Chemical Changes of Australian Coking Coals from Different Basins

Nov 8, 2016 - Chemical changes of eight Australian coking coal samples from six different basins with various ranks (RvMax from 0.87 to 1.52) and mace...
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Chemical Changes of Australian Coking Coals from Different Basins with Various Ranks and Maceral Compositions: Linking to Both Physical and Thermal Changes Wei Xie,* Terry Wall, John Lucas, Merrick Mahoney, and Rohan Stanger Department of Chemical Engineering, University of Newcastle, Callaghan, New South Wales 2308, Australia ABSTRACT: Chemical changes of eight Australian coking coal samples from six different basins with various ranks (RvMax from 0.87 to 1.52) and maceral constituents (vitrinite from 48.2 to 76.5%) were dynamically investigated to explore the mechanism of coal coking linking to the synchronized physical and thermal changes. Volatile release during coal pyrolysis was monitored using a novel technique of dynamic elemental thermal analysis (DETA) that is able to differentiate tar and gas evolution in terms of carbon and hydrogen compositions. Condensed coal tars were characterized both chemically and thermally using the DETA and laser desorption ionization−time of flight−mass spectrometry (LDI−TOF−MS) techniques. Coal pyrolysis experiments were conducted at a heating rate of 5 °C/min from room temperature (25 °C) to 1000 °C with a top coal particle size of 212 μm, which kept the same experimental conditions as the physical and thermal measurements. The results indicated that, overall, the volatile evolution rates decreased with the coal rank but increased with the vitrinite content for the tested coals. This chemical observation between different coals is consistent with swelling and exothermic heat, except for medium-rank coal samples C and D from basin III that evolved a similar amount of gas and tar to the comparative rank coal sample E, but showed lower swelling and smaller exothermic heat than expected. A comparison of the hydrogen/carbon ratio (tar H/C) showed that the volatile tars evolved from the coals C, D, and G that originated from basin III contain more molecular hydrogen than that from the comparative rank coals. These volatile tars appeared to be like those from lower rank coals. Condensed tar analysis showed that volatile tars produced from all coals are in a high molecular weight distribution between 200 and 600 Da. Lower and mediumrank coals that gave higher swelling showed an elongated distribution toward a higher molecular weight, while medium-rank coals C and D from basin III and higher rank coals that gave lower swelling did not. The results implied that the molecular weight distribution and the H/C ratio in the volatile tar might affect its utilization for driving thermo-swelling.

1. INTRODUCTION 1.1. Physical, Chemical, and Thermal Changes of Heating Coal. Coke is commercially produced in coke oven batteries by carbonization of coal from room temperature (25 °C) to 1000 °C in the absence of oxygen. During coking, coal passes through a softening phase to form a metaplastic material, followed by solidification occurring at ∼550 °C with the formation of semi-coke.1 With an increasing temperature, pyrolysis proceeds with consecutive physical, chemical, and thermal changes to form coke at ∼1000 °C. The phase changes of heating coal are usually accompanied by swelling (400−550 °C) and contraction (550−1000 °C), linking to thermochemical reactions as a function of the temperature.2,3 The first exothermic reaction occurs because of the depolymerization of organic molecules between 400 and 550 °C, which causes the main evolution of gases and tars, while the secondary exothermic reaction because of repolymerization between 550 and 1000 °C results in the cross-linking with the release of light gases (mainly CO and H2).3 These physical, thermal, and chemical changes during the transformation of coal to coke can be affected by coal rank,4 coal maceral contents,3 mineral constituents,5 and coking conditions, such as the particle size, bulk density, coking temperature, and heating rate.1 1.2. Effect of Coal Maceral Compositions and Rank on Physical, Chemical, and Thermal Changes of Heating Coal. Solomon et al.2 and Saxena et al.6 suggested that the decomposition of the coal initiates with the breakage of the © XXXX American Chemical Society

