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360 CHAPTER 14 1958 Guide hydeous combustion theory since most investigators who have studied the former have observed that the formation of aldehydes is one of the steps in the combustion process. It has been well established that fuel oils 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 Fig. 3. Illustration op Blue and Yellow Flame Combustion 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 is complete 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 substances, have all been identified as intermediate products in certain reactions. The chain reaction theory, in reality embraces and elaborates on the aldehydeous combustion theory. Oil Burning Indexes A number of indexes have been used, or proposed, as an indication of the burning qualities of fuel oils based on one or more physical measure ments made on the oil. These may be summarized as follows: A. Indexes based on a single physical test: (1) API gravity7, (2) Aniline point', (3) Institute of Petroleum smoke test, (4) Carbon-Hydrogen ratio based on flue gas analysis or ultimate analysis, and (5) Percent aromatics determined by sulfuric acid absorption tests'. B. Indexes based on two or more physical tests: (1) Diesel index' based on API gravity and aniline point, (2) Institute of Petroleum cetane number 10 based on the API gravity and 50 percent distillation point, (3) Universal Oil Products characteri zation factor" based on specific gravity and average boiling point, (4) Burning index1' based on API gravity and 50 percent distillation point, and (5) Estimated CarbonHydrogen ratio" based on API gravity aniline point, and boiling point. Various investigators have shown correlation between one or more of these indexes and the performance of fuel oils in oil burners. Experiments Fuels and Combustion 361 conducted with the Oil Heat Institute Reference Test Unit14 indicated good correlation between the smoking tendency of fuel oils and API gravity, burning index, Diesel index, and hydrogen-carbon ratio for a limited num ber of oils in laboratory apparatus simulating a pressure-atomizing burner. These results are shown graphically in Fig. 4. Smoking tendency is given here in terms of smoke spot reflectance, the light reflectance of a smokesoiled filter paper. A high reflectance, relative to a clean filter paper, indicates low smoking tendency. COs/U is the observed COi divided by the ultimate or maximum theoretical C02 expressed as a percentage. Reid and Hersberger12 have related burning qualities and burning index for various oils in a wall-flame burner. Cauley and Delgass13 cite test results o Smoke Spot Reflectance for 80% of Ultimate CO* if u oo a> z 4 ao: ho u. UJ o~Uw_-l oq: 70 60 60 -1-50 HYOROGEN-CARBON RATIO BURNING INDEX Fig. 4. Correlation op Burning Qualities op Fuel Oils with Four Combustion Indexes on combustion indexes obtained with vaporizing burners. The present experimental data are probably too meager as yet to correlate adequately any one of these indexes with burning qualities of oil fuel for all types of burners. Few attempts have been made to suggest limits for any of these fuel oil indexes for particular applications, even though correlations be tween them and burning qualities have been observed. In other words, none of the above mentioned indexeshas yet gained sufficiently wide usage 0 rePlace the grading of oils by Commercial Standard CS12-48. Experiments have shown that thermal decomposition or cracking of yarocarbons begins at a temperature of approximately 680 F at atmosenc Pressure, although the temperature of cracking varies somewhat ove and below this value. Thus pure distillate fuel oils, whose end point in'68 no*\ e.xceed this temperature, can usually be completely evaporated vaponzmg-typg oil burners at atmospheric pressure without leaving a iaue or without cracking of the hydrocarbons. Fuel oils that cannot