Emissions: Measuring NOx

Measuring NO_x

The traditional way to find the value of NO_x is to take a sample of the gases exiting the stack and measure the level of NO. It is then necessary to multiply this value by 1.1 to account for the additional NO₂, as NO_x is the amount of NO added to the amount of NO₂. This method of measurement is acceptable depending upon the process for EPA reporting, but it is not the most accurate way to measure the quantity as the ratio of NO to NO₂ may vary. Measuring both NO and NO₂ (True NO_x) is a much more accurate way to determine NO_x than measuring NO only.

O₂ Reference

The O₂ reference is a standard that has been set to help monitor NO_x emissions. This standard calculates NO_x emissions based on a set oxygen level, to standardize the monitoring and reporting of total amounts of NO_x emitted. The O₂ reference is effective in removing any attempts of diluting emissions, which can make NO_x levels appear lower than they actually are.

Low NO_x

Low NO_x is typically defined as any value of NO_x that is under 500 ppm. The most common values are between 9 and 300 ppm. It is most imperative to measure True NO_x (NO+NO₂) when dealing with these low values to reduce the error since a few ppm error accounts for a significant amount of the Total NO_x.

True NO_x

True NO_x is a method of measuring the value of NO_x emissions without using any factors or presumptions. The concentrations of NO and NO₂ are measured separately with high accuracy sensors and the values are then added together to find the True “Total” NO_x value. It is especially important to measure True NO_x when dealing with low NO_x emissions to reduce error.

True NO_x vs. NO_xreturn

Measuring True NO_xreturn

There are specific instruments that quantify the level of NO and NO₂ in a sample. Before being able to measure NO_x, it is important to locate a sample that is representative of the emissions being released into the atmosphere. Therefore, it is important to take the sample from the right location and to be sure that there are no gas leaks.

Problems With Measuring NO₂return

It is very important that when taking NO₂ measurements all precautions are taken to ensure that the integrity of the data is kept. This can be achieved by making sure that condensation does not build up, as NO₂ is very water-soluble. If there is condensation, as much as 50% of the NO₂ can dissolve out of the gas phase into the condensate, affecting the readings greatly.

Best Method for Measuring NO₂return

Given the solubility of the NO₂ gas, it is imperative that the integrity of the gas sample is maintained and all the water is eliminated under a controlled environment minimizing any contact with the sample gas; this can be achieved by using a sampling system that includes the following components:

Heated Probe

A heated probe allows for flue gas sampling without condensation, which maximizes the accuracy of the measurement. With the heated probe, the combustion gases cannot dissolve into the condensate. Therefore, the sample remains representative of the emissions in the stack.

Heated Sample Line

A heated sample line is used to keep the gas sample above to dew point to prevent the absorption of gases into the water phase. This provides conditions for more accurate measurements as gases are not lost into the condensate.

Chiller

An internal chiller dries the flue gas to avoid dilution of NO₂ and SO₂ into the condensate. This allows for a sample that is most representative of the emissions being released, as none of the gases are lost into the water phase.

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