Autoignition

The autoignition temperature (AIT) of a vapor, sometimes called the spontaneous ignition temperature (SIT), is the temperature at which the vapor ignites spontaneously from the energy of the environment. The autoignition temperature is a function of the concentration of vapor, volume of vapor, pressure of the system, presence of catalytic material, and flow conditions. It is essential to experimentally determine AITs at conditions as close as possible to process conditions. 

Composition affects the AIT rich or lean mixtures have higher AITs. Larger system volumes decrease AITs an increase in pressure decreases AITs and increases in oxygen concentration decrease AITs. This strong dependence on conditions illustrates the importance of exercising caution when using AIT data. 

An increase in temperature tends to widen the flammable range, reducing the LFL. For example, the LFL for methane in air is commonly quoted as 5%. As the temperature of methane increases to autoignition temperature, the LFL falls to around 3%. Stronger ignition sources can ignite leaner mixtures. Flammability limits also depend on the type of atmosphere. Flammability limits are much wider in oxygen, chlorine, and other oxidizers than in air (NFPA, 1997). 

The flammability limit of a mixture of fuels may be calculated using the following equation  

is the lower flammable limit of fuel component i (%vol) Flammability limits can be narrowed by the addition of inert gases such as nitrogen or carbon dioxide. 

The autoignition temperature of a substance is the lowest temperature at which a solid, liquid, or gas will spontaneously ignite resulting in self-sustained combustion without the need for an external ignition source. A material released from a process above its autoignition temperature will ignite. Autoignition temperatures of some common materials are shown in Table B-2. 

Ignition of a mixture above its autoignition temperature is not instantaneous. The ignition time delay may be a fraction of a second for temperatures well in excess of the autoignition temperature a few minutes when the material is just above its autoignition temperature. 

Raising a mixture of fuel and oxidizer to a given temperature might result in a combustion reaction according to the Arrhenius rate equation, Equation (4.1). This will depend on the ability to sustain a critical temperature and on the concentration of fuel and oxidizer. As the reaction proceeds, we use up both fuel and oxidizer, so the rate will slow down according to Arrhenius. Consequently, at some point, combustion will cease. Let us ignore the effect of concentration, i.e. we will take a zeroth-order reaction, and examine the concept of a critical temperature for combustion. We follow an approach due to Semenov [3], 

The heat added term is given in terms of a convective heat transfer coefficient, h, and surface area, S, 

Actually, we expect T 7 X due to the release of chemical energy, so this is a heat loss. We write this loss as 

The critical condition is given by point c with Tc being the critical temperature. It should be noted that the critical heat loss rate, Ql2, depends not just on the vessel size but also on the surrounding temperature, T. Hence, the slope and the T intersection of Q in Equation (4.4), as tangent to Qr, will give a different Tc. We shall see that the Arrhenius character of the reaction rate will lead to T0 7X. This is called the autoignition temperature. The mathematical analysis is due to Semenov [3]. 

An approximate mathematical analysis is considered to estimate the functional dependence of the gas properties and vessel size on the autoignition temperature. We anticipate that Tc will be close to 7X, so we write 

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Autoignition temperature. The autoignition temperature of a material is the temperature at which it will ignite spontaneously in air without any external source of ignition. 

Combustion of a flammable gas-air mixture occurs if the composition of the mixture lies in the flammable range and if there is a source of ignition. Alternatively, combustion of the mixture occurs without a source of ignition if the mixture is heated up to its autoignition temperature. 

Unlike spark-induced combustion engines requiring fuel that resists autoignition, diesel engines require motor fuels, for vhich the reference compound is cetane, that are capable of auto-igniting easily. Additives improving the cetane number will promote the oxidation of paraffins. The only compound used is ethyl-2-hexyl nitrate. 

The autoignition temperature is the minimum temperature required for self-sustained combustion in the absence of an external ignition source. The value depends on specified test conditions. Tht flammable (explosive) limits specify the range of concentration of the vapor in air (in percent by volume) for which a flame can propagate. Below the lower flammable limit, the gas mixture is too lean to burn above the flammable limit, the mixture is too rich. Additional compounds can be found in National Fire Protection Association, National Fire Protection Handbook, 14th ed., 1991. 

Substance Autoignition temperature, °C Flammable (explosive) limits, percent by volume of fuel (25°C, 760 mm) ... 

Flash points and autoignition temperatures are given in Table 11. The vapor can travel along the ground to an ignition source. In the event of fire, foam, carbon dioxide, and dry chemical are preferred extinguishers. The lower and upper explosion limits are 1% and 7%. 

Table 11. Flash Points and Autoignition Temperatures of the Cg Aromatic Compounds...

The engine was the first to incorporate compression ignition of alcohols (7). Low cetane fuels can autoignite, however, provided the in-cylinder temperature is high enough and fuel injection correcdy timed. This compression ignition works for methanol as well as ethanol and gasoline. 

Syltherm XLT is a polydimethylsiloxane intended for Hquid-phase systems which operate at low temperatures. Syltherm 800 is a modified dimethylsiloxane polymer intended for Hquid-phase systems. The recommended maximum fluid temperature is greater than the autoignition temperature. 

Some of the tests and criterion used to define fire resistance may be found in the Hterature (9). Additionally, the compression—ignition and hot manifold tests as defined in MIL-H-19457 and MIL-H-5606, respectively the Wick test as defined by Federal Standards 791, Method 352 flash point and fire point as defined in ASTM D92 autoignition temperature as defined in ASTM D2155 and linear flame propagation rate are defined in ASTM D5306 are used. 

Properties of the principal hydrocarbons found in commercial hexane are shown in Table 9. The flash point of / -hexane is —21.7 °C and the autoignition temperature is 225°C. The explosive limits of hexane vapor in air are 1.1—7.5%. Above 2°C the equiUbrium mixture of hexane and air above the Hquid is too rich to fall within these limits (42). 

Undesirable combustible gases and vapors can be destroyed by heating to the autoignition temperature in the presence of sufficient oxygen to ensure complete oxidation to CO2 and H2O. Gas incinerators are appHed to streams that are high energy, eg, pentane, or are too dilute to support combustion by themselves. The gas composition is limited typicaUy to 25% or less of the lower explosive limit. Gases that are sufficiendy concentrated to support... 

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