How can Watney get so much hydrogen in the air?

Question

This does not make sense to me.

Mark Watney needs water. He uses an iridium dish to decompose hydrazine into a gaseous mix of hydrogen and nitrogen (2:1 ratio) that he feeds into a hand made oven. Exact quote: "Soon I had short bursts of flame sputtering from the chimney." For some reason, hydrogen does not fully combust and contaminates the air. In the end there is seven times more the hydrogen in the air than oxygen (64% volume). It should have exploded when reached 4% concentration.

Also, there is a lot of nitrogen generated at the iridium dish, yet nitrogen is only at 22% in the end, even though part of hydrogen is utilized to produce water

One more thing, he produces hundreds of liters of water from CO2 (via regenerated O2), but it should have produced about 100kg of carbon - where did it go to? How does his oxidizer work? I haven't read beyond this point so I wonder if he had an idea to feed that carbon to his plants.

Answer 1

You have confused "explosive mixture" with "exploding mixture". The explosive mixture range for a hydrogen/air mixture is 4% to 75%. However, just because a substance is explosive does not mean that it will spontaneously explode. In the case of hydrogen, a spark or flame is required, and presumably Watney is producing neither. Apparently, high-pressure hydrogen jets can self-ignite, but that does not apply. Likewise, there are reports that hydrogen-air mixtures can self-ignite in the presence of a catalyst such as platinum, but that does not seem appropriate either.

The autoignition temperature for hydrogen is 500 C, so as long as Watney does not provide an open flame or produce sparks, and has no local temperatures in excess of 500 C, he is theoretically safe.

That said, I'd be very surprised if there were not spark sources such as ventilation motors in his habitat.

Answer 2

Just because the hydrogen has the capability to combust, does not mean it will do so without provocation.

Hydrogen needs a spark or an open flame to be ignited, unless it reaches 500 C, which, as Watney was still alive, we can assume it didn't.

The hydrogen that was in the air seperated from the group before reaching the flame provided by Watney, and there was a constant flow of O2 there preventing the Hydrogen from combusting on it.

The reason that the hydrogen content is so low is because of the Atmospheric Regulator, a wonderful piece of technology that has the capability to freeze separate the air in the Hab.

The regulator uses freezeseparation to sort out the gasses. When it decides there’s too much oxygen, it starts collecting air in a tank and cooling it to 90 kelvin. That makes the oxygen turn to liquid, but leaves the nitrogen (condensation point: 77K) still gaseous. Then it stores the O2 - Mark Watney - The Martian .

So this is why the Hydrogen content was so high whilst the O2 and N2 concentrations remained within normality. Note that he could not do this with the hydrogen as it's freeze temperature is much too low.

The way he turned the CO2 into pure O2 is a bit more complicated, however.

The AR (Atmospheric Regulator) stores the CO2 into tanks, which then go to the Oxygenator, a fancy device which takes CO2, liberates the O2, and vents the remainder (carbon) outside.