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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 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
DCS-III Programmable Controller
Plant Wide Controller
PCC-III Multiple Loop Controller Draft Control
Operator Interface
JC-10D Process Bargraph Display
PCC-III Faceplate Display
SCADA/Flex Distributed Control Station
LCD Message Display
OIT10 Operator Interface Terminal
Sensors
HD-A1 Tank Gauge Leak Detector
Pressure Sensor
Outdoor Air Temperature Sensor
Tank Gauge Level Sensor
ZP Oxygen Probe
PCC-300 EPA Opacity Monitor
JC-30D Opacity 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 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.
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 Typical Ultimate Analysis of Common Fuels Percent by Weight #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
---
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
Common Combustion Reactions 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
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 Burners Burner Fuel-Air Ratio 100 Air %
90
Oxygen %
80 70
Air %
60 50 40 30 20 10 0 0
10
20
30
40
50 Fuel %
60
70
80
90
100
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
A 100 degree increase in stack temperature
Higher for dirty boilers, higher loads and increased excess air levels 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
Dry, cold air has more O2 per cubic ft
20% less at 95% RH, 120OF, and 29.9 mm Hg 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 25%
Typical Combustion Control System "Errors" (Expressed in % Excess Air Required) 20%
20%
15%
14% 14%
Jackshaft and Parallel Positioning Type Systems Fully Metered Systems
10%
5%
5%
5% 2%
0%
2%
3%
2% 2% 0%
Burner Requirments
Humidity
Draft Pressure
Fuel BTU/lb Changes
Air Temperature
2% 0%
Hysterisis
2%
2%
0%
0%
0%
Air Pressure
Fuel Pressure Changes
Fuel Temperature Changes
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
% 27%
% 6%
#/hr 20,300
#/HR 841
BTU 342,070
Fuel Equivalent to Lost BTU's #/hr 14.3
Total Fuel Lost
Annualized Additional Fuel cost
% 1.7%
US$ $ 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 Strategies
Single Point Positioning (Jackshaft)
Parallel Positioning
Fuel and air are tied mechanically Simple, low cost, safe, requires extra excess air 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
Jackshaft Strategy 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.
All control errors affect this system. Typically, 20 - 50 % 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.
Jackshaft Strategy D ru m P re s s u re
STEAM FT
100 FU EL V LV
ACK
D IS
ALARM RUN
PV
SP
AUTO M AN
AUTO MA N
LO O P
REM LO C
O U T
P C C - III F IR IN G R A T E
F u e l A c tu a to r O IL
G AS
Jackshaft Strategy 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
Parallel Positioning Strategy Separate actuators are used to position fuel and air final devices, flows are unknown. Fuel to air ratio can be varied automatically
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. Many of the same applications, limitations and improvements described in the Single Point Positioning section also apply to Parallel Positioning
Parallel Positioning Strategy Drum Pressure
STEAM
100
FT
100
FUEL VLV
AIR DAMPER
ACK
DIS
ACK
ALARM
ALARM
RUN
RUN
DIS
Air Actuator
PV
SP
OUT
AUTO MAN
AUTO MAN
AUTO MAN
AUTO MAN
LOOP
REM LOC
LOOP
REM LOC
PV
SP
OUT
PCC - III
PCC - III
FIRING RATE
AIR FLOW
Fuel Actuator OIL
GAS
Parallel Positioning Strategy Advantages
Disadvantages
Allows electronic characterization Changes in fuel or air pressure, temperature, of fuel-air ratio viscosity, density, humidity affect 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
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 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 F u e l G a s F lo w
D ru m P re s s u re
C o m b u s tio n A ir F lo w
F u e l O il F lo w
STEAM
100
100
FU EL VLV
A IR D A M P E R
ACK
D IS
ACK
A LARM
ALARM
RUN
RUN
FT
FT
D IS
A ir A c tu a to r
PV
FT
O IL FT
G AS
SP
AUTO M AN
AUTO M AN
AUTO M AN
AUTO M AN
LO O P
REM LO C
LO O P
REM LO C
O U T
PV
SP
O U T
P C C - III
P C C - III
F IR IN G R A T E
A IR F L O W
F u e l A c tu a to r
Fully Metered Strategy Advantages
Disadvantages
Corrects for control valve, damper drive and pressure regulator Hysteresis
Installation is more costly.
Compensates for flow variations.
With no oxygen trim….For all types of flow meters, the fuel Btu value and air oxygen content must be assumed.
Applicable to multiple burners. Allows simultaneous firing of oil and gas.
Jackshaft Positioning Application Specifics Dual Fuel Firing Low-fire changeover only Full Load Simultaneous Firing Single/Multiple Burners Single Burner Multiple Burners Furnace Conditions Pressurized Balanced Draft (FD & ID Fans are used) Air Heater Type Ljungstrom (Rotary) Tubular Stack Options Independent Common & slight effect on furnace pressure Common & significant effect on furnace pressure F.D. Fan Location Integral with windbox Remote Air Composition Constant Variable but slight Variable & significant Fuel Composition Clean Variations Boiler Performance Monitoring Fuel Consumption Efficiency by “Losses” Method Efficiency by Input - Output Method
Parallel Positioning
Fully Metered
Option Option Option Not Recommended Not Recommended Option Option Option Option Not Recommended Not Recommended Option Option Option Option Not Recommended Not Recommended Option Not Recommended Not Recommended Option Option Option Option Option Option
Option Option
Option Option
Not Recommended Not Recommended Option
Option Option Not Recommended Option
Option Option
Option Option Option Option Option Option Not Recommended Not Recommended Option Option Option Option Not Recommended Not Recommended Option NO YES
NO Option
YES YES
NO
NO
Option
Comparison
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.
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.
Saving Fuel with Combustion Control
Oxygen Trim of air flow
Variable speed drive of combustion air fan
Applicable to any control strategy Should be applied to any large boiler Oxygen readout is valuable even if trim is impractical 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 Corporation 31-35 South Street • Danbury • CT T: (203) 743-6741 • F: (203) 798-7313 www.preferred-mfg.com
