Carbon isotopic signature of coal-derived methane emissions to the atmosphere: from coalification to alteration

Abstract

Currently, the atmospheric methane burden is rising rapidly, but the extent to which shifts in coal production contribute to this rise is not known. Coalbed methane emissions into the atmosphere are poorly characterised, and this study provides representative δ¹³C_CH₄ signatures of methane emissions from specific coalfields. Integrated methane emissions from both underground and opencast coal mines in the UK, Australia and Poland were sampled and isotopically characterised. Progression in coal rank and secondary biogenic production of methane due to incursion of water are suggested as the processes affecting the isotopic composition of coal-derived methane. An averaged value of -65[per thousand] has been assigned to bituminous coal exploited in open cast mines and of -55[per thousand] in deep mines, whereas values of -40 and -30[per thousand] can be allocated to anthracite opencast and deep mines respectively. However, the isotopic signatures that are included in global atmospheric modelling of coal emissions should be region- or nation-specific, as greater detail is needed, given the wide global variation in coal type.

Atmos. Chem. Phys., 16, 1366913680, 2016 www.atmos-chem-phys.net/16/13669/2016/ doi:10.5194/acp-16-13669-2016 Author(s) 2016. CC Attribution 3.0 License.

Giulia Zazzeri1, Dave Lowry1, Rebecca E. Fisher1, James L. France1,2, Mathias Lanoisell1, Bryce F. J. Kelly4, Jaroslaw M. Necki3, Charlotte P. Iverach4, Elisa Ginty4, Miroslaw Zimnoch3, Alina Jasek3, and Euan G. Nisbet1

1Royal Holloway University of London, Egham Hill, Egham, Surrey TW20 0EX, UK

2University of East Anglia, Norwich Research Park, Norwich, Norfolk NR4 7TJ, UK

3AGH-University of Science and Technology, Al. Mickiewicza 30, Krakw, Poland

4Connected Waters Initiative Research Centre, UNSW, Sydney, Australia

Correspondence to: Giulia Zazzeri ([email protected]) and Euan G. Nisbet ([email protected])

Received: 17 March 2016 Published in Atmos. Chem. Phys. Discuss.: 30 March 2016 Revised: 16 September 2016 Accepted: 25 September 2016 Published: 3 November 2016

Abstract. Currently, the atmospheric methane burden is rising rapidly, but the extent to which shifts in coal production contribute to this rise is not known. Coalbed methane emissions into the atmosphere are poorly characterised, and this study provides representative 13CCH4 signatures of methane emissions from specic coalelds. Integrated methane emissions from both underground and opencast coal mines in the UK, Australia and Poland were sampled and isotopically characterised. Progression in coal rank and secondary biogenic production of methane due to incursion of water are suggested as the processes affecting the isotopic composition of coal-derived methane. An averaged value of 65

has been assigned to bituminous coal exploited in open cast mines and of 55 in deep mines, whereas values of 40

and 30 can be allocated to anthracite opencast and deep

mines respectively. However, the isotopic signatures that are included in global atmospheric modelling of coal emissions should be region- or nation-specic, as greater detail is needed, given the wide global variation in coal type.

1 Introduction

Methane emissions from the energy sector have been driven in recent years using the impact of a shift from coal to natural gas, in the US and Europe, whereas in China coal production has increased over this century. Currently, the atmospheric methane burden is rising rapidly (Nisbet et al., 2014), but the extent to which shifts in coal production contribute to this

Carbon isotopic signature of coal-derived methane emissions to the atmosphere: from coalication to alteration

rise is not known. Coalbed methane emissions into the atmosphere are poorly characterised, as they are dispersed over large areas and continue even after a mines closure (IPCC, 2006). Methane is emitted in coal processing (crushing and pulverisation) and, during the initial removal of the overburden, it can be diluted and emitted through ventilation shafts in underground coal mines or directly emitted to the atmosphere from open-cut coal mining, where releases may occur as a result of deterioration of the coal seam.

For the UN Framework Convention on Climate Change, national emissions are estimated by a bottom-up approach, based upon a general equation where the coal production data are multiplied by an emission factor that takes into account the mines gassiness, which in turn is related to the depth of the mine and the coal rank (i.e. carbon content of coal) (US EPA, 2013). These modelled estimates are often reported without an error assessment; therefore the level of accuracy of the emissions is not known. Top-down assessment of methane emissions can be made by chemical transport models constrained by atmospheric measurements (Bousquet et al., 2006; Locatelli et al., 2013). However, this top-down approach provides the total amount of methane emissions into the atmosphere, which has to be distributed among the different methane sources in order to quantify each source contribution.

