Article pubs.acs.org/EF
Thermo-swelling Properties of Particle Size Cuts of Coal Maceral Concentrates Wei Xie,* Rohan Stanger, John Lucas, Merrick Mahoney, Liza Elliott, Jianglong Yu, and Terry Wall Department of Chemical Engineering, University of Newcastle, Callaghan NSW 2308, Australia ABSTRACT: A suite of coal maceral concentrates were prepared from a single coal using a water-based method for two discrete particle size fractions. Coal macerals were produced with the vitrinite content varying from 91.2% to 26.1% for the 106−212 μm particle size fraction and 96.0% to 38.2% for the 212−500 μm particle size fraction. Thermo-swelling of coal maceral concentrates were evaluated by the Computer Aided Thermal Analysis (CATA) technique with extended volumetric measurements. This novel CATA technique allows the apparent specific heat, thermal conductivity, and transient volumetric swelling of the coal sample to be measured simultaneously, as a function of temperature. The experimental results indicated that the maximum swelling (∼510 °C) and high-temperature contraction (600−1000 °C), as well as exothermic heat during primary devolatilization and thermal conductivity at maximum swelling all increased with vitrinite content, with the data inferring a linear relationship, and was independent of particle size, when coal maceral concentrates contain more than 63% vitrinite. A hypothesis for intragranular (within particles) and intergranular (between particles) swelling was used to explain the association of swelling with vitrinite content and particle size. Optical microscopic results of the final pyrolytic residues of different coal maceral concentrates support this hypothesis.
1. INTRODUCTION Previously, an amount of work has focusing on understanding the correlations between swelling and thermoplasticity. During coal pyrolysis, the weakest bridges those between ring systems in coal molecules can break to produce molecular fragments (depolymerization) between 200 °C and 500 °C,1,2 which leads to the formation of gases and tars. Meanwhile, the coal particles soften and fuse to form a plastic material in nature, called Metaplast.3,4 The meta-plastic material exhibits fluidlike properties, which affects the permeability of gas flowing through the heating of coal pellets.4−6 Consequently, gases are trapped in the particle pores, forming gas bubbles. With the growth of gas bubbles, the internal pressure in the particles increases, which causes the occurrence of swelling.5,7,8 The swelling rate rapidly increases with the evolution of volatiles and bubble growth and coalescence. With increasing temperature, the foam structure is formed and the internal gas pressure is higher than the strength of the shell, and bubble rupture occurs;8 as a result, the permeability dramatically increases.5 Around 500 °C, the rate of devolatilization decreases, and the corresponding plasticity decreases and finally irreversibly disappears. The decrease in plasticity is normally described as resolidification, and the temperature at which the plasticity disappears can be regarded as the resolidification temperature.9 The swelling reaches the maximum and is generally constant before the contraction initiated.5 Above 600 °C, there is some rearrangement of the carbon atoms; CO is released as an important constituent between 600 °C and 800 °C, while H2 passes through a very characteristic maximum at temperatures of ∼750 °C.5,9 Meanwhile, the foam structure is not stable and, with the generation of volatile (mainly H2 and CO), gas bubble is easy to rupture again; therefore, high-temperature shrinkage relating to secondary devolatilization occurs.5,8 The porous coke is formed at ∼1000 °C with the completeness of secondary devolatilization. © 2015 American Chemical Society
During the transformation of coal to coke, it involves a sequence of endothermic and exothermic reactions relating to coal devolatilization. Using Computer-Aided Thermal Analysis (CATA), Strezov et al.10 suggested that the primary devolatilization (420−520 °C) and the secondary (550−1000 °C) devolatilization are the major exothermic processes,10,11 although the primary exothermic stage may include some endothermic behavior due to volatile vaporization.5 The major endothermic processes are related to the removal of moisture (110 °C), a preplastic transition involving structural relaxation with early release of CO2 and CO (380−420 °C), and the onset of the secondary devolatilization (520−580 °C). In terms of different coals, Strezov et al.11 reported that coking coals showed larger exothermic heats of devolatilization than thermal coals, indicating the correlation between coal thermoplasticity and heats of primary reactions. The observation that higher fluidity coking coal showed larger thermal conductivity11 is also evidence that thermal conductivity is related to plasticity. In relation to the measured volumetric change, Xie et al.5,12 concluded that swelling occurred at the first exothermic stage with primary devolatilization, while contraction was observed at the secondary exothermic process with the secondary devolatilization. Loison et al.9 discussed the swelling mechanism of heating coal based on the hypothesis of intragranular (within particles) and intergranular (between particles) swelling. Intragranular swelling, which represents the swelling within coal particles, is dependent on the coal type, particle size, and heating rate, while the intergranular swelling, which represents the swelling between coal particles, is dependent on the packed bed of Received: May 20, 2015 Revised: July 22, 2015 Published: July 26, 2015 4893
DOI: 10.1021/acs.energyfuels.5b01122 Energy Fuels 2015, 29, 4893−4901
Article
Energy & Fuels
higher thermal conductivity, and greater volumetric swelling than inertinite-rich concentrates during primary devolatilization (400−600 °C). However, some concentrates that had the same vitrinite contents but different particle size distributions showed different high-temperature contraction and exothermic heat between 600 and 1000 °C. To reduce the influence of size classification, an initial suite of maceral concentrates were produced from a single particle size fraction (106−212 μm) and thermally tested.5,23 Significant differences in thermal behavior between maceral concentrates were found to occur in terms of thermoplasticity, volatile release, and tar characteristics. A recent publication demonstrated that the blend of vitrinite and inertinite impacts the degree to which volatiles are utilized for thermo-swelling.5 Particle size is also expected to influence this behavior, with larger inertinite particles providing alternative diffusion pathways for escaping volatiles. Our previous work included coal macerals with size ranges of 0−212 μm and 106−212 μm. In practice, coal charged in the industrial coke oven has a top size up to 3 mm.15 The competitive impact between particle sizes and maceral constituents is not well-known yet. Therefore, this paper has applied the water-based separation approach to include particles of larger size (212−500 μm) and compared the separation performance to laboratory-size (106−212 μm) particles. Using the CATA technique, the comparison in thermo-swelling properties for various size cuts of maceral concentrates has been applied to evaluate the influence both of maceral constituents and particle sizes on coke formation. The swelling mechanism of heating coal maceral concentrates has been clarified based on intragranular (within particles) and intergranular (between particles) swelling.
the coal and the pressure caused by gaseous bubble on the solid phase.9 This can be explained based on the development of plasticity and gas diffusion. Habermehl et al.3 summarized that the visible softening of a heating coal under microscopic observation initiates with the deformation of the individual particles that move into the interparticle space. At the beginning of softening, part of gases and tars can vaporize and be transported out of the coal particles as they are formed. With the rapid development of gaseous bubbles inside particles, the diffusion of gaseous bubbles in the plastic particles becomes limited, because of the increased plasticity. The larger the particle, the more difficult it becomes for the gas to diffuse.13 With increasing temperature, the external boundary of the plastic particles deform due to the gas pressure building up inside the particles, so that the particles move closer together. The observation for the gradual evanishment of long cracks in briquetting coal during pyrolysis between 500 °C and 600 °C also proved this hypothesis.13 Therefore, the increasing plasticity and gas pressure may cause not only intragranular swelling but also intergranular swelling. Under coking, thermo-swelling of heating coal is mainly dominated by its chemical compositions such as maceral constituents. The macerals are classified mainly in three groupsi.e., liptinite (exinite), vitrinite, and inertinite which influence, to a great extent, both coal devolatilization and the chemical and mechanical properties of the resulting coke.14−16 Liptinite is the lightest and the most fluid maceral during coking. Vitrinite appears to be cement which surrounds the other macerals and mineral matter and is easily fractured. It has more aliphatic C−H and hydrogen bonding and lower aromaticity than inertinite.4,9 It swells and agglomerates during the Metaplast phase. Inertinite contains relatively high aromaticity and is rich in carbon, but poor in hydrogen and volatile matter. It remains more thermal stable at low temperature, because of the strong aromatic rings. However, these rings decompose further at the second devolatilization when the temperature is above 600 °C. Therefore, it was suggested that the liptinite and vitrinite components are the reactive components during coal carbonization. They are related to the development of fluidity and swelling. By comparison, inertinite is considered to behave more or less as inert additives during carbonization; it shows little fluidity and swelling.4,9 Current methods of maceral