franco b wrote: A hopper can also help in the burning of volatiles by partially coking the fed coal and capturing the heat normally wasted by feeding cold coal in a batch.
Let me put a myth to rest here. The heating of coal in a hopper does not come for free-- it takes energy to do that, in fact the same amount of energy if the coal was heated to the same temperature in the firebox or in a hod on the floor. Anthracite has an extremely low percentage of volatiles so not much to be gained there either.
Mostly agree. The difference is how and where the coal is heated. Assuming the hopper has been topped up at the previous shake, then at the current shake very hot and partially de'gassed or coked coal is fed to the fire, leading to rapid ignition and fewer volatiles to burn,since they have already been partially burned. This leads to a hotter steadier fire than if a cold batch of coal were fed. That hotter fire has a much better chance of burning any CO that has been forming in the already burning mass of coal. When feeding a batch of cold coal the fire immediately cools and depending on the size of the batch can take quite a while to stabilize the heat output again. Meanwhile a great deal of the volatiles go up the chimney along with increased amounts of CO unburnt until enough heat is again established.
The percentage of volatiles in coal is irrelevant, what is relevant is the percentage of heat they contain. While only about 4 percent they make up about 20 percent of the heat in a pound of coal. Just look at your stack temperature when they start burning.
I think the Chubby is a dandy stove. I like the look and the design. The suspended fire pot has the effect of insulating it from the sides with an air gap leading to a hotter fire and better combustion. Running it at its sweet spot as you seem to be doing gets the most out of it. If fed in smaller batches which you may already be doing I think it can do even better.