Energy-efficient CO2 liquefaction for oxy-combustion power plant with ASU-CPU integration enhanced by cascaded sub-ambient energy utilization and waste heat recovery

- Novel design for integrating the sub-ambient subsystems of oxycombustion is proposed.
- High temperature differences between systems used for efficient power recovery.
- Equation based optimization reduces power requirement by 15.2% over prior designs.

In the oxycombustion process, the cost of carbon capture is directly related to the energy requirements of the Air Separation Unit (ASU) and CO2 Processing Unit (CPU). In this paper, an integrated sub-ambient process is proposed to reduce the energy expense of these processes. Intermediate recuperative Rankine cycles (IRRC) driven by cryogenic energy from liquid N2 in the ASU are able to utilize waste heat from condensing CO2 and compression process in the CPU at high efficiency. Consequently, the high boiling point temperature difference between N2 and CO2 is converted from a problem of highly irreversible heat transfer to the benefit of efficient power recovery. Furthermore, a novel CPU process featuring post-separation direct pumping of CO2 is introduced to achieve 15.2% savings in power. Finally, the efficacy of these new processes is demonstrated and further improved by optimization of the ASU-CPU coupled system, based on an equation oriented modeling framework and large scale nonlinear programming (NLP) technology.