Presentation on theme: "Preferred Utilities Manufacturing Corp"— Presentation transcript:
1 Preferred Utilities Manufacturing Corp Combustion Theory BoilerEfficiency And ControlPreferred Utilities Manufacturing Corporation31-35 South St. • Danbury • CTT: (203) F: (203)
2 Overview Introduction Combustion Basics Efficiency Calculations Control Strategy Advantages and DisadvantagesSummary
3 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
4 Instrumentation & Control Products DCS-IIIProgrammable ControllerPlant Wide ControllerPCC-IIIMultiple Loop ControllerDraft Control
5 Distributed Control Station Operator InterfaceJC-10D ProcessBargraph DisplayPCC-IIIFaceplate DisplaySCADA/FlexDistributed Control StationLCDMessage DisplayOIT10 OperatorInterface Terminal
6 Sensors HD-A1 Tank Gauge Leak Detector Pressure Sensor Outdoor Air Temperature SensorTank GaugeLevel SensorZP Oxygen ProbePCC-300 EPAOpacity MonitorJC-30DOpacity Monitor
8 PCC-III Combustion Experience Boiler Specific...Operator FriendlyF(x) Characterizers with “Learn” ModeBuilt In Boiler EfficiencyConstructed For Boiler Front Mounting120 Vac Inputs for Direct BMS InterfaceTriac Outputs to Drive Electric ActuatorsFree Standard Combustion BlockwarePreferred has over 30 years experience in design, manufacturing, and field service for digital combustion control.There are many digital controller manufacturers, but NONE have Preferred’s in-depth and ongoing combustion control experience.
9 UtilitySaverTM Burner Control Fuel and Electrical Savings…The 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.
15 Combustion Basics What 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?
16 Most Fuels are Hydrocarbons Common 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
17 Typical Ultimate Analysis of Common Fuels Common Fuels AnalysisTypical Ultimate Analysis of Common FuelsPercent by Weight
18 Composition of (Dry) Air By 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
19 Common Combustion Reactions Neglecting H2O in AirNeglecting NOx, Other minor reactionsSimplifying percentages:4N2 + O H2 Þ 2H2O + 4N2 + Heat4N2 + O C Þ CO2 + 4N2 + Heat4N2 + O S Þ SO2 + 4N2 + Heat
20 Common Combustion Reactions For MethaneCH4 + 2O2 Þ CO2 + 2H2O + HeatÞTherefore:#O2 Required = 64# Fuel = 16Therefore #O2/#Fuel =4/1 or 4
21 Boiler Efficiency and Control Boiler efficiency is computed “by losses”Understanding 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
22 Boiler Efficiency “by Losses” Conservation of EnergyFuel 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 loss”Radiation loss may be 3% at full inputMiscellaneous losses might be 1 to 2%
24 Stack Losses Latent heat of water vapor in stack Fixed 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
25 Radiation Loss Generally a fixed BTU / hour heat loss As 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
26 Miscellaneous Losses Consist of: Generally on the order of one percent blow down lossesunburned fuel losses (carbon in ash or CO)Generally on the order of one percent
30 Effects of Stack Temperature Generally, 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”
32 Oxygen and Air Required for Gas To 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%
33 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
34 Combustion Control Objectives Maintain 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
35 Related and Interactive Loops Feedwater 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
36 Variations in Air Composition “Standard” air has 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
37 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 errors…HysteresisFlow 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
40 Combustion Control Strategies Single 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
41 Jackshaft StrategyOne actuator controls fuel and air via linkage. It is assumed that a given position will always provide a particular fuel flow and air flow.All control errors affect this system. Typically, % extra excess air must be supplied to the burner to account for control inaccuracies.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.
43 Jackshaft Strategy Disadvantages Advantages Fuel valves and fan damper must be physically close togetherChanges 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 correctionAdvantagesSimplicityProvides large turndownInexpensive
44 Parallel Positioning Strategy Separate actuators are used to position fuel and air final devices, flows are unknown. Fuel to air ratio can be varied automaticallyCross 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.Many of the same applications, limitations and improvements described in the Single Point Positioning section also apply to Parallel Positioning
46 Parallel Positioning Strategy AdvantagesAllows electronic characterization of fuel-air ratioAdapts to boilers with remote F.D. fans and / or variable speed drivesProvides large turndownAllows low fire changeover between fuelsOxygen 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 actuatorsOxygen Trim limit prevents adequate correction
47 Fully Metered Strategy Both 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.
49 Fully Metered Strategy AdvantagesCorrects for control valve, damper drive and pressure regulator HysteresisCompensates for flow variations.Applicable to multiple burners.Allows simultaneous firing of oil and gas.DisadvantagesInstallation is more costly.With no oxygen trim….For all types of flow meters, the fuel Btu value and air oxygen content must be assumed.By summing the air requirements for eachFuel to air ratios near “ideal” can be maintainedActual flows can be cross limited for safety
51 Other Control Loops that Impact Control of Fuel and Draft ControlFeedwater Control
52 Draft Control Changing furnace draft can change air flow Changed 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
54 Types of Draft Control Self contained units such as Preferred JC-20 “Sequencing” 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
55 Feedwater Control Benefits of stable water level control high 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
56 Simple Feedwater Control Strategies On-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 “reset”suitable for steady loads
57 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
58 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 can’t compensate for poor design or condition of boiler
59 Two Element Feedwater Control Control 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
61 Three Element Feedwater Control Water 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
63 Auxiliary Controller Functions Calculation 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
64 Data Acquisition for Combustion Allows remote operation of controllersReduces 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
65 New Advances in Combustion Control These features offers help firing systems meet emissions goals.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.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.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.
66 Saving Fuel with Combustion Control Oxygen 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 savingsEconomic Boiler Dispatch
67 Oxygen Trim Strategies Mechanical 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
69 Oxygen Trim Cautions Replace 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.
70 Variable Speed Fan Drives Applicable 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
71 Summary Combustion control is a specialty field Each application has unique requirementsEach system should balance:efficiency of operationinstalled costsafety and reliabilityPreferred Instruments is leader in the field of special combustion control systems
72 Preferred Utilities Manufacturing Corp For further information, contact...Preferred Utilities Manufacturing Corporation31-35 South Street • Danbury • CTT: (203) • F: (203)