Ammonia—a
renewable fuel made from sun, air, and water—could power the globe
without carbon
By Robert F. ServiceJul. 12, 2018 , 2:00 PM
SYDNEY, BRISBANE, AND MELBOURNE, AUSTRALIA—The ancient, arid
landscapes of Australia are fertile ground for new growth, says
Douglas MacFarlane, a chemist at Monash University in suburban
Melbourne: vast forests of windmills and solar panels. More sunlight
per square meter strikes the country than just about any other, and
powerful winds buffet its south and west coasts. All told, Australia
boasts a renewable energy potential of 25,000 gigawatts, one of the
highest in the world and about four times the planet's installed
electricity production capacity. Yet with a small population and few
ways to store or export the energy, its renewable bounty is largely
untapped.
That's where MacFarlane comes in. For the past 4 years, he has been
working on a fuel cell that can convert renewable electricity into a
carbon-free fuel: ammonia. Fuel cells typically use the energy stored
in chemical bonds to make electricity; MacFarlane's operates in
reverse. In his third-floor laboratory, he shows off one of the
devices, about the size of a hockey puck and clad in stainless steel.
Two plastic tubes on its backside feed it nitrogen gas and water, and
a power cord supplies electricity. Through a third tube on its front,
it silently exhales gaseous ammonia, all without the heat, pressure,
and carbon emissions normally needed to make the chemical. "This is
breathing nitrogen in and breathing ammonia out," MacFarlane says,
beaming like a proud father.
Companies around the world already produce $60 billion worth of
ammonia every year, primarily as fertilizer, and MacFarlane's gizmo
may allow them to make it more efficiently and cleanly. But he has
ambitions to do much more than help farmers. By converting renewable
electricity into an energy-rich gas that can easily be cooled and
squeezed into a liquid fuel, MacFarlane's fuel cell effectively
bottles sunshine and wind, turning them into a commodity that can be
shipped anywhere in the world and converted back into electricity or
hydrogen gas to power fuel cell vehicles. The gas bubbling out of the
fuel cell is colorless, but environmentally, MacFarlane says, ammonia
is as green as can be. "Liquid ammonia is liquid energy," he says.
"It's the sustainable technology we need."
Ammonia—one nitrogen atom bonded to three hydrogen atoms—may not seem
like an ideal fuel: The chemical, used in household cleaners, smells
foul and is toxic. But its energy density by volume is nearly double
that of liquid hydrogen—its primary competitor as a green alternative
fuel—and it is easier to ship and distribute. "You can store it, ship
it, burn it, and convert it back into hydrogen and nitrogen," says Tim
Hughes, an energy storage researcher with manufacturing giant Siemens
in Oxford, U.K. "In many ways, it's ideal."
Researchers around the globe are chasing the same vision of an
"ammonia economy," and Australia is positioning itself to lead it.
"It's just beginning," says Alan Finkel, Australia's chief scientist
who is based in Canberra. Federal politicians have yet to offer any
major legislation in support of renewable ammonia, Finkel says,
perhaps understandable in a country long wedded to exporting coal and
natural gas. But last year, the Australian Renewable Energy Agency
declared that creating an export economy for renewables is one of its
priorities. This year, the agency announced AU$20 million in initial
funds to support renewable export technologies, including shipping
ammonia.
Australia's windy coasts offer a
bounty of energy, which it might one day export as a carbon-free fuel.
COAST PROTECTION BOARD, SOUTH AUSTRALIA
In Australia's states, politicians see renewable ammonia as a
potential source of local jobs and tax revenues, says Brett Cooper,
chairman of Renewable Hydrogen, a renewable fuels consulting firm in
Sydney. In Queensland, officials are discussing creating an ammonia
export terminal in the port city of Gladstone, already a hub for
shipping liquefied natural gas to Asia. In February, the state of
South Australia awarded AU$12 million in grants and loans to a
renewable ammonia project. And last year, an international consortium
announced plans to build a US$10 billion combined wind and solar plant
known as the Asian Renewable Energy Hub in Western Australia state.
