Expert: George Maxwell - 12/5/2007

Question
QUESTION: If i were to use combustion in a sealed environment by inserting a hydrogen/oxygen mixture would there be any need for an exhaust? Would any excess fumes or gasses form/need to escape?

ANSWER: Hi, and thank you for your question.

The answer, generally, is yes: the physical force produced by the hydrogen/oxygen explosion is a result of the very hot gases (steam and CO2) produced. These forces are very great, and without sufficient exhausing, your container will tear or shatter. In deonstrating this reaction to students in a soda bottle, I've occasionally had the bottles explode even though they were open - the "exhaust" at the top wasn't big enough to cope with the volume of gases producd.

You could, of course, try a non-explosive mixture of Hydrogen and Oxygen (e.g. with little oxygen), or (more normlly), you could introduce a continuous stream of both gases in to the container and burn them at a controlled rate. You'd still need an exhaust, though, or the produced CO2 / Steam will smother the flame.

The only Hydrogen-based fuel system that I know of where you wouln't need an exhaust is the fuel cell. Here, the gases (Hydrogen and Oxygen) are passed over a catalyst that causes them to react to form water. The reaction is elctrochemical (e.g. as in a battery) rather than purely chemical (e.g. as in burning a fuel), so the gases don't burn / explode. The only "exhaust" you need is to remove the water produced.

Hop this helps!

---------- FOLLOW-UP ----------

QUESTION: Thank you for your answer it was most helpful. If CO2 and steam forms that must mean that the water and oxygen components still remain separate. As far as I was aware 'fire' was the result of fusion of oxygen to the fuel provided. In the hydrogen scenario where does the carbon bonding come from, as in fossil fuels I assumed that the carbon element was a result of the bondage between the fossil fuel and oxygen.

Answer
Thank you for the follow up.

You're quite right that, in a "pure" eviornment, Hydrogen and Oxygen will only produce steam, and in any case, steam is the majority product.

The small amounts of CO2 (and other decomposition gases) ar a result of the fact that the H2 /02 Flame is so hot - the container itself is partally oxidised by the heat in the manner you describe. Going back to the soda bottle example, I often find the plastic has not only melted, but also charred after the explosion.

Sorry, should have made that clearer before -my apologies.