Thermo-swelling Behavior of Australian Coking Coals from Different

Subscriber access provided by University of Otago Library

Article

Thermo-swelling behaviour of Australian coking coals from different basins - relating to rank and maceral compositions Wei Xie, Terry F. Wall, John A. Lucas, Merrick R. Mahoney, and Rohan J. Stanger Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01683 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Thermo-swelling behavior of Australian coking coals from different basins - relating to rank and maceral compositions Wei Xie,* Terry Wall, John Lucas, Merrick Mahoney, and Rohan Stanger Department of Chemical Engineering, University of Newcastle, Callaghan NSW 2308, Australia

ABSTRACT Eight Australian coking coal samples (RvMax from 0.8 to1.6) from six different basins were employed to investigate the effect of coal maceral compositions and rank on thermo-swelling behavior. 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. Thermoplasticity of coal was evaluated based on the change of permeability of coal pellets, plastic range, swelling factor (defined by maximum swelling%/vitrinite%) and heat of exothermic reactions during the primary devolatilization. The endothermic and exothermic processes were identified by the estimate of the apparent specific heat using the Computer Aided Thermal Analysis (CATA) technique, while swelling and permeability of gas flowing through coal pellets were simultaneously investigated with extended volumetric measurement at downstream and pressure sensor at upstream, respectively. Volatile evolution profiles of heating coal samples with temperature were obtained using Thermogravimetric Analysis (TGA). Overall, the low rank coals with high vitrinite contents showed larger exothermic heats, higher volatile evolution rates and higher swelling factors than the high rank coals with low vitrinite contents. These lower rank coals also initiated a decrease in permeability at lower temperatures, showed large plastic ranges and resulted in larger pore size of semi-coke. When accounting for total fusible maceral (vitrinite plus fusible semi-inert), rather than vitrinite content alone, the trend of swelling factor decreasing with coal rank became clearer. However, it was found that coals from basin III showed significantly lower swelling factor than the comparative rank coals, and the heat of exothermic reactions showed the similar trend, despite these coals having similar volatile evolution rate with the comparative rank coals. The reasons for this have been discussed based on the thermoplasticity, which suggests that not only the rank and maceral compositions, also the origin and chemistry of the coal may affect thermo-plasticity of coking coal.

* Corresponding author: Email: [email protected] ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

1. INTRODUCTION 1.1 Physical and Chemical Changes Coke is commercially produced in coke oven batteries by carbonization of the coals or their blends from room temperature (25 °C) to 1000 °C in the absence of oxygen. During coking, the heating coal undergoes devolatilization accompanied by softening and fusion (300-550 °C), forming what is referred to as the metaplast.1,2 This metaplastic material affects mass transport of volatiles within the coal pellets.2-4 With the development of fluidity, the permeability of the coal pellet decreases; making it difficult for volatiles (gas and vaporized tar) to escape through the metaplast.3 Consequently, gas bubbles form, which causes the metaplastic material to swell.3,

5-6

with increasing temperature,

continuous swelling proceeds and completes at the maximum gas evolution rate as the resolidification occurs.3 This thermo-swelling event ultimately produces the porous structure which governs the final coke strength. Above 550 °C, the primary devolatilization is completed as cross-linking reactions dominate, resulting in light gas evolution (mainly H2 and CO).3

1.2 Thermal Change The development of thermoplasticity during the transformation of coal to coke is always accompanied by consecutive thermal events, which has been previously investigated in the literature. Using the Computer Aided Thermal Analysis (CATA) technique, Strezov et al.7 have quantified five pyrolytic regions during coal coking. The first endothermic dehydration region between 100 and 200 °C was attributed to removal of moisture, while the second endothermic region between 380 and 420 °C was found to correlate with a pre-plastic transition involving structural relaxation with the early release of CO2 and CO. The first exothermic region between 420 and 550 °C was related to the primary devolatilization with the development of fluidity of the coal. The fourth region (550-620 °C) was attributed to another endothermic reaction relating to tar vaporization and the onset of the secondary devolatilization. The fifth and largest region involved an exothermic reaction with the release of H2 and a contraction of carbon hexagonal planes during the secondary devolatilization. They also reported that the heat of the exothermic reactions was between 15 and 45 MJ/m3 during the primary 2 ACS Paragon Plus Environment

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

devolatilization (420-550 °C), and between 15 and 25 MJ/m3 during the secondary devolatilization (650-925 °C).8 The initial thermal conductivity for different coals was in the range between 0.17 and 0.31 W/(m K). With the development of metaplast, thermal conductivity increased rapidly with the changes of fluidity and the structure of coal pellets. These heat transfer effects can alter the thermal profile inside a coke oven, particularly around the plastic layer.

1.3 Effect of Coal Maceral Compositions and Rank on Thermoplasticity. Coal is generally classified by its rank which is a way of expressing the progressive metamorphism of coal from lignite (low in rank), sub-bituminous coal, bituminous coal to meta-anthracite (high in rank).2 Coal rank is based on heating value and percentage of fixed carbon that increase from lignite to low-volatile bituminous coal as the percentages of moisture and volatile matter decrease. Microscopically, coal is composed of mainly three maceral groups of liptinite, vitrinite and inertinite and minerals.2 The reflectance of vitrinite is related to the rank of the coal, higher reflectance being associated with more severely metamorphosed coals. Therefore, the vitrinite mean maximum reflectance (RvMax) is practically considered an indication of coal rank. During coking, the development of thermoplasticity of a coal or the blends of several coals could vary dramatically along with coal rank, therefore, coal rank could significantly influence coke strength. Zhang et al.9 suggested that a coke with good thermal properties can be made from a coal with a mean vitrinite reflectance of 1.1-1.2%. Huntington10 summarized that high volatile bituminous coal with RvMax of 11.15, medium volatile bituminous coal with RvMax of 1.2-1.5 and low volatile bituminous coal with RvMax of 1.5-1.65 are suitable for coking, which are expected to be able to be used for making medium or good coke.

Coal maceral compositions in the coal or the blends of several coals can significantly affect the development of thermoplasticity, such as fluidity, permeability, swelling and heat of reaction, as well as coke structure and thus influence coke strength.2,3 In terms of different maceral compositions, in general, liptinite and vitrinite are fusible, causing high fluidity and decreasing the permeability of metaplast. Our previous work has found that the higher the vitrinite content, the higher swelling, more 3 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dramatic change in permeability, larger exothermic heat could be observed during metaplast.3 Inertinite compositions may be fusible or infusible, inertinite that remains infusible can provide a pathway for volatile diffusion and retain their solid structure within a coke matrix to provide structural strength.2,3 While inertinite that is fusible behaves similarly to vitrinite, increasing thermoplasticity and thus affecting coke strength differently. As coal macerals may contain various sub-maceral compositions, thermoplasticity of coal maceral compositions may vary from coal to coal along with coal rank and origin. Therefore, the objectives of this work are to investigate the impact of coal maceral compositions and rank on thermo-swelling behavior of a suite of Australian coking coals that originated from different basins. Australian coking coals contain large proportions of semi-inerts, the fusibility of these semi-inert may vary along with coal rank and the origin.11,12 As a result, some of the semi-inert may be able to improve coking performance while some of them may show a reverse tendency.12,13 Therefore, not only vitrinite, also the effect of fusible inertinite on thermo-swelling of heating coal is investigated in this work. Eight Australian coking coals from six different basins with rank reflectance varying from 0.8% to 1.5%, vitrinite content varying from 48% to 76% and fusible inertinite content varying from 8% to 20% were used in this study. The focus is to use the CATA and TGA techniques to characterise the changes of permeability, swelling, heat of reaction and the devolatilization of the selected coals, and to correlate the thermo-swelling of the heating coal with the development of coke structure. Previous work with CATA on maceral concentrates from a single coal has shown that thermo-swelling behaviour is highly non-linear with respect to vitrinite content below ~65%.14 Furthermore, the utilisation of volatile gas to drive swelling is relatively low and highly non-linear across the plastic temperature range.3 This suggests that the diffusion of volatiles is critical in determining bubble formation3 as is the nature of the plastic phase. Here we extend this thermo-swelling methodology towards an industrial suite of coking coals with variable amounts of fusible semi-fusinite to determine the impact of coal origin on thermoplasticity

4 ACS Paragon Plus Environment

Page 4 of 27

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2. METHODOLOGY 2.1 Computer Aided Thermal Analysis (CATA) for Apparent Specific Heat and Thermal Conductivity The experimental set-up uses the principle of the CATA technique which has been used previously.7 A specific modification has been used in this work to measure volumetric swelling of the heating coal sample and pressure drop of gas flowing through coal pellet, as a function of temperature.3 The heating chamber is shown in Figure 1. The sample (~ 2 g by mass) with a top particle size of 212 µm was packed in a quartz tube with a length of 20 mm and a diameter of 11.80 mm. Pyrolytic experiments were conducted from room temperature to 1000 °C at a heating rate of 5 °C/min under an inert atmosphere with an argon flow of 30 mL/min. The temperatures of the graphite heating element, surface, and center of the sample were measured for estimating the apparent specific heat and thermal conductivity based on one-dimensional heat conduction eq 1, which uses a calibrated heat flux. The heat flux to the sample surface was calibrated using the apparent thermal resistance of a graphite sheath surrounding the central quartz tube, which was determined beforehand with a copper cylinder of known dimensions. Details for how to estimate the apparent volumetric specific heat and thermal conductivity can be seen in our previous work.3,14,15

ρC P

∂T  1  ∂  ∂T  (1) =  λ  r  ∂t  r  ∂r  ∂r 

In eq 1, where ρ is the density of the sample (kg/m3), CP is the specific heat (J/(kg K)), λ the thermal conductivity (W/(m K)), T the temperature (K), t the time, and r the radius (m).

