combustion 1 presentation1

8/13/2019 Combustion 1 Presentation1

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5-7 Combustion Chemistry and Products of CombustionFor all fuels, the actual chemical process is the oxidation of thehydrogen and the oxygen in the fuel by combining them with oxygen

from the air. The nitrogen from the air and any other non-combustibles in the fuel pass through the process with essentially nochemical change. A minimal amount of nitrogen in the air combineswith oxygen to form nitrous oxides (NOx), which pollute the air. Somefuels contain a small percentage of sulphur, which-when burned-results in sulphur oxides that pollute the air. These may also corrodethe boiler if the flue gas containing them is allowed to cool below thedew point. Figure 5-10 demonstrates the basic chemical process andthe chemical elements and compounds involved in complete and

incomplete combustion. For any fuel, a precise amount ofcombustion air is needed to furnish the oxygen for completecombustion of that fuel's carbon and hydrogen.The precise amountof combustion air is called thc theoretical air for that particular fuel.If

the fuel analysis is known, the theoretical air requirements can becalculated easily.

THE CONTROL OF BOILERS 2nd EditionSAM G. DUKELOW



The amounts of carbon and oxygen for completecombustion of carbon are represented by theformula:



Weights equivalent to the molecular weight in poundscombine. One molecule of carbon containing one atom ofcarbon combines with one molecule of oxygen containingtwo atomsof oxygen to form one molecule of carbondioxide containing one atom of carbon and two atoms ofoxygen.



As with the carbon combustion, weights equivalent tothe molecular weights in pounds combine. Twomolecules of hydrogen, each containing two atoms ofhydrogen, and one molecule of two atoms of oxygenmake two molecules of water, with a total of four atomsof hydrogen and two atoms of oxygen.

A simple example of the many incomplete combustionreactions resulting in intermediate hydrocarboncompounds is the partial combustion of carbon, resultingin carbon monoxide rather than carbon dioxide.



In this case some of the potential heat energy from thecarbon remains in the carbon monoxide.



With the right conditions of time, temperature, andturbulence, and by adding more oxygen to the carbonmonoxide, the carbon monoxide will further oxidize tocarbon dioxide, releasing the second part of the heatenergy from the original carbon.



The common chemical reactions in combustion are shown in Table 5-7, with the heat energy resulting from the combustion reaction.Figures 5-9 and 5- 10 and Table 5-7 identify those products ofcombustion that are produced by the oxidation of the hydrogen,carbon, or sulfur present in the fuel. As indicated, thecombustion process produces heat, but a low percentage of the heatproduced is not useful in transferring heat to the boiler water. As

hydrogen combines with oxygen during the combus-tion process toform water, the combustion temperature vaporizes the water intosuperheated steam. This vaporization absorbs the latent heat forproducing the steam from the hot combustion gases. As the gases

pass through the boiler and exit from the system, the gases retainthe vaporized water in the form of superheated steam, and the latentheat and any remaining sensible heat are lost from the process.



The amount of latent heat loss is determined by the hydrogencontent of the fuel. If the fuel is natural gas and thus is higherin % hydrogen, the latent heat loss is greater than if the fuel

were coal, which is lower in % hydrogen. The effect on boilerefficiency for different fuels is shown in Figure 5- 1 1 .



Since this latent heat is not useful to a combustionprocess, the fuel is said to have a “gross” and a “net”

heating value or a higher (HHV) or a lower (LHV)heating value. It is important to keep in mind thatcombustion air must be furnished for the totalcombustion or on the basis of the HHV, while only theLHV has any effect on the heat transfer of the system.Figure 5-12 demonstrates with a coal analysis how thedifference between these two heating values can becalculated.



5-8 Theoretical Air Requirements and Relationship to Heat ofCombustionUsing the combustion chemistry formulas, if the fuel analysis isknown, the theoretical amount of oxygen can be calculated. Theamount of oxygen can easily be converted to a quantity ofcombustion air due to the known content of oxygen in air. Anexample of this calculation using a formula developed from the

combustion equations and the known content of oxygen in air isgiven in Figure 5-13. In this example the amount of airtheoretically required to produce 10,000 Btu is also shown. Table5-8 is a tabulation of combustion constants that is useful insimplifying such calculations. Figure 5-14 demonstrates using thetable of combustion constants for a gaseous fuel of 85 percentmethane and 15 percent ethane.



Note that the amount of combustion air required toproduce 10,000 Btu is nearly the same for coal andnatural gas. If the reciprocals are taken, the result isBtu/lb of air. Table 5-9 shows that for coal, oil, or gasthe Btu/lb of air is approximately the same evcnthough the Btu/lb of the fuels is radically different.

The difference between the Btu/lb of air on a “net” basis for these fuels is smaller than that shown in thetable. The fact that combustion air requirements canbe closely approximated, based on the heat

requirement, is an important concept used in theapplication of combustion control logic.



