Article pubs.acs.org/EF
Investigation of Coke Quality Variation between Heat-Recovery and Byproduct Cokemaking Technology Mhlwazi S. Nyathi,*,† Richard Kruse,† Maria Mastalerz,‡ and David L. Bish§ †
ArcelorMittal USA, Global R&D, East Chicago, Indiana 46312, United States Indiana Geological Survey, Indiana University, Bloomington, Indiana 47405, United States § Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405, United States ‡
ABSTRACT: Coke structural properties that lead to coke quality difference between heat recovery and byproduct cokemaking technology are investigated. Coke fingers collected from two zones (bottom center and top center) of a commercial heat recovery oven were analyzed and compared to the coke produced from the same coal blend using a moveable-wall slot oven. Coke fingers were studied at set intervals, starting from the floor of the heat recovery oven through the top of the bed, and from the slot oven wall to wall. Coke strength after reaction (CSR) test, gas adsorption techniques, optical microscopy, and X-ray diffraction analyses were used to study the coke samples. Although exhibiting lower overall CSR compared to the heat-recovery oven coke, the slot oven produced a more uniform coke quality along the coke cake width, because of the narrow width and bilateral heating of the slot oven. The top-section coke of the heat-recovery oven displayed unique structural characteristics ascribed to the availability of free space atop this coke and the use of radiant heat to drive its coking process. The heat-recovery oven bottom center and slot-oven coke displayed low surface area and total porosity, indicating restrictive coke cake expansion. Limited swelling is attributed to the coal charge weight overlying the heat-recovery oven bottom coke and the constrained nature of the slot-oven chamber. However, the shorter coking time used in the slot oven disadvantageously deprives the slot-oven coke of sufficient time to fully develop its carbon structure; therefore, the heat-recovery oven cokes demonstrated better carbon structural development. For the slot-oven coke, carbon structural development seems to have a stronger impact on CSR than surface area does, while for the heat-recovery oven top center coke, surface area appears to be the parameter that most affects CSR.
1. INTRODUCTION The byproduct cokemaking technology, which uses batteries of slot ovens, has enjoyed decades of successful use as a predominant method of metallurgical coke production in U.S. coke manufacturing plants. However, the heat-recovery cokemaking technology is increasingly garnering attention and application as an alternative method, a move necessitated by tightening environmental legislations, among other factors.1,2 With coal being an organically complex material consisting primarily of hydrocarbons, emissions associated with coal preparation fugitive particulates, coal charging, oven leaks, coke pushing, coke quenching, and byproduct recovery processes adversely impact human health and the environment. The U.S. Environmental Protection Agency (EPA) established the maximum achievable control technology and emission standards, which requires batteries to comply with source specific emission standards. In view of byproduct cokemaking technology environmental pollutants, the 1990 Clean Air Act Amendments encourages the adoption of nonrecovery/heat-recovery cokemaking technology, as this technology meets the emission standards.2 Because of differences in design between heat-recovery ovens and slot ovens, operational conditions in these two technologies are significantly different. Industry observers report a retrospective coke quality, as expressed by coke strength after reaction (CSR), variation that overrides coal properties of blend used between these two technologies. While metallurgical coke quality can be measured and expressed using various methods and terms, © XXXX American Chemical Society
CSR is arguably one of the most commonly used indicators of coke quality.3,4 In byproduct cokemaking technology, the oven is relatively narrow and has a vertical orientation; thus, the coal bed is heated from the sides by conduction in an oxygen-deficient environment. The off-gas is collected and sent to a chemical plant, where byproducts such as light oil, tar, and benzol are recovered. On the other hand, heat-recovery cokemaking does not form byproducts, because the off-gas from the coal charge is partially combusted in the free space above a relatively thick horizontally oriented coal bed prior to being fully combusted in the sole flues below the oven floor. Hence, the carbonization process proceeds from the top of the oven by radiant heat transfer and from the bottom of the oven by heat conduction through the sole flues. The coal charge is constrained by flue walls in byproduct ovens, whereas in a heat-recovery oven, the coal bed is not constrained and, thus, can swell upward into free space above the charge. These differences in oven design mean that the heat-recovery oven coke is produced from a thicker coal bed under unconstrained conditions, using longer coking cycles and achieving higher final temperatures, in comparison to byproduct oven coke. The ultimate consequence of these design differences is the discrepancy in CSR of coke produced using these two oven types. Received: October 28, 2016 Revised: December 15, 2016
A
DOI: 10.1021/acs.energyfuels.6b02817 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Differences in coke quality between these two technologies have implications on, among other things, measures to effectively design coal blends and predict CSR for heat-recovery oven coke. The existing coke quality prediction models were developed for and verified against byproduct ovens.4 Understanding the nature and cause of the differences in coke quality between cokes produced in these technologies will enable a flexible and economical designing of coal blends for heat-recovery ovens. Improved knowledge in this regard will enable cokemakers to incorporate the advantages afforded by the fact that, in heatrecovery ovens, the unconstrained nature of the bed eliminates the oven coking pressure concerns typically encountered in slot ovens. Understanding these differences is also important for developing a path toward re-evaluating existing CSR prediction models followed by, if necessary, either modifying or replacing these models. Furthermore, the vast majority of literature on the cokemaking process relates to and originates from byproduct cokemaking technology,5−11 whereas a small portion of the literature specifically refers to heat-recovery technology.12,13 For optimal operation and efficient application of this technology, it is vital to conduct studies that provide insights into fundamental differences between cokes produced by these two technologies. As part of our continuing study on heat-recovery cokemaking technology, the aim of this investigation is to develop an understanding of the nature and cause of the differences in CSR between cokes produced in a heat-recovery oven and conventional byproduct oven technology.
