Method and arrangement for expanding a gas stream comprising carbon dioxide

Abstract

The present disclosure relates to a method and an arrangement for expanding a gas stream comprising carbon dioxide, CO 2 . The method includes: removing CO 2 from a process gas to produce a CO 2 lean gas stream comprising residual CO 2 ; monitoring a temperature of the gas stream downstream of an expander; controlling a pressure of the gas stream downstream of the expander by means of a pressure controller; and overriding the pressure controller when the temperature downstream of the expander is below a predefined minimum temperature; whereby deposition of solid CO 2 from the residual CO 2 in the gas stream is avoided. The disclosure also relates to a gas cleaning system and a power plant, such as an oxy-combustion power plant, associated with the arrangement.

1 . A method of expanding a gas stream having carbon dioxide, CO 2 , the method comprising:
 removing CO 2  from a process gas to produce a CO 2  lean gas stream comprising residual CO 2 ;   monitoring a temperature of the gas stream upstream of an expander,   controlling a pressure of the gas stream upstream of the expander by means of a first pressure controller,   overriding the first pressure controller when the temperature upstream of the expander is below a first predefined minimum temperature; and/or   monitoring a temperature of the gas stream downstream of an expander,   controlling a pressure of the gas stream downstream of the expander by means of a second pressure controller, and   overriding the second pressure controller when the temperature downstream of the expander is below a second predefined minimum temperature;   whereby deposition of solid CO 2  from the residual CO 2  in the gas stream is avoided.   
     
     
         2 . The method of  claim 1 , wherein the first and/or second predefined minimum temperature is within the range of from −35° C. to −57° C., such as from −50° C. to −56° C. 
     
     
         3 . The method of  claim 1 , wherein the pressure of the gas stream upstream of the expander is within the range of 2-4 MPa. 
     
     
         4 . The method of  claim 1 , wherein the pressure of the gas stream downstream of the expander is within the range of 0.1-1.5 MPa, such as 0.5-0.8 MPa. 
     
     
         5 . The method of  claim 1 , wherein the second pressure controller is upstream of a second expander. 
     
     
         6 . The method of  claim 5 , further comprising:
 monitoring a temperature of the gas stream downstream of the second expander;   controlling a pressure of the gas stream downstream of the second expander by means of a third pressure controller; and   overriding the third pressure controller when the temperature downstream of the second expander is below a third predefined minimum temperature.   
     
     
         7 . The method of  claim 1 , further comprising:
 venting the gas stream to the atmosphere.   
     
     
         8 . The method of  claim 1 , wherein the CO 2  lean gas stream comprises between 1-50 mol-% CO 2 , such as between 25-33 mol-%. 
     
     
         9 . The method of  claim 1 , wherein the process gas is a flue gas from a power plant, such as an oxy-combustion power plant. 
     
     
         10 . The method of  claim 1 , wherein the CO 2  removed from the process gas forms a CO 2  rich fluid stream comprising at least 80 mol-% CO 2 , such as at least 95 mol-% CO 2 . 
     
     
         11 . An arrangement for expanding a gas stream comprising carbon dioxide, CO 2 , the arrangement comprising:
 means for removing CO 2  from a process gas to produce a CO 2  lean gas stream comprising residual CO 2 ; and   means for monitoring a temperature of the gas stream upstream of an expander,   means for controlling a pressure of the gas stream upstream of the expander by means of a first pressure controller, and   means for overriding the first pressure controller when the temperature upstream of the expander is below a first predefined minimum temperature; and/or   means for monitoring a temperature of the gas stream downstream of an expander,   means for controlling a pressure of the gas stream downstream of the expander by means of a second pressure controller, and   means for overriding the second pressure controller when the temperature downstream of the expander is below a second predefined minimum temperature;   such that the arrangement is configured to avoid deposition of solid CO 2  from the residual CO 2  in the gas stream.   
     
     
         12 . The arrangement of  claim 11 , wherein the arrangement is associated with a gas cleaning system, the system comprising means for removing particles, sulphur oxides, nitrogen oxides, mercury, and/or water. 
     
     
         13 . The arrangement of  claim 11 , wherein the arrangement is associated with a power plant, such as an oxy-combustion power plant, such that the process gas is a flue gas of said power plant. 
     
     
         14 . A gas cleaning system comprising the arrangement of  claim 11 . 
     
     
         15 . An oxy-combustion power plant comprising the arrangement of  claim 11 .