System and method for sequestering emissions from engines

Abstract

A system is provided for sequestering carbon dioxide emitted in exhaust gas from an engine. The exhaust gas may be cooled using heat exchange techniques, for example reverse flow heat exchangers. Carbon dioxide may be separated from the exhaust gases, for example using fractional distillation, heat exchangers, and turbo chargers. The separated carbon dioxide may be stored, for example using injection deep into the earth for enhancing natural gas and oil extraction. Alternatively, the separated carbon dioxide may be cooled to solid carbon dioxide and stored, for example, in an LNG tank for use as ballast onboard a ship or for use in industry.

1 . A liquefied natural gas system for providing power and sequestering emitted carbon dioxide, the system comprising:
 an engine configured to use natural gas for fuel and emit carbon dioxide gas in an exhaust gas;   a liquefied natural gas tank configured to store liquefied natural gas and solid carbon dioxide;   a first reverse flow heat exchanger in fluid communication with the engine, the first reverse flow heat exchanger configured for exchanging heat between cold natural gas and the emitted carbon dioxide gas to warm the cold natural gas for fuel for the engine and to cool the emitted carbon dioxide gas to cold carbon dioxide;   a second reverse flow heat exchanger in fluid communication with the cold carbon dioxide in the first reverse flow heat exchanger and liquefied natural gas in the liquefied natural gas tank, the second reverse flow heat exchanger configured for exchanging heat between the liquefied natural gas and the cold carbon dioxide to provide heat to the liquefied natural gas for a phase change from liquid natural gas to cold natural gas, and to remove heat from the cold carbon dioxide for a phase change from gas to solid carbon dioxide for storage in the liquefied natural gas tank; and   a refrigerator configured to receive a portion of the cold natural gas from the liquefied natural gas tank, to condense the cold natural gas to liquefied natural gas, and to provide the liquefied natural gas to the liquefied natural gas tank.   
     
     
         2 . The system of  claim 1 , wherein the second reverse flow heat exchanger is disposed in the liquefied natural gas tank. 
     
     
         3 . The system of  claim 1 , wherein the refrigerator is further configured to receive energy from the engine and use the received energy to condense the portion of the cold natural gas to liquefied natural gas temperature. 
     
     
         4 . The system of  claim 1 , further comprising a separator in fluid communication with the first reverse flow heat exchanger and the second reverse flow heat exchanger, the separator configured to receive the exhaust gas including nitrogen and the cold carbon dioxide from the first reverse flow heat exchanger, to separate the cold carbon dioxide from the nitrogen and to communicate the cold carbon dioxide to the second reverse flow heat exchanger. 
     
     
         5 . The system of  claim 4 , further comprising a reverse flow environmental heat exchanger in fluid communication with the first reverse flow heat exchanger, the environmental heat exchanger configured for exchanging heat between ambient temperature natural gas and warm carbon dioxide to heat the ambient temperature natural gas to warm natural gas and to cool the warm carbon dioxide to about ambient temperature. 
     
     
         6 . The system of  claim 5 , further comprising a reverse flow regenerative heat exchanger in fluid communication with the environmental heat exchanger and the engine, the regenerative heat exchanger configured for exchanging heat between warm natural gas and hot carbon dioxide to heat the warm natural gas to about engine temperature and to cool the hot carbon dioxide to warm carbon dioxide. 
     
     
         7 . A method for storing carbon dioxide exhausted from a natural gas engine, the method comprising:
 burning natural gas received from a liquefied natural gas tank in the natural gas engine to produce exhaust gas including carbon dioxide and nitrogen;   compressing the exhaust gas;   cooling the compressed exhaust gas to a liquid carbon dioxide temperature;   separating the liquid carbon dioxide and the nitrogen;   decompressing the separated carbon dioxide;   exchanging heat between the decompressed carbon dioxide and the liquefied natural gas to heat the liquefied natural gas to cold natural gas and to cool the decompressed carbon dioxide to below solid carbon dioxide temperature;   storing the solid carbon dioxide in the liquefied natural gas tank;   providing a first portion of the cold natural gas to the natural gas engine;   condensing a second portion of the cold natural gas to liquefied natural gas; and   storing liquefied natural gas in the liquefied natural gas tank.   
     
