The method in a recovery boiler comprises estimating the first melting temperature T.sub.0 of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) content of the fly ash; measuring or estimating the temperature T.sub.ss of superheated steam; evaluating a temperature difference T.sub.D1 between the first melting temperature T.sub.0 and the temperature T.sub.ss of the superheated steam, the temperature difference T.sub.D1 providing an estimate of the risk of corrosion; and selecting a control action for influencing the temperature difference T.sub.D1. Alternatively or additionally, the method comprises estimating the sticky temperature T.sub.STK of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) and chlorine (Cl) contents of the fly ash; measuring or estimating the temperature T.sub.FG of the flue gases; evaluating a temperature difference T.sub.D2 between the sticky temperature T.sub.STK and the temperature T.sub.FG of the flue gases; the temperature difference T.sub.D2 providing an estimate of the risk of plugging; and selecting a control action for influencing the temperature difference T.sub.D2.

The method in a recovery boiler comprises estimating the first melting temperature T.sub.0 of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) content of the fly ash; measuring or estimating the temperature T.sub.ss of superheated steam; evaluating a temperature difference T.sub.D1 between the first melting temperature T.sub.0 and the temperature T.sub.ss of the superheated steam, the temperature difference T.sub.D1 providing an estimate of the risk of corrosion; and selecting a control action for influencing the temperature difference T.sub.D1. Alternatively or additionally, the method comprises estimating the sticky temperature T.sub.STK of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) and chlorine (Cl) contents of the fly ash; measuring or estimating the temperature T.sub.FG of the flue gases; evaluating a temperature difference T.sub.D2 between the sticky temperature T.sub.STK and the temperature T.sub.FG of the flue gases; the temperature difference T.sub.D2 providing an estimate of the risk of plugging; and selecting a control action for influencing the temperature difference T.sub.D2.

There is provided a combined cycle power plant in which a high-pressure steam turbine and an intermediate-pressure steam turbine can operate in a state where amounts of thermal effect thereof are close to a limit value, and capable of reducing start-up time. A combined cycle power plant includes: an exhaust heat recovery boiler that includes a high-pressure superheater which superheats steam for a high-pressure steam turbine, and a reheater which reheats steam for an intermediate-pressure steam turbine; bypass pipes through which steam bypasses the high-pressure superheater and the reheater; bypass valves that regulate flow rates of steam which flows through the bypass pipes; and a bypass controller that controls the bypass valves such that a difference between thermal effect-amount margins of the turbines is decreased.

There is provided a combined cycle power plant in which a high-pressure steam turbine and an intermediate-pressure steam turbine can operate in a state where amounts of thermal effect thereof are close to a limit value, and capable of reducing start-up time. A combined cycle power plant includes: an exhaust heat recovery boiler that includes a high-pressure superheater which superheats steam for a high-pressure steam turbine, and a reheater which reheats steam for an intermediate-pressure steam turbine; bypass pipes through which steam bypasses the high-pressure superheater and the reheater; bypass valves that regulate flow rates of steam which flows through the bypass pipes; and a bypass controller that controls the bypass valves such that a difference between thermal effect-amount margins of the turbines is decreased.

The method in a recovery boiler comprises estimating the first melting temperature T.sub.0 of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) content of the fly ash; measuring or estimating the temperature T.sub.ss of superheated steam; evaluating a temperature difference T.sub.D1 between the first melting temperature T.sub.0 and the temperature T.sub.ss of the superheated steam, the temperature difference T.sub.D1 providing an estimate of the risk of corrosion; and selecting a control action for influencing the temperature difference T.sub.D1. Alternatively or additionally, the method comprises estimating the sticky temperature T.sub.STK of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) and chlorine (Cl) contents of the fly ash; measuring or estimating the temperature T.sub.FG of the flue gases; evaluating a temperature difference T.sub.D2 between the sticky temperature T.sub.STK and the temperature T.sub.FG of the flue gases; the temperature difference T.sub.D2 providing an estimate of the risk of plugging; and selecting a control action for influencing the temperature difference T.sub.D2.

The method in a recovery boiler comprises estimating the first melting temperature T.sub.0 of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) content of the fly ash; measuring or estimating the temperature T.sub.ss of superheated steam; evaluating a temperature difference T.sub.D1 between the first melting temperature T.sub.0 and the temperature T.sub.ss of the superheated steam, the temperature difference T.sub.D1 providing an estimate of the risk of corrosion; and selecting a control action for influencing the temperature difference T.sub.D1. Alternatively or additionally, the method comprises estimating the sticky temperature T.sub.STK of the fly ash depositing on heat transfer surfaces, the estimating being based on potassium (K) and chlorine (Cl) contents of the fly ash; measuring or estimating the temperature T.sub.FG of the flue gases; evaluating a temperature difference T.sub.D2 between the sticky temperature T.sub.STK and the temperature T.sub.FG of the flue gases; the temperature difference T.sub.D2 providing an estimate of the risk of plugging; and selecting a control action for influencing the temperature difference T.sub.D2.

A back-up boiler system for a solar thermal power plant (201) for transferring solar energy into electricity, said back-up boiler system comprising a combustion chamber (70) and a convection section (80) in fluid connection with said combustion chamber (70), wherein in the convection section (80) at least a first heat exchanger (92) is provided for heating a molten salts mixture of the solar thermal power plant and a second heat exchanger (90) for pre-heating boiler feed water of the solar thermal power plant, wherein the back-up boiler system (25) is configured to allow selection between only providing heat to the first heat exchanger (92), only providing heat to the second heat exchanger (90) and providing heat to both heat exchangers (90, 92), preferably dependent on availability of solar radiation and/or dependent on demand of power generation. The invention also relates to a solar thermal power plant (201) for transferring solar energy into electricity and a method for operating a solar thermal power plant.

A back-up boiler system for a solar thermal power plant (201) for transferring solar energy into electricity, said back-up boiler system comprising a combustion chamber (70) and a convection section (80) in fluid connection with said combustion chamber (70), wherein in the convection section (80) at least a first heat exchanger (92) is provided for heating a molten salts mixture of the solar thermal power plant and a second heat exchanger (90) for pre-heating boiler feed water of the solar thermal power plant, wherein the back-up boiler system (25) is configured to allow selection between only providing heat to the first heat exchanger (92), only providing heat to the second heat exchanger (90) and providing heat to both heat exchangers (90, 92), preferably dependent on availability of solar radiation and/or dependent on demand of power generation. The invention also relates to a solar thermal power plant (201) for transferring solar energy into electricity and a method for operating a solar thermal power plant.

There is provided a combined cycle power plant in which a high-pressure steam turbine and an intermediate-pressure steam turbine can operate in a state where amounts of thermal effect thereof are close to a limit value, and capable of reducing start-up time. A combined cycle power plant includes: an exhaust heat recovery boiler that includes a high-pressure superheater which superheats steam for a high-pressure steam turbine, and a reheater which reheats steam for an intermediate-pressure steam turbine; bypass pipes through which steam bypasses the high-pressure superheater and the reheater; bypass valves that regulate flow rates of steam which flows through the bypass pipes; and a bypass controller that controls the bypass valves such that a difference between thermal effect-amount margins of the turbines is decreased.