Molecular Dynamics Characteristics Of The Pyrolysis And Combustion Of Stearic Acid

The conflict between the wide application of stearic acid in industrial production and its high risk of dust explosion makes the risk assessment and safety prevention of stearic acid in production and use of great significance.Therefore,it is necessary to explore the pyrolysis and combustion of stearic acid from the perspective of complex reaction mechanisms using the molecular dynamics method.In this paper,the pyrolysis and combustion of stearic acid were conducted through ReaxFF molecular dynamics simulation.The initial decomposition paths of the stearic acid molecule were summarized in detail and the evolutions of the main products were obtained.Furthermore,the reaction mechanisms of pyrolysis and combustion of stearic acid were described.Finally,the experimental tests were carried out on the thermal decomposition properties and combustion flame characteristics of stearic acid.In the pyrolysis of stearic acid,the main products were obtained as C2H4,H2,C2H2,CH4,H2O,CO2,CO,etc.,and the initial decomposition apparent activation energy was 175.749 kJ·mol-1,which was consistent with the results of TG-IR tests.As the density increased,the initial decomposition temperature of stearic acid decreased.With the increase in temperature,the initial decomposition of stearic acid and the formation of products were accelerated.The main pyrolysis mechanism was that the stearic acid molecules through dissociation of C-C bonds generated small molecules such as C2H4,CH4,C2H2,C3H6,etc.Moreover,H2O was produced by the combination of ·H and·OH,and CO and CO2 were mainly derived from the paths that-COOH directly removed-OH and H atom.In the combustion of stearic acid,the main products were obtained as C2H4,H2O,CO2,CO,CH2O,etc.,and the apparent activation energies of the initial decomposition and the intermediate reaction stage of stearic acid combustion were 94.273 kJ·mol-1 and 234.239 kJ·mol-1,respectively.As the density and temperature increased,the initial decomposition of stearic acid and the formation of products were promoted.Under the theoretical oxygen content,the combustion reaction rate was the fastest.The flame propagation process of stearic acid combustion can be divided into three stages:flame growth stages,stable flame stage,and flame decay stages.The three flame growth stages correspond to three flame propagation velocity peaks,which moved toward higher concentrations with the flame propagation stages.Stearic acid first absorbed energy and vaporizes after ignition,and reacted with O2 to trigger the initial decomposition of stearic acid,resulting in a combustion flame and a release of energy.The unburned stearic acid was further ignited,making the flame grow and spread steadily.After the initial decomposition of stearic acid,the initial decomposition products were further reacted with oxidizing groups such as O2 and ·HO2,and continuously produced main combustion products such as H2O,CH2O,CO,and CO2.Finally,the reactions were completed and the flame was gradually extinguished.The combustions of octadecane,stearyl alcohol,and stearic acid were contrasted,and the influences of-OH and-COOH on the reaction rate,activation energy,and decomposition paths were discussed.In the initial decomposition stage,the combustion reaction rate of C18H36O2 was the fastest,followed by C18H38O.However,the combustion of C18H38 required a higher temperature.The order of activation energy in the initial decomposition stage was C18H38>C18H38>C18H36O2.The main paths in the initial decomposition of C18H38,C18H38O,and C18H36O2 were dominated by the direct participation of O2 in the reaction to generate ·HO2 radicals and the breakage of C-C bonds.The presence of-OH and-COOH increased the proportion of O2 involved reaction and decreased the breakage of C-C bonds.