Residual fuel oil is the residual oil of other petroleum products. Residual fuel oil must be free from tendency to corrosion, acid, solid particles and other bond solid materials that may clog or damage the burner. Residual fuel oil also must be free from chemicals that can form flux and crust on the wall boiler furnace.

Some residual fuel oil contains vanadium and sodium in extremely small quantities. This material can result in corrosion when used as fuel for steam boiler or gas turbine. This problem can be solved by using additive magnesium, calcium, zinc, and others. The heating value of residual fuel oil is ranging from 6.260.000 to 6.450.000 Btu/bb.

The viscosity of residual fuel oil is the most important thing to be considered because of the difficulty in handling and atomizing of thick / viscous oil. The advantages of usage residual fuel oil compared with coal are as follows:
1. The weight of residual fuel oil 20% lighter and its volume 50% smaller with same heating value.
2. Destruction does not occur in storage.
3. Free from sudden fire.
4. Residual fuel oil can be distributed to combustion furnace so without using man power.
5. Residual fuel oil has higher combustion efficiency and does not produce smoke.
6. Residual fuel oil does not generate dust or ash which is harmless to surrounding places.
7. Reducing the cost of cleaning combustion furnace.
8. Excess water is needed to perform smaller combustion.
9. Residual fuel oil with high sulfur levels when used in the steam boiler will tend to be corrosive. Therefore the temperature of flue gas from combustion process that passes through air heater and economizer must be sufficiently high so as not to corrode materials. Residual fuel oil with sulfur content of 6% requires temperature 60oF higher than residual fuel oil with sulfur content of 2%.