weaker bonded aliphatic bridges to form small molecules. The chemical change is accompanied by softening with the development of fluidity.1,3 Microscopically, coal macerals are classified mainly as three groups, i.e., liptinite (exinite), vitrinite, and inertinite.7 The molecular characteristics, such as the ratio of aliphatic components to aromatic compounds, between coal macerals may result in the differences in the evolution of volatile (gas and tar) and the development of fluidity.7,8 Using Fourier transform infrared spectroscopy (FTIR), Xie et al.9 found that the inertinite spectrum shows less aliphatic C−H absorption but a considerably stronger aromatic C−H signal than liptinite and vitrinite spectra. Similarly, Kidena et al.10 and Sun et al.11 reported that vitrinite-rich samples contain more long aliphatic chains and bridges, a greater number of substituents on aromatic rings, and lower aromaticity than inertinite-rich samples. Xie et al.3 found that the vitrinite concentrates evolved higher yields of gas and tar, showed larger exothermic heat, and resulted in higher fluidity than the inertinite concentrates that were separated from the same coal. Inertinite material tends to remain solid to combine with the fusible macerals to form a metaplastic material.12,13 However, some inertinite may show a certain extent of fluidity because some of the semi-inert may be fusible, behaving like vitrinite.14 Received: July 10, 2016 Revised: November 2, 2016 Published: November 8, 2016 A

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Figure 1. DETA apparatus for measurements of chemical compositions (CHNOS) of tars and gases during evolution.

fusible inertinite) content initiated the decrease of permeability at lower temperatures and showed larger exothermic heats and higher volatile evolution rates, corresponding to higher swelling than the higher rank coals with lower fusible maceral content. However, it was also found that the coals named C, D, and G from basin III showed significantly lower exothermic heat and swelling than expected. Also, these coals showed smaller plastic ranges and less porous semi-coke than the comparative rank coals. The reason for this has been discussed on the basis of the thermoplasticity of heating coal, which indicated that the evolved volatiles were not used to the same extent for driving swelling. However, the reason for this is unclear. The objectives of this work are to study the effect of coal rank and maceral compositions on chemical changes of the heated coal. More specifically, this study is focused on the evolution of volatiles and the characterization of condensed tar. To achieve this, a novel technique [dynamic elemental thermal analysis (DETA)] was employed to investigate the evolution of volatiles from the coal (as elemental C and H). This method can isolate gas only and tar only streams as well as analyze the condensable liquid tar. A laser desorption ionization−time of flight−mass spectrometry (LDI−TOF−MS) technique was also used to identify the molecular weight distribution of condensed tar. The intention of this work is to understand the nature of the coal volatiles in relation to previously measured thermo-swelling behavior.

Coal rank represents the extent of metamorphic alteration from lignite to meta-anthracite. The heat value and the percentage of fixed carbon increase from lignite to low-volatile bituminous coal as the percentages of moisture and volatile matter decrease. Microscopically, reflectance of vitrinite is used as the indicator of coal rank; with increasing rank, aromaticity and molecular cluster size increase and the reflectance also correspondingly increases. The evolution rate and compositions of volatiles may vary from coals with different ranks.4 The lowrank coals (60−75% C) may attain the same total volatile yields as the high-volatile bituminous coals (80−80% C), while the total volatile yields may decrease for the higher rank coals (>85% C). The low-rank coals may generate a high yield of gases but a low yield of tars, while high-rank coals may generate a low yield of gases and a low or moderate yield of tars.2,15 In addition, the molecular structure of liquid tar may vary from different ranked coals. Using gas chromatography (GC, for molecular mass materials smaller than 200 Da) and field ionization mass spectrometry (for the molecular weight range from 200 to 1000 Da), Nomura et al.16 suggested that the relatively smaller molecules (detected at 400−800 Da) play an important role in the appearance of a plastic state. For the higher rank coal, the liquid tar showed a lower ratio of aliphatic components to aromatic compounds and wider molecular weight distribution. Meanwhile, the aromatic clusters, which are the frame of the fluid matrix, are larger in higher rank coals than those in lower rank coals. This is consistent with other observations. On the basis of the chemical percolation devolatilization (CPD) model and 13C nuclear magnetic resonance (NMR) estimates, Fletcher et al.17 suggested that the high-rank coals have lower side-chain molecular weights compared to the low-rank coals. The NMR data also correspond to the observation that more aliphatic −CH2− is seen in FTIR spectra in low-rank coal tars than that in highrank coal tars.18 1.3. Summary of the Thermo-swelling Behavior of Eight Australian Coals with Various Rank and Maceral Compositions. This work is a continuation of a study that evaluates the influence of coal maceral composition and rank on the thermo-swelling behavior of eight Australian coking coals originated from different basins. Thermo-swelling of heating coals as a function of the temperature was investigated at a heating rate of 5 °C/min using the computer-aided thermal analysis (CATA) technique; the results were presented in a separate publication and are briefly summarized here. Overall, the lower rank coals with higher fusible maceral (vitrinite plus