separation are unsuitable for coking studies, relying on grinding to ultrafine particle size (density gradient centrifugation (DGC)17,18 or using organic/ inorganic liquids.20,21 DGC requires coal particles ∼10 μm in size to get almost-pure vitrinite or inertinite;9 however, these fine particles are not suitable for further coking analysis. Dense media, such as carbon tetrachloride and bromoform17,19 and cerium chloride,17,22 may require repetitive cycles to obtain a particular level of vitrinite- and inertinite-rich concentrates by varying the specific gravity of the density media. Also, the dense media may potentially influence the coking behavior. A novel technique called the “reflux classifier” was recently employed for coal maceral separation.12 It is a water-based method for producing concentrates. As a result, the coking properties of coal will not be affected by dense media. Using the reflux classifier, Xie et al.12 successfully separated a suit of coal maceral concentrates with different vitrinite and inertinite contents from a single coal (laboratory coal particle size 0−212 μm). Using the CATA technique, it was found that vitrinite-rich concentrates showed larger exothermic heats,
2. COAL SELECTION AND COAL MACERALS SEPARATION 2.1. Coal Macerals Separation. Not all single coking coals are able to be transformed to good coke with high coke strength for the utilization in the blast furnace. Coals charged in the coke ovens are usually blends of several different coking coals. It has been recognized that coking coals with ranks (RvMax) between 0.6 and 1.7 may be suitable for cokemaking, in particular, those with mean maximum vitrinite reflectance of 1.2% to 1.3% may produce strong coke.24 However, the limited resources of coking coals with these ranks are promoting the exploration of increasing the proportion of coking coals out of a mean maximum vitrinite reflectance of 1.2%−1.3% in the blends of coals charged in the coke oven. The increased proportion of these types of coking coals and/or their maceral concentrates in the blends will effectively enhance the utilization of poor coking coal. Therefore, this work has selected a bituminous coking coal with a mean maximum vitrinite reflectance of RvMax 1.59%. The selected parent coal contains vitrinite 73.6% and inertinite 26.4% (mmf). Other results, such as Gieseler plastometer, dilatation, proximate, ultimate, and petrography analyses, are summarized in Table 1. Before maceral separation, the selected coal samples had been crushed to a particle size of 63%. While largeparticle-size concentrates have higher ash contents than smallparticle-size samples when the vitrinite content is below 63%.
Figure 6. Swelling for coal maceral concentrates with a particle size of 212−500 μm at a heating rate of 10 °C/min.
concentrates began to swell at lower temperatures and, at temperatures between 400 °C and 550 °C, produced greater swelling than the inertinite-rich concentrates. Maximum swelling was 372% and 446% for the highest vitrinite concentrates with particle sizes of 106−212 μm and 212−500
Figure 4. Relationship between vitrinite content and ash content for particle size cuts of samples; the vertical dotted line represents concentrates with a vitrinite content of 63%.
Figure 7. Swelling for coal maceral concentrates with a particle size of 106−212 μm at a heating rate of 10 °C/min. 4896
DOI: 10.1021/acs.energyfuels.5b01122 Energy Fuels 2015, 29, 4893−4901
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Energy & Fuels μm, respectively. Also, the high-temperature contraction between 600 °C and 1000 °C increased with vitrinite content for all heating concentrates. 4.2.1.2. Endothermic and Exothermic Reactions and Thermal Conductivity. Endothermic and exothermic reactions for all particle size cuts of coal maceral concentrates were identified by measuring the apparent specific heat, as shown in Figures 8 and 9 (to easily distinguish between the maceral
Figures 10 and 11 show that, for all concentrates, the estimated thermal conductivities were ∼0.16−0.18 W/(m K)
Figure 10. Thermal conductivity for coal maceral concentrates with a particle size of 212−500 μm at a heating rate of 10 °C/min (to easily distinguish between the maceral curves, this figure has been scaled in the temperature range of 350−750 °C rather than 0−1000 °C, which does not affect our explanation in the thermoplasticity). Figure 8. Apparent specific heat for coal maceral concentrates with a particle size of 212−500 μm at a heating rate of 10 °C/min (to easily distinguish between the maceral curves, this figure has been scaled in the temperature range of 350−750 °C rather than 0−1000 °C, which does not affect our explanation in the thermoplasticity).
Figure 11. Thermal conductivity for coal maceral concentrates with a particle size of 106−212 μm at a heating rate of 10 °C/min (to easily distinguish between the maceral curves, this figure has been scaled in the temperature range of 350−750 °C rather than 0−1000 °C, which does not affect our explanation in the thermoplasticity).
