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Improvement on slurry ability and combustion dynamics of low quality coals with ultra-high ash content
Journal Pre-proof Improvement on slurry ability and combustion dynamics of low quality coals with ultra-high ash content Shunxuan Hu, Junguo Li, Xin Yang, Yumeng Chen, Fenghai Li, Jianfei Wang, Changning Wu, Li Weng, Ke Liu
PII:
S0263-8762(20)30072-1
DOI:
https://doi.org/10.1016/j.cherd.2020.02.011
Reference:
CHERD 3998
To appear in:
Chemical Engineering Research and Design
Received Date:
22 November 2019
Revised Date:
2 February 2020
Accepted Date:
9 February 2020
Please cite this article as: Hu S, Li J, Yang X, Chen Y, Li F, Wang J, Wu C, Weng L, Liu K, Improvement on slurry ability and combustion dynamics of low quality coals with ultra-high ash content, Chemical Engineering Research and Design (2020), doi: https://doi.org/10.1016/j.cherd.2020.02.011
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Improvement on slurry ability and combustion dynamics of low quality coals with ultra-high ash content
Shunxuan Hu a, b, Junguo Li c, Xin Yang d, Yumeng Chen b, Fenghai Li e, Jianfei Wang f
, Changning Wu c,*, Li Weng b, c, Ke Liu b, c,*
a
School of Chemistry and Chemical Engineering, Harbin Institute of Technology,
b
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Harbin 150001, China
Department of Chemistry, Southern University of Science and Technology, Shenzhen
518055, China
Clean Energy Institute, Academy for Advanced Interdisciplinary Studies, Southern
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c
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University of Science and Technology, Shenzhen 518055, China.
Department of Engineering Science, University of Oxford, Oxford, OX2 0ES, UK
e
School of Chemistry and Chemical Engineering, Heze University, Heze, 274015,
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d
f
School of Chemistry and Chemical Engineering, Kunming University, Kunming,
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650214, China
Corresponding authors
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*
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China
Email: [email protected] (K. Liu); [email protected] (C. Wu). Address：Southern University of Science and Technology, 1088 Xueyuan Avenue, Shenzhen 518055, China.
Highlights
Low quality coal could prepare high concentration CWS after upgrading. Solid content increases by 6% using upgraded coals obtained from a novel flotation. Change in the form of water distribution determines the apparent viscosity of
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CWS.
Combustion characteristics of upgraded coals are different comparing to raw
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coals.
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Abstract
It is a significant subject that low quality coals could be used for high concentration
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slurry preparation because of the large reserve and low price. Nevertheless, due to the
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microstructure and surface properties of low quality coals, they are difficult to prepare qualified coal water slurry (CWS) directly. In this study, an improved flotation
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technique developed was employed to upgrade two low quality coals, then Jinda raw coal (JD-RC) and its upgraded coal (JD-UC), Jisuo raw coal (JS-RC) and its upgraded
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coal (JS-UC) were applied to CWS preparation and combustion dynamics analysis. Results showed that after two samples of low quality coals were upgraded, the maximum slurry concentration could be increased by 6%. It was attributed to the variation of free water content, which was mainly influenced by composition and pore structure of coal particles. Because of the remarkable decrement on specific surface
area and porosity after flotation, the ignition temperature and apparent activation energy of CWS prepared by JD-UC were increased comparing to JD-RC. However, the ignition temperature and apparent activation energy of CWS prepared by JS-UC were lower than JS-RC, which was resulted from the higher combustible content in JS-UC. The results of this study provided a method for the preparation of high concentration CWS using low quality coals with ultra-high ash content.
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Keywords: Low quality coals; Flotation; Coal water slurry; Slurry ability; Combustion performance
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Nomenclature
coal water slurry
JD-RC
Jinda raw coal
JD-UC
Jinda upgraded coal
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CWS
JS-UC
Jisuo upgraded coal scanning electron microscope
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SEM
Jisuo raw coal
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JS-RC
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SEM-EDS
scanning electron microscope-energy dispersive spectrometer
BET
Brunauer Emmett Teller
BJH
Barrett Joyner Halenda
FT-IR
fourier transform infrared spectroscopy
TG
thermogravimetry
differential thermogravimetry
α
conversion ratio, %
T
reaction temperature
A
pre-exponential factor, s-1
β
heating rate, K/min
E
apparent activation energy, KJ/mol
R
molar gas constant, 8.314 KJ/( mol·K)
n
reaction order
w0
sample weight before the reaction
wt
sample weight at the time of t
w∞
sample weight after the reaction
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DTG
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1 Introduction
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Coal will continue being an important energy source in the 21st century due to a higher reserve/production (R/P) ratio than petroleum and natural gas especially in
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America, China, Russia, Australia, Canada and India (BP, 2018; Wang et al., 2018a). Technologies for the efficient utilization of coal resource have attracted much
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attention and played a vital role in the current energy policy (Jacobson, 2009; Li et al., 2018a). In terms of energy saving and emission reduction, coal water slurry (CWS) technology allows coal to be utilized in a cleaner and more efficient way because of its convenient transportation and higher combustion efficiency (Huang et al., 2013; Lee et al., 2011; Xu et al., 2009; Zhang et al., 2017; Zhang et al., 2016). In addition,
as one of the ideal alternatives to fuel oil, the qualified CWS products should possess a high slurry concentration and a low viscosity i.e., the solid content in suspension with an apparent viscosity of 1000 mPa s at a shear rate of 100 s-1 is regarded as the maximum slurry concentration for atomization combustion (Li et al., 2018b; Valiullin and Strizhak, 2017; Wang et al., 2019). Considering the high price and unstable supply of bituminous coals, it is attractive for researchers and industrial users to
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prepare CWS by using low quality coals. Nevertheless, the high mineral content of
low quality coals make it difficult to prepare a high calorific CWS directly (Bae et al.,
2014; Chai et al., 2018; Gong et al., 2013; Mukherjee et al., 2015). Therefore, it is
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necessary to upgrade low quality coals before CWS preparation (Gui et al., 2015;
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Vapur et al., 2010).
Flotation, which is widely used in coal beneficiation for low quality coals in
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industry, is principally based on the differences in the surface hydrophobicity between
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the combustible components and minerals (Bournival et al., 2019; Hacifazlioglu and Toroglu, 2011). Recently, increasing attention has been paid on improving flotation
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processes for low quality coals. Vapur et al. reported that the coal ash content was decreased from 33.4% to 17.9% with a combustible recovery of 95%, as the coal
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flotation parameters optimized for particles size, pulp density, the dosage of reagents and the feed rate of wash water (Vapur et al., 2010). Dickinson et al. proposed a two-stage fast flotation to beneficiate a low quality coal with ash content of 42%, achieving a fully deslimed product of 7.8% ash and combustible recovery up to 80% (Jiang et al., 2016). Gui et al. also developed a two-stage process for successfully
intensifying the flotation process of low-rank coal, the results indicated that ash content of clean coal products was around 10% and the recovery of combustible matter was up to 90% (Gui et al., 2017; Xing et al., 2015). Therefore, flotation could efficiently change chemical component and surface properties of coal, which have great influences on the preparation and combustion of CWS. Both B. K. Sahoo et al. and Sang Jun Yoon et al. found that the apparent viscosity of CWS was increased as a
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function of mineral matter, which meant that the inherent minerals in coal particles
made it unmanageable to prepare CWS with large solid loading (Sahoo et al., 2010; Yoon et al., 2010). Zeki Aktas et al. observed that the demineralized coal was more
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suitable for high concentration CWS preparation, and the low viscosity slurries could
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be obtained by enhancing the electrostatic repulsion forces of coal particles (Aktaş and T., 2000). Xing Chen et al. provided a novel method to enhance the surface
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hydrophobicity of coal particles by using the second fluid, and the maximum slurry
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concentration of CWS was increased from 53.4 wt.% to 56.3 wt.% (Chen et al., 2019). The chemical component and surface properties of coal particles were modified by
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flotation techniques, but there is limited study on characterization and combustion performance of CWS prepared by upgraded coals obtained from flotation processes.
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In the previous study, the developments and limitations of low quality coal
beneficiation techniques had be reviewed and the feasibility of upgraded coal for high concentration CWS preparation had been investigated (Hu et al., 2019; Wang et al., 2018a). Therefore, this paper aimed to reveal the difference of slurry preparation and combustion performance caused by coal quality change after flotation treatment, and
the researches were focused on (i) the microstructure and surface properties variation of upgraded coal after floatation and (ii) the maximum slurry concentration and combustion behavior of the CWS prepared by upgraded coals. Two kinds of low-quality coals (JD-RC and JS-RC) were selected as the raw materials in coal flotation to produce upgraded coals (JD-UC and JS-UC) for CWS preparation, and the measurement of SEM-EDS, BET, FTIR, TG, zeta potential, contact angle were
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applied for characterization and analysis of coal properties. The findings of this research might be of great potential to provide a direct solution on the efficient
utilization of low quality coals and address the raw materials supply for CWS
Experimental
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preparation.
