Production of High-strength Cokes from Non- and Slightly Caking Coals. Part II: Application of Sequence of Fine Pulverization of Coal, Briquetting and Carbonization to Single Coals and Binary Blends

Abstract

Sequential coal briquetting and carbonization was applied to preparation of cokes from 9 non- or slightly caking coals with carbon contents (f_C) of 67–85 wt%-daf. Coal pulverization to sizes of <106 µm and briquetting at 40°C enabled to prepare cokes with tensile strength (σ) over 10 MPa from 4 coals with f_C of 82–85 or 67 wt%-daf. Then, by introducing fine pulverization to sizes of < 10 µm before the briquetting, 7 coals were converted successfully into cokes with σ = 11–25 MPa. Increasing the briquetting temperature to 240°C further increased σ to 19–35 MPa for all the 9 coals. It was thus demonstrated that the hot briquetting of finely pulverized coal was a method to prepare high strength coke regardless of the rank of parent coal. Cokes were also prepared from 14 binary coal blends. All the cokes prepared by applying the fine pulverization and hot briquetting had σ of 20–35 MPa, which agreed well with that calculated by weighted average of those from the component coals. On the other hand, positive and also negative synergistic effects of blending occurred when blends were briquetted at 40°C. Characteristics of bonding/coalescence among particles of different types of coals were responsible for such synergies.

1. Introduction

It is desired to produce coke having mechanical strength high enough to be served in blast furnace from non- and slightly caking coal and also without either binder or caking coal for sustainable ironmaking. In the previous part (Part I) of this series paper,¹⁾ the present authors demonstrated preparation of cokes with cold tensile strength over 10 MPa, from non-caking subbituminous and bituminous coals by a sequence of pulverization (to sizes of < 106 μm), binderless hot briquetting at temperature of 100–240°C, and carbonization with heating rate of up to 30°C/min. Taken together with the authors’ previous reports on high-strength coke preparation from lignites^2,3,4,5) it was demonstrated that so-called ‘caking property’ of coal was not necessary for preparing high strength coke. In addition, the authors reported that fine pulverization of coal to its sizes smaller than 10 or 5 μm before the briquetting greatly increased the strength of resulting coke to even above 25 MPa.

In continuation of the above-mentioned previous study¹⁾ the authors have been investigating the pulverization-briquetting-carbonization of non- and slightly caking coals expanding the range of coal rank. The primary purpose of this study was to examine the applicability of not only hot but also cold briquetting of non- and slightly caking coals that had been pulverized or finely pulverized. The secondary purpose was to investigate the effectiveness of blending coals expecting synergistic effects of two different coals (i.e., binary blending) on the strength and other properties of resulting coke.

2. Experimental

2.1. Coal Sample

Nine different types of coals ranging lignite to bituminous ranks were used. Table 1 lists the coals and their properties. Among the nine coals, two slightly caking coals (TW and DT) had maximum Gieseler fluidities (MF) of 1.1. Other two coals (BG and NL) had slight fluidity with MF = 0.4–0.6, while the others showed no fluidity. All the coals were pulverized to particle sizes smaller than 106 μm and dried at 40°C under vacuum until the moisture content decreased to ca. 10 wt%-wet. Every coal sample thus prepared was subjected to fine pulverization by ball milling for 10 h. Details of the milling were reported in the previous part of this series paper.¹⁾ As reported in the previous part, the particle sizes after 10 h ball milling were smaller than 10 μm regardless of the coal type. The coal sample with and without the fine pulverization are hereafter referred to as W-FP X and W/O-FP X (X; coal ID as indicated in Table 1), respectively, if necessary to distinguish one from the other. This study also employed binary blends, each of which was prepared by subjecting two different coal samples (mass ratio; 50:50) to the ball milling for 10 h together with each other. In other words, binary coal blends were prepared not by post-fine-pulverization mixing but in-situ pulverization/mixing. A coal blend of coal X and Y will be denoted by X+Y.

