Abstract:
The world's ever growing demand for energy has resulted in increased consumption of fossil fuels for electricity generation. The emissions from this combustion have contributed to increasing ambient levels of carbon dioxide in the atmosphere. Many efforts have been made to curb and reduce carbon dioxide emissions in the most efficient manner. The computer process modeling software CHEMCAD was used to model a natural gas combined cycle powerplant for carbon capture and sequestration. Equipment for two proven carbon capture techniques, oxy-combustion and post-combustion amine scrubbing, were modeled. The necessary components modeled included an air separation unit, powerplant, amine scrubbing unit, and a carbon dioxide compression and drying unit. The oxygen concentration in the oxidizer supplied to the powerplant was varied from ambient air, 21%, to nearly pure oxygen, 99.6%. Exhaust gas recirculation was incorporated to maintain a constant combustion temperature. At ambient conditions no air separation unit was necessary and all carbon capture was provided by the amine scrubbing unit. At concentrations ranging from 22 - 99% both oxy-combustion and amine scrubbing techniques are used at inversely varying degrees. At 99.6%, no amine scrubbing unit was necessary. As the oxygen concentration was varied operational parameters were investigated with the goal of identifying optimum operational conditions. Across the varying oxygen concentrations, the First Law efficiency losses ranged from 3.3 - 13.6%. The optimal operational point occurred when ambient air was supplied and exhaust gas recirculation was utilized for flame temperature control. A Second Law efficiency of 52.2% was maximized at an oxygen concentration of 22%. This corresponds to a 2.28% reduction in Second Law efficiency. An exergy analysis of each component identified the air separation unit as the component where the most improvements are possible. At 99% oxygen concentration, the Second Law efficiency of the air separation unit was 3%. Through modeling a natural gas combined cycle powerplant for carbon capture and varying the oxy-combustion characteristics, valuable information was gained in the understanding of operational losses associated with carbon capture.
