preferred utilities manufacturing corp preferred utilities manufacturing corporation 31-35 south st....
TRANSCRIPT
Preferred Utilities Manufacturing Corp
Preferred Utilities Manufacturing Corporation31-35 South St. • Danbury • CT
T: (203) 743-6741 F: (203) 798-7313www.preferred-mfg.com
Combustion Theory Boiler
Efficiency And Control
Overview
Introduction Combustion Basics Efficiency Calculations Control Strategy Advantages and
Disadvantages Summary
Preferred Utilities Manufacturing Corp.
Over 80 Years of Combustion Experience Custom Engineered Combustion Solutions Package Burners for Residual Oil, Distillate Oil and Natural
Gas Fuel Handling Systems for Residual Oil Burners Fuel Handling Systems for Distillate Oil Burners Diesel Engine Fuel Management Systems Combustion Control Systems Burner Management Systems Data Acquisition Systems
Instrumentation & Control Products
PCC-IIIMultiple Loop Controller
Plant Wide Controller
DCS-IIIProgrammable Controller
Draft Control
Operator Interface
LCDMessage Display OIT10 Operator
Interface Terminal
PCC-IIIFaceplate Display
JC-10D ProcessBargraph Display
SCADA/FlexDistributed Control Station
Sensors
Tank GaugeLevel Sensor
HD-A1 Tank GaugeLeak Detector
PressureSensor
Outdoor AirTemperature Sensor
ZP Oxygen Probe PCC-300 EPAOpacity Monitor
JC-30DOpacity Monitor
Boiler Room Fire Safety
PCC-III Combustion Experience
Boiler Specific... Operator Friendly F(x) Characterizers with “Learn” Mode Built In Boiler Efficiency Constructed For Boiler Front Mounting 120 Vac Inputs for Direct BMS Interface Triac Outputs to Drive Electric Actuators Free Standard Combustion Blockware
There are many digital controller manufacturers, but NONE have Preferred’s in-depth and ongoing combustion control experience.
UtilitySaverTM Burner Control
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.
Fuel and Electrical Savings…
BurnerMate Touch Screen
Fully Integrated Touch Screen…
BurnerMate Touch Screen
DCS-III Controller…
BurnerMate TS
Advanced
Communication…
BurnerMate Touch Screen
Easy Operation…
BurnerMate Touch Screen
Easy Setup…
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?
Most Fuels are Hydrocarbons
Common fuels have “typical” analysis can be used for most combustion calculations especially for natural gas also number 2 fuel oil
Residual oil can be approximated with a typical fuel oil analysis
Wood, coal, waste require a case by case chemical analysis for combustion calculations
Common Fuels Analysis
#2 Fuel Oil #4 Fuel Oil #6 Fuel Oil Natural Gas Coal Wood
Hydrogen 12.6 11.8 9.7 23.5 5.0 5.7
Carbon 87.3 87.9 87.1 75.2 75.0 53.9
Nitrogen 0.02 0.1 0.5 1.3 1.5 25.3
Oxygen --- --- 1.5 --- 6.7 13.1
Sulfur 0.1 0.2 0.3 --- 2.3 trace
Ash --- --- 0.2 --- 7.0 2.0
Water --- --- .7 --- 2.5 ---
Typical Ultimate Analysis of Common Fuels
Percent by Weight
Composition of (Dry) Air
By Volume 20.95% Oxygen, O2
79.05% Nitrogen, N2
By Weight 23.14% Oxygen 76.86% Nitrogen
Can be up to 9% H2O by volume in Summer
Traces of Argon and CO2
Neglecting H2O in Air Neglecting NOx, Other minor reactions Simplifying percentages:
4N2 + O2 + 2H2 2H2O + 4N2 + Heat
4N2 + O2 + C CO2 + 4N2 + Heat
4N2 + O2 + S SO2 + 4N2 + Heat
Common Combustion Reactions
For Methane
CH4 + 2O2 CO2 + 2H2O + Heat
16 + 64 44 + 36
Therefore:
#O2 Required = 64
# Fuel = 16
Therefore #O2/#Fuel =4/1 or 4
Common Combustion Reactions
Boiler Efficiency and Control
Boiler efficiency is computed “by losses” Understanding of efficiency calculations
helps in choosing the proper control strategy Energy “traps” such as economizers can
provide a payback Preferred Instruments has over 75 years of
combustion experience to help optimize boiler efficiency
Boiler Efficiency “by Losses”
Conservation of Energy Fuel energy in equals heat energy out Energy leaves in steam or in losses Efficiency = 100% minus all losses
Typical boiler efficiency is 80% to 85% The remaining 15% to 20% is lost Largest loss is a typical 15% “stack loss” Radiation loss may be 3% at full input Miscellaneous losses might be 1 to 2%
