The Influence of CO on the Carbon Isotopic Composition of CH4 in Closed Pyrolysis Experiment With Co

Vol 23 No 4CHINESE JOURNAL OF GEOCHEMISTRY2004The Influence of co on the carbon IsotopicComposition of Ch in Closed PyrolysisExperiment With CoalLIU Quanyou(刘全有) and Liu Wenhui(刘文汇State Key Laboratory of Gas Geochemistry, Lanzhou Institute of GeologyChinese academy of Sciences, Lanzhou 730000, China)Abstract: A low-mature coal (R, =0. 4%, from the Manjiaer depression, Tarim Basin, Chi-na)was subjected to closed system pyrolysis, in sealed gold tubes, under isothermal temperature conditions. The carbon isotopic compositions of the pyrolyst fractions( hydrocarbon, CO 2Co, etc.)at two temperature points(350%C and 550C)were measured. The results showedthat8CcHA value is generally heavier at 350%C than that at 550C, because the high abun-dance of Co generated at low temperature would greatly influence 8CCH, value, and the reten-tion time of CO in gas chromatograph is close to that of CHa. But Co is formed through chemicalreaction of the oxygen-containing functional group-C=0, e. g. lactones, ketones, ether, etat low temperature, while CO, comes mainly from decarboxylizationThe carbon isotopic composition of coal gas from Lanzhou Coal Gas Works was definitelyfferent from that of thermally pyrolysed products from coal. The8 CcHA value of coal gas wasabnormally heavier than 8'Cco. At the same time, the reversed sequence(8C1>8C2) of8C and 8C? happened. The bond energy of free ions generally decides the sequence ofgeneration of hydrocarbon fractions according to the chemical structure, whereas the stability ofpyrolysate fractions and their carbon isotope fractionation are affected by the C-C bond energy.Key words low rank coal vitrinite: thermal simulating experiment; coal gas; carbonisotope fractionation1 IntroductionThe carbon isotopic composition of natural gases is regarded as a contant tracer used in the geohemical research of natural gases. Based on the carbon isotopic composition, the genetic types ofnatural gases are basically divided to determine the maturation of natural gases and make gas-sourcecorrelations( Stahl, 1973; Stahl and Carey, 1975; Stahl et al., 1977; James, 1983; Schoell1983, 1988; Dai Jinxing et al., 1992; Fu Jiamo et al., 1992 Xu yongchang, 1994). The formation of natural gases could be deduced in terms of carbon isotopic variations in pyrolysis experiments, because the carbon isotopic composition can reflect parent source rock type and its thermal e-volution( Fu Jiamo et al., 1992; Xu Yongchang, 1994; Huang Difan et al., 1995; Tang et al.2000). Therefore, great attention should always be paid to carbon isotopic composition in naturalgas research. Previous studies showed that the carbon isotopic composition of methane(8Cbecame gradually heavier with the parent source rock eve中国煤化工 ivy hydrocarbons( James, 1983; Schoell, 1983 ) The study of carbon isotYHCNMHGrolysis experiISSN10009426This study was supported by the National Basic Research(973 )Program of China(No. 2001 CB209102)360CHINESE JOURNAL OF GEOCHEMISTRYVol 23ment is helpful to probe the carbon isotopic composition of gases produced during thermal evolutionof the parent source rocks, and natural gas typesThe pyrolysis gases are composed of hydrocarbon and non-hydrocarbon gases(e.g. CO2, COetc.). The pyrolysis products were injected into an MAT-251 mass spectrometer to measure theircarbon isotopes, with analytical error( standard deviation) less than 0. 3%0. The hydrocarbon gaseswhich dont come up to the lower limit of mass spectrometric measurement cannot be detected, because of a high abundance of CO,, Co and a very low abundance of hydrocarbon gases at low pyrol-ysis temperature. At the same time, the peaks of CO and CH, were hard to separate from each otherin chromatograph separation on the MAT-251 mass spectrometer because of their close retentiontime. So, the measured 8CcH, was influenced, to some extent by the abundance of CO under thesame experimental conditions, especially at low temperature. Actually, 8CcH, values measured onthe Mat-251 mass spectrometer are the bulk values of carbon isotopes of CO and CH4( Table 1). Inprevious pyrolysis experiments, the carbon isotopic composition of CHa often showed a trend of beingfirst heavy and then light at low temperatures, finally the carbon isotopic composition of Cha becameheavy once again for high-rank coals( Peters et al., 1981; Schoell, 1983; Liu Quanyou, 2001Tang et al., 2000). This result is always inconsistent with the