Carbon and oxygen isotopic composition of coal and carbon dioxide

COGEL-02657; No of Pages 8 International Journal of Coal Geology xxx (2016) xxx–xxx

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International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo

Carbon and oxygen isotopic composition of coal and carbon dioxide derived from laboratory coal combustion: A preliminary study Peter D. Warwick ⁎, Leslie F. Ruppert U.S. Geological Survey, MS 956, Reston, VA 20192, USA

a r t i c l e

i n f o

Article history: Received 29 January 2016 Received in revised form 3 June 2016 Accepted 10 June 2016 Available online xxxx Keywords: Coal Atmospheric coal combustion Carbon dioxide Carbon and oxygen isotopes

a b s t r a c t The concentration of carbon dioxide (CO2) in the atmosphere has dramatically increased from the start of the industrial revolution in the mid-1700s to present levels exceeding 400 ppm. Carbon dioxide derived from fossil fuel combustion is a greenhouse gas and a major contributor to on-going climate change. Carbon and oxygen stable isotope geochemistry is a useful tool to help model and predict the contributions of anthropogenic sources of CO2 in the global carbon cycle. Surprisingly few studies have addressed the carbon and oxygen isotopic composition of CO2 derived from coal combustion. The goal of this study is to document the relationships between the carbon and oxygen isotope signatures of coal and signatures of the CO2 produced from laboratory coal combustion in atmospheric conditions. Six coal samples were selected that represent various geologic ages (Carboniferous to Tertiary) and coal ranks (lignite to bituminous). Duplicate splits of the six coal samples were ignited and partially combusted in the laboratory at atmospheric conditions. The resulting coal-combustion gases were collected and the molecular composition of the collected gases and isotopic analyses of δ13C of CO2, δ13C of CH4, and δ18O of CO2 were analysed by a commercial laboratory. Splits (~1 g) of the un-combusted dried ground coal samples were analyzed for δ13C and δ18O by the U.S. Geological Survey Reston Stable Isotope Laboratory. The major findings of this preliminary work indicate that the isotopic signatures of δ13C (relative to the Vienna Pee Dee Belemnite scale, VPDB) of CO2 resulting from coal combustion are similar to the δ13CVPDB signature of the bulk coal (−28.46 to −23.86 ‰) and are not similar to atmospheric δ13CVPDB of CO2 (~ −8 ‰, see http:// www.esrl.noaa.gov/gmd/outreach/isotopes/c13tellsus.html). The δ18O values of bulk coal are strongly correlated to the coal dry ash yields and appear to have little or no influence on the δ18O values of CO2 resulting from coal combustion in open atmospheric conditions. There is a wide range of δ13C values of coal reported in the literature and the δ13C values from this study generally follow reported ranges for higher plants over geologic time. The values of δ18O (relative to Vienna Standard Mean Ocean Water) of CO2 derived from atmospheric combustion of coal and other high-carbon fuels (peat and coal) range from +19.03 to +27.03‰ and are similar to atmospheric oxygen δ18OVSMOW values which average +23.8‰. Further work is needed on a broader set of samples to better define the relationships between coal composition and combustion-derived gases. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction The concentration of carbon dioxide (CO2) in the atmosphere has dramatically increased from about 270 ppm (ppm) at the start of the industrial revolution in the mid-1700s (Joos and Spahni, 2008) to levels exceeding 409 ppm in April 2016 (Scripps, 2016). Carbon dioxide is a greenhouse gas and is a major contributor to on-going climate change (Intergovernmental Panel on Climate Change, 2014). The primary source of anthropogenic CO2 has been from the combustion of fossil fuels associated with industrial development (Le Quéré et al., 2014). In 2014, coal combustion from stationary electric power generation ⁎ Corresponding author. E-mail address: [email protected] (P.D. Warwick).

facilities in the United States emitted to the atmosphere 1570 million metric tonnes (MMT) of CO2 equivalent or about 30% of all greenhouse gas emissions related to fossil fuel combustion in the United States (U.S. Environmental Protection Agency, 2016). Carbon and oxygen stable isotope geochemistry has been used to model and predict the contributions of anthropogenic sources of CO2 in the global carbon cycle (Keeling, 1958, 1961; Francey and Tans, 1987; Gruber, 2001; Cuntz et al., 2003; Hoag et al., 2005; Affek and Eiler, 2006; Affek et al., 2007; Horváth et al., 2012). Surprisingly few studies have addressed the carbon and oxygen isotopic composition of CO2 derived from coal combustion from naturally burning underground coal fires (Gleason and Kyser, 1984) or from laboratory combustion experiments (Schumacher et al., 2011). Schumacher et al. (2011) used only high purity (99.99%) oxygen for their laboratory coal combustion

http://dx.doi.org/10.1016/j.coal.2016.06.009 0166-5162/Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Warwick, P.D., Ruppert, L.F., Carbon and oxygen isotopic composition of coal and carbon dioxide derived from laboratory coal combustion: A preliminary study, Int. J. Coal Geol. (2016), http://dx.doi.org/10.1016/j.coal.2016.06.009

