Diffusion carbon combustion

Koylii, U.O. and Feath, G.M. Carbon monoxide and soot emissions from liquid-fueled buoyant turbulent diffusion flames. Combustion and Flame, 1991. 87, 61-76. 

As is seen in Section B.4, if the reaction rate at the surface and the gas pressure are high enough, then the burning rate is controlled by the rate of diffusion in the gas. The occurrence of this diffusion-controlled regime is well established for carbon combustion [31], [37]-[39]. The following analysis will be restricted to this limit, which ceases to apply if the dimensions of the carbon materials become too small [39], [40], [41]. 

Some coals contain an ash in addition to carbon, moisture, and volatiles. To obtain a conservative estimate, one should assume that a porous ash shell is retained during the burning of the combustible material. This ash may, of course, have a catalytic effect on the heterogeneous carbon combustion reactions however, it is a cause for additional diffusion resistance. 

The oxidation reactions of carbon and sulfur on hydroprocessing catalysts seem to be kinetically controlled by oxygen diffusion inside the catalyst porosity. Figure 3 shows the carbon and sulfur removal for Cat C which contains a very high amount of nickel and molybdenum, and an appreciable load of carbon. It is clear that the sulfur elimination occurs at higher temperatures than for the other catalysts and is simultaneous to carbon combustion. A tentative explanation of this phenomenon would be that the diffusion of oxygen in the microporosity is limited by coke deposit which needs to be at least partly removed to allow complete sulfur oxidation. 

Figure 4 Dependence of burnoff time on initial carbon level, for diffusion controlled combustion (silica-alwnina cracking catalyst, 700°C) (from Weisz and Goodwin [11]).

The most widely used model is a gas-solid reaction, assuming a carbon spherical particle. The most common cases are the regeneration and diesel soot combustion. In this model, carbon combustion is admitted and a cinder layer remains. Therefore, there is gas diffusion through the cinder layer and reaction on the carbon particle surface, which moves inward until total consumption. CO2 is formed during combustion and must diffuse through the cinder in the opposite direction, as shown in Figure 19.12. 

The mechanism of carbon combustion in FBC is assxjmed to be diffusion controlled in most of the modelling efforts as discussed in the section on Char-Oxygen Reaction, This is true only for large particles (> 300 microns) at high temperatures (> 1200 K). Feed coal contains a wide range of sizes, and assimiing a diffusion controlled kinetics for all particle sizes wo iLd lead to overestimation of the combustion rate. 

Lee, G.W., Jumg, J., and Hwang, J. (2004). Formation of nickel-catalyzed multi-waUed carbon nanotubes and nanofibers on a substrate using an ethylene inverse diffusion flame. Combust Flame 139 167-175. 

The formation of carbon black in a candle flame was the subject of a series of lectures in the 1860s by Michael Faraday at the Royal Institution in London (23). Faraday described the nature of the diffusion flame, the products of combustion, the decomposition of the paraffin wax to form hydrogen and carbon, the luminosity of the flame because of incandescent carbon particles, and the destmctive oxidation of the carbon by the air surrounding the flame. Since Faraday s time, many theories have been proposed to account for carbon formation in a diffusion flame, but controversy still exists regarding the mechanism (24). 

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

The violent or explosive reactions which carbon tetrachloride, chloroform, etc., exhibit on direct local contact with gaseous fluorine [1], can be moderated by suitable dilution, catalysis and diffused contact [2], Combustion of perfluorocy-clobutane-fluorine mixtures was detonative between 9.04 and 57.9 vol% of the halocarbon [3], Iodoform reacts very violently with fluorine owing to its high iodine content [4], Explosive properties of mixtures with 1,2-dichlorotetrafluoroethane have been studied [5],... 

A dry combustion-direct injection apparatus was applied to water samples by Van Hall et al. [51 ]. The carbon dioxide was measured with a non-dispersive infrared gas analyser. Later developments included a total carbon analyser [97], a diffusion unit for the elimination of carbonates [98], and finally a dual tube which measured total carbon by combustion through one pathway and carbonate carbon through another. Total organic carbon was then calculated as the difference between the two measurements [99]. 

Gershey et al. [58] have pointed out that persulfate and photo-oxidation procedures will determine only that portion of the volatile organics not lost during the removal of inorganic carbonate [30,79,92,181]. Loss of the volatile fraction may be reduced by use of a modified decarbonation procedure such as one based on diffusion [98]. Dry combustion techniques that use freeze-drying or evaporation will result in the complete loss of the volatile fraction [72,79, 92,93],... 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

FIGURE 9.18 Schematic of the double-film model of carbon particle combustion, whereby carbon monoxide produced at the particle surface is oxidized to carbon dioxide in a boundary layer flame, consuming the oxygen that is diffusing toward the particle. 

The oxygen diffuses through the boundary layer to the particle surface and countercurrent diffusion of char combustion products (carbon monoxide and carbon dioxide), see Figure 56. [73,77]... 

Step one is, oxygen diffusion in the porous system of the particle inwards to the char combustion front and the reaction site, (2) adsorption of oxygen to the active sites on the intraparticle char phase, (3) oxidation reaction with carbon, and (4) desorption of... 

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