Thermal Stability And Decomposition Mechanism Of Working Fluid HFO-1336mzz(Z)

Organic Rankine Cycle(ORC)is an effective way to convert waste heat into electricity and has received widespread attentions and researches.The working fluid may be decomposed during the operation in the medium-high temperature ORC.Once the working fluid is decomposed,it will affect the performance of the system and even cause a safety accident,so the thermal stability of working fluid need to be investigated.As a new type working fluids,HFO working fluids have equivalent or even superior performance in ORC compared to conventional working fluids and this type of working fluids also have better environmental protection characteristics.Therefore,the application of HFO working fluids in ORC has been studied by many researchers.However,the studies of HFO working fluids thermal stability and thermal decomposition mechanism were scarce.In this paper,the thermal stability and thermal decomposition mechanism of HFO working fluids,especially HFO-1336mzz(Z),were mainly studied through experimental and theoretical methods.The safe use temperatures at different conditions were obtained.The thermal decomposition and oxidation decomposition mechanism of HFO-1336mzz(Z),affect mechanism of lubricant and copper surface on the thermal decomposition of HFO-1336mzz(Z)were investigated by using reactive molecular dynamics Reax FF force field and density functional theory(DFT).In addition,the thermal decomposition mechanism of n-pentane has also been theoretically studied.And the affect mechanism of copper surface on the thermal decomposition of HFC-245fa was revealed.The main conclusions of this article were as follows:(1)An experimental system was established to effectively measure the thermal stability and pyrolysis products of HFO working fluids,and the decomposition temperature of HFO-1336mzz(Z)was measured using the fluoride ion method.The safe use temperatures of HFO-1336mzz(Z)under different pressures were determined.The results showed that pressure has a significant effect on thermal stability of HFO-1336mzz(Z),this factor should be considered when selecting HFO refrigerants as working fluids.In addition,the dissociation products of HFO-1336mzz(Z)were measured.(2)The initial reactions of n-pentane thermal decomposition were mainly the cleavage of C-C bonds.Among them,the breakage of C2-C3 bond was the dominant route.The main products of n-pentane thermal decomposition were hydrogen,methane,ethane,ethylene,acetylene,propane and propylene.Based on the kinetic analysis of n-pentane thermal decomposition,the apparent activation energy and pre-exponential factor were calculated.(3)The effect of lubricant on the thermal stability of HFO-1336mzz(Z)was investigated.The dissociation temperature range of HFO-1336mzz(Z)with lubricant was determined and the safe use temperature was obtained.Reax FF force field was carried out to investigate the affect mechanism of Polyol Ester(POE)lubricant on the thermal decomposition mechanism of HFO working fluids.The results indicated that the thermal stability of POE lubricant was worse than that of HFO-1336mzz(Z).It first decomposes through the cleavage of the C-O bond and generates the oleic acid chain(C₄H₇O₂ radical),and then form small molecule and radical products.The radicals obtained from the dissociation of PEC4 will react with HFO-1336mzz(Z)to dissociate HFO-1336mzz(Z).Therefore,the thermal stability of HFO working fluids and lubricant systems mainly depends on the thermal decomposition temperature of the lubricant.(4)Copper can obviously promote the dissociation of HFO-1336mzz(Z).The difficulty of the chemical bonds of HFO-1336mzz(Z)decomposing on the copper surface was as follows:C=C bond<C-F bond<C-H bond<C-C bond.For the thermal decomposition of HFO-1336mzz(Z),the catalytic activity of three copper surfaces was as follows:Cu(1 1 1)surface<Cu(1 1 0)surface<Cu(1 0 0)surface.The presence of oxygen atom enhances the interactions of the HFC-245fa with the Cu(1 1 1)surface,Cu(1 1 1)surface can distinctly facilitate the HFC-245fa decomposition,and the pre-covered oxygen atom has contributed to the decomposition of HFC-245fa molecule.Therefore,when HFO-1336mzz(Z)or HFC-245fa as the working fluid in ORC system,copper could be used as a material for components with lower heat temperatures(lower than the decomposition temperature of HFO-1336mzz(Z)or HFC-245fa)and should be avoided in the high temperatures components of ORC system(such as evaporator).(5)The presence of H₂O molecule in reaction system can provide enough H atoms to convert the excessive F atoms into HF molecule to promote the HFO-1336mzz(Z)decomposition rate.HF,COF₂ and CO₂ were final products in the O₂/H₂O environment.When a complete decomposition reaction of HFO-1336mzz(Z)molecule happens,the chemical equation can be expressed as follows:CF₃CHCHCF₃+3O₂=2HF+2COF₂+2CO₂(in the absence of H₂O)and CF₃CHCHCF₃+3O₂+x H₂O=2(1+x)HF+(2-x)COF₂+(2+x)CO₂(in the presence of H₂O).An experimental system that can effectively measure the thermal stability of HFO working fluids was built in this dissertation,and the theoretical research methods such as reactive molecular dynamics and density functional theory were introduced to investigate the thermal stability and thermal decomposition mechanisms of HFO working fluids,the affect mechanism of ORC materials on the thermal decomposition of fluorine-containing working fluids and the oxidation decomposition mechanism of HFO working fluids.It has important theoretical significance and reference value for the research on thermal stability of HFO working fluids and subsequent engineering applications.