Influence of Heating Conditions on the Strength of Coke Produced from Slightly-Caking Coal Containing Chemically-Loaded Thermoplastic Components

Abstract

In this work, we studies the production of higher-strength coke from chemically-loaded coal in which noncovalent-bonds between O-functional groups in coal are cleaved by pyridine and HPC-derived thermoplastic components are introduced into the pores produced by swelling. The effect of heating rate up to thermoplasticity temperatures of coal on coke strength is first investigated. To examine synergistic effects due to further fluidity enhancements caused by the increased proximity of coal to thermoplastic components during carbonization, the influence of heating rate on coke-strength prepared from pelleted-coal also examined, as described above, to clarify the optimal heating conditions for yielding high-strength coke from slightly-caking coal. An investigation of the use of a SUS-tube to produce high-strength coke from slightly-caking coal with chemically-loaded HPC pyridine-soluble components reveals that high-strength coke may be obtained by 20°C/min to 400°C and then continuing to heat at 3°C/min to 1000°C. On the other hand, when producing coke from formed specimens consisting of slightly-caking coal with chemically-loaded HPC pyridine- soluble components, we exhibit that, by heating first at 20°C/min to 500–600°C and then heating at 3°C/min to 900°C, it is possible to produce coke whose strength rivals that of coke produced by carbonization at 3°C/min of strongly-caking coal. In addition, in producing high-strength coke from formed slightly-caking coal, an optimal amount of additive is present for all types of additive considered — HPC physical blend, chemically-loaded pyridine-soluble HPC and physical blend of pyridine-insoluble HPC components — and, with chemically-loaded pyridine-soluble HPC, it is possible to prepare particularly high-strength coke.

1. Introduction

In recent years, growing demand for iron and steel in Asian countries has sharply increased the demand for strongly-caking coal, which is a raw materials needed to make metallurgical coke, with prices rising steadily year after year. Moreover, while this trend may subside temporarily, in the long run it is expected only to increase. For this reason, it is of crucial importance to develop technologies to expand the range of resources available for coke production — that is, to develop technologies capable of producing coke simply and reliably, with quality comparable to that of the coke in use today, from a broad range of raw materials, including slightly-caking coal, non-caking coal, bituminous coal and brown coal. A key challenge that must be addressed is that the presence of significant amounts of low-quality coal among the inputs to a coke production process has the effect of reducing thermoplasticity, in which case it may not be possible to produce coke of sufficient strength. A strategy used to improve coke strength in such cases is the addition of pitch or other caking agents to enhance the fluidity of coal blends containing low-quality coal.

In recent years, these factors have motivated the development of methods for producing coke from mixed coals to which Hyper Coal (HPC) — a substance with outstanding thermoplasticity — has been added as a binder to increase coke strength.¹⁾ Although the fluidity and permeability of HPC vary depending on the type of coal used to produce it, the extraordinary thermoplasticity of this additive serve to enhance fluidity of coal blends; moreover, because HPC penetrates into gaps between particles of the surrounding coal mixture and fuses there — even at low temperatures — it is thought to play a role in enhancing cohesion by filling interparticle gaps, as well as increasing fused components.²⁾ On the other hand, it is well known that low-quality coal contains large numbers of O-functional groups and that reactions occurring during the carbonization process result in the formation of oxygen-oxygen interlinkages, which may prevent the material from exhibiting thermoplasticity at all.^3,4) Suppressing the cross-linking reaction is thus another key challenge for the use of low-quality coal.

Based on these reports, we have investigated methods for exploiting the specific effects of pyridine on coal — namely, solvent swelling and the cleavage of noncovalent bonds — to suppress cross-linking reactions during carbonization and amplify thermoplastic components, facilitating the development of methods for producing high-strength coke from low-quality coal. In particular, it was demonstrated that coke manufactured from a specially-prepared input material — in which pyridine-soluble HPC components are chemically loaded into the coal pores produced by pyridine treatment — is stronger than that produced from the unloaded raw coal.⁵⁾ This result was attributed to a synergistic interplay of two mechanisms: (a) pyridine-induced cleavage of noncovalent bonds between O-functional groups in coal, and (b) suppression of cross-linking reactions during carbonization, and amplification of fluidity due to increased proximity between coal and thermoplastic components, resulting from the introduction of HPC-derived thermoplastic components into the pores produced by pyridine-induced swelling.