Affolter et al. (2011) Not published Affolter et al. (2011) Not published Warwick et al. (2005) Warwick et al. (2005)

ID = identification; Lig = lignite; OH = Ohio; NM = New Mexico; MS = Mississippi; TX = Texas.

1.74 0.49 0.46 0.35 0.49 0.51 1.44 0.55 0.22 0.03 0.01 0.01 0.59 0.04 0.02 0.01 0.01 0.01 na 8587 9158 4956 9494 8447 10.65 42.19 27.34 18.57 7.52 11.11 6.95 7.96 9.81 44.11 34.46 37.73 3.77 1.08 0.7 0.39 0.51 0.53 1.24 0.99 1.1 0.64 1.16 1.04 67.1 45.06 48.9 33.68 51.77 46.59 4.72 3.48 3.85 7.09 6.49 6.43 46 30.80 33.90 22.54 35.09 29.44 na 25.96 30.5 23.81 33.92 32.01 9.99 41.43 24.2 10.57 5.61 7.68 6.19 1.81 11.5 43.08 25.38 30.87 5.04 na 9.88 30.55 19.22 25.79 1.21 na 1.78 18.04 7.63 6.85 1.21 na na na 29.75 30.37

Pyrite Sulfate

Forms of sulfur (AR) Calorific value

Ash (dry) Oxygen Sulfur Nitrogen Carbon

Ultimate (AR)

Hydrogen Ash

Volatile matter

Fixed carbon Total moisture

Air dry loss Residual moisture Equilibrium moisture

na = not available; AR = as received; OH = Ohio; NM = New Mexico; MS = Mississippi; TX = Texas.

Associated report

Bituminous Bituminous Subbituminous Lignite Lig-subbituminous Lig-subbituminous

07018-01314GBC SBT-19-R7 E-0709002-063 MS-02-DU USGS-PA-2-CN6 USGS-PA-2-CN2

Rank

07018-01314GBC SBT-19-R7 E-0709002-063 MS-02-DU PA-2-CN6 PA-2-CN2

Pennsylvanian OH Permian India Cretaceous NM Paleocene MS Paleocene TX 1 Paleocene TX 2

Field ID

Pennsylvanian OH Permian India Cretaceous NM Paleocene MS Paleocene TX 1 Paleocene TX 2

Field identification

Sample

Table 2 Proximate and ultimate analyses of coal samples used in study. All values in weight percent, except calorific values.

Table 1 Coal samples used in this study.

Proximate (AR)

2. Methods To compare the carbon and oxygen isotopic signatures of coal and CO2 from coal combustion, six coal samples were selected from the U.S. Geological Survey (USGS) coal storage archives in Reston, Virginia. The samples were selected to represent various geologic ages and coal ranks (Table 1). All samples were stored in plastic bags and were air dried and ground to b2 mm in size before they were archived. The coal samples ranged in age from Carboniferous to Paleogene and coal ranks ranged from lignite to bituminous. Proximate and ultimate analyses of the samples conducted by Geochemical Testing in Somerset, Pennsylvania, were available from previous studies and are presented on Table 2. All analyses followed ASTM coal analytical standards available at the time of the analyses (see http://www.astm.org/Standards/ coal-and-gas-standards.html). Duplicate sets of splits (1 to 3 g) of the six coal samples were ignited on 11 February 2015 (run 1), and 31 March 2016 (run 2), with a Bunsen burner and partially combusted at atmospheric conditions following a modified method described by Connecticut Energy Education (2015) and similar laboratory combustion methods described by Horváth et al. (2012) and Liu et al. (2014). The Bunsen burner was removed after coal ignition, to prevent natural gas combustion from contributing CO2 to the coal combustion gases. The ignition of the duplicate set of coal samples allowed for a measure of repeatability of the methods and results. Approximate combustion temperatures (all b500 ℃) for each sample were measured for the run 2 samples at the top of the smoldering coal pile using an Oakton Mini InfraPro™ 6 noncontact

