Document dJydMBO1N4Ko79r4E6a6D7v5

490 CHAPTER 33 1960 Guide Table 7____General Data of Combustible Elements and Compounds SvbdaiKo Sulfur (to SO,) Sulfur (to SO,) Commercial Butane Molecular Symbol _ -- -- -- CO CH, C*H, CtH, C,H, H, H*S C,H, C4H,o __ -- Chemical Reaction of Comburiioo Ignition r*mparafur** F Oeg Calorific Value Theoretical Oxygen and Air Requirement, Bta per Lb Btu per Cu Ft* Lb per Lb Cu Ft per Cu Ft Gross Net Grow o, Air o* Air 2C + O, * 2CO 2C + 20, = 2CO* S + O, - SO, 2S + 30, * 2SO, 2C0 + O, = 2CO, CH4 + 20, - CO, + 2H,0 2C,H, -f 60, - 4CO, + 2H,0 C,H4 + 30, * 2C0, + 2H,0 2C,H, + 70, - 4CO, + 6H*0 2H, + O, - 2H,0 2H,S + 30, * 2H0 + 2S0* C,H, + 50, * 3C0, + 4H,0 2CH|* 4- 130, - 8C0, + 10H,0 -- -- _. -- -- -- 1166-1319 1260-1380 763-824 986-1123 990-1120 1063-1166 599-608 950-1080 890-1020 920-1020 900-1000 3950 14093 3983 5940 4347 23879 21500 21644 22320 60958* 7100 21661 21308 21560 21180 - -- -- -- -- 21520 20776 20295 20432 51571* 6545 19944 19680 19865 19591 - 1.33 5.76 - -- 2.66 11.53 -- -- -- 1.00 4.29 -- -- 1.50 ---- 321.8 0.57 2.47 0.5 2.39 1013.2 3.99 17.27 2.0 9.53 1499 3.07 13.30 2.5 11.91 1613.8 3.42 14.81 3.0 14.29 1792 3.73 16.12 3.5 16.68 325 7.94 34.34 0.5 2.39 647 1.41 6.10 1.5 7.15 2590 3.63 15.70 5.0 |23.82 3370 3.58 15.49 6.5 30.97 2520 3.60 15.58 4.9 b.4 3260 3.54 15.3 6.3 30.0 * Value* in table chiefly from pace >1 of FW Fine Gasu published by American Gas Aeaodation. * Gu measoxnd at 60 P and 30 in. He- * Value from NatiocaJ Bureau of Standard*. of beating equipment must be sufficiently flexible in per formance to provide complete combustion with not only variations in the quality of a fuel but also changes in the rate at which it is supplied. This situation makes it advis able, from a practical standpoint, to insure complete com bustion but not perfect combustion in the sense expressed above. To attempt to do so would undoubtedly result eventu ally in unsatisfactory performance especially from a safety standpoint. Consequently, common types of heating equip ment are usually designed, installed, and adjusted to operate with-some excess air. The exact percentage of such air de pends on the type of fuel being utilized, as well as antici pated variations in its quantity and quality. Despite these practical limitations, however, it should not be inferred that common types of fuels cannot be utilized economically. Ref erence to flue loss charts such as Figs. 4 and 5 for gas burn ing equipment and to the air requirements discussion in Chapter 34 shows that reasonable quantities of excess air do not reduce operating efficiencies appreciably. Oxygen combines with the combustible elements and com pounds of any fuel in accordance with fixed laws. The reac tions and resultant products of perfect combustion, of com mon fuel constituents are shown in Table 7. All of the oxygen required for combustion is normally obtained from the surrounding air, which is a mechanical mixture of nitro gen and oxygen with small amounts of carbon dioxide, water vapor and inert gases. For practical combustion calcula tions, air is considered to consist of 20.9 percent oxygen and 79.1 percent nitrogen by volume, or 23.15 percent oxygen and 76.85 percent nitrogen by weight. The nitrogen, being inert, passes through the reaction without change. Table 7 gives the air quantities corresponding to the oxygen required for perfect combustion. Air supplied to the combustion reaction is in most in stances introduced in two ways. Primary air is introduced through or with the fuel, and secondary air is supplied to the flames issuing from the fuel. Incomplete combustion is obtained when any of the com bustible elements are not completely oxidized in the combus tion reaction. This condition not only represents inefficient use of the fuel but also presents a hazard because carbon monoxide is usually one of the products of incomplete com bustion. For example, a hydrocarbon may not oxidize com pletely to carbon dioxide and water, as indicated in Table 7, but may also form alcohols, ketones, aldehydes, or carbon monoxide depending on where and how the reaction is in terrupted. Too low a temperature {such as may be caused by flame impingement on a cold surface), a poor oxygen supply to the flames (due to insufficient or poorly located air supply, or smothering by products of combustion not properly vented), or insufficient mixing of the air and fuel, are the primary causes of incomplete combustion. Heat of Combustion As previously stated, the process of combustion results in the evolution of heat. The heat generated by the com plete combustion of a unit of fuel is constant for a given combination of combustible elements and compounds, and is known as the heat of combustion, calorific value, or heat ing value of the fuel. The heat of combustion of the several substances found in common fuels is given in Table 7. The calorific value of a fuel may be determined either by direct measurement of the heat evolved during combustion in a calorimeter, or it may be computed from the ultimate analysis and the heat of combustion of the several chemical elements in the fuel. When the heating value