to conduct CSR test per increment. Nonetheless, analysis at 50 mm increments is a preferred option, because it produces a greater number of data points. Further sample preparation and testing for CSR was done in accordance with ASTM Standard D5341. For comparison with other coke properties, CSR values for the 50 mm segments were extrapolated from second- or third-order polynomial functions that were fit to the 75 mm CSR data. R2 values for these fit curves ranged from 1.000 to 0.998. Surface area was determined by low-pressure nitrogen adsorption on degassed samples, using a Micromeritics Model ASAP-2020 apparatus, at the boiling temperature of liquid nitrogen, as detailed elsewhere,12,14 Coke surface area was obtained from the isotherms by applying the Brunauer−Emmett−Teller (BET) equation. Mercury intrusion technique was used to determine total porosity of the coke samples, by measuring the cumulative volume of the displaced mercury with each pressure increase and interpreting the curve using the Archimedes principle and Boyle’s law. The petrographic analysis of coke samples was performed using a Leica microscope equipped with reflected white-light illumination, a polarizer, a 40× oil immersion objective, and an analyzer. To study the porous structure of the samples, 60 images per sample were collected and analyzed using the Leica image analysis suite and ImageJ. The Leica image analysis suite was used to measure cell wall thickness, while the ImageJ program was used to measure the pore size. Quantitative analysis of coke carbon forms was performed following the standard procedure outlined in ASTM Standard D5061, which involved a point-count technique for determining the volume percentage of various textural domains in a metallurgical coke. The isotropic and anisotropic domains identified and quantified are according to the classification established by Gray and Devanney15 (Table 1). The point-count analysis is a well-established procedure
2. MATERIALS AND METHODS
Table 1. Classification of Binder Phase Carbon Formsa
2.1. Coking Runs. The heat-recovery coke was produced in a selected oven of a commercial plant battery operating 48-h coking cycle ovens. At the end of the coking cycle of the selected oven, pushing was interrupted by stopping and retracting the ram as it reached the halfway position into the oven. This allowed for preservation of an intact section of the coke bed with individual coke fingers retaining their relative position and orientation of production. The coke bed was quenched in place in prevention of the disintegration of the coke bed block and coke fingers. The top center (TC) and bottom center (BC), as separated by the center fissure, were sampled for intact coke fingers to be tested for CSR and characterized for their structural properties. The slot-oven coke was produced in a movable wall oven (MWO) using the coal stock used in a heat-recovery coke oven. The slot-oven coke run was top-charged with an oven bulk density similar to that used in the commercial heat-recovery oven and a gross coking time of 22 h. After quenching, intact coke fingers were collected for analysis. Similarly to heat-recovery oven sampling, coke fingers were collected at the center of the oven, thus eliminating coke produced near the oven doors and its oven edge effect. 2.2. Sample Preparation and Analytical Methods. Coke fingers were cut into 50 mm segments along the length of the coke finger, such that the analysis could be done incrementally across the coke finger. This sequential slicing along coke fingers enables evaluation of quality variation across the height of the heat-recovery oven bed, systematically from oven floor to the top, and across the slot-oven coke cake from one wall to another. The 50 mm segments are identified by the distance of their midpoint position from heat-recovery oven floor (i.e., 25, 75, 125, 175, 225, 275, 325, 375 mm, etc.), from the sole traveling upward. For slot-oven coke, the identification is by distance from oven wall moving horizontally across oven width. Each segment was crushed to a particle size of 2.8−4.7 mm,14 from which a representative sample was tested for surface area, porosity, petrography, and crystallization. For CSR testing, coke fingers were sliced into 75 mm segments, and then combined to make a bulk sample for each increment. Sectioning at 75 mm instead of 50 mm was done out of necessity, because of the large amount of sample needed for the CSR test. Slicing coke fingers at 50 mm increments would have produced an insufficient amount of the material
isotropic incipient anisotropic circular anisotropic fine circular anisotropic medium circular anisotropic coarse lenticular anisotropic fine lenticular anisotropic medium lenticular anisotropic coarse ribbon anisotropic fine ribbon anisotropic medium ribbon anisotropic coarse pyrolytic carbon a
width, μm
length to width
0.5 0.5−1.0 1.0−1.5 1.5−2.0 1.0−3.0 3.0−8.0 8.0−12.0 2.0−12.0 12.0−25.0 >25
L=W L=W L=W L < 2W L > 2W; L < 4W L > 2W; L < 4W L > 2W; L < 4W L > 4W L > 4W L > 4W
L = length, W = width.
whereby the sample pellet is systematically traversed and the textural component under the crosshair are counted according to their classification. Up to 500 points per sample were counted, from which the contribution of each carbon form in the structural makeup of the coke was computed. X-ray diffraction (XRD) analysis of coke samples was carried out using a Bruker D8 Advance X-ray diffractometer equipped with a Sol-X solid-state detector, and a Cu X-ray tube operated at 40 kV and 30 mA. Powdered samples packed in a sample holder mount were scanned from 2° to 90°, using a count time of 2 s per 0.02 step size. The Rietveld refinement method encompassing the TOPAS software was used to acquire coke crystallite height.
3. RESULTS AND DISCUSSION The differences in operational conditions, such as coal charge dimensions, coking rates, gross coking times, and off-gas flow patterns, between heat-recovery and slot-oven cokemaking technology lead to variation in coke quality between these two types. As a measure of coke quality, CSR determines the coke B
DOI: 10.1021/acs.energyfuels.6b02817 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels resistance to excessive degradation under thermal and chemical gasification conditions within the blast furnace.3,4 As noted in the experimental section, CSR was measured across the coke bed at 75 mm increments. Interpolation and extrapolation was applied to the data to obtain CSR values at 50 mm intervals (Table 2). All following discussions on CSR in this paper refer to the interpolated and extrapolated CSR data. Table 2. Coke Strength after Reaction (CSR) Values for Coke Fingers; Heat-Recovery Oven Top Center (TC), Bottom Center (BC), and Slot-Oven Coke Measured section BC
TC
coke bed height (width), mm 37.5 112.5 187.5 262.5
Interpolated and Extrapolated CSR
distance from center fissure, mm
Heat-Recovery Oven 65.0 63.6 61.9 57.2
25 75 125 175 225 275
65.1 64.1 63.3 62.2 60.0 55.9
average
61.9
average
61.8
337.5 412.5 487.5 562.5 637.5 712.5
57.3 61.3 62.3 60.6 56.6 50.5
325 375 425 475 525 575 625 675 725
56.3 59.7 61.7 62.3 61.8 60.1 57.4 53.8 49.4
average
58.1
25 75 125 175 225 275 325 375
60.3 60.3 58.2 53.8 53.8 58.2 60.3 60.3
average
58.1
average 37.5 112.5 187.5 262.5 337.5 412.5
average
58.1 Slot Oven 59.9 60.0 55.1 55.1 60.0 59.9
58.3
Figure 1. CSR variation (●) along heat-recovery oven coke bed height from oven floor to the top of the bed, and (▲) along the slot-oven coke cake width from oven wall to oven wall.
CSR
decrease in CSR toward the top of heat-recovery bed, the heatrecovery TC coke has average CSR identical to that of slot-oven coke (58.1). However, the slot-oven coke, expectedly, has a lower overall CSR than the heat-recovery coke (BC and TC combined). Average CSR across the length of the heat-recovery oven bed and slot-oven cake are 59.5 and 58.1, respectively. Furthermore, because of its shorter length and the bilateral electrical heating profile of the moveable wall oven, the slot-oven coke is more homogeneous in quality than the heat-recovery coke. Homogeneity of the slot-oven coke is shown by its smaller CSR range. The slot-oven coke CSR increases from 53.8 to 60.3 for a range of 6.5 points, whereas the heat-recovery oven coke varies considerably in CSR across the oven chamber, increasing from 49.4 to 65.1, a range of 15.7 points. Despite exhibiting better CSR, the heat-recovery oven coke has a broader quality distribution. It is generally of practical importance to produce coke of homogeneous quality. However, it must be noted that the broad CSR distribution observed here is based on limited experimental sampling. Ideally, in addition to studying CSR distribution along the slot-oven width, quality distribution would be studied along the height and length of the bed. Similarly, CSR distribution in heat-recovery oven bed would be best reflected if studied along the length of the oven, in addition to oven height. Nonetheless, pertaining to quality variation along the coke finger, the slot-oven coke appears to be more uniform in quality. Furthermore, at this point, it is important to note that, while sampling at the center of each oven ensures that the sample quality is representative of each oven type performance, the limitations of comparing a commercial oven to a pilot scale oven are recognized in discussing these results. Unfortunately, conducting a test in an identical oven scale for both technologies was neither practical nor possible at the time that this work was carried out. Nonetheless, the data presented in this work provide important insights into understanding coke structural properties, accounting for coke quality difference between these two oven types. The universal consensus among researchers identifies pore structure, carbon forms, and chemistry as coke properties that have a direct impact on CSR.19,20 In this investigation, we focus our attention to coke pore structure and carbon forms, excluding coke chemistry, because it is almost exclusively determined by the coal blend and less dependent on the type of oven used. The coke surface area available for solution loss, as determined by coke porosity and microfissuring, is known to influence CSR.19−22 An increase in surface area enhances the diffusion of
As expected,16−18 CSR tests show that there are appreciable differences in CSR trends between cokes produced in a heatrecovery oven and a slot oven (Figure 1). The CSR trend in the heat-recovery oven decreases unidirectionally from the oven floor to the tar line at the center fissure where the top section of the bed begins. From the center fissure, CSR increases progressively, reaching a maximum midway into the top-center coke section prior to decreasing toward the cauliflower end at the top of the bed. Displaying a trend almost similar to that of heatrecovery oven bottom coke section, the slot-oven coke has a CSR trend that is unidirectional along the cake width, decreasing from cauliflower ends at the oven walls to the tar line at the oven center. In terms of average CSR along the cake width and bed width, slot-oven coke has a lower overall CSR (58.1) than that of heat-recovery oven BC coke (61.8) (Table 2). Because of a C
DOI: 10.1021/acs.energyfuels.6b02817 Energy Fuels XXXX, XXX, XXX−XXX
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across the length of the coke finger. Instead, it had the smallest surface area at the tar line and a relatively large surface area at the cauliflower ends at the oven walls. Moreover, the slot-oven coke does not have the highest overall surface area. The slot-oven coke has a lower average surface area (1.04 m2/g) than heat recovery TC coke (3.25 m2/g), but higher than BC coke (0.90 m2/g) (Table 3). This unexpected behavior demonstrated by the slot-
carbon dioxide into the interior parts of a coke lump. Therefore, surface area is an important property as a proxy for determination of coke reactivity.23 The surface area trend increases from the floor of the heat-recovery oven toward the center line, then shows a curving trend on the top coke section, decreasing from the tar line and then curving upward as it approaches the top of the bed (Figure 2). Thus, the highest surface area in a heat-
Table 3. Structural Parameters for Coke Fingers; Bottom Center (BC), Top Center (TC), and Slot-Oven Coke section BC
Figure 2. Variation in surface area (●) along the height of the heatrecovery oven coke bed from the floor of the oven to the top of the bed, and (▲) along the slot-oven coke cake from oven wall to oven wall.
