An-approach-for-upgrading-lignite-to-improve-slurryability--Blending_2021_En.pdf

An approach for upgrading lignite to improve slurryability: Blending
with direct coal liquefaction residue under microwave-assisted
pyrolysis
Suqian Gu a
, Zhiqiang Xu a, *
, Yangguang Ren b, **
, Yanan Tu a
, Meijie Sun a
, Xiangyang Liu a
a
School of Chemical and Environmental Engineering, China University of Mining & Technology (Beijing), Beijing, 100083, China
b
College of Chemical and Biological Engineering, Shandong University of Science and Technology, Qingdao, 266590, China
a r t i c l e i n f o
Article history:
Received 7 October 2020
Received in revised form
13 January 2021
Accepted 26 January 2021
Available online 1 February 2021
Keywords:
Lignite
Direct coal liquefaction residue
Microwave-assisted pyrolysis
Lignite water slurry
a b s t r a c t
As the composition of direct coal liquefaction residue (DCLR) is complex and difficult to handle, more
importantly its dielectric properties are excellent, it is used as consumable wave-absorbents to enhance
the microwave pyrolysis of lignite for slurryability improvement. The effects of the different additions of
DCLR on the evolution of pyrolysis products were studied by using gas chromatography, Fourier trans-
form infrared spectroscopy (FTIR), Raman spectroscopy, and low-temperature N2 adsorption analysis.
The results indicated that microwave-assisted pyrolysis with DCLR was a promising method for
improving slurryability of lignite and an effective method for utilizing DCLR. The introduced DCLR
accelerated the heating rate of the process of microwave upgrading lignite, facilitating the decomposition
of active oxygen-containing functional groups and aliphatic hydrocarbons and then their transformation
into stable ether groups and aromatic structures. In the meantime, it was converted into gaseous
products, mainly composed of H2, CO, and CH4, and solid products with high quality. Additionally, the
cyclization and aromatization of organic structures were improved, as well as the order degree of aro-
matic systems, especially with 12 wt% of DCLR added. Furthermore, the existing and newly formed
structures of micropores and mesopores in the upgraded lignite (UL) were remarkably reduced with the
increase of DCLR contents. The re-absorption capacity was dramatically reduced and the slurryability of
UL was improved as a result of the changes in chemical properties and pore structures. The maximum
solid concentration of lignite water slurry (LWS) increased from 41.73 wt% to 65.42 wt% with lower
pseudo-plasticity and static stability.
© 2021 Elsevier Ltd. All rights reserved.
1. Introduction
Lignite, as fossil fuel with abundant reserves (about 45% of
global coal reserves) or raw material for other products, remains
the cheapest and most easily obtained energy source in the world
[1e4]. Directtransportation, storage, and utilization of lignite are
not safe and economical because of its complex chemical compo-
sitions and structures, such as high volatile content, abundant
active oxygen functional groups, and developed pore structures,
offering it strong re-absorption capacity and the characteristics of
spontaneous combustion [5,6]. At present, the preparation of
lignite into LWS, which is transported through pipelines and then
converted into other chemical feedstock, is one of the most
economical and effective ways to improve its utilization rate
[7e10]. However, it is disappointing that the LWS prepared directly
from lignite has low solid concentration (not exceeding 50 wt%)
due to the high moisture content, developed pore structures, and
complex macromolecular network structure of lignite [11]. There-
fore, an efficient and economical method is urgently needed to
increase the slurryability of lignite to facilitate the industrial
application of LWS.
To date, the heating methods including hot-air drying [12], hy-
drothermal treatment [13], fluidized-bed drying process [14], and
the non-heating techniques such as exploring high-performance
dispersant [15], low-rank coal flotation [16], and mixing high-
carbon substance [17,18], are insufficient to change the coal
* Corresponding author. School of Chemical and Environmental Engineering,
China University of Mining & Technology (Beijing), Beijing, 100083, China.
** Corresponding author. College of Chemical and Biological Engineering, Shan-
dong University of Science and Technology, Qingdao, 266590, China.
E-mail addresses: xzq@cumtb.edu.cn (Z. Xu), renyg54@sdust.edu.cn (Y. Ren).
Contents lists available at ScienceDirect
Energy
journal homepage: www.elsevier.com/locate/energy
https://doi.org/10.1016/j.energy.2021.120012
0360-5442/© 2021 Elsevier Ltd. All rights reserved.
Energy 222 (2021) 120012

quality (e.g. chemical properties and microstructure features)
which affect its slurryability, of lignite. Compared with these
methods, microwave pyrolysis technique, which utilizes micro-
wave irradiation to achieve pyrolysis reaction and change the
specific physical and chemical properties of coal, is considered as
one of the most effective means to improve the slurryability of
lignite [11,19e22]. The main advantages of the microwave pyrolysis
technique are rapid treatment, timely heating, thoroughness, and
higher level of safety and automation. It has been demonstrated
that microwave pyrolysis is suitable for various industrial pro-
cesses, including waste recycling [23,24], biomass pyrolysis [25,26],
hazardous waste processing [27], and mineral processing [28e30].
Nevertheless, the dielectric properties of lignite are relatively spe-
cial. There are polarizable water and polar functional groups in
lignite, while its internal structure is highly disordered, leading to
the insufficient interaction of most components within lignite with
microwaves in time. Therefore, the heating rate of dry lignite by
microwave is slow so that it is hard to reach the temperature
required for pyrolysis. In order to improve this situation,
microwave-assisted pyrolysis technique with the introduction of
strong dielectric materials has been developed [26,31e35].
Microwave-assisted pyrolysis with the introduction of strong
dielectric materials is a novel approach that has achieved good
results in promoting rapid pyrolysis of low-rank coals
[29,30,32,34e36]. These dielectric materials can be divided into
three categories: inorganic metal oxides (e.g., Fe3O4 [21] and V2O5
and CuO [33]), inorganic salts (e.g., NaCl [37], K2CO3 and CaCl2 [35]),
and carbonaceous materials (e.g., activated carbon [31], graphite
[38], char [39] and petroleum coke [40]). While these three types of
heterogeneous absorbents show excellent performance in raising
temperature and reducing energy consumption. However, the
deactivation of inorganic metal oxides at high temperatures is the
main obstacle to their utilization in catalytic pyrolysis, and inor-
ganic salts can easily stick into lignite after dehydration. Accord-
ingly, these two types of absorbing materials are formidable to be
separated from the soild products, inevitably resulting in the UL
with higher ash content and lower calorific value. Differently, the
carbonaceous absorbents with a high carbon content and graphi-
tization degree, which is similar to coal, are parts of the upgraded
products and do not need to be separated from UL. Thus, carbo-
naceous materials have become a hot spot in the research of mi-
crowave absorbents for lignite in recent years. In Ref. [41], Liu et al.
found that charcoal could rapidly promote the pyrolysis tempera-
ture of coal in microwave field, which could be used as microwave
absorbent to enhance the absorbing ability of coal. In Ref. [39], char
was utilized as an ideal microwave absorbent to improve the vol-
atile removal of Indonesian lignite during microwave-assisted py-
rolysis. The study of the physical and chemical properties of UL
suggested that char absorbent played multiple roles in accelerating
the heating rate, reducing the volatile compounds, and increasing
the apparent aromaticity of Indonesian lignite was irradiated at
800 W for 30 min. These various changes on the physicochemical
properties of UL improved the slurryability of Indonesian lignite.
Besides, in Ref. [40], Ren et al. studied the effect of different pe-
troleum coke loadings, from 0 g to 30 g with a constant amount of
lignite (120 g), on the slurryability of lignite, as well as the adverse
factors that affect the slurryability, including volatiles, water, and
hydrophilic oxygen functional groups. The slurryability was
increased most significantly with the petroleum coke loading of
12 g and LWS with high solid concentration of 67.51 wt% was ob-
tained under this condition. It was emphasized that petroleum coke
was high in carbon content, and the solid concentration of LWS
could be further increased without separation of the petroleum
coke after microwave upgrading. Although great progress has been
made in this field, these carbonaceous microwave-assisted
absorbents remain expensive for the application in industrial
products, so it is hard to obtain economic benefits from UL.
Therefore, the premise of large-scale industrial application of this
technique is to explore more suitable materials with high carbon
content and strong dielectric properties.
During the direct coal liquefaction process, DCLR is produced as
an undesirable by-product that accounts for 20e30% of the lique-
fied feedstock. It not only contains coke, heavy oils, and asphalt
components, but also exists benzene series, polycyclic aromatic
compounds, and sulfur-containing catalysts [42,43]. The combus-
tion and traditional utilization methods are not suitable for DCLR
because its injurious ingredients are difficult to handle and harmful
to the environment [44e47]. Exploring clean and efficient ways to
utilize DCLR has always been a challenge facing the industry.
However, a process to combine DCLR and microwave pyrolysis
lignite may be a feasible method, because the following two ben-
efits can be expected: (1) the DCLR has the same compositions as
carbonaceous wave-absorbents and thus it seems to be an ideal
ingredient for lignite pyrolysis under microwave irradiation; (2) the
DCLR can be converted cleanly and efficiently in the form of high
value-added gas during the microwave pyrolysis.
To the best of our knowledge, the paper is the first to provide a
new treatment method that uses microwave-assisted pyrolysis to
perform both the upgrading of lignite and the utilization of DCLR. In
this research, the DCLR was used to improve microwave upgrading
efficiency and changed the physical and chemical properties of UL
that affect its slurryability. The DCLR was eventually converted into
gases with highly added value after pyrolysis. Moreover, the
changes in the chemical composition, coal structure, and interface
characteristics of lignite and the gas products before and after
introducing DCLR were systematically studied through various
analytical techniques, as well as the microscopic mechanism of
these slurryability-affecting changes on UL. The significant contri-
butions in this work were that the physical and chemical properties
of lignite that affect its slurryability were changed more thor-
oughly; the DCLR as consumable wave-absorbent, with the prop-
erties of higher conversion efficiency as well as simpler and less
time-consuming operation, was confirmed.
2. Experimental section
2.1. Materials
The lignite from Inner Mongolia and the direct liquefaction
residue of bituminous coal from Shenhua Coal to Oil chemical plant
(Shanxi, China) were selected as the experimental research objects.
The particle sizes of lignite and DCLR were controlled in the range
of 0.3e3 mm and 0e0.15 mm respectively by crushing, screening,
and ball milling. The parameters (ε0, ε
00
and tand) that represent the
dielectric properties of the materials are carried out under a vector
network analyzer (E5071C, Keysight Technologies, USA), as shown
in Table 1. The ε0 was related to the microwave energy storage ca-
pacity, the ε
00
was associated with the capacity to dissipate the
stored microwave energy into thermal energy, and tand refers to
the efficiency of converting microwave energy into thermal energy.
For lignite, the values of ε0, ε
00
, and tand were much smaller than that
of DCLR, meaning that DCLR could preferably absorb microwave
energy and convert it into heat to assist the heating of lignite during
the microwave pyrolysis process.
