An experimental study on pyrolysis and oxidation of CH2Cl2 and CH3Cl in oxygen/hydrogen or oxygen/methane mixtures and argon bath gas was carried out at 1 atmosphere pressure in tubular flow reactors. Degradation of CH2Cl2, or CH3CI, along with the formation and destruction of intermediate and final products was analyzed systematically over 873 to 1273°K, with average residence times of 0.2 to 2.0 seconds.
Thermochemical parameters: enthalpy, entropy, and heat capacities for many chloro-oxy-carbon products and intermediates are calculated using the techniques of group additivity and the THERM computer code. Kinetic analysis on the reactions of hydroxy radical with vinyl chloride are performed using thermochemical analysis and a statistical chemical activation formalism based on the Quantum Kassel Theory for the addition reactions. The two abstraction paths have been also analyzed by using Evans-Polanyi relation for activation energy and Transition State Theory for pre-exponentials. Good agreement with the experimental data in the literature was obtained.
A nonlinear group additivity formalism to estimate the normal boiling points has been developed because boiling points are important to calculate critical properties needed for flame modeling. The model is straightforward and applies to compounds with a wide range of molecular weight, varied functional groups, and complex structures. We further utilize the proposed model for normal boiling points and adapt Joback's method into Benson type groups to calculate critical properties (Tc, Pc, Vc). Transport coefficients such as Lennard Jones Parameters (collision diameter and well depth), polarizability, and rotational relaxation collision numbers can also be estimated. The same group information (input data) needed for thermo properties estimation is then used to estimate transport properties required in flame modeling.
A detailed kinetic reaction mechanism based upon fundamental thermochemical and kinetic principles, Transition State Theory and evaluated literature rate constant data is developed. The mechanism is used to model results obtained from our experiments, in addition to results from other studies, on the thermal reactions of CH2Cl2 and/or CH3Cl. Comparison of the model to experimental data of other researchers for a wide range of conditions (tubular flow reactor, flat flame, perfect stirred reactor) showed good agreement in most cases.
Sensitivity analysis determined important reactions in the mechanism to several "target" products including reactions effective in inhibiting CO conversion to CO2. The results indicate that the reaction OH + HCl ---> H2O + Cl is a major cause of OH loss. This decrease in OH effectively stops CO burnout. In addition, the reaction H + HCl --> H2 + Cl is also important when H2 concentrations are very low. Sensitivity analysis also indicates that the reaction OH + OH <---> H2O + O, which usually forms H2O during hydrocarbon incineration, reacts in the reverse direction when HCI is present at concentrations comparable to CO, due to the large extent of OH depletion. The addition of moderate levels of high temperature steam are predicted to help CO conversion by shifting the above equilibria to more OH.
Knowledge and application of the reaction mechanisms to emulation of incineration operation allows calculation of modifications to incinerator design and/or feed to minimize pollutant formation. We predict that adding high temperature steam to the incinerators will improve Cl conversion to HCl by shifting the equilibrium of the OH + OH = H2O + O reaction to the left. The viability of computer modeling is illustrated as a diagnostic for understanding and for improvement or optimization in combustion processes with assumed ideal mixing.
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