I don’t think ammonium hydroxyfluoride exists in the first place :). Do
you mean hydrofluoride – NH4HF2? I doubt it can work as fuel. It’s
non-flammable, and although it can decompose to gases at high
temperature (expanding gas can power something), the costs would
probably be enormous and the products are extremely corrosive and toxic.
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Chemical engineers do it in packed beds.