Emissions from ship exhaust gases can be categorized into emissions which are directly harmful to human beings on the one hand and to the environment on the other. The first category includes nitrogen oxide (NOX), sulphur oxide (SOX) and particle emissions (PM). Carbon dioxide (CO2) emissions relate to the combustion process and are environmentally harmful with regard to the man-made climate change.
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.
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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 (NOX%). 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 NOX 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")
- 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.
- Diesel cycle: LSHFO/MGO
SOx compliance is ensured by the low SOx content of the fuel. EGR/SCR equipment is required for Tier III compliance. SCR increases the CO2emissions.
- Diesel cycle: LNG
LNG is sulphur-free so there are no SOx emissions. The effort required to achieve Tier III compliance is lower than for oil fuel, but EGR/SCR equipment is still needed.
- 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.
- 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 NOX compliance can be achieved with internal means (dry low NOX 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.