Second law analysis of premixed and non-premixed oxy-fuel combustion cycles utilizing oxygen separation membranes

Abstract

Two basic oxy-combustion cycles were investigated under premixed and non-premixed combustion conditions and the results were compared in terms of exergy destruction and first and second law efficiencies. An air separation unit (ASU) was used for oxygen separation from the feeding air in the premixed combustion cycle. In the non-premixed combustion cycle, CO2/H2O splitting membrane reactors were utilized, where oxygen separation and in situ oxy-combustion processes occur within the reactor. A gas turbine cycle, working on conventional air combustion of methane, was selected as the reference base case. Commercial process simulator Aspen Hysys V7.3 was used to model and simulate the different systems. The work proposed novel cycle designs for higher cycle efficiency under oxy-combustion conditions. Cycle performance using ion transport membrane (ITM) and ASU was evaluated and compared. Losses in the ASU and the condenser were identified to be the main reason for lower efficiencies and, hence, the systems were modified to include heat recuperation cycles. Additional two modified oxy-combustion cycle designs were proposed. First law and second law efficiencies of the modified premixed cycle were found to be 34.1% and 47%, compared to 35.1% and 44% for the reference air-combustion cycle. The overall thermal and second law efficiencies of the modified non-premixed cycle were the highest among all cycles with 37.8% and 50.4% efficiencies. The effects of hydrogen addition on the efficiencies of the premixed system were evaluated. It was found that hydrogen addition results in increased first and second law efficiencies of the cycle; however, the increase is only marginal.