If shipping is to decarbonise within the next few decades, technology will have to be developed at a rate probably faster than at any previous time. With a ship’s lifespan reckoned to be between twenty and thirty years depending upon type, only around 4% of the world fleet in numerical terms is new ships at any given time.
Currently the only ships being built that are completely carbon-free are the very small number of purely battery powered ships. These are mostly ferries or similar specialist craft and will have no perceptible impact on reducing carbon emissions from shipping as a whole.
Until the IMO adopted its ambitious zero emission plans, LNG was being heavily promoted as the best alternative to oil fuels because of its lower emissions of NOx, SOx and CO2 but as many have pointed out since April 2018, LNG can only ever be a transitional fuel to a carbon-free shipping future. None of the other fuels being touted as an alternative – methanol, ethane, propane and butane are carbon free and neither are biofuels even if officially policy is to say that they are.
In the future, fuel cells running on liquid hydrogen do promise a carbon free route but despite numerous projects to develop them, progress so far has been agonisingly slow. A few prototypes have been fitted but aside from some very small passenger craft there are no commercially viable ships sailing on fuel cells alone.
Until quite recently there had been a lull in activity around fuel cells for marine purposes but there has been a reawakening of interest with the passenger sector at the fore. Royal Caribbean is the first to commit with fuel technology being used on the Icon Class ships, the first of which is due to begin construction this year. Even so the fuel cell on the ships are not planned to be the primary power source which will be LNG-fuelled engines.
Because fuel cells would seem to be the only practical alternative to internal combustion engines, nobody seriously expects the diesel engine to be usurped from its position of dominance for many years yet. Assuming that ten years is necessary for fuel cell technology to reach a point where it could be considered as the sole power source for a ship, and another ten years or so for production capacity to be ramped up, we are looking at 2040 before fuel cells enter the mainstream.
Conceivably then the carbon free future will depend upon fuels for diesel engines that contain no carbon. It is possible to burn liquid hydrogen in an internal combustion engine but the storage system involves lower cryogenic temperature even than for LNG. More to the point, hydrogen has the ability to dissipate through metals and causing metals to become brittle – not ideal for engines which are primarily made from metals.
There is one fuel – ammonia – that is liquid at normal temperature and which contains no carbon. Ammonia which has the chemical formula NH3 which would suggest that the exhaust gases from its combustion will be water vapour and NOx although if the ignition temperature can ne kept low enough, NOx will not form and nitrogen will be emitted instead.
There are engines which can and do run on ammonia and there is research being undertaken by engine makers with an interest in marine. In 2008, Caterpillar filed a patent (US Patent US20100019506A1) which described an ammonia fuelled engine and ancillary systems. In the patent description, Caterpillar specifically mentioned the search for zero carbon fuels, but it also described some of the problems relating to using ammonia as a fuel.
Caterpillar said the characteristics of ammonia fuel, such as zero CO2 emission, relatively high energy density, well-established production infrastructure, and competitive cost, have made ammonia an attractive alternative fuel for combustion engines.
On the negative side, when ammonia is combusted, the combustion produces a flame with a relatively low propagation speed. In other words, the combustion rate of ammonia is low. This low combustion rate of ammonia causes combustion to be inconsistent under low engine load and/or high engine speed operating conditions. Most existing combustion engines that use ammonia as engine fuel typically require a combustion promoter (i.e., a second fuel such as gasoline, hydrogen, diesel, etc.) for ignition, operation at low engine loads and/or high engine speed. However, the requirement for the combustion promoter fuel fluctuates with varying engine loads and engine speed, which can cause control issues. Furthermore, the use of dual fuels generally requires dual fuel storage systems, dual delivery systems, and dual injection systems, thus adding additional weight, complexity, and cost to the engine system. To eliminate the use of combustion promoter fuel, combustion engines that burn ammonia alone as engine fuel have been explored.
Another engine maker with an interest in ammonia is MAN Energy Solutions which has been conducting research in co-operation with Alfa Laval with particular regard to LPG fuel conditioning. In April Alfa Laval issued a press release describing the testing of a MAN B&W ME-LGIP engine running on propane and the Alfa Laval Fuel Conditioning Module (FCM). The set up was also evaluated for use with ammonia. Apparently, propane has some of the same problems as ammonia when used in a two-stroke engine. A full technical paper about the Alfa Laval FCM LPG and the results achieved at the MAN test facility will be presented at the CIMAC Conference 2019, taking place in Vancouver, Canada, 10–14 June.
There are also projects and research involving producing sufficient quantities of Ammonia for it to be able to be used on a large scale as a fuel for marine and shore situations. Some of these involve manufacturing ammonia by combining hydrogen produced by electrolysis of water using surplus renewable energy with nitrogen from the air. Whether this is economically viable remains to be determined.
Producing ammonia is one aspect but solving the problem of poor combustion under changeable load conditions and engine speed has still to be tackled. One idea that could overcome this may be worthy of consideration. Hybrid ships using batteries are now accepted technology. It would surely not be beyond the abilities of engineers to devise a system whereby the ammonia powered engine runs as a genset at optimal conditions as regards load and speed eliminating the combustion problems. The electrical power produced could be used to charge batteries which would in turn power electric propulsion motors without problems associated with changing load demands.
The scenario described may be somewhat naïve in trusting that ammonia can be produced cheaply using renewable energy but it is ideas such as this that need to be fully investigated and proved viable or not if the IMO’s ambitions are to be achieved. It also requires considerable investment by engine makers and others with no guarantee that the concepts will be taken up by ship operators or even permitted by regulators, highlighting another obstacle in turning ambition into reality.