Well, looking at the grates,, and mostly the almost flat shape of the circular firbox.. burning coal in this firebox is going to be really tough..
If you go to some of the stove sites for coal stoves and boilers and look at the way the firebox is designed, I think you can see what I mean..http://www.harmanstoves.comhttp://www.hitzer.comhttp://www.alternateheatingsystems.com
You will see that a firebox designed to burn coal has vertical, firebrick lined sides, and the whole bottom of the firebox is a shaker grate system.
Coal is very tempermental to burn, unlike wood. Coal must be burnt in a firebox that is designed to burn coal.. you can burn wood in virtually any hand feed coal stove, but not the reverse.
You could go out and buy a bunch of firebrick and try to make a box around the grates in your boiler, and try to burn a smaller, deeper bed of coal.. but I don't think it will work..
think of a milk crate, made of steel, line the crate with firebrick, and make the grate on the bottom movable to shake the ash down,, this would be a small but good coal firebox.
You didn't mention the type of coal you are trying to burn,, if it is Anthracite, hard coal.. I don't think you will have much success. Bituminous coal or soft coal burns a little bit easier, a bit more like wood, but it still needs the above described firebox to burn well and properly.