Weighing The Pros and Cons of Hydrogen and Ammonia As Fuel Source

• The power source is a trade-off between space and various factors.
• It depends on the weight of the fuel and power generator and the total capacity of the ship.
• Powering the ship must be deducted from the freight earned for carrying cargo.
• HFO is the normal fuel of choice for ships because it is relatively cheaper.
• Methanol is a good substitute for HFO and ICE is preferred as the most efficient means of using fuel.
• Ammonia is twice as energy-rich as liquid hydrogen by weight.

According to an article published in ShipInsight and authored by Malcolm Latarche, when selecting the power source for a ship there is always a trade-off between the space and weight of the fuel and power generator and the total capacity of the ship.

Cost of operating a ship

Most ships are built for carrying cargo and since the expense of powering the ship must be deducted from the freight earned for carrying cargo, the cost of the former needs to be minimized and the revenue from the latter maximized.

Generally speaking for the heavy cargo lifters such as tankers and bulkers, this ratio is much more important than for container ships which normally carry lighter cargoes and have much more space above deck to carry cargo.

HFO – a normal choice of fuel

HFO has become the normal fuel of choice for ships because it is relatively cheaper than most of the alternatives, it is easy to handle and in terms of energy density, it is better than most of the alternatives. For sure, LNG has better energy when measured in weight terms, but it requires 50% more space than HFO for the same level of energy and needs to be stored at a temperature of minus 162°C.

In recent years new fuels have entered the mix available to shipping. Methanol and LPG are already being used in a small number of vessels. Methanol is claimed to be a good substitute for HFO, and it is true that it can be stored at ambient temperature and is in many respects similar to HFO being a liquid and not requiring any pressurization or refrigeration.

Internal Combustion Engine most preferred

Thus far all of the fuels used are those which are able to be burned either to create steam – as in LNG carriers with steam turbines – or more typically in an internal combustion engine (ICE) using either the Diesel or Otto cycles. The ICE is preferred as it is the most efficient means of using fuel even though little more than half of the thermal energy is converted into usable power for the ship. Except in a few experimental or prototype situations, all of the fuels also contain carbon.

While it would be possible to produce heat – and therefore power – by means other than combustion, for example, heat-producing chemical reactions such as oxidization, the means to do that are not developed on a scale suitable for ship power production or have other obstacles.

Weights and measures

Currently this leaves the only hydrogen as a suitable fuel for ships equipped with ICE power production. Hydrogen can be used directly in an engine but many consider that use of a hydrogen carrier such as ammonia holds more promise. Pure hydrogen stored at ambient temperature as gas would take up so much space that it is not considered feasible and therefore the idea is that liquid hydrogen would be the best alternative.

Liquid hydrogen has three times the energy of HFO by weight but even as a liquid is low in volume energy density requiring four to five times the volume of HFO with an equivalent energy capacity. As a liquid, hydrogen would either have to be refrigerated to minus 253°C or pressurized at around 700bar. Neither of these is exactly attractive propositions. Hydrogen also has other undesirable properties as contact with it can make metals brittle and as gas can diffuse through many materials.

Importance of pure hydrogen

Many of the problems inherent in hydrogen as a marine fuel could be overcome by using it not as pure hydrogen but as a component of a more acceptable chemical. Ammonia for example is comprised of one atom of nitrogen and three of hydrogen. Since it contains no carbon, it will not produce CO₂ when burned and is therefore considered one means of reaching the IMO’s decarbonization targets.

Ammonia – good or bad?

Ammonia is not an energy-rich choice having a lower energy density in both weight and volume terms than methanol. It is however twice as energy-rich as liquid hydrogen by weight. By weight, it has less than half the energy density of HFO and by volume just a third. It does present fewer storage issues than hydrogen as it can be stored at ambient temperature under a pressure of around 10bar or without pressure refrigerated to minus 34°C. Ammonia also has other undesirable properties. It is toxic even at relatively low levels so care is needed in containment systems and it could pose problems for crew and salvors in a damaged situation.

Ammonia has an attraction that it is readily available because of the large scale production of it for fertilizer use. That said, the production process is very energy-intensive and currently uses fossil fuels. Global production of ammonia under present conditions produces around 66% of the CO₂ produced by shipping and by that factor could not really be considered as a good environmental choice to replace HFO. Hydrogen production, transport, and storage is also highly energy-intensive and for both fuels, ultimate comparisons with current fossil fuels used in the marine will only be valid if the full environmental cost of all fuels is weighed for the whole production and use chain.

Less energy-intense

If a less energy-intense means of producing ammonia is developed – there are projects in this regard underway – the scenario changes. Ammonia has been used to power ICE but appears to work best under stable conditions that might not be achievable onboard a ship where demand is constantly changing. This could be overcome by optimizing the combustion process – something that Wärtsilä has announced it is researching – or addition of a battery into the loop for peak shaving.

If the combustion process can be properly managed, little if any greenhouse gases will be produced but the chemical formula of ammonia NH₃ highlights the presence of nitrogen. When combusted, the hydrogen in ammonia will combine with oxygen in the air to form water vapor but the nitrogen presence both from the fuel itself and as a major constituent of air will produce NOx which is itself a GHG.

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