2.2 Permeability and Swelling In Figure 1, the coal bed was restrained on the left hand side and allowed to expand only on the right hand side measured with a linear variable differential transducer (LVDT). A well-calibrated pressure sensor was connected at the gas inlet (the left side) to measure the pressure drop through the heating coal pellets. The LVDT readings as well as pressure drop were monitored each second, along with the


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

temperature. All experiments were started under the same slight compression of the well-calibrated spring. The transient horizontal swelling or contraction of samples was determined by the measured value of the LVDT. Swelling was calculated by the transient compressed value (∆L) of the spring and the original length (L0) of the sample, namely, swelling (% ) =

∆L × 100 L0

(2)

Coal bed permeability can be defined by a lumped index of how well fluid passes through coal.16 With the increase of fluidity, the permeability across the heating coal pellet decreases; it becomes difficult for gases and tars to escape from the plastic coal. The change of bed permeability with temperature can be described based on permeability coefficient k (m2) of gas flowing through coal pellets, which was estimated according to Darcy’s law.17,18 ∆P 1 = µU (3) L k

∆P is the pressure drop of gas flowing through the coal sample expressed in Pa, µ is the viscosity of the carrier gas argon expressed in Pa s, changes of argon viscosity with temperature was based on data in Perry’s chemical engineer’s handbook.19 U is the carrier gas velocity in m/s, L is the transient length of the coal bed in m, which was calculated according to the LVDT measurement. Downstream pressure was assumed to be atmospheric.

2.3 Thermogravimetric Analysis (TGA) for Volatile Evolution Volatile evolution rate of heating coal sample was evaluated using a thermogravimetric analyzer Q50 manufactured by TA Instruments. For each test, about 10 mg of coal sample was heated at 5 °C/min from room temperature to 1000 °C under nitrogen atmosphere with a flow rate of 50 mL/min. This heating rate is the same with the thermo-swelling analysis.

3. SAMPLE Eight coal samples were employed to study the thermo-swelling behaviour during coking. These coals originated from different six basins, coal A from basin I, coal B from basin II, coals C, D and G from 6 ACS Paragon Plus Environment

Page 6 of 27

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

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 3 groups based on the rank. Each group contained 2 or 3 coals from 2 or 3 different basins. The ranks of the coals were in the ranges of RvMax 0.8-1.1 for A and B, 1.1-1.3 for C, D and E, and 1.3-1.5 for F, G and H. Table 1 summarises the results for proximate, ultimate, petrography and Gieseler plastometer analyses of these samples.

4. RESULTS 4.1 Permeability and Swelling For the purpose of easily distinguishing the results between coals, the estimated permeability for these coals are shown in three groups according to the rank of RvMax, which is reported in Figure 2. The swelling and swelling rate of individual coals are presented in Figure 3 and 4, respectively. Figure 2 indicates that the estimated permeability showed a linear increase with temperature prior to 360-435 °C. The slow increase in permeability is considered thermal expansion of heated gas rather than decomposition of coal. The hotter the gas, the greater the volumetric flow is, and hence velocity through the packed bed. As the coal softened, the estimated permeability tended to decrease because of the restriction of gas flow. Since the experimental set-up in Figure 1 allows the heating coal to swell during softening, the estimated permeability showed a slight increase with the development of swelling, as seen in Figures 2-4. As the thermo-swelling proceeded, several peaks and troughs in permeability appeared along with the development of metaplast, which is different from work elsewhere.3,18 However, the change of the estimated permeability was consistent with the swelling rate that also showed several peaks and troughs, as seen in Figure 4. The permeability rapidly increased when the swelling was complete. Above 580 °C, the estimated permeability slowly increased with temperature, which indicated the completeness of thermoplasticity. Samples quenched at this temperature were able to be removed easily from the quartz tube, indicating that an initial contraction


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

had occurred, although at a low extent. At higher temperature (above 650 °C), contraction proceeded with the final coke formed ~1000 °C. The instantaneous permeability and swelling with temperature varied dramatically from coal to coal. Overall, the lower rank coals with higher vitrinite contents showed more distinct changes in permeability at the initial stage of pyrolysis (360-470 °C) than the higher rank coals with lower vitrinite contents, except for coals C, D and G that originated from basin III. This trend can be clearly observed by comparing the permeability change and the swelling rate of these coals, as seen in Figure 2 and Figure 4. Coal samples C and D showed a much lower permeability than coal E over a broader temperature range, even though they had a similar rank. Coal G gave extremely low swelling and also showed a slight decrease in permeability during this temperature range. Figures 2-4 also show the difference in critical temperatures for the changes in permeability and swelling of heating coals. The onset temperature for permeability change was impacted by coal rank and inertinite content. Within each comparative rank group, the coals that showed lower swelling rate initiated the swelling and the rapid decrease in permeability at higher temperatures, indicating a level of consistency in the changes of permeability and swelling of heating coal.

