Increasing amounts of ships exhaust gases emitted worldwide forced the International Maritime Organization to issue some restricted maritime legislation for reducing the adverse environmental impacts arising from such emissions.
Consequently, ships emission reduction became one of the technical and economical challenges that facing the ships’ operators.
Different strategies that can be used to reduce those emissions, especially nitrogen oxides and sulphur oxides have come up recently. The strategies included: applying reduction technologies onboard, using of alternative fuels, and follows one of fuel saving strategies.
Using of selective catalytic reduction and sea water scrubbing appeared as the best reduction technologies onboard ships. Moreover, among the various proposed alternative fuels, natural gas, in its liquid state; has the priority to be used instead of conventional fuels.
Applying one of those strategies is the matter of ship type and working area. The proposed methods have been investigated at a high-speed craft operating in the oceans and the results were satisfactory from the point of view of environment and economic issues, and reflected the importance of applying those strategies.
Technical measures to cut air pollution from ships significantly are implementable and outweigh the costs involved. These measures include the adoption of cleaner fuels, adding closed-loop ‘scrubbers’ or other exhaust gas cleaning devices to ships (for SOx), SCR systems (for NOx), slow steaming, and wider use of alternative sources of energy including wind power and port-side electricity.
Using low-sulphur fuels: this is the easiest way of reducing air pollutants from ships. Shipping fuels in use outside sulphur emission control areas contain up to 3,500 times the sulphur content of fuels used by road transport in Europe. Low-sulphur fuels can make the ship’s engine run smoother and more efficiently with less operating problems and maintenance costs. Last but not least, using low-sulphur (non-residual) fuels reduces other pollutant emissions, such as black carbon which is a potent global-warming agent.
Scrubbers: an alternative compliance option to burning low-sulphur fuels approved by the IMO and the EU is for ships to install scrubbers. These could cut emissions of SO2 by 99% and considerably reduce emissions of other polluting particles. There are, however, concerns regarding wash-water discharges from open-loop scrubbers which deposit them in open seas and closed-water areas. This leads to higher pH levels in surrounding waters causing additional environmental concerns. Hence, open-loop scrubbers are not a sustainable alternative compliance method for marine sulphur standards.
Internal engine modifications, such as water injection and exhaust gas recirculation (EGR – for 4-stroke engines): these are techniques to prevent NOx production during the combustion process. However, Tier III standard cannot be met by these methods alone.
Selective catalytic reduction (SCR): a system to treat exhaust gases after their production but before they are actually emitted. SCR is very effective in reducing NOx emissions far beyond Tier III. It’s already used by many ships worldwide and works better with low-sulphur fuels.
Gas or duel-fuel engines: ship engines can work with liquefied natural gas (LNG) which doesn’t contain sulphur and therefore has SO2 emissions close to zero. Gas engines also dramatically reduce other PM and BC emissions. Although it’s easier to fit new ships with such engines, a few conversions have already taken place. LNG can also reduce NOx emissions; it has been shown to reach Tier III levels, hence providing a good solution for ship air pollution.
Air pollution from ships continues to increase as the sector grows. Land-based emissions – SOx and NOx – on the other hand, particularly from fixed installations, have been reduced dramatically at great cost. NOx from shipping is set to exceed NOx from all EU land-based sources in the coming decade.
Through chemical reactions in the air, SO2 and NOx is converted into fine particles, sulphate and nitrate aerosols. In addition to the particles directly emitted by ships such as black carbon, these secondary particles increase the health impacts of shipping pollution.
Implementing the global 0.5% sulphur standard for marine fuels from 2020, as adopted by the IMO in 2008 and reconfirmed in 2016, is expected to save 40,000 lives a year globally from lung cancer and cardio vascular deceases (lives saved exclude child asthma and morbidity).
Sea News Feature, July 12