Chemical structure of coal is evolutionary changed during pyrolysis that accompanies gas release. The chemical structural change and gas formation profiles play important roles in determining caking property and physical properties such as strength and size of the resultant coke. However, analyses of volatile components and structural analysis of solid char have been mostly performed individually, and it is difficult to combine both and to obtain quantitative understanding on the thermal decomposition of coal at mechanistic level. In this study, simultaneous analyses of solid chemical structures of the heat treated coals and gas formation profiles were conducted for two kinds of coals that were pyrolyzed at an identical condition. On-line gas analysis with a quadrupole mass spectrometer and spectroscopic methods (NMR and FT-IR) were employed for quantitative evaluation of gas formation characteristics and solid chemical structure, respectively. The information obtained were then integrated to acquire new insight for coal pyrolysis mechanism. Here an approach to quantify the transferable hydrogen that contributes to stabilize radicals formed in pyrolyzing coal was proposed. It includes the quantitative assessment of aromatic cluster growth, decomposition of hydroxyls, and releases of hydrogen and pyrolytic water into gas phase. The proposed approach suggested that a bituminous coal that exhibits plasticity during pyrolysis had 3.5 mol/kg-coal transferable hydrogen, whereas the amount of transferable hydrogen of the sub-bituminous coal, a non-caking coal, was 1.3 mol/kg-coal, during pyrolysis up to 500°C.
A subbituminous coal was treated with a wash oil or 1-methyl naphthalene (MN), for 1 hr or 2 hr at 673 K or 693 K under nitrogen, in order to chemically upgrade the low-rank coal. In case of wash oil as the treated solvent, the extraction yield at 673 K reached 66 mass%, daf after recovering the solvent, however, some of wash oil remained in the extract because the wash oil contained polar compounds of 8.5 masst%. The extraction yield with MN at 673 K was 41.2 mass%, daf and it decreased with an increase in the severity of the extraction condition; it was 36.3 mass%, daf at 693 K for 2 hr.
The more polar solvent which contained polar compounds of 21.7 mass%, called the Super-Solvent (SS), was successfully produced by separation of the wash oil with methanol/water (3:1 by weight) mixture. The extract obtained from the extraction with SS showed similar degree of the H/C and O/C atomic ratios as bituminous coals, indicating that chemically upgrading reactions might occurred by the solvent treatment.
After the solvent treatment with MN at 693 K for 2 hr, hexane was used as the washing solvent, resulting that the ratio of hexane soluble fraction was a few. Thus, the solvent upgraded coal originated from a low-rank coal was successfully produced in high yield. The H/C and O/C ratios of the solvent upgraded coal became the similar level as those of caking coals. Decarboxylation and aromatization reactions might occur during the solvent treatment.
Japanese steel industries, which are importing all coal resources required, are facing the necessity of increasing usable coal resources due to the recent rapid decrease of high-grade coking coal reserve and increase in its price. Low rank coals such as brown coals and subbituminous coals are promising substitutes for coking coals since they are abundant and cost-effective. However, the low rank coals generally have no thermoplasticity which is required for producing a coke with high mechanical strength using conventional coke ovens. In this work we have proposed to pretreat low rank coals by oxidative degradation reactions with aqueous oxidant such as hydrogen peroxide in order to convert them into thermoplastic coals suitable for coke making. An Australian brown coal was treated with hydrogen peroxide aqueous solution either at room temperature for 4, 8 and 24 h, or at 60°C for 0.5, 1 and 2 h. The treated coals were then pelletized at room temperature and carbonized at 900°C to obtain cokes. It was found that the strength of the resulting cokes increased with increasing the oxidation time at 60°C and reached the level of a commercial coke, indicating that the treatment gave thermoplasticity to the coal. It was thus suggested that the wet oxidation was effective as a pretreatment for producing cokes from low rank coals.
The authors have previously proposed to pretreat low rank coals with aqueous oxidant such as hydrogen peroxide in order to convert them by oxidative degradation reactions into thermoplastic coals suitable for coke making. The treatment with hydrogen peroxide aqueous solution at 60°C improved a brown coal’s thermoplasticity and high-strength cokes were successfully prepared from the dried treated coal. In this study effect of water content of the treated coal on its thermoplastic behavior was investigated. The water-containing treated coal melted and re-solidified at less than 200°C whereas the dried treated coal only softened at around this temperature. Apparent viscosity of the wet treated coal was dependent only on temperature and water content and could be lowered to as low as 2.5 × 103 Pa s. The apparent viscosity of the treated coal containing a certain amount of water decreased with increasing temperature. The apparent viscosity at a certain temperature decreased with the increase of water content, reached a minimum value at around 40 wt% of water content on dry basis, and increased with the further increase of water content. As expected from the low apparent viscosity of the wet treated coal, the possibility was shown to utilize the treated coal as a binder for producing cokes from slightly- or non- coking coals.
In order to produce high-strength coke from low-quality coals, noncovalent bonds between O-functional groups in coal were cleaved by pyridine containing HPC pyridine soluble and HPC-derived thermoplastic components were introduced into the pores formed by swelling; thus, the synergistic effect during carbonization of the suppression of cross-linking reactions and the fluidity amplification due to close placement of coal and thermoplastic components was investigated. When HPC was extracted with pyridine, a decrease in O-functional groups was observed in the pyridine-soluble and pyridine-insoluble components. When HPC was extracted with MeOH, on the other hand, O-functional groups in HPC selectively moved into the soluble components. When non- or slightly-caking coal was chemically-modified with the prepared HPC pyridine-soluble components by utilizing the solvent-swelling effect of pyridine, the fluidity improved compared with the coals physically mixed with the soluble components or HPC. On the other hand, the fluidity of the chemically-modified sample with the MeOH-soluble components hardly changed from that of the original sample, and no effect of the modification with the thermoplastic component was observed. Furthermore, it was clarified that higher-strength coke can be produced from the chemically-modified sample with the HPC pyridine-soluble components than from the original coal or the physically mixed coal with the soluble components. The contraction behavior during carbonization of the chemically-modified sample with the soluble components and that of the original coal was investigated; as a result, a large difference was not observed between these two. Thus, it was found that high-strength coke can be produced from low-quality coals by the present method.
The addition of coal tar pitch mainly composed of polycyclic aromatic hydrocarbons (PAHs), increases the dilatation of coal. We investigated the effect of PAHs on the maximum dilatation (MD) of hard coking coals and semi-soft coking coals. Twenty-six kinds of PAHs and aliphatic hydrocarbon were used as additive agents.
The use of a PAH with a molecular weight in the range 152.19–178.23 as an additive agent almost unchanged the MD. When a PAH with a molecular weight in the range 178.23–378.47 was added to the coal, the MD of the coal increased. The increase in MD achieved with the addition of 9,10-dihydrophenanthrene was larger than that obtained with the addition of phenanthrene. This is likely because the hydrogen atoms in the ninth and tenth positions in 9,10-dihydrophenanthrene inhibit the polymerization of the coal structure. The MD was hardly affected by the addition of a linear aliphatic hydrocarbon (C28H58) but MD decreased due to the addition of anthraquinone with carbonyl carbon. When a PAH with non-planar structure was added to the coal, the increase in MD was smaller than when a PAH with planar structure was added.
Coal fluidity is an important parameter in coal blending techniques for coke making because it strongly influences coke qualities. On the other hand, recently, the amount of high fluidity coal has been limited. