Alternatives for power supply to natural-gas export compressors combined with heat production evaluated with respect to exergy utilization and CO2 emissions

The supply of process steam in combination with power for natural-gas export compressors was investigated using exergy analysis. The existing system with three 12.32 MW direct drive gas turbines each with a HRSG delivering 19.2 kg/s high-pressure steam was compared with an alternative where the gas turbines were replaced with new turbines. The exergy efficiencies were 46.7% and 48.6%, respectively, for the two cases. A second alternative with electric motors and a new CHP was investigated in three variants, all with some surplus electricity production. All variants gave higher exergy efficiencies than the other alternatives, from 51.5% to 53.6%. A third alternative with electric motors, stand-alone boilers and purchase of electricity was also analyzed, considering different origins of the electricity. This alternative gave the lowest exergy efficiencies, from 37.1% to 41.4% for different variants. In accordance with the exergy utilization, the CO2 emissions per unit of exergy delivered were the lowest for the second alternative, while the total emissions were the highest for the third alternative. However, the domestic emissions, important in relation to international CO2 agreements, were shown to be the lowest for the stand-alone boiler in combination with imported electricity.