Although most of the project's 9000 megawatts of electricity would
flow through an undersea cable to power millions of homes in
Indonesia, some of that power could be used to generate ammonia for
long-distance export. "Ammonia is the key enabler for exporting
renewables," says David Harris, research director for low-emissions
technologies at Australia's Commonwealth Scientific and Industrial
Research Organisation (CSIRO) Energy in Pullenvale. "It's the bridge
to a whole new world."
First, however, the evangelists for renewable ammonia will have to
displace one of the modern world's biggest, dirtiest, and most
time-honored industrial processes: something called Haber-Bosch.
The ammonia factory, a metallic metropolis of pipes and tanks, sits
where the red rocks of Western Australia's Pilbara Desert meet the
ocean. Owned by Yara, the world's biggest producer of ammonia, and
completed in 2006, the plant is still gleaming. It is at the
technological vanguard and is one of the largest ammonia plants in the
world. Yet at its core are steel reactors that still use a century-old
recipe for making ammonia.
Until 1909, nitrogen-fixing bacteria made most of the ammonia on the
planet. But that year, German scientist Fritz Haber found a reaction
that, with the aid of iron catalysts, could split the tough chemical
bond that holds together molecules of nitrogen, N2, and combine the
atoms with hydrogen to make ammonia. The reaction takes brute force—up
to 250 atmospheres of pressure in the tall, narrow steel reactors—a
process first industrialized by German chemist Carl Bosch. The process
is fairly efficient; about 60% of the energy put into the plant ends
up being stored in the ammonia's bonds. Scaled up to factories the
size of Yara's, the process can produce vast amounts of ammonia.
Today, the facility makes and ships 850,000 metric tons of ammonia per
year—more than double the weight of the Empire State Building.
Most is used as fertilizer. Plants crave nitrogen, used in building
proteins and DNA, and ammonia delivers it in a biologically available
form. Haber-Bosch reactors can churn out ammonia much faster than
natural processes can, and in recent decades the technology has
enabled farmers to feed the world's exploding population. It's
estimated that at least half the nitrogen in the human body today
comes from a synthetic ammonia plant.
Haber-Bosch led to the Green Revolution, but the process is anything
but green. It requires a source of hydrogen gas (H2), which is
stripped away from natural gas or coal in a reaction using
pressurized, super-heated steam. Carbon dioxide (CO2) is left behind,
accounting for about half the emissions from the overall process. The
second feedstock, N2, is easily separated from air, which is 78%
nitrogen. But generating the pressure needed to meld hydrogen and
nitrogen in the reactors consumes more fossil fuels, which means more
CO2. The emissions add up: Ammonia production consumes about 2% of the
world's energy and generates 1% of its CO2.
A green way to make ammonia
Reverse fuel cells can use renewable power to make ammonia from air
and water, a far more environmentally friendly technique than the
industrial Haber-Bosch process. Renewable ammonia could serve as
fertilizer—ammonia's traditional role—or as an energy-dense fuel.
Industrial ammonia Most of the world’s ammonia is synthesized using
Haber–Bosch, a century-old process that is fast and fairly efficient.
But the factories emit vast amounts of carbon dioxide (CO2).
Yara is
taking a first step toward greening that process with a pilot plant,
set to open in 2019, that will sit next to the existing Pilbara
factory. Instead of relying on natural gas to make H2, the new add-on
will feed power from a 2.5-megawatt solar array into a bank of
electrolyzers, which split water into H2 and O2. The facility will
still rely on the Haber-Bosch reaction to combine the hydrogen with
nitrogen to make ammonia. But the solar-powered hydrogen source cuts
total CO2 emissions from the process roughly in half.
Other
projects are following suit. The state of South Australia announced
plans in February to build a AU$180 million ammonia plant, again
relying on electrolyzers powered by renewable energy. Slated to open
in 2020, the plant would be a regional source of fertilizer and liquid
ammonia, which can be burned in a turbine or run through a fuel cell
to make electricity. The supply of liquid energy will help stabilize
the grid in South Australia, which suffered a debilitating blackout in
2016.
Ammonia
made this way should attract buyers in places such as the European
Union and California, which have created incentives to buy greener
fuels. And as the market grows, so will the distribution routes for
importing ammonia and the technologies for using it, Harris says. By
then, fuel cells like MacFarlane's could be ready to displace
Haber-Bosch itself—and the half-green approach to ammonia production
could become fully green.
Instead
of applying fearsome heat and pressure, reverse fuel cells make
ammonia by deftly wrangling ions and electrons. As in a battery being
charged, charged ions flow between two electrodes supplied with
electricity. The anode, covered with a catalyst, splits water
molecules into O2, hydrogen ions, and electrons. The protons flow
through an electrolyte and a proton-permeable membrane to the cathode,
while the electrons make the journey through a wire. At the cathode,
catalysts split N2 molecules and prompt the hydrogen ions and
electrons to react with nitrogen and make ammonia.
At
present, the yields are modest. At room temperature and pressure, the
fuel cell reactions generally have efficiencies of between 1% and 15%,
and the throughput is a trickle. But MacFarlane has found a way to
boost efficiencies by changing the electrolyte. In the water-based
electrolyte that many groups use, water molecules sometimes react with
electrons at the cathode, stealing electrons that would otherwise go
into making ammonia. "We're constantly fighting having the electrons
going into hydrogen," MacFarlane says.
A component in a reverse fuel cell
uses renewable power to knit together water and nitrogen to make
ammonia.
STEVEN MORTON/FELLOW OF THE ROYAL PHOTOGRAPHIC SOCIETY
To
minimize that competition, he opted for what's called an ionic liquid
electrolyte. That approach allows more N2 and less water to sit near
the catalysts on the cathode, boosting the ammonia production. As a
result, the efficiency of the fuel cell skyrocketed from below 15% to
60%, he and his colleagues reported last year in Energy &
Environmental Science. The result has since improved to 70%,
MacFarlane says—but with a tradeoff. The ionic liquid in his fuel cell
is goopy, 10 times more viscous than water. Protons have to slog their
way to the cathode, slowing the rate of ammonia production. "That
hurts us," MacFarlane says.
To speed things up, MacFarlane and his colleagues are toying with
their ionic liquids. In a study published in April in ACS Energy
Letters, they report devising one rich in fluorine, which helps
protons pass more easily and speeds ammonia production by a factor of
10. But the production rate still needs to rise by orders of magnitude
before his cells can meet targets, set for the field by the U.S.
Department of Energy (DOE), that would begin to challenge Haber-Bosch.
Next to Monash University, Sarb Giddey and his colleagues at the
Clayton offices of CSIRO Energy are making ammonia with their
"membrane reactor." It relies on high temperatures and modest
pressures—far less than those in a Haber-Bosch reactor—that, compared
to MacFarlane's cell, boost throughput while sacrificing efficiency.
The reactor designs call for a pair of concentric long metallic tubes,
heated to 450°C. Into the narrow gap between the tubes flows H2, which
could be made by a solar- or wind-powered electrolyzer. Catalysts
lining the gap split the H2 molecules into individual hydrogen atoms,
which modest pressures then force through the atomic lattice of the
inner tube wall to its hollow core, where piped-in N2 molecules await.
A catalytically active metal such as palladium lines the inner
surface, splitting the N2 and coaxing the hydrogen and nitrogen to
combine into ammonia—much faster than in MacFarlane's cell. So far
only a small fraction of the input H2 reacts in any given pass—another
knock to the reactor's efficiency.
Other approaches are in the works. At the Colorado School of Mines in
Golden, researchers led by Ryan O'Hayre are developing button-size
reverse fuel cells. Made from ceramics to withstand high operating
temperatures, the cell can synthesize ammonia at record rates—about
500 times faster than MacFarlane's fuel cell. Like Giddey's membrane
reactors, the ceramic fuel cells sacrifice some efficiency for output.
Even so, O'Hayre says, they still need to improve production rates by
another factor of 70 to meet the DOE targets. "We have a lot of
ideas," O'Hayre says.
Whether any of those approaches will wind up being both efficient and
fast is still unknown. "The community is still trying to figure out
what direction to go," says Lauren Greenlee, a chemical engineer at
the University of Arkansas in Fayetteville. Grigorii Soloveichik, a
manager in Washington, D.C., for the DOE's Advanced Research Projects
Agency-Energy program on making renewable fuels, agrees. "To make
[green] ammonia is not hard," he says. "Making it economically on a
large scale is hard."
"It looks like there’s enough
interest to get this industry started."
