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Effect of coal properties on the strength of coke under simulated blast furnace conditions
Fuel 237 (2019) 775–785
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Full Length Article
Effect of coal properties on the strength of coke under simulated blast furnace conditions
T
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Xing Xinga, , Harold Rogersb, Paul Zullic, Kim Hockingsd, Oleg Ostrovskia a
School of Materials Science and Engineering, UNSW Sydney, Kensington, NSW 2052, Australia School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia c School of Mechanical, Materials & Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia d BHP, Brisbane, QLD 4000, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal properties Coke strength Blast furnace conditions
Eight cokes made from coals/blend with different properties (vitrinite mean maximum reflectance = 0.90–1.66, logarithm Gieseler maximum fluidity = 4.16–1.30) were subjected to gasification and annealing simulating the conditions within an ironmaking blast furnace (BF). The specific methodology utilised included gasification of coke with BF gas-temperature profile from 900 to 1400 °C (corresponding from the thermal reserve zone to the cohesive zone) and annealing of coke up to 2000 °C (corresponding to the raceway region). The coke microstrength and macrostrength were determined using ultra-micro indentation and tensile test to understand the effect of coal precursor properties on the strength of the resulting coke and the changes when processed under the simulated BF conditions. Under the high temperatures in the simulated BF processes, the cokes from different coals showed significant differences in their properties, even though most of them had similar Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR) values. Coke microtextures experienced significant reflectance loss and structure change upon simulated BF gasification and annealing conditions. The decreases in mean maximum reflectance and bireflectance were more severe for the coke produced from the high rank coal. The cokes made from coals with higher rank and lower Gieseler maximum fluidity exhibited greater change in their microstructure upon high temperatures. As a result, microstrength of cokes produced from these coals decreased more than that of coke made from the parent coals with a lower rank and higher Gieseler maximum fluidity; this tendency was more significant in the Reactive Maceral Derived Components (RMDC) than Inert Maceral Derived Components (IMDC) microtextural type. Degradation of macrostrength of cokes produced from coals with higher rank and lower maximum fluidity was also more severe due to the greater decrease in their microstrength.
1. Introduction Functionally, metallurgical coke fed to the BF fulfils three major roles providing fuel, reducing agent and structural support media. Of these three roles, the first two can be largely substituted by the tuyere injection of other fuels, including oil, pulverised coal, natural gas, and plastic wastes [1,2]. However, only coke fulfils the burden support role in the BF. The pulverized coal injection (PCI) significantly decreases the coke rate (weight of coke required to produce 1 ton of hot metal); simultaneously, coke residence time is significantly increased, which means that coke is subjected to longer periods under the mechanical, thermal and chemical stresses in the BF. Therefore, coke is required to maintain adequate strength to support the iron-bearing burden and provide a permeable matrix necessary for slag and metal to pass down
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into the hearth and for hot gases to pass upwards into the stack. Coal rank (typically characterised by vitrinite mean maximum reflectance, MMR) and caking property (typically characterised using Gieseler maximum fluidity) are two of the most important coal properties in deciding the application [3]. The coal rank correlates strongly with volatile matter content, calorific value of the coal and other chemical parameters that reflect the changes taking place in the coal molecular structure during maturation. The fluidity of coal plastic stage is a major factor in determining what proportions of a coal is used in a blend for cokmaking. The coke strength largely depends on the properties of its coal precursor. One of the approaches to predict coke cold strength was based on the correlation established in MOF diagram between petrographic data (Rmax) and rheological properties (Gieseler maximum fluidity) of coals [4]. Miura suggested that the coke cold
Corresponding author. E-mail address: [email protected] (X. Xing).
https://doi.org/10.1016/j.fuel.2018.10.069 Received 2 July 2018; Received in revised form 29 August 2018; Accepted 10 October 2018 Available online 16 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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subsequently stabilised by three drops through 4 m onto cement. All produced coke samples were crushed to a size range of +19 to 21 mm for the bench-scale testing and processing. The proximate analyses of the cokes were conducted following Australian Standard AS 1038.3-2000. The CRI and CSR were determined following Australian Standard AS 1038.13-1990. The cold strength of coke was measured using I-drum tumbler test. A 150 g coke sample with a particle size of −21 + 19 mm was tumbled in an I-drum tumbler (130 mm internal diameter, 700 mm length) rotated at 20 revolutions/minutes for 30 min. The tumbled sample was sieved over a 10 mm perforated plate laboratory sieve; the weight percentage of +10 mm of the original sample was reported as I-drum tumbler strength and noted as I600. The ash composition of cokes was analysed using XRF. A summary of the properties of produced cokes are presented in Table 2. The difference in Gieseler maximum fluidity of the parent coals/ blend did not cause significant variation in coke cold strength in this study. Except for Coke B, all the cokes had comparatively low CRI value in the range of 17.2–27.8%. Coke B had significantly higher reactivity (CRI = 46.7%) and much lower strength after reaction (CSR = 31.9%) than other coke samples. The high reactivity of Coke B was partially attributed to its ash composition and microtextural types. Compared to other cokes, ash of Coke B had much greater concentration of Fe2O3, which could catalyse the solution loss reaction during the CRI test. Microtextures of cokes includes RMDC and IMDC. The MMR of the parent coals investigated in this study covered a wide range, and the microtexture types of RMDC varied significantly among the cokes. The optical microscopy assessments were based on the classification described by Gray [21]. Optical microscopical inspections in plain ‘white’ light, were employed at magnifications of 50–1000 times and the classifications applied in respect of the shape, form and size of isochromatic units. Coke A was produced from coal with the lowest MMR among eight samples, the fused components of Coke A mainly consisted of fine circular anisotropic with minor incipient anisotropic and rare isotropic microtextural types; the cokes made from coals with a higher rank tend to have microtextural types with larger size isochromatic units as their RMDC microtextures; the MMR of parent coal of Coke H was 1.66, the fused components of this coke were dominated by the medium to coarse lenticular anisotropic and fine to coarse ribbon anisotropic microtextural types (Fig. 1).
strength (JIS indices, DI30/15) was mainly governed by the parent coal rank when the Gieseler maximum fluidity of coal was in the range of 200–1000 ddpm [5]. A tendency of coke JIS drum index to increase as maximum fluidity increased were reported in previous studies [6,7]. Nakamura et al. [8] reported that the CSR reached the maximum in the region of prime coking coal with mean reflectance of 1.2–1.3%. Zhang et al. [9] also found that the coke with high CSR could be produced from a coal with random reflectance of 1.1–1.2%, further increases in inertinite content or rank of the parent coal caused a decrease in the CSR value of the formed cokes [10–12]. Pusz et al. [13] suggested that the higher inertinite content in parent coals could increase the CRI and decrease the CSR of the formed coke, and this influence was greater for the middle rank coals than those from lower ranks; the technological properties, e.g. the CRI and CSR indices, of coke could be predicted by its reflectance parameters. The correlation between parent coal properties (mean vitrinite reflectance and Gieseler maximum fluidity) and resulting CSR has been intensively studied with models developed to predict coke CSR [5,9,14]. However, the behaviours of coke under the BF operating conditions has not been comprehensively investigated in the previous studies. In the lower region of the BF, the temperature increases from 1200 °C to approximately 2000 °C (close to the typical raceway flame temperature); the gas atmosphere and flow dynamics also change significantly, which significantly influence the strength of coke. The previous studies on the coke extracted from operating BF mainly focused on the significant graphitisation of coke experienced after descent to the tuyere level [15–17]. Gornostayev et al. [18] observed the graphite crystals with an ideal hexagonal prism shape formed on the surface of BF cokes. Wang et al [19] suggested that the temperature, amount of iron in contact with coke and residence time of coke in the hottest regions of the furnace are important factors in the catalytic graphitisation process. However, very few studies have been conducted to systematically investigate the effect of the parent coal properties on the behaviours of coke strength under the more realistic BF conditions. In this study, a series of cokes were prepared from coals/blend with different ranks and caking properties carbonised in a 400-kg pilot scale coke oven. All the cokes were subjected to conditions simulating BF gas compositiontemperature profiles to 1400 °C, or gasification with subsequent annealing under N2 at 2000 °C (temperature of the region close to the raceway in BF). This paper aims to study the effect of precursor coal rank and caking property on the micro- and macro-strength of resulting cokes under simulated BF conditions and to understand the mechanism of different behaviours of cokes produced from various parent coals in the BF ironmaking process.
2.2. Treatments of cokes under the simulated BF conditions A 200 g sample of each coke (+19–21 mm particle size range) was contained in a silicon carbide reaction vessel and gasified in a vertical furnace. Most importantly, the gasification of the coke samples in this study was conducted under a BF gas composition-temperature profile, simulating conditions from the thermal reserve zone through to cohesive zone. The composition of the CO-CO2-N2 in the gas mixture was varied with temperature (Fig. 2). The actual gas composition-temperature profile used was designed based on the vertical probing of a BF, as reported by van der Velden et al. [22]. The feed coke sample prior to any treatment was labelled as Coke 1. The gasification commenced at 900 °C and stopped once temperature reached 1100 °C (Coke 2) and 1400 °C (Coke 3). Coke samples reacted under the conditions for Cokes 3 were then kept at the final conditions for additional 2 h, the samples after gasification were subsequently annealed under N2 at 2000 °C for 2 h in a graphite furnace. Such gasified and annealed coke samples were labelled as Cokes 4. The gasification with subsequent annealing of coke simulated the proposed conditions of coke passage to the tuyere level of BF. The weight losses of samples in gasification and gasification with subsequent annealing were measured by weighing the samples before and after each treatment.