For methane emissions from fossil fuels (coal and natural gas), the source partitioning is mainly bottom-up, based on energy use statistics and local inventories, which might be highly uncertain. Conversely, the top-down study of the

Published by Copernicus Publications on behalf of the European Geosciences Union.

13670 G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions

carbon isotopic composition of methane, which is indicative of the methane origin, provides a valuable constraint on the budget appraisal, allowing different sources in a source mix to be distinguished and their individual strength to be evaluated (Hein et al., 1997; Liptay et al., 1998; Lowry et al., 2001; Townsend-Small et al., 2012).

Measurements of methane mole fractions can be complemented in atmospheric models by typical 13CCH4 signatures of the main methane sources in order to estimate global and regional methane emissions and assess emissions scenarios throughout the past years (Fung et al., 1991; Miller, 2004; Whiticar and Schaefer, 2007; Bousquet et al., 2006;Monteil et al., 2011; Mikaloff Fletcher et al., 2014). However, even though isotopic values are fairly distinctive, for specic methanogenic processes, the variety of production pathways and local environmental conditions that discriminate the methane formation process leads to a wide range of 13CCH4 values. The global isotopic range for coal is very large, from 80 to 17 (Rice, 1993), but it can be nar

rowed down when a specic basin is studied. While there are several studies of isotopic composition of methane generated from coal in Australia, the USA and China (Smith et al., 1981; Dai et al., 1987; Rice et al., 1989; Aravena et al., 2003; Flores et al., 2008; Papendick et al., 2011), there is a signicant lack of information about the isotopic characterisation of methane emissions from coal mines in Europe.

The purpose of this study was to determine links between 13CCH4 signatures and coal rank and mining setting, and to provide representative 13C signatures to be used in atmospheric models in order to produce more accurate methane emission estimates for the coal exploitation sector.

Process of coalication and parameters affecting the 13C signature of methane emissions

The process of coalication involves both biochemical and geochemical reactions. The vegetal matter rstly decays anaerobically under water; the simple molecules derived from initial decomposition (i.e. acetate, CO2, H2, NH+4,

HS, long chain fatty acids) are metabolised by fermentative archaea, which produce methane via two methanogenic paths: acetoclastic reaction or CO2 reduction (Whiticar, 1999). Different pathways lead to diverse 13CCH4 isotopic signatures methane from acetate is 13C enriched relative to methane from CO2 reduction, ranging from 65 to 50

and 110 to 50 respectively (Levin et al., 1993; Wal

dron et al., 1998). With increasing burial and temperatures, coal is subjected to thermal maturation, which implicates more geochemical changes.

As coalication proceeds, the carbon content increases, accompanied by a relative depletion in volatile compounds, such as hydrogen and oxygen, emitted in the form of water, methane, carbon dioxide and higher hydrocarbons through decarboxylation and dehydration reactions (Stach and Murchison, 1982). At higher degrees of coalication

and temperature, the liquid hydrocarbons formed in previous stages are thermally cracked to methane, increasing the amount of methane produced (Faiz and Hendry, 2006). Peat and brown coal represent the rst stage of the coalication process. The vertical pressure exerted by accumulating sediments converts peat into lignite. The intensication of the pressure and heat results in the transition from lignite to bituminous coal, and eventually to anthracite, the highest rank of coal (OKeefe et al., 2013).

During peat and brown coal stages, primary biogenic methane is formed and it is mainly dissolved in water or released during burial, as coal is not appropriately structured for gas retention (Kotarba and Rice, 2001). At more mature stages, thermogenic methane is produced by thermal modication of sedimentary organic matter, which occurs at great depths and intense heat. Following the basin uplift, methane production can be triggered in the shallower sediments by the meteoric water inow into the coal (secondary biogenic gas) (Rice, 1993; Scott et al., 1994).

The isotopic signature of the methane produced during the coalication process is controlled by the methane origin pathway (Whiticar, 1996). Thermogenic methane is isotopically enriched in 13C ( 13C > 50 ) compared to bio

genic methane, as methanogens preferentially use the lightest isotopes due to the lower bond energy (Rice, 1993). Intermediate isotopic compositions of methane might reect a mixing between microbial and thermogenic gases or secondary processes. Indeed, many controlling factors co-drive the fractionation process, and several contentions about their leverage still persist in literature. Deines (1980) asserts that no signicant trend is observed in the isotopic signature of methane in relation to the degree of coalication. Conversely, Chung et al. (1979) observed that the composition of the parent material does affect the isotopic composition of the methane accumulated. While the link between isotopic composition and coal rank is not that straightforward, studies carried out in different worldwide coal seams conrm a stronger relationship between coalbed gas composition and depth. Rice (1993), using data from Australian, Chinese and German coalbeds, shows that shallow coalbeds tend to contain relatively isotopically lighter methane when compared to those at greater depths. In the presence of intrusions of meteoric water, secondary biogenic methane, being isotopically lighter, can be generated and mixed with the thermogenic gas previously produced. Colombo et al. (1966) documented a distinct depth correlation in the Ruhr coal basin in Germany, with methane becoming more 13C-depleted towards the surface zone, independently from coalication patterns. This tendency can be explained either by bacterial methanogenesis or by secondary processes such as absorption/desorption of methane. Also Scott (2002), in a study about coal seams in the Bowen Basin, Australia, ascribes the progressive methane 13C-enrichment with depth to the meteoritic recharge in the shallowest seams, associated with a

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ing methods (Frodsham and Gayer, 1999). Emissions from two deep mines, Unity and Aberpergwm, were investigated, where mine shafts reach depths up to approximately 750 m (http://www.wales-underground.org.uk/pit/geology.shtml

Web End =http://www.wales-underground.org.uk/pit/geology.shtml ).

2.1.2 Upper Silesian Coal Basin in Poland

The Upper Silesian Coal Basin extends from Poland to the Czech Republic and is one of the largest coal basins in Europe, with an area of 7400 km2 (Jureczka and Kotas, 1995),

contributing 1.3 % to global coal-derived methane emissions (US EPA, 2012). Emissions from the Silesian Region of Poland are estimated to be in the range of 4501350 Gg annually (Patyska, 2013). The Upper Carboniferous coal-bearing strata of this region are associated with gas deposits of both thermogenic and microbial origin, and the methane content and spatial distribution are coal rank related (Kedzior, 2009) i.e. the sorption capacity of coal in the basin is found to increase with coal rank. Most of the methane generated during the bituminous stage in the coalication process has escaped from the coal source following basin uplift during the Palaeo-gene (Kotarba and Rice, 2001) and diffused through fractures and faults occurring in tectonic zones. The late-stage gas generated by microbial reduction of CO2 in the coal seams at the top of the Carboniferous sequence accumulated under clay deposition in the Miocene (Kedzior, 2009).

2.1.3 Australia: Hunter Coaleld (Sydney Basin)

The Hunter Coaleld is part of the Sydney Basin, on the east coast of New South Wales in Australia, and consists of three major coal measures. The deepest is the early Permian Greta Coal Measures, which is overlain by the late Permian Whittingham Coal Measures and upper Newcastle Coal Measures. Throughout the Hunter coaleld all sedimentary strata are gently folded, and the same coal seam can be mined at the ground surface and at depths of several hundred metres.In the region surveyed, both the opencast and underground mines are extracting coal from the Whittingham coal measures, which are generally high volatile bituminous coals, although some medium to low bituminous coals are extracted (Ward and Kelly, 2013). The mining operation is on a much larger scale than in the UK the total coal production for the Hunter Coaleld was 123.63 Mt in 2011, of which 88.24 Mt was saleable (State of New South Wales, 2013) and methane emissions are estimated to contribute 4.6 % to global coal-derived methane (US EPA, 2012). Several studies have attested to the dispersion of thermogenic methane formed at higher degrees of coalication (i.e. high temperatures and pressures) during the uplift and the subsequent erosion of the basin, followed by the replenishment of the unsaturated basin with more recently formed methane of biogenic origin (Faiz and Hendry, 2006; Burra, 2010). Gas emplacement is related to sorption capacity of coal strata, in particular to the pore pressure regime, which is inuenced by the local geologi-

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G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions 13671

higher bacterial activity and a preferential stripping of 13C

CH4 by water ow.

The migration of methane from the primary zone as a consequence of local pressure release can affect the isotopic composition, since 12CH4 diffuses and desorbs more readily than 13CH4 (Deines, 1980), but the fractionation effect due to migration is less than 1 (Fuex, 1980). A much larger variation in the isotopic composition is associated with different methanogenic pathways (variation of 30 ) and ther

mal maturation stage (variation of 25 ) (Clayton, 1998).

The measurement of deuterium, coupled with 13CCH4 values, would help to distinguish the pathways of secondary biogenic methane generation, acetoclastic reactions or CO2 reduction (Faiz and Hendry, 2006), but such distinction is beyond the scope of this study.

Overall, the 13C values of methane from coal show an extremely wide range, and understanding the processes driving the methane isotopic composition requires a focus on the particular set of geological conditions in each sedimentary basin.