Meanwhile, as a technique for increasing the percentage of low-quality coal input to a coke production process while simultaneously increasing the strength of the coke produced, researchers have used methods such as SCOPE21 for rapidly heating the temperature region of coal thermoplasticity, while conducting detailed investigations of the corresponding effects; this has revealed that increasing the rate of temperature increase in the fluidity region is an effective method for producing coke from low-quality coal.^6,7,8) Also, it has long been known that pelletization is an effective method for converting non-/slightly-caking coal into coke, and many studies have investigated formed coke;^{9,10,11,12,13,14)} among the findings that have been reported are the following. (1) Ensuring large particle-to-particle contact area is important for promoting binding between coal particles during the coke production process. (2) This, in turn, requires that the elasticity of the particles be low and that their plasticity be high; it is desirable for particles to submit readily to plastic deformation. (3) The plasticity of coal is affected by processes such as adsorption or immersion in organic solvents.^15,16,17) Although the extent of this effect depends on factors such as the particular organic solvent and coal rank, the use of pyridine with coal rank corresponding to non-/slightly-caking coal (80 mass%-daf) causes a major reduction in Knoop hardness.¹⁵⁾ Thus, plasticity increases, formability increases, and, in consequence, the strength (Shore hardness) of the resulting coke increases.¹⁵⁾

In view of the above, it is reasonable to expect that a two-step procedure — including both (1) a pyridine treatment intended to induce cleavage of noncovalent bonds in coal and to produce pores due to swelling, and (2) a pelletization process involving a proximity treatment using HPC pyridine-soluble components — prior to coke production will result in the production of higher-strength coke. The objective of this study is thus to produce higher-strength coke from an input material in which pyridine has induced cleavage of noncovalent bonds between O-functional groups in coal and an HPC-derived thermoplastic component has been introduced into the pores produced by swelling. In addition, we first examined (i) the possibility of the high-strength coke production from the sample in which HPC pyridine soluble and coal are brought close proximity (HPC pyridine soluble chemically loaded coals). And then we investigated the optimum heating conditions for high-strength coke production from the HPC pyridine soluble chemically loaded coals.

2. Experiment

2.1. Specimens

The primary specimens used are coal samples (GA, TA, KP) and HPC (< 250 μm), of varying particle sizes (< 250 μm and 0.5–1.0 mm), provided from the Study Group of Technique Elements for New Cokemaking Process. Table 1 lists the analytical values and Gieseler fluidity properties of the coal specimens used. For TA and GA, the values of the initial softening temperature (IST) are 390 and 410°C, respectively. TA exhibits maximum fluidity (MF) in the vicinity of 425°C, while for GA this temperature is near 460°C. The materials resolidified at temperatures in the range 450–490°C. KP did not exhibit a transition to fluidity. The MF values for GA, TA and KP were 2.1, 1.1 and 0 log(ddpm), respectively.

Table 1. Analyses of samples used in this study.

Coal	Elemental analysis					Proximate analysis			Gieseler fluidity property
	mass%-daf					mass%-dry			Temperature, °C				Maximum fluidity
	C	H	N	S	Oa	Ash	VMb	FCa,c	ISTd	MFTe	RSTf	FTRg	ddpm	log(ddpm)
KP	73.6	5.5	1.5	0.7	18.7	5.8	42.9	51.3	–	–	–	–	–	–
TA	83.5	5.1	2.0	0.4	6.0	6.2	37.5	56.3	388	426	453	65	14	1.1
GA	88.1	5.0	1.8	0.6	4.5	11.3	23.4	65.3	410	460	490	80	130	2.1
HPC	88.0	4.9	1.6	0.7	5.1	1.6	46.5	51.9	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.

^aEstimated by difference. ^bVolatile matter. ^cFixed carbon. ^dInitial softening temperature. ^eMaximum fluidity temperature.

^fResolidification temperature. ^gFluidity temperature range.

2.2. Preparation of Coal with Chemically-Loaded HPC Pyridine-Soluble Components

Preparation of coal containing chemically-loaded HPC pyridine-soluble components was followed previous reports to. The detailed procedure was as reported in Ref. 5. HPC was first extracted by pyridine at room temperature, then coal was added to a solution of pyridine-soluble HPC and stir at room temperature for 24 h to achieve swelling and loading of the pyridine-soluble components. Then, pyridine was eliminated via pressure-reduced heating to prepare samples containing chemically-loaded pyridine-soluble HPC. The amount of loaded soluble HPC components lies in the range 0-50 mass% with respect to coal.

