flue gas loses.pdf
TRANSCRIPT
-
7/28/2019 flue gas loses.pdf
1/91
-
7/28/2019 flue gas loses.pdf
2/91
2008Prof. Sorour
Flue gas losses
This chapter is devoted to study thereasons and methods of reducing theheat losses that are carried by the flue
gases for different reasons and those dueto combustion and gas side factors ingeneral.
-
7/28/2019 flue gas loses.pdf
3/91
2008Prof. Sorour
Flue gas losses
These losses are:
1- Heat carried away in chimney gases
a- High excess air
b- High flue gas temperature
2- Loss due to incomplete combustion,insufficient air supply
3- Loss due to moisture in the air4- Too high rate of combustion
-
7/28/2019 flue gas loses.pdf
4/91
2008Prof. Sorour
Flue gas temperature The two basic causes of high flue gas
temperature:
Insufficient heat transfer surfaces
Fouling of heat transfer surfaces
-
7/28/2019 flue gas loses.pdf
5/91
2008Prof. Sorour
Flue gas temperature
Good practice dictates that stack temperaturebe kept as low as possible without causingcold end corrosion.
Temperature higher than those required by theboiler manufacture, are caused by:
Excessive draft
dirty, carbon- covered heating surfaces Poor design of H.E. surfaces and lack of sufficient
baffling.
Undersized furnace.
Incorrect or defective combustion.
Over firing of boiler or furnace.
Improper adjustment of draft regulator.
-
7/28/2019 flue gas loses.pdf
6/91
2008Prof. Sorour
Flue gas temperature
Solutions
Repair the broken or corroded baffle thatcould be bypassing the hot gas to the stack.
Adjust draft.
Decrease soot deposits on heating surfacesby improving fuel combustion efficiency.
Note: the net stack temperature is obtained by
subtracting the ambient temperature from theflue-gas temperature.
-
7/28/2019 flue gas loses.pdf
7/91
-
7/28/2019 flue gas loses.pdf
8/91
2008Prof. Sorour
Smoke in flue gas
Causes:
Excessive air leaks. Improper preheat Improper fuel atomization.
Worn, clogged, or incorrect nozzle Improperly, adjusted oil pressure to
nozzle
Leaking cut-off valve, allowing after-drip of fuel oil.
-
7/28/2019 flue gas loses.pdf
9/91
2008Prof. Sorour
Smoke in flue gas Smoke Test: A total of 110.39 cubic inch of smoke- laden flue
products are drawn through a 0.049 inch squarearea of a standard guide filter paper.
The color of the resultant smoke stain on the
filter paper is matched to the closest color spoton the standard graduated smoke scale.
The results are interrupted according to 1 - 9color scale the low number mean little or nosmoke. Using this scale, the following smokecolors are acceptable for fuel oil
Fuel oilNo 2: 3 or less
Fuel oil No6: 4 or less
-
7/28/2019 flue gas loses.pdf
10/91
2008Prof. Sorour
Smoke in flue gas
Solutions
Balancing the CO2 with more air, until thesmoke is 4 or less.
Controlling fuel-oil temperature so that therecommended viscosity will be reached.
Treating fuel oil properly to keep the burnerclean and improve atomization.
Determining any mechanical problemsthrough further testing.
-
7/28/2019 flue gas loses.pdf
11/91
2008Prof. Sorour
Firing ControlsFiring Controls
Three major parameters that need
to be controlled and monitored are:
1.1. Fuel gas/Fuel oil pressureFuel gas/Fuel oil pressure2.2. Excess airExcess air
3.3. Draft in the furnaceDraft in the furnace
-
7/28/2019 flue gas loses.pdf
12/91
2008Prof. Sorour
Excess Air Control
Excess air control essentially involvesExcess air control essentially involvesanswering three basic questionsanswering three basic questions
1.1. How much excess air is provided?How much excess air is provided?
2. How much excess air should be provided?
3. How efficient is the burning equipment?
-
7/28/2019 flue gas loses.pdf
13/91
2008Prof. Sorour
Correct Excess Air
Good flame lengthMaximum flame temperature
Burner with correct excess air.
