Thanks for the info. So if it's largely a refining problem, is anything lost if bunker fuel is simply outlawed? (Besides lower cost anyway?) Is bunker fuel an inescapable result/byproduct of petroleum refining, or can it be refined further into less-polluting, more desirable fuels?
It's complicated... crude oil is a mix of hydrocarbons with carbon chain lengths between say 5 and 40+, plus various impurities such as sulfur. It is refined into gasoline (~7-12 carbon atoms per molecule), diesel (~12-18) etc, with bunker fuel containing the heaviest fractions. It is possible to crack the heavy fractions into lighter ones, and this is commonly done at most refineries.
However, what to do with the impurities? You can take them out, but then disposal becomes a problem (see image below for example). So, if possible, refiners tend to leave them in the product.
