Abstract
In present, many industries including steel mills, refineries, chemical plants, emit low-pressure saturated steam and low temperature waste heat due to processes in need. These vapors and waste heat are difficult to recover because of their relatively low pressure and temperature. But the average pressure and temperature of the waste heat are still 5 ~ 20 atm and greater than 200 ℃. If the energy of the waste heat can be recovered, the energy consumption can be reduced. Steam turbines can utilize low and medium pressure steam in the steam Rankine cycle, but they still discharge high temperature steam. This study combines the steam Rankine cycle and the organic Rankine cycle into a combined cycle in order to utilize waste heat and improve the efficiency. Because not all the waste heat discharged from the factory is clean high temperature and high pressure steam, this study distinguishes between two heat sources: Model Ⅰ includes pure steam as the heat source that can be directly transported to the steam turbine; Model Ⅱ includes the heat source of impurities and the need to exchange heat with the steam Rankine cycle. Model Ⅰ and Model Ⅱ have been verified with literature [10-11] and varied the parameters of waste heat source, compressor inlet pressure and different organic working fluids to simulate the combined cycle in different conditions. The combined cycle has been compared with the steam Rankine cycle and the organic Rankine cycle. The results show that by using a combined cycle, the output power can be increased by 70% in Model Ⅰ, and both the efficiency and output power can be increased by 65% in Model Ⅱ.