4.2 Specific Heat and Thermal Conductivity Figure 5 presents the estimated apparent volumetric specific heat of heating coal samples from 25 to 1000 °C at a heating rate of 5 °C/min. The results indicated that the apparent volumetric specific heats for the eight coal samples are in the range 1250-1700 kJ/(m3 K) at 150 °C and 1450-1950 kJ/(m3 K) at 300 °C. As the packed densities of coals were ~910 kg/m3, the estimated apparent mass specific heats are ~1.7 kJ/(kg K), which are very similar to the results reported by Hanrot et al20 and Strezov et al.7,8 They reported that the apparent specific heat of coal is in the range of 1.0-1.4 kJ/(kg K) at 25 °C and up to 1.3-1.8 kJ/(kg K) at 300 °C. The peaks in the apparent specific heat represent the endothermic processes, while the troughs of the apparent specific heat indicate the exothermic reactions stages. The endothermic processes during coal pyrolysis showed three stages: moisture and the bonded secondary water release (below 180 °C), 8 ACS Paragon Plus Environment

Page 8 of 27

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

irreversible pre-pyrolytic physical transition relating to the primary devolatilization (380-420 °C), tar vaporization and the commencement of the secondary devolatilization (580-650 °C). In comparison, the primary devolatilization (420-580 °C) and the secondary devolatilization (650-1000 °C) processes are considered to be the main exothermic reactions stages. The exothermic heats for all coal samples during the primary devolatilization are given in each comparative rank group in Figure 6. Differential specific heat was generated based on the assumed real Cp and the estimated apparent Cp. Details of this calculation can be found in elsewhere.8 Overall, the lower rank coals with higher vitrinite content showed larger exothermic heat during the primary devolatilization. However, the medium rank coal E showed larger exothermic heat than the coals C and D originated from basin III. This is consistent with the higher swelling and indicates the link with heat of exothermicity during devolatilization. The measured thermal conductivities for the eight coal samples are shown in Figure 7. They are in the range of 0.11-0.20 W/(m K) from room temperature up to 400 °C, which is in agreement with the earlier reported data by Butorin et al 21 and Strezov et al.7 The estimated thermal conductivity initially increased linearly with temperature prior to the onset of pyrolysis. Between 430 and 550 °C, a significant increase can be observed. The change of thermal conductivity appears to be related to the level of thermoplasticity. The later increase in thermal conductivity at temperatures above maximum swelling suggests it may be more closely related to the cross-linking and graphitisation than depolymerisation and tar release.3,15 In terms of the eight coal samples, the lower rank coals A and B showed larger increases in thermal conductivity at lower temperatures compared to the higher rank coals F and H. Coals C, D and G originated from basin III showed a slower change in thermal conductivity at higher temperatures than other coals. Therefore, the trend in the change of thermal conductivity for these coals was consistent with that of permeability, swelling and exothermic heat between 350 and 550 °C.

4.3 TGA for Volatiles Evolution Weight loss for all samples as a function of temperature during pyrolysis is given in Figure 8 (a), the 9 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

corresponding volatile release (TG-DTG) rate is presented in Figure 8 (b). The results indicated that the lower rank coals with high swelling and large heat of devolatilization showed higher weight loss compared to the higher rank coals with low swelling and small heat of devolatilization. Coals B, C, D, and G showed lower weight loss rate than other coals in their individual comparative rank groups. The initial temperatures for the onset of coal devolatilization increased with coal rank, which can be seen clearly in TG-DTG. This is consistent with the changes of permeability and swelling with temperature in section 4.1.

4.4 Coke Properties 4.4.1 Density Figure 9 presents the density of coke buttons that were obtained at 1000 °C. The density was estimated based on the mass (ash free) and volume of coke button, therefore, it is an indication of the total porosity of coke button. Overall, the density of the coke buttons showed the similar correlation as swelling against coal rank and vitrinite content, namely, the lower rank coal with high vitrinite content produced a lower density coke button, indicating a more porous structure. Since the coke button was produced at 1000 °C while maximum swelling occurred ~500 °C, the consistency of coke density with maximum swelling indicated that high temperature contraction did not significantly affect total porosity of the final coke buttons. Coke button produced from coal samples C, D and G that originated from basin III showed much higher coke density than expected, which is also consistent with the measured maximum swelling.

