Determination of Volatile Hydrocarbons in Coals andShales Using Supercritical Fluid Extraction and

Chromatography

Wenbao Li,† Iulia M. Lazar, Yanjian J. Wan, Steven J. Butala, Yufeng Shen,Abdul Malik,‡ and Milton L. Lee*

Department of Chemistry and Biochemistry, Brigham Young University,Provo, Utah 84602-5700

Received October 9, 1996. Revised Manuscript Received June 25, 1997X

Conventional analytical techniques, such as headspace gas chromatography and Soxhletextraction, can provide compositional information for the gaseous (C1-5) and heavy (C15+)hydrocarbon constituents, respectively. The volatile (C6-14) hydrocarbons, if present, usually goundetected because of volatility fractionation and loss. In this study, supercritical CO2 was usedto extract the C6-C14 volatile hydrocarbons from pulverized coal samples. Capillary column gaschromatography/mass spectrometry was used to identify the mixture components, and packedcapillary column supercritical fluid chromatography was used to separate and quantify thealiphatic and aromatic hydrocarbon class fractions. It was found that the compositions of thelight hydrocarbon fractions included several homologous series of normal and branched aliphatichydrocarbons, cyclic and aromatic hydrocarbons, and alkyl-substituted benzenes and naphtha-lenes; the concentrations of these volatile hydrocarbons ranged between 0.01 and 0.2 wt % of thebulk material for different coal and shale samples.

Introduction

The extraction of coals with supercritical fluids (SFE),also referred to as supercritical gas extraction, has beenapplied for a variety of reasons during the past 20years.1,2 The primary reasons include the productionof liquid fuels from coal, the elucidation of coal structureand mechanisms of coalification, and the selectiveremoval of sulfur from coal. When SFE has beenapplied to coal, usually an organic solvent under super-critical conditions was used. The process is analogousto both solvent extraction and distillation; supercriticalgas extraction is usually carried out at 350-450 °C andat a pressure of 10-20 MPa.2 Therefore, supercriticalgas extraction generally extracts heavy materials aswell as volatile components, similar to an organicsolvent, and results in possible thermal degradation ofthe coal macromolecular network.3

SFE with CO2 offers several advantages over conven-tional supercritical gas extraction and solvent extrac-tion. Supercritical CO2 has higher diffusivity and lowerviscosity compared to liquid solvents, which shouldresult in improved mass transfer properties duringextraction. The solvent strength of supercritical CO2is dependent on its temperature and pressure, whichcan be easily manipulated to extract certain classes ofcompounds. Carbon dioxide is relatively nonreactive,

nonpolar, nontoxic, available in purified form, and hasa low critical temperature. These properties makesupercritical CO2 an ideal vehicle for extraction ofnonpolar hydrocarbons and allow the extraction to beperformed at relatively low temperatures to avoid anypossible thermal degradation. Supercritical CO2 hasbeen used successfully to extract polychlorinated bi-phenyls, polycyclic aromatic hydrocarbons, and aliphatichydrocarbons from different matrices.4-7

The object of this study was to develop a method forextracting and analyzing relatively light hydrocarbons(C6-14) from coal which would provide for (a) quantita-tive trapping of the volatile extracts and (b) rapid group-type analysis of the collected fractions. Conventionalanalytical techniques such as headspace gas chroma-tography and solvent extraction can only provide infor-mation for the gaseous (C1-5) and heavy hydrocarbon(C15+) constituents.8,9 The C6-14 fraction usually goesundetected because of volatility loss. In order to addressthis problem, a method for using supercritical CO2 toextract the C6-14 hydrocarbons from coal, followed bygroup-type separation of the extracts with packedcapillary column supercritical fluid chromatography(SFC), was developed in this study. Twenty different

* Author to whom correspondence should be addressed.† Current address: Haskell Laboratory, DuPont Central Research

and Development, P.O. Box 50, Newark, DE 19714.‡ Current address: Department of Chemistry, University of South

Florida, Tampa, FL 33620.X Abstract published in Advance ACS Abstracts, August 15, 1997.(1) Kershaw, J. R. J. Supercrit. Fluids 1989, 2, 35-45.(2) Olcay, A. InNew Trends in Coal Science; Yurum, Y., Ed. Kluwer

Academic Publishers: New York, 1988; pp 401-415.(3) Chang, H. C. K. Ph.D. Dissertation, Brigham Young University,

1989.

