Crude Oil is refined into many product streams - HFO is what remains.
Info Centre >> Fuel and Lube Oil Technical Manual >> 1. The Basic Building Blocks >> 1.3 Residual Fuel Oil (HFO)
The great majority of the fuel oil used by the world’s merchant fleet is residual fuel oil. This is also true for the vast majority of large diesel engines operated on land. By definition, residual fuel oils are the products remaining from the refinery processes after all the distillate or lighter fractions have been removed. These residues are complex mixtures which depend upon the source of crude oils processed and the complexity of the refinery.
In the early days of refining, fuel oil was derived from the residue of the atmospheric or vacuum distillation process. Generally the product entering the fuel oil market was of a consistent quality and there were few problems. As demand for distillate products increased, refiners introduced secondary refining processes which had an effect on the characteristics of fuel oil in the market place.
The variation of product demand in individual countries is extremely wide and these cannot be accommodated by crude oil selection alone because of the sheer volume required. Further, within any single geographical area, various crude oil sources are used and numerous refinery process configurations are employed. The result of this is that on a world-wide basis, fuel for the industrial and marine markets is subject to considerable variation in its properties. Whilst this has basically always been the case, the variations can be more pronounced than they have been in the past.
Some of the effects of the secondary refining processes are: