Document 6RLw5eqwvmOLnXZ9eMrOwzb9m

CHAPTER 14 1954 Guide 3O3U41 between domestic grades and between commercial and industrial grades. ` The relation between the API gravity of fuel oils and their palorific value is shown in Table 5. Grades No. 1 and No. 2 are used predominantly in domestic heating equipment whereas grades 5 and 6 are used in commercial and industrial burners. Grade 6 usually requires preheating to increase its fluidity and to permit atomization, whereas grade 5 is used in some burners without preheating. Grade 4 fuel oil does not require preheating and can be burned satisfactorily in a limited number of domestic burners. COMBUSTION OF FUEL OILS Many theories have been advanced during the past century to explain the mechanism of combustion of hydrocarbons in oil burners and other devices used for producing heat or light. These theories have been modi fied from time to time to agree with hew experimental evidence. Much Table 5. Approximate Gravity and Calorific Value of Standard Grades of Fuel Oil Commercial Standard No. Approximate Gravity Range Calorific Value Btu Per Gallon 1 2 .4 5 6 35-45 26-40 12-25 10-23 8-17.5 138,800-132,900 144,300-135,800 153.000- 145,000 154,600-146,200 156.000- 149,700 still remains unknown about the process of decomposition and combustion! of hydrocarbons. >> Only three theories will be discussed here: (1) the carbonic combustion theory, (2) the aldehydeous combustion theory, and (3) the chain reaction theTohrye. carbonic combustion theory postulates that thermal destruction! of hydrocarbon molecules is likely to occur if (1) the oil is suddenly exposed' to intense heat without allowing time for previous evaporation, (2) the' oil and air are inadequately mixed, and (3) there is no preheating of the' air or mixture. According to this theory the hydrocarbons would ti' thermally decomposed into hydrocarbons of lower molecular weight along, with some free carbon atoms released under the conditions just describe? The free carbon atoms may produce smoky combustion while those carbon atoms that are oxidized to carbon dioxide will produce a yellow luminous flamThee. aldehydeous combustion theory is based on the evidence that al4< hydes, alcohols, and possibly peroxides are formed as intermediate product; when hydrocarbons are decomposed and oxidized to the final products: combustion. The formation of formaldehyde is certain since it can'', identified in the flue gases from blue flame oil burners when insuffici!; combustion air is provided. Alcohols have been identified by certain'^1! vestigators during the oxidation of methane and ethane. Aldehyde1 combustion is illustrated by the blue flame oil burner and the condition, conducive to this type of hydrocarbon decomposition consist of (1) allow# the oil time and opportunity to evaporate completely prior to combustiojV (2) mixing the air and oil vapor thoroughly before combustion, and (3)Pf|' heBatliunegatnhde yaeirlloowr tfhlaemmeicxotumreb.ustion can be demonstrated by the apparj'|| Fuels and Combustion 335 illustrated in Fig. 3. If methane is burned in an atmosphere of air, as in burner A, a yellow flame will result, whereas the introduction of the air for combustion in the center of a stream of methane, as in burner B, will result in blue flame combustion. . As the center of the flame in burner A is exposed to intense radiation the methane is thermally decomposed and liberates carbon particles which emit a yellow luminous flame during oxida tion. In burner B the center of the'flame cone is filled with air which can not decompose under heat, the methane gas at the zone of contact with the air is only moderately heated because of outward radiation, and the air is preheated in the center as it approaches the flame; each of these conditions tend to produce aldehydeous combustion. It is probable that the chain reaction theory is an extension of the alde hydeous combustion theory since most investigators who have studied the former have observed that the formation of aldehydes is one of the steps n the combustion process. It has been well established that fuel oils Fig. 3. Illustration of Blue and Yellow Flame Combustion must be gasified before combustion can occur, and that molecules of a hydrocarbon and oxygen do not combine directly with each other to form carbon dioxide and water vapor, but pass through intermediate reactions in the process. Lewis and von Elbe,5 Pease, and others6 have advanced the theory that the reactions between hydrocarbons and oxygen are probably chain reac tions. This theory postulates that a great many different reactions take place simultaneously or progressively between molecules, atoms, and radi cals in a mixture of hydrocarbons and oxygen. Some of these reactions produce particles or substances that tend to accelerate the reactions while others tend to slow down the process. Also, temperature, pressure, light and certain catalytic agents all may affect the speed and nature of these processes. The kind of intermediate products formed before combustion ls cmplete depends on the physical conditions mentioned, as well as the molecular structure of the particular hydrocarbon participating in the re action. Aldehydes, methyl and ethyl alcohols, formic acid, and other Vostances, have all been identified as intermediate products in certain orwk nS' ^le chain reaction theory, in reality embraces and elaborates the aldehydeous combustion theory. 0*1 Burning Indexes (jl^,m*ml>er of indexes have been used, or proposed, as an indication of Qj j rning qualities of fuel oils based on one or more physical measure' ts made on the oil. These may be summarized as follows: