What Are NOx and SOx and Why Are They Important?

The terms "NOx" and "SOx" are often seen in emissions measurement specifications, but many people are unsure of their accurate definitions. The terms come from the study of atmospheric chemistry, and each refers to a family of compounds.

Oxides of Nitrogen (NOx)
NOx refers to the total concentration of the most important oxides of nitrogen that are emitted by combustion sources: nitric oxide (NO) and nitrogen dioxide (NO₂). The principal sources of NOx in a combustion process are fuel NOx and thermal NOx. As you might guess from the name, fuel NOx results from the oxidation of nitrogen atoms bound in the fuel. Thermal NOx occurs when nitrogen molecules in the air react with oxygen in the high temperature of the combustion zone.

It is appropriate to treat the two species together because most of the NOx emitted by a typical combustion source such as a power plant is in the form of NO. However, this species oxidizes readily, so it is quickly converted to NO₂ once it is in the atmosphere. When the concentrations of the two species are expressed as parts-per-million (ppm), the NOx concentration is calculated as:

ppm NOx = ppm NO + ppm NO₂

There are two common sources of confusion when discussing NOx. These are the presence of other nitrogen oxides and the calculation of NOx concentration in mass units of mg/m3. Although their concentrations in flue gases are very small, other oxides of nitrogen are important in atmospheric chemistry. Therefore, the term NOy is used to describe all oxidized nitrogen species, including N₂O and N₂O₂, as well as NO and NO₂.




















Calculating the mass concentration of NOx is complicated because a molecule of NO₂ weighs more than a molecule of NO. Converting a concentration in ppm to a concentration in mg/m³ involves a factor that depends on the molecular weight of the gas in question.

mg/m³ NO = ppm NO x 1.34

mg/m³ NO₂ = ppm NO₂ x 2.05

Because all NOx ends up as NO₂ in the atmosphere, it is usual to apply the conversion factor for NO₂.

mg/m³ NOx = ppm NOx x 2.05

Because of this factor, we have the rather surprising fact that: 

mg/m³ NOx ≠ mg/m³ NO + mg/m³ NO₂

Oxides of Sulphur (SOx)
As with nitrogen, many oxides of sulphur are important in atmospheric chemistry, but only two are commonly present in flue gases. These are sulphur dioxide (SO₂) and sulphur trioxide (SO₃). The SO₂ results from the oxidation of sulphur in fuels such as coal and oil, which occurs in the combustion zone. A small fraction, typically around 1%, of the SO₂ is further oxidized to SO₃. By analogy with NOx, atmospheric chemists use the term SOx to refer to all oxides of sulphur in a gas mixture. 

There are many sensitive and reliable methods to measure SO₂ in flue gases, but SO₃ measurements are much more difficult. Given the low concentration of SO₃ in typical flue gases relative to SO₂, most emissions regulations do not prescribe an emission limit value. Therefore, there is little practical need to make a true measurement of SOx, and so most flue gas measuring equipment measures SO₂ alone.

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