Carbon intensity and which fuel has the most carbon content?

Answer

Carbon Intensity (CI) is a lifecycle metric that measures the mass of greenhouse gases—expressed in grams of carbon dioxide equivalent (g CO2​eq)—emitted per unit of energy produced (MJ) or per unit of transport work executed (dwt⋅nautical mile).

Under MARPOL Annex VI and the IMO Lifecycle Assessment (LCA) Guidelines (Res. MEPC.391(81)), Carbon Intensity is evaluated across two distinct boundaries:

1. Well-to-Tank (WtT): Emissions generated from feedstock extraction, processing, refining, and transport to the ship.

2. Tank-to-Wake (TtW): Emissions generated directly on board during fuel combustion/utilization.

3. Well-to-Wake (WtW): Total lifecycle carbon intensity (WtW=WtT+TtW).

It is crucial to separate Carbon Content (the physical mass percentage of carbon atoms within the fuel molecule) from Carbon Intensity (the actual environmental impact on a lifecycle basis):

• Carbon Content (%C by weight): Determines the immediate CO2​ conversion factor (CF​) when burned.

• Carbon Intensity (g CO2​eq/MJ): Determines the overall emissions impact per unit of usable energy, taking into account heat content (Lower Heating Value - LHV) and upstream production emissions.