2. Materials and methods 2.1. Materials This work examined eight cokes manufactured in a 400-kg pilot oven from coals/blend with vitrinite mean maximum reflectance (MMR) in the range of 0.90–1.66 and logarithm Gieseler maximum fluidity (log MF) in the range of 4.16–1.30. Generally, the logarithm Gieseler maximum fluidity of the coals/blend involved in this study decreased linearly with their increasing rank. Apart from Coke D, all the other cokes were produced from single coals; Coke D was produced from a blend of three coals. The properties of coals/blend are summarised in Table 1. The production of cokes was conducted in a pilotscale moving wall coke oven [20]. The carbonization was carried out in the oven chamber with a dimension of 885 mm × 445 mm × 825 mm. The charge bulk density was 825 kg/m3 (dry basis), the charge moisture was controlled at ∼5%. The oven initial flue temperature was 770 °C and the total coking time was 20 h. The charge centre temperature at the end of coking was around 1030 °C. At the end of coking, the cokes were pushed out and water quenched, the coke samples were
2.3. Microstrength of cokes The microstrength of cokes was measured using a UMIS2000 ultra 776
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Table 1 Characterisation of parent coal/blend samples. Coal (Identifier)
Coal A
Coal B
Coal C
Coal D
Coal E
Coal F
Coal G
Coal H
MMR log MF Ash, % (ad) Volatile Matter, % (ad) Inertinite, %
0.90 4.16 7.3 34.1 16.0
1.04 2.88 8.1 26.9 41.2
1.23 3.23 10.7 23.6 27.9
1.26 2.74 10.2 23.8 –
1.30 2.93 9.9 22.3 34.0
1.39 2.60 10.9 19.7 24.2
1.60 1.72 10.2 17.2 23.4
1.66 1.30 9.3 17.8 21.2
Ash Composition, % SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Modified Basicity Index (MBI)
52.3 37.1 4.21 1.26 0.44 0.47 0.74 0.88
51.9 25.7 11.2 3.2 1.2 0.4 1.6 2.51
66.6 24.8 2.13 1.43 0.57 0.38 1.24 0.88
59.8 26.0 3.09 2.51 0.67 0.16 1.62 1.26
62.5 25.0 3.86 1.91 0.82 0.23 1.26 1.18
69.1 23.2 2.19 0.93 0.55 1.24 0.48 0.79
53.2 29.5 7.30 2.53 0.72 0.49 1.03 1.80
51.3 35.0 3.72 2.87 0.84 1.03 1.16 1.26
MBI = [(100 × Ash)/(100 − Volatile Matter)] [(Na2O + K2O + CaO + MgO + Fe2O3)/(SiO2 + Al2O3)]. Table 2 Characterisation of produced coke samples. Coke (Identifier)
Coke A
Coke B
Coke C
Coke D
Coke E
Coke F
Coke G
Coke H
Ash, % (ad) Volatile Matter, % (ad) CSR, % CRI, % I600, %
11.0 0.3 55.2 27.8 84.3
11.9 0.3 31.9 46.7 85.0
14.2 < 0.1 68.8 21.4 85.8
12.3 0.8 71.7 17.2 83.7
12.8 0.6 69.5 24.5 87.5
13.5 0.1 69.7 22.2 86.7
12.1 1.5 62.7 24.6 82.0
12.0 0.4 69.5 20.1 81.1
Ash composition, % SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Modified Basicity Index (MBI)
51.5 38.5 3.14 1.56 0.36 0.40 0.83 0.77
51.4 23.1 14.3 3.68 0.82 0.39 1.42 3.30
63.6 23.9 4.49 1.91 0.60 1.49 1.20 1.57
58.3 25.3 4.63 2.71 0.73 0.48 1.66 1.51
56.8 25.0 7.09 3.58 0.68 0.46 0.93 2.01
68.3 23.7 3.13 1.04 0.55 0.55 0.82 0.89
55.1 25.7 9.18 3.24 0.56 0.48 1.23 2.23
50.1 33.6 5.09 3.05 0.83 1.16 1.20 1.63
2.5. Microstructure of cokes
micro indentation system. This system was designed to carry out the mechanical properties measurement using an ultra-low load with penetration depths of around 1 µm. This allowed the mechanical properties of individual microtextural type grains to be determined separately. The measurements were carried out on IMDC and RMDC microtextural types. 10 measurements for each type of coke microtexture were carried out across several coke pellets. Hardness, H, was determined from the loading-unloading curve, obtained in the micro-indentation tests with a Berkovich indenter by increasing the applied load to 100 mN and measuring the depth of penetration of the diamond indenter. The hardness was calculated according to the method developed by Oliver and Pharr [23].
Microstructure of cokes was studied using X-ray diffraction (XRD). XRD was used to determine the coke crystallinity, which was characterized by the crystallite size, Lc, or stack height, calculated using the Scherrer equation [25]. XRD spectra of powdered samples were obtained using a Philips X’Pert Multipurpose X-ray Diffraction System (MPD). Copper Kα radiation (45 kV, 40 mA) was used as the X-ray source. Samples were scanned in the range of 2θ from 10 to 50° with a step size of 0.02° and 0.6 s scanning time at each step. 2.6. Quantitative petrographic analysis of cokes Quantitative petrographic analysis was carried out at Pearson Coal Petrography Inc. (Victoria, BC, Canada). The Pearson petrographic analysis is an automated method for the quantitative measurement of coke reflectance/bireflectance over comparatively large (250–1600 mm2) polished coke surfaces. Polished coke samples (mounted in red colored resin) were imaged on a Zeiss Universal reflected light microscope equipped with auto-focus, controlled rotating polarizer and Manzhauser scanning stage. Monochromatic green incident light (546 nm) and a 25 × oil immersion objective was used. For each stage position the polarizer was stepped through 180° in 18° increments and a digital image (2 × 2 megapixel monochrome CCD camera) acquired at each polarizer position. Digital images were subsequently stitched together to a form a composite image of the lump with apparent maximum and minimum reflectances available on a pixel-by-pixel basis. The optical system was calibrated with a Klein & Becker strontium titanate reflectance standard (Reflectance = 5.41%)
2.4. Macrostrength of cokes The macrostrength of cokes was measured using tensile testing on an Instron 1185 screw universal testing machine with 5 kN load cell. The rate of load application was standardised at a machine crosshead speed of 0.5 mm/min. Cylindrical coke pellets were prepared by core drilling coke lumps and trimming cores to give pellets of 8 mm diameter and 8 mm long. 50 test specimens were examined for the tensile strength determination. The tensile strength of cokes was determined using the diametrical-compression test [24]. A compressive load was applied across the diameter of the coke pellet; the fracture of coke pellet occurring along the diameter as a result of the development of a tensile stress at right angles. The tensile strength was calculated from the applied load at breakage and the dimensions of the cylinder sample [24].
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Fig. 1. Representative RMDC microtextures of feed cokes from coals/blend with different ranks.
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Fig. 2. Gas-composition and temperature profile under the simulated BF conditions.
gasification to 1400 °C; upon the gasification with subsequent annealing at 2000 °C, the development of sinuous microfissures became more extensive. Quantitative petrographic analysis was applied to Coke B and H, which were from coals with MMR of 1.04 and 1.66, respectively. The mean maximum reflectance (Ro max), mean minimum reflectance (Ro min) and bireflectance (Rbi = Ro max–Ro min) of feed cokes and cokes subjected to simulated BF conditions are presented in Table 3. Feed Coke B had lower mean maximum reflectance and higher mean minimum reflectance than feed Coke H, therefore, its bireflectance was also lower than feed Coke H. After gasification to 1400 °C, the decreases in the mean maximum reflectance and bireflectance of Coke B were 0.22 and 0.45%, respectively. Gasification with subsequent annealing at 2000 °C brought more significant reductions in the petrographic parameters of Coke B, the decreases in mean maximum reflectance and bireflectance caused by high temperature annealing were 1.91 and 0.64%, respectively. The coke produced from high rank coal (Coke H) returned a similar trend in the reflectance parameters but at much higher values than those found for Coke B, implied the greater change in its microstructure upon simulated BF gasification and annealing. The losses in mean maximum reflectance and bireflectance of Coke H after gasification plus annealing at 2000 °C were 4.18 and 2.62%, respectively.
