CO₂

Greenhouse gas (GHG) emissions are measured as CO₂-equivalent emissions. Of all relevant fossil fuels, LNG produces the lowest CO₂ emissions. However, the release of unburned methane (so-called methane slip) could reduce its benefit over HFO and MGO, because methane (CH₄) has 25 to 30 times the GHG effect compared to CO₂. Nevertheless, engine manufacturers claim that the Tank-to-Propeller CO₂-equivalent emissions of Otto-cycle dual-fuel (DF) and pure gas engines are 10 to 20% below the emissions of oil-fuelled engines. Diesel-cycle gas DF engines have very low methane slip. This is also the case for the COGES system as proposed by the PERFECt Ship concept.

To fulfil the ambitions of the IMO Greenhouse Gas Strategy, shipping will need to introduce carbon-neutral fuels in the long run. These fuels can be produced from hydrogen and carbon dioxide. The hydrogen will be produced from water electrolysis, where the electrical energy comes from renewable sources. The price of these PtL and PtG fuels will be much higher compared to today’s ship fuel price.

If the shipping sector resorts to synthetic fuels produced from hydrogen and CO₂ using renewable energy, the available alternatives can be liquefied methane (which is very similar to LNG) and diesel-like fuels.
>> Download a copy of the PERFECt ship report

NO_x

The value for HFO-fuelled Tier II diesel engines is used as a baseline (100%). The values are only comparable when assuming the same rotational speed. The bars on the right-hand side of the diagram represent the potential emissions reduction through switching from Tier II to Tier III (NO_X%). It is obvious that for all fuels given in the below figure, diesel-cycle engines must be equipped with exhaust gas treatment systems to comply with the IMO Tier III limits. Only Otto-cycle engines burning LNG or hydrogen have the potential to remain within the Tier III limits without requiring exhaust gas treatment. This means that in most cases a switch of fuel is not sufficient to comply with the Tier III NO_X limits.

Overall emission behaviour

Ship propulsion concepts differ in their principal emission behaviour. This is illustrated in the figure to the right of the diesel-cycle and Otto-cycle engines as well as the gas steam turbine concept as applied in the PERFECt Ship project.(for explanation of Diesel- and Otto-Cycle comp. Annex 1 in white paper "Alternative fuels and technologies for greener shipping")

1. Diesel cycle: HFO
   The IMO rules can be fulfilled when applying additional technical means, but at the cost of added fuel consumption and increased CO₂ emissions caused by the scrubber and exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) equipment.
   
   
2. Diesel cycle: LSHFO/MGO
   SO_x compliance is ensured by the low SO_x content of the fuel. EGR/SCR equipment is required for Tier III compliance. SCR increases the CO₂emissions.
   
   
3. Diesel cycle: LNG
   LNG is sulphur-free so there are no SO_x emissions. The effort required to achieve Tier III compliance is lower than for oil fuel, but EGR/SCR equipment is still needed.
   
   
4. Otto cycle: LNG
   Otto-cycle medium and low-speed engines (pure gas and DF engines, comp. also Annex in white paper "Alternative fuels and technologies for greener shipping) can meet Tier III requirements without additional exhaust gas treatment. Methane slip compromises the benefit in terms of CO₂ reduction, so the maximum 28 to 30 per cent improvement cannot be achieved. Engine manufacturers indicate potential CO₂ reduction values of 10 to 20 per cent over similar oil-fuelled engines.
   
   
5. The COGES project is illustrated for comparison. It should be noted that it can only achieve efficiency improvements and a CO₂ emission reduction similar to piston engines if the power demand is high enough (30 to 35 MW as an approximate lower limit). If this condition is met, Tier III NO_X compliance can be achieved with internal means (dry low NO_X burner) when operating on oil or gas. Methane slip does not occur. All things considered, the emissions of COGES systems as proposed in the PERFECt Ship project meet all foreseeable IMO limits. No external exhaust gas cleaning is needed.