Why does burning coal and oil release sulfur dioxide?

Burning coal and oil releases sulfur dioxide because these fossil fuels naturally contain sulfur compounds. During the process of combustion, the sulfur present in the fuel reacts with oxygen to form sulfur dioxide ($S{O}_2$).123

Coal contains sulfur in various forms, including sulfide minerals like pyrite and marcasite, sulfate minerals such as gypsum, and organically bound sulfur. Similarly, crude oil contains sulfur as a nonhydrocarbon constituent, which can vary significantly depending on the source of the oil.45

Because sulfur dioxide is a major air pollutant that contributes to acid rain, industrial facilities often use flue gas desulfurization (FGD) systems to capture the gas before it is released into the atmosphere.612