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158 Chapter 8 1945 Guide AIR AND COMBUSTION The weight of air required for the perfect combustion of a pound of fuel may be determined by use of the ultimate analysis of the fuel as applied to Equations 2 to 4. The various elements are expressed in percentages by weight. Solid and Liquid Fuels: ''Pounds air required per pound fuel = 34.56 + (h -- ^J Gaseous Fuels: Pounds air required per pound fuel = 2.46 CO + 34.56 Ht + 17.28 CHt + 13.29 C?H, + 14.81 Ciff, + 16.13 CJH, + 6.10 H,S - 4.32 O, (2) ._. w' When the analysis is given on a volumetric basis the equation is expressed as follows: Cubic feet air required per cubic foot gas = 2.39 (CO + H?) + 9.56 CH, 411.98 C,H, + 14.35 C,H, + 16.74 CJI, - 4.78 O, (4) Equations 5.and 6 may be used as approximate methods of determining the theoretical air requirement for any fuel. Pounds air required per pound fuel = 0.755 X value, <^u per pound) 1000 Cubic feet air required per unit fuel = P^r un*0 100 (g) Approximate values for the theoretical air required for different fuels are given in Table 3. It is customary to make use of the analysis of the products of com bustion to determine the amount of flue gas produced and the actual Table 3. Approximate Theoretical Air Requirements Solid Fuel Anthracite....... ............ Semi-bituminous coal...................... Bituminous coal .. I.temite . ..... ... ......... Coke.. AmPounds Per Pound Fuel 9.6 11.2 10.3 62 11.2 .- Funl Oil Commercial Standard No. 1 Commercial Standard No. 2. ....... Commercial Standard No. 3 . Commercial Standard No. 5 Commercial Standard No. fi AmPounds Per Gallon Fuel 102.6 104.5 106.5 112.0 114.2 Gaseous Fuels Natural gas. .. Mixed, natural and water pas Carbureted water gas Water gas. coke ... Coke oven gas............................................ Am PCubic Feet ee Cubic Foot Gas 10.0 4.4 4.4 2.1 5.2 Combustion and Fuels 159 amount of air supplied for combustion. The analysis of flue gases has been well described in various publications of the Bureau of Mines and in the literature and the details of Orsat manipulation need not be considered in this discussion. (See Chapter 34.) The weight of dry flue gas per pound of fuel burned is used in com bustion loss calculations and may be determined by Equation 7. 11 CO, + 8 0, + 7 (CO + Ni) ,, ,, Pounds dry flue gas per pound fuel 3 (CO, + CO) (7) Values for CO?, 0?, CO and Nt 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. EXCSS 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 8 will give reasonably accurate results, for most solid and liquid fuels, for determining the amount of air supplied per pound of fuel. Pounds dry air supplied per pound of fuel 3.04 1V, (CO, + CO) (8) Values for CO?, 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. ,, . Air supplied -- Theoretical air Per c^ent excess air = ----------- =Tqh-e--o--r-ert-i--cail--a=i-r------------ (9) Since the calculation is usually made from Orsat readings, Equation 10 will be found to be a convenient statement of this relationship. 100(0,-^) Per cent excess air =--------------- ------- -------- 7 N, X 0.264 - (o, - -- (10) In this formula the symbols represent volumetric percentages of the flue gas constituents as determined by analysis. . The amount of excess air in its relation to the percentage of CO? is shown by the curves in Fig. 1 for several fuels. These are approximate values. It should be noted that in hand-fired furnaces with long periods between firings the combustion goes through a cycle in each period and the quantity of excess air present varies. Due to the different carbon-hydrogen ratios of the different fuels the maximum CO? attainable varies. Representative values for perfect com bustion of several fuels are .given in Table 4. In considering the factor of excess air it should be noted that a'deficien cy of air supply will result in combustible products passing to the stack unburned. An excess of air absorbs heat from the products of combustion and results in a greater loss of sensible heat to the stack. An excess of air is always required, however, to eliminate combustible losses occasioned