As others have said, if it's a woodburner only don't use any other fuel in it.
Even in a multifuel stove you should never use ordinary household coal. Here's why:
Well, even with a multi-fuel stove, burning household coal is not a good idea. If you check your manual, we would expect it to warn against the use of household coal. It might not tell you why you shouldn’t use it.
The main reason is that, when coal is added to a fire it doesn’t start to burn immediately. Before it catches light, large quantities of smoke are released. This thick yellow-grey smoke is highly volatile. It will will fill your stove and flue system and, once up to temperature can create an explosive flash.
The force of this will often crack the glass in your stove door or cause damage to your flue. Even if you’re fortunate enough to avoid an explosive flash, the volatiles released by the coal will still catch fire. They will burn intensely for around 30 minutes until the coal settles into a more controlled burn.
This extreme initial burn will start to damage the inside of your stove. Examples of premature wear you will see include twisted baffles, misshaped grates, pitted and cracked liners and warped fuel retainers.
blog.gr8fires.co.uk/2013/04/24/can-you-use-household-coal-in-wood-burning-or-multi-fuel-stoves/