Thermodynamic analysis and optimization of an oxy-combustion combined cycle power plant based on a membrane reactor equipped with a high-temperature ion transport membrane ITM

Abstract

This paper presents an advanced zero emission power plant (AZEP) which is a combined cycle power plant with a membrane reactor equipped with a high-temperature ion transport membrane (ITM). The membrane reactor, which is used as a replacement for a combustor in the gas turbine, combines three functions: combustion of fuel, oxygen separation from the air in the ITM, and heating of the oxygen-depleted air. Due to the membrane reactor, the AZEP does not need an energy-consuming external air separation unit, which is a significant advantage over other types of power plants which utilize oxy-combustion technology. The presented thermodynamic model for AZEP with an advanced numeric model of the ITM allows the user to perform thermodynamic analyses for a wide range of AZEP operating parameters and to select their optimal values, without the need to determine the geometric parameters of the ITM and heat exchangers. The obtained results allow one to indicate the basic features of this carbon capture technology. The selection of AZEP operating parameters is related to the balance between the power plant’s efficiency maximization and the limitation of the ITM and heat exchangers surface areas. This manuscript also presents an analysis of the possible AZEP plant development. The development model of the E-AZEP plant assumes a number of improvements in the parameters of the ITM and the entire membrane reactor, with respect to the AZEP plant basic model. The ITM assumes an improvement in the ionic conductivity (σion) by approx. 45%, by changing the value of the conductivity coefficient (C2) as well as increasing the maximum operating temperature of the membrane from 900 °C to 1000 °C. The work demonstrated the great influence of the membrane surface. In the model of the membrane reactor, a higher flue gas temperature was assumed at the combustion chamber outlet, equal to t1g = 1600 °C, which together with the assumption of temperature approximation ΔThe.HHX = 20 K gives an air temperature t3a = 1580 °C. With the current technological possibilities, AZEP plants are competitive compared to alternative solutions, but they do not have a significant advantage over them. However, overcoming the presented limitations may allow us to achieve an advantage in terms of achieved efficiency compared to alternative CO2 capture technologies. Achieving the air temperature at the inlet to the expander, at a level close to the temperatures used in J-class gas turbines (E-AZEP plant case), allows for a decrease in the efficiency when compared to modern combined cycle power plants, at a level of 3% points.