Chemical branched-chain ignition

As described in the Introductory Chapter, attention was focused [1] prior to 1961 mainly on the morphology of the cool-flame and ignition regions, rates were followed by pressure change, and essentially chemical techniques were used for product analysis. The acceptance of free radicals, followed by the masterly and elegant Semenov theory [2], which established the principles of branched chain reactions, provided the foundation for modern interpretations of hydrocarbon oxidation. This chapter builds on these early ideas, and pioneering experiments such as those carried out by Knox and Wells [3] and Zeelenberg and Bickel [4], to provide a detailed account of the reactions, thermochemistry and detailed mechanisms involved in the gas-phase chemistry of hydrocarbon oxidation. 

The problems of simultaneously treating spatial distributions of both temperature and concentration are currently the concern of the chemical engineer in his treatment of catalyst particles, catalyst beds, and tubular reactors. These treatments are still concerned with systems that are kineticaliy simple. The need for a unified theory of ignition has been highlighted by contemporary studies of gas-phase oxidations, many features being revealed that neither thermal theory, nor branched-chain theory for that matter, can resolve alone. A successful theoretical basis for such reactions necessarily involves the treatment of both the enorgy balance and mass balance equations. Such equations are invariably coupled and cannot be solved independently of each other. However, much information is offered by the phase-plane analj s of the syst (e.g. stability of equilibrium solutions, existence of oscillations) without the need for a formal solution of the balance equations. 

See N. N. Semenov, Some Problems in Chemical Kinetics and Reactivity, translation by M. Boudart (Princeton, N.J. Princeton University Press, 1958), Vol. 2, Chapter 9 for a general discussion of ignition in branching-chain systems. 

The calculated boundary of the change in the dominant chemical reaction mechanism is confined inside the shaded area between fines A and B in Fig. 6.4. To the left of the separating zone (sometimes called the forth ignition limit [12]), branching-chain reactions dominate. For the first time, the possibility of the growth of a self-ignition delay with a pressure rise in a combustible mixture was indicated by the improved expression for t, in [17]. 

The chemical kinetics occur at a finite rate, with a certain time required for reactions to proceed. As the frequency decreases, providing more time at relatively higher temperature and pressure within each cycle, there is time for the chain-branching free-radical species to build up to levels that trigger an ignition. As the frequency increases, the time available... 

As shown in Fig. 13.4, after the initial low temperature reactions, which are mainly due to chain-branching, there is a temperature region, called the cool-flame region, where reverse reactions lead to a very slow burning process. This can result in a small temperature increase and hence in an increase of the ignition delay. Subsequently, after the cool-flame regime, other reaction paths dominate the chemical processes and release sufficient heat such that the associated temperature increase leads to the high-temperature reactions, that is, the actual combustion process. 

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