4.4.2 Optical Microscopic Analysis A semi-coke button was prepared for each coal using the same conditions as the CATA method in order to compare the coke structure development during swelling. Figures 10-13 show the optical microscopic images of the cross-sectioned semi-coke buttons that were prepared at 550 °C (after maximum swelling but prior to high temperature contraction). The images are arranged approximately from low rank to high rank coal with maximum swelling noted in the caption. In terms of the two low


Page 10 of 27

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

rank coals A and B in Figure 10, the large pore development is clearly visible, to the degree that the outer cross-section boundary is absent because of bubble growth against the quartz tube wall. Figure 11 shows the optical microscopic structure of semi-cokes from the medium rank coal samples C, D and E. The cross-sections of these semi-cokes clearly show that there are only small pores formed for coal samples C and D, while larger pores formed in coal sample E, indicating higher fluidity and bubbling occurred in coal E. Semi-coke from coal sample C shows limited porosity, while semi-coke from coal sample D (derived from the same parent coal with coal sample C but higher vitrinite content) displays minor levels of porosity. This is consistent with the measured swelling. Figure 12 shows the cross-sections of semi-cokes for the higher rank coals F, G and H. Again, significant variation in semi-coke structure is evident. Semi-coke from coal sample G shows very limited porosity, while semi-coke from coal H shows a well-developed pore structure. Coals D and H show minor indicators of a diffusion boundary layer, as seen in Figure 13. Coal F shows a semi-coke structure with the widest variation in pore development with a thick boundary layer extending from the tube wall and a highly porous middle section. The low porosity close to the wall is an expected phenomenon based on the mass diffusion of light gases and tars. This gas phase diffusion is similar with volatile evolution in an industrial coke oven, in which coking starts from both sides of the parallel walls. The softening coal forms a metaplast and this metaplast moves from the hot side towards the centre of the coke oven, which is called a moving plastic layer. The gas diffusion boundary layer lies at the front and rear of the moving plastic layer and the relative thickness of this boundary layer will ultimately affect the final coke structure.22 Volatiles close to the boundary of the packed bed are able to diffuse more readily through the softening coal to the tube wall to escape. This acts to lower the local concentration of volatiles in the plastic medium near the wall and reduce bubble formation and growth. However, if the heating coal shows high fluidity, a thick plastic layer between hot side and cool side will form. Therefore, the gas phase is entrapped in metaplast, gas bubbles may form, which results in large pores in the semi-coke.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5. DISCUSSIONS 5.1 Thermoplasticity Using the CATA technique, it was previously shown that the initial temperature of permeability change (between 360 °C and 435 °C) was an indication of the softening temperature obtained from Gieseler plastometer test.3 The completeness of swelling is regarded as the solidification temperature.3 Therefore, in this work, temperatures between initial change in permeability and maximum swelling were considered the plastic range, during which the coal underwent softening and resolidification to form a porous semi-coke. Figure 14 gives one example of how the plastic range was identified based on the changes of permeability and swelling with temperature for coal B. Based on the assumption above, the plastic ranges for all coal samples were estimated based on the results in Figures 2-3, and presented in Figure 15. Overall, low rank coal samples with high vitrinite contents initiated softening at lower temperatures (360-470 °C) and showed larger plastic ranges except for coal samples C, D and G that originated from basin III. Coal samples C and D initiated softening at higher temperatures and showed smaller plastic range than the comparatively ranked coal sample E; although coal sample D contains higher vitrinite than Coal E. This indicates that the origin of coal sample may also significantly affect the development of thermoplasticity during cokemaking.13 For all coal samples, the onset temperature of softening showed a similar trend with that of swelling, namely, the higher temperature for the onset of softening, the higher temperature for the onset of swelling This indicates the driving influence of softening on swelling. However, the temperature for the onset of swelling showed negative correlation with the maximum swelling, and the maximum swelling was not linearly related to the plastic range. This is because swelling also relates to the characteristics of metaplast, such as fluidity, and the diffusion of gas phase through the metaplast.


Page 12 of 27

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5.2 Effect of Maceral Composition and Coal Rank on Swelling and Heat of the Primary Devolatilization Liptinite and vitrinite soften and combine with the infusible inert particles to form a meta-plastic material. The entrapped volatile species formed during decomposition may produce a higher internal pressure in the metaplast, resulting in thermo-swelling of the packed bed. The influence of inertinite on thermoplasticity is complex. Inertinite that is fusible may behave similarly to vitrinite, while inertinite that remains infusible can retain their solid structure in the meta-plastic material.13 These solid inertinites may act as a diffusion pathway for gas flow and become the skeleton of the porous coke. Coal sample G contained higher infusible inertinite than coal sample H and initiated softening at higher temperature. Furthermore, Coal G showed a narrower plastic range, lower fluidity and swelling. However, reverse results were observed between coal samples D and E; where coal E contained a higher proportion of fusible inerts. The influence of coal rank and the total fusible macerals (including vitrinite and fusible inertinite) on swelling was investigated by using a normalised swelling factor; defined as maximum swelling divided by vitrinite or total fusible macerals. The results are presented in Figure 16. Overall, the lower the rank, the higher the swelling factor was. This rank dependant trend became particularly clear when accounting for fusible inertinite (as expected). In other words, swelling per fusible maceral decreased with increasing coal rank. However, coal samples C, D and G originated from basin III did not fit this trend. They showed noticeably lower swelling factors compared to other coal samples with the similar rank. An important distinction is that volatiles that evolved between 110 °C and maximum swelling temperature (based on TGA tests) in coal samples C and D are higher than that in the higher rank coal samples F and H, as seen in Figure 17. Given that the gas and tar evolution occurred over a similar temperature range (200-600 °C) for all coals, this further suggests that not only the diffusion of gas phase (gas and tar), the fluidity of the softening coal may also impact swelling. Our previous work14 found that for coal macerals concentrated from the same coal, the higher fluidity of vitrinite concentrates, the higher proportion of volatile was used for driving swelling. Further identification such as the proportion of gas and tar in volatile and the molecular weight distribution of tar and 13 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