2.2. Microwave-assisted pyrolysis
The microwave pyrolysis test rig was mainly composed of three
parts: an automated laboratory microwave oven, a material
S. Gu, Z. Xu, Y. Ren et al. Energy 222 (2021) 120012
2

reaction device, and a gas pipeline system. The laboratory micro-
wave oven, with the frequency of 2450 ± 50 MHz, the power range
of 0e1800 W, and the cavity size of 520 680 460 mm, was
utilized. The material reaction device was made of quartz material,
consisting of three mouths for nitrogen input, the infrared detector
for material temperature, and pyrolysis gas outlet respectively,
which was used for the material reaction. 160 g lignite was evenly
mingled with 0 wt%, 4 wt%, 8 wt%, 12 wt%, 16 wt%, and 20 wt% of
DCLR in the reactor respectively, and then the carrier gas (nitrogen)
with a flow rate of 0.4 L/min was introduced into the reactor to
prevent lignite combustion and accelerate the release of pyrolysis
gas. All samples were irradiated at 1000 W for 30 min (the final
temperature was maintained at 700 C), which was chosen as the
experimental condition based on the initial exploration. Then the
solid products were separated to obtain the UL samples with
different DCLR contents (defined as S10-0, S10-4, S10-8, S10-12, S10-16,
and S10-20) for further analysis. Meanwhile, the composition of the
collected pyrolysis gases (defined as G10-0, G10-4, G10-8, G10-12, G10-16,
and G10-20) was assessed by a gas chromatograph (Agilent 7890A
GC, USA) to analyze the products from DCLR conversion. Since the
evaluation of the performance and the reliability of the test method
was crucial in the research process [48], at least 15 parallel tests
were carried out for the microwave pyrolysis experiment under
each condition to analyze the performance of the method and meet
the LWS preparation of UL. The reliability of the method was
evaluated by the solid yield of UL [40], and the relative error of the
solid yield under the same condition was less than 2.5%.
2.3. Characterization and analysis of upgraded lignite
Due to the complexity of the coal structure, it was difficult to
determine the evolution of the chemical properties of UL using one
single technology [49,50]. Fourier transform infrared spectroscopy
(FTIR; NICOLET iS10) and Raman spectroscopy (Raman; HORIBA
XploRA PLUS) were used to analyze the mineral compositions, coal
structure, and carbonization degree of UL samples. The powdered
lignite and KBr were mixed at a mass ratio of 1:200 to prepare
tablets for testing. 56 UL samples (4 for each composition) were
used for FTIR and Roman tests. The measuring range, resolution,
and scanning number of infrared spectra were 4000e400 cm1
,
4 cm1
, and 32 times, respectively. Similarly, the Raman spectra in
the range of 1800e800 cm1
were generated by collecting 10 scans
with a laser at 532 nm. Besides, the FTIR and Raman spectra were
curve-fitted by the professional software (Origin Lab, Peak Fit v4)
for further semiquantitative analysis.
A nitrogen adsorption instrument (JW-BK122W) was used for
conducting desorption/adsorption tests under relative pressure of
103
e1 at 77 K to analyze the pore structure of UL samples. The
tests were repeated at least twice, with the difference between the
last digit of the two results no more than 3. Otherwise, the third test
was needed.
2.4. Preparation and determination of LWSs properties
The solid concentration and rheological behavior of LWS were
closely related to its energy density, transportation, and
atomization, while the storage and effective utilization of LWS were
directly affected by its stability [8,10,51]. The average particle sizes
of lignite and UL were controlled at 45.78 ± 2 mm for the prepa-
ration of LWSs. The lignite powder, deionized water, and 1% sodium
methylene dinaphthalene sulfonate (NNO) were preliminarily
mixed at a low speed (350e500 r/min) for 5 min and then stirred at
a higher speed (1000 r/min) for 10 min to prepare LWSs. All tests
were repeated by conducting three sets of parallel tests to obtain
the final results.
2.4.1. Solid concentration and rheology measurements
The solid concentration (u) of LWSs was assessed by the water
analytic apparatus (MA35, Sartorius, Germany). As the solid con-
centration was increased with the viscosity (h), the maximum solid
concentration (umax) of LWS was denoted as the solid concentration
when the shear rate was 100 s1
and the apparent viscosity was
1000 mPa‧s to meet the required viscosity of transportation and
utilization [51].
The viscosity of LWSs was determined with a rotational
viscometer (NXS-11B). Since LWSs belonged to non-Newtonian
fluid that the viscosity was independent of shear time, their rheo-
logical properties could be characterized with the following for-
mula, as shown in Eq. (1).
tyx ¼ ty þ hgn
(1)
where tyx, ty, h, g and n are shear stress (Pa), yield stress (Pa),
viscosity (Pa$s1
), shear rate (s1
) and fluidity index, respectively.
Among these parameters, ty and n depended on the properties of
the non-Newtonian fluid, and the smaller n value represented the
weaker pseudo-plasticity of LWS [10].
2.4.2. Static stability analysis
The static stability of LWSs prepared from UL samples within 7
days was determined by the stability kinetic index (TSI) of Turbis-
can LAB™ which was capable of measuring the slurry without
damaging the sample, avoiding the interference of subjective fac-
tors, and showing the cause of instability (flocculation, aggregation
or migration). The TSI values were obtained by accumulating the
difference between the result of each scan and the previous scan,
rather than relying on the number of samples. The formulation is
shown in Eq. (2).
TSI ¼
X
i
P
hjscaniðhÞ scani1ðhÞj
H
(2)
where h and H are the scan height and the total height of the
sample respectively. Thus, smaller TSI value indicated better static
stability of the samples.
3. Results and discussion
3.1. Changes in heating characteristics of upgraded lignite
Fig.1 illustrates the heating rate of the UL samples with different
DCLR additions. As the irradiation time extended, the microwave-
assisted pyrolysis occurred after 15 min evidently, especially for
Table 1
Dielectric properties of lignite and DCLR samples.
Samples Dielectric constant (ε0) Dissipation factor (ε
00
) Dielectric loss tangent (tand)
Lignite 4.30 0.254 0.059
DCLR 6.55 0.937 0.143
Note: The experiment was repeated three times, and the standard deviations of ε0 and ε
00
were less than 0.15 and 0.02 respectively.