TC
recovery oven is located in the upper zones of the heat-recovery coke bed. By contrast, the surface area of the slot-oven coke increases slowly from the tar line to cauliflower end at the oven walls, a trend curiously different from that of surface area in heatrecovery oven. The high surface area at the top of the heatrecovery bed is attributed to the unconstrained nature of the bed, which can swell into the free space above the bed. During thermal cracking and vaporization reactions, coal decomposition is accompanied by a release of volatiles and gas products as the coal softens into a plastic state. As the coal softens, bubbles form within coal particles, enhancing the fusion of coal particles. If gas evacuation proceeds slower than gas generation, the plastic layer swells with the growth of bubbles, because of gas entrapment; therefore, the permeability of the plastic phase is crucial for determination of coal swelling and the subsequent pore structure.8−11 In heat-recovery ovens, there is no physical pressure on top of the bed; thus, in the early stages of a coking cycle, the plastic zone can expand and push toward the oven crown. The expansion of the coke bed inadvertently affects the dynamics of pore structural development near the top of the bed. High surface area in this region is a consequence of this expansion. As the freedom to expand diminishes, the surface area decreases gradually toward the oven center. In contrast, the bottom section of the heat-recovery coke is confined by the physical presence of the top section of the bed; thus, it exhibits a relatively smaller surface area. Also, the route of the escaping gases in the bottom section of the bed is not through the already formed coke, but is upward through the coal charge and from the sides. By the same logic, a possible explanation for the high surface area in the cauliflower end of the slot-oven coke is that the escape pathway for the off-gas is through the semicoke toward the oven wall,8,16 which causes moderate expansion of the semicoke during the early phase of the cycle. Based on the understanding that a high surface area results in a decrease in CSR, the slot-oven coke was expected to have the largest surface area values across the length of its coke fingers, compared to heat-recovery oven cokes, because it had a low CSR
pore area, μm2
cell wall, μm
crystallite height, nm
Heat-Recovery Oven 0.24 49.6 0.39 42.6 0.50 47.6 0.47 51.0 1.10 48.6 2.71 55.8
626 548 489 533 413 455
24.6 25.0 28.7 33.6 32.5 31.4
4.6 4.6 4.7 4.6 4.4 4.4
average
0.90
49.1
510
29.3
4.5
325 375 425 475 525 575 625 675 725
5.06 2.48 1.57 1.31 1.10 0.87 1.35 7.37 8.16
51.8 51.8 48.8 49.7 55.3 54.5 53.2 49.6 60.8
418 504 638 605 716 768 802 710 626
29.3 27.6 27.0 24.8 24.0 20.8 19.8 18.0 14.9
4.6 4.8 5.6 6.0 6.8 7.5 8.5 10.9 16.0
average
3.25
643
22.9
7.8
25 75 125 175 225 275 325 375
2.13 0.78 0.72 0.54 0.54 0.72 0.78 2.13
52.8 Slot Oven 48.7 47.0 48.9 47.1 47.1 48.9 47.0 48.7
783 704 707 638 638 707 704 783
19.2 26.9 26.3 27.7 27.7 26.3 26.9 19.2
5.0 4.4 3.6 3.7 3.7 3.6 4.4 5.0
average
1.04
47.9
708
25.0
4.1
coke bed height (width), mm 25 75 125 175 225 275
surface area, m2/g
total porosity, %
oven coke shows a weak relationship between surface area and CSR for slot-oven coke, while the heat-recovery oven coke shows a strong relationship between these two parameters, as expected (Figure 3). While the relationship observed in the slot-oven coke is unexpected, a recent study reported a positive relationship between CRI and surface area for cokes prepared using three Polish coals; however, the dependence between these parameters was not exactly linear.24 Coke total porosity accounts for all pore sizes, microfissures, and cracks. Therefore, it indicates accessibility of oxidizing gases into the interior regions of the coke where gasification occurs.6,25 Studies have suggested that there is a relationship between crack path, pore orientation, and quality of coke.26,27 Both the heatrecovery bed and slot-oven cake do not show a clear total porosity trend along the height of the coke bed (Figure 4). However, the slot-oven coke has lower total porosity, compared to heat-recovery oven cokes (slot-oven coke (47.9%) < BC D
DOI: 10.1021/acs.energyfuels.6b02817 Energy Fuels XXXX, XXX, XXX−XXX
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thickness, whereby the thinning of the cell-wall is indicative of pore enlargement.27,28 Cell-wall thickness underscores the susceptibility of the coke structural framework to crack or resist cracking during handling and its ability to carry the blast furnace burden without crumbling into fines. The slot-oven coke has a thinner average pore cell-wall (25.0 μm) than the BC coke finger (29.3 μm), but thicker than the TC coke (23.6 μm) (Table 3). The thinner cell wall in the heat-recovery TC section is due to the pore growth attained during the excessive expansion of the bed during the early stages of coking cycle. The thicker cell wall in the heat-recovery BC section is consistent with a smaller average pore size in this coke. The heat-recovery BC section coke