Boiler Energy Balance
Stack Losses
Latent heat of water vapor in stack Fixed amount depending on hydrogen in fuel About 5% of fuel input for fuel oil About 9% of fuel input for natural gas Assumes a non-condensing boiler (typical)
Sensible heat of stack gasses Typically around 10% of fuel input Increased mass flow and stack temperature
increase the loss
Radiation Loss
Generally a fixed BTU / hour heat loss As a percentage, is greater at low fire Depends on the boiler construction Is generally about a 3% loss at high fire Would be 12% loss at 25% of fuel input
Miscellaneous Losses
Consist of: blow down losses unburned fuel losses (carbon in ash or CO)
Generally on the order of one percent
Excess Air Required for Burners
Excess Air Required for BurnersBurner Fuel-Air Ratio
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100
Fuel %
Air
%
Air %
Oxygen %
Excess Versus Deficient Air
Effects of Stack Temperature
Generally, stack temperature is: Steam temperature plus 100 to 200 degrees F
» Rule of thumb – watertube-150, firetube-100F Higher for dirty boilers, higher loads and increased
excess air levels A 100 degree increase in stack temperature
Costs about 2.5% in energy losses May mean the boiler needs serious maintenance
Economizers are useful on medium and high pressure boilers as an energy “trap”
Efficiency Calculation Charts
Oxygen and Air Required for Gas
To release 1 million BTU with gas 42 lbs. of gas are burned 168 lbs. of oxygen are required no excess air 725 lbs. of combustion air 767 lbs. of stack gasses are produced
5% to 20% excess air is required by burner Each additional 10% increase in excess air:
Adds 73 lbs. of stack gasses Reduces efficiency by 1% to 1.5%
Cost of Inefficiency
The combined effects of extra excess air and the resulting increase in stack temperature: Could mean a 2% to 10% efficiency drop Reducing this “extra” excess air saves fuel Savings = (Fuel Cost)*[(1/old eff)-(1/new eff)]
For a facility with a 30,000 pph steam load 10% to 60% Extra Excess Air Represents From
$6,000 to $35,000 in potential savings per year Running 20 hours, 300 days, $4.65 per MM Btu
Combustion Control Objectives
Maintain proper fuel to air ratio at all times Too little air causes unburned fuel losses Too much air causes excessive stack losses Improper fuel air ratio can be DANGEROUS
Always keep fuel to air ratio SAFE Interface with burner management for:
Purge Low fire light off Modulate fuel and air when safe to do so
Related and Interactive Loops
Feedwater Flow feedwater is usually cooler than water in boiler adding large amounts of water cools the boiler cooling the boiler causes the firing rate to increase
Furnace Draft changing pressure in furnace changes air flow changed air flow upsets fuel to air ratio
Variations in Air Composition
“Standard” air has 0.0177 LB. O2 per FT3
Hot, humid air has less O2 per cubic ft 20% less at 95% RH, 120OF, and 29.9 mm Hg
Dry, cold air has more O2 per cubic ft 10% more at 0% RH, 32OF, and 30.5 mm Hg
Combustion controls must: Adapt to changing air composition (O2 trim), or Allow at least 20% extra excess air at “standard”
conditions
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…
Hysteresis Flow transmitter can not measure fuel Btu flow rate (Btu /
hr) Oxygen content per cubic foot of air changes with
humidity, temperature and pressure Fuel 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 ParallelPositioning TypeSystemsFully Metered Systems
Additional Errors Due To Jackshaft and ParallelPoitioning Control Method
Control System Errors
For example a 600 BHP boiler, delivering 20kpph of 15 psi saturated steam has thefollowing additional operating cost due to excess combustion air:
Excess Air Excess O2 Air Flow TheoreticalFuel flow
Lost BTU'sUp Stack
FuelEquivalent
to LostBTU's
Total FuelLost
AnnualizedAdditional 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 operatingat full load for 20hrs/day & 300days/year. Excess air also causes additional forced draftfan horsepower costs.
