By-products of burning Natural Gas

Why does burning the gas create humidity? where does the humidity come from, water in the air? and where does it go, does it float up above the flame? is there any way to trap it?

Not exactly the byproducts, but simply the products of the reaction, on a chemical point of view. "natural gas" is methane CH4, and what you call "burning" is a combustion, it means : a reaction with the dioxygen O2 in the air when the mixture of air and methane is heated (with an electrical spark typically or with the flame of a match). The equation of this reaction is the following :

CH4 + 2 O2 -> CO2 + 2 H2O
As in every chemical reaction, there is the same number of atoms of each type (C, O and H) on both sides. So your question "where the water comes from" has no sense for modern chemists who know that water is H2O, because the answer would be that the H atoms of the water come from the natural gas and the oxygen atom from the dioxygen of the air. Note also that some oxygen from the air is consumed during the reaction : that's why it is dangerous to burn anything in a room that is not ventilated.

At the temperature of the flame, water is no longer a liquid but a gas (that we call steam). So it will mix with the other gases of the air and the CO2 from the reaction and most of it will be lost... but if you approach a cold object of a flame when burning methane, you may see some water condensing on the surface !

On the distinction between "products" and "byproducts" : we call "byproducts" the products of any secondary reaction, different of what we want to obtain. In the case of the combustion of methane (or any hydrocarbon like coal, petrol, etc...), we want the combustion to be "complete", which means that every atoms of the starting hydrocarbon are as much oxidised as possible. But in some bad conditions (not enough O2, too low temperature, impurities in the hydrocarbon...) the combustion will not be complete. In this case the byproducts of the burning of natural gas may be carbon C (graphite), a black solid which will cause fumes, or in the worse case carbon monoxide CO, a very toxic gas which doesn't smell anything but will stick on your hemoglobin and kill you in a short time.

Trapping the CO2 from burning fossil fuels cost effectively is the real challenge. Water vapour sorts itself out naturally.

Originally published at:  http://www.gatago.com/sci/chem/