The IMO Regulations governing the emissions from ships have focused on limiting the sulphur content of the fuel oil utilized by the vessel.
However, the regulations do allow for alternatives.
Increasingly, the use of Exhaust Gas Cleaning Systems (EGCS) or “Scrubbers”.
Scrubbers allow the vessel to burn heavy fuel oil (no limit on sulphur content).  The emissions are then ’scrubbed’ before being released into the environment removing the harmful gases.

Attached (and link provided) is a LAMA Circular detailing IMO guidelines for the installation, certification, and use of Scrubbers.

LAMA Circular 01_2019 Sulphur Emissions and Scrubbers.pdf