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Table of contentschapter one - COMBUSTION EFFICIENCYIntroduction . 3Combustion Theory and Stoichiometric Combustion . 3Why Use Air Instead of Pure Oxygen . 3The Importance of Excess Air . 4Unburned Fuel Loss . 4Combustion Efficiency - Measuring Oxygen and Combustibles . 6chapter two - OXYGEN MEASUREMENTIntroduction . 7Zirconium Oxide Cell . 7The Nernst Equation . 8Advantages of Zirconium Oxide Technology . 9Zirconium Oxide Sampling Techniques . 9Insitu Analyzer . 9Close-coupled Extractive Analyzer . 10Convective Analyzer (Hybrid Model) . 10Extractive Analyzers . 11Other Benefits of Zirconium Oxide Cell . 11Equivalent Combustibles . 11Paramagnetic Oxygen Sensor . 11Wet Electrochemical Cell . 12chapter three - COMBUSTIBLES MEASUREMENTIntroduction . 13Catalytic Combustibles Detector . 13Infrared Carbon Monoxide Measurement . 14Extractive Infrared CO Analyzers . 14Across-the-Stack Infrared CO Analyzers . 15Wet Electrochemical Cell . 16chapter 4 - PRE-COMBUSTION VS. FLUE GAS MEASUREMENTIntroduction . 17Premix Analysis - How it Works . 17Combustion Air Requirement Index (CARI) . 17appendix - COMBUSTION EFFICIENCY TABLESConversions . 18How to Use Tables . 18Natural Gas Tables . 19No. 2 Oil Tables . 23No. 6 Oil Tables . 27Bituminous Coal Tables . 312

chapter oneCOMBUSTION EFFICIENCYTotal efficiency is defined as the effectiveness of any combustion apparatus to convert the internal energy contained in the fuelinto heat energy for use by the process. Anyheat losses lower the efficiency of the process. Radiant losses from heat escapingthrough the surfaces of the boiler is oneexample of efficiency losses. Combustionefficiency is the total energy contained perunit of fuel minus the energy carried awayby the flue gas and unburned fuel exitingthe stack.Combustion efficiency losses are a bigpart of total efficiency losses. Before making large capital investments to improveboiler performance, make sure you maximize combustion efficiency. The best wayto maximize combustion efficiency is tomeasure oxygen and combustibles in theflue gas on a continuous basis.Combustion Theory andStoichiometric CombustionThe three essential components ofcombustion are fuel, oxygen, and heat.Stochiometric combustion is defined ashaving just the right amount of oxygen andfuel mixture so the most heat is released.In most fossil fuels, the chemical elementsthat react with oxygen to release heat arecarbon and hydrogen.1Stoichiometric reactions for pure carbon,oxygen, and hydrogen are as follows:C O2 CO2 14,093 Btu/lb.H2 1/2 O2 H2O 61,100 Btu/lb.For these stoichiometric combustion reactions, only heat and CO2 or H2O result.Common fuels consist of compoundscontaining certain amounts of hydrogenand carbon. These fuels are commonlycalled hydrocarbons. For example, meth-1Sulfur is rarely used as a fuel, but will react with oxygen as follows:S O2 SO2 3,983 Btu/lb.3ane (CH4) is a hydrocarbon gas that burnsas follows:CH4 2O2 CO2 2H2O 1,013 Btu/ft.3Why Use Air Instead of Pure OxygenAir contains about 21% oxygen and 79%nitrogen by volume, and is readily available.Pure oxygen must be processed, and onmost applications the cost to process oxygen outweighs the benefit of increased combustion control. When you use air insteadof oxygen, one cubic foot of methane (atstandard temperature and pressure) willburn completely with 9.53 cubic feet of airas shown below:CH4 2O2 7.53N2 CO2 2H2O 7.53N2 1,013 Btu/ft.3The ratio of 9.53 cubic feet of air to onecubic foot of methane is known as the stoichiometric air/fuel ratio. The heat releasedwhen the fuel burns is known as the heat ofcombustion. Table 1-1 and Table 1-2 list thestoichiometric air/fuel ratios and heats ofcombustion for several common fuels.Ideally, you want to provide just the rightamount of air to completely burn all the fuel.Table 1-1 — Combustion ranges for selectedgaseous fuelsFuelStoichiometricAir/fuel ratio(ft3 air/ft3 fuel)Heat ofCombustion(Btu/ft3)Hydrogen (H2)Carbon Monoxide (CO)Methane (CH4)Propane (C3H8)Natural GasCoke Oven Gas2.382.389.5323.829.4 - 11.03.5 - 5.532532210132590950-1150400-600Table 1-2 — Combustion ranges for selected solid andliquid fuelsFuelCarbon (C)Sulfur (S)No. 2 OilNo. 6 OilBituminous CoalStoichiometricAir/fuel ratio(ft3 air/lb. fuel)Heat 0933,98318,500-19,80017,500-19,00012,000-14,000

