Under MARPOL Annex VI, the IMO has so far regulated NOx and SOx and also through the EEDI rules some effort to control CO2 is also in hand. All of the regulations around these components of engine exhausts have been devised independent of each other.
In meeting the targets for one, there has often been a detrimental effect upon another, notably the increase in CO2 caused by meeting NOx targets. Operating a scrubber to remove SOx will also increase CO2 output because of the pumps needed for the wash water.
However, scrubbers have a secondary function that could address one of the expected future needs under MARPOL Annex VI. So far particulate matter (PM) and black carbon have not come under any regulatory requirements, but they are included in the list of substances to be regulated.
PM especially the material around 2.5microns known as PM2.5 have a negative effect on human health. Marine engines have been blamed for producing enough PM2.5 to cause the premature deaths of tens of thousands of humans each year. What is not often mentioned at the same time is that the marine engine exhaust is just one source of PM2.5.
A United Nations Economic Commission for Europe (UNECE) report entitled DIESEL ENGINES EXHAUSTS: MYTHS AND REALITIES produced in 2013, shows that within the EU, only 2% of PM2.5 emissions are attributable to ‘Other Transport’ which would include rail, sea and air. Around 16% came from road transport, energy production and distribution amounted to 13%, Industrial processes accounted for 11% and at 52% by far the biggest contributor was that labelled household, commercial and institutional.
Nevertheless, shipping will almost certainly be under pressure to reduce its output of PM in the not too distant future. Most of the blame for PMs from maritime activity is attributed to diesel engines and to the burning of heavy fuel in particular. It is true that HFO does produce more particulates than other fuel types especially LNG which is considered the cleanest, but the situation regarding comparison with other fuels is not so clear cut.
In the 2009 scientific paper by Hulda Winnes and Erik Fridell Particle Emissions from Ships: Dependence on Fuel Type, published in the Journal of the Air & Waste Management Association, the authors describe the results of some field trials.
In the tests, measurements were carried out on the 4,500kW four-stroke main engine on-board a product tanker. Both HFO and MGO were tested on the same engine at comparable load settings. All components of exhaust gas were test and while total PM emissions were generally higher for HFO than for MGO; for the smallest size-fraction measured the opposite was observed. This finding emphasises that to minimise negative health effects of particles from ships, further regulation may be needed to reduce small-sized particles but a fuel shift to low sulphur fuel alone does not seem to accomplish this reduction.
When a scrubber is installed and in operation on a ship, the scrubbing action reduces not only SOx production but also captures most of the particulates of all sizes along with other contaminants.
According to some proponents of scrubbers that is a strong argument in their favour and installation of a system could not only allow compliance with the 2020 sulphur cap but also be a possible means of meeting any future regulation on particulates.