How big is the 'heat sink' you are stuffing 16 tons of coal into. It has to be over 5000 sq ft, OR it has no roof, only walls.
WOW! I have 10,000 sq ft buildings that an AA 260 does on 12 tons.....hot water too. (10 apts,) An AHS 260 is doing 9400 sq ft easily and AA and AHS 130 are doing over 4000 sq ft on 8 tons annually. EFM 520's and a 900 easily do similar sizes with the same coal usage. I'd look over your total set-up, check the draft, reset the damper if necessary....or you are heating more than any of us is 'guesstimating'. 16 tons would equate to about 3000 gallons of oil.....that is no small usage number!
My vote is also to purchase a separate oil boiler and not go to the trouble of adding the option to your coal burner. Plenty of very good used equipment that a newbie changes out because some 'slickster' sold him a 94.5% efficient condensing gas boiler. Don't reinvent the wheel, there are many who passed before us who knew MUCH more than we ever will.