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Preferred Utilities Manufacturing Corp Preferred Utilities Manufacturing Corporation 31-35 South St. Danbury CT T: (203) 743-6741 F: (203) 798-7313

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Presentation on theme: "Preferred Utilities Manufacturing Corp Preferred Utilities Manufacturing Corporation 31-35 South St. Danbury CT T: (203) 743-6741 F: (203) 798-7313"— Presentation transcript:

1 Preferred Utilities Manufacturing Corp Preferred Utilities Manufacturing Corporation 31-35 South St. Danbury CT T: (203) 743-6741 F: (203) 798-7313 Combustion Theory Boiler Efficiency And Control

2 Overview q Introduction q Combustion Basics q Efficiency Calculations q Control Strategy Advantages and Disadvantages q Summary

3 Preferred Utilities Manufacturing Corp. q Over 80 Years of Combustion Experience q Custom Engineered Combustion Solutions q Package Burners for Residual Oil, Distillate Oil and Natural Gas q Fuel Handling Systems for Residual Oil Burners q Fuel Handling Systems for Distillate Oil Burners q Diesel Engine Fuel Management Systems q Combustion Control Systems q Burner Management Systems q Data Acquisition Systems

4 Instrumentation & Control Products PCC-III Multiple Loop Controller Plant Wide Controller DCS-III Programmable Controller Draft Control

5 Operator Interface LCD Message Display OIT10 Operator Interface Terminal PCC-III Faceplate Display JC-10D Process Bargraph Display SCADA/Flex Distributed Control Station

6 Sensors Tank Gauge Level Sensor HD-A1 Tank Gauge Leak Detector Pressure Sensor Outdoor Air Temperature Sensor ZP Oxygen ProbePCC-300 EPA Opacity Monitor JC-30D Opacity Monitor

7 Boiler Room Fire Safety

8 PCC-III Combustion Experience Boiler Specific... 4 Operator Friendly 4 F(x) Characterizers with Learn Mode 4 Built In Boiler Efficiency 4 Constructed For Boiler Front Mounting 4 120 Vac Inputs for Direct BMS Interface 4 Triac Outputs to Drive Electric Actuators 4 Free Standard Combustion Blockware 4 There are many digital controller manufacturers, but NONE have Preferreds in-depth and ongoing combustion control experience.

9 UtilitySaver TM Burner Control q The UtilitySaver includes firing rate control with both oxygen trim and variable speed fan combustion air flow control. q UtilitySaver fuel and electrical savings can pay for the installed system in two years or less. Fuel and Electrical Savings…

10 BurnerMate Touch Screen Fully Integrated Touch Screen…

11 BurnerMate Touch Screen DCS-III Controller…

12 BurnerMate TS Advanced Communication…

13 BurnerMate Touch Screen Easy Operation…

14 BurnerMate Touch Screen Easy Setup…

15 Combustion Basics q What is fuel made of? q What is air made of? q What happens when fuel is burned? q Where does the energy go? q What comes out the smoke stack?

16 Most Fuels are Hydrocarbons q Common fuels have typical analysis n can be used for most combustion calculations n especially for natural gas n also number 2 fuel oil q Residual oil can be approximated with a typical fuel oil analysis q Wood, coal, waste require a case by case chemical analysis for combustion calculations

17 Common Fuels Analysis Typical Ultimate Analysis of Common Fuels Percent by Weight

18 Composition of (Dry) Air q By Volume n 20.95% Oxygen, O 2 n 79.05% Nitrogen, N 2 q By Weight n 23.14% Oxygen n 76.86% Nitrogen q Can be up to 9% H 2 O by volume in Summer q Traces of Argon and CO 2

19 q Neglecting H2O in Air q Neglecting NOx, Other minor reactions q Simplifying percentages: 4N 2 + O 2 + 2H 2 2H 2 O + 4N 2 + Heat 4N 2 + O 2 + C CO 2 + 4N 2 + Heat 4N 2 + O 2 + S SO 2 + 4N 2 + Heat Common Combustion Reactions

20 q For Methane CH 4 + 2O 2 CO 2 + 2H 2 O + Heat 16 + 64 44 + 36 Therefore: #O2 Required = 64 # Fuel = 16 Therefore #O2/#Fuel =4/1 or 4 Common Combustion Reactions

21 Boiler Efficiency and Control q Boiler efficiency is computed by losses q Understanding of efficiency calculations helps in choosing the proper control strategy q Energy traps such as economizers can provide a payback q Preferred Instruments has over 75 years of combustion experience to help optimize boiler efficiency