2.1 Coal preparation
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In order to achieve the maximum liberation degree of combustible components and
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minerals, the comminution of low quality coals were implemented in a horizontal high speed stirred mill with a grinding volume of 20 L and very high power intensities up
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to 300 kW/m3. This system could be operated to obtain upgraded coal products, so called clean solid fuel (CSF) with the ash content of 6-10%, with high combustible
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recovery of 80-85%.
The upgraded coals had been collected as follows: firstly, low quality coals were
grounded to be within a narrow size distribution (in the range of 1-100 µm); secondly, C11-C17 alkanes and micro bubbles were adsorbed by the surface of those fine coal particles at a shear rate of 10,000 rpm in a pre-mineralization equipment, in order to
increase the separation efficiency of the organic-rich combustible particles and inorganic-rich non-combustible mineral particles; finally, the slurry with 10% solid loading was fed into a flotation column, which was filled with water, and the upgraded coal with low ash content was collected as the clean products at the top of system. In this study, two low quality coals (JD-RC and JS-RC) and their upgraded coals
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(JD-UC and JS-UC) obtained by flotation process were used as the raw materials for comparisons. The particle size had been measured by a laser particle size analyzer (BECKMAN LS-13-320), as shown in Fig. 1.
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The results of proximate analysis and ultimate analysis on these coal samples were
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listed in Table 1. The ash content was decreased from 57.71% in JD-RC to 9.20% in JD-UC, and the removal ratio was 84.06%. In addition, the ash was decreased from
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31.40% in JS-RC to 6.98% in JS-UC, and the removal ratio was 77.77%. Meanwhile,
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the sulfur content in the upgrade coals had been reduced by 20-30%. 2.2 Characterization of coal after flotation
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2.2.1 SEM-EDS
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MERLIN SEM (ZEISS, Germany) equipped with an energy dispersive spectroscope (EDS) was applied to determine the surface morphology and elemental composition of intruded ultrafine coal particles. During the characterization, the surface of coal samples had been sputter-coated with a gold film before SEM analysis. The SEM-EDS measurement procedure was as follows: firstly, the coal samples were subsequently placed in the sample bin of the SEM-EDS system after the
instrument and cooling water were activated; secondly, the SEM was configured to provide an electron beam at accelerating voltages up to 20 kV; thirdly, the opportune scanning speed, magnification and brightness values were identified to ensure image quality; finally, the elemental spectra were analyzed by the associate EDS software.
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2.2.2 BET The specific surface area and pore size distribution measurement of coal samples were measured by an automatic N2 adsorption apparatus (ASAP2020, Micromeritics,
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America). The experimental procedure was as follows: first, 0.15-0.2 g of dry-basis
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coal sample was put into the sample tube and then degassed at 180 °C for 6 h; then an N2 adsorption/desorption detection was performed by using the Brunauer Emmett
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Teller (BET) and Barrett Joyner Halenda (BJH) method to calculate the specific
2.2.3 FT-IR
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surface area and porosity data.
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The surface functional groups of raw intruded coals were investigated by Fourier-transform infrared spectroscopy (FTIR, Nicolet iS10, Thermo Scientific,
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USA). The homogeneous mixture, including 1 mg of pulverized coal and 100 mg of KBr, was placed into a dye with a pressure of 20 MPa to prepare the pellet for FTIR measurement. The spectrum of coal samples was described from 4000 to 400 cm-1 at a resolution of 4 cm-1 and redacted from 32 scans with excluding background values. 2.2.4 Contact angle
Hydrophobicity of coal particles were analyzed by a contact angle analyzer with an accuracy of 0.1° on the sessile drop method. The pulverized coal with a mass of approximately 0.4 g was pressed under a pressure larger than 30 MPa by a tablet machine for 15 min to form a pellet. The images after the drop contacted coal pellet were recorded by a high-speed camera at different times to ascertain contact angles and droplet spreading ability on coal surface.
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2.2.5 Zeta Potential
The relationship between zeta potential of fine coal particles and pH value was determined by ZetaPALS analyzer (Brookhaven, USA). Dilute CWSs (0.2g
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pulverized coal in 100ml deionized water without any dispersant) were conducted by
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the ultrasonic treatment for 30min. In order to promote the settlement of coarse particles, the suspension was centrifuged at 1000 r/min for 5 min. Afterwards, the
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upper suspensions were used to perform zeta potential measurement, and the pH value
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of suspensions was adjusted from 3 to 11 with an appropriate amount of HCl or NaOH addition. The pH values were measured by a PHS-2 pH meter. Each sample
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was tested 5 times and the average value of the zeta potential was calculated. 2.3 CWS Characterization
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2.3.1 CWS preparation and viscosity measurement CWS was prepared as follows: first, a specified quantity of deionized water was
added into a 200 mL beaker, and the corresponding amount of coal powder was slowly poured into the beaker with continuously stirring by a high speed disperser; second, the mixing was lasted for 5 min at 11200 rpm to ensure the uniformity of
CWS. The apparent viscosity of CWS was measured by using a rotary viscometer (NXS-4C, Chengdu Instrument factory, China) at 25±1 ℃. The measurement of apparent viscosity was repeated for six times at a shear rate of 100 s-1. 2.3.2 Free water content of CWS measurement The free water content of CWS prepared by different coals were characterized by a thermogravimetric analyzer (TGA2, METTLER TOLEDO, Switzerland). CWSs with
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the solid concentration of 50 wt.% were used for free water content measurement. Firstly, the CWS sample was hold for 5 min at 30 ℃ in TG; and then it was heated
from 30 to 140 ℃ under N2 atmosphere at a heating rate of 1 ℃/min. The extreme
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point of moisture drying rate during the process was determined as the distinction
2.4.1 TG and DTG
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2.4 Combustion dynamics of CWS
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between free and bound water (Zhou et al., 2008).
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The combustion kinetics experiments of CWS with 50 wt.% solid loading were characterized by the same thermogravimetric analyzer. The combustion curves of
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slurry samples prepared by different coals were investigated at the heating rate of 10℃/min, 20℃/min, 30℃/min, 40℃/min and 50℃/min, while the air flow rate was 60
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ml/min and the weight of CWS sample was maintained around 15 mg for all measurements. 2.4.2 Identification of ignition temperature The ignition temperature reflects the difficulty of fuel to initiate the combustion. The ignition temperature of coal samples was determined by TG-DTG method (Cheng
et al., 2008). 2.4.3 Apparent activation energy calculation The activation energy E, as one of primary dynamics parameters, described the combustion performance in essence. According to the law of mass action and Arrhenius equation (Ullah et al., 2018), the non-isothermal TGA dates could be fitted by the equation as follows: (1)
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d A RTE e ( 1 n) dT
here, α, T, A, β denotes the conversion ratio, the reaction temperature (K), the
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pre-exponential factor (s-1) and the heating rate (K/min), respectively. While E, R, n represents the apparent activation energy (kJ/mol), the molar gas constant (8.314
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kJ/(mol·K)) and the reaction order, respectively.
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The conversion rate α of coal combustion reaction could be calculated as follows：
w0 wt w0 w
(2)
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=
where, w0, wt, w∞ refer to sample weight before the reaction, at the time of t and
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after the reaction, respectively.
The reaction order n of coal combustion was 1, so formula (1) could be rewritten as
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follows:
l n (1 ln( T2
) E ) RT
AR ln E
A linear regression was fit to the date on “ ln(
(3)
ln(1 ) ) ” versus “-1/T”, then the T2
apparent activation energy E (KJ/mol) of coal samples at different heating rates could be obtained.
3 Results and discussion 3.1 The microstructure of coal 3.1.1 SEM-EDS analysis Fig. 2 (a-d) showed the surface morphology and element composition of different coal particles by SEM-EDS. As can be seen from Fig. 2 (a-2) and (b-2), the low quality coals of JD-RC and JS-RC showed the abundant pore structures and uneven
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surfaces. As illustrated in Fig. 2 (c-2) and (d-2), the surfaces of upgraded coal particles (JD-UC and JS-UC) were smoother and the micro-pores of them were smaller after flotation treatment. Because the content of ash was reduced, the
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attachment of ash on coal particles was decreased. It led to the relatively smooth
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surfaces of coal particles (Xia et al., 2018). Due to the lower ash content of JD-UC and JS-UC, their surface was relatively smooth. In addition, the surfactant in flotation
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process might promote the agglomeration between coal particles, and it resulted in
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smaller micro-pores (Forsmo et al., 2008).
