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Do I Need a NOx Converter On My Flue Gas Monitor?

Most combustion sources must monitor their emissions of NOx (oxides of nitrogen) and report them to the local environmental protection authority. 

Depending on the relevant regulations and the nature of the combustion process, the measurement may use a continuous emissions monitoring system (CEMS), a portable gas analyzer or even a periodic measurement made by a stack tester. 

The requirement is to report the combined total of nitric oxide (NO) and nitrogen dioxide (NO2) which comprise the NOx. Many common analytical techniques, such as chemiluminescent monitors and electrochemical sensors, are specific to NO. These instruments often use a catalytic NOx converter to reduce the NO2 to NO so that the single-species analyzer can measure all of the NOx. 

If we look at the original source of the NOx, we find that it is formed through two principal mechanisms in a combustion source:

1. Fuel NOx forms when nitrogen bound in the fuel oxidizes alongside the carbon and hydrogen that are its principal combustible components. 

2.
Thermal NOx is a result of nitrogen molecules in the air splitting in the high temperature of the flame and reacts with oxygen in the air. This reaction takes place much more rapidly when the peak temperature of the flame is above 1500 °C (roughly 2700 °F).

In both cases, the reaction produced NO, which oxidizes further to produce NO2 as the combustion gases move through the process. Once the gases are emitted to the atmosphere, the oxidation continues so that all of the NOx ends up as NO2.

 The majority of the NOx present at the stack exit is usually present as NO, and many sources emit around 95% of their NOx in this form. Because of this, we can often estimate the NOx emissions by applying a small mathematical correction the NO measurement, avoiding the need to measure NO2. Because the correction is rather small, the uncertainties remain within acceptable limits and the measurement is accepted by many regulators, including the UK Environment Agency. 

Of course, this approach is not acceptable in all territories and all processes. The principal exception is a gas turbine whose NO2 emissions are comparable to its NO, and it is important to measure both species ion such applications.

In conclusion, many processes can report their NOx emissions by measuring NO alone and applying a mathematical correction. This calculation can be applied automatically by all AMETEK Land NO monitors, including the Lancom and FGA series.

 

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