If HPC pyridine soluble was introduced into coal structure via solvent swelling of coal, the loaded component must not elute during pyridine extraction of chemically-loaded sample. However, elution of loaded component was observed, because HPC pyridine soluble also deposits on coal particles surface and inherent pores in coal or expanded pores by pyridine swelling. Therefore, it was not possible to obtain chemical evidence that HPC pyridine soluble was introduced into the coal structure by this method. As is well-known, although HPC consists of hundreds to thousands of molecular weights,¹⁸⁾ HPC pyridine soluble is composed with lower molecular weight than original HPC, because DTG curve of the soluble gave the main peaks at around 250 and 450°C (HPC also provides the main peaks at same temperatures), and the former peak does not observed for HPC pyridine insoluble.⁵⁾ In addition, it was found that the interaction of HPC pyridine soluble and coal occurs during heat treatment of chemically loaded coal prepared by this method. Now, although we cannot obtain the direct evidence of HPC pyridine soluble introduced into coal structure produced during solvent swelling, the lighter component in the prepared HPC pyridine soluble may be loaded into coal structure. At least, it is suggested that the HPC pyridine soluble is introduced into the originally-present pores in the coal and the pores expanded by swelling, from the DTG results above-mentioned.

2.3. Carbonization

SUS tube was used for coke preparation.¹⁸⁾ After charging the tube [dimensions: 20 mm (diameter) × 50 mm (height)] with specimens of particle size 0.5–1.0 mm, sample was applied with a load of 200 g and carbonized for 30 min in a muffle furnace under N₂ gas flow at 1000°C. Table 2 lists the heating conditions applied when the tube was used. As indicated in Table 2 (Cases 1–4), two heating rate of 3 and 20°C/min were considered. In Case 1, the sample was heated at the constant rate of 3°C/min to 1000°C, while in Cases 2–4 the sample first heated at 20°C/min to an intermediate temperature in the range 400–600°C, then continue heating at 3°C/min to 1000°C. Note that the rate of 20°C/min follows the heating pattern used in the formed coke making process of the Japan Iron and Steel Federation.⁹⁾ The intermediate temperature range (400–600°C) was chosen because many raw coals have fluidity regions in this range.¹⁹⁾

Table 2. Heat conditions used in this study for coke production.

Heat condition	Heat condition at first step			Heat condition at second step		Holding time, min
	Heating rate, °C/min	Temperature range, °C		Heating rate, °C/min	Temperature range, °C
Case 1a	3	20–1000		–	–	30
Case 2a	20	20–400	→	3	400–1000	30
Case 3a	20	20–500	→	3	500–1000	30
Case 4a	20	20–600	→	3	600–1000	30
Case 5b	3	20–900		–	–	30
Case 6b	20	20–400	→	3	400–900	30
Case 7b	20	20–500	→	3	500–900	30
Case 8b	20	20–600	→	3	600–900	30

^aCoke preparation with a SUS tube. ^bFormed coke preparation.

To prepare formed coke, the particle size < 250 μm of coal was used, as well as our prepared specimens. After pelletizing samples with an applied uniaxial pressure of 30 MPa, the sample was carbonized for 30 min at 900°C using the heating patterns described above. It has been reported that mechanical pressure during the pyrolysis is enhanced caking properties of slightly-coking coal due to diffusion control of volatile components out of coal particles and inhibition of cross-linking reaction.²⁰⁾ In addition, the HPC pyridine soluble deposited on the surface of the coal particles are prepared by the method described in Section 2.2. The pre-forming treatment was carried out to produce high-strength coke. The pre-forming aimed to increase the fluidity by proximity between the chemically modified coal particles, increasing the mobile component donating from HPC pyridine soluble to the slightly-caking coal, and inhibition of cross-linking reaction during carbonization. The heating conditions used to prepare formed coke are listed as Cases 5–8 of Table 2. For brevity, the heating at rates of 3 and 20°C/min was denoted as low-speed heating and high-speed heating. Also, coke prepared from pelleted coal was denoted as formed coke. The strength of prepared coke samples was characterized destructively by using a tensile and compression testing machine (Minebea) to measure maximum loads, after which the indirect tensile strength was calculated using the following equation:   

$${\mathrm{f}}_{\mathrm{t}}=2\mathrm{P}/\pi \mathrm{d}\mathrm{l}$$

where f_t is the tensile strength in MPa, P is the maximum load in N, and d and l are the sample diameter and height in mm, respectively.

