Concentration limits combustion

Flammability and Explosivity. Ozone is endothermic, thus it can bum or detonate by itself and represents the simplest combustible and explosive system. The concentration threshold for spark-initiated explosion ofHquid ozone in oxygen at —183° is 18.6 mol % O the concentration limit... 

Elammability Limits The minimum and maximum concentrations of combustible material in a homogeneous mixture with gaseous oxidizer that will propagate a flame. 

Flammable Limits in Air - This is a concentration expressed as percent by volume of the chemical in air, whereby spontaneous combustion will be supported. The lowest concentration where combustion will be supported is known as the lower flammability limit (LFL) or lower explosion limit (LEL). LEL and LFL are considered interchangeable. The upper concentration limit is the UFL (Upper Flammability Limit) or UEL (Upper Explosion Limit). 

FIRE SIMULATOR predicts the effects of fire growth in a 1-room, 2-vent compartment with sprinkler and detector. It predicts temperature and smoke properties (Oj/CO/COj concentrations and optical densities), heat transfer through room walls and ceilings, sprinkler/heat and smoke detector activation time, heating history of sprinkler/heat detector links, smoke detector response, sprinkler activation, ceiling jet temperature and velocity history (at specified radius from the flre i, sprinkler suppression rate of fire, time to flashover, post-flashover burning rates and duration, doors and windows which open and close, forced ventilation, post-flashover ventilation-limited combustion, lower flammability limit, smoke emissivity, and generation rates of CO/CO, pro iri i post-flashover. 

Combustible Concentration Reduction The technique of maintaining the concentration of combustible material in a closed space below the lower flammable limit. 

All combustible solids can create a dust explosion hazard if dispersed in air as a line dust within certain concentration limits. Refer to Table 5.2. The hazard increases with decreasing size. 

The data reported in Table VI for Halon 1301 concentration required for flame extinguishment vary in the range of 3 to %, which is comparable to the range found for ordinary combustibles. Thus maintenance of Halon 1301 concentrations in excess of is expected to extinguish fires in the ignition zone for the FRC materials this concentration limit satisfies the current Halon 1301 requirements for fire suppression systems for tracked vehicles. 

Lower Explosive Limit (LEL) - The minimum concentration of combustible gas or vapor in air below which propagation of flame does not occur on contact with an ignition source. 

There are several standard tests for determining the flammability of materials (ASTM, 2004). For example, the upper and lower concentration limits for the flammability of chemicals and waste can be determined by standard test methods (ASTM D4982, E681), as can the combustibility and the flash point (ASTM D1310, E176, E502). With these definitions in mind it is possible to divide ignitable materials into two subclasses ... 

Flammable limits—The minimum (lower flammable limit, FFF) and maximum (upper flammable limit, UFF) concentration of combustible vapor in air that will propagate a flame. 

For determination of concentration limits for combustion of fuel gases in air (or oxygen) burning can be conducted either in glass tubes open at one end or in apparatus illustrated in Fig of Ref 6 and Fig 28 of Ref 15, p 120, reproduced here as Fig A. For determination of lower limit, the concentration of combustible gases is decreased until flame ceases to appear, while for determination of upper limits, the concentr ation is increased. 

What problems face the theory of combustion The theory of combustion must be transformed into a chapter of physical chemistry. Basic questions must be answered will a compound of a given composition be combustible, what will be the rate of combustion of an explosive mixture, what peculiarities and shapes of flames should we expect We shall not be satisfied with an answer based on analogy with other known cases of combustion. The phenomena must be reduced to their original causes. Such original causes for combustion are chemical reaction, heat transfer, transport of matter by diffusion, and gas motion. A direct calculation of flame velocity using data on elementary chemical reaction events and thermal constants was first carried out for the reaction of hydrogen with bromine in 1942. The problem of the possibility of combustion (the concentration limit) was reduced for the first time to thermal calculations for mixtures of carbon monoxide with air. Peculiar forms of propagation near boundaries which arise when normal combustion is precluded or unstable were explained in terms of the physical characteristics of mixtures. 

However, no.theory of the concentration limit has existed until recently. The opinion was expressed that the limits correspond to the composition at which the flame velocity goes to zero (Bunte, Jahn) since the combustion temperature is equal to the ignition temperature (Jouguet). 

Experiments by Payman showed that the most diluted mixtures which are still combustible yield a flame propagation velocity in a pipe of order 12-20 cm/sec, and Wheeler s empirical rule gives a velocity of 15 cm/sec. The corresponding flame propagation velocity for a gas (after accounting for the influence of convection) is of order 3-8 cm/sec. Thus, naive notions of the concentration limits are refuted by both theory and experiment. 

Thus, considering the low accuracy of the calculations and the inexactness of experimental determination of concentration limits, the agreement must be deemed satisfactory. In evaluating the character of the agreement, it should be kept in mind that we calculate the concentration limits from completely independent data on the combustion velocity, gas radiation and thermo-chemical constants. Meanwhile, to date, calculations have not gone beyond determination of limits for mixtures of several fuels on the basis of the limits of the individual fuels (Le Chatelier s rule). 

Thus we have been able, not only to establish the physical nature of the concentration limit, but also to relate it in a number of cases to the chemistry of combustion reactions. 

Besides the equilibrium radiation considered in this article, the concentration limits can also be affected by chemiluminescence which arises if, in the combustion, chemical compounds form with a non-equilibrium energy distribution along the degrees of freedom of molecules or an atom. The chemiluminescence itself cannot lead to the appearance of a flame propagation limit if only one specific energy fraction is emitted. However, forced luminescence in an optical resonator (in combustion lasers) can lead to quenching of the flame. 

The data shown in Fig. 20.4 are for tests in which dusts with narrow size distributions were used. Hertzberg and Cashdollar (1987) pointed out that with broad size distributions, strong size dependencies tend to be blurred, hut the trends should remain the same. With small mean particle sizes, all flammable material is volatilized from the particles, and the combustion process is homogeneous. As the particle size increases, only the surface material is volatilized so that more dust is required overall to sustain combustion. Finally, above some upper limit of particle size, the volatilization rate is such that a minimum concentration of combustible volatile material is not achieved. 

Limiting oxidant concentration for combustion is dependent on the type of dust and type of inert gas used. Once the LOC of the dust has been determined for the inert gas that will be used, the inert gas needs to be introduced into the vessel. Successful inert gas blanketing will only be possible if the entire volume of the vessel is inerted and the inert atmosphere is maintained throughout even when the vessel is opened during the addition of solids and/or liquids to the vessel. 

There are two composition limits of flammability for air and a gaseous fuel under specified conditions [76]. The lower limit is the minimum concentration of combustible gas that will support combustion, while the higher limit is the maximum concentration. Table 5.1 shows the lower and higher limits for pure hydrocarbons in air at room temperature and atmospheric pressure (RTP) [76]. For methane in air, the flammability limit is 5—15 mol%. For ethane in air, the limits are 2.9—13.0 mol%. The limits become lower with increasing molecular weight. It also is interesting to note that the limits are the same for n-pentane and isopentane, and also for -butane and isobutane. 

Rapid combustion of fine soUd particles. Upper and lower concentration limits of the dust... 

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