New Insights into the Nitrogen Isotope Compositions in Coals from the

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New insights into the nitrogen isotope compositions in coals from the Huainan coalfield, Anhui Province, China: Influence of the distribution of nitrogen forms Dianshi Ding, Guijian Liu, Biao Fu, and Wenjun Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02467 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Energy & Fuels

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New insights into the nitrogen isotope compositions in coals from the Huainan coalfield,

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Anhui Province, China: Influence of the distribution of nitrogen forms Dianshi Ding a,b, Guijian Liu a,b*, Biao Fu a, Wenjun Wangc

3 a

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CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and

Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China b

State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, The Chinese Academy of Sciences, Xi’an, Shaanxi 710075, China

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c

School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, Anhui 230000, China

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*Corresponding author. Liu Guijian. Tel: +86-551-3603714; fax: +86-551-3621485.

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E-mail address: [email protected] (G. J. Liu)

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Abstract: A combination of nitrogen isotope analysis and X-ray photoelectron spectroscopy

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(XPS) techniques were used to characterize nitrogen isotopic compositions and organic

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nitrogen forms in a total of eleven coal samples from China. A range of δ15N values between

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-2.9‰ and +3.6‰ was observed in all coals, and the N-5 peak is predominant. The results

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show that a correlation exists between the δ15N value and the proportions of the N-5 peak and

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the sum of the N-6 and N-Q peaks. This suggests that the nitrogen isotopic composition of

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coal may be controlled by the distribution of nitrogen forms. The pyridinic-structure nitrogen

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is enriched in 15N compared with the pyrrolic-structure nitrogen.

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Key words: Coal; Nitrogen isotope; Nitrogen forms; XPS; China

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1 Introduction

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NOx has an adverse effect on the environment and human health1-2. Since the Industrial

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Revolution, anthropogenic NOx emissions have grown greatly3 and now have risen more than

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natural NOx emissions4, which is mainly from the increase of fossil fuel combustion, such as

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coal4. NOx can be produced by reaction of coal nitrogen with combustion oxygen5-6.

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The nitrogen isotopic composition of NOx (δ15N-NOx) is used potentially as an

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environmental tracer7. Atmospheric nitrogen isotope ratio (δ15N) measurements on NOx

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suggest that significant differences may exist between NOx derived from coal and that from

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other fuel sources8-10. Snape et al.10 suggested that much of the isotopic fractionation that

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takes place between coal nitrogen and NOx takes place in the formation of char, of which the

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extent is related to the reactivity of coals. Further, it is possible that coal nitrogen forms11 and

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their isotopic composition10 could influence δ15N-NOx values from coal combustion.

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Therefore, understanding nitrogen isotopic composition in coal is significant environmentally,

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and will be an important development to help guarantee combustor performance and aid

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combustion modeling. Nitrogen in coal consists mainly of organic nitrogen, although in

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high-rank coals clay minerals could store NH4+ in interlayers or absorbed on the mineral

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surface12-13. The research about the organic nitrogen chemistry in coal is still limited14-17.

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Much less information is available on nitrogen isotopic composition in coal18-21. The

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range of δ15N values for Australian coals was +0.8‰ to +3.7‰19. In a study of various

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Canadian coals, Whiticar20 found a narrow range of δ15N values between -0.2‰ to +1.4‰,

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except that two coals were more 15N depleted with δ15N values of -2.4‰ and -3.2‰. Ader et

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al.18 investigated organic nitrogen in anthracite, and showed that Pennsylvanian (America)

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δ15N values ranged from +4.1‰ to +5.4‰, and Bramsche (Germany) values ranged from

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+2.7‰ to +3.7‰. Xiao and Liu21 reported that the nitrogen isotopic ratios in Chinese coals

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ranged from -6‰ to +4‰, and approximately 73% of this ranges from -3‰ to +2‰.

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The variations in nitrogen isotopic ratio in coal appear to be related to maceral

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compositions20,

22-23

. Whiticar20 found that two vitrinite samples had low δ15N values

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(approximately -2‰ and -3‰) while δ15N values of their respective bulk coal samples were

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approximately 0‰ and +1‰. Rimmer et al.22 suggested slightly enriched

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relative to inertinite. Valentim et al.23 observed the macrinite-rich coal (~63%) was lower in

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δ15N (-2.8‰) than the vitrinite-rich coal (~92%, +3.1‰) and semifusinite + fusinite rich coal

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(~25%, +2.9‰) from the Peach Orchard coals in America. However, coal rank does not seem

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to influence δ15N value below the meta-anthracite rank18, 20, 24.