Figure 1-2 — Flue gas constituents vs. excess air formethane combustion% FLUE GAS CONCENTRATIONSBut this proves elusive for a number of reasons, including inadequate mixing of air andfuel, burner performance, fluctuating operating and ambient conditions, and burnerwear and tear. To ensure that the fuel isburned with little or no combustibles, someamount of excess air is provided. To ensure no more excess air than required isused, you measure excess oxygen in theflue gas. To ensure the amount of hydrogen or carbon monoxide in the flue gas isminimized, combustibles are measured.N2H2OCO H2O2-20The Importance of Excess AirFlue gas heat loss is the single largest energyloss in a combustion process. It is impossibleto eliminate all flue gas heat loss because theproducts of combustion are heated by the combustion process. But flue gas heat loss can beminimized by reducing the amount of excessair supplied to the burner.Flue gas heat loss increases with bothincreasing excess air and temperatures asshown in Figure 1-1.Figure 1-1 — Flue gas heat loss increases with bothincreasing excess air and temperatures% FLUE GAS HEAT LOSS (CH4 FUEL)40T 3520406080100% EXCESS AIRSince the oxygen in the flue gas is directly related to the amount of excess airsupplied (Figure 1-2), an oxygen flue gasanalyzer is the best way to effectively measure and control the amount of excess airin the flue gas and the associated heat loss.2Zirconium oxide O2 flue gas analyzers arethe preferred combustion control analysismethod. Oxygen analyzers, including thoseusing zirconium oxide technology, paramagnetic, and wet electrochemical cells, arediscussed further in Chapter 2.0 FUnburned Fuel Loss FFor combustion efficiency, you never wantto operate a burner with less air than is required for stoichiometric combustion. Notonly does this result in a smoking stack, butit significantly reduces the total energy released in the combustion process due tounburned fuel. Figure 1-3 shows how theenergy released per cubic foot of methanefalls off with decreasing combustion air.10030T 07002500 FT 5200050100% EXCESS AIR1502002In the past, CO2 analyzers were also used. But as seen in Figure1-2, a particular CO2 level in flue gas can indicate either an excessfuel or excess air condition. This dual meaning is unacceptable incombustion control systems, and therefore CO2 analyzers are nolonger used for combustion efficiency purposes.4

Figure 1-3 — Available heat drops sharply withdeficient air supplyIf a burner is operated with a deficiency of air,or the air and fuel are mixed improperly, all thefuel will not burn. As a result, carbon monoxideand hydrogen will appear in the products of combustion. Carbon monoxide (CO) and hydrogen(H 2), collectively referred to as combustibles,result from incomplete combustion. When insufficient excess air is available, the amount of combustibles in the flue gas increases dramatically.In practice, some trace levels of unburned fuelappear in the flue gas stream even with someamount of excess air, due to imperfect mixing offuel and air at the burner or other burner conditions (see Figure 1-4). As a result, combustionprocesses are not operated at the stoichiometricpoint. Instead, combustion processes are operated with sufficient excess air to keep the amountof combustibles minimized. Combustibles levels ofa few hundred parts per million (ppm) in the fluegas have an insignificant effect on efficiency.But what is the correct O2 level? For every combustion process, the optimum amount of excessair depends on several variables including the typeof fuel, the load, and the size and condition of theburner (see Figure 1-5). There is no single O2 levelwhich is right for all processes.Figure 1-5 — Optimum efficiency rangeFigure 1-4 — Actual vs. theoretical combustiblesCombustibles detectors can measure both hydrogen and carbon monoxide in flue gas, with accuracies of 100 ppm or better. For detection ofnatural gas during purge-down and light-off cycles,special versions of detectors are available for %methane. Infrared or electrochemical cell analyzers measure only carbon monoxide, but are usedto measure total combustibles in some cases. Combustibles detectors and CO analyzers are discussed in detail in Chapter 3.5

Combustion Efficiency Measuring Oxygen and CombustiblesFigure 1-6 — Total combustion efficiency losses is sumof unburned fuel loss and heat lossesNOx ReductionIn addition to maximizing efficiency, low excessair firing has the benefit of reducing NOx emissions(see Figure 1-7). This should be the first step in anyNOx reduction strategy.Fiugure 1-7 — NOx Reduction300100250802006015040100PPM NOxNOx ReductionPPM COIn the past, it was not practical to continuouslycalculate unburned fuel loss and flue gas heat lossto maintain the most efficient level of excess air.Combustion efficiency tables were used to determine optimal control points in the burner designstage. These tables help you to determine the expected efficiency of a boiler or furnace, and to assist in making cost/benefit decisions. The tables inthe appendix provide efficiency curves for variousfuels. These tables can be used to make a roughestimate of the efficiency improvements that canbe made by decreasing the amount of excess airand the level of combustibles in the flue gas in yoursystem. You then continuously monitor the processusing a flue gas analyzer to fine tune your system.Measuring oxygen alone is insufficient for combustion efficiency purposes because of ever-changing boiler conditions that affect the amount of combustibles in the flue gas. Similarly, measuring combustibles alone doesn’t provide sufficient detail tomake continuous adjustments to the process. Tomaintain the highest combustion efficiency level ona continuous basis, both oxygen and combustiblesin the flue gas need to be measured.This leads to the fundamental principle of combustion efficiency: “Combustion efficiency is maximized when the correct amount of excess air issupplied so that the sum of energy losses from bothunburned fuel loss and flue gas heat loss is minimized.” By measuring the concentrations of oxygen and combustibles, both unburned fuel loss andflue gas heat loss can be minimized (see Figure1-6).Combined oxygen and combustibles analyzersenable both measurements to be made at a singlesample point. From this, the supply of excess aircan be controlled on a continuous basis, minimizing heat loss and unburned fuel loss, and therefore ensuring the most efficient operation of yourboiler.20500012345% O2CombustiblesNox6

chapter twoOXYGEN MEASUREMENTOxygen concentration in flue gas is an excellent indicator of excess air in the flue gas.The existing technologies used to measureexcess air in flue gas are the zirconium oxide cell, the paramagnetic oxygen cell, andthe wet electrochemical cell.Zirconium oxide analyzers indicate netoxygen; that is, the oxygen remaining afterburning with whatever free combustibles arepresent around the hot zirconium oxide cell.Paramagnetic and wet electrochemical celloxygen analyzers measure gross oxygen.For combustion efficiency applications, thedifference between net and gross measurements are small since combustibles aregenerally in the ppm range, while oxygen isusually in the percent range.Differences may also occur between thetechnologies because zirconium oxide analyzers can measure oxygen on a wet basiswhere the flue gas contains water vapor.The other measuring techniques all requirecool, dry samples, and measure on a drybasis. For example, assume you have aflue gas containing 5% O2, 10% H2O, balance nitrogen (85%). If the water (H2O) isremoved from the sample to make a dry reading, oxygen would read as 5.5% O2 (5% of100% vs. 5% of 90%).There should be no cause for alarm because of wet vs. dry or gross vs. net measurements. There is no right or wrongmethod. All are valid conventions. It is important only to know which convention isbeing used and to be consistent.With zirconium oxide, because it can bea wet measurement, it is important to keepthe sample above the acid dewpoint to prevent condensation or corrosion in thesample line. Typically, this is not an issuesince the process and the sensor operateat temperatures greater than the aciddewpoint.Zirconium oxide analyzers now usemicroprocessor-based control units (seeFigure 2-1) that include automatic calibrations, RS-485 two-way communications,current outputs, and alarms. Calibration,maintenance, and repair are user friendly.The sensor can be calibrated with the pushof a key, or can even be calibrated automatically at timed intervals that require nooperator intervention.Maintenance and repair of these newersystems is made easier by a self-diagnostic system that, through the use of texthelp messages, can tell an operator whataction needs to be taken or what itemneeds to be replaced. Components areusually modular, making repairs and replacements easier.Zirconium Oxide CellThe zirconium oxide cell is the most prevalent technology for continuous monitoringof flue gases. The sensor was developed inthe mid-1960s in conjunction with the U.S.Space Program’s Apollo mission. Becauseof its inherent ability to make oxygen measurements in hot, dirty gases withoutsample conditioning, it was quickly acceptedby industrial users.7Figure 2-1 — Thermox Series 2000 microprocessor-based control unit.

