Scholarly article on topic 'Carbon and oxygen isotopic composition of coal and carbon dioxide derived from laboratory coal combustion: A preliminary study'

Carbon and oxygen isotopic composition of coal and carbon dioxide derived from laboratory coal combustion: A preliminary study Academic research paper on "Earth and related environmental sciences"

CC BY-NC-ND
0
0
Share paper
Keywords
{Coal / "Atmospheric coal combustion" / "Carbon dioxide" / "Carbon and oxygen isotopes"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Peter D. Warwick, Leslie F. Ruppert

Abstract 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 400ppm. 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 δ 13C of CO2, δ 13C of CH4, and δ 18O of CO2 were analysed by a commercial laboratory. Splits (~1g) of the un-combusted dried ground coal samples were analyzed for δ 13C and δ 18O by the U.S. Geological Survey Reston Stable Isotope Laboratory. The major findings of this preliminary work indicate that the isotopic signatures of δ 13C (relative to the Vienna Pee Dee Belemnite scale, VPDB) of CO2 resulting from coal combustion are similar to the δ 13CVPDB signature of the bulk coal (−28.46 to −23.86‰) and are not similar to atmospheric δ 13CVPDB of CO2 (~−8‰, see http://www.esrl.noaa.gov/gmd/outreach/isotopes/c13tellsus.html). The δ 18O values of bulk coal are strongly correlated to the coal dry ash yields and appear to have little or no influence on the δ 18O values of CO2 resulting from coal combustion in open atmospheric conditions. There is a wide range of δ 13C values of coal reported in the literature and the δ 13C values from this study generally follow reported ranges for higher plants over geologic time. The values of δ 18O (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 δ 18OVSMOW 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.

Academic research paper on topic "Carbon and oxygen isotopic composition of coal and carbon dioxide derived from laboratory coal combustion: A preliminary study"

ARTICLE IN PRESS

COGEL-02657; No of Pages 8

International Journal of Coal Geology xxx (2016) xxx-xxx

Carbon and oxygen isotopic composition of coal and carbon dioxide derived from laboratory coal combustion: A preliminary study

Peter D. Warwick *, Leslie F. Ruppert

US. Geological Survey, MS 956, Reston, VA 20192, USA

ARTICLE INFO

ABSTRACT

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

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 S13C of CO2, S13C of CH4, and S18O of CO2 were analysed by a commercial laboratory. Splits (~1 g) of the un-combusted dried ground coal samples were analyzed for S13C and S18O by the U.S. Geological Survey Reston Stable Isotope Laboratory.

The major findings of this preliminary work indicate that the isotopic signatures of S13C (relative to the Vienna Pee Dee Belemnite scale, VPDB) of CO2 resulting from coal combustion are similar to the S13CVPDB signature of

the bulk coal (— 28.46 to — 23.86 %o) and are not similar to atmospheric 613CypDB of CO2 (--8 %o, see http://

www.esrl.noaa.gov/gmd/outreach/isotopes/c13tellsus.html). The S18O values of bulk coal are strongly correlated to the coal dry ash yields and appear to have little or no influence on the S18O values of CO2 resulting from coal combustion in open atmospheric conditions. There is a wide range of S13C values of coal reported in the literature and the S13C values from this study generally follow reported ranges for higher plants over geologic time. The values of S18O (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 S18Ovsmow 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 Quere et al., 2014). In 2014, coal combustion from stationary electric power generation

* Corresponding author. E-mail address: pwarwick@usgs.gov (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; Horvâth 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/).

ARTICLE IN PRESS

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

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.

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 <2 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 Horvath 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 <500 °C) 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

Table 1

Coal samples used in this study.

Sample

Field ID

Associated report

Pennsylvanian OH Permian India Cretaceous NM Paleocene MS Paleocene TX1 Paleocene TX 2

07018-01314GBC

SBT-19-R7

E-0709002-063

MS-02-DU

PA-2-CN6

PA-2-CN2

Bituminous Bituminous Subbituminous Lignite

Lig-subbituminous Lig-subbituminous

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.

id ist esi oi

•c £

o o o o o

o o o o o

0. 0. 0. 0. 0. 0.

