Method of operating an oxy-fuel boiler system

Abstract

The present disclosure relates to a method of operating a boiler system that includes an oxyfuel boiler in which an oxygen stream and a fuel stream are combusted to generate a stream of flue gas, an oxygen gas source producing the stream of oxygen for the boiler, and a gas processing unit for cleaning and compressing at least a portion of the stream of flue gas generated in the boiler for producing a stream of pressurized fluid comprising carbon dioxide. The method includes operating the boiler system, at least for a period of time, in an evaporation mode, in which a stream of oxygen from the oxygen gas source in liquid form is evaporated prior to being introduced in the boiler by transferring heat energy to the stream of liquid oxygen from a first stream of carbon dioxide of 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.

1 . A method of operating a boiler system including an oxyfuel boiler in which an oxygen stream and a fuel stream are combusted to generate a stream of flue gas, an oxygen gas source producing the stream of oxygen for the boiler, and a gas processing unit for cleaning and compressing at least a portion of the stream of flue gas generated in the boiler for producing a stream of pressurized fluid comprising carbon dioxide, the method comprising:
 operating the boiler system, at least for a period of time, in an evaporation mode, in which a stream of oxygen from the oxygen gas source in liquid form is evaporated prior to being introduced in the boiler by transferring heat energy to the stream of liquid oxygen from a first stream of carbon dioxide of the gas processing unit.   
     
     
         2 . The method according to  claim 1 , further comprising operating the boiler system, at least for a period of time, in a condensing mode, in which a stream of gaseous oxygen from the oxygen gas source is condensed prior to being forwarded to a liquid oxygen storage unit by transferring heat energy from the stream of gaseous oxygen to a second stream of carbon dioxide of the gas processing unit. 
     
     
         3 . The method according to  claim 2 , further comprising transferring, in the condensing mode, heat energy from the gaseous oxygen to the second stream of carbon dioxide which has the form of a stream of pressurized carbon dioxide from the gas processing unit, wherein the pressurized carbon dioxide is made to expand before the transfer of heat energy. 
     
     
         4 . A 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,   controlling the boiler system to operate in the evaporation mode when the boiler system operates at the first load, and controlling the boiler system to stop operation in the evaporation mode when the boiler system operates at the second load.   
     
     
         5 . The method according to  claim 2 , further comprising:
 establishing whether the boiler system operates at a third load or at a fourth load,   wherein the fourth load is a lower load than the third load,   controlling the boiler system to operate in the condensing mode when the boiler system operates at the fourth load, and   controlling the boiler system to stop operation in the condensing mode when the boiler system operates at the third load,   wherein a stock of liquid oxygen is built up when the boiler system operates at the fourth load.   
     
     
         6 . The method according to  claim 5 , wherein the fourth load is a lower load than the first load. 
     
     
         7 . The method according to  claim 5 , further comprising controlling the boiler system to avoid, at least for a period of time, simultaneously operating in the evaporation mode and in the condensing mode,
 wherein a stock of liquid oxygen is built up in the condensing mode when operating at the fourth load, and that said stock of liquid oxygen is evaporated in the evaporation mode for use in the boiler when operating at the first load, wherein the fourth load is a lower load than the first load.   
     
     
         8 . The method according to  claim 1 , wherein an intermediary heat transfer fluid is utilized in the evaporation mode for transferring heat to the stream of liquid oxygen from the stream of carbon dioxide. 
     
     
         9 . The method according to  claim 1 , further comprising transferring, in the evaporation mode, heat directly to the liquid oxygen from the stream of carbon dioxide. 
     
     
         10 . A boiler system comprising
 an oxyfuel boiler in which a stream of oxygen and a fuel are combusted to generate a stream of flue gas,   an oxygen gas source producing the stream of oxygen for the boiler,   a gas cleaning system for cleaning at least a portion of flue gas generated in the boiler,   a flue gas condenser for condensing the cleaned flue gas,   a gas processing unit for further cleaning and compression of the flue gas for producing a stream of pressurized fluid comprising carbon dioxide,   a first heat exchanging system, operative at least for a period of time in an evaporation mode, in which a stream of oxygen from the oxygen gas source in liquid form is evaporated prior to being introduced in the boiler by transferring heat energy to the stream of liquid oxygen from a first stream of carbon dioxide of the gas processing unit.   
     
     
         11 . The boiler system according to  claim 10 , further comprising a second heat exchanging system operative at least for a period of time in a condensing mode, in which a stream of gaseous oxygen from the oxygen gas source is condensed prior to being forwarded to a liquid oxygen storage unit by transferring heat energy from the stream of gaseous oxygen to a second stream of carbon dioxide of the gas processing unit. 
     
     
         12 . The boiler system according to  claim 11 , further comprising a controlling device which controls the heat transfer systems to operate in the evaporation mode or condensation mode, respectively, based on a measured load on the boiler system. 
     
     
         13 . The boiler system according to  claim 10 , further comprising an oxygen storage unit for storing oxygen from the oxygen storage unit in liquid form. 
     
     
         14 . The boiler system according  claim 11 , wherein the second heat exchanging system comprises a heat exchanger in which a compressed liquid carbon dioxide stream is evaporated by heat energy transferred from condensation of a gaseous oxygen stream from the oxygen gas source. 
     
     
         15 . An oxy-fuel combustion power plant comprising the system according to  claim 10 .