120The sensing element itself is a closed-endtube or disk made from ceramic zirconiumoxide stabilized with an oxide of yttrium orcalcium. Porous platinum coatings on theinside and outside serve as a catalyst andas electrodes. At high temperatures (generally above 1200 F, 650 C), oxygen molecules coming in contact with the platinumelectrodes near the sensor become ionic.As long as the oxygen partial pressures oneither side of the cell are equal, the movement is random and no net flow of ions occurs. If, however, gases having differentoxygen partial pressures are on either sideof the cell, a potentiometric voltage is produced (See Figure 2-2). The magnitude ofthis voltage is a function of the ratio of thetwo oxygen partial pressures. If the oxygenpartial pressure of one gas is known, thevoltage produced by the cell indicates theFigure 2-2 — Zirconium oxide cell principle of operationCELL OUTPUT—MILLIVOLTSThe Nernst Equation100806040 200- 205020105210.50.20.1% OXYGEN CONCENTRATIONFigure 2-3 — Cell output vs. oxygen concentrationoxygen content of the other gas (See Figure 2-3). A reference gas, usually air, is usedfor one of the gases.Since the voltage of the cell is temperature dependent, the cell is maintained at aconstant temperature. Some newer hightemperature insitu models use the heat fromthe process to heat the sensor, and the process temperature is continuously measuredand used in the software calculation. Theoxygen content is then determined from theNernst equation:RT O1E ln4FO2where R and F are constants, T is absolute temperature, and O1 and O2 are theoxygen partial pressures on either side ofthe cell. For example, if air is the referencegas, and the cell temperature is 735 C, theequation becomes:0.209E 0.050 logO2The cell produces zero voltage when thesame amount of oxygen is on both sides,and the voltage increases as the oxygenconcentration of the sample decreases.The voltage created by the difference inthe sample gas and the reference air iscarried by a cable to the microprocessorcontrol unit, where it is linearized to anoutput signal.8

Advantages of ZirconiumOxide TechnologyThe zirconium oxide cell has several advantages over other oxygen-sensing methods. First, since the cell operates at hightemperatures, there is no need to cool ordry the flue gas before it is analyzed. Mostzirconium oxide analyzers make direct oxygen measurements on the stack with nothing more than a filter to keep ash or particulate away from the cell. This dramatically improves the response time. The cellis not affected by vibration, and unlike othertechniques, the output actually increaseswith decreasing oxygen concentration. Inaddition, the cell has a virtually unlimitedshelf life.Zirconium Oxide Sampling TechniquesFour different sampling techniques are usedto measure flue gas with a zirconium oxidesensor. These sampling techniques are theinsitu, close-coupled extractive, extractive,and convective.Insitu AnalyzerAs its name implies, an insitu analyzer (SeeFigure 2-4) places the zirconium oxide celldirectly in the flow of the flue gas. The zirconium oxide cell is located at the end of astainless-steel probe inserted into the stack,and is inserted from a few inches to a fewfeet in the process, depending on theapplication.A heating element, in conjunction with athermocouple, controls the cell temperatureto ensure proper operation.Flue gas diffuses into the probe openingand comes in contact with the zirconiumoxide cell by diffusion.The compact design of an insitu analyzermakes it a good choice for many industrialapplications where oxygen measurementsalone are adequate. However, the insitudesign does not lend itself to combustiblesmeasurements necessary for combustionefficiency. And because all its analyzingcomponents are located directly in thestack, the insitu cannot be used in applications with temperatures above 1250 F.One other drawback to older insitu models has been difficulty of servicing. Whenan insitu probe stopped functioning, it hadto be taken completely off line and shippedback to its manufacturer for repairs. Newermodels, however, employ a modular construction of the internal components so thecell, furnace, and thermocouple can be removed and repaired in the field while leaving the outer protection tube in the process.Parts can be unscrewed and replaced inminutes instead of the weeks needed for afactory repair (see Figure 2-4).Figure 2-4 — Cross-section of Insitu probe9