Combustion Control Strategies
Single Point Positioning (Jackshaft) Fuel and air are tied mechanically Simple, low cost, safe, requires extra excess air
Parallel Positioning Fuel valve and air damper are positioned separately Allows oxygen trim of air flow
Fully Metered Fuel and air FLOW (not valve position) are controlled
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
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
STEAM
Fue l A ctua tor
PCC - III
ACK
LOOP
AUTOMAN
AUTOMAN
DIS
REMLOC
ALARM
RUN
PV SP OUT
FIRING RATE
100FU E L V LV
FT
D rum P ressure
OIL
GAS
Jackshaft Strategy
AdvantagesSimplicity
Provides large turndown
Inexpensive
DisadvantagesFuel 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
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
Many of the same applications, limitations and improvements described in the Single Point Positioning section also apply to Parallel Positioning
Parallel Positioning Strategy
STEAM
Air Actuator
Fuel Actuator
PCC - III
ACK
LOOP
AUTOMAN
AUTOMAN
DIS
REMLOC
ALARM
RUN
PV SP OUT
AIR FLOW
100AIR DAMPER
PCC - III
ACK
LOOP
AUTOMAN
AUTOMAN
DIS
REMLOC
ALARM
RUN
PV SP OUT
FIRING RATE
100FUEL VLV
FT
Drum Pressure
OIL
GAS
AdvantagesAllows 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
DisadvantagesChanges 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
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.
Fully Metered Strategy
STEAM
A ir A ctua tor
Fue l A ctua tor
Fue l O il F low
Fue l G as F low
PCC - III
ACK
LOOP
AUTOMAN
AUTOMAN
DIS
REMLOC
ALARM
RUN
PV SP OUT
AIR FLOW
100A IR D A M P E R
PCC - III
ACK
LOOP
AUTOMAN
AUTOMAN
DIS
REMLOC
ALARM
RUN
PV SP OUT
FIRING RATE
100FU E L V LV
FT
FT
FT
FT
D rum P ressure
C om bustion A ir F low
OIL
GAS
AdvantagesCorrects 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
Comparison Jackshaft
Positioning Parallel
Positioning Fully Metered
Application Specifics
Dual Fuel Firing Low-fire changeover only Option Option Option
Full Load Simultaneous Firing
Not Recommended Not Recommended Option
Single/Multiple Burners Single Burner Option Option Option
Multiple Burners Not Recommended Not Recommended Option Furnace Conditions
Pressurized Option Option Option Balanced Draft
(FD & ID Fans are used) Not Recommended Not Recommended Option
Air Heater Type Ljungstrom (Rotary) Not Recommended Not Recommended Option
Tubular Option Option Option Stack Options
Independent Option Option Option Common & slight effect
on furnace pressure Option Option Option
Common & significant effect on furnace
pressure
Not Recommended Not Recommended Option
F.D. Fan Location Integral with windbox Option Option Option
Remote Not Recommended Option Option Air Composition
Constant Option Option Option Variable but slight Option Option Option
Variable & significant Not Recommended Not Recommended Option Fuel Composition
Clean Option Option Option Variations Not Recommended Not Recommended Option
Boiler Performance Monitoring
Fuel Consumption NO NO YES Efficiency by
“Losses” Method YES Option YES
Efficiency by Input - Output Method
NO NO Option
Other Control Loops that Impact
Control of Fuel and Draft Control Feedwater Control
Draft Control
Changing furnace draft can change air flow Changed air flow effects efficiency Changed air flow effects emissions Draft Control keeps furnace pressure constant Draft Control becomes extremely important:
When multiple boilers share a stack Stack is very high Induced FGR is used for NOx control
Draft Control Schematic
Types of Draft Control
Self contained units such as Preferred JC-20 “Sequencing” closes damper when boiler is off Saves energy Draft sensing diaphragm and logic in one unit
Micro-processor controllers for tighter control Feedforward based on firing rate True PID control of furnace draft
Feedwater Control
Benefits of stable water level control high and low water trips are avoided water carryover in steam is minimized steam pressure stays more nearly constant
Swinging feedwater flow can: cause pressure swings cause firing rate to hunt create extra wear and tear on valves and linkage waste fuel
