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Abstract

Studies have been made of the combustion of relatively high molecular weight alkanes (especially n-decane) at sub-atmospheric pressures and in the temperature range 480 to 620°K, Special attention has been paid to the variation with n-decane: air ratio of the spontaneous ignition temperature limits and the effect on these limits of replacing some or all of the n-decane by a highly branched alkane, 2,2,5-trimethlhexane. In addition, the variation with molecular weight and molecular structure of the alkane of the spontaneous ignition temperature limits has been investigated. Studies have also been made of the initial conditions required to cause spontaneous ignition in asymmetrically heated n-decane-air mixtures.

The Introduction (Section 1) outlines the hazards associated with unwanted ignition in hydrocarbon fuels, with particular reference to spontaneous ignition. Methods of classifying flammable and explosive gas mixtures are briefly reviewed, and ways in which the ignition tendency of these mixtures may be reduced are described in terms of the influence of experimental conditions on thermal balance and on the chemistry of the combustion reactions.

The Experimental Section (Section 2) describes the vacuum apparatus and the procedures used. Temperature and pressure changes accompanying combustion reactions were monitored using fine thermocouples and a strain-gauge pressure transducer respectively. Asymmetric heating of n decane-air mixtures was achieved using a hot tube contained in a cooler spherical vessel.

The Results Section (Section 3) describes the temperature/ pressure ignition profiles for n-decane-air mixtures, and the slow-combustion/cool-flame boundaries obtained with several other alkanes. Overall changes in the nature of the combustion reactions are related to the variation with initial temperature and pressure of the induction period preceding and pressure change accompanying the passage of cool flames. Temperature profiles across the vertical diameter of the reaction vessel show the existence of maximum stable temperature rises at the centre of the vessel under slow-combustion/ cool-flame boundary conditions for n-decane-air mixtures; these measurements also indicate the extent of natural convection in such mixtures.

For the hot-tube-in-a-sphere studies, the minimum tube temperature required to cause spontaneous ignition of n-decane in air is shown to depend on vessel temperature and tube diameter. Induction periods preceding, and pressure changes accompanying ignition are shown to be very sensitive to changes in tube temperature.

Finally, in the Discussion Section (Section 4) it is shown that lobes occurring on the two-stage ignition profile for n-decane-air mixtures probably result from changes with temperature of the chain-branching reactions and that the overall activation energy is related to these reactions. The ignitability of fuel-air mixtures containing n-decane and 2, 2, 5-trimethylhexane is shown to be influenced largely by the more readily ignitable component, n-decane.

Under conditions where heat transfer in the reacting gases takes place purely by conduction, cool-flame limits are in excellent agreement with predictions according to the conductive theory of thermal ignition. A computer programme based on thermal ignition theory is presented which predicts reasonably accurately for alkanes with carbon number not greater than ten the autoignition temperatures at atmospheric pressure, and the limiting conditions for cool flames. ‘When convective heat transfer is significant, the critical condition for ignition is a function of the Rayleigh number of the gas. The overall activation energy for cool-flame combustion is shown to be a parameter characteristic of the ignitability of an alkane.

Spontaneous ignition in asymmetrically heated gaseous reactants is unlikely to occur unless a critical amount of heat is supplied to the gas which is consequently raised above a critical temperature.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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