Fuel oil is derived from crude petroleum and is the most widely used liquid fuel for power generation. Distillate and residual fuel oils are the two major categories of this type of fuel.

Fuel oils are classified by grade numbers, with No. 1 and No. 2 being distillate fuel oils and No. 5 and No. 6 being residual (heavy) fuel oils (such as are preheated and fired in power plants). The Mexican oil, called “combustóleo” has similar characteristics to residual fuel oil No.6 [46–48].

Distillate fuel oils contain less than 0.3 % sulfur (by weight), are more volatile and less viscous than residual oils and have negligible nitrogen and ash contents.

Distillate fuel oils are commonly used in domestic and small commercial applications and include kerosene and diesel. On the other hand, residual fuel oils are highly viscous and may need heating for easy handling and proper use in combustion.

Residual fuel oils contain important amounts of ash, sulfur and nitrogen and are mainly used in industrial and large commercial applications.

Due to differences in composition and combustion characteristics, distillate and residual oils result in different emissions upon combustion. For example, particulate matter emissions from distillate oils are lower than from residual fuel oils.

On the other hand, No. 6 fuel oil generally has a higher sulfur content, and because the emissions of sulfur oxides are directly related to the sulfur content of the oil, emissions from No. 6 fuel oil are more polluting than those from distillate oils.

Other pollutants generated during combustion of fuel oils are nitrogen oxides, carbon monoxide, greenhouse gases, volatile compounds (like unburned hydrocarbons) and toxic trace metals.