SORCOMBUSTIBLESDETECTORCALIBRATIONGAS LINEBOILER WALLPRIMARYLOOPASPIRATORAIR INLETFigure 2-5 — Close-coupled extractive oxygen/combustibles analyzerClose-coupled Extractive AnalyzerA close-coupled extractive probe (see Figure 2-5) uses the force of an air-drivenaspirator to pull flue gas into the analyzer.The sensor is located just outside the process wall with a probe that extends intothe flue gas to extract a sample. The fluegas is then returned to the process afterbeing measured.Flue gas is pulled through the primarysample loop by an aspirator. The flue gasenters the pipe to fill the vacuum createdby the aspirator, and about five percent of itis lifted into the secondary loop using convection - this is where the cell and furnaceare located. This analyzer yields the fastest response to process changes becauseof the aspirator, and can work in processesup to 3200 F (1760 C). Since the sensor islocated so close to the stack and is heated,no sample line conditioning is needed. Thefurnace provides the ability to tightly control temperature, which improves accuracyover an insitu measurement. The closecoupled extractive analyzer is ideal for relatively clean-burning applications, such asnatural gas and lighter grades of oil, andcan be equipped with a combustibles detector (see Chapter 3).Convective Analyzer (Hybrid Model)This type of analyzer (See Figure 2-6) usesconvection to bring the sample flue gas tothe zirconium oxide cell, which is locatedjust outside the process wall.Since hot air rises, the temperature-controlled furnace and oxygen-sensing cell areplaced above the level of the gas inlet pipe.As gas near the cell is heated, it rises upand out of the cell housing, and is replacedby gas being drawn out of the filter chamber and into the inlet pipe. The gas that hasleft then cools off on its way back into thefilter chamber. Process gas is constantlydiffusing in and out of the filter chamber.The intake area of a convective analyzeris surrounded by a filter. Since gases diffusethrough the filter and are then drawn into theanalyzer by convection, no flow through thefilter occurs and thus no particles can enterthe filter to plug it. This makes it ideal forhigh particulate applications such as coalfired boilers, cement kilns, waste incinerators, and recovery boilers.The convective analyzer can be used forprocesses up to 2800 F (1537 C). Becauseit works on a diffusion principle, and thesample path is longer than that of an insituprobe, the response time is slower thanthe close-coupled extractive technique, MPLE GASINLETPOROUSCERAMIC FILTEROXYGENSENSORCALIBRATIONGAS PORTSAMPLE GASEXHAUSTFigure 2-6 — Convective oxygen/combustibles analyzer10

900800700CELL mVis comparable in response time to the insitudesign since it also uses diffusion. However,the tighter temperature control of the furnace and the ability to add a combustiblesdetector as an option outweighs the slowerresponse time.600500Extractive Analyzers400Extractive analyzers, unlike close-coupledextractive or convective analyzers, do notreturn the sample to the gas stream (seeFigure 2-7). This is because the gas is often extracted as far as 100 feet or morefrom the stack for analysis. Either wet ordry readings are possible with extractivezirconium oxide analyzers. Zirconium oxide extractive analyzers can also be placedon the stack if desired, since they requireno sample conditioning.300Other Benefits of Zirconium Oxide CellIn the absence of molecular oxygen, the zirconium oxide cell responds to the minuteamount of oxygen produced by the dissociation of water and carbon dioxide at thehigh cell operating temperature. This dissociation is inhibited by the presence ofcombustibles (carbon monoxide and hydrogen) in the sample gas. As the combustiblesconcentration increases, the oxygen concentration decreases, and the output signal from the zirconium oxide cell increases.2001000108642% OXYGEN02468 10% COMBUSTIBLESFigure 2-8 — Cell output vs. net oxygen/net combustibles (methane fuel)This means that the zirconium oxide celldoes not stop responding when there is nonet oxygen in the flue gas. In fact, it becomes a sensor of net combustibles.Equivalent CombustiblesFigure 2-8 shows how the voltage generatedby the cell increases sharply as flue gaschanges from a net oxygen to a net combustibles condition. This property of thezirconium oxide cell is extremely usefulon some combustion processes becauseit permits measurement on both sides ofstoichiometric combustion, either excessair or excess fuel. This is beneficial, forexample, on a two stage burner where thefirst stage is kept reducing and the second stage is oxidizing.Equivalent combustibles are used for lowlevels of free oxygen in the flue gas. It usesa summation of the zirconia cell and thecombustibles detector to ensure combustibles readings are accurate and continuousunder all process conditions.Paramagnetic Oxygen SensorFigure 2-7 — Extractive oxygen analyzer11The paramagnetic sensor takes advantageof the fact that oxygen molecules are

