Combustion of Metal Particles

The most important process with regard to metal oxidation and combustion is the formation of metal oxides on the surface of the metal particles. Some metal particles become coated with an oxide layer that surrounds the unreacted metal, whereas in other cases finely divided metal oxides are formed that are expelled from the surface of the metal particle. When solid shells of metal oxides are formed, no additional supply of the oxidizer fragments (or molecules) to the metal particles is possible. The oxidation process is thus interrupted and incomplete combustion occurs. On the other hand, when the metal oxide is expelled from the surface, oxidizer fragments continue to be supplied to the underlying unreacted surface of the metal. 

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The combustion temperature of metallized pyrolants is high due to the high heat of combustion of metal particles, and the molecular mass of the combustion pro-... 

When a pyrolant is composed of metallic particles and an oxidizer component, both gaseous molecules and metal oxides are formed as combustion products. Since the metal oxides are produced in the form of condensed-phase particles, the equation of state shown in Eq. (10.1) is no longer valid to evaluate the pressure in the cham-... 

The chosen combinations of these chemicals and metals depend on the requirements of the specific application. Gasless combustion prevents pressure increase in a closed combustion chamber. Some combinations of metal particles and metal oxide particles or of metal particles and crystalline oxidizers are chosen as chemical ingredients of gasless pyrolants. On the other hand, hydrocarbon polymers are used to obtain combustion products of low molecular mass, such as H2O, CO, CO2, and H2. High pressure is thus obtained by the combustion of hydrocarbon polymers. Table 10.6 shows the chemical ingredients used to formulate various types of pyrolants. 

AP composite propellants without aluminum particles are termed reduced-smoke propellants and are employed in tactical missiles to conceal their launch site and flight trajectory. No visible smoke is formed when the relative humidity of the atmosphere is less than about 40%. However, since high-frequency combustion oscillation tends to occur in the combustion chamber in the absence of solid particles that serve to absorb the oscillatory energy, a mass fraction of 0.01-0.05 of metallic particles is still required for the reduced-smoke propellants. These particles and/or their oxide particles generate thin smoke trails. The white smoke trail includes the white fog generated by the HCl molecules and the condensed water vapor of the humid atmosphere. 

There is a large class of industrially important heterogeneous reactions in which a gas or a liquid is brought into contact with a solid and reacts with the solid transforming it into a product. Among the most important are the reduction of iron oxide to metallic iron in a blast furnace the combustion of coal particles in a pulverised fuel boiler and the incineration of solid wastes. These examples also happen to be some of the most complex chemically. Further simple examples are the roasting of sulphide ores such as zinc blende ... 

Say, for example, it is not possible to suspend the necessary amount of particles in a thixotropic system, or that similarly in the hybrid it was not possible to burn the metal efficiently because the large quantities of metal particles must be projected into the flame from a decomposing surface creating only a small amount of gaseous components. Then it is possible to place small quantities of metal in both the liquid and solid to obtain higher combustion efficiency and thus performance. Figure IV.B. 1. represents this approach as the gelled hybrid (F). 

The luminosity of a flame can be greatly increased by the introduction of solid particles which become incandescent,1 and the rapid combustion of such substances as give non-volatile solid oxidation products is usually accompanied by brilliant luminosity. A familiar example is the combustion of metallic magnesium. But hydrogen burns in oxygen under pressure with high luminosity, so that solids are not essential to the phenomenon. 

Fig. 47. Effect of metal particle size on combustion velocity in solid flame systems.

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