78647 cm T-

mc^^t^cN

coi— mmm 0. 2. 7. 8. 7.

r^qt^rninin 3. . 0. 0. 0. 0.

. .0 .9 .6 .7

in co rn 1 '¡t^mm

in oo m is ^ m

.7 .4 .8 .0 .4 .4 4. 3. 3. 7. 6. 6.

co ^ in o ^r co o rn (N in cti 433232

G) in co q ro in O rn rn (N

n23233

^ o m r>.

t— 00 00 N

co in q rn co r^ r^ rn in o

mtNffl co m (Nt^

n 9 3 2

"rBa 8- 9 9 2-

-2 -2 A- A-

0SS -0 G G

ns a o o o

<u <u rt rt rt PPCPPP

ARTICLE IN PRESS

P.D. Warwick LF. Ruppert / International Journal of Coal Geology xxx (2016) xxx-xxx 3

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 iso-topic 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 S13CVPDB-CO2, 513Cvpdb-ch4, (gas chromatography-combustion-isotope ratio mass spectrometry; precision ±0.3%), and 52HVSMOW-CH4 (gas chromatography/pyrolysis/isotope ratio mass spectrometry; precision ±5.0%o). 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 613CVPDB and 618OVSMOW by the Reston Stable Isotope Laboratory (RSIL) at USGS in Reston, Virginia (Table 4). Analyses of S13C of the coal samples follow Revesz 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

available for oxygen isotope analysis in coal, the coal samples were analyzed along with water references directly and the S18O of coal data are normalized to the VSMOW-SLAP scale. Although there are no suitable isotopic reference materials available for S18O 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 618OVSMOW 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 S18O 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 S18O 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 (S18O = + 29.79%) that were sealed in silver tubes were interspersed among the coal samples for normalization. The S18O of coal results represent the average of two analyses with uncertainty < 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.

ARTICLE IN PRESS

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

^ in 000 000

O T- T- To o o o

.0.0 0. 0.

.0.0.0.0 0. 0. 0. 0.

.0.0 0. 0.

.0.0.0.0 0. 0. 0. 0.

COLnin^inmin^CNCO

oooooooooo . . . . . . . . . . 0. . . . . . . . . .

tommio^^ro't ^ (N oo 0000^-^-00 ooo oooooooo ooo OOOOOOOO-TjOOO 0. . . . . . . . n 0. 0. 0.

^nin^r-CO'-tNCOl.Ot'NtO

oooo^-ooo^-o^-o . . . . . . . . . . . . 0. . . . . . . . . . . .

OOOO^-CNOOO^-O OOOOOOOOOOO' . . . . . . . . . . . 0. . . . . . . . . . .

(Ni-Or-OtN(NfO

. . . . . . . . . 0. 0. 0. 0. 0. 0. 0. 0. 0.

lOooiNOiNoo^r-m^i m

3 2 4 3 8 4 3 7 8 19 0

----'vTrOOMNliM

.0 .0 0. 0.

0. 0. 0. 0. 0. 0. 0. 0. 0.

9 7 8 6 4 6 16 0 3 7 9

COCNCOt^CNCOt^C^int^CNO

.0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.

^mcocooinint^cot^roco toomooaii-iooomoorn MrorOM^inOiN^l^iN'.O

.0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.

rOCNin^COCO^-CNCOfOinCO

oomi'toomfSMNmr-0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.

4 .8 7 .8 0 .9 6 .4 in .9 in .7 9 .2 3 .0 6 .3 4 .8 8 .9 LT .2

0. 0. 2. 3. 0. 2.

2 6 3 9 9 in 7 9 8

.0 .7 m .0 .6 m CN m .9 .4 .9

9. 7 8. 7 9. 7 9. 7 9. 7 8. 7 8. 7 8. 7 8. 7 6. 7 8. 7 6. 7

in O in .1.9 .1

4. . .

