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

Abstract

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.

Material Flow Analysis and Life Cycle Assessment of Polyethylene Terephthalate and Polyolefin Plastics Supply Chains in the United States

“…Similar to the U.S. baseline systems analysis presented in this study, analyses of future scenarios for a circular economy of plastics should integrate diverse data sets and models, combining MFA, process simulation and optimization, TEA, LCA, transportation logistics, and market dynamics ,, and may also include optimization approaches to interrogate system parameters and trade-offs among indicators of sustainability. ,− …”

Critical Evaluation of Life Cycle Assessment Analyses of Plastic Waste Pyrolysis

“…Out of the 18 studies presented in Table , 5 of them claimed the use of confidential information, − making the independent critical evaluation of the proposed LCA analyses difficult. Other studies are based on available laboratory-scale data, − with no evident connection to actual large-scale industrial performance, making it difficult to ensure that the published data would eventually be useful to represent actual large-scale chemical recycling operations. Some studies have extrapolated the data using process simulators but representing the pyrolysis unit as conversion, yield, or stoichiometric reactors, which still rely on the experimental data and have no kinetic model.…”

“…Additionally, sometimes the underlying assumptions of the proposed methodological procedures are not evident. For example Zhao and You (2021) used experimental data collected through pyrolysis experiments performed with 2 g of high-density polyethylene (HDPE) and considered that the results would remain the same for reactors of 2 t/h, although it is not well-known how product yields and compositions in small scale reactors would change in large scale ones due to mass and heat transfer effects and possible modifications of the reactor configuration. Furthermore, it is also known that upstream and downstream processes also affect the product quality and LCA scores. ,− For instance, heating a plastic material in an extruder can be less energetically demanding and can remove hydrochloric acid.…”

Non-thermal Plasma-Assisted Deconstruction of High-Density Polyethylene to Hydrogen and Light Hydrocarbons over Hollow ZSM-5 Microspheres

“…Notwithstanding, the abolition of plastic waste by burning is a trade-off with a significantly large amount of CO 2 emitted. Identifying potential markets with highcapacity needs of recycled HDPE is challenging, 11 and therefore, novel approaches, which can generate high addedvalue products, are highly desired to spur the interest in the reuse of HDPE waste. Advanced upcycling involves the efficient conversion of waste plastic to value-added products such as hydrogen, light hydrocarbons, and monomers, which can be used either as direct fuels for power generation or as intermediates for the synthesis of chemicals.…”