Abstract
The design of efficient Organic Rankine Cycle (ORC) units for the usage of industrial waste heat at high temperatures requires direct contact evaporators without intermediate thermal oil circuits. Therefore, the thermal stability of high-temperature working fluids gains importance. In this study, the thermal degradation of hexamethyldisiloxane (MM) is investigated in an electrically heated tube. Qualitative results concerning remarks on degradation products as well as quantitative results like the annual degradation rate are presented. It is shown that MM is stable up to a temperature of 300 °C with annual degradation rates of less than 3.5%. Furthermore, the break of a silicon–carbon bond can be a main chemical reaction that influences the thermal degradation. Finally, it is discussed how the results may impact the future design of ORC units.
Highlights
IntroductionDue to the high temperature of the firing, an intermediate thermal oil circuit is used to prevent the organic working fluid from degradation [1,2,3]
The Organic Rankine cycle (ORC) have been extensively used in biomass fired power plants.Due to the high temperature of the firing, an intermediate thermal oil circuit is used to prevent the organic working fluid from degradation [1,2,3]
Si–C bond bond and and the the subsequent formation of methyl radicals can be seen as the start reaction of thermal degradation
Summary
Due to the high temperature of the firing, an intermediate thermal oil circuit is used to prevent the organic working fluid from degradation [1,2,3]. Synthetic oils with degradation temperatures of more than 300 ̋ C [4] are used to avoid thermal stress of the ORC working fluid. In the last years, ORC has become popular in other high-temperature applications as well. Waste heat recovery from industrial processes and from diesel engines in stationary applications and heavy duty trucks are discussed. Linear siloxanes like hexamethyldisiloxane (MM) are promising working fluids for such high-temperature applications. For waste heat recovery in heavy duty trucks, the engine compartment package is crucial and mostly does not allow an additional circuit
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