Thermal Decomposition Of HFO-1234yf Through ReaxFF Molecular Dynamics Simulation

With the increasing pressure to address the issue of global warming,the European Parliament has made an F-gas regulation that bans the refrigerants with global warming potential?GWP?over 150 using in air conditioners for new model automobiles.According to the Intergovernmental Panel on Climate Change 4th Assessment Report?AR4?,R-134a has a 100-year direct global warm potential of 1430 and will be replaced by HFO-1234yf?2,3,3,3-tetrafluoropropene,CH2=CFCF3?.HFO-1234yf has similar vapor pressure and thermodynamic properties compared to R-134a and its GWP 4satisfies the EU regulation requirements.HFO-1234yf also has exceptional Life Cycle Climate Performance?LCCP?compared with R-134a and CO2.With the difference of refrigeration cycle,ORC cycle often occurs over-temperature phenomenon which may lead to pyrolysis of working fluid.It is classified as mildly flammable refrigerants.However,the thermal stability and pyrolysis properties of HFO-1234yf have not been studied by scholars.Therefore,a systemic study of the thermal decomposition process is conducted by ReaxFF-MD simulation.The decomposition properties and process are examined by analyzing the trajectory in ReaxFF MD simulation and the detailed contents are as follow.?1?The initial reactions of HFO-1234yf include the cleavage of C-C bond,C-H bond and C-F bond.HF molecule is the main product and four formation pathways are observed.The temperature has a great influence on the HFO-1234yf pyrolysis process.?2?The high temperature in simulated system promotes the formation of carbon clusters.CF₃H?CF?CH?CF?CF and H₂ are also obtained in pyrolysis system.?3?The initial reactions of HFO-1234yf pyrolysis are started at 2100 K after the simulated system reached thermal equilibrium for 1 ns isothermal simulations.The initial reactions including the cleavage of C-C bond,C-H bond and C-F bond have released numerous free radicals?·CF3,·F and·H?.The high concentrations of free radical·F and·H from initial reactions can increase the quantity of HF and accelerate the reaction rate of HFO-1234yf pyrolysis.The oxidation reactions are started at 2500 K and produce intermediates including H2O,CF3OH and FCOOH which affect the generations of final products.?4?The oxidation process is mainly induced by·OH,·F and·H free radical.The reaction from·CF₃ to CF₃O·is critical for the reaction chain transfer which is primarily carried by the conversion from HOO·to·OH radical.The reaction activity of oxidation is closely associated with the concentration of free radicals?FCOO·,·F,·H,HOO·and·OH?.The generations of CO₂ are associated with free radicals of·CF=O and·CH=O.?5?HF,COF2 and CO2 are the main products and the results agree with the data of related experiments.Ten formation pathways of primary products are observed.The generation of COF2 from CF3OH through an intramolecular elimination reaction is the key step for HFO-1234yf oxidation.?6?In the simulated system with water and oxygen,HF molecular is the main products.The COF2 molecular decrease as the water molecular increases and the water promotes the conversion process from COF2 to CO2.