What is

Combustion?
Combustion occurs when fossil fuels, such as
natural gas, fuel oil, coal or gasoline, react with
oxygen in the air to produce heat. The heat from
burning fossil fuels is used for industrial processes,
environmental heating or to expand gases in a
cylinder and push a piston. Boilers, furnaces and
engines are important users of fossil fuels.
• Fossil fuels are hydrocarbons, meaning they are composed primarily
of carbon and hydrogen. When fossil fuels are burned, carbon dioxide
(CO2) and water (H2O) are the principle chemical products, formed
from the reactants carbon and hydrogen in the fuel and oxygen (O2)
in the air.

• The simplest example of hydrocarbon fuel combustion is the reaction

of methane (CH4), the largest component of natural gas, with O2 in
the air. When this reaction is balanced, or stoichiometric, each
molecule of methane reacts with two molecules of O2 producing one
molecule of CO2 and two molecules of H2O. When this occurs, energy
is released as heat.
The combining of oxygen in the air and carbon in the fuel to form carbon dioxide and generate heat is a complex
process, requiring the right mixing turbulence, sufficient activation temperature and enough time for the
reactants to come into contact and combine. Unless combustion is properly controlled, high concentrations of
undesirable products can form. Carbon monoxide (CO) and soot, for example, result from poor fuel and air
mixing or too little air. Other undesirable products, such as nitrogen oxides (NO, NO2), form in excessive
amounts when the burner flame temperature is too high. If a fuel contains sulfur, sulfur dioxide (SO2) gas is
formed. For solid fuels such as coal and wood, ash forms from incombustible materials in the fuel.
 Combustion Analysis
• Combustion analysis is part of a process intended to improve fuel economy,
reduce undesirable exhaust emissions and improve the safety of fuel burning
equipment. Combustion analysis begins with the measurement of flue gas
concentrations and gas temperature, and may include the measurement of draft
pressure and soot level.

• To measure gas concentration, a probe is inserted into the exhaust flue and a gas
sample drawn out. Exhaust gas temperature is measured using a thermocouple
positioned to measure the highest exhaust gas temperature. Soot is measured
from a gas sample drawn off the exhaust flue. Draft is the differential pressure
between the inside and outside of the exhaust flue.
• Once these measurements are made, the data is interpreted using calculated
combustion parameters such as combustion efficiency and excess air. A more in
depth analysis will examine the concentration of the undesirable products
described earlier.
 Why perform
Combustion Analysis?
Improve Fuel Efficiency
The largest sources of boiler heat losses are shown Figure 2. Heat energy leaving
the system exhaust flue (or stack) is often the largest single source of lost fuel
energy and is made up of the Dry Gas loss and Latent Heat Loss. Although some
flue loss is unavoidable, an equipment tune-up using combustion analysis data
can often significantly reduce this source of heat loss and save fuel costs by
improving fuel efficiency. Table 1 gives examples of yearly cost savings that can
be realized by improving equipment efficiency by five percent.
Reduce Emissions
Carbon monoxide, sulfur dioxide, nitrogen oxides and particles are undesirable
emissions associated with burning fossil fuels. These compounds are toxic,
contribute to acid rain and smog and can ultimately cause respiratory
problems. Federal and state laws govern the permissible emission rates for
these pollutants under the guidance of the Clean Air Act and oversight of the
federal Environmental Protection Agency (EPA). State and local environmental
agencies also exert authority in regulating the emissions of these pollutants.
Combustion analysis is performed to monitor toxic and acid rain forming
emissions in order to meet these federal, state and local regulations.
Improve Safety
Good equipment maintenance practice, which includes combustion analysis, enables
the boiler technician to fully verify and maintain the equipment operating specifications
for safe and efficient operation. Many boiler manufacturers suggest that flue gas
analysis be performed at least monthly. Boiler adjustments that affect combustion will
tend to drift with time. Wind conditions and seasonal changes in temperature and
barometric pressure can cause the excess air in a system to fluctuate several percent. A
reduction in excess air can cause, in turn, a rapid increase of highly toxic carbon
monoxide and explosive gases, resulting in rapid deterioration in system safety and
efficiency. Low draft pressures in the flue can further result in these combustion gases
building up in the combustion chamber or being vented indoors. Excessive draft
pressures in the flue also can cause turbulence in the system. This can prevent
complete combustion and pull explosive gases into the flue or cause flame
impingement and damage in the combustion chamber and to the heat exchanger
material
What’s Measured?
Combustion analysis involves the measurement of gas concentrations,
temperatures and pressure for boiler tune-ups, emissions checks and safety
improvements. Parameters that are commonly examined include:

• Oxygen (O2)
• Carbon Monoxide (CO)
• Carbon Dioxide (CO2)
• Exhaust gas temperature
• Supplied combustion air temperature
• Draft • Nitric Oxide (NO)
• Nitrogen Dioxide (NO2)
• Sulfur Dioxide (SO2)
 Oxygen, Carbon Monoxide and Carbon Dioxide
As described earlier, simple combustion involves the reaction of oxygen in the air
with carbon and hydrogen in the fuel, to form carbon dioxide and water and
produce heat. Under ideal conditions, the only gases in the exhaust flue are CO2,
water vapor and nitrogen from the combustion air. When O2 appears in the flue
exhaust, it usually means that more air (20.9 percent of which is O2) was supplied
than was needed for complete combustion to occur. Some O2 is left over. In other
words, the measurement of O2 gas in the flue indicates that extra combustion air,
or Excess Air, was supplied to the combustion reaction.

This is demonstrated in Figure 3 where the bar on the right represents the exhaust
gas composition.
When too little air is supplied to the burner, there is not enough oxygen to completely form
CO2 with all the carbon in the fuel. Instead, some oxygen combines with carbon to form
carbon monoxide (CO). CO is a highly toxic gas associated with incomplete combustion and
efforts must be made to minimize its formation. This effort goes hand-in-hand with improving
fuel efficiency and reducing soot generation. This formation of CO gas is illustrated in Figure 4.
As a rule, the most efficient and cost-effective use of fuel takes place when the
CO2 concentration in the exhaust is maximized. Theoretically, this occurs when
there is just enough O2 in the supplied air to react with all the carbon in the
fuel supplied. This quantity of supplied air is often referred to as the theoretical
air.
The theoretical air required for the combustion reaction depends on fuel
composition and the rate at which the fuel is used (e.g. pounds per hour, cubic
feet per minute, etc.). In real-world combustion, factors such as the condition of
the burner and the burner design also influence the amount of air that is needed.
The theoretical air is rarely enough.

The general relationship between the O2 supplied and the concentration of CO2
and CO in the exhaust is illustrated in Figure 5. As the air level is increased and
approaches 100% of the theoretical air, the concentration of CO molecules
decreases rapidly as they pick up additional oxygen atoms and form CO2. Still
more combustion air and CO2 reaches a maximum value. After that, air begins to
dilute the exhaust gases, causing the CO2 concentration to drop. The maximum
value of CO2 is dependent on the type of fuel used.
Temperature and Draft
Exhaust Gas Temperature and Supplied Combustion Air Temperature

Heat leaving the exhaust flue with the hot gases is not transferred to do useful work, such as
producing steam. This heat loss becomes a major cause of lower fuel efficiency. Because the
heat content of the exhaust gas is proportional to its temperature, the fuel efficiency drops as
the temperature increases. An example of efficiency loss due to the increase in stack gas
temperature is shown in Figure 6.

When determining heat loss from the flue, the temperature of the supply air is subtracted
from the flue gas temperature. This gives the net temperature and accounts for the heat
supplied to the system by the supply air.

