Carbonaceous particle surface

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 post-combustion gases. As mentioned in the previous chapter, the soot that penetrates the reaction zone of a co-annular diffusion flame normally bums if the temperatures remain above 1300K. This soot combustion process takes place by surface oxidation. 

Time-Resolved Laser-Induced Incandescence (by Prof. Alfred Leipertz et al.) introduces an online characterization technique (time-resolved laser-induced incandescence, TIRE-LII) for nano-scaled particles, including measurements of particle size and size distribution, particle mass concentration and specific surface area, with emphasis on carbonaceous particles. Measurements are based on the time-resolved thermal radiation signals from nanoparticles after they have been heated by high-energetic laser pulse up to incandescence or sublimation. The technique has been applied in in situ monitoring soot formation and oxidation in combustion, diesel raw exhaust, carbon black formation, and in metal and metal oxide process control. 

The major contributors to radiation are soot, carbon dioxide, water vapor, inorganic particulates and other intermediate products whose concentrations depend upon the particular fuel. The presence of solid particles such as ash and carbonaceous material affects the radiation heat transport as they are continuous emitters, absorbers, and scatterers of radiation. Carbonaceous particles tend to be large relative to the wavelength of radiation and have surfaces with high absorptivity. 

Near rich limits of hydrocarbon flames, soot is sometimes produced in the flame. The carbonaceous particles—or any other solid particles— easily can be the most powerful radiators of energy from the flame. The function k(t) is difficult to compute for soot radiation for use in equation (21) because it depends on the histories of number densities and of size distributions of the particles produced for example, an approximate formula for Ip for spherical particles of radius with number density surface emissivity 6, and surface temperature is Ip = Tl nrle ns) [50]. These parameters depend on the chemical kinetics of soot production—a complicated subject. Currently it is uncertain whether any of the tabulated flammability limits are due mainly to radiant loss (since convective and diffusive phenomena will be seen below to represent more attractive alternatives), but if any of them are, then the rich limits of sooting hydrocarbon flames almost certainly can be attributed to radiant loss from soot. 

Measurements of spheroidal carbonaceous particles made by Solovieva et al. (2002) in surface sediments of lakes on the same transects as those studied here corroborate a wider extent of particulate fallout around Vorkuta. This class of particle, which is an emission product of coal combustion, was present at sites 1.1 and 1.6 at higher concentrations than at any of the other remote sites examined in the study area (e.g. sites 3.2 and 2.2). 

Figure 4. SEM photograph of a spheroidal carbonaceous particle from (he combustion of coal, showing a convoluted surface texture, previously thought typical of oil combustion and used in fuel-type apportionment ofSCPs.

Rose, N. L, S. Juggins, J. Watt R. W. Battarbee, 1994. Fuel-type characterization of spheroidal carbonaceous particles using surface chemistry. Ambio. 23 296-299. 

It should be borne in mind that there are limitations in the theory developed by Coelho etal. [131] and Bekki et al. [132], The uptake on the soot surface is assumed to be a pseudo-first-order process in the framework, but uptake can be a complex multistep process [134,135], depending on the environmental conditions. Also, carbonaceous particles can contain a very large organic fraction that is mixed with an insoluble fraction composed predominantly of elemental carbon. The organic fraction is also strongly reactive [136]. 

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