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Impact of coal pyrolysis products as rheological additive on thermoplasticity of a coking coal Wei Xie, Rohan J. Stanger, Quang Anh Tran, Nathan D Smith, Terry F. Wall, and John A. Lucas Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03232 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018
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Energy & Fuels
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Impact of coal pyrolysis products as rheological additive
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on thermoplasticity of a coking coal Wei Xie*a, Rohan Stangera, Quang Anh Trana, Nathan Smithb Terry Walla, John Lucasa
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a
Chemical Engineering, The University of Newcastle, Callaghan NSW 2308, Australia
b
Analytical & Biomolecular Research Facility, The University of Newcastle, Callaghan, NSW 2308, Australia
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Abstract
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Thermoplasticity is a determining factor for the development of coke structure and coke strength.
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Mobile phase, whether vaporizable or not, may significantly affect thermoplasticity during coal
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coking. This work studies the effect of the mobile phase, including the volatile tar and extractable
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metaplast generated from one bituminous coking coal, on the thermo-swelling of the raw coal. The
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volatile tar was collected when the raw coal was heated from room temperature to 450 °C at a heating
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rate of 5 °C/min while the metaplast was extracted from the heated char. Molecular properties of the
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tar and metaplast were characterized using a Laser Desorption Ionization-Time of Flight-Mass
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Spectrometry technique (LDI-TOF-MS). Thermo-swelling of the raw coal and its blends with the
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volatile tar and extractable metaplast was investigated using a Computer Aided Thermal Analysis
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(CATA). Volatile (C and H) evolution rate of the heating coal samples was tracked using a Dynamic
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Elemental Thermal Analysis (DETA), and the weight loss rate was investigated using
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Thermogravimetric Analysis (TGA). It was found that the extracted metaplast has a higher molecular
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weight distribution than the volatile tar. The swelling and thermal changes of the heating coal
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increased with the addition of tar or metaplast. The weight loss rate prior to coal swelling increased
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with the additives, while the raw coal showed a higher volatiles release after maximum swelling than
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the blends. The addition of metaplast into the raw coal led to greater swelling, increased exothermicity
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and resulted in a higher thermal conductivity than the addition of volatile tar during the primary
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devolatilization, particularly when the additive was 20 wt% in the blend. Different influences of
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thermoplasticity of the blends indicated that the interactions between the additive and the coal are 1 ACS Paragon Plus Environment
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affected by molecular weight distribution of the additive. These findings will aid in the selection of
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additive for improving thermoplasticity of low-caking coals to benefit coke production.
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1. Introduction
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Coke is commercially produced in coke oven batteries by carbonization of coal from room
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temperature (25 °C) to 1000 °C in the absence of oxygen.1,2 Coal evolves volatiles (light gas and
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vaporizable tar) during primary devolatilization (350-550 °C) and light gas, mainly CO and H2, during
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secondary devolatilization (600-800 °C).1,2 Devolatilization is usually accompanied by the exothermic
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and endothermic reactions.3-5 These thermal and chemical changes lead to physical changes, such as
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softening (300-400 °C), re-solidification (550-650 °C), swelling (400-550 °C) and contraction (550-
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1000 °C)2,3. An initial exothermic reaction between 400 and 550 °C causes the main evolution of
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gases and tars.3-5 Coal tar generated during coal coking is one of the determining factors in affecting
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the thermoplasticity of coking coal. Tar comprised of small molecular weight compounds can
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vaporize to promote volumetric swelling.3 In comparison, tar with high molecular weight compounds
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tends to remain in the heating coal, combining the infusible components to form a plastic immediate
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phase called metaplast, which may affect the escape of the gas phase, gas bubbling, and thus
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volumetric swelling.6-8
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Previously, for the purpose of investigating the influence of the mobile phase on the development of
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thermoplasticity during coal coking, coal tar or the metaplast was solvent extracted. A number of
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solvents have been used to extract the metaplast compositions in the heating coal, such as carbon
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disulphide–pyridine,9 carbon disulphide–NMP (1-methyl-2-pyrrolidinone),10,11 dichloromethane,12
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quinolone,13 tetrahydrofuran (THF),14 and tetralin.15 Because of its high boiling point (115 °C) of
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pyridine, the molecular weight distribution for pyridine extracts is thought to reflect the secondary
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polymerization of the extract in boiling pyridine solution rather than the extraction of the higher coal
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molecular weight fractions.16 By contrast, THF has a boiling point of 66 °C, and is suggested to
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extract only metaplastic materials. Therefore, in this study, THF is used to extract coal metaplast.
