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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