Process Integration in Coal based Oxy-Combustion Power Plants

The oxy-combustion technology is ready for demonstration [1]. However, the energy penalty and investment cost related to CO2 capture are still too high to implement this technology in the power industry. The thermal efficiency penalty is currently around 10% points, and as discussed in [2] it is commonly expected to be reduced to below 6% points in the near future. Improvements on the sub-units, such as the air separation unit (ASU), the CO2 compression and purification unit (CPU), the burner and the steam cycle, have drawn considerable attention in the past. In addition, process integration is expected to take an important part in reducing the energy consumption and the cost. One example is to optimize the purity of the O2 to be supplied for combustion with respect to the energy consumption in the ASU and the CPU. Another example is to recover O2 and/or CO2 from the inerts (O2, N2, Ar and CO2) separated from the CPU. A third example is to integrate the compression heat from the ASU and/or the CPU with the steam cycle, i.e. using the compression heat to preheat the boiler feedwater. Our previous study includes the following work: (1) A detailed thermodynamic study has been performed on an entire oxy-combustion process. As a result of the study, various measures to improve the plant performance have been proposed. The minimum thermal efficiency penalty related to CO2 capture is calculated to be 3.4% points based on the second law of thermodynamics [3]. (2) A recuperative vapor recompression (RVRC) air distillation scheme has been developed for O2 supply. The advantage is that the (significant) compression of the O2 in the air feed is avoided. The total energy consumption for the ASU is reduced by 5% compared to traditional dual-reboiler ASUs [4]. (3) A techno-economic study has been performed to improve the CO2 conditioning process [5]. The two-stage flash process has been proven to be more cost effective compared to the cases of one-stage flash and three-stage flash. However, a higher CO2 recovery rate is obtained in the three-stage flash case. In addition, alternatives for utilizing the compression heat from the ASU and the CPU have been investigated [2]. A promising alternative is to use the compression heat for preheating the boiler feedwater. This paper introduces our recent work on this topic.