Table 1. Properties of coals.

Coal	C content wt%-daf	Ash content wt%-db	Log (MF/ddpm)
LY	67.0	0.5	0.00
CV	76.3	13.0	0.00
KP	74.0	6.7	0.00
MB	78.6	10.2	0.00
DT	79.6	21.1	1.1
TW	80.3	5.0	1.1
BG	82.5	8.1	0.60
NL	84.0	11.5	0.40
IS	84.6	12.3	0.00

2.2. Briquetting and Carbonization

Both briquetting of coal sample and subsequent carbonization were performed according to the procedure developed by the present authors.^2,3) In this study, the mechanical pressure for the briquetting was fixed at 128 MPa. The briquetting temperature (T_b) was 40°C or 240°C. The briquetting with T_b = 40°C and 240°C will often be referred to as cold briquetting and hot briquetting, respectively. The main reason why the cold briquetting was performed not at ambient temperatures but at 40°C was that the latter temperature gave better reproducibility of temperature independent of ‘ambient’ one. The heating rate, final carbonization temperature (T_c) and holding time at T_c were 5°C/min, 1000°C and 10 min, respectively. Resulting cokes were in form of disc with typical thickness and diameter of 12±0.5 mm and 5.0±1 mm, respectively.

2.3. Characterization of Briquette and Coke

Properties of briquette and coke, such as bulk density (briquette; ρ_b, coke; ρ_c), tensile strength (σ) were measured by the same methods as reported previously.²⁾ After tensile strength measurements, fractured surfaces of the broken specimens were polished and then observed by a scanning electron microscopy (SEM). Details of the coke characterization were reported previously.¹⁾

3. Results and Discussion

3.1. Cokes from Single Coals

Figure 1 summarizes the strength of cokes prepared from single W/O-FP and W-FP coals showing the effect of the fine pulverization (FP) on the coke strength and also that of T_b. Figure 2 plots coke strength (σ) against the carbon (C) content of coal for the three series, i.e., cokes from W/O-FP coals with T_b = 40°C, those from W-FP coals with T_b = 40°C and those from W-FP coals with T_b = 240°C. σ of cokes from W/O-FP coals with T_b = 40°C distributes over a range from 0.2 to 18.4 MPa. Cokes with σ ≥ 10 MPa were prepared from the lignite (LY) and bituminous coals (BG, IS and NL) having carbon contents more than 82 wt%-daf. On the other hand, the cold briquetting of W/O-FP coals with C contents of 74–79 wt%-daf (CV, KP, and MB) resulted in cokes with σ < 2 MPa. The FP increased σ greatly or moderately for the coals with C contents of > 76 wt%-daf, except TW (80.3 wt%-daf). There seemed to be no or very little positive effect of the FP on the cokes from KP and LY. Then, applying the FP and T_b = 240°C enabled to prepare strong cokes with σ of 18–35 MPa from all the coals. Thus, within the variety of coal types employed in this study, the FP, or otherwise, that together with hot briquetting, successfully produced coke with σ > 18 MPa from all the subbituminous and bituminous coals as well as lignite.

Fig. 1.

Effect of fine pulverization of coal on σ of coke. W/O-FP; coal without fine pulverization, W-FP; finely pulverized (ball-milled) coal. T_b = 40 or 240°C. The nine single coals are put according to the order of σ for W/O-FP and T_b = 40°C.

Fig. 2.

Plots of σ against carbon content of coal for cokes prepared from W/O-FP and W-FP coals with T_b = 40°C or 240°C. (Online version in color.)

