Anal. Chem. 1997, 69, 2030-2034
A Gas Chromatographic Separation for the H and C Stable Isotope Ratio Determination of Coal Compounds Diano Antenucci,‡ Jean-Marie Bouquegneau,§ Alain Brasseur,† Patrick Dauby,§ Rene´ Le´tolle,⊥ Christiane Jacquemin,‡ and Jean-Paul Pirard*,†
Laboratoire de Ge´ nie Chimique, Universite´ de Lie` ge, B6a, B-4000 Lie` ge, Belgium, Institut Scientifique de Service Public, rue du Che´ ra, 200, B-4000 Lie` ge, Belgium, Laboratoire d’Oce´ anologie, Universite´ de Lie` ge, B6, B-4000 Lie` ge, Belgium, and Universite´ Pierre et Marie Curie, Laboratoire de Ge´ ologie Applique´ e, Case 123, Tour 26, 5e e´ tage, 4, place Jussieu, F-75252 Paris Cedex 05, France
A new, completely automated gas chromatography technique has been developed to separate the different gaseous compounds produced during underground coal gasification for their 13C/12C and D/H isotope ratio measurements. The technique was designed for separation and collection of H2, CO, CO2, H2O, H2S, CH4, and heavier hydrocarbons. These gaseous compounds are perfectly separated by the gas-phase chromatograph and quantitatively sent to seven combustion and collection lines. H2, CO, CH4, and heavier hydrocarbons are quantitatively oxidized to CO2 and/or H2O. The isotopic analyses are performed by the sealed-tube method. The zinc method is used for reduction of both water and H2S to hydrogen for D/H analysis. Including all preparation steps, the reproducibility of isotope abundance values, for a quantity higher than or equal to 0.1 mL of individual components in a mixture (5 mL of gases being initially injected in the gas chromatograph), is (0.1‰ for δ13CPDB and (6‰ for δDSMOW. Traditional mining methods are no longer economical for the working of coal resources at great depth. Underground coal gasification thus appears to be an interesting alternative solution. Besides previous experiments in the United States1 and in Europe, only one other experiment was performed, at Thulin, Belgium,2 and another is still in progress at Alcorisa, Spain. Underground coal gasification consists of in situ converting the coal into a gaseous combustible by combustion with an O2-H2O mixture injected from the surface through the injection well. The produced gas is recovered through the production well. The typical composition of this gas mainly involves CO, CO2, H2, H2O, N2, O2, CH4, and heavier hydrocarbons. Sulfur and nitrogen present in coal as organic or inorganic compounds are found in the produced gas principally as H2S and NH3. In situ direct measurement methods are not easy and, in any case, are expensive. That is the reason why indirect measurement methods †
Laboratoire de Ge´nie Chimique, Universite´ de Lie`ge. Institut Scientifique de Service Public. Laboratoire d’Oce´anologie, Universite´ de Lie´ge. ⊥ Laboratoire de Ge ´ ologie Applique´e, Universite´ Pierre et Marie Curie. (1) Cena, R.; Britten, J.; Thorsness, G. Proceedings of the 14th Underground Coal Gasification Symposium, Chicago, IL, 15-18 August 1988; pp 200-212. (2) Patigny, J.; Li, T. K.; Ledent, P.; Chandelle, V.; Depouhon, F.; Mostade, M. Proceedings of the 13th Annual Underground Coal Gasification Symposium, Laramie, WY, 24-26 August 1987; pp 28-40. ‡ §
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should be developed in order to estimate some physical parameters characterizing the processes, especially temperatures and fluid flows. During the underground gasification at Thulin, Dufaux et al.3,4 and Chandelle et al.5 used the isotope ratios of carbon and hydrogen in order to estimate in situ temperatures and model the gasifier. The stable isotope method is based on the small, temperature-related variations in the isotope abundances of any molecule involved in a chemical process. This well-known phenomenon has not, however, been supported by sufficient kinetic data set. Nevertheless, Richet et al.6 consider that the isotopic distribution should obey the laws of classic thermodynamics. Clayton et al.7 have demonstrated that, for pressures below 1 GPa, the isotopic exchange could be considered as an equilibrium that is only temperature dependent. According to this hypothesis, Bottinga8 calculated the variations of equilibrium constants against temperature by the laws of statistical thermodynamics. These variations, coupled with the mass and heat balances of the gasifier, enable the calculations of temperatures reached in situ (Le´tolle et al.,9 Pirard et al.10). These authors have shown that the δ13C and δD measurements in the CO/CO2 system and in CH4/H2 and H2O/H2 systems, respectively, are valuable indicators for temperature estimations. The isotope abundance measurements implicate the quantitative separation as well as the collection of the involved gases. Gas chromatography isotope ratio mass spectrometry (GCIRMS) allows the 13C/12C measurements to be made for hydrocarbons (Fisons Instruments, Finnigan MAT, and Europa Scientific), but this technique cannot routinely provide D/H measurements at the present time. It is well-known that a quantitative