I tried burning coal?

I tried burning coal for the first time in my wood/coal boiler. got a nice bed of coals started and added the rest of the 40lb bag. it was burning great. I was impressed until i got up this morning. I checked it and it was out. There was about a 1/3 the bag left and it was whitish yellow. Was this because my automatic damper on the boiler closed and snuffed out my burn? Any tips on how to do this right?