(4) Hawthorne, S. B.; Krieger, M. S.; Miller, D. Anal. Chem. 1989,61, 736-740.

(5) Cross, R. F.; Ezzell, J. L.; Porter, N. L.; Richter, B. E. Am. Lab.1994, Aug, 12-17.

(6) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M. Anal. Chem.1993, 65, 2549-2551.

(7) Brooks, M. W.; Uden, P. C. J. Chromatogr. 1993, 637, 175-179.(8) Rao, B. R. Determination of the Maximum Emissions from

Storage Tanks for Heavy Fuel Oil. In Applied Headspace Gas Chro-matography; Kolb, B., Ed., Heydon and Sons, Ltd.: New York, 1980;Chapter 7.

(9) Chang, H-C. K.; Bartle, K. D.; Markides, K. E.; Lee, M. L.Structural Comparison of Low Molecular-Weight Extractable Com-pounds in Different Rank Coals using Capillary Column Gas Chro-matography In Advances in Coal Spectroscopy; Meuzelar, H., Ed.,Plenum Press: New York, 1991.

945Energy & Fuels 1997, 11, 945-950

S0887-0624(96)00176-4 CCC: $14.00 © 1997 American Chemical Society

US coals and shale samples covering a broad range ofgeologic age were analyzed using this new method.

Experimental Section

Supercritical Fluid Extraction. SFE with neat CO2

(Scott Specialty Gases, Plumsteadville, PA) was performed off-line using a Lee Scientific Model 501 SFC system (Dionex,Sunnyvale, CA). A 55 mm × 9.2 mm i.d. stainless steelextraction cell rated to 34.5 MPa and containing a volume of3.5 mL (Dionex, Salt Lake Technical Center, Salt Lake City,UT) was used for all extractions within a pressure range of10.1-35.5 MPa. The collection vials were home-built from 1/8in. stainless steel tubing, with dimensions of 250 mm × 3.0mm i.d. A simplified schematic diagram of the SFE system isshown in Figure 1. In the case that there was not enoughcoal sample to fill the extraction cell, silanized glass beads (25µm diameter; Supelco, Bellefonte, PA) was used to take up theremaining cell void volume. In order to further minimize anydead volume, a 200 µm i.d. fused silica capillary (PolymicroTechnologies, Phoenix, AZ) was used after the extraction cellto conduct the extraction fluid to the collection vial. A 15 µmi.d. fused silica capillary (Polymicro Technologies) was usedas a flow restrictor. Spectral grade carbon disulfide (EMScience, Gibbstown, NJ), maintained at -5 °C with a coolingsystem, was used for collection of extracts.

Supercritical Fluid Chromatography. The SFC ap-paratus used for group-type separation was the same as isdescribed in ref 10. Timed-split injection with 1.2 s valve-opentime was used for sample introduction. Several fused silicacapillary columns (98 cm × 200 µm i.d., Polymicro Technolo-gies) packed with 10 µm (60 Å) silica particles (KeystoneScientific, Bellefonte, PA) were used to prepare the analyticalcolumns. Each column was prepared using a CO2 slurrypacking method.11 A 35 cm × 10 µm i.d. deactivated fusedsilica capillary was used as a flow restrictor for the SFC.Group separations were conducted at a constant temperatureof 45 °C and constant pressure of 25.3 MPa.Gas Chromatography/Mass Spectrometry. Individual

components in the extracts were identified using an HP5890gas chromatograph equipped with an HP5970 mass selectivedetector (Hewlett-Packard, Wilmington, DE). A 25 m × 200µm i.d. fused silica open tubular column coated with SE-S4(df ) 0.25 µm) was used as the separation column. High-purityhelium (99.99%) was used as carrier gas. The column tem-perature was programmed from 40 to 300 °C at 2.5 °C/minafter an initial 10 min isothermal period. Peak assignmentswere made by comparison of sample component retentiontimes, elution patterns, and mass spectra with those ofstandard compounds.Standard Chemicals and Materials. All chemical stan-

