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334 CHAPTER 16 1949' Guide is determined in a caloiimeter the water vapor is condensed and the latent heat of vaporization is included in the heating value of the fuel. Theheatiiig value so determined is termed the gross or higher heating value and this is what is ordinarily meant when the heating value of a fuel is specified. In burning the fuel, however, the products of combustion are not cooled to the dew-point and the higher heating value cannot be utilized. When combustion is complete, the carbon in the fuel unites with oxygen to form carbon dioxide, C02, the hydrogen unites with oxygen to form water vapor, H20, and the nitrogen, being inert, passes through the re action without change. When combustion is incomplete, some of the carbon may unite with oxygen to form carbon monoxide, CO, and some of the hydrogen and hydrocarbon gases may not be burned at all. When carbon monoxide or other combustible gases are present in the flue gases, considerably less heat is produced per unit of fuel consumed, and a lower combustion efficiency is obtained.. Incomplete combustion may result' from any or all of the following three conditions: 1. Inadequate air supply. 2. Insufficient mixing of air and gases. . 3. A temperature too low to produce ignition or maintain combustion. AIR REQUIRED FOR COMBUSTION The weight of air required for perfect combustion of a pound of fuel may be determined by use of the ultimate analysis of the fuel as applied to Equations 1 and 2. The various elements are expressed in percentages by weight. Solid and Liquid Fuels: [f+MKlPounds air required per pound fuel = 34.56 <> Gaseous Fuels: Pounds air required per pound fuel = 2.46 CO + 34.56 Hi + 17.28 CHt -f13.29 Ciffj ,4: 14.81 Cjit + 16.13 Ctff -f 6.i0 HtS -- 4.32 02 '' When the analysis is given on a volumetric basis the equation is ex pressed as follows: Cubic feet air required per cubic foot gas = 2.39 (CO + Hi) + 9.56 CHt + 11.98 CVHJ +44.35 CtH, + 16.74\C,ff, - 4.78 02 A' Equations 4 and 5 may be used as approximate methods of determining the theoretical air requirement for any fuel. Heating value (Btu per pound) . , . Pounds air required per pound fuel = 0:755 X -------- :w . . ., Heating valuefBtu per unit) Cubic feet air required per unit fuel =. --------- ;-- ^ .--:------r-- r XUU ,. w Approximate values for the theoretical air required for different fuels are given in Table 8. . . It is customary to make use of the analysis: of' the products of combus tion to determine the amount of flue gas produced and the actual amount of air supplied for combustion. The analysis of flue games'has'been well Fuels and Combustion Table 8. Approximate Theoretical Air Requirements 335 Sous Fuel Pounm Am Per Pound Fuel 9.6 11.2 10.3 6.2 11.2 Fuel Oil Commercial Standard No. 3__________ ________________ Commercial Standard No. 6... Pounds Am Psa Gallon Fuel 102.6 104.5 106.5 112.0 114.2 Gaseous Fuels Mixed, natural and water gas_____________________ _____ Coke oven gas..--...____________________________________ AmCubic Feet Pbb Cubic Foot Gas 10.0 4.4 4.4 2.1 5.2 described in various publications of the U. S. Bureau of Mines and in the literature and the details of Orsat manipulation need not be considered in this discussion. (See Chapter 11.) The weight of dry flue gas per pound of fuel burned is used in combustion loss calculations and may be determined by Equation 6. ,, ,, ,, , , , -nC0, + 80,+.7(C0 + lVt).. ,, Pounds dry flue gas per pound fuel = -------~ 3 (CO + CO)---------- X C (6) Values for C02, 02, CO, and N2 are percentages by volume from the flue gas analysis and C is the weight of. carbon burned per pound of fuel .corr rected for carbon in the ash. ; ,. EXCESS AIR Since one measure of the efficiency of combustion is the relation existing between the amount of air theoretically required for perfect combustion and the amount of air actually supplied, a method of determining the latter factor is of value. Equation 7 will give reasonably accurate results, for most solid and liquid fuels, for determining the amount of air supplied per pound of fuel. 3.04 Aft Pounds dry air supplied per pound of fuel (COt + CO) X c (7) Values for C02, CO, and N are percentages by volume from the flue gas analysis and C is the weight of carbon burned per pound of fuel corrected for carbon in the ash. The difference between the air actually supplied for combustion and the theoretical air required is known as excess air. -( ")XPer cent excess air Air supplied -- Theoretical air\ 100 Theoretical air (8)