At high concentrations we can see NOx emissions with our naked eye as a characteristic yellow-brown plume. For lower concentrations a chemiluminescence analyser is ideal.
There are several techniques for NOx measurement including NDIR, FTIR and electrochemical detectors. However, the most common technique is chemiluminescence.
Many industries in Australia burn gas, oil, coal, biomass or waste materials to generate heat or power. In most states, the stack emissions from these combustion sources is regulated to control the amount of NOx pollutant emissions. Chemiluminescence analysers are some of the most common types of analytical equipment used in this continuous emissions monitoring (CEMS) application.
The use of selective catalytic reduction (SCR) to mitigate NOx emissions is becoming more common. When such technology is employed, NOx levels will be measured before and after the SCR unit. The initial measurement influences the dosing of urea through a process control loop. Urea is the reducing agent that helps convert the NOx to molecular nitrogen gas. The final measurement confirms that the reduction process has taken place and provides a record for regulatory authorities to confirm that the stack emissions are compliant with local regulations. The same analyser can be used for both measurements because the detection range of a chemiluminescence instrument is very broad. The instrument will be calibrated at regular intervals using certified CEMS calibration gas mixtures which are available from the Coregas specialty gases portfolio.
In the automotive industry, vehicle exhaust emissions are almost always measured with chemiluminescence instrumentation. It is a rapid response online technique that can quickly respond to changes in engine performance and exhaust gas composition. Most car engines in use today have been developed in engine development and testing facilities using this type of instrumentation. Type testing of production models also involves exhaust emissions testing. For the engine test bed, where untreated engine-out emissions may be measured, NOx levels are moderately high - perhaps between 100 and 800 ppm for a diesel engine depending on the rpm speed and engine temperature. For automotive type-testing, exhaust NOx emissions post a modern SCR after-treatment system will typically be in the range of 5 to 100 ppm.
In ambient air quality measurement, the NOx levels are lower still at around 50 ppb. Despite this vast difference in concentration, the chemiluminescence technique is favoured for each of these applications. Coregas is also able to manufacture certified calibration gas mixtures to calibrate the chemiluminescence instrumentation at each of these levels.