separation and collection of gas compounds, for 13C/12C and D/H ratios (3) Dufaux, A.; Gaveau, B.; Le´tolle, R.; Mostade, M.; Noe¨l, M.; Pirard, J. P. Fuel 1990, 69, 624-632. (4) Dufaux, A.; Gaveau, B.; Le´tolle, R.; Mostade, M.; Noe¨l, M.; Pirard, J. P. Fuel 1990, 69, 1454-1456. (5) Chandelle, V.; Jacquemin, C.; Le´tolle, R.; Mostade, M.; Pirard, J. P.; Somers, Y. Fuel 1993, 72, 949-963. (6) Richet, P.; Javoy, M.; Bottinga, Y. Annu. Rev. Earth Planet. Sci. 1977, 5, 65-110. (7) Clayton, R. N.; Goldsmith, J. R.; Karel, K. J.; Mayeda, T. K.; Newton, R. C. Geochim. Cosmochim. Acta 1975, 39, 1197-1201. (8) Bottinga, Y. Geochim. Cosmochim. Acta 1969, 33, 49-49. (9) Le´tolle, R.; Mostade, M.; Pirard, J. P. Entropie 1991, 166, 3-12. (10) Pirard, J. P.; Antenucci, D.; Renard, X. In Les isotopes stables. Applicationss Production; Goldstein, S., Louvet, P., Soulie´, E., Eds.; Centre d’e´tude de Saclay: Saclay, France, 1994; pp 141-149. S0003-2700(96)01125-0 CCC: $14.00
© 1997 American Chemical Society
Figure 1. General scheme of the gas chromatography and separation lines. MS, molecular sieve; PQS, Porapak QS column; TCD, thermal conductivity detector; V, vacuum.
measurements, can be done either through a manual gas separation chain, a time-consuming technique, or through preparative chromatography. The latter technique has been used in the work of Dumke et al.11 for separating and collecting CO2, CH4, C2H6, and C3H8. To use the isotopic exchange method in the framework of underground coal gasification trials, a new preparative gas chromatographic separation has been developed to separate and collect the different compounds in gas mixtures before determination of isotope ratios of both carbon and hydrogen compounds. This paper deals with the conception and perfecting of this new completely automated gas chromatographic separation technique, allowing the separation and collection of H2, O2, N2, CO, CO2, H2O, H2S, CH4, and heavier hydrocarbons. EXPERIMENTAL SECTION A general scheme of the gas separation lines is shown in Figure 1. It comprises the gas chromatographic separation as well as the combustion and/or collecting lines. The GC 8000 Special gas chromatograph (Micromass) is equipped with a thermal conductivity detector (TCD) and three packed columns: namely, two Porapak QS (2 m length) and one 5 Å molecular sieve (2 m length) are connected in series. A general scheme of the chromatographic columns’ ordering is given (11) Dumke, I.; Faber, E.; Poggenburg, J. Anal. Chem. 1989, 61, 2149-2154.
in Figure 2. Helium is the carrier gas, and the flow rate was 30 mL/min. The three columns were set to a temperature of 60 °C. At this temperature, H2O is so strongly adsorbed on the Porapak column that it cannot be eluted. Consequently, after the complete separation of all gaseous compounds except heavier hydrocarbons, the temperature is raised to 110 °C for H2O eluting. Five milliliters of the gas mixture is introduced in the first Porapak QS column (called back-flush column) through the sampling valve (Figure 1). While H2, O2, N2, CO, CH4, H2O, CO2, and H2S are oriented through the B valve toward the second Porapak QS column, the heavier hydrocarbons are retained on the back-flush column. These heavier hydrocarbons will be sent toward a specific combustion line after the complete separation of the other gaseous compounds. The separation of H2O, CO2, and H2S is fully performed with the second Porapak QS column, wherein H2, O2, N2, CO, and CH4 are perfectly separated by the molecular sieve. Since CO2, H2S, and H2O are irrevocably adsorbed on the molecular sieve, this column must be bypassed by means of a C valve when these gases come out from the second Porapak column. At the line end, each gas is detected by the TCD, which is linked to a computer which orders a multiport valve orienting the separated gases toward specific lines. Each line is kept under helium flow in order to avoid pollution by air. After separation, CO2, H2S, and H2O are directly collected in cold traps (6 mm o.d., Duran glass), wherein CO is first oxidized to CO2, Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
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450 °C
H2 + CuO 98 H2O + Cu An oxygen flow can be sent in each combustion line through the multiport valve for the regeneration of the oxidant. The regeneration is done every week and lasts 20 min. Methods for reducing water to hydrogen for isotope analysis are mostly based on the reaction with hot uranium (Bigeleisen et al.,13 Friedman and Smith14 ) or zinc (Friedman15 ) in flow systems or closed ampules (Rolle et al.,16 Coleman et al.17). Recently, Tanweer and Han18 has developed a new technique for the quantitative conversion of water to hydrogen with manganese as reducing agent. The method used in the present work is based on that of Coleman et al. Namely, a small portion of water (10 µL) is reduced with zinc shot (supplied by UCB, Belgium) in a sealed tube according to the reaction 450 °C
H2O + Zn 98 H2 + ZnO
Figure 2. Chromatographic columns ordering and separation steps.
