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June
15, 2019
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Credit: Siemens
The Siemens green ammonia test
plant uses wind power to convert hydrogen and nitrogen to ammonia.
The
Haber-Bosch process, which converts hydrogen and nitrogen to
ammonia, could be one of the most important industrial chemical
reactions ever developed. The process made ammonia fertilizer widely
available, helping cause a world population boom as yields from
agriculture increased rapidly in a short time.
Globally, ammonia production plants made
157.3 million metric tons (t) of the compound in 2010, according to
the Institute for Industrial Productivity’s Industrial Efficiency
Technology Database. Between 75 and 90% of this ammonia goes toward
making fertilizer, and about 50% of the world’s food production
relies on ammonia fertilizer.
The rest of the ammonia helps make
pharmaceuticals, plastics, textiles, explosives, and other
chemicals. Almost every synthetic product we use containing nitrogen
atoms comes to us through the Haber-Bosch process in some way, says
Karthish Manthiram, a chemical engineer from the Massachusetts
Institute of Technology. “All those nitrogen atoms came from
ammonia, which means that there is this enormous carbon dioxide
footprint embedded in all the different products that we use.”
That massive carbon footprint exists
because although the Haber-Bosch process represents a huge
technological advancement, it’s always been an energy-hungry one.
The reaction, which runs at temperatures around 500 °C and at
pressures up to about 20 MPa, sucks up about 1% of the world’s total
energy production. It belched up to about 451 million t of CO2
in 2010, according to the Institute for Industrial Productivity.
That total accounts for roughly 1% of global annual CO2
emissions, more than any other industrial chemical-making reaction
The carbon footprint of ammonia
synthesis goes well beyond its energy demands. Hydrogen used
for the reaction comes from natural gas, coal, or oil through
processes that release CO2. According to a 2013
joint report from the International Energy Agency, the
International Council of Chemical Associations, and the
Society for Chemical Engineering and Biotechnology, CO2
emissions from hydrogen production account for more than half
of those from the entire ammonia production process. In total,
from hydrocarbon feedstocks to NH3 synthesis, every
NH3 molecule generated releases one molecule of CO2
as a coproduct.
And our hunger for ammonia
fertilizer is increasing. According to the Food and
Agriculture Organization of the United Nations, nitrogen
fertilizer demand is projected to increase from 110 million t
in 2015 to almost 119 million t by 2020.
Chemists and engineers across the
world are trying to make ammonia synthesis sustainable. Some
are working to power the reaction with renewable energy
sources and to generate hydrogen without fossil fuels. Others
want to find a more efficient reaction than Haber-Bosch to
synthesize ammonia. The researchers admit that progress has
been slow but worth it.
“Ammonia as it’s produced today for
fertilizers is effectively a fossil-fuel product,” says
Douglas MacFarlane, an electrochemist from Monash University.
“Most of our food comes from fertilizers. Therefore, our food
is effectively a fossil-fuel product. And that’s not
sustainable.”
At green ammonia plants around the
world, including in Japan, England, Australia, and the US,
researchers have been experimenting with using renewable
energy and feedstocks to make the valuable chemical on small
scales. These companies mostly use the conventional Haber-Bosch
process, but instead of relying on fossil fuels to generate
hydrogen and power the reactions, they’re using water
electrolysis and alternative energy sources.
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Ammonia by the numbers
157.3 million:
Metric tons of NH3 produced
worldwide in 2010
451 million:
Metric tons
of CO2 emitted by NH3 synthesis worldwide
in 2010.
~1%:
Percentage of global CO2 emissions that come from NH3
synthesis.
Sources: Institute for
Industrial Productivity. |
Since last year, the Japanese company JGC has been trying these
approaches at a trial plant at the Fukushima Renewable Energy
Institute. Through a national program called the SIP Energy
Carriers, the company has teamed up with the National Institute of
Advanced Industrial Science and Technology (AIST) to get the green
ammonia demonstration plant up and running. It can run on solar
power, produces hydrogen through water electrolysis, and operates a
Haber-Bosch-type reaction using a new ruthenium catalyst that JGC
developed with AIST.
