Small once-through boiler

A once-through boiler provided with a radiant heat transfer section for heating a plurality of vertical water tubes positioned in a peripheral zone of said radiant heat transfer section and arranged in a row of circular formation comprising:

• a regenerative combustion system positioned in an upper portion of a boiler body;
• the regenerative combustion system including a combustion air supply system, a flue gas exhaust system, a fuel supply system, a regenerator and changeover means for changing over a passage for a combustion air flow and a passage for a combustion exhaust gas flow, the regenerator having first and second portions adapted to be alternately in heat-transferable contact with the combustion air flow to be supplied to said radiant heat transfer section and the combustion exhaust gas flow extracted therefrom so that the combustion air is heated by the combustion exhaust gas;
• means for connecting the water tubes with each other so as to define the radiant heat transfer section inside of the circular row of the water tubes and a convection heat transfer section in which a combustion gas flow heats the water tube while moving upward therethrough;
• said convection heat transfer section is formed between the water tubes and a wall surface of the boiler body;
• means for introducing the combustion gas from the radiant heat transfer section into a lower portion of the convection heat transfer section;
• and means for recirculating the combustion gas from the convection heat transfer section to an upper area of said radiant heat transfer section so that an exhaust gas recirculation flow is generated in the radiant heat transfer section.

A method of heating a plurality of water tubes in a combustion chamber of a once-through type boiler which includes a regenerative combustion system having a regenerator, which comprises:

• introducing a combustion air flow through a first part of the regenerator into said combustion chamber so as to preheat the combustion air and exhausting a combustion exhaust gas flow through a second part of the regenerator to heat the second part;
• feeding a fuel to the combustion air to generate a combustion reaction in said combustion chamber to take a radiant heat transfer action on water tubes arranged in a periphery of the combustion chamber;
• splitting a downward combustion gas flow in a lower area of the combustion chamber into a first gas flow upwardly flowing in the combustion chamber and a second gas flow introduced through a first opening into a convection heat transfer section, so that the second gas flow upwardly passes through said convection heat transfer section, wherein said first opening is formed between lower portions of the water tubes and said convection heat transfer section is formed between the water tubes and a wall surface of the boiler body;
• introducing said second gas flow into said combustion chamber through a second opening, which is formed between upper portions of said water tubes, whereby a temperature of an upper area of the combustion chamber is lowered by a recirculation of said second gas flow flowing out from the second opening;
• and introducing the combustion air flow through said second part of said regenerator into said combustion chamber so as to preheat the combustion air and exhausting the combustion exhaust gas flow through said first part of the regenerator to heat the first part.