(454e) Solvolysis Liquefaction of Low-Rank Coals and Polyolefin Waste-Plastics-Derived Liquid into Mesophase Pitch Intermediates to Manufacture Value-Added Carbon Materials

As primary packaging materials, polyolefin plastics (PP, HDPE, and LDPE) account for more than 40% of plastic waste discharge but are very hard to reuse. It is more economical to depolymerize the waste polyolefins and convert them into value-added chemicals and fuels. Currently, most attempts at depolymerization of polyolefins involve pyrolysis, which affords waxes and light hydrocarbon mixtures that can be used as an alternative to petroleum-derived liquid fuel. However, the depolymerization of polyolefins into liquid fuel requires an expensive catalytic process with low yield and selectivity, while the polyolefin-derived liquid is economically non-competitive with petroleum gasoline, and the further burning of polyolefin liquid would eventually release the carbon into the atmosphere as CO₂. Therefore, stabilizing the carbons in polyolefin waste plastics in more valuable carbon materials, like carbon fibers, graphite electrodes, or carbon foam, can be a profitable and environmentally-friendly method to utilize the polyolefin waste plastics and reduce plastic waste discharge.

In our previous study, a mild solvolysis liquefaction method converted low-rank bituminous steam coal into a hydrogenated heavy tar intermediate to manufacture value-added mesophase pitch materials with relatively high yield and excellent mesophase formation performance [1]. However, the liquefaction process requires the application of a hydrogen vehicle solvent with a high boiling point, making recycling and reusing the solvent costly. Meanwhile, previous studies also found that the non-catalytic hydrogenolysis of polyolefins leads to an aromatic-rich and naphthenic-rich liquid with high heavy asphaltene and pre-asphaltene fractions. Based on these characteristics of the liquid, it has the potential to be used as a hydrogen vehicle solvent in solvolysis liquefaction. Therefore, depolymerizing polyolefin waste plastics into a hydrogen vehicle solvent, then converting a mixture of polyolefin-derived liquid and low-rank coals into precursors of value-added carbon materials appears very promising.

In this study, various polyolefin waste plastics were hydrogenolyzed into a waste-polyolefin-derived liquid (WPDL) under modified conditions [2]. The obtained WPDL was used as an alternative solvent, in lieu of the pure chemical hydrogen vehicle solvent, in the solvolysis liquefaction of low-rank bituminous steam coal. The liquefaction process was optimized based on the yield and composition of the target product, polyolefin-coal-liquefaction heavy tar (PCL-HT). Solvent fractionation, TGA, FTIR, and ¹H NMR methods were applied to characterize the PCL-HT derived at different conditions. PCL-HT with a high content of heavy fraction and good hydrogen transferability was selected and thermally upgraded into value-added mesophase pitch. The produced mesophase pitch was characterized by DMA, XRD, and polarizing microscopy. The results indicated that the WPDL could replace the pure hydrogen vehicle solvent and refine the composition of the heavy tar derived from the solvolysis liquefaction of low-rank coals. Our research indicates that this method can utilize both polyolefin waste plastics and low-rank coal resources to generate value-added product precursors, especially from different coal-producing areas of the United States.

Reference

[1] Wang W, Preciado I, Malzahn J, Eddings E. Mild Solvolysis Liquefaction of Low-Rank Coal into a Feedstock of Value-Added Carbon Materials. 2021 AIChE Annual Meeting. AIChE; 2021.

[2] Wang W, Preciado I, Eddings E. Thermochemical Co-Conversion of Waste Polyolefins with Low-Rank Aromatic-Rich Hydrocarbons into an Intermediate of High-Quality Anisotropic Pitch. 2021 AIChE Annual Meeting. AIChE; 2021.