A compact combustor-boiler construction is described that during operation will emit substantially no nitrogen oxides, while burning a fuel/air mixture essentially to completion. Nested cooled porous plug burners with a common fuel/air supply chamber are flanked by inner and outer burned gas cooler units with the coolant flow paths through the burners being connected in series with a flow path through the outer and inner cooler units in sequence whereby the coolant (working fluid) is sequentially exposed to liquid heating, nucleate boiling, film boiling and, preferably, super-heating.