Dancing Blue Flames

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?

You'll get green flame with copper in the fire. Could be some wire mixed in the with coal? Wouldn't be the first time I've seen or heard of that happening.

I have also gotten blue/green flames with the oxford coal. It burns with a yellow flame at first then after a bit it will have a few dancing green flames then they will turn bluish. I figured it was just CO(burns blue) and some hydrocarbons(burns yellow) blue and yellow make green.