Oxy-fuel boiler system and its operation

Abstract

The present disclosure relates to a method of operating a boiler system that includes an oxy-fuel boiler in which an oxygen stream and a fuel stream are combusted to generate a flue gas stream. An air separation unit produces the oxygen stream for the oxy-fuel boiler. A gas processing unit cleans and compresses at least a portion of the flue gas stream generated in the oxy-fuel boiler producing a pressurized carbon dioxide stream. The method includes operating the boiler system, at least for a period of time, in a recirculation mode, during which a carbon dioxide stream from a CO 2 compression unit within the gas processing unit is evaporated in the air separation unit and forwarded as a stream to the gas processing unit. The present disclosure further relates to a boiler system for an oxy-fuel process as well as to a power plant comprising such a system. The present disclosure also relates to the use of a carbon dioxide containing stream as a refrigerant.

1 . A method of operating a boiler system having an oxy-fuel boiler in which an oxygen stream and a fuel stream are combusted to generate a flue gas stream, an air separation unit producing the oxygen stream for the oxy-fuel boiler, and a gas processing unit for cleaning and compressing at least a portion of the flue gas stream generated in the oxy-fuel boiler to produce a pressurized carbon dioxide stream, the method comprising:
 operating the boiler system, at least for a period of time, in a recirculation mode, during which a carbon dioxide stream from a CO 2  compression unit within the gas processing unit is evaporated in the air separation unit and forwarded as a stream to the gas processing unit.   
     
     
         2 . The method according to  claim 1 , wherein in the recirculation mode, the carbon dioxide stream is made to expand before entry into the air separation unit as a stream. 
     
     
         3 . The method according to  claim 1 , further comprising:
 establishing whether the boiler system operates at a first load or at a second load,   wherein the second load is a lower load than the first load, and   controlling the boiler system to operate in the recirculation mode when the boiler system operates at the second load, and controlling the boiler system to stop operation in the recirculation mode when the boiler system operates at the first load.   
     
     
         4 . The method according to  claim 1 , further comprising controlling the recirculation mode of the boiler system using a controlling device and a regulating device. 
     
     
         5 . The method according to  claim 4 , wherein the regulating device is also responsible for expanding the carbon dioxide stream before entry into the air separation unit as the stream. 
     
     
         6 . The method according to  claim 1 , wherein the carbon dioxide stream from the CO 2  compression unit is a part of the pressurized carbon dioxide stream exiting the gas processing unit or is taken from the CO 2  compression unit before the pressurized carbon dioxide stream is exiting the gas processing unit. 
     
     
         7 . The method according to  claim 1 , wherein the stream is forwarded to the inlet of the CO 2  compression unit or to a stream containing carbon dioxide entering the CO 2  compression unit. 
     
     
         8 . A boiler system comprising
 an oxyfuel boiler in which a stream of oxygen and a fuel are combusted to generate a flue gas stream,   an air separation unit producing the stream of oxygen for the boiler,   a gas processing unit for further cleaning and compression of the flue gas for producing a stream of pressurized fluid comprising carbon dioxide,   a recirculation system, operable at least for a period of time in a recirculation mode, in which a carbon dioxide stream from a CO 2  compression unit within the gas processing unit is evaporated in the air separation unit and forwarded as a stream to the gas processing unit.   
     
     
         9 . The boiler system according to  claim 8 , further comprising a controlling device and a regulating device which controls the recirculation system to be operable in the recirculation mode based on a measured load on the boiler system. 
     
     
         10 . The boiler system according to  claim 9 , wherein the regulating device is also responsible for expanding the carbon dioxide stream before entry into the air separation unit as the stream. 
     
     
         11 . The boiler system according to  claim 8 , wherein the carbon dioxide stream from the CO 2  compression unit is a part of the pressurized carbon dioxide stream exiting the gas processing unit or is taken from the CO 2  compression unit before the pressurized carbon dioxide stream is exiting the gas processing unit. 
     
     
         12 . The boiler system according to  claim 8 , wherein the stream is forwarded to the inlet of the CO 2  compression unit or to a stream containing carbon dioxide entering the CO 2  compression unit. 
     
     
         13 . The boiler system according to  claim 8 , wherein the gas processing unit apart from the CO 2  compression unit further comprises a flue gas compression unit and a carbon dioxide separation unit. 
     
     
         14 . An oxy-fuel combustion power plant comprising the system according to  claim 8 . 
     
     
         15 . Use of a carbon dioxide stream from a CO 2  compression unit within a gas processing unit of a boiler system, said carbon dioxide stream, as a refrigerant in an air separation unit of said boiler system, wherein the carbon dioxide is withdrawing heat energy from the air separation unit, said carbon dioxide being evaporated and a resulting evaporated carbon dioxide stream is forwarded back to the gas processing unit.