Thermal stability and pyrolysis mechanism of decamethyltetrasiloxane (MD2M) as a working fluid for organic Rankine cycle

Abstract

The thermal stability of working fluids is a crucial property studied in the selection of organic Rankine cycle fluids, as they may undergo decomposition at elevated temperatures. In previous studies, siloxanes have been identified as promising choices for ORCs. However, research on the thermal stability of siloxanes in ORCs has been relatively limited. This study investigates the thermal stability and pyrolysis mechanism of MD2M through a combination of experiment, DFT simulation, and ReaxFF-MD calculation. The experiment revealed that MD2M exhibits poor thermal stability, with a decomposition rate of approximately 1.820% at 200°C in 72h. Consequently, it is unsuitable for operating in ORCs at temperatures of 200°C and above. The primary gas products in the pyrolysis of MD2M include CH4, C2H6, C2H4, CO, and CO2, among others. ReaxFF-MD and DFT elucidated the thermal decomposition mechanism of MD2M. The Gibbs free energy barriers for Si-C bond cleavage reactions are relatively lowest, measured at 352.98 and 341.33 kJ∙mol-1, respectively. Simultaneously, methylation of the terminal Si atom is likely to represent the primary reaction pathway for the initial decomposition.