Acetylene-Hydrogen Mixes

Acetylene’s main safety concern – and it’s a big one – is its bad habit of self-decomposition, explosively, via either of these reactions:

2 C2H2 —> H2 + 2 C, T = 1,200 degrees C

2 C2H2 —> CH4 + 3 C, T = 300 degrees C

Thus extreme care is taken to stop this decomposition process from being triggered. Raising the gas pressure to ~ 2 atmospheres is enough to trigger it in some situations, so acetylene gas supply has to be metered and kept below this acetylene partial pressure at STP. Even a valve closing and causing a shockwave can make it go “bang”. Acetylene carrying pipes have been constructed will some very special safety features to avoid self-decomposition detonations.

However Russian researchers have found that adding other gases – surprisingly one of which is hydrogen – will reduce the decomposition risk. Add enough hydrogen and acetylene won’t self-decompose.

Oddly enough it can still react with the hydrogen via the following (for example):

C2H2 + 8 H2 —> 2 CH4 + 5 H2 + energy (resulting gas temp ~1,140 K)

But starting that reaction needs a triggering energy input that’s higher than the mere nudge that decomposition of acetylene requires.

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