Some heat loss is unavoidable. The temperature in the flue needs to remain high enough to
avoid condensation inside the stack. One process for recovering some of the heat loss in the
exhaust is to use the hot flue gases to preheat the supply air before it is introduced into the
combustion chamber.
Draft
Draft refers to the flow of gases through the heat generating equipment, beginning with
the introduction of air at the back of the burner. Once combustion occurs, the heated gas
leaves the combustion chamber, passes heat exchangers and exits the exhaust stack.
Depending upon the design of the equipment, draft is natural, meaning combustion air is
pulled in by buoyant heated gases venting up the stack, or it is mechanical, meaning air is
pushed or pulled through the system by a fan. Often, draft relies on a combination of
both natural and mechanical means.

Adequate draft is typically verified by measuring the pressure in the exhaust stack. The
manufacturer of the fuel burning equipment provides specifications for the required draft
pressure and locations for making the draft measurement. Measurement is important
since environmental influences such as changes in barometric pressure and ambient
temperature can influence the flow. Typical draft pressures are in the range of –0.5 to 0.5
inches of water column.
Excessive draft can prevent heat transfer to the system and increase the flue
temperature if the excess air percentage is not elevated. If the excess air increases
from the high draft, the flue temperature will decrease. Low draft pressures can
cause temperatures in the flue to decrease, allowing water vapor to condense in
the flue, forming acid and damaging the system.
 Nitrogen Oxides (NOx)

Nitrogen oxides, principally nitric oxide (NO) and nitrogen dioxide (NO2), are
pollutant gases that contribute to the formation of acid rain, ozone and smog.
Nitrogen oxides result when oxygen combines with nitrogen in the air or in the fuel.
NO is generated first at high flame temperatures, then oxidizes further to form NO2
at cooler temperatures in the stack or after being exhausted. The NO
concentration is often measured alone, and the NO2 concentration is generally
assumed to comprise an additional five percent of the total nitrogen oxides. The
nitrogen oxide gas concentrations are sometimes combined and referred to as the
NOX concentration.
 Sulfur Dioxide (SO2)

Sulfur dioxide combines with water vapor in the exhaust to form a sulfuric acid
mist. Airborne sulfuric acid is a pollutant in fog, smog, acid rain and snow, ending
up in the soil and ground water. Sulfur dioxide itself is corrosive and harmful to the
environment. Sulfur dioxide occurs when the fuel contains sulfur and where the
emission levels are directly related to the amount of sulfur in the fuel. The most
cost-effective way to reduce sulfur emissions is to select a low-sulfur or de-sulfured
fuel.
 Hydrocarbons (HCs)/Volatile Organic Compounds
(VOCs)

