AUTOMATED MONITORING OF STACK GAS EMISSIONS BY EDXRF

Transcription

1 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol AUTOMATED MONITORING OF STACK GAS EMISSIONS BY EDXRF 0. Haupt, R. Harrnel, C. Schafer2 and W. Dannecker University of Hamburg, Institute of Inorganic and Applied Chemistry, Martin-Luther-King-Platz 6, D Hamburg, Germany 2Spectro Analytical Instruments, BoschstraRe 10, D Kleve, Germany ABSTRACT A continuously working sampling and analysing device for monitoring of hazardous air pollutant (HAP) metals in the form of particulate matter was developed and tested at an industrial metal smelter INTRODUCTION Monitoring and control of stack gas emissions from industrial furnaces are important fields in environmental analysis. Stack gas emissions in this case means the emission of toxic elements in the form of particulate matter. The toxic and ecological harmful effects of emitted heavy metals are well known and were taken into account by the German law (TA-Luft [l] and 17NmSchV [2]) and the EPA urban air toxic program for hazardous air pollutants (HAP) [3]. The concentration of heavy metals in the emissions is usually low because of good cleaning devices. Small sized particles with an aerodynamic diameter (AD) below 2.5 microns are poorly separated (and have a tendency to accumulate in biosystems) while carrying significant metal loads. Several authors used optical emission spectrometry with an inductive coupled plasma (ICP- OES) for continuous emission monitoring [4][5][6]. This technique shows mndamental problems relating to the isokinetic sampling, the highly changing particle sizes, the stack gas temperatures and stack moisture which have an effect on the plasma temperature and thus on the achieved intensities and determined element concentrations. Also, the argon needed is too expensive for a continuous monitoring system. The main disadvantage is the destruction of the samples during the analysis. The goal here is an automated monitoring system which combines an aerosol sampler with a rapid element analysis. Usually aerosols were collected on membrane or fiber filter substrates. In cooperation with SPECTRO A. I. (Kleve, Germany) we built up an automated and continuously working sampling and analysis system called X-DUST. For precipitating the stack gas aerosols and analyzing them by a fast, nondestructive and robust method like energydispersive x-ray fluorescence spectroscopy, a newly developed quartz fiber filter tape named MK 370 was used [7]. Quartz fiber filter materials have a lot of advantages, specifically the low blank values for most elements, the resistance to hot and corrosive gases and the high loading capacity. A disadvantage of the fiber filters is the high mass per unit area, which leads to a higher background in x-ray fluorescence analysis. INSTRUMENTAL The combined sampling and analysis system X-DUST shown in Figure I, provides a multielement analysis of toxic elements within 30 minutes after sampling from the stack gas channel. For the entire system we had to develop and combine several components in close cooperation with industrial partners. Stack gas samples are taken by a heated titanium probe

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol while using an isokinetic flow control unit from the PAUL GOTHE company (Bochum, Germany) to ensure representative particle sampling. Due to the high water load of stack gases the temperature of the probe and the titanium filter holder in the sampling unit are kept at high temperatures, up to 120 degrees centigrade, to prevent condensation. The filter medium is a trace clean quartz fiber tape developed in cooperation with MUNKTELL FILTER AB (Sweden). At the end of the preadjusted sampling period the filter tape with the aerosol loaded filter area of about 7 cm2 (30 mm in diameter) is moved automatically into the energy dispersive x-ray fluorescence spectrometer for element analysis. The excitation source of the spectrometer unit is a 400 W end-window Rh-tube with an angle of 40 to the filter tape surface. The detection system is a silicon drift chamber called,,xflash detector manufactured by RGNTEC (Berlin, Germany) and positioned vertical to the excited filter surface. The detector and the x-ray tube are adapted in cooperation with SPECTRO A.I. onto the sampling unit - a reconstructed emission sampling system F 904 by VEREWA (Hamburg, Germany). For optimum excitation of most elements different absorption-filter materials can be inserted between x-ray source and aerosol loaded filter. The distance between sampling position and analysis system is exactly 100 mm, therefore it is possible to get about 400 samples while using a 40 meter quartz fiber tape. If the sampling period is 30 minutes there is a weekly maintenance interval required. 1 heated titanium probe 6 heated titanium blocks 11 drying agent 2 flange 7 rhodium-end window tube 12 automatic isokinetic regulate 3 stack gas channel 8 XFlashdetector 13 bypass 4 sampling device 9 personal computer 14 pump 5 quartz fibre filter band 10 gas cooling device 15 volume Fig. 1: Emission sampling and analysis device X-DUST. After passing the filter (5) the water vapor of the stack gas sample is removed in a titanium condenser (10). The dry gas sample then passes the isokinetic control unit consisting of a pressure gauge, control circuitry (12) by-pass valve (13) and pump (14). Finally, the sample gas volume is measured by a flow meter (15) and then converted to standard temperature volume. The entire system is software controlled by a PC (9). The data collected by the spectrometer are evaluated based on calibrations with reference filter materials. The obtained concentrations in ng/cm2 are converted into rig/m3 based on the standardized sample gas volume.