3

the samples with DCLR. As shown in Fig. 1, the heating curves of the
mixtures could be divided into four stages: the rapid dehydration
stage (0e120 C), the slow drying stage (120e280 C), the rapid
pyrolysis stage (280e700 C), and the constant temperature stage
(near 700 C). During the rapid dehydration stage, the difference in
heating rate between samples was very small, because water
molecules with best dielectric properties absorbed the majority
microwave energy. After dehydration, the heating rate was reduced
and the wave-absorbing ability of dry lignite was weak. In this case,
the introduced DCLR in the system assisted the temperature rise
during the slow drying stage because of its better dielectric prop-
erties. This slow drying stage was one of the most important stages
affected by DCLR as well. It is emphasized that the softening of
DCLR occurred at 180 C, and the melting of DCLR happened at
about 320 C [52]. In addition, the pyrolysis reaction of DCLR at
350 C, transformed it from the liquid into gases with highly added
value. During the microwave-assisted pyrolysis, DCLR was pyro-
lyzed and gasified, with the heat transferring to the surrounding
lignite at the same time. The interaction between microwaves,
lignite, and DCLR under microwave irradiation is shown in Fig. 2.
Among these samples, the heating rates were the highest for the
samples S10-12 and S10-16, and then it decreased with excessive
DCLR contents. Therefore, the addition of 12 wt% ~16 wt% DCLR in
the system was the optimal for accelerating the microwave-assisted
pyrolysis for lignite.
3.2. Main compositions and yield of pyrolysis gases
Fig. 3 shows that the gaseous products generated from the py-
rolysis of the mixture are mainly composed of CO2, CH4, CO, H2, and
CnHm. The majority of CO2 was emitted as a result of the decom-
position of unstable carboxyl functional groups when the temper-
ature was above 200 C. The CH4 was formed by the breakage of
side chains, branches, and bridge structures containing methylene
groups in the mixture at 580 C. The CO mainly came from three
sources: the decomposition of the carbonyl and ether groups (e.g.,
the cracking of carbonyls at about 400 C), the cleavage of other
oxygen-containing heterocycles at over 500 C, and the Boudouard
reaction between CO2 and C when the temperature was above
Fig. 1. Influence of DCLR contents on the heating rate of UL samples.
Fig. 2. Mechanism of microwaves interaction with lignite and DCLR under microwave irradiation.
Fig. 3. Influence of DCLR contents on the composition and content of gas products.
4

650 C [40,53]. The H2 was originated from the decomposition of
aliphatic and aromatic structures in the mixture and the conden-
sation reaction of coke within higher temperature range. As can be
seen from Fig. 3, the proportion of combustible gases (CH4, CO, and
H2) was gradually increased, from 73.64 vol% (13.56 L, S10-0) to
80.07 vol% (13.56 þ 9.83 L, S10-16), and then reduced to 78.21 vol%
(13.56 þ 8.46 L, S10-20) as the content of DCLR was increased. The
significant increase of the gas products was primarily due to the
rapid pyrolysis and volatilization of organic components in DCLR,
whereas the decrease of the gas products was because the vola-
tilization of excessive DCLR weakened the upgrading effect of mi-
crowave on lignite. Among the gas samples produced by the
pyrolysis of mixture, G10-16 with longer high-temperature duration
had the lowest proportion of CO2. While as the temperature
dropped from 700 C to 550 C continuously in the constant tem-
perature stage, G10-20 produced more CH4 and CO2. In addition, the
appearance of unreacted oily substances was found in the S10-16 and
S10-20 samples. Therefore, when 16 wt% DCLR was used in the
upgrading of lignite, the highest content of combustible gases was
obtained in the gas products after pyrolysis due to the highest
heating rate.
3.3. Chemical and physical properties of upgraded lignite
3.3.1. Chemical compositions of upgraded lignite
The proximate analysis and ultimate analysis of lignite, DCLR,
and UL samples are listed in Table 2. It is obvious that DCLR has
much higher carbon content and much lower re-absorption ca-
pacity and oxygen content than lignite. After being irradiated, the
contents of equilibrium re-adsorption moisture and volatile of
lignite decreased from 8.30 wt%, 48.07 wt% to 4.81 wt%, 29.23 wt%
respectively, while its calorific value increased by 2.92 MJ/kg. These
changes have invariably been emphasized for the lignite upgrading
process, resulting in the avoidance of spontaneous combustion
during storage and transportation in this process [20,36,54]. As the
DCLR contents increased from 0 wt% to 20 wt%, the equilibrium re-
adsorption moisture of UL declined continuously, with the lowest
volatile matter and the highest carbon content at S10-12. In addition,
it was found that the excessive DCLR was not conducive to
upgrading lignite as the excessive DCLR preferentially absorbed
most microwaves that accelerated its own rapid pyrolysis and
volatilization, thereby taking away a lot of heat in the mixture.
Hydrogen, oxygen, and carbon were the main elements in the
macromolecular structure of lignite, among which oxygen is the
most important heteroatom and hydrogen was related to the
coalification degree of coal. Both of the two elements mainly
existed on the side chains and oxygen-containing functional groups
of lignite molecules [31], and their amount gradually decreased
with the release of volatiles. In contrast, the carbon element, which
remained the main source of calorific value from lignite, was mostly
retained in the UL. Meanwhile, the oxygen-carbon atomic ratio (O/
C) and hydrogen-carbon atomic ratio (H/C) of UL significantly
declined, especially in S10-12, indicating that the coal quality of UL
was improved greatly. Therefore, the introduced DCLR could
significantly facilitate the reduction of the re-adsorption capacity
and volatile content, as well as the improvement of the coal quality.
