Mechanism of CO2-regulated pyrolysis behavior and thermal stability enhancement of zeotropic R1243zf mixture for organic rankine cycle

To enhance the thermal stability of the low-GWP working fluid R1243zf in medium-to-high temperature Organic Rankine Cycle systems, the pyrolysis behavior of zeotropic R1243zf/CO2 mixture and the inhibition mechanism of CO2 were systematically investigated. A multi-scale approach combining experimental characterization, reactive force field molecular dynamics simulations, and Density Functional Theory calculations was employed to reveal the thermal stability enhancement and the underlying microscale mechanisms. Experimental results demonstrate that the addition of CO2 increases the pyrolysis onset temperature from 180 to 200 °C for pure R1243zf to 240–260 °C for the mixture. CO is identified in the gaseous products, while the formation of the corrosive product HF is suppressed. Reactive force field molecular dynamics simulations further indicate that CO2 effectively captures carbon radicals generated during the pyrolysis process, reducing the formation of C1–C4 clusters to 16.7 %–47.3 % of the level in the pure system and delaying HF generation. Density Functional Theory calculations identify C–C bond cleavage as the primary initial reaction pathway (energy barrier: 399.16 kJ mol−1) and clarify the competitive reaction mechanism involving CO2 leading to CO formation. Kinetic analysis confirms that as the mass fraction of CO2 increases from 0 to 0.75, the apparent activation energy for the decomposition of R1243zf rises significantly from 108.65 kJ mol−1 to 459.25 kJ mol−1. This work provides a theoretical basis and design guidance for the safe application of the zeotropic R1243zf/CO2 mixture in medium-high temperature Organic Rankine Cycle systems.