dards (99%) were purchased from Aldrich (Milwaukee, WI) andused as received. They include n-alkanes from C7 to C25,methylcyclohexane, cis-1,2-dimethylcyclohexane, cis- and trans-1,4-dimethylcyclohexane, 1,4-dimethyl-1-cyclohexene, propyl-cyclohexane, ethylcyclopentane, cycloheptatriene, bicylo[2.2.1]-hepta-2,5-diene, 1-octene, toluene, 2-ethyltoluene, mesitylene,sec-butylbenzene, 1,2,3,4-tetrahydronaphthalene, 2-methyl-naphthalene, phenanthrene, biphenyl, p-terphenyl, acenaph-thylene, fluorene, and 2-ethyltoluene. Sources of the coal andshale samples are listed in Table 1.Analytical Procedure. Before performing an extraction,

the extraction cell was washed with a series of organic solvents(tetrahydrofuran, methylene chloride, and carbon disulfide)and then dried at 120 °C in an oven. This procedure was

(10) Li, W.; Malik, A.; Lee, M. L.; Jones, B. A.; Porter, N. L.; Richter,B. E. Anal. Chem. 1995, 67, 647-654.

(11) Malik, A.; Li, W.; Lee, M. L. J. Microcol. Sep. 1993, 5, 361-369.

Table 1. Hydrocarbon Group-Type Quantitation of Low Molecular Weight Hydrocarbons (C6-C14) in Selected UnitedStates Coals and Interbedded Shale Kerogensa,b

sample location (county, state) water (%) aliphatic (%) aromatic (%)

Beulah-Zap coalc Mercer, ND 28.5 0.01 jWyodak-Anderson coalc Campbell, WY 21.6 0.04 0.06Blind Canyon coalc Emery, UT 3.3 0.20 0.08Basal Fruitlandd San Juan, NM 0.5 0.13 0.04Basal Fruitland shale kerogend San Juan, NM 14.8 0.07 0.02Intermediate Fruitland coale La Plata, CO 0.0 0.05 jIntermediate Fruitland shale kerogene La Plata, CO k 0.12 0.03Basal Fruitland coalc La Plata, CO 0.4 0.05 0.01Basal Fruitland shale kerogenc La Plata, CO k 0.11 0.03Basal Fruitland coalf La Plata, CO k 0.08 0.01Basal Fruitland shale kerogenf La Plata, CO k 0.14 0.02Illinois No. 6 coalc St. Clair, IL 4.5 0.03 0.01Pittsburgh No. 8 coalc Greene, PA 0.4 0.15 0.09Lewiston-Stockton coalc Logan, WV 0.3 0.04 0.01Upper Freeport coalc Indiana, PA 0.3 0.07 jPottsville coalg Jefferson, AL 0.1 0.03 jPottsville coalh Jefferson, AL 0.2 0.04 jPottsville coali Jefferson, AL 0.1 0.03 jPottsville shale kerogeni Jefferson, AL 0.6 0.04 0.01Pocahontas No. 3 coalc Buchanan, VA k 0.05 0.08a Based on wt % of total coal or shale. b Determined from supercritical fluid chromatographic peak area measurements. Values contain

minor contributions from hydrocarbons larger than C15. c Premium coal sample from Argonne National Laboratory. d Hamilton No. 3well, San Juan Basin (30 T32N R10W). e Valencia Canyon Southern Ute No. 32-1 well, San Juan Basin (32 T33N R11W). f Southern UteTribal H-1 well, San Juan Basin (18 T32N R10W). g Pratt, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). h Mary Lee,Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). i Black Creek, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W).j Means not detected or less than 0.01%. k Information unavailable.

Figure 1. Schematic diagram of the supercritical fluidextraction system.

