(80c) Adiabatic Compression As an Open Heat Pump for Process Integration within Oxy-Combustion Coal-Based Power Plants

Coal was responsible for 41% of global energy related CO₂ emissions in 2005 and is expected to be responsible for 44% in 2030 [1]. Power generation is the largest application of coal and amounts to more than 40% of the annual coal consumption in the world [1]. Thus coal based power plants are extremely interesting candidates when Carbon Capture and Storage (CCS) is going to be implemented. For coal-based power plants, oxy-combustion is a promising technology to enable the capture of CO₂, since the reduction in power efficiency and the additional investment cost are less than (or at least comparable to) other CO₂ capture technologies. The possibility of co-capturing SOx and NOx also makes oxy-combustion competitive. In addition, some other environmental impacts (rather than the global warming impact) related to CCS are smaller for oxy-combustion than pre- and post-combustion.

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 oxy-combustion technology is ready for demonstration in both new power plants and for retrofitting existing facilities [2]. The main challenge for implementing this technology is the large power penalty caused by the air separation unit (ASU) and the CO₂ compression and purification unit (CPU). A 500 MWe power plant will consume around 10,000 tons of O₂ per day. Cryogenic air separation through distillation is the only commercially available technology for large volume O₂ production. 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) [3]. The ASU and the CPU contribute 6.6 and 3.7 percentage points respectively. It is expected that the total efficiency penalty will be reduced to less than 6% points in the future [4], while the theoretical minimum is 3.4% points when all the unit operations in the ASU and the CPU are assumed reversible [5]. Process integration is expected to play an important role in reducing this power efficiency penalty.

Both the ASU and the CPU are subambient separation processes. Mechanical energy (work) is consumed by compressors in the refrigeration processes. The compression heat is normally removed by cooling water to lower the operating temperature, and 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 from the power cycle. Adiabatic compression may be more favorable than multi-stage compression with interstage cooling when the compression heat is utilized for preheating the boiler feedwater. In this case, compression heat is upgraded (the temperature is lifted) through adiabatic compression, which acts as an open heat pump. As a result, the boiler feedwater is preheated to a higher temperature. The extraction of steam from the higher pressure levels can be reduced, resulting in more power generation from the steam turbines. On the other hand, however, more work is consumed in the adiabatic compression case.

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. Such heat integration has been investigated by several case studies. The features of the open heat pumps will be addressed. The results show that adiabatic compression may be a more favorable compression scheme for the ASU. The thermal efficiency of the power plant increases by 0.78% points when the compression heat from the ASU and the CPU is integrated.

References

[1]   EIA, International Energy Outlook 2008.

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

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

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

[5]   C. Fu, T. Gundersen. Exergy analysis of an oxy-combustion process for coal-fired power plants with CO₂ capture. Submitted to Fuel (under review), June 2011.