how much water forms during combustion ?

When you burn fossil fuels or hydrocarbon fuels, the exothermic combustion reaction of Hydrogen in the fuel and Oxygen in the air produces water. This water goes out as vapour along with the flue gas. In coal firing, moisture in the coal augments the water quantity.

The quantity of water produced is much more than the steam cycle make-up quantity in fossil fuel power plants. If one can economically capture this water and use it for steam cycle make up and other utility applications in the power plant, it can greatly
reduce the pressure on regional water balances. This will be a boon for low rainfall regions and desert economies.

How Much Water?
The combustion of hydrogen, the exothermic reaction, produces heat and water. The water quantity produced is almost nine times the weight of hydrogen.

Bituminous coal contains around 3 % hydrogen which along with a 12 % moisture in coal produces almost 0.388 kilogram of water per kilogram of coal. A 500 MW plant consuming around 300 tons of coal per hour produces 116 tons of water per hour. Even with a fifty percent yield, a flue gas water recovery system can meet the steam cycle makeup water requirement.