metaplast may provide an insight into gas diffusion and bubbling,23,24 which will provide support in explaining the swelling mechanism, fluidity of metaplast and the development of porous structure of coke. Similarly, the relationship between coal rank and heat of the primary devolatilization is given in Figure 18. There is a clear trend for lower rank coals to exhibit higher exothermic heat except for coals C, D and G from basin III that appeared to be behaving like coals of higher rank from other basins with relatively lower exothermic heat. This is consistent with lower swelling factors in Figure 16. The estimated exothermic heat and thermal conductivity of heating coal are related to coal devolatilization and in particular thermo-swelling. Coals with larger exothermic heat and higher thermal conductivity usually correspond to higher volatile evolution rates and higher swelling. Coke buttons produced from these coals showed large pore sizes and thin pore walls, implying significant bubble growth and low permeability. Coals that showed relatively low exothermic heat and thermal conductivity are consistent with low fluidity and swelling, as well as small pore size with visible particles left in coke buttons. This link between thermal change with the measured physical and chemical changes was also observed in earlier work,3,8 indicating the estimated thermal changes may be used to identify the coking behavior of heating coal. However, the complex chemical characteristics of coal may affect the decomposition of macromolecule of coal, as well as the characteristics of the fragments that affect the fluidity of metaplast. For instance, coals C, D and G that originated from basin III showed unusual physical and chemical changes, which needs further study, particularly, in the chemistry area.

5.3 Thermoplasticity Relating to the Development of Coke Structure This work has also demonstrated the correlation of thermoplasticity with the development of coke structure for the tested coals. Larger pore sizes in the final coke button were observed in coals with higher swelling. Thermo-swelling is a combination of sufficient fluid phase and volatile gas production. However, our previous work has shown that the volatile release is a complex mixture of 14 ACS Paragon Plus Environment

Page 14 of 27

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

condensable tars species and light gases.3 These volatiles are released sequentially with a degree of overlap as depolymerisation processes (tar formation) are overtaken by cross-linking processes (light gas production). Our work has also shown that a portion of the condensable tar fraction is capable of being vaporised within the fluid forming temperature region, suggesting that they may contribute to the fluid phase.3 Gas production (tars and light gases) initially forms because of successive decomposition of coal macromolecules within coal pellets. With increasing temperature, coal pellets swell because of gas bubble nucleation, growth and coalescence within metaplast, which results in the formation of porous structure of semi-coke at resolidification temperature.4 According to Hays et al.,25 the pore development of semi-coke within the plastic range includes mainly four stages: (1) intraparticulate pores grow, (2) interparticulate voids are completely filled, (3) post-fusion pores grow to maximum size relating to swelling, and (4) a decrease in pore size leading to the contraction of the structure near the resolidification temperature. Clearly, the vitrinite rich lower rank coal samples A and B as well as the medium rank coal sample E initiated softening (identified by the change of permeability) and volatiles release at lower temperatures with higher softening extents and volatile evolution rates compared to other coal samples, which led to higher swelling. These coals exhibited larger pores in the semi-coke buttons, indicating significant bubble growth and coalescence. It could be expected that such high porosity would lead to lower coke strength and this was observed in removing the semi-coke buttons after heating. By contrast, the medium rank coal samples C and D, and high rank coal sample G derived from basin III initiated softening at higher temperatures with narrower plastic ranges, resulting in lower swelling. These coals exhibited smaller pore size in their semi-coke buttons. The higher rank coal samples F and H both showed larger pores than semi-coke buttons formed from coal samples C, D and G, which is consistent with higher swelling. A thick boundary layer and a highly porous middle section were observed in semi-coke button of coal F, indicating that volatile gas diffusion plays a critical role in determining coke structure for this coal. This boundary layer was not observed in semicoke buttons formed from coal H, corresponding to lower swelling.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6. CONCLUSIONS This study has used the novel CATA technique to investigate the impact of coal maceral compositions and rank on thermo-swelling behavior of a suite of Australian coking coals originated from different basins. Overall, the measured maximum swelling, volatile evolution and exothermic heat between 300 and 550 °C decreased with coal rank but increased with vitrinite content and this increasing trend became clearer when accounting for total fusible macerals (vitrintie plus fusible inertinite). The lower rank coals with higher vitrinite contents initiated themoplasticity change at lower temperature and showed larger plastic range than the higher rank coals with lower vitrinite contents. However, thermoplasticity of the coal was significantly affected by the origin of the coal. Coal samples C, D and G that were originated from basin III showed lower swelling, smaller exothermic heat and narrower plastic ranges than expected. For all tested coals, the estimated heat of exothermic reactions and thermal conductivity during the primary devolatilization were consistent with swelling, namely, large exothermic heat and higher thermal conductivity corresponded to high swelling. The volatile evolution rate corresponded with swelling extent for all coals except those from a specific basin. Coal samples C, D and G originated from basin III showed similar volatile evolution with the comparative rank coal, but much lower swelling than expected. These coals showed similar swelling behaviour to those of higher rank. Pore structure of semi-coke was found to be consistent with the development of thermoplasticity. Large pore structures corresponded to high swelling coals with large exothermic heat.