Here we analyse the isotopic signatures of methane plumes emitted to the atmosphere, from the dominant bituminous and anthracite mines in Europe and Australia, of both deep and open-cut types, to test the theory of isotopic change due to coal rank.

2 Material and methods

2.1 Coal basins investigated and type of coal exploited

2.1.1 English and Welsh coal mines

Coal-derived methane emissions in the UK are estimated at66 Gg in 2013 (National Atmospheric Emissions Inventory, http://naei.defra.gov.uk

Web End =http://naei.defra.gov.uk ), representing 0.4 % of global emissions from mining activity (US EPA, 2012). The major coal-elds of England and Wales belong to the same stage in the regional stratigraphy of north-western Europe (Westphalian Stage, Upper Carboniferous). Mining has ceased in many areas. Of those remaining, some are located in South Yorkshire (Hateld and Maltby collieries), where 50 % of the coal-elds output came from the Barnsley seam, which includes soft coal overlaying a semi-anthracitic coal and bituminous coal in the bottom portion.

The coal of the South Wales Basin exhibits a well-dened regional progression in rank, which varies from highly volatile bituminous coal in the southern and eastern margins to anthracite in the north-western part, and the main coal-bearing units reach 2.75 km in thickness toward the southwest of the coaleld (Alderton et al., 2004). The coal is now preferentially extracted in opencast mines, as the extensive exploitation of the coal has left the accessible resources within highly deformed structures (e.g. thrust overlaps, vertical faults) that cannot be worked by underground min-

13672 G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions

cal features (compressional or extensional) of the basin. The south of the Hunter Coaleld is characterised by higher gas content and enhanced permeability than the northern area, with a large potential for methane production, mainly biogenic (Pinetown, 2014).

2.2 Sampling and measurement methodology

For isotopic characterisation of the methane sources, integrated methane emissions were assessed through detection of the off-site downwind plume. In fact, even when emissions are focused on dened locations, such as vent pipes in underground mines, the methane provenance cannot be localised, since most of collieries are not accessible. For sample collection and measurements of methane emissions downwind of coal mines in the UK and Australia, the mobile system described by Zazzeri et al. (2015) has been implemented.The system utilises a Picarro G2301 CRDS (cavity ring-down spectroscopy) within the survey vehicle, for continuous CH4 and CO2 mole fraction measurements, and a mobile module including air inlet, sonic anemometer and GPS receiver on the roof of the vehicle. The entire system is controlled by a laptop, which allows methane mole fractions and the methane plume outline to be displayed in real time on a Google Earth platform during the survey to direct plume sampling. When the plume was encountered, the vehicle was stopped and air samples collected in 3 L Tedlar bags, using a diaphragm pump connected to the air inlet. Samples were taken at different locations along the plume transect in order to obtain a wide range of methane mole fractions and isotopic signatures in the collected air. The Upper Silesian Basin was surveyed with a Picarro 2101-i measuring continuous CO2 and CH4 mole fractions and 13CCO2 isotopic ratio. Samples were collected on site for analysis of 13CCH4 isotopic ratio.

The carbon isotopic ratio ( 13CCH4) of bag samples was measured in the greenhouse gas laboratory at RHUL (Royal

Holloway University of London) in triplicate to high precision (0.05 ) by continuous ow gas chromatography

isotope ratio mass spectrometry (CF GC-IRMS) (Fisher et al., 2006). CH4 and CO2 mole fractions of samples were measured independently in the laboratory with a Picarro G1301 CRDS analyser, calibrated against the NOAA (National Oceanic and Atmospheric Administration) WMO-2004A and WMO-X2007 reference scales respectively. The 13CCH4 signature for each emission plume was calculated using the Keeling plot approach, according to which the 13C

isotopic composition of samples and the inverse of the relative mole fractions have a linear relationship, with an intercept that represents the isotopic signature of the source (Pataki et al., 2003). Source signatures were provided with the relative uncertainty, computed using the BCES (Bivariate Correlated Errors and intrinsic Scatter) estimator (Akritas and Bershady, 1996), which accounts for correlated errors between two variables and calculates the error on the slope and intercept of the best interpolation line. Mole fraction data

and co-located coordinates were used to map the mole fraction variability using the ArcGIS software.

