbon. Ethylene was also a major product of PE pyrolysis accounting for 29% of the input plastic carbon. Only about 19% of the input plastic carbon was in the oil product. Ex situ catalytic pyrolysis was performed over HY, a large pore zeolite. When the catalysis step was performed at 250 °C, the oil product phase separated into a largely biomass derived fraction and an oil derived fraction, where the oil derived fraction was not wax but liquid as a result of a shift to a slightly lower average carbon chain length. When the catalysis was performed at 300 °C, there was a shift in reactivity for the blends compared with switchgrass only, decreasing CO formation and resulting in an oil rich in alkyl benzenes, alkyl naphthalenes and alkyl phenols.