Off-line system for measurement of nitrous oxide concentration in gases leaving the irradiation chamber

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Pergamon

Radiat. Phys. Chem. Vol. 45, No. 6, pp. 1035 1038, 1995

0969-806X(94)00166-9

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-806X/95 $9.50 + 0 . 0 0

O F F - L I N E SYSTEM F O R M E A S U R E M E N T OF N I T R O U S O X I D E C O N C E N T R A T I O N IN GASES L E A V I N G THE IRRADIATION CHAMBER J. LICKI, 1 A. G. CHMIELEWSKI 2 and B. RADZIO 1 qnstitute of Atomic Energy, 05-400 Otwock-Swierk, Poland and 2Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland Abstract--The N20 concentration in the gas leaving the irradiation chamber was determined by off-line gas chromatographic analysis. The grab sample system involved the scrubber with a 1.0 N NaOH solution and the drying columns. The paper presents preliminary results of N20 concentration measurement for two different gas irradiation doses with other technological parameters of the pilot installation being constant.

INTRODUCTION

Nitrous oxide (N20) occurs in the troposphere at background concentration of about 300ppb and measurement over the past two decades have shown that its concentration is rising at a rate of about 0.2~).4% per year. The molecules, being chemically stable in the troposphere (life time of approximately 150 years), if transported to the stratosphere undergo conversion to NO, which subsequently reacts with 03. Hence, an increase in N20 emission is followed by further 03 depletion in the stratosphere. Anthropogenic sources, including the denitrification of chemical fertilizers and the combustion of fossil fuels are estimated to comprise of one-third of the total N zO produced. Nitrous oxide is also emitted from different technologies, for example in the deNOx technologies. In the deNO.~ process exploiting NH 3 for nitrogen oxide reduction, secondary undesired reactions are observed: (a) For selective catalytic reduction (SCR) 2NH 3 + 202 ----~N20 + 3H20 (b) For selective noncatalytic reduction (SNCR) 2NH3 + 202---~ N20 + 3H20 (c) For electron beam flue gas treatment (EB process) (Suzuki et al., 1980) NO2 + N ---*N20 + O NH 3 + OH ~ NH2 + H20 NO2 + NH2---~ N20 + H20 In the case of SNCR system (especially urea based systems) ammonia decomposition reaction results in emission of N20, 10-15% of the processed nitrogen oxides. If an SNCR system reduces NOx emission by 50%, the net NOx reduction considering N20 emission is only 35-40%. The SCR system converts less

than 1% of NOx to N20 (Cochran and Harpenan, 1993), but this number increases if the activity of the catalyser decreases during the time of operation. The emission of N20 in the EB process is observed and results in up to 10% NOx reduction in the case of a single irradiation process. One of the further tasks in the optimization studies planned at Kaw~czyn pilot plant (Chmielewski et al., 1992) is to reduce formation of undesired molecules. This paper presents system for reliable and accurate N20 concentration measurement in the gas leaving the irradiation chamber and preliminary experimental results. APPROACH

Nitrous oxide concentration in flue gases can be determined by such analytical techniques as: tunable diode laser spectroscopy (TLDS), Fourier transform infrared spectroscopy (FTIR), nondispersive infrared analysis (NDIR) and gas chromatography (GC). The TDLS and FTIR have the potential of providing the fine spectral resolution. For field applications only GC and NDIR methods are suitable. The NDIR method is less sensitive than GC but its great advantage is direct and continuous monitoring. The off-line GC measurement may lead to significant errors due to the formation of N20 in the sampling container. Recently, Muzio and Kramlich (1988; see also Muzio et al., 1989; Montgomery et al., 1989) have shown theoretically and experimentally that storing combustion products containing a mixture of SO2, NOx and water for a long period is responsible for the formation of several hundred ppm artifact N 2O in the container due to reaction: H20

2NO + SO2 - - ~ N2 O + SO 3 Thus, one guide to obtain a valid sample would be to perform the N 2O analysis on-line, avoiding storage of

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1. Stack wall 2. Heated sampling probe 3. Inlet ceramic filter 4. Outlet ceramic filter 5. Heated sampling line 6. SO2 removal 7. H20 removal 8. Diaphragm pump 9. Sample valve (three way) 10. Sampling tank, 1 liter stainless steel 11. Mercury manometer 12. Pump valve (three way) 13. Vacuum pump Fig. 1. Nitrous oxides (N20) sampling apparatus.

sample. Experimental results (Muzio et al., 1989) indicate that drying the sample gas to a 0°C dewpoint before its introduction in the container reduces, but does not eliminate entirely N 2 0 formation at the container. Valid grab samples can be obtained by either removing the SO 2 from the combustion products before the gas enters the sample container, achieved by scrubbing the sample gas through a 1.0 N N a O H solution or introducing of the aqueous solution of a 10 N N a O H to sample container prior to sampling.

