The relation of cool flames and auto-ignition phenomena to process safety at elevated pressure and temperature.
The cool-flame phenomenon can occur in fuel-oxygen (air) mixtures within the flammable range and outside the flammable range, at fuel-rich compositions, at temperatures below the auto-ignition temperature (AIT). It is caused by chemical reactions occurring spontaneously at relatively low temperatures and is favoured by elevated pressure. The hazards that cool flames generate are described. These vary from spoiling a product specification through contamination and explosive decomposition of condensed peroxides to the appearance of unexpected normal (hot) flame (two-stage ignition).
Chemiluminescence spectra from cool and blue flames: Electronically excited formaldehyde
Resolved emission spectra of electronically excited formaldehyde (1A2 → 1A1) have been obtained from two-stage ignition in a Vertical Tube Reactor combustion flow system, using acetaldehyde and n-butane as fuels. The very low intensity UV-visible emission from both the cool flame reaction zone (200–400°C) and blue flame reaction zone (400–800°C) is due to formaldehyde chemiluminescence. The “continuum” observed by previous researchers is due to formaldehyde band overlap, with typical hot flame emission from radicals, CO flame bands and continuum, and black body radiation being absent except for minor contributions in the acetaldehyde blue flame (800°C). Differences in the spectra band structure and underlying ‘continuum’ are due to the temperatures of the various flames. The cool flame formaldehyde emission yield per reacting fuel molecule is ∼10−8 for the fuels studied.
The Autoignition Chemistry of Paraffinic Fuels and Pro-Knock and Anti-Knock Additives: A Detailed Chemical Kinetic Study
A numerical model is used to examine the chemical kinetic processes which lead to knocking in spark-ignition internal combustion engines. The construction and validation of the model is described in detail, including the low temperature reaction paths involving alkylperoxy radical isomerization. The numerical model is then applied to C{sub 1} to C{sub 7} paraffinic hydrocarbon fuels, and a correlation is developed between the Research Octane Number (RON) and the computed time of ignition for each fuel. Octane number is shown to depend on the rates of OH radical production through isomerization reactions, and factors influencing the rate of isomerization such as fuel molecule size and structure are interpreted in terms of the kinetic model. The knock behavior of fuel mixtures is examined, and the manner in which pro-knock and anti-knock additives influence ignition is studied numerically. The kinetics of methyl tert-butyl ether (MTBE) is discussed in particular detail. 28 refs., 5 figs., 5 tabs.