As shown in Fig. 2 (a-3) and (c-3), the C content on coal surface was increased
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from 30.48% in JD-RC to 79.62% in JD-UC, while the Al and Si were decreased from 7.78% and 11.85% in JD-RC to 1.08% and 1.32% in JD-UC, respectively.
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Correspondingly, the O content was decreased from 44.66% to 17.92%. Also, it was noted that the Fe and Ca were disappeared on the coal surface of JD-UC. From Fig. 2 (b-3) and (d-3), similar variances in the elemental composition were observed and the final elemental composition in JS-UC was in a similar level to JD-UC. The above results showed that the content of Si, Al, Fe and Ca on the upgraded coal
surface were decreased or disappeared, which was mainly due to the substantial reduction in the mineral content. According to SEM-EDS of the selected microregion in Fig. 2 and the ultimate analysis in Table 1, the C content was increased but the O content was decreased after flotation, which implied that the coal quality (related to the C/O ratio) was improved. Therefore, the low quality coal could be upgraded to
3.1.2 Specific surface area and porosity analysis by BET
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clean solid fuel with a higher calorific value by the flotation process.
The pore structure characteristics and specific surface area of coal surface considerably influence the subsequent applications including CWS preparation and
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combustion (Cheng et al., 2008; Fei et al., 2017). As shown in Fig. 3, the adsorption
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and desorption lines of the four coal samples tend to coincide in the lower relative pressure range, which indicated that the pores in the small aperture range were mostly
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closed impermeable pores at one end. Meanwhile, the curves were raised slowly in the
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lower range of relative pressure, which suggested that the multilayer adsorption had occurred (Wang X, 2008). The adsorption curves were increased sharply at the
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relatively high pressure and the adsorption capacity did not reach saturation although the saturation vapor pressure was finally reached. It predicted that the capillary
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condensation and multilayer adsorption took place together (Fuller, 2007). The desorption curves of all samples showed that it was decreased sharply around relative pressure (P/P0) of 0.5, which was radically striated by the abundant inkpot holes in coal particles (Jin et al., 2012). When the relative pressure went down to 0.5, the sudden release of gas in inkpot holes led to an inflection point.
It could be seen from Fig. 3 (a-1) and (b-1) that the regression trend of adsorption and desorption was similar for raw coals and upgraded coals, which meant that the type of pore in coal particle did not change by the flotation process. However, the adsorption capacity of low quality coals (JD-RC and JS-RC) were greater than that of upgraded coal (JD-UC and JS-UC) at the same pressure. It suggested that the particle porosity was reduced by the flotation process.
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BJH method was used to determine pore volume distribution as a function of pore diameter (0.1-600 nm). From Fig. 3 (a-2) and (b-2), the pore volume of low quality
coals was larger than that of upgraded coals at the same pore diameter, which
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demonstrated that the pore volume of coal samples was decreased by flotation process,
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and the result was in line with conclusion drawn from Fig. 3 (a-1) and (b-1). The BET surface area of JD-RC, JD-UC, JS-RC and JS-UC was 14.5 m2/g, 3.8
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m2/g, 5.4 m2/g and 3.4 m2/g, respectively, and the pore volume of JD-RC, JD-UC,
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JS-RC and JS-UC was 3.56×10-2 cm3/g, 9.85×10-3 cm3/g, 1.52×10-2 cm3/g and 1.02×10-2 cm3/g, respectively. Compared with low quality coals (JD-RC and JS-RC),
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the specific surface area and porosity of upgraded coals (JD-UC and JS-UC) were obviously reduced. The main reason might be the removal of mineral components in
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coal particles, so the coal surface became smoother (it could also be explained from Fig. 2) and the specific surface area descended (Xia et al., 2018). Moreover, the addition of the collector in the flotation process could promote the agglomeration of fine particles during flotation process, mainly resulted in a decrease of porosity in coals.
3.2 Surface properties of coal 3.2.1 Surface functional group detection by FT-IR Fig. 4 showed the infrared spectra of low quality coals (JD-RC and JS-RC) and their upgraded coals (JD-UC and JS-UC). The relationship between the absorbance and functional groups was identified (Mengüg, 1993; Wang et al., 2018b; Xia et al., 2017; Xia et al., 2019). The transmittance band around 3695 and 3619 cm-1 could be
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attributed to hydroxyl (-OH) with stretching vibration in kaolinite. The peak around
913 cm-1 could be corresponded to the bending vibration of Al-OH. While symmetric stretching vibration of Si-O groups in quartz were absorbed near 797 cm-1 and the
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band around 695 cm-1 was due to the stronger stretching vibration of Si-O group. The
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transmittance band around 537 cm-1 was attributed to the bending vibration of Al-O-Si
vibration of Si-O bond.
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group. The bands observed around 470 cm-1 and 430 cm-1 were assigned to bending
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Combined with the results in Table 1, all of above groups were more pronounced in the low quality coals due to the higher mineral content. It could be seen from the
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infrared spectrum that, characteristic absorption peaks of characteristic absorption peak in kaolin and quartz were either decreased or disappeared in the upgraded coals.
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3.2.2 Contact Angle
The spreading ability and contact angle of water droplets on different coal surfaces
could be seen from Fig. 5. It could be found that the contact angles of coal samples i.e., JD-RC and JS-RC, were significantly decreased as a function of contact time due to the strong surface hydrophilicity. However, the values of contact angle of JD-UC
and JS-UC were increased. Meanwhile, the contact angle of JD-UC and JS-UC were relatively stable with the contact time, which indicated that the surface hydrophobicity of coals had been significantly enhanced through the flotation process. The hydrophobicity of coal surface was subject to composition and surface properties (Rios et al., 2006; Zhou et al., 2018). Abundant mineral content in JD-RC and JS-RC might indeed be responsible for the receding contact angles observed in
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Fig. 5. It appeared that more water molecules tend to interacting with mineral due to
its more polar hydrophilic groups (Chau, 2009). The contact angles of upgraded coals
were larger than the raw coals and they exhibited relatively small changes as function
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of contact time. The main reason for the difference was attributed to the removal of
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most mineral components and surface modification of collector by the flotation
3.2.3 Zeta Potential
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treatment (Moncayo-Riascos and Hoyos, 2017).
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Zeta potential of different coal samples has a substantial effect on coal processing technology and applications. The zeta potential values on different types of coal as a
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function of pH were described in Fig. 6. All coal samples were proven to have the negative charge in the measured pH range (3 to 11). The zeta potential values of all
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coal samples were decreased with the increasing in pH value. The polar groups on coal surface, such as -COOH and -OH, might get protonated or ionized as the pH value variation in solution (Das et al., 2008). Positive charge, which was generated by the protonated carboxyl group at the acidic pH ( RCOOH H RCOOH 2 ), would result in the decrement of the absolute value of zeta potential. In addition, the negative
charge, which was caused by dissociated carboxyl group at alkaline pH ( RCOOH OH - RCOO- H 2O ), could promote the increment of the absolute value of zeta potential. These charges accounted for its zeta potential when electric field was applied. More importantly, the zeta potential measurements showed that the absolute value of upgraded coals (JD-UC and JS-UC) were more negatively charged than that of raw
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coals (JD-RC and JS-RC) at an acidic range of pH. Owing to the high price metal ions
(Ca2+, Al3+, Fe3+) precipitated from the mineral content of raw coals, the negative charge on the surface of coal particles had been neutralized at acidic conditions
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(Yukselen-Aksoy and Kaya, 2003). For CWS preparation, upgraded coals would be
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more dispersed than raw coals in the slurry at acidic environment because of higher electrostatic repulsion, which implied that upgraded coals would be better raw
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3.3 Slurry ability
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materials for CWS preparation.
3.3.1 The maximum concentration of CWS prepared by different coals
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The apparent viscosity of coal water slurries prepared by JD-RC, JS-RC, JD-UC and JS-UC were measured at a shear rate of 100 s-1 by varying coal loading in the
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range of 50-66% without any additives. The apparent viscosity of those four slurries with different solid loadings was shown in Fig. 7, and it was increased with solid content for all slurries. As depicted in Fig. 7, the solid concentration at an actually acceptable apparent viscosity of (1000±100) mPa·s were approximately 54% for JD-RC and 58% for
JS-RC. It was far away from the industrial application considering the high ash content in low quality coals and low coal loadings for CWS preparation by them. However, the maximum solid concentrations of CWS prepared by JD-UC and JS-UC are 60% and 64%, respectively. It suggested that the slurry ability of CWS prepared by upgraded coals were much better than that by raw coals. In general, the lower apparent viscosity of CWS could be attributed to more free
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water around coal particles. Firstly, the strong interaction of hydrophilic ash might
resulted in more water absorbed by minerals to form gel in coal particles (Das et al., 2009). Meanwhile, more water might be enclosed inside the coal particle and bounded
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by surface oxygen-containing functional groups on coal surface, the decrease of free
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water content led to a significant increase in slurry viscosity, which could be the inherent mechanisms underlying the effect on coal slurry performance (Das et al.,
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2009). Thus, the distribution of free water and bound water on slurries with same coal
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loading were investigated as follows.