Figure 9. Apparent specific heat for coal maceral concentrates with a particle size of 106−212 μm at a heating rate of 10 °C/min (to easily distinguish between the maceral curves, this figure has been scaled in the temperature range of 350−750 °C rather than 0−1000 °C, which does not affect our explanation in the thermoplasticity).
over the range of 25−475 °C. With increasing temperature, a distinct increase in thermal conductivity occurred between 475 °C and 550 °C for vitrinite-rich concentrates, while no significant increase was observed for inertinite-rich concentrates. The changes in thermal conductivities for different concentrates are consistent with swelling and exothermic reactions, as seen in Figures 6−9. For vitrinite-rich concentrates, a high swelling and a large exothermic trough correspond to a sharp increase in thermal conductivity between 400 °C and 550 °C. At the onset of the secondary exothermic reaction, the thermal conductivities for all coal maceral concentrates rapidly increased with temperature; the reason may be related to structural change and thermal inversion between measured surface and center temperatures, which were discussed in our previous paper.12 4.2.2. Effect of Both Maceral Content and Particle Size. 4.2.2.1. Swelling. Figure 12 shows the relationships between maximum swelling (∼510 °C at a heating rate of 10 °C/min) and vitrinite content for coal maceral concentrates with particle sizes 106−212 and 212−500 μm. For all concentrates with vitrinite contents of >63%, the maximum swelling increases
curves, these figures have been scaled in the temperature range of 350−750 °C, rather than 0−1000 °C, which does not affect our explanation in the thermoplasticity (the same has been done with Figures 10 and 11)). The peak of apparent specific heat represents an endothermic reaction zone, while the trough indicates an exothermic reaction zone.5,10 Overall, the vitriniterich concentrates have larger primary exothermic troughs (400−550 °C) than the inertinite-rich concentrates. A large exothermic trough corresponds to a high extent of swelling in Figures 6 and 7, which is consistent with our previous finding.5 Prior to the primary exothermic reaction, all concentrates underwent an endothermic stage between 380 °C and 480 °C, which can be identified by the endothermic peak. This endothermic peak extends to a higher temperature, with increasing inertinite content. For instance, for coal maceral concentrates with a particle size of 106−212 μm, the 86.4% vitrinite concentrate produced this endothermic peak at 425 °C, while it reached 475 °C for the 41.2% vitrinite concentrate. 4897
DOI: 10.1021/acs.energyfuels.5b01122 Energy Fuels 2015, 29, 4893−4901
Article
Energy & Fuels
was determined as the temperature when the primary endothermic reaction was complete. The end temperature of the exothermic reaction was determined as the temperature for the onset of the secondary endothermic reaction between 550 °C and 600 °C. An example of how the primary exothermic reaction of a vitrinite-rich concentrate (86.4% vitrinite, 106− 212 μm) was calculated is shown in Figure 14. This method for
Figure 12. Relationship between vitrinite content and maximum swelling (∼510 °C at 10 °C/min) for particle size cuts of coal maceral concentrates.
with vitrinite content independent of particle size, with a linear dependence inferred from Figure 12. Similarly, the hightemperature contraction (600−1000 °C) also increases with the vitrinite content, as seen in Figure 13. In addition to particle Figure 14. Example for estimating exothermic heat during the primary devolatilization (420−540 °C) for the 86.4% vitrinite concentrate with 106−212 μm size range at a heating rate of 10 °C/min.
calculating the heat of the primary exothermic reactions applied to all coal maceral concentrates that have vitrinite contents above 63%. As coal maceral concentrates with lower vitrinite contents (63%, which was consistent with the relationships between maximum swelling at ∼510 °C (Figure 12) and hightemperature contraction from 600 °C to 1000 °C (Figure 13) with vitrinite content. Coke lengths for all samples were close to the original packed coal lengths (20 mm) when the vitrinite content was below 60%, which was consistent with the observations that there was little swelling for inertinite-rich concentrates. However, they all agglomerated because of the softening of vitrinite components in these concentrates.
from high-vitrinite concentrates were more porous than those formed from the low-vitrinite concentrates. In terms of coal maceral concentrates with the vitrinite content above 90%, there was no distinct difference in coke density due to the high degree of swelling and low ash content. However, for coal maceral concentrates with vitrinite contents of 63% vitrinite, the Metaplast is involved in coalescence of all particles and, therefore, is independent of coal particle size. Therefore, the measured volumetric increase in these maceral concentrates is dominated by intergranular swelling, increases with vitrinite content, and is independent of particle size, because of the high extent of plasticity. However, when the vitrinite content is