The zinc shot is cleaned according to the procedure proposed by Tanweer,19 i.e., washed in 0.15 M nitric acid, rinsed with demineralized water, dried under vacuum, and outgassed under vacuum at 300 °C for 2 h. The reduction of H2S to H2 with hot uranium has been described by Bigeleisen et al.,13 and Roth20 reduced H2S with copper (Cu) at 300 °C. No other methods have been found in the literature. However, by analogy with the water reduction on Zn, we have found that the following reaction is complete and permits δD measurements: 450 °C
and CH4 to CO2 and H2O before trapping. The trapped CO2, H2S, and H2O are then transferred from the cold traps to glass ampules (6 mm o.d., 20 cm length, Duran glass) under the action of vacuum and heat gradient, and then the glass ampules are sealed. As H2 cannot be kept in a cold trap except with liquid helium, this gas is converted into H2O, which is collected in the same way in an ampule. H2O and H2S, trapped in closed ampules, are afterward reduced to H2 for D/H analysis. The transformation of all components into H2 and CO2 is necessary for the D/H and 13C/ 12C analysis by IR-MS (Micromass Optima). The oxidation of H , 2 CO, and CH4 is carried out in a heated reactor using CuO as catalyst. The conversion of CO to CO2 is performed according to the well-known reaction 450 °C
CO + CuO 98 CO2 + Cu
The combustion of CH4 is made according to the reaction described by Ebel,12 880 °C
CH4 + 4CuO 98 CO2 + 2H2O + 4Cu
The oxidation of H2 to H2O is carried out with CuO according to the reaction (12) Ebel, S. Z. Anal. Chem. 1973, 264, 16-28.
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H2S + Zn 98 H2 + ZnS RESULTS AND DISCUSSION Several mixtures of reference gases from Air Products were used to test and improve the preparative gas chromatography technique (Table 1). For programming the opening and cutting-off sequence of the different valves, the retention time of each compound must be accurately determined. Taking into account the necessity to bypass the molecular sieve for CO2, H2S, and H2O, two series of analyses were carried out. The first series was performed with all the gases without the use of the molecular sieve. The second series was carried out with CO, O2, N2, H2, and CH4 using the molecular sieve. The complete numerical results are shown in Table 2. This two-step process enables us to define the sequence of the valves allowing the best separation of all gases involved in the process. The reference time corresponds to the opening of the sampling valve. The sample is sent to the back-flush column. This column retains water and heavier hydrocarbons, while the other gases are oriented toward the second Porapak column. (13) Bigeleisen, J.; Perlman, M. L.; Prosser, H. C. Anal. Chem. 1952, 24, 13561357. (14) Friedman, I.; Smith, R. L. Geochim. Cosmochim. Acta 1958, 15, 218-218. (15) Friedman, I. Geochim. Cosmochim. Acta 1953, 4, 89-103. (16) Rolle, W.; Bigl, F.; Haase, G.; Runge, A.; Hu ¨ bner, H. Isotopenpraxis 1969, 5, 35-36. (17) Coleman, M.; Shepherd, T.; Durham, J.; Rouse, E.; Moore, G. R. Anal. Chem. 1982, 54, 993-995. (18) Tanweer, A.; Han, L.-I. Isotopes Environ. Health Stud. 1996, 32, 97-103. (19) Tanweer, A. Anal. Chem. 1990, 62, 2158-2160. (20) Roth, E. C. R. Acad. Sci. Paris 1956, 242, 3097-3100.
Table 3. δ13CPDB Measurements (‰) for Gas 1 (CO2-CH4-O2), Gas 4 (CO2-CO), and Gas 7 (CO2)
Table 1. Theorical Composition of the Reference Gases Used for the Analyses (Vol %) gas 1 2 3 4 5 6 7 8 9
O2
CO
10 2 15 50 30
CO2 30 1 10 50 30
CH4
C3H8
C4H10
60 1 20
N2
H2
H2S
96 55
40 30
70
40
30
gas 7
CO2
CO2a
CO
CO2
CO2a
-29.87 -29.89 -29.95
-35.49 -35.29 -35.42
-29.53 -29.57 -29.55
-29.46 -29.73 -29.61
-26.29 -26.35 -26.14
-30.48 -30.50 -30.50
-30.46 -30.47
Gas analyzed without chromatographic separation.