3.3.2 Free water content of CWS prepared by different coals
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It is well-known that there are two dominant types of water around coal particles in CWS: bound water and free water. The bound water, which was absorbed by the
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hydrophilic functional groups of particle surface or filled in inner hole of particle, generally considered to be a solid or quasi-solid phase. Due to the strong interaction between bound water and particles, more heat energy is needed for their evaporation (Lu et al., 2018; Zhou et al., 2008). Many studies had reported that the free water, filled in the void gap between coal particles, was one of primary factors in dictating
the lubricity among particles in suspension (Mukherjee and Pisupati, 2016). The enough free water in CWS suspension was critical on contributing to the viscosity reduction and fluidity of CWS with suitable coal loadings. As observed from Fig. 8, the drying-rate curves of CWS prepared by different coals were obtained by TGA results, and the corresponding equipoise water content of the inflection point in Fig. 8 was the free water content in CWS. The free water contents
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of slurries with 50% solid concentration were 70.48% for JD-RC, 76.62% for JS-RC,
79.01% for JD-UC and 81.19% for JS-UC. The JD-RC slurry had the lowest free
water content while JS-UC slurry had the highest free water content. Fig. 8 indicated
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that the free water contents of CWS prepared by upgraded coals increased obviously,
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comparing with that of CWS prepared by low quality coals. The ratio of free water to total water demonstrated the water trapping capacity of the carbonaceous solid and the
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apparent viscosity of slurries was influenced by the availability of free water directly.
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Therefore, the slurry performance of upgraded coals was better than that of raw coals (as shown in Fig. 7).
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SEM-EDS and BET in section 3.1 indicated that the Al, Si content and porosity of both upgraded coals were decreased. The measurement of FT-IR and contact angles in
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section 3.2.1 and 3.2.2 showed the removal of mineral species and improvement of particle surface after flotation Zeta potential of coal particles suggested that stronger electrostatic repulsion of upgraded coals at an acidic range of pH. The above results demonstrated that the free water content of upgraded coals was higher than that of low quality coals at same solid content. In another word, upgraded coals should be more
suitable for preparation of CWS with high coal loading. 3.4 Combustion Dynamics The process for CWS combustion includes evaporation of water, volatile release, ignition, char combustion and burn out (Cheng et al., 2008). The combustion properties of different CWS samples at different heating rates were identified by the TG and DTG curves, as illustrated in Fig. 9.
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According to the data analysis from Fig. 9, the ignition temperature of all coal
samples at different heating rates could be obtained, as shown in Fig. 10. The ignition temperature of all samples was increased with the rising of heating rate. It was
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attributed to the limited oxygen concentration around coal samples which might
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constrain coal combustion reaction although ambient temperature had risen rapidly during the process of rapid heating; for another reason the higher the heating rate, the
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more volatiles released per unit time, but the volatiles released from coal particles
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seriously hinder the thermal energy transportation from heating gas to inner particle (Shuang et al., 2010).
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As shown in Fig. 10, with a heating rate of 10, 20, 30, 40, 50 ℃/min, the ignition temperature of JD-RC was 420.40, 434.85, 438.56, 440.80 and 445.92 ℃, but the
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ignition temperature of JD-UC was 422.10, 437.840, 440.89, 443.59 and 449.72 ℃. The ignition temperature of JD-RC was always lower than that of JD-UC coal. The main reason might be the larger specific surface area and richer pore structure (as described in section 3.1.2) of JD-RC, which was very helpful for oxygen diffusion inside coal particles (Cheng et al., 2016; Yuan-gang et al., 2010). As depicted in Fig.
10, the ignition temperature of JS-RC was 435.05, 448.36, 453.05, 454.08 and 456.40 ℃ at different heating rate, while the ignition temperature of JS-UC was 433.02, 440.42, 444.00, 448.27 and 452.49 ℃. It was found that the ignition temperature of JS-UC was lower than that of JS-RC at the same heating rate, which was due to the higher volatile and fixed carbon content of JS-UC, as shown in Table 1. More volatiles in JS-UC could contribute to ignition, meanwhile the higher fixed
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carbon content could provide more carbon active sites to react with oxygen molecules (Katalambula et al., 1997). It should be noted that there was less difference on the
specific surface area and porosity of JS-RC and JS-UC (comparing with JD-RC and
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JD-UC), which was demonstrated in section 3.1.2.
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As shown in Fig. 11, the apparent activation energy of JD-RC, JD-UC, JS-RC and JS-UC were decreased gradually with the increasing of heating rate. This might be
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that the higher heating rate enhanced the whole reaction process. Meanwhile, the
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apparent activation energy of JD-UC coal was higher than JD-RC, which was mainly attributed to larger specific surface area and richer pore structure of JD-RC as
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depicted in section 3.1.2. However, the apparent activation energy of JS-UC coal was lower than JS-RC, the main reason might be that the content of combustible body in
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JS-UC was much higher than that in raw coal from Table 1, so the interface area between the hydrocarbon in coal particles and the diffused O2 was enlarged, resulting in an enhancement in combustion reaction (Wang et al., 2011). 4 Conclusion In this study, the slurry ability and combustion dynamics of two low quality coals
and their upgraded coals by flotation had been carefully investigated based on systematical characterizations. SEM-EDS showed that the surfaces of both upgraded coals were smoother and the Al, Si content on the surface were decreased significantly. BET determined that the specific surface area and porosity of upgraded coals went down. Infrared spectrum showed that the characteristic absorption peaks of kaolinite and quartz on the upgraded coal surface were obviously weakened or disappeared.
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The hydrophobicity of particle surface was significantly increased after flotation according to contact angle measurement. Zeta potential tests showed electrostatic
repulsion between upgraded coal particles was greater than that of raw coals at pH of
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7. Results of viscosity measurement showed that the slurry maximum concentration of
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CWS prepared by both upgraded coals were approximately 6% more than that by low quality coals. The main reason was that the content of free water around coal particles
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was changed. The ignition temperature and apparent activation energy of CWS
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prepared by JD-UC was greater comparing to JD-RC, while that by JS-UC was lower comparing to JS-RC.
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Funding sources
This research was supported by Guangdong Innovative and Entrepreneurial Research
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Team Program (No. 2016ZT06N532) with Shenzhen Government Related Supporting Fund (No. KYTDPT20181011104002), Shenzhen Clean Energy Research Institute Project (No. CERI-KY-2019-003). The authors would also like to thank the financial supports from Shenzhen High-Level Professional Program (No. 20160802681J) and a Low Quality Coal Program initiated by Development and Reform Commission of
Shenzhen Municipality.
Declaration of interests 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.
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Figure captions
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Fig. 1. Particle size distribution of raw coals and upgraded coals.
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Fig. 2. Surface morphology and element content analysis of different coal particles
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by SEM-EDS: (a) JD-RC, (b) JS-RC, (c) JD-UC, and (d) JS-UC.
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Fig. 3. Adsorption capacity and pore volume distribution curve of different coal: (a)
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JD-RC and JD-UC, (b) JS-RC and JS-UC.
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Fig. 4. Surface functional groups of different coals.
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Fig. 5. Water contact angles of JD-RC, JD-UC, JS-RC and JS-UC coal samples.
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Fig. 6. Zeta potential of raw coals and upgraded coals as a function of pH value.
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Fig. 7. Effect of solid content on apparent viscosity of CWS prepared by different
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coal.
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Fig. 8. Free water to total water ratio of 50% CWS prepared by raw coals and upgraded coals.
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Fig. 9. TG and DTG curves of JD-RC, JD-UC, JS-RC and JS-UC with the heating
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rate of 10, 20, 30. 40, 50 ℃/min.
Fig. 10. Ignition temperature of raw coals and upgraded coals with different heating rates.
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Fig. 11. Apparent activation energy of different coals at different heating rates.
