Welcome to the forum. I just started burning anthracite in October, and it’s been a great learning experience. Having a place like this with friendly, helpful people has really good me hooked to the fourm, and anthracite.
Your main question asks how much coal you can put in, and also you asked how the coal would act in the firebox.
I have an older larger stove and I was told from the past owner I can fill it up to the top of the firebrick. From the way yours looks, I think it would be the same. You won’t have to do that in this weather, where you might be able to keep it in half the firebox.
After you get going, your firebox you’ll have layers of different coal. Bottom will be dieing coals, and ash. Middle will be red hot coals burning making tons of heat. The top might have some black on it(which is good if you want a long burn), and will still make blue flame. I normally add coal 3 times a day to my fire, 6am, 6pm, 10pm.
Before you fire this up you need to make sure you have proper draft. The draft is very important with a coal fire. It’s good that there is a top exhaust on this burner, because I think fly ash could build up in the bottom exhaust plugging it. Coal burns from bottom to the top, and air needs to pass from the bottom to top for your fire to burn well. So my opinion is that if the bottom getting plugged, that might not be horrible. Only time will tell.
The grates look like they just spin. Do they open up a little to allow bigger solid ash pieces to fall down? If it just spins, you might have to do some poking to break up your ash. Coal ash isn’t just a fine powder like wood ash.
If you don’t have an ash pan you can get a some sheet metal and bend it without a break using clamps, two 2x4’s, and a hammer. Then just some self tapping metal sheet metal screws to keep it together. You might be able to find some type of metal mixing bowl that would be the size you need, just make sure its heavy metal that would take the heat.
Read up on how to light coal there are plenty of how to’s on that here. You basically need to start a good hot kindling wood fire first. I add some charcoal in because I know that will get hot and stay hot for a while. Once your wood fire is getting hot, after maybe 10 minutes and slowly start adding coal. Maybe a coal shovel full every 5-10 minutes until that catches, (I still add fresh kindling too during the fire few shovels) then you can add more every 30 minutes until maybe half the firebox. The whole time I’m closing off the damper to the flue, from almost wide open to nearly closed (I have good draft from my chimney). I am also closing my combustion air draft. A stack thermometer is a must, and you should have that before you fire it up(Only $10-$20 from any stove shop). My stack temp will go up to 400f – 500f when I’m starting it up… but it will work back down to around 300 f if I want good heat. It’s only around 200 today with just enough of a fire for the fire not to go out.
Do you plan on using anthracite? If so what size?
How does the combustion air enter the stove, from the side or the front door?