Chapter 9
Combustion of nonvolatile fuels
Abstract
The combustion of nonvolatile fuels including metals, carbon char, and soot is presented. For metals, the criteria for vapor phase versus heterogeneous combustion are discussed. Thermodynamic analyses of metal oxidizer systems are performed with attention given to the effects of pressure and oxidizer composition. The analyses are then extended to the combustion synthesis of materials. Particle combustion of nonvolatile fuels is discussed, particularly with regard to size and the burning limits of diffusion and kinetic control. Specific examples include boron and carbon combustion. Finally, the combustion of coal, including devolatilization and char combustion, is explained along with gasification and oxy-combustion.
Keywords
Carbon and coal combustion; Combustion synthesis; Metal combustion; Soot oxidation9.1. Carbon Char, Soot, and Metal Combustion
The final stages of coal combustion produce a nonvolatile char which must be consumed to obtain good combustion efficiencies. The combustion of this char—a factor that has not yet been considered—is essentially a surface burning process similar to that occurring when carbon graphite burns. Coal is a natural solid fuel that contains carbon, moisture, ash-generating minerals, and a large number of different hydrocarbons that volatilize when combustion is initiated. The volatiles in coal contribute a substantial amount to the overall energy release. But the volatiles burn rapidly compared to the solid carbonaceous char that remains after devolatilization. It is the surface burning rate of this remaining nonvolatile solid carbonaceous fuel that determines the efficiency of the coal combustion process.
Similarly, the emission of soot from many practical devices, as well as from flames, is determined by the rate of oxidation of these carbonaceous particles as they pass through a flame zone and into the postcombustion gases. As noted in the previous chapter, the soot that penetrates the reaction zone of a coannular diffusion flame normally burns if the temperatures remain above 1300 K. This soot combustion process takes place by surface oxidation.
Heterogeneous surface burning also arises in the combustion of many metals. Since the energy release in combustion of metals is large, many metals are used as additives in solid propellants. Indeed, in the presence of high oxygen concentrations, metal containers, particularly aluminum, have been known to burn, thereby leading to serious conflagrations.
Not all metals burn heterogeneously. The determination of which metals will burn in a heterogeneous combustion mode can be made from a knowledge of the thermodynamic and physical properties of the metal and its oxide product [1].
In the field of high-temperature combustion synthesis, metals have been reacted with nitrogen, both in the gaseous and in the liquid phases, to form refractory nitrides [2]. In most cases, this nitriding process is heterogeneous.
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.