Life Cycle Assessment of Waste Oil-to-Jet Fuel Production via the HEFA Process with Integrated Co-generation

Technical Summary: Life Cycle Assessment of Waste Oil-to-Jet Fuel Production via the HEFA Process with Integrated Co-generation

Problem Statement
The production of Sustainable Aviation Fuel (SAF) from waste oils via the Hydroprocessed Esters and Fatty Acids (HEFA) process currently faces challenges regarding its reliance on external electricity and hydrogen, which can undermine its overall environmental sustainability. As the global energy sector transitions toward decarbonization, there is a critical need to quantitatively evaluate how integrating combined heat and power (co-generation) and substituting hydrogen sources affect the life cycle environmental performance of SAF production. This study addresses the gap in understanding the trade-offs between conventional HEFA pathways and an integrated HEFA-PG (Power and Gas) system under future energy transition scenarios, specifically within the context of China's evolving power grid and hydrogen production landscape.

Methodology
The study employs a Cradle-to-Grave Life Cycle Assessment (LCA) following the ISO 14040 framework. The functional unit is defined as the production of 100 tonnes of bio-jet fuel.

• System Boundaries: The analysis covers waste oil collection and transportation, pretreatment, production (hydrogenation and hydroprocessing), distribution, and end-use. Two pathways are modeled: a conventional HEFA process and an HEFA-PG process integrated with a combined heat and power (CHP) unit.
• Simulation and Data: Process simulations were conducted using Aspen Plus to determine material and energy flows. Life Cycle Inventory (LCI) data were compiled based on these simulations and regional parameters for the Beijing–Tianjin–Hebei region.
• Impact Assessment: The ReCiPe 2016 method was utilized to evaluate environmental impacts across six midpoint indicators (Global Warming Potential [GWP], Particulate Matter Formation Potential [PMFP], Terrestrial Acidification Potential [TAP], Freshwater Eutrophication Potential [FEP], Human Toxicity Potential [HTP], and Fossil Fuel Depletion Potential [FFP]) and three endpoint indicators (Resource Availability, Ecosystem Diversity, and Human Health).
• Allocation and Scenarios: Mass-based allocation was applied to the conventional HEFA process for by-products, while the HEFA-PG process utilized an avoided burden approach for internally generated electricity. Sensitivity analyses were performed to project environmental performance under power grid decarbonization scenarios (2030–2060) and to compare natural gas reforming hydrogen against wind-powered electrolytic hydrogen.

Key Contributions and Results
The study provides a comparative analysis of the environmental burdens of conventional HEFA, integrated HEFA-PG, and fossil-based aviation kerosene.

1. Performance of HEFA-PG vs. Conventional HEFA:

   • The HEFA-PG process demonstrates significant environmental advantages over the conventional HEFA route. By generating internal green electricity, the HEFA-PG system achieves a net GWP reduction to approximately 66% of the conventional HEFA baseline.
   • The integrated co-generation stage provides an offset effect of roughly 35% for FEP, HTP, and FFP indicators.
   • In terms of ozone formation, the HEFA-PG process reduces contributions from 80% (in conventional HEFA) to approximately 10%, significantly mitigating ground-level ozone formation.

2. Comparison with Fossil Aviation Kerosene:

   • Both bio-jet fuel pathways exhibit far superior environmental performance compared to fossil aviation kerosene. The GWP of HEFA and HEFA-PG pathways is reduced to 13.3% and 7.2% of the fossil baseline, respectively.
   • The HEFA-PG pathway shows the lowest environmental burden across all categories, including TAP and PMFP, followed by HEFA, with fossil fuel being the highest.

3. Impact of Power Grid Decarbonization (2030–2060):

   • As the power grid becomes cleaner, the GWP of the conventional HEFA process gradually declines due to lower carbon intensity in purchased electricity.
   • Conversely, the GWP advantage of the HEFA-PG process diminishes over time. As the grid decarbonizes, the carbon credit benefit from displacing grid electricity decreases, causing the HEFA-PG GWP to eventually surpass that of the conventional HEFA process by 2060.
   • However, the HEFA-PG process maintains a robust advantage in POFP (reductions >80%) and remains less sensitive to grid changes regarding HTP, as its toxicity burden is concentrated in feedstock logistics and hydrogen production rather than electricity generation.

4. Green Hydrogen Substitution:

   • Replacing natural gas reforming hydrogen with wind-powered electrolytic hydrogen further reduces GWP for both pathways.
   • Critical Trade-off: Despite carbon benefits, electrolytic hydrogen leads to a Human Toxicity Potential (HTP) that is more than four times higher than natural gas reforming. This increase is primarily attributed to the environmental loads associated with off-site sulfide tailings generated during the manufacturing of wind power equipment.
   • Under current technical conditions, natural gas reforming hydrogen presents superior comprehensive environmental performance within the HEFA-PG system. The toxicity drawbacks of electrolytic hydrogen can only be alleviated through cleaner manufacturing of wind equipment or revised allocation of environmental burdens among co-products.

Significance and Claims
The paper claims that the HEFA-PG process is a highly effective design for near-to-medium-term energy transitions, offering substantial emission reduction benefits through co-generated electricity. However, the study emphasizes that the long-term environmental superiority of this integrated system is contingent upon the pace of power grid decarbonization.

Furthermore, the research highlights a critical nuance in "green" hydrogen adoption: while wind-powered electrolysis reduces carbon footprints, it may inadvertently increase human toxicity risks due to upstream manufacturing impacts. Consequently, the authors assert that under current technological constraints, natural gas reforming remains the more environmentally balanced hydrogen source for SAF production. The study identifies feedstock collection/transportation and hydrogen production as the core environmental hotspots, suggesting that future improvements must focus on cleaner wind equipment manufacturing and optimized co-product allocation strategies to fully realize the potential of green hydrogen in the SAF sector.