Regulation 14 of MARPOL Annex VI places limits on the sulphur content of fuel as a way of defining SOx emissions and unlike Regulation 13 for NOx, specific emissions limits (grams SOx per kilowatt hour) are not given. In order to clarify that using fuels with an allowable sulphur content is not the only method of compliance, Regulation 4 of Annex VI confirms that an Administration can allow alternatives, including “any fitting, material, appliance or apparatus… if such… methods are at least as effective in terms of emissions reduction as that required by the Annex ” and as such the regulation is titled Equivalents. This means that both inside and outside of ECAs approved abatement technologies can be used to reduce SOx emissions to a level that would be produced by the sulphur-in-fuel limits.
In 2004, with the impending entry into force of MARPOL Annex VI, the development of Guidelines for Exhaust Gas Cleaning Systems was raised from a low to high priority by IMO and an initial version was adopted in 2005 – IMO Resolution MEPC.130(53). Marine Exhaust Gas Cleaning Systems typically use water to remove sulphur oxides and particulate matter from exhaust streams, however the engineering technology used by different manufacturers has varied considerably. The Guidelines for Exhaust Gas Cleaning Systems have therefore been performance rather than design-based from the outset and contain 2 methods of achieving compliance with Regulation 14. The methods can be summarised as:
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- ‘Scheme A’ – initial certification of exhaust gas cleaning performance, followed by periodic survey, with continuous monitoring of key system operating parameters and daily emission checks to confirm performance in service; and
- ‘Scheme B’ – performance confirmation by continuous monitoring of exhaust emissions using an approved system, which is also subject to periodic survey, with daily checks of key system operating parameters.
Under both schemes discharges of ‘washwater’ to sea must be monitored and importantly rather than monitoring the specific emissions rate of SO2 in g/kW h, the ratio of parts per million-sulphur dioxide to percentage-carbon dioxide (SO2 ppm/CO2 %) is allowed.
As practical experience has grown, the Guidelines for Exhaust Gas Cleaning Systems have been reviewed with a particular focus on washwater discharges. This enabled an updated version to be adopted in 2008 – IMO Resolution MEPC.170(57), which contained extensive revisions to improve the structure and logic of the document and washwater discharge criteria. It was agreed that the washwater criteria ” should be revised in the future as more data becomes available on the contents of the discharge and its effects, taking into account any advice given by GESAMP”, The Joint Group of Experts on Scientific Aspects of Marine Environmental Protection – an advisory body to the United Nations. It was also agreed later in 2008 that 170(57) should remain valid until the revised MARPOL Annex VI entered info force in July 2010.
In 2009, a third iteration of the Guidelines for Exhaust Gas Cleaning Systems, – IMO Resolution MEPC.184(59), was adopted and replaced 170(57) in July 2010. This reflected changes to Annex VI and included SO2/CO2 ratios relating to various levels of sulphur-in-fuel, as the requirement to determine a specific SOx emissions value in g/kW h was no longer required. It was once again agreed that the washwater discharge criteria should continue to be reviewed taking into account advice received from GESAMP.
The fifth and latest iteration is MEPC.340(77), which was adopted on 26 November 2021.
