You don't even have to put a naked flame nearby. Coal Dust spontaneously ignites at room temperature. So Urist McMiner goes on break, comes back to a burning Coal Seam.
...new modding project?
Hmmm, well...
If you wanted to represent coal mining accidents with reasonable accuracy, you'd need to represent the various gasses, and the chances of both worked coal and the as yet unworked seam itself igniting.
The gasses are Firedamp (methane & other natural gas), Blackdamp (CO2 & Nitrogen), Whitedamp (CO), Stinkdamp (H2S, Hydrogen Sulphide, rotten egg small) and Afterdamp (essentially Black & White damps mixed).
Of those, 3 (Fire, White & Stink) Explode. They will all (in high enough concentration) cause Oxygen Deprivation, asphyxiation & death. Most of them will also react with anything else you take nearby - including water.
I honestly don't know if it's possible to set light to unworked mineral blocks in DF (i.e. bits that are still terrain), but worked coal will already burn. In addition, mining out coal causes Coal Dust, which also explodes. Coal Dust explosions can be prevented by keeping the area wet - I have no idea how you would go about representing that.
Coal Mining accidents are deadly because if just one thing goes wrong, you tend to get a disaster dominoes effect happen. Firedamp used to be a big killer not because it created big explosions - it didn't - but because it could set off the Coal Dust, as the shockwave took it airborne & the heat dried it rapidly. The Coal Dust explosion was much bigger, which then set light to the seam, which would then ignite further pockets of gas.
Also, Coal Deposits that are near Iron Deposits are
bad news. Swing a pick at a Coal face, hit a tiny inclusion of Iron Ore, get a spark with even a tiny amount of Firedamp nearby... I'm sure you can see where that one is going.
In short, accurately representing Coal Mining in the game would include
enormous opportunities for ‼FUN‼
Bonus points if you can mod in canaries as a safety feature though.
(EDITED because I actually went and looked things up on the other wiki to refresh my chemistry knowledge... oh DF, what have you done?)