Parameter of installation Flow rate of flue gases Gas temperature NH3 flow SO~ NO% H20° CO/CO2 02 Electron beam dose

EXPERIMENTAL

Grab sampling system for N20 determination Gas samples were taken at the outlet of the irradiation chamber. The extractive system incorporated sample probe tipped with set of two ceramic filters (coarse and fine) and 10 m long sampling line (Fig. 1). The probe, both filters and sampling line were kept at elevated temperature (180°C). First, the heated gas samples passed the scrubber with a 1.0 N N a O H solution (SO 2 removal), next the drying columns filled

Table 1. Samplingposition Inlet to installation Outlet from irradiation chamber Inlet to irradiation chamber Gas composition measurement Inlet to installation Inlet to irradiation chamber Outlet from irradiation chamber Both ELV-3 scanners

Instrumentation Segment orifice Platinumresistance thermometer Dr/iger flowmeterK-29.1 TIV 29 Thermoelectron Model 40 Tbermoelectron Model 10A/R Off-line measurement manual method Shimadzu NDIR analyzer CGT-10-1A Schimadze Paramagnetic analyzer POT-101 Monitoring system of each accelerator

Measurement of N20 in gases

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with P205 (H20 removal). Finally, they were admitted to the evacuated stainless steel containers. So prepared gas samples were sent for GC analysis. Each samples were analyzed within 1 h of collection.

Gas chromatography CO

The gas chromatograph Model 201 Pye Unicam equipped with TC D detector was used. Nitrous oxide was separated on a 3 m × 4 mm o.d. column packed with Porapak Q + 20 cm Porapak R and operated at 303°K. The carrier gas, helium was applied with flow rate 30 ml/min. The T D C detector was operated at 473°K. The chromatograph was calibrated using 102 ppm standard mixture of N 2 0 in nitrogen (BOC Special Gases, Morden, U.K.).

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Grab sampling configuration Direct

SO2 and H20 removed

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CO2

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0 2 + N2

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Fig. 3. Chromatogram of the sample gas taken at outlet of the irradiation chamber using sampling configuration involving SO2 and H20 removal process. Gas composition was the same as in Fig. 2. Dose 0.8 Mrad; N20 13.5 ppm.

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On-line instrumentation The technological parameters of pilot installation were monitored on-line (Licki et al., 1992) and the equipment used for this is listed in Table 1. RESULTS

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Fig. 2. Chromatograms of the sample gas taken at outlet of the irradiation chamber using two gas sampling configurations: one direct and other with SO2 and H20 removal. Gas composition: CO2 7.6% ; 0 2 10.7% ; H2O° 8.5% ; SO~ 450 ppm; NH 3stoichiometry 0.8: flow rate 10,000 m3/h; dose 1.6 Mrad; N20 25.0 ppm.

Figure 2 presents chromatograms of sample gas taken at outlet of the irradiation chamber using two gas sampling configurations. The left chromatogram was obtained from the sample gases collected directly to the container. The N 2 0 peak partially overlaps with CO2 peak. In this case the precision of N 2 0 measurement is small. The right chromatogram was obtained from the sample gases collected with SO2 and H 2 0 removed in the scrubber and drying columns respectively. The N 2 0 peak is well separated

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from CO 2 peak. It is related to the partial absorption of CO2 in the N a O H solution. Direct indication of the last one is more narrow CO2 peak in the right chromatogram. The N 2 0 concentration calculated from the right chromatogram is 25.0 ppm. In this case, the irradiation dose was 1.6 Mrad. When the dose was two times lower with other technological parameters of installation left unchanged, the N2 O concentration decreased to 13.5 ppm (Fig. 3). CONCLUSION The utilization of the scrubber with a 1.0 N N a O H solution and the drying columns in the grab sample system gave adequate separation of N 2 0 peak from CO2 peak in the chromatogram and more exact measurement of N 2 0 concentration. The SO2 and H 2 0 were removed from the gas sample before gas enters the sample container. Then, an artifact N 2 0 molecules formation is prevented in the container. So sample gas may be reliably analyzed later (within unrestricted time). Such prepared off-line systems will be used in the optimization studies which are performed at the

Polish pilot plant. One reason for multistage irradiation process is to obtain as small as possible undesired N 2 0 emission. The paper presents the results of preliminary measurements of N 2 0 concentration in the gas leaving the irradiation chamber with one step irradiation. REFERENCES

Suzuki N., Nishimura K. and Tokunaga O. (1980) J. Nucl. Sci. Teehnol. 17, 622. Chmielewski A. G., Iller E., Zimek Z. and Licki J. (1992) Radiat. Phys. Chem. 40, 321. Cochran J. R. and Harpenan P. A. (1993) Alternative NO X emission reduction system comparison: Coal fired power plant upgrade, 15-17 June, 1993, Warsaw, Poland. Licki J., Chmielewski A. G., Zakrzewska-Trznadel G. and Frank N. W. (1992) Monitoring and control systems for an eb flue gas treatment pilot plant--Part I. Analytical system and methods. Radiat. Phys. Chem. 40, 331. Montgomery T. A., Samuelson G. S. and Muzio L. J. (1989) Continuous infrared analysis of N20 in combustion products. J.A.P.C.A. 39, 721. Muzio L. J. and Kramlich J. C. (1988) An artifact in the measurement of N 20 from combustion sources. Geophys. Res. Lett 15, 1369. Muzio L. J., Kramlich J. C., Cole J. A., McCarty J. M. and Lyon R. K. (1989) Errors in grab sample measurements of N20 from combustion sources. J.A.P.C.A. 39, 287.