The problem is simple to understand. Molecules of carbon and
other greenhouse gases absorb heat. The more greenhouse gases emitted into the
atmosphere, the warmer the atmosphere becomes, exacerbating global climate
change. Solving the problem is not so simple, especially with regards to
aviation -- the source of two-percent of the annual greenhouse gas emissions
from human activity. While biofuels have proven to be an effective, renewable,
low-carbon alternative to gasoline and diesel, jet fuels pose unique
challenges. These challenges have now been met with a new technique developed
by researchers at the Energy Biosciences Institute (EBI), a partnership led by
the University of California (UC) Berkeley that includes Lawrence Berkeley
National Laboratory (Berkeley Lab) and the University of Illinois at
Urbana-Champaign, and the BP energy company.
"We've combined chemical catalysis with life-cycle
greenhouse gas modeling to create a new process for producing bio-based
aviation fuel as well as automotive lubricant base oils," says Alexis
Bell, a chemical engineer with joint appointments at Berkeley Lab and UC
Berkeley. "The recyclable catalysts we developed are capable of converting
sugarcane biomass into a new class of aviation fuel and lubricants with
superior cold-flow properties, density and viscosity that could achieve net
life-cycle greenhouse gas savings of up to 80-percent." These challenges
have now been met with a new technique developed by researchers at the Energy
Biosciences Institute (EBI), a partnership led by the University of California
(UC) Berkeley that includes Lawrence Berkeley National Laboratory (Berkeley
Lab) and the University of Illinois at Urbana-Champaign, and the BP energy
company.
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| Alex Bell, a leading authority on catalysis in biofuels. Credit:Image courtesy of DOE/Lawrence Berkeley National Laboratory |
Bell is one of three corresponding authors of a paper
describing this research in the Proceedings of the National Academy of Sciences
(PNAS). The paper is titled "Novel pathways for fuels and lubricants from
biomass optimized using life-cycle greenhouse gas assessment." Corinne
Scown, a research scientist with Berkeley Lab's Energy Analysis and Environmental
Impacts Division, and Dean Toste, a chemist with joint appointments at Berkeley
Lab and UC Berkeley, are the other two corresponding authors. Additional
authors are are Madhesan Balakrishnan, Eric Sacia, Sanil Sreekumar, Gorkem
Gunbas and Amit Gokhale.
The concentrations of carbon and other greenhouse gases in
Earth's atmosphere are now at their highest levels in the past three million
years, primarily as a result of the burning of petroleum and other fossil
fuels. Biofuels synthesized from the sugars in plant biomass help mitigate
climate change. However, jet fuels have stringent requirements that must be
met.
"Jet fuels must be oxygen-free, have the right boiling
point distribution and lubricity, and a very low pour point, meaning the fuel
can't become gelatinous in the cold temperatures of the stratosphere,"
Bell says. "Biofuel solutions, such as farnesane, mixed directly with
petroleum jet fuel have been tested, but offer only modest greenhouse gas
reduction benefits. Ours is the first process to generate true drop-in aviation
biofuels."
Scown cites the Intergovernmental Panel on Climate Change
(IPCC) on the importance of drop-in aviation biofuels.
"In a 2014 report, the IPCC pointed out that drop-in
biofuels are the only viable alternative to conventional jet fuels," she
says. "If we want to reduce our dependence on petroleum, air travel is
going to require renewable liquid fuels because batteries and fuel cells simply
aren't practical."
The process developed at EBI can be used to selectively
upgrade alkyl methyl ketones derived from sugarcane biomass into trimer
condensates with better than 95-percent yields. These condensates are then
hydro-deoxygenated into a new class of cycloalkane compounds that contain a
cyclohexane ring and a quaternary carbon atom. These cycloalkane compounds can
be tailored for the production of either jet fuel, or automotive lubricant base
oils. Lubricant base oils can produce even more greenhouse gas emissions on a
per-mass basis than petroleum-derived fuels if even a fraction of the lubricant
is repurposed as fuel. The ability of the EBI process to yield jet fuel or
lubricants should be a significant advantage for biorefineries.
"Sugarcane biorefineries today produce ethanol, sugar
and electricity," says PNAS paper co-author Gokhale, a chemical engineer,
who is managing the research project from BP's side. "Expanding the
product slate to include aviation fuels and lubricant base oils could allow for
operators to manage their market risks better, which is exactly how
petrochemical refinery complexes operate today. Rather than optimize for one
product, they try to optimize the overall product slate."
Adds Scown, "Another important advantage offered by our
process is that it enables refineries to convert a portion of the bagasse, the
fibrous residue that remains after juice is extracted from sugarcane stalk,
into fuels and other products. The rest of the waste biomass can be combusted
to produce process heat and electricity to operate the refinery." This new
EBI process for making jet fuel and lubricants could also be used to make
diesel and additives for gasoline as Gokhale explains.
"With some minimal modifications to both the catalysts
and the reaction schemes we can produce drop-in diesel as well," he says.
"We're planning further studies on this."
Although the goal of this study was to develop a strategy
for the flexible production of jet fuels and lubricant base oils in a Brazilian
sugarcane refinery, the strategy behind the process could also be applied to
biomass from other non-food plants and agricultural waste that are fermented by
genetically engineered microbes.
"Although there are some additional technical challenges
associated with using sugars derived entirely from biomass feedstocks like
Miscanthus and switchgrass, there is no fundamental reason why we could not
produce similar outputs, albeit in different proportions," Scown says.
"We expect that further research will make this option increasingly
attractive."
In their PNAS paper the authors acknowledge that the
commercial implementation of their proposed process would include financial
implications that extend beyond greenhouse gas emission reductions but hold
that there still important incentives to encourage investments.
"We've shown in this study that biorefineries can use
inexpensive catalysts to produce a suite of hydrocarbon fuels and
lubricants," Scown says. "By strategically piecing together biological
and thermochemical processes, biorefineries can also operate without any
fossil-derived inputs."
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