The International Maritime Organization rule, IMO 2020 established strict regulations on marine vessel sulfur emissions, there are really two common ways to comply with regulation. Either limit the sulfur content in the fuel being used or add scrubbing units to the exhaust system to capture the excess sulfur oxide being expelled from the vessel’s exhaust.
For vessels not using scrubbing units on the exhaust systems, they must utilize a fuel 500ppm (parts per million) sulfur content or less, which is a considerable jump from the old “bunker fuel” previously used, which could contain up to 2000ppm sulfur.