     
         8 . The method of  claim 7 , wherein compressing the exhaust gas is performed using a compressor and decompressing the separated nitrogen is performed using a first turbine configured to drive the compressor. 
     
     
         9 . The method of  claim 8 , further comprising decompressing the separated carbon dioxide using a second turbine configured to drive the compressor. 
     
     
         10 . The method of  claim 7 , wherein cooling the second portion of the cold natural gas to liquefied natural gas temperature comprises receiving energy from the engine and using the received energy for cooling the portion of the cold natural gas. 
     
     
         11 . The method of  claim 7 , wherein cooling the second portion of the cold natural gas to liquefied natural gas temperature comprises exchanging heat between the portion of the cold natural gas and liquid oxygen. 
     
     
         12 . The method of  claim 7 , wherein exchanging heat between the liquefied natural gas and the cold carbon dioxide is performed inside the liquefied natural gas tank. 
     
     
         13 . A system for storing solid carbon dioxide, the system comprising:
 a tank configured to store liquefied natural gas, the tank in fluid communication with exhaust gas including carbon dioxide gas emitted from a natural gas engine;   a heat exchanger disposed in the tank and in fluid communication with the carbon dioxide gas, the heat exchanger configured for removing heat from the carbon dioxide gas and condensing the carbon dioxide gas to solid carbon dioxide, the heat exchanger further configured for providing the removed heat to the liquefied natural gas in the tank to convert a portion of the liquefied natural gas to natural gas, the tank configured for storing the solid carbon dioxide;   a manifold configured to communicate a first portion of the natural gas from the tank to the natural gas engine; and   a refrigerator in fluid communication with the tank, the refrigerator configured to receive a second portion of the natural gas from the tank and to condense the second portion of the natural gas to liquefied natural gas, the tank further configured to receive the condensed liquefied natural gas from the refrigerator.   
     
     
         14 . The system of  claim 13 , wherein the heat exchanger comprises a nozzle configured to introduce the carbon dioxide gas into the liquefied natural gas in the tank. 
     
     
         15 . The system of  claim 13 , wherein the refrigerator comprises liquid oxygen and a heat exchanger configured to exchange heat between the cold natural gas and the liquid oxygen. 
     
     
         16 . The system of  claim 13 , further comprising a liquid oxygen heat exchanger configured to exchange heat between carbon dioxide gas and liquid oxygen. 
     
     
         17 . A method for storing ballast in a ship, the method comprising:
 removing liquefied natural gas from a liquefied natural gas tank;   burning the removed liquefied natural gas in an engine to produce energy for driving the ship and to produce exhaust gas including carbon dioxide gas;   cooling the carbon dioxide gas to produce solid carbon dioxide; and   storing the solid carbon dioxide as ballast in place of the removed liquefied natural gas in the liquefied natural gas tank.   
     
     
         18 . The method of  claim 17 , further comprising separating the carbon dioxide gas from nitrogen gas in the exhaust gas using a compressor, a turbine to drive the compressor, and a heat exchanger. 
     
     
         19 . The method of  claim 17 , further comprising:
 compressing the exhaust gas;   cooling the compressed exhaust gas to liquid carbon dioxide temperature;   separating the liquid carbon dioxide from nitrogen gas in the exhaust gas;   exchanging heat between the compressed exhaust gas and the separated liquid carbon dioxide to cool the compressed exhaust gas;   exchanging heat between the compressed exhaust gas and the separated nitrogen gas to cool the compressed exhaust gas; and   driving a turbo charger using the separated nitrogen and carbon dioxide, the turbo charger configured for compressing the exhaust gas.   
     
     
         20 . The method of  claim 19 , further comprising sequestering a portion of the solid carbon dioxide in the ocean at a depth of at least 500 feet below the surface.