90206288 .6 .6 .7 .6 .2 .3 .3 .4

COiNf^r-^'îfO^C^O^U^

3 3 4 3 2 3 3 12 10 2 .9 .9 .9 .9 .9 .9 .9 .9 9 .9 9 .9

0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.

mO^mm^-Ln^-C^ 0. 0. 0. 0. 0. 0. 0. 0. 0.

dddddddddddd

33 66 00

BB GG 44

0 0 - -9999 0011

7 7 - -00TT

U 6 6 2 2 -D N N N N 2- -C -C -C -C -0 -2- -2- -2- -2-S A- A- A- AM P P P P

-2 2 2-

ia ia mi mi

uueeee oonnnnnn eeeeeeee cccccccc aaoooooo et et le le le le le le

CCPPPPPP

are generally greater than 1000 C). 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 S13CvpDb of the coal combustion CO2 ranged from — 26.94 to — 24.16%» (Table 4; Fig. 3) and those for 618OVSMOW 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 613CVPDB and lighter values of 618OVSMOW. Eight coal combustion gases (3 from run 1, and 5 from run 2) produced sufficient quantities of CH4 for the measurement of S13CVPDB of CH4, and these values ranged from — 33.62% to — 16.95%. Two of the collected gas samples from run 2 yielded S2Hvsmow_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 613CVPDB of the coal samples range from — 28.46 to — 23.86%. The S18O 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 ofvarious 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 S18O values of the produced CO2. The samples were combusted in high-purity oxygen with an isotopic signature of S18O = + 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 fraction-ation 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 S18O signature of the combusted organic material may influence the S18O signature of the resulting combustion-derived CO2; however, they did not measure S18O 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 S13C 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). Grocke (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 613CVPDB of coal samples analyzed in this study (Table 4)

ARTICLE IN PRESS

P.D. Warwick LF. Ruppert / International Journal of Coal Geology xxx (2016) xxx-xxx 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

0i3C 0 CVPDB Mass fraction of C (%) 618°VSMOW Mass fraction of O (%) CO2 Ô13cvpdb Ô18°VSMOW CH4 Ô13CVPDB Ô2hvsmow

Pennsylvanian OH E-0709002-063 1 — 26.57 54.6 10.29 15.5 -26.13 24.93 - 25.89 na

Pennsylvanian OH E-0709002-063-2 2 -25.2 21.89 - 30.57 na

Permian India SBT-19-R7 1 — 23.86 48.0 02.96 09.2 -26.24 26.04 na na

Permian India SBT-19-R7-2 2 -25.68 26.05 -16.95 na

Cretaceous NM 07018-01314GBC 1 — 24.09 69.4 11.82 11.3 - 26.48 24.03 - 24.78 na

Cretaceous NM 07018-01314GBC-2 2 - 24.95 19.03 - 25.39 na

Paleocene MS MS-02-DU 1 — 25.73 42.0 14.77 25.5 - 25.74 26.67 na na

Paleocene MS MS-02-DU-2 2 -24.16 23.04 na na

Paleocene TX1 PA-2-CN6 1 — 28.46 62.7 14.12 21.4 - 26.94 26.46 na na

Paleocene TX 1 PA-2-CN6-2 2 -26.13 20.65 - 33.62 - 239.8

Paleocene TX 2 PA-2-CN2 1 — 26.77 59.7 13.27 21.0 -26.01 27.03 -17.46 na

Paleocene TX 2 PA-2-CN2-2 2 - 25.23 21.12 - 31.11 - 243

na = not analyzed.

generally follow the plant and coal isotopic trend described by Grocke (2002).

The coal combustion-derived S13Cvpdb-co2 signatures are similar to that of the S13Cvpdb of the bulk coal (- 28.46 to - 23.86%; Table 4; Fig. 3) and are not similar to modern atmospheric 613Cvpdb of CO2 ( — 8.2 to — 6.7%; Coplen et al., 2002). The coal partial-combustion-derived 613Cvpdb-ch4 and S2Hsmow-ch4 values fall within the range of thermogenic natural gas described by Whiticar (1996) and the 613Cvpdb-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 S18O (Schumacher et al., 2011). The S18O values of the CO2 collected from run 2 are more depleted than CO2 S18O values from run 1 (Fig. 5). There is also enrichment in the S13CVPDB-cO2 values from those of run 1 to run 2, a trend that was also reported by Schumacher

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 613Cvpdb values and 618OVSMOW of CO2 for combustion-derived CO2 and that of atmospheric O2 is shown on Fig. 5. The values of 618OVSMOW of CO2 (+19.03 to +27.03%, Table 4) from the coal combustion gases from this study are similar to atmospheric oxygen S18O 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 (S18O = +27.2%) for 10 charcoal, peat, and coal samples and reported results for the combustion derived 618OVSMOW of CO2 to be 10 to 20% less than atmospheric oxygen S18O values (Fig. 5). The peat and coal samples combusted in the atmosphere by Schumacher et al. (2011) have simular 618OVSMOW

-t— CO2 (R1, %) -♦-CO2 (R2, %) CO (R1, %) CO (R2, %) -,k-CH4 (R1, %) -a-CH4 (R2, %)

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.

ARTICLE IN PRESS

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

Coal S13C (fc) S13C-CO2 (R1, fc) -A- S13C-CO2 (R2, fc) ■ S13C-CO4 (R1, fc) • S13C-CO4 (R2, fc)

Coal sample

Fig. 3. Plot of the isotopic signatures of 613Cvpdb 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 S O values obtained in this study (Fig. 5). The S Cwdb 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 S18Ovsmow of CO2 values similar to that of the atmosphere and range between + 19 to +27%o. In contrast, Gleasonand Kyser (1984) report that combustion gases from an underground coal fire had S18OVSMOW of CO2

values of —17% and suggested the oxygen isotope values were similar to the groundwater (—15% S18OVSMOW) of their study area in Utah.

Correlation coefficients were calculated for selected data presented in Tables 2 and 4. The S13Cvpdb values of the bulk coal correlate with the carbon values from the original coal samples (R2 = 0.652). The S18OVSMOW-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 S18OVSMOW values of combustion derived CO2

Co2 S18O (R1, fc)

—Co2 S18O (R2, fc)

Coal S18O (fc)

-0- Atms S18O (fc) CO2 (Coplen et al. 2002)

Coal sample

Fig. 4. Plot of the isotopic signatures of 018OVPDB for the original coal samples and carbon dioxide (CO2) derived from coal sample combustion. The value of oxygen isotopic signature of atmospheric (Atms) CO2 (018OVSMOw = + 23.88) is from Brand et al. (2014). R1 = run 1; R2 = run 2.

ARTICLE IN PRESS

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

1 Atmosphere O2 1

Peat ■ ■ 1

Coal Charcoal (500 oC) „ , '' " 1 ^ Charcoal ' N

J ^ Coal Coal coal Charcoal (750 oC) ; m \ Peat j I \ 1

Lignite 1 \ 1 \ :

Peat \ /

Atmospheric combustion

High-purity oxygen combustion

♦ This study, run 1 ▲ This study, run 2 ■ Schumacher et al. (2011)

S180Usmow №>)

Fig. 5. Plot of 618Ovsmow -slap of CO2 derived from combustion of coal and other carbon-rich fuels and S13Cvpdb values of the combustion-derived CO2. Note that samples combusted in atmospheric conditions cluster around the average atmospheric oxygen 618OVSMOW value +23.8%o (vertical dashed line). Schumacher et al. (2011) combusted coal, peat, and charcoal in high-purity oxygen (618OVSMOW = +27.2&0 and the resulting values of 618OVSMOW of CO2 are 10 to 20%o less than atmospheric oxygen 618O 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 618OVSMOw results (Table 4) determined by the online TC/EA method in bulk coal are preliminary. There is no published method available for 618OVSMOW determination in bulk coal samples that contain different amounts of incombustible material (dry ash) by on-line TC/EA. The 618OVSMOW 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 S18O values than the other bulk coal samples. Minerals generally have lighter S18O values than that of plants (Coplen et al., 2002); Brand et al., 2014). For an accurate determination of 618OVSMOW 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 S18O 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 S18O analyses.

Further work is needed on a broader set of coal and combustion gas samples to support our findings. Gas samples collected from industrial-scale 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 S13C and S18O 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

1 0 20 30 40

Ash (dry) weight percent

Fig. 6. Plot of 618OVSMOW signatures and dry ash yields of the bulk coal.

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:

• There is a wide range of 613CVPDB 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 S13CVPDB of CO2 resulting from laboratory coal partial combustion are similar to and probably derived from the original S13C signatures of the coal.

• The preliminary 618OVSMOW values of coal show a strong correlation to the coal dry ash yields and appeared to have little or no influence on the 618OVSMOW values of CO2 resulting from coal combustion. Further work is needed to validate the analytical method for

ARTICLE IN PRESS

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

S18OVSMOW determination of bulk coal with different ash yields by on-line TC/EA.

• The S13CVPDB 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 CO2 samples.

• The values of 618OVSMOW 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 618OVSMOW values which average +23.8%.

• Developing coal reference materials for S18O 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 CO2 needs to be better characterized and compared to the results of laboratory coal combustion studies.

Acknowledgements

The authors wish to thank Haiping Qi (USGS, Reston Stable Isotope Laboratory) for conducting analytical work for this study and for many discussions and reviews of the manuscript. The authors also wish to thank Allan Kolker, USGS, and anonymous reviewers for their thoughtful comments on the manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

References

Affek, H.P., Eiler, J.M., 2006. Abundance of mass 47 CO2 in urban air, car exhaust, and human breath. Geochim. Cosmochim Acta 70,1-12. http://dx.doi.org/10.1016/j.gca. 2005.08.02.

Affek, H.P., Xu, X., Eiler, J.M., 2007. Seasonal and diurnal variations of 13C18O16O in air: initial observations from Pasadena, CA. Geochim. Cosmochim. Acta 71,5033-5043. http://dx.doi.org/10.1016/j.gca.2007.08.014. Affolter, R.H., Groves, S., Betterton, W.J., Benzel, E., Conrad, K.L., Swanson, S.M., Ruppert, L.F., Clough, J.G., Belkin, H.E., Kolker, A., Hower, J.C., 2011. Geochemical database of feed coal and coal combustion products (CCps) from five power plants in the United States. U.S. Geol. Sur. Data Series, Pamphlet (19 pp. http://pubs.usgs.gov/ds/ 635).

ASTM D3588-98, 2011. Standard Practice for Calculating Heat Value, Compressibility Factor, and Relative Density of Gaseous Fuels. ASTM International, West Conshohocken, PA (www.astm.org). Bechtel, A., Gratzer, R., Sachsenhofer, R.F., Gusterhuber, J., Lücke, A., Püttmann, W., 2008. Biomarker and carbon isotope variation in coal and fossil wood of Central Europe through the Cenozoic. Palaeogeogr. Palaeoclimatol. Palaeoecol. 262,166-175. Bernard, B.B., Brooks, J.M., Sackett, W.M., 1978. Light hydrocarbons in recent Texas

continental shelf and slope sediments. J. Geophys. Res. 83, 4053-4061. Brand, W.A., Coplen, T.B., Aerts-Bijma, AT., Böhlke, J.K., Gehre, M., Geilmann, H., Gröning, M., Jansen, H.G., Meijer, H.AJ., Mroczkowski, S.J., Qi, H., Soergel, K., Stuart-Williams, H., Weise, S., Werner, RA., 2009. Comprehensive inter-laboratory calibration of reference materials for 81sO versus VSMOW using various on-line high-temperature conversion techniques. Rapid Commun. Mass Spectrom. 23, 999-1019. http://dx.doi.org/10.1002/rcm.3958. Brand, W.A., Coplen, T.B., Vogl, J., Rosner, M., Prohaska, T., 2014. Assessment of international reference materials for isotope-ratio analysis (IUPAC technical report). Pure Appl. Chem. 86 (3), 425-467. http://dx.doi.org/10.1515/pac-2013-1023. Connecticut Energy Education, 2015. CO2 Emissions from Burning Fossil Fuels: Connecticut Energy Education Website. (http://www.ctenergyeducation.com/lesson.htm? id=mzlkvdao).