2. METHODOLOGY 2.1. DETA. DETA is based on the concept of converting the volatile matter (gas and tar) into combustion products as they are evolved using a custom-built O2 lance. By analysis of the combustion products, the results can be used to back calculate the elemental streams CHNOS as the gas and tar are evolved from the heating coal. About 500 mg of coal sample (wet basis) was weighed in a crucible for each test. The top size of the coal was 212 μm for all tested coals. The crucible with the coal sample was sitting in a quartz tube for pyrolysis under an inert atmosphere with an argon flow of 100 mL/min, and the heating rate was the same with thermophysical analysis by CATA at 5 °C/min. The experimental setup contains two modes, as shown in Figure 1. In mode 1, volatile material released from the heating coal samples was carried out by argon to meet the heated O2 lance at 900 °C. The combustion products were analyzed using a LiCor A CO2/ H2O infrared analyzer and a Testo 350XL flue gas analyzer (O2, CO, H2, NO, NO2, SO2, and hydrocarbons). The second mode is to combust the light gases using a second reactor outside the furnace, with the liquid tar being condensed by an ice trap placed after the furnace. Therefore, the dynamic elemental compositions of tar were mathematically calculated on the basis of the difference between total B

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ash volatile matter fixed carbon carbon hydrogen nitrogen sulfur + oxygen RvMax (%) liptinite (mmf) vitrinite (mmf) inertinite (fusible) (mmf) softening temperature (°C) maximum fluidity temperature (°C) solidification temperature (°C) plastic range (°C) maximum fluidity (log 10) maximum swelling (%) exothermic heat in the plastic range (MJ/m3)

coal name

B 11.6a 27.5a 60.9a 88.50 5.30 1.86 4.35 1. 11 1.1 67.7 23.8 (14.4)14 N/Ac N/Ac N/Ac N/Ac N/Ac 191.0 42.9

A 10.2a 33.5a 56.3a 85.51 5.69 2.26 6.54 0.87 1.6 76.5 16.0 (8.6)14 N/Ac N/Ac N/Ac N/Ac N/Ac 225.0 134.3

C 6.8 28.6 64.6 87.69b 4.83b 1.31b 6.18b 1.19 0 48.2 47.7 (N/A) 435 450 465 30 0 5.2 12.0

D 5.2 24.0 70.8 88.09 4.91 1.68 5.32 1.18 0 55.6 39.6 (14.7)14 435 450 470 35 0.3 19.2 11.8

E 9.5 22.8 67.7 88.40 4.74 1.86 5.00 1.3 0 47.2 45.3 (20.2)14 420 465 495 75 2.2 131.0 35.5

F 11.1 19.0 69.9 90.00 4.66 1.86 3.48 1.5 0 63.0 31.7 (N/A) 430 470 500 70 2.0 60.5 10.3

G 8.0 19.6 72.4 90.77b 4.68b 1.44b 3.11b 1.46 0 53.4 39.9 (19.0)14 450 475 490 40 0.8 8.2 5.7

H 11.0 19.2 69.8 88.92 4.87 1.95 4.26 1.52 0 72.9 18.2 (13.4)14 445 480 500 55 1.3 35.7 11.1

Proximate analysis for coals A and B was tested on the basis of ASTM E1131-08 on thermogravimetric analysis (TGA). bUltimate analysis for coals C and G was obtained by DETA tests.3 cGieseler plastometer analysis for coals A and B was not obtained because of insufficient samples from the industrial coal supplier. Fusibility of inertinite compositions was determined by coal grain analysis (CGA) with coal particles of 0−16 mm,14 and these data for coals C and F were not provided in the reference. dSwelling and exothermic heat from the thermo-swelling study of Australian coking coals.

a

swelling (CATA)d exothermic heat (CATA)d

Gieseler plastometer (As 1038.12.4.1)

petrography analysis

ultimate analysis (wt %, daf)

proximate analysis (wt %, db)

Table 1. Proximate, Ultimate, Petrography, Gieseler Plastometer, Swelling, and Exothermic Heat Analyses of Coal Samples

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Figure 2. Total volatiles of eight coal samples with the temperature at a heating rate of 5 °C/min in terms of total hydrogen and carbon (derived from DETA combustion of volatiles).