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The impact of coal tar pitch on thermal swelling and coking property has been widely studied in the
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literature. Previous work reported that coal tar pitch can improve swelling (Audibert-Arnu total
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dilatation) and fluidity (maximum fluidity, ddpm) of coking coals, and increase the plastic zone
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because of a decrease of the softening temperature.17,18 Chang et al.19 found that the addition of 5-10
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wt% extracts that were extracted by creosote oil at 360 °C can increase the fluidity and plastic range
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of coals. They suggested that the extracts can increase coke strength by acting as a binder. Similarly,
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Nomura and Arima20 reported that the coal tar pitch can act as a binder that generates gas to interact
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with coal before coal starts to soften, which leads to enhancement of caking property. Zubkova et al.21
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found that the coal tar pitch can affect coking by acting as an external plasticizer. They diffused
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between the macromolecules of coals, which thus weakened the interaction of macromolecules of
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coals and favoured their higher mobility during swelling of the coal particles. The molecular
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compositions of the addition may affect caking property and coke strength. Koyano et al.22 found that
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the addition of large aromatic-ring compounds (i.e., coronene, perylene and naphtha[2,3-a]pyrene)
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can significantly increase coke strength because they co-fused with the coal particles. However, 3-ring
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compounds with lower boiling points, and a straight chain compound, did not increase coke strength.
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However, to what extent the molecular weight distribution effect on thermo-swelling change, such as
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the endo/exothermicity, fluidity, and swelling, during coal coking is still unclear. The objectives of
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this work are to understand the impact of molecular weight distribution of the vaporizable tar and the
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extractable metaplast on thermo-swelling of a coking coal. Our previous work found that the
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molecular weight distribution of the THF extracts from the semi-coke changed with pyrolysis
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temperature, from an initial upper limit of 500 Da (350 °C) to up to 1800 Da (400-450 °C) and then
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back (500 °C). Therefore, in this study, the pyrolysis by-products at 450 °C was extracted, which
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includes a large range of molecules.
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2. Methodology
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2.1 Computer Aided Thermal Analysis (CATA)
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The thermo-swelling of the heating coal was investigated on a novel CATA experimental set-up, this
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technique has been used in our previous publications.3-5 It can directly measure the instant volumetric
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swelling and pressure drop of gas flowing through a coal pellet during pyrolysis.3 The measured
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pressure drop was used to estimate the permeability through the heating coal pellet based on Darcy’s
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law.23 The schematic of the experimental setup is shown in Figure 1. Pyrolytic experiments were
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conducted from 25 to 1000 °C at a heating rate of 5 °C/min under an argon flow of 30 mL/min. For
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each test, ~ 2 g sample with a top particle size of 0.5 mm was packed in a quartz tube with a length of
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20 mm and a diameter of 11.80 mm. Coal particles with a top size of 0.5 mm were applied in this
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study because this particle size is closer to industrial coal charge (2-3 mm)24 in a coke oven. A
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calculated Biot number of Bi=0.15 for this system suggests that convective influences can be
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considered negligible. With a slow heating rate of 5 °C/min, a maximum temperature gradient of 8 °C
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(average 5 °C) was measured across the packed bed creating relatively isothermal conditions within
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the sample. During coking tests, three temperatures, graphite of the heating element, surface and
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center of the sample, were measured for estimating the apparent specific heat and thermal
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conductivity based on one-dimensional heat conduction eq 1, which uses a calibrated heat flux. The
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heat flux to the sample surface was calibrated using the apparent thermal resistance of a graphite
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sheath surrounding the central quartz tube, which was determined beforehand with a copper cylinder
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of known dimensions. Details for how to estimate the apparent volumetric specific heat and thermal
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conductivity can be seen in our previous publications.3-5, 25
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ρC P
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In eq 1, where ρ is the density of the sample (kg/m3), CP is the specific heat (J/(kg K)), λ the thermal
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conductivity (W/(m K)), T the temperature (K), t the time, and r the radius (m).
