1、28.1CHAPTER 28COMBUSTION AND FUELSPrinciples of Combustion. 28.1Fuel Classification . 28.5Gaseous Fuels 28.5Liquid Fuels . 28.6Solid Fuels 28.8Combustion Calculations . 28.10Efficiency Calculations. 28.13Combustion Considerations . 28.14PRINCIPLES OF COMBUSTIONOMBUSTION is a chemical reaction in whi
2、ch an oxidantCreacts rapidly with a fuel to liberate stored energy as thermalenergy, generally in the form of high-temperature gases. Smallamounts of electromagnetic energy (light), electric energy (free ionsand electrons), and mechanical energy (noise) are also produced dur-ing combustion. Except i
3、n special applications, the oxidant for com-bustion is oxygen in the air. The oxidation normally occurs with thefuel in vapor form. One notable exception is oxidation of solid car-bon, which occurs directly with the solid phase.Conventional fuels contain primarily hydrogen and carbon, in ele-mental
4、form or in various compounds (hydrocarbons). Their com-plete combustion produces mainly carbon dioxide (CO2) and water(H2O); however, small quantities of carbon monoxide (CO) and par-tially reacted flue gas constituents (gases and liquid or solid aerosols)may form. Most conventional fuels also conta
5、in small amounts of sul-fur, which is oxidized to sulfur dioxide (SO2) or sulfur trioxide (SO3)during combustion, and noncombustible substances such as mineralmatter (ash), water, and inert gases. Flue gas is the product of com-plete or incomplete combustion and includes excess air (if present),but
6、not dilution air (air added to flue gas downstream of the combus-tion process, such as through the relief opening of a draft hood).Fuel combustion rate depends on the (1) rate of chemical reactionof combustible fuel constituents with oxygen, (2) rate at which oxy-gen is supplied to the fuel (mixing
7、of air and fuel), and (3) tempera-ture in the combustion region. The reaction rate is fixed by fuelselection. Increasing the mixing rate or temperature increases thecombustion rate.With complete combustion of hydrocarbon fuels, all hydrogenand carbon in the fuel are oxidized to H2O and CO2. Generall
8、y, com-plete combustion requires excess oxygen or excess air beyond theamount theoretically required to oxidize the fuel. Excess air is usu-ally expressed as a percentage of the air required to completely oxi-dize the fuel.In stoichiometric combustion of a hydrocarbon fuel, fuel isreacted with the e
9、xact amount of oxygen required to oxidize allcarbon, hydrogen, and sulfur in the fuel to CO2, H2O, and SO2.Therefore, exhaust gas from stoichiometric combustion theoreti-cally contains no incompletely oxidized fuel constituents and nounreacted oxygen (i.e., no carbon monoxide and no excess air oroxy
10、gen). The percentage of CO2contained in products of stoichio-metric combustion is the maximum attainable and is referred to asthe stoichiometric CO2, ultimate CO2, or maximum theoreticalpercentage of CO2.Stoichiometric combustion is seldom realized in practice becauseof imperfect mixing and finite r
11、eaction rates. For economy andsafety, most combustion equipment should operate with some excessair. This ensures that fuel is not wasted and that combustion is com-plete despite variations in fuel properties and supply rates of fueland air. The amount of excess air to be supplied to any combustioneq
12、uipment depends on (1) expected variations in fuel properties andin fuel and air supply rates, (2) equipment application, (3) degree ofoperator supervision required or available, and (4) control require-ments. For maximum efficiency, combustion at low excess air is de-sirable.Incomplete combustion o