22 Boiler Efficiency by Losses q Conservation of Energy n Fuel energy in equals heat energy out n Energy leaves in steam or in losses n Efficiency = 100% minus all losses q Typical boiler efficiency is 80% to 85% n The remaining 15% to 20% is lost n Largest loss is a typical 15% stack loss n Radiation loss may be 3% at full input n Miscellaneous losses might be 1 to 2%

23 Boiler Energy Balance

24 Stack Losses q Latent heat of water vapor in stack n Fixed amount depending on hydrogen in fuel n About 5% of fuel input for fuel oil n About 9% of fuel input for natural gas n Assumes a non-condensing boiler (typical) q Sensible heat of stack gasses n Typically around 10% of fuel input n Increased mass flow and stack temperature increase the loss

25 Radiation Loss q Generally a fixed BTU / hour heat loss q As a percentage, is greater at low fire q Depends on the boiler construction q Is generally about a 3% loss at high fire q Would be 12% loss at 25% of fuel input

26 Miscellaneous Losses q Consist of: n blow down losses n unburned fuel losses (carbon in ash or CO) q Generally on the order of one percent

27 Excess Air Required for Burners


29 Excess Versus Deficient Air

30 Effects of Stack Temperature q Generally, stack temperature is: n Steam temperature plus 100 to 200 degrees F »Rule of thumb – watertube-150, firetube-100F n Higher for dirty boilers, higher loads and increased excess air levels q A 100 degree increase in stack temperature n Costs about 2.5% in energy losses n May mean the boiler needs serious maintenance q Economizers are useful on medium and high pressure boilers as an energy trap

31 Efficiency Calculation Charts

32 Oxygen and Air Required for Gas q To release 1 million BTU with gas n 42 lbs. of gas are burned n 168 lbs. of oxygen are required no excess air n 725 lbs. of combustion air n 767 lbs. of stack gasses are produced q 5% to 20% excess air is required by burner q Each additional 10% increase in excess air: n Adds 73 lbs. of stack gasses n Reduces efficiency by 1% to 1.5%

33 Cost of Inefficiency q The combined effects of extra excess air and the resulting increase in stack temperature: n Could mean a 2% to 10% efficiency drop n Reducing this extra excess air saves fuel n Savings = (Fuel Cost)*[(1/old eff)-(1/new eff)] q For a facility with a 30,000 pph steam load n 10% to 60% Extra Excess Air Represents From $6,000 to $35,000 in potential savings per year n Running 20 hours, 300 days, $4.65 per MM Btu

34 Combustion Control Objectives q Maintain proper fuel to air ratio at all times n Too little air causes unburned fuel losses n Too much air causes excessive stack losses n Improper fuel air ratio can be DANGEROUS q Always keep fuel to air ratio SAFE q Interface with burner management for: n Purge n Low fire light off n Modulate fuel and air when safe to do so

35 Related and Interactive Loops q Feedwater Flow n feedwater is usually cooler than water in boiler n adding large amounts of water cools the boiler n cooling the boiler causes the firing rate to increase q Furnace Draft n changing pressure in furnace changes air flow n changed air flow upsets fuel to air ratio

36 Variations in Air Composition q Standard air has 0.0177 LB. O 2 per FT 3 q Hot, humid air has less O 2 per cubic ft n 20% less at 95% RH, 120 O F, and 29.9 mm Hg q Dry, cold air has more O 2 per cubic ft n 10% more at 0% RH, 32 O F, and 30.5 mm Hg q Combustion controls must: n Adapt to changing air composition (O 2 trim), or n Allow at least 20% extra excess air at standard conditions

37 Control System Errors Combustion 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… n Hysteresis n Flow transmitter can not measure fuel Btu flow rate (Btu / hr) n Oxygen content per cubic foot of air changes with humidity, temperature and pressure n Fuel flow for a given valve position varies with temperature and pressure

38 Control System Errors


40 Combustion Control Strategies q Single Point Positioning (Jackshaft) n Fuel and air are tied mechanically n Simple, low cost, safe, requires extra excess air q Parallel Positioning n Fuel valve and air damper are positioned separately n Allows oxygen trim of air flow q Fully Metered n Fuel and air FLOW (not valve position) are controlled

41 n All 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. Jackshaft Strategy n Oxygen trim systems can reduce the extra excess air to 10% n Suitable for firetube boilers and small watertube boilers. Used when annual fuel expense is too small to justify a more elaborate system.

42 Jackshaft Strategy

43 Advantages Simplicity Provides large turndown Inexpensive Disadvantages Fuel 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

44 n Cross 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 automatically Parallel Positioning Strategy n Many of the same applications, limitations and improvements described in the Single Point Positioning section also apply to Parallel Positioning

45 Parallel Positioning Strategy

46 Advantages Allows 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 accomplish Disadvantages 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. Position feed back is expensive for pneumatic actuators Oxygen Trim limit prevents adequate correction Parallel Positioning Strategy

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. n Fuel and Air Flow setpoints are Cross Limited using fuel and air flows. n 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.

