NOx is produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures. The NOx group constitutes Nitrogen Dioxide (NO2) , nitrous acid and nitric acid.
NOx is emitted by automobiles, as well as industrial sources such as power plants, industrial boilers, cement kilns, and turbines
NOx causes respiratory problems in human beings leading to premature death in certain cases , deterioration in water quality due to oxygen depletion, form acid rain which affects ecosystem, causes smog formation and reduce visibility.
The main challenge in measuring NOx is selecting right type of measuring method which meets the requisite performance for the intended application it is used for.
Selecting right type NOx analyser measuring principle along with right type of sample handling system that is most suited for one’s application is biggest challenge as it is seen that many clients world-wide did not made right choice , which lead to :-
a) Frequent Maintenance
b) Drift in reading over time
c) Regular Manual Calibration
d) Failing to meet regulatory performance levels
e) Absence of remote operation - control, diagnostic and calibration added to operational cost
f) Wrong choice in selecting In Situ / Extractive type sample measurement leading to early deterioration in analyser performance
NOx can be measured by adopting any of the following methods: -
a) Infrared which works on IR absorption by gas principle at a particular wavelength. This technique is costly and there might be some interference of signal from presence of other gases in sample.
a) Chemiluminescence which works on reaction between NO and ozone (O3) . Here NO2 has to be converted to NO first using an additional convertor.
Lack of sufficient selectivity and sensitivity to various physicochemical factors.
a) Electrochemical which work on oxidisation of gas principle is of low cost and with lower levels of performance compared to other two measuring methods. Sensor need frequent calibration and have shorten life span needing regular replacement
From the practical experience and several laboratories research worldwide following guidelines have been proposed in selecting the measuring method to be used.
a) Infrared generally to be adopted for meeting regulatory NORMS
b) Chemiluminescence is preferred for developing new technologies in laboratories and more suitable for measuring low level of NOx as it is more sensitive compared to IR method
c) Electrochemical are used where average accuracy, low-cost solution is acceptable