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346 CHAPTER 14 , 1953 Guide Table 8. Approximate Air Requirements for Theoretically Perfect Combustion of Fuels* Type of Fuel Air Required for Perfect Combustion Lbs per Lb Fuel. , Cu Ft per TJnitb Fuel Approxi mate Preci- SION, Per Cent Exceptions Solid Btu per lb X 0.00073 Btu per lb. X 0.0097 Liquid Btu per lb X 0.00071 Btu per lb X 0.0094 Gas Btu per lb X 0.00067 Btu per cu ft X 0.0089 3 Fuels containing more than 30% water 3 Results low for gasoline and kerosene 5 Gases of 300 Btu per cu ft or less -. -* Values in table taken from page 276 of Gaseous Fuels, 1948, published by American Gae Association. b Units for solid and liquid fuels in pounds, for gas in cubic feet. 2. Fuel Oil (Pounds air per gallon): Commercial Standard No. 1. 102.6: No. 2, 105.5; No. 5, 112; No. 6, 114.2. 3. Gaseous Fuels (Cubic feet of air per cubic foot): Natural, 10.0;-Mixed Natural and Manufactured, 8.0; Manufactured, 4.7, Propane, 23.8, Butane, 31.0. COMBUSTION EFFICIENCY FROM THE FLUE GAS ANALYSIS Excess Air A commonly.employed index of efficiency of combustion is the relation existing between the amount of air theoretically required for perfect com bustion and the amount of air actually supplied. Since the difference between air supplied for combustion and theoretical air required is charac terized as excess air, its percentage may be calculated by use of the follow ing equation, ' /Air supplied -- Theoretical air\ __ .. Percent excess air = ( --------- ----------- ----------------------- ) X 100 (5) ' V Theoretical air f The amount of dry air supplied per pound of fuel burned may be obtained from Equation 6 which has reasonable precision for most solid and liquid fuels. Values for GOi, CO and N2 are percentages by volume from the flue gas analysis, and C is the weight of carbon burned per pound of fuel, cor rected for carbon in the ash. . 3.04N, Pounds dry air supplied per pound of fuel (CO, + CO) x (6) Because excess air calculations are almost invariably made from Orsat analysis results, and theoretical air requirements are not always known, another convenient method of expressing the relation of Equation 5 is as follows: 100(0, - CO/2) Percent excess air = Nt X 0.264 - (0, - CO/2) (7) As measurement standards for gaseous fuels are almost universally ex pressed in cubic feet, Equation 8 may be employed for computing excess air on a percentage basis for gases.21 Percent excess air (V - CO,) P X 100 - CO, A (8) Fuels and Combustion 347 Table. 9. Approximate Maximum Theoretical CO, Values, and :CO, Values for Various Fuels with Different Percentages of Excess Air Type of Fuel * -Maximum Theoretical or Percent COj Percent COj at Given -Excess . Air Values 20% 40% 60% Coke Anthracite Bituminous Coal No. 1 and 2 Fuel Oil 21.0 20.2 18.2 15.0 17.5 16.8 15.1 12.3 15.0 14.4 12.9 10.5 13.0. 12.6 li:3 9.1 No. 6 Fuel Oil Natural Gas Carburetted Water Gas Coke Oven Gas 16.5 12.1 17.2 11.2 13.6 9.9 14.2 9.2 11.6 8.4 12.1 7.8 10.1 7.3 10.6 6:8 . Mixed Gas (Natural and Carbu retted Water Gas) Propane Gas (Commercial) Butane Gas (Commercial) 15.3 13.9 14.1 12.5 . 11.4 11.6 10.5 9.6 9.8 9.1 8.4 8.5 where V = ultimate carbon dioxide, percent of flue gases resulting from perfect.com bustion. CO, = carbon dioxide content of flue gases, percent. P = dry products from perfect combustion, cubic feet per cubic foot of gas burned. A = air theoretically required for complete combustion, cubic feet per cubic foot of gas burned. As the ratio of P/A is approximately 0.9 for most city gases, a value of 90 may be substituted for 100 P in Equation 8 for rough calculation. A Carbon-hydrogen ratios of different fuels vary considerably, hence the maximum or ultimate C02 attainable also varies. Where they are un known, theoretical maximum CO2 values may be calculated from a flue gas analysis by use of Equation 9. HA , __ __ % CO, in flue gas sample X 100 Maximum theoretical % CO2 = --------------7---------------------------r-- f Os in same sample \ )100-1 ------------------------ -- I V 0.21 , (9) Approximate maximum CO2 values for perfect combustion of several common types of fuel are shown in Table 9 together with values of C02 that will be attained with different amounts of excess air. Desirable values to be attained in practice depend upon the fuel, the method of firing, and other considerations. In general, fuels burned in suspension, such as gas, oil, and pulverized coal, can be burned with a lower amount of excess air than fuels burned on grates. To produce heat efficiently by burning any common fuel a number of basic requirements must be met: (1) adequate heat absorbing surface of proper shape and construction is necessary in the appliance, (2) heat trans fer surfaces must be clean, (3) a minimum amount of excess air must be present, (4) air employed for combustion and combustible gases must be properly mixed, and (5) flue gas losses must be reduced to a safe minimum. If insufficient heating surface is employed, or if heat transfer surfaces are covered with soot, ash, or scale, flue losses will generally be excessive