(kJ, AR)

experiments. The carbon and oxygen isotopic signature of CO2 produced from industrial-scale use of coal combustion for electric power generation has not been reported in the scientific literature. To prevent CO2 release to the atmosphere, anthropogenic CO2 resulting from industrial-scale coal combustion can be captured and safely injected into and stored in underground reservoirs or it can be used in CO2-enhanced oil recovery (EOR) operations (Grobe et al., 2010). The isotopic signatures of carbon and oxygen of the injected CO2 (Johnson et al., 2011) as well of those of noble gases (Gilfillan and Haszeldine, 2011), have been used to distinguish injected CO2 from background CO2 that may be present or dissolved in fluids in the reservoir. The goal of this research is to document baseline relationships between carbon and oxygen isotopes of coal and that of the CO2 produced from laboratory atmospheric, or open, combustion of the coal. The partial combustion of the coal samples described in this report were done under uncontrolled laboratory conditions and the results should be considered preliminary; however, the results of this work may help to characterize the isotopic signatures of CO2 produced from coal combustion in atmospheric conditions; for example, CO2 sourced from domestic coal combustion and natural surface and underground coal fires. The results may also be helpful to distinguish CO2 produced from industrial coal combustion from various other CO2 sources (both naturally occurring and anthropogenic), and may be used to better model anthropogenic CO2 distributions in the atmosphere and anthropogenic CO2 that has been injected into subsurface geologic reservoirs.

Organic

P.D. Warwick, L.F. Ruppert / International Journal of Coal Geology xxx (2016) xxx–xxx

Sample number

2



thermometer with an accuracy of ±2 °C. Air flow at the fume hood face was set at 30.5 m per minute. The resulting combustion coal gases were collected using an inverted glass funnel connected by a hose to a hand pump and Isotech Tbag™ Gas Bags (Fig. 1A & B). Isotech Laboratories, Inc. of Champaign, Illinois analyzed the produced gases for gas composition (mol.%; accurate to within 2%) and compound-specific isotopes (Tables 3 and 4). Isotech reported the isotopic signatures of carbon and oxygen relative to the Vienna Pee Dee Belemnite (VPDB) scale, and hydrogen isotope relative to Vienna Standard Mean Ocean Water and Standard Light Antarctic Precipitation (VSMOW‐SLAP) scale. According to Isotech Laboratories, Inc. (2012), the molecular composition of sample gases are determined using Shimadzu 2010 or Shimadzu 2014 gas chromatographs (GCs). The compound-specific isotopes analyzed for this study include δ13CVPDB-CO2, δ13CVPDB-CH4, (gas chromatography–combustion–isotope ratio mass spectrometry; precision ± 0.3‰), and δ2HVSMOW-CH4 (gas chromatography/pyrolysis/isotope ratio mass spectrometry; precision ±5.0‰). For the latter analyses Isotech used SRI 8610C gas chromatographs and “continuous flow” systems consisting of an Agilent GC combustion unit interfaced with a mass spectrometer (Delta V Plus or Delta Plus Advantage). A Finnigan MAT Delta S Isotope Ratio Mass Spectrometer was used for the measurement of 13C/12C and 18O/16O in CO2 (Isotech Laboratories, Inc., 2012). The oxygen isotopic values were converted from the VPDB scale to the VSMOW-SLAP scale following methods described in Coplen et al. (2002) and Brand et al. (2014). Calculations for gas calorific values (kilojoules converted from British thermal units) and specific gravity follow ASTM D3588-98 (2011). Chemical compositions were normalized to 100%. Single splits (~1 g) of the un-combusted dried ground coal samples were analyzed for δ13CVPDB and δ18OVSMOW by the Reston Stable Isotope Laboratory (RSIL) at USGS in Reston, Virginia (Table 4). Analyses of δ13C of the coal samples follow Révész et al. (2012). Two to five aliquots of each sample were analyzed for carbon isotope composition. The average standard deviation is better than 0.2‰ (1 sigma). For oxygen isotope analysis, because there is no international isotopic reference material

