Don't know what type of stove you have, but keep in mind that it's not enough just to put an elevated grate in the stove, like some fireplaces use. You actually have to have air coming up thru the grates and not able to bypass around the grates sides. And you need a mechanism to shake the ashes out several times a day, usually before loading fresh coal.
On my bit. coal, I can get by with quite a bit of secondary air entering above the fire to help burn the volatiles. But I still need the majority of the air coming up thru the coals.
You'll need something like the pic below. Look at some stoves at the harman dealer or other stove shops for ideas too.http://nepacrossroads.com/download/file.php?id=1381