With emissions regulation focusing on exhaust components such as SOx, NOx and CO2, the only way to easily achieve compliance without any additional machinery needed on board I to use a fuel which does not contain the chemicals that produce those components during combustion.
The oxygen and nitrogen that are present in those undesirable emissions mostly come from the air that supports combustion and so cannot easily be eliminated. Air is comprised of approximately 78% nitrogen and 21% oxygen with the remaining 1% being mostly Argon and minor trace elements and compounds. Mineral fuels such as oil and LNG contain varying amounts of hydrogen and carbon which are the energy sources of those fuels. There are other trace elements but sulphur which can run as 4% or more in HFO (MARPOL rules currently allow a maximum of 3.5%) is the biggest problem.
Sulphur is found in most fuels when extracted but the refining process removes most of it from all but HFO. The IMO’s decision to reduce the permitted level in fuels used outside of ECAs to just 0.5% in 2020 has more or less resolved the problem of SOx from shipping in theory although only events after 2020 will show how effective the cut will be.
Because distillates, LNG and some other less common fuels contain virtually no sulphur, they are being considered as the main alternatives to HFO from 2020. Even before the IMO adopted the 2020 date for removing sulphur from fuels, LNG had been promoted as a cleaner alternative to HFO because of its low NOx emissions.
There has definitely been a growing trend towards LNG that accelerated after the 2020 decision was made in 2016 but its usefulness for meeting the SOx issue has been countered by the IMO’s decarbonisation of shipping ambitions. Even environmental organisations are now questioning the long-term use of LNG in shipping.
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 fuel that some are demanding that shipping should be using. Its formula – CH4 – highlights that most of the energy from LNG will come from the carbon atoms but it still contains one atom of carbon. Similarly, related gases such as ethane, butane and propane have more hydrogen atoms than carbon but all contain even more carbon relative to the hydrogen than LNG.
The related alcohol fuels – methanol, ethanol and so on, have relatively less carbon in their composition because of the hydroxy group. Methanol has also attracted attention as a potential marine fuel because it is liquid and has less practical issues than LNG which must be stored under cryogenic conditions.
Methanol has in fact already found a use as a marine fuel in a number of ships being built with MAN ME-LGI engines. At MSC 99 in May this year, the IMO recognised that methanol may well feature in the future fuel mix and invited the ISO to develop a standard for methyl/ethyl alcohol as a marine fuel and a standard for methyl/ethyl alcohol fuel couplings.
As shipping is urged to decarbonise, the search for alternate fuels 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.
All of the fuels such as LNG and methanol will undoubtedly have a role to play but It is hydrogen that is being promoted as the fuel for the future. Hydrogen is extremely common but not as a naturally occurring element so it must be produced by processes involving chemical compounds. As regards its use as a marine fuel there are far more problems to overcome before it is likely to be accepted as its proponents would hope.
Hydrogen has obstacles to overcome
Hydrogen 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 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 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.
The first is the CMB Hydroville a small 14m shuttle ferry operating in Belgium. The vessel is powered by two H2ICED engines which are standard diesel engines modified to run on hydrogen. The engines have been in use in road vehicles for more than a decade and are similar to those used in the BMW Hydrogen 7 concept car of which 100 were built in 2006 for BMW executives. Another small ferry is being built under the Golden Gate Zero Emission Marine Project. The vessel which will operate in San Francisco Bay will be powered by a fuel cell.