[26,27]. In this study the cokes produced from coals with low and high ranks (Cokes B and H) were subjected to quantitative petrographic analysis to determine the change of their petrography upon simulated BF conditions. 2.7. Porosity of cokes 40 specimens for each tested coke were mounted in epoxy resin blocks and polished to provide samples for image analysis. A representative analysis was based on 40 images captured across 40 individual specimens by a Nikon Model EPIPHOT 600 microscope equipped with a Nikon digital camera. Since the large pore were of most interest in relation to the coke strength degradation, a low power objective lens (magnification 5 times) was used under which small pores (< 12 µm) were not detectable. The captured images were converted from grey scale to binary (black or white) for each pixel using image analysis software developed by NIH. After conversion, the pores and walls of the cokes were represented by black and white areas, respectively. The total porosity of cokes was calculated using ImageJ as the area fraction of the black area [28]. 3. Results and discussion 3.1. Microtextures of cokes subject to simulate BF conditions
3.2. Effect of coal properties on coke microstrength subjected to simulated BF conditions
The microtextures of Cokes B and H subjected to simulated BF conditions are shown in Fig. 3. Compared to the feed cokes (Fig. 1), loss in coke reflectance was observed in the coke after gasification and gasification with subsequent annealing. Reflectance loss of coke upon BF treatments was also commonly overserved on other cokes involved in this study. The loss of coke reflectance was consistently observed in the cokes representing ‘raceway’ samples from blast furnace tuyere probe [29,30] and cokes retrieved from pilot scale hot model raceway test rigs [31]. Beside the reflectance loss, some pockmarks were observed on the Coke B after gasification with subsequent annealing at 2000 °C, these pockmarks, some as voids, up to 10 µm, and rarely to 15 µm in long dimension with irregular outline were distributed as low reflectance domains on the RMDC components. For Coke H, some sinuous microfissures, typically up to 100 µm long by 5 µm wide, formed between the lamellae of the ribbon anisotropic microtextural types after
The hardness measured on the cokes subjected to gasification and gasification with subsequent annealing under the simulated BF conditions is presented in Table 4. The average relative standard deviation (RSD) for all hardness measurements was 0.26. The hardness of IMDC microtextural type in the feed cokes was in the range of 5.87 to 6.57 GPa, depending on the type of the parent coals/blend. The hardness of RMDC microtextural type of feed cokes was in the range of 3.42–5.65 GPa, which was generally lower than that of IMDC. Overall, gasification and annealing under simulated BF conditions caused a decrease in hardness of both IMDC and RMDC microtextural types. Gasification to 1100 °C caused a marginal drop in coke microstrength (as this temperature was not significantly distinguished from the coking temperature). Further gasification to 1400 °C only caused a slight 779
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Fig. 3. Representative RMDC microtextures of Cokes B and H subjected to simulated BF conditions.
case of coke gasified to 1600 °C, the hardness of coke IMDC microtextural type only changed slightly upon treatment; the hardness of RMDC microtextural type decreased from 3.8 GPa to 1.5 GPa. It should be noted that the temperature in the BF bosh region and the gasification temperature in their experiments were much lower than that in the raceway and bird’s nest regions [22], which resulted in the more moderate degradation of coke microstrength in their study. The degradation of coke microstrength was calculated as follow:
Table 3 Petrographic parameters of Cokes B and H, %. Coke (Identifier)
Ro max
Ro min
Rbi
Coke Coke Coke Coke Coke Coke
7.36 7.14 5.45 9.43 8.62 5.25
5.23 5.45 3.97 4.24 4.20 2.66
2.13 1.68 1.49 5.20 4.43 2.58
B1 (Feed coke) B3 (Gasification to 1400 °C) B4 (Gasification + Annealing at 2000 °C) H1 (Feed coke) H3 (Gasification to 1400 °C) H4 (Gasification + Annealing at 2000 °C)
Microstrength degradation = change in IMDC microstrength but a more significant change in RMDC microstrength. Gasification with annealing at 2000 °C caused a significant degradation in the coke microstrength of both IMDC and RMDC with a stronger effect on RMDC; e.g. after annealing at 2000 °C, the hardness of RMDC in Coke H decreased to 0.28 GPa. A similar tendency was found by Nightingale et al. [32], who studied the coke extracted form BF bosh region and the coke treated following simulated BF gas-temperature profile to 1600 °C. In the case of bosh coke, the microstrength of IMDC microtextural type remained almost unchanged from that of the feed coke; however, the hardness of RMDC microtextural type decreased from 3.8 GPa to 1.8 GPa. In the
H o−H ' × 100% Ho
(1)
where H° is the hardness of the feed coke, and H′ is the hardness of the coke after gasification with subsequent annealing at 2000 °C (Coke 4). Fig. 4 shows that the degradation of coke microstrength depends on the rank and caking property of coal precursors. RMDC microtexture in cokes produced from coals with lower rank and higher maximum fluidity had stronger resistance to degradation upon high temperature annealing. Annealing at 2000 °C caused 78% hardness decrease in the fused components of Coke A (from the coal with lowest rank and highest Gieseler maximum fluidity) compared to 92% for Coke H. The strong correlation of MMR and log MF of parent coals with coke hardness degradation was observed for RMDC; however, the
Table 4 Hardness of cokes subjected to simulated BF conditions, GPa. Coke (Identifier)
A
B
C
D
E
F
G
H
IMDC Coke 1 Coke 2 Coke 3 Coke 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
6.48 6.25 6.17 4.25
6.31 6.35 6.27 4.07
6.21 5.93 5.61 4.11
6.21 5.89 5.65 3.90
6.57 5.91 5.10 3.91
6.42 6.23 5.76 3.92
5.96 5.78 5.21 3.82
5.87 5.94 4.56 3.68
RMDC Coke 1 Coke 2 Coke 3 Coke 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
5.65 5.60 3.51 1.25
5.56 5.49 3.29 1.05
4.43 4.24 3.02 0.70
4.82 4.23 2.69 0.64
4.76 4.39 2.95 0.64
4.10 3.98 2.79 0.43
4.05 3.67 2.72 0.33
3.42 3.25 2.46 0.28
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Fig. 4. Correlation between microstrength degradation and (a) MMR (b) log MF of parent coals.
correlations for IMDC microtexture were very poor at R2 = 0.16 and 0.11, respectively. That was due to the RMDC microtextural type was predominantly formed through a fluid stage of vitrinite maceral where the MMR and log MF were measured; however, IMDC was formed from unfused macerals in the parent coals, the MMR and log MF measurements were not able to reflect the properties of IMDC microtextural types in the formed coke.
3.3. Mechanism of microstrength degradation of cokes under the simulated BF conditions The difference in the microstrength degradation of cokes produced from precursors with varied properties could be attributed to the different changes in their microstructure upon simulated BF conditions. The microstructure of coke samples, represented by the crystallite size, Lc, under simulated BF conditions is shown in Table 5. The crystallite size of the feed cokes was in the range 17–28 Å. Treatment temperature had a strong effect on the growth of coke crystallite size. Gasification to 1100 °C (similar to the cokemaking temperature) had a small effect on the coke crystallinity, which slightly increased in comparison with the feed cokes. At 1400 °C, there was a further increase in the coke crystallite size to the range of 29–41 Å; however, annealing at high temperatures up to 2000 °C caused a significant increase of the coke crystallite size (91–163 Å). Gupta et al. [33] reported similar non-linear increase of coke crystallite size with temperature in the range of 1000–1600 °C. The previous XRD study by Wang et al. [19] on the coke extracted from different regions of BF tuyere level showed that the crystallite size, Lc, was 76 Å for the coke collected at the end of deadman region, and it increased significantly to 176 Å for the coke sampled from the raceway region. Similar results were also obtained in the study of the coke sampled from BF at BlueScope Steel [32]. The XRD results obtained in this study verify the ability of current methodology to simulate the BF low region conditions. XRD results showed that the gasification to 1100 °C did not cause significant modification in crystallite size, the remarkable changes in the coke carbon structure occurred after annealing at 2000 °C. These data indicate that the coke carbon structure changes significantly upon the higher temperatures within the BF. The correlation between the
Fig. 5. Correlation of Lc and hardness of metallurgical cokes.
coke microstrength and the carbon crystallite size presented in Fig. 5 indicates that the hardness of both RMDC and IMDC microtextures decreases with growth in crystallite size. The hardness of the coke matrix decreases significantly during the initial stage of the carbon crystallite enlargement; this degradation is lessened as the crystallite grows to a larger size; with further increase of the carbon crystallite size, slight degradation takes place in the coke matrix. Fig. 5 shows non-linear correlation between coke graphitisation, determined using XRD, and coke hardness. With the high temperature in the gasification and annealing under the simulated BF conditions, the microstructure of coke carbon transforms from the crosslinked nongraphitic structure towards a columnar graphitic structure with paralleled layers, the resistance toward fracture of coke matrix degrades during this process [34–36]. In the early stage of coke graphitisation, the base structure units (BSUs) in the carbon structure associate into distorted columns; the interfaces between BSUs disappear, and the initial cross-linked microstructure of coke carbon changes significantly, accompanied with slow enlargement of carbon crystallites during this stage [37,38]. Therefore, the coke microstrength degrades significantly with slight growth of carbon crystallite. Above 1500 °C, the initial crosslinked structure has already been destroyed and all the BSUs are
Table 5 Crystallinity size (Lc) of cokes subjected to simulated BF conditions, Å. Coke (Identifier)
A
B
C
D
E
F
G
H
Coke Coke Coke Coke
17.9 18.3 32.3 91.3
16.9 21.5 29.3 103.5
21.8 23.3 34.1 129.4
28.2 28.2 39.3 155.3
22.0 22.3 34.1 124.2
19.9 23.5 35.5 147.9
19.5 20.0 34.5 163.4
27.7 29.3 40.8 163.4
1 2 3 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
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Fig. 6. Correlation between crystallite growth and (a) MMR (b) log MF of parent coals.
study. Therefore, this assumption should be confirmed by further systemic study of coke made from different blends. In general, cokes made from coals with higher rank and lower maximum fluidity had a higher propensity for graphitisation. It has been found that the dimensions of the coke isochromatic units depend on the properties of its coal precursor, the higher the rank of parent coal, the larger and more elongate the isochromatic units in the coke fused components [39]. The graphitisation process is limited by the geometrical factor identified as the size of the isochromatic units in the RMDC; the smaller the areas, the lower ability to graphitisation [37]. Fig. 1 shows that the RMDC of Coke A is dominated by the fine circular anisotropic with minor incipient anisotropic and rare isotropic microtextural types, which limits its capacity to grow a large crystallite. In this study, the graphitisation of Coke A upon high temperature annealing was lower than of other cokes, meaning a smaller change in microstructure from the initial cross-linked structure. Therefore, the degradation of its microstrength was not as significant as that of other cokes; on the other hand, the greater change in the microstructure of cokes made from higher rank coals upon BF high temperatures caused more severe degradation in their microstrength, which also had potential to affect their macrostrength degradation.
stacked into parallel distorted columns. The further increase in temperature promotes the rapid coalescence of the distort columns into large carbon crystallites; however, the parallel structure is less changed in this stage [37,38]. Therefore, the coke microstrength only decreases slightly even though the carbon crystallite grows significantly in this stage. The XRD analysis indicates that the carbon structure of all the cokes in this study transform towards the structure of graphite upon treatments of the simulated BF conditions. However, the effect of increasing temperature on the growth of crystallite size shows a significant difference amongst the cokes produced from different precursors. For example, the crystallite size of Coke H, produced from coal with MMR of 1.66 and log MF of 1.30, increased to 163.4 Å after annealing at 2000 °C; however, the crystallite size of Coke A (MMR = 0.90, log MF = 4.16), reached to only 91.3 Å. A measure of this growth in crystallite size, ΔLc, is calculated as the difference between sizes of feed coke and coke after annealing at 2000 °C. This difference is plotted against the rank and logarithm Gieseler maximum fluidity of the parent coals in Fig. 6. Previous study on the coke graphitisation in the BF suggested that temperature, amount of iron in contact with coke and residence time of coke in the hottest regions of BF are important factors in the catalytic graphitisation process [19]. The treatment temperatures and residence time at each temperature were consistent for all the coke samples in current study; rank, caking property and ash composition of parent coals/blend were variables. The coke ash composition analysis indicated that the concentration of iron contained minerals varied from coke samples in this study; however, it did not show strong correlation with the growth of coke crystallite size. Therefore, the rank and caking property of parent coals/blend were the major factors affecting the microstructure change of cokes. It should be noted that the Coke D in this study was produced from a blend of three individual coals, the crystallite size growth of Coke D upon gasification with subsequent annealing was 127.1 Å, which was above the trendline in Fig. 6a. This implied that the interactions between coal constituents in the blend during carbonisation could have a potential effect on the behaviours of produced coke upon simulated BF conditions. It should be noted that Coke D was the only available coke produced from blend for current
3.4. Effect of coal properties on coke macrostrength subject to simulated BF conditions The tensile strengths of cokes subjected to gasification and annealing under the simulated BF conditions are presented in Table 6. The average relative standard deviation (RSD) for tensile strength measurements was 0.29. Feed Cokes A had the highest tensile strength of 5.6 MPa, the lowest value of 4.5 MPa was measured for Coke H. The tensile test results indicated that cokes experienced some degree of degradation upon gasification, this degradation increased substantially with post annealing at 2000 °C. The strength degradation of coke upon high temperature was also observed by Haapakangas et al. [40,41], who determined the coke strength using Gleeble system under the different temperatures. Compared to the room temperature, 9% decrease in coke strength was found at 1000 °C; 27% decrease at 1600 °C, and 36% decrease at 1750 °C.
Table 6 Tensile strength of cokes subjected to simulated BF conditions, MPa. Coke (Identifier)
A
B
C
D
E
F
G
H
Coke Coke Coke Coke
5.57 5.10 4.91 4.46
5.49 5.12 4.53 3.93
4.90 4.87 4.81 3.23
4.68 4.24 4.19 2.90
5.14 5.06 5.11 3.45
5.51 5.22 4.91 3.13
4.62 4.28 3.86 2.37
4.47 4.12 4.05 2.33
1 2 3 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
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Table 7 Porosity of cokes subjected to simulated BF conditions, %. Coke (Identifier)
A
B
C
D
E
F
G
H
Coke Coke Coke Coke
62.4 62.9 65.1 66.2
57.9 60.8 63.1 66.8
54.3 53.6 54.9 60.0
59.2 61.1 61.1 65.9
57.8 58.4 58.3 61.7
55.1 56.4 56.4 63.1
66.3 67.2 68.7 73.6
63.9 64.0 64.6 68.6
1 2 3 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
found between coke weight loss and macrostrength degradation (Fig. 7) as the coke macrostrength was also affected by coke microstrength, which was independent of weight loss, but related to coke microstructure. The empirical relationship between coke strength, its porosity and matrix properties established in previous investigations [42–44] was based on equation derived by Kundsen [45]. In current study, a relationship between coke porosity, microstrength and macrostrength is given by equation (3)
The most significant degradation caused by gasification was observed for Coke B, which had the highest CRI and lowest CSR in the standard test (Table 2). The tensile strength of Coke B degraded by 17% after gasification to 1400 °C; however, after gasification with subsequent annealing of this coke at 2000 °C, the degradation of Coke B only increased slightly to 28%; the degradation of Coke A, which was made from coal with MMR of 0.90 and log MF of 4.16, was even lower as 20%. Degradation of other cokes after gasification and annealing at 2000 °C was more significant (e.g. ∼48% for Coke H). The final tensile strength of Coke A after gasification plus annealing at high temperature was 4.5 MPa, which was the strongest among the eight cokes tested. It should be noted that, although five of the cokes in this study had similar CSR (Table 2), their relative performance upon gasification with subsequent annealing at 2000 °C was significantly different. For example, for Cokes E and H, which had exactly the same CSR, the relative degradation in their tensile strength after gasification plus annealing at high temperature was 33% and 48%, respectively; and Coke H became 32% weaker than Coke E after gasification and annealing under the simulated BF conditions. As a porous material, coke macrostrength is related to the microstrength of the coke matrix and the porosity. The porosity of cokes subjected to simulated BF conditions is presented in Table 7. The average relative standard deviation (RSD) for porosity measurements was 0.09. Feed cokes porosity ranged between 54 and 66%. Compared with other cokes, gasification to1400 °C caused a higher porosity increase for Coke B (relative increase of 9%) due to its higher reactivity. Annealing at 2000 °C after gasification caused significant development of porosity in the cokes tested (relative increase of 6–15%), with the largest relative increase found in Coke B as its larger increase during the gasification stage. The degradation of coke macrostrength was calculated as follow:
Macrostrength degradation =
T o−T ' × 100% To
T = A × HB × exp (C × P )
(3)
where T is the tensile strength, H is the coke hardness and P is the coke porosity. A, B and C are constants, determined by the regression analysis to fit the calculated tensile strengths to the measured data with maximum correlation coefficient R2. The constants A, B and C in Eq. (3) were found as 5.2, 0.4 and −1.3, respectively. Empirical Eq. (3) indicated that coke macrostrength decreased with its microstrength but was inversely affected by the coke porosity. Therefore, the mechanism of coke degradation under the simulated BF conditions in current study could be attributed to the deterioration of coke matrix and the enlargement of coke porosity caused by gasification and high temperature annealing in the BF. In this study, the degradation in the coke microstrength of RMDC, the dominate component of the coke microtexture, was above 80%; the degradation of IMDC was also above 35% under the simulated BF gasification and annealing conditions; however, the enlargement of coke porosity was only in the range of 6–15%. Therefore, the macrostrength degradation of cokes under the BF-like conditions in this study was significantly dependent on the decrease in the microstrength of individual cokes. Fig. 4 shows that coke microstrength is strongly correlated with the properties of its precursor; therefore, the correlations of coke macrostrength degradation with the MMR and log MF of parent coals/blend are established and presented in Fig. 8. The cokes made from the coals with higher rank and lower maximum fluidity have more severe macrostrength degradation under the simulated BF conditions due to their greater growth of crystallite size and more significant degradation of microstrength upon gasification with subsequent annealing at high temperature (2000 °C).
(2)
where T° is the tensile strength of feed coke, and T′ is the tensile strength of coke after gasification with subsequent annealing at 2000 °C. Generally good correlation was found between coke weight loss and the relative increase of porosity upon gasification plus annealing under the BF conditions; however, no direct correlation was
4. Conclusions The effect of properties of coal precursors on the micro- and macrostrength of cokes produced in a pilot oven was studied upon the gasification and annealing test conditions which more closely simulated those found in a BF. The specific methodology utilised included gasification of coke with BF gas-temperature profile from 900 to 1400 °C (corresponding from the thermal reserve zone to the cohesive zone) and annealing of coke up to 2000 °C (corresponding to the raceway region). General agreement was achieved between the properties for the cokes subjected to the simulated BF conditions and those for the cokes sampled from operating BF. The major findings are: 1. Cokes made from coals with different rank and carking property showed significant differences in their strength under the simulated BF test conditions, even though most of them had similar CSR/CRI values.
Fig. 7. Correlation between coke weight loss with porosity increase and tensile strength degradation. 783
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Fig. 8. Correlation between macrostrength degradation and (a) MMR (b) log MF of parent coals.
2. Coke microtextures experienced significant reflectance loss and structure change upon simulated BF gasification and annealing conditions. The decreases in mean maximum reflectance and bireflectance were more severe for the coke produced from the high rank coal. 3. High temperature annealing under the simulated BF conditions significantly decreased the coke microstrength of both IMDC and RMDC microtextural types. All the cokes had similar degradation in their IMDC microtextural types; however, at high temperature, degradation in the RMDC was more severe in the cokes produced from coals with higher rank and lower Gieseler maximum fluidity. 4. Under the simulated BF conditions, the degree of graphitisation of cokes increased with increasing treatment temperature, which indicated that the coke carbon structure transformed from a nongraphitic structure toward a graphitic structure. The cokes made from coals with higher rank and lower Gieseler maximum fluidity had much higher graphitisation development upon the annealing at high temperature. The more significant degradation in the microstrength of cokes produced from these coals was attributed to the greater change in their microstructure toward graphitic structure upon high temperature. 5. The cokes made from coals with higher rank and lower Gieseler maximum fluidity had more significant degradation in their macrostrength due to their greater microstructure change and more significant microstrength degradation upon gasification and annealing under the BF conditions (2000 °C).
