The use of oxy-fuel for coal-fired power generation is attracting global attention due to its application to carbon capture schemes. Coal combustion in nitrogen depleted air produces a flue gas stream which contains less impurities and particulate matter than standard air-combustion (Spero CS, 2007). This allows for easier flue-gas cleaning and hence cost reductions associated with reaching desired CO2 purity flue gas for Carbon Capture and Storage (CSS). Additionally, a large volume percentage drop in the oxidizing gas stream subsequent to nitrogen separation allows for smaller unit sizes, and hence the promise of lower capital investments for an oxy-fuel power plant.
Existing methods of oxygen production are all currently bound by high capital costs and high operational energy consumption, thus incurring large cost and efficiency penalties for an associated plant. The promise of a 35% cost reduction for implementing a membrane system over existing oxygen production technologies (Stiegel et al., 2004) spurs interest in membranes as a viable replacement of mature technologies such as cryogenic distillation and swing adsorption.
The advantage of membrane technology for gas separation is that energy intensive phase changes are not required, and permeate flow is steady. Much promise is focused on the recent advances in dense ceramic ionic conducting membranes because they offer a theoretical permeate flow of 100% O2 (given no defects or leaks). The ionic transport mechanism through the dense membrane structure initiates only at temperatures exceeding 600 oC (A. Leo, 2006), and hence large energy penalties will be incurred in pre-heating the air. This high temperature separation parameter is a material property of the membrane, and hence current research direction is aimed at investigating ways to reduce this.
As a result of these membranes being in the research and development stage, this thesis is of a hypothetical nature, and aims to provide information of power plant efficiencies for a range of membrane operating conditions (temperature and pressure). Data pertaining to these operating conditions was experimentally determined in the Films and Inorganic Membrane Laboratory of the Chemical Engineering Division of the University of Queensland throughout the duration of this thesis. Plant efficiencies are calculated using process modeling software to simulate a super-critical oxy-fuel coal power plant (net output 500 MW).
It was found that the optimal plant thermal efficiencies ranged from 32-37% for a membrane module operating between 650 – 800 oC at 5 atmospheres. The associated module sizes vary from 4 – 12 m3 and are presented in Table 12: Optimal ASU Operating Conditions, page 40.