steinkebunch wrote:I burn WY bituminous coal, and would like to do a better job of buring the volatiles/smoke. I built my own stove patterned off the Harman Mark III - works good, just still have trouble with volatiles.
To burn volatiles you need high heat and air at the right place. They should be burned as gas and before precipitating as bits of carbon or smoke. To accomplish this various downdraft stoves have been designed to force the flue gas down to the hottest part of the stove which is the burning bed, before exiting. A combustion chamber that is hot enough will do a better job without the need for downdraft. The trick is to separate the needs of the combustion chamber from the heat absorbing portion of the stove.
Early Victorian boilers were simply a long tank with a fire under one end. The flue gas traveled the length of the tank heating the water. They were big and not efficient. With the advent of the Scotch Marine design the fire was moved to a combustion chamber in front of the tank and flue gas was directed through a series of tubes that went through the tank. Better combustion and much more compact boiler. Combustion was separated from heat absorption.
In an effort to design a wood stove that would accomplish both a hotter chamber and separate heat exchange I came up with a simple design that worked very well. I did not go further with it because it did not hold enough wood. It might though be just the thing for bit coal which has similar problems burning volatiles as wood.
It is 3 concentric cylinders or any shape you like. The inner cylinder is the combustion chamber which in my stove was stainless steel but in a coal stove would be fire brick all the way to the top. The inner cylinder has slots at the top to act as flue passage to the next cylinder which causes the flue gas to descend on the outside of the inner chamber keeping it very hot. It descends to the bottom and then exits to the outer heat exchange surface of the outer cylinder and out the flue. Standard grate and feed at the top side with provision for secondary air through the feed door. The combustion area should be incandescent.