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Full Length Article
Effect of coal properties on the strength of coke under simulated blast furnace conditions
T
⁎
Xing Xinga, , Harold Rogersb, Paul Zullic, Kim Hockingsd, Oleg Ostrovskia a
School of Materials Science and Engineering, UNSW Sydney, Kensington, NSW 2052, Australia School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia c School of Mechanical, Materials & Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia d BHP, Brisbane, QLD 4000, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal properties Coke strength Blast furnace conditions
Eight cokes made from coals/blend with different properties (vitrinite mean maximum reflectance = 0.90–1.66, logarithm Gieseler maximum fluidity = 4.16–1.30) were subjected to gasification and annealing simulating the conditions within an ironmaking blast furnace (BF). The specific methodology utilised included gasification of coke with BF gas-temperature profile from 900 to 1400 °C (corresponding from the thermal reserve zone to the cohesive zone) and annealing of coke up to 2000 °C (corresponding to the raceway region). The coke microstrength and macrostrength were determined using ultra-micro indentation and tensile test to understand the effect of coal precursor properties on the strength of the resulting coke and the changes when processed under the simulated BF conditions. Under the high temperatures in the simulated BF processes, the cokes from different coals showed significant differences in their properties, even though most of them had similar Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR) values. Coke microtextures experienced significant reflectance loss and structure change upon simulated BF gasification and annealing conditions. The decreases in mean maximum reflectance and bireflectance were more severe for the coke produced from the high rank coal. The cokes made from coals with higher rank and lower Gieseler maximum fluidity exhibited greater change in their microstructure upon high temperatures. As a result, microstrength of cokes produced from these coals decreased more than that of coke made from the parent coals with a lower rank and higher Gieseler maximum fluidity; this tendency was more significant in the Reactive Maceral Derived Components (RMDC) than Inert Maceral Derived Components (IMDC) microtextural type. Degradation of macrostrength of cokes produced from coals with higher rank and lower maximum fluidity was also more severe due to the greater decrease in their microstrength.
1. Introduction Functionally, metallurgical coke fed to the BF fulfils three major roles providing fuel, reducing agent and structural support media. Of these three roles, the first two can be largely substituted by the tuyere injection of other fuels, including oil, pulverised coal, natural gas, and plastic wastes [1,2]. However, only coke fulfils the burden support role in the BF. The pulverized coal injection (PCI) significantly decreases the coke rate (weight of coke required to produce 1 ton of hot metal); simultaneously, coke residence time is significantly increased, which means that coke is subjected to longer periods under the mechanical, thermal and chemical stresses in the BF. Therefore, coke is required to maintain adequate strength to support the iron-bearing burden and provide a permeable matrix necessary for slag and metal to pass down
⁎
into the hearth and for hot gases to pass upwards into the stack. Coal rank (typically characterised by vitrinite mean maximum reflectance, MMR) and caking property (typically characterised using Gieseler maximum fluidity) are two of the most important coal properties in deciding the application [3]. The coal rank correlates strongly with volatile matter content, calorific value of the coal and other chemical parameters that reflect the changes taking place in the coal molecular structure during maturation. The fluidity of coal plastic stage is a major factor in determining what proportions of a coal is used in a blend for cokmaking. The coke strength largely depends on the properties of its coal precursor. One of the approaches to predict coke cold strength was based on the correlation established in MOF diagram between petrographic data (Rmax) and rheological properties (Gieseler maximum fluidity) of coals [4]. Miura suggested that the coke cold
Corresponding author. E-mail address: [email protected] (X. Xing).
https://doi.org/10.1016/j.fuel.2018.10.069 Received 2 July 2018; Received in revised form 29 August 2018; Accepted 10 October 2018 Available online 16 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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subsequently stabilised by three drops through 4 m onto cement. All produced coke samples were crushed to a size range of +19 to 21 mm for the bench-scale testing and processing. The proximate analyses of the cokes were conducted following Australian Standard AS 1038.3-2000. The CRI and CSR were determined following Australian Standard AS 1038.13-1990. The cold strength of coke was measured using I-drum tumbler test. A 150 g coke sample with a particle size of −21 + 19 mm was tumbled in an I-drum tumbler (130 mm internal diameter, 700 mm length) rotated at 20 revolutions/minutes for 30 min. The tumbled sample was sieved over a 10 mm perforated plate laboratory sieve; the weight percentage of +10 mm of the original sample was reported as I-drum tumbler strength and noted as I600. The ash composition of cokes was analysed using XRF. A summary of the properties of produced cokes are presented in Table 2. The difference in Gieseler maximum fluidity of the parent coals/ blend did not cause significant variation in coke cold strength in this study. Except for Coke B, all the cokes had comparatively low CRI value in the range of 17.2–27.8%. Coke B had significantly higher reactivity (CRI = 46.7%) and much lower strength after reaction (CSR = 31.9%) than other coke samples. The high reactivity of Coke B was partially attributed to its ash composition and microtextural types. Compared to other cokes, ash of Coke B had much greater concentration of Fe2O3, which could catalyse the solution loss reaction during the CRI test. Microtextures of cokes includes RMDC and IMDC. The MMR of the parent coals investigated in this study covered a wide range, and the microtexture types of RMDC varied significantly among the cokes. The optical microscopy assessments were based on the classification described by Gray [21]. Optical microscopical inspections in plain ‘white’ light, were employed at magnifications of 50–1000 times and the classifications applied in respect of the shape, form and size of isochromatic units. Coke A was produced from coal with the lowest MMR among eight samples, the fused components of Coke A mainly consisted of fine circular anisotropic with minor incipient anisotropic and rare isotropic microtextural types; the cokes made from coals with a higher rank tend to have microtextural types with larger size isochromatic units as their RMDC microtextures; the MMR of parent coal of Coke H was 1.66, the fused components of this coke were dominated by the medium to coarse lenticular anisotropic and fine to coarse ribbon anisotropic microtextural types (Fig. 1).
strength (JIS indices, DI30/15) was mainly governed by the parent coal rank when the Gieseler maximum fluidity of coal was in the range of 200–1000 ddpm [5]. A tendency of coke JIS drum index to increase as maximum fluidity increased were reported in previous studies [6,7]. Nakamura et al. [8] reported that the CSR reached the maximum in the region of prime coking coal with mean reflectance of 1.2–1.3%. Zhang et al. [9] also found that the coke with high CSR could be produced from a coal with random reflectance of 1.1–1.2%, further increases in inertinite content or rank of the parent coal caused a decrease in the CSR value of the formed cokes [10–12]. Pusz et al. [13] suggested that the higher inertinite content in parent coals could increase the CRI and decrease the CSR of the formed coke, and this influence was greater for the middle rank coals than those from lower ranks; the technological properties, e.g. the CRI and CSR indices, of coke could be predicted by its reflectance parameters. The correlation between parent coal properties (mean vitrinite reflectance and Gieseler maximum fluidity) and resulting CSR has been intensively studied with models developed to predict coke CSR [5,9,14]. However, the behaviours of coke under the BF operating conditions has not been comprehensively investigated in the previous studies. In the lower region of the BF, the temperature increases from 1200 °C to approximately 2000 °C (close to the typical raceway flame temperature); the gas atmosphere and flow dynamics also change significantly, which significantly influence the strength of coke. The previous studies on the coke extracted from operating BF mainly focused on the significant graphitisation of coke experienced after descent to the tuyere level [15–17]. Gornostayev et al. [18] observed the graphite crystals with an ideal hexagonal prism shape formed on the surface of BF cokes. Wang et al [19] suggested that the temperature, amount of iron in contact with coke and residence time of coke in the hottest regions of the furnace are important factors in the catalytic graphitisation process. However, very few studies have been conducted to systematically investigate the effect of the parent coal properties on the behaviours of coke strength under the more realistic BF conditions. In this study, a series of cokes were prepared from coals/blend with different ranks and caking properties carbonised in a 400-kg pilot scale coke oven. All the cokes were subjected to conditions simulating BF gas compositiontemperature profiles to 1400 °C, or gasification with subsequent annealing under N2 at 2000 °C (temperature of the region close to the raceway in BF). This paper aims to study the effect of precursor coal rank and caking property on the micro- and macro-strength of resulting cokes under simulated BF conditions and to understand the mechanism of different behaviours of cokes produced from various parent coals in the BF ironmaking process.
2.2. Treatments of cokes under the simulated BF conditions A 200 g sample of each coke (+19–21 mm particle size range) was contained in a silicon carbide reaction vessel and gasified in a vertical furnace. Most importantly, the gasification of the coke samples in this study was conducted under a BF gas composition-temperature profile, simulating conditions from the thermal reserve zone through to cohesive zone. The composition of the CO-CO2-N2 in the gas mixture was varied with temperature (Fig. 2). The actual gas composition-temperature profile used was designed based on the vertical probing of a BF, as reported by van der Velden et al. [22]. The feed coke sample prior to any treatment was labelled as Coke 1. The gasification commenced at 900 °C and stopped once temperature reached 1100 °C (Coke 2) and 1400 °C (Coke 3). Coke samples reacted under the conditions for Cokes 3 were then kept at the final conditions for additional 2 h, the samples after gasification were subsequently annealed under N2 at 2000 °C for 2 h in a graphite furnace. Such gasified and annealed coke samples were labelled as Cokes 4. The gasification with subsequent annealing of coke simulated the proposed conditions of coke passage to the tuyere level of BF. The weight losses of samples in gasification and gasification with subsequent annealing were measured by weighing the samples before and after each treatment.