Organic compounds are sometimes present in the combustion exhaust products

because of incomplete combustion. Hydrocarbons (HCs), or volatile organic
compounds (VOCs), are best reduced through proper burner maintenance and by
maintaining the proper air/fuel mixture.
 Soot
Soot is the black smoke commonly seen in the exhaust of diesel trucks, and is
present whenever fuel oils or solid fuels are burned. Excessive soot is undesirable
because it indicates poor combustion and is responsible for coating internal heat
transfer surfaces, preventing good thermal conductivity. Over time, serious damage
to the heat exchanger can occur. Soot is primarily unburned carbon, and is formed
for the same reasons CO is formed—insufficient combustion air, poor mixing and
low flame temperature. As with CO, it is usually impossible or impractical to
entirely eliminate soot formation for some fuel types.
Measurement Tools
 Manual Gas Measurements
The Orsat analyzer is a gas concentration analysis tool typically used to manually
sample CO2, O2 and CO from the flue of a combustion system. The Orsat analyzer
determines the gas concentrations from a sample of gas extracted from the flue
and bubbled through solutions of reagents that selectively absorb each gas. By
measuring the decrease in gas volume over the liquid reagents, the amount of gas
absorbed is indicated. From this information, stack gas concentration is calculated.
Manual gas measurements are time consuming and do not accurately reflect real-
time adjustments made to a system.
 Portable Electronic Instruments
In recent years, electronic instruments such as the CA-CALC™ Combustion Analyzer
from TSI Incorporated (see Figure 7) have been developed to analyze combustion
routinely for tune-ups, maintenance and emissions monitoring. These instruments
are extractive. They remove a sample from the stack or flue with a vacuum pump
and then analyze the sample using electrochemical gas sensors. Thermocouples are
used for stack and combustion air temperature measurements, and a pressure
transducer is used for the draft measurement. An on-board computer performs the
common combustion calculations, eliminating the need to use tables or perform
tedious calculations. Electronic instruments show the results of boiler adjustments
in real-time and give more accurate information to help ensure that a system has
been tuned properly.
 Continuous Emission Monitors
Continuous emission monitors, or CEMS, are a class of electronic instruments
designed to measure exhaust stack gases and temperature continuously. CEMs are
sometimes used for combustion control, but typically are used for monitoring
pollutant gas emissions as required by government regulations. CEMs can use both
extractive and in-situ (sensors in the stack) sampling methods, and employ a variety
of electronic sensor technologies for gas detection. CEMs are used most often on
larger installations or when required by regulatory agencies.
 Using The
Measurements
Once flue gas and temperature measurements are made, combustion
parameters are calculated to help evaluate the operating
performance of the furnace or boiler. Typical combustion parameters
include:

• Excess Air
• Carbon Dioxide
• Combustion Efficiency
• O2 Reference
• Emissions Conversions
 Excess Air
Insufficient combustion air causes a reduction in fuel efficiency, creates highly toxic
carbon monoxide gas and produces soot. To ensure there is enough oxygen to
completely react with the fuel, extra combustion air is usually supplied. This extra
air, called “Excess Air,” is expressed as the percent air above the amount
theoretically needed for complete combustion. In real-world combustion, the excess
air required for gaseous fuels is typically about 15 percent. Significantly more may
be needed for liquid and solid fuels.

A good estimate of excess air can be determined using the following formula. This
calculation uses the oxygen concentration measured in the exhaust. If the CO
concentration is very high, it may also be included in the excess air calculation. This
is shown in Appendix C, “Calculations.”
Although required, higher excess air comes with a price—it wastes fuel. There are a
number of reasons why this occurs but, stated simply, supply air cools the
combustion system by absorbing heat and transporting it out the exhaust flue. The
more air, the more the cooling. Consider, too, that nitrogen, which makes up about
eighty percent of the air, plays no role chemically to produce heat. It does,
however, add significantly to the weight of gas that absorbs heat energy. Figure 8
illustrates how increasing excess air reduces combustion efficiency.
Using too much excess air is one of the principal causes of poor fuel economy. For
this reason, optimizing excess air usage can be one of the simplest ways to achieve
significant fuel savings.

NOTE: Excess Air and Fuel/Air Mixing

Adding additional excess air is often done to reduce the CO concentration. Too much
excess air can actually have the reverse effect of increasing CO. This results when
fuel and air no longer mix properly in the burner, reducing the time of contact
between oxygen and fuel and inhibiting a complete reaction.
 Calculating the Carbon Dioxide Concentration
Carbon dioxide (CO2) forms when carbon in the fuel combines with O2 in the
combustion air. When there is just enough O2 supplied to react with the carbon in
the fuel, the CO2 concentration in the stack exhaust is at its highest level. This is
generally at or close to the ideal operating condition for the heat generating
equipment. This was shown in Figure 5.