shares a similar experience of limited swelling due to physical constraint as slot-oven coke. However, the shared experience is not reflected by the coke average pore size, because the similarities between pore structural traits of slot-oven coke and heatrecovery BC coke are dilapidated by differences in off-gas escape routes. The exit path of gases in the bottom zone of the heatrecovery oven is upward through the coal bed, instead of downward through the semicoke toward the oven sole, whereas in the slot-oven the evacuating gases migrate through the semicoke, thus causing pore coalescence. In addition to promoting coalescence, the migrating gases in the slot-oven semicoke possibly distort/deform the bubbles, resulting in pore elongation and formation of fissurization sites, thus increasing the propensity for the formation of connected pores and weakened structural framework by reason of unfavorably shaped pores. The integrity of the carbon matrix also plays an important role on the reactivity of the coke. The coke reactivity for solution loss is known to decrease with an increase in the order of carbon anisotropy.5,33 Carbon matrix development can be expressed by the extent of crystallization and the development of optical textural components. An increase in crystallite height implies an improvement in the degree of carbon structural ordering and, by extension, improved coke quality.29,30 XRD analysis shows that, in the heat-recovery oven coke, the crystallite height remains relatively the same from the floor of the oven to the center fissure and then increases considerably from the center fissure to the top of the bed (see Figure 5). On the other hand, the slot-oven coke crystallite height decreases from cauliflower ends at oven walls to the tarline at the oven center. The decrease in crystallite height toward the oven center is partially explained by the maximum temperature gradient along the slot-oven width and heatrecovery oven height. The highest maximum temperatures are in regions nearest to the sources of heat; therefore, the maximum
Figure 3. Relationship between CSR and surface area: (▲) bottom center (BC), (■) top center (TC), and (×) slot-oven coke.
Figure 4. Total porosity variation (●) along the heat-recovery oven coke bed height from the floor of the oven to the top of the bed, and (▲) along the slot-oven coke cake from oven wall to oven wall.
(49.1%) < TC (52.8%)). Porosity development is governed by the interaction between the evolving gases and the plastic matter during the coking cycle and is influenced by the rate of volatile release and fluid rheology. Expansion and contraction are a function of plastic zone gas pressure, coking rate, and coking time.6 Therefore, the pore structural evolution is dependent on these variables, in addition to coal blend properties. It is suggested that the total porosity of the slot-oven coke is, accordingly, kept minimal by oven wall constraints, which limit cake swelling and shorter coking time, which does not allow for further pore growth. Lower total porosity in the heat-recovery BC section, compared to the TC section, is probably due to the presence of weight of the coal charge overlying and exerting physical pressure, thus limiting the swelling of this coke. The influence of physical pressure and routes for escaping gases is also shown by coke average pore size. Although having the lowest total porosity, the slot-oven coke has a pore area (708 μm2) that is relatively larger than the TC coke (643 μm2) and BC coke (510 μm2) (Table 3). Under the pressure created by a constrained oven chamber, the slot-oven coke may have experienced the coalescence of gas bubbles to form larger pores. The combined effect of bubbles coalescing and the escaping gases going through the semicoke into the slot-oven wall possibly resulted in the observed relatively large average pore area. The lower average pore area in the BC coke is ascribed to the influence of weight of the overlying coal charge. Complementary to pore enlargement is the pore cell-wall
Figure 5. Crystallite height variation (●) along the heat-recovery oven coke bed height from the floor of the oven to the top of the bed, and (▲) along the slot-oven coke cake from oven wall to oven wall. E
DOI: 10.1021/acs.energyfuels.6b02817 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
undergo solution loss at different rates, with inert and isotropic components being the most reactive.5,33,34 Displaying yet another similarity in behavior, the heat-recovery BC coke and slot-oven coke show the effect of longer local soak times on the development of carbon forms (see Table 4). In the heat-recovery BC coke, this is evidenced by a decrease in the proportion of anisotropic ribbon and pyrolytic carbon, while the concentration of isotropic and incipient carbon increases, from the cauliflower end at the floor of the oven to the tar line at the center fissure. For the slot-oven coke, the concentration of anisotropic lenticular carbon increases along the length of the finger at the expense of anisotropic ribbon carbon. The trend observed in BC and