Figure 2-9 — Paramagnetic oxygen sensorstrongly influenced by a magnetic field.Although there are several variations, themost common design uses two diamagnetic, nitrogen-filled quartz spheres connected by a quartz rod to form a dumbbellshape. This is supported by a torsionalsuspension, and is located in a strongnon-uniform magnetic field (see Figure 29). Because the spheres are diamagnetic,they will swing away from the strong magnetic field until the torsional forces equalthe magnetic forces.If a gas containing oxygen passes throughthe cavity that contains the dumbbell, themagnetic field changes, and the dumbbellmoves. This movement can be detected optically or electronically, and is a measure ofthe oxygen concentration in the sample gas.Because of the delicate nature of the dumbbell assembly, paramagnetic analyzers arebest suited for laboratory applications. Whenused on flue gas, a fairly complex samplingand cleaning system is required to ensurethat the gas is clean, dry, and cool before itis introduced into the analyzer.with an aqueous electrolyte. Oxygen molecules diffuse through a membrane to thecathode where a chemical reaction occursthat uses electrons from the oxygen moleculeto release hydroxyl ions (OH-) into the electrolyte. At the anode, which is typically leador cadmium, the hydroxyl ions react with theanode material, oxidizing it and releasingelectrons. Since electrons are released atthe anode and accepted at the cathode, thecell is essentially a battery with an electricalcurrent that is directly proportional to the flowof oxygen through the membrane.The best wet cells are packaged into neat,compact plastic cylinders containing themembrane, electrodes, and electrolyte.When the anode material is depleted, thesecylinders can simply be removed, discarded, and replaced. A cross-section ofsuch a cell is shown in Figure 2-10. Thiskind of wet cell is ideal for use in portableoxygen analyzers because it is lightweightand requires only battery power. For permanent installations, however, a sampling/cooling system must be installed betweenthe analyzer and the combustion process.Without sample conditioning, the cell membrane quickly becomes coated and ceasesto function. Care must also be taken sincethe anode of the cell oxidizes rapidly whenexposed to air. Usually, cells are stored inair-tight packages. Finally, the response timeis extremely slow compared to zirconiumoxide cells.ELECTROLYTECATHODEWet Electrochemical CellWet electrochemical cells, of which there aremany designs, use two electrodes in contactANODEFigure 2-10 — Typical wet electrochemical cell12

chapter threeCOMBUSTIBLES MEASUREMENTIncomplete combustion results in combustibles, consisting of hydrogen (H2) and carbon monoxide (CO), in the flue gas. Morethan a few 100 ppm of combustibles resultsin wasted fuel, soot formation, and reducedheat transfer efficiency. In addition, highconcentrations of combustibles create anenvironmental concern and a potentiallyexplosive condition.Hydrogen measurement has often beenignored in flue gas analysis, with the focus instead being on CO. For combustion efficiency purposes, though, you needto be able to detect total combustibles,which includes both carbon monoxide andhydrogen.The three prevalent methods for on-linemonitoring of combustibles in flue gas arewith a catalytic element, wet electrochemical cell, and non-dispersive infrared absorption. However, wet electrochemicalcells and infrared technologies me

boiler performance, make sure you maxi-mize combustion efficiency. The best way to maximize combustion efficiency is to measure oxygen and combustibles in the flue gas on a continuous basis. Combustion Theory and Stoichiometric Combustion The three essential components of combustion are fuel,

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