3 Results and discussion

While most of the emissions from deep mines come specically from ventilation shafts, which are point sources, emissions from open-cut mines are widespread, and difcult to estimate. However, the objective of this study is not the quantication of emissions, but the assessment of the overall signature of methane released into the atmosphere, made through the sampling of integrated emissions from the whole area. Therefore, even though on-site access to collieries was not possible, by driving around the contiguous area, methane emissions could be intercepted and their mole fractions measured. Table 1 summarises 13CCH4 signatures of all the coal mines and coal basins surveyed with the Picarro mobile system.

3.1 English coal mines

Methane plumes from Hateld colliery, one of the few British deep mines still open at the time of this study, were detected during surveys on 10 July and 26 September 2013.The methane desorbed from coal heaps within the colliery area and methane vented from shafts might explain the relatively high mole fractions measured (up to 7 ppm). Keeling plots based on the samples collected (4 on the rst and 5 on the second survey) give intercept values of 48.3 0.2 and 48.8 0.3 (2 SD).

Maltby colliery is one of the largest and deepest mines in England and was closed in March 2013. The coal mine methane was extracted and used for electricity, but over 3 ppm mole fractions were detected in ambient air during both surveys in July and September 2013, giving evidence of methane releases from the recovery system and ventilation shafts. The Keeling plot intercepts based on the samples collected in July and in September are 45.9 0.3 and 45.4 0.2 (2 SD) respectively, 3 heavier than the

isotopic source signatures calculated for Hateld Colliery on the same sampling days. Both source signatures are in good agreement with the value of 44.1 observed for desorbed

methane of the Barnsley coal seam, in a study conducted by Hitchman et al. (1990b).

Kellingley colliery is a still open deep mine situated in North Yorkshire. A surface drainage plant for electrical power generation has been implemented (Holloway et al., 2005), and methane releases from the power plant might justify the mole fractions peak of 9 ppm that was observed while driving on the northern side of the coal mine on 10 July 2013.The Keeling plot analysis of the samples collected indicates a source signature of 46.5 0.3 (2 SD).

The highest mole fractions detected around Thoresby mine, in Nottinghamshire, approached 5 ppm. Nine samples

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G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions 13673

Table 1. 13CCH4 signatures for all of the coal mines and coal basins surveyed with the Picarro mobile system. Errors in the 13C signatures are calculated as 2 standard deviations.

Sampling site Country Coal seam Coal type Mine type Sampling date

13C Signatures []

Samples number used in Keeling plots

Kellingley Colliery (active)

North Yorkshire/UK Beeston and Silkstone coal seams

Bituminous coal

Deep mine Sep

2013

46.50.3 5

Maltby Colliery (closed since March 2013)

South Yorkshire/UK Barnsley and Parkgate seams, up to 991 m in depth (McEvoy et al., 2004)

Bituminous coal

Deep mine Jul

2013 Sep 2013

45.90.3

45.40.2

3

4

Hateld Colliery (active)

South Yorkshire/UK Barnsley ( 401465 m),

Dunsil (414376 m)

High Hazel ( 313284 m)

seams (Hill, 2001)

Bituminous coal

Deep mine Jul

2013 Sep 2013

48.30.2

48.80.3

4

5

Thoresby Colliery (active)

Nottinghamshire/UK Parkgate seam, 650 m in

depth

Bituminous coal

Deep mine Nov

2013

51.20.3 9

Daw Mill Colliery (closed since March 2013)

Warwickshire/UK Warwickshire Thick seam, from 500 to 1000 m in depth

Bituminous coal

Deep mine Nov

2013

51.40.2 4

Cwmllynfell Colliery (active)

Wales/UK South Wales Coaleld Anthracite Surface mine

Oct2013

41.20.9 3

Abercrave Colliery (active)

Wales/UK South Wales Coaleld Anthracite Surface mine

Oct2013

41.40.5 5

33.31.8 5

Aberpergwm Colliery (closed since December 2012)

Wales/UK South Wales Coaleld Anthracite Deep mine Oct

2013

Unity Colliery (active)

Wales/UK South Wales Coaleld Anthracite Deep mine Oct

2013

30.91.4 3

Hunter Coaleld (active)

Australia Whittingham Coal Measures Bituminous coal

Surface and deep mines

Mar2014

66.41.3 12

Bulga Colliery (active)

Australia Blakeeld South seam, from

130 to 510 m in depth (Bulga Underground Operation mining, 2015)

Bituminous coal

Deep mine Jan

2016

60.80.3 10

Upper Silesian Basin (active)

Poland 502 (590634 m) and 510 seam

(763812 m)

Bituminous coal

Deep mine Jun

2013

50.91.2 8

Only the coaleld is indicated, as the name of the seams exploited could not be retrieved.

were collected, giving a source signature of 51.2 0.3

(2 SD).