3

available for oxygen isotope analysis in coal, the coal samples were analyzed along with water references directly and the δ18O of coal data are normalized to the VSMOW-SLAP scale. Although there are no suitable isotopic reference materials available for δ18O analysis of coal, the direct use of VSMOW and other water reference standards sealed in silver tubes (Qi et al., 2010) has been demonstrated as the most accurate and effective method in δ18OVSMOW determination of O-bearing materials (Brand et al., 2009). Most of the isotopic reference materials for hydrogen and oxygen currently available at the time of this publication are all calibrated against VSMOW and SLAP waters (Brand et al., 2009; Coplen and Qi, 2012). An online continuous-flow technique for automated δ18O determinations using an isotope-ratio mass spectrometer connected to a high-temperature conversion system (HTC or temperature conversion/elemental analysis, TC/EA) was used for the δ18O measurements in the coal samples. The HTC reactor temperature was operated at 1350 °C and the gas chromatograph (GC) column temperature was set to 80 °C. Approximately 0.5 to 1.3 mg bulk coal sample was weighed out and wrapped into a silver capsule for analysis. The coal samples were introduced into a high temperature reactor and the converted gases (H2, CO) from the coal samples were separated by the GC column, and introduced into a Delta + XP isotope-ratio mass spectrometer via a ConFlo IV interface. International water reference VSMOW and laboratory reference water UC03 (δ18O = +29.79‰) that were sealed in silver tubes were interspersed among the coal samples for normalization. The δ18O of coal results represent the average of two analyses with uncertainty b 0.25‰ (Table 4). 3. Results The coal samples used in this study were partially combusted. At the time of gas sample collection, the underside of the ground coal pile ignited by the Bunsen burner was glowing whereas the top of the pile was smoldering. Temperatures measured by the laser thermometer at the top of the smouldering coal pile for run 2 samples ranged from 325 to 485 °C (note that coal combustion temperatures in power plants

Fig. 1. Photographs of laboratory coal combustion methods. A) Ground coal samples were ignited with a Bunsen burner and partially combusted in laboratory atmospheric conditions. B) The resulting coal gases were collected using an inverted glass funnel connected by a hose to a hand pump and gas sample bags.


ID = identification; nd = not detected; na = not analyzed; mol. = mole; C1 = methane; C2 = ethane; C2H4 = ethylene; C3 = propane; iC4 = isobutane; nC4 = normal butane; iC5 = isopentane; nC5 = normal pentane; C6 + = hexane and molecular weight hydrocarbons.

1.009 1.005 1.004 1.001 1.000 1.003 1.010 1.016 1.019 1.011 1.022 1.02 0.0142 0.0085 0.0140 0.0109 0.0154 0.0080 0.0151 0.0082 0.0074 0.0129 0.0089 0.0152 0.0016 0.0015 0.0015 0.0019 0.0015 0.0013 0.0005 0.0009 0.0002 0.0026 0.0007 0.0025 0.0006 0.0005 0.0005 0.0006 0.0014 0.0014 0.0003 0.0004 nd 0.0009 0.0002 0.0008 0.0041 0.0036 0.0058 0.0045 0.0113 0.0035 0.0018 0.0024 0.0188 0.0062 0.0177 0.0064 0.0009 0.0007 0.0007 0.0008 0.0018 0.0020 0.0004 0.0007 0.0004 0.0013 0.0009 0.0011 0.0148 0.0161 0.0158 0.0144 0.0215 0.0173 0.0074 0.0145 0.0086 0.0264 0.0204 0.0343 0.0269 0.0227 0.0368 0.0276 0.0524 0.0366 0.0171 0.0296 0.0450 0.0473 0.0627 0.0609 0.0364 0.0305 0.0356 0.0386 0.0490 0.0515 0.0065 0.0207 0.0108 0.0757 0.0283 0.0636 E-0709002-063 E-0709002-063-2 SBT-19-R7 SBT-19-R7-2 07018-01314GBC 07018-01314GBC-2 MS-02-DU MS-02-DU-2 PA-2-CN6 PA-2-CN6-2 PA-2-CN2 PA-2-CN2-2 Pennsylvanian OH Pennsylvanian OH-2 Permian India Permian India-2 Cretaceous NM Cretaceous NM-2 Paleocene MS Paleocene MS-2 Paleocene TX 1 Paleocene TX 1-2 Paleocene TX 2 Paleocene TX 2-2

nd nd nd nd nd nd nd nd nd nd nd nd

0.345 0.042 0.404 0.342 0.566 0.121 0.541 0.109 0.957 0.147 0.750 0.14

0.937 0.939 0.945 0.935 0.929 0.939 0.939 0.912 0.929 0.919 0.906 0.926

13.41 17.26 14.02 15.94 14.04 17.56 12.82 14.93 7.65 17.24 8.00 14.7

4.15 1.90 3.15 2.04 2.69 1.60 4.72 4.60 8.26 3.32 8.38 5.48

79.02 78.76 79.31 79.03 79.41 78.69 78.59 78.21 78.55 76.97 78.49 76.98

1.84 0.87 1.90 1.46 1.95 0.75 2.29 1.03 3.36 0.84 2.98 1.25

0.183 0.132 0.145 0.144 0.208 0.206 0.0514 0.1220 0.128 0.373 0.155 0.316

0.0136 0.0128 0.0142 0.0130 0.0482 0.0148 0.0034 0.0071 0.0785 0.0214 0.199 0.0203

Specific gravity mol.%

C6 + nC5

mol.% mol.%

iC5 nC4

mol.% mol.%

iC4 C3H6

mol.% mol.% mol.% mol.% mol.%

C3 C2H4 C2 C1 CO

mol.% mol.%

N2 CO2

mol.% mol.%

O2 Ar

mol.%

H2

mol.%

He

mol.%

Sample number Sample name

Table 3 Molecular composition of the collected gases. Sample numbers ending in “-2” were used in run 2.

13 7 13 11 16 8 12 7 20 12 23 13

P.D. Warwick, L.F. Ruppert / International Journal of Coal Geology xxx (2016) xxx–xxx Calorific value (kJ)

4

are generally greater than 1000 ℃). All gases produced from the combusted coal samples were dominated by atmospheric nitrogen (N2; 77 to 79 mol.%) and oxygen (O2; 7.6 to 17 mol.%) because the coal had been partially combusted at atmospheric conditions (Table 3). Other gases included CO2 (1.6 to 8.4 mol.%), carbon monoxide (CO; 0.75 to 3.4 mol.%) and methane (CH4; 0.05 to 0.37 mol.%) and trace amounts of heavier hydrocarbons (Table 3; Fig. 2). The gases collected during the second combustion run generally contained less CO2 and CO perhaps due to different combustions conditions during the two runs. The gas samples from the lignite and sub-bituminous coal samples produced more CO2 and CO than the gases derived from the bituminous coal samples (Fig. 2). The isotopic results of the δ13CVPDB of the coal combustion CO2 ranged from − 26.94 to − 24.16‰ (Table 4; Fig. 3) and those for δ18OVSMOW of CO2 ranged from +19.03 to +27.03‰ (Table 4; Fig. 4). The CO2 gases collected during the second combustion run had slightly heavier values of δ13CVPDB and lighter values of δ 18O VSMOW. Eight coal combustion gases (3 from run 1, and 5 from run 2) produced sufficient quantities of CH4 for the measurement of δ13 CVPDB of CH4 , and these values ranged from − 33.62‰ to − 16.95‰. Two of the collected gas samples from run 2 yielded δ2HVSMOW-CH4 values of − 243‰ and − 239.8‰. The carbon and oxygen isotope results of the bulk coal samples are shown on Table 4. The results for δ13CVPDB of the coal samples range from − 28.46 to − 23.86‰. The δ18O of the coal samples ranged from + 2.96 to + 14.77‰. Because the coal samples were stored in plastic bags and periodically exposed to atmosphere during previous analytical procedures, the samples were somewhat pre-oxidized by long-term exposure to atmospheric oxygen, and their bulk oxygen isotopic signature may not be identical to that of freshly mined coal that is usually subject to industrial combustion. 4. Discussion The gases collected from the duplicate combustion runs indicate that the analytical results are generally reproducible. Schumacher et al. (2011) combusted samples of various organic materials (leaves, wood, peat, and coal) using controlled combustion temperatures (range 450 to 750 °C) in the laboratory to study the effects of fuel type, fuel particle size, combustion temperature, oxygen availability, and fuel water content on the δ18O values of the produced CO2. The samples were combusted in high-purity oxygen with an isotopic signature of δ18O = +27.2‰; however, to compare the results to those from atmospheric combustion, two samples (a charcoal and a peat) were combusted using laboratory atmosphere (Fig. 5). Schumacher et al. (2011) described the influences on carbon and oxygen isotopic fractionation during the combustion process and reported the major influences on the isotopic composition of combustion gases include the temperature of combustion, the carbon isotopic signature of the combusted fuel material, fuel particle size, and water content of the fuel material. Schumacher et al. (2011) also suggested the δ18O signature of the combusted organic material may influence the δ18O signature of the resulting combustion-derived CO2; however, they did not measure δ18O of the coal samples used in their study. Schumacher et al. (2011) chose to use high-purity oxygen for their combustion experiments because of the dampening effect of nitrogen and water vapor on the combustion process in natural atmosphere. Atmospheric components may also react with the combustion gases. Although preliminary, the results of our study may help to better characterize the oxygen and carbon isotopic character of CO2 derived from atmospheric coal combustion. The δ13C signature of bulk coal has been well studied and reported in the literature (Jeffery et al., 1955; Gleason and Kyser, 1984; Holmes and Brownfield, 1992; Rimmer et al., 2006; Elswick et al., 2007; Bechtel et al., 2008; Singh et al., 2012). Gröcke (2002) has compared the carbon isotope composition of ancient atmospheric CO2 to that of organic matter derived from higher-plants and suggests that both vary over geologic time. The δ13CVPDB of coal samples analyzed in this study (Table 4)