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N in vitrinite

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Nitrogen in coal is virtually exclusively organic nitrogen25. Organic nitrogen presents

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within the carbon matrix principally as pyrrolic (N-5), pyridinic (N-6), and quaternary

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nitrogen (N-Q) forms25-29. There is little or no systematic variation of nitrogen forms with


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rank25. Although weak trends were observed in some individual studies24, 29, 30, Thomas25

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suggested that these trends disappeared when the data were combined. However, over a wide

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range of Cdaf content (65-95 wt.% on dry-ash-free basis), the nitrogen forms are composed of

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50-80% pyrrolic nitrogen, 20-40% pyridinic nitrogen, and 0-20% quaternary nitrogen in

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general23.

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X-ray photoelectron spectroscopy (XPS) techniques is an effective method to study

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composition and functional groups of organic compounds31-34. The technique has been

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successfully and extensively used to determine the occurrence and distribution of organic

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nitrogen forms in coal24, 26, 28-30, 35-38.

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Data of nitrogen isotopic composition of coal are insufficient and up to now there have

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been limited nitrogen isotopic studies of coal on nitrogen forms basis. The purpose of this

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study was to examine the influence of nitrogen forms on the nitrogen isotopic composition of

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coal. Samples from Chinese Huainan coals were chosen, and we determined δ15N values and

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assigned nitrogen organic forms using isotope ratio mass spectrometry analysis and XPS

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technique, respectively.

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2 Samples and methods

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2.1 Samples and coal quality analyses

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The coal samples utilized in this study were obtained from the Huainan coalfield of

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Anhui Province, which is a main coal production location in China39 (Fig. 1). A total of eleven

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coals were sampled following Chinese National Standard GB/T 482-2008, which cover two

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minable coal seams (No. 16 and 17) in the Permian Upper Shihezi formation from seven coal

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mines (Fig. 1). It is expected that the source of organic matter, age, burial depth and thermal

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maturation for the coals from closely-spaced areas are similar. All coals were sampled from

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the zones without the intrusion to eliminate igneous influences. Each sample of approximately

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1 kg was stored in plastic bags to minimize contamination and oxidation. All samples were

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air-dried, ground, homogenized, and sifted a 200-mesh sieve for further experiments.

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Proximate analysis, including the determination of moisture, ash yield, and volatile

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matter, was performed according to ASTM Standard D3173-11 (2011), D3174 (2011), and

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D3175-11 (2011), respectively. Ultimate analysis, including C, H, N and O contents, was

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conducted using a Vario EL III elemental analyser (Elementar Analysensysteme GmbH,


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Hanau, Germany) at Instruments’ Center for Physical Science of USTC. The determination of

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maceral group composition and minerals of the coals were performed by counting points of

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the polished coals under a reflecting microscope based on Chinese National Standard GB/T

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8899-2013. The acceptable measurement precision was within ± 5%. In order to guarantee the

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precision and reproducibility, all samples were measured in triplicate.

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2.2 X-ray photoelectron spectroscopy (XPS) analysis

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XPS analysis was performed based on Chinese National Standard GB/T 19500-2004 by

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using a Thermo-VG Scientific ESCALAB 250 X-ray photoelectron spectrometer at the

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University of Science and Technology of China’s (USTC) Instruments’ Center for Physical

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Science. Powdered samples were pressure-mounted onto conducting indium foil. An

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achromatic Al (Kα) radiation source running at 15 kV was operated and the Constant

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Analyzer Energy mode with 30 eV pass energy was used. The C 1s level binding energy at

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284.6 eV was used as internal standard in order to calibrate the binding energy scale. XPS

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analysis was conducted in triplicate. The repeatability of peak position of XPS measurement

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can be achieved within ±0.1 eV.

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The data was processed in the spectra curve resolution by software XPSPEAK based on

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least squares curve fitting using the procedure with typically 30% Lorentzian and 70%

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Gaussian functions, fixing Full Width at Half Maximum at 1.7 eV, and defining but not fixing

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the N peaks23, 29-30. Three peaks in the N 1s spectra were initially measured in the samples,

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including 1) N-5 peak at 400.6 eV; 2) N-6 peak at 398.8 eV; and 3) N-Q peak at 401.3 eV.

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2.3 Nitrogen isotopic analysis

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Nitrogen isotopic analysis was conducted at Scientific Instruments Sharing Platform,

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Third Institute of Oceanography, State Oceanic Administration using elemental analyzer/

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isotope ratio mass spectrometry (EA/IRMS, Finnigan Delta V Advantage interfaced with

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Flash EA 1112 HT, Thermo Fisher Scientific, Bremen, Germany). First, the prepared samples

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were treated with excessive HCl (4M) for 24 h at 25 ºC, and were washed to neutral using

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Millipore water. Then dried sample was weighed about 10 mg into tin capsules, and

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combusted at 960 ºC to obtain N2 gas. The N2 was separated by Porapak QS gas

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chromatography columns at 50 ºC with a He flow at 90 ml/min. The results are represented

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with delta (δ) notation relative to air standard:


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δ15N (‰) = (Vc/Vs - 1) × 1000

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where Vc is 15N/14N value of the measured coal, and Vs is 15N/14N value of air standard. The

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measured δ15N values were standardized using certified reference materials Acetanilide#1

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(δ15N = + 1.18‰). In order to ensure the reproducibility and precision, all samples were

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measured in triplicate. Standard deviation for analysis was within ± 0.25‰.

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3 Results and discussion

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3.1 Coal quality

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The results of proximate and ultimate analyses in 12 coal samples were tabulated in

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Table 1. The range of moisture is 0.7%-2.1%. The coals are ultra-low moisture coal based on

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Chinese National Standard for moisture (MT/850-2000, ≤ 6.00% for ultra-low moisture coal).

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The ash yields of the coals vary from 13.4% to 26.6%, indicating that most samples are

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low-ash to middle-ash coals according to Chinese National Standard (GB/T 15224.1-2010,

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10.01-20.00% for low-ash coal and 20.01-30.00% for middle-ash coal). The volatile matter

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yields range from 18.1% to 36.8%. According to Chinese National Standard (MT/849-2000,

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10.00-20.00% for low-volatile coal, 20.00-28.00% for medium-volatile coal and 28.00-37.00%

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for medium-high-volatile coal), the coals span from low-volatile coal to medium-high-volatile

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coal. Table 2 lists the proportions of maceral group compositions and minerals in the coals.

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Vitrinite is the most abundant and accounts for > 55% of the entire maceral compositions.

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3.2 Nitrogen forms

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The distribution of each nitrogen form as a proportion of total nitrogen in the coal

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samples resulting from XPS analysis were summarized in Table 3. The N-5 peak is

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predominant in the spectra of all studied coals (Fig. 2), and corresponds to pyrrolic nitrogen26,

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29

, which is bonded to one hydrogen atom and two carbon atoms in a five-membered ring24, 28,

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30

(Fig. 3). This is consistent with the results published on other coals24, 26, 28-29. The N-6 peak

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is the secondary peak and presents in all coal samples (Fig. 2), which corresponds to pyridinic

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nitrogen bonded to two carbon atoms24, 28, 30 (Fig. 3). However, the fraction of N-Q is very

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small, and even little or no N-Q peak appears in some coals (Fig. 2). The identification of

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N-Q is a little bit difficult. In general, the N-Q peak represents “protonated” quaternary

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nitrogen in coal with Cdaf less than about 90 wt.%, which represents a structure of pyridinic

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nitrogen protonated by oxygen functional groups (for example, hydroxyl or carboxyl


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groups)24, 30, 40 (Fig. 3). In coal with Cdaf more than about 90 wt.%, the N-Q peak represents

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nitrogen atoms incorporated in a grapheme layer and is called “graphitic” quaternary

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nitrogen24, 30 (Fig. 3). The N-Q can convert to N-6 due to the loss of oxygen functional groups

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undergone heat-treatment. Kelemen et al.27 found that the sum of N-6 and N-Q is preserved

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during pyrolysis. In this studies, the Cdaf content of the coal samples ranged from 61.02 wt.%

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to 84.53 wt.%, and thus it can be inferred that the N-Q peak corresponds to “protonated”

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quaternary nitrogen in the coal samples. The relative amounts of these forms do not vary with

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Cdaf content (Fig. 4), which is in agreement with the proposal of Thomas25.

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XPS is a surface-sensitive spectroscopy technique. However, there is a reasonable

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correlation between the surface and bulk concentrations for coals41. The total nitrogen

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concentration from XPS analysis corresponds closely (within 20%) with that from elemental

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analysis in the samples (Table 4), indicating that the surface compositions of organic nitrogen

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in the samples is possibly comparable to that of the bulk26.

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3.3 Nitrogen isotopic composition

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The coal samples exhibit a range of δ15N values between -2.9‰ and +3.6‰ (Table 3),

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within the range for Chinese coals (-6‰ to +4‰)21. It appears that the δ15N values do not

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systematically change with the variation of the Cdaf contents (Fig. 5a), which is in agreement

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with previous studies24. The δ15N value increases roughly with increasing Ndaf content (Fig.

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5b). The nitrogen isotopic composition of coal may be dependent on maceral

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compositions22-23. In our coals, however, the δ15N value does not show considerable changes

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with the variations of maceral compositions (Fig. 6).

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The δ15N value was plotted in Fig. 7 as a function of the proportion of pyrrolic nitrogen

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(N-5) and pyridinic nitrogen (N-6). There is a significant correlation between nitrogen isotope

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ratio and the proportion of pyrrolic nitrogen in the coals. The δ15N value decreases with

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increasing proportion of pyrrolic nitrogen (Fig. 7a). However, no obvious correlation was

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observed between the δ15N value and the proportion of pyridinic nitrogen (Fig. 7b). The N-Q

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peak, in our coals, was assigned to “protonated” quaternary nitrogen with protonated

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pyridinic structure (Fig. 3). It is to note that although the binding energy positions of pyridinic

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nitrogen and “protonated” quaternary nitrogen are different, in the view of molecular structure

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both of them are pyridinic-structure nitrogen (Fig. 3). In this case, nitrogen in our coals can be


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classified as pyrrolic-structure nitrogen and pyridinic-structure nitrogen. The cross-plots (Fig.

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7) of δ15N value versus pyrrolic-structure nitrogen (N-5) and pyridinic-structure nitrogen (N-6

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+ N-Q) show significant correlations, and suggest that nitrogen-containing molecules in coal

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may influence overall bulk nitrogen isotopic composition.

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The pyrrolic-structure nitrogen mainly resulted from the transformation of amide26. The

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pyridinic-structure nitrogen could result from the transformation of amide and the thermal

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conversion of pyrrole (pyrrolic-structure nitrogen)26,35,42. During the process of thermal

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conversion, it takes less energy to break 14N-C bonds than 15N-C bonds, and 14N could be lost

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easily. Thus, it is likely that 15N is enriched in pyridinic-structure nitrogen compared to parent

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pyrrolic-structure nitrogen due to thermal reactions. In addition, bacterial activity during the

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peat stage and early diagenesis may be also an important factor in the formation of

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pyrrolic-structure and pyridinic-structure nitrogen26. The bacterial alteration for the

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transformation of amides into pyrrole

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(pyridinic-structure nitrogen) may have resulted in different nitrogen isotopic fractionation.

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However, the specific process for bacterial effect is not clear, and further study is needed to

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test this speculation.

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4 Conclusions

(pyrrolic-structure nitrogen)

and

pyridine

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The nitrogen isotopic compositions and nitrogen forms of Chinese coals were examined.

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A correlation exists between δ15N value and the proportion of the N-5 peak and the sum of the

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N-6 and N-Q peaks. This indicates that the nitrogen isotopic composition of coal may be

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controlled by the distribution of nitrogen forms. The pyridinic-structure nitrogen is enriched

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in

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macromolecules. The nitrogen isotopic signature of different nitrogen-containing molecules

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may exist difference. Further detailed research is expected to establish the change mechanism

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of nitrogen isotopic composition in the transformation process between nitrogen-containing

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molecules in coal.

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Acknowledgments

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N compared with pyrrolic-structure nitrogen. Coal consists of complex organic

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This work was supported the National Natural Science Foundation of China (41672144).

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We acknowledge editors and reviewers for improving the language of the paper and for


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in-depth discussion.

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Table 1 Proximate analysis and ultimate analysis (%) of the studied coals.