3.3.2. FTIR analysis of upgraded lignite
The infrared spectra of lignite and UL samples are illustrated in
Fig. 4. The coal samples with different molecular structures and
oxygen-containing functional groups had various infrared spectra,
which were mainly manifested at wavenumbers of
3550e3200 cm1
, 2923 cm1
, 2850 cm1
, 1703 cm1
, 1590 cm1
,
1427 cm1
, 1108 cm1
, 1034 cm1
and 700e900 cm1
. The assign-
ments of the peaks in the FTIR of lignite samples are based on the
literature [34,50,55e57], as presented in Table 3. The peaks at
3600e3000 cm1
referred to the hydroxyl group in alcohol, water
clusters, and phenol and the peaks at 1709 cm1
referred to eC]O
in carboxylic acids. At these positions, lignite, S10-0, and S10-4 had
Table 2
Proximate analysis and ultimate analysis of lignite, DCLR, and UL samples.
Samples Proximate analysis (%) Qb,d (MJ/kg) Ultimate analysis (%) AO/C AH/C
Meq Ad Vdaf FCdaf Cdaf Ndaf Hdaf Sdaf Odaf
Lignite 8.30 17.53 48.07 51.93 20.68 65.64 0.70 5.23 0.92 27.51 0.314 0.957
DCLR 0.02 14.40 49.54 50.46 31.29 90.60 1.03 4.58 2.38 1.42 0.010 0.610
S10-0 4.81 20.55 29.23 70.77 23.60 78.30 0.89 4.19 0.77 15.85 0.152 0.642
S10-4 2.70 19.52 26.17 73.83 23.64 78.95 0.87 3.92 0.72 15.53 0.148 0.596
S10-8 2.53 20.43 25.76 74.24 23.82 83.11 0.84 3.79 0.97 11.28 0.102 0.547
S10-12 2.36 20.60 21.95 78.05 24.70 84.66 0.91 3.42 1.07 9.94 0.088 0.485
S10-16 2.27 20.72 25.55 74.45 24.42 83.62 0.96 3.53 1.19 10.71 0.096 0.506
S10-20 2.02 20.55 28.16 71.84 24.21 83.23 0.96 3.54 1.18 11.08 0.100 0.511
Note: M e moisture content; A e ash content; V e volatile content; FC e fixed carbon; C e carbon; N e nitrogen; H e hydrogen; S e sulfur; O e oxygen; Qb e bomb calorific
value; eq e equilibrium re-adsorption moisture in a 48.5% ± 0.5% ambient humidity; d e dry basis; daf e dry ash-free basis; AO/C, AH/C represent the ratio of oxygen to carbon
and hydrogen to carbon, respectively. The data in this table are the results of three parallel experiments, all with deviations less than 0.01.
Fig. 4. Original infrared spectra of lignite and UL samples.
5

significantly higher peak intensity than other samples, which was
consistent with the results of higher oxygen content in Table 2. The
peaks of lignite at 2923 cm1
and 2850 cm1
, representing asym-
metric and symmetric stretching vibrations of methylene structure
respectively, were significantly stronger than those of UL samples,
indicating the presence of more aliphatic hydrocarbon branches
and alkyl side chains in lignite. The peaks of aluminosilicate and
quartz in UL near 1103e1000 cm1
, which corresponded to the
higher ash contents in proximate analysis, had higher intensity
compared with lignite. In addition, the intensity of peaks for UL
samples in the range of 700e900 cm1
which belongs to ]CeH in
aromatic structures varied greatly. These results suggested that the
molecular structures of UL have been changed to varying degrees
after pyrolysis. Moreover, this reaction process was promoted
especially when DCLR was added into the system.
To further perform the semi-quantitative analysis of the mo-
lecular structures and oxygen-containing functional groups of the
coal, the infrared spectra of UL samples in the range of
4000e400 cm1
were curve-fitted by the professional software
(Origin Lab, Peak Fit v4). According to the chemical structure of the
coal [34,55], the infrared spectrum was divided into four regions:
(1) 3600e3000 cm1
: eOH and aromatic CeH bond stretching
vibration zone; (2) 3000e2800 cm1
: aliphatic hydro-carbon
stretching vibration zone; (3) 1800e1000 cm1
: carbon-oxygen
and carbon-carbon double bond stretching vibration zone; (4)
900e700 cm1
: out-of-plane deformation vibrations of aromatic
carbon-hydrogen bonds. The baseline correction, peak splitting,
and fitting analysis were performed for the spectra and the results
of each region in the infrared spectra are shown in Fig. 5. The results
showed that the curves were fitted well with the original ones, with
the relevant parameters, including height, width, position, and area
of the peaks, of each molecular structure and the functional group
being obtained. Thus, the changes of characteristic parameters
related to coal structure could be compared and analyzed via
calculating the representative peak areas of the corresponding
samples.
The macromolecular structures of lignite contained the nucleus
with condensed aromatic structure, abundant oxygen-containing
functional groups, and alkyl side chains on its periphery. Accord-
ing to the studies by Zhang et al. [58] and Ren et al. [20], the
oxygen-containing functional groups that affected the slurryability
(maximum solid concentration and fluidity) of lignite were mainly
hydroxyl and carboxyl groups because they were easier to be
combined with water molecules in the slurry via hydrogen bonds.
Therefore, the following investigation of this paper would mainly
focus on the analysis of the changes in hydroxyl and carboxyl
functional groups in UL.
It has been shown in previous research that the microwave
treatment could break the hydrogen bonds associated with hy-
droxyl groups in lignite, promoting the conversion of hydroxyl
groups into inert ether groups [19,34]. Hence, the ratio of the
characteristic peak area of the hydroxyl group to the ether group
(AOH=AO) is used to indicate the conversion of oxygen func-
tional groups in UL, as shown in Eq. (3).
AOH
AO
¼
I35503200
I13301000
(3)
The chemical reaction that results in the loss of content of hy-
droxyl groups is described by Eq. (4), [59].
Table 3
Parameters allocation of fitted peaks from lignite’s FTIR spectrum.
