In shipping’s search for carbon-free fuels, the answer may be blowing in the wind
What’s the story?
MAN Energy Solutions has announced it will spend the next two to three years developing a two-stroke engine – more accurately, modifying its existing dual-fuel concept – that burns ammonia as fuel.
How did it happen?
Windfarm developer Siemens Gamesa approached MAN when it began considering producing ammonia at its windfarms. It wanted to make sure it had a consumer. Discussions about Siemens’ role are continuing, but MAN will go ahead.
What is the potential?
Ammonia, which is used as a fertiliser, cleaning agent and refrigerant, is shipped in industrial quantities by LPG carriers. MAN already has an engine that enables these ships to burn LPG: its ME-LGIP two-stroke, which was launched in September. Developing that engine to burn ammonia would bring more fuel flexibility to the LPG carrier fleet. The engine could eventually be used by other ship types, given a clean and abundant supply of ammonia. A project slated for April investigating the fuel also includes a container-ship owner.
Why use ammonia as fuel?
It’s clean, with not a carbon atom in sight, the only significant emission is NOx. But MAN notes that it is easy to clean exhaust gases using ammonia as a catalyst in a selective catalytic reduction unit, leaving just nitrogen and water vapour.
"Ammonia does not need to be compressed and is liquid at normal temperatures under low pressures – another reason why it could be a good shipping fuel"
But doesn’t ammonia come from natural gas?
Today, mostly. Methane-rich natural gas from coal, oil and gas operations is converted to hydrogen, by steam reforming, and then to ammonia by the Haber-Bosch process. So although ammonia burns cleanly at the funnel, there is a carbon footprint.
That is about to change. Renewable electricity can power an alternative method of hydrogen creation, using electrolysis to extract hydrogen from water. Combine this with nitrogen extracted from the air and you have truly clean ammonia with no carbon footprint.
You need hydrogen before you get ammonia? Why not just use that?
Hydrogen is a potential clean fuel for shortsea and coastal shipping applications, particularly as an energy source for fuel cells. But Siemens has calculated that it is more efficient to convert hydrogen into ammonia than to either liquefy or compress hydrogen after it is produced. Ammonia does not need to be compressed and is liquid at normal temperatures under relatively low pressures – another reason why it could be a good shipping fuel.
What needs to change for MAN’s engine to burn ammonia?
Not much. A fuel gas system would supply ammonia at around 20 bar. Injection will need to be considered, but is likely to be around 600 bar, which is the pressure used for injecting LPG today.
Kyushu University in Japan is already studying combustion and heat transfer characteristics. These will enable MAN to programme software controlling fuel injection, as well as informing any optimisation of nozzle design.
Another obstacle is the use of a toxic substance as a fuel, which is currently prohibited under the IGC Code governing the bulk carriage of low-flashpoint fuels. MAN will work with class to make sure it can burn ammonia on LPG carriers.
What is the potential supply of clean ammonia?
Siemens and MAN calculate that every 10 MW of installed wind turbine power could also generate 1 MW of ship propulsion. Total offshore wind capacity reached 18.8 GW by 2018, according to the Global Wind Energy Council, and is set to grow six-fold by 2027. That could be enough for ammonia to become a significant part of shipping’s fuel mix as it seeks to cut its greenhouse gas emissions.