Investigation of first-stage liquefaction of coal with model plastic waste mixtures

A series of liquefaction tests have been conducted using coal, polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), in various combinations and proportions. These tests were done in batch microautoclaves, 1-L semibatch reactors, and a small-scale continuous unit. Results on individual plastics showed that PE and PP were converted primarily into aliphatic hydrocarbons, PS into alkyl benzenes, and PET into benzenes, ethane, and carbon dioxide. PE was by far the most difficult of the model plastics to convert. In two-component and multicomponent tests, tetrahydrofuran (THF) conversions could be estimated from the behavior of the individual components under similar conditions. Results were highly sensitive to conditions, especially those of atmosphere and temperature. Higher conversions could be obtained with higher temperatures, provided that retrograde reactions that re-polymerize the products were minimized. Retrograde reactions could be suppressed via the use of a synthesis gas atmosphere and/or the absence of added solvent. Acidic catalysts, such as molecular sieves, also led to higher conversions, but only in systems where coal was absent.