Zone, ignition

Pyrolysis commences at bed surface temperatures in the range of 150-300°C [22,23]. Almost simultaneously, flaming combustion takes place in the combustion system above the fuel bed (see Figure 58C). At t2 the pyrolysis is sustained by heat from over-bed flames. The heat is transported by radiation. At times ts to t4 the dominant heat source has changed to the char combustion zone (ignition front) instead. The heat from the ignition front is also transported by means of conduction and radiation. 

The diesel engine takes in and compresses the air. The fuel is injected into the cylinder in atomized form at the end of the compression stroke and is vaporized in the air. Ignition begins by auto-ignition in one or several zones in the combustion chamber where the conditions of temperature, pressure and concentration combine to enable combustion to start. 

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

The in situ combustion method of enhanced oil recovery through air injection (28,273,274) is a chemically complex process. There are three types of in situ combustion dry, reverse, and wet. In the first, air injection results in ignition of cmde oil and continued air injection moves the combustion front toward production wells. Temperatures can reach 300—650°C. Ahead of the combustion front is a 90—180°C steam 2one, the temperature of which depends on pressure in the oil reservoir. Zones of hot water, hydrocarbon gases, and finally oil propagate ahead of the steam 2one to the production well. 

For Hquid fuels, ignition delay times are of the order 50 ]ls at 700 K and 10 ]ls at 800 K. At low temperatures most of the ignition delay is the result of slow, free-radical reactions, and a distinction between the initiation and explosion periods within the ignition delay time can be made. With increasing ignition temperature for a given mixture, these times become comparable and at temperatures as high as 1500 K, both times may be of the order of lO " s. Consequently, the reaction zone in the flame of a mixture is observed to be one continuous event (12—14). 

Correct selection of heating equipment and zoning of electrical equipment to reduce the chance of an ignition source arising. 

PR valve risers in flammable service should also be sized such that exit velocities are at least 30 m/s under all foreseable contingencies (except fire) which would cause the valve to release. On the basis of experimental work and plant experience, this minimum velocity, in conjunction with the riser elevation requirements, has been shown to ensure effective dispersion. Entrainment of air and dilution result in a limited flammable zone, with a negligible probability of this zone reaching any equipment which could constimte an ignition source. 

Flame Front That portion of the flame reaction zone moving into the nnbnrned gas where the bulk of the reaction occurs and the medium reaches its ignition temperature. 

Gluh-verlust, m. loss on ignition, -waohs, n. gilder s wax. -wem, m. mulled wine, -zone, /. zone of incandescence, -ziinder, m. electric igniter. 

In the search for a better approach, investigators realized that the ignition of a combustible material requires the initiation of exothermic chemical reactions such that the rate of heat generation exceeds the rate of energy loss from the ignition reaction zone. Once this condition is achieved, the reaction rates will continue to accelerate because of the exponential dependence of reaction rate on temperature. The basic problem is then one of critical reaction rates which are determined by local reactant concentrations and local temperatures. This approach is essentially an outgrowth of the bulk thermal-explosion theory reported by Fra nk-Kamenetskii (F2). 

A third alternative has been proposed by Anderson and Brown (A6, A9) as an outgrowth of their research on the ignition of composite propellants. Their ignition studies suggest significant contributions to the overall combustion process from the solid phase. Two exothermic reaction zones contributing to combustion are considered, as shown schematically in Fig. 19. 

These zones are (1) the ignition front zone at the propellant surface and in the subsurface layers, and (2) the luminous combustion zone in the gas phase. 

When pressure-decay rates less than critical are employed, the gas-phase combustion zone is removed from the propellant surface and extinguished, but not the ignition from within the condensed phase. Therefore, the temperature of the surface material will be above the autoignition temperature, and steady-state combustion will eventually be initiated. This mechanism is consistent with the observation that the luminosity of the combustion zone can vanish without combustion having been completely terminated. 

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