3. Results and Discussion

3.1. Influence of Heating Conditions on the Strength of Coke

Figures 1 and 2 present strengths of coke samples prepared from GA and TA under the various heating conditions and cross-sectional images after the tensile and compression tests, respectively. For Case 1, in which the sample was heated to 1000°C at 3°C/min, the strengths of coke samples prepared from GA and TA were 3.3 and 1.2 MPa, respectively. The strength of coke from pyridine-treated TA coal was 0.4 MPa, less than the untreated case. The strength of coke from the coal physically mixed with HPC pyridine-soluble components was 1.1 MPa, close to the value obtained for the untreated case. The highest strength (4.5 MPa) was observed for coke prepared from the chemically loaded sample with the pyridine-soluble components; this sample also revealed a dense cross section (Fig. 2). Meanwhile, for all samples prepared under the conditions of Cases 2–4 (which involved heating at 20°C/min to an intermediate temperature in the range 400–600°C, then continuing at 3°C/min to 1000°C), the strength tended to decrease as the intermediate temperature increased. As is evident in the photograph of Fig. 2, for the GA samples there is some loss due to swelling, while in the cross sections of the coke from the chemically-loaded coal we see numerous foam-like structures arising from excessive swelling.

Fig. 1.

Effect of heating conditions on coke strength of GA and TA samples: (a) Case 1, (b) Case 2, (c) Case 3 and (d) Case 4.

Fig. 2.

Cross-sectional images after the tensile and compression tests of coke prepared from GA and TA samples under the conditions of Cases 1–4.

Figure 3 shows the observed yield for coke prepared from the GA and TA specimens of Fig. 1, together with the influence of heating conditions on bulk density (Fig. 3(a)) and the relationship between strength and bulk density (Fig. 3(b)). The left and right vertical axes of Fig. 3(a) indicate bulk density and coke yield, respectively, while the horizontal axis indicates the intermediate temperature to which samples were heated at 20°C/min; the curves represent the variation of density and yield with temperature. Data points plotted at the far left (intermediate temperature 0°C) indicate results for Case 1, in which samples were heated to 1000°C at a constant rate of 3°C/min. Other data points indicate results for Cases 2–4, in which samples were heated at 20°C/min to intermediate temperatures in the range 400–600°C (indicated by the location of data points on the horizontal axis), then heated at 3°C/min to 1000°C. Whereas the observed yields are roughly constant across all sets of experimental conditions, the density tends to decrease as the temperature and the rate of heating increase. The extent of this reduction in density was particularly prominent for Cases 3 and 4, in which samples were heated at 20°C/min to temperatures of 500–600°C. In addition, Fig. 3(b) shows that, although the data are somewhat scattered, there is some correlation between bulk density and strength. It is well known that increasing the rate of heating in the thermoplastic temperature region of coal tends to increase fluidity.^6,7,8,21) Therefore, the decreased strength observed for Cases 2–4 may be said to arise from a decrease in bulk density (the formation of foam-like structures) due to excessive fluidity. Our findings thus demonstrate that the decreased strength observed at a higher temperature and faster heating rate is due primarily to excessive swelling — and thus that we must take care to suppress swelling when conducting strength tests to obtain accurate assessments of the net strength of the coke specimens prepared in Fig. 1.

Fig. 3.

Change in bulk density and coke yield under different heating conditions (a) and relationship between bulk density and indirect tensile strength (b). (Data points plotted at the far left (intermediate temperature 0°C) indicate results for Case 1, in which samples were heated to 1000°C at a constant rate of 3°C/min.)

To this end, Fig. 4(a) shows the results of strength tests for coke prepared from KP — a non-caking coal — blended with TA containing chemically supported soluble components. For comparison, the results for GA/KP mixture has also included in this plot. For GA/KP, it was found that the coke strength peaks at a maximum of 1.0 MPa at a GA amount of 30 mass% with respect to KP; increasing the GA amount to 50 mass% results in significantly reduced strength (< 0.2 MPa). For coke prepared under the conditions of Case 2 from chemically loaded TA with HPC pyridine-soluble components/KP, the strength increases up to a blend ratio of 80 mass%, peaking at 2.8 MPa. In contrast, for coke prepared from the same input material under the conditions of Case 3 or 4, a peak in strength is observed at a blend ratio of 70 mass%, but the value of this maximum strength is only 0.5–0.75 MPa, less than the strength observed for Case 2. Figure 4(b) shows coke-yield and bulk-density data for the specimens of Fig. 4(a). For GA/KP, the yield increases as the KP blend ratio increases; in contrast, for chemically loaded TA with HPC pyridine-soluble components/KP the yield remains essentially constant under all heating conditions. The curves of bulk density behave roughly similarly to the strength curves for all samples and all heating conditions. Thus, it was concluded that adding excessive amounts of chemically loaded TA with the pyridine-soluble components to KP — beyond the optimal blend ratio at which strength is maximized — increases the formation of foam-like structures, resulting in decreased strength. Based on these results, the following conclusion was obtained: when using the SUS tube to produce high-strength coke from chemically loaded TA with the pyridine-soluble components, the optimal heating schedule is to (a) heat at high speed (20°C/min) through the thermoplasticity temperature range of the coal species, then (b) reduce the rate of heating at the Case 2 intermediate temperature of 400°C and subsequently continue heating at low speed (3°C/min) to 1000°C. In Case 3 and 4, although the cokes were prepared by blending chemically loaded coal and non-caking coal, it became low strength due to formation of form-like structure. We have not found any methods for suppressing this excessive melting (a method for evaluating the coke strength prepared from chemically modified coal under Case 3 and 4 conditions). In Case 3 and 4 conditions, strength of coke prepared from chemically loaded coal may change. Thus, evaluating of coke strength prepared from chemically loaded coal under Case 3 and 4 is important, and the details will be subject in future work.

