Sulphur oxides (SOx) are mainly formed from combustion of fuel containing sulphur. We currently see a trend towards reduced SOx emissions from land-based sources, and recent international regulations aim to also reduce the SOx emissions from shipping accordingly by putting limits on the fuel sulphur content.
Requirements
The revised Annex VI to MARPOL was adopted by IMO on 10 October 2008. The sulphur oxide (SOx) emissions from ships will in general be controlled by setting a limit for the sulphur content of marine fuel oils. Within specified Emission Control Areas (ECAs), the sulphur limit will be even stricter. The 2015 ECA sulphur limit will have severe implications for ship owners and charterers as well as for entire supply chains. In addition, local limits are enforced in certain areas such as in EU ports and California, and new local rules may emerge.
Based on a review of existing marine engine technology and expected technology development, ship owners currently mainly have three choices if they wish to continue sailing in ECAs from 2015: switch to low sulphur fuel, install an exhaust gas scrubber, or switch to liquefied natural gas (LNG) as fuel.
Marine Gas Oil (MGO) and Marine Diesel Oil (MDO) can be supplied with a sulphur content of below 0.10%. Switching to such fuels only requires minor modifications to the fuel system on board the ships. On the negative side, the availability of low sulphur fuel is already limited and rising demand is expected to increase this fuel's price uncertainty.
An exhaust gas scrubber can be installed to remove sulphur from the engine exhaust gas using sea water and/or chemicals. Dry scrubbers are also available. Scrubbers are generally bulky and require significant alterations on board: additional tanks, pipes, pumps and a wash water treatment system. The sulphur-rich sludge produced is categorised as special waste, to be disposed of at dedicated shore facilities. Moreover, scrubbers increase the power consumption, thereby increasing total CO2 emissions. Scrubbers can be retrofitted to ensure ECA compliance for existing ships although there is still some uncertainty about the consequences of scaling up such installations for large diesel engines, e.g. in the range of 50 MW and upwards.
The third alternative is to use LNG as fuel. Natural gas is the cleanest fossil fuel available and, when fuelling a ship with LNG, no additional abatement measures are required in order to meet the ECA SOx requirements. However, an LNG-fuelled ship requires purpose-built or modified engines and a specially designed system including gas tanks, a vaporiser and double-insulated piping with gas sensors and shut-down arrangements. Available space for cylindrical LNG fuel tanks on board ships has been a key challenge, but new prismatic tanks are expected to simplify this issue. DNV Class rules are in place to ensure safe LNG fuel operations.
| Solution | Pros | Cons |
| Scrubber | Can use cheaper, high sulphur fuel Fuel available | Takes up space Significant investment cost No significant reduction of NOx Requires additional energy during operation Discharge of water |
| LNG | Currently cheaper fuel, but future price development is uncertain | Retrofit difficult |
| Distilate fuel | No or little modifications and investment needed | Higher fuel cost Prices likely to increase Fuel availability uncertain Wear and tear |
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