Jili Hou, Yue Ma, Shuyuan Li, Jian Shi, Lu He, Jia Li: Full Length Article
Jili Hou, Yue Ma, Shuyuan Li, Jian Shi, Lu He, Jia Li: Full Length Article
Jili Hou, Yue Ma, Shuyuan Li, Jian Shi, Lu He, Jia Li: Full Length Article
Fuel
journal homepage: www.elsevier.com/locate/fuel
A R T I C LE I N FO A B S T R A C T
Keywords: Transformation of sulfur and nitrogen and properties of pyrolysates (semicoke, retorting gas, coal tar) during
Coal Shenmu coal pyrolysis were investigated using XPS, TG-FTIR-MS, GC–MS and GC-PFPD. The results showed that
Pyrolysis total sulfur content in the semicoke decreased gradually and turned to sulfate sulfur and thiophenic sulfur with
S/N transformation increasing pyrolysis temperature. In Shenmu coal, predominant nitrogen species consisted of pyrrolic and
XPS
pyridinic compounds. Total nitrogen content was decreased, resulting from mutual conversion of different ni-
TG/GC-FTIR-MS
trogen compounds. The composition of retorting gas from TG-FTIR indicated that H2S began to release at 400 °C
and reached maximum at about 600 °C. This trend was consistent with the release of carbonyl sulfide
(500–650 °C) and SO2. GC-PFPD analysis revealed that detected sulfur compounds in coal tar were benzothio-
phene, dibenzothiophene and thiophene with the contents of 0.141%, 0.110% and 0.0004%, respectively. In
coal tar, 57 kinds of nitrogen-containing compounds were identified using GC–MS, in which basic nitrogen was
0.90% and non-basic nitrogen was 0.23%.
⁎
Corresponding author.
E-mail address: syli@cup.edu.cn (S. Li).
https://doi.org/10.1016/j.fuel.2018.05.046
Received 6 July 2017; Received in revised form 8 May 2018; Accepted 9 May 2018
0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
J. Hou et al. Fuel 231 (2018) 134–144
Table 1
Analysis of proximate, ultimate and sulfur speciation of different samples.
Sample Proximate analysis (%) Ultimate analysis, wad/% Sulfur speciation analysis
SM coal 9.65 30.44 4.99 54.92 71.50 5.02 7.39 0.34 1.11 0.34 0.04 0.19 0.11
Semicoke 4.48 8.70 8.38 78.44 80.73 2.45 2.84 0.30 0.82 0.30 0.10 0.07 0.13
Note: M, moisture; V, volatile; A, ash; FC, fixed carbon; ad, air-dried basis. O determined by difference.
Table 2 was put into small scale retorting reactor. After programmed heating
Yields of pyrolysates of SM coal at different pyrolysis temperatures (wt, %). process, the reactor reached to different temperatures (300, 400, 500,
T, °C Gas Semi-coke Tar Water 600, 700 °C). 20 min at each temperature was necessary to ensure that
decomposable volatiles can be maximum. After coal pyrolysis, solid
300 2.64 92.54 0.01 4.81 product obtained in the retort was semi-coke. Other pyrolysates (tar,
400 4.20 84.12 6.26 5.42
water, and retorting gas) would pass through the outlet tube into a
500 9.17 72.58 9.01 9.24
600 9.61 70.23 10.07 10.09
conical flask that was cooled using an ice-water bath. Tar and water
700 9.75 70.14 10.03 10.08 were remained in the conical flask. Retorting gas was collected using a
vacuum gas bag for further testing. The yields of pyrolysates (gas, semi-
coke and tar) at different pyrolysis temperature are listed in Table 2.
Table 3 The retorting gas was analyzed using Agilent-6890 gas chromato-
XPS results of SM coal and semicokes at different temperatures. grapher. The chromatographic conditions are as follows. Initial column
Sample Content, w% temperature was 50 °C. 3 min was needed at this temperature. And then
temperature was increased to 100 °C within 10 min using a heating rate
C N O S of 5 °C/min and to 180 °C using a heating rate of 10 °C/min. Finally
3 min was needed at 180 °C. Carrier gas flow rates in flame ionization
SM coal 59.20 4.87 35.70 0.23
Semicoke-300 °C 69.45 2.58 27.78 0.19 detector (FID) were 40, 650, and 20 mL/min for H2, air, and N2, re-
Semicoke-400 °C 69.74 1.94 28.18 0.14 spectively. The front inlet pressure was 0.05 MPa and back inlet pres-
Semicoke-500 °C 70.33 1.36 28.22 0.09 sure was 0.10 MPa. Temperatures of injection port and detector for FID
Semicoke-600 °C 70.04 1.69 28.15 0.12 and thermal conductivity detector (TCD) were 50 and 250 °C, respec-
Semicoke-700 °C 70.76 2.01 27.03 0.20
tively.
Note: as XPS cannot detect H, so put C, N, O and S normalized to 1: C + N The basic and non-basic nitrogen compounds were analyzed using
+O + S = 100%. Agilent 6890/5975 GC–MS. The chromatographic conditions are as
follows. HP-5MS quartz capillary chromatographic column
separately, there were less studies about the comprehensive analysis of (60 m × 0.25 mm × 0.25 μm) was used. Gasification chamber tem-
coal pyrolysis. In the present work, several analytical methods (XPS, perature is 300 °C. The flow rate of He, as a carrier gas, was 1 mL/min.
TG-FTIR-MS, GC–MS and GC-PFPD) were used to analyze transforma- Temperature was increased from initial 50 °C to 120 °C using a heating
tion characteristics and distribution of sulfur and nitrogen in the solid, rate of 20 °C/min, to 250 °C using a heating rate of 4 °C/min and to
liquid and gas phases at different temperatures during SM coal pyr- 310 °C using a heating rate of 3 °C/min. The mass spectrum conditions
olysis. The information obtained could help to understand removal are as follows: ion source of electron impact (EI), ionizing voltage of
mechanism of sulfur and nitrogen for clean coal utilization and design 70 eV, interface temperature of 260 °C, ion source temperature of
of commercial coal retorting plants. 200 °C.
The distribution of sulfur compounds in coal tar was determined
using SP3420 capillary gas phase configuration 5380 with pulse flame
2. Materials and methods
photometric detector (PFPD). Chromatographic analysis conditions are
as follows: column HP-5, 30 m × 0.25 mm i.d. × 0.25 μm, injection
2.1. Materials
port temperature of 300 °C, and detector temperature of 250 °C. Column
temperature was maintained at 50 °C for 3 min, and then rose to 300 °C
The raw coal was from Shenmu (SM coal), Shanxi province of China.
using a heating rate of 5 °C/min. The shunt ratio is 50:1. The flow rates
Crushed samples with a size fraction of 0–0.2 mm were selected.
of nitrogen, air 1, air 2 and H2, as carrier gases are 1 mL/min, 10 mL/
Proximate and ultimate analyses were conducted following ASTM
min, 14 mL/min and 11.5 mL/min, respectively. The injection volume
Standards D3173-03, D3175-02, D3174-04 and D3176-15. Total sulfur
is 0.4 μL.
content and sulfur forms were determined following ASTM Standards
X-ray photoelectron spectroscopy ESCALab 250Xi (XPS) was used in
D3177-02 and D2492-02 (2012), respectively. The data in table 1
this study. According to binding energy of sulfur and nitrogen com-
clearly shows that SM coal is a typical bituminous coal with high vo-
pounds [23–25], sulfur (2p) spectra were resolved using components at
latile of 30.44% and high carbon content of 71.50%. After 550 °C, vo-
the peaks of 2p1/2 and 2p3/2 with separated energy of 1.18 eV, fixed
latile decreased to ∼22%, and semicoke with low S/N content was
energy positions and full width at half maximum (fwhm) value. Ni-
obtained. Some harmful elements have been removed to some extent
trogen (1s) spectra were resolved using four peaks at fixed energy po-
followed by the release of volatiles during coal pyrolysis. Besides, re-
sition and fwhm of 1.4 ± 0.1 eV. Experiments on the generation of
sults of sulfur speciation analysis show that semicoke contains about
sulfurous and nitrogenous gaseous products were carried out using TG-
40% organic sulfur and 60% inorganic sulfur. Sulfate sulfur content
FTIR-MS (TG: NETZSCH TG209F1 Libra, FTIR: Bruker ALPHA FTIR,
increased after coal pyrolysis.
MS: MSD Agilent). In each experiment, the sample (25 ± 0.01 mg) was
put into alumina crucible of TG. The sample was heated from room
2.2. Apparatus and procedure temperature to 650 °C with a heating rate 10 °C/min for pyrolysis in
argon atmosphere. The flow rates of protection gas and purge gas were
The coal sample (50 ± 0.5 g) with a particle size of less than 3 mm 60 mL/min and 20 mL/min, respectively. In order to get a better
135
J. Hou et al. Fuel 231 (2018) 134–144
Fig. 1. Curve-fitting of XPS-S2p spectra for SM coal and semicoke at different temperatures.
analytical result, the resolution of FTIR was set as 2 cm−1. Time re- (163.3 ± 0.4 eV), thiophenic sulfur (164.1 ± 0.2 eV), sulfoxide sulfur
solution was 8 s and number of scans of infrared instrument was 16. (166.0 ± 0.5 eV), sulfone sulfur (168.0 ± 0.5 eV) and sulfate sulfur
(169.5 ± 0.5 eV). The lower and higher fwhm values are approxi-
3. Results and discussion mately 1.1 ± 0.1 eV and 1.5 eV [26–28]. Nitrogen (1s) spectra were
resolved using four peaks at fixed energy positions of 398.8 ± 0.4,
3.1. The species of sulfur and nitrogen compounds in semicoke at different 400.2 ± 0.3, 401.4 ± 0.3 eV and 402.9 ± 0.5 eV, corresponding to
pyrolysis temperatures the forms of pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), qua-
ternary nitrogen (N-Q) and oxidized nitrogen (N-X), respectively
The species of sulfur and nitrogen compounds in SM coal and [29,30].
semicokes were investigated using XPS. According to binding energy Table 3 listed the contents of surface elements of SM coal and semi-
signal, different sulfur compounds were identified using the following cokes at different temperature through XPS analysis. From Table 3 it
values of pyritic sulfur (162.5 ± 0.3 eV), organic sulfide was shown that concentrations of nitrogen and sulfur on semicoke
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J. Hou et al. Fuel 231 (2018) 134–144
Fig. 2. The contents of sulfur species of SM coal and semicoke at different temperatures.
surface were decreased with increasing temperature when temperature 3.1.1. The species of sulfur compounds in semicoke at different pyrolysis
was below 500 °C, due to the reaction of free sulfur radical and free temperatures
hydrogen radical produced from organic matter pyrolysis. After 500 °C, Fig. 1 shows a curve-fitting of XPS-S2p spectra of sulfur species on
the generation rate of free sulfur radical in coal exceeds the rate of the surface of SM coal and semicoke at different temperature. Ac-
hydrogen supplied, resulting in increasing sulfur content. cording to the peak numbers in Fig. 1, XPS spectra fitting results of six
samples are basically consistent with each other. The sulfur species of
different samples were changed with rising pyrolysis temperature,
which is mainly caused by the mutual transformation and
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J. Hou et al. Fuel 231 (2018) 134–144
decomposition between different sulfur forms. In addition, sulfur exists with rising temperature, which was mainly from the oxidation of sulf-
mainly in coal in the forms of organic sulfur. Specifically the highest oxide sulfur. And then sulfone sulfur began to be decomposed. Its
content is thiophenic sulfur (54.54%), followed by sulfoxide sulfur content decreased to 8.86% at 700 °C as the continued increasing
(13.51%) and pyritic sulfur (10.90%). The contents of sulfate sulfur, temperature.