5

Table 4 Carbon and oxygen isotope compositions in bulk coal and gases collected from combusted coal, all isotopic values given in per mil (‰). Sample

Sample number

Run

Bulk coal

Collected gases from partially combusted coal

δ13CVPDB

Pennsylvanian OH Pennsylvanian OH Permian India Permian India Cretaceous NM Cretaceous NM Paleocene MS Paleocene MS Paleocene TX 1 Paleocene TX 1 Paleocene TX 2 Paleocene TX 2

E-0709002-063 E-0709002-063-2 SBT-19-R7 SBT-19-R7-2 07018-01314GBC 07018-01314GBC-2 MS-02-DU MS-02-DU-2 PA-2-CN6 PA-2-CN6-2 PA-2-CN2 PA-2-CN2-2

1 2 1 2 1 2 1 2 1 2 1 2

Mass fraction of C (%)

δ18OVSMOW

−26.57

54.6

10.29

15.5

−23.86

48.0

02.96

09.2

−24.09

69.4

11.82

11.3

−25.73

42.0

14.77

25.5

−28.46

62.7

14.12

21.4

−26.77

59.7

13.27

21.0

Mass fraction of O (%)

CO2

CH4

δ13CVPDB

δ18OVSMOW

δ13CVPDB

δ2HVSMOW

−26.13 −25.2 −26.24 −25.68 −26.48 −24.95 −25.74 −24.16 −26.94 −26.13 −26.01 −25.23

24.93 21.89 26.04 26.05 24.03 19.03 26.67 23.04 26.46 20.65 27.03 21.12

−25.89 −30.57 na −16.95 −24.78 −25.39 na na na −33.62 −17.46 −31.11

na na na na na na na na na −239.8 na −243

na = not analyzed.

et al. (2011) for their coal and charcoal samples, respectively, combusted at 500 and 750 °C in high-purity oxygen. According to Weather Underground (www.weatherunderground) for the Washington Dulles International Airport station (KIAD), about 10 km west of the Reston VA USGS laboratories, the humidity on the day and time of run 1 was 40% and of run 2 was 53%. Although the laboratory building has heating and air conditioning systems, the laboratory humidity does vary according to outside conditions (Kolker and Huggins, 2007). We did not measure the humidity of the laboratory air during the combustion runs. For comparison purposes, a plot of the δ13CVPDB values and δ18OVSMOW of CO2 for combustion-derived CO2 and that of atmospheric O2 is shown on Fig. 5. The values of δ18OVSMOW of CO2 (+ 19.03 to + 27.03‰, Table 4) from the coal combustion gases from this study are similar to atmospheric oxygen δ18O values which average + 23.8‰ (Coplen et al., 2002) (Fig. 5), which is to be expected as the samples were combusted in an open system. Schumacher et al. (2011) used controlled temperature combustion in high-purity oxygen (δ18O = +27.2‰) for 10 charcoal, peat, and coal samples and reported results for the combustion derived δ18OVSMOW of CO2 to be 10 to 20‰ less than atmospheric oxygen δ18O values (Fig. 5). The peat and coal samples combusted in the atmosphere by Schumacher et al. (2011) have simular δ18OVSMOW

generally follow the plant and coal isotopic trend described by Gröcke (2002). The coal combustion-derived δ13CVPDB-CO2 signatures are similar to that of the δ13CVPDB of the bulk coal (− 28.46 to − 23.86‰; Table 4; Fig. 3) and are not similar to modern atmospheric δ13CVPDB of CO2 (− 8.2 to − 6.7‰; Coplen et al., 2002). The coal partial-combustionderived δ13CVPDB-CH4 and δ2HSMOW-CH4 values fall within the range of thermogenic natural gas described by Whiticar (1996) and the δ13CVPDB-CH4 values compared to the ratio of methane and higher hydrocarbon composition (using a Bernard diagram, Bernard et al., 1978; Whiticar, 1999) indicates they were sourced from Type III kerogen (coal). A greater amount of methane was captured from the combustion gases of the second combustion run than from run 1, and may indicate that run 2 was conducted at slightly lower combustion temperatures, or that atmospheric humidity during run 2 may have inhibited the combustion temperatures. Combustion at decreased temperatures (below 450–500 °C) would produce greater amounts of methane than at higher combustion temperatures and would cause the resulting CO2 to be more depleted in δ18O (Schumacher et al., 2011). The δ18O values of the CO2 collected from run 2 are more depleted than CO2 δ18O values from run 1 (Fig. 5). There is also enrichment in the δ13CVPDB-CO2 values from those of run 1 to run 2, a trend that was also reported by Schumacher

