Technology Development for Supercritical Carbon Dioxide (SCO2) Based Power Cycles
The Advanced Turbines Program at NETL will conduct R&D for directly and indirectly heated supercritical carbon dioxide (CO2) based power cycles for fossil fuel applications. The focus will be on components for indirectly heated fossil fuel power cycles with turbine inlet temperature at or above 1400 ºF (760 ºC) and oxy-fuel combustion for directly heated supercritical CO2 based power cycles.
The supercritical carbon dioxide power cycle operates in a manner similar to other turbine cycles, but it uses CO2 as the working fluid in the turbomachinery. The cycle is operated above the critical point of CO2 so that it does not change phases (from liquid to gas), but rather undergoes drastic density changes over small ranges of temperature and pressure. This allows a large amount of energy to be extracted at high temperature from equipment that is relatively small in size. SCO2 turbines can have a gas path diameter as small as a few inches compared to several feet for utility scale combustion turbines or steam turbines.
The cycle envisioned for the first fossil-based indirectly heated is a non-condensing closed-loop Brayton cycle with heat addition and rejection on either side of the expander, like that in Figure 1. In this cycle, the CO2 is heated indirectly from a heat source through a heat exchanger, not unlike the way steam would be heated in a conventional boiler. Energy is extracted from the CO2 as it is expanded in the turbine. Remaining heat is extracted in one or more highly efficient heat recuperators to preheat the CO2 going back to the main heat source. These recuperators help increase the overall efficiency of the cycle by limiting heat rejection from the cycle.
Figure 1. Closed Loop SCO2 Recompression Brayton Cycle Flow Diagram
(click to enlarge)
Directly fired SCO2 cycles combust fossil fuels with oxygen and the resulting steam/CO2 mixture is used to drive the turbine. Figure 2 illustrates a potential configuration of this cycle. In this particular cycle, the remaining heat in the steam/CO2 mixture is recuperated to preheat the cooled and compressed CO2 that is used as the combustion diluent. The mixture is cooled further cooled to condense the water out and then compressed for storage.
Figure 2. Oxygen Fueled Directly Heated SCO2 Cycle
(click to enlarge)
Fossil fuels, particularly coal, can provide an ideal heat source for SCO2 cycles. Studies suggest that a supercritical CO2 oxy-fuel PFBC system has the potential to significantly increase efficiency by 9 percentage points over other pulverized coal oxy-fuel combustion configurations with a 20 percent lower levelized COE and the potential for near 100 percent CO2 capture. Water consumption and other emission profiles are also very attractive for this cycle. There are also opportunities to advance these performance numbers with higher firing temperatures made possible by advanced airfoil cooling technology.