Ship emissions in the future: Review

This review covers the ship exhaust emissions already regulated (NOx and SOx), those anticipated to be regulated in the future (PM, PN, BC, methane) and some less common emission species (formaldehyde, NO2, N2O, NH3). Furthermore, technologies to reduce emissions are discussed.
LNG powered ships have shown reduction of SOx and PM emissions of almost 100% and NOx emissions about 90% compared to marine fuel oils. Methanol as marine fuel has slightly lower emission reduction potential than LNG. Exhaust treatment with SOx scrubber removes SOx emissions almost completely, while SCR is efficient for reduction of NOx. SOx scrubber, SCR and DOC indicated potential to remove also e.g. PM emission to some extent. Combination of distillate fuel, SCR and particulate filter could reduce almost 100% of SOx and PM emissions, and more than 90% of NOx emissions. However, particulate filters are not in commercial use for marine diesel engines using marine fuels, yet.
When approaching climate-neutral shipping, renewable fuels (methane, methanol, distillate-type, hydrogen of renewable origin) and batteries are potential options. LNG has also GHG mitigation potential, however, compensated to some extent by methane emissions.
There are several technologies capable to meet tightening emission regulations set for the ship exhaust emissions, although many technologies have their limitations.