Peak# Center
(cm1
)
Assignments Area
(A)
1e4 3550
e3200
v: eOH in alcohol, water clusters, and phenol 0.9608
5 3100
e3000
v: CH in aromatic structure 0.0392
6 2955 as, v: CH3 in aliphatic structure 0.4666
7 2923 as, v: CH2 in aliphatic structure 1.0280
8 2899 v: CH in aliphatic structure 0.4570
9 2882 s, v: CH3 in aliphatic structure 0.0802
10 2855 s, v: CH2 in aliphatic structure 0.9589
11 1750 v: eC]O in aliphatic series 0.0125
12 1709 v: eC]O in carboxylic acids 0.0852
13 1660 v: highly conjugated eC]O 0.1264
14 1610 v: C]C in aromatic rings 0.1800
15 1561 v: C]C in aromatic rings; as, v: COO in
carboxylates
0.1727
16 1503 v: C]C in aromatic rings 0.0944
17 1445 as, d: CH3 in aliphatic structure 0.1231
18 1386 s, b: CH3 in aliphatic structure 0.1344
19 1333 v: ArdOdC 0.1034
20 1280 as, v: CeOeC in cyclic ethers 0.1147
21 1233 as, v: CeOeC in aromatic ethers 0.0946
22 1189 v: CeO in phenols, ethers 0.0829
23 1154 v: CeO in phenols, ethers 0.0738
24e27 1103
e1000
as, v: SieOeSi in quartz; v: SieOeC in
aluminosilicates
0.2898
28e33 900
e700
o, d: ]CeH in aromatic structures with
isolated aromatic hydrogens (1H), two
adjacent hydrogens per ring (2H), and five
adjacent hydrogens per ring (5H); o, b:
alkanes side rings (CH2)n 4.
1.8128
Note: v: stretching vibration; d: deformation vibration; s: symmetric; as: asym-
metric; b: bending vibration; o: out-of-plane.
(4)
6

As can be seen in Fig. 4, the area of peaks ranging from
1800 cm1
to 1550 cm1
in UL samples declined gradually, sug-
gesting the progressive loss of oxygen-containing functional groups
dominated by carboxyl groups. The decomposition of these
oxygen-containing functional groups could be described as the
following equation:
Ac¼o
Aar
¼
I17101700
I16601590
(5)
The potential chemical reactions that caused the decomposition
of oxygen-containing functional groups are presented in Eq. (6) and
Eq. (7), [34,60].
Fig. 5. Representative ﬁtting results for the infrared spectra of lignite.
(6)
(7)
7

Therefore, the moisture-holding capacity and re-absorption
characteristics of UL were significantly weakened due to the
decrease of the hydroxyl and carboxyl functional groups content,
which was verified by the data in Table 2.
In Fig. 6, the contents of active oxygen-containing functional
groups in UL samples are compared. The decrease in AOH= AO
and AC¼O=Aar ratios was more obvious for S10-0 than lignite and
these ratios further declined in UL samples upgraded with DCLR.
This result was consistent with the work done by Li et al. [34].
Microwaves could produce dielectric response within polar groups
rapidly, which broke the associative hydrogen bonds in the active
oxygen-containing functional groups. As a result, the hydroxyl
groups gradually transformed to stable ether bonds during the
microwave pyrolysis process, while the carboxyl groups were
decomposed into CO2. This data illustrated that the introduced
DCLR could accelerate the decomposition and the subsequent
conversion of active oxygen-containing functional groups in lignite.
However, the trend of the two ratios for UL (S10-16, S10-20) showed a
gradual increase with further increase of DCLR contents. Thus,
adding a certain amount of DCLR could facilitate the conversion
and/or decomposition of active oxygen-containing functional
groups into stable molecular structures under microwave
irradiation.
The contents of aliphatic and aromatic structures in UL could
intuitively reflect the changes of the coalification degree. The
length and branching degree aliphatic chains in coal were charac-
terized by the ratio of methylene to methyl (CH2=CH3), and the ratio
of aromatic hydrocarbons to aliphatic hydrocarbons (Aar= Aal)
which was usually expressed as the aromaticity and coal rank [19],
as shown in Eq. (8) and Eq. (10).
CH2
CH3
¼
I29232922
I29552953
(8)
where a lower ratio implied shorter aliphatic chains length and
higher branching degree. The reaction in Eq. (9) might happen
during this progress.
Aal
Aar
¼
I30002800
I838722
(10)
where a higher ratio denoted more aromaticity and higher coal
rank.
The changes in aliphatic and aromatic structures in UL samples
are illustrated in Fig. 7. The ratios of CH2=CH3 and Aar=Aal in S10-
0 dropped dramatically after 30 min of microwave treatment. This
was primarily because those short-chain hydrocarbons formed
with the decomposition of the methylene bridge bonds and alkyl
side chains were added to the unsaturated sites in the aromatic
structure [50]. Meanwhile, the latter ratio reflected the evolution of
the aromatic structure during the pyrolysis process. As the DCLR
contents increased, the Aar=Aal ratio of UL became lower and was
the lowest for S10-12, suggesting the UL was forming a more stable
and compact molecular structure through generating new aromatic
structure. Therefore, S10-12 had the highest coalification degree.
This result demonstrated that in microwave irradiation system an
appropriate addition of DCLR could enhance the decomposition of
aliphatic branches and alkyl side chains, as well as the enrichment
of the aromatic structures to obtain UL with higher maturity.
Fig. 6. Influence of DCLR contents on active oxygen-containing functional groups in
UL.
Fig. 7. Influence of DCLR contents on aliphatic and aromatic structures in UL.
(9)
8

3.3.3. Roman spectrum analysis of upgraded lignite
Compared with the high-rank coal, the skeleton structures of
carbon in lignite were mainly composed of aromatic ring structures
with different sizes which were connected by aliphatic chains and/
or oxygen-containing functional groups and/or alkyl side chains.
During the microwave treatment with the temperature from 25 C
to 700 C, the molecular structure of lignite changed greatly.