During the sampling of methane plumes from Daw Mill

colliery, the highest methane mole fractions ( 5 ppm) were

recorded close to the edge of the colliery, whereas, driving downwind of the site at further distances, only background values were measured. Indeed, the estimated methane content of the coal seam exploited in Daw Mill colliery, closed in March 2013, is low (typically about 1.7 m3 t1) and, furthermore, a ventilation system is implemented, so that the coal mine methane potential is curtailed (Drake, 1983). Keel-

ing plot analysis reveals a source signature of 51.40.2

(2 SD).

3.2 Welsh coal mines

Methane emissions from coal mines in Wales were sampled in order to characterise methane releases from a different rank of coal. The area investigated extended from Cwmllynfell to Merthyr Tydl. The South Wales Coaleld is estimated to have the highest measured seam gas content in the UK (Creedy, 1991).

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13674 G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions

Figure 1. Methane mole fractions recorded in the Hunter Coaleld on 12 March 2014 (a) and during a more detailed survey of the area highlighted by the black square on 18 March 2014 (b). Source: Esri, DigitalGlobe, GeoEye, i-cubed, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo and the GIS User Community.

3.2.1 Deep mines

The deep mine Aberpergwm, which closed in December 2012, did not operate coal mine methane schemes and methane was vented up to the surface as part of standard operation systems (Holloway et al., 2005). This is consistent with methane mole fractions peaks of 6 ppm observed when approaching the colliery. Air samples were also collected near Unity deep mine, Wales largest drift mine, which reopened in 2007 and is located in the town of Cwmgwrach, only 1.5 km away from Aberpergwm. Isotopic source signatures of 33.31.8 and 30.91.4 result from the Keel

ing plots based on samples collected respectively near Aberpergwm and Unity colliery, both highly 13C enriched relative to all English collieries.

3.2.2 Opencast mines

The Picarro mobile system was driven around opencast mines at Cwmllynfell and Abercrave, in the Swansea Valley. Up to 3 ppm methane mole fractions were recorded near the two mines, which were closed in the 1960s as drift mines and are currently exploited as opencast mines. Our measurements conrm that they are still emitting methane, even though

the methane mole fractions recorded downwind of the mines were not signicantly above background. 13C signatures between 41.40.5 and 41.20.9 (2 SD) result from iso

topic analysis of samples collected downwind of the opencast mines, approximately 10 lower than the isotopic signature characterising Welsh deep anthracite mines.

3.3 Polish coal mine

A Picarro mobile survey of the Upper Silesian Basin took place on 10 June 2013 and 12 air samples were collected for isotopic analysis. All the mines in this area are deep mines, exploiting the coal at depths ranging from 300 to 900 m. The Keeling plot analysis includes eight samples collected around the area of Radoszowy, downwind of the KWK Wujek deep mine shafts. Methane mole fractions in the range of 35 ppm were measured in the majority of the area of Katowice and over 20 ppm mole fractions were detected when transecting the plume originating from the exhaust shafts, which conrms the high level of methane that the mine contains. A source signature of 50.9 0.6 (2 SD) was calculated

by Keeling plot analysis, which is consistent with the values obtained for the deep mined English bituminous coal.

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G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions 13675

Figure 2. Keeling plots based on samples collected around English and Welsh coal mines (a), coal mines in Poland (b) and Australia (c). Errors on the y axis are within 0.05 and on the x axis within 0.0001 ppm1, and are not noticeable on the graph.

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13676 G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions

Table 2. Literature-derived carbon isotopic ranges for methane from CBM (Coalbed Methane) wells in a wide range of sedimentary basins. Errors are not included as measurement precisions for the ranges of analytical techniques used are not specied.

Site Coal rank 13C () Author 13C () measured in this study

Powder River Basin, USA Sub-bituminous coal 79.5 to 56.8 Bates et al. (2011)

Australia From brown coal to low volatile bituminous coal

73.0 to 43.5 Smith et al. (1981) 66.4 1.3 to55.5 1.3

Velenje Basin, Slovenia Lignite 71.8 to 43.4 Kandu et al. (2015)

Powder River Basin, USA Sub-bituminous coal 68.4 to 59.5 Flores et al. (2008)

Illinois Basin, USA Bituminous coal 66.6 to 56.8 Str apo et al. (2007)

Elk Valley Coaleld, Canada Bituminous coal 65.4 to 51.8 Aravena et al. (2003)

Bowen Basin, Australia Bituminous coal 62.2 to 48 Kinnon et al. (2010)

Queensland Basin, Australia From sub-bituminous tohigh volatile bituminous

57.3 to 54.2 Papendick et al. (2011)

Tara region, Queensland Basin, Australia

From sub-bituminous tohigh volatile bituminous

56.7 Maher et al. (2014)

San Juan Basin, New Mexico and Colorado

High-volatile bituminous coal 43.6 to 40.5 Rice et al. (1989)

Michigan Basin, USA Bituminous coal 56 to 47 Martini et al. (1998)

UK, Barnsley seam Bituminous coal 44.1 Hitchman et al.