48 Fully Metered Strategy

49 Advantages Corrects 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. Fully Metered Strategy

50 Comparison

51 Other Control Loops that Impact Control of Fuel and q Draft Control q Feedwater Control

52 Draft Control q Changing furnace draft can change air flow q Changed air flow effects efficiency q Changed air flow effects emissions q Draft Control keeps furnace pressure constant q Draft Control becomes extremely important: n When multiple boilers share a stack n Stack is very high n Induced FGR is used for NOx control

53 Draft Control Schematic

54 Types of Draft Control q Self contained units such as Preferred JC-20 n Sequencing closes damper when boiler is off n Saves energy n Draft sensing diaphragm and logic in one unit q Micro-processor controllers for tighter control n Feedforward based on firing rate n True PID control of furnace draft

55 Feedwater Control q Benefits of stable water level control n high and low water trips are avoided n water carryover in steam is minimized n steam pressure stays more nearly constant q Swinging feedwater flow can: n cause pressure swings n cause firing rate to hunt n create extra wear and tear on valves and linkage n waste fuel

56 Simple Feedwater Control Strategies q On-off control n typically used on small firetube units q Single Element Feedwater Control n opening of valve is influenced by change in level n typical of older thermo-hydraulic systems n thermo-hydraulic systems are proportional only n use of PID controller can add reset n suitable for steady loads

57 Shrink and Swell q Momentary drum level upsets in water tube boilers when the steam load swings q Increase in load causes swell: n drops pressure in boiler n increases size of steam bubbles in watertubes n causes more water to flash to steam n causes the actual level in the drum to rise while the total amount of water actually drops n single element will close the valve, not open it

58 Shrink and Swell, cont. q Drop in load causes: n pressure to rise n some steam to condense n size of remaining bubbles to shrink n water level in drum drops n actual amount of water might be rising q Controls reduce impact of shrink and swell n controls cant compensate for poor design or condition of boiler

59 Two Element Feedwater Control q Control on water level and steam flow n drop in level increases valve opening n rise in steam flow increases valve opening n reduces impact of shrink and swell n better for swinging loads q PID control with steam flow feed-forward which can be characterized to match the valve trim q Requires a steady feedwater supply pressure

60 Two Element Feedwater Control

61 Three Element Feedwater Control q Water level, steam flow and feedwater determine controller output signal q Two PID loops in cascade configuration: n hold drum level at setpoint n hold feedwater flow to match steam flow q Very stable level control q Keeps water inventory constant during periods of shrink and swell

62 Three Element Feedwater

63 Auxiliary Controller Functions q Calculation of pressure compensated steam flow q Compensation of drum level signal for changing water density in steam drum q Totalization of steam flow q Totalization of feedwater flow q Alarms for high and low water levels

64 Data Acquisition for Combustion q Allows remote operation of controllers q Reduces manpower requirements in plant q Provides historical data n Trend data to replace strip or circular charts n Reports to document plant operation q Can compare energy usage per degree day n From year to year n From building to building n Allows energy wasting trends to be spotted

65 New Advances in Combustion Control These features offers help firing systems meet emissions goals. n 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. n 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. n 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 q Oxygen Trim of air flow n Applicable to any control strategy n Should be applied to any large boiler n Oxygen readout is valuable even if trim is impractical q Variable speed drive of combustion air fan n Can generate considerable horsepower savings n Applicable to any control strategy q Economic Boiler Dispatch

67 Oxygen Trim Strategies q Mechanical trim devices for single point positioning n Can vary the air damper position n Can vary the fuel pressure q Biasing the air damper actuator position for parallel positioning control q Changing the fuel to air ratio in metering systems q Changing the fan speed in systems with VFD

68 Oxygen Trim for Jackshaft System

69 Oxygen Trim Cautions q Replace worn dampers and linkage FIRST! q Use only proven analyzers for the signal q Use only proven controllers and control strategies to accomplish the trim q Budget calibration and probe replacement.

70 Variable Speed Fan Drives q Applicable to parallel positioning or metering control strategies q Can generate considerable electricity savings n For a 40,000 pph boiler running at 50% load: n Savings could be up to $12,000 per year n R.O.I. could be as low as 1.5 years q Might be a candidate for a utility company rebate

71 Summary q Combustion control is a specialty field q Each application has unique requirements q Each system should balance: n efficiency of operation n installed cost n safety and reliability q Preferred Instruments is leader in the field of special combustion control systems

72 Preferred Utilities Manufacturing Corp For further information, contact... Preferred Utilities Manufacturing Corporation 31-35 South Street Danbury CT T: (203) 743-6741 F: (203) 798-7313

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