Method and a system for maintaining steam temperature with decreased loads of a steam turbine power plant comprising a fluidized bed boiler

A method of maintaining the temperature of steam supplied to a steam turbine of a steam turbine power plant, which power plant further comprises a circulating fluidized bed boiler comprising a furnace and a fluidized bed superheater configured to superheat steam supplied to the steam turbine by transferring thermal energy to said steam from fluidized bed material, the method comprising:

• selecting a trigger load for the power plant, which trigger load is less than the full rated load of the power plant and greater than the minimum rated viable load of the power plant,
• determining the load of the power plant,
• when the load of the power plant is at or above the trigger load, superheating steam with the fluidized bed superheater such that the temperature of the superheated steam entering the steam turbine is at or near its maximum temperature, and
• when the load of the power plant is below the trigger load:
• injecting combustible gas into the fluidized bed material outside the furnace, the combustible gas being ignited by the fluidized bed material when the combustible gas comes into contact with the fluidized bed material having a temperature at or above a minimum temperature required to ignite the combustible gas,
• superheating steam with the fluidized bed superheater,
• additionally heating the fluidized bed material outside the furnace such that the temperature of the superheated steam entering the steam turbine is at or near its maximum temperature, wherein the additional heating of the fluidized bed material is brought about by combustion of the injected combustible gas, and
• conveying the superheated steam from the fluidized bed superheater to the steam turbine.