(1990a, b)

48.8 0.3 to 45.4 0.2

Upper Silesian Coal Basin, Poland

Sub-bituminous coal toanthracite

79.9 to 44.5 Kotarba and Rice

(2001)

50.9 0.6

Eastern China Sub-bituminous to anthracite 66.9 to 24.9 Dai et al. (1987)

Ruhr basin, Germany Bituminous coal, anthracite 37

(60 to 14)

Deines (1980)

Western Germany High-volatile bituminous to anthracite

70.4 to 16.8 Colombo et al. (1966)

Eastern Ordos Basin, China Bituminous coal 42.3 to 39.8 Guo et al. (2012)

South-eastern Ordos Basin, China

Low-volatile bituminous and semi-anthracite

48.1 to 32.8 Yao et al. (2013)

Qinshui, China Anthracite 41.4 to 34.0 Qin et al. (2006)

Qinshui Basin, China Semi-anthracite to anthracite 36.7 to 26.6 Su et al. (2005)

Sichuan Basin, China Bituminous to anthracite 36.9 to 30.8 Dai et al. (2012)

Xiong et al. (2004)

Wales, UK Anthracite 33.3 1.8 to

30.9 1.4

3.4 Australia: Hunter Coaleld

On 12 and 18 March 2014, 12 samples in total were collected along the route in the Hunter Coaleld, where the bituminous coal strata of the Sydney Basin are extracted, both in open-cast and underground mines. The methane plume width was in the range of 70 km (Fig. 1). A maximum mole fraction of13.5 ppm was measured near a vent shaft associated with the Ravensworth underground mine (see white star in Fig. 1b).The source signature calculated by the Keeling plot analysis based on all the samples collected during both surveys (grey markers in Fig. 2c) is 66.41.3 (2 SD) and this signature

likely includes a mixture of methane derived both from underground and opencast mines. Two samples collected down-wind of the Ravensworth ventilation shaft (red pushpins in Fig. 1b) fall off the Keeling plot trend for March 2014 and

are not included in the calculation, because they are likely dominated by methane from the vent shaft and are not representative of the regional mixed isotopic signature.

In January 2016, 10 samples were collected downwind of a ventilation fan in the Bulga mine (second white star in Fig. 1b). The Keeling plot for these (black circles in Fig. 2c) indicates a 13C source signature of 60.8 0.3 (2 SD).

The samples collected next to the Ravensworth ventilation shaft in 2014 t on this Keeling plot, suggesting that the 13CCH4 isotopic signature of emissions from underground works in the Hunter Coaleld is consistent.

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G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions 13677

Figure 3. Coal maturation process. The 13CCH4 signatures are based on the literature values in Table 2 and on the results of this study.

3.5 New representative 13CCH4 isotopic signatures for coal-derived methane

The 13CCH4 isotopic values for coal have been found to be characteristic of single basins, but general assumptions can be made to characterise coal mines worldwide. Table 2 provides the literature 13CCH4 isotopic values characteristic of specic coal basins. The isotopic signatures of emissions from English bituminous coal are consistent with the range of 49 to 31 suggested by Colombo et al. (1966) for in

situ coalbed methane in the Ruhr basin in Germany, which contains the most important German bituminous coal of Upper Carboniferous age and low volatile anthracite (Thomas, 2002). Progression in coal rank might explain the value of

50 for emissions from English underground mined bituminous coal and 30 for anthracite deep mines, followed

by a 510 13C-depletion likely caused by the incursion of meteoric water in the basin and the subsequent production of secondary biogenic methane (Scott et al., 1994), resulting in

40 for methane plumes from Welsh open-cut anthracite mines.

The link between coal rank and 13CCH4 isotopic signature is appreciable in the study of UK coal mines, but differences in the 13C / 12C isotopic ratio within the same coal sequences could be ascribed to other parameters, such as the depth at which the coal is mined. As methane migrates towards the surface, diffusion or microbial fractionation may occur, especially if there is water ingress (see Sect. 1). In particular, emissions from Thoresby are more 13C depleted than those measured around Maltby, although both mines exploit the Parkgate seam (see Table 1), meaning that different isotopic signatures cannot be entirely linked to the coal rank. Biogenic methane produced in a later stage due to water intrusion might have been mixed with the original thermogenic methane formed during the coalication process.