Figure 3. Example for mathematical calculation of “dynamic tar by difference” between total volatile and “gas only” for coal B. volatiles and gas only. Mode 1 was also used to characterize the condensed tar. After the condenser was washed out with acetone, the dissolved coal tars were reloaded into the crucible and heated in mode 1. This allows the H and C elements of tar to be quantified as it revaporized. 2.2. LDI−TOF−MS. Liquid tar samples were analyzed using LDI− TOF−MS technique. A volume of 0.8 μL of liquid (acetone + tars) was deposited, matrix-free, onto a ground steel target plate, and the solvent was allowed to evaporate at ambient temperature. Samples were analyzed using a smartbeam II laser (Nd:YAG, 355 nm) in positive, reflectron mode. A total of 2000 laser shots were accumulated for each sample at 500 shot intervals from random positions in the sample slot. The laser power was kept at the 60% energy level. The detected mass was set from 20 to 7980 Da, the limitation of the detector in reflectron mode. This LDI−TOF−MS operational mode and sampling scheme have also been used to observe the changes in solvent-extracted material from heated coal.19,20

the properties of these coal samples. Results for swelling and exothermic heat identified by CATA from the thermo-swelling study are also included.

4. RESULTS 4.1. Total Volatiles by DETA. This section uses the DETA technique to investigate the evolution rate of total volatiles, gases only, and tars only formed during coal coking. Figure 2 gives a comparison of the hydrogen and carbon concentrations produced by combusting the total volatiles as they evolved from heating coal samples at a heating rate of 5 °C/min from 25 to 1000 °C. Because the dominant elements released during the transformation of coal to coke are carbon and hydrogen, the transient concentrations of H and C represented the evolution rate of total volatile per gram of sample as a function of the temperature. For all heated coal samples, three peaks for the hydrogen evolution profile were observed. The first peak that appeared at 110 °C corresponded to moisture release; however, Figure 2 only shows the temperature between 250 and 750 °C to compare the difference in C and H evolutions between different coals during devolatilization. The second peak occurred between 450 and 520 °C, relating to the primary devolatilization. The third peak observed at 745 °C was attributed to the secondary devolatilization. For the carbon profile, there was only one peak between 450 and 520 °C, which corresponded to the primary devolatilization. The total volatile evolution profile indicated by the measured hydrogen and carbon concentrations between 300 and 580 °C corresponds to the observed physical and thermal changes that were presented in the thermo-swelling study and summarized

3. SAMPLE The same eight coal samples from the thermo-swelling study were used in this work. All coals have conserved the same names as the thermo-swelling study. These coals originated from six different basins: coal A from basin I, coal B from basin II, coals C, D, and G from basin III, coal E from basin IV, coal F from basin V, and coal H from basin VI. Samples C and D were derived from the same parent coal by combining different amounts of vitrinite- and inertine-rich fractions obtained from washing the coal. These samples were divided into three groups based on the rank. Each group contained two or three coals from two or three different basins. The ranks of the coals were in the ranges of RvMax of 0.8−1.1 for A and B, RvMax of 1.1−1.3 for C−E, and RvMax of 1.3−1.5 for F−H. Table 1 summarizes D

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Figure 4. Gas only (H and C) for eight coal samples at a heating rate of 5 °C/min.

Figure 5. Dynamic tar (H and C) for eight coals at a heating rate of 5 °C/min.

consisted predominantly of H2 and a small amount of H2O (observed in the condensed fraction as dynamic tar H). Between different coal samples, the evolution of gas only and tar only with the temperature followed the same trends with the development of total volatiles. The evolution of tar as measured by DETA (i.e., dynamic tar) has a clear influence of coal rank, with lower rank coals producing larger amounts of tar. This influence is summarized in Figure 6, which shows the integrated H and C yields in Figures 4 and 5 between 280 and 600 °C, reporting on a molar basis (as opposed to a mass basis of coal) and representing the

in the Introduction and Table 1. The general trend is that the high evolution rates of C and H for total volatiles are consistent with the high swelling and the large exothermic heat during the primary devolatilization, except for coal samples C, D, and G originating from basin III that developed much lower swelling and smaller exothermic heat than expected. Previous work3 indicated that the main gaseous compositions during the secondary devolatilization (occurring at temperatures above the plastic region) are H2 and CO; however, the measured H and C at ∼750 °C indicate that there was only a low concentration of CO for these tested coal samples. 4.2. Differentiating Tar and Gas Release by DETA. The measured release of volatile may be further divided into “gases only” and “tars only” using the DETA technique. When the liquid tars are condensed out in a condenser and the evolved gases are combusted in the secondary reactor (in mode 2), the total volatiles may be further deconstructed into “gas only” and calculated “tar only” fractions or “dynamic tars by difference”. All samples for gas only combustion were heated at the same heating rate of 5 °C/min, which is consistent with the total volatile combustion. One example about how to mathematically calculate “dynamic tars by difference” for coal B is shown in Figure 3. The comparisons for “gas only” and “tar only” between different coals are shown in Figures 4 and 5, respectively. For all coal samples, it can be observed that the initial volatile evolution consisted predominantly of carbon heavy condensable tar species initiating between 230 and 290 °C, with hydrogen-rich light gases beginning between 350 and 390 °C, as seen in Figures 4 and 5. The maximum evolution rates of tar and gas were observed between 450 and 525 °C. Above this temperature, the evolution of tar ceased and the volatiles

Figure 6. Total amount of hydrogen and carbon reporting to evolved tar from coal and overall elemental ratio of tar H/C (indicating bulk aromaticity). Results determined from integrating DETA “dynamic tar by difference” between 280 and 600 °C are based on the original mass of coal. Trend lines are fitted without the coals that showed lower swelling than other coals in the same rank groups to show difference. E

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Figure 7. LDI−TOF−MS results for condensed tar collected from heating coals at a heating rate of 5 °C/min from 25 to 1000 °C.

total amount of the condensable tar fraction from the original coal sample. Both tar H and tar C showed a downward inflecting trend with increasing rank. The ratio of these bulk tar H to tar C numbers indicated the overall aromaticity of the condensed tar phase. This ratio also trended downward with a higher rank, suggesting an increase in aromaticity. The medium-rank coal samples C and D as well as the high-rank coal sample G originated from basin III appeared to have a higher tar H/C than expected, indicating that condensed tar from these coals is more aliphatic than other coals with a similar rank. This unique chemical difference of condensable tar corresponds to the observation of lower swelling, and a smaller exothermic heat of these coals originated from basin III. 4.3. Condensed Tar Properties by LDI−TOF−MS. The molecular weight distribution of condensed coal tars during pyrolysis up to 1000 °C was analyzed using LDI−TOF−MS, as seen in Figure 7. In each panel, the intensity has been normalized by the largest peak signal for comparison. For all condensed coal tars, molecular weight was mainly in the range

of 200−600 Da. In an individual group, coal tars from the lowrank coals A and B showed a similar molecular weight distribution, and similar properties were also observed between tars from high-rank coals F−H. However, for coals C−E with rank RvMax of 1.1−1.3, condensed coal tar from coal E has a larger molecular weight distribution than that from coals C and D. Differences in molecular weight distribution of coal tar (right side in Figure 7) in each rank group indicated the potential differences in chemical components. These different curves in molecular weight distribution have been generated by subtracting one spectrum from another. In the case of both the high- and low-rank groupings, this difference is mainly because of minor changes in chemistry, resulting in a difference curve that resides within the whole distribution curve. For the midrank group, coal E has a higher molecular weight distribution, which results in the difference curve being shifted toward a higher molecular weight, containing more material with a peak ∼400 Da. An overall observation of these LDI−TOF−MS spectra is that the molecular weight distribution appears to take F

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Figure 8. LDI−TOF−MS results for condensed tar collected from the same coal heated to different temperatures (500 and 1000 °C) at a heating rate of 5 °C/min.

For the purpose of comparing the properties of the evolved volatile tar formed before and after swelling, Figure 8 compares the molecular weight distribution of condensed coal tar collected for each single coal at 500 °C (near the maximum swelling) and 1000 °C. For all coals, condensed coal tar collected at 1000 °C showed a larger molecular weight than that at 500 °C. This may be attributed to two possible reasons. The first reason is that the heating coal evolved heavier tar between 500 and 600 °C.20 The second reason is that the

two distinct shapes: coals A, B, and E show an elongated tail tending toward the higher molecular weight range, while in coals D, F, G, and H, this elongation is absent. Coals A and B have the highest volatile release (Figure 2), while coal E cannot be suggested to have varied significantly in either gas or tar evolution from coal D to justify the presence of the elongated dalton tail. Therefore, it can be concluded that this higher dalton fraction is a reflection of the distribution within the coals rather than an enhanced evaporative mechanism. G

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Figure 9. Analysis of condensed tar by DETA. All results have been scaled to make peak carbon equal 2000 ppm. The H results have used the same scaling factors and are comparable across all coals.

