A multi-stage heating apparatus having a first stage combustion chamber (14), a second stage combustion chamber (30), a primary porous matrix chamber (13) disposed between the first and second stage combustion chambers, a second porous matrix chamber (38), heat exchanger tubes for transfer of heat from the combustion products to a working fluid, a recirculation device (18) for recirculation combustion products in the first and second stage combustion chambers to the root of flames therein, an economizer (43) for preheating a working fluid and through a condenser for removing moisture in the combustion products prior to being exhausted.
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