13、ccurs when a fuel element is not com-pletely oxidized during combustion. For example, a hydrocarbonmay not completely oxidize to carbon dioxide and water, but mayform partially oxidized compounds, such as carbon monoxide, al-dehydes, and ketones. Conditions that promote incomplete com-bustion includ
14、e (1) insufficient air and fuel mixing (causing localfuel-rich and fuel-lean zones), (2) insufficient air supply to the flame(providing less than the required amount of oxygen), (3) insufficientreactant residence time in the flame (preventing completion of com-bustion reactions), (4) flame impingeme
15、nt on a cold surface (quench-ing combustion reactions), or (5) flame temperature that is too low(slowing combustion reactions).Incomplete combustion uses fuel inefficiently, can be hazardousbecause of carbon monoxide production, and contributes to airpollution.Combustion ReactionsThe reaction of oxy
16、gen with combustible elements and com-pounds in fuels occurs according to fixed chemical principles,including Chemical reaction equationsLaw of matter conservation: the mass of each element in the reac-tion products must equal the mass of that element in the reactantsLaw of combining masses: chemica
17、l compounds are formed byelements combining in fixed mass relationshipsChemical reaction ratesOxygen for combustion is normally obtained from air, which isa mixture of nitrogen, oxygen, small amounts of water vapor, car-bon dioxide, and inert gases. For practical combustion calculations,dry air cons
18、ists of 20.95% oxygen and 79.05% inert gases (nitro-gen, argon, etc.) by volume, or 23.15% oxygen and 76.85% inertgases by mass. For calculation purposes, nitrogen is assumed topass through the combustion process unchanged (although smallquantities of nitrogen oxides form). Table 1 lists oxygen and
19、airrequirements for stoichiometric combustion and the products ofstoichiometric combustion of some pure combustible materials (orconstituents) found in common fuels.Flammability LimitsFuel burns in a self-sustained reaction only when the volumepercentages of fuel and air in a mixture at standard tem
20、perature andpressure are within the upper and lower flammability limits (UFLand LFL), also called explosive limits (UEL and LEL; see Table 2).Both temperature and pressure affect these limits. As mixture tem-perature increases, the upper limit increases and the lower limit de-creases. As the pressur
21、e of the mixture decreases below atmosphericThe preparation of this chapter is assigned to TC 6.10, Fuels and Combus-tion.28.2 2013 ASHRAE HandbookFundamentalspressure, the upper limit decreases and the lower limit increases.However, as pressure increases above atmospheric, the upper limitincreases
22、and the lower limit is relatively constant.Ignition TemperatureIgnition temperature is the lowest temperature at which heat isgenerated by combustion faster than it is lost to the surroundingsand combustion becomes self-propagating. (See Table 2). The fuel/air mixture will not burn freely and contin
23、uously below the ignitiontemperature unless heat is supplied, but chemical reaction betweenthe fuel and air may occur. Ignition temperature is affected by alarge number of factors.The ignition temperature and flammability limits of a fuel/airmixture, together, are a measure of the potential for igni
24、tion (GasEngineers Handbook 1965).Combustion ModesCombustion reactions occur in either continuous or pulse flamemodes. Continuous combustion burns fuel in a sustained manneras long as fuel and air are continuously fed to the combustion zoneand the fuel/air mixture is within the flammability limits.
