(41d) Heat Integration in Coal Based Oxy-Combustion Power Plants

Oxy-combustion is a competitive technology to enable the capture of CO₂ from coal based power plants. The reduction in power efficiency and the increment of investment cost are less than (or at least comparable to) other CO₂ capture technologies. In addition, the possibility of co-capturing SOx and NOx is attractive when oxy-combustion is applied in coal based power plants. The core concept of oxy-combustion is to use high purity oxygen instead of air for the combustion process so that the flue gas is composed mainly of CO₂ and H₂O. The CO₂ can be separated by condensing the H₂O and then purified by chilling. After being compressed to a dense phase, the CO₂ can be transported to appropriate storage sites.

The oxy-combustion technology is ready for demonstration in both new and retrofitting power plants [1]. The energy penalty and investment cost related to CO₂ capture are the main barriers to implement this technology. Commercially available air separation technologies for high volume oxygen production which can be applied in oxy-combustion plants are based on cryogenic distillation. When a traditional double-column distillation cycle is applied for the supply of O₂ in a coal based oxy-combustion power plant, the total thermal efficiency penalty related to CO₂ capture is around 10.3% points (based on the higher heating value) [2]. The cryogenic air separation unit (ASU) contributes 6.6% points and the CO₂ compression and purification unit (CPU) contributes 3.7% points. The total efficiency penalty is expected to be reduced to less than 6% points in the future [3], while the theoretical minimum is 3.4% points when all the unit operations in the ASU and the CPU are assumed reversible [2].

For oxy-combustion power plants, the required O₂ purity is around 95 mole% and the supply pressure is slightly above atmospheric pressure. These new specifications on the O₂ product provide considerable opportunities for improving the ASUs. When the technology is shifted from thermally-coupled distillation to vapor recompression distillation, the energy consumption can be reduced by 7.5% [4]. In addition, when the principle of distributed reboiling is applied and combined with vapor recompression distillation, the energy consumption can be reduced by 17.1%. Thus the thermal efficiency penalty related to CO₂ capture can be reduced from 10.3 to 9.2% points. 

Both the ASU and the CPU are sub-ambient separation processes. Mechanical work is consumed by the compressors in the refrigeration processes. The compression heat is generally removed by cooling water to lower the operating temperature, thus reduce the work consumption in the compressors. An alternative option for the compression heat from the ASU and the CPU is to be integrated with the steam cycle by partially preheating the boiler feedwater, so that less steam is extracted. A preliminary study shows that such integration increases the thermal efficiency by around 1.5% points [5], thus the efficiency penalty related to CO₂ capture is expected to be reduced to around 7.7% points. 

This paper presents a detailed study on the integration of the compression heat with the steam cycle. The integration potential is first investigated in the case when interstage cooling is applied in the compression processes. In this case, the compression work in the ASU and the CPU is not influenced by the integration. Then fully and/or partially adiabatic compression is applied instead of interstage cooling, so that the temperature of the compression heat is lifted. The upgraded compression heat can be used to preheat the boiler feedwater in the steam cycle to a higher temperature, thus less steam will be extracted at the higher pressure levels. However, more work will be consumed in the compression processes due to higher operating temperatures. In addition, the compressor efficiency should be lower in this case since the mass flow through the compressors is smaller at higher temperatures. Thus the integration problem becomes much more complex when fully and/or partially adiabatic compression is applied. Such integration issues will be discussed in this paper.

References

[1] IEAGHG, Greenhouse News, 2011:  NO. 104.  

[2] C. Fu, T. Gundersen. Heat integration of an oxy-combustion process for coal-fired power plants with CO₂ capture by pinch analysis. Chem. Eng. Transactions, 2010: 21, 181-186.

[3] J.P. Tranier, R. Dubettier, N. Perrin. Air separation unit for oxy-coal combustion systems. 1st International Oxyfuel Combustion Conference, 7-11 September, 2009, Cottbus, Germany.

[4] C. Fu, T. Gundersen, D. Eimer. Air separation. GB patent, application number: GB1112988.9, 2011.

[5] T. H. Zeiner. Process integration potentials in coal based power plants using oxy-combustion. Master thesis, Department of Energy and Process Engineering, Norwegian University of Science and Technology, 2012.