OCR Text |
Show CURRENT M E T H O D S O F M E A S U R I N G N O x IN FLUE GASES When testing actual boilers, heaters or furnaces in the field, it is usually necessary to determine the immediate affect on N O x due to an operating or equipment modification. Therefore, speedy, continuous results are required. On the other hand, officially recognized compliance testing must follow specified procedures which at this time includes wet analysis for nitrogen oxides concentration. This is slow and cumber* some and rely heavily on the quality and experience of the measurement team. Sampling for SO2 and N O x is simpler than particulate sampling, since isokinetic flow conditions are not required, and official procedures do not mandate any special sampling technique. Nitrogen oxides measurement instrumentation currently in use includes colorimetric, photometric, electrochemical and chemiluminescent detectors. There are two types widely accepted at this time: (1) The optical/photometric split beam analyzer, measuring the difference of radiation by the sample at two wave lengths in the UV visible region. (2) The chemiluminescent reaction anojyzer, where nitric oxide is detected by combining with ozone to produce nitrogen dioxide in the electronically excited state, accompanied by photon emission which is proportional to the mass flow rate of N O into the analyzer. Nitrogen dioxide is entirely converted to nitric oxide through a catalyst converter prior to analyzing the sample in order to produce total N O x concentration results. The authors have had more actual experience with the chemiluminescent type analyzer, and believe the accuracy of the results obtained are excellent, say + 2 %. These continuous stack analyzers have not been accepted for official compliance tests to date. 13/8 |