Utilization of Low-Grade Heat and Process Integration in Oxy-Combustion Coal Based Power Plants

CO2 Capture and Storage (CCS) is a technology that collects and concentrates the CO2 emitted from large point sources, transports it to a suitable storage location and stores it away from the atmosphere for a long time. CCS is expected to play a pivotal role in stabilizing the atmospheric greenhouse gas levels within acceptable limits. It has been estimated that the average contribution of CCS in total emission reduction would range from 15% to 54% for stabilization targets of 750 ppmv to 450 ppmv CO2 respectively[1]. The deployment of CCS could help bring down the overall cost of mitigation of climate change in the longer run[2].

Oxy-combustion is one of the three main capture routes that is considered to be promising and in the demonstration phase. This particular method that burns the fuel in oxygen instead of air has several environmental and economic advantages[3, 4]. The upstream production of oxygen and the downstream purification and compression (CPU) are responsible for an efficiency penalty of around 10 percentage points[4]. Novel concepts such as pressurized coal combustion improves the overall efficiency of an oxy-combustion power plant with CO2 capture and helps reduce the capture penalty[5]. Even with pressurized coal combustion, the efficiency penalty is still considerable and needs to be reduced in order to make the oxy-combustion technology commercially viable in the near future.

Operating the boiler at a higher pressure also helps recover more latent heat from the flue gases before it enters the CPU for final purification and compression. This low grade heat is usually recovered by employing special types of heat exchangers that are resistant to acid corrosion. They are made of highly inert materials such as Teflon and are called acid condensers or plastic heat exchangers. The most common way of utilizing this heat is to heat the boiler feed water in the steam cycle. This reduces the amount of steam extracted from the turbines and hence increases the gross power produced in the steam turbines. Apart from this method, low temperature power cycles such as Organic Rankine Cycles or CO2 Brayton cycles can be used to improve the overall efficiency of the power plant[6]. The rising cost of electricity could justify the additional capital investment required for this method.

Both methods mentioned above for utilization of the low grade heat are designed independently of the steam cycle and the integration with the rest of the power plant is not optimized. Thus, development of a more tightly integrated power plant using mathematical modelling and optimization principles could yield better solutions at a much lower capital cost[7]. In this context, a systematic approach is required to achieve optimal utilization of the available low-grade heat. Simulation of an oxy-combustion coal based power plant with CO2capture is presented here along with various integration options and their efficiency improvement potential. Also, the current work based on mathematical modelling is discussed in brief.

 References:

[1] T. Morita, N. Nakicenovic, J. Robinson, Overview of mitigation scenarios for global climate stabilization based on new IPCC emissions scenarios, Environmental Economics and Policy Studies, 3 (2000) 65-88.

[2] J.A. Edmonds, P. Freund, J.J. Dooley, The role of carbon management technologies in addressing atmospheric stabilization of greenhouse gases., in:  Proceedings of the Fifth International Conference on Greenhouse Gas Control Technologies, 2000.

[3] F. Petrakopoulou, G. Tsatsaronis, A. Boyano, T. Morosuk, Exergoeconomic and Exergoenvironmental evaluation of power plants including CO2 capture, Chemical Engineering Research and Design, 89 (2011) 1461-1469.

[4] M. Kanniche, R. Gros-Bonnivard, P. Jaud, J. Valle-Marcos, J.-M. Amann, C. Bouallou, Pre-combustion, post-combustion and oxy-combustion in thermal power plant for CO2 capture, Applied Thermal Engineering, 30 (2010) 53-62.

[5] R. Soundararajan, T. Gundersen, Coal based power plants using oxy-combustion for CO2 capture: Pressurized coal combustion to reduce capture penalty, in:  Chemical Engineering Transactions, Vol. 29, 2012, pp. 187-192.

[6] J.P. Roy, M.K. Mishra, A. Misra, Parametric optimization and performance analysis of a waste heat recovery system using Organic Rankine Cycle, Energy, 35 (2010) 5049-5062.

[7] X. Luo, B. Zhang, Y. Chen, S. Mo, Heat integration of regenerative rankine cycle and process surplus heat through graphical targeting and mathematical modeling technique, Energy, 45 (2012) 556-569.