organic sulfide sulfur and sulfone sulfur are 8.57%, 6.23% and 6.25%, Fig. 2 (6) shows that content of sulfate sulfur increased rapidly with
respectively. rising temperature. This was mainly caused by the oxidizing reaction of
Fig. 2 shows the contents of sulfur species of SM coal and semicokes ferric sulfide in coal with some oxygen-containing functional groups
at different pyrolysis temperatures through XPS spectrographic fitting and oxidized state sulfur. Probably, there is another reason that sulfates
analysis. were gradually transferred from center to surface of the sample as
From Fig. 2 (1), with the increasing temperature, the sulfur com- temperature increased.
pounds in coal was decomposed into ferrous sulfide and active sulfur.
The active sulfur could be combined with the hydrogen radical to
generate H2S or combined with the organic matter in semicoke to be 3.1.2. The species of nitrogen compounds in semicoke at different pyrolysis
converted into new organic sulfur. Small amount of ferric sulfide would temperatures
be oxidated to sulfates. Pyrites would be broken down almost com- Previous work showed that four types of nitrogen compounds exist
pletely at 700 °C [31–33]. Besides, XRD results in Fig. 3 shows that FeS2 in coal, including N-6, N-5, N-Q and N-X. Nitrogen exists predominately
gradually decomposed into Fe(1-x)S with increasing temperature at early in the forms of pyrrolic and pyridinic compounds, in which content of
stage, and then to generate FeS [34] after 500 °C. It cannot be detected pyrrolic nitrogen is greater than pyridinic nitrogen [26]. Fig. 4 gives the
after 700 °C, which was consistent with results of XPS. results of curve-fitting of XPS-N1s spectra for SM coal and semi-coke at
Fig. 2 (2) shows that content of organic sulfide in pyrolysis process different temperatures. Four types of nitrogen species were identified
decreases with increasing temperature. The aliphatic sulfide was de- by the peaks at different position. And their relative contents have been
composed between 300 °C and 400 °C. Cyclic sulfide and aromatic sul- obtained, indicating that contents were N-6 (15.79%), N-5 (69.08%), N-
fide were cracked between 500 °C and 600 °C. After 500 °C, the sulfide Q (9.21%) and N-X (5.92%). Pyrrole and pyridine become main species
sulfur was basically completely decomposed. of nitrogen compounds in SM coal.
As is shown in Fig. 2 (3), the content of thiophenic sulfur increased Fig. 5 shows the contents of various nitrogen species in SM coal and
with increasing temperature. There was a slight reduction at about semicoke at different pyrolysis temperatures. From Fig. 5 (1), it was
500 °C, due to a small amount of thiophene formed during coal tar found that N-6 has a trend of decreasing at first and then increasing
production [35]. Coal tar generation was almost complete after 600 °C. with rising temperature. Before 300 °C, the content of pyridine in raw
After 600 °C, the increase of thiophene content was mainly caused by coal and semicoke changed slightly. From 300 °C to 500 °C, there was a
condensation reaction of organic sulfide sulfur in coal. significant decrease due to combination of pyridine nitrogen with some
Fig. 2 (4) shows that content of sulfoxide sulfur decreases with in- oxygen-containing functional groups in coal to form oxidative nitrogen.
creasing temperature and basically disappears at 700 °C. The decreasing After 500 °C, some unstable N-5 species were converted to N-6. Because
sulfoxide sulfur contents before and after 550 °C were resulted mainly of high temperature, N-O bonds in the nitrogen species would be
from the removal of aliphatic sulfoxide and aromatic sulfoxide, re- broken to release a small amount of N-6, resulting in the increasing
spectively. Sulfoxide could be oxidized to sulfone by oxygen-containing content of total amount of N-6.
