Which process is essential for coal combustion to produce sulfur dioxide?

In the combustion of coal, the formation of sulfur dioxide is primarily a result of the oxidation of sulfur compounds present in coal. During the combustion process, when coal is burned at high temperatures, the sulfur in the coal reacts with oxygen from the air. This oxidation reaction transforms sulfur into sulfur dioxide (SO₂), a significant pollutant that contributes to atmospheric issues such as acid rain.

The other processes mentioned do not play a direct role in the production of sulfur dioxide during coal combustion. For example, while carbon does undergo oxidation to carbon dioxide, this process is related to energy production and not to sulfur dioxide formation. The decomposition of water is not relevant to the combustion of coal, nor does it contribute to sulfur dioxide production. Lastly, the combination of nitrogen with oxygen primarily leads to the formation of nitrogen oxides, which is a different set of reactions altogether. Thus, the correct answer emphasizes the critical role of the oxidation of sulfur compounds in the generation of sulfur dioxide during coal combustion.