946 Energy & Fuels, Vol. 11, No. 5, 1997 Li et al.

utilized to remove polar, intermediate, and nonpolar residues.The cell was filled with a pulverized sample (200 mesh) andthen installed in the extraction system. The mass of coalextracted was determined by mass difference (i.e., subtractingthe mass of the empty cell and the mass of the glass beadsfrom the total mass to give the mass of coal extracted). Theextraction was conducted dynamically at 120 °C and 20.3 MPafor 2 h. A 0.75-1.0 mL volume of carbon disulfide waspreloaded into the collection vial and placed in a cooling bath.After the system was depressurized, the extract was trans-ferred to a 2 mL vial and analyzed by GC/MS and SFC.Compounds identified by GC/MS are listed in Table 2. TheSFC group-type analysis was conducted as described in ref 10,and quantitation was based on standard calibration.Standard solutions (concentrations ranging from 5 µg mL-1

to 10 mg mL-1) containing equal amounts of n-alkanes fromC6 to C14 were prepared in carbon disulfide and used forcalibrating the SFC for aliphatic hydrocarbons; standardsolutions of 2-ethyltoluene were used for calibration foraromatic hydrocarbons. For the aliphatic hydrocarbon quan-titation, it was found necessary to correct the peak area asthe CS2 solvent coeluted with the aliphatic fraction. This wasaccomplished by measuring the pure CS2 peak area 10 timesand then subtracting the average value from the aliphatic-CS2 peak.

Results and Discussion

It is well-known that coal is a porous material12containing macropores (diameters greater than 40 nm),mesopores (diameters between 2 and 40 nm), andmicropores (diameters less than 2 nm). Walker et al.13reported that CO2 can be easily taken up by themicropores, which constitute most of the surface area.Hence, based on kinetic considerations only, CO2 shouldbe an ideal solvent for the extraction of coal matrices.The extraction efficiency and selectivity obviously de-pend on the CO2 solvating power which is somewhatdependent on CO2 density, temperature, and extractiontime. The extraction conditions were selected to maxi-mize the extraction of the C6-14 hydrocarbons, and atthe same time, to minimize the extraction of highermolecular weight hydrocarbons and polar compounds.The selection of the extraction temperature is of

primary importance, since temperature considerablyaffects the supercritical fluid density. Increasing thetemperature increases the solute solubility and volatility

and also increases the possibilities of degradation andextraction of heavy hydrocarbons. Experiments wereconducted at 120, 150, 200, and 300 °C for a BlindCanyon coal. It was found that by simply increasingthe temperature, the content of lower molecular weighthydrocarbons did not change significantly (<20%), whilethe content of higher molecular weight hydrocarbonsincreased. Therefore, 120 °C, which is higher than theboiling point of water, was selected. This temperatureprovided sufficient extraction power and also avoidedmost problems associated with plugging of the restrictorwith ice.The extraction pressure is as important as the tem-

perature. Changing the pressure also changes theextraction properties. At 120 °C, pressures of 70, 150,200, and 300 atm were tested for extraction efficiencyusing Stockton and Pittsburgh No. 8 coals. The extrac-tion time used was approximately 2 h. A higherpressure resulted in higher extraction efficiency, butalso in greater extraction of heavy hydrocarbons. Apressure of 200 atm was selected for extraction of thesamples.Experiments demonstrated that proper flow restric-

tion after the collection vial was critical for successfulextractions. Coals usually contain waxy material thateasily precipitates in the capillary tubing from theextraction cell to the collection vial. A restrictor afterthe collection vial helps to maintain the CO2 solvatingpower and eliminate precipitation and subsequent plug-ging of the tubing. Furthermore, it was found thatoccasional heating of the transfer lines was sometimesrequired to eliminate wax buildup in the lines.The completeness of extraction depends on the ex-

traction conditions. In order to optimize the extractionconditions, extraction times of 1, 3, 5, 10, and 15 h at120 °C and 200 atm, and for flow rates in the range of35-50 mLmin-1 (gaseous CO2 at atmospheric pressure)through a 15 µm i.d. restrictor, were studied using a

(12) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire, R. J. TheStructure and Reaction Processes of Coal; Plenum Press: New York,1994; p 157.

(13) Walker, P. L.; Verma, S. K.; Rivera-Utrilla, J.; Davis, A. Fuel1988, 67, 1615-1623.

