Adapting Gas Turbines to Zero Emission Oxy-Fuel Power Plants

Future power plants will require some type of carbon capture and storage (CCS) system to mitigate carbon dioxide (CO₂) emissions. The most promising technologies for CCS are: oxy-fuel (O-F) combustion, pre-combustion capture, and post-combustion capture. This paper discusses the recent work conducted by Siemens Power Generation, Florida Turbine Technologies, Inc. (FTT) and Clean Energy Systems, Inc. (CES) in adapting high temperature gas turbines to use CES’s drive gases in high-efficiency O-F zero emission power plants (ZEPPs). CES’s O-F cycle features high-pressure combustion of fuel with oxygen (O₂) in the presence of recycled coolant (water, steam or CO₂) to produce drive gases composed predominantly of steam and CO₂. This cycle provides the unique capability to capture nearly pure CO₂ and trace by-products by simple condensation of the steam. An attractive O-F power cycle uses high, intermediate and low pressure turbines (HPT, IPT and LPT, respectively). The HPT may be based on either current commercial or advanced steam turbine technology. Low pressure steam turbine technology is readily applicable to the LPT. To achieve high efficiencies, an IPT is necessary and efficiency increases with inlet temperature. The high-temperature IPT’s necessitate advanced turbine materials and cooling technology. O-F plants have an abundance of water, cool steam ∼200°C (400°F) and CO₂ that can be used as cooling fluids within the combustor and IPT systems. For the “First Generation” ZEPP, a General Electric J79 turbine, minus the compressor, to be driven directly by CES’s 170 MW_t high-pressure oxy-fuel combustor (gas generator), has been adapted. A modest inlet gas temperature of 760°C (1400°F) was selected to eliminate the need for turbine cooling. The J79 turbine operating on natural gas delivers 32 MW_e and incorporates a single-stage free-turbine that generates an additional 11 MW_e. When an HPT and an LPT are added, the net output power (accounting for losses) becomes 60 MW_e at 30% efficiency based on lower heating value (LHV), including the parasitic loads for O₂ separation and compression and for CO₂ capture and compression to 151.5 bar (2200 psia). For an inlet temperature of 927°C (1700°F), the nominal value, the net output power is 70 MW_e at 34% efficiency (LHV). FTT and CES are evaluating a “Second Generation” IPT with a gas inlet temperature of 1260°C (2300°F). Predicted performance values for these plants incorporating the HPT, IPT and the LPT are: output power of approximately 100–200 MW_e with an efficiency of 40 to 45%. The “Third Generation” IPT for 2015⁺ power plants will be based on the development of very high temperature turbines having an inlet temperature goal of 1760°C (3200°F). Recent DOE/CES studies project such plants will have LHV efficiencies in the 50% range for natural gas and HHV efficiencies near 40% for gasified coal.