The maximum possible CO2 exhaust concentration depends ultimately on the

carbon content of the fuel being burned. This number, the CO2 maximum, appears
often in combustion calculations, such as the one below for determining the
percent of CO2 in the exhaust. Maximum CO2 concentrations for a variety of fuels
are presented in Appendix D, “Fuel Specifications.”
NOTE: Using Measured O2 to Determine CO2
Using the O2 concentration to determine the concentration of CO2 has advantages
compared to measuring CO2 directly. As indicated in Figure 5, the same CO2
concentration is possible when there is too little air supplied (less than 100%
theoretical air) or too much air (greater than 100% theoretical air). When CO2 is
measured alone, it is not possible to tell if the mix of exhaust gases is represented
by those to the left of the CO2 peak or those to the right of the CO2 peak. When to
the left of the peak, high levels of toxic CO are present, resulting in a potentially
dangerous operating condition. By detecting sufficient O2 in the exhaust, the
combustion reaction stays on the right side of the CO2 peak, minimizing the
formation of CO.
 Determining Combustion Efficiency
Combustion efficiency is a measure of how effectively energy from the fuel is
converted into useful energy (e.g. to create steam). Combustion efficiency is
determined by subtracting the heat content of the exhaust gases, expressed as a
percentage of the fuel’s heating value, from the total fuel-heat potential, or 100%,
as shown in the formula below.
Stack heat losses are calculated using gas concentration and temperature
measurements from combustion analysis, and using the fuel’s specifications for
chemical composition and heat content. These fuel specifications are unique
properties of the fuel, determined from chemical analysis by the fuel supplier.

Stack heat losses are primarily from the heated dry exhaust gases (CO2, N2, O2)
and from water vapor formed from the reaction of hydrogen in the fuel with O2 in
the air (refer to Figure 2). When water goes through a phase change from liquid to
vapor, it absorbs a tremendous amount of heat energy in the process. This “heat of
vaporization,” or latent heat, is usually not recovered. The white cloud seen exiting
a stack on a cold day is mostly condensing water vapor giving up its latent heat to
the atmosphere.
Modern portable analyzers, like TSI’s CA-CALC Combustion Analyzers, automatically
perform combustion efficiency calculations, typically with better accuracy than
manual calculations or charts. Table 2 below shows the combustion efficiency for
fuel oil under various conditions.
NOTE: Other Definitions of Efficiency

It is important to recognize that other definitions of efficiency are often used to

describe furnace or boiler performance in addition to combustion efficiency.
Thermal efficiency or boiler efficiency are examples. These may include or exclude
sources of heat loss as part of their calculation. Combustion efficiency, for example,
does not include losses from radiation or steam leaks. Sometimes the latent heat of
water formation is not included. There are also fundamental differences between
calculations performed in the U.S. and those performed in some European
countries. Appendix B, “Fuel Specifications,” touches on this briefly in the
discussion of heating values used in the efficiency calculation. When comparing the
performance of equipment from different manufacturers, it is important to know
how an efficiency number is calculated.
NOTE: Why use Combustion Efficiency?

Combustion efficiency, determined from combustion analysis, is a cost-effective

way to improve equipment operation and reduce fuel expense. The stack losses
used in combustion efficiency calculations are simple to determine using
combustion analysis. Other losses, such as those from steam leaks, radiation or
boiler blow-down, are much more difficult to assess. Stack losses are typically the
largest source of energy waste. If the equipment is properly maintained, losses
such as those from steam leaks are minimal. Convection and radiation losses are
often small also, and usually unavoidable.
 The O2 Reference
As discussed earlier, excess air is supplied to the combustion process to ensure that there
is enough oxygen to completely react with the fuel. Excess air is measured in the flue as a
percentage of O2. This excess air dilutes the concentration of other gases measured.