5-9 The Requirement of Excess Combustion AirIn actual practice gas-, oil-, coal-burning, and other systemsdo not do a perfcct job of mixing the fuel and air evenunder the best achievable conditions of turbulence.Additionally, cqmplete mixing may take too much time-sothat the gases pass to a lower temperature area not hotenough to complete the combustion-before the process iscompleted.If only the amount of theoretical air werefurnished, some fuel would not burn, the combustionwould be incomplete, and the heat in the unburned fuel

would be lost. To assurc complete combustion, additionalcombustion air is furnished so that every molecule of thefuel can easily find the proper number of oxygen moleculesto complete the combustion.



This additional amount of combustion air that is furnishedto complete the combustion process is called excess air.Excess air plus theoretical air is called total air. Having thisnecessary excess air means that some of the oxygen willnot be used and will leave the boiler in the flue gases, asshown in Figure 3-4, which describes the fuel and air

chemicals’ mass balance. The oxygen portion of the fluegas can be used to determine the percentage of excess air.If the percentage of excess air is increased, flametemperature is reduced and the boiler heat transfer rate is

reduced. The usual effect of this change is the increase inthe flue gas temperature, as shown in Figure 5-15.



Measurements of either percentage of carbon dioxide or thepercentage of oxygen in the flue gas or both are used to

determine percentage of excess air, but the percentage ofoxygen is preferred for the following reasons:(1) Oxygen is part of the air-if oxygen is zero, then excess air iszero. The presence of oxygen always indicates that some

percentage of excess air is present.(2) The percent of carbon dioxide rises to a maximum atminimum excess air and then decreases as air is furtherreduced. It is thus possible, with the same percentage ofcarbon dioxide, to have two different percentages of totalcombustion air. For this reason, the per-centage of carbondioxide cannot be used alone as a flue gas analysis input to acombustion control system.



(3) To determine excess air with the same precision,greater precision of measurement is required for thepercentage of carbon dioxide method than for thepercentage of oxygen method.(4) The relationship between the percentage of oxygenand the percentage of excess air changes little as fuelanalysis or type of fuel changes, while the percentage ofcarbon dioxideto- excess air relationship variesconsiderably as the percentage of carbon-to-hydrogenratio of the fuel changes.



The heat loss in the flue gases essentially depends uponthe difference between the temperature of the flue gasesand that of the incoming combustion air, the amount ofexcess air, and the fuel analysis. There is an optimumamount of excess air because less air will mean unburned

fuel from incomplete combustion, and more air will meancomplete combustion but more heat loss in the flue gasdue to the greater mass of the flue gases.



The amount of excess air required depends upon the typeof fuel, burner design, fuel characteristics andpreparation, furnace design, capacity as a percent ofmaximum, and other factors. The amount for anyinstallation should be determined by testing thatparticular unit.An approximate amount of excess air

required for full capacity is shown in Table 5-10.



The charts in Figures 5-16, 5-17, and 5-18 show therelationship between the flue gas analysis by volume andthe percentage of excess air for natural gas, fuel oil, andcoal. While these curves are for fuels with specific fuelanalyses, the curves for % oxygen vs. excess air are quitesimilar, while the % carbon dioxide vs. excess air curves are

quite different for the different fuels.



These curves also show the difference between the fluegas analysis depending upon the presence or removal of

the water vapor that is formed by the combustionprocess. This difference is important to note for tworeasons. Since approximately 1970, flue gas analyzers for% oxygen using the zirconium oxide fuel cell principle

have been marketed. This type of % oxygen analyzer,which analyzes the flue gas on the “wet” basis, is now thestandard method for permanently installed flue gasanalysis equipment. On the other hand, 7% oxygen vs.excess air formulas, including the one in the text above,are based on the “dry” basis, which was universally useduntil the early 1970s.



In addition, these newer zirconium oxide analyzersnormally measure the % oxygen on a “net” basis. Ifcombustible gases such as CO are present, the hightemperature and catalytic action of the measuring cellcomplete the combustion by subtracting a portion or allof the oxygen passing the analysis cell. It is thus not

necessary to subtract C02 from the % oxygen before it isused in the older formulas. Since these formulas are onthe dry basis, however, it is necessary to convert the wetbasis analyzer readings to dry basis before using them in

the older equations for combustion calculations.



In analyzing flue gases to determine % excess air, it isuseful to have a check on the accuracy of the analysis.Figure 5-19, which is based on dry basis analysis, can beused for this purpose. By drawing a straight line, asshown, between the % oxygen and the % carbon dioxidevalues, there is an intersection with the hydrogen-carbonratio line.



When using a particular fuel of a certain H/C ratio, theintersection should always be at the same point on the

line. If it is not, the fuel has changed or the results of theanalysis are incorrect. If the fuel analysis is known, themeasurement of % oxygen can be used to determine thecorrect % carbon dioxide, or the % carbon dioxide can beused to determine the correct % oxygen. If boiler testsare being made and the fuel analysis is constant, anydata (within limits of measurement accuracy) thatdoesn't measure up to this kind of examination shouldbe thrown out.

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