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Application of Rapid Infrared Spectrometry to Air Pollution Research Joseph R. Comberiati Morgantown Energy Research Center, Bureau of Mines, United States Department of the Interior, Morgantown, W.Va. LABORATORY RESEARCH of methods being studied for the removal of sulfur dioxide in stack gases from coal-burning electric powerplants requires the application of rapid and continuous analysis for type and concentration of constituents. Several methods under investigation for removing sulfur dioxide (1-4) from gas streams, for instance, involve chemical transformation of sulfur dioxide into other gases that must subsequently be identified and their concentration established. Slow-scan standard spectrophotometers and a 1-compound, single-wavelength infrared monitor in general use are not adequate. Moreover, infrared spectra of pure permanent gases are not as widely available in the literature as are spectra of liquid compounds (5-8). Also, few collections of pure compound gases are available. Pierson and coworkers (9) compiled infrared spectra of 66 gases and vapors. Accordingly, application was made of a rapid-scan infrared spectrophotometer that gives a complete spectrum of gases in 12.5 seconds, thus providing positive identification of all gaseous compounds, except the elemental gases, without flow interruption and without time-consuming delay for sampling and analysis. EXPERIMENTAL
Equipment. Several fast-scan infrared spectrophotometers are marketed that could have been used for this investigation, but a Beckman IR-102 was available and suitable for the purpose. (Trade names are included to facilitate understanding and do not imply endorsement by the Bureau of Mines.) The IR-102 is a single-beam instrument with automatic repetitive scanning in 5 or 12.5 seconds of wavelengths in three divisions-2.5 to 4.5 microns; 4.4 to 8.0 microns; and 7.9 to 14.5 microns-with interference filters. The sample cell has a gold-plated interior, 30-cm pathlength, holds 9 cc, and is used in a flow-through manner. Coupled to the spectrophotometer is a high-speed hot-stylus oscillographic recorder. (1) D. Bienstock, L. W. Brunn, E. M. Murphy, and H. E. Benson, BuMines Inf. Circ. 7836 (1958). (2) P. G. Marvin and J. Jonakin, Chem. Eng., 77 (9), 173 (1970). (3) J. G. Stites, Jr., W. R. Horlacker, Jr., J. L. Bachofer, Jr., and J. S . Bartman, Chem. Eng. Progr., 65 (lo), 74 (1969). (4) S. T. Cuffe, R. W. Gerstle, A. A. Oming, and C . H. Schwartz, J. Air Poll. Control Ass., 19 (9), 353 (1964). (5) S. Ochiai and S. Yoshio, Bunseki Kagaku, 17, 1025 (1968); Chem. Abstr., 69 (26), 109597~(1968). (6) R. Cameroni and A. Albasini, I1 Farmaco, 19 (5), 227 (1964); Chem. Abstr., 64, 1248a (1966). (7) J. W. Birkeland and J. H. Shaw, J. Opt. Soc. Amer., 49, 637 (1959). (8) R. L. Bowman and J. H. Shaw, Appl. Opt., 2,176 (1963). (9) R. H. Pierson, A. N. Fletcher, and E. St. C. Gantz, ANAL. CHEM.,28, 1218 (1956).
Both instruments rest on a table fitted with 5-inch diameter wheels and two shelves containing accessories and tubing. The assembly is compact and can readily be moved from one laboratory to another. A simple switching arrangement permits monitoring of three gas streams: helium to provide an inert atmosphere in the sample cell for a background trace, gas to a reaction vessel, and gas from a reaction vessel. Gases and Gas Mixtures. Standard gas mixtures were prepared gravimetrically rather than by pressure, although other mixtures were also made by pressure for comparative purposes. Individual gases were obtained in lecture size bottles with grade and purity as follows: SO2, commercial grade, 99.9%; CHI, CP grade, 99.1 %; CO?,bone-dry grade, 99.8 %; CO, CP grade, 99.5 %; and COS, 96 %. Qualitative Analysis. Since the spectrophotometer is a single-beam instrument, bands in the spectrum from CO? and moisture in the air are corrected by means of a background trace before and after each analysis. Spectra are obtained by simultaneously passing the pure gas and dilution gas (helium or nitrogen) through the gas cell in a flow-through manner. Scanning is begun and the dilution gas flow is adjusted until suitable spectra are obtained. Spectra are recorded for all constituents in the gas stream except the elemental gases. The complete spectrum is quite small-2 inches X 5 '/z inches-so an illuminated magnifier and a millimeter rule are used to determine the wavelengths of the bands. Quantitative Analysis. The small spectra makes it essential that measurement of band absorbance be accurate. Therefore, part of the standard infrared absorbance scale is redrawn, photographed, and reduced to the size of the instrument spectra. A transparent plastic overlay is then placed over the spectra and the absorbance of the individual band is measured. With the aid of the illuminated magnifier, readings can be obtained with an accuracy of 1 0 . 0 5 to *O,l and good reproducibility. After the spectra of gas mixtures of known concentrations are obtained and the band absorbance is measured, the latter values are plotted against corresponding concentrations to make a calibration curve. Gases in trace amounts can be determined if multiple interference is no problem and only simple mixtures with little overlapping of bands are analyzed. RESULTS AND DISCUSSION
Figures 1 ( A ) and 2 (C) show spectra of sulfur dioxide and carbonyl sulfide, each diluted with helium. (Background traces of all spectra were raised to avoid overlap of the two traces.) Apparent bands in the 4 to 5 micron and the 8micron regions are actually the result of filter changes. Table I gives infrared absorption bands for the common gases found in flue and producer gas, except for elemental gases that do not absorb in the 2.5 to 14.5 micron range. Only the bands used to identify the gases are listed. Relative
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Table I. Absorption Bands for Constituents in Flue and Producer Gases
Gaseous compound
Wavelength,
Band vibrationa C AS AS AS
SO2
cos
1,340 (s) 1,160 (m) 1,130 (m) 3,030 (m) 2,940 (m) 1,360 (s) 1,330 (s) 3,850 (w) 3 9 700 (w) 2,320 (s) 2,170 (s) 2,130 (s) 2,860 (m) 2,020 (s)
3.30 (m) 3.40 (m) 7 . 3 5 (s) 7.50 (s) 2.60 (w) 2.70 (w) 4.30 (s) 4.60 (s) 4.70 (s) 3.50 (m) 4.95 (s) 9.80 (m) 12.00 (s) 4.60 (w) 6.55 (s)
c
AS (C=O) AS (S=C=O) ss (S=C=O)
cs2
2,470 (w) 1,380 (s)
8.85 (m)
AS S S
co
4.05 (w) 7.25 (s)
(Not resolved)
AS(CH3) AS (CHI) SB (CHI) SB (CH3)
coz
Frequency, cm-L
7.45 (s) 8.60 (m)
ss ss
CHd
/J
AS
3
4
4
5
5
6
6
7
7
8
8
4
5
3
4
5
1,020 (m) 830 (s) 2,170 (w) 1,530 (s)
C, combination; AS, asymmetric stretching; SS, symmetric stretching; SB, symmetric bending; S, stretching.