10
30
Table 2. Absolute Retention Times with and without the Molecular Sieve at 60 °C (Carrier Gas, He at 30 mL/min) retention times (min)
H2 O2 N2 CO CH4 CO2 H2S
gas 4 CH4
a
99.999 90
gas 1 CO2
without molecular sieve
with molecular sieve
3.5 4.2 4.2 4.2 6.4 10.4 35.3
3.5 7.3 15.3 25.3 20.5 10.4a 35.3a
a The molecular sieve is bypassed between 8.3 and 14 min and again after 30 min.
Figure 3. Typical chromatogram obtained by the separation of H2O2-CO2-N2-CH4-CO-H2 at 60 °C.
Within 8 min, H2, O2, N2, CO, and CH4 reach the molecular sieve. H2 and O2 are completely eluted after 3.5 and 7.3 min, respectively. After 8.3 min, the C valve is switched and the molecular sieve bypassed, and then CO2 passes and is completely eluted after 14 min. The C valve switches again, and N2, CH4, and CO are then separated. After 30 min, as H2S is coming out, the molecular sieve is bypassed again. At 35 min, the temperature is raised to 110 °C, and at 40 min, H2O is eluated. The B valve is switched in order to remove the hydrocarbons heavier than CH4 from the first Porapak column. The chromatogram shown in Figure 3 displays the perfect separation of all gases. Other isothermal conditions have been tested: 50, 70, and 80 °C. The experiments obviously show that these temperatures are not suitable for the separation of H2, O2, N2, CO, CO2, H2S, H2O, CH4, and heavier hydrocarbons.
Table 4. δDSMOW Measurements (‰) for Gas 1 (CO2-CH4-O2), Gas 6 (H2-N2), Gas 8 (H2S-N2) and Water gas 1, CH4
gas 6, H2
gas 8, H2S
water
-117 -106 -110
-658 -670 -663
-188 -195 -196
-31 -27 -28
The problem arises from the fact that the retention times with or without the molecular sieve lead to the cutting or the overlapping of several peaks. This problem can also be observed when a huge quantity of an individual component has to be separated. Both temperature and carrier gas flow have been optimized in order to avoid it. The ideal temperature for the best separation of the gases mentioned above must be neither below nor higher than 60 °C. During the detection of each component, the multiport valve permits them to be directed to their specific combustion line and/ or cold trap. It is imperious to convert completely CH4, CO, and H2 into CO2 and H2O. The combustion products CO2 and H2O have been injected in a second chromatograph previously calibrated. Incidentally, we verified that the quantities of CO2 and H2O so formed correspond exactly to the quantities of the starting gases, CH4, CO, and H2. This test is a proof of the perfect separation and the complete oxidation of the starting gases. It must be noticed that the second chromatograph also enabled us to verify the extremely low level of external contamination in each combustion line (less than 0.05% of air). Reducing H2O to H2 in the glass ampule at 450 °C (with cleaned zinc shot, 1 g) is complete after 2 h. The same transformation of H2S into H2 (with Zn, 2 g) is complete after 2 h. It must be noted that the reduction of H2S with Zn starts at room temperature. The results of isotope ratio mass spectrometry are given in Tables 3 and 4 for δ13CPDB and δDSMOW measurements, respectively. As shown in Table 3, the mean reproducibility for the δ13CPDB is (0.1‰. The result is recorded for whatever can be the origin of CO2 (CO, CO2, CH4). It is also of interest to note that the δ13C measurements from CO2 analyzed with or without chromatographic separation are the same. This demonstrates that, in our experimental conditions, no isotopic exchanges occur during the chromatographic separation. Table 4 shows that the mean reproducibility for δDSMOW of compounds such as CH4, H2, H2S, and H2O is 6‰. These reproducibilities are observed for a quantity of individual components higher than or equal to 2% (0.1 mL) in the starting mixture (5 mL). Below this limit of 2%, it is observed that the reproducibility becomes less satisfactory. However, these values, in good agreement with those published by Dumke et al.,11 are Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
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very reasonable, particularly when taking into account the huge range of δD in coal compounds. ACKNOWLEDGMENT The authors are grateful to the European Coal and Steel Community for financial support (Coal Research Project 7220/ EC/210 and 7220/ED/201). Special thanks are due to Pr. E. Faber (Federal Institute for Geosciences and Natural Resources (BGR), Hannover, FRG) for valuable discussions and to Dr. Ir.
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H. Jacobs (Interscience SPRL, Louvain-la-Neuve, Belgium) for technical assistance.
Received for review November 7, 1996. Accepted March 27, 1997.X AC961125I X
Abstract published in Advance ACS Abstracts, May 1, 1997.