David Harris, CSIRO Energy
However distant, the prospect of Asia-bound tankers, full of green
Australian ammonia, raises the next question. "Once you get ammonia to
market, how do you get the energy out of it?" asks Michael Dolan, a
chemist at CSIRO Energy in Brisbane.
The simplest option, Dolan says, is to use the green ammonia as
fertilizer, like today's ammonia but without the carbon penalty.
Beyond that, ammonia could be converted into electricity in a power
plant customized to burn ammonia, or in a traditional fuel cell, as
the South Australia plant plans to do. But currently, ammonia's
highest value is as a rich source of hydrogen, used to power fuel cell
vehicles. Whereas ammonia fertilizer sells for about $750 a ton,
hydrogen for fuel cell vehicles can go for more than 10 times that
amount.
In the United States, fuel cell cars seem all but dead, vanquished by
battery-powered vehicles. But Japan is still backing fuel cells
heavily. The country has spent more than US$12 billion on hydrogen
technology as part of its strategy to reduce fossil fuel imports and
meet its commitment to reduce CO2 emissions under the Paris climate
accord. Today the country has only about 2500 fuel cell vehicles on
the road. But by 2030 Japanese officials expect 800,000. And the
nation is eyeing ammonia as a way to fuel them.
Converting hydrogen into ammonia only to convert it back again might
seem strange. But hydrogen is hard to ship: It has to be liquefied by
chilling it to temperatures below −253°C, using up a third of its
energy content. Ammonia, by contrast, liquefies at −10°C under a bit
of pressure. The energy penalty of converting the hydrogen to ammonia
and back is roughly the same as chilling hydrogen, Dolan says—and
because far more infrastructure already exists for handling and
transporting ammonia, he says, ammonia is the safer bet.
That last step—stripping hydrogen off ammonia molecules—is what Dolan
and his colleagues are working on. In a cavernous metal warehouse on
the CSIRO campus that has long been used to study coal combustion, two
of Dolan's colleagues are assembling a 2-meter-tall reactor that is
dwarfed by a nearby coal reactor. When switched on, the reactor will
"crack" ammonia into its two constituents: H2, to be gathered up for
sale, and N2, to waft back into the air.
That reactor is basically a larger version of Giddey's membrane
reactor, operating in reverse. Only here, gaseous ammonia is piped
into the space between two concentric metal tubes. Heat, pressure, and
metal catalysts break apart ammonia molecules and push hydrogen atoms
toward the tube's hollow core, where they combine to make H2 that's
sucked out and stored.
Ultimately, Dolan says, the reactor will produce 15 kilograms per day
of 99.9999% pure hydrogen, enough to power a few fuel cell cars. Next
month, he plans to demonstrate the reactor to automakers, using it to
fill tanks in a Toyota Mirai and Hyundai Nexo, two fuel cell cars. He
says his team is in late-stage discussions with a company to build a
commercial pilot plant around the technology. "This is a very
important piece of the jigsaw puzzle," Cooper says.
Beyond 2030, Japan will likely import between $10 billion and $20
billion of hydrogen each year, according to a renewable energy roadmap
recently published by Japan's Ministry of Economy, Trade and Industry.
Japan, Singapore, and South Korea have all begun discussions with
Australian officials about setting up ports for importing renewably
produced hydrogen or ammonia. "How it all comes together economically,
I don't know," Harris says. "But it looks like there's enough interest
to get this industry started."
Cooper knows how he wants it to end. Over coffee on a rainy morning in
Sydney, he describes his futuristic vision for renewable ammonia. When
he squints, he can see, maybe 30 years down the road, Australia's
coast dotted with supertankers, docked at offshore rigs. But they
wouldn't be filling up with oil. Seafloor powerlines would carry
renewable electricity to the rigs from wind and solar farms on shore.
On board, one device would use the electricity to desalinate seawater
and pass the fresh water to electrolyzers to produce hydrogen. Another
device would filter nitrogen from the sky. Reverse fuel cells would
knit the two together into ammonia for loading on the tankers—a bounty
of energy from the sun, air, and sea.
It's the dream that nuclear fusion never reached, he says:
inexhaustible carbon-free power, only this time from ammonia. "It can
never run out, and there is no carbon in the system."
