Optimization of main steam temperature and pressure under various operating conditions in sCO<sub>2</sub> coal-fired cycle

<p indent="0mm">Regarding supercritical carbon dioxide (sCO₂) coal-fired cycles, optimizing the main steam parameters can notably enhance the cycle efficiency. Aiming at clarifying the trend of optimization results under various operating conditions, a comprehensive model covering the boiler, turbine, compressor and heat exchanger was established in this study, and the relationships between main steam temperature (<italic>T</italic>₅), main steam pressure (<italic>P</italic>₅), cycle thermal efficiency (<italic>η</italic>_th) and capacity (<italic>W</italic>_net) and mass flow rate (<italic>m</italic>) were analyzed under various operating conditions. The results demonstrate that for <italic>W</italic>_net = <sc>300 MWe, </sc><italic>η</italic>_th monotonically increases from 47.5% to 50.4% as <italic>T</italic>₅ is increased from 570°C to 650°C, which is mainly because of the decrease in boiler losses. With the increase in <italic>P</italic>₅ from 20 to <sc>35 MPa, </sc><italic>η</italic>_th initially increases until it peaks at 49.6% at <italic>P</italic>₅ = <sc>30 MPa,</sc> after which it begins to exhibit a decreasing trend. This is attributed to the trade-off between the positive effect of boiler pressure drop reduction and the negative effect of turbomachinery efficiency reduction. At <italic>m </italic>= <sc>2500 kg/s,</sc> as <italic>P</italic>₅ is increased, <italic>η</italic>_th also increases and then decreases until it peaks to 46.9% at <italic>P</italic>₅ = <sc>29 MPa;</sc> this is attributed to the trade-off between the work done by the turbine and the work consumed by the compressor. With <italic>W</italic>_net (100–<sc>1000 MWe), </sc><italic>m</italic><sc> (500–4500 kg/s),</sc> and <italic>T</italic>₅ (570°C–650°C) monotonically increasing, <italic>P</italic>_5,op monotonically increases from 26.3, 20.2, and 26.9 MPa to <sc>37.2</sc>, 35.8, and <sc>29.5 MPa,</sc> respectively, all of which are ascribed to the more notable positive effect of <italic>P</italic>₅ increase, while the negative effect is relatively stable. The conclusions of this paper can help extend the optimization results of specific conditions to different conditions, providing an important reference for the optimization design of supercritical CO₂ coal-fired cycles.