Help With Coal Stove Insert

I was given a Sierra Turbo Burn 4500 woodstove fireplace insert which also has a small cast iron box used for coal burning. I have had many problems trying to get the coal box to burn coal effectively, so I removed the box and put a layer of steel railroad plates on each side of the grate over the fire brick so as to build a coal fire over the entire area. I suspected that the coal box was way too small to have a long lasting fire in it.

Here's where I need help....How should I build the coal fire to hold at least throughout the night? I cannot maintain a fire for more than 3 hours without tending to it...adding coal, shaking grates. Is it even possible in this stove to be able to do that?

My grates are only about 9x16" in the center of the stove only. By not using the coal box I am able to pile coal higher over this grate area and off to the sides. Is there a danger in building the coal fire 4"-6" high throughout the entire bottom floor of the stove?

I use anthracite nut coal and can make a beautiful fire with blue flame but cannot keep it throughtout the night or even while I go out for a few hrs. everything on this stove is manual operation. It has an ash pan with opening for draft and also damper on top of stove. What position should these be kept in?

I do not have a liner in the fireplace chimney.

If anyone out there has experience with these older Sierra stoves, please reply. It is most appreciated!!! I see they do not make this model anymore and I cannot find a dealer in my area to talk to about this.

Do you have a picture of your stove you could post?