3
3
8
1 0 1 2 1 4
8 1 0 1 2 1 4
6
8
7
7
6
8
8
8
1 0 1 2 1 4
1 0 1 2 1 4
MICRONS
Figure 2. (C) Carbonyl sulfide (impurities carbon dioxide and carbon disulfide) (D) Gas mixture No. 4 used as standard for quantitative analysis
3
4
5
3
4
5
6
6
7
7
8
8
8
1 0 1 2 1 4
MICRONS
MICRONS
Figure 1. ( A ) Sulfur dioxide (impurity carbon dioxide) ( B ) Gas mixture No. 1 used as standard for quantitative analysis
Figure 3. Reaction of SO, with CO in the presence of catalyst: ( E ) inlet gas stream, (F)outlet gas stream
intensities are designated as strong (s), medium (m), or weak (w). Included in the list is carbon disulfide, a common impurity of carbonyl sulfide. Spectra were made for oxides of nitrogen and several other gases, but none were found in the gases that were monitored. Figures 1 ( B ) and 2 (D)show traces for two gas mixtures that were used as standards for quantitative analysis and converted into calibration charts. Charts were made for all gases of interest. A linear relationship usually existed over a small range, e.g., four gas mixtures containing concentrations 0 showed a of sulfur dioxide from = 300 ppm up to ~ 4 0 0 ppm straight line relationship for sulfur dioxide’s most intense 1498
bands at 7.25 and 7.45 microns. The relationship above this concentration was not determined for sulfur dioxide. Accuracy of the infrared analyzer for sulfur dioxide was determined by comparing with analyses of a flue gas mixture of unknown sulfur dioxide concentration by gas chromatograph and mass spectrometer. Results of these analyses were as follows: gas chromatograph, 2500 ppm; mass spectrometer, 2300 ppm; infrared analyzer, 2580 ppm. Application of Method. Infrared monitoring was conducted of the inlet and outlet gas streams in three different research investigations of sulfur dioxide removal from flue gas by chemical reaction or absorption. Spectra of gases in these three studies are shown in Figures 3-5, with com-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
r
I
I'
1 3
4
5
U
L j
I
L
L
I . L U L L
6
7
8
8 1 0 1 2 1 4
3
4
5
3
4
5
6
7
8
8
101214
6 7 8
8
101214
MICRONS
MICRONS
Figure 4, Reaction of SOz with FeS: (G)inlet gas stream,
Figure 5. Absorption of SO? on eastern oil shales: ( I ) inlet gas stream, ( J ) outlet gas stream
( H )outlet gas stream
parison of inlet and outlet gases given in each instance. It is quite apparent that chemical reaction took place. In Figure 3, for example, carbon dioxide, carbon monoxide, and sulfur dioxide are present in the inlet gas stream; in the outlet stream, the carbon dioxide concentration is about the same, but the carbon monoxide content is about one fourth of what it was before. Moreover, carbonyl sulfide bands are present but the sulfur dioxide bands are not. In this work, the reaction between flue gas and producer gas was being investigated. The objective was to precipitate elemental sulfur and convert carbon monoxide to carbon dioxide. In the initial experiments, a small amount of sulfur was formed, but 4 to 6 hours was required for each experiment. The first few tests with the in-line infrared analyzer showed that the carbon monoxide was completely converted to carbon dioxide within a few minutes and the sulfur dioxide concentration remained relatively unchanged. In a later test under different conditions, the spectra shown in Figure 3 (F)was obtained after two minutes of operation, In this case, sulfur dioxide was converted to carbonyl sulfide, as evidenced by the bands at 4.95 and 12.00 microns. The test was discontinued because the objective was not met, thus saving 4 to 6 hours of running time. Figure 4 shows the sulfur dioxide (diluted with nitrogen) concentration of inlet and outlet gas streams to be about 3700
and 370 ppm, respectively-a reduction of about 90 per cent. Figure 5 shows a small decrease in sulfur dioxide content, representing a little more than 25 per cent removal. CONCLUSIONS
Qualitative and quantitative analyses of gas mixtures are obtainable in 5 to 10 minutes by repeated, rapid scanning with an infrared spectrometer. Rapid analysis permits evaluation of chemical reactions during an experiment and provides a basis for stopping the experiment or continuing it. Portability of the analytical system makes possible the monitoring of research studies in different laboratories. Practical use of the method is limited to laboratory research studies with clean dry gases; it cannot be used in the plant to analyze actual stack gases. Analyses at temperatures up to 100 "C resulted in good spectra, although analyses have been made at temperatures up to 300 "C. At the higher temperature, the silicone gaskets had to be replaced with aluminum gaskets. Sensitivity of the instrument was lower, however, and the background trace dropped down to less than 50 per cent transmittance. RECEIVED for review January 28, 1971. Accepted May 25, 1971.
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