Table 2. General Composition of the Supercritical CO2Extracts of Coal and Shale Samples

peak no. compounds peak no. compounds

O n-alkane 14 C5-cyclohexane0 branched alkane 15 naphthalene1 C1-cyclohexane 16 C6-benzene2 C2-cyclopentane 17 C1-naphthalene3 C3-cyclopentane 18 biphenyl4 toluene 19 C2-naphthalene5 C2-cyclohexane 20 C1-biphenyl6 C4-cyclopentane 21 C3-naphthalene7 C3-cyclohexane 22 C2-biphenyl8 C2-benzene 23 C4-naphthalene9 C3-cyclohexane 24 C5-naphthalene10 C3-benzene 25 C6-naphthalene11 C4-cyclohexane 26 pristane12 C4-benzene 27 phytane13 C5-benzene

Figure 2. Determination of time for supercritical fluidextraction completeness.

Figure 3. Determination of collection efficiency after super-critical fluid extraction.

Volatile Hydrocarbons in Coals and Shales Energy & Fuels, Vol. 11, No. 5, 1997 947

Pittsburgh No. 8 coal. The extraction was performedin the dynamic mode, and the extracts were collectedat the selected time intervals and injected into the GCfor analysis. Figure 2 shows the relationship betweenthe sum of the peak areas (total extract) and theextraction time. It can be seen that the longer theextraction time, the greater the amount extracted.Approximately 90% of the estimated total C6-14 hydro-carbons could be extracted after 2 h. However, a minorbut detectable amount of material still could be ex-tracted after 10 h. Therefore, considering the analysistime versus extraction efficiency, a 2 h extraction timewas considered to be adequate for this study. Bycomparison, this time is much shorter than conventionalSoxhlet extraction which typically takes between 24 and96 h for adequate extraction. Indeed, it was demon-

strated by Given,14 citing Vahrman, that several weeksof Soxhlet extraction may be required for near completeremoval of the bitumen.The collection efficiency is influenced by the config-

uration of the collection vial, the collection solvent, thetemperature, and the collection procedure. In thisstudy, the collection vial was made from long, narrowstainless steel tubing because of both safety and trap-ping efficiency considerations. To handle volatile hy-drocarbons, the collection vial had to be sealed, and theCO2 released through the proper restrictor. Since thevial was maintained under high liquid CO2 pressure, itwas fabricated out of stainless steel.The collection solvent was selected based on a number

(14) Given, P. H.; Marzec, A.; Burton, W. A.; Lynch, L. J.; Gerstein,P. C. Fuel 1986, 65, 155-163.

Figure 4. Capillary column gas chromatogram of the supercritical CO2 extract of a Pittsburgh No. 8 coal. Conditions: 25 m ×200 µm i.d. capillary column coated with 0.25 µm film of SE-54, helium carrier gas, temperature programmed from 40 to 300 °Cat 2.5 °C min-1 after a 10 min initial isothermal period, 1 µL splitless injection, FID. Peak identifications are listed in Table 1.

Figure 5. Capillary column gas chromatogram of the supercritical CO2 extract of a Basal Fruitland (Southern Ute Tribal H-1well) shale. Conditions: same as described in Figure 4. Peak identifications are listed in Table 1.

948 Energy & Fuels, Vol. 11, No. 5, 1997 Li et al.

of properties, including its response in the flame ioniza-tion detector (FID). Carbon disulfide has a low responsein the FID and is available in high purity. This helpedto reduce any interference from the solvent peak duringquantitation.The collection temperature was evaluated for the

range of -40 to 10 °C. The lowest temperature (-40°C) was achieved using a dry ice and ethanol solution;all other temperatures were achieved using a coolingbath with polyethylene glycol/water solution. Very lowtemperatures (-40 °C) led to frequent plugging of therestrictor, while temperatures higher than 10 °C did notfavor the collection of volatile compounds. We foundthat a collection temperature in the range of 0 to -5 °Cwas adequate to trap the low molecular weight com-pounds from the extraction.In order to avoid any volatile losses during collection