Table
Table 1. Proximate and ultimate analysis (wt.%) of coal samples. Proximate analysis
Coal
Ultimate analysis(daf)
sample
Mar
JD-RC
1.93 57.71 19.18 23.11
70.64 4.73 23.68 0.95 2.57
JD-UC
1.87
88.47 5.50
JS-RC
1.26 31.40 30.06 38.54
83.50 5.46 10.03 1.02 3.80
JS-UC
1.27
87.93 5.79
Ad
Vd
FCd
9.20 34.19 56.61 6.98 39.02 54.00
C
H
O*
N
St
4.79 1.24 1.87 5.22 1.06 2.99
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Note: ar, as received base; d, dry base; daf, dry and ash free base; *, by difference
PII:
S0263-8762(20)30072-1
DOI:
https://doi.org/10.1016/j.cherd.2020.02.011
Reference:
CHERD 3998
To appear in:
Chemical Engineering Research and Design
Received Date:
22 November 2019
Revised Date:
2 February 2020
Accepted Date:
9 February 2020
Please cite this article as: Hu S, Li J, Yang X, Chen Y, Li F, Wang J, Wu C, Weng L, Liu K, Improvement on slurry ability and combustion dynamics of low quality coals with ultra-high ash content, Chemical Engineering Research and Design (2020), doi: https://doi.org/10.1016/j.cherd.2020.02.011
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Improvement on slurry ability and combustion dynamics of low quality coals with ultra-high ash content
Shunxuan Hu a, b, Junguo Li c, Xin Yang d, Yumeng Chen b, Fenghai Li e, Jianfei Wang f
, Changning Wu c,*, Li Weng b, c, Ke Liu b, c,*
a
School of Chemistry and Chemical Engineering, Harbin Institute of Technology,
b
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Harbin 150001, China
Department of Chemistry, Southern University of Science and Technology, Shenzhen
518055, China
Clean Energy Institute, Academy for Advanced Interdisciplinary Studies, Southern
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c
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University of Science and Technology, Shenzhen 518055, China.
Department of Engineering Science, University of Oxford, Oxford, OX2 0ES, UK
e
School of Chemistry and Chemical Engineering, Heze University, Heze, 274015,
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d
f
School of Chemistry and Chemical Engineering, Kunming University, Kunming,
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650214, China
Corresponding authors
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*
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China
Email: [email protected] (K. Liu); [email protected] (C. Wu). Address：Southern University of Science and Technology, 1088 Xueyuan Avenue, Shenzhen 518055, China.
Highlights
Low quality coal could prepare high concentration CWS after upgrading. Solid content increases by 6% using upgraded coals obtained from a novel flotation. Change in the form of water distribution determines the apparent viscosity of
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CWS.
Combustion characteristics of upgraded coals are different comparing to raw
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coals.
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Abstract
It is a significant subject that low quality coals could be used for high concentration
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slurry preparation because of the large reserve and low price. Nevertheless, due to the
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microstructure and surface properties of low quality coals, they are difficult to prepare qualified coal water slurry (CWS) directly. In this study, an improved flotation
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technique developed was employed to upgrade two low quality coals, then Jinda raw coal (JD-RC) and its upgraded coal (JD-UC), Jisuo raw coal (JS-RC) and its upgraded
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coal (JS-UC) were applied to CWS preparation and combustion dynamics analysis. Results showed that after two samples of low quality coals were upgraded, the maximum slurry concentration could be increased by 6%. It was attributed to the variation of free water content, which was mainly influenced by composition and pore structure of coal particles. Because of the remarkable decrement on specific surface
area and porosity after flotation, the ignition temperature and apparent activation energy of CWS prepared by JD-UC were increased comparing to JD-RC. However, the ignition temperature and apparent activation energy of CWS prepared by JS-UC were lower than JS-RC, which was resulted from the higher combustible content in JS-UC. The results of this study provided a method for the preparation of high concentration CWS using low quality coals with ultra-high ash content.
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Keywords: Low quality coals; Flotation; Coal water slurry; Slurry ability; Combustion performance
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Nomenclature
coal water slurry
JD-RC
Jinda raw coal
JD-UC
Jinda upgraded coal
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CWS
JS-UC
Jisuo upgraded coal scanning electron microscope
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SEM
Jisuo raw coal
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JS-RC
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SEM-EDS
scanning electron microscope-energy dispersive spectrometer
BET
Brunauer Emmett Teller
BJH
Barrett Joyner Halenda
FT-IR
fourier transform infrared spectroscopy
TG
thermogravimetry
differential thermogravimetry
α
conversion ratio, %
T
reaction temperature
A
pre-exponential factor, s-1
β
heating rate, K/min
E
apparent activation energy, KJ/mol
R
molar gas constant, 8.314 KJ/( mol·K)
n
reaction order
w0
sample weight before the reaction
wt
sample weight at the time of t
w∞
sample weight after the reaction
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DTG
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1 Introduction
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Coal will continue being an important energy source in the 21st century due to a higher reserve/production (R/P) ratio than petroleum and natural gas especially in
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America, China, Russia, Australia, Canada and India (BP, 2018; Wang et al., 2018a). Technologies for the efficient utilization of coal resource have attracted much
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attention and played a vital role in the current energy policy (Jacobson, 2009; Li et al., 2018a). In terms of energy saving and emission reduction, coal water slurry (CWS) technology allows coal to be utilized in a cleaner and more efficient way because of its convenient transportation and higher combustion efficiency (Huang et al., 2013; Lee et al., 2011; Xu et al., 2009; Zhang et al., 2017; Zhang et al., 2016). In addition,
as one of the ideal alternatives to fuel oil, the qualified CWS products should possess a high slurry concentration and a low viscosity i.e., the solid content in suspension with an apparent viscosity of 1000 mPa s at a shear rate of 100 s-1 is regarded as the maximum slurry concentration for atomization combustion (Li et al., 2018b; Valiullin and Strizhak, 2017; Wang et al., 2019). Considering the high price and unstable supply of bituminous coals, it is attractive for researchers and industrial users to
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prepare CWS by using low quality coals. Nevertheless, the high mineral content of
low quality coals make it difficult to prepare a high calorific CWS directly (Bae et al.,
2014; Chai et al., 2018; Gong et al., 2013; Mukherjee et al., 2015). Therefore, it is
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necessary to upgrade low quality coals before CWS preparation (Gui et al., 2015;
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Vapur et al., 2010).
Flotation, which is widely used in coal beneficiation for low quality coals in
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industry, is principally based on the differences in the surface hydrophobicity between
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the combustible components and minerals (Bournival et al., 2019; Hacifazlioglu and Toroglu, 2011). Recently, increasing attention has been paid on improving flotation
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processes for low quality coals. Vapur et al. reported that the coal ash content was decreased from 33.4% to 17.9% with a combustible recovery of 95%, as the coal
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flotation parameters optimized for particles size, pulp density, the dosage of reagents and the feed rate of wash water (Vapur et al., 2010). Dickinson et al. proposed a two-stage fast flotation to beneficiate a low quality coal with ash content of 42%, achieving a fully deslimed product of 7.8% ash and combustible recovery up to 80% (Jiang et al., 2016). Gui et al. also developed a two-stage process for successfully
intensifying the flotation process of low-rank coal, the results indicated that ash content of clean coal products was around 10% and the recovery of combustible matter was up to 90% (Gui et al., 2017; Xing et al., 2015). Therefore, flotation could efficiently change chemical component and surface properties of coal, which have great influences on the preparation and combustion of CWS. Both B. K. Sahoo et al. and Sang Jun Yoon et al. found that the apparent viscosity of CWS was increased as a
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function of mineral matter, which meant that the inherent minerals in coal particles
made it unmanageable to prepare CWS with large solid loading (Sahoo et al., 2010; Yoon et al., 2010). Zeki Aktas et al. observed that the demineralized coal was more
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suitable for high concentration CWS preparation, and the low viscosity slurries could
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be obtained by enhancing the electrostatic repulsion forces of coal particles (Aktaş and T., 2000). Xing Chen et al. provided a novel method to enhance the surface
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hydrophobicity of coal particles by using the second fluid, and the maximum slurry
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concentration of CWS was increased from 53.4 wt.% to 56.3 wt.% (Chen et al., 2019). The chemical component and surface properties of coal particles were modified by
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flotation techniques, but there is limited study on characterization and combustion performance of CWS prepared by upgraded coals obtained from flotation processes.
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In the previous study, the developments and limitations of low quality coal
beneficiation techniques had be reviewed and the feasibility of upgraded coal for high concentration CWS preparation had been investigated (Hu et al., 2019; Wang et al., 2018a). Therefore, this paper aimed to reveal the difference of slurry preparation and combustion performance caused by coal quality change after flotation treatment, and
the researches were focused on (i) the microstructure and surface properties variation of upgraded coal after floatation and (ii) the maximum slurry concentration and combustion behavior of the CWS prepared by upgraded coals. Two kinds of low-quality coals (JD-RC and JS-RC) were selected as the raw materials in coal flotation to produce upgraded coals (JD-UC and JS-UC) for CWS preparation, and the measurement of SEM-EDS, BET, FTIR, TG, zeta potential, contact angle were
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applied for characterization and analysis of coal properties. The findings of this research might be of great potential to provide a direct solution on the efficient
utilization of low quality coals and address the raw materials supply for CWS
Experimental
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2
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preparation.
