With the recent concern over the depletion of high-quality coking coal used for coke production, it is desirable to develop a simple method to convert low-grade coals with poor coking properties into high-quality coking coals. In this study we have focused on the possibility that polymerization reactions caused by the recombination of coal-inherent radicals may occur when coal is heated, which can deteriorate the softening and melting properties of coal. Aiming at removing such radicals, we have proposed a pretreatment method for improving coking property of low-grade coals, in which low-grade coals are treated with a reductant such as formic acid at around 60°C. It was shown that the treatment by either aqueous formic acid or formic acid vapor significantly improved the coals’ thermoplastic performance and enhanced the strength of the resulting coke even though consumption of the reductant formic acid was small enough to hardly change the elemental composition of the coals. We have also succeeded in restoring the coking property of weathered coking coal.

The three-dimensional aggregated structural models of two types of coals, A and B, were constructed. It is found that the density and the T_g of the models were qualitatively consistent with values obtained experimentally. The T_g of coals A and B calculated using the model structures are 315°C and 328°C, respectively. The effect of temperature on the distribution of cohesive energy was quantitatively elucidated using the three-dimensional aggregated structural models. The cohesive energy density (CED) of coal B was greater than that of coal A at temperatures <T_g. However, at temperatures >T_g, the CED of coal A is comparable to that of coal B. This implies that the molecules are more strongly aggregated in coal B than in coal A at low temperatures due to hydrogen bonding, and the intermolecular interaction is considered to have gradually relaxed above the T_g. It is concluded that differences in the molecular and cohesive structures of the coals led to differences in the distribution of van der Waals energy and electric energy at different temperatures. The van der Waals energy changed from attraction to repulsion at about 450°C and 285°C in coal A and B, respectively. Electric energy remained an attractive force as the temperature increased. The mechanism of the 13°C difference in the calculated T_g of each coal can be explained by the temperature change in the intermolecular force distribution. Therefore, this three-dimensional aggregated structural model can be used to understand the thermal behavior of aggregated molecules, such as coal thermoplasticity.

Researchers have thoroughly studied coal pyrolysis over a long period, while the analysis of volatile evolution and the chemical structural changes of solid char were carried out individually in most of the studies. In this work, we quantified the chemical reactions to explain the different physical phenomena, such as softening and caking properties, exhibited by different ranks of coals during pyrolysis. Four typical carbonaceous feedstocks (bituminous, sub-bituminous, lignite coals, and biomass) were selected as test samples. The authors analyzed the generated gas during pyrolysis by using a quadrupole mass spectrometer (Q-MS) and the chemical structure of the pyrolyzed char via spectroscopic methods (NMR, FT-IR, CHNS, and XPS) to gain new insights into the pyrolysis mechanism of the carbonaceous feedstocks. Transferable hydrogen was introduced to define the hydrogen used to stable the free radicals formed during pyrolysis, which can be obtained by quantifying the conversion routes of hydrogen. The hydrogen released for the growth of aromatic clusters has three pathways, namely, (1) consumption by the hydrodeoxygenation reaction to produce pyrolytic vapor, (2) release as gaseous H₂, and (3) transferable hydrogen. The calculation shows that the amount of transferable hydrogen during pyrolysis decreases as the coal rank gets lower. For pyrolysis up to 500°C, the amount of transferable hydrogen is 3.96, 2.32, and 1.36 mol/kg-coal for bituminous, sub-bituminous, and lignite coals, respectively. On the other hand, the transferable hydrogen of biomass needs to be further considered in terms of the effect of cellulose and hemicellulose’s structure.

In our previous paper, a new measurement method for the coal adhesion property called “surface tension of semi-coke” was devised. The surface tension of a semi-coke sample obtained by heat treatment of a coal sample at 500°C was measured as a unique adhesion property. Conventionally, it has been thought that adhesion is dominant under a low MF (Gieseler maximum fluidity) condition. Moreover, it is important for effective coal utilization to develop a technique that enables production of high strength coke under low MF conditions, which has been thought to deteriorate coke strength. However, in the previous paper, the effect of surface tension on coke strength was investigated only under a single MF condition without changing the level of MF.

In this paper, the effects of surface tension on coke strength under adhesion dominant conditions (low MF and high TI (total inert content)) were investigated. As a result, it was found that the effect of surface tension on coke strength was significant when MF was low or TI was high. Therefore, it is considered that high strength coke can be produced even under low-grade conditions (low MF or high TI) by controlling surface tension. Finally, based on the results, the concept of the conventional MOF diagram was extended. This technique enables effective selection and utilization of coal resources.

