Comparative 3-E (Energy, Exergy & Environmental) Analysis of Oxy-Coal and Air-Coal Combustion based 500 MWe Supercritical Steam Power Plants with CO2 Capture

Abstract

The comparative performance study is carried out for 500 MW Supercritical (SupC) Oxy-Coal Combustion (OCC) and Air-Coal Combustion (ACC) power plants with membrane-based CO2 capture at the fixed furnace temperature. The proposed configurations are modelled using a computer-based analysis software 'Cycle-Tempo' at different operating conditions, and the detailed thermodynamic study is done by considering Energy, Exergy, and Environmental (3-E) analysis. The result shows that the net energy and exergy efficiencies of ACC power plants with CO2 capture are about 35.07 % and 30.88 %, respectively, which are about 6.44 % and 5.77 % points, respectively higher than that of OCC power plant. Auxiliary power consumption of OCC based power plant is almost 1.97 times more than that of the ACC based plant due to huge energy utilization in the Air Separation Unit (ASU) of OCC plant which leads to performance reduction in OCC plant. However, environmental benefit of OCC based power plant is more than that of ACC based power plant with respect to CO2 emission. OCC plant emits about 0.164 kg/kWh of CO2 which is approximately 16.75 times lower than the CO2 emission in ACC based power plant. It is also analyzed that the performance of the CO2 Capture Unit (CCU) for the OCC based plant is about 3.65 times higher than the ACC based power plant due to higher concentration of CO2 (nearly 80.63%) in the flue gas emitting from OCC plant. The study also reveals that the auxiliary power consumption per kg of CO2 capture of the OCC based plant is about 0.142 kWh/kg, which is approximately 0.06 times lower than the ACC based plant. The higher performance of the OCC based power plant is found at lower value of flue gas recirculation due to the fact that reduction in exergy destruction at the mixing zone of the combustor is higher than the increase in exergy destruction of the heat exchangers at higher furnace exit temperature. But the metallurgical temperature limit of boiler tube materials restricts the use of the higher value of furnace temperature. OCC based power plant with CO2 capture can be preferred over ACC based plant with CO2 capture due to higher environmental benefits towards mitigating CO2, the key greenhouse gas on earth in spite of exhibiting lesser energy and exergy efficiencies.