Rate of combustion

Testing procedure. Horizontally held specimen is ignited at one end. The rate of combustion is calculated from the time required for the flame to pass a 25 mm 

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Mechanisms and Rates of Combustion. AH soHd fuels and wastes bum according to a general global mechanism (Fig. 2). The soHd particle is first heated. FoHowing heating, the particle dries as the moisture bound in the pore stmcture and on the surface of the particle evaporates. Only after moisture evolution does pyrolysis initiate to any great extent. The pyrolysis process is foHowed by char oxidation, which completes the process. 

Many finely divided metal powders in suspension in air are potential e] losion hazards, and causes for ignition of such dust clouds are numerous [Hartmann and Greenwald, Min. MetalL, 26, 331 (1945)]. Concentration of the dust in air and its particle size are important fac tors that determine explosibility. Below a lower Umit of concentration, no explosion can result because the heat of combustion is insufficient to propagate it. Above a maximum limiting concentration, an explosion cannot be produced because insufficient oxygen is available. The finer the particles, the more easily is ignition accomplished and the more rapid is the rate of combustion. This is illustrated in Fig. 20-7. 

In large industrial installations, the latter is the most common arrangement. The burner has a profile plate that controls the rate of combustion air. The warm air delivery fan may be either centrifugal or axial. 

It is assumed that the target surface faces toward the radiation source so that it receives the maximum incident flux. The rate of combustion depends on the release. For a pool fire of a fuel with a boiling point above the ambient temperature (Tg), the combustion rate can be estimated by the empirical relation ... 

Duiser (1989) calculates emissive power from rate of combustion and released heat. As a conservative estimate, he uses a radiation fraction (/) of 0.35. He proposed the following equation for calculating the emissive power of a pool fire ... 

The major mechanism of a vapor cloud explosion, the feedback in the interaction of combustion, flow, and turbulence, can be readily found in this mathematical model. The combustion rate, which is primarily determined by the turbulence properties, is a source term in the conservation equation for the fuel-mass fraction. The attendant energy release results in a distribution of internal energy which is described by the equation for conservation of energy. This internal energy distribution is translated into a pressure field which drives the flow field through momentum equations. The flow field acts as source term in the turbulence model, which results in a turbulent-flow structure. Finally, the turbulence properties, together with the composition, determine the rate of combustion. This completes the circle, the feedback in the process of turbulent, premixed combustion in gas explosions. The set of equations has been solved with various numerical methods e.g., SIMPLE (Patankar 1980) SOLA-ICE (Cloutman et al. 1976). 

The rate of combustion is set equal to the rate of heat applied to warm the entrained air plus the radiative heat losses ... 

The output from each case produces a wealth of information, including distribution of pressure, combustion products, rates of combustion, velocity components, etc. The results of each case will be summarized by presenting the pressure time histories at the eight locations that were presented in Figure 2 together with the flame speed along some selected directions. Some contour plots will also be presented. 

Gas turbines and power stations are particularly prone to generate NOx and the search for the low-NOx burner that will operate at high efficiency (i.e. with low hydrocarbon emissions) continues. The principle of the low-NOx burner is to slow the rate of combustion by dividing it into several stages by the gradual mixing of the combustion gases with the stoichiometric air volume. 

Effect of oxygen concentration on rate of combustion, (a) Steel wool in the flame reads with atmospheric oxygen, (b) In a stream of pure oxygen, the steel wool bums much faster. 

Give several ways by which the rate of combustion in a candle flame might be increased. State why the rate would be increased. 

CA 78, 161665 (1973) A math analysis of the theory is presented on the basis of the combustion rate, the thermal conductivity, the heat capacity, the surface temp of the proplnt grains, and other factors. Expts were made to determine the relation of the combustion rate to acceleration for various proplnts. The rate of combustion at 70 atm was compared with the initial rate. The. relation of the critical pressure of transitional laminar combustion to acceleration, and the dependence of the combustion rate of nitroglycol to the pressure at various acceleration rates were determined. Exptl observations were compared with results of theoretical calcns... 

The concept of the controlled chemical fireball is said to have significantly advanced the potential effectiveness of incendiary weapons. Theoretical studies have indicated that a great many applications may be possible, since, by changing the blend of the chemical constituents so as to achieve predetermined rates of combustion, the incendiary agent may be adapted to various operational requirements... 

The powdered material burns readily on exposure to air [1], and the rate of combustion of air suspensions has been studied with respect to concentrations of hydride and moisture in the air [2],... 

In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance... 

Piobert s law of 1839 states that burning takes place by parallel layers where the surface of the grain regresses, layer by layer, normal to the surface at every point . Thus the combustion gases flow in the opposite direction to the rate of combustion progress (or surface regression). 

Figure 25.12 Rate of combustion of pure carbon particles this figure is adapted from Yagi and Kunii (1955).

The impact of temperature on the rate of combustion is exponential. The rate increases by a factor of 2.4 going from 1200 to 1300°F. However, the rate increases by factor of 7.2 going from 1200 to 1400°F. The impact of carbon concentration on catalyst is also nonlinear. The relative amount of residence time required to decrease carbon concentration by 0.1% increases by a factor of 10 from an initial concentration of 1.0-0.15 wt%. The impact of oxygen partial pressure is linear. The unit feed rate will also inflnence coke burning kinetics. As feed is increased, the coke production will increase requiring more air for combustion. Since the bed level is constant, the air residence time in the bed will decrease causing the O2 concentration in the dilute phase to increase. This will lead to afterbum, which is defined as the combustion of CO to CO2 in the dilute phase or in the cyclones of the regenerator. 

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