2.1 Coal preparation
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In order to achieve the maximum liberation degree of combustible components and
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minerals, the comminution of low quality coals were implemented in a horizontal high speed stirred mill with a grinding volume of 20 L and very high power intensities up
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to 300 kW/m3. This system could be operated to obtain upgraded coal products, so called clean solid fuel (CSF) with the ash content of 6-10%, with high combustible
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recovery of 80-85%.
The upgraded coals had been collected as follows: firstly, low quality coals were
grounded to be within a narrow size distribution (in the range of 1-100 µm); secondly, C11-C17 alkanes and micro bubbles were adsorbed by the surface of those fine coal particles at a shear rate of 10,000 rpm in a pre-mineralization equipment, in order to
increase the separation efficiency of the organic-rich combustible particles and inorganic-rich non-combustible mineral particles; finally, the slurry with 10% solid loading was fed into a flotation column, which was filled with water, and the upgraded coal with low ash content was collected as the clean products at the top of system. In this study, two low quality coals (JD-RC and JS-RC) and their upgraded coals
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(JD-UC and JS-UC) obtained by flotation process were used as the raw materials for comparisons. The particle size had been measured by a laser particle size analyzer (BECKMAN LS-13-320), as shown in Fig. 1.
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The results of proximate analysis and ultimate analysis on these coal samples were
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listed in Table 1. The ash content was decreased from 57.71% in JD-RC to 9.20% in JD-UC, and the removal ratio was 84.06%. In addition, the ash was decreased from
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31.40% in JS-RC to 6.98% in JS-UC, and the removal ratio was 77.77%. Meanwhile,
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the sulfur content in the upgrade coals had been reduced by 20-30%. 2.2 Characterization of coal after flotation
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2.2.1 SEM-EDS
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MERLIN SEM (ZEISS, Germany) equipped with an energy dispersive spectroscope (EDS) was applied to determine the surface morphology and elemental composition of intruded ultrafine coal particles. During the characterization, the surface of coal samples had been sputter-coated with a gold film before SEM analysis. The SEM-EDS measurement procedure was as follows: firstly, the coal samples were subsequently placed in the sample bin of the SEM-EDS system after the
instrument and cooling water were activated; secondly, the SEM was configured to provide an electron beam at accelerating voltages up to 20 kV; thirdly, the opportune scanning speed, magnification and brightness values were identified to ensure image quality; finally, the elemental spectra were analyzed by the associate EDS software.
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2.2.2 BET The specific surface area and pore size distribution measurement of coal samples were measured by an automatic N2 adsorption apparatus (ASAP2020, Micromeritics,
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America). The experimental procedure was as follows: first, 0.15-0.2 g of dry-basis
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coal sample was put into the sample tube and then degassed at 180 °C for 6 h; then an N2 adsorption/desorption detection was performed by using the Brunauer Emmett
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Teller (BET) and Barrett Joyner Halenda (BJH) method to calculate the specific
2.2.3 FT-IR
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surface area and porosity data.
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The surface functional groups of raw intruded coals were investigated by Fourier-transform infrared spectroscopy (FTIR, Nicolet iS10, Thermo Scientific,
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USA). The homogeneous mixture, including 1 mg of pulverized coal and 100 mg of KBr, was placed into a dye with a pressure of 20 MPa to prepare the pellet for FTIR measurement. The spectrum of coal samples was described from 4000 to 400 cm-1 at a resolution of 4 cm-1 and redacted from 32 scans with excluding background values. 2.2.4 Contact angle
Hydrophobicity of coal particles were analyzed by a contact angle analyzer with an accuracy of 0.1° on the sessile drop method. The pulverized coal with a mass of approximately 0.4 g was pressed under a pressure larger than 30 MPa by a tablet machine for 15 min to form a pellet. The images after the drop contacted coal pellet were recorded by a high-speed camera at different times to ascertain contact angles and droplet spreading ability on coal surface.
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2.2.5 Zeta Potential
The relationship between zeta potential of fine coal particles and pH value was determined by ZetaPALS analyzer (Brookhaven, USA). Dilute CWSs (0.2g
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pulverized coal in 100ml deionized water without any dispersant) were conducted by
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the ultrasonic treatment for 30min. In order to promote the settlement of coarse particles, the suspension was centrifuged at 1000 r/min for 5 min. Afterwards, the
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upper suspensions were used to perform zeta potential measurement, and the pH value
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of suspensions was adjusted from 3 to 11 with an appropriate amount of HCl or NaOH addition. The pH values were measured by a PHS-2 pH meter. Each sample
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was tested 5 times and the average value of the zeta potential was calculated. 2.3 CWS Characterization
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2.3.1 CWS preparation and viscosity measurement CWS was prepared as follows: first, a specified quantity of deionized water was
added into a 200 mL beaker, and the corresponding amount of coal powder was slowly poured into the beaker with continuously stirring by a high speed disperser; second, the mixing was lasted for 5 min at 11200 rpm to ensure the uniformity of
CWS. The apparent viscosity of CWS was measured by using a rotary viscometer (NXS-4C, Chengdu Instrument factory, China) at 25±1 ℃. The measurement of apparent viscosity was repeated for six times at a shear rate of 100 s-1. 2.3.2 Free water content of CWS measurement The free water content of CWS prepared by different coals were characterized by a thermogravimetric analyzer (TGA2, METTLER TOLEDO, Switzerland). CWSs with
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the solid concentration of 50 wt.% were used for free water content measurement. Firstly, the CWS sample was hold for 5 min at 30 ℃ in TG; and then it was heated
from 30 to 140 ℃ under N2 atmosphere at a heating rate of 1 ℃/min. The extreme
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point of moisture drying rate during the process was determined as the distinction
2.4.1 TG and DTG
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2.4 Combustion dynamics of CWS
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between free and bound water (Zhou et al., 2008).
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The combustion kinetics experiments of CWS with 50 wt.% solid loading were characterized by the same thermogravimetric analyzer. The combustion curves of
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slurry samples prepared by different coals were investigated at the heating rate of 10℃/min, 20℃/min, 30℃/min, 40℃/min and 50℃/min, while the air flow rate was 60
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ml/min and the weight of CWS sample was maintained around 15 mg for all measurements. 2.4.2 Identification of ignition temperature The ignition temperature reflects the difficulty of fuel to initiate the combustion. The ignition temperature of coal samples was determined by TG-DTG method (Cheng
et al., 2008). 2.4.3 Apparent activation energy calculation The activation energy E, as one of primary dynamics parameters, described the combustion performance in essence. According to the law of mass action and Arrhenius equation (Ullah et al., 2018), the non-isothermal TGA dates could be fitted by the equation as follows: (1)
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d A RTE e ( 1 n) dT
here, α, T, A, β denotes the conversion ratio, the reaction temperature (K), the
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pre-exponential factor (s-1) and the heating rate (K/min), respectively. While E, R, n represents the apparent activation energy (kJ/mol), the molar gas constant (8.314
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kJ/(mol·K)) and the reaction order, respectively.
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The conversion rate α of coal combustion reaction could be calculated as follows：
w0 wt w0 w
(2)
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=
where, w0, wt, w∞ refer to sample weight before the reaction, at the time of t and
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after the reaction, respectively.
The reaction order n of coal combustion was 1, so formula (1) could be rewritten as
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follows:
l n (1 ln( T2
) E ) RT
AR ln E
A linear regression was fit to the date on “ ln(
(3)
ln(1 ) ) ” versus “-1/T”, then the T2
apparent activation energy E (KJ/mol) of coal samples at different heating rates could be obtained.
3 Results and discussion 3.1 The microstructure of coal 3.1.1 SEM-EDS analysis Fig. 2 (a-d) showed the surface morphology and element composition of different coal particles by SEM-EDS. As can be seen from Fig. 2 (a-2) and (b-2), the low quality coals of JD-RC and JS-RC showed the abundant pore structures and uneven
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surfaces. As illustrated in Fig. 2 (c-2) and (d-2), the surfaces of upgraded coal particles (JD-UC and JS-UC) were smoother and the micro-pores of them were smaller after flotation treatment. Because the content of ash was reduced, the
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attachment of ash on coal particles was decreased. It led to the relatively smooth
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surfaces of coal particles (Xia et al., 2018). Due to the lower ash content of JD-UC and JS-UC, their surface was relatively smooth. In addition, the surfactant in flotation
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process might promote the agglomeration between coal particles, and it resulted in
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smaller micro-pores (Forsmo et al., 2008).
As shown in Fig. 2 (a-3) and (c-3), the C content on coal surface was increased
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from 30.48% in JD-RC to 79.62% in JD-UC, while the Al and Si were decreased from 7.78% and 11.85% in JD-RC to 1.08% and 1.32% in JD-UC, respectively.