4 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol CALIBRATION For the calibration of the X-DUST spectrometer we produced multielement standard materials by precipitating multielement standard solutions with predetermined element concentrations onto quartz fiber filters. For this task we used an aerosol generator described in earlier publications [S][9]. The aerosol loaded reference materials were first measured by the x-ray fluorescence spectrometer of the X-DUST. The normalized intensities (counts s-l ma- ) from the XRF spectrometer were plotted against the elemental concentrations (ng/cm2) determined by optical emission spectrometry using inductive coupled plasma (ICP-OES), mass spectrometry using inductive coupled plasma (ICP-MS) and atomic absorption spectroscopy (AAS) after dissolving the filters by oxidative digestion [lo]. Table I displays the calibration ranges (ng/cm2), the limits of detection (ng/cm2) using the IUPAC-definition [ll], the limits of detection (ng/m3) calculated for a sampling volume of 1.9 m3/h and a filter area of cm2, and the coefficients of correlation (r) for filter calibration method of the X-DUST. Additionally the blank values of the used quartz fiber filter tape MK 370 (ng/cm2) are given. Tab. 1: Calibration parameters of X-DUST. Element Calibration Limits of Limits of Blank Value coefficients Range Detection Detection2 MK370 of correlation [@cm21 [ng/cm2] [ ug/m3] [@cm21 r Cl , K ,os < Ca , Ti , < V , Cl Cr , Mn , Cl Fe , co , Ni , cu , Zn , As , Se , Cd , < Sn , < Sb , < Tl , < Pb Calculated with IUPAC-definition [ Calculated for a sampling volume of 1.9 mvh and a filter area of cm2 In Figure 2 the calibration curve for Cu on the quartz fiber filter tape MK 370 for the X-DUST x-ray fluorescence spectrometer is given together with the coefficient of correlation Y, the slope a as a value of sensitivity and the y-intercept b.

5 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Calibration Curve of Copper r = a = b = 0.29 n n,/, m /' / /' / _, /',/'. 30- ZO- /' / /",'/ /"./ v,/y,/,,' IO- A / I I I I Concentration [ng/cm2] Fig. 2: Calibration curve of Cu on quartz fiber filter MK 370 for the X-DUST. Most elements have a wide calibration range of about two orders of magnitude and a sufficient coefficient of correlation. Sampling The first field experiment with the X-DUST was carried out between 22 and 24 of May in 1998 at the NORDDEUTSCHE AFFINERIE (NA). It is one of the biggest copper smelters in Europe with tons a year output of % pure copper. The NA was founded in 1866 near downtown Hamburg (Germany) directly at the harbor. Additional products are, for example, Ag, Au, Pt, Pb, Ni, Sb and last but not least, tons a year of 98 % pure sulfuric acid. The sampling site was at the electric oven of the metal smelter directly behind a formation of tube filters. These tube filters are the first cleaning stage for separation of coarse particles. It is essential to point out that the main cleaning facilities for the stack gases, like washers and different types of cyclones are installed behind our sampling site. So the results determined by our sampling and analysis system listed in this paper are not the concentrations being released into the ambient air. Within two days about 100 samples with sampling periods of 30 minutes were collected. The sampling parameters are given in Table 2