However, it was difficult to obtain a quantitative relationship be-
tween the Raman spectra and the structural parameters from the
two broad peaks in the original curve since lignite was a kind of
disordered carbon material. Therefore, the Roman spectra of UL in
the range of 1800e800 cm1
were curve-fitted by the Gauss
method in professional software Peak Fit v4. The results are shown
in Fig. 8 (b), (c), (d), (e), (f), (g) and (h). According to the results of
previous studies [49,56], the distribution of the 10 Gaussian bands
fitted by the original curve is exhibited in Fig. 8 (a). The six bands of
G, GR, VL, VR, D, and S represented the core skeleton structure of
lignite macromolecules while the four bands of GL, SL, SR, and R
were related to the peripheral side chains and oxygen-containing
functional groups. After pyrolysis, the area ratios of the G and D
bands became larger than those of lignite and S10-0, indicating that
the introduced DCLR in lignite microwave upgrading could pro-
mote the “graphitization” process. However, further analysis was
needed for the “graphitization” degree between ULs with different
additives.
As shown in Fig. 9, Raman parameters, including ID/IG, I(GR、VL、
VR)/ID and ID/IS, which represent the aromatic rings, the ratio of
small to large rings in the aromatic system, and the alkyl-aryl car-
bon respectively, are used in this work. As more and more macro-
cyclic aromatic structures were formed by small aromatic clusters
in dehydrogenation and polycondensation reactions, the ID/IG ratio
of UL increased from 0.42 to 0.46 while the I(GR、VL、VR)/ID value
reduced from 3.39 to 2.96. However, an intensely wide bandwidth
of D in the Raman spectrum of S10-0 indicated that various sizes of
aromatic rings were presented in UL but they were still far from
forming graphite crystals. This phenomenon was mainly because
the coalification degree of lignite was too low to graphitize since
Fig. 8. Roman spectra analysis of UL samples. (a) Summary of Raman spectral fitting band assignment. (b), (c), (d), (e), (f), (g), and (h) Roman spectra of UL samples with the
corresponding fitted results.
Fig. 9. The transformed ID/IG, I(GR、VL、VR)/ID and ID/IS values of UL samples.
9

the graphitization required longer time in the high-temperature
condition. Similarly, in the study of microwave heating process
the structure and dielectric properties of low-rank coals, Liu et al.
[61] found that carbon did not undergo a real graphitization
transformation, and there was just a process of “similar graphiti-
zation” occurring. When DCLR content further increased, the values
of ID/IG and I(GR、VL、VR)/ID reached 0.63 (the highest value) and 2.34
(the lowest value) for S10-12, respectively. This mainly resulted from
the auxiliary heating by DCLR. At the same time, it was also man-
ifested that the size and quantity of the aromatic ring system of UL
were increased, with more macrocyclic structures generated. Be-
sides, the rapid formation of alkyl-aryl CeC bonds in UL after the
decarboxylation reaction and/or the loss of other oxygen-
containing functional groups during the pyrolysis process caused
an obvious increasing trend in the ID/IS value (the highest point was
still reached near S10-12), and the result was consistent with the
trend in Fig. 6. Therefore, these findings suggested that the for-
mation of the aromatic ring structure that was more closely ar-
ranged in UL was enhanced by the effect of the addition of DCLR
during microwave-assisted pyrolysis.
3.3.4. Pore structure analysis of upgraded lignite
Since lignite had porous structure, the number and structures of
pores, especially mesopores (2e50 nm) and micropores (2 nm)
structures, had great influence on its slurryability and industrial
applications [39,62]. Fig. 10 (a) illustrates the BET surface area, pore
volume, and average pore diameter of lignite and UL samples. After
microwave pyrolysis, the porosity of S10-0 increased significantly.
Conversely, as the DCLR contents increased, the BET surface area
and pore volume of UL decreased rapidly, from 14.36 m2
*g1
and
0.022 m3
*g1
(S10-0) to 1.83 m2
*g1
and 0.011 m3
*g1
(S10-20)
respectively, while the average pore diameter increased from
7.94 nm to 12.80 nm. As a result, the pyrolysis of DCLR changed the
pore structure of UL. With the reduction of micropores and meso-
pores structures, as shown in Fig. 10 (b). Zhou et al. [39] believed
that the removal of moisture and volatiles caused the exposure of
the original pores, and the pressure difference between the porous
networks and the container environment could induce vast new
fractures and pores. However, the unreacted DCLR, which
possessed stronger adsorption capacity in the form of liquid phase
in the high-temperature environment, preferentially adhered to
the mesopores and micropores, resulting in the reduction of porous
structure of UL. This finding exhibited another difference from
other microwave absorbents. Therefore, DCLR, as a microwave
absorbent, was beneficial to reducing the unfavorable factors (the
contents of mesopores and micropores) that affected the slurry-
ability of UL.
Fig. 10. Influence of DCLR contents on the BET surface area, pore volume, and average
pore diameter pore structures of UL samples as well as the corresponding pore
diameter distribution.
Fig. 11. Relationship between apparent viscosity and solid concentration of LWSs
prepared from UL samples.
Fig. 12. Changes in the rheological behavior of LWSs prepared from UL samples at
25 C.
10

3.4. Properties of LWSs
3.4.1. Solid concentration and rheological behavior of LWSs
Fig. 11 shows the relationship between h and u of LWSs. With
the continuous increase of DCLR, the u increased from 41.73 wt%
(lignite) to 65.42 wt% (S10-16) and then decreased to 64.38 wt% (S10-
20), mainly attributed to the changes in coal quality and interfacial
characteristics of UL, such as chemical compositions, oxygen-
containing functional groups, pore structure. Compared with the
pyrolysis of pristine lignite, the increase in coal quality and the
reduction of hydrophilic oxygen-containing functional groups, as
well as the pore structure content of the UL samples upgraded with
DCLR have been improved, which reduced its re-absorption ca-
pacity and water-holding capacity and resulted in significant
improvement of the solid concentration. Additionally, when DCLR
addition content increased from 12 wt% to 20 wt%, the promotion
effect of lignite was gradually weakened, whereas the umax of LWS
prepared from S10-16 reached 65.42 wt%. As can be seen in Fig. 10,
the contents of micropores and mesopores in S10-16 with strong
adsorption properties for water and dispersant in the slurry were
obviously reduced compared with other samples. Therefore, the
solid concentration of LWS prepared from S10-16 could be increased
further and was the highest among the samples as well.
