SOx (sulphur oxides) limitations (Regulation 14):

• Global sulphur cap: the sulphur content of fuel oil used on board ships must not exceed 0.50% m/m (from 1 January 2020), reduced from the earlier 3.50% cap.
• In Emission Control Areas (ECAs - Baltic, North Sea, North American, US Caribbean): the sulphur content must not exceed 0.10% m/m (from 1 January 2015).
• Compliance can be achieved by using compliant low-sulphur fuel or by an approved equivalent method (e.g. an exhaust gas cleaning system/scrubber that achieves an equivalent SOx emission reduction).
• The fuel oil must be documented (bunker delivery note, the sulphur content) and the ship must have the designated fuel oil sampling points; the fuel oil quality and the sulphur content are verified by sampling/testing.

NOx limitations (Regulation 13):

• NOx emission limits (g/kWh) are set by engine speed and Tier (Tier I, II, III) as described in the detailed answers (Tier I from 2000, Tier II from 2011, Tier III within NOx ECAs from 2016/2021). The limits are: for n<130 rpm: Tier I 17.0, Tier II 14.4, Tier III 3.4; for n>=2000 rpm: Tier I 9.8, Tier II 7.7, Tier III 2.0; interpolated between.
• Engines must be certified under the NOx Technical Code (EIAPP) and carry the NOx Technical File; Tier III applies within the NOx Emission Control Areas (North American, US Caribbean, Baltic, North Sea).

SOx reduction:

• Use of compliant low-sulphur fuel (0.50% global / 0.10% ECA) - the primary measure.
• Exhaust gas cleaning systems (scrubbers - open/closed/hybrid) that remove SOx from the exhaust, allowing the use of higher-sulphur fuel while meeting the equivalent SOx emission.
• Fuel switching/management (change to low-sulphur fuel before entering an ECA, with the required changeover time and record).
• Operational: optimise fuel consumption (less fuel = less SOx), and use of shore power in port.

NOx reduction:

• Engine design/optimisation: retarded injection timing, optimised combustion, fuel injection rate shaping, and the use of the engine's certified settings.
• Selective Catalytic Reduction (SCR): injecting urea/ammonia into the exhaust to convert NOx to nitrogen and water - the most effective method to meet Tier III.
• Exhaust Gas Recirculation (EGR): recirculating a portion of the exhaust to reduce combustion temperature and NOx.
• Water-in-fuel emulsification / water injection: adding water to the fuel or combustion to lower the flame temperature and NOx.
• Operational: operate the engine within its certified parameters, maintain the emission control devices, and use the appropriate fuel/engine settings; in ECAs, ensure the Tier III method (e.g. SCR) is operational and the consumables (urea) are available.