that is and the boiler get heat out of the flues? still if there is 300deg at the stack isnt that all leakage energy(the heat in the smoke(exhaust))
in the link i posted above, it is a vortex tube diagram, it is currently used for spot weld tech, its theory is that it takes in an ambient temperature and with a forced flow(compression) it creates a delta between temperatures on the outputs(ie it gets hot on one side and cold on the other), if we were to take one of these and replace the stack, it would do the same with the squirrel pressure, create a delta hot and cold side(in the system ambient would be 500deg, while hotside goes up 100deg, cold side goes down 100deg. we could then feedback the boosted temp back in to the collector(boiler) or to combustion chamber, and feed out the cold side out the stack, this is not to replace a flew, but to feedback the losses, also i would fatten up the chamber up a bit(diameter) as a collector for the cooled sooting....... how would you feel about that(in theory!)?
also i was reading that soot is usually a hydrocarbon compound, which means incomplete reactions, feeding it back to chamber with additional O2 should ignite to CO2 +energy(may need higher temp to "spark"),,, just a thought?