Hello James, welcome to the forum. I did a google search for your outside boiler, and it appears that the boiler has a grate system and an ashpan. So burning coal is workable. What I can't tell is if the firebox has firebrick on each side of the grate system??
In a big firebox like that, I'd try some stove size coal. The pieces range from the largest you get with nut coal up to about the size of a fist or even a paperback book. With the larger pieces, you may have a hotter fire, and the burn time may be different compared to burning nut coal.
Take a photo of the grate system you have, and post it with a photo of your boiler on the 'pictures of your stove' thread.
Bye the way, please turn off your 'caps lock' key on your keyboard. On internet forums typing is all-caps is considered yelling, and it is hard on the eyes.
Give us a report how you like the stove size coal, I liked it in my outside boiler. I used a mix of nut and stove. Often a layer of stove, then a layer of nut over the top to get a longer burn for all night.
Take care,, Greg L