-
7/28/2019 flue gas loses.pdf
14/91
2008Prof. Sorour
A visual on the flame is used to adjust the
flame color and flame height based on thefuel pressure.
Once the flame is set correctly, the damper isadjusted for the correct draft.
Burner with correct excess air.
-
7/28/2019 flue gas loses.pdf
15/91
2008Prof. Sorour
Finally, the secondary air supply is adjustedto give the desired oxygen reading or O2setpoint.
When set correctly, and with good air-fuel
mixing, the burner will produce the maximumflame temperature in a compact flame.
Burner with correct
excess air.
-
7/28/2019 flue gas loses.pdf
16/91
2008Prof. Sorour
The less secondary air that is needed, thebetter the efficiency.
At optimum efficiency, the flue gas willcontain a minimum of oxygen together withlevels of combustibles (CO and H2) in the100 to 200 ppm range and a minimum of
NOx.
Burner with correctexcess air.
-
7/28/2019 flue gas loses.pdf
17/91
2008Prof. Sorour
Burner with too much
excess air.
Short flame
Cooler temperature
Wasted heat
Increased NOx
-
7/28/2019 flue gas loses.pdf
18/91
2008Prof. Sorour
NOx production increases
with increasing excess air Too much excess air reduces flame
temperature and drops efficiency. In most companies this is the biggest source
of heater inefficiency and NOx production.
-
7/28/2019 flue gas loses.pdf
19/91
2008Prof. Sorour
Burner with insufficientexcess air.
Long flame
Cooler temperature
No NOx
Very inefficient
-
7/28/2019 flue gas loses.pdf
20/91
2008Prof. Sorour
The large amount of CO and H2 formed as a result
of the incomplete combustion makes the burnerextremely inefficient
This reduces the flame temperature and might
encourage the operator to increase fuel flowthus making matters worse.
This condition may not be noticed because leakage
in the convection section can hide insufficientair getting to the burner.
Burner with insufficient excess air.
-
7/28/2019 flue gas loses.pdf
21/91
2008Prof. Sorour
WHAT IS THE CORRECT O2 SETPOINT?
The Importance of Oxygen and Combustibles
-
7/28/2019 flue gas loses.pdf
22/91
2008Prof. Sorour
Reducing the excess air or oxygen to theminimum safe level is the most important stepin reducing energy consumption.
There is no single O2 level that is right for all
burners. The optimum oxygen depends on theload, the burner design, the type of fuel, andthe burner performance.
Reducing oxygen while measuring the ppmcombustibles allows the correct operating pointto be determined.
WHAT IS THE CORRECT O2 SETPOINT?
The Importance of Oxygen and Combustibles
-
7/28/2019 flue gas loses.pdf
23/91
2008Prof. Sorour
Without combustibles, it is not possible tofind the optimum set point, since you cannotknow when to stop.
With the combustibles detector, the oxygencan be reduced safely until the combustiblesstarts to increase.
WHAT IS THE CORRECT O2 SETPOINT?
The Importance of Oxygen and Combustibles
-
7/28/2019 flue gas loses.pdf
24/91
2008Prof. Sorour
Flue gas analysis provides the answer tothe first question.
The oxygen concentration in the flue gasprovides an indication of the excess air
supplied to the combustion process.
WHAT IS THE CORRECT O2 SETPOINT?
The Importance of Oxygen and Combustibles
-
7/28/2019 flue gas loses.pdf
25/91
2008Prof. Sorour
How much excess air is provided?
Excess Air vs. Oxygen Content in Flue Gas
-
7/28/2019 flue gas loses.pdf
26/91
2008Prof. Sorour
Excess Air Control
Optimum excess air is the minimum
excess air because minimizes the heat loss to the flue gases,
minimizes the cooling effect on the flame, improves the heat transfer
-
7/28/2019 flue gas loses.pdf
27/91
2008Prof. Sorour
With less than the minimum excess air, the
unburned fuel will start appearing in the fluegas due to insufficient air.
Minimum excess air should be specified bythe burner vendor and should be verified
during burner testing.
Excess Air Control
-
7/28/2019 flue gas loses.pdf
28/91
2008Prof. Sorour
BURNERS
The performance of a burner directlyaffects combustion efficiency because ofthe excess air required to obtain complete
combustion at the burner. A poorly adjusted burner, or one incapable
of efficiently mixing fuel and air at all load
ranges, will increase excess air and wastefuel.
-
7/28/2019 flue gas loses.pdf
29/91
2008Prof. Sorour
BURNERS
Provided that the burners are clean and wellmaintained, the air to fuel ratio controls onmodern combustion systems should be able
to maintain the recommended excess airthrough much of the turndown ratio of theburner, although the excess air will increase
at low turndown ratios.
-
7/28/2019 flue gas loses.pdf
30/91
2008Prof. Sorour
BURNERS
Experience, however, shows that many burnersare incorrectly adjusted.
Changes that take place due to wear on cams,linkages, pins, etc, often results in a change in
air /fuel ratio and consequent loss in efficiency.
It is also difficult to properly set the combustionair damper position for manually controlledburners in the absence of instruments formeasuring flue gas composition.
-
7/28/2019 flue gas loses.pdf
31/91
2008Prof. Sorour
BURNERS
For example a scored or scratched burner tip on apressure jet oil burner can lead to incorrect atomizationof the fuel: some droplets may be small, some large.
This results in inadequate mixing of fuel and air, andexcessive formation of carbon monoxide. It is thuspossible to have high oxygen levels in the flue gas (ie
high excess air) at the same time as having high carbonmonoxide.
A similar effect can be caused by a burner tip which hasbeen over-enthusiastic cleaning using abrasive toolsof the small hole or jet nozzle in the centre, whosedimensions are critical for the proper atomization of agiven fuel oil.
-
7/28/2019 flue gas loses.pdf
32/91
2008Prof. Sorour
BURNERS It is therefore important to understand that attaining
the optimum excess air rate may be prevented by theuse of damaged or incorrectly adjusted burner parts.
Whenever high oxygen levels are found inconjunction with high combustibles (ie carbonmonoxide, or in extreme case smoke) the
mechanical integrity of the burner and air distributionsystem is suspect and should be checked.
-
7/28/2019 flue gas loses.pdf
33/91
2008Prof. Sorour
BURNERS An efficient natural gas burner requires only 2% to 3%
excess oxygen, or 10% to 15% excess air in the fluegas, to burn fuel without forming excessive carbonmonoxide.
Recently most new good gas burners exhibit turndown
ratios of 10:1 or 12:1 with little or no loss incombustion efficiency.
A higher turndown ratio reduces burner starts,
provides better load control, saves wear and tear onburner, reduce refractory wear, reduces purge airrequirement, and provides fuel savings
-
7/28/2019 flue gas loses.pdf
34/91
2008Prof. Sorour
BURNERS Dont confuse turndown to fully modulating
burners.
Having a fully modulating burner with only atypical turndown of 4 will not benefit andwill cost much more.
-
7/28/2019 flue gas loses.pdf
35/91
2008Prof. Sorour
Effect of Turndown on fuel cost
This figure show how the turndown ratio
impacts the fuel cost for a small 100Hp boiler.When you combine the effect of low excess airand high turndown, the operating cost savingsrange from 10% to 15%
E l
-
7/28/2019 flue gas loses.pdf
36/91
2008Prof. Sorour
Example
Consider a 50,000 PPH (22.7ton/hr22.7ton/hr) processboiler with a combustion efficiency of 79%79% (E1).
The boiler annually consumes 500,000 million500,000 millionBtuBtu (MMBtu) of natural gas. At a price of$8.00/MMBtu$8.00/MMBtu, the annual fuel cost is $4 million$4 million.
What are the savings from an energy efficient
burner that improves combustion efficiency by 1%,1%,2%, 3%?2%, 3%?
Cost savingsCost savings
= Fuel consumption x Fuel Price x (1= Fuel consumption x Fuel Price x (1-- E1/E2)E1/E2)
-
7/28/2019 flue gas loses.pdf
37/91
2008Prof. Sorour
Energy Savings due to the installation of
an energy Efficient burner
146,32018,2903
89,76012,3452
50,0006,2501
Annual Dollarsavings
$
Annual Energysavings,
MMBtu/hr
Burner combustionefficiency
improvement %
-
7/28/2019 flue gas loses.pdf
38/91
2008Prof. Sorour
If the installed cost is $ 75,000 for a new
burner that provides an efficiencyimprovement of 2%,
Simple payback=Simple payback= $ 75,000$ 75,000/ $/ $98,76098,760
== 0.76 year0.76 year
Energy Savings due to the installation of
an energy Efficient burner
S t d ti
-
7/28/2019 flue gas loses.pdf
39/91
2008Prof. Sorour
Suggested action:
Perform burner maintenance and tune yourburner.
Conduct combustion efficiency test in full and inpart load.
If excess oxygen exceed 3% or combustion
efficiencies values are low, considermodernizing the fuel/ air control to include solidstate sensors and controls without linkage.
Also consider installing improved processcontrols, an oxygen trim system, or a newenergy- efficient burner
-
7/28/2019 flue gas loses.pdf
40/91
2008Prof. Sorour
Suggested action:
A new energy efficient burner should be
considered if repair costs becomeexcessive, reliability becomes an issue,and energy savings are guaranteed.
Install a smaller burner on a boiler that isoversized relative to its steam load
-
7/28/2019 flue gas loses.pdf
41/91
2008Prof. Sorour
Oxygen Control O2 Trim
When boiler burners are manually tunedon a periodic basis, they are typicallyadjusted to about 3% excess oxygen
which is about 15% excess air.
This is because there are many ambient
and atmospheric conditions that can affectoxygen/air supply.
-
7/28/2019 flue gas loses.pdf
42/91
2008Prof. Sorour
Oxygen Control O2 Trim
For example, colder air is denser and contains
more oxygen than warm air; wind speedaffects every chimney/flue/stack differently;and barometric pressure further affects draft.
Therefore, an excess oxygen/air setting at thetime of tuning assumes there will still be
enough oxygen available for completecombustion when conditions worsen
-
7/28/2019 flue gas loses.pdf
43/91
2008Prof. Sorour
Oxygen Control O2 Trim
-
7/28/2019 flue gas loses.pdf
44/91
2008Prof. Sorour
Oxygen Control O2 Trim
From an efficiency standpoint, the excessO2 means there is more air in the
combustion stream than there needs to be.
That air also contains moisture and it all is
heated and then lost up the stack.
The amount of excess O2 is about directly
proportional to the efficiency lost; that is,3% excess O2 means 3% efficiency drop.
-
7/28/2019 flue gas loses.pdf
45/91
2008Prof. Sorour
Oxygen Control O2 Trim Although it may be possible to monitor and adjust
the burner on a daily basis, it is not practical. Automatic O2 systems continuously monitor the
flue gases and adjust the burner air supply, theyare generically called O2 Trim Systems.
An electronic sensor is inserted into the boilerflue, ahead of any dampers or other sources ofair leakage into the boiler or flue.
The sensor is connected to a control panel that
measures oxygen and sends a signal to a controldamper on the burner air supply.
-
7/28/2019 flue gas loses.pdf
46/91
2008Prof. Sorour
Oxygen Control O2 TrimThere are other advantages of the installationof an O2 Trim package in addition to fuel
savings.
Combustion efficiency computation per fuel to alert theowner when service is required on the burner.
Flue gas temperature monitoring and alarms, alerts theowner when the boiler tubes are fouled (A 22Ctemperature rise above design results in a 1% fuelincrease.) and shut down due to high flue gas
temperature. O2 monitoring and alarm due to low excess air or
combustibles.
-
7/28/2019 flue gas loses.pdf
47/91
2008Prof. Sorour
Oxygen Control O2 Trim
There are two types of approaches for O2 trim.
