Answer
When an examiner asks this, clarify whether they mean absolute carbon mass fraction (CF) according to IMO regulatory factors or physical carbon chain complexity.
IMO Carbon Factor (CF) Perspective
Under MARPOL Annex VI (and the IMO LCA Guidelines), the carbon factor CF is expressed in g CO2/g Fuel. Interestingly, lighter distillate fuels have a slightly higher pure carbon mass fraction than residual oils because residuals contain significant amounts of sulfur, ash, heavy metals, and nitrogen, which displace pure carbon weight:
| Marine Fuel Type | IMO CF Factor (g CO2/g Fuel) |
|---|---|
| Marine Gas Oil / Diesel Oil (MGO/MDO) | 3.206 (Highest absolute carbon ratio) |
| Light Fuel Oil (LFO) | 3.151 |
| Heavy Fuel Oil (HFO) | 3.114 |
| LPG (Butane) | 3.030 |
| LPG (Propane) | 3.000 |
| LNG (Methane) | 2.750 (Lowest among fossil fuels) |
Takeaway for Oral Panel: By pure mass fraction (CF), MGO/Diesel has the highest carbon-to-mass ratio. However, from a lifecycle or heavy hydrocarbon perspective, HFO contains long-chain complex hydrocarbons with higher dense aromatic rings, releasing more particulates and harmful emissions per unit of energy.