Not all of the challenge of reducing
transportation-related greenhouse gas (GHG) emissions is found at the
end of automobile tailpipes. The aviation sector currently accounts
for more than ten percent of U.S. transportation-related GHG
emissions. To address this issue, the White House recently announced a
Sustainable Aviation Fuel (SAF) Grand Challenge to generate at least 3
billion gallons of SAFs by 2030 and, by 2050, sufficient SAFs to meet
100 percent of U.S. aviation fuel demand, currently projected to be
around 35 billion gallons annually. Meanwhile, the International Air
Transportation Association (IATA), which represents major global
airlines, has committed to net-zero carbon emissions from global air
transportation by 2050.
Replacing fossil jet fuels with SAFs has been recognized as a crucial
and promising strategy to help the aviation industry meet its
emissions reduction goals, diversify fuel supply, and enhance energy
security. Major U.S. airlines have also pledged to work with the U.S.
federal government and other stakeholders to rapidly advance the
commercialization, deployment, and expansion of SAFs.
The technology needed for SAF production already exists, including
ethanol-to-jet (ETJ) conversion technologies. Producing ethanol from
corn is a mature technology, and life-cycle GHG emissions of corn
ethanol have decreased by roughly a quarter in the past 15 years
, excluding land-use change emissions. Compared to petroleum jet fuel,
SAF produced from today’s corn ethanol also already offers a 15
percent lower carbon intensity. However, to achieve the industry’s
net-zero objectives, additional significant GHG reductions are still
needed.
To assess whether and how the ethanol-to-jet (ETJ) pathway may reach
the net-zero emission target, Argonne scientists performed a
life-cycle analysis (LCA) of corn to ethanol and then to jet fuel,
using the Greenhouse gases, Regulated Emissions, and Energy use in
Technologies (GREET)
® model developed at Argonne. The GREET simulation covers the entire
ETJ pathway, including corn farming, corn ethanol refining, jet fuel
upgrading, and fuel transportation and consumption.
Starting with the current state of the domestic ethanol industry, the
simulation evaluated a range of technologies and GHG management
options applicable to corn farming, ethanol production, and jet
conversion in an attempt to identify key GHG emissions sources in the
corn-to-ethanol pathway. The simulation revealed ethanol plants and
corn farming to be two major sources of corn ethanol GHG emissions
(Figure 1).
Figure 1. Share of corn ethanol to jet
GHG emissions by source, including both corn-to-ethanol and
ethanol-to-jet processes (the chart does not include 13.4 g CO2e/MJ
jet credits from distillers’ grains with solubles). Image courtesy of
Argonne National Laboratory
For both ethanol refining and jet upgrading,
energy use (e.g., natural gas for steam generation) helps drive GHG
emissions. For this reason, the analysis evaluated potential options
to displace natural gas (NG). At the ethanol plants, displacing half
of NG with syngas from biomass through gasification may decrease GHG
emissions of ETJ by 12 g CO2e/MJ (Figure 2). Replacing the other half
of NG demand at both ethanol plants and jet upgrading facilities with
renewable natural gas (RNG) from animal wastes could reduce GHG
emissions by 33 g CO2e/MJ on average (Figure 2).
Since CO2 from the corn fermentation unit is the CO2 uptaken from the
air by corn plants in the cornfield, capture and injection of the
fermentation CO2 into geologic formations may generate a significant
GHG credit, causing net GHG emissions of ETJ to be negative (-16.8 g
CO2e/MJ Jet).
In addition to refining, farming is another promising area for jet
fuel decarbonization. The analysis includes several smart farming
practices such as variable nitrogen applications with 4R (right time,
right place, right form, and right rate), and enhanced efficiency
fertilizers help reduce nitrogen fertilizer inputs to corn farms and
reduce N2O emissions from the fertilizers. “Green” ammonia with
renewable electricity and renewable hydrogen further reduce nitrogen
fertilizer GHG footprint. Finally, sustainable farming practices to
increase soil organic carbon contents have the largest GHG reductions
in corn farming. Combining all these measures, GHG emissions of ETJ
can be reduced to –44.8 g CO2e/MJ, which is 153% lower than the
petroleum jet (Figure 2).
Figure 2. Life-cycle GHG emissions (g CO2e/MJ)
of corn to ethanol and then to jet pathway compared to petroleum jet.
Analysis was conducted using the recently released GREET 2021.
Indirect emission credits in hashed bars include avoided emissions
(e.g., avoided methane emissions from animal waste) and carbon
sequestration credits. Image courtesy of Argonne National Laboratory
Overall, the analysis reveals there is
great potential to produce SAFs with potentially zero or
negative GHG emissions, through a combination of cleaner
production technologies and sustainable farming practices. More
R&D, incentives, and coordinated efforts would be needed to
speed up the deployment of these technologies. Argonne’s GREET
model can help guide the biofuel and aviation industry to both
identify carbon reduction opportunities and to move in the right
direction.Thanks to Hui
Xu, Uisung Lee, and Michael Wang at Argonne National Laboratory
for their work as contributing authors on this story.