Local regulatory agencies have guidelines for monitoring NO, NO2, CO, and SO2 gases.
Generally, it is required that the concentration of these gases be corrected for the diluting
effects of excess air. The amount of excess air is determined from the O2 concentration
measured in the flue. The measured O2 concentration, together with the O2 reference
value is used in the equation below to obtain the corrected gas concentration. O2
reference values of 3 and 6 percent are often used, giving a corrected gas concentration
equivalent to that at oxygen concentrations of 3 or 6 percent. When an O2 reference of
zero is used, the gas concentration is referred to as undiluted or air free.
To obtain the O2 referenced concentration of gasses in the flue, the following
equation is used:
 Emission Conversions
A measure of the toxic gas concentration in parts per million (PPM) or percent does
not indicate the actual weight of pollutant entering the atmosphere. The EPA
requires the conversion of pollutant concentrations to pounds per million Btu of
fuel consumed (lb/MBtu). This is done so the weight of pollutants can be readily
determined from the pollutant concentration and the rate of fuel usage. EPA
Method 19 has equations for performing the conversions and presents fuel-specific
conversion factors for use in performing the calculations.
 Making the
Measurements
 Taking Gas Samples
Extracting flue gases to measure their concentration is done using a stainless
sampling probe inserted through a small hole in the exhaust flue. Probe placement
is important, and several factors must be considered when choosing a sampling
location.

To get the most accurate measurement, the gas sampling probe must be placed
prior to any draft damper or diverter, so that the gases are not diluted, and as close
to the equipment breach as possible so the gases have not cooled in the flue. If
there is a stack economizer or similar device, the measurement should be taken
just downstream of the installed device.
Figures 9 and 10 show two examples of recommended insertion points for gas
sampling probes.

