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Oxides of Nitrogen (NOx)

The primary nitrogen pollutants produced by combustion are nitric oxide (NO) and nitrogen dioxide (NO2) and are generally referred to collectively as NOx.  Increasing evidence suggests that NOx has a direct negative effect on the human respiratory system and when exhausted into the atmosphere, reacts with moisture to produce ozone and acid rain. 

For years it has been commonly accepted that NO constituted about 95% of NOx with NO2 making up the other 5%.  More recent studies have found this may not be the case and, as such, some jurisdictions have begun to require separate measurements of NO and NO2.  Before investing in an instrument, contact your local authorities to determine which method of sampling is required.  Instruments which measure both NO and NO2 are considerably more expensive. 

Instruments which measure NOx generally read in Parts Per Million (PPM).  Because the excess air level in the flue gases dilutes the NOx percentage, many authorities of jurisdiction have chosen a standardized flue gas oxygen reading to which NOx readings are corrected.  For most space or process heating boilers, that level is 3%. 

To standardize readings from a flue gas sample use the formula:  

NOx ppm corrected to 3% O2 = Actual NOx ppm Reading X 17.9

                                                                                            (20.9 - Actual O2 Reading) 

Some emissions standards require levels in pounds of NOx per million Btu’s fired (Lbs. NOx/MBtu) or other units of measurement. 

POLLUTANT CONVERSIONS

To convert from PPM to any of the units below:

multiply PPM by the number in the correct column and row

Fuel

Pollutant

LB/MBTU

MG/NM3

MG/KG

G/GJ

Nat Gas

NOx

0.00129

2.053

20.788

0.556

Oil (#2, #6)

NOx

0.00134

2.053

24.850

0.582

Definitions (all numbers apply to values as corrected to 3% excess Oxygen)

 LB/MBTU   pounds of pollutants per Million BTU (British Thermal Unit)

MG/NM3      mlligrams of pollutant per Normal cubic meter of gas supplied

                     Normal means at standard temperature and pressure

MG/KG       milligrams of pollutant per Kilogram of fuel burned

G/GJ            grams of pollutant per Giga Joule (109 Joule)


NOx emissions are formed in one of three ways:

                   Thermal NOx is produced when nitrogen and oxygen in the combustion air supply combine at high flame temperatures.  Thermal NOX is generally produced during the combustion of both gases and fuel oils.

                   Fuel NOx is produced when nitrogen in the fuel combines with the excess oxygen in the combustion air and is only a problem with fuel oils containing fuel bound nitrogen.

                  Prompt NOx is formed during the early, low temperature states of combustion and is insignificant.

NOx control technologies vary widely depending on the required emissions standards in different jurisdictions and dictate the most cost effective strategy available for NOx reduction.

                    Reducing the amount of O2 available to bind with nitrogen during the combustion process is probably the least expensive strategy to implement.  This entails the use of a combustion analyzer to adjust the fuel/air mixture such that the amount of O2 as measured in the flue gas sample is minimized (and still within the manufacturers’ specifications).  Tuning up the boiler in this manner can potentially reduce the NOx production by as much as 10%.

                    Burning low nitrogen fuel oils which contain approximately 18% less nitrogen can reduce NOx emissions by as much as 50%.

                    Injecting water or steam into the flame, reduces flame temperature and thus lowers overall NOx production by as much as 80% for gas.  However, this technique can result in lowering boiler efficiency by as much as 10% depending on the amount of steam or water injected.  Increasing the amount of moisture in the flue gases may also lead to condensation and consequent damage to boiler and flue passageways.

                    Flue gas recirculation (FGR) is one of the more commonly used methods to reduce NOx emissions and involves pulling relatively cool combustion gases from the vent system and mixing it with combustion air.  Flue gases are composed of inert gases such as water vapor, carbon dioxide and nitrogen which take heat away from the combustion process and lower flame temperatures.

Determining the correct amount of recirculated flue gases requires that a combustion test be performed on the boiler breach which measures both flue gas and recirculated flue gas O2.  Then a sample is extracted from the point at which recirculated flue gases mix with the incoming combustion air (often referred to as the ‘windbox’) and an O2 level at that point recorded.  A chart available from the burner manufacturer is then used to calculate the percentage of recirculated flue gases.

A sample of a manufacturer’s chart for determining the percentage of Flue Gas Recirculation:

O2

Reading

in Flue

6%

FGR

   7%

    FGR

8%

FGR

9%

FGR

10%

FGR

11%

FGR

12%

FGR

13%

FGR

O2 Reading in Windbox

2.4

19.9

19.7

19.5

19.4

19.2

19.1

18.9

18.8

2.6

19.9

19.7

19.5

19.4

19.2

19.1

18.9

18.8

2.7

19.9

19.7

19.6

19.4

19.2

19.1

19.0

18.8

2.9

19.9

19.7

19.6

19.4

19.3

19.1

19.0

18.8

3.0

19.9

19.7

19.6

19.4

19.3

19.1

19.0

18.8

3.2

19.9

19.7

19.6

19.4

19.3

19.1

19.0

18.9

3.3

19.9

19.8

19.6

19.4

19.3

19.2

19.0

18.9

3.5

19.9

19.8

19.6

19.5

19.3

19.2

19.0

18.9

Flue gas recirculation is capable of reducing NOx emissions by as much as 75%.