Differences in methane emissions and their isotopic signature between opencast and deep mines have been assessed by surveying both surface and underground mines, in the Welsh anthracite belt, and in the Hunter Coaleld. The shallower deposits are more exposed to the weathering and meteoric water, most likely associated with the production of some

isotopically lighter microbial methane. Mole fractions up to2.5 ppm were measured around opencast mines in the Hunter

Coaleld, in Australia, within a methane plume of more than 70 km width. The highest methane mole fractions were consistently measured downwind of vent shafts in underground mines. The difference in the source isotopic signature for methane emissions between the two types of mining, both in the Hunter Coaleld (from 61 to 66 ) and in Wales

(from 31 to 41 ), reects the isotopic shift of 510

that might be attributed to the occurrence of secondary biogenic methane.

The 13C signatures for coalbed methane emissions from the Upper Silesian Basin are highly variable, with the most

13C-depleted methane associated with diffusion processes or secondary microbial methane generation (Kotarba and Rice, 2001), but the value of 51 measured for methane emis

sions from the KWK Wujek deep mine is consistent with the value of 50 inferred for emissions from English bitumi

nous coal extracted in underground mines.

Figure 3 shows the 13CCH4 signatures for each coal type, based on the results of this study and isotopic values found in literature.

4 Conclusions

By measuring the isotopic signatures of methane plumes from a representative spread of coal types and depths, we show that the 13C isotopic value to be included in regional and global atmospheric models for the estimate of methane emissions from the coal sector must be chosen according to coal rank and type of mining (opencast or underground). For low-resolution methane modelling studies, our measurements suggest an averaged value of 65 for bituminous

coal exploited in open cast mines and of 55 or less neg

ative, in deep mines, whereas values of 40 and 30 can

be assigned to anthracite opencast and deep mines respectively. However, the ranges of isotopic signatures of methane emitted from each of these categories are wide (see Fig. 3). Further measurements would be required in other coal mining areas, especially China, to determine appropriate val-

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13678 G. Zazzeri et al.: Carbon isotopic signature of coal-derived methane emissions

ues for global modelling of methane isotopes. The overall bulk isotopic ratio of Chinas very large coal production is still very poorly quantied, but could be less depleted than

40 , as indicated by unpublished RHUL studies of winter air samples collected in Hong Kong with back trajectories that cross industrial China from Mt Waliguan in NW China.

Global methane budget models that incorporate isotopes have used a 13C signature of 35 for coal or a value 40 for total fossil fuels (e.g. Hein et al., 1997; Mikaloff Fletcher et al., 2004; Bousquet et al., 2006; Monteil et al., 2011), but, given the relative rarity of anthracite global coal reserves and the dominance of bituminous coal (1 and 53 % respectively) (World Coal Institute, 2015), it seems likely that a global average emission from coal mining activities will be lighter, with the 50 recorded for deep-mined bi

tuminous coal in Europe being a closer estimate. However, for detailed global modelling of atmospheric methane, isotopic signatures of coal emissions should be region or nation specic, as greater detail is needed given the wide global variation. The assignment of an incorrect global mean, or use of a correct global mean that is inappropriate for regional-scale modelling, might lead to incorrect emissions estimates or source apportionment. The new scheme gives the possibility for an educated estimate of the 13C signature of emission to the atmosphere to be made for an individual coal basin or nation, given information on the type of coal being mined and the method of extraction.

In conclusion, high-precision measurements of 13C in

plumes of methane emitted to the atmosphere from a range of coal mining activities have been used to constrain the isotopic range for specic ranks of coal and mine type, offering more representative isotopic signatures for use in methane budget assessment at regional and global scales.

5 Data availability

The isotopic ratios and mole fraction measurements used for the Keeling plots are included in the Supplement.

The Supplement related to this article is available online at http://dx.doi.org/10.5194/acp-16-13669-2016-supplement

Web End =doi:10.5194/acp-16-13669-2016-supplement .

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Acknowledgements. Giulia Zazzeri would like to thank Royal Holloway, University of London for the provision of a Crossland scholarship and a contribution from the Department of Earth Sciences from 2011 to 2014. The analysis of samples from Poland was funded through the European Communitys Seventh Framework Programme (FP7/2007-2013) in the InGOS project under grant agreement no. 284274. Sampling in Australia was possible due to a grant from the Cotton Research and Development Corporation.

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