Supercritical CO2


| Working fluid of the future?


Over the last two decades, there has been a significant R&D effort directed at power generation cycles with supercritical carbon dioxide as the working fluid. Due to the unique properties of CO2


at pressures and temperatures


above its critical point, it is possible to design compact turbomachinery with high efficiency for diverse applications ranging from nuclear power to waste heat recovery. An important imminent development is the entry into operation, expected shortly, of the USDOE funded 10 MWe STEP Demo sCO2


pilot plant facility on the campus of Southwest


Research Institute, San Antonio, TX, USA, with project participants including GE and GTI Energy. The potential of the oxycombustion sCO2


Allam–Fetvedt Cycle (with its inherent carbon capture capabilities, eliminating the need for expensive chemical process plant add-ons) is also attracting considerable interest


S. Can (John) Gülen Bechtel Fellow, Bechtel Infrastructure & Power, Inc The supercritical CO2 (sCO2 ) cycle is a closed,


Brayton cycle. If the heat rejection part of the cycle is below the critical point and entails condensation of CO2


, it is referred to as sCO2


Rankine cycle. Its emergence as a potential rival to the conventional steam Rankine cycle for electric power generation can be traced to the late 1960s. A key modification to the recuperated cycle is splitting the hot CO2


flow downstream


of the recuperator to enable reduced cycle heat rejection for higher efficiency. At present, this is the most widely investigated sCO2


cycle


variant – known as the “part-“ or “split-flow” re-compression cycle – with a cycle pressure ratio (PR) of about 3:1 and cycle maximum temperatures up to 760°C.


Figure 1. Schematic


description of the semi-closed oxy-combustion Allam cycle


recuperated heat engine cycle with the working fluid state-points (partially or wholly) above the critical point of carbon dioxide (73.8 bar and 31.1°C). If the heat rejection part of the cycle is in the supercritical region, it is referred to as sCO2


at the discharge of the re-compressor. One effect of this modification is to increase the temperature range of the cycle heat addition, which makes it more suitable to a bottoming cycle, ie, heat recovery applications. The other effect of the partial cooling modification is an increase in cycle PR, from 3:1 to about 4:1 by enabling lower turbine exhaust pressure. Overall, though, everything else being the same, split-flow re-compression has comparable efficiency but the partial cooling cycle boosts plant net output significantly.


A further modification of the simple recuperated cycle, known as partial cooling, entails the addition of a pre-cooler and pre- compressor upstream of the re-compressor. Flow split takes place at the discharge of the pre- compressor. The net effect of the pre-cooler and pre-compressor is to reduce the temperature of sCO2


Until the early 2000s, sCO2 power cycle technology was dormant due to limited


availability of and/or experience with high temperature–pressure materials and high- performance heat exchangers (with low pressure loss and high effectiveness, ie, above 95%) requisite for acceptable cycle efficiency with low cycle PR. Since then, there has been a veritable renaissance in sCO2


power generation cycles


for applications including nuclear, concentrated solar, fossil-fired (coal and natural gas), and waste heat recovery. So far, the experience has been limited to small-scale pilot and/or demonstration facilities with indirect heat addition (ie, no combustion, fully closed cycle).


The most intriguing application of sCO2 technology is the semi-closed variant with direct heat addition via oxycombustion, utilising natural gas or syngas (generated by gasification). Two early variants of oxycombustion cycles were the Graz and Matiant cycles, which did not proceed beyond paper concepts. The Matiant cycle is the more cumbersome of the two and comprises an sCO2 a regenerative CO2


Graz cycle is simpler in that it is similar in basic


construction to the simple recuperation sCO2 cycle. Combustion products consisting of CO2 and H2


O are expanded in a turbine, followed by


a condenser where steam is condensed and CO2 is vented, compressed, and transported to a site for utilisation or sequestration. Condensate is pumped and recycled to the combustor where it acts as the moderator in the stoichiometric combustion process with O2


. Another version


of the Graz cycle was the basis for a demo plant with a General Electric J79 turbine, minus the compressor, to be driven directly by a 170 MWt high-pressure, oxycombustor.


Allam–Fetvedt Cycle A variation of the cycles described above, sometimes referred to as water cycles because of recycled H2


is the Allam cycle, where the combustion moderator is recycled CO2


Allam cycle, CO2 at 300 bar. In the constitutes (nominally) 95% of


the fluid flow in the combustor (by mass) with the rest, 5%, made up of oxygen and fuel. The resulting combustion product is roughly 90%(v) CO2


. Oxygen for combustion is generated by a cryogenic air separation unit (ASU). Carbon


10 | January/February 2023| www.modernpowersystems.com


Rankine cycle combined with Brayton cycle with reheat. The


O used as combustion moderator,


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