2. Materials and methods 2.1. Materials This work examined eight cokes manufactured in a 400-kg pilot oven from coals/blend with vitrinite mean maximum reflectance (MMR) in the range of 0.90–1.66 and logarithm Gieseler maximum fluidity (log MF) in the range of 4.16–1.30. Generally, the logarithm Gieseler maximum fluidity of the coals/blend involved in this study decreased linearly with their increasing rank. Apart from Coke D, all the other cokes were produced from single coals; Coke D was produced from a blend of three coals. The properties of coals/blend are summarised in Table 1. The production of cokes was conducted in a pilotscale moving wall coke oven [20]. The carbonization was carried out in the oven chamber with a dimension of 885 mm × 445 mm × 825 mm. The charge bulk density was 825 kg/m3 (dry basis), the charge moisture was controlled at ∼5%. The oven initial flue temperature was 770 °C and the total coking time was 20 h. The charge centre temperature at the end of coking was around 1030 °C. At the end of coking, the cokes were pushed out and water quenched, the coke samples were
2.3. Microstrength of cokes The microstrength of cokes was measured using a UMIS2000 ultra 776
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Table 1 Characterisation of parent coal/blend samples. Coal (Identifier)
Coal A
Coal B
Coal C
Coal D
Coal E
Coal F
Coal G
Coal H
MMR log MF Ash, % (ad) Volatile Matter, % (ad) Inertinite, %
0.90 4.16 7.3 34.1 16.0
1.04 2.88 8.1 26.9 41.2
1.23 3.23 10.7 23.6 27.9
1.26 2.74 10.2 23.8 –
1.30 2.93 9.9 22.3 34.0
1.39 2.60 10.9 19.7 24.2
1.60 1.72 10.2 17.2 23.4
1.66 1.30 9.3 17.8 21.2
Ash Composition, % SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Modified Basicity Index (MBI)
52.3 37.1 4.21 1.26 0.44 0.47 0.74 0.88
51.9 25.7 11.2 3.2 1.2 0.4 1.6 2.51
66.6 24.8 2.13 1.43 0.57 0.38 1.24 0.88
59.8 26.0 3.09 2.51 0.67 0.16 1.62 1.26
62.5 25.0 3.86 1.91 0.82 0.23 1.26 1.18
69.1 23.2 2.19 0.93 0.55 1.24 0.48 0.79
53.2 29.5 7.30 2.53 0.72 0.49 1.03 1.80
51.3 35.0 3.72 2.87 0.84 1.03 1.16 1.26
MBI = [(100 × Ash)/(100 − Volatile Matter)] [(Na2O + K2O + CaO + MgO + Fe2O3)/(SiO2 + Al2O3)]. Table 2 Characterisation of produced coke samples. Coke (Identifier)
Coke A
Coke B
Coke C
Coke D
Coke E
Coke F
Coke G
Coke H
Ash, % (ad) Volatile Matter, % (ad) CSR, % CRI, % I600, %
11.0 0.3 55.2 27.8 84.3
11.9 0.3 31.9 46.7 85.0
14.2 < 0.1 68.8 21.4 85.8
12.3 0.8 71.7 17.2 83.7
12.8 0.6 69.5 24.5 87.5
13.5 0.1 69.7 22.2 86.7
12.1 1.5 62.7 24.6 82.0
12.0 0.4 69.5 20.1 81.1
Ash composition, % SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Modified Basicity Index (MBI)
51.5 38.5 3.14 1.56 0.36 0.40 0.83 0.77
51.4 23.1 14.3 3.68 0.82 0.39 1.42 3.30
63.6 23.9 4.49 1.91 0.60 1.49 1.20 1.57
58.3 25.3 4.63 2.71 0.73 0.48 1.66 1.51
56.8 25.0 7.09 3.58 0.68 0.46 0.93 2.01
68.3 23.7 3.13 1.04 0.55 0.55 0.82 0.89
55.1 25.7 9.18 3.24 0.56 0.48 1.23 2.23
50.1 33.6 5.09 3.05 0.83 1.16 1.20 1.63
2.5. Microstructure of cokes
micro indentation system. This system was designed to carry out the mechanical properties measurement using an ultra-low load with penetration depths of around 1 µm. This allowed the mechanical properties of individual microtextural type grains to be determined separately. The measurements were carried out on IMDC and RMDC microtextural types. 10 measurements for each type of coke microtexture were carried out across several coke pellets. Hardness, H, was determined from the loading-unloading curve, obtained in the micro-indentation tests with a Berkovich indenter by increasing the applied load to 100 mN and measuring the depth of penetration of the diamond indenter. The hardness was calculated according to the method developed by Oliver and Pharr [23].
Microstructure of cokes was studied using X-ray diffraction (XRD). XRD was used to determine the coke crystallinity, which was characterized by the crystallite size, Lc, or stack height, calculated using the Scherrer equation [25]. XRD spectra of powdered samples were obtained using a Philips X’Pert Multipurpose X-ray Diffraction System (MPD). Copper Kα radiation (45 kV, 40 mA) was used as the X-ray source. Samples were scanned in the range of 2θ from 10 to 50° with a step size of 0.02° and 0.6 s scanning time at each step. 2.6. Quantitative petrographic analysis of cokes Quantitative petrographic analysis was carried out at Pearson Coal Petrography Inc. (Victoria, BC, Canada). The Pearson petrographic analysis is an automated method for the quantitative measurement of coke reflectance/bireflectance over comparatively large (250–1600 mm2) polished coke surfaces. Polished coke samples (mounted in red colored resin) were imaged on a Zeiss Universal reflected light microscope equipped with auto-focus, controlled rotating polarizer and Manzhauser scanning stage. Monochromatic green incident light (546 nm) and a 25 × oil immersion objective was used. For each stage position the polarizer was stepped through 180° in 18° increments and a digital image (2 × 2 megapixel monochrome CCD camera) acquired at each polarizer position. Digital images were subsequently stitched together to a form a composite image of the lump with apparent maximum and minimum reflectances available on a pixel-by-pixel basis. The optical system was calibrated with a Klein & Becker strontium titanate reflectance standard (Reflectance = 5.41%)
2.4. Macrostrength of cokes The macrostrength of cokes was measured using tensile testing on an Instron 1185 screw universal testing machine with 5 kN load cell. The rate of load application was standardised at a machine crosshead speed of 0.5 mm/min. Cylindrical coke pellets were prepared by core drilling coke lumps and trimming cores to give pellets of 8 mm diameter and 8 mm long. 50 test specimens were examined for the tensile strength determination. The tensile strength of cokes was determined using the diametrical-compression test [24]. A compressive load was applied across the diameter of the coke pellet; the fracture of coke pellet occurring along the diameter as a result of the development of a tensile stress at right angles. The tensile strength was calculated from the applied load at breakage and the dimensions of the cylinder sample [24].
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Fig. 1. Representative RMDC microtextures of feed cokes from coals/blend with different ranks.
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Fig. 2. Gas-composition and temperature profile under the simulated BF conditions.
gasification to 1400 °C; upon the gasification with subsequent annealing at 2000 °C, the development of sinuous microfissures became more extensive. Quantitative petrographic analysis was applied to Coke B and H, which were from coals with MMR of 1.04 and 1.66, respectively. The mean maximum reflectance (Ro max), mean minimum reflectance (Ro min) and bireflectance (Rbi = Ro max–Ro min) of feed cokes and cokes subjected to simulated BF conditions are presented in Table 3. Feed Coke B had lower mean maximum reflectance and higher mean minimum reflectance than feed Coke H, therefore, its bireflectance was also lower than feed Coke H. After gasification to 1400 °C, the decreases in the mean maximum reflectance and bireflectance of Coke B were 0.22 and 0.45%, respectively. Gasification with subsequent annealing at 2000 °C brought more significant reductions in the petrographic parameters of Coke B, the decreases in mean maximum reflectance and bireflectance caused by high temperature annealing were 1.91 and 0.64%, respectively. The coke produced from high rank coal (Coke H) returned a similar trend in the reflectance parameters but at much higher values than those found for Coke B, implied the greater change in its microstructure upon simulated BF gasification and annealing. The losses in mean maximum reflectance and bireflectance of Coke H after gasification plus annealing at 2000 °C were 4.18 and 2.62%, respectively.
[26,27]. In this study the cokes produced from coals with low and high ranks (Cokes B and H) were subjected to quantitative petrographic analysis to determine the change of their petrography upon simulated BF conditions. 2.7. Porosity of cokes 40 specimens for each tested coke were mounted in epoxy resin blocks and polished to provide samples for image analysis. A representative analysis was based on 40 images captured across 40 individual specimens by a Nikon Model EPIPHOT 600 microscope equipped with a Nikon digital camera. Since the large pore were of most interest in relation to the coke strength degradation, a low power objective lens (magnification 5 times) was used under which small pores (< 12 µm) were not detectable. The captured images were converted from grey scale to binary (black or white) for each pixel using image analysis software developed by NIH. After conversion, the pores and walls of the cokes were represented by black and white areas, respectively. The total porosity of cokes was calculated using ImageJ as the area fraction of the black area [28]. 3. Results and discussion 3.1. Microtextures of cokes subject to simulate BF conditions
3.2. Effect of coal properties on coke microstrength subjected to simulated BF conditions
The microtextures of Cokes B and H subjected to simulated BF conditions are shown in Fig. 3. Compared to the feed cokes (Fig. 1), loss in coke reflectance was observed in the coke after gasification and gasification with subsequent annealing. Reflectance loss of coke upon BF treatments was also commonly overserved on other cokes involved in this study. The loss of coke reflectance was consistently observed in the cokes representing ‘raceway’ samples from blast furnace tuyere probe [29,30] and cokes retrieved from pilot scale hot model raceway test rigs [31]. Beside the reflectance loss, some pockmarks were observed on the Coke B after gasification with subsequent annealing at 2000 °C, these pockmarks, some as voids, up to 10 µm, and rarely to 15 µm in long dimension with irregular outline were distributed as low reflectance domains on the RMDC components. For Coke H, some sinuous microfissures, typically up to 100 µm long by 5 µm wide, formed between the lamellae of the ribbon anisotropic microtextural types after
The hardness measured on the cokes subjected to gasification and gasification with subsequent annealing under the simulated BF conditions is presented in Table 4. The average relative standard deviation (RSD) for all hardness measurements was 0.26. The hardness of IMDC microtextural type in the feed cokes was in the range of 5.87 to 6.57 GPa, depending on the type of the parent coals/blend. The hardness of RMDC microtextural type of feed cokes was in the range of 3.42–5.65 GPa, which was generally lower than that of IMDC. Overall, gasification and annealing under simulated BF conditions caused a decrease in hardness of both IMDC and RMDC microtextural types. Gasification to 1100 °C caused a marginal drop in coke microstrength (as this temperature was not significantly distinguished from the coking temperature). Further gasification to 1400 °C only caused a slight 779
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Fig. 3. Representative RMDC microtextures of Cokes B and H subjected to simulated BF conditions.