Simple Feedwater Control Strategies
On-off control typically used on small firetube units
Single Element Feedwater Control opening of valve is influenced by change in level typical of older thermo-hydraulic systems thermo-hydraulic systems are proportional only use of PID controller can add “reset” suitable for steady loads
Shrink and Swell
Momentary drum level upsets in water tube boilers when the steam load swings
Increase in load causes swell: drops pressure in boiler increases size of steam bubbles in watertubes causes more water to flash to steam causes the actual level in the drum to rise while the
total amount of water actually drops single element will close the valve, not open it
Shrink and Swell, cont.
Drop in load causes: pressure to rise some steam to condense size of remaining bubbles to shrink water level in drum drops actual amount of water might be rising
Controls reduce impact of shrink and swell controls can’t compensate for poor design or
condition of boiler
Two Element Feedwater Control
Control on water level and steam flow drop in level increases valve opening rise in steam flow increases valve opening reduces impact of shrink and swell better for swinging loads
PID control with steam flow feed-forward which can be characterized to match the valve trim
Requires a steady feedwater supply pressure
Two Element Feedwater Control
Three Element Feedwater Control
Water level, steam flow and feedwater determine controller output signal
Two PID loops in cascade configuration: hold drum level at setpoint hold feedwater flow to match steam flow
Very stable level control Keeps water inventory constant during
periods of shrink and swell
Three Element Feedwater
Auxiliary Controller Functions
Calculation of pressure compensated steam flow
Compensation of drum level signal for changing water density in steam drum
Totalization of steam flow Totalization of feedwater flow Alarms for high and low water levels
Data Acquisition for Combustion
Allows remote operation of controllers Reduces manpower requirements in plant Provides historical data
Trend data to replace strip or circular charts Reports to document plant operation
Can compare energy usage per degree day From year to year From building to building Allows energy wasting trends to be spotted
New Advances in Combustion Control
These 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 Control
Oxygen Trim of air flow Applicable to any control strategy Should be applied to any large boiler Oxygen readout is valuable even if trim is
impractical Variable speed drive of combustion air fan
Can generate considerable horsepower savings Applicable to any control strategy
Economic Boiler Dispatch
Oxygen Trim Strategies
Mechanical trim devices for single point positioning Can vary the air damper position Can vary the fuel pressure
Biasing the air damper actuator position for parallel positioning control
Changing the fuel to air ratio in metering systems
Changing the fan speed in systems with VFD
Oxygen Trim for Jackshaft System
Oxygen Trim Cautions
Replace worn dampers and linkage FIRST! Use only proven analyzers for the signal Use only proven controllers and control
strategies to accomplish the trim Budget calibration and probe replacement.
Variable Speed Fan Drives
Applicable to parallel “positioning” or metering control strategies
Can generate considerable electricity savings For a 40,000 pph boiler running at 50% load: Savings could be up to $12,000 per year R.O.I. could be as low as 1.5 years
Might be a candidate for a utility company rebate
Summary
Combustion control is a specialty field Each application has unique requirements Each system should balance:
efficiency of operation installed cost safety and reliability
Preferred Instruments is leader in the field of special combustion control systems
Preferred Utilities Manufacturing Corp
For further information, contact...
Preferred Utilities Manufacturing Corporation31-35 South Street • Danbury • CT
T: (203) 743-6741 • F: (203) 798-7313www.preferred-mfg.com