Fingerprinting pollutant discharges from synfuels plants A field-tested sampling and analytical approach may help to determine each source that contributes to atmospheric loading at a given receptor site
Karl J. Bombaugh Ken W.Lee
Radian Corporation Austin, Tex. 78766 Aerosol formation in the air is a significant environmental problem, particularly in highly industrialized metropolitan areas. In order to cope with this problem, a considerable effort has been expended to identify atmospheric pollutants that contribute to the formation of aerosols and to develop methods for tracing them to their respective sources. Until now, aerosol monitoring programs have been limited to “massloading” measurements, whereby only the quantity of aerosols is measured. Unfortunately, in situations in which multiple and varied sources contribute to an aerosol loading, it is difficult to relate a given mass-loading measurement to any particular contributing source. Consequently, the usefulness of the measurement is severely limited. This limitation can be overcome by developing “fingerprints,” or identifiers, for pollutant discharges that can be used to establish a relationship between a contributing pollutant and its discharge source. However, the development of reliable identification methods will generally require a sound knowledge of the chemicals and processes involved. Many chemical processes generate groups of compounds that are characteristic of cither the processes or the feedstocks being used. These compounds frequently cause environmental concern because they may be contained in the process’s discharges and become environmental pollutants. As a group, these compounds are signifi1142
Environmental Science 8. Technology
cant because they can be used as identifiers to trace discharges. To be used as an identifier, any characteristic group of compounds must be distinguishable from its background. A simple method of making the required distinction is by the use of a gas chromatograph (GC) with selective detection. By-products from many industrial processes contain high levels of the same compounds that are discharged
Source apporfionment Sowce apportionment” determining the share of pol &at each of several sources co to the total atmos
tion between a collected organ lutant and its emission some. icie addresses the crit uch methods by offering that has been field-tes mainly at Kosovo, Yugoslavk, wever, it should be noted e of its high discharge le? and its isolated location. the Kos plant‘s discharges could be trace their source without difficulty. In I ideal situations, such as contra plants operating in highly diversi industrlal complexes, defining pc transport could be much IT cult and could place great ds on the scientist’s skills. m Y these difficult situations Wet rlnting approach is n ,,,
from those processes as pollutants. Therefore, by analyzing appropriate by-products from an emission source, it should be possible to develop a profile that is characteristic of the discharges from that source. Such a profile can be used as a fingerprint to establish a connection between a collected aerosol and its source.
The Kosovo study Since the profile can be defined without complete speciation and quantification, this approach is extremely cost-effective. It was used in an ambient aerosol study that was conducted at an industrial complex in the Kosovo region of Yugoslavia ( E S & T , November 1980, p. 1285) ( I ) . The complex contained a commercial coal gasification plant with ancillary gas and water extraction facilities. It also contained an ammonia-based fertilizer plant, an 800-MW coal-fired power plant, a Fleissner coal drying plant, and other support facil-
by-prod The first task in the fingerprinting approach is to select a by-product or a group of by-products that is repre sentative of the plant’s pollutant di5 charge and that meets the following criteria for providing an identifier: The representative by-product should contain a compound group that is uniquely characteristic of the source. The unique grwp must ba distinguishable hom a complex background matrlx. The distinguishing group must
0013-936X/81~0915-1142501.25/0 0 1981 American Chemical Society
InrtrUlnOnt: Hewlen-PackardModel 5710 Column: 3.1 m x 4 mm 1.d. 10% OV-1OT
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Volume 15. Number 10.October 1981
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ities, e.g., a steam-generating plant. The Kosovo coal gasification plant is of environmental interest because it utilizes Lurgi processes (2). Although this plant lacks the environmental controls that may be considered essential to a US. plant, the overall composition of its discharge is chemically representative of the Lurgi gasification process. The Lurgi gasification process is of particular interest because it represents a commercially feasible technology for the gasification and indirect liquefaction of coal. Because this process is receiving worldwide consideration for use as a means of producing both fuels and chemical feedstocks, a timely characterization of the potential environmental pollutants produced by the process is warranted. Knowledge about the composition of its process discharges can be of real value in tracking its pollutants and determining their ultimate fate in the environment. Most of the organic species in the discharges from a Lurgi-type process result either from combustion or coal pyrolysis, also known as devolatilization. The combustion products are primarily fixed gases and light hydrocarbons. The pyrolysis products are largely light aromatics, including benzene, alkyl benzenes, polycyclic aromatics, and