Ammonia could be the answer to shipping's hydrogen challenge

Ammonia is the third-most transported chemical globally. Photo: Ben Ostrowsky
Ammonia is the third-most transported chemical globally. Photo: Ben Ostrowsky
Ammonia molecule. Image: Colin Behrens
Ammonia molecule. Image: Colin Behrens
Ammonia could make for a switch in fuel cell chemistry. Photo: GenCell Energy
Ammonia could make for a switch in fuel cell chemistry. Photo: GenCell Energy
Diagram of alkaline fuel cell.  GenCell Energy
Diagram of alkaline fuel cell. GenCell Energy

There are already plans on the table to create green, carbon-free hydrogen from renewables, so the maritime industry has begun its search for the next step, one which would allow H2 to be turned into a more easily storable, transportable form of fuel. Ammonia could be the answer, writes Stevie Knight.

Surprisingly, ammonia’s volumetric energy density is higher than liquefied hydrogen, says Grzegorz Pawelec of Hydrogen Europe: at 3.5kWh of energy per litre it takes up less space than LH2, working out at three times the volume of MGO. Admittedly, there’s a trade-off in energy lost to processing, but the benefits are arguably greater than the losses: ammonia poses a lower fire risk as it has a narrower flammability range and higher ignition temperature than hydrogen. Alongside this, it’s far, far easier to handle: it only requires 10 bar pressure or low-level refrigeration (below -33C) to keep it liquid.

Further, it’s already widely used in everything from fertilisers to plastics and pharmaceuticals, and as Gennadi Finkelshtain, CTO of GenCell Energy points out, “transcontinental pipelines and production of ammonia in quantities of more than 180m tonnes per annum” also mean there’s a lot of existing handling experience – and the common, hydrocarbon-derived variety is readily sourced.

However, utilising ammonia pushes the best-known Permeable Membrane (PEM) cells out of the development loop. These have benefitted from a great deal of automotive innovation and are fast on load uptake, but PEM cells are particularly susceptible to poisoning, demanding a completely pure hydrogen feed. “The problem is that even after cracking ammonia onboard to return the hydrogen, you have traces of it left in the H2,” says Pawelec, necessitating a purification unit. Beside the space, this extra process means that operating PEMFCs on ammonia becomes “energy prohibitive” says Finkelshtain.

An ammonia future therefore demands a fundamental change in approach, and the answer may be to switch the underpinning chemistry. Utilising an alkaline, liquid electrolyte rather than a solid acid medium has major advantages: ion transport – the main driver of the process – is more efficient and excess heat is likewise better shifted away from the electrodes. Further, anodic and cathodic reactions require lower activation energy, allowing the use of non-noble metal catalysts which in turn brings down costs.

Moreover, Finkelshtain explains that alkaline fuel cells (AFCs) have a big advantage: they are capable of handling the mixture of hydrogen, nitrogen and those trace amounts of ammonia that result from cracking the NH3, without requiring further purification procedures.

GenCell is therefore planning to offer an array of commercial, boxed solutions for off-grid and backup energy, including the remote telecoms and high voltage substation markets. Its A5 system, which includes a heat cracker and modestly-sized battery for peak shaving immediate electrical demand, is presently undergoing intensive trials at the company’s field test facility and is expected to demonstrate a capacity for operating across a wide range of weather conditions.

However, the most important point is that pairing it with a simple, low-pressure, liquefied ammonia cylinder gives you everything you need for an independent, off-grid energy source making it potentially interesting for future maritime development. While current offerings yield around 5kW per unit, Finkelshtain underlines that all it would take to scale it up and reduce the footprint for an onboard application is “the right partners”.

There are other fuel cell applications that could make use of ammonia: “Cruise ships can be like a floating town for a thousand or more people, with a power requirement reaching up to 50MW,” says Olivier Bucheli of SOLIDpower.

The solution here could well be solid oxide fuel cells (SOFC), which are robust enough to cope with less-than-pure hydrogen.

