Conversion Factors

Correcting for Oxygen Concentration
In order to compare NOx concentrations on an equivalent basis, the field measured readings are corrected to a reference oxygen concentration. This reference concentration is typically 3% oxygen (dry volume basis) for boiler applications. If this correction is not applied, the NOx concentrations could simply be reduced by dilution with air although this would not reduce the quantity of emissions into the air.

The following is the equation for correcting for oxygen concentration:

PPM corrected = PPM measured x Multiplier

Multiplier = ( 20.9 – % OxygenReference) / (20.9 – % OxygenMeasured)

A table of multipliers for correcting to 3% Oxygen (dry volume basis) is summarized below:

%Oxygen
0
1
2
3
4
5
6
7
8
9
10
Multiplier
0.86
0.90
0.95
1.00
1.06
1.13
1.20
1.29
1.39
1.50
1.64

Unit Conversion Factors

NOX is measured and reported in several units including parts per million (volume basis) in exhaust gas, pounds per million BTU of fuel input, grams per Joule of fuel input. Some conversion factors are presented below for converting between units. The factors may vary slightly depending upon the assumed fuel composition. For regulatory compliance purposes, it is recommended that the reader consult with the documents of the applicable regulatory authority for the “official” factors for that jurisdiction.

From “Boiler Emissions Reference Guide” (by Cleaver-Brooks):

For Natural Gas:

NOx: ppm (3% reference) = pounds per million BTU x 850

For Light and Heavy Fuel Oil:

NOx: ppm (3% reference) = pounds per million BTU x 750

SOx: ppm (3% reference) = pounds per million BTU x 540

From “National Emission Guideline for Commercial/Industrial Boilers and Heaters – N306” (by Canadian Council of Ministers of the Environment):

For Natural Gas:

NOx:

Grams NOx (as NO2) per Gigajoule (g/GJ) x 1.907
= ppm (volume, dry at 25°C) NOx (as NO2) at 3% Oxygen

For Light and Heavy Fuel Oil:

NOx:

Grams NOx (as NO2) per Gigajoule (g/GJ) x 1.808
= ppm (volume, dry at 25°C) NOx (as NO2) at 3% Oxygen

General Conversion Factors:

Mass per fuel input:

Grams per Gigajoule x 0.00233 = pounds per million BTU

Mass:

Pounds x 0.454 = kilograms

Energy:

Gigajoule per hour x 0.9483 = million BTU per hour