6 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Tab. 2: Sampling parameters at the NORDDEUTSCHE sampling date number of samples sampling period stack gas flow avrg. Stack gas velocity sampling volume filter diameter / area Temperature in stack gas Temperature at heated titanium blocks AFFINERIE. 5/22/l 998 to 5/24/l minutes m3/h 5.46 m/s 1.9 m3/h 30 mm / 7,069 cm2 78 C 70 C The split sampling volume was about 950 liters within half an hour. This means a sample flow of 1.9 m3/h obtained by isokinetic sampling using a probe with 10 mm in diameter. The stack gas temperature was about 78 degrees centigrade and to prevent condensation on the filter material the temperature at the titanium blocks was set in advance to 70 degrees centigrade. After determination of the element concentrations by the integrated spectrometer the aerosol samples on the quartz fiber filter tape were automatically covered with a MYLAR foil to enable further investigations with other analytical methods. RESULTS In our laboratory all samples were punched out of the filter tape to reanalyze them with the laboratory energy dispersive x-ray fluorescence spectrometer X-LAB (SPECTRO A.I.). To give an idea how the MK 370 quartz fiber filter material looks, Figure 3 displays one of the aerosol loaded filters with a diameter of 30 mm. The structure on the filter surface results from the production process of this material. Further investigations by scanning electron microscopy (SEM Model 44 CAM SCAN, UK) were carried out, and in Figure 4 a magnification of a MK 370 blank filter is given. The diameters of the quartz fibers are up to 1.5 urn and the fiber length can be up to centimeters. Figure 5 shows a Fig. 3: Aerosol loaded filter MK 370 (32 mm in diameter) magnification of the aerosol loaded filter sample,,na21.49 taken out of the stack gas channel at the NORDDEUTSCHE AFFINERIE.

7 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Fig.4: 6000x magnification of MK 370 blank. Fig.5: 6000x magnification of,,na The optical single particle size is up to 1 microns but the single particles can accumulate to agglomerations with diameters of 1 urn to 10 urn. The particles were separated onto the filter almost by impact processes and penetrated into the first 100 to 150 urn of the 650 urn thick filter material. Figure 6 displays the two spectra of the same typical real filter sample from May 23 at 5:34 a.m. received by the X-DUST while using two different absorption filter materials for optimum excitation of each element in one measuring method. First absorption lilter is a MO-filter for excitation of K-lines from potassium to bromine and the L-line of lead using a measuring time of 700 seconds. Second one is a Ta-filter for excitation of K-lines from palladium to barium using a measuring time of 1000 seconds. While analyzing this sample in the spectrometer, the next aerosol sample was taken out of the stack gas isokinetically and precipitated on the filter material at the sampling position of the combined system. Rh-tube, MO-filter, 30 kv, 10 ma I Rh-tube, Ta-filter, 50 kv, 6 ma WI,, I,,, I I -! energy [kev] 1adI,,,, 1,, I I I, I energy[kev] - _ -..,e Fig. 6: Two spectra ofthe same tilter sample trom May 23 at 5:34 a.m.. Industrial furnaces like metal smelters have to comply with the emission limits set by the technical advisory for air (TA-Luft) in the German law for toxic elements in the year Table 3 gives the limits of emission (LOE) for different element groups in ug/m3. The regulations classify the following elements and their compounds in three classes.

8 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol The first class combines cadmium, mercury, thallium and their compounds with a LOE of 200 ug/m3. Second class is arsenic, cobalt, nickel, selenium, tellurium and their compounds with a LOE of 1000 ug/m3. Last but not least, the LOE for the sum of antimony, lead, chromium copper, manganese and their compounds with a value of 5000 ug/m3. The calculated detection limits given in Table 1 in ug/m3 are four to five orders of magnitude below the limits of emission set in the German regulation act. Figure 7 displays the results of the field experiment. First, the half hour values for the copper concentration determined,,on-line by the X-DUST during the sampling period (May 22 5: 11 a.m. to 24 4:54 a.m.) are shown. The averaged element concentration over the entire sampling period was about 50 ug/m3. Obviously there are two high values, first on May 22 at 5:07 p.m., second on May 23 at 3 :56 Tab.3: TA-LuR (1986). element group LOE [ug / m3] class I (by a mass flow 2 1 g/h) Cd, Hg, Tl and their compounds total < 200 class II (by a mass flow 2 5 g/h) As, Co, Ni, Se, Te and their compounds class III (by a mass flow 2 25 g/h) Sb, Pb, Cr, CN-, F-, Cu, Mn, Pt, Pd, Rh, V, Sn and their compounds 100 i Emission of C Monitored On-line by X-DUST total < 1000 total < :ll x : i g 75 2 E date/time Emission of C Monitored Off-line by X-LAB ITO : , , statelbime lsoklnetlc Sampling Volume 11 n _,,/-\ \I+ h/v;- 1 I I \ d --J-- / i , : : dedtlme Fig. 7: Results of the emitted Cu concentrations obtained by X-DUST and X-LAB; isokinetic sampling volume. p.m. and there is a plateau in front of the higher value at May 22. For further evaluation of I/