“The major advantage of our process is
that hydrogen is produced at a much lower pressure than the
conventional process,” says Mototaka Kai, project manager at the
plant. The hydrogen pressure is around 5 MPa, Mototaka says, which
is around one-third to one-quarter that of a traditional Haber-Bosch
plant. This lower pressure has two advantages. The reaction is safer
because it’s operating at a lower pressure. Plus, the plant requires
less energy to pressurize the system. Currently, the plant produces
20–50 kg of ammonia per day.
Siemens in the UK is working with
researchers at the University of Oxford, the UK’s Science and
Technology Facilities Council, and Cardiff University to run a
demonstration plant using the typical Haber-Bosch process, powering
it with wind. Ian Wilkinson, program manager in corporate technology
at Siemens, names two reasons the firm chose to use only mature
technology available today to run its plant. First, Siemens wants to
show that it can produce ammonia renewably, in a way that it can
quickly scale up. The company also views the plant as a test system
for ongoing technology development, including Haber-Bosch catalyst
development and ammonia combustion tests.
The plan has worked so far. The small
plant, set up in shipping containers, takes electricity from a wind
turbine, runs it through a hydrogen electrolysis unit, and then uses
the resulting hydrogen to synthesize ammonia. If the company runs
the plant continuously, it gets 30 kg of ammonia a day, Wilkinson
says. “It’s a small-scale, proof-of-principle system,” he says,
noting that the only thing in the plant that the firm didn’t buy off
the shelf is the synthesis loop in which the actual Haber-Bosch
reaction takes place. “We had to build our own. You can’t buy them
this small,” he says.
Ammonia synthesis at a wind farm could
help solve one of the biggest problems with renewable energy
sources—they produce energy intermittently. The sun doesn’t always
shine and the wind doesn’t always blow, so how do you generate
electricity consistently? Burning ammonia produced renewably may be
one answer, Wilkinson says. Both Siemens and JGC are interested in
green ammonia production not just to make fertilizer but also to
synthesize a carbon-free fuel. Similar to gasoline, ammonia can be
shipped and stored, and it is easier to deal with than gaseous
hydrogen, another possible carbon-free fuel.
“Ammonia is what I like to call a nexus
molecule,” Manthiram says. “It’s useful as a fertilizer. It’s useful
for food. It’s useful for energy storage.” Electricity generated
through renewable sources can combine nitrogen from the air and
hydrogen from water to make a transportable fuel, he says. And
companies already ship ammonia across oceans for current uses,
MacFarlane says. “That technology is well understood in large
quantities of ammonia.”
But no matter how these companies plan to
use the ammonia produced by their green plants, they’re still mostly
using Haber-Bosch to synthesize the molecule. The reaction involves
combining hydrogen and nitrogen gas over an iron catalyst, at high
temperatures and pressures. And it isn’t efficient, MacFarlane says.
Each metric ton of ammonia packs about 5 MW h of energy. “The best,
most efficient Haber-Bosch plants work at around 10 MW h per metric
ton of ammonia,” MacFarlane says. “So we’re approximately only 50%
efficient. It’s wasting a lot of energy for what you get.”
![](images/images_05_27_2022/Pic2.jpg)
Credit: Rong Cai
Nitrogenase is the only enzyme
known to reduce N2 to NH3 at ambient
temperature and pressure. The enzyme breaks down the energy-rich
adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and uses
that energy to power the transfer of electrons from an iron-sulfur
cluster (cyan/yellow) through a phosphorus cluster (pink/yellow) to
the iron-molybdenum catalytic cofactor (orange), where N2
gets reduced.
Switching to renewable feedstocks and
energy sources is a good solution in the short term, Manthiram says,
because companies can effectively combine current renewable energy
technologies with Haber-Bosch. But to improve the sustainability of
ammonia synthesis over the long term, scientists have to change the
game entirely.
“Many people are looking at alternatives
to Haber-Bosch,” says Shelley Minteer, a bioelectrochemist at the
University of Utah. “How can we do something at low temperatures and
atmospheric pressure or near atmospheric pressure?”
Research in the field has taken off since
about 2015, perhaps because of expanded funding availability as
federal agencies have started to focus on the topic, says Lauren
Greenlee, a chemical engineer at the University of Arkansas.
