Power generation with CO_2 capture: Technology for CO_2 purification

摘要

The goal of this paper is to find methodologies for removing a selection of impurities (H_2O, O_2, Ar, N_2, SO_X and NO_X) from CO_2 present in the flue gas of two oxy-combustion power plants fired with either natural gas (467 MW) or pulverized fuel (596 MW). The resulting purified stream, containing mainly CO_2, is assumed to be stored in an aquifer or utilized for enhanced oil recovery (EOR) purposes. Focus has been given to power cycle efficiency i.e.: work and heat requirements for the purification process, CO_2 purity and recovery factor (kg of CO_2 that is sent to storage per kg of CO_2 in the flue gas). Two different methodologies (here called Case I and Case II) for flue gas purification have been developed, both based on phase separation using simple flash units (Case I) or a distillation column (Case II). In both cases purified flue gas is liquefied and its pressure brought to 110 atm prior to storage. Case I: A simple flue gas separation takes place by means of two flash units integratedin the CO_2 compression process. Heat in the process is removed by evaporating the purified liquid CO_2 streams coming out from both flashes. Case I shows a good performance when dealing with flue gases with low concentration of impurities. CO_2 fraction after purification is over 96% with a CO_2 recovery factor of 96.2% for the NG-fired flue gas and 88.1% for the PF-fired flue gas. Impurities removal together with flue gas compression and liquefaction reduces power plant output of 4.8% for the NG-fired flue gas and 11.6% for the PF-fired flue gas. The total amount of work requirement per kg stored CO_2 is 453 kJ for the NG-fired flue gas and 586 kJ for the PF-fired flue gas. Case II: Impurities are removed from the flue gas in a distillation column.Two refrigeration loops (ethane and propane) have been used in order to partially liquefy the flue gas and for heat removal from a partial condenser. Case II can remove higher amounts of impurities than Case I. CO_2 purity prior to storage is over 99%; CO_2 recovery factor is somewhat lower than in Case 1:95.4% for the NG-fired flue gas and 86.9% for the PF-fired flue gas, reduction in the power plant output is similar to Case I.Due to the lower CO_2 recovery factor the total amount of work per kg stored CO_2 is somewhat higher for Case II: 457 kj for the NG-fired flue gas and 603 kj for the PF-fired flue gas.