Thermal Stability Of Organic Rankine Cycle Working Fluid And The Impact On System Performance

The efficiency of utilization of the high temperature waste energy has been rising yearly at home and abroad, while the utilization efficiency of low temperature wastes heat energy remains low. Waste heat recourses of a temperature below500℃widely exist in industrial producing processes, which can be used by means of Organic Rankine Cycle (ORC) effectively and efficiently, with promotion in energy utilization efficiency and reduction in pollution of the environment. The organic Rankine cycle power generation technology employs heat exchange between organic working fluids of low temperature and hot waste heat resources, by what organic working fluids turned into steam of high temperature and high pressure. The steam enters into turbine expansion machine, drives turbine to rotate and makes the generator to work, with power output in the end. But, the low-temperature working fluids decompose thermally under continuous cycle of long term at low temperature, resulting in deviations of the system in the performance from the designation at first. So, it is a problem in urgent for application of ORC in low-temperature field to define the thermal stability of organic Rankine cycle fluids.According to the structure and principle of the the organic Rankine’s cycle power generation system, and by Peng-Robinson (PR) equation of state and Vander Waals mixed covolume rules, this paper constructed vapor liquid phase equilibrium between pure working medium and multivariate mixed working medium on the basis of the liquid phase equilibrium theory, and calculation model of thermodynamic parameters, was established, in which enthalpy h, than-entropy s were involved. Further, thermodynamics, heat transfer and fluid mechanics were integrated, and an analysis model thermal efficiency, exergy efficiency and energy was established for organic Rankine’s low temperature waste heat power generation cycle.Then, from the molecular structure of the selected organic working medium, pentane were calculated to break down into methane, ethane and propylene and ethylene by means of gibbs free energy ion principle at375℃heat source. Experiments worked out in high temperature reaction kettles as follow, samples by time-sampling during the decomposition were detected by gas chromatograph comparing with pure working medium. The results went consistent between experiments and theory calculation of decompositions, and so provided evidence of pseudo-first-order reaction equation for pentane pyrolysis theoretically and experimentally.In the end, we calculated efficiency of organic Rankine’s cycle of pentane and mixed working medium from decomposition of pentane at375℃, on basis of analysis model of the performance of organic Rankine’s cycle circulation system. Through comparison and analysis, it is found that the thermal decomposition of organic working medium results in deviations of whole organic Rankine’s circulation system from the design condition, with reductions in system efficiency,0.5%in amount.