Coplen, T.B., Qi, H., 2012. USGS42 and USGS43: human-hair stable hydrogen and oxygen isotopic reference materials and analytical methods for forensic science and implications for published measurement results. Forensic Sci. Int. 214,135-141. http://dx. doi.org/10.1016/j.forsciint.2011.07.035. Coplen, T.B., Hopple, JA, Böhlke, JK, Peiser, H.S., Rieder, S.E., Krouse, H.R., Rosman, K.J.R., Ding, T., Vocke Jr., R.D., Revesz, K.M., Lamberty, A., Taylor, P., De Bievre, P., 2002. Compilation of minimum and maximum isotope ratios of selected elements in naturally occurring terrestrial materials and reagents. U.S. Geol. Sur. Water Resources Investigations Report 2001 -4222 (98 pp., available at http://pubs.usgs. gov/wri/wri014222/).

Cuntz, M., Ciais, P., Hoffmann, G., Knorr, W., 2003. A comprehensive global three-dimensional model of 81SO in atmospheric CO2: 1. Validation of surface processes. J. Geophys. Res. 108, ACH1-ACH23.

Elswick, E.R., Hower, J.C., Carmo, A.M., Sun, T., Mardon, S.M., 2007. Sulfur and carbon isotope geochemistry of coal and derived coal-combustion by-products: an example from an Eastern Kentucky mine and power plant. Appl. Geochem. 22, 2065-2077.

Francey, R.J., Tans, P.P., 1987. Latitudinal variation in oxygen-18 of atmospheric CO2. Nature 327, 495-497.

Gilfillan, S.M.V., Haszeldine, R.S., 2011. Report of noble gas, carbon stable isotope and HCO3- measurements from the Kerr Quarter and surrounding area, Goodwater, Saskatchewan. In: Sherk, G.W. (Ed.), The Kerr Investigation: Final Report, Vol. IPAC-CO2 Research Inc., Regina (available at http://www.geos.ed.ac.uk/homes/sgilfil1/ Kerrreport.pdf).

Gleason, J.D., Kyser, T.K., 1984. Stable isotope compositions of gases and vegetation near naturally burning coal. Nature 307, 254-257.

Grobe, M., Pashin, J.C., Dodge, R.L. (Eds.), 2010. Carbon dioxide sequestration in geological media—state of the scienceAAPG Stud. Geol. 59 (715 pp.).

Gröcke, D.R., 2002. The carbon isotope composition of ancient CO2 based on higher-plant organic matter. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 360, 633-658. http://dx. doi.org/10.1098/rsta.2001.0965.

Gruber, N., 2001. An improved estimate of the isotopic air-sea disequilibrium of CO2: implications for the oceanic uptake of anthropogenic CO2. Geophys. Res. Lett. 28,555.

Hoag, K.J., Still, C.J., Fung, I.Y., Boering, K.A., 2005. Triple oxygen isotope composition of tropospheric carbon dioxide as a tracer of terrestrial gross carbon fluxes. Geophys. Res. Lett. 32, L02802. http://dx.doi.org/10.1029/2004GL021011.

Holmes, C.W., Brownfield, M.E., 1992. Distribution of carbon and sulfur isotopes in Upper Cretaceous coal of northwestern Colorado. Geol. Soc. Am. Spec. Pap. 267, 57-68.

Horvâth, B., Hofmann, M.E.G., Pack A., 2012. On the triple oxygen isotope composition of carbon dioxide from some combustion processes. Geochim. Cosmochim. Acta 95, 160-168. http : //dx.doi.org/10.1016/j.gca.2012.07.021.

Intergovernmental Panel on Climate Change, 2014. Climate change 2014: mitigation of climate change. In: Edenhofer, O., Pichs-Madruga, R., Sokona, Y., Farahani, E., Kadner, S., Seyboth, K., Adler, A., Baum, I., Brunner, S., Eickemeier, P., Kriemann, B., Savolainen, J., Schlömer, S., von Stechow, C., Zwickel, T., Minx, J.C. (Eds.), Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom, and New York, New York.

Isotech Laboratories, Inc., 2012. Quality Assurance Plan. Isotech Laboratories, Inc., Champaign, IL (available upon request, 27 p.).

Jeffery, P.M., Compston, W., Greenhalgh, D., De Laeter, J., 1955. On the carbon-13 abundance of limestones and coals. Geochim. Cosmochim. Acta 7, 255-286.