temperature region of tar evolution between 230 and 600 °C, this suggests that the first half of the LDI−TOF−MS dalton distribution (i.e., 200−340 Da) consists of tars evolved close to their boiling point. Those of higher molar mass are likely to have been removed through an evaporative mechanism (i.e., removed prior to their boiling point), entrained in the light gas and boiling volatiles. 4.4. Condensed Tar Properties by DETA. Condensed tars were collected in the condenser from each “gas only” experiment using acetone as a solvent. After the solvent was allowed to vaporize, this condensed coal tar sample was then reloaded into the crucible and reheated in the DETA system, which allows the condensed tars to be revaporized. As vaporized tar migrates through the plastic layer in an industrial coke oven, this simulates the effect of the tar behavior in a coke oven. The imprecise nature of this methodology means that the variable amount of tars loaded into the crucible was a function of the initial amount generated by the coal and the proportion reloaded. Figure 9 shows the DETA results for each condensed coal tar sample. To allow for a comparison between samples, each carbon profile has been scaled, so that peak carbon in each case equals 2000 ppm, and this same scaling factor has then been applied to the hydrogen profile for each coal. An important feature of all tar samples is that the majority of tars was revaporized prior to the plastic temperature region of the parent coal (notionally between 360 and 500 °C). On the carbon profile, there are several visible peaks that are centered at ∼200 and 300−350 °C, which correspond to the existence of

evolved tars between 350 and 500 °C underwent cross-linking reactions as they were subsequently condensed and revaporized out of the furnace as the coal was heated to 1000 °C. The molecular weight distributions from LDI−TOF−MS spectra showed that the collected tars from all coals contain a relatively high range of molar mass species. These tars have been collected downstream of the heating sample and, hence, have been removed from the sample as a gaseous volatile component. To provide a perspective on the size of collected tars, two well-known coal tars pyrene and chrysene (both fourringed aromatics) are shown below.

These compounds represent the low end of the LDI−TOF− MS spectra (200 g/mol) with a boiling point of ∼400 °C. The most abundant peak occurred at 340 g/mol in all coal tars, which corresponds to an estimated boiling point of approximately 623 °C using the same boiling point correlation.21 Given that the DETA system has identified the H

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Figure 10. Critical temperature for gas and tar evolution linking to the physical and thermal changes.

Figure 11. Utilization of the released volatile for driving swelling. Results are plotted in rank groups. Coals A, B, E, F, and H showed proportional change in volatile evolution during swelling. Coals C, D, and G displayed non-proportional behavior. The plots only include gas and tar evolution during swelling. The rapid decrease of gas and tar evolution after the maximum swelling is not displayed.

chemical side chains is the cause of the minor differences in peak temperatures. The hydrogen profile has the potential to be impacted by pyrolysis water below 150 °C, although in the case of tar from coal A, there appears significant water content, which overwhelms the signal up to 200 °C. A comparison of

two- and three-ringed aromatic tar species and their derivatives. The addition of methyl or ethyl groups (−CH3 and −CH2CH3) or oxygenated groups (−OH and −COOH) to such ringed systems can have a considerable impact on the boiling point, and it is likely that the distribution of such I

