Other than CO2 ships also produce a number of atmospheric emissions which are considered harmful
Other than CO2 ships also produce a number of atmospheric emissions which are considered harmful. They are products of combustion, and the most significant are sulphur oxides (SOx and nitrous oxides NOx), along with tiny waste particles known as particulates. All are now being limited by international, regional and national laws and will be subject to increasing severity of regulations over the next few years. Emission Control Areas, where these limits apply, might be expected to spread around the world in the future.
What practical means can be undertaken to reduce, or even prevent any of these emissions entering the atmosphere? In the case of sulphur, the quality of the fuel that is used can be changed to low sulphur oil or distillates, which will make a marked difference. Alternatively, the emissions can be scrubbed to clean them before they pass into the atmosphere. Or it might be feasible to use liquefied natural gas which burns cleanly, or even biofuels, although there are issues about the use of products which would otherwise have been available for food, and objections to the felling of tropical rainforests for the planting of oil palms.
Engine manufacturers have been working hard to address these problems, coincidentally producing very much more efficient marine engines which, by burning less fuel in the first place, go some way to solving emission problems. The problem of NOx can be reduced substantially by means of exhaust gas recirculation, a system which, rather than pumping the exhausts straight into the atmosphere cleans, cools and recirculates the gases back into the engine. This reduces the amount of NOx that is generated in the combustion chamber. Tests have confirmed that if just 20% of the exhaust gas is recirculated, there is a 50% reduction in the amount of NOx produced. Waste heat recovery, also reduces other emissions, and increases engine efficiency.
The big challenge is less to do with the design of new ships and new engines, but with the existing fleet, ships which were built according to all the regulations that obtained at the time, and with the expectation of a 20-25 year life. In the past, such as when fuel prices quadrupled in the 70s making steam turbine ships uneconomic, it was possible to re-engine, but this is obviously vastly expensive. It is however possible to “retrofit” exhaust gas cleaning systems to existing machinery, that will enable tighter emission controls to be complied with. And while ships built before certain dates may be allowed to operate to the term of their natural lives without modification, it is possible that pressure from the users of ships who might wish to be seen using “cleaner” and greener ships might encourage changes.