10 9 8

Mole percent

7 6 CO2 (R1, %) 5

CO2 (R2, %)

4

CO (R1, %)

3

CO (R2, %) CH4 (R1, %)

2

CH4 (R2, %) 1

2 eo ce

ne

TX

TX Pa l

Pa l

eo

ce ne

ce ne eo Pa l

ac e re t

1

S M

M N s ou

n ia Pe r

m

C

Pe n

ns

yl

va n

ia

n

O

In di

H

a

0

Coal sample Fig. 2. Plot of the mole percent composition of coal combustion carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4). R1 = run 1; R2 = run 2.


6

P.D. Warwick, L.F. Ruppert / International Journal of Coal Geology xxx (2016) xxx–xxx -16

-18

-20

-22

δ13CVPDB (‰)

Coal δ13C (‰) -24 δ13C-CO2 (R1, ‰) -26 δ13C-CO2 (R2, ‰) -28 13 δ C-CO4 (R1, ‰)

-30

δ13C-CO4 (R2, ‰)

-32

2 TX

1 Pa

le

le

oc en

oc en

e

e

TX

M S oc le Pa

Pa

Pe

C

re

Pe

ta

ce o

en

us

e

N

In d rm ia n

ia n an lv sy nn

M

ia

O H

-34

Coal sample Fig. 3. Plot of the isotopic signatures of δ13CVPDB for the original coal samples and carbon dioxide (CO2) and methane (CH4) of the gases derived from coal sample combustion. R1 = run 1; R2 = run 2.

of CO2 as the δ18O values obtained in this study (Fig. 5). The δ13CVPDB values of CO2 from the carbon-rich samples of Schumacher et al. (2011) and this study are simular and range between − 27 to − 22.5‰ (Fig. 5). These results indicate that atmospheric combustion of carbon-rich fuel (charcoal, peat, and coal) will result in δ18OVSMOW of CO2 values similar to that of the atmosphere and range between +19 to +27‰. In contrast, Gleason and Kyser (1984) report that combustion gases from an underground coal fire had δ18OVSMOW of CO2

values of −17‰ and suggested the oxygen isotope values were similar to the groundwater (−15‰ δ18OVSMOW) of their study area in Utah. Correlation coefficients were calculated for selected data presented in Tables 2 and 4. The δ13CVPDB values of the bulk coal correlate with the carbon values from the original coal samples (R2 = 0.652). The δ18OVSMOW-coal signatures derived from bulk coal correlate (R2 = 0.792) with the coal dry ash yields (7.52 to 42.19%) (Fig. 6) and do not correlate with the δ18OVSMOW values of combustion derived CO2

30 28 26

Co2 δ18O (R1, ‰)

24 22

Co2 δ18O (R2, ‰)

δ18OVSMOW (‰)

20 18

Coal δ18O (‰)

16 14

Atms δ18O (‰)

12

CO2 (Coplen et al. 2002)

10 8 6 4 2

2 TX

1 en e oc le Pa

oc e Pa le

oc e le Pa

ne

ne

TX

M

S

M N ou s C

re ta

ce

ia rm Pe

Pe nn

sy

lv

an

ia

n

n

In

O

di

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a

0

Coal sample Fig. 4. Plot of the isotopic signatures of δ18OVPDB for the original coal samples and carbon dioxide (CO2) derived from coal sample combustion. The value of oxygen isotopic signature of atmospheric (Atms) CO2 (δ18OVSMOW = +23.88) is from Brand et al. (2014). R1 = run 1; R2 = run 2.



