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