phenols, plus heterocyclic compounds, such as thiophenes, pyridines, and quinolines. These compounds are present as isomeric species within each class of compound. Virtually all of these compounds originate in the gasifier and are separated into by-product fractions later in the process. Since the separations are based on physical parameters, there is considerable overlap in the composition of the by-products. Their components are unique to the coal gasification-devolatilization process and are, therefore, different from the components in the waste discharges from other types of processes within the Kosovo complex. Because of the overlap in the composition of these by-products, similar compounds will be found in many of the gasification plants’ major atmospheric discharge streams (3-5). The Lurgi process, as used at Kosovo, produces four major liquid byproducts: naphtha, medium oil, tar, and crude phenol. Naphtha consists primarily of light aromatics and organic sulfur species such as thiophenes. Medium oil consists primarily of C ~ - C aromatics I~ and includes arrays of sulfur and nitrogen heterocyclic compounds. Tar is a complex mixture of strongly associated aromatic substances which also contains heteroat1144
EnvironmentalScience 8 Technology
oms, such as oxygen, nitrogen, and sulfur. Although it contains a moderate amount of light and intermediate boiling-range aromatics, most of its components are poorly volatile and are not easily separated by chromatographic techniques. Crude phenol originates in the Phenasolvan plant, where it is separated from the phenolic wastewater by extraction with diisopropyl ether. The extracting solvent is recovered by distillation which leaves a by-product, “crude phenol,” which contains a mixture of phenols, alkyl phenols, and nitrogen-based compounds such as alkyl
pyridines and quinolines. Crude phenol contains virtually no alkyl aromatics, but may contain some polynuclear aromatic substances that are dissolved in the organic matrix. Gas chromatograms of the four major by-products are shown in Figure 1. Based on a common GC retention time scale, there is considerable overlap in their respective boiling ranges. Of the four major by-products, medium oil was selected for the fingerprinting study because it was the most representative of the organic materials being discharged to the atmosphere (3). Further, the boiling
Chromatograms of sulfur species in KOSOVOmedium dl and in th downwind sample'
or distinctive group of compounds and a complex background matrix. Gas 'chromatography offers a range of selective detectors, such as the sulfurswcific and the nitrogen-specific GC detector, as well as a mass Bpectrometer (MS), which uses either selective ion monitoring or selective ion scanning. Other fingerprinting techniques could include high-pressure liquid chromatography (HPLC) with selective or simultaneous multiple-waveDeveloping a fingerprint length detection, fluorescence detecA by-product's fingerprint can be tion, or amperometric detection, as developed by any analytical technique well as a combination of chemical and that can differentiate between a unique instrumental techniques.
range of its components was reasonably representative of the range of substances that could be collected by the vapor traps. The vapor traps used Tenax GC resin tocollect the airborne pollutants. Tar was also used to provide a fingerprint for the aerosols that were collected by high-volume filters, because the filterable particles discharged from the plant contained s i g nificant amounts of tar ( 5 ) .
I n this work, a GC (6) was used in both the sulfur and the nitrogen modes and a GC/MS was used with selective ion scanning. Chromatograms of byproducts obtained by these techniques were compared with those of the organic substances recovered from the ambient sample collections. Aerosol collection The Kosovo test was conducted over a 16-day period, during which ambient aerosols were collected at five stations located on a perimeter approximately 1-2 km outside the plant boundaries. Aerosols and vapors were collected concurrently on high-volume quartzfiber filters and in Tenax GC cartridges. The collected organics were separated from the collection matrices by both thermal desorption and solvent extraction for subsequent identification and quantification. The detailed results of the aerosol characterization have been reported separately (I). Initially, the organic substances collected from the Kosovo site were characterized by a GC in conjunction with a flame ionization detector (FID). The sample vapors were desorbed from the collection resin thermally and introduced into the chromatograph by a purge-and-cryogenic-trapping technique. Because the FID responds to organic carbon, these chromatograms provided a reasonably quantitative indication of all organic matter collected. Elution from the column was in boiling point order, so the chromatogram also produced an indication of the boiling range of the collected organics. By comparing the FID chromatogram of the sample with those of the by-products (Figure I), it was possible to select the by-product that was most applicable for use as a fingerprint. A comparison of the sample chromatogram and the selected by-product is shown in Figure 2. The top chromatogram was obtained with a sample that was collected downwind from the plant. The other was obtained with a sample of medium oil. From the identified peaks in each chromatogram, it is evident that the two samples have similar boiling rangesand indeed may contain many of the same components. However, the FID chromatogram does not provide an adequate fingerprint because it is not sufficiently discriminating. More selective information was obtained with a GC equipped with a sulfur-specific detector. This detector provides a quantitative response to organic sulfur and provides a very large discrimination ratio (50 0001) between organic sulfur and hydrocarVolume 15, Number IO, October 1981
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bons. Since interference from the hydrocarbon background matrix is minimal, sulfur species can be detected even at trace levels. Sulfur-specific chromatograms of the collected organics and the byproduct medium oil are shown in Figure 3. A similarity between the two chromatograms is readily apparent. A relationship exists between a component’s boiling point and its specific retention volume on a given substrate. By use of this principle, a calibration curve was prepared on the basis of hydrocarbon standards. Since many organic sulfur species, such as mercaptans and thiophenes, exhibit the same polarity as hydrocarbons, it is reasonable to assume that their individual retention characteristics could be used with this model. Thus. thiophene was readily identified as the peak in Figure 3 having the same retention as benzene (0 on the chromatogram in Figure 1). By deduction, the structure of methyl thiophene, which has two isomers, was tentatively assigned peak graup No. 1; similarly, the peak cluster labeled “2” 1148 Environmental Science 8 Technology
was tentatively attributed to dimethyl and an ethyl thiophene (four dimethyl isomers and two ethyl isomers are possible); and subsequently, peak cluster No. 3 was tentatively labeled trimethyl thiophene. With theaidofacalibrationcurve, as shown in Figure 4, the retention times of the major peaks were entered on the ordinate and the boiling points of the various thiophenes were entered on the abscissa. Their points of intersection were plotted in the graph. The remarkable fit obtained indicates that the proposed assignments are highly probable. These assignments were further supported by peak enrichment with known materials and selected peaks were ultimately confirmed by GCIMS. The remarkable facet of this approach is that it enabled the initial characterization to be completed in a very short time. Known materials or tedious search techniques that are normally required for sulfur species on GC/MS were not needed. In this respect, the technique was extremely cost-effective.
From the comparison of the chromatograms of theambient sample with that of medium oil and hy utilizing the tentative compositional assignments, it is reasonable to conclude that the two materials have a similar or common origin. For example, the chromatograms in Figure 5 , obtained with the “blank” resin and an upwind sample, show that the collected species did not originate upwind of the plant or with the collecting resin. The results obtained with the sulfur-specific detector were supported by results obtained for nitrogen species by using the Hall detector in the nitrogen-specific mode. As with the sulfur species, there was a similarity between the two chromatograms. Peaks were tentatively identified as alkylpyridine-type compounds by the same projected boiling-point method that was used for the sulfur species. However, a different retention curve was used here because a different GC column was required to separate the nitrogen-containing species. Tentative identities of the respective peak groups were supported by peak enrichment
and by GC/MS. Superimposed chromatograms identify those peaks that arecommon to both chromatograms. A large number of similar peaks indicate that the chromatogram of medium oil that is obtained with the nitrogen-specific detector clearly constitutes identifiers of the gaseous discharges from the Kosovo Lurgi gasification process. When used in conjunction with the sulfur-specific chromatogram, this peak similarity provides unequivocal evidence that the collected organics originated at the gasification plant. A representative fingerprint of the polynuclear aromatic hydrocarbons (PAHs) in Kosovo aerosols was peovided by the by-product tar. One component in this group is benzo[a]pyrene. Since the isomers comprising this group are comparatively difficult to separate, they tend to travel through the various plant processes, as well as through the air, as a group. Consequently, a GC/MS selective ion chromatogram provided a fingerprint to trace these hazardous PAHs to their source.
GURE 5
hmmatograms of a blank resin and of a sample collected upwind
upwnd sample
Using chemical reactions In cases in which selective detection is not applicable, other avenues are open to the environmental chemist. For example, a selective chemical reaction can provide a means of distinguishing between a specific class of compounds and its background. This principle can be demonstrated by comparing chromatograms of a phenolic fraction from two Kosovo by-products with those of a third by-product, crude phenol. The phenolic fraction is isolated from the predominantly hydrocarbon matrix of the medium 'oil and the tar by an acid-base extraction in which the phenols are converted to their sodium salts. In this form, they are readily extractable from the matrix as a group. Such chromatograms show a distinct similarity in that they all exhibit the same peak clusters. Each cluster represents a group of alkyl phenols that are present in each by-product. However, this similarity between crude phenol and the other Kosovo byproducts would not be recognizable without the selectivity introduced by the chemical separation (Figure 1). Fingerprint comparison A very important question is: What is the relationship between the fingerprints from two different gasification processes? Unfortunately, only limited information is available to answer the question. However, infor-
