I’ve long been a hydrogen fuel-cell skeptic. But after laying out my case against “the fuel of the future” in a recent drive comparison of the Toyota Mirai and Hyundai Nexo, a reader from Down Under pointed me to recent research by Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) that promises to address many of my concerns regarding hydrogen distribution by making hydrogen from ammonia at the pump.
These days hydrogen is typically shipped in ready-to-use liquid or gas form, but the liquifying process consumes 30 percent of its eventual energy content, and further losses occur from inevitable boil-off in transit. What the Aussies are proposing is to transport it as liquid ammonia and then convert it to hydrogen at the point of sale. Fun fact: The density of hydrogen in liquid ammonia is about 45 percent greater than in pure liquid hydrogen(!).
Oz has access to far more renewable energy than it knows what to do with. It’s the globe’s most solar-energy-rich country, receiving between 7-8 kW-hr/square meter of solar irradiation per day across the entire continent; there’s also abundant ocean tide energy and plenty of wind, as well. So the government is keen to export that green energy, and easily transportable liquid ammonia produced without generating any CO2 looks like a great way to do so.
Most ammonia produced today involves hydrocarbon feedstocks and hence produces CO2. CSIRO proposes producing hydrogen by electrolyzing water and combining it with nitrogen separated from air. These gases are then compressed and fed into the same Haber–Bosch synthesis reactor used for hydrocarbon-based ammonia production (which involves iron-based catalysts, temperatures of 750-930 degrees, and pressures of 2,200-3,600 psi). Total energy input is roughly 10-12 kW-hr/kilogram of ammonia—all of it clean.
There are no ocean-going hydrogen tanker ships, but ammonia is routinely shipped by sea. Now CSIRO, in conjunction with Fortescue Metals Group, has developed a novel two-step process to convert ammonia into pure hydrogen gas.
The chemical reaction starts off in a ruthenium catalyst that cracks the ammonia into hydrogen and nitrogen at about 840 degrees. Then it flows through a second set of tubes involving a vanadium membrane, through which H2 separates from N2 at 640 degrees. The hydrogen then gets bubbled through a water bath to remove any remaining ammonia gas, which is toxic to a proton-exchange membrane fuel cell. The proof-of-concept prototype has produced 99.99-percent pure hydrogen. A commercial-scale version is currently under development.
The ammonia cracking process obviously requires energy, and it also involves losses—some ammonia boils off or fails to crack, and some hydrogen escapes, as well, plus it must then be pressurized for dispensing into a vehicle.
The American Chemical Society conducted research into the “round-trip efficiency,” or net useful propulsion energy available at the wheels of the fuel-cell vehicle divided by the total energy required to produce, transport, compress, and dispense the hydrogen. It found the process efficiencies to be as follows: ammonia production: 58.8 percent; ammonia cracker/separator: 75.9 percent; hydrogen compression/dispensing: 88.0 percent; fuel-cell vehicle: 48.0 percent. Overall RTE: 19 percent.
Research by the California Fuel Cell Partnership pegs California hydrogen production/distribution efficiency at 65 percent, with vehicle efficiency averages ranging from 36 to 44 percent, for RTE of 23 to 29 percent. But remember that in the Aussie analysis all the hydrogen/ammonia production energy was renewable and carbon-free. Only about a third of California’s hydrogen currently comes from carbon-free renewable sources.
I just feel better knowing that, in this scenario, every hydrogen atom can be happily married to a nitrogen right up until it’s ready to board my FCEV, shortly after which it will be reunited with its true love, oxygen. These happier hydrogens are almost never plotting an escape, and they seem way greener than the ones we’re driving around using as fuel nowadays.
Read more stories from Frank Markus here:
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