Low-pressure hydrothermal processing of mixed polyolefin wastes into clean fuels

•A new LP-HTP method was developed for converting mixed polyolefins to clean fuels.•Reaction pathways for conversion of PE/PP mixtures in LP-HTP were found.•LP-HTP saves 90% of capital costs and 80% of energy compared to SWL.•Energy efficiency and GHG emissions are better than producing fuels from crude oil.•LP-HTP can produce 190 Mt of fuels annually and save 1.5 billion BOE energy. The amount of polyolefin waste, which is 63% of the total plastic waste, has grown exponentially over the past six decades. Its degradation products pose a serious threat to ecosystems and human health. Polyolefins can be converted to clean oils or fuels by using the supercritical water liquefaction (SWL) method at high pressures (≥23 MPa), which require high capital and energy costs. A new low-pressure (~2 MPa) hydrothermal processing (LP-HTP) method is developed to convert polyolefin waste with 50% or more polypropylene into oils with 87% yield at 450 °C and 45 min. The oils, which are C4 to C25 hydrocarbons, can be distilled into clean gasoline and ultra-low sulfur diesel. With this method, up to 190 million tons of fuels can be produced annually from polyolefin waste, resulting in savings of 92% of the energy and 71% of the GHG emissions compared to conventional methods for producing fuels.