Recently, the environment has indicated an increase in the price of coking coal. Thus, in order to produce high strength coke at a low production cost, the application of blending technology to various types of coal is required. Coal dilatation is an important factor in determining coke strength. It is well known that the dilation values of blended coal containing semi-soft coking coals does not match with the weighted average value of coking coal and semi-soft coking coal; it denotes a value lower than the weighted average value. Therefore, in this study, to develop and apply coal blending technology to various types of semi-soft coal, we investigated the inhibitory influence of kind and particle size of semi-soft coking coals on the dilatation of hard coking coal.

Subsequently, we observed that the inhibitory effect was stronger when the specific dilatation volume of the blended coal was low. This is because when the semi-soft coking coal was resolidified, the specific dilatation volume of the semi-soft coking coal or/and hard coking coal was minimal. Additionally, the inhibitory influence was further increased by the decrease in the grain size of the semi-soft coking coal. These results suggest that the inhibitory influence on the dilatation of hard coking coal is dominated by the pore structure of the semi-coke derived from the semi-soft coking coal.

Coal particles with a high water content tend to coalesce and grow into quasi-particles. As a result, the strength of the coke produced by carbonizing the coal is weak, which causes a problem in the operation of the blast furnace as the following process. It has been reported that mixing of blended coal is effective to disintegrate quasi-particles and produce high strength coke. In this paper, the suitable operating conditions of an actual-scale mixer are investigated by applying the discrete element method (DEM).

First, mixing tests with a laboratory-scale mixer and corresponding DEM simulations are performed. The mixer is a horizontal vessel having a rotating main shaft with blades. In the middle of the mixer, there are choppers rotating at high speed, which contribute to disintegration of quasi-particles. In the experiments, test coal which includes tracer coal painted with fluorescent paint is mixed, and images of the mixed coal sample are taken under ultraviolet light. From the images, the area ratio of particles over 1 mm is evaluated as the non-mixing degree. In the DEM simulation, coal particles which receive a prescribed high collision stress are considered as disintegrated during mixing. The ratio of non-disintegrated particles to the total is evaluated as the non-mixing degree. Validation of the DEM evaluation is performed by comparing the results of the experiment and simulation.

Then, actual-scale DEM simulations are performed by varying the operating conditions. Based on the results, feasible and suitable operating conditions are suggested.

Briquette blending aims to increase coke strength by increasing the bulk density of the coal charge by blending in high-density briquettes. This technique tends to decrease the bulk density of the powder coal portion of the coal blend. In this study, we attempted to elucidate the mechanism behind this decrease in the bulk density of the powder coal portion owing to briquette blending. We conducted drop tests of blended coal that included briquettes, observed the behaviour of the dropped coal using a high-speed camera, and then performed quantitative estimation of the change in bulk density by observing the resulting coal packing structure using an X-ray CT system and image analysis. The bulk density of the powder coal portion decreased owing to the formation of localised low-bulk-density regions around the briquettes. Two types of low-bulk-density regions exist. In the first case, the scattering of the powder coal by the impact of the falling briquette forms gaps, which remain in the form of large voids under the briquettes after charging. The second type is presumably due to the large difference in size between the briquette and powder coal, which causes a wall effect between them. We then used a newly developed image analysis process to estimate the widths of the two types of low-bulk-density regions quantitatively as 8–10 mm and 5–6 mm. This study demonstrated how briquette blending creates anisotropic low-bulk-density regions around the briquettes, which leads to a decrease in the bulk density of the powder coal portion.

The method for separating the active component and the inertinite, respectively, in a lump coal was investigated, and it was confirmed that they could be separated from each other from microscopic observation. Then the methods for measuring the contraction of the active component and the inertinite were investigated. The author made a rod-shaped sample from a lump semicoke of the active component and the inertinite heated to 773 K for the measurement of contraction. Using rod-shaped samples had good results. The contraction of the active component was higher than that of the inert, confirming the trend that the higher the degree of coal carbonization, the lower the contraction of the active component.

In recent years, three-dimensional (3D) measurements of actual coke particles have been conducted using a laser scanner in order to acquire knowledge about the gas/liquid permeability of blast furnaces. In order to ensure representativeness, a large number of coke particles need to be measured during actual operation. Therefore, we measured over 100 coke particles using a medical X-ray computed tomography (CT) scanner and obtained 3D shape information of each particle using image analysis. The validity of the proposed method was confirmed by comparing the analysis results with the actual measurement results from sieve separation. In this study, we mainly focused on the sphericity and flattening ratio as 3D shape indices. A 10 kg sample contains coke with a wide distribution of sizes and shapes, and the standard error in addition to the mean value should be considered when comparing samples with different production conditions. The results of the analysis targeting two samples with different manufacturing conditions showed that the sphericity was greatly affected by the impact of the transportation process and closely related surface breakage. Furthermore, the flattening ratio was greatly affected by the fissures formed during the carbonization process, which is closely related to the furnace temperature and volatile matter of the blended coal. This study shows that a medical X-ray CT scanner is a useful and practical tool for acquiring 3D shape of coke particles.

The strength, pore structure, and material constants of coke prepared from caking coal (Coke A) and non- or slightly caking coal (Coke C) were experimentally and numerically investigated with a particular focus on those values at high temperatures. Coke A showed higher strength and lower porosity than Coke C. The pore structure imaged by X-ray computed tomography was translated to the finite element mesh with the image-based modeling, and the stress analysis based on the finite element method was performed to calculate the mode value of maximum principal stress at different Young’s modulus and Poisson’s ratio. Young’s modulus of Coke A and Coke C at a constant Poisson’s ratio decreased and increased, respectively by heating. When the temperature increased, the compression stress of Coke A increased. The result indicated that the coke strength could be increased by heating because of the decrease in apparent Young’s modulus, accompanied by the occurrence of creep.

This study investigated the mechanisms by which rapid-heating and/or high-pressure conditions can improve the thermal fluidity of coal, using a synthetic compound as a model for low-molecular weight (Mw) compounds in coal. The compound had one quinoline ring and two naphthalene rings, C₃₆H₃₃N, Mw of 479, and a boiling point (bp) of ~520°C. Rapid heating (> 10°C/min) overcame the evaporation rate of the compound, whereas high pressure (> 1 MPa) increased the bp and suppressed the pyrolysis reaction. These conditions allowed the compound to remain until temperatures > 400°C, although it completely evaporated at 370°C under general heating conditions in a coke oven (3°C/min and 0.1 MPa). The effects of increasing the heating rate from 3 to 10°C/min at 0.1 MPa corresponded to the effects of increasing the pressure from 0.1 to 1.0 MPa at 3°C/min. The compound remaining at temperatures > 370°C can act as a mobile phase and hydrogen donor, thereby increasing the fluidity of coal. It can also serve as a reactant in the coking reaction and increase the coke yield.

We studied the pressurized carbonization conditions that can produce high-strength coke at low temperature (800°C) from formed coals containing a large amount of non-caking or slightly-caking coal. It was found that as the rapid heating temperature range (room temperature → 450–600°C) increased, the amount of adherence between coal particles increased and the coke strength increased. The effect of the heating rate (10–40°C/min) became significant when the heating temperature exceeded 500°C, and increased with increasing heating rate. The strength also increased with increasing pressure, and the optimal pressure under the present conditions was 1 MPa.

In this study, a solvent based chemical upgrading process for coal was applied for chemical upgrading of biomass. Biomass was solvent treated at 320, 350, 380, 400, and 420°C, for 2 h with 1-methylnapthlene (1-MN) solvent. Upgraded biomass samples were obtained by removing 1-MN from the solvent treated samples using hexane washing approach. A part of 1-MN soluble fraction was also soluble in the hexane. n-Hexane soluble fraction obtained after the n-Hexane and the 1-MN recovery (solute) was combined to obtain upgraded biomass. H/C and O/C of the upgraded biomass (UB) samples were close to the values in the region of caking coals utilized in coke-making process suggesting chemical upgrading of the biomass. Solute fraction from the biomass was higher in comparison to that from the coal. The GCMS analysis showed the n-Hexane soluble may have compounds with boiling points lower than 1-MN. The carbon balance was lower than 100% may be because of the loss of the n-Hexane soluble biomass compounds with boiling points lower than 1-MN during the 1-MN separation process. Effect of addition of upgraded biomass to coal was investigated. Fracture strength and bulk density showed improvement due to the addition of upgraded biomass. Change in 1-MN solvent characteristics was investigated by recycling the used solvent. H/C, O/C and solute yields were nearly same for each recycle run.

We investigated the optimal heating conditions (heating rate, rapid heating temperature range, and pressure) and blending ratio that can produce high-strength coke by using formed coals containing biomass and modified-biomass. As a result, the coke strength was increased by broadening the rapid heating temperature range. Furthermore, it was found that the coke strength increased as the heating rate increased, with the optimal heating conditions being a heating rate of 40°C/min and rapid heating temperature range of room temperature to 600°C. When biomass was added to coal sample, the coke strength was degraded compared to the case of only coal even under optimal heating conditions. However, when modified-biomass was added, the degree of decrease in the coke strength was smaller, and there was a trend for the indirect tensile strength to increase with increasing pressure. The optimal pressure within the experimental range was 1 MPa. In addition, the coke with a high-strength of 6 to 12 MPa could be produced by increasing the modified-biomass blending ratio.

In the steel industry, promoting the use of low-grade coal is important because of the depletion of coking coal. To compensate for the lack of caking property of low-quality coals, binders must be added to produce coke from coal blends. Lignin is an aromatic polymer and one of the main components, accounting for 30% of wood. The chemical structure and characteristics of lignin differ depending on the raw woody biomass and the extraction method used. In this study, to find biomass-derived binders, alkali lignin, sulfuric acid lignin, kraft lignin, organic solvent lignin, phenolated lignin (lignophenol), and torrefied lignophenol were blended at a ratio of 3 mass% with coal powder (coking coal: thermal coal = 1:1). The blended powder was carbonized to form coke, and its bulk density and strength were measured. With the addition of lignin, the bulk density of coke was almost the same or decreased, and the strength decreased significantly, regardless of the lignin type. Lignophenol was expected to be a potential binder because it was observed to be thermoplastic at the melting temperature of coking coal by an in situ camera; however, it largely reduced the fluidity of the coal and the coke strength. The lignin samples did not behave like asphalt pitch, and it was concluded that it was difficult to use lignin as a binder for blended coal.

In the steel industry, the development of a coke production technology that uses low-grade coal is desired. Decarbonization by the use of plant biomass is also an important option. A candidate is formed coke, which is produced by forming non-coking coal using a binder, followed by carbonization. Kraft lignin, by-product in pulp and paper mills is a potential binder because it is an aromatic polymer dissolved from wood. In this study, thermal coal powder was blended with softwood kraft lignin powder at ratios from 0 to 20% and was briquetted by cold-pressing or hot-pressing. Then, the briquette was carbonized at 1000°C to produce formed coke. To verify whether the formed coke has sufficient strength for use in blast furnace, the product was subject to indirect tensile test. The tensile strength of the carbonized formed coke increased with increasing lignin content; however, a formed coke exceeding the target tensile strength of 5 MPa could not be obtained by cold forming. The briquetting temperature was the determinant strengthening factor of the formed coke. Formed coke produced by hot-pressing at temperatures higher than 150°C with 10% lignin at a pressing pressure of 150 MPa successfully attained the target strength. The tensile strength of the formed coke did not always correspond to the density of the corresponding coke, indicating that carbonized products originating from lignin play a role in strongly binding coal particles. Softwood kraft lignin seems a good binding material for low-grade coal to produce formed coke with high strength.

Torrefaction, pulverization, hot briquetting, and carbonization in sequence successfully produce high-strength coke from woody biomass. This method was further improved by introducing washing of torrefied biomass with acidic water from torrefaction before briquetting. The primary purpose of the washing was to remove alkali, and alkaline-earth metallic species of which catalyses were responsible for high reactivity of the coke. The acidic water (AW) from 275°C torrefaction of Japanese cedar contained 12, 0.9, and 39.4 mass% of acetic and formic acids, and the other organic compounds, respectively. A simulated AW (SAW) was prepared with the same composition as that of AW. SAW with pH of 1.95 removed 96–97% of K, Mg, and Ca and 48% of Na from the torrefied cedar. These removal rates were higher than those by washing with an aqueous solution of acetic acid, hydrogen chloride, or oxalic acid with pH of 2.35, 1.05, or 0.77, respectively. Organic compounds dissolved in SAW helped water and acids penetrate the matrix of the cedar. The washing with SAW increased the tensile strength of coke from 16 to 21 MPa by promoting volumetric shrinkage of the briquette during the carbonization and then particle bonding and coalescence. More importantly, the washing greatly reduced coke reactivity. The times required for gasifying 50% and 99% of coke with 50 kPa CO₂ at 900°C, t_0.50 and t_0.99, respectively, were extended by factors of 24 and 46, respectively. It was thus demonstrated that the coke reactivity was controllable over such a wide range.

The mechanisms of expansion and shrinkage behavior of the briquettes of coal or torrefied biomass were investigated by measuring the weight loss of coal and torrefied biomass and the expansion and shrinkage of the briquettes. In the case of torrefied biomass, particularly large expansion occurred at temperatures below 600°C. In the case of hot briquetting, the expansion was small below the briquetting temperature, suggesting that the expansion originated from relaxation of residual stresses during briquetting process. When coal and biomass were blended, the maximum strain was slightly close to the briquette of coal, and the temperature at which the maximum strain was taken was almost the same as that of the briquette of torrefied biomass, and in both cases, additivity was not valid. The expansion and shrinkage behavior of the briquettes was modeled as thermal expansion unique to the material, expansion due to relaxation of stresses accumulated during molding, and shrinkage due to the release of volatiles.