natural result that the carbon isotopiccomposition of methane becomes heavier gradually with the evolution of parent source rocks. Previous explanation is usually based on the heavy carbon isotopic composition of remnant-absorbed hydrocarbons in the original sample dismounted from the primary pyrolysis stage and inhomogeneousmaterial( Peters et al., 1981)Table 1. Carbon isotopes of pyrolysis gasesT(℃)Rn(%)8"C2(%)8"C(%)8"c(%)0.7420.526.5Vitrinite7431.4These samples were measured on an MAT-251 mass spectrometer; PDB. C %oThe carbon isotopic composition of methane measured is true because the abundance of co isnegligible in nature gases( Xu Yongchang, 1994 ). But in pyrolysis experiments large amounts ofCo were liberated from organic matter at low temperature, especially at 400C. Sometimes, theabundance of CO in pyrolysis is higher than that of CHA. So, it is difficult to precisely measure thecarbon isotopic composition of methane due to the partial overlap of Co peak and CH, peak. Therefore, the influence of CO on the measurement of carbon isotopic composition of methane is very precise. The lower the temperature is, the more obvious this influence will be. This influence woulddisappear"with decreasing CO abundance and greatly increasing CHa abundance with the rise of2 Experimental device and pyrolysis product analysisA low-mature coal(R =0.4%, from the Manjiaer depression, Tarim Basin, China),wassubjected to closed system pyrolysis, in sealed goldtemperature conditieranging from 250 to 550C at temperature intervals中国煤化工 hours per samyBut only two pyrolysis temperatures(350%C and 550CN MH Glected to elucidate theinfluence of CO on the carbon isotopic composition of methane due to high analytical expenditureThe vitrinite was separated by the heavy liquid method at Peking University, with purity being moreNo 4CHINESE JOURNAL OF GEOCHEMISTRY361than 95%, total organic carbon(TOC)at least 66. 12%. Table 2 presents the geochemical charac-teristics of original sample. Yet, coal gas for house use was interestingly chosen in order to make acomparison with the pyrolysis gasTable 2. Geochemical characteristics of coal samples from the Tarim BasinWell NoStratigraphical positionRock typR。(%)TOC(%)Tma(℃)Well Huavingca3075-307767.41The analytical method used for laboratory pyrolysis is described by Liu quanyou et al.( 2001)Figure I shows the process of collecting pyrolysis gas and its measurement procedure. The pyrolysisproducts were analyzed on an HP5890-11 gas chromatograph. Column: CFAIl PORAPAK Q(d 3mm×1.5m)and5 A molecule sieve(φ3mm×3m); detector:FID(250℃); analytical conditions: initial temperature: 40C; initial time: 5 min; heating rate: 70C/ min; end temperature130C; retention time 8 min; the temperature of CH, transferor: 380C; column temperature:400; carrier gas: highly pure H,. Analytical error is less than 5%0. The involvement of a trace amount of air in the sealed gold tube is inevitable. Moreover, the gas chromatograph detected all thepyrolysis gases. However, the involved air can be deducted according to the constant ratio of Ar inthe air. Then, the pyrolysis gas fractions were gained by normalizing the remained gas. Table 3 liststhe chemical compositions of pyrolysis products from the Tarim Basin coal and coal gas from theLanzhou Coal Gas WorksThe carbon isotopic compositionsof the pyrolysis products were measuredon an MAT-252 mass spectrometer atthe State Key Laboratory of Gas Geo-hemistry, Chinese Academy of Sciences. because the MAT-252 massspectrometer can wonderfully measurethe carbon isotopes of Co and CH,, Saltwater Vacuum purpwhich are generally overlapped in the2gas chromatogram column of the MAT-251 mass spectrometer. The analyticalconditions are: gas chromatograph col-atwater poolumn:5 m filled column; the temperature ranging from -10-120Ci heat-Sketch of collection and determination of pyrolysis gases. Iing rate: 5C/min; carrier gas: pure -9. valves. I Vessel with scale: Il. vessel; I. reactor: MHe with high pressure. Analytical errorpyrolysis fraction collector.is less than 0. 3%0. Every pyrolysisfraction was measured three times to obtain the average value. Table 3 shows the carbon isotopiccompositions of pyrolysis products and coal gas from the Lanzhou Coal Gas Works3 Experimental resultsThe dominant pyrolysis fractions are CO2, CO, andlittle amount of ethane at 350C( Liu Quanyou et al., 20H中国煤化工=0.6%, thCNMHGof the pyrolysisfractions indicated that the organic matter didn t reach maturity at this temperature. Table 3 presents the carbon isotopic compositions of pyrolysis fractions of coal and vitrinite362CHINESE JOURNAL OF GEOCHEMISTRYVol 238Cco>8CTable 3. Chemical fractions of pyrolysis products from coal and coal gas samplesTemperatureR(%)CO2(%)CO(%)C3(%)C2。