Automated Monitoring of Stack Gas Emissions by EDXRF

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-20 146 Hamburg, Germany

2Spectro Analytical Instruments, BoschstraRe 10, D-47533 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 energy- dispersive 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

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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.


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 7.069 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 200 - 24000 14,62 0.054 96 0.9982 K 1700 - 20000 19,os 0.071 <20 0.9975 Ca 200 - 25000 19,64 0.073 130 0.9988 Ti 25 - 25000 21,90 0.082 <54 0.9992 V 10 - 11500 4,30 0.016 Cl2 0.9999 Cr 30 - 6500 2,30 0.009 13 0.9996 Mn 60 - 6500 2,22 0.008 Cl2 0.9994 Fe 270 - 24000 11,33 0.042 287 0.9978 co 35 - 13200 0,89 0.003 27 0.9995 Ni 40 - 4200 0,49 0.002 32 0.9952 cu 20 - 4000 2,04 0.008 17 0.9956 Zn 300 - 6400 3,22 0.012 35 0.9991 As 200 - 35000 4,12 0.015 6 0.9992 Se 15 - 660 3,15 0.012 10 0.9954 Cd 50 - 50000 27,45 0.102 <30 0.9995 Sn 400 - 14000 40,88 0.152 <40 0.9990 Sb 150 - 1500 41,50 0.154 <40 0.8689 Tl 20 - 60 2,34 0.009 <23 0.6850 Pb 25 - 5500 8167 0.032

’ Calculated with IUPAC-definition [ 111 2 Calculated for a sampling volume of 1.9 mVh and a filter area of 7.069 cm2

29 0.9987

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.


Calibration Curve of Copper

50-

40- r = 0.996 a = 0.013 b = 0.29

/’ _,

/' ,/' .

/ /'

/' n ,‘m

n

,/,,'

,/

30-

v ,/Y

/" ./

ZO- ,'/ /"

/' /

IO-

A /

I I I I 0 1000 2000 3000 4000

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 365.000 tons a year output of 99.99 % 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, 750.000 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


5/22/l 998 to 5/24/l 998

100

Tab. 2: Sampling parameters at the NORDDEUTSCHE AFFINERIE.

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

30 minutes 12500 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

Fig. 3: Aerosol loaded filter MK 370 (32 mm in diameter)

centimeters. Figure 5 shows a

magnification of the aerosol loaded filter sample ,,NA21.49“ taken out of the stack gas channel

at the NORDDEUTSCHE AFFINERIE.


Fig.4: 6000x magnification of MK 370 blank. Fig.5: 6000x magnification of ,,NA2 1.49“.

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

- _ -. . ,e Fig. 6: Two spectra ofthe same tilter sample trom May 23 at 5:34 a.m..

WI, , I , , , I I - ! ’ 2 4 6 8 10 12 14 16

energy [keV]

Rh-tube, Ta-filter, 50 kV, 6 mA

I

1adI , , , , 1 , , I I I, I- 10 12 14 16 18 20 22 24 26

energy[keV]

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 1986.

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.


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.

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

total < 1000

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

total < 5000

Emission of C” Monitored On-line by X-DUST

100

i 4 7.5

I/ 60

25

0 22 05.199806:ll 22.051998 1707 x3.05.1998 05:02 23.06.1998 16-58 24.05.1998 0‘154

date/time

Emission of C” Monitored Off-line by X-LAB

Figure 7 displays the results of the field experiment. First, the half

hour values for the

100

i g 75

50

25

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

0 22 05.1998 05’11 22.051998 ITO7 23.05.1998 05:02 23.05.1998 ,658 24 06.1998 0‘1-54

statelbime

lsoklnetlc Sampling Volume

2 E

,000

11 n _, ,/-\

900

\I+

h/v;- \ d

--J--

800 ‘/ i 1 I

700 4 I

600 22 05,998 05’11 22.05.1998 17:07 23.051998 05:02 2305.1998 16’56 24.05.1996 0‘154

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 3 750 =L

500

250

0 552 l&m3 (X-

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

analyzing sys- Emission of Elements in TA-Luft Class 3:

V, Cr, Mn, Cu, Sn, Sb, Rh, Pd, Pt, Pb; LOE: 5000 pg/m3 tems is 0.928

due to the great

difference be-

tween the results

for the very high

XDUST

X-LAB .__-.

loaded filter of sample

4047 ug/m3 (X-

DUST) com-

pared with

22.05.1998 05.11 22 05 1998 17:07 23.05.1998 05 02 23.05.1998 16:58 24 05 1998 04:54

date/time LAB). Figure 8 summarizes the emission rates

FIG. 8: Summarized values of emission tar class 3 elements. for all elements mentioned in

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.


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 27.02.1986. (GMBI. S. 95)

[2] Bundes-Immissionsschutzgesetz-Verordnung, 17. BImSchV, Bundesminister fir Umwelt und Reaktorsicherheit, Bonn 23.11.1990. (BGBl. I s,2545,2832)(BGBl. III 2129-8-1-17)

[3] Revised Standards for Hazardous Waste Combustors. Proposed Rules. Fed. Regist. 61 (1996) 17357-17536

[4] M.D. Seltzer, Applied Spectroscopy, 52 (1998) 195-199

[5] A.M. Gomes, J.P. Sarrette, L. Madon, A. Almi, Spectrochimica Acta, Part B 51 (1996)1695-1705

[6] M.D. Seltzer, G. A. Meyer, Emviron. Sci. Technol., 31 (1997) 2665-2672

[7] T. Stahlschmidt, 0. Haupt, M. Schulz, W. Dannecker, Gefahrstofi Reinhaltung der Lz@; 58 (1998) 199-204

[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) 712-724


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Automated Monitoring of Stack Gas Emissions by EDXRF