Thermodynamic System Studies for a Natural Gas Combined Cycle (NGCC) Plant with CO2 Capture and Hydrogen Storage with Metal Hydrides

Abstract

Flexibility in natural gas combined cycle power plants (NGCC) with pre-combustion CO2 capture could be introduced with co-production of hydrogen and subsequent hydrogen storage with metal hydrides (MH). The current work presents a thermodynamic analysis and comparison between steady state ASPEN Plus models of a reference case NGCC plant with no capture and H2 storage, an NGCC plant with pre-combustion capture using gas heated - auto thermal reformer (GHR-ATR) combined with a sorption enhanced water gas shift (SEWGS) unit and a NGCC model with capture and a metal hydride (MH) based hydrogen storage unit. Results have been presented for a high temperature hydride (MgH2) and a medium temperature hydride (Na3AlH6). A net plant efficiency of about 48% was achieved for the system with only capture. Addition of hydrogen storage in this system shows that such plants can be still operated at comparable time based average efficiency of about 47% with an appropriate heat integration in the system. Operating the metal hydride at different temperatures does not reflect reduction in efficiency but a lower equilibrium temperature seems more beneficial particularly for a practical economizer in the HRSG. It is concluded that MH based hydrogen storage in NGCC plants with pre-combustion CO2 capture is a promising option to manage fluctuations in power demand and further investigations are required particularly on heat integration, effect of H2 purification technologies and economic assessment.