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interval. For coal E, the end of swelling showed an increase in both gas and tar utilization, which is similar to the nonlinear profile of coals C, D, and G and the initial swelling of coal H. Within this temperature region, the amount of volatiles required to drive swelling is greater. This suggests that one of two mechanisms is dominating: (a) either the plastic medium is undergoing a rise in viscosity or (b) the diffusion of volatiles is enhanced through the pore network. Coal D required a significant proportion of volatiles to drive a relatively small amount of swelling. Such poor use of volatiles in swelling is particularly unusual given that its total fusible content (i.e., vitrinite plus fusible inertinite) is more (70.3%) than that of coal E (67.4%). 5.3. Association of Molecular Weight Distribution of Tar with Thermoplasticity. Swelling of heating coal is caused by gas bubbles in the metaplast. Diffusion of gas-phase volatiles (both gas and tar) through the metaplast is a complex concept, which can be affected by both molecular weight distribution and overall viscosity effects (i.e., temperature and solid content). For tested coals, condensed coal tar collected at 1000 °C showed a high molecular weight distribution between 200 and 600 Da, which implied that the metaplast of the coal was also composed of the fragments with a high molecular weight distribution.20 Our previous work proved that the nonvaporized liquid in the metaplast was composed of heavier molecules compared to vaporized tar.20 Another work8 found that tetrahydrofuran (THF) extracts from heated coal showed a significant variation in molecular weight distribution with the progress of the metaplast, shifting to heavier compounds from the onset of softening to the acceleration of swelling and then shifting back to lighter compounds when the swelling was complete. Such a change of molecular weight distribution of the metaplast is also synonymous with the development of fluidity during coal, which affects the release of volatile.8,20 The molecular weight distribution of volatile tar may affect its release through the metaplast. Previous work by Stanger et al.8 and Nomura at al.16 considered that volatile tars contain base aromatic units with additional methyl groups (i.e., −CH3). DETA results showed that volatile tar evolution from coals C, D, and G contains a higher aliphatic ratio than that from the comparative rank coals E, F, and H. The LDI−TOF−MS spectra indicated that these tars were composed of a lighter molecular weight distribution. Therefore, it could be inferred that the lighter molecular weight distribution of volatile tar from coals C and D contains less base aromatic and more additional methyl groups (i.e., −CH3) than the heavier molecular weight distribution from coal E. In addition, coals C and D contain a higher proportion of inertinite, which resulted in a low fluidity of the metaplast. Therefore, the relatively poor use of volatiles in driving thermo-swelling for coals C and D might be attributed to the enhanced diffusion of the light volatile tar with limited volatile bubble formation in the metaplast. A particular subset of the coal samples are two maceral concentrates from the same parent coal. Coal samples C and D originated from the same parent coal, but sample D contains higher vitrinite than sample C. The DETA results showed that volatile tar from sample D contains higher H (Figure 6), and LDI−TOF−MS spectra indicated that this tar is composed of lighter molecules (Figure 7) than those from sample C. The tar revaporization results indicated that sample D showed a higher amount of material revaporized above 350 °C than that of

the tars from coals D and G shows a greater abundance of material centered around the 350 °C peak than the comparative rank coals E and H, respectively, which might be relevant to the lower fluidity and swelling of coals D and G that originated from basin III. For all coal tar revaporization profiles in Figure 9, they exhibited a relatively constant H/C ratio (between 1.0 and 1.5) in the temperature region between 250 and 450 °C. Above this temperature range, this ratio begins to inflect upward, indicating an increasing amount of H most likely released as H2 and an indicator of coking reactions rather than being directly revaporized. This can be proven by the residue in the crucible obtained when the experiments were completed. Such a prevalence for coking reactions over tar vaporization is a potential reason for the lack of material that can be thermally removed at temperatures close to the plastic region (360−500 °C) of the original coal. From the LDI−TOF−MS molecular weight distributions, the peak daltons of 340 Da corresponded with an estimated boiling point of 623 °C, while the lowest detected peak was 200 Da, corresponding to 388 °C (i.e., pyrene). Thus, from the chemical analysis, there is an expectation that a significant amount of vaporizable tar species exist with a boiling point inside the plastic temperature region. This discrepancy between the chemical and thermal analyses of the collected coal tars also suggests that coking mechanisms become increasingly dominant above 400 °C.

5. DISCUSSION 5.1. Correlation of Chemical Events with Thermal and Physical Events. Figure 10 represents the correlation of the chemical event with thermal and physical events for all heating coals. In general, these events initiated from chemical changes with tar and gas evolution (indicated by the measured H), followed by softening, exothermicity, and swelling. This is consistent with previous finding for maceral concentrates separated from the same coal.3,12 In terms of different coals, the temperatures for the onsets of these chemical, physical, and thermal events increased with the coal rank. However, coals C, D, and G from basin III showed larger temperature intervals between the onsets of gas evolution and swelling than the comparative rank coals, which indicated that the gas phase was easier to diffuse through these coals than that from the comparative rank coals. 5.2. Effect of the Volatile Evolution Rate on Swelling. The chemical changes here have already demonstrated that such rank dependence of physical and thermal changes is associated with the higher volatile release. However, coal samples of C, D, and G that originated from basin III showed noticeably lower swelling and smaller exothermic heat than expected, although coal samples C and D showed similar gas and tar evolution with coal sample E that had a similar rank. This suggests that the gases and tars evolved from the softened coal matrix are not being used to the same extent to drive swelling. Given that swelling is related to both volatile (gas and tar) release and fluidity of the metaplast, differentiating the impacts separately is challenging. Figure 11 shows the rate of volatile release (indicated by C evolution) as a function of swelling. Coals A, B, E, F, and H showed a relatively linear relationship, indicating that the flow of volatiles (both gas and tar) was being used to drive swelling in a relatively consistent way. This indicates that the rate of bubble growth is proportional to volatile evolution and further suggests that the plastic medium is also relatively consistent across this J