25、Continu-ous combustion is more common than pulse combustion and is usedin most fuel-burning equipment.Pulse combustion is an acoustically resonant process that burnsvarious fuels in small, discrete fuel/air mixture volumes in a veryrapid series of combustions.The introduction of fuel and air into th
26、e pulse combustor is con-trolled by mechanical or aerodynamic valves. Typical combustorsconsist of one or more valves, a combustion chamber, an exit pipe,and a control system (ignition means, fuel-metering devices, etc.).Typically, combustors for warm-air furnaces, hot-water boilers,and commercial c
27、ooking equipment use mechanical valves.Aerodynamic valves are usually used in higher-pressure applica-tions, such as thrust engines. Separate valves for air and fuel, asingle valve for premixed air and fuel, or multiple valves ofeither type can be used. Premix valve systems may require aflame trap a
28、t the combustion chamber entrance to prevent flash-back.Table 1 Combustion Reactions of Common Fuel ConstituentsConstituentMole-cular Formula Combustion ReactionsStoichiometric Oxygen and Air Requirements Flue Gas from Stoichiometric Combustion with Airlb/lb Fuelaft3/ft3Fuel Ulti-mate CO2,%Dew Point
29、,cFft3/ft3 Fuel lb/lb FuelO2Air O2Air CO2H2OCO2H2OCarbon (to CO) C C + 0.5O2 CO 1.33 5.75bb Carbon (to CO2)C C + O2CO22.66 11.51bb29.30 3.664 Carbon monoxide CO CO + 0.5O2CO20.57 2.47 0.50 2.39 34.70 1.0 1.571 Hydrogen H2H2+ 0.5O2H2O 7.94 34.28 0.50 2.39 162 1.0 8.937Methane CH4CH4+ 2O2CO2+ 2H2O 3.9
30、9 17.24 2.00 9.57 11.73 139 1.0 2.0 2.744 2.246Ethane C2H6C2H6+ 3.5O22CO2+ 3H2O 3.72 16.09 3.50 16.75 13.18 134 2.0 3.0 2.927 1.798Propane C3H8C3H8+ 5O23CO2+ 4H2O 3.63 15.68 5.00 23.95 13.75 131 3.0 4.0 2.994 1.634Butane C4H10C4H10+ 6.5O24CO2+ 5H2O 3.58 15.47 6.50 31.14 14.05 129 4.0 5.0 3.029 1.550
31、Alkanes CnH2n+2CnH2n + 2+ (1.5n + 0.5)O2nCO2+ (n + 1)H2O 1.5n+ 0.57.18n+ 2.39128 to 127nn + 1 44.01n 18.01(n + 1)14.026n + 2.016 14.026n + 2.016Ethylene C2H4C2H4+ 3O22CO2+ 2H2O 3.42 14.78 3.00 14.38 15.05 125 2.0 2.0 3.138 1.285Propylene C3H6C3H6+ 4.5O23CO2+ 3H2O 3.42 14.78 4.50 21.53 15.05 125 3.0
32、3.0 3.138 1.285Alkenes CnH2nCnH2n+ 1.5nO2nCO2+ nH2O 3.42 14.78 1.50n 7.18n 15.05 125 nn 3.138 1.285Acetylene C2H2C2H2+ 2.5O22CO2+ H2O 3.07 13.27 2.50 11.96 17.53 103 2.0 1.0 3.834 0.692Alkynes CnH2mCnH2m+ (n + 0.5m)O2nCO2+ mH2On + 0.5m 4.78n+ 2.39m nm 22.005n 9.008m6.005n + 1.008m 6.005n + 1.008mSOx
33、H2OSOxH2OSulfur (to SO2) S S + O2SO21.00 4.31bb1.0SO2 1.998 (SO2)Sulfur (to SO3) S S + 1.5O2SO31.50 6.47bb1.0SO3 2.497 (SO3Hydrogen sulfide H2SH2S + 1.5O2SO2+ H2O 1.41 6.08 1.50 7.18 125 1.0SO21.0 1.880 (SO2)0.528Adapted, in part, from Gas Engineers Handbook (1965).aAtomic masses: H = 1.008, C = 12.
