The traditional way to find the value of NOx 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 NOx 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 NOx) is a much more accurate way to determine NOx than measuring NO only.
- O2 Reference
- The O2 reference is a standard that has been set to help monitor NOx emissions. This standard calculates NOx emissions based on a set oxygen level, to standardize the monitoring and reporting of total amounts of NOx emitted. The O2 reference is effective in removing any attempts of diluting emissions, which can make NOx levels appear lower than they actually are.
- Low NOx
- Low NOx is typically defined as any value of NOx that is under 500 ppm. The most common values are between 9 and 300 ppm. It is most imperative to measure True NOx (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 NOx.
- True NOx
- True NOx is a method of measuring the value of NOx 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” NOx value. It is especially important to measure True NOx when dealing with low NOx emissions to reduce error.
True NOx vs. NOx
Measuring True NOx
There are specific instruments that quantify the level of NO and NO2 in a sample. Before being able to measure NOx, 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 NO2
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 NO2
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.
- 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.