Ignition And Kinetic Modeling Study Of CH₃NHCH₃ And CH₃NHCH₃/CH₄ Mixtures

Biomass fuels and propellants usually contain nitrogen,which contributes significantly to the emission of NOx in combustion.Therefore,it is desirable to understand the combustion chemistry of nitrogen-containing compounds and their effects on oxidation of hydrocarbons,such as CH4.However,ignition delay time data for these systems are insufficient.The primary objectives of this dissertation are to provide new ignition delay times by shock tube,to calculate the rate constants for some key elementary reactions by quantum chemistry method,to modify or assemble kinetic models,and to gain the chemical kinetic insights into the ignition process for CH3NHCH3 and CH3NHCH3/CH4 mixtures.Ignition delay times of CH3NHCH3/O2/Ar mixtures are measured behind reflected shock waves in the temperature range of 1040-1600 K.We investigate the effect of different pressures(4,8,and 18 atm)and equivalence ratios(0.5,1,and 2)on ignition.We first examine a recently developed CH3NHCH3 kinetic model,and then modify this model by adding the hydrogen ion reactions from CH3NHCH3 by NO2 and HO2.The rate constants of the hydrogen ions by NO2 are estimated by analogy to the hydrogen abstraction from CH3NH2 by HO2 and those by HO2 are estimated by analogy to the hydrogen abstraction from CH3OH by HO2.The modified model well predicts the present measurements.On basis of this modified model,kinetic analyses including sensitivity and reaction pathway are performed to provide insights into the chemical kinetics of CH3NHCH3 ignition.At low temperatures,CH3NHCH3 is mainly consumed by hydrogen abstraction reactions,and its unimolecular decomposition reaction becomes more important with increasing temperature.We investigate hydrogen abstraction reactions from CH3NH2,(CH3)2NH,and(CH3)3N by HO2 radicals via quantum chemistry calculations.In geometry optimizations,frequencies,and intrinsic reaction coordinate calculations,the B3LYP/6-31G(2df,p)method is employed.The G4 method is used in energy and thermodynamical property calculations.With one-dimensional hindered rotor treatment and Eckart tunneling correction,rate constants are determined by the calculations based on variation transition state theory and conventional transition state theory.A branching ratio analysis at each reaction site for every reaction system is also conducted.We measure ignition delay times of stoichiometric CH3NHCH3/CH4/O2/Ar mixtures in the temperature range of 1100-2000 K,with a shock tube.We explore different pressures(4,8,and 18 atm)and CH3NHCH3 blending ratios(0,0.05,0.1,0.2,0.5,and 1).CH4 ignition is promoted by CH3NHCH3 addition.Through multiple linear regression,correlations for the measured ignition delay times are inferred.A recently developed CH3NHCH3 kinetic mode are examined,and a new model for CH3NHCH3/CH4/O2/Ar mixtures is assembled and validated against the present measurements.To gain insights into the effect of CH3NHCH3 on CH4 igni-tion,sensitivity and rate of production analyses have been conducted.In addition,perturbation of the radical pool(H,CH3,OH,and H02)are also discussed.Hydrogen abstraction from four amines including methylamine,ethylamine,propylamine,and butylamine,by H02 radicals,is investigated with quantum chemistry calculations.Geom-etry optimizations,frequencies,and connections between transition states and corresponding local minima are calculated using B3LYP method with 6-31G(2df,p)as basis set.The potential energy surfaces are constructed with the G4 composite method,which is also used to determine the thermodynamic properties of the selected reactants and products.Rate constants are calcu-lated using variational transition state theory and conventional transition state theory,including one-dimensional hindered rotor treatment and Eckart tunneling correction.The calculated rate constants are further fitted into an Arrhenius form,and a branching ratio analysis for each reac-tion system is also performed.