Coal needs primary air from directly underneath, having some of it sitting on top of ashes piled up on the auger will result the that coal directly above the auger not burning due to a lack of primary air.Teddy wrote: Unhippy: I thought about a chain grate stoker but would like to keep this thing a lot less mechanically complicated. I was thinking that if the gap between the bottom edges of the side grates were big enough to allow clinkers to pass through to the ash auger, the ashes piled up on the auger would stop raw coal from "falling through". I thought that the bottom edge of the grates would sit on top of the trough the auger sits in. I could make the trough deep enough that the auger wouldn't get too hot. If need be, put a "clinker breaker" between the grates at the bottom operated from the outside? Again I would like to know what diameter an ash/clinker auger ought to be? 3"? 4"?
Clinkers don't form in nice little lumps that will slide down the grates and into the ash auger.....if your firing hard your likely to end up with one stonking great clinker that takes up most of the grate surface
I think some of the problems you are having understanding how coal burns, is that you are trying to treat it like pieces of ultra-hard hardwood 2 inches across.....It has needs and burn characteristics that are totally different to wood.
The closest analogy I can come up with is the difference between tuning a high performance petrol engine and a high performance diesel engine....both have roughly the same type of parts in the same places, but the difference between them means they have to be treated totally differently.
Before you dismiss a travelling grate stoker out of hand, count up how many moving parts, actuators and timers etc you would have in anything you design or modify.....by the time you get clinker breakers and ash augers going it makes the chain-grate look pretty simple....and they can be run on natural draught if they are designed for it and the coal you have wouldn't need resized either.