Carbon dioxide separator, fuel cell system including same, and method of operating the fuel cell system

Abstract

A system and method in which a high temperature fuel cell stack exhaust stream is recycled back into the fuel inlet stream of the high temperature fuel cell stack. The recycled stream may be sent to a carbon dioxide separator that separates carbon dioxide from the fuel exhaust stream. The carbon dioxide separator may include a carbon dioxide separation membrane, an oxygen blocking membrane, and a water blocking membrane.

1 . A carbon dioxide separator for a fuel cell system, the carbon dioxide separator comprising:
 a sweep channel configured to receive a sweep gas;   a feed channel configured to receive a fuel exhaust stream of the fuel cell system;   a carbon dioxide separation membrane disposed between the sweep channel and the feed channel, the carbon dioxide separation membrane having a higher permeability to carbon dioxide and water than to hydrogen gas, carbon monoxide, and nitrogen gas; and   a first blocking layer disposed on a first side of the carbon dioxide separation membrane and comprising a first water blocking layer that is hydrophobic.   
     
     
         2 . The carbon dioxide separator of  claim 1 , wherein the first water blocking layer has a higher permeability to carbon dioxide than to water. 
     
     
         3 . The carbon dioxide separator of  claim 1 , wherein the first water blocking layer comprises porous polytetrafluoroethylene (PTFE) or a porous polysulfone. 
     
     
         4 . The carbon dioxide separator of  claim 1 , wherein the carbon dioxide separator has an oxygen gas permeability ranging from about 0% to about 25%, and a water permeability ranging from about 20% to about 40%. 
     
     
         5 . The carbon dioxide separator of  claim 1 , wherein the carbon dioxide separation membrane comprises a (2-aminoisobutyric acid)-potassium salt (AIBA-K). 
     
     
         6 . The carbon dioxide separator of  claim 1 , wherein the carbon dioxide separation membrane comprises a glycine-potassium salt (glycine-K). 
     
     
         7 . The carbon dioxide separator of  claim 1 , further comprising a second blocking layer disposed on an opposing second side of the carbon dioxide separation membrane and comprising a second water blocking layer that is hydrophobic. 
     
     
         8 . The carbon dioxide separator of  claim 7 , wherein the first side of the carbon dioxide separation membrane faces the sweep channel and the second side of the carbon dioxide separation membrane faces the feed channel. 
     
     
         9 . The carbon dioxide separator of  claim 1 , wherein the carbon dioxide separation membrane comprises active amine sites. 
     
     
         10 . A fuel cell system, comprising:
 a fuel cell stack;   the carbon dioxide separator of  claim 1 , the carbon dioxide separator being configured to remove carbon dioxide from a fuel exhaust stream output from the fuel cell stack and to create a purified recycled fuel exhaust stream by purifying the fuel exhaust stream;   a first recycling conduit fluidly connecting a fuel exhaust outlet of the fuel cell stack to the feed channel and configured to provide the recycled fuel exhaust stream to the carbon dioxide separator, by diverting at least a portion of a fuel exhaust stream output by the fuel cell stack; and   a second recycling conduit fluidly connecting a feed channel outlet of the carbon dioxide separator to a fuel inlet conduit connected to the fuel cell stack, the second recycling conduit adapted to provide the purified recycled fuel exhaust stream to the fuel inlet conduit.   
     
     
         11 . The fuel cell system of  claim 10 , wherein an inlet of the sweep channel and an inlet of the feed channel are disposed on the same side of the carbon dioxide separator, such that the sweep gas and the fuel exhaust stream are configured to flow through the carbon dioxide separator in the same direction. 
     
     
         12 . The fuel cell system of  claim 10 , further comprising a sweep gas humidifier fluidly connected to the sweep gas channel. 
     
     
         13 . The fuel cell system of  claim 12 , further comprising:
 a humidity or dew point detector configured to detect a humidity or dew point of the sweep gas; and   a control unit configured to control the sweep gas humidifier in accordance with the detected humidity or dew point.