A PC unit with a complete set of advanced criteria-emissions controls is shown in Fig. 2.3. It can be viewed as consisting of three blocks: the boiler block, the steam-cycle steam-turbine block, and the flue gas clean-up block as shown in Fig. 2.4. The design and operating conditions of the steam-cycle block largely determines the generating efficiency of the unit. For most existing PC units, the design and operating conditions of the steam cycle is below the critical point of water, which is referred to as subcritical operation. Operation above the critical point of water is referred to as supercritical operation. Ultra-supercritical is used to denote operation
above a somewhat arbitrary set of operating parameters in the supercritical region. These ranges and typical generating efficiencies are summarized below.
For PC units, typical operating conditions and overall electrical generating efficiency are:
• Subcritical Unit
• Steam-cycle operation to 550°C (1,025°F) and 2,400 psi
• 33-37% overall generating efficiency (HHV)
• Supercritical Unit
• Steam-cycle operation to 565°C (1,050°F) and 3,530 psi
• 37-42% overall generating efficiency (HHV)
• Ultra-Supercritical Unit
• Steam-cycle operation 600-620°C (1,110-1,150°F) and 4,650 psi
• 42-45% overall generating efficiency (HHV)
Moving from subcritical to ultra-supercritical generating conditions reduces coal consumption by over 20% per kWe-h of electricity generated. Moving from subcritical generating conditions to typical supercritical generating conditions can reduce coal consumption by over 10% per kWe-h of electricity generated. Obviously, the higher the generating efficiency the lower the CO2 emissions per kWe-h of electricity generated. At a minimum, units need to be designed and operated at the highest efficiency that is economically justified to reduce CO2 emissions. Current R&D programs are focusing on developing and proving materials and operating conditions above current ultra-supercritical conditions that could provide even higher PC generating efficiency. The next step in USCPC is 650°C (1,200°F) with generating efficiency exceeding 45%, with the next tranche being to 760°C (1,400°F) with efficiencies exceeding 48%. Materials properties, fabrication, and maintenance currently limit reaching these latter conditions.
The U.S. coal fleet consist largely subcritical generating units, with a limited number of supercritical units. Interest in supercritical technology in the U.S. has recently increased. India and China have built almost exclusively subcritical technology, but both countries have begun to construct a mix of sub and supercritical units. Meanwhile, Europe and Japan have built about a dozen ultra-supercritical units during the last decade . Using modern materials technology, these units have reliability records equal to subcritical unit operation. The U.S. is behind in PC generating efficiency with a fleet average of about 33% .
A marked reduction of CO2 emissions from PC power generation would require CO2 capture from the flue gas, involving addition of another unit to the flue gas train. Today, the choice for CO2 capture technology for PC generation would be amine absorption. Amine CO2 capture is commercially proven in smaller-scale applications, including recovery of CO2 from the flue gas of several smaller units for beverage, food and other industrial uses. The application of CO2 capture from power plant flue gas is illustrated in Fig. 2.5. CO2 is captured in the amine solution and then must be recovered from the solution. A large amount of energy is required to recover the CO2 from the amine solution, regenerating the solution to capture more CO2. A smaller amount of energy is needed to compress the CO2 to a supercritical fluid. An energy diagram illustrating the parasitic energy requirements for CO2 capture from a subcritical PC unit is shown in Fig. 2.6. For PC generating units that are designed for capture, the generating efficiency is reduced by about 9-11% points independent of steam cycle type. For subcritical, supercritical, and ultra-supercritical units estimated generating efficiency reductions are from 34% to 25%, from about 39% to 29%, and from 43% to 34% respectively for example. To maintain constant electrical output requires a 38-40% increase in coal consumption when a CO2 capture designed unit is compared with a non-capture designed unit [10, 11, 13].
The energy comparison illustrated in Fig. 2.6 is for units that are designed specifically for capture or no-capture, and thus, all the components are of optimum
CO2 Recovery 34.3 (Heat)
I Compressor „ 2 1 r Recovery
CO2 Recovery 34.3 (Heat)
I Compressor „ 2 1 r Recovery
size and performance to provide the maximum total unit efficiency. If a unit designed for no-capture is retrofitted for capture at a later date, the efficiency penalty for CO2 capture is larger because some of the components become suboptimum. This will be addressed further in the discussion of retrofitting.
Other approaches to CO2 capture are being examined. For example, the use of chilled ammonia absorption is claimed to significantly reduce these energy requirements and is being evaluated on a 1.7 MWe system at a 1,224 MWe commercial coal-fired generating station in Wisconsin [16, 17]. Additional approaches being pursued include unique framework solids, algal systems, frosting, and other adsorbents. These are further from economic evaluation or demonstration. Improvements can be expected for absorption and adsorption systems, but there are physiochemical and thermodynamic limitations to how large these improvements will be.
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