(99e) Carbon Footprint Assessment of Production of Ethylene from U.S. Corn Ethanol

Ethylene is one of the most common feedstocks used in production of plastics such as polyethylene (PE). Currently, ethylene and PE are produced almost exclusively from fossil resources.

The purpose of this study was to evaluate the potential carbon footprint estimate of producing polymer-grade ethylene by using U.S. corn ethanol as feedstock. The functional unit for this assessment was 1 kg of ethylene and the system boundary was cradle-to-ethylene manufacturing output gate.

The system boundary of corn ethanol derived ethylene included ethanol production and conversion to ethylene. The ethanol production from corn is divided into corn farming, ethanol manufacturing, and ethanol transportation stages. Previous studies from Argonne National Laboratory (ANL) and the Greenhouse gases, Regulated Emissions, and Energy use in Technologies model version 2022 (GREET 2022) were leveraged for data for these steps in the ethanol value chain. Sensitivity analyses were conducted to understand the impact of variability in the carbon intensity (CI) of ethanol due to Life Cycle Assessment (LCA) allocation methods, sustainable farming, and fuel switching, and its influence on ethylene carbon footprint.

The carbon footprint analysis covered bio-based ethylene production pathways via corn ethanol dehydration from the following routes:

1) Stand-alone ethanol-to-ethylene processes

2) A co-processing route via fluid catalytic cracking (FCC) process wherein corn ethanol was co-processed with vacuum gas oil (VGO)

Process inventory data for stand-alone processes was derived from published literature sources (secondary data). The inventory for co-processing ethanol to produce ethylene was generated based on primary data, modeled/simulation data (benchmarking and internal calculations) and secondary data. Additionally, background global warming potential (GWP) values for process inputs were derived from ANL GREET 2022. One challenge associated with co-processing is the estimation of the carbon footprint of products with bio-based content due to the combination of the feedstocks utilized (bio- and fossil-based). For this study, laboratory data based on carbon-14 (C-14) analysis was used to generate process yields for corn ethanol feed. In the case of products containing both bio-derived and fossil components, C-14 content is used to calculate how much of the product is derived from bio- vs. fossil-derived feedstocks. Additionally, modeled yields based on co-processing yields of ethanol and VGO and base FCC unit yields was used to generate bio-ethylene yields for comparison purposes (Modeled Yields Case).

The conventional, fossil-based ethylene production pathway was included as reference case for comparison with corn-ethanol derived, bio-based ethylene. For this analysis, the steam cracking LCA developed by ANL for GREET 2022 was leveraged to represent a carbon footprint estimated range for fossil-based ethylene. The range was based on feedstock choices for steam cracking such as naphtha and ethane.

This study demonstrated that corn ethanol-based ethylene showed lower carbon footprint estimate when compared to fossil-based ethylene production (103% - 127% reduction in the base case scenarios). This reduction is due to the addition of biogenic CO₂ sequestration credits to the bio-based product. The study also evaluated the impact of low CI ethanol on the carbon footprint of bio-ethylene. For example, substituting natural gas used in ethanol production with renewable natural gas (RNG) from dairy and swine manure and utilizing heat and power from corn stover collected during farming reduced the carbon footprint estimate of bio-ethylene compared to the baseline corn ethanol production. This reduction is due to the addition of biogenic CO₂ sequestration credits and avoided emissions of current waste management practices by use of RNG.