Fig. 4.

In indirect tensile strength (a), bulk density (b) or yield (c) of coke prepared from different blend ratios of GA or chemically-loaded TA to KP.

3.2. Influence of Heating Conditions on the Strength of Formed Coke

Figure 5 compares the indirect tensile strength of formed coke specimens prepared from TA samples under various heating conditions. Figure 6 shows the data of Fig. 5 organized by specimen to compare results for different heating conditions. For formed coke prepared from GA, the strength decreases (from 13 to 1.5 MPa) as the intermediate temperature increases. For formed coke from TA, the trend is reversed, with the strength increasing from 0.5 to 4.5 MPa. Increased strength accompanying increasing intermediate temperatures were also observed for pyridine-treated TA, 15 mass% physically mixed TA with the HPC pyridine-soluble components and 15 mass% chemically loaded TA with the HPC pyridine-soluble components; the strengths observed for the case of heating at 3°C/min (1.0, 2.5, 2.0 MPa) increased respectively to 2.0, 11, 12 MPa under the conditions of Case 8. For 30 mass% physical mixture of the pyridine-soluble components, a maximum strength of 8.5 MPa was attained under the conditions of Case 7; this fell slightly to 7.5 MPa under the conditions of Case 8. Similarly, for 30 mass% chemically loaded TA with the pyridine-soluble components, the maximum strength (10 MPa) was observed for Case 6, while the strength for Cases 7–8 lay in the range 5.5–6.5 MPa.

Fig. 5.

Indirect tensile strength of formed coke prepared from TA samples under the conditions of Case 5 (a), Case 6 (b), Case 7 (c) and Case 8 (d).

Fig. 6.

Indirect tensile strength of formed coke prepared from GA (a), TA (b), pyridine treated (c), physical mixture (15 mass%) (d), physical mixture (30 mass%) (e), chemically-loaded (15 mass%) (f) and chemically-loaded (30 mass%) (g).

The strength of coke production by pelletizing were larger than that by SUS tube, and high-strength was observed for HPC pyridine soluble chemically loaded coal. According previous work,²⁰⁾ when coal pyrolysis is performed during applying mechanical pressure, the coal particles are consolidated by loading, and the melting/anastomoses of coal particles proceed. As a result, the diffusion of volatile matter to outside of coal particles is suppressed. In addition, the volatile matter remaining in the particles promotes the softening and melting of the coal and at the same time increases the diffusion resistance of the volatile matter. And, these phenomena acts as factor of suppression of the diffusion of volatile matter to the outside of coal particles. Moreover, the cross-linking reaction that occurs at 350 to 450°C is suppressed because the volatile matter (tar) retained in coal particles are stabilized by hydrogen-donating components to form lighter-molecules, and coal fluidity is proceeded by light component.²⁰⁾ There is a difference between pre- and in-situ pressurization, in this study, similar phenomena may occur. In addition, HPC pyridine soluble loaded sample has more light molecular than other sample. Therefore, it is suggested that the above phenomenon is promoted by the proximity of the HPC pyridine soluble and the coal by pelletizing, and, at the same time, the effect of the heating rate described in Fig. 8 in Section 3.2 effectively worked, and, as a result, high strength coke may be produced.

Fig. 8.

Relationship between indirect tensile strength and bulk density: (a) comparison among samples, (b) comparison among heating conditions.