functional groups on the surface of coal. From Fig. 5 (2), N-5 increased with increasing temperature because
From Fig. 2 (5), it was shown that sulfone sulfur content increased N-Q was converted to N-5 before 500 °C. Compared to pyridine, thermal
stability of pyrrole was slightly poor. With rising temperature, the
138
J. Hou et al. Fuel 231 (2018) 134–144
Fig. 4. Curve-fitting of XPS-N1s spectra for SM coal and semicoke at different temperatures.
content of N-5 reduced while N-6 increased in the semicoke, indicating slightly before 300 °C. After 300 °C, N-X content was gradually in-
that transformation reactions happened between N-5 and N-6. This creased because of the combination of N-6 with oxygen-containing
conclusion was consistent with the results of Nelson [36] and other functional groups to generate N-X species. N-O bond was broken to
studies [37–39]. produce N-6 after 600 °C. N-X compound was gradually decreased. It
Results from Fig. 5 (3) shows that N-Q decreased with increasing will be completely decomposed after 900 °C [40].
temperature. At 700 °C, the content of N-Q in semicoke was decreased
to 2.41%. N-Q compounds would be converted to pyrrole at low tem-
perature and decomposed after 600 °C.
From Fig. 5 (4), N-X compounds were found to be increased first,
and then to be decreased with rising temperature. N-X content changed
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J. Hou et al. Fuel 231 (2018) 134–144
Fig. 5. The contents of nitrogen species of SM coal and semicoke at different temperatures.
Table 4
Composition of retorting gas at different pyrolysis temperatures (%).
T, °C H2 CO CO2 CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8 C5H12 C5H10
400 1.71 16.61 49.87 18.82 5.28 1.45 2.22 1.59 0.7 0.97 0.32 0.46
500 8.64 16.06 27.98 34.00 6.25 1.37 2.01 1.33 0.73 0.79 0.42 0.42
550 15.94 16.83 21.75 35.64 4.97 1.06 1.44 1.00 0.44 0.30 0.20 0.43
600 19.50 16.93 19.65 35.47 4.07 0.87 1.23 0.81 0.47 0.47 0.29 0.25
650 26.29 15.86 15.51 35.54 3.39 0.77 0.94 0.71 0.28 0.39 0.13 0.19
3.2. Analysis of the components of retorting gas at different temperatures Because different compound has different energy needed to be de-
composed, formation of retorting gas will show certain regularity with
3.2.1. Analysis of main components of retorting gas at different the changes of pyrolysis temperature. Using formation characteristics of
temperatures gaseous products, the pyrolysis mechanism of coal can be reasonably
The retorting gas produced during coal pyrolysis mainly contains understood.
CH4, C2H6, C2H4, C3H8, C3H6, C4H10, C4H8, CO2, CO and H2. Among One small amounts of retorting gas were generated before 400 °C.
them, CH4、CO2、CO and H2 have the larger contents. The main gas Gas production was found to be increased quickly after 400 °C. Fig. 6
components and their formation behavior were obtained at different shows H2 production at different temperature. When pyrolysis tem-
pyrolysis temperature using gas chromatography. Results are shown in perature reached 500 °C, the formation rate of H2 was obviously ac-
Table 4. celerated. CO2 content decreased as the temperature increased, in-
Literatures [41–43] indicated that retorting gas was mainly derived dicating that CO2 was mainly generated at low temperature.
from organic matter decomposition. CO2 was mainly generated from The changes of CO content were not obvious with rising tempera-
decarboxylation reaction of carboxylic acid compounds. CO was mainly ture, indicating that temperature has a little effect on CO generation
produced from decomposition of non-carboxylic acid compounds, such during whole pyrolysis process. The content of CH4 increased with in-
as, ethers, phenols, ketones, etc. H2, CH4 and C2+ were derived from creasing temperature. The yield of C2+ components was smaller than
decomposition of aliphatic compounds and aromatic side chains. that of H2 and CH4. It can be seen from the formation mechanism of
140
J. Hou et al. Fuel 231 (2018) 134–144
141
J. Hou et al. Fuel 231 (2018) 134–144
shown in Fig. 8.