7

-20

Atmosphere O2 -21

-22 Peat

-23

δ13CVPDB (‰)

Coal Charcoal o (500 C) -24 Charcoal -25

Charcoal

Coal

Coal Charcoal (750 oC)

Peat

This study, run 1 This study, run 2

Lignite

-26

Schumacher et al. (2011) Peat

-27

High-purity oxygen combustion

-28

Atmospheric combustion

-29

-30 0

5

10

15

20

25

30

δ OVSMOW (‰) 18

Fig. 5. Plot of δ18OVSMOW-SLAP of CO2 derived from combustion of coal and other carbon-rich fuels and δ13CVPDB values of the combustion-derived CO2. Note that samples combusted in atmospheric conditions cluster around the average atmospheric oxygen δ18OVSMOW value +23.8‰ (vertical dashed line). Schumacher et al. (2011) combusted coal, peat, and charcoal in high-purity oxygen (δ18OVSMOW = +27.2‰) and the resulting values of δ18OVSMOW of CO2 are 10 to 20‰ less than atmospheric oxygen δ18O values. Dashed arrows indicate the fractionation offsets that occurred between samples of run 1 and run 2 of this study, and samples combusted at different temperatures by Schumacher et al. (2011). Combustion temperatures are in parentheses.

(R2 = 0.066). The δ18OVSMOW results (Table 4) determined by the online TC/EA method in bulk coal are preliminary. There is no published method available for δ18OVSMOW determination in bulk coal samples that contain different amounts of incombustible material (dry ash) by on-line TC/EA. The δ18OVSMOW values obtained from TC/EA should reflect the total oxygen within the bulk coal samples, which includes oxygen from trapped moisture, organics, and various oxygen-bearing minerals. However, whether the oxygen from bulk coal samples was released completely or not by this method (TC/EA reactor temperature set at 1350 °C) needs to be further investigated. Nevertheless, the preliminary data shown that the coal samples with the greatest ash yield (for example the Permian India coal sample, Table 2, Fig. 6) have more negative δ18O values than the other bulk coal samples. Minerals generally have lighter δ18O values than that of plants (Coplen et al., 2002); Brand et al., 2014). For an accurate determination of δ18OVSMOW in bulk coal samples with different dry ash yields, more work is needed

to develop a method by using on-line TC/EA techniques. Having coal reference materials for δ18O measurements is desirable to calibrate analyses with other coals with different ash yields. We are currently working with the RSIL to develop coal reference materials for future δ18O analyses. Further work is needed on a broader set of coal and combustion gas samples to support our findings. Gas samples collected from industrialscale coal combustion facilities need to be characterized and are expected to have isotopic signatures influenced by the CO2-capture process, particularly the oxygen isotopes because the oxygen in CO2 rapidly exchanges with the oxygen in water (Johnson et al., 2011). Combustion gases collected from underground coal fires also need to be better characterized for their δ13C and δ18O values of CO2, so that these data can be used to better track the various natural and anthropogenic contributions of CO2 to the atmosphere. 5. Conclusions This preliminary study compares the carbon and oxygen isotopic signatures of coal and CO2 from atmospheric laboratory coal partial combustion. Duplicate splits of six available archived coal samples of various geologic ages and ranks were combusted in uncontrolled atmospheric conditions in the laboratory. The major findings of this work are summarized below:

16

δ18OVSMOW (‰) in bulk coal

14 12

R² = 0.7917

10 8 6 4 2 0 0

10

20

30

40

Ash (dry) weight percent Fig. 6. Plot of δ18OVSMOW signatures and dry ash yields of the bulk coal.

50

• There is a wide range of δ13CVPDB values of coal reported in the literature and the values obtained in this study generally follow previously reported ranges for higher plants over geologic time. • The isotopic signatures of δ13CVPDB of CO2 resulting from laboratory coal partial combustion are similar to and probably derived from the original δ13C signatures of the coal. • The preliminary δ18OVSMOW values of coal show a strong correlation to the coal dry ash yields and appeared to have little or no influence on the δ18OVSMOW values of CO2 resulting from coal combustion. Further work is needed to validate the analytical method for


8

•

•

•

•


δ18 O VSMOW determination of bulk coal with different ash yields by on-line TC/EA. The δ 13 C VPDB values of the combustion-derived CO2 moderately correlate with the carbon values for the original coal samples. This correlation needs to be further evaluated with new analyses of a diverse set of coal and combustion-derived CO 2 samples. The values of δ 18 O VSMOW of CO 2 derived from atmospheric combustion of coal and other high-carbon fuels (peat and coal) range from + 19.03 to + 27.03‰ and are similar to atmospheric oxygen δ 18 O VSMOW values which average + 23.8‰. Developing coal reference materials for δ 18 O measurements is needed to calibrate analyses with other coals with different ash yields by on-line TC/EA method. Further work is needed on a broader set of coal and coal combustion gas samples to support these findings. The isotopic composition of industrial CO 2 needs to be better characterized and compared to the results of laboratory coal combustion studies.

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