Figure 7 shows the variation in bulk density and yield for formed coke prepared from TA samples under the various heating conditions corresponding to Figs. 5 and 6. As seen in Fig. 7(a), the density is significantly reduced for two specimens in particular: GA, for which a significant drop was observed in strength for Case 8 in Figs. 5, 6, and 30 mass% chemically loaded TA with the pyridine-soluble components. Thus, the formation of foam-like structures attribute the drop in strength for these two specimens due to excessive swelling. In contrast, in cases where the strength increased with increasing rate of heating and intermediate temperature, the bulk density either remained essentially unchanged or increased. Meanwhile, the yield (Fig. 7(b)) remained essentially constant for all specimens and heating conditions. These findings demonstrate the possibility of producing high-strength formed coke by increasing the rate of heating in the thermoplastic temperature range of the input coal. In the present study, it was found that the optimal heating schedule for producing high-strength formed coke from slightly-caking coal with chemically-loaded HPC pyridine-soluble components is first to heat at 20°C/min to an intermediate temperature of 500–600°C, then switch to low-speed heating and continue at a heating rate of 3°C/min.

Fig. 7.

Bulk density (a) and coke yield of formed coke prepared from TA sample under different heating conditions. (Data points plotted at the far left (intermediate temperature 0°C) indicate results for Case 5, in which samples were heated to 1000°C at a constant rate of 3°C/min.)

Figure 8 shows the relationship between strength and bulk density corresponding to Figs. 6 and 7(a). In Fig. 8(a), in which results are organized by specimen, we see that, with the exception of GA, density increases with increasing strength. In Fig. 8(b), in which results are organized by heating condition, strength increases with increasing density was observed. These results demonstrate that, for chemically loaded coals with HPC pyridine-soluble components, the soluble component can serve as a binder to ensure a highly dense structure of coke during carbonization, resulting in increased strength.

As shown in Fig. 4(a), the largest coke strength was observed at mixture ratio of chemically modified TA to KP equals 80 mass% at Case 2 conditions. It may be possible that an increase in the coke strength occurs by (i) suppression of cross-linking reaction during carbonization due to cleavage of a part of noncovalent bond in the coal in the process of HPC pyridine soluble chemically modification of TA, (ii) thermal relaxation of coal aggregation structure by relaxation of noncovalent bands due to rapid heat treatment, or (iii) an increase in fluidity due to HPC pyridine soluble loading for TA. As is well known, the heat treatment with heating rate of several thousand °C/min is effective for inhibition of cross-linking reaction during coal carbonization.²²⁾ Therefore, at the heating rate of about 20°C/min under the present experimental conditions, the extent of non-covalent relaxation during carbonization of (ii) is considered to be small, but there is a possibility that an increase in coal fluidity has occurred. According previous reports, it is reported that an increase in heating rate from 3°C/min to 8, 10, 30, 50, 100, or 1000°C/min increases coal fluidity at gieseler fluidity measurement or viscoelasticity test.^23,24,25) Moreover, non-caking coal, which does not show the fluidity at 3°C/min of gieseler fluidity examination, provides the fluidity at 8°C/min of same measurement method.²³⁾ It is believed that rapid heating treatment increases mobile components (high mobility low molecular) due to thermal relaxation of coal structure via suppression of cross-linking reaction, resulting coal fluidity is promoted, and it becomes possible to manufacture high-strength coke.^26,27) In addition, IST, MFT, and RST shift to high temperature at large heating rate comparing with small heating rate conditions. Therefore, it is suggested that the increase in coke strength in Case 2 may be caused by suppression of cross-linking formation by increasing the heating rate up to 400°C and thermoplasticity amplification effect by HPC pyridine soluble loading to coal above 400°C in carbonization. On the other hand, the heating condition of Case 3 and Case 4 covers the softening temperature range (400–600°C) of TA, which is observed at gieseler fluidity examination at 3°C/min. Therefore, it is estimated that decrease in coke strength prepared at Case 3 or Case 4 occurs by formation of formed structure due to increased fluidity by suppression of cross-linking reaction and rapid increase in thermoplastic amount/quality of TA/KP/HPC pyridine soluble loaded to coal during rapid heat treatment in the FTR. As the resulting, these large thermoplasticity leads excessive fluidity and is produced foam-like structures in coke.