case of coke gasified to 1600 °C, the hardness of coke IMDC microtextural type only changed slightly upon treatment; the hardness of RMDC microtextural type decreased from 3.8 GPa to 1.5 GPa. It should be noted that the temperature in the BF bosh region and the gasification temperature in their experiments were much lower than that in the raceway and bird’s nest regions [22], which resulted in the more moderate degradation of coke microstrength in their study. The degradation of coke microstrength was calculated as follow:
Table 3 Petrographic parameters of Cokes B and H, %. Coke (Identifier)
Ro max
Ro min
Rbi
Coke Coke Coke Coke Coke Coke
7.36 7.14 5.45 9.43 8.62 5.25
5.23 5.45 3.97 4.24 4.20 2.66
2.13 1.68 1.49 5.20 4.43 2.58
B1 (Feed coke) B3 (Gasification to 1400 °C) B4 (Gasification + Annealing at 2000 °C) H1 (Feed coke) H3 (Gasification to 1400 °C) H4 (Gasification + Annealing at 2000 °C)
Microstrength degradation = change in IMDC microstrength but a more significant change in RMDC microstrength. Gasification with annealing at 2000 °C caused a significant degradation in the coke microstrength of both IMDC and RMDC with a stronger effect on RMDC; e.g. after annealing at 2000 °C, the hardness of RMDC in Coke H decreased to 0.28 GPa. A similar tendency was found by Nightingale et al. [32], who studied the coke extracted form BF bosh region and the coke treated following simulated BF gas-temperature profile to 1600 °C. In the case of bosh coke, the microstrength of IMDC microtextural type remained almost unchanged from that of the feed coke; however, the hardness of RMDC microtextural type decreased from 3.8 GPa to 1.8 GPa. In the
H o−H ' × 100% Ho
(1)
where H° is the hardness of the feed coke, and H′ is the hardness of the coke after gasification with subsequent annealing at 2000 °C (Coke 4). Fig. 4 shows that the degradation of coke microstrength depends on the rank and caking property of coal precursors. RMDC microtexture in cokes produced from coals with lower rank and higher maximum fluidity had stronger resistance to degradation upon high temperature annealing. Annealing at 2000 °C caused 78% hardness decrease in the fused components of Coke A (from the coal with lowest rank and highest Gieseler maximum fluidity) compared to 92% for Coke H. The strong correlation of MMR and log MF of parent coals with coke hardness degradation was observed for RMDC; however, the
Table 4 Hardness of cokes subjected to simulated BF conditions, GPa. Coke (Identifier)
A
B
C
D
E
F
G
H
IMDC Coke 1 Coke 2 Coke 3 Coke 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
6.48 6.25 6.17 4.25
6.31 6.35 6.27 4.07
6.21 5.93 5.61 4.11
6.21 5.89 5.65 3.90
6.57 5.91 5.10 3.91
6.42 6.23 5.76 3.92
5.96 5.78 5.21 3.82
5.87 5.94 4.56 3.68
RMDC Coke 1 Coke 2 Coke 3 Coke 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
5.65 5.60 3.51 1.25
5.56 5.49 3.29 1.05
4.43 4.24 3.02 0.70
4.82 4.23 2.69 0.64
4.76 4.39 2.95 0.64
4.10 3.98 2.79 0.43
4.05 3.67 2.72 0.33
3.42 3.25 2.46 0.28
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Fig. 4. Correlation between microstrength degradation and (a) MMR (b) log MF of parent coals.
correlations for IMDC microtexture were very poor at R2 = 0.16 and 0.11, respectively. That was due to the RMDC microtextural type was predominantly formed through a fluid stage of vitrinite maceral where the MMR and log MF were measured; however, IMDC was formed from unfused macerals in the parent coals, the MMR and log MF measurements were not able to reflect the properties of IMDC microtextural types in the formed coke.
3.3. Mechanism of microstrength degradation of cokes under the simulated BF conditions The difference in the microstrength degradation of cokes produced from precursors with varied properties could be attributed to the different changes in their microstructure upon simulated BF conditions. The microstructure of coke samples, represented by the crystallite size, Lc, under simulated BF conditions is shown in Table 5. The crystallite size of the feed cokes was in the range 17–28 Å. Treatment temperature had a strong effect on the growth of coke crystallite size. Gasification to 1100 °C (similar to the cokemaking temperature) had a small effect on the coke crystallinity, which slightly increased in comparison with the feed cokes. At 1400 °C, there was a further increase in the coke crystallite size to the range of 29–41 Å; however, annealing at high temperatures up to 2000 °C caused a significant increase of the coke crystallite size (91–163 Å). Gupta et al. [33] reported similar non-linear increase of coke crystallite size with temperature in the range of 1000–1600 °C. The previous XRD study by Wang et al. [19] on the coke extracted from different regions of BF tuyere level showed that the crystallite size, Lc, was 76 Å for the coke collected at the end of deadman region, and it increased significantly to 176 Å for the coke sampled from the raceway region. Similar results were also obtained in the study of the coke sampled from BF at BlueScope Steel [32]. The XRD results obtained in this study verify the ability of current methodology to simulate the BF low region conditions. XRD results showed that the gasification to 1100 °C did not cause significant modification in crystallite size, the remarkable changes in the coke carbon structure occurred after annealing at 2000 °C. These data indicate that the coke carbon structure changes significantly upon the higher temperatures within the BF. The correlation between the
Fig. 5. Correlation of Lc and hardness of metallurgical cokes.
coke microstrength and the carbon crystallite size presented in Fig. 5 indicates that the hardness of both RMDC and IMDC microtextures decreases with growth in crystallite size. The hardness of the coke matrix decreases significantly during the initial stage of the carbon crystallite enlargement; this degradation is lessened as the crystallite grows to a larger size; with further increase of the carbon crystallite size, slight degradation takes place in the coke matrix. Fig. 5 shows non-linear correlation between coke graphitisation, determined using XRD, and coke hardness. With the high temperature in the gasification and annealing under the simulated BF conditions, the microstructure of coke carbon transforms from the crosslinked nongraphitic structure towards a columnar graphitic structure with paralleled layers, the resistance toward fracture of coke matrix degrades during this process [34–36]. In the early stage of coke graphitisation, the base structure units (BSUs) in the carbon structure associate into distorted columns; the interfaces between BSUs disappear, and the initial cross-linked microstructure of coke carbon changes significantly, accompanied with slow enlargement of carbon crystallites during this stage [37,38]. Therefore, the coke microstrength degrades significantly with slight growth of carbon crystallite. Above 1500 °C, the initial crosslinked structure has already been destroyed and all the BSUs are
Table 5 Crystallinity size (Lc) of cokes subjected to simulated BF conditions, Å. Coke (Identifier)
A
B
C
D
E
F
G
H
Coke Coke Coke Coke
17.9 18.3 32.3 91.3
16.9 21.5 29.3 103.5
21.8 23.3 34.1 129.4
28.2 28.2 39.3 155.3
22.0 22.3 34.1 124.2
19.9 23.5 35.5 147.9
19.5 20.0 34.5 163.4
27.7 29.3 40.8 163.4
1 2 3 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
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Fig. 6. Correlation between crystallite growth and (a) MMR (b) log MF of parent coals.
study. Therefore, this assumption should be confirmed by further systemic study of coke made from different blends. In general, cokes made from coals with higher rank and lower maximum fluidity had a higher propensity for graphitisation. It has been found that the dimensions of the coke isochromatic units depend on the properties of its coal precursor, the higher the rank of parent coal, the larger and more elongate the isochromatic units in the coke fused components [39]. The graphitisation process is limited by the geometrical factor identified as the size of the isochromatic units in the RMDC; the smaller the areas, the lower ability to graphitisation [37]. Fig. 1 shows that the RMDC of Coke A is dominated by the fine circular anisotropic with minor incipient anisotropic and rare isotropic microtextural types, which limits its capacity to grow a large crystallite. In this study, the graphitisation of Coke A upon high temperature annealing was lower than of other cokes, meaning a smaller change in microstructure from the initial cross-linked structure. Therefore, the degradation of its microstrength was not as significant as that of other cokes; on the other hand, the greater change in the microstructure of cokes made from higher rank coals upon BF high temperatures caused more severe degradation in their microstrength, which also had potential to affect their macrostrength degradation.