Researchers are trying a wide range of approaches: electrochemistry,
electrocatalysis,
photocatalysis, and photoelectrocatalysis. And they’re even
taking inspiration from biochemistry. “That diversity as a field
that’s growing so quickly is actually fantastic because then you’re
able to learn from each other what works and what doesn’t,” Minteer
says.
Electrochemical reduction of nitrogen to
ammonia over a catalyst has captured the imagination of many
scientists. The chemists apply a voltage across an electrochemical
cell to drive both water oxidation and nitrogen reduction
simultaneously. The catalyst at the anode oxidizes water to form
hydrogen ions, which migrate to the cathode, where a different
catalyst reduces nitrogen to ammonia. Scientists have developed
numerous electrochemical ammonia-synthesis catalysts, including
noble-metal nanostructures, metal oxides, metal nitrides, metal
sulfides, nitrogen- and boron-doped carbon, and lithium metal.
“What’s enticing about [electrochemistry]
is that you can get your hydrogen atoms directly from water
molecules without having to go through molecular hydrogen,” Greenlee
says. “If, in theory, your electrochemical process is being driven
by renewable energy, you eliminate the need for fossil fuels both
from an energy input standpoint for electricity, but also from a
hydrogen production standpoint.” This method also avoids the need to
do electrolysis as a separate step and has the potential to operate
at low pressure and possibly low temperature, she says. There are “a
lot of pieces of it that are really positive, if we can get it to
work.”
Electrochemistry also presents a good way
to solve a trade-off between reaction rates and yields that chemists
must face when running the Haber-Bosch reaction, Manthiram says. The
reaction has good yields at very low temperatures, he says, but the
rate is sluggish. To speed it up, chemists raise the temperature.
But at those high temperatures, the reaction’s thermodynamics
change, and the yield goes down. So chemists raise the pressure to
bring the yields back up. What’s special about an electrochemical
system is that chemists can increase voltage instead of pressure,
Manthiram says. “What normally takes hundreds of bar pressure to
achieve can be done with fractions of a volt.”
Current process
Today, ammonia synthesis starts
with generating hydrogen gas from fossil-fuel feedstocks. A reformer
turns the feedstocks into a mixture of gases called synthesis gas (syngas),
which includes hydrogen. A CO shift converter combines water and the
carbon monoxide from syngas to form CO2 and more
hydrogen, and then acid gas removal isolates the hydrogen for
ammonia synthesis. This process releases CO2 at various
steps along the way.
![](images/images_05_27_2022/Pic3.jpg)
One of the other possible advantages of
the electrochemical approach is that the reaction system can be
small. A device under development in MacFarlane’s lab is about the
size of a cell phone. The idea is that it could synthesize ammonia
for fertilizer on the scale of a farm or greenhouse, so the material
could be used right where it’s made, eliminating the need for
transport, MacFarlane says.
Meanwhile, other researchers are looking
to nature to understand how to efficiently reduce nitrogen to
ammonia. Some bacteria use large protein complexes called
nitrogenases to grab nitrogen out of the air and make ammonia.
Minteer and her team have been studying this system to connect these
bacterial enzymes to electrodes to create new electrocatalysts. But
they still have a long way to go, Minteer says. Their systems do
more proton reduction than ammonia production. The goal is to get to
the point where they’re making 99% ammonia and 1% hydrogen. Right
now, their systems make about 40% ammonia and 60% hydrogen, she
says.
![](images/images_05_27_2022/Pic4.jpg)
Credit: Douglas MacFarlane
This device, developed by
Douglas MacFarlane and coworkers at Monash University, can convert
hydrogen and nitrogen to ammonia inside a cell phone–sized
package.
One of the other possible advantages of
the electrochemical approach is that the reaction system can be
small. A device under development in MacFarlane’s lab is about the
size of a cell phone. The idea is that it could synthesize ammonia
for fertilizer on the scale of a farm or greenhouse, so the material
could be used right where it’s made, eliminating the need for
transport, MacFarlane says.
Meanwhile, other researchers are looking
to nature to understand how to efficiently reduce nitrogen to
ammonia. Some bacteria use large protein complexes called
nitrogenases to grab nitrogen out of the air and make ammonia.
