Combustion particle

The primary consideration in the burning of a metal particle in air is the limitation of the temperature attained by the boiling of the resultant oxide. 

Two schemes for particle combustion have been proposed which differ mainly in the consideration of the condensed oxide formed by the combustion reaction. 

For vapour phase combustion, the burning rate of spherical droplets can be expressed as in equation (5.3)  

For example, titanium is a non-volatile metal with a melting point of about 1660 °C and boiling point approaching 3320 °C. The oxide Ti02 has melting and boiling points of 1870 and 3827 °C, respectively. In the 

The kinetics that control the small droplet reaction are characterised by the dissolution of titanium oxide which, in turn, exposes further, unoxidised metal. Interestingly, this process appears to be independent of the type of oxidant, whether it be potassium perchlorate, potassium nitrate or atmospheric oxygen. 

Titanium is therefore an important ingredient in fountain compositions. It is characterised as a non-volatile metal with non-volatile oxides. The particles are easily ignited, even in the form of large flitters , and once ignited they grow progressively brighter and finally explode in a spectacular star formation. 

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Aluminum-containing propellants deflver less than the calculated impulse because of two-phase flow losses in the nozzle caused by aluminum oxide particles. Combustion of the aluminum must occur in the residence time in the chamber to meet impulse expectations. As the residence time increases, the unbumed metal decreases, and the specific impulse increases. The soHd reaction products also show a velocity lag during nozzle expansion, and may fail to attain thermal equiUbrium with the gas exhaust. An overall efficiency loss of 5 to 8% from theoretical may result from these phenomena. However, these losses are more than offset by the increase in energy produced by metal oxidation (85—87). 

As illustrated on Figure 7.8, the DPF regeneration step is just a soot particles combustion reaction (soot is mainly composed by a carbon matrix), which requires a temperature in the range of 600°C and oxygen presence in the exhaust gases. 

As mentioned in the previous section, the condition for vapor phase combustion versus heterogeneous combustion may be influenced by pressure by its effect on the flame temperature (Tvol or Td) as well as by its effect on the vaporization temperature of the metal reactant (Th). For aluminum combustion in pure oxygen, combustion for all practical conditions occurs in the vapor phase. In air, this transition would be expected to occur near 200 atm as shown in Fig. 9.15 where for pressures greater than —200 atm, the vaporization temperature of pure aluminum exceeds the adiabatic flame temperature. This condition is only indicative of that which will occur in real particle combustion systems as some reactant vaporization will occur at temperatures below the boiling point... 

FIGURE 9.17 Schematic of the single-film model of carbon particle combustion, whereby oxygen and carbon monoxide counterdiffuse through an unreactive boundary layer. 

With the importance of the devolatilization process to solid particle combustion and the complexity of the chemical and physical processes involved in devolatilization, a wide variety of models have been developed to describe this process. The simplest models use a single or multiple Arrhenius rates to describe the rate of evolution of volatiles from coal. The single Arrhenius rate model assumes that the devolatilization rate is first-order with respect to the volatile matter remaining in the char [40] ... 

At low temperatures (T<1320 °C) and small particles, combustion regime (I) prevails [11,74,75]. Regime (I) is controlled by chemical kinetics intraparticle (reaction control), see Figure 55. The oxygen content is constant at any radius inside the particle since the rate of diffusion is fast compared to the rate of heterogeneous reaction. The particle then burns with reducing density and a constant diameter, see Figure 55. 

At higher temperatures (T>1320 °C) and larger particles, combustion regime (II) prevails [75], Regime (II) is controlled by both intraparticle diffusion and chemical kinetics. In this case the density and diameter decrease, see Figure 55. 

Saastamoinen J.J. and Richard J.R., The Simultaneous Drying and Pyrolysis of Solid-Fuel Particles , Combustion and Flame 106, 288-300(1996)... 

Beshty B.S., A Mathematical Model for the Combustion of a Porous Carbon Particle , Combustion and Flame 32, 295-311(1978). 

Newtons theory of refraction was based on the presumed attraction of the particles of transparent bodies for the particles of light passing near them. Since objects containing sulphurous particles (combustibles) have greater power of refraction than other bodies, they must have a greater force of attraction, a proposition Newton supports by citing the fact that the concentrated rays of sunlight attract the sulphurous particles from combustible bodies as flame. 

The process of particle combustion depends on the physical and chemical nature of the solid as it heats and burns. Coal is a complex material of volatile and nonvolatile components which becomes increasingly porous during volatilization of low-boiling constituents in burning. The crucial practical questions for boiler design concern whether pulverized fuel combustion is controlled by oxidizer diffusion or by chemical kinetics. 

Adopting the approach developed above for the char particles combustion, the size distribution function of limestone particles as a result of sulfation reaction in the overflow stream which is the same as in the bed is given by. 

When o)o/v < < 1, as for the case of most hydrocarbons burning in air, the dimensionless driving force for particle combustion can be hnearized as ... 

Test calculations have been performed for the single char particle combustion and compared with the experimental data of Van Der Honing [9], The rate constant derived by Winter et al. [7] for the char formed from sewage sludge is currently adopted in the mechanism. 

Aho, M.J., HSmalainen, J.P., Tummavuori, J.L. (1993) Importance of Solid Fuel Properties to Nitrogen Oxide Formation Through HCN and NHj in Smalt Particle Combustion. Combust. Flame, 95, 22-30. 

Particle temperature is obtained from the particle energy equation, which includes the effects of particle mass loss, particle combustion and heat loss to the surroundings. 

Aho, M, HamSleinen, J. Tummavuori, J. (1993) Importance of solid fuel properties to nitrogen oxide formation through HCN and NHj in small particle combustion. Combustion and Flame, 95, pp. 22-30. 

Miller R. S. and Bellan J. (1996) Analysis of reaction products and conversion time in the pyrolysis of cellulose and wood particles. Combust. Sci. and Tech., 119,331-73... 

Simmons W.W., Ragland K.W. (1985). Single Particle Combustion Analysis of Wood, In Proc. Fundamentals of Thermochemical Biomass Conversion , OPEREND R.P. MILNE T.A., MUDGE LK. ed. Elsevier Applied Science pub., Londres, UK, 777-792. 

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