Optimization of Operating Profit with Variable Load for CO₂ Capture by Aqueous Piperazine on a 460 MW NGCC

A rating model of the piperazine with the advanced stripper process downstream of a 460 MW natural gas combined cycle (NGCC) was developed in previous work, which included all process units and ancillary equipment. Surrogate rating models were used to study and optimize the operating profit at different gas turbine loads and electricity pricing scenarios. The analysis used a fixed CO2 value of $75/tonne and natural gas prices between $1 and $5/MMBTU. For an electricity price defined by the NGCC heat rate, the capture plant remained profitable when turning down the gas turbine from 95 to 50%, despite a decrease of 4220 $/h in variable profit. At a gas price above $3/MMBTU, the system maintained >98% CO2 removal, a heat duty of 2.27 GJ/tonne, and a parasitic load on the power plant below 7.3%. The breakeven natural gas price, below which it was optimal to maximize heat duty, was around $2/MMBTU. This operating condition was only optimal for a total period of a year and three months over the past decade when using the Henry Hub natural gas spot price for comparison. A scenario with high electricity prices showed that the breakeven electricity price for the capture plant was $457/MWh at an ambient temperature of 18.3 °C and a gas price of $3/MMBTU. This was due to the inclusion of a standalone boiler, which minimized the parasitic load on the power plant. All of the maximum scarcity prices in the Southwest Power Pool (SPP) between 2021 and 2023 were below this value. A scenario with negative electricity prices due to high wind power generation showed that, at a gas load of 95% and a natural gas price of $3/MMBTU, the capture plant was able to decrease the breakeven electricity price of the power plant from $21/MWh to −$10/MWh. Turning down the gas turbine to 50% reduced the breakeven price to −$20.6/MWh.