Bunker fuel is not a waste product left over from the refining process of crude oil.
Refineries produce very little waste.
Virtually all of the crude that goes into the refinery comes out as a saleable product. The source material for bunker fuel is the residue left over from the distilling stages of the refining process. This residue is processed further to generate valuable products, including material used to blend marine fuels. Upon arrival at a refinery, crude oil is put into settling tanks to separate out the water and salt that all oil contains. Left untreated, the salt would cause severe corrosion problems in the refinery, and could poison expensive catalysts.