NOTE: Special Care for Emission Gas Measurements

For emission gas measurements, consideration must be given to the fact that as
water vapor cools and condenses in the stack, highly soluble NO2 and SO2 gases
may be scrubbed out by dissolving in the water droplets. Sampling must be done
before condensation begins.
Condensation is a problem in the gas sample line as well as in the flue, causing a
loss of NO2 and SO2. Measurement errors due to the loss of SO2 and NO2 are
lessened by reducing the time water and gases are in contact, and by reducing the
contact surface area between the water and gases. This is achieved by quickly
removing moisture from the gas sample. If necessary, the use of an ice bath or
Peltier cooler, followed by immediate collection of the condensed water in a
reservoir away from the gas stream, is beneficial. In addition, when sampling SO2
and NO2, use of a non-reactive, non-wetting material such as Teflon for the sample
tubing is essential. Wetting of the interior surfaces, associated with other tubing
materials, significantly increases the contact surface area between gas and water,
resulting in gas loss from absorption. When a SO2 or NO2 gas sample must be
transported more than a few feet before the water is removed, the sample lines
are usually heated (250–300°F) to keep water in a vapor state.
 Making Temperature and Draft Measurements
As described earlier, measurements of the stack gas temperature and the
combustion air temperature are required to establish the heat loss from the
exhaust gases and determine combustion efficiency. Since stack temperatures can
exceed 1,000 degrees Fahrenheit, a bimetallic thermocouple probe is typically used
to measure the stack temperature. This thermocouple probe is placed at the point
of highest exhaust gas temperature at the base of the flue and toward the center
for small ducts. If the stack gas temperature is underestimated, the operating
efficiency will be overstated. When an economizer or air heater is used, stack
temperature is measured after these devices. Figure 10 shows locations for
measuring both stack and combustion air temperatures. Combustion air
temperature is measured outside the equipment in these examples
Draft is a measurement to ensure the combustion gases are being properly
exhausted. Draft is measured using a manometer or electronic pressure transducer.
The equipment manufacturer can provide the recommended draft pressure for
specific equipment and where to take the measurement. Draft is usually measured
in the same location as the stack temperature relative to the ambient space. When
a draft diverter or draft hood is in the stack, a second measurement should be
taken downstream of the device.
 Soot Measurements
Soot is most commonly measured during equipment tune-up and maintenance by
extracting a sample of the exhaust gases using a manual sampling pump. The
sample is taken from the same location as the stack temperature measurements
shown in Figure 10. A specific number of strokes on the pump (recommended by
the pump manufacturer) is required to draw a known gas volume through a glass
fiber filter. Once complete, the filter color is compared to a scale displaying shades
of gray corresponding to known soot concentrations.
Sample Boiler
Tune-Up Procedure
NOTE: These abbreviated procedures are presented here for
illustration purposes only and are not meant to substitute for the
actual procedures. When performing tuning or other equipment
maintenance, always follow the equipment manufacturer’s
recommendations first. Maintenance, including tune-ups, must
always be performed by a qualified technician or engineer.
 Tune-up Procedure—Using an Electronic Combustion
Analyzer
1. Insert the combustion analyzer gas sampling probe into the flue. The probe
should be as close to the equipment breach as possible, upstream of any
diverter or draft damper and downstream of the last heat exchanger or heat
recovery coil.
2. If ambient air is used as the combustion air, an additional temperature probe is
not required. An ambient temperature probe is located inside the combustion
analyzer.
3. If the combustion air is ducted directly to the combustion chamber or
preheated, place a combustion supply air temperature probe into the ductwork
just prior to the combustion chamber inlet.
4. Without the gas sampling probe connected to the instrument, turn on the
combustion analyzer. Follow the steps on the analyzer to determine if a baseline
calibration is required to zero out any sensors that may have drifted too far off
zero. If you are using TSI’s CA-CALC Combustion Analyzer, it will give you an
alarm message when a baseline calibration is recommended. Caution— baseline
calibration requires ambient air to be fresh air.
5. After you have passed or completed a required baseline calibration and there
were no errors, the gas sampling probe can be connected to the instrument.
6. Be sure the gas sampling pump is on and all temperature probes are attached.
The electronic analyzer is now ready for use to tune the system to optimize
combustion efficiency and to monitor any emission gases for which the
appropriate sensor is installed in the combustion analyzer.
7. Combustion analysis should be performed on a warmed up unit at firing rates typical
of normal operation. For systems with high, low and additional firing rates,
adjustments should be made at all firing rates whenever possible.
8. Place the combustion analyzer in a location where the screen is easy to view, free
from the modulation controller or linkages to be adjusted.
9. Put the burner control system into manual mode and test several firing rates for
combustion efficiency and emission gas concentrations.
a. Observe the values on the combustion analyzer. If the percentage of O2 in the
stack is at the lower end of the expected minimum values (as specified by equipment
manufacturer) and the CO emissions are low with no smoke being generated, the
burner is probably tuned at or near optimum efficiency at this firing rate.
b. Observe the values on the combustion analyzer. If the combustion equipment has
been tuned previously, compare the previous combustion efficiency to the
combustion efficiency of the current measurement. A significant difference indicates
burner adjustments or repair might be appropriate to improve combustion
efficiency.
10. If tuning the equipment is required, adjust the mechanical linkages to decrease
the excess air until CO is about 400 PPM or to the maximum allowable
concentration stated in the local code. If a bad flame develops or significant
smoking occurs prior to reaching 400 PPM CO, stop making adjustments at this
point. This will verify that too much excess air is not lowering efficiency.
11. Observe the O2 level on the combustion analyzer at these settings. Add oxygen
sufficient to satisfy local codes to the observed oxygen level and reset the
burner to operate automatically at this higher stack gas oxygen level.
12. Compare the measured value of oxygen at this burner setting to the minimum
value of excess oxygen recommended by the local authority. If the minimum
value measured is significantly higher than the minimum value recommended
by the local authority, then the burner may need additional maintenance or
replacement.
13. When an adjustment has been completed, verify the new adjustment has not
had an adverse effect on the other firing rates that have already been adjusted.
If it has, settings should optimize conditions at the predominant firing rate and
the combustion controls need to be readjusted at the affected firing rates.
14. After adjustments have been completed at all firing rates, the controls should
be modulated or staged through all combustion firing rates to ensure proper
operation. The control settings at firing rates most typical of operation should be
recorded. The measurements by the combustion analyzer should be data logged
into memory for future reference and for generating printouts and reports.

Additional adjustments may be needed to balance the emissions with the

combustion efficiency.