Post combustion CO₂ capture in power plant using low temperature steam upgraded by double absorption heat transformer

In CO₂ capture retrofit unit of existing coal-fired power plants, energy level mismatch between extraction steam from turbines and CO₂ regeneration process always results in large exergy destruction and low thermal efficiency. Thus, a new CO₂ capture system driven by double absorption heat transformer is proposed. Through the absorption heat transformer, low-temperature steam is upgraded into a higher energy level to match the temperature of CO₂ regeneration. Also, flue gas heat is partly recovered to preheat the circulating water from CO₂ capture process to further decrease system energy penalty. Aspen Plus 11.0 is used to simulate the system and parameters of key processes are validated by experimental values. It is shown that with 90% CO₂ capture, the thermal efficiency of the power plant with proposed CO₂ capture system is enhanced by 1.25 percentage points compared with traditional method. And the efficiency enhancement of the proposed system has a trend of increase first and then decrease with CO₂ capture rate growth. For a 350 MW coal-fired power plant, the optimum CO₂ capture rate is 53.65% and the corresponding efficiency enhancement is 2.06 percentage points. Exergy analysis shows that the exergy destruction in CO₂ separation and steam condensation process can decrease by 49.5% in the proposed system, and thereby the exergy efficiency is 1.85 percentage points higher than the conventional method. Furthermore, the cost of CO₂ avoided and cost of electricity of the proposed system will be reduced by 10.7 $/t-CO₂ and 1.9 $/MW h, respectively.