It doesn’t exist. Really, truly, Mr. Dickens, it does not happen. At all. Ever. No matter what this Coroner in Galway says.
Sheesh. What happens in “Spontaneous Human Combustion”–three things are necessary. 1. the human needs to be drunk or drugged. 2. the person was probably smoking, or had a candle or some form of open flame. 3. The person was fattish, with clothing that can act as a wick. When fat gets hot, it melts. When human fat gets hot and runs onto absorbent fabric, what you have is a wick. Put the wick on a stuporous individual, and let it smoulder, eventually the person will go away and then go out.
I wouldn’t make a cunning plan on it happening, but remember that very fat people have set crematoria on fire (grease fires). It’s not so good a story as spontaneous combustion, but since the human body is 70% water, it’s much more likely.