At 700°C these run hotter than most other varieties but don’t require an external cracker, the heat can be used to separate the ammonia into hydrogen and nitrogen either inside, or in a close-coupled area just outside the casing.

More, the excess thermal energy can be utilised by standard waste heat recovery systems, and it may assist transition to cleaner ships. “Installing fuel cells usually means you’re removing an engine, so you’re down on a heat source. But you can use the 250°C output to help generate hot water and steam for the boilers,” says Bucheli.

There are some operational points to consider that make this particular technology suitable for larger ships. Unlike the more common PEM cells which can respond in a few seconds, starting up an SOFC installation from cold takes time, around eight hours in all. However, it can ‘hot ramp’ up or down, he explains: “Once going, SOFC’s can move from 50% load to full load in only 15 minutes.” So, if there’s a fairly constant baseline draw, as there will be on a cruise ship, “you just let it run”.

While SOLIDpower is putting together a 60kW containerised unit, a more integrated solution has distinct advantages.

Fuel cells don’t need the same kind of space around them as combustion engines, so they can be stacked together... or separated. “As a result, you can move away from one or two large engine rooms to five or more different zones,” says Bucheli. Not only does this give the designers a chance to reclaim some of the void areas around the ship but clever positioning could cut down the distance between source and consumer. “There’s less copper and lower resistance, so there are fewer losses,” he explains, similar efficiencies likewise applying to heat transport.

BACKUP

Redundancy requirements alter the picture even further.

Firstly, a fuel cell’s behaviour is almost the inverse of a combustion engine, as it is better utilised at part loads – this can be as low as 25%. It’s only when a fuel cell reaches around 70% to 80% of its capacity that its efficiency really starts tailing away.

Therefore, a certain level of redundancy will result from the installation being tailored for these lower loads, leaving a lot of overhead room for greater demand: unlike gensets, most peaks probably won’t require switching on another unit.

Secondly, because they are modular in essence “you can lose one block out of five from a fuel cell stack, and the others will still work” Bucheli explains.

Still, he believes that the most advanced applications will, like GenCell’s offering, see a pairing of fuel cells and batteries to cover “the few minutes” left by the fairly slow FC response, rather than relying on gensets. It’s a sensible solution for a greening cruise industry: if there’s some notice given on a cold start, the energy storage can be sized to cover a relatively limited periodic draw.

There are considerations but according to a paper by Nick Ash and Tim Scarbrough of Ricardo Energy & Environment, the outlines for ammonia as a fuel already exist: they say that as bulk ammonia transport vessels (usually liquefied petroleum gas-carriers) are designed according to the requirements of the 2014 IGC Code, only “minor adjustments would be required to equip vessels to operate with ammonia as a fuel”.

However, while they write that, “existing safety principles and systems used throughout the global ammonia industry would also need to be deployed on ships such as gas detection systems and appropriate chemically resistant protective clothing”, others have pointed out it is toxic, so particular care will be necessary for isolating and venting both for maintenance and emergency access. Further, while industry is used to moving vast quantities – it’s the third-most transported chemical globally – bunkering events, though smaller in volume, will be more frequent and require a whole new layer of safety kit and procedures.

Spills into marine environment are also a concern, though the jury’s out about just how detrimental this would be as so much depends on the quantities released and the conditions it meets.

LNG... AND OTHERS

Others segments are also investigating the potential of SOFC cells as they can be used with other, less-than-pure gases. For example, LNG could also be put through the system, albeit via steam reforming and trapping of the released carbon.

The interest is definitely rising: there are already a couple of European and a handful of Korean ship builders “expressing an interest in running fuel cells from LNG” says Bucheli. This kind of system may lead to future green-tech development as it yields around 60% electrical efficiency: a little more than ammonia. Further, although fossil-derived LNG is presently dominant, a synthetic version may prove attractive as the infrastructure is already well developed along the main shipping routes.

There’s also methanol: this too can be created from hydrogen, combined with carbon dioxide, and again it can be run through an SOFC with some pre-treatment. So can diethyl ether (DMT).

