The coal spontaneously combusts on contact.
Please watch the video first, then explain why immersing carbon fiber over-wrapped helium tanks into tanks of liquid oxygen in the presence of additional sources of energy (e.g. expansion, contraction, friction, vibration) is not likely to catch fire, and is likely to be safe and reliable for human-rated spaceflight.
edit: I'll also reproduce part of the block quote from this answer where the idea of putting a combustible material in direct contact with LOX is roundly balked-at:
Finally, for completeness, page 146 of Clark's book contains a rather disturbing mention of LOX + liquid methane as a proposed monopropellant:
"If Tannenbaum's mixtures were bad, that proposed at a monopropellant conference in October 1957 by an optimist from Air Products, Inc., was enough to raise the hair on the head of anybody in the propellant business. He suggested that a mixture of liquid oxygen and liquid methane would be an extra high-energy monopropellant, and had even worked out the phase diagrams of the system.* How he avoided suicide (the first rule in handling liquid oxygen is that you never, never let it come in contact with a potential fuel) is an interesting question, particularly as JPL later demonstrated that you could make the mixture detonate merely by shining a bright light on it. Nevertheless, ten years later I read an article seriously proposing an oxygen-methane monopropellant! Apparently junior engineers are allergic to the history of their own business."
I think we can see why this one never took off.
“When you think about it boys”, he said brokenly, “that’s what holds us together more than anything else, except maybe gravity. We few, we happy few, we band of brothers - joined in the serious business of keeping our food, shelter, clothing and loved ones from combining with oxygen.”
--Kurt Vonnegut, God Bless You, Mr. Rosewater (1965)