(%)N2(%)85.2880.60.702.49Vitrinite5001.941.774.0144.327.63catalyst, on March 27, 2001Table 4. Carbon isotopic compositions of pyrolysis gas and coal ga24.832.6-26,4-29.5Vitrinite-17.422.5pal gas Room temperature-132 These samples were measured on MAT-25spectrometer, PDB. C%cThis relationship among 8Cco,,8 CCHa and8CCaH is entirely consistent with that of natu-ral gases except for 8Cco. However, the phenomenon that 8Cco lies in between8C8CcH suggested the parent material of CO formation is different from that of CO, and CHa, and8CcH, values would be influenced by a high abundance of Co in the pyrolysis products because oftheir close retention time. In order to elucidate the influence of carbon isotopic composition of CO on8C1,8CI values calculated by the following equation are compared with the carbon isotope da-ta measured on an MAT-251 mass spectrometer. The lost pyrolysis gas can be negligible relative tothe total amount of the total pyrolysis gas8"C1=Vcm×8"C1/(Vcn,+Vo)+Vo×8"Co/(Vcn,+Ⅴo)(VcH, X8CI+Vco X8CCO)/(VcH,+Vco)where 8CI (%0): calculated carbon isotopic composition of methaneVCH,(ml g toc): total volume of methane produced by unit organic carbonVco(ml g TOC): total volume of carbon monoxide produced by unit organic carbon;8CI (%0): carbon isotopic composition of methane measured on MAT-252 mass spectrometer8Cco(%0): isotopic composition of carbon monoxide measured on MAT-252 mass spectrometerThe equation of transferred error is as followsσy=(σy+σywhere, or is the total error, oy and oyz denote the individual unit errors, respectivelyThe systematic errors should be equal as the pyrolysis fractions and carbon isotopes were determinedon the same analytical instrument. ThereforeThen, the following equation is obtainedwhere o,: precision of HP5890-II gas chromatograp中国煤化工g, precision of MAT 252 mass spectrometer,CNMHGa: bulk ratio of CHa or Co to the total of CHa andb: carbon isotope ratio of CH, or CONo 4CHINESE JOURNAL OF GEOCHEMSTRY363Then the calculated error is0.84%The above calculated result shows that the 8C,' is mostly consistent with the8C, measuredon the MAT-251 mass spectrometer in the limit errors. The calculated carbon isotopic composition ofethane from coal samples is -27 2% at 350%C, whereas 8 C, is -26. 5%o from MAT-251 massspectrometric measurement; 8C calculated from vitrinite is -29 5%0 at 350C, 8 CI29. 7%0; at 500C8C1 calculated from vitrinite -30. 6%0, 8C1-29 7%00. The error in-volved in the measurement may be large at 500C because an amount of methane generated is approximately 95 times higher than that of carbon monoxide whose abundance is so low that the 8Cfrom vitrinite at 500C would be less affected by 8Cco.8C, measured on the MAT-251 massspectrometer is 4%0 higher than 8CI measured on the MAT-252 mass spectrometer at 350C, butat 500C8C1 measured on the MAT-251 mass spectrometer is about 1% higher than 8C,meas-ured on the MAt-252. It is shown that the influence of cO on8C tends to reduce with the evolution of coal and increasing methane abundanceThe carbon isotopic composition of coal gas from Lanzhou Coal Gas Works is markedly differentfrom that of pyrolysis gas. The results show that 8CCHA, which is abnormally heavy, is -23. 1%easier than8Cco, and Cah4 Fractionation mechanisms of carbon isotopesIn natural gases, the mixing and the selective oxidation of bacteria can cause a reverse se-quence of carbon isotopes of gaseous hydrocarbons, which doesnt happen in the coal gas prepara-tion process and thermal simulation experiment( Ding Fucheng et al., 1991). So the existence ofcarbon isotope reduction and reverse sequence in low-temperature thermal simulation is related to experimental analysis. Lignite is a typical humic organic matter, which has a short side alkyl chainoxygenous group that would break off gradually and form instable free groups in the process ofthermal evolution of organic matter( Harry et al., 1986; Ding Fucheng et al., 1991). These freegroups can form new stable compounds by trapping other free ions, for example, H and 0 fromthe surrounding media. According to the chemical structure, the sequence of hydrocarbon generationwould be determined by the energy that controlled the sequence of the free groups breaking and falling off from the big molecule skeletons of coal( Huang Difan