Coals contains different percentages of Sulphur. So, you must analyze a sample at laborautary...
When you fired it, Sulphur reacts and burn with oxygen, and compose sulphurdioxide and sulphurtrioxide. (SO2 & SO3)
Generally the %95-97 of sulphur composes to SO2 (Sulphur DiOxide) and the small part composes to SO3 (Sulphur TriOxide)
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