DETERMINATION OF THE MASS CONCENTRATION OF NITROGEN OXIDES IN THE WORK PLACE AIR BY CHEMILUMINESCENCE METHOD UDC:546.17/.19:614.8 Ralević Gordana, Oravec Miroljub

A rapid, senstive method for NO2 analysis utilizing the chemiluminescence reaction of NO and O3 is provided by the use of the Scott Model 125 Chemiluminescense NO/NOI, Analyzer. Measurement of NO2 in the air sample by reducing it to NO by passage tgrough a converter before it enters the reaction chamber. Thermal converter will also convert, partially or completly, NH3 (and other N-componds) depending on converter type and temperature. In the air at many work places in the fertilizer factory, NH3 and NO2 may be present simultaneously and NH3 may cause interference when NO2 determine by this method. The aim of this study was to determine the influence of the converter temperature to the conversion of NO2 and NH3 to NO. 1. OPERATION The principles of operation is based upon the NO/O3 chemiluminescent reactions in wich O3 react with O3 react with NO to yield O2 and electronically exicited NO2. The transition of excited NO2 to graund state NO2 yields a detectable light emission. This light emission is then measured using a photomultiplier and associated electronic. Received July 6, 1998 G. RALEVIĆ, M. ORAVEC 84 Fig. 1. Flow diagram 2. DETERMINATION NO CONCENTRATION The method allso utilizes the principle that thermal decomposition of NO2 is complete at about 540°C (1100°F). Alenght of coiled seamless stainless steel tubing heated, using a low voltage, high current power supply, to a temperature od 700°C (1300°F) provides sufficient residence time at a sample flow rate of 10 scfh for essentialy complete conversion of NO2 to NO. a) Analyzer unconverted NOx (bypass converter) and record the true level ofNO indicated on the readout meter. b) Divert NOx throught the Thermal Converter (changes NOx to the total NO), then direct to the analyzer. Record the level of total NO indicated on the readout meter. c) The difference in the level of total NO and true NO becomes the calculated level of NO2 in the sample gas. In the fertilizers factory there is a constant control of the working area pollution by measuring NO2, NH3, dust, etc. concentrations. The presence of NH3 in the working area makes it difficult to determine NO2 by chemiluminescence method because at high temperatures of the converter NH3, itself, is partially converted into NO. This distrubance caused by ammonia is especially emphasized when very low ppm concentrations of NO2 are to be determined. (MAC of NO2 in the working area is 1 ppm). The objective of this paper is to examine the possibility of measuring NO2 at lower working temperatures of the converter when the influence of NH3 would be reduced or totally eliminated. For this puprpse some calibration mixtures were prepared with different concentrations of NO2, i.e. NH3 into the air, and conversion level of NO2 i.e. NH3 was determined at 6 different converter temperatures. A permeation tube EL-SRT-2 (KIN-TEK Laboratories Inc.) was used for the preparation of these calibration mixtures of NO2. The same tube was used for the calibration mixtyures of 5.5 ppm and 11 ppm NH3. The mixtures with higher concentrations were obtained by blending the instrument air and pure NH3 in Teflon bags of 501. The results are given as diagrams and as tables. Determination of the Mass Concentration of Nitrogen Oxides in the Work Place Air... 85 Table 1. Calibration mixture 1 ppm NO2 1,5 ppm NO2 3 ppm NO2 6 ppm NO2 12 ppm NO2 Convertor t°C a b a b a b a b a b 700 0.96 100 1.47 100 2.90 100 6.10 100 12.10 100 650 0.57 59.40 1.21 82.30 2.47 85.20 5.50 90.15 11.20 92.60 600 0.39 40.60 0.96 65.30 1.78 61.40 4.60 75.40 10.00 82.60 550 0.23 24.00 0.74 50.30 1.45 50.00 3.60 59.00 8.50 70.25 500 0.16 16.70 0.62 42.20 1.30 44.80 2.90 47.50 7.45 61.60 400 0.13 13.50 0.55 37.40 1.15 39.70 2.25 36.90 5.60 46.30 a-NO2 concentration (ppm) b-conversion level NO2 to NO (%) Table 2. Calibration mixture 1 ppm NO2 1,5 ppm NO2 3 ppm NO2 6 ppm NO2 12 ppm NO2 Convertor t°C a b a b a b a b a b 700 3.00 54.55 2.10 19.10 8.70 39.55 20.50 37.30 39.50 35.90 650 1.50 13.60 7.90 335.90 20.20 36.70 43.10 39.20 600 2.40 43.65 0.72 6.55 6.60 30.00 18.60 33.80 44.70 40.65 550 0.40 3.65 5.15 23.40 16.20 29.45 40.20 36.55 500 1.00 18.20 0.23 2.10 3.55 16.15 10.00 18.80 21.20 19.30 400 0.13 1.20 1.55 7.00 3.60 6.60 6.10 5.50 a-NO2 concentration (ppm) b-conversion level NH3 to NO (%) 3. CONCLUSIONS The conversion level for low NO2 concentration to NO depends on the converter temperature and these analyses must be carried out at the converter operating temeperature (700°C), when the conversion level is 100%. Gas mixtures containing higher concentration of NO2 (<10 ppm) indicated to the less temperature dependance of the conversion level, and consequently, it would be, probably, possible to determine thesse concentrations by the temperatures below refered tot he rated converter temperature (700°C). The conversion level of NH3 to NO on the rated temperature (700°C) amounts 30% to 55% depending on ammonia concentration. The conversion level of various G. RALEVIĆ, M. ORAVEC 86 concentrations of ammonia into NO by the lower converter temperatures (bellow 500°C) is <15%, but on these temperatures the conversion of NO2 into NO is not complete (between 40% and 55%).