9 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol these results each filter sample was punched out of the quartz fiber filter tape and additional analysis of these filter samples were carried out,,off-line by the laboratory x-ray fluorescence spectrometer X-LAB in our institute. The concentrations obtained by the X-LAB spectrometer are also given in Figure 7. The consequence was that there were obviously some missing filter samples caused by transportation problems of the sampling device. Because of these transportation problems the same filter area was covered a few times and the spectrometer of the X-DUST analyzed the previous sample again and again. After a while the X-DUST restarted the transportation and finally moved the multiple covered filter area into the spectrometer of the X-DUST resulting in higher values. On May 23 around 4 pm another irregular Cu concentration was observed which cannot be explained by a sampling error. We learned from the technical manager of the plant that this was due to a change in the melting process. At the end of the copper melting process at 3 p.m. on 23 May the slag was removed. In order to do this, the oven needs to be shut down which is reflected in the isokinetic sampling volume from the stack gas channel registered by the X- DUST (Figure 7). Restarting the process with a new batch of materials obviously causes short term releases of high Cu emissions. More importantly though, the figures show very good agreement for the results obtained by X- DUST and X-LAB, even for low concentrations. The coefficient of correlation for both i =L 500 DUST) compared with l&m3 (X : : :54 LAB). Figure 8 date/time summarizes the emission rates FIG. 8: Summarized values of emission tar class 3 elements. for all elements mentioned in 250 Emission of Elements in TA-Luft Class 3: V, Cr, Mn, Cu, Sn, Sb, Rh, Pd, Pt, Pb; LOE: 5000 pg/m3 XDUST X-LAB. -. analyzing systems is due to the great difference be- tween the results for the very high loaded filter sample of 4047 ug/m3 (X- class 3 of the TA-Luft observed by the X-DUST and the X-LAB. The data agree well with the production process. Besides controlling and measuring metal emission rates for regulatory purposes the data can also be valuable in order to monitor critical steps of the production process and for trouble shooting.

10 Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol CONCLUSIONS An automated monitoring system for emission sampling of stack gas aerosols was developed. The combination of an isokinetic sampling device and nondestructive analyzing system by x-ray fluorescence analysis in one instrument was successfully tested at a stack gas channel of a copper smelter. The results of the X-DUST showed that this system allows the observation of industrial furnaces for regulatory purposes and for trouble shooting. The particles were precipitated,,out of stack on a quartz fiber filter tape MK 370 with low blanks and high resistance to hot and aggressive gases. Due to the nondestructive analyzing method the aerosol samples can be used for further analysis like SEM, AAS, ICP and so on. The detection limits of the X-DUST were sufficient for quantitative analysis at four to five orders of magnitude below the limits of emission set in the German regulation act. ACKNOWLEDGEMENT Many thanks to the DLR (Deutsche Forschungsanstalt fir Luft- und Raumfahrt ev) and the BMBF (Bundes-Ministerium f%r Bildung und Forschung) for financial support. REFERENCES [l] Technische Anleitung zur Reinhaltung der Luft-TA Luft, Bonn (GMBI. S. 95) [2] Bundes-Immissionsschutzgesetz-Verordnung, 17. BImSchV, Bundesminister fir Umwelt und Reaktorsicherheit, Bonn (BGBl. I s,2545,2832)(bgbl. III ) [3] Revised Standards for Hazardous Waste Combustors. Proposed Rules. Fed. Regist. 61 (1996) [4] M.D. Seltzer, Applied Spectroscopy, 52 (1998) [5] A.M. Gomes, J.P. Sarrette, L. Madon, A. Almi, Spectrochimica Acta, Part B 51 (1996) [6] M.D. Seltzer, G. A. Meyer, Emviron. Sci. Technol., 31 (1997) [7] T. Stahlschmidt, 0. Haupt, M. Schulz, W. Dannecker, Gefahrstofi Reinhaltung der Lz@; 58 (1998) [S] 0. Haupt, B. Klaue, C. Schafer, W. Dannecker, X-Ray Spectrom. 24 (1995) 267 [9] 0. Haupt, C. Schafer, S. Strauss, W. Dannecker, Fresenius Z. Anal. Chem. 355 (1996)375 [lo] M. Kriews, Ph. D. Thesis, University of Hamburg, Germany (1992) [l l] J.D. Winefordner, G.L. Long, Anal. Chem., 55 (1983)