Thermodynamic Assessment of Membrane-Assisted Premixed and Non-Premixed Oxy-Fuel Combustion Power Cycles

Abstract

This study focuses on the investigations of gas turbine power generation system that works on oxy-combustion technology utilizing membrane-assisted oxygen separation. The two investigated systems are (i) a premixed oxy-combustion power generation cycle utilizing an ion transport membrane (ITM)-based air separation unit (ASU) which selectively allows oxygen to permeate from the feeding air and (ii) a non-premixed oxy-fuel combustion power cycle, where oxygen separation takes place, with cogeneration of hydrogen in an integrated combustor. A gas turbine combined cycle that works on conventional air–methane combustion was considered as the base case for this work. Commercial software package Hysys V8 was utilized to conduct the process simulation for the proposed cycles. The two novel cycle designs were proposed and evaluated in comparison with that of the conventional cycle. The first law efficiency of the premixed combustion power cycle was calculated to be 45.9%, a loss of 2.4% as an energy penalty for the oxygen separation. The non-premixed cycle had the lowest first law efficiency of 39.6%, which was 8.7% lower than the efficiency of the base cycle. The lower effectiveness of the cycle could be attributed to the highly endothermic H2O splitting reaction for oxygen production. High irreversibility in the H2O-splitter and the reactor was identified as the main cause of exergy losses. The overall second law efficiency of the non-premixed power cycle was around 50% lesser than that of the other cycles. The energy penalty related to air separation is dominated as the parameter that reduces the efficiencies of the oxy-fuel combustion cycles; however, the premixed combustion cycle performance was found to be comparable to that of the conventional air-combustion cycle.