Techno-economic analysis of combined cycle power plants integrated with chemical looping reforming and CO2 capture

Abstract

The current thesis is a part of the EU FP7 Project titled NanoSim. It focuses on techno-economic analysis of combined cycle power plants with integrated pre-combustion CO2 capture and reforming of Natural Gas (NG). The process consists of reforming of NG, Water-Gas Shift (WGS) reactors, CO2 capture and compression section, and a hydrogen-fueled combined cycle power plant. Two reactor concepts for reforming of NG, Chemical Looping Reforming (CLR) and Gas Switching Reforming (GSR), were considered in this thesis. The respective integrated processes are denoted as CLR-CC and GSR-CC. Both the CLR and GSR involve gas-solid reactions and use a metallic oxygen carrier for the reforming of NG. Exergy analysis carried out shows that CLR has a better thermodynamic potential when compared to the traditional gas-gas partial oxidation process.

The design pressure in the CLR was found to be an important parameter in the CLR-CC process that effects the process design and integration. Hence, the CLR-CC process was designed and analysed at different design pressures in the CLR between 5 to 30 bar. The net electrical efficiency of the process increases with an increase in pressure. Anyhow, beyond a pressure of 18 bar, which is also the pressure of the air bleed from the compressor discharge of the selected gas turbine system (F-class gas turbine system in this case), an additional air compressor is required with relatively lower gain in the net electrical efficiency. It was also understood that the reforming and water-gas shift reactions are exothermic, and the heat recovery from these reaction steps to produce steam for the steam cycle in the power plant affects the net electrical efficiency. Different options for heat integration were analysed without modifying the basic design of the Heat Recovery Steam Generator (HRSG). The net electrical efficiency of the CLR-CC process was estimated to be between 40.6 and 46.5%. Producing high-pressure steam instead of low-pressure steam from heat recovery from reforming and water-gas shift reactions, and integrating with HRSG shows a difference of 4%-points in the net electrical efficiency. To carry out the techno-economic analysis of the CLR-CC, a 1D model (includes kinetics of gas-solid reactions and hydrodynamics in the reactor) of the CLR developed in MATLAB was linked with the steady state process models for WGS, CO2 capture and compression section in Aspen Hysys V8.6, and the steady state combined cycle power plant model in Thermoflex component of the Thermoflow Suite V26. The multi-scale model linking approach was established to link the dynamic 1D model of the CLR with the steady state process models for a smooth interaction and flow of process data between them. With the help of this linking approach, a sensitivity study for the effect of air flowrate in the oxidation reactor, steam/carbon ratio in the fuel reactor and the oxidation reactor outlet temperature of the CLR, on the net electrical efficiency was carried out. The levelised cost of electricity (LCOE) of the CLR-CC was also estimated and it was found that it is highly sensitive to the fuel cost followed by the process contingency costs (capital costs accounting for maturity of the process technology). The LCOE of the CLR-CC process lies between 75.3 and 144.8 $/MWh. The CO2 avoidance rates of more than 85% is possible in CLR-CC.

Techno-economic assessment of the GSR-CC process was carried out and the net electrical efficiency, CO2 avoidance rates and LCOE were estimated. Sensitivity studies with respect to oxygen carrier utilization and steam/carbon ratio in the GSR is presented in the thesis. The net electrical efficiency of the GSR-CC process lies between 45.1 and 46.2% with CO2 avoidance rates of more than 95%. A case without the WGS in the GSR-CC was also studied and the net electrical efficiency was estimated to be around 47.3%. The LCOE of the GSR-CC process is found to be highly sensitive to the fuel cost and can be as low as 80 $/MWh when the NG price is 4.5 $/GJ-LHV (when compared to 9.8 $/GJ-LHV considered in analysis of GSR-CC). There is still scope to improve and optimize the CLR-CC and GSR-CC processes. Further research on these processes can help in improving the techno-economic behavior and make it competitive against the post-combustion capture Technologies.