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Correspondingly, the O content was decreased from 44.66% to 17.92%. Also, it was noted that the Fe and Ca were disappeared on the coal surface of JD-UC. From Fig. 2 (b-3) and (d-3), similar variances in the elemental composition were observed and the final elemental composition in JS-UC was in a similar level to JD-UC. The above results showed that the content of Si, Al, Fe and Ca on the upgraded coal
surface were decreased or disappeared, which was mainly due to the substantial reduction in the mineral content. According to SEM-EDS of the selected microregion in Fig. 2 and the ultimate analysis in Table 1, the C content was increased but the O content was decreased after flotation, which implied that the coal quality (related to the C/O ratio) was improved. Therefore, the low quality coal could be upgraded to
3.1.2 Specific surface area and porosity analysis by BET
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clean solid fuel with a higher calorific value by the flotation process.
The pore structure characteristics and specific surface area of coal surface considerably influence the subsequent applications including CWS preparation and
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combustion (Cheng et al., 2008; Fei et al., 2017). As shown in Fig. 3, the adsorption
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and desorption lines of the four coal samples tend to coincide in the lower relative pressure range, which indicated that the pores in the small aperture range were mostly
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closed impermeable pores at one end. Meanwhile, the curves were raised slowly in the
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lower range of relative pressure, which suggested that the multilayer adsorption had occurred (Wang X, 2008). The adsorption curves were increased sharply at the
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relatively high pressure and the adsorption capacity did not reach saturation although the saturation vapor pressure was finally reached. It predicted that the capillary
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condensation and multilayer adsorption took place together (Fuller, 2007). The desorption curves of all samples showed that it was decreased sharply around relative pressure (P/P0) of 0.5, which was radically striated by the abundant inkpot holes in coal particles (Jin et al., 2012). When the relative pressure went down to 0.5, the sudden release of gas in inkpot holes led to an inflection point.
It could be seen from Fig. 3 (a-1) and (b-1) that the regression trend of adsorption and desorption was similar for raw coals and upgraded coals, which meant that the type of pore in coal particle did not change by the flotation process. However, the adsorption capacity of low quality coals (JD-RC and JS-RC) were greater than that of upgraded coal (JD-UC and JS-UC) at the same pressure. It suggested that the particle porosity was reduced by the flotation process.
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BJH method was used to determine pore volume distribution as a function of pore diameter (0.1-600 nm). From Fig. 3 (a-2) and (b-2), the pore volume of low quality
coals was larger than that of upgraded coals at the same pore diameter, which
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demonstrated that the pore volume of coal samples was decreased by flotation process,
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and the result was in line with conclusion drawn from Fig. 3 (a-1) and (b-1). The BET surface area of JD-RC, JD-UC, JS-RC and JS-UC was 14.5 m2/g, 3.8
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m2/g, 5.4 m2/g and 3.4 m2/g, respectively, and the pore volume of JD-RC, JD-UC,
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JS-RC and JS-UC was 3.56×10-2 cm3/g, 9.85×10-3 cm3/g, 1.52×10-2 cm3/g and 1.02×10-2 cm3/g, respectively. Compared with low quality coals (JD-RC and JS-RC),
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the specific surface area and porosity of upgraded coals (JD-UC and JS-UC) were obviously reduced. The main reason might be the removal of mineral components in
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coal particles, so the coal surface became smoother (it could also be explained from Fig. 2) and the specific surface area descended (Xia et al., 2018). Moreover, the addition of the collector in the flotation process could promote the agglomeration of fine particles during flotation process, mainly resulted in a decrease of porosity in coals.
3.2 Surface properties of coal 3.2.1 Surface functional group detection by FT-IR Fig. 4 showed the infrared spectra of low quality coals (JD-RC and JS-RC) and their upgraded coals (JD-UC and JS-UC). The relationship between the absorbance and functional groups was identified (Mengüg, 1993; Wang et al., 2018b; Xia et al., 2017; Xia et al., 2019). The transmittance band around 3695 and 3619 cm-1 could be
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attributed to hydroxyl (-OH) with stretching vibration in kaolinite. The peak around
913 cm-1 could be corresponded to the bending vibration of Al-OH. While symmetric stretching vibration of Si-O groups in quartz were absorbed near 797 cm-1 and the
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band around 695 cm-1 was due to the stronger stretching vibration of Si-O group. The
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transmittance band around 537 cm-1 was attributed to the bending vibration of Al-O-Si
vibration of Si-O bond.
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group. The bands observed around 470 cm-1 and 430 cm-1 were assigned to bending
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Combined with the results in Table 1, all of above groups were more pronounced in the low quality coals due to the higher mineral content. It could be seen from the
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infrared spectrum that, characteristic absorption peaks of characteristic absorption peak in kaolin and quartz were either decreased or disappeared in the upgraded coals.
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3.2.2 Contact Angle
The spreading ability and contact angle of water droplets on different coal surfaces
could be seen from Fig. 5. It could be found that the contact angles of coal samples i.e., JD-RC and JS-RC, were significantly decreased as a function of contact time due to the strong surface hydrophilicity. However, the values of contact angle of JD-UC
and JS-UC were increased. Meanwhile, the contact angle of JD-UC and JS-UC were relatively stable with the contact time, which indicated that the surface hydrophobicity of coals had been significantly enhanced through the flotation process. The hydrophobicity of coal surface was subject to composition and surface properties (Rios et al., 2006; Zhou et al., 2018). Abundant mineral content in JD-RC and JS-RC might indeed be responsible for the receding contact angles observed in
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Fig. 5. It appeared that more water molecules tend to interacting with mineral due to
its more polar hydrophilic groups (Chau, 2009). The contact angles of upgraded coals
were larger than the raw coals and they exhibited relatively small changes as function
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of contact time. The main reason for the difference was attributed to the removal of
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most mineral components and surface modification of collector by the flotation
3.2.3 Zeta Potential
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treatment (Moncayo-Riascos and Hoyos, 2017).
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Zeta potential of different coal samples has a substantial effect on coal processing technology and applications. The zeta potential values on different types of coal as a
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function of pH were described in Fig. 6. All coal samples were proven to have the negative charge in the measured pH range (3 to 11). The zeta potential values of all
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coal samples were decreased with the increasing in pH value. The polar groups on coal surface, such as -COOH and -OH, might get protonated or ionized as the pH value variation in solution (Das et al., 2008). Positive charge, which was generated by the protonated carboxyl group at the acidic pH ( RCOOH H RCOOH 2 ), would result in the decrement of the absolute value of zeta potential. In addition, the negative
charge, which was caused by dissociated carboxyl group at alkaline pH ( RCOOH OH - RCOO- H 2O ), could promote the increment of the absolute value of zeta potential. These charges accounted for its zeta potential when electric field was applied. More importantly, the zeta potential measurements showed that the absolute value of upgraded coals (JD-UC and JS-UC) were more negatively charged than that of raw
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coals (JD-RC and JS-RC) at an acidic range of pH. Owing to the high price metal ions
(Ca2+, Al3+, Fe3+) precipitated from the mineral content of raw coals, the negative charge on the surface of coal particles had been neutralized at acidic conditions
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(Yukselen-Aksoy and Kaya, 2003). For CWS preparation, upgraded coals would be
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more dispersed than raw coals in the slurry at acidic environment because of higher electrostatic repulsion, which implied that upgraded coals would be better raw
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3.3 Slurry ability
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materials for CWS preparation.
3.3.1 The maximum concentration of CWS prepared by different coals
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The apparent viscosity of coal water slurries prepared by JD-RC, JS-RC, JD-UC and JS-UC were measured at a shear rate of 100 s-1 by varying coal loading in the
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range of 50-66% without any additives. The apparent viscosity of those four slurries with different solid loadings was shown in Fig. 7, and it was increased with solid content for all slurries. As depicted in Fig. 7, the solid concentration at an actually acceptable apparent viscosity of (1000±100) mPa·s were approximately 54% for JD-RC and 58% for
JS-RC. It was far away from the industrial application considering the high ash content in low quality coals and low coal loadings for CWS preparation by them. However, the maximum solid concentrations of CWS prepared by JD-UC and JS-UC are 60% and 64%, respectively. It suggested that the slurry ability of CWS prepared by upgraded coals were much better than that by raw coals. In general, the lower apparent viscosity of CWS could be attributed to more free
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water around coal particles. Firstly, the strong interaction of hydrophilic ash might
resulted in more water absorbed by minerals to form gel in coal particles (Das et al., 2009). Meanwhile, more water might be enclosed inside the coal particle and bounded
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by surface oxygen-containing functional groups on coal surface, the decrease of free
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water content led to a significant increase in slurry viscosity, which could be the inherent mechanisms underlying the effect on coal slurry performance (Das et al.,
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2009). Thus, the distribution of free water and bound water on slurries with same coal
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loading were investigated as follows.
