Advanced ultra supercritical (A-USC) systems operate at high temperature and pressure conditions, increasing plant efficiency, but requiring the use of expensive advanced materials with high-temperature resistance and strength such as nickel alloys. This study is a part of EPRI’s Fossil Fleet Program activities aimed at presenting the best near-term economic case for pulverized coal (PC) with fully integrated post-combustion CO2 capture (PCC). Previous studies in this series have focused on integration of the best available PCC technology to provide the minimum performance penalty. This study, however, focuses entirely on new design concepts that may well provide the highest efficiency and lowest cost for a baseline A-USC PC plant.
The high cost of A-USC system materials is critical to overall plant economics, particularly when using a conventional boiler layout. These costs can be attributed to the long vertical runs and cost of affected piping such as final superheater and reheater tubes, sub-headers, manifolds, and main transportation pipework. An industry move up to the A-USC plant operating temperature range of 1300°F (~704°C) and beyond requires innovative boiler/plant designs aimed at minimizing material use and, consequently, capital costs. The overarching goal is to align the cost of electricity generated from these high efficiency plants with that of lower efficiency conventional plants.
The key aim of this study was to evaluate the design, performance, capital cost, and operation and maintenance costs for three A-USC PC system cases, based on different boiler/plant arrangements, in order to quantify and understand any potential cost savings between novel and conventional designs. Following are the cases considered:
Case 1—Baseline for the A-USC PC plant: This case is based on conventional steam generator and steam turbine arrangements, differing from the USC PC plant by the use of nickel alloy components where required.
Case 2—Inverted boiler design concept: This concept reduces the length of the main steam pipes—from the boiler superheater to the steam turbine check valves—by lowering the elevation of the superheater outlet close to the height of the steam turbine, which is conventionally located at ground floor.
Case 3—Split turbine design concept: This concept shortens the steam pipeline length by raising the high pressure and intermediate pressure component section of the steam turbine. These components are raised to the elevation of the main steam and hot reheat steam boiler outlets on a concrete slab supported by a steel structure while maintaining a conventional boiler arrangement.
The project team evaluated performances and cost of three greenfield A-USC PC plants, located in Kenosha, Wisconsin. Design and costing knowledge related to the high-temperature materials and innovative arrangements were developed from the literature data available on A-USC PC systems, with an independent assessment of performance and cost for the different alternatives. For the more conventional process areas of the plants, inputs were obtained from various vendors or licensors from previous EPRI studies or were referenced from public studies.
Using the same Alloy 617 for the high temperature and pressure components, and obtaining steam conditions of 5235 psig at 1260°F/1295°F (~682°C/~702°C), for all three cases considered resulted in the following:
An estimated net electrical efficiency of 41% (higher heating value, or HHV) for the A-USC plants (slightly more than 2 percentage points of improvement over current USC designs)
No significant performance advantage for the inverted or split turbine cases over the conventional base case A-USC plant arrangement
Notable reductions in length of superheater and hot reheat nickel alloy piping for both the inverted and split turbine cases, yet capital costs still remain too high
The study indicates the need for more radical designs to A-USC plants than the conservative approach presently investigated. Additional “unconventional” steam generator designs with specific materials selections, steam conditions, and equipment arrangements need to be considered, evaluated, and optimized to mitigate the high nickel alloy costs of new build A-USC plants. Current budget constraints limit the number of cases that could be investigated here to three, but others may be considered in future.
Applications, Value, and Use
Controlling CO2 emissions while maintaining competitive electricity prices and sustaining economic growth presents unprecedented economic and technical challenges. This work is intended to discover ways in which new A-USC PC plants can be designed to be competitive. This report adds to the series of evaluations that EPRI has undertaken on the potential for improving cost and performance of baseline coal power while optimizing PCC system integration.