In fact, it’s possible to see that in future, this technology may yield a certain level of fuel flexibility: “Methanol could be run alongside DMT, while LNG and LPG could pair up,” concludes Bucheli.

THE SLOW PATH FOR H2 ADOPTION

When it comes to a sensible way to utilise hydrogen, the big issue is not the lack of alternatives, but rather, the number of potential development paths.

It makes it more difficult for any one technology to gain traction. So, as Olivier Bucheli argues, “before we decide on the silver bullet... we should let the market develop further”.

There’s a lot to recommend taking it slowly, and some are looking toward using hydrogen or hydrogen derivatives in ‘old technology applications’: MAN, for example, is looking at tweaking existing dual-fuel engines to run on ammonia.

COMBUSTION

There are pragmatic reasons for this approach. Firstly, it yields a greater fuel flexibility, and as Joe Pratt of GGZEM explains, while smaller craft see a larger difference between putting hydrogen through standard fuel cell and burning it, the advantage is somewhat lower for larger, long distance vessels “just because the big, marine two-stroke engines are very efficient as they are”. However, Pratt points out, “there are more reasons behind technology choice than efficiency alone”, adding that as people are now actively searching for low or zero emission solutions, this element is steadily rising up the agenda.

Re-enter our old friend green ammonia, giving carbon-sensitive shipping yet another option. Pawelec outlines the sums that owners need to bear in mind: “Standard combustion of MGO equals high energy density, but low overall efficiency - up to 45% but as poor as 30% for low-power units. Ammonia through an internal combustion engine generally gives you efficiency comparable to MGO but lower energy density... by a factor of 2.8.” However you have to add NOx mitigation as the peak flame temperature is hot enough to fuse the nitrogen and oxygen out of the air in the cylinder.

On the other hand, ammonia run through an SOFC yields the same energy density as burning it, (again, 2.8 times lower than MGO) “but a high efficiency, around 60%”, adds Pawalec.

Simply put, you may need more fuel storage, but you get some way to clean running from burning green-sourced ammonia, though you can go all the way to zero emissions and get far more ‘bang-for-your-buck’ if you don’t actually bang it, but feed a fuel cell instead.

But even if green ammonia does gain adherents, the hydrogen story will have more than strand. That’s because even straightforward H2 doesn’t always require outsized tanks.

According to Pawelec, “working out what fuel space you need onboard is not a simple case of comparing energy densities, there are more factors to consider”.

Here, route becomes important. Hydrogen Europe’s recently-developed calculation tool was used to study the choices facing a fairly typical box feeder operation: the 141m Neuburg’s calls take it between Holland, Sweden, Poland, the UK and Spain. It currently utilises a Caterpillar 8,400 BHP (6,260kW) main engine, has 880 m3 HFO tanks, and consumption running to about 36 tonnes a day.

Replicating that with compressed hydrogen at 700 bar increases fuel volume five times, to about 4,700 m3, costing the ship 15% of its payload capacity. Even liquid H2 would result in a 7.2% loss.

But, “you have to ask whether all that fuel is actually needed onboard” says Pawelec. In fact, the vessel’s fuel tanks currently allow it approximately 25 days of sailing without bunkering. Therefore, there’s certainly room to play with the operation: “For example, reducing the amount of fuel to a level required for seven days of continuous sailing enables the use of compressed hydrogen for a loss of only 25 TEU, that’s 3% of the cargo capacity and just four containers for liquefied hydrogen, under 1% of the cargo.” It comes to even less if using ammonia.

While the fuel has to be available en route and the ship would consequently lose some operational flexibility, he believes the benefits, zero emissions and its tie-in to the market could well outweigh the issues.

If this sounds familiar, it should: “It’s exactly the same thing that’s happening in the case of LNG,” says Pawelec. “Although LNG has a lower energy density than MGO, the tank sizes don’t vary that much. What’s happening is the ship owners are deciding to sacrifice some of the excess autonomy in exchange for not having to reduce payload capacity.”

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