The rheological curves of each slurry with h of 1000 mPa‧s are
selected for comparison in Fig. 12, and the fitted parameters are
shown in Table 4. All LWSs were pseudoplastic fluids, with the
shear-thinning characteristic of which h decreased with the
increasing tyx. However, the pseudo-plasticity of the LWS with high
u became weaker. At the same time, the LWS prepared from UL
samples had better fluidity due to their higher n. The reason for this
was that the rheology of the slurry was primarily related to the
molecular space structure, hydrophilic oxygen-containing func-
tional groups, and the pore structure of lignite samples. Lignite
possessed macromolecular network structures composed of
several aromatic rings, abundant aliphatic branch chains, alkyl side
chains, and oxygen-containing functional groups, etc. The space
structure was complex, leading to the requirement of higher initial
shear stress to prepare LWS. Nevertheless, UL samples with rela-
tively simple structures after microwave pyrolysis were more hy-
drophobic and generated more water as flowing medium in the
LWS system. Thus, the pseudo-plasticity of LWSs was weaker,
especially for S10-16 samples that contained oily substances.
3.4.2. Static stability of LWSs
Fig. 13 is the static stability index (TSI) change of the LWSs
evaluated by Turbiscan LAB™ software within 7 days. According to
the calculation results, the stability of LWSs was gradually weak-
ened with the extension of the storage time, while the TSI values of
the S10-16 and S10-20 samples increased significantly. Moreover, the
static stability of LWSs was ranked from high to low as follows:
lignite S10-0 S10-4 S10-8 S10-12 S10-16 S10-20. However, there
was no obvious water separation and/or sediment in these samples
after 7 days, except for LWS prepared from S10-20, indicating that
residual DCLR was detrimental for the stability of the LWS.
4. Conclusion
This work is devoted to exploiting a new processing method by
adding DCLR with excellent dielectric properties for facilitating the
microwave upgrade of lignite and the effective utilization of DCLR,
which combines the upgrading of low-rank coal and the pyrolysis
of DCLR under microwave irradiation. This approach remedies the
shortcomings of the previous methods in terms of obtaining lignite
with better slurryability in an more economical and effective way
(e.g., higher heating rate and recovery rate). These results provide
an experimental reference with crucial scientific research, eco-
nomic value, and environmental protection properties, for
upgrading lignite and utilizing DCLR under microwave irradiation.
The key conclusions from this work are presented as follows:
1) In the case of the addition of 12 wt% ~16 wt% DCLR into the
system, the pyrolysis reaction of mixed materials was the most
sufficient under microwave treatment, among which the heat-
ing rate of UL was the highest.
2) Under the synergistic effect of microwave irradiation and DCLR,
the active oxygen-containing functional groups, alkyl side
chains and aliphatic structure in lignite were gradually
decomposed and/or converted into stable molecular structures.
Meanwhile, DCLR, especially with the addition of 12 wt%, facil-
itated the cyclization and aromatization of organic structure and
the ordering degree of the aromatic system in UL, leading to the
improvement of its coal quality and maturity.
3) The pyrolyzed DCLR dramatically reduced the existing and
newly formed micropores and mesopores structures in lignite
during microwave-assisted pyrolysis. Consequently, with the
DCLR contents increased, the BET surface area and the pore
volume of UL samples decreased rapidly.
4) The maximum solid concentration of LWS prepared from S10-16
was the highest, with the value up to 65.42 wt%, and showed
lower pseudo-plasticity and static stability.
Therefore, microwave-assisted pyrolysis with DCLR is an effec-
tive approach for improving slurryability of lignite via improving
the upgrading efficiency and modifying its physicochemical
Table 4
The solid concentration, rheology, and static stability parameters of LWS.
LWS u (wt %) s h (mPa$s1
) n R2
n
TSImax
Lignite 41.73 0.11 1067.3 0.655 0.9977 0.073
S10-0 60.55 0.09 1004.8 0.906 0.9999 0.161
S10-4 61.53 0.08 1007.9 0.958 0.9983 0.183
S10-8 62.20 0.11 998.6 0.970 0.9988 0.210
S10-12 64.10 0.10 996.9 0.985 0.9983 0.225
S10-16 65.42 0.08 1001.1 0.984 0.9996 0.297
S10-20 64.38 0.07 992.9 0.970 0.9984 0.383
Note:s refers to the standard deviation of u, and R2
n is the fitting degree of n.
Fig. 13. Changes in static stability index (TSI) of LWSs prepared from UL samples in 7
days.
11

properties. It is also an excellent way to utilize DCLR cleanly and
efficiently.
Credit author statement
Suqian Gu: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Data Curation, Writing - Original Draft.
Zhiqiang Xu: Conceptualization, Writing - Review Editing, Su-
pervision, Funding acquisition. Yangguang Ren: Conceptualization,
Validation, Writing - Review Editing. Yanan Tu: Project admin-
istration, Resources. Meijie Sun: Resources. Xiangyang Liu:
Investigation.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgment
This research was financially supported by the China National
Nature Science Foundation (Nos. 51974325). the Yue Qi Distin-
guished Scholar Project, China University of Mining Technology,
Beijing, and the Natural Science Foundation of Shandong Province
(ZR2020QE140).
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