NREL Produces Ethylene via Photosynthesis
September 25, 2012
Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory
(NREL) have demonstrated a better way to use photosynthesis to produce ethylene, a
breakthrough that could change the way materials, chemicals, and transportation fuels
are made, and help clean the air.
The organism – Synechocystis sp. PCC 6803 – produced ethylene at a high rate and is
still being improved. The laboratory demonstrated rate of 170 milligrams of ethylene
per liter per day is greater than the rates reported for the photosynthetic production
by microorganisms of ethanol, butanol or other algae biofuels.
The process does not release carbon dioxide into the atmosphere. Conversely, the process
recycles carbon dioxide, a greenhouse gas, since the organism utilizes the gas as
part of its metabolic cycle.
Ethylene is the most widely produced petrochemical feedstock in the world. But currently
it is produced only from fossil fuels, and its production is the industry’s largest
emitter of carbon dioxide. Steam cracking of long-chain hydrocarbons from petroleum
produces 1.5 to 3 tons of carbon dioxide for every ton of ethylene produced.
The NREL process, by contrast, produces ethylene by using carbon dioxide, which is
food for the bacteria. That could mean a savings of six tons of carbon dioxide emissions
for every ton of ethylene produced -- the three tons that would be emitted by tapping
fossil fuels and another three tons absorbed by the bacteria.
NREL principal investigator, Jianping Yu, says it’s the difference between using old
photons and new photons. Ethylene from old photons is the ethylene produced from fossil
fuels, derived from photosynthetic organisms that captured the sun’s energy millions
of years ago. The NREL process uses new photons that are currently hitting plants,
algae and bacteria capable of producing fuels directly.
Ten years ago, a group of Japanese scientists led by Takahira Ogawa at Sojo University
was the first to try to produce ethylene via photosynthetic conversion in the cyanobacterium
Synechococcus 7942. But by the fourth generation, the bacteria were defunct, producing
no ethylene at all, Yu said.
NREL turned to a different cyanobacterium, Synechocystis 6803, which scientists had
been researching for a long time, knowing how to change its DNA sequences. They manipulated
the sequence to design an ethylene-producing gene to be more stable and more active
than the original version.
This process resulted in an organism that uses carbon dioxide and water to produce
ethylene, but doesn’t lose its ability to produce ethylene over time. The product
ethylene is non-toxic to the producing microorganisms and is not a food source for
other organisms that could potentially contaminate an industrial process.
“Our peak productivity is higher than a number of other technologies, including ethanol,
butanol, and isoprene,” Yu said. “We overcame problems encountered by past researchers.
Our process doesn’t produce toxins such as cyanide and it is more stable than past
efforts. And it isn’t going to be a food buffet for other organisms.”
After the culture reaches maximum growth, it’s possible that it could keep producing
for months at a time, said Rich Bolin, who is a member of NREL’s partnerships group.
The ethylene gas it produces naturally leaves the organism, spurring the organism
to keep producing more.
The ethylene would be produced in an enclosed photobioreactor containing seawater
enriched with nitrogen and phosphorous. The ethylene gas would rise and be captured
from the reactor’s head space. It could then undergo further processing, including
a catalytic polymer process to produce fuels and chemicals. The continuous production
system improves the energy conversion efficiency and reduces the operational cost.
NREL is initiating discussions with potential industry partners to help move the process
to commercial scale. Interested companies include those in the business of producing
ethylene or - transportation fuels, as well as firms that build photobioreactors.
“Separations in biotechnology are complicated and costly,” said Jim Brainard, director
of NREL’s Biosciences Center. “The nice thing about this system is that it is a gas
that just separates from the culture media and rises to the head space. That’s a huge
advantage over having to destroy the valuable culture that is taking carbon dioxide
and light and water to make your product. It’s much easier than a liquid-liquid separation
like in ethanol.”
NREL is the U.S. Department of Energy's primary national laboratory for renewable
energy and energy efficiency research and development. NREL is operated for DOE by
the Alliance for Sustainable Energy, LLC.