Lower price of residual fuel oils typically attracts consumers to use them

Residual fuel oil is what is left after making everything else

Residual fuel oil is what is left after making everything else, i.e. gasoline, jet fuel, gas oil, lubricants and asphalt, hence the name: residue. Most of the residue left after crude oil distillation is pumped to the vacuum distillation or vacuum flasher unit, leaving an asphaltic type of residue left.

Residual fuel oils have always commanded lower prices than all other fuels

Residual fuel oils have always commanded lower prices than all other fuels for two reasons; one reason is physical, the other commercial. Moving and consuming residual fuel oil requires special equipment. Transportation via pipelines and storage must be done heated so the residual fuel does no solidify. Commercially, the residual fuel oil is a leftover from producing all other fuels, which makes the residual disposal an important commercial issue.

The lower price of residual fuel oils typically attracted consumers to use them

The traditional use for residual fuel oils was boiler fuel, the simplest possible way to burn fuels. The heated fuel is sprayed into a large boiler furnace where the constant fire vaporises the fuel droplets and consequently burn them. Eventually mechanical engineers developed the diesel engine to make use of this heavier, cheaper residual fuel oil.

In short: Residual fuel oil is lower priced compared to other fuels, boilers and diesel engines consume residual fuel oil.