As shown in Figs. 1 and 2, the effects of FP and T_b on σ strongly depended on the coal type in a complex manner, which is here considered. The briquetting of W/O-FP and W-FP LY lignites gave briquettes with the same densities (ρ_b), 1.27 g/cm³, and resulted in cokes with very similar densities (ρ_c) of 1.22–1.23 g/cm³ and σ of 10–11 MPa (see Circle A in Fig. 2). It was believed that LY accepted particles deformation at a certain level that was necessary for briquette densification regardless of particle size range. No influence of FP on ρ_c (1.10 g/cm³) was also the case of W/O-FP and W-FP KP coals, which were converted to coke with σ as small as 1.6 MPa (see Circle B in Fig. 2). The FP increased ρ_c of KP coke (1.12 to 1.18 g/cm³), but without contributing to increase in σ. ρ_b and ρ_c for the individual cokes are listed in Table 2 together with σ and coke yield.

Table 2. Properties of briquette and coke and its yield.

Coal	Tb [°C]	σ [MPa]	ρb [g/cm3]	ρc [g/cm3]	Coke yield [wt%-db]	Coke yield [wt%-daf]
CV,W/ O-FP	40	0.2	1.11	1.02	69.5	64.9
CV,W-FP	40	8.3	1.21	1.27	69.7	65.1
CV,W-FP	240	18.8	1.23	1.35	67.9	63.0
MB,W/ O-FP	40	0.7	1.15	1.08	70.0	66.6
MB,W-FP	40	15.1	1.20	1.29	74.1	71.1
MB,W-FP	240	22.6	1.25	1.40	72.1	68.9
KP,W/ O-FP	40	1.6	1.10	1.12	65.4	63.0
KP,W-FP	40	1.6	1.10	1.18	63.8	61.2
KP,W-FP	240	27.4	1.23	1.40	63.5	60.9
TW,W/ O-FP	40	2.8	1.09	1.08	70.0	68.4
TW,W-FP	40	4.4	1.14	1.22	72.4	71.0
TW,W-FP	240	25.7	1.20	1.35	70.9	69.3
DT,W/ O-FP	40	5.5	1.12	1.10	72.1	64.6
DT,W-FP	40	18.4	1.19	1.24	72.9	65.7
DT,W-FP	240	24.3	1.23	1.40	71.3	63.6
LY,W/ O-FP	40	10.0	1.27	1.22	54.9	54.7
LY,W-FP	40	10.7	1.27	1.23	54.9	54.7
LY,W-FP	240	35.1	1.29	1.34	55.9	55.6
BG,W/ O-FP	40	12.2	1.13	1.21	72.9	70.5
BG,W-FP	40	18.2	1.18	1.35	73.7	71.4
BG,W-FP	240	27.9	1.22	1.33	72.6	70.2
IS,W/ O-FP	40	12.8	1.18	1.31	80.8	78.2
IS,W-FP	40	24.6	1.22	1.39	82.1	79.5
IS,W-FP	240	33.1	1.27	1.48	78.5	75.5
NL,W/ O-FP	40	18.4	1.20	1.35	79.0	76.3
NL,W-FP	40	24.9	1.24	1.41	78.8	76.1
NL,W-FP	240	28.0	1.29	1.49	77.1	74.1

It is known that ‘microhardness’ of coal strongly depends on its type and roughly correlated with properties such as ultimate analysis^6,7) and reflectance.⁸⁾ According to Krevelen,⁷⁾ microhardness of coal is maximized at carbon content of 75–80 wt%-daf. It was estimated that KP had a highest hardness among the coals investigated in this study, and the densification of its particles was difficult even after FP. The FP had a positive effect on ρ_c, while leaving σ as low as 1.6 MPa. The density of coke is widely and reasonably recognized to be a good parameter for its mechanical strength.^1,2,3,4,5) The result from the KP coke, however, showed that such recognition did not directly apply.

σ’s of the cokes from W/O-FP CV, DT, MB and TW were as low as 0.2–5.5 MPa, and this was explainable by microhardness of those coals with C contents of 75–80 wt%-daf.⁷⁾ Different from KP, the FP of those coals brought about more or less increase in σ. In particular, σ’s of CV and MB cokes were greatly increased by the FP. In fact, the FP increased ρ_b by 0.05–0.10 g/cm³, and ρ_c more significantly by 0.14–0.25 g/cm³. The microhardness of those coals was estimated to be not so high as to that of KP. The FP of BG, NL and IS increased σ’s of the resulting cokes, as expected from that those three coals with C contents more than 82 wt%-daf had lower microhardness than the other coals with the lower C contents.

Figure 2 also shows that increasing T_b from 40°C to 240°C increased σ of coke for all the W-FP coals. This was primarily due to increase in ρ_b by 0.02–0.13 g/cm³. It is seen that the increases in σ of the W-FP LY and W-FP KP were significant. It was believed that W-FP KP, heated at 240°C, finally underwent particles’ deformation losing its original hardness (i.e., resistance to deformation), obeying the mechanical pressure. Increasing T_b caused increases in ρ_b and ρ_c for W-FP KP as much as 0.13 and 0.22 g/cm³, respectively. The increasing T_b did not increase ρ_b of W-FP LY briquette (increase; only 0.022 g/cm³), but it caused subsequent ρ_c increase from 1.23 to 1.34 g/cm³. It was thus difficult to correlate the ρ_b increase with that of ρ_c. It was suggested that the briquette, if prepared with higher T_b or with T_b over a critical temperature, carried less residual stress than that prepared at T_b as low as 40°C. It is well agreed that stress relaxation of becomes more extensive and rapid as the temperature increases.⁹⁾

Figure 3 exhibits SEM photographs of polished fractured surfaces from cokes prepared from W-FP coals (except W-FP IS) with T_b = 40°C. The porous nature of coke is qualitatively evaluated from the photographs. It is clearly seen that particles’ bonding and coalescence had least developed for the KP coke. This is in good agreement with that the coke had smallest σ (1.6 MPa) among the cokes from the W-FP coals. It is also noticed that two cokes from CV and TW had higher porosity than the other five cokes with greater σ, although particles’ bonding/coalescence had developed more extensively than the KP coke. The other cokes from LY, MB, DT, BG and NL seemed to have more or less similar porosity with relatively well developed continuous dense phases. Figure 4 demonstrates a marked difference in the internal structure between the FP-KP cokes prepared by applying T_b = 40°C and 240°C. The difference explains well the above-mentioned significant increase in σ as well as ρ_c by increasing T_b.

Fig. 3.

SEM photographs of fractured and polished surfaces of cokes prepared from 8 different finely pulverized coals with T_b = 40°C.

Fig. 4.

SEM photographs of fractured and polished surfaces of cokes prepared from FP KP coal with T_b = 40°C (left) and T_b = 240°C (right).

3.2. Cokes from Binary Coal Blends

This section discusses synergistic effects of blending two W-FP coals on the strength of resulting coke. Figure 5 compares σ’s of cokes from five binary blends, KP+LY, KP+NL, KP+MB, KP+CV and KP+TW. The synergistic effect of blending is evaluated simply by comparing σ of blend-derived coke, σ_c,b, with coke-mass-based weighted average of those of cokes from the individual components, C1 and C2 coals, i.e.,   

$${\sigma }_{\text{c},\text{b},\text{calc}}=({\sigma }_{\text{c},\text{C}1}{Y}_{\text{c},\text{C}1}+{\sigma }_{\text{c},\text{C}2}{Y}_{\text{c},\text{C}2})/\left(Y_{\text{c},\text{C}1}+Y_{\text{c},\text{C}2}\right)$$

Fig. 5.

Effect of blending W-FP KP with another W-FP coal (LY, NL, MB, CV or TW) on the strength of coke. Black- and grey-colored bars indicate T_b = 240°C and T_b = 40°C, respectively. +: A positive synergistic effect of blending on σ_c,b is clear. ++; σ_c,b is clearly greater than those for both single coals.

Y_C1 and Y_C2 are the coke yields from C1 and C2 coals, respectively, on dry coal mass bases. σ_c,C1 and σ_c,C2 are σ’s of the cokes from C1 and C2, respectively.¹⁰⁾ The ratio of σ_c,b to σ_c,b,calc, is employed as a simple measure for the synergistic effect of blending on the coke strength.   

$$r_{\sigma }={\sigma }_{\text{c},\text{b}}/{\sigma }_{\text{c},\text{b},\text{calc}}$$

Synergy is also considered on the bulk density of coke, according to its primary importance in the coke strength.^1,2,5) The density of coke from a blend can be calculated by the following equation if there is no synergistic effect of blending on the density.   

$${\rho }_{\text{c},\text{b},\text{calc}}=\left(Y_{\text{c},\text{C}1}+Y_{\text{c},\text{C}2}\right)/(Y_{\text{c},\text{C}1}/{\rho }_{\text{c},\text{C}1}+Y_{\text{c},\text{C}2}/{\rho }_{\text{c},\text{C}2})$$

The density ratio is defined in the same way as r_σ.   

$$r_{\rho }={\rho }_{\mathrm{c},\mathrm{b}}/{\rho }_{\mathrm{c},\mathrm{b},\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{c}}$$

For every of the five blend-derived cokes with T_b = 240°C, σ_c,b was similar to σ_c,b,calc. Positive synergies are seen for KP+LY and KP+NL blends, but σ_c,b’s are still similar to the corresponding σ_c,C1 and σ_c,C2. In fact, r_σ’s were in a range of 0.94–1.14, while r_ρ’s were in that of 0.98–1.00. Thus, there was apparently no significant synergistic effect of the blending on the coke strength. Sakamoto and Igawa prepared cokes from single caking and slightly-caking coals and their binary blends by simulating a coke-oven method, and investigated synergistic effects of blending on coke properties.¹⁰⁾ They reported significantly positive effects of blending caking and slightly-caking coals on coke strength (exactly saying, a type of drum index).¹⁰⁾ This would be arisen mainly from difficulty of producing high strength coke only from slightly caking coal by the coke oven method. In contrast, the combination of the FP and briquetting at 240°C enabled to prepare cokes with σ within a relatively narrow range of 19–35 MPa, in which the maximum to minimum ratio was as small as 1.9. Thus, roughly saying, σ of cokes from different coals were more or less similar to each other. Sakamoto also reported very weak synergistic effects on the coke strength for blends of caking coals and those of slightly caking coals.¹⁰⁾

On the other hand, strong and positive synergies occurred for the five blend-derived cokes with T_b = 40°C. For these cokes, σ_c,b/σ_c,b,calc was in a range of 1.4–3.2. In particular, σ_c,b was greater than both σ_c,C1 and σ_c,C2 for the KP+CV and KP+TW blends, as indicated by ‘++’ in Fig. 5. Such positive synergistic effects are explained qualitatively but well by Fig. 6. The polished fractured surface of coke from the KP (σ = 1.6 MPa) is typical to that consisting mainly of particles with neither significant bonding nor coalescence. On the other hand, those of cokes from the other single coals consist of flat (smooth) parts and recesses arising from nonporous and porous parts, respectively. It is noted that the surfaces of the individual blend-derived cokes are all similar to those from not the KP but its partner coal. In other words, the characteristics of the KP-derived coal had largely been lost in the blend-derived coke. It was found that r_ρ’s for the five blend-derived cokes were 1.04–1.07. This indicated the blending promoted densification of coke. One of the main reasons for the promoted densification of coke seemed to be that of briquette. The measured briquette densities were greater systematically than those calculated in the same manner as for coke by 4–5%. It was then believed that ‘softer’ particles of the partner coal underwent deformation forming contacts with or even bonding to ‘harder’ particles of KP in the briquetting. The SEM pictures of the blend-derived cokes, in particular, those of the KP+NL and KP+LY-derived coals demonstrate that bonding and coalescence of particles of different coals.

Fig. 6.

SEM photographs of polished fractured surfaces of cokes from FP-KP, FP-LY, FP-NL, FP-CV and FP-TW, and those from blends of FP-KP and another.

Figure 7 shows σ’s of cokes from the other nine blends together with those from the component coals. The cokes with T_b = 240°C had r_σ ranging from 0.93–1.04. This trend is in good agreement with that shown in Fig. 5. On the other hand, three of the blend-derived cokes with T_b = 40°C (indicated by ‘+’) had r_σ = 1.4–1.8, while other four (indicated by ‘–’) had r_σ = 0.4–0.8. Thus, both positive and negative synergistic effects occurred. Figure 8 displays polished fractured surfaces of the cokes from blends of TW with CV, MB and LY with r_σ = 1.4–1.8 and r_σ = 0.99–1.00. It seemed for CV+TW and MB+TW that the blending slightly promoted formation and connection of flat areas on the polished surface, which are attributed to particles’ bonding/coalescence. The result of the LY+TW blending is similar to those containing KP (see Fig. 6), but it caused formation of grater and connected pores. This could be a reason of r_σ limited at 1.4.

Fig. 7.

Comparison of σ of coke from binary blend with those from two component coals in the same manner as in Fig. 6. – : A negative synergistic effect of blending on σ_blend is clear.

Fig. 8.

SEM photographs of polished fractured surfaces of cokes from CV+TW, MB+TW and LY+TW blends with r_σ = 1.4–1.8 and r_ρ = 0.99–1.00.

As shown in Fig. 9, the DT+MB and CV+MB blending suppressed particles bonding/coalescence, and this resulted in r_σ = 0.4 and 0.8, respectively. This figure also shows that the MB+BG and LY+NL blending enhanced particles’ bonding/coalescence while forming greater pores with no significant synergistic effect on the coke density, i.e., r_ρ = 0.99. The cases of the MB+BG and LY+NL were similar to that of LY+TW although its r_σ was over 1. Thus, simultaneous promotion of particles’ bonding/coalescence and pore enlargement (probably due to pores’ coalescence simultaneously with that of dense parts) can not only caused positive but also negative synergistic effects on σ depending on development of the above-mentioned two events. Figure 10 exhibits another example of such ‘simultaneous promotion’, that is NL+TW blending, of that r_σ and r_ρ were both 1.00 resulting from a balance between the positive and negative effects as above.

Fig. 9.

SEM photographs of polished fractured surfaces of cokes from DT+MB, CV+MB and MB+BG and LY+NL blends with r_σ = 0.4–0.8 and r_ρ = 0.98–0.99.

Fig. 10.

SEM photographs of polished fractured surfaces of cokes from TW, NL and NL+TW with r_σ = 1.00 and r_ρ = 1.00.

Figure 11 graphically shows relationships between r_ρ and r_σ for the 14 blend-derived cokes. Based on the results shown in Figs. 5, 6, 7, 8, 9, 10, the blend-derived cokes with T_b = 40°C are classified into 4 groups.

Fig. 11.

Relationship between r_ρ and r_σ. Group 1: KP+TW, KP+CV, KP+LY, KP+NL, KP+MB, Group 2: CV+TW, MB+TW, BG+DT, Group 3: LY+TW, NL+TW, MB+BG, LY+NL, Group 4: MB+DT, MB+CV.

Group 1: The blending KP with another coal makes particular characteristics of KP, i.e., suppressed particles bonding/coalescence, disappear causing r_ρ > 1 and r_σ > 1 simultaneously.

Group 2: The blending results in intermediate structure of coke between the two component coals, bringing about r_σ > 1 while r_ρ slightly smaller than unity.

Group 3: The blending promotes particles’ bonding and coalescence, developing continuous dense phase but also forming greater pores. This results in r_ρ slightly smaller than 1. r_σ > 1 or r_σ < 1 is determined by relative extents of the phenomena that positively and negatively influence the coke strength.

Group 4: The blending suppresses particles’ bonding and coalescence, resulting in r_ρ < 1 and r_σ < 1 simultaneously. The component coals have similar C contents of 75–80 wt%-daf, but their particles have low affinity of bonding.

An important and common feature among Groups 1–3 was that bonding and coalescence occurred and developed among particles of different types of not only slightly caking coals but also non-caking coals. The r_σ vs r_ρ plots also show that the result of neither positive nor negative effect of blending cannot be explained simply by that on the briquette/coke density. Though not shown here, r_σ vs r_ρ plots were also made for the blend-derived cokes with T_b = 240°C. r_ρ distributed over a range from 0.97 and 1.03, but r_ρ was concentrated within a very narrow range of 0.9–1.1. Thus, the synergistic effect on the coke density is, even if important, just one of the factors or an indirect factor for that on the coke strength. As suggested in this paper, it is necessary to understand characteristics and mechanism of bonding and coalescence among particles of different coals, and also resulting formation/growth and disappearance/shrinkage of pores during the carbonization.

It was suspected that the blending of coals had induced synergistic effect on the coke yield, and it influenced the coke properties, because particles of two different coals were in very close contact with each other. The synergistic effect was evaluated according to ∆Y_c which was defined by the following equation.   

$$\mathrm{\Delta }{Y}_{\text{c}}=Y_{\text{c}}–Y_{\text{c},\text{calc}}=Y_{\text{c}}–\left(Y_{\text{c},\text{C}1}+Y_{\text{c},\text{C}2}\right)/2$$

Y_c, Y_c,C1 andY_c,C2 are the coke yields from the blend and the two component coals (C1 and C2) with the same mass, respectively. ∆Y_c’s for all the blends are shown in Fig. 12. There occurred synergistic effect on the coke yield systematically, but |∆Y_c| was as small as 0.2–1.1 wt%. Interestingly, it seemed that blending W-FP KP with another caused positive synergistic effects, in other words, promoted co-carbonization suppressing volatile release. Positive ∆Y_c was also confirmed for the blends of involving W-FP LY. On the other hand, blending two W-FP coals with C contents of >76 wt%-daf tended to promoted volatile release rather than co-carbonization. Relationships of ∆Y_c with parameters such as r_ρ and r_σ were investigated, but no meaningful correlation was found. Thus, either of enhanced carbonization or volatiles release did not seem to be a factor crucial for the coke strength.

Fig. 12.

∆Y_c for the 14 binary blends of W-FP coals.

4. Conclusions

Coke production from nine non- or slightly coals was studied and the followings have been demonstrated.

(1) The sequence of pulverization to particle sizes of < 106 μm, briquetting at 40°C and carbonization at 1000°C produced cokes with σ of 10 MPa or greater from four coals with C contents of 82–85 and 67 wt%-daf, but not for five coals with C contents of 74–80 wt%-daf.

(2) Replacing the pulverization of the above sequence by the fine pulverization enabled preparation of cokes with σ >10 MPa from seven coals including three of those with C contents of 74–80 wt%-daf.

(3) Further change of a briquetting condition, i.e., increase in T_b from 40 to 240°C, produced cokes with σ of 19–35 MPa from all the 9 coals.

(4) σ of the cokes from the 14 binary W-FP coal blends were in a range of 24–35 MPa when T_b = 240°C was applied. Those measured σ’s were in good agreement with weight-averaged σ’s of cokes from the single component coals.

(5) The briquetting of the binary blends at 40°C caused a variety of synergistic effects, in other words, both positive and negative effects, on r_ρ and r_σ of resulting cokes. σ of the coke from W-FP KP was as low as 1.6 MPa, but it was increased to 9–20 MPa by taking advantage of the binary blending.

Acknowledgment

A part of this work was financially supported by The Iron and Steel Institute of Japan for 2015–2018. The authors are grateful to Cooperative Research Program of Network Joint Research Center for Materials and Devices that has been supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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