DOI: 10.1021/acs.energyfuels.6b01684 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

swelling than the comparative rank coals. The temperature range for the onsets of volatile (gas and tar) evolution and swelling for coals C, D, and G is larger than expected in comparison to other coals with a similar rank, which is also evidence that coals C, D, and G used less volatile to drive swelling. For all tested coal samples, volatile tars showed a high molecular weight distribution between 200 and 600 Da and heavy condensable tar (450−800 Da) formed between 500 and 1000 °C. The coals that showed high swelling and large exothermic heat, such as coals A, B, and E, evolved tars with an elongated distribution toward a higher molecular weight distribution than other coals that showed low swelling and small exothermic heat, such as coals C, D, G, F, and H. Further analysis indicated that the condensable tars from the low swelling coals C, D, and G are more aliphatic. In addition, the high contents of inertinite compositions in coals C, D, and G decreased the fluidity of the metaplast. Therefore, the poor use of volatiles in swelling for coals C and D might be attributed to the enhanced diffusion of the light volatile tar in the metaplast.

sample C. This material could conceivably be present in the metaplast and, hence, add to fluidity. 5.4. Coal Properties Relating to Industrial Utilization. This work has demonstrated the impact of coal rank and maceral compositions on chemical changes linking to the physical and thermal behavior for a suite of coking coal originated from different basins. Overall, the low-rank coals with high fusible maceral (vitrinite plus fusible inertinite) contents showed increased levels of swelling, exothermic heat, and volatile evolution (both tar and gas). However, it was found that the origin of the coal could also significantly affect the physical, chemical, and thermal changes of heating coal. Coal samples C, D, and G originated from basin III showed lower than expected swelling and exothermic heat, although they evolved a similar volatile with the comparative rank coals. Thermo-swelling has also shown that coals C, D, and G stand apart in terms of lower exothermic heat during the plastic region and a smaller plastic temperature range than those of the comparative rank coals. Within the plastic range, coals C, D, and G used smaller amounts of volatiles to drive swelling compared to similarly ranked coals. These factors clearly impact bubble growth and the development of porosity in the plastic region. The optical microscopy of the coke buttons showed that the coals C, D, and G displayed the lowest level of porosity across the coal suite. As found here, this low swelling and porosity is not simply because of the lower flow of volatiles but that less volatiles are being used to drive swelling. The mechanistic reasons for this phenomenon are likely related to the nature of the metaplastic material (i.e., viscosity) and the rate of diffusion of volatiles. Thermoplastic coals A, B, and to a lesser extent E are plastic across a wide temperature range and have a high rate of volatile release, consequently leading to significant swelling. In a coke oven, it would be expected that such coals have a wider plastic layer and have a larger pore size (because of bubble coalescence). By comparison, the studied coals C, D, and G from basin III would be expected to have a more narrower plastic layer, where the diffusion boundary layer comprises of a larger proportion. The effect of a narrow plastic layer and a large diffusion boundary would effectively retard bubble growth, because the concentration of dissolved volatile species in the liquid is lower. Overall, it is speculated that this propensity to reduce swelling would make these particular coals from basin III a useful blend component to reduce high-fluidity blends, where less porosity development is desirable.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 61-2-4921-6457. Fax: 61-2-4921-6521. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Australian Coal Association Research Program (ACARP) for funding this work.

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REFERENCES

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6. CONCLUSION This study has used the DETA and LDI−TOF−MS techniques to investigate the volatile evolution (both gas and tar) rate of heating coal and identified the aliphatic and aromatic ratio and the molecular weight distribution of the condensable coal tar. The results have been used to clarify the impact of coal maceral compositions and rank on chemical changes linking to the thermo-swelling behavior that was investigated by CATA. The results obtained in this study have provided a fundamental explanation for the measured thermo-swelling behavior of a suite of tested coal. The volatile evolution rate (of both “gas only” and “tar only”) increased with the vitrinite content but decrease with the coal rank, which is consistent with the general trend of thermo-swelling of the tested coals. However, the CATA results indicated that coals C, D, and G that originated from basin III showed lower thermo-swelling than expected. The relationship between volatile evolution and swelling is evidence that these coals used less volatile to drive K

DOI: 10.1021/acs.energyfuels.6b01684 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b01684 Energy Fuels XXXX, XXX, XXX−XXX