slotoven coke carbon forms is also due to a dissimilarity in coking rate along the coal bed. Given a natural thermal gradient showing a gradual decrease in temperature toward oven center, the coking rate has a tendency to slow toward the oven interior, a variation that is accompanied by a narrowing of a plastic mass as it migrates inward the oven. A thicker plastic zone favors the formation of less-reactive carbons forms. The heat-recovery TC section coke does not show a clear trend of carbon forms development along the axis of the finger. However, there is a notably higher contribution of pyrolytic carbon in the TC coke than there is in the BC and slot-oven coke, indicating that the escape pathway for off-gases is through the top portion of the heat-recovery oven. On an average basis, petrographic analysis indicates that the slot-oven coke has a smaller proportion of less-reactive carbon forms (63.1%), compared to BC coke (73.1%) and TC coke (67.3%). In this study, the percentage of less-reactive carbon forms is given by adding the contributions of anisotropic lenticular, anisotropic ribbon, and pyrolytic carbon together. The deficiency observed in less-reactive carbons of slot-oven coke is attributable to a shorter coking time. The development of carbon forms is reliant on the availability of time for isochromatic units to coalesce or grow into larger domains. It is worth noting that the third data point (49.0%) in the slot-oven coke is likely erroneous (Table 4). This is similar to the correlation between crystallite height and CSR, while the BC shows a positive relationship between the percentage of less-reactive carbon forms and CSR (Figure 7), the TC coke shows a weak relationship between these parameters. Poor correlation observed in the slot-oven coke is caused by the erroneous data point. When this data point is excluded, the slot-oven displays a positive relationship between CSR and the percentage of lessreactive carbon forms. The behavior shown by the TC coke is not consistent with other studies, which suggests an increase in CSR as the carbon forms develop. Considering that the BC and slotoven coke displayed relatively low surface area and total porosity values, compared to TC coke, we suggest that, if a heat-recovery coke has a high surface area available for solution loss, surface area is more likely to take a dominant role in controlling coke reactivity, relative to carbon structure. Notwithstanding some similarities in pore structure with the BC coke, the slot-oven coke has a less desirable carbon structure, because of the shorter coking time used in the slot oven.
temperature declines toward the oven interior. Thus, the presence of lower degrees of crystallization near the oven center is explainable on this account.9,31 However, the slot-oven and BC coke experience only a slight variation in crystallite height along the length of the coke finger. The slot-oven coke has a smaller average crystallite size (4.1 nm) relative to BC coke (4.5 nm) and TC coke (7.8 nm) (Table 3). Higher crystallite height in the heat-recovery TC coke is indicative of the relative amount of time this coke was exposed to escaping off-gases. Prolonged exposure resulted in the deposition of a considerable amount of pyrolytic carbon in the TC coke. Another possible explanation for the higher degree of crystallization in the TC coke is its heating mechanism, which resulted in the top section of the heatrecovery oven coking at a faster rate than the bottom section. The TC coke is produced via radiant heating of the coal charge while the BC and slot-oven coke are produced using a conductive heating mechanism. Despite having an equally long residence time, the heat-recovery BC coke exhibits a relatively low degree of crystallization. This is unexplainable at the moment, although the slower coking rate in the bottom section of the heat-recovery bed likely resulted to a thinner plastic mass, thus impacting crystallite height. A smaller crystallite height in the slot-oven coke is symptomatic of a shorter coking time used in the slot-oven coking run. The shorter coking time used in the slot-oven limits the amount of time the aromatic layers have to mobilize and stack into larger crystals. Molecular geometry investigations surmise that highly oriented molecular domains tend to have large crystals that are spatially less turbostratic, resulting in improved coke quality.32 Based on the perception that large crystallite height increases resistance to oxidizing gases and also indicates a decrease in the presence of reactive functional groups, a positive relationship was expected between the extent of crystallization and CSR. In the case of slot-oven and heat-recovery oven BC section coke, the relationship between CSR and crystallite height is weak (R2 = 0.51 and 0.67, respectively). Although weak, the relationship indicates a positive correlation between these two parameters (see Figure 6). In contrast, for the TC coke, there is a poor
Figure 6. Relationship between CSR and crystallite height: (▲) bottom center (BC), (■) top center (TC), and (×) slot-oven coke.
