Based on the theory of nonisothermal chain reactions developed in the studies presented in the previous chapters, an experimentally verified explanation is given for the anomalous behavior of pyrolysis and combustion of a technically important product—hydrazine. The revealed kinetic features of N2H3 radicals, which led to the unusual properties of the hydrazine-oxygen ignition peninsula, are also explained. The mechanism of easy ignition and explosion has been clarified.
In this chapter, we will discuss the issues of inhibitor consumption outside the ignition region and the advantages of the method for determining rate constants from ignition limits, as well as a method for determining rate constants for reactions of displacement of atoms and from a molecule by an attacking atom.
Contradictions between generally accepted ideas about a one-stage reaction underlying detonation and experimental data, which existed before the appearance of the author’s works are listed. Publications that deny the chain nature of reactions in gas detonation are mentioned. The necessity of a reaction of free atoms and radicals for the realization of explosion and detonation is explained. A test bench with a shock tube is described, on which experiments were carried out to identify the chain nature of reactions in detonation. Results are presented showing that, contrary to generally accepted ideas about the one-stage model of detonation reactions, chemical processes in the detonation of gases are chain processes with all the characteristic properties of this class of reactions. A stationary detonation wave was split into a combustion wave and a shock wave, and the stationary detonation velocity was varied by inhibition. It is noted that the results of identifying combustion, explosion, and detonation are a priority of Russia and Russian Academy of Sciences.
It is shown that the features of chain processes are determined mainly by the fact that most of the internal energy of the initial reagents is converted into the energy of free valences of atoms and radicals. This results in high concentrations of active particles, which react with the initial reagents at higher rates, providing intensive self-heating, which additionally accelerates the process. It is noted that, without considering the chain nature of reactions, it is impossible to explain ignition upon heating, since the high activation energies of reactions of only molecular reagents exclude an increase in the rate during heating. Taking into account the chain nature of combustion reactions made it possible to explain ignition upon heating and develop a theory of the phenomenon. An explanation is given to the most important characteristic of gas combustion: the length of reaction chains, which determines the role of the chain mechanism in combustion, and the dependence of kinetics on temperature. A solution to the problems of the third flammability limit, confirmed by kinetic and spectroscopic experiments, was obtained, as well as the evidence of the presence of HO2 radicals in the flame, their active role in the heterogeneous development of reaction chains and in combustion in general. The possibility of replacing the solution of diffusion differential equations by effective kinetic parameters when describing combustion has been experimentally proven.
The formation of atoms and radicals in the flame in concentrations by many orders of magnitude higher than the equilibrium values at combustion temperatures is explained. Results that prove the important role of homogeneous and heterogeneous reactions between atoms and radicals in combustion are presented. The decisive role of previously unknown reactions of heterogeneous chain propagation in combustion is illustrated by experimental results. The reasons for the misconceptions of several foreign and Russian authors who tried to deny chain combustion are explained.
The widely discussed phenomenon of flame propagation without self-heating is considered using as examples the combustion of carbon disulfide and the decomposition of nitrogen trichloride. It is shown that, in the nonthermal propagation of a carbon disulfide flame and in its combustion, a determining role is played by a new type of elemental reaction: the displacement of an atom from a molecule by an attacking atomic reactant. For nitrogen trichloride as an example, an unambiguous quantitative relationship between the flame speed and the nonlinear branching rate constant is shown. In both processes, the proposed mechanisms are confirmed by the identification, using EPR and optical spectroscopy, of atoms and radicals that play the main role in the process. The equations corresponding to the identified mechanisms of the processes are confirmed by experiments.
The rarely considered fundamental difference between the temperature dependences of the reaction rate and the rate constant is emphasized. The small change in the rate of reactions with high activation energies, contrary to existing ideas, is illustrated. It was shown that the main reason for the deviation between the calculated and experimental rates is the neglect of the temperature dependence of the reagent concentrations during the reaction. It has been shown that the main reason for the deviation between the calculated and experimental rate values is the neglect of the temperature dependence of the reactant concentrations in the course the reaction. The concept of the temperature rate constant is introduced: the change in the rate with a unit change in temperature, i.e., the temperature derivative of the rate constant. It is shown that this characteristic determines the competition between the stages of a complex process under nonisothermal conditions. The law of temperature dependence of the chain process was discovered, and its agreement with experiment was verified. The difference between the self-acceleration of a reaction from an increase in temperature and from the multiplication of active particles is explained. An experimental illustration is provided. The difference between the temperature dependences of the reaction rates in a gas heated from outside before and after the onset of ignition is explained. Based on experimental data, the determining role of the hydrogen atom concentrations in the combustion rate at hundredths of atmospheric pressure and at atmospheric pressure is quantitatively demonstrated. This demonstrates the determining role in combustion of the conversion of a significant part of the enthalpy of the initial reagents into the free-valence energy.
In this chapter, we will consider the mechanism of inhibition efficiency, stability of inhibitors, which determines the reliability and duration of ignition prevention, and the mechanism of synergy between the combined action of an inhibitor and inert gases. Special consideration will be given to the effect of inhibitors on flame propagation and detonation.
The theory of ignition induction periods, developed by the author, is presented. Quantitative confirmation of the theory by experiment at different pressures is provided. The same experimental data also confirm the theory of heterogeneous development of reaction chains.
The existence of two regimes of chain combustion is predicted, the phenomenon of explosion with a chain mechanism is explained, and the condition for the transition of combustion to this regime is formulated. The abrupt changes in the kinetic curves during the transition of combustion to this regime are illustrated and explained. The results of an experimental study of the transition from combustion to explosion are presented; it is found out that the ignition peninsulas, which are presented in courses and monographs on chemical kinetics as kinetically homogeneous regions, actually consist of two regions that are different in all reaction characteristics: the combustion region and the explosion region. Experimental evidence is given of the chain nature of combustion in an explosion under conditions of cumulation and control of such an explosion by means of inhibitors.
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