34、01, O = 16.00, S = 32.06.bVolume ratios are not given for fuels that do not exist in vapor form at reasonable temperatures or pressure.cDew point is determined from Figure 2.Table 2 Flammability Limits and Ignition Temperatures of Common Fuels in Fuel/Air MixturesSubstanceMolecular FormulaLower Flam
35、mability Limit, %Upper Flammability Limit, %Ignition Temperature, F ReferencesCarbon C 1220 Hartman (1958)Carbon monoxide CO 12.5 74 1128 Scott et al. (1948)Hydrogen H24.0 75.0 968 Zabetakis (1956)Methane CH45.0 15.0 1301 Gas Engineers Handbook (1965)Ethane C2H63.0 12.5 968 to 1166 Trinks (1947)Prop
36、ane C3H82.1 10.1 871 NFPA (1962)n-Butane C4H101.86 8.41 761 NFPA (1962)Ethylene C2H42.75 28.6 914 Scott et al. (1948)Propylene C3H62.00 11.1 856 Scott et al. (1948)Acetylene C2H22.50 81 763 to 824 Trinks (1947)Sulfur S 374 Hartman (1958)Hydrogen sulfide H2S 4.3 45.50 558 Scott et al. (1948)Flammabil
37、ity limits adapted from Coward and Jones (1952). All values corrected to 60F, 30 in. Hg, dry.Combustion and Fuels 28.3In a mechanically valved pulse combustor, air and fuel are forcedinto the combustion chamber through the valves under pressuresless than 0.5 psi. An ignition source, such as a spark,
38、 ignites the fuel/air mixture, causing a positive pressure build-up in the combustionchamber. The positive pressure causes the valves to close, leavingonly the exit pipe of the combustion chamber as a pressure reliefopening. Combustion chamber and exit pipe geometry determinethe resonant frequency o
39、f the combustor.The pressure wave from initial combustion travels down the exitpipe at sonic velocity. As this wave exits the combustion chamber,most of the flue gases present in the chamber are carried with it intothe exit pipe. Flue gases remaining in the combustion chamber beginto cool immediatel
40、y. Contraction of cooling gases and momentum ofgases in the exit pipe create a vacuum inside the chamber that opensthe valves and allows more fuel and air into the chamber. While thefresh charge of fuel/air enters the chamber, the pressure wave reachesthe end of the exit pipe and is partially reflec
41、ted from the open end ofthe pipe. The fresh fuel/air charge is ignited by residual combustionand/or heat. The resulting combustion starts another cycle.Typical pulse combustors operate at 30 to 100 cycles per secondand emit resonant sound, which must be considered in their appli-cation. The pulses p
42、roduce high convective heat transfer rates.Heating ValueCombustion produces thermal energy (heat). The quantity ofheat generated by complete combustion of a unit of specific fuel isconstant and is called the heating value, heat of combustion, orcaloric value of that fuel. A fuels heating value can b
43、e determinedby measuring the heat evolved during combustion of a known quan-tity of the fuel in a calorimeter, or it can be estimated from quanti-tative chemical analysis of the fuel and the heating values of thevarious chemical elements in the fuel. For information on calculat-ing heating values, s
44、ee the sections on Characteristics of Fuel Oilsand Characteristics of Coal.Higher heating value (HHV), gross heating value, or totalheating value includes the latent heat of vaporization and is deter-mined when water vapor in the fuel combustion products is cooledand condensed at standard temperatur
45、e and pressure. Conversely,lower heating value (LHV) or net heating value does not includelatent heat of vaporization. In the United States, when the heatingvalue of a fuel is specified without designating higher or lower, itgenerally means the higher heating value. (LHV is mainly used forinternal c
46、ombustion engine fuels.)Heating values are usually expressed in Btu/ft3for gaseous fuels,Btu/gal for liquid fuels, and Btu/lb for solid fuels. Heating valuesare always given in relation to standard temperature and pressure,usually 60, 68, or 77F and 14.735 psia (30.00 in. Hg), depending onthe partic
47、ular industry practice. Heating values in the United Statesand Canada are based on standard conditions of 60F (520R) and14.735 psia (30.00 in. Hg), dry. Heating values of several substancesin common fuels are listed in Table 3.With incomplete combustion, not all fuel is completely oxidized,and the h
48、eat produced is less than the heating value of the fuel.Therefore, the quantity of heat produced per unit of fuel consumeddecreases (lower combustion efficiency).Not all heat produced during combustion can be used effectively.The greatest heat loss is the thermal energy of the increased tem-perature
49、 of hot exhaust gases above the temperature of incoming airand fuel. Other heat losses include radiation and convection heattransfer from the outer walls of combustion equipment to the envi-ronment.Altitude CompensationAir at altitudes above sea level is less dense and has less mass ofoxygen per unit volume. The volume concentration of oxygen,however, remains the same as sea level. Therefore, combustion ataltitudes above sea level has less available oxygen to burn with thefuel unless compensation is m
