A method and apparatus for heating a fluid and treating a combustion products waste stream includes two or more nozzles discharging into a mixing chamber, and an outlet of the mixing chamber discharging to a gas-liquid separator. A liquid output of the gas-liquid separator may be treated to remove carbonaceous or other impurities. The nozzles may include an annular nozzle, Fisenko nozzle, and/or Laval nozzle arranged in a transonic jet module. A heated input liquid may be accelerated to sonic velocity in a main nozzle, causing boiling due to pressure drop prior to mixing with a combustion product stream in the mixing chamber. Heat may be recovered from a mixture discharged from the mixing chamber. Carbonic, sulfuric, or other combustion impurities may be captured by dissolving in water or other solvent in the transonic jet module and then recovered or otherwise used in a liquid stream from the separator.