Johnson, G., Mayer, B., Nightingale, M., Shevalier, M., Hutcheon, I., 2011. Using oxygen isotope ratios to quantitatively assess trapping mechanisms during CO2 injection into geological reservoirs: the Pembina case study. Chem. Geol. 283, 185-193.

Joos, F., Spahni, R., 2008. Rates of change in natural and anthropogenic radiative forcing over the past 20,000 years. Proc. Natl. Acad. Sci. U. S. A. 105,1425-1430.

Keeling, C.D., 1958. The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas. Geochim. Cosmochim. Acta 13, 322-334.

Keeling, C.D., 1961. The concentration and isotopic abundances of carbon dioxide in rural and marine air. Geochim. Cosmochim. Acta 24, 277-298.

Kolker, A., Huggins, F.E., 2007. Progressive oxidation of pyrite in five bituminous coal samples: An As XANES and 57Fe Mössbauer study. Appl. Geochem. 22, 778-787.

Le Quéré, et al., 2014. Global carbon budget 2013. Earth Syst. Sci. Data 6. http://dx.doi.org/ 10.5194/essd-6-235-2014 (335-263).

Liu, G., Li, J., Xu, H., Wu, D., Liu, Y., Yang, H., 2014. Isotopic compositions of elemental carbon in smoke and ash derived from crop straw combustion. Atmos. Environ. 92, 303-308. http://dx.doi.org/10.1016/j.atmosenv.2014.04.042.

Qi, H.P., Gröning, M., Coplen, T.B., Buck, B., Mroczkowski, S.J., Brand, W.A., Geilmann, H., Gehre, M., 2010. Novel silver tubing method for quantitative introduction of water into high temperature conversion systems for stable hydrogen and oxygen isotopic measurements. Rapid Commun. Mass Spectrom. 24,1821-1827. http://dx.doi.org/ 10.1002/rcm.4559.

Révész, K., Qi, H., Coplen, T.B., 2012. Determination of the S15N and S13C of total nitrogen and carbon in solids; RSIL lab code 1832, chap. 5 of stable isotope-ratio methods sec. C of In: Révész, K., Coplen, T.B. (Eds.), Methods of the Reston Stable Isotope Laboratory (Slightly Revised From Version 1.1 Released in 2007). U.S. Geol. Sur. Techniques and Methods, Book 10 (31 pp., http://pubs.usgs.gov/tm/2006/tm10c5/. (Supersedes versions 1.0 and 1.1 released in 2006 and 2007, respectively.)).

Rimmer, S.M., Rowe, H.D., Taulbee, D.N., Hower, J.C., 2006. Influence of maceral content on S13C and S15N in a Middle Pennsylvanian coal. Chem. Geol. 225, 77-90.

Schumacher, M., Werner, R.A., Meijer, H.A.J., Jansen, H.G., Brand, W.A., Geilmann, H., Neubert, R.E.M., 2011. Oxygen isotopic signature of CO2 from combustion processes. Atmos. Chem. Phys. 11,1473-1490.

Scripps, 2016. The Keeling Curve. (http://keelingcurve.ucsd.edu/ (last access: 4 May 2016)).

Singh, P.K., Singh, M.P., Prachiti, P.K., Kalpana, M.S., Manikyamba, C., Lakshminarayana, G., Singh, A.K., Naik A.S., 2012. Petrographic characteristics and carbon isotopic composition of Permian coal: implications on depositional environment of Sattupalli coalfield, Godavari Valley, India. Int. J. Coal Geol. 90-01, 34-42.

U.S. Environmental Protection Agency, 2016. Inventory of U.S. greenhouse gas emissions and sinks: 1990-2014. U.S. Environmental Protection Agency EPA 430-R-16-002, variously paginated https://www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html.

Warwick P.D., SanFilipo, J.R., Karlsen, A.W., Barker, C.E., 2005. Results of coalbed methane drilling in Panola County, Texas. U.S. Geol. Sur. Open-File Report 2005-1046 155 (http://pubs.usgs.gov/of/2005/1046/).

Whiticar, M.J., 1996. Stable isotope geochemistry of coals, humic kerogens and related gases. Int. J. Coal Geol. 32,191-215.

Whiticar, M.J., 1999. Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chem. Geol. 161, 291-314.