3.3. Influence of Additive Amount on the Strength of Formed Coke

To assess the influence of heating conditions and the amount of caking additive on the strength of formed coke, we change in variation in coke properties with heating conditions and amounts of caking additive was investigated. Figure 9(a) shows the influence of heating conditions on the strength of coke prepared from a specimen consisting of TA in which HPC has been physically blended. Note that the TA used in these experiments and the TA used in Figs. 5, 6, 7, 8 were taken from different rods; a comparison of the strength of coke prepared from TA specimens with no HPC additive indicates that some weathering has occurred. However, we conducted our investigations on the assumption that weathering would not impact the results of an assessment of the influence of caking additive amounts and heating conditions on the strength of formed coke. In Case 5, the strength of coke initially increases as the amount of HPC additive increases, peaking at a maximum strength of 5.0–5.5 MPa at an additive amount of 30–40 mass% and thereafter falling to 3.0 MPa. In contrast, for Case 6 the strength peaked at 5.0 MPa for 20–30 mass%, falling to 1.0 MPa at 40 mass%. For Cases 7 and 8, the strength vs. additive amount trend was similar: a maximum strength of 5.5 MPa was observed at 10 mass%, with the strength subsequently decreasing as the amount of additive increased. From the variation in bulk density with additive amount shown in Fig. 9(b), it was concluded that decreasing strength accompanying increased additive amount is due to the formation of excessive foam-like structures due to increased fusibility. The yield remained essentially constant independent of additive amount. This indicates that HPC interacts with TA and is retained in the solid phase. Thus it have been found that, by increasing rate of heating and intermediate temperature, it is possible to produce high-strength coke even with a small amount of HPC additive.

Fig. 9.

Indirect tensile strength (a), and bulk density and yield (b) of coke prepared under different heating conditions against physically mixed amount of HPC to weathered TA.

Figure 10 shows the strength, bulk density and yield of coke prepared under various heating conditions from weathered TA containing various amounts of chemically-loaded HPC pyridine-soluble components. The TA used for these tests was taken from rods different from those used for HPC additive tests, and exhibited some weathering. In the low-speed heating scenario of Case 5, the coke strength was 9.5 MPa at additive amount 50 mass% (Fig. 10(a)). For Case 6, a maximum strength of 9.5 MPa was observed at additive amount 40 mass%. On the other hand, for the cases involving heating at 20°C/min to 500–600°C (Cases 7 and 8), a maximum coke strength of approximately 9.0 MPa was observed at a amount of 30 mass%; thus, in these cases the maximum strength is attained at lower amounts of additive compared to the previous two cases. Although this behavior is similar to that observed for the case of HPC additives, the required amount of additive is greater here than in that case. From the variation in bulk density vs. additive amount shown in Fig. 10(b), it was concluded that the decrease in coke strength following the attainment of maximum strength is due to the proliferation of foam-like structures, as in the case of HPC additive. The yield lies in the range 60–70 mass% and were almost constant, irrespective for addition amounts. The fact that the yield remains essentially constant as the additive amount increases indicates that the pyridine-soluble components interact with TA.

Fig. 10.

Changes in indirect tensile strength (a), and bulk density and coke yield (b) of coke prepared under different heating conditions against chemically loaded amount of HPC pyridine-soluble components to weathered TA.

As shown in Fig. 10, the curves observed at Case 5 and Case 6 were different with those of Case 7 and Case 8. In the Case 5 of heating rate of 3°C/min, it is suggested that large amount of HPC pyridine soluble is needed to TA fluidity due to formation of cross-linking reaction during carbonization. On the other hand, in the Case 6, which is heat condition of 20°C/min up to 400°C (before IST observed at gieseler fluidity examination for TA), there is a possibility that cross-linking reaction occurring below 400°C is suppressed by rapid heat treatment.^{22,23,24,25,26,27)} In addition, the thermoplasticity is also increased by mobile components from HPC pyridine soluble loaded. Thus, HPC pyridine soluble loading amount to TA in the Case 6 is smaller than that in the Case 5. In the Case 7 and Case 8 of heating rate of 20°C/min up to 500–600°C, which cover FTR observed at gieseler fluidity examination of TA at 3°C/min, thermoplasticity amplification effect occurs by suppression of cross-linking and thermal structure relaxation of TA and HPC pyridine soluble loaded by rapid heat treatment. In other word, it is important that rapid heat treatment of fluidity range observed at gieseler fluidity examination of 3°C/min to obtain the high strength coke Therefore, the curves of Case 7 and Case 8 in Fig. 10 are similar, and the highest coke strength is observed at small HPC pyridine soluble loading amount.