et al., 1995). The chain with lessbond-energy breaks off first, and then new compounds will be formed so that the carbon isotopiccomposition of methane formed early is relatively light due to the increase of bond-energy of C-2CC-C, C-C in turn( Peters et al., 1981; Galimov et al., 1985; Tang et al., 2000; JiangFeng et al., 2000). Moreover, the carbon isotopic composition of methane becomes heavier little byttle with the thermal evolution of organic matter( Peters et al., 1981The free groups formed in organic cracking include-COOH, -COH,-CO-,-OCH3,-0-,-OHetc. All these carbon bonds that have mixed atoms are most instable, falling off easily to form oxy-gen-bearing compounds( Galimov et al., 1985; Tang et al., 2000; Fu Jiamo et al., 1992; HuangDifan et al., 1995). Generally, it is considered that CO 2 is mostly formed from the pyrolysis of car-boxyl, ester and other groups, but CO from oxygen-contones, ketones, ether, etc.( Fu Jiamo et al., 1992)中国煤化工ore abundant aneeavier the mixed atoms, the heavier the carbon isotopCNMH GGalimov et al.1985). o the carbon isotopic composition of carboxyl, ester, and groups are heavier than those oflactone, ketone, ether and other groups. The carbon isotopic composition of CO2 derived from car364CHINESE JOURNAL OF GEOCHEMISTRYVol 23boxyl, ester and groups is heavier than that of Co formed from lactone, ketone, ether and othergroups. Theoretical calculations are consistent with the pyrolysis experimental resultsThe coal gas is cracked under the action of nickel-vanadium catalyst in the presence of suffi-cient water. The products are dominated by Co and CHa( Table 3). In coal gas, 8 CCH23. 1%0, is heavier than 8Cco, -25 5%0. The carbon isotopic composition of methane becom-ing heavier is the result of cracking of positive carbon ions( Sackett, 1978),i. e., free groupcracking is converted to positive carbon ion cracking( Harry et al., 1986)Carbon isotope fractionation is controlled by dynamic proAs the bond-energy ofC-Hhigher than that ofC-H, the compounds with high bond energy are more stable. So the compoundswith higher bond-energy would be formed preferentially in the reaction( Tang et al., 2000; JiaFeng et al., 2000). Therefore, the heavier positive carbon ions formed as a result of catalyzatieand cracking would combine preferentially with H to form CH4, thus 8CCH, would be heavierthan 8Cco, leading to an invert-sequence of 8CI>8C2. Generally, there are three explanations for this invert-sequence of 8C1>8C2 of methane homologies: 1) mixing of gases of different maturities; 2)abiogenic gas; 3)heavier carbon isotope of methane remnants that were generatedfrom organic cracking at high-temperature, and the cracked fragments of -CH, were condensed toform C2H6. The ethane formed by two methyls is lighter carbon-isotopically than the methane, be-cause carbon-bond cracking takes place inC-C bond first so that the C is relatively enriched inmethyl derived from cracking of methaneApplied chemistry of coal indicates that it would be possible that water may take part in the fol-wing reactions in the carbon matter system Elliott, 1981; Yang Tianyu and Wang Hanyun1987; Wang Xinzhou et al., 1996)2C+0,=2C02C0 +2H2=CH4 +cO2C+H,0=CO+HCO+H2O=H,+CO,CO +C=2C0nCO +2nH,=C,H2n +nH2OAlkali and alkali-earth metal elements can, as the catalysts for the above reactions, cause donot require so harsh reaction conditions, because coal is high in oxygen and often contains alkali andalkali-earth metal elements that combine with functional groups in the molecules of vitrinite5 ConclusionsThe carbon isotopic composition of CHa at low pyrolysis temperature is heavier than that of Chin natural gases, mostly because of the influence of CO with heavier carbon isotopic compositionThe peaks of CO and CH, in pyrolysis products would partly overlap in the MaT-251 mass spectrometer measurement because of their close retention time in gas chromatograph column. There is somedifference in parent material between CO2 from early decarboxylation and Co from the oxygen-containing functional group-C=O, e.g. lactone, ketone, ether and other groups. The forming mecha-nism of coal gas is different from that of coal gas generated in pvrolvsis experimentsCatalyzing and cracking of coal gas takes place中国煤化工 nickel and vanadiumare used as catalysts in the presence of much water.CN MH Gn convert free groupsinto positive carbon ions, thus making the carbon isotopic composition of methane heavier than thatof carbon monoxide. 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