AUTHOR INFORMATION Corresponding Author

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

ACKNOWLEDGEMENT The authors gratefully acknowledge ACARP for funding this work. 16 ACS Paragon Plus Environment

Page 16 of 27

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

8. REFERENCES (1)

Habermehl , D.; Orywal, F.; Beyer, H-d. Plastic properties of coal. In Elliott, M. A. Editor. Chemistry of coal utilization, Wileyinterscience: New York, Chichester, Brisbane. Toronto, 1981; pp 317-368.

(2)

Van Krevelen, D. W. Coal, Typology-Chemistry-Physics-Constitution; Elsevier: Amsterdam,1993.

(3)

Xie, W.; Stanger, R.; Lucas, J.; Wall, T.; Mahoney, M. Fuel 2015, 147, 1-8.

(4)

Steel, K. M.; Diaz, M. C.; Duffy, J. J.; Snape, C. E.; Mahoney, M. R. Fuel 2014, 129, 102-110.

(5)

Yang, H.; Li, S.; Fletcher, T. H.; Dong, M.; Zhou, W. Energy Fuels 2014, 28, 3511-3518.

(6)

Yang, H.; Li, S.; Fletcher, T. H.; Dong, M. Energy Fuels 2014, 28, 7216-7226.

(7)

Strezov, V.; Lucas, J. A.; Strezov, L. Metall. Mater. Trans. B 2000, 31B, 1125-1131.

(8)

Strezov, V.; Morrison, A.; Nelson, P. F. ACARP Final Report; No. C15067, Macquarie Univeristy, Sydney, NSW, Australia, 2007.

(9)

Zhang, Q.; Wu, X.; Feng, A.; Shi, M. Fuel Process. Technol. 2004, 86 (1), 1-11.

(10) Huntington, H. D. Iron and steel Engineer 1997, 74 (11), 28-33. (11) O’Brien, G.; Mahoney, M.; Pickup, A.; Warren, K. ACARP Final Report No. C16047, CSIRO, Clayton South Victoria, Australia, 2010. (12) Mahoney, M.; O’Brien, G.; Hapugoda, P.; Warren, K.; Riley, D.; Lu, L. M. ACARP Final Report No. C20008, The University of Newcastle, 2013. (13) Warren, K.; Krahenbuhl, G.; Mahonery, M.; O’Brien, G.; Hapugoda, P. ACARP Final Report NO. C21059, CSIRO, 2014. (14) Xie, W.; Stanger, R.; Lucas, J.; Mahoney.; Elliott, L.; Yu, J. L.; Wall, T. Energy Fuels 2015, 29, 4893-4901. (15) Xie, W.; Stanger, R..; Lucas, J.; Wall, T.; Mahoney, M. Fuel 2013, 103, 1023-1031. (16) Shen, J.; Qin, Y.; Wang, G. X.; Fu, C.T.; Lei, B. International Journal of Coal Geology 2011, 86 (2-3), 266-275. (17) Casal, M. D.; Díaz-Faes, E.; Alvarez, R.; Díez, M. A.; Barriocanal, C. Fuel 2006, 85 (3), 281-288. (18) Nomura, S.; Mahoney, M.; Fukuda, K.; Kato, K.; Bas, A. L.; McGuire, S. Fuel 2010, 89 (7), 1549-1556. (19) Green, D. W.; Perry, R. H. Perry's Chemical Engineers' handbook. 8th ed. New York: Mcgraw-hill; 2008. (20) Hanrot, F.; Ablitzer, D.; Houzelot, J. L.; Dirand, M. Fuel, 1994, 73 (2), 305-309. (21) Butorin, V. I.; Matveeva, G. N. Coke and Chemistry 1975, 10, 20-24. (22) Osinski, E. J., Barr, P. V.; Brimavombe, J. k. Ironmaking and Steelmaking 1993, 20, 350. (23) Tran, Q. A.; Stanger, R.; Xie, W.; Smith, N.; Lucas, J.; Wall, T. Energy Fuels 2016, 30 (5), 3906-3916. (24) Stanger, R.; Tran, Q. A.; Xie, W.; Smith, N.; Lucas, J.; Yu, J.; Kennedy, E.; Stockenhuber, M.; Wall, T. Fuel 2016, 165, 33-40. (25) Hays, D.; Patrick, J.W.; Walker, A. Fuel 1976, 55, 297-302.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

