Fluidized bed boiler plant and a method of preheating combustion gas in a fluidized bed boiler plant

A method of preheating combustion gas in a fluidized bed boiler plant, the fluidized bed boiler plant comprising a furnace and a combustion gas channel for combusting fuel by combustion gas fed to the furnace through the combustion gas channel, and a water-steam cycle comprising an evaporator section, a superheater section in flow connection with the evaporator section comprising a last superheater, a steam turbine, and a superheating path for conveying steam from the evaporator section via the superheater section to the turbine, a first combustion gas preheater arranged in the combustion gas channel, a second combustion gas preheater arranged in the combustion gas channel and a steam extraction line attached in flow connection with the second combustion gas preheater and with the superheating path in a location upstream of the last superheater, the method of preheating combustion gas comprising:

• transferring heat to the combustion gas in the first combustion gas preheater;
• conveying steam from the superheating path from a location upstream of the last superheater via the steam extraction line to the second combustion gas preheater;
• and
• transferring heat from the steam to the combustion gas in the second combustion gas preheater.