of a fuel is determined in a calorimeter, the water vapor is condensed and the latent heat of vaporization is included in the heating value of the fuel. The heating value so determined is termed the gross or higher heating value, and this is what is ordi narily meant when the heating value of a fuel is specified. When combustion is complete, the carbon in the fuel unites with oxygen to form carbon dioxide, CO., the hydro gen unites with oxygen to form water vapor, H.O, and the nitrogen, being inert, passes through the reaction 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 Fuels and Combustion 491 burned at all. When carbon monoxide or other combustible gases are present in the flue gases, there is a loss of heat produced per unit of fuel consumed, and consequently, a lower combustion efficiency is obtained. Incomplete combus tion may result from any or all of the following three condi tions: (1) inadequate air supply, (2) insufficient mixing of air and gases, and (3) a temperature too low to produce ignition or maintain combustion. AIR REQUIRE) FOR PERFECT COMBUSTION Air requirements for combustion of solid and liquid fuels are ordinarily expressed in pounds. On the other hand simi lar requirements for gaseous fuels are usually stated on a cubic foot basis. For solid and gaseous fuels this method-of treatment corresponds to the standards of measurement commonly employed by these two industries. The weight of air required for perfect combustion per .pound of solid or liquid fuel may be calculated after substi tution of the proper percentages by weight of the various elements obtained from an ultimate analysis of the fuel in Equation 2. For gaseous fuels see Equation 3 for volume of air required. Solid or Liquid Fuels: Pounds air required per pound fuel M0 + 1 8 / + J 134.56 fC . (r* 0\ S'! For Gaseous Fuels: Cubic feet air required per cubic foot gas - 2.39 (CO + H,) + 9.53 CH* + 16.68 C,H + 23.82 CjHi + 30.97 C4H,, + 11.91 C,H, + 14.29 C*H4 + 7.15-HjS -- 4.78 O* + 30.47 Illuminants (3) Gaseous fuels may contain a wide variety of components classified as iUuminants, which are not separated by the usual methods of gas analysis. The principal iUuminants in addition to ethylene (C*H) and acetylene (C*H.) which are included in Equation 3, with the air required per cubic foot of gas are: propylene, 21.44; butylene, 2858; pentene, 35.73; benzene, 35.73; toluene, 42.88; and xylene, 50.02. Since toluene and xylene are normally scrubbed from the gas before distribution, they may be disregarded in computing air required for a fuel gas. An approximate value of 30.47 (as shown in Equation 3) may, therefore, be employed. If ethylene and acetylene are included as iUuminants, it is sug gested that the value 19.65 be used. If it is desired to make the gas calculations on a weight basis the equation is expressed as follows: Pounds air required per pound fuel = 2.47 CO + 3454 H, + 17.27 CH, + 16.12 C*H. + 15.70 C,Hf + 15.49 C4H,, + 13-30 C*H, + 1481 C,H4 + 6.10 HS - 4.32 O, (4) Where approximate results only, are desired, values ap pearing in Table 8 may be substituted for Equations 2, 3, and 4; or the air required for perfect combustion may be estimated by assuming that 0.9 cu ft air is required per 100 Btu of fuel. Where extreme precision is involved it is suggested that the reader refer to scientific literature published on the sub ject of combustion and related topics by the various indus tries-concerned. If approximate values for theoretical air requirements suffice, or if complete information on the fuel is not available, the following values should also be found helpful: 1. Solid Fuels (Pounds air per pound fuel): Anthracite, 9.6: Semi-Bituminous, 11-2; Bituminous 10.3; Lignite 62; and Coke 112. 2. Fuel Oil (Pounds air per gallon): Commercial Standard No. I, 102.6; No. 2, 1055; No. 5, 112; No. 6, 1142. 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 theoreti cally required for perfect combustion and the amount of air actually supplied. Since the difference between air supplied for combustion and theoretical air required is characterized as excess air, its percentage may be calculated by use of the following equation, Percent excess air -( -)Air supplied -- Theoretical ai X 100 (5) Theoretical air 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 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. Pounds dry air supplied per pound of fuel 3.Q4N, (CO, + CO) X C (6) Because excess air calculations are almost invariably made from Oreat analysis results, and theoretical air requirements Table 8 .... Approximate Air Requirements for Theoretically Perfect Combustion of Fuels* Type of Air Required for Perfect Coaburfioa Lb per Lb Fuel Cu Ft per Unit* Fuel Approxi mate Predrioo, % Solid Btu per lb Btu per lb X 0.00073 X 0.0097 Liquid Btu per lb Btu per lb X 0.00071 X 0.0094 Gas Btu per lb Btu per cu ft X 0.00067 X 0.0089 3 3 5 Fuels contain ing more than 30% water Results low for gasoline and kerosine Gases of 300 Btu per cu ft or less * Values in table taken bom pace >70 of Gate*** FurU, IMS, published by h Unit* tor solid and liquid fuels in pounds, for cos in cubic feet.