I have been burning Ohio bituminous coal and wood mixed in an outdoor wood boiler with success for a couple years now. I have noticed that at reload time I have blue and sometimes green flames dancing in the coal (as in hot coals) bed. I wonder if it’s a phenomenon associated with the wood coal mix or if it is primarily coming from the coal. I know the Anthracite guy's have laid claim to the term "Blue Ladies". I can’t help but call these flames the "Blue Biddies”
. Any coal burners got any ideas on the blue/green flames?