Fuels and Lubricants

The chemistry of decarbonisation

Malcolm Latarche
Malcolm Latarche

26 June 2019

The chemistry of decarbonisation

Rudolf Diesel intended the engines that bear his name to run on a wide variety of fuels and in fact his first two engines were fuelled by coal dust but were unsuccessful and Diesel turned instead to oil. Less than 20 years after Diesel patented his engine, the first ships powered by them were brought into service. The fuel used then was a variant of modern MDO with heavier oils coming later.

Modern shipowners enjoy a choice of fuels that their predecessors could only dream of with the available fuel mix today including all the standard types of mineral oils such as HFO, MDO and MGO together with vegetable and animal-based bio-fuels, LNG, ethane, methanol and to a lesser extent battery power. Hydrogen is tipped as the next fuel type – although maybe not in an internal combustion engine – and ammonia is also being touted as a future fuel.

Most of the newer fuel types available have come about as a result of emission regulation. LNG would likely not have taken off for ship types other than LNG carriers were it not for the Norwegian NOx tax, even if today it is seen as being a solution to SOx and CO2 regulation as well as NOx. In advance of the 2020 cap on sulphur content being reduced from its current 3.5% to 0.5%, the latest trend by the oil majors is towards developing a new range of oil fuels with ultra-low sulphur content. The adoption at MEPC 72 of an ambitious strategy that would aim for a 50% reduction in so-called greenhouse gases (GHG) from shipping by 2050 followed by a complete decarbonisation policy thereafter would, on the face of it, see fuel oils displaced in favour of supposedly cleaner fuels such as LNG and methanol although that view has held sway for almost a decade and has not yet proved to be correct.

In many minds, the term ‘greenhouse gases’ refers almost exclusively to CO2 but the IPCC (the UN body overseeing the debate on climate change) lists a great many more. The IPCC lists water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) as the primary greenhouse gases in the Earth’s atmosphere and the IMO itself in its latest GHG strategy recognises that methane slip from LNG-fuelled power systems is a problem that needs to be dealt with.

The inclusion of water vapour as a GHG may come as a surprise to most but many studies recognise it as the most common of the GHG and highly potent if short-lived. In the era of fossil fuels the attention has been focussed on CO2, but if there were to be a big switch to fuel cells and hydrogen in internal combustion engines then it is possible that something as innocuous as water vapour will be a matter requiring attention. Burning one tonne of a typical fuel oil produces around 3.5 tonnes of CO2 but burning the same amount of hydrogen would produce 18 tonnes of water or water vapour.

The common thread that connects all of the fuels used by or touted for ship use is that they contain the easily combustible elements of carbon and hydrogen in varying ratios. Oil fuels usually have twice as many hydrogen atoms as they have carbon atoms but because the atomic weights of the elements are so different an oil fuel is typically 82% by mass of carbon and 12% by mass of hydrogen – the remainder being other elements and compounds.

LNG as pure methane has one carbon atom and four hydrogen atoms. Ethane and propane have a higher carbon content than methane but are still less carbon rich than oil.

Hydrogen and ammonia both contain no carbon. Hydrogen is of course an element so contains only hydrogen and a single molecule of ammonia has one nitrogen atom and three hydrogen atoms.

Hydrogen and ammonia can both be used in internal combustion engines but there are difficulties associated with their use in this way.