The nature of carbon combustion

Carbon combustion is an interesting illustration of gas-solid reactions 

The nature of carbon combustion depends strongly on the temperature of the fuel. At temperatures above 4000K, sublimation of solid carbon becomes nonnegligible at atmospheric pressure, and gas-phase combustion processes may be important. At lower temperatures, oxygen 

FIGURE 3.3. Diagram of a rectangular duct with carbon oxidation occurring at its walls. 

A number of complications lead to differences in experimental results. Aside from possible catalytic influences of impurities in the carbon, there are different structural types of carbon that have been studied. These include amorphous carbon (typically randomly oriented graphite crystals) 

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]. 

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With regards to the overall balance of combustion, the chemical structure of the motor or heating fuel, e.g., the number of carbon atoms in tbe chain and the nature of the bonding, does not play a direct role the only important item is the overall composition, that is, the contents of carbon, hydrogen, and — eventually— oxygen in the case of alcohols or ethers added to the fuel. 

At still higher temperatures, when sufficient oxygen is present, combustion and "hot" flames are observed the principal products are carbon oxides and water. Key variables that determine the reaction characteristics are fuel-to-oxidant ratio, pressure, reactor configuration and residence time, and the nature of the surface exposed to the reaction 2one. The chemistry of hot flames, which occur in the high temperature region, has been extensively discussed (60-62) (see Col ustion science and technology). 

Carbonyl sulfide occurs as a by-product ia the manufacture of carbon disulfide and is an impurity ia some natural gases, ia many manufactured fuel gases and refinery gases, and ia combustion products of sulfur-containing fuels (25). It tends to be concentrated ia the propane fraction ia gas fractionation an amine sweetening process is needed to remove it. 

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). 

The most likely substitute for coal is natural gas, which, as noted earlier, releases about 55 percent of the amount of carbon dioxide that coal, on the average, does. In addition, it produces far fewer other pollutants, such as sulfates and polycyclic hydrocarbons, than coal and oil yield on combustion. 

The process will adversely affect air quality by releasing nitrogen oxides, sulfur oxides, carbon monoxides and other particulates into the atmosphere. Better control of the conversion conditions and better control of emissions can make the process cleaner, yet technology cannot do anything to curb carbon emissions. Since much of the carbon in coal is converted to carbon dioxide in the synthesis process, and is not part of the synthetic fuel itself, the amount of carbon dioxide that will be released to the environment during combustion is 50 to 100 percent more than coal, and around three times more than natural gas. 

Future legislation will stimulate burner development in the areas of carbon monoxide, NOx and particulate generation. Techniques will include flue-gas recirculation, staged combustion, and additives to reduce the NOx and more sophisticated controls. Controls over the sulfur generated do not affect burner design greatly since the sulfur dioxide is a natural product of combustion and can only be reduced by lower fuel sulfur contents or sulfur removal from the exhaust gases. 

When a composite propellant composed of ammonium perchlorate (AP) and a hydrocarbon polymer burns in a rocket motor, HCl, CO2, H2O, and N2 are the major combustion products and small amounts of radicals such as OH, H, and CH are also formed. These products are smokeless in nature and the formation of carbon particles is not seen. The exhaust plume emits weak visible light, but no afterburning occurs because AP composite propellants are stoichiometrically balanced mixtures and, in general, no diffusional flames are generated. 

Carbon dioxide is a gas at room temperature. Below -78°C it is a solid and is commonly referred to as dry ice. At that temperature it sublimes and changes directly from a solid to a vapor. Because of this unique property, as well as its non-combustible nature, it is a common refrigerant and inert blanket. Table 3.4 shows the uses of carbon dioxide in all its forms liquid, solid, and gas. Refrigeration using dry ice is especially important in the food industry. Beverage carbonation for soft drinks is a very big application. In oil and gas recovery carbon dioxide competes with nitrogen as an inert atmosphere for oil wells. 

Combustion, Flames and Explosion in Gases , 2nd Editn, Academic Press, NY (1961), 322—23 (Dust quenching occurs at a critical value of the surface area of the dust per unit vol of the suspension, and depends on the nature of the salt. Better results are obtained with salts having a mp under 200°. Alkali halides are better than carbonates, potassium better than sodium, fluoride better than iodide and better than chlor-ide. If the dust concentration is high enough, even detonation waves can be extinguished)... 

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