(158c) Optimal Integration of Compression Heat Within Oxy-Combustion Coal-Based 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. The emissions of both SOx and NOx can also be reduced when the oxy-combustion technology is applied [1]. 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.

The main challenge for implementing the oxy-combustion technology is that a large power penalty is caused by the air separation unit (ASU) and the CO₂ compression and purification unit (CPU). Due to the high volumes of gases to be processed, sub-ambient separation is used in the two units. The thermal efficiency penalty related to CO₂ capture is around 10% points (based on the higher heating value) [2]. Mechanical work is consumed by the compressors for producing the refrigeration energy. In order to reduce the compression work, multi-stage compression with interstage cooling can be applied. An interesting way of utilizing the compression heat is to preheat the boiler feedwater (BFW) in the steam cycle, so that less steam is extracted for the regenerative preheating of the BFW and thus more steam expands through the steam turbines for power generation [3]. 

When the compression heat is integrated with the steam cycle, multi-stage compression with interstage cooling may not be necessary. The compression heat can be upgraded (the temperature is lifted) by fully or partially adiabatic compression in order to preheat the BFW to a higher temperature. The extraction of steam at the higher pressure levels can be reduced, resulting in more power generated from the steam turbines. On the other hand, however, more work is consumed in the adibatic compression case. Thus there exists a trade-off between the work consumed in the compression processes and the power generated from the steam turbines. Optimal compression schemes should be tailored for the heat integration. 

This paper presents a mathematical optimization study of the heat integration. Two challenges will be addressed: (1) how to integrate a heat stream with the steam cycle, and (2) how to optimize the compression schemes. The influences of the compressor efficiency and the temperature differences in the gas/water heat exchangers on the plant performance will be investigated. 

References

[1] B.J.P. Buhre, L.K. Elliott, C.D. Sheng, R.P. Gupta, T.F. Wall. Oxy-fuel combustion technology for coal-fired power generation. Progress in Energy and Combustion Science, 31 (2010) 1068-1076.

[2] C. Fu. Process integration in coal based oxy-combustion power plants with CO₂ capture. PhD thesis, Norwegian University of Science and Technology, 2012.

[3] C. Fu, T. Gundersen. Heat integration in coal based oxy-combustion power plants. AIChE Annual Meeting, 28 October - 2 November, 2012, Pittsburgh, USA.