3.3.2 Free water content of CWS prepared by different coals
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It is well-known that there are two dominant types of water around coal particles in CWS: bound water and free water. The bound water, which was absorbed by the
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hydrophilic functional groups of particle surface or filled in inner hole of particle, generally considered to be a solid or quasi-solid phase. Due to the strong interaction between bound water and particles, more heat energy is needed for their evaporation (Lu et al., 2018; Zhou et al., 2008). Many studies had reported that the free water, filled in the void gap between coal particles, was one of primary factors in dictating
the lubricity among particles in suspension (Mukherjee and Pisupati, 2016). The enough free water in CWS suspension was critical on contributing to the viscosity reduction and fluidity of CWS with suitable coal loadings. As observed from Fig. 8, the drying-rate curves of CWS prepared by different coals were obtained by TGA results, and the corresponding equipoise water content of the inflection point in Fig. 8 was the free water content in CWS. The free water contents
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of slurries with 50% solid concentration were 70.48% for JD-RC, 76.62% for JS-RC,
79.01% for JD-UC and 81.19% for JS-UC. The JD-RC slurry had the lowest free
water content while JS-UC slurry had the highest free water content. Fig. 8 indicated
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that the free water contents of CWS prepared by upgraded coals increased obviously,
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comparing with that of CWS prepared by low quality coals. The ratio of free water to total water demonstrated the water trapping capacity of the carbonaceous solid and the
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apparent viscosity of slurries was influenced by the availability of free water directly.
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Therefore, the slurry performance of upgraded coals was better than that of raw coals (as shown in Fig. 7).
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SEM-EDS and BET in section 3.1 indicated that the Al, Si content and porosity of both upgraded coals were decreased. The measurement of FT-IR and contact angles in
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section 3.2.1 and 3.2.2 showed the removal of mineral species and improvement of particle surface after flotation Zeta potential of coal particles suggested that stronger electrostatic repulsion of upgraded coals at an acidic range of pH. The above results demonstrated that the free water content of upgraded coals was higher than that of low quality coals at same solid content. In another word, upgraded coals should be more
suitable for preparation of CWS with high coal loading. 3.4 Combustion Dynamics The process for CWS combustion includes evaporation of water, volatile release, ignition, char combustion and burn out (Cheng et al., 2008). The combustion properties of different CWS samples at different heating rates were identified by the TG and DTG curves, as illustrated in Fig. 9.
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According to the data analysis from Fig. 9, the ignition temperature of all coal
samples at different heating rates could be obtained, as shown in Fig. 10. The ignition temperature of all samples was increased with the rising of heating rate. It was
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attributed to the limited oxygen concentration around coal samples which might
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constrain coal combustion reaction although ambient temperature had risen rapidly during the process of rapid heating; for another reason the higher the heating rate, the
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more volatiles released per unit time, but the volatiles released from coal particles
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seriously hinder the thermal energy transportation from heating gas to inner particle (Shuang et al., 2010).
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As shown in Fig. 10, with a heating rate of 10, 20, 30, 40, 50 ℃/min, the ignition temperature of JD-RC was 420.40, 434.85, 438.56, 440.80 and 445.92 ℃, but the
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ignition temperature of JD-UC was 422.10, 437.840, 440.89, 443.59 and 449.72 ℃. The ignition temperature of JD-RC was always lower than that of JD-UC coal. The main reason might be the larger specific surface area and richer pore structure (as described in section 3.1.2) of JD-RC, which was very helpful for oxygen diffusion inside coal particles (Cheng et al., 2016; Yuan-gang et al., 2010). As depicted in Fig.
10, the ignition temperature of JS-RC was 435.05, 448.36, 453.05, 454.08 and 456.40 ℃ at different heating rate, while the ignition temperature of JS-UC was 433.02, 440.42, 444.00, 448.27 and 452.49 ℃. It was found that the ignition temperature of JS-UC was lower than that of JS-RC at the same heating rate, which was due to the higher volatile and fixed carbon content of JS-UC, as shown in Table 1. More volatiles in JS-UC could contribute to ignition, meanwhile the higher fixed
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carbon content could provide more carbon active sites to react with oxygen molecules (Katalambula et al., 1997). It should be noted that there was less difference on the
specific surface area and porosity of JS-RC and JS-UC (comparing with JD-RC and
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JD-UC), which was demonstrated in section 3.1.2.
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As shown in Fig. 11, the apparent activation energy of JD-RC, JD-UC, JS-RC and JS-UC were decreased gradually with the increasing of heating rate. This might be
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that the higher heating rate enhanced the whole reaction process. Meanwhile, the
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apparent activation energy of JD-UC coal was higher than JD-RC, which was mainly attributed to larger specific surface area and richer pore structure of JD-RC as
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depicted in section 3.1.2. However, the apparent activation energy of JS-UC coal was lower than JS-RC, the main reason might be that the content of combustible body in
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JS-UC was much higher than that in raw coal from Table 1, so the interface area between the hydrocarbon in coal particles and the diffused O2 was enlarged, resulting in an enhancement in combustion reaction (Wang et al., 2011). 4 Conclusion In this study, the slurry ability and combustion dynamics of two low quality coals
and their upgraded coals by flotation had been carefully investigated based on systematical characterizations. SEM-EDS showed that the surfaces of both upgraded coals were smoother and the Al, Si content on the surface were decreased significantly. BET determined that the specific surface area and porosity of upgraded coals went down. Infrared spectrum showed that the characteristic absorption peaks of kaolinite and quartz on the upgraded coal surface were obviously weakened or disappeared.
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The hydrophobicity of particle surface was significantly increased after flotation according to contact angle measurement. Zeta potential tests showed electrostatic
repulsion between upgraded coal particles was greater than that of raw coals at pH of
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7. Results of viscosity measurement showed that the slurry maximum concentration of
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CWS prepared by both upgraded coals were approximately 6% more than that by low quality coals. The main reason was that the content of free water around coal particles
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was changed. The ignition temperature and apparent activation energy of CWS
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prepared by JD-UC was greater comparing to JD-RC, while that by JS-UC was lower comparing to JS-RC.
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Funding sources
This research was supported by Guangdong Innovative and Entrepreneurial Research
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Team Program (No. 2016ZT06N532) with Shenzhen Government Related Supporting Fund (No. KYTDPT20181011104002), Shenzhen Clean Energy Research Institute Project (No. CERI-KY-2019-003). The authors would also like to thank the financial supports from Shenzhen High-Level Professional Program (No. 20160802681J) and a Low Quality Coal Program initiated by Development and Reform Commission of
Shenzhen Municipality.
Declaration of interests 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.
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Figure captions
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Fig. 1. Particle size distribution of raw coals and upgraded coals.
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Fig. 2. Surface morphology and element content analysis of different coal particles
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by SEM-EDS: (a) JD-RC, (b) JS-RC, (c) JD-UC, and (d) JS-UC.
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Fig. 3. Adsorption capacity and pore volume distribution curve of different coal: (a)
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JD-RC and JD-UC, (b) JS-RC and JS-UC.
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Fig. 4. Surface functional groups of different coals.
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Fig. 5. Water contact angles of JD-RC, JD-UC, JS-RC and JS-UC coal samples.
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Fig. 6. Zeta potential of raw coals and upgraded coals as a function of pH value.
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Fig. 7. Effect of solid content on apparent viscosity of CWS prepared by different
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coal.
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Fig. 8. Free water to total water ratio of 50% CWS prepared by raw coals and upgraded coals.
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Fig. 9. TG and DTG curves of JD-RC, JD-UC, JS-RC and JS-UC with the heating
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rate of 10, 20, 30. 40, 50 ℃/min.
Fig. 10. Ignition temperature of raw coals and upgraded coals with different heating rates.
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Fig. 11. Apparent activation energy of different coals at different heating rates.
Table
Table 1. Proximate and ultimate analysis (wt.%) of coal samples. Proximate analysis
Coal
Ultimate analysis(daf)
sample
Mar
JD-RC
1.93 57.71 19.18 23.11
70.64 4.73 23.68 0.95 2.57
JD-UC
1.87
88.47 5.50
JS-RC
1.26 31.40 30.06 38.54
83.50 5.46 10.03 1.02 3.80
JS-UC
1.27
87.93 5.79
Ad
Vd
FCd
9.20 34.19 56.61 6.98 39.02 54.00
C
H
O*
N
St
4.79 1.24 1.87 5.22 1.06 2.99
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Note: ar, as received base; d, dry base; daf, dry and ash free base; *, by difference