stacked into parallel distorted columns. The further increase in temperature promotes the rapid coalescence of the distort columns into large carbon crystallites; however, the parallel structure is less changed in this stage [37,38]. Therefore, the coke microstrength only decreases slightly even though the carbon crystallite grows significantly in this stage. The XRD analysis indicates that the carbon structure of all the cokes in this study transform towards the structure of graphite upon treatments of the simulated BF conditions. However, the effect of increasing temperature on the growth of crystallite size shows a significant difference amongst the cokes produced from different precursors. For example, the crystallite size of Coke H, produced from coal with MMR of 1.66 and log MF of 1.30, increased to 163.4 Å after annealing at 2000 °C; however, the crystallite size of Coke A (MMR = 0.90, log MF = 4.16), reached to only 91.3 Å. A measure of this growth in crystallite size, ΔLc, is calculated as the difference between sizes of feed coke and coke after annealing at 2000 °C. This difference is plotted against the rank and logarithm Gieseler maximum fluidity of the parent coals in Fig. 6. Previous study on the coke graphitisation in the BF suggested that temperature, amount of iron in contact with coke and residence time of coke in the hottest regions of BF are important factors in the catalytic graphitisation process [19]. The treatment temperatures and residence time at each temperature were consistent for all the coke samples in current study; rank, caking property and ash composition of parent coals/blend were variables. The coke ash composition analysis indicated that the concentration of iron contained minerals varied from coke samples in this study; however, it did not show strong correlation with the growth of coke crystallite size. Therefore, the rank and caking property of parent coals/blend were the major factors affecting the microstructure change of cokes. It should be noted that the Coke D in this study was produced from a blend of three individual coals, the crystallite size growth of Coke D upon gasification with subsequent annealing was 127.1 Å, which was above the trendline in Fig. 6a. This implied that the interactions between coal constituents in the blend during carbonisation could have a potential effect on the behaviours of produced coke upon simulated BF conditions. It should be noted that Coke D was the only available coke produced from blend for current
3.4. Effect of coal properties on coke macrostrength subject to simulated BF conditions The tensile strengths of cokes subjected to gasification and annealing under the simulated BF conditions are presented in Table 6. The average relative standard deviation (RSD) for tensile strength measurements was 0.29. Feed Cokes A had the highest tensile strength of 5.6 MPa, the lowest value of 4.5 MPa was measured for Coke H. The tensile test results indicated that cokes experienced some degree of degradation upon gasification, this degradation increased substantially with post annealing at 2000 °C. The strength degradation of coke upon high temperature was also observed by Haapakangas et al. [40,41], who determined the coke strength using Gleeble system under the different temperatures. Compared to the room temperature, 9% decrease in coke strength was found at 1000 °C; 27% decrease at 1600 °C, and 36% decrease at 1750 °C.
Table 6 Tensile strength of cokes subjected to simulated BF conditions, MPa. Coke (Identifier)
A
B
C
D
E
F
G
H
Coke Coke Coke Coke
5.57 5.10 4.91 4.46
5.49 5.12 4.53 3.93
4.90 4.87 4.81 3.23
4.68 4.24 4.19 2.90
5.14 5.06 5.11 3.45
5.51 5.22 4.91 3.13
4.62 4.28 3.86 2.37
4.47 4.12 4.05 2.33
1 2 3 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
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Table 7 Porosity of cokes subjected to simulated BF conditions, %. Coke (Identifier)
A
B
C
D
E
F
G
H
Coke Coke Coke Coke
62.4 62.9 65.1 66.2
57.9 60.8 63.1 66.8
54.3 53.6 54.9 60.0
59.2 61.1 61.1 65.9
57.8 58.4 58.3 61.7
55.1 56.4 56.4 63.1
66.3 67.2 68.7 73.6
63.9 64.0 64.6 68.6
1 2 3 4
(Feed coke) (Gasification to 1100 °C) (Gasification to 1400 °C) (Gasification + Annealing at 2000 °C)
found between coke weight loss and macrostrength degradation (Fig. 7) as the coke macrostrength was also affected by coke microstrength, which was independent of weight loss, but related to coke microstructure. The empirical relationship between coke strength, its porosity and matrix properties established in previous investigations [42–44] was based on equation derived by Kundsen [45]. In current study, a relationship between coke porosity, microstrength and macrostrength is given by equation (3)
The most significant degradation caused by gasification was observed for Coke B, which had the highest CRI and lowest CSR in the standard test (Table 2). The tensile strength of Coke B degraded by 17% after gasification to 1400 °C; however, after gasification with subsequent annealing of this coke at 2000 °C, the degradation of Coke B only increased slightly to 28%; the degradation of Coke A, which was made from coal with MMR of 0.90 and log MF of 4.16, was even lower as 20%. Degradation of other cokes after gasification and annealing at 2000 °C was more significant (e.g. ∼48% for Coke H). The final tensile strength of Coke A after gasification plus annealing at high temperature was 4.5 MPa, which was the strongest among the eight cokes tested. It should be noted that, although five of the cokes in this study had similar CSR (Table 2), their relative performance upon gasification with subsequent annealing at 2000 °C was significantly different. For example, for Cokes E and H, which had exactly the same CSR, the relative degradation in their tensile strength after gasification plus annealing at high temperature was 33% and 48%, respectively; and Coke H became 32% weaker than Coke E after gasification and annealing under the simulated BF conditions. As a porous material, coke macrostrength is related to the microstrength of the coke matrix and the porosity. The porosity of cokes subjected to simulated BF conditions is presented in Table 7. The average relative standard deviation (RSD) for porosity measurements was 0.09. Feed cokes porosity ranged between 54 and 66%. Compared with other cokes, gasification to1400 °C caused a higher porosity increase for Coke B (relative increase of 9%) due to its higher reactivity. Annealing at 2000 °C after gasification caused significant development of porosity in the cokes tested (relative increase of 6–15%), with the largest relative increase found in Coke B as its larger increase during the gasification stage. The degradation of coke macrostrength was calculated as follow:
Macrostrength degradation =
T o−T ' × 100% To
T = A × HB × exp (C × P )
(3)
where T is the tensile strength, H is the coke hardness and P is the coke porosity. A, B and C are constants, determined by the regression analysis to fit the calculated tensile strengths to the measured data with maximum correlation coefficient R2. The constants A, B and C in Eq. (3) were found as 5.2, 0.4 and −1.3, respectively. Empirical Eq. (3) indicated that coke macrostrength decreased with its microstrength but was inversely affected by the coke porosity. Therefore, the mechanism of coke degradation under the simulated BF conditions in current study could be attributed to the deterioration of coke matrix and the enlargement of coke porosity caused by gasification and high temperature annealing in the BF. In this study, the degradation in the coke microstrength of RMDC, the dominate component of the coke microtexture, was above 80%; the degradation of IMDC was also above 35% under the simulated BF gasification and annealing conditions; however, the enlargement of coke porosity was only in the range of 6–15%. Therefore, the macrostrength degradation of cokes under the BF-like conditions in this study was significantly dependent on the decrease in the microstrength of individual cokes. Fig. 4 shows that coke microstrength is strongly correlated with the properties of its precursor; therefore, the correlations of coke macrostrength degradation with the MMR and log MF of parent coals/blend are established and presented in Fig. 8. The cokes made from the coals with higher rank and lower maximum fluidity have more severe macrostrength degradation under the simulated BF conditions due to their greater growth of crystallite size and more significant degradation of microstrength upon gasification with subsequent annealing at high temperature (2000 °C).
(2)
where T° is the tensile strength of feed coke, and T′ is the tensile strength of coke after gasification with subsequent annealing at 2000 °C. Generally good correlation was found between coke weight loss and the relative increase of porosity upon gasification plus annealing under the BF conditions; however, no direct correlation was
4. Conclusions The effect of properties of coal precursors on the micro- and macrostrength of cokes produced in a pilot oven was studied upon the gasification and annealing test conditions which more closely simulated those found in a BF. The specific methodology utilised included gasification of coke with BF gas-temperature profile from 900 to 1400 °C (corresponding from the thermal reserve zone to the cohesive zone) and annealing of coke up to 2000 °C (corresponding to the raceway region). General agreement was achieved between the properties for the cokes subjected to the simulated BF conditions and those for the cokes sampled from operating BF. The major findings are: 1. Cokes made from coals with different rank and carking property showed significant differences in their strength under the simulated BF test conditions, even though most of them had similar CSR/CRI values.
Fig. 7. Correlation between coke weight loss with porosity increase and tensile strength degradation. 783
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Fig. 8. Correlation between macrostrength degradation and (a) MMR (b) log MF of parent coals.
2. Coke microtextures experienced significant reflectance loss and structure change upon simulated BF gasification and annealing conditions. The decreases in mean maximum reflectance and bireflectance were more severe for the coke produced from the high rank coal. 3. High temperature annealing under the simulated BF conditions significantly decreased the coke microstrength of both IMDC and RMDC microtextural types. All the cokes had similar degradation in their IMDC microtextural types; however, at high temperature, degradation in the RMDC was more severe in the cokes produced from coals with higher rank and lower Gieseler maximum fluidity. 4. Under the simulated BF conditions, the degree of graphitisation of cokes increased with increasing treatment temperature, which indicated that the coke carbon structure transformed from a nongraphitic structure toward a graphitic structure. The cokes made from coals with higher rank and lower Gieseler maximum fluidity had much higher graphitisation development upon the annealing at high temperature. The more significant degradation in the microstrength of cokes produced from these coals was attributed to the greater change in their microstructure toward graphitic structure upon high temperature. 5. The cokes made from coals with higher rank and lower Gieseler maximum fluidity had more significant degradation in their macrostrength due to their greater microstructure change and more significant microstrength degradation upon gasification and annealing under the BF conditions (2000 °C).
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