When coal is burned in air, the primary gaseous product formed is carbon dioxide (CO2). Carbon dioxide is released as a byproduct of the combustion reaction between carbon (found in coal) and oxygen (present in the air).

The overall chemical reaction for the combustion of coal can be represented as follows:

```

C(coal) + O2(air) -> CO2(gas) + H2O(water vapor) + Other byproducts

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During coal combustion, other combustion products such as sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), water vapor (H2O), and ash particles are also produced. However, carbon dioxide is the main gaseous product that contributes significantly to greenhouse gas emissions and climate change.