Methods to Measure Carbon Monoxide and Nitrogen Oxide

In industrial facilities, it’s required to run Continuous Emission Monitoring Systems (CEMS). They monitor and provide information about flue gases, for combustion control and following the permitted emission standards. Some of these facilities have combustion systems which produce a massive amount of toxic air.

Normally, CEMS continuously monitor the number of poisonous air pollutants emitted. The best samples of these are Carbon Monoxide (CO) and Nitrogen Oxide (NOx). These 2 pollutants are tested regularly because they could cause casualties. In connection with this, there are 3 different ways to conduct CO CEMS and NOx CEMS to avoid the dangers caused by the exceeded limits.

Full Extractive

This method provides optimal control of sample conditioning which gives accurate analysis. This process uses a sample portion of gas and carries it to an analyzer. However, transporting the sample can be tricky. It contains moisture that results in transport difficulties. The lines used in fully extractive systems are heated to prevent the sample from cooling beyond its dew point, which will cause condensation.

The good thing about this is that it has 2 types of analysis: wet and dry. If the sample is measured with the presence of moisture, it’s called wet analysis. If the gas is cooled in a controlled temperature before being sent to the analyzer, it’s called dry analysis.

For example, in the case of carbon monoxide, it’s normally measured by infrared, which causes the interference of water vapor. The moisture is then removed, and it undergoes dry analysis. The same goes for nitrogen oxide. However, wet analysis is performed with NOx if there are no hydrocarbons in the flue gas.

Dilution Probes

This approach uses a small portion of gas and lowers its dew point to the lowest possible level, making it impossible for moisture to enter the sample before transferring to the analyzer. There are 2 techniques to dilute a sample. The first one is “in-stack dilution” wherein the taken gas is diluted with dry air with the use of dilution probe inside the stack. The ratio is controlled using a critical orifice inside the probe, maintaining critical flow as a set pressure drop is achieved. On the other hand, “out-of-stack dilution” is diluting the air outside of the stack, controlled by a critical orifice in the box.


This analysis consists of measuring and analyzing the air at the desired point. There are 2 analyzations under this method. The first one is performed within a very short distance from the monitoring instrument, which is called the “in-situ point.” The other one is fulfilled over a distance or a gas stream which is called the “in-situ path.”

Conducting CO CEMS and NOx CEMS to determine the amount of carbon monoxide and nitrogen oxide particles in a gas is a way to avoid air poisoning and promote safety. It should be conducted regularly since it’s regulated by the state, and it encourages a healthier environment in industrial facilities.

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