1- Single point (jackshaft) positioning with atrim actuator.
2- Parallel positioning (metering), separateactuators for the fuel valve(s) and FD damper.
O C t l
-
7/28/2019 flue gas loses.pdf
48/91
2008Prof. Sorour
Oxygen Control
The most common method today is parallelpositioning. The components include:
1-Controller:
2- Pressure or temperature sensor,
3 - O2 analyzer
4- Fuel valve actuator(s) (servomotors).
5- Air damper actuator (servomotor).
-
7/28/2019 flue gas loses.pdf
49/91
2008Prof. Sorour
Oxygen Control The Controller accepts inputs from the fuel
and air actuators, O2 analyzer, flue gastemperature sensor and a header pressure ortemperature sensor.
The controller will interface with the burner
management system for purge, low fire, fuelselect and other functions.
Excess Air Effect on Efficiency
-
7/28/2019 flue gas loses.pdf
50/91
2008Prof. Sorour
Excess Air Effect on Efficiency
72.01%6.5
74.03%6.0
75.32%5.5
77.31%5.0
78.60%4.579.72%4.0
80.71%3.5
81.68%3.0
82.37%2.5
83.08%2.0
Natural GasExcess Oxygen %
E ti ti S i f O T i
-
7/28/2019 flue gas loses.pdf
51/91
2008Prof. Sorour
Estimating Savings from O2 Trim
Fuel Savings = 1.0 - (Starting Efficiency / Ending
Efficiency)
Example:
4.5% Excess Oxygen reduced to 2.0%Fuel saving =1.0 - (0.7972 / 0.8308)
= 0.04044 = 4.04%
E ti ti S i f O T i
-
7/28/2019 flue gas loses.pdf
52/91
2008Prof. Sorour
Estimating Savings from O2 Trim
NOTE
Because some boilers operate with a veryhigh percentage of excess oxygen, it iscommon for the first year savings to besubstantially higher than this.
Much of that savings can be attributed to amore reasonable manual tuning of the boiler,and not necessarily from the installation ofan automatic O2 control system.
Well-tuned boilers can expect savings of 2 -4%
Estimating Payback from the Installation
-
7/28/2019 flue gas loses.pdf
53/91
2008Prof. Sorour
Estimating Payback from the Installation
of O2 Trim
The equipment costs for O2
Trimvaries only a little with boiler size.
Investment Costs will vary mainly due
to the torque requirements for theservomotors and types of O2
analyzers. Installation costs are highly variable.
Estimating Payback from the Installation
-
7/28/2019 flue gas loses.pdf
54/91
2008Prof. Sorour
Estimating Payback from the Installation
of O2 Trim
For O2 trim systems, 100 to 600 HP boilers, the
investment costs are: Equipment: Controller in cabinet, pressure
sensor, O2 analyzer, flue gas temperature
sensor, servomotors (actuators) for fuel valves(2) and FD damper.
Equipment cost range is $ 10,000 to $ 11,000.
Installation: $ 5,000 to 7,000. Start-up & Training Services: $ 2,500 to $ 4,000.
Estimating Payback from the Installation
-
7/28/2019 flue gas loses.pdf
55/91
2008Prof. Sorour
Estimating Payback from the Installation
of O2 Trim
For O2 Trim Systems, 600 HP to 100,000 lbsper hour boilers (45 ton/hr), the investment
costs are: Equipment: Controller in cabinet, pressure
transmitter, O2 analyzer, flue gas temperaturesensor, actuators for fuel valves (2) and FDdamper. Equipment cost range is $ 11,000 to$ 17,000.
Installation costs of $ 7,000 to $ 12,000.
Start-up & Training Services: $ 2,500 to $4,000.
Example
-
7/28/2019 flue gas loses.pdf
56/91
2008Prof. Sorour
Example
Assume a 500 hp boiler with an input of20,000,000 BTUs, operating 8,000 hours
per year with a 50% load factor could save avery conservative 2% with the installation ofan O2 Trim system:
20 MMBTUs x 8,000 hrs x 50% x 2% =2,560 MMBTUs or 2,560 MCF Natural Gas
per Year.
