Is ammonia (NH3) to key to making hydrogen a useful transportation fuel and a replacement fuel in fossil-fired power plants? Hydrogen is also ideal for use in electrochemical fuel cells. Ammonia and hydrogen, both together and separate, have been getting a lot of buzz lately.
The international bank HSBC recently predicted that ammonia produced from natural gas will overcome a chief problem with hydrogen as a transport fuel: the difficulty of moving the fundamental element from where it is produced to where it is used. HSBC said, “A key challenge in developing the hydrogen economy comes from the high cost of transportation, which could be as much as three times the cost of production, Methods of storing hydrogen for transportation include conversion to methanol, in metal hydrides, and in pressurized or liquefied form.”
HSBC said, “Ammonia is, in our view, the most likely vehicle of choice for hydrogen transportation because of its high storage density. Ammonia is easier to liquefy — it liquefies at minus 33 degrees Celsius — and contains 1.7 times more hydrogen per cubic meter than liquefied hydrogen.”
Ammonia can be burned in power plants, either along with natural gas, or directly. Power Magazine in November reported that Japan’s JERA, a joint venture of TEPCO and Chubu Electric, plans to ramp down its 2.2 GW supercritical coal plants by 2030, converting them eventually to ammonia and hydrogen fuel in ultrasupercritical configurations.
From an environmental standpoint, a problem with ammonia is that conventional, large-scale production requires natural gas. Ammonia is among the most widely produced industrial chemicals in the world. The conventional production involves steam reforming of natural gas to produce hydrogen. Then the hydrogen is combined with nitrogen, common in the air, through the Haber-Bosch process, using a metal catalyst under high temperature and pressure.
Separating the hydrogen from the nitrogen atoms, reverse engineering ammonia, is tricky. As a Department of Energy white paper several years ago noted, looking at the possibility of on-board conversion of ammonia to hydrogen for transportation uses, cited “high operating temperature (>500° C); longevity and reliability of catalysts and other components (at high temperatures and in the presence of impurities); start-up time (to get the system up to operating temperature); purification requirements (to prevent ammonia poisoning of fuel cells); complexity of the overall system; energy efficiency (on-board ammonia would have to be burned in the cracking process); cost (currently ~$100K for 1-3 g H2/s stationary units); and reactor weight and volume (commercial units with sufficient throughput currently weigh about 2000-5000 kg and are about 3000-6000 liters in size).”
As a result, the 2015 DOE concluded that it “does not plan to fund R&D to improve ammonia fuel processing technologies for use on board light weight vehicles at the present time.”
Recently, several studies have suggested ways to make “green” ammonia. An engineering study from the University of New South Wales in Sydney, Australia, reported in Science Daily, claims it has found “a way to make ‘green’ ammonia from air, water and renewable electricity that does not require the high temperatures, high pressure and huge infrastructure currently needed to produce this essential compound.” The researchers have demonstrated the concept at the laboratory scale, saying it “has to potential to play a role in the global transition towards a hydrogen economy, where ammonia is increasingly seen as a solution to the problem of storing and transporting hydrogen energy.”
About the same time, Science Daily reported, researchers at Northwestern University in Illinois said they have also found a way to produce “green” method of converting ammonia into hydrogen, the other end of the ammonia to hydrogen promise.
Northwestern lead author Sossina Haile said, “The bane for hydrogen fuel cells has been the lack of delivery infrastructure. It’s difficult and expensive to transport hydrogen, but an extensive ammonia delivery system already exists. There are pipelines for it. We deliver lots of ammonia all over the world for fertilizer. If you give us ammonia, the electrochemical systems we developed can convert that ammonia to fuel-cell-ready, clean hydrogen on-site at any scale.”
Haile’s team have a process that produces hydrogen from ammonia at 250 degrees C, compared to around 500 C for conventional ammonia-to-hydrogen technology, using an electrochemical cell with a proton exchange membrane and a catalyst the splits the ammonia into nitrogen and hydrogen. The hydrogen is converted into protons, which then are driven across the membrane.
A caution about over-optimism. As folks who followed the synfuels issue in the 1980s will recall, promising bench-scale research often fails to scale up, due to science, engineering, and economics.
–Kennedy Maize
kenmaize@gmail.com