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Preferred Utilities Manufacturing Corp. Combustion Theory Boiler Efficiency And Control. Preferred Utilities Manufacturing Corporation 31-35 South St. • Danbury • CT T: (203) 743-6741 F: (203) 798-7313 www.preferred-mfg.com. Overview. Introduction Combustion Basics - PowerPoint PPT Presentation
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Preferred Utilities Manufacturing CorpPreferred Utilities Manufacturing Corporation31-35 South St. Danbury CTT: (203) 743-6741 F: (203) 798-7313www.preferred-mfg.comCombustion Theory Boiler Efficiency And Control
OverviewIntroductionCombustion BasicsEfficiency CalculationsControl Strategy Advantages and DisadvantagesSummary
Preferred Utilities Manufacturing Corp.Over 80 Years of Combustion ExperienceCustom Engineered Combustion SolutionsPackage Burners for Residual Oil, Distillate Oil and Natural GasFuel Handling Systems for Residual Oil BurnersFuel Handling Systems for Distillate Oil BurnersDiesel Engine Fuel Management SystemsCombustion Control SystemsBurner Management SystemsData Acquisition Systems
Instrumentation & Control ProductsPCC-IIIMultiple Loop ControllerPlant Wide ControllerDCS-IIIProgrammable ControllerDraft Control
Operator InterfaceLCDMessage DisplayOIT10 OperatorInterface TerminalPCC-IIIFaceplate DisplayJC-10D ProcessBargraph DisplaySCADA/FlexDistributed Control Station
SensorsTank GaugeLevel Sensor HD-A1 Tank GaugeLeak DetectorPressureSensorOutdoor AirTemperature SensorZP Oxygen ProbePCC-300 EPAOpacity MonitorJC-30DOpacity Monitor
Boiler Room Fire Safety
PCC-III Combustion ExperienceBoiler Specific...Operator FriendlyF(x) Characterizers with Learn Mode Built In Boiler EfficiencyConstructed For Boiler Front Mounting120 Vac Inputs for Direct BMS InterfaceTriac Outputs to Drive Electric ActuatorsFree Standard Combustion BlockwareThere are many digital controller manufacturers, but NONE have Preferreds in-depth and ongoing combustion control experience.
UtilitySaverTM Burner ControlThe UtilitySaver includes firing rate control with both oxygen trim and variable speed fan combustion air flow control.UtilitySaver fuel and electrical savings can pay for the installed system in two years or less.Fuel and Electrical Savings
BurnerMate Touch ScreenFully Integrated Touch Screen
BurnerMate Touch ScreenDCS-III Controller
BurnerMate TSAdvanced Communication
BurnerMate Touch ScreenEasy Operation
BurnerMate Touch ScreenEasy Setup
Combustion BasicsWhat is fuel made of?What is air made of?What happens when fuel is burned?Where does the energy go?What comes out the smoke stack?
Most Fuels are HydrocarbonsCommon fuels have typical analysiscan be used for most combustion calculationsespecially for natural gasalso number 2 fuel oilResidual oil can be approximated with a typical fuel oil analysisWood, coal, waste require a case by case chemical analysis for combustion calculations
Common Fuels AnalysisTypical Ultimate Analysis of Common FuelsPercent by Weight
Composition of (Dry) AirBy Volume20.95% Oxygen, O279.05% Nitrogen, N2By Weight23.14% Oxygen76.86% NitrogenCan be up to 9% H2O by volume in SummerTraces of Argon and CO2
Common Combustion ReactionsNeglecting H2O in AirNeglecting NOx, Other minor reactionsSimplifying percentages:
4N2 + O2 + 2H2 2H2O + 4N2 + Heat4N2 + O2 + C CO2 + 4N2 + Heat4N2 + O2 + S SO2 + 4N2 + Heat
Common Combustion ReactionsFor Methane
CH4 + 2O2 CO2 + 2H2O + Heat16 + 64 44 + 36Therefore:#O2 Required = 64# Fuel = 16Therefore #O2/#Fuel =4/1 or 4
Boiler Efficiency and ControlBoiler efficiency is computed by lossesUnderstanding of efficiency calculations helps in choosing the proper control strategyEnergy traps such as economizers can provide a paybackPreferred Instruments has over 75 years of combustion experience to help optimize boiler efficiency
Boiler Efficiency by LossesConservation of Energy Fuel energy in equals heat energy outEnergy leaves in steam or in lossesEfficiency = 100% minus all lossesTypical boiler efficiency is 80% to 85%The remaining 15% to 20% is lostLargest loss is a typical 15% stack lossRadiation loss may be 3% at full inputMiscellaneous losses might be 1 to 2%
Boiler Energy Balance
Stack LossesLatent heat of water vapor in stackFixed amount depending on hydrogen in fuelAbout 5% of fuel input for fuel oilAbout 9% of fuel input for natural gasAssumes a non-condensing boiler (typical)Sensible heat of stack gassesTypically around 10% of fuel inputIncreased mass flow and stack temperature increase the loss
Radiation LossGenerally a fixed BTU / hour heat lossAs a percentage, is greater at low fireDepends on the boiler constructionIs generally about a 3% loss at high fireWould be 12% loss at 25% of fuel input
Miscellaneous LossesConsist of:blow down lossesunburned fuel losses (carbon in ash or CO)Generally on the order of one percent
Excess Air Required for Burners
Excess Air Required for Burners
Chart1
20.182
22.773
23.961
31.242
40.531
52.525
61.321
74.523
81.722
9927
Air %
Oxygen %
Fuel %
Air %
Burner Fuel-Air Ratio
Sheet1
Using the PCC III Characterizer Block (ie, F(x) )
for Burner Fuel-Air Ratio, and Oxygen Trim Control
f(x)PointFuelSCFHFuel %Air %Oxygen %Oxygen %
1720010.320.18.282
21480021.122.77.373
32150030.723.96.161
42760039.431.24.242
53510050.140.53.131
64280061.152.52.525
74930070.461.32.121
85680081.174.52.323
96390091.381.72.222
106970099.6992.727
70000
Sheet1
Air %
Oxygen %
Fuel %
Air %
Burner Fuel-Air Ratio
Sheet2
Sheet3
Excess Versus Deficient Air
Effects of Stack TemperatureGenerally, stack temperature is:Steam temperature plus 100 to 200 degrees FRule of thumb watertube-150, firetube-100FHigher for dirty boilers, higher loads and increased excess air levelsA 100 degree increase in stack temperatureCosts about 2.5% in energy lossesMay mean the boiler needs serious maintenanceEconomizers are useful on medium and high pressure boilers as an energy trap
Efficiency Calculation Charts
Oxygen and Air Required for GasTo release 1 million BTU with gas42 lbs. of gas are burned168 lbs. of oxygen are required no excess air725 lbs. of combustion air767 lbs. of stack gasses are produced5% to 20% excess air is required by burnerEach additional 10% increase in excess air:Adds 73 lbs. of stack gassesReduces efficiency by 1% to 1.5%
Cost of InefficiencyThe combined effects of extra excess air and the resulting increase in stack temperature:Could mean a 2% to 10% efficiency dropReducing this extra excess air saves fuelSavings = (Fuel Cost)*[(1/old eff)-(1/new eff)] For a facility with a 30,000 pph steam load10% to 60% Extra Excess Air Represents From $6,000 to $35,000 in potential savings per yearRunning 20 hours, 300 days, $4.65 per MM Btu
Combustion Control ObjectivesMaintain proper fuel to air ratio at all timesToo little air causes unburned fuel lossesToo much air causes excessive stack lossesImproper fuel air ratio can be DANGEROUSAlways keep fuel to air ratio SAFEInterface with burner management for:PurgeLow fire light offModulate fuel and air when safe to do so
Related and Interactive LoopsFeedwater Flowfeedwater is usually cooler than water in boileradding large amounts of water cools the boilercooling the boiler causes the firing rate to increaseFurnace Draftchanging pressure in furnace changes air flowchanged air flow upsets fuel to air ratio
Variations in Air CompositionStandard air has 0.0177 LB. O2 per FT3Hot, humid air has less O2 per cubic ft20% less at 95% RH, 120OF, and 29.9 mm HgDry, cold air has more O2 per cubic ft10% more at 0% RH, 32OF, and 30.5 mm HgCombustion controls must:Adapt to changing air composition (O2 trim), orAllow at least 20% extra excess air at standard conditions
Control System ErrorsCombustion control system can not perfectly regulate fuel and oxygen flows. Therefore, extra excess air must be supplied to the burner to account for control system errorsHysteresisFlow transmitter can not measure fuel Btu flow rate (Btu / hr)Oxygen content per cubic foot of air changes with humidity, temperature and pressureFuel flow for a given valve position varies with temperature and pressure
Control System Errors
Typical Combustion Control System "Errors"
(Expressed in % Excess Air Required)
5%
14%
2%
2%
20%
3%
2%
2%
2%
14%
2%
2%
0%
0%
0%
0%
0%
5%
0%
5%
10%
15%
20%
25%
Burner
Requirments
Humidity
Draft Pressure
Fuel BTU/lb
Changes
Air
Temperature
Hysterisis
Air Pressure
Fuel Pressure
Changes
Fuel
Temperature
Changes
Jackshaft and Parallel
Positioning Type
Systems
Fully Metered Systems
Additional Errors Due To Jackshaft and Parallel
Poitioning Control Method
Control System Errors
For example a 600 BHP boiler, delivering 20kpph of 15 psi saturated steam has the following additional operating cost due to excess combustion air:
Excess Air
Excess O2
Air Flow
Theoretical Fuel flow
Lost BTU's Up Stack
Fuel Equivalent to Lost BTU's
Total Fuel Lost
Annualized Additional Fuel cost
%
%
#/hr
#/HR
BTU
#/hr
%
US$
27%
6%
20,300
841
342,070
14.3
1.7%
$ 9,543
The fuel savings are calculated using a fuel cost of $4.65/MMBTU and a boiler operating at full load for 20hrs/day & 300days/year. Excess air also causes additional forced draft fan horsepower costs.
Combustion Control StrategiesSingle Point Positioning (Jackshaft)Fuel and air are tied mechanicallySimple, low cost, safe, requires extra excess airParallel PositioningFuel valve and air damper are positioned separatelyAllows oxygen trim of air flowFully MeteredFuel and air FLOW (not valve position) are controlled
Jackshaft StrategyAll control errors affect this system. Typically, 20 - 50 % extra excess air must be supplied to the burner to account for control inaccuracies.One actuator controls fuel and air via linkage. It is assumed that a given position will always provide a particular fuel flow and air flow.Oxygen trim systems can reduce the extra excess air to 10%Suitable for firetube boilers and small watertube boilers. Used when annual fuel expense is too small to justify a more elaborate system.
Jackshaft Strategy
Jackshaft StrategyAdvantagesSimplicityProvides large turndownInexpensiveDisadvantagesFuel valves and fan damper must be physically close together
Changes in fuel or air pressure, temperature, viscosity, density, humidity affect fuel-air ratio.
Only one fuel may be burned at a time.
Not applicable to multiple burners.
Not applicable to variable speed fan drives.
Oxygen Trim is difficult to apply, trim limit prevents adequate correction
Parallel Positioning StrategyCross Limiting is employed for safety and to prevent combustibles or smoke during load changes. Cross Limiting requires and accurate position feedback signal from each actuator. A failure of either actuator or feedback pot will force the air damper open and the fuel valve to minimum position.Separate actuators are used to position fuel and air final devices, flows are unknown. Fuel to air ratio can be varied automaticallyMany of the same applications, limitations and improvements described in the Single Point Positioning section also apply to Parallel Positioning
Parallel Positioning Strategy
Parallel Positioning StrategyAdvantagesAllows electronic characterization of fuel-air ratio
Adapts to boilers with remote F.D. fans and / or variable speed drives
Provides large turndown
Allows low fire changeover between fuels
Oxygen trim is easy to accomplishDisadvantagesChanges in fuel or air pressure, temperature, viscosity, density, humidity affect fuel-air ratio.
Only one fuel may be burned at a time.
Not applicable to multiple burners.
Position feed back is expensive for pneumatic actuators
Oxygen Trim limit prevents adequate correction
Fully Metered StrategyBoth the fuel flow and the combustion air flow are measured. Separate PID controllers are used for both fuel and air flow control. Demand from a Boiler Sub-master is used to develop both a fuel flow and air flow setpoint.Fuel and Air Flow setpoints are Cross Limited using fuel and air flows. Oxygen trim control logic is easily added as an option. Flue gas oxygen is measured and compared against setpoint to continuously adjust (trim) the fuel / air ratio. The excess air adjustment allows the boiler to operate safely and reliably at reduced levels of excess air throughout the operating range of the boiler. This reduction in excess air can result in fuel savings of 2% to 4%. The flue gas excess oxygen setpoint is based on boiler firing rate or an operator set value.
Fully Metered Strategy
Fully Metered StrategyAdvantagesCorrects for control valve, damper drive and pressure regulator Hysteresis
Compensates for flow variations.
Applicable to multiple burners.
Allows simultaneous firing of oil and gas.
Disadvantages
Installation is more costly.
With no oxygen trim.For all types of flow meters, the fuel Btu value and air oxygen content must be assumed.
Comparison
Other Control Loops that ImpactControl of Fuel and Draft ControlFeedwater Control
Draft ControlChanging furnace draft can change air flowChanged air flow effects efficiencyChanged air flow effects emissionsDraft Control keeps furnace pressure constantDraft Control becomes extremely important:When multiple boilers share a stackStack is very highInduced FGR is used for NOx control
Draft Control Schematic
Types of Draft ControlSelf contained units such as Preferred JC-20Sequencing closes damper when boiler is offSaves energyDraft sensing diaphragm and logic in one unitMicro-processor controllers for tighter controlFeedforward based on firing rateTrue PID control of furnace draft
Feedwater ControlBenefits of stable water level controlhigh and low water trips are avoidedwater carryover in steam is minimizedsteam pressure stays more nearly constantSwinging feedwater flow can:cause pressure swingscause firing rate to huntcreate extra wear and tear on valves and linkagewaste fuel
Simple Feedwater Control StrategiesOn-off controltypically used on small firetube unitsSingle Element Feedwater Controlopening of valve is influenced by change in leveltypical of older thermo-hydraulic systemsthermo-hydraulic systems are proportional onlyuse of PID controller can add resetsuitable for steady loads
Shrink and SwellMomentary drum level upsets in water tube boilers when the steam load swingsIncrease in load causes swell:drops pressure in boilerincreases size of steam bubbles in watertubescauses more water to flash to steamcauses the actual level in the drum to rise while the total amount of water actually dropssingle element will close the valve, not open it
Shrink and Swell, cont.Drop in load causes:pressure to risesome steam to condensesize of remaining bubbles to shrinkwater level in drum dropsactual amount of water might be risingControls reduce impact of shrink and swellcontrols cant compensate for poor design or condition of boiler
Two Element Feedwater ControlControl on water level and steam flowdrop in level increases valve openingrise in steam flow increases valve openingreduces impact of shrink and swellbetter for swinging loadsPID control with steam flow feed-forward which can be characterized to match the valve trimRequires a steady feedwater supply pressure
Two Element Feedwater Control
Three Element Feedwater ControlWater level, steam flow and feedwater determine controller output signalTwo PID loops in cascade configuration:hold drum level at setpointhold feedwater flow to match steam flowVery stable level controlKeeps water inventory constant during periods of shrink and swell
Three Element Feedwater
Auxiliary Controller FunctionsCalculation of pressure compensated steam flowCompensation of drum level signal for changing water density in steam drumTotalization of steam flowTotalization of feedwater flowAlarms for high and low water levels
Data Acquisition for CombustionAllows remote operation of controllers Reduces manpower requirements in plantProvides historical dataTrend data to replace strip or circular chartsReports to document plant operationCan compare energy usage per degree dayFrom year to yearFrom building to buildingAllows energy wasting trends to be spotted
New Advances in Combustion ControlThese features offers help firing systems meet emissions goals.To enable improved burner turndown, Combustrol provides automatic switching to positioning control of the air control damper whenever the firing rate of the unit is below the turndown range of the air flow transmitter. Combustrol's fully metered combustion control strategy includes differential cross limiting of fuel and air flows. This feature adds an addition level of protection to the conventional air flow and fuel flow cross limiting combustion control scheme by preventing the air fuel ratio from becoming too air rich as well as too fuel rich. For rapid boiler load response, the air flow control output is the sum of the air flow controller output and an air flow demand feedfoward index.
Saving Fuel with Combustion ControlOxygen Trim of air flowApplicable to any control strategyShould be applied to any large boilerOxygen readout is valuable even if trim is impracticalVariable speed drive of combustion air fanCan generate considerable horsepower savingsApplicable to any control strategyEconomic Boiler Dispatch
Oxygen Trim StrategiesMechanical trim devices for single point positioningCan vary the air damper positionCan vary the fuel pressureBiasing the air damper actuator position for parallel positioning controlChanging the fuel to air ratio in metering systemsChanging the fan speed in systems with VFD
Oxygen Trim for Jackshaft System
Oxygen Trim CautionsReplace worn dampers and linkage FIRST!Use only proven analyzers for the signalUse only proven controllers and control strategies to accomplish the trimBudget calibration and probe replacement.
Variable Speed Fan DrivesApplicable to parallel positioning or metering control strategiesCan generate considerable electricity savingsFor a 40,000 pph boiler running at 50% load:Savings could be up to $12,000 per yearR.O.I. could be as low as 1.5 yearsMight be a candidate for a utility company rebate
SummaryCombustion control is a specialty fieldEach application has unique requirementsEach system should balance:efficiency of operationinstalled costsafety and reliabilityPreferred Instruments is leader in the field of special combustion control systems
Preferred Utilities Manufacturing CorpFor further information, contact...Preferred Utilities Manufacturing Corporation31-35 South Street Danbury CTT: (203) 743-6741 F: (203) 798-7313www.preferred-mfg.com
Preferred has over 30 years experience in design, manufacturing, and field service for digital combustion control.
By summing the air requirements for eachFuel to air ratios near ideal can be maintainedActual flows can be cross limited for safety