sorry, i forgot about my post down here in the bitumionus section. here's a few things i noticed just from the pic; the grates are too narrow, the brick too steep, and the firebrick doesn't extend high enough. the width of the grates adn the incline of the firebrick MAY not be a problem if the coal you're burning is low or very low ash, less than 10%, for this reason i would reccomend not getting ROM coal even though it's cheaper but rather nut, possibly larger, egg may work as long as it doesn't clinker. i would highly reccomend extending the firebrick higher however, not because i'm worried about protecting the steel, but rather the heat needs to be kept in the coal bed to achieve a nice long burn time vs. opening the furnace finding a bunch of burned out coke, also if the heat is kept in the bed better you will have much less trouble with mid-phase bridgeing, rather in an evenly heated bed you will end up with short duration bridging followed by the coke cracking and shrinking allowing the coal bed to continue burning well without human interference. ideally you would also have some kind of brick arch or preheated secondary air directed at the top of the coal bed, this increases efficiency and helps alleviate "bridging". whether it's possible to easily incorporate any of these suggestions into your boiler; i'm not sure it depends on many factors, including your ability to wield etc.
If you need a good source of coal let me know via pm.
post more pics if you can, i'm interested to see the rest of the boiler. i sincerely hope for your sake that those are shaker/sliding grates.