Combustion point

Spontaneous combustion can arise from exothermic chemical reactions of some materials, e.g., with water, or through the action of microorganisms. Microorganism growth can heat a medium to about 70°C where, although the heat is sufficient to kill the microorganisms, the increased temperature may be sufficient to rapidly increase the rate of some other chemical reaction which subsequently raises the temperature of the medium to the combustion point (Cross and Farrer, 1982). 

Combustion Point— The temperature at which a given fuel can burst into flame. 

Physical methods for characterising lipids comprised determination of melting and freezing point, density, hardness, viscosity, surface and interfacial tension, solubility, flash point and combustion point. These classicaF methods have been described in detail in the works of T. P. Hilditch [59] and H. P. Kaufmann [77]. 

For inslance, in excess of 10% of the heating value of methane can be lost in this manner, while the stack gases remain clear. However, if heavy fuel oil is burned, the flue gas will quickly turn black as the combustion air rate falls below the absolute combustion point. For an extreme contrast, the combustion of hydrogen will never turn a stack black, regardless of combustion air insufficiency. 

One of the more common factors causing high oxygen levels in flue gas at the absolute combustion point is poor air-fuel mixing. To promote air-fuel mixing, the following parameters should act as a guide ... 

FIGURE lS-5 Absolute combustion point (courtesy Oil Gas Journal)... 

The heater outlet temperature declines as air flow is diminished past the absolute combustion point. That is, products of the partial combustion of hydrocarbons (aldehydes, ketones, CO, etc.) are formed, and the heating value of the fuel gas is effectively reduced. 

The conclusion from the preceding example is that excess air is often used to prevent overheating of the combustion zone, and the oxygen requirements to reach the absolute combustion point are, in this case, irrelevant. 

Example 6.4 The process in Fig. 6.2 is to have its hot utility supplied by a furnace. The theoretical flame temperature for combustion is 1800°C, and the acid dew point for the flue gas is 160°C. Ambient temperature is 10°C. Assume = 10°C for process-to-process heat transfer but = 30°C for flue-gas-to-process heat transfer. A high value for for flue-gas-to-process heat... 

The problem of explosion of a vapor cloud is not only that it is potentially very destructive but also that it may occur some distance from the point of vapor release and may thus threaten a considerable area. If the explosion occurs in an unconfined vapor cloud, the energy in the blast wave is generally only a small fraction of the energy theoretically available from the combustion of all the material that constitutes the cloud. The ratio of the actual energy released to that theoretically available from the heat of combustion is referred to as the explosion efficiency. Explosion efficiencies are typically in the range of 1 to 10 percent. A value of 3 percent is often assumed. 

Example 9.1 A process involves the use of benzene as a liquid under pressure. The temperature can be varied over a range. Compare the fire and explosion hazards of operating with a liquid process inventory of 1000 kmol at 100 and 150°C based on the theoretical combustion energy resulting from catastrophic failure of the equipment. The normal boiling point of benzene is 80°C, the latent heat of vaporization is 31,000 kJ kmol the specific heat capacity is 150 kJkmoh °C , and the heat of combustion is 3.2 x 10 kJkmok. ... 

All petroleum energy products, as distinct and dissimilar as they can be, are subjected to the process of flame combustion. It is helpful at this point to bring to mind some definitions and general laws of thermochemistry. 

The gasoline end point should not exceed a given value, currently established for Europe at 215°C. In fact the presence of too-heavy fractions leads to incomplete combustion and to a number of accompanying problems ... 

For optimum combustion, the fuel should vaporize rapidly and mix intimately with the air. Even though the design of the injection system and combustion chamber play a very important role, properties such as volatility, surface tension, and fuel viscosity also affect the quality of atomization and penetration of the fuel. These considerations justify setting specifications for the density (between 0.775 and 0.840 kg/1), the distillation curve (greater than 10% distilled at 204°C, end point less than 288°C) and the kinematic viscosity (less than 8 mm /s at -20°C). 

In order to maintain high energy efficiency and ensure a long service life of the materials of construction in the combustion chamber, turbine and jet nozzle, a clean burning flame must be obtained that minimizes the heat exchange by radiation and limits the formation of carbon deposits. These qualities are determined by two procedures that determine respectively the smoke point and the luminometer index. 

The second indication is a faint smoke-like cloudiness in the zone of the tube which is being heated by the Bunsen this is readily visible as the interior of the tube is normally quite clear and bright. This is a later stage of development of the flash-back than the rise of pressure, already mentioned, and should be counteracted by moving the Bunsen immediately to the point of the combustion tube where heating was commenced. In either case the Bunsen should then be moved slowly forwards as before. A flash-back is attended by the deposition of carbon particles, carried back by the explosion wave, on the cold walls of the tube. Care should be taken that these are completely burnt off as the Bunsen is slowly moved forward again. 

Revised material in Section 5 includes an extensive tabulation of binary and ternary azeotropes comprising approximately 850 entries. Over 975 compounds have values listed for viscosity, dielectric constant, dipole moment, and surface tension. Whenever possible, data for viscosity and dielectric constant are provided at two temperatures to permit interpolation for intermediate temperatures and also to permit limited extrapolation of the data. The dipole moments are often listed for different physical states. Values for surface tension can be calculated over a range of temperatures from two constants that can be fitted into a linear equation. Also extensively revised and expanded are the properties of combustible mixtures in air. A table of triple points has been added. 

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