4. CONCLUSIONS In this study, we sought to develop and enhance a detailed understanding of fundamental structural and operational differences that lead to differences in coke strength after reaction (CSR) between cokes that are produced from the same coal blend, using byproduct and heat-recovery cokemaking technology. As expected, the moveable wall slot-oven coke displayed a lower overall CSR than the heat-recovery coke. However,
relationship between these parameters. Thus, the relationship between the degree of crystallization and the degree of coke reactivity is complicated; the extent of crystallization is probably less effective in indicating the reactivity of coke produced via radiant heating mechanism. Petrographic analysis is another method of analyzing the magnitude of carbon structural ordering. Different carbon forms F
DOI: 10.1021/acs.energyfuels.6b02817 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 4. Petrographic Properties for Coke Fingers; Bottom Center (BC), Top Center (TC), and Slot-Oven Coke Carbon Forma (%) section BC
TC
a
coke bed height (width), mm
isotropic + incipient, %
C
L
Heat-Recovery Oven 1.7 17.7 2.6 24.0 5.7 19.0 6.9 42.6 6.2 29.8 14.0 40.0
R
PC
L + R + PC, %
filler phase, %
59.8 57.2 52.0 33.7 33.2 23.0
3.5 3.0 0.0 0.0 0.0 0.0
81.0 84.2 71.0 76.3 63.0 63.0
11.8 10.2 15.3 8.8 17.5 10.0
25 75 125 175 225 275
5.5 3.0 8.0 7.9 13.4 13.0
average
8.5
6.2
28.9
43.2
1.1
73.1
12.3
325 375 425 475 525 575 625 675 725
4.6 10.0 2.9 11.2 6.1 3.0 4.0 1.0 6.0
6.9 16.0 17.7 4.1 11.2 16.2 7.9 8.9 6.0
50.9 42.0 40.1 40.8 41.8 39.4 35.6 45.5 36.0
24.8 14.0 27.5 25.5 15.3 17.2 18.8 22.8 21.0
0.0 0.0 1.0 1.0 4.1 16.2 1.0 4.0 19.0
75.7 56.0 68.6 67.3 61.2 72.8 55.4 72.3 76.0
12.7 18.0 10.8 17.3 21.4 8.1 32.7 17.8 12.0
average
5.4
41.3
20.8
5.1
67.3
16.8
25 75 125 175 225 275 325 375 average
4.0 6.0 20.0 9.3 9.3 20.0 6.0 4.0 9.8
10.5 Slot Oven 13.0 7.0 10.0 10.4 10.4 10.0 7.0 13.0 10.1
39.0 59.0 41.0 51.8 51.8 41.0 59.0 39.0 47.7
16.0 11.0 8.0 14.5 14.5 8.0 11.0 16.0 12.4
12.0 0.0 0.0 0.0 0.0 0.0 0.0 12.0 3.0
67.0 70.0 49.0 66.3 66.3 49.0 70.0 67.0 63.1
16.0 17.0 21.0 14.1 14.1 21.0 17.0 16.0 17.0
C, L, R, and PC denotes circular, lenticular, ribbon, and pyrolytic carbon, respectively.
in both the slot-oven coke and the heat-recovery bottom center coke are attributable to the presence of physical pressure and the use of a conduction heating mechanism to produce these cokes. The slot-oven wall lateral pressure and the off-gases escape pathway resulted in low surface area and low total porosity in the slot-oven coke. Similarly, limited swelling that was attributed to the coal charge weight overlying the heat-recovery oven bottom coke and off-gas route in the bottom section of the oven resulted in low surface area and total porosity in heat-recovery bottom center coke. However, slot-oven coke developed an unfavorable pore structure, because of off-gases passing the semicoke, thus promoting pore coalescence and, possibly, pore distortion. Moreover, a shorter coking time in slot-oven coke resulted in noticeably weaker development of slot-oven carbon structure compared to heat-recovery oven coke. The heat-recovery oven top center coke shows that, when a large amount of surface area is available for solution loss, the significance of carbon forms, with regard to impacting coke reactivity, diminishes.
Figure 7. Relationship between CSR and concentration of less-reactive carbon forms: (▲) bottom center (BC), (■) top center (TC), and (×) slot-oven coke.
■
because of the narrower width of the slot oven and its bilateral heating system, the slot-oven coke had a more homogeneous CSR along the length of the coke fingers than did the heatrecovery oven coke. The heat-recovery oven top-center coke displayed distinct characteristics, because of the absence of physical pressure and the use of radiant heat in producing this coke. On the other hand, porous structural similarities observed
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Mhlwazi S. Nyathi: 0000-0003-1763-8089 G
DOI: 10.1021/acs.energyfuels.6b02817 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Notes
(33) Fujita, H.; Hijiriyama, M.; Nishida, S. Fuel 1983, 62, 875−879. (34) Chiu, Y. F. Ironmaking Steelmaking 1982, 9, 193−199.
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS The authors would like to thank ArcelorMittal USA, SunCoke Energy, and Indiana Geological Survey for supporting this work. REFERENCES
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DOI: 10.1021/acs.energyfuels.6b02817 Energy Fuels XXXX, XXX, XXX−XXX