∂T 1 ∂ ∂T (1) = λ r ∂t r ∂r ∂r
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2.2 Permeability and Swelling
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During heating, the coal sample was restrained on the left hand side and allowed to expand only on
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the right hand side, measured with a linear variable differential transducer (LVDT). A well-calibrated
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pressure sensor was connected at the gas inlet (left side) to measure the pressure drop through the
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heating coal pellets. The LVDT readings and the pressure drop were monitored every second along
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with temperature. All experiments were started under the same slight compression of the calibrated
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spring. The transient horizontal swelling or contraction of samples was determined by the measured
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value of the LVDT. Swelling was calculated by the transient compressed value (∆L) of the spring and
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the original length (L0) of the sample, namely,
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swelling (% ) =
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Coal bed permeability can be defined by a lumped index of how well fluid passes through coal.26 With
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the increase of fluidity, the permeability across the heating coal pellet decreases, therefore, it becomes
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difficult for gases and tars to escape from the plastic coal. The change of bed permeability with
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temperature can be described based on permeability coefficient k (m2) of gas flowing through coal
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pellets, which was estimated according to the Darcy’s law.23, 26
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∆P 1 = µU (3) L k
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∆P is the pressure drop of gas flowing through the coal sample expressed in Pa, µ is the viscosity of
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the carrier gas argon expressed in Pa s, changes of argon viscosity with temperature was based on data
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in Perry’s chemical engineer’s handbook.27 U is the carrier gas velocity in m/s, L is the transient
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length of the coal bed in m, which was calculated according to the LVDT measurement. Downstream
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pressure was assumed to be atmospheric.
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2.3 Dynamic Elemental Thermal Analysis
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The carbon and hydrogen evolution rate of the sample was measured using Dynamic Elemental
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Thermal Analysis (DETA). The evolved volatiles were combusted by a custom heated O2 lance
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placed at downstream of the heating coal. The combustion products, including CO2/CO, H2O, H2 and
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O2, were analysed after cooling and provided the elemental composition of the total volatiles. Figure 2
∆L × 100 L0
(2)
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shows the experimental setup. Details of the DETA working principle can be found in our previous
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work.3,25
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2.4 Thermogravimetric Analysis (TGA)
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A thermogravimetric analyser Q50 manufactured by TA Instruments was used for Thermogravimetric
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analysis (TGA). The TGA Q-50 operates in the temperature range from ambient to 1000°C and has an
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isothermal temperature accuracy of ±1°C and isothermal temperature precision of ±0.01°C. It has a
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weight capacity of 1.0 g, a sensitivity of 0.1 µg and a precision of ± 0.01%. For all tests in this work,
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10 mg of the sample was heated on ceramic crucibles at 5 °C/min from 25 to 1000 °C and held for 30
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mins under nitrogen with a flow rate of 50 mL/min.
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2.5 Laser Desorption/ Ionization Time-of-Flight Mass Spectrometry
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The molecular weight distribution of the volatile tars and the THF extractable metaplast was acquired
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using laser desorption/ ionization time of flight mass spectrometry technique (LDI-TOF-MS). The
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LDI-TOF-MS experiments were conducted on Bruker Daltonics UltrafleXtreme MALDI TOF/TOF
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Mass Spectrometer. A sample volume of 0.8 µL was deposited on a ground steel target plate without
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matrix assistance. Smartbeam II laser (Nd:YAG, 335 nm) regulated at 80% energy level was used to
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generate 500 laser shots in positive, reflectron mode at one position in the sample slot. By repeating
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the shooting process four times at random positions, an accumulation of 2000 laser shots was acquired
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to generate sample spectra. The mass detector was set to detect the molecular weight (MW) from 20
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to its detection limit in reflectron mode, 7980 Da.
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3. Sample
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Table 1 summarises the results of the proximate, petrographic and Gieseler plastometer analyses of
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this coal.
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Three additives were produced using pyrolysis products from the parent coal; i) light molecular range
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(