Minteer and her team have been studying this system to connect these
bacterial enzymes to electrodes to create new electrocatalysts. But
they still have a long way to go, Minteer says. Their systems do
more proton reduction than ammonia production. The goal is to get to
the point where they’re making 99% ammonia and 1% hydrogen. Right
now, their systems make about 40% ammonia and 60% hydrogen, she
says.Besides this selectivity
issue, scientists also have to worry about.
How long these catalysts last,
MacFarlane says, and it’s something that many groups are not
thinking about yet. For a new ammonia production system to be
practical, such as in an electrochemical device like the one his
group is working on, catalysts will need to remain active and
viable for years, even if the system could be taken apart and
refurbished, he says. “Catalyst lifetime is a challenge that’s yet
to be clearly identified and understood.” Most people are not
publishing data on lifetimes, but the longest he’s seen is about a
day, he says.
The road to Haber-Bosch-free ammonia is
long, Minteer says. Whether it’s an electrocatalysis,
photocatalysis, or biocatalysis system, any promising lab-scale
reaction will still take at least a decade or two to make
commercial scale, she says.
Searching for alternatives to Haber-Bosch
is also risky, Manthiram says, because what scientists are
pursuing now may not pan out. But with ammonia production touching
so many things that we use every day, including our food and
pharmaceuticals, scientists need to find a way to make these
lab-scale systems work on larger scales, he says. “It’s hard to
imagine a world where we’re just going to be OK with the way that
we make ammonia today.”
Errors from the air: The trials and tribulations of developing
ammonia catalysts
When Shelley Minteer at the University of
Utah first got started studying how bacterial enzymes called
nitrogenases produce ammonia, she noticed something funny. Some days,
the complexes wouldn’t produce ammonia. On other days, they’d produce
a lot. The culprit? The cleaning lady.
“We would see spikes in production the
days she cleaned the floor,” Minteer says.
Nitrogen and ammonia are all around us.
Nitrogen makes up 78% of the air we breathe, and nitrogen-containing
molecules like ammonia are in numerous plastics, textiles—and cleaning
supplies. These molecules can stick to tubing, gloves, and glassware.
“It‘s very difficult, if not impossible, to get all of the
contaminants of ammonia out of all the samples,” Minteer says.
Contaminants also include other nitrogen-containing compounds, such as
nitrites and nitrates, which can easily react to make ammonia.
The field of new ammonia-producing
catalysts is still young, says Lauren Greenlee, a chemical engineer at
the University of Arkansas. “The catalysts just are not very
efficient.” Scientists make small amounts of a catalyst and then test
it in small-scale setups. “The problem is that the amounts of ammonia
that are actually produced by many electrocatalysts are not much
larger than what you might measure in the background.”
So how do you know if the ammonia you’re
measuring actually came from your catalyst instead of from
contaminants in the lab? Without proper controls, you don’t, Minteer
says. If part of that ammonia is coming from the background,
scientists might think that their catalyst is working well when it may
not be.
Currently, journals don’t require data on
specific control experiments to publish data from an ammonia-producing
catalyst. Whether the journals should require those controls is a
matter of debate in the community. “I’ve talked to some people who
have argued that we should wait and not do controls,” Greenlee says.
Maybe, these members of the field argue, the catalyst community will
move forward, and catalysts will get more efficient so that the
difference between what the catalyst is producing and the amount of
ambient ammonia will become larger.
While that may happen, that wait-and-see
approach has issues, Greenlee says. If a group reports a
high-performing catalyst, other researchers may start working with it,
thinking that it’s an improvement. “But what if that’s not the right
direction to go because the group didn’t measure their background
correctly?” Greenlee asks.
Greenlee thinks that researchers should
run controls and take background measurements for every catalyst on
every day they run experiments. Such controls would include running
experiments with isotopically labeled molecules as a final evaluation
of successful catalysts so scientists know where the nitrogen in
ammonia came from. Papers should also report the results from these
control experiments. “Even if a lab is doing appropriate controls,
it’s very hard to tell as a reviewer” because they’re not adequately
reported in the paper, she says.
“There are surely errors made in the history
of science,” says Karthish Manthiram, a chemical engineer at the
Massachusetts Institute of Technology. “As long as everyone admits to
their errors, we all move forward together.”
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509 995 1879 cell, Pacific.
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