Consequential Life Cycle Assessment and Optimization of High-Density Polyethylene Plastic Waste Chemical Recycling

This work develops a consequential life cycle optimization (CLCO) framework that integrates the superstructure optimization, consequential life cycle assessment (CLCA) approach, market equilibrium models, and techno-economic assessment methodology to determine the economically and environmentally optimal waste high- density polyethylene ( HDPE) chemical recycling technology pathway, which manufactures chemical and energy products that cause market dynamics. System expansion in CLCA can quantify the environmental consequences of an increment of feedstock suppliers' and downstream product consumers' processes as well as a decrement of feedstock consumers' and downstream product suppliers' processes. These market dynamics are ignored when performing attributional life cycle assessment (ALCA). The CLCO problem is formulated as a multi-objective mixed-integer nonlinear fractional programming problem and solved by an optimization algorithm that integrates the inexact parametric algorithm and branch-and-refine algorithm. The environmental and economic objectives are minimizing the unit consequential life cycle environmental impacts and maximizing the unit net present value, respectively. Market dynamics results show that consuming natural gas in the waste HDPE chemical recycling process increases the natural gas' market price and supply by 0.1%, while onsite manufacturing propylene decreases propylene market price by 5.46%, decreases propylene supply by 8.8%, and increases the propylene demand by 10.2%. Disparities and comprehensiveness are two effects of system expansion. Disparities are illustrated by a 14.22% decrease in greenhouse gas (GHG) emissions and a 60.37% reduction of photochemical oxidant formation compared to ALCA results. Comprehensiveness of system expansion is reflected by the diverse results of the environmental consequences of each consumer or marginal supplier's process of feedstocks and products. Specifically, the substitution of 1-butene supplier's process by onsite production from chemical recycling reduces 19.68% of total photochemical oxidant formation. Onsite use of natural gas from chemical recycling increases 37.14% of total ionizing radiation.