Figure 11 shows the strength, bulk density and yield of coke prepared under various heating conditions from weathered TA with various amounts of physically-mixed HPC pyridine-insoluble components. The TA used for these tests was taken from rods different from those used for Fig. 10. For Case 5, the coke strength at an additive amount of 50 mass% was only 3.0 MPa, a small value. For Case 6 at 50 mass% the strength rises to 7.0 MPa, while for Cases 7 and 8 a maximum strength of 7.0 MPa is observed at 30 mass% additive amount, after which the strength decreases due to the proliferation of foam-like structures (Fig. 11(b)). In addition, the coke strength for zero additive amount in Fig. 11(a) is greater than that shown in Fig. 10(a). From these observations, it was concluded that, despite the different rods used, the strength of coke prepared from specimens with the pyridine-insoluble additives is less than that prepared from specimens with chemically-loaded pyridine-soluble components. As shown in Fig. 11(a), increasing extent of coke strength by HPC pyridine insoluble addition was smaller than that of HPC pyridine soluble loading. It is estimated that this distinction occurs due to the difference between heavy fraction, mobility, and molecular distribution of HPC pyridine soluble and insoluble.

Fig. 11.

Indirect tensile strength (a), and bulk density and yield (b) of coke prepared under different heating conditions against physically mixed amount of HPC pyridine-insoluble components to weathered TA.

According to the self-continuous-dissolution model.²⁸⁾ although TA coal begins to soften at about 400°C, the mobile components that form initially tend to lack sufficient quality and amount; this is believed to suppress the subsequent continuous dissolution of the immobile components. Results of thermogravimetric analysis indicate that low-molecular- weight components is more abundant in HPC pyridine-soluble than in the pyridine-insoluble, whereupon it is hypothesized that softening also begins to occur at low temperatures.⁵⁾ It is possible that, by chemically-loading HPC pyridine-soluble components — in which low-molecular-weight components are plentiful — not only amplify the initial mobile components of TA but also contribute to the dissolution of immobile components at later stages, resulting in improved fluidity and greater coke strength. On the other hand, the pyridine-insoluble components are present in abundance in high-mass components of HPC, and the higher temperatures at which the fluidity region occurs ensure that the types of continuous dissolution phenomena envisioned by the continuous self-dissolution model are not occurring in this case; consequently, the total fluidity is low (because the components that soften at early stages are insufficiently abundant), ensuring that the coke is of lower strength than that produced from coal in which the soluble components are loaded. Meanwhile, in cases where untreated HPC is used, the peak strength is observed at blend ratios where the amount of additive is less than in cases of specimens with chemically-loaded pyridine-soluble components. This is because HPC consists of a series of fluid components with abundant thermoplasticity, and may be understood as arising from the ability of HPC to dissolve both the initial mobile components required for thermoplasticity of TA and the immobile components present at later stages. Because the coal specimens used in our tests were not identical rods, simply comparing the absolute strengths of prepared coke samples cannot be done. However, a comparison of the relative improvements in the coke strength shown in Figs. 9, 10, 11 — that is, the strength of the coke produced by adding each caking agent vis-a-vis the strength of the coke prepared from TA alone without any additives — shows that the improvement factors are largest for the specimens in which HPC pyridine-soluble components are chemically loaded. This indicates that the preparation techniques used in this method — including the formation of pores due to solvent swelling (with cleavage of noncovalent bonds in coal) and the enhanced proximity resulting from the filling of those pores by thermoplastic components — are effective methods for producing high-strength formed coke.

4. Conclusions

(1) When using a SUS tube to produce high-strength coke from slightly-caking coal with chemically-loaded HPC pyridine-soluble components, the optimal heating schedule is to (a) heat at high speed (20°C/min) through the thermoplasticity temperature range of the coal species, then (b) reduce the rate of heating at the intermediate temperature of 400°C and thereafter continue heating at low speed (3°C/min) over the temperature range 400–1000°C.

(2) In producing high-strength formed coke from pelleted specimens containing the chemically-loaded pyridine-soluble components, the optimal heating schedule is first to heat at 20°C/min to an intermediate temperature of 500–600°C, then switch to low-speed heating and continue at a heating rate of 3°C/min.

(3) By increasing the rate of heating and the intermediate temperature in the thermoplastic temperature region of coal, it is possible to produce high-strength formed coke with smaller amounts of HPC added to TA, and this is true in all cases considered: physical blend of HPC, chemically-loaded pyridine-soluble HPC and physical blend of pyridine-insoluble HPC components.

(4) In producing high-strength formed coke from TA coal, an optimal amount of additive exists for all cases investigated and, with chemically-supported pyridine-soluble HPC, it is possible to prepare particularly high-strength coke.

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