and to further verify the collection efficiency, threecollection vials were connected to each other in seriesusing 50 µm i.d. fused silica capillary tubing. Each vialwas filled with the same amount of CS2 and immersedin the cooling bath. The 15 µm i.d. restrictor tubing wasattached to the outlet of the last vial. After theextraction was finished, the solution in each vial wasinjected into the GC and analyzed. Figure 3 shows therelative total peak area versus collection vial number.It can be seen that more than 95% of the solute wascollected in the first vial, while less than 4% wascollected in the other vials together. All further extrac-tions were conducted using only one collection vial.Peak Identification Using GC/MS. Identification

of individual C6-15 hydrocarbons in the coal and shaleextracts (Table 2) was accomplished by comparing GCretention times and mass spectra of the resolved sample

components with those of standard compounds. Thecompounds identified include normal hydrocarbons,branched and cyclic aliphatic hydrocarbons, and aro-matic hydrocarbons with and without alkyl substitution.Representative gas chromatograms of the coal and

shale extracts are shown in Figures 4 and 5, respec-tively. From these chromatograms, it can be seen thatthe extracts are very complex mixtures. Althoughcapillary GC was used to separate the compounds inthe extracts, it is still very difficult to resolve andidentify all of the isomers.There was a minor contribution by C15+ hydrocarbons

to the total mass extracted. In principle, a correctioncould be made by subtracting the appropriate peakareas. However, considering the fact that the totalcontribution of C6 to C14 to the original coal mass wasless than 0.2%, no correction was carried out.Group-Type Separation Using Packed Capillary

Column SFC. The separation and quantitation of thealiphatic and aromatic hydrocarbon groups in the coaland shale extracts were accomplished using a newly

Figure 6. Supercritical fluid chromatogram for the group-type separation of the supercritical CO2 extract of a PittsburghNo. 8 coal. Conditions: 52 cm × 200 µm i.d. capillary columnpacked with 10 µm (60 Å) silica particles, 45 °C, 150 atm, CO2,0.2 µL splitless injection, 35 cm × 10 µm i.d. capillary linearrestrictor, FID.

Figure 7. Supercritical fluid chromatogram for the group-type separation of the supercritical CO2 extract of a BasalFruitland shale. Conditions: same as described in Figure 6.

Figure 8. Calibration curve for a mixture of standardaliphatic hydrocarbons using supercritical fluid chromatogra-phy. Conditions: same as described in Figure 6.

Volatile Hydrocarbons in Coals and Shales Energy & Fuels, Vol. 11, No. 5, 1997 949

developed supercritical fluid chromatography methodwhich utilizes packed capillary columns.10 Accordingto this method, all of the aliphatic hydrocarbons presentin the extracts were eluted as a single chromatographicpeak before the aromatic compounds. The aromaticcompounds were eluted in several peaks depending onthe number of aromatic rings in the compounds. Thisprovided a simple and easy method to quantify the totalC6-14 aliphatic and aromatic hydrocarbons from acalibration curve. Figures 6 and 7 represent typicalSFC chromatograms for group separations of coal and

shale samples, respectively. Figures 8 and 9 give thecalibration curves for the aliphatic and aromatic hydro-carbon groups, respectively. The correlation betweenpeak area and concentration is very strong, with acorrelation coefficient >0.9990. Peak areas were usedfor quantitation, and different types of hydrocarbonswere assumed to have equal detector responses.Twenty different coal and shale samples were ex-

tracted using supercritical CO2. These extracts werethen separated into aliphatic and aromatic groups usingpacked capillary column SFC. Each result is theaverage of three repeated injections. The estimatedstandard error for the three repeat injections was (10%.As can be seen from these data, the contents of lowmolecular weight aliphatic hydrocarbons in the coal andshale samples ranged between 0.02 and 0.20 wt % ofthe bulk source rock material. The contents of aromatichydrocarbons ranged from 0.01 to 0.1%. The C6-14hydrocarbons in the shale samples have a narrowerrange and greater abundance than those in the coalsamples.

Acknowledgment. This work was funded by theGas Research Institute, Contract Number 5091-260-2239.

EF960176F

Figure 9. Calibration curve for standard 2-ethyltoluene.Conditions: same as described in Figure 6.

950 Energy & Fuels, Vol. 11, No. 5, 1997 Li et al.