As shipping is urged to decarbonise, the search for alternate fuels for ships has stepped up a pace. Fossil fuels have great advantages over most of the available alternatives in that they are relatively cheap and safe in use if the undesirable pollution is taken out of the equation and the technology for using them is mature although improvements are constantly being made to improve performance.
As an alternative to oil fuels, LNG has the advantage of producing no SOx and lower CO2 and NOx emissions, but it is not by any means the clean fuels for ships that some are demanding that shipping should be using. It is hydrogen that is being promoted as the fuel for the future on ships but there are far more problems to overcome before it is likely to be accepted as its proponents would hope.
Fuels for ships – Hydrogen
Hydrogen for ships can be used in two ways to produce power; in fuel cells or burnt in an internal combustion engine. There are several types of fuel cell but most work along the same principle of combining hydrogen with oxygen to produce electricity and water. In effect it is the reverse of electrolysis whereby hydrogen and oxygen are produced by passing a current through an electrolyte such as brine. In an internal combustion engine, hydrogen can be burned in the presence of air just as oil or LNG is but in this case it will produce NOx as one part of the exhaust gas stream.
Although neither method of using hydrogen produces CO2 it should not be forgotten that the most common greenhouse gas is water vapour. In fact some scientific studies claim that 95% of all GHGs is water vapour thus hydrogen as a fuel for ships may be less of a problem but is not without some undesirable effects. It is also important to remember that hydrogen not freely available without a great deal of energy being used to produce it.
Hydrogen has a very high energy content by mass, but it is a very light gas with 1Kg occupying 5.4M3 at STP. Thus a large amount of storage space will be needed for gaseous hydrogen. Liquid hydrogen is better but requires extremely low temperatures (-253°C) and pressures. This is much more challenging even than LNG. The density of liquid hydrogen is only 70.99 g/L (at 20 K), a relative density of just 0.07. Although the specific energy of liquid hydrogen is around twice that of other fuels, this gives it a remarkably low volumetric energy density.
As a fuel for ships hydrogen also has other undesirable properties. The hydrogen atom is very small and can penetrate into almost all materials including steel and aluminium. When this happens, the affected material can become brittle and prone to damage. Since hydrogen has a very low flash point and is highly explosive/flammable, the method of storage on a ship which is subject to many stresses is something that will pose problems far more than on land.
There is a great deal of momentum behind the future use of hydrogen and it is very likely that it will be used at some point. However, to consider it as the best method to decarbonise shipping in a very short space of time is probably premature. Its first uses will doubtless be in small craft and ferries where some initial breakthroughs have already been made.