Residual Fuel Oil has maximum sulfur content determined by the reactions that takes place as a result of major environmental concerns and it happens to be the heaviest among the range of different grades of fuels.[RFO] is the lower grade of fuel oil which contains the undistilled residue from atmospheric or vacuum distillation of crude oil and maybe be called Bunker Fuel oil ,or heavy fuel oil for it ability to power ocean-going ships and also known as No.6 oil.