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mation presented in Figure 6 is a step in that direction. The cMomatograms in Figure 6 show a comparison of the nitrogen species in samples of water-soluble organics from two different gasification plants that were using different feed coals. One sample was taken from a Lurgi plant that was using Kosovo lignite; the other was taken from a Chapman-Wilputte gasifier (Kingsport, Tenn.) that was using Virginia bituminous coal (7). Both samples were taken from the hot gas - auench. liquor. The result of this comparison is informative. The chromatograms have 1148 Environmental Science 8 Technology
many peaks in common, indicating that the two samples may contain many of the same compounds. However, there appear to be some quantitative differences between the two, which may either relate to differences in their respective processes or to their feed coals. A major difference is evident in the greater proportion of a group of lower-boiling compounds (pyridines) in the sample from the Kosovo Lurgi plant that. cannot be explained by differences in evaporation or extraction. Because of such differences, the nittogen-specific chromatogram of the by-products or waste.s from the one plant can be distin-
guished from the other. On the basis of this result, it is conceivable that each gasification plant will produce fingerprints that are characteristic of the processes and the feed coals being used. Other important questions are: What is the relationship between fingerprints from two different types of sources, such as a coal gasification plant and a petroleum refinery, and will it be possible to differentiate between their fingerprints? Again, information with which to answer these questions is limited, but the data that are available seem to show a distinguishable difference between their profiles. For example, with the aid of a GC/MS with selective ion monitoring, Gallegos prepared a profile of sulfur species in heavy gasoline (8). This profile appeared to be different from those of the gasification products. Earlier, using a coulometric detector, Martin and Grant prepared sulfurspecific chromatograms of a variety of petroleum refinery products which showed that these products contained isomer distributions of mercaptans and thiophenes (9). Although these refinery products contained basically the same types of compounds as the gasification products, their profiles appeared to be different. A closer comparison (for example, chromatograms developed under identical conditions) of the products and discharges from these two types of sources will be needed to determine whether distinction between their emissions is possible when sulfur profiles are used as an identifier. However, profiles of other groups of compounds may be used in conjunction with the sulfur species to increase discrimination. For example, the phenols and the nitrogen heterocyclics, both produced in comparatively large quantities in the coal gasification/ devolatilization reaction, can provide added dimensions to the definition of the emission profile of the gasification process; these added dimensions may provide the basis for differentiating between chemically similar emissions from different sources. Acknowledgments
This work was sponsored by the US.Environmental Protection Agency. The authors thank Mr. T. Kelly Janes of the Industrial and Environmental Research Laboratory and Mr. Ronald K.Patterson of the Environmental Science Research Laboratory for their contributions to this program. The authors also thank L. D. Ogle and D. S. Lewis for their work in gas chromatography and C. H. Williams for his GC/MS contributions.
Before publication. this article was read and commented on by Charles C. Coutant, O a k Ridge National Laboratory. Oak Ridge, Tcnn. 37830. and William H. Glaze. University of Texas-Dallas. Richardson, Trx. 75080.
References
From new sources of energy to new ideas about the origin of life, from computers to recombinant DNA 1980 was an exciting
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year for research in chemistry.
( I ) Bombaugh. K. J.. CI al. "Aerosol CharacIcri7alion of Ambicnt Air N u r a Commercii1 Lurgl Coal Gaaliwiion Plant". EPA Keplrl Nu 600 1-80-117: Kosovo. Yugocla\ia.
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"What's Happening in Chemistry"
(2) Kudolph. P F. H "Lurgi Procr~rin Coal Garifii~lionin Fncrg) Technology Hand. huk".Considine. D.M . . I d . ; Mv(irw.H~lI. Inc.. N W York. 1911.pp. I R U ?on (31 Bwnbdugh. K. J.;CorbFlt. W k. "Kosovo tiaitficaiion Tcil hOgr3m Kcdi,-Part II. Ddu Andl)sis and Inierpretauun": EPA 600 7-79-217,S)mpium on Fnwonmcnial Aspecls 01 Fuel Conversion Tcchnulugy I V : April 1919. Hollyuood. Fla , IV7Y. pp. I h l -201 (4) Rombaugh. K. J.:Corbctt. W. E.; Matwon. M D. "Lnvironmcntal Assmmcnl: Source Tesl and l\alustion Repori Lurgi (Korovo) Medium-Rtu Gmficauon. Phase I":FPA6U0/1-19-190;August 1919 ( 5 ) Bmbaugh. K. J , e t a1 "An Environmcntall) Baied Evalusiion of the Mullimcdia Diiuhargci from the Korovo Lure) Coal (iwiliraim S)stem". S)mposwm on Fnwronmcni~l A,pecls of l'ucl Convcr\ion Tcchnoldgy V Si. I.ouis. M o , Sept. lb-19. tYhV
( 6 ) Andcnon. R J ,Hall, R.C. "Odfcrcntial Flcctrolyiic Conductivity Octwtor for GC. Design and Applications American Labora. tar)". Tracer. Inc; Austin. Tcx..February 1980. (7) Rombaugh. K. J. "Analysirof GrabSample, from Iired.Bed Coal Garilicaiion Pre C C ~ ~ C S " : EPA R c p r t 600/1-11-141: Dec c m k r 1911 ( 8 ) Gallcgm. E. J. A n d . Chem 1975. 47.