From Fig. 9 it was shown that main sulfur compounds in coal tar are
thiophenes (Ts), benzothiophenes (BTs) and dibenzothiophenes (DBTs)
while mercaptan, thoiether and other chain sulfide compounds were
not detected. Low content of thiophenes was found. XPS results in
Fig. 2(3) indicated that content of thiophenes in semicoke increased
with increasing pyrolysis temperature. Small amount of thiophenes was
formed in coal tar at around 500 °C, which was consistent with the
result of GC-PFPD detection.
Table 6 shows the content of sulfur compounds in SM coal tar.
Sulfur compounds in coal tar mainly include thiophene, benzothio-
phene and dibenzothiophene with the corresponding contents of
0.141%, 0.110% and 0.0004%, respectively.
The pyrolysates (tar, semicoke and gas) were obtained after SM coal
pyrolysis at 550 °C. Calculated results of sulfur and nitrogen transfor-
mation in Fig. 10 showed that about 62% sulfur was remained in the
semicoke, in which thiophenic sulfur and sulfate sulfur were main
products. Around 38% sulfur was removed from raw coal during the
pyrolysis. 43.7% sulfur was transferred into coal tar with main forms of
benzothiophene (52%), dibenzothiophene (41%), thiophene (0.15%)
and the other (6.85%). 56.2% sulfur was transferred into retorting gas
with main forms of H2S, carbonyl sulfur and a small amount of SO2. The
remaining 0.1% sulfur was possibly in pyrolysis water or lost during
pyrolysis process. For nitrogen transformation, 74% nitrogen was en-
riched in semicoke with the forms of pyridinic nitrogen (26%) and
pyrrolic nitrogen (74%). The removed nitrogen was mainly transferred
into coal tar, including basic nitrogen and non-basic nitrogen com-
pounds. The remaining nitrogen was released as the gaseous products
(NO, NO2 and NH3).
Thiophenes 0.0004 C4-benzothiophene 0.038 (1) In the semicoke, as a solid product, contents of sulfate and thio-
Benzothiophene 0.005 C5-benzothiophene 0.033 phenic sulfur increased while contents of pyrite, sulfoxide and or-
C1-benzothiophene 0.014 dibenzothiophene 0.012 ganic sulfide decreased with increasing pyrolysis temperature.
C2-benzothiophene 0.015 C1, C2-benzothiophene 0.098 Sulfone content increased before 600 °C and then decreased after
C3-benzothiophene 0.036
600 °C.
(2) In the semicoke, N-6 content increased first and then decreased
3.3. Analysis of sulfur and nitrogen compounds in SM coal tar with rising temperature. The contents of N-5, N-Q and N-X de-
creased as the temperature increased during coal pyrolysis.
3.3.1. GC-PFPD detection analysis of sulfur compounds in SM coal tar (3) In gaseous products, H2S, COS and SO2 were mainly produced by
Results from table 5 show that SM coal tar produced at 600 °C has the reactions of different forms of sulfur and free sulfur radicals
low sulfur content of 0.27%. Gas chromatography-pulsed flame pho- with hydrogen radicals, carbon monoxide or free oxygen radicals.
tometric detector (GC-PFPD) was used for the qualitative and quanti- Nitrogenous compounds (NO, NO2 and NH3) were hardly detected
tative analysis of sulfur compounds in coal tar, and the results were in pyrolysis gases due to water interference.
(4) 37% sulfur and 26% nitrogen were transferred into coal tar and
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J. Hou et al. Fuel 231 (2018) 134–144
Table 7
GC–MS results of nitrogen compounds in SM coal tar.
Relative content/% Molecular formula Possible compound Relative content/% Molecular formula Possible compound
0.079 C10H13N
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