For the common liquid fuels, co, emissions are largely related to the efficiency of the engine and ship. They can be reduced by about 20%if natural gas is used as fuel mainly because of the chemical composition. Ro-Ro marine gas engines in service have demonstrated this, together with a reduction in NOx of about 95% without further exhaust clean up and an absence of SOx and particulates. Ro-Ro aero-derivative gas turbines for marine propulsion can also burn natural gas very efficiently with even less undurned hydrocarbons.