Table. 1 – Proximate, Ultimate, Petrography, Gieseler Plastometer Analysis of Coal Samples

Coal name Proximate Analysis (wt %, db)

Ultimate Analysis (wt %, daf)

Petrography Analysis

Gieseler

A

B

C

D

E

F

G

H

Ash

10.2a

11.6a

6.8

5.2

9.5

11.1

8.0

11.0

Volatile Matter

33.5 a

27.5a

28.6

24.0

22.8

19.0

19.6

19.2

Fixed Carbon

56.3 a

60.9a

64.6

70.8

67.7

69.9

72.4

69.8

Carbon

85.51

88.50

87.69b

88.09

88.40

90.00

90.77b

88.92

Hydrogen

5.69

5.30

4.83b

4.91

4.74

4.66

4.68b

4.87

Nitrogen

2.26

1.86

1.31b

1.68

1.86

1.86

1.44b

1.95

Sulfur+oxygen

6.54

4.35

6.18b

5.32

5.00

3.48

3.11b

4.26

RvMax (%)

0.87

1. 11

1.19

1.18

1.3

1.5

1.46

1.52

Liptinite (mmf)

1.6

1.1

0

0

0

0

0

0

Vitrinite (mmf)

76.5

67.7

48.2

55.6

47.2

63.0

53.4

72.9

Inertinite (fusible) (mmf)

16.0

23.8

47.7

39.6

45.3

31.7

39.9

18.2

(8.6)13

(14.4)13

N/A

(14.7)13

(20.2)13

N/A

(19.0)13

(13.4)13

softening temperature, °C

N/Ac

N/Ac

435

435

420

430

450

445

maximum fluidity temperature, °C

N/Ac

N/Ac

450

450

465

470

475

480

solidification temperature, °C

N/Ac

N/Ac

465

470

495

500

490

500

plastic range, °C

N/Ac

N/Ac

30

35

75

70

40

55

maximum fluidity (Log10)

N/Ac

N/Ac

0

0.3

2.2

2.0

0.8

1.3

Plastometer (As 1038.12.4.1)

a Proximate analysis for coals A and B was tested based on ASTM E1131-08 (2014) on TGA. bUltimate analysis for coals C and G was obtained by DETA test.3 cGieseler plastometer analysis for coals A and B was not obtained because of insufficient samples from industrial coal supplier. Fusibility of inertinite compositions was determined by Goal Grain Analysis (CGA) with coal particles 0-16 mm,13 and this data for coals C and F was not provided in the reference.


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 1. Schematic of the experimental setup with pressure sensor and spring loaded on LVDT for the CATA technique.

Figure 2. Permeability of heating coal samples with temperature at a heating rate of 5 °C/min.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Swelling of heating coal samples with temperature at a heating rate of 5 °C/min.

Figure 4. Swelling rate of coal samples with temperature at a heating rate of 5 °C/min.


Page 20 of 27

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 5. Apparent specific heat of eight coal samples at a heating rate of 5 °C/min.

Figure 6. Differential specific heat (real Cp-apparent Cp) for heating coals describing exothermic heat during the primary devolatilization.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Thermal conductivities of eight coal samples with temperature at a heating rate of 5 °C/min.

Figure 8 (a) and (b).Weight loss (TGA) of eight coal samples with temperature at a heating rate of 5 °C/min.

Figure 9. Comparison of the densities of coke buttons from coals with different maceral compositions and ranks.


Page 22 of 27

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 10. Cross-sections of coke buttons for the lower rank high swelling coals A and B (A: 225% and B: 195% maximum swelling).

Figure 11. Cross-sections of coke buttons for medium rank coals (C: 5.2%, D: 19.2% and E: 131% maximum swelling).


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 12. Cross-sections of coke buttons from higher rank coals (F: 60.5%, G: 8.2% and H: 35.7% maximum swelling). The dotted line in the magnified image of F designates the diffusion boundary layer.


Page 24 of 27

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 13. Magnified cross-section images of coals D and H, showing evidence of diffusional boundary layer (notionally indicated by the dotted line).

Figure 14. One example for the measured changes of permeability of gas flowing through coal pellets and swelling versus temperature for identifying plastic range of coal B.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 15. Critical temperature for thermoplasticity of heating coal samples.

Figure 16. Comparison of the influence of vitrinite and fusible inertinite on swelling factor for all coal samples, fusibility of inertinite of these coals had been tested by Warren et al.13 the data for coals C and F was not provided.


Page 26 of 27

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 17. Relationship between released volatile and measured maximum swelling, volatile release is defined here as being material lost between 110 °C and maximum swelling.

Figure 18. Heat of the primary devolatilization versus coal rank, exothermic heat is plotted on positive axis (i.e., -∆H) and has been determined by integration between apparent specific heat and real specific heat, details for how to integrate exothermic heat can be found in elsewhere.7,14


Thermo-swelling Behavior of Australian Coking Coals from Different

Recommend Documents