-
7/28/2019 flue gas loses.pdf
57/91
2008Prof. Sorour
Example: If Natural Gas cost is $7.00 per MCF, 2,560 x
$7 = $17,920 per Year Savings.
Payback would be in the 1 - 2 Year Range,depending on Installation Costs.
Note that if savings were 4%, the paybackcould drop to less than a one year payback.
-
7/28/2019 flue gas loses.pdf
58/91
2008Prof. Sorour
Hays Cleveland O2 Trim System.
Air Temp and Pressure
-
7/28/2019 flue gas loses.pdf
59/91
2008Prof. Sorour
Air Temp and Pressure
Boiler air/fuel adjustment is critical to
efficient boiler operations.
Many boilers are manually adjusted peryear and then left to operate through awide variation in temperature and
barometric weather conditions.
-
7/28/2019 flue gas loses.pdf
60/91
2008Prof. Sorour
Air Temp and Pressure The amount of oxygen in the air is directly
related to air density and temperature. Conventional boiler controls are set
according to air volume.
New boiler controls that adjust air volumeaccording to a continuous oxygen monitoring
of the flue gas is one way to maintain correctair/fuel ratios for peak combustion efficiency.
Effects of Air Properties on Oxygen Content
-
7/28/2019 flue gas loses.pdf
61/91
34.5%31 in.
19.%30 in.
15% - Initial Setting29 in.
11%28 in.
7.0%27 in.
1.1%120 F- 49 C9.6%100 F- 37.5 C15.0% - Initial Setting80 F -26.5 C20.2%
60 F -15.5 CExcess Air PercentageBarometric PressureAir Temperature
-
7/28/2019 flue gas loses.pdf
62/91
Flue Gas Analysis
Flue Gas Analysis
-
7/28/2019 flue gas loses.pdf
63/91
2008Prof. Sorour
Flue Gas Analysis
Flue gas analysis is used both for efficiency andemissions purposes.
Due to tightening environmental regulations,monitoring may be mandatory.
The instruments used for analysis can beinexpensive small hand-held devices thatproduce reasonable accuracy, to larger
permanently installed units that are capable ofproducing lab quality results on a continuousbasis.
-
7/28/2019 flue gas loses.pdf
64/91
2008Prof. Sorour
Flue Gas AnalysisThe type of flue gas analysis performed and the
equipment required will be determined by: Is the primary need for efficiency, environmental
regulation or both
Will analysis be spot-checked on a periodicbasis or is continuous monitoring required
What gases/emissions must be monitored
What accuracy is required
Flue Gas Analysis
-
7/28/2019 flue gas loses.pdf
65/91
2008Prof. Sorour
ue Gas a ys s
The components that are of interestbecause they affect efficiency are primarily
oxygen (O) and carbon (C).
The components that are of interest from an
environmental perspective are nitrous-oxides (NOx), carbon-dioxide (CO2),carbon-monoxide (CO) and sulphur-dioxide
(SO2)
-
7/28/2019 flue gas loses.pdf
66/91
2008Prof. Sorour
Flue Gas Analysis Flue Gas Analysis is performed by inserting a probe
into the flue of boiler between the last heat
exchanger and draft diverter.
This is known as 'in-situ' testing.
It is also necessary to take a combustion airtemperature measurement or ambient temperature ifthat is the source of combustion air.
Depending on the gas being measured, most probestoday are either infrared or some sort ofelectrochemical.
-
7/28/2019 flue gas loses.pdf
67/91
2008Prof. Sorour
Flue Gas Analysis Not all analyzers use 'in-situ' probes.
Some units pump flue gases through atube to the instrument.
This is more typical of continuousmonitoring equipment and for very largesystems where it is not easy to reach a
spot in the flue to insert a probe or locate aportable meter.
Flue Gas Analysis
-
7/28/2019 flue gas loses.pdf
68/91
2008Prof. Sorour
y
The tube may need to be heated in order to keepthe product to be measured from condensing
out, such as NO2, SO2, and HCL.
In other cases, the flue gases must be cooledand dried to prevent moisture damage to the
probe.
Instruments use a device called a 'Peltier
Cooler' which is an electrochemical device thatproduces a cool surface that condenses anymoisture out of the flue gas before it reachesthe measuring sensor.
Flue Gas Analysis
-
7/28/2019 flue gas loses.pdf
69/91
2008Prof. Sorour
y
Measurement values are either provided in
part per million (ppm) or percentage (%)depending on the size of the reading.
Larger numbers, such as oxygen andcarbon-dioxide are generally provided inpercentage and small numbers, such as
NOx and carbon-monoxide are provided inppm.
-
7/28/2019 flue gas loses.pdf
70/91
2008Prof. Sorour
Impact on Efficiency
-
7/28/2019 flue gas loses.pdf
71/91
2008Prof. Sorour
Impact on Efficiency The most common measurement for the
purposes of efficiency is oxygen (O) orcarbon-dioxide (CO2) and temperature.
For a given fuel type, it is possible tocalculate the percent O if CO2 is
measured, or to calculate CO2 if O ismeasured.
-
7/28/2019 flue gas loses.pdf
72/91
2008Prof. Sorour
Impact on Efficiency Another common indicator is CO; if O is
high, there won't be any significant CO.
Measurable CO means the unit is starved forO or there is something very wrong with theflame.
The O and/or CO2 measurement along withthe temperature of the flue gas and the
combustion air temperature are the neededvariables to determine combustionefficiency.
Impact on Efficiency
-
7/28/2019 flue gas loses.pdf
73/91
2008Prof. Sorour
Electronic instruments are programmed to
calculate and display efficiency directly.
Impact on Efficiency
-
7/28/2019 flue gas loses.pdf
74/91
2008Prof. Sorour
Impact on Efficiency
-
7/28/2019 flue gas loses.pdf
75/91
2008Prof. Sorour
For the convective heat transfer:
(where t1, is the temperature of the gases leaving the radiant section and
entering the tubes). The thermal resistances of the metal tube wall and thewater film on the outside surface of the tube are negligible and hence wecan assume that the inside surface of the tube is approximately equal tothe saturation temperature of the steam, ts,.
The increase in excess air ratio will increase theconvective heat transfer but the reduced flame
temperature will reduce the radiant heat transfer.
Impact on Efficiency
-
7/28/2019 flue gas loses.pdf
76/91
2008Prof. Sorour
(Therefore the LMTDis given by(t1 tG )/ln {(t 1 - tg)/( t G - tg) , and then substituting inthe equation above we have:
Impact on Efficiency
-
7/28/2019 flue gas loses.pdf
77/91
2008Prof. Sorour
The heat transfer by convection is controlled bythe heat transfer coefficient for the inside of the
tube which varies with the mass flow rate to thepower 0.8. Hence for the case where only themass flow rate changes (i.e. neglecting changesin properties due to the change in thetemperatures ) we have
(where k is a constant).
Impact on Efficiency
-
7/28/2019 flue gas loses.pdf
78/91
2008Prof. Sorour
For the case of an increase in excess air the
temperature t1 will be reduced. However it canbe seen that as the mass flow of gasesincreases k/mG
o.2 which offsets the decrease in
t1 and the resultant is an increase in flue gastemperature.
-
7/28/2019 flue gas loses.pdf
79/91
2008Prof. Sorour
Impact on Efficiency As the mass flow is increased through the
outlet is increased. The mass amounts isanalogous to the amount of excess airused by the gas burner.
Emission's Impact on the Environment
-
7/28/2019 flue gas loses.pdf
80/91
2008Prof. Sorour
p
The emission of primary concern from thecombustion of natural gas is NOx.
Typical uncontrolled NOx from natural gasboilers are 70 to well over 100 ppm.
Regulations in the world have tightenedsubstantially over recent years, requiringsome boilers to operate at less than 9 ppm.
Several other have requirements for under 50ppm for larger boilers.
Methane Detection During Start-up
-
7/28/2019 flue gas loses.pdf
81/91
2008Prof. Sorour
As an additional precaution during the purge-down and light-off cycle, the methane detectorwill detect any natural gas or other fuels which
have leaked into the firebox and could causean explosion. If the boiler is started up only rarely, the
methane detector would have limited use.
When the boiler is started-up frequently, thenthe methane measurement gives additionalpeace of mind.
The methane part is used only during thepurge-down/light-off cycle with natural gas-fired boilers.
Methane Detection During Start-up
-
7/28/2019 flue gas loses.pdf
82/91
2008Prof. Sorour
./ combustibles analyzer2coupled O-Close
-
7/28/2019 flue gas loses.pdf
83/91
2008Prof. Sorour
Convection loop of WDG-IVCM O2, combustibles and methane analyzer.
Using Thermox Oxygen And ppm CombustiblesAnalyzers For Efficiency And Nox Reduction: Portable
-
7/28/2019 flue gas loses.pdf
84/91
2008Prof. Sorour
Versus Fixed (Dry Versus Wet O2)
Do not try to use the oxygen value
determined from a portable analyzer asthe set point for the fixed oxygen analyzer.
The portable, typically fuel cell orparamagnetic based, measures on a dry
basis since the water must be removedbefore it hits the cell.
Using Thermox Oxygen And ppm CombustiblesAnalyzers For Efficiency And Nox Reduction: Portable
-
7/28/2019 flue gas loses.pdf
85/91
2008Prof. Sorour
Versus Fixed (Dry Versus Wet O2)
The fixed analyzer, normally zirconium
oxide based, measures the flue gas as is,including the water and thus measures ona wet basis.
The dry is always higher than the wet andit can be a significant difference.
Using Thermox Oxygen And ppm CombustiblesAnalyzers For Efficiency And Nox Reduction: Portable
-
7/28/2019 flue gas loses.pdf
86/91
2008Prof. Sorour
Versus Fixed (Dry Versus Wet O2)
For example if you have 20% moisture inFor example if you have 20% moisture in
the flue gas and 3% Othe flue gas and 3% O22 , the fixed, the fixedanalyzer will read 3%, whereas theanalyzer will read 3%, whereas theportable will read 3.75%.portable will read 3.75%.
Neither is right or wrong.Neither is right or wrong. They are just different ways of looking atThey are just different ways of looking at
the same thing and in fact this principle isthe same thing and in fact this principle is
used to measure flue gas moisture.used to measure flue gas moisture.
Using Thermox Oxygen And ppm CombustiblesAnalyzers For Efficiency And Nox Reduction: Portable
V Fi d (D V W O2)
-
7/28/2019 flue gas loses.pdf
87/91
2008Prof. Sorour
Versus Fixed (Dry Versus Wet O2)
If you use a portable to determine theIf you use a portable to determine the
optimum Ooptimum O22 setpointsetpoint, and then set the O, and then set the O22control at this level, the boiler will run at toocontrol at this level, the boiler will run at toohigh an Ohigh an O22 level.level.
ThermoxThermox oxygen analyzers are designed foroxygen analyzers are designed fordirect installation at the high temperaturedirect installation at the high temperatureradiant section, typically 1500radiant section, typically 1500 -- 20002000F (815F (815 --
10931093C)C)
Flue Gas Analysis Table
-
7/28/2019 flue gas loses.pdf
88/91
2008Prof. Sorour
Using a Flue Gas Analyzer or any meterdesigned to measure oxygen or carbon-monoxide, and taking the flue gas temperatureand the temperature of the combustion air, thefollowing Table can be used to determine
combustion efficiency when operating onnatural gas.
The Temperature Column is the NET
Difference between Flue Gas and CombustionAir Temperatures.
-
7/28/2019 flue gas loses.pdf
89/91
Flue Gas Analysis
-
7/28/2019 flue gas loses.pdf
90/91
2008Prof. Sorour
Burner Performance
-
7/28/2019 flue gas loses.pdf
91/91
2008Prof. Sorour
PPM ofcombustibles in
flue gas
Loss due tocombustibles
Loss due toexcess air