Analysis of a combined ORC system utilizing LNG cold energy and waste heat of gas turbine exhaust

Since the temperature difference between liquefied natural gas (LNG, 111K) and gas turbine (GT) exhaust gas (673K) is too large, the difference between the evaporation temperature and the condensation temperature in a single organic Rankine cycle (ORC) will increase the cycle pressure ratio and the cost of the equipment. A combined ORC energy recovery system which can realize better temperature matching of different cycles and cascade utilization of the waste energy is proposed. The system consists of three cycles: cycle I and cycle II are coupled to efficiently utilize most of the LNG cold energy and cycle III can utilize the remaining cold energy after LNG gasification. 19 working fluids are selected for comparison based on the thermal-physical properties and safety level. By comparing the performance of the fluids in the cycles by MATLAB and REFPROP9.0, propane is selected as the working fluid in cycle I, R141b in cycle II, and toluene in cycle III. Thermodynamic laws are adopted to analyse thermal performance of the system and the key parameters are determined to optimize the cold energy utilization and maximize the total output power. The results show that this system in a gas turbine power plant with 3 Mt/a LNG can achieve output work of 43.8MW and the exergy efficiency, cold exergy recovery rate and heat exergy recovery rate are 36.9%, 83.0% and 72.4%, respectively. The flue gas heat exchangers account for the largest parts of exergy loss.