flue gas loses.pdf

7/28/2019 flue gas loses.pdf

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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.


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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


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Flue gas temperature The two basic causes of high flue gas

temperature:

Insufficient heat transfer surfaces

Fouling of heat transfer surfaces


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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.


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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.


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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.


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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


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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.


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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


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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?


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Correct Excess Air

Good flame lengthMaximum flame temperature

Burner with correct excess air.


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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.


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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.


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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.


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Burner with too much

excess air.

Short flame

Cooler temperature

Wasted heat

Increased NOx


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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.


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Burner with insufficientexcess air.

Long flame

Cooler temperature

No NOx

Very inefficient


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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.


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WHAT IS THE CORRECT O2 SETPOINT?

The Importance of Oxygen and Combustibles


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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.




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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.




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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.




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How much excess air is provided?

Excess Air vs. Oxygen Content in Flue Gas


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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


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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


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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.


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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.


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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.


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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.


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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.


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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


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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.


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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


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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)


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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 %


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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


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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


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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


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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.


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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


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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.


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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.


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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.


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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


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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).


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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


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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


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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


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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


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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.



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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.



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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


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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.


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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.


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Hays Cleveland O2 Trim System.

Air Temp and Pressure


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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.


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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


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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


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Flue Gas Analysis

Flue Gas Analysis


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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.


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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


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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)


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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.


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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


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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


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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.


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Impact on Efficiency


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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.


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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.



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Electronic instruments are programmed to

calculate and display efficiency directly.



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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.



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(Therefore the LMTDis given by(t1 tG )/ln {(t 1 - tg)/( t G - tg) , and then substituting inthe equation above we have:



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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).



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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.


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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


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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


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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


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./ combustibles analyzer2coupled O-Close


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Convection loop of WDG-IVCM O2, combustibles and methane analyzer.

Using Thermox Oxygen And ppm CombustiblesAnalyzers For Efficiency And Nox Reduction: Portable


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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.



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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.



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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.


V Fi d (D V W O2)


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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


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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.


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Flue Gas Analysis


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Burner Performance


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PPM ofcombustibles in

flue gas

Loss due tocombustibles

Loss due toexcess air

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