Sample PB-16 PY-16 GQ-17 XQ-16 XQ-17 ZA-16 ZA-17 PS-16 PS-17 ZU-16 ZU-17 342

a

Proximate analysisa

Ultimate analysisa

Mad

Ad

Vdaf

Cdaf

Hdaf

Odaf

Ndaf

1.3 1.7 0.7 1.9 1.2 1.6 1.3 1.3 1.6 1.0 1.0

24.9 13.4 14.8 26.6 24.7 19.1 20.5 18.8 16.7 23.0 15.8

35.8 22.4 19.5 29.2 35.1 37.6 24.3 19.2 36.8 18.1 18.1

66.03 75.72 84.39 71.11 68.15 67.14 79.42 72.85 61.02 84.53 83.88

4.96 3.55 3.19 4.09 4.55 4.47 3.72 3.62 5.27 4.57 3.10

23.35 18.40 8.72 20.84 22.06 24.00 12.29 19.82 27.76 6.56 8.99

1.22 1.08 1.01 1.49 1.21 1.24 1.39 0.90 1.21 1.32 1.05

±ca.5%; ad: air-dry basis; d: dry-basis; daf: dry-ash-free basis


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Table 2 Proportions (%) of macerals and minerals of the studied coals.

Sample PB-16 PY-16 GQ-17 XQ-16 XQ-17 ZA-16 ZA-17 PS-16 PS-17 ZU-16 ZU-17

Vitrinite 53.0 55.0 57.5 60.7 56.2 65.5 58.7 55.2 64.7 67.0 55.4

Inertinite 28.0 30.2 25.9 26.2 30.5 19.9 25.6 29.6 20.9 20.4 26.6

Liptinite 9.7 8.5 9.2 7.1 8.5 8.0 6.2 7.7 7.0 5.7 9.2

Organic matter 90.7 93.7 92.5 94.1 95.2 93.5 90.6 92.5 92.7 93.1 91.2

Clay 9.0 5.8 7.1 5.5 4.1 6.2 8.9 6.9 6.9 6.5 8.3

Sulfide nd nd nd nd nd nd nd nd nd nd nd

nd: not detected


Carbonate 0.3 0.4 0.4 0.4 0.6 0.3 0.5 0.5 0.4 0.5 0.4

Oxides and others nd nd nd nd 0.1 nd nd 0.1 nd nd 0.1

Mineral matter 9.3 6.3 7.5 5.9 4.8 6.5 9.4 7.5 7.3 7.0 8.8

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Table 3 Results from XPS N 1s spectra and nitrogen isotope ratio (‰) of the studied coals.


Nitrogen forms (% of total nitrogen) N-5

N-6

N-Q

63 69 68 55 68 50 58 76 71 63 64

37 31 20 33 32 32 42 24 29 37 36

0 0 12 12 0 19 0 0 0 0 0


δ15N 0.6 -0.4 -1.2 2.7 0.1 3.6 2.1 -2.6 -2.9 0.8 1.4

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Table 4 Comparison between XPS and elemental analysis results for total nitrogen of the studied coals.


Total nitrogen (peer 100 carbons) Elemental analysis (EA)

XPS

1.85 1.43 1.20 2.10 1.78 1.85 1.75 1.24 1.98 1.56 1.25

1.74 1.85 1.47 1.94 1.86 2.18 1.76 1.42 2.07 1.45 1.58


|XPS-EA|/XPS*100% 6 23 19 8 5 15 1 13 4 8 21

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Fig. 1 Location map of the Huainan coalfield and the profiles of sampling coal seams.


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Fig. 2 XPS N 1s spectra and curve resolution of the studied coals.


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Fig. 3 Structures of major nitrogen forms presenting in coal. Modified from the literature reported by (Boudou et al. 24; Pels et al. 28; Ding et al. 30)


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Fig. 4 Proportions of N-5 and N-6 vs. Cdaf content for the studied coals.


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Fig. 5 Cdaf content and Ndaf content vs. /15N value for the studied coals.


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Fig. 6 Proportions of macerals vs. δ15N value for the studied coals.


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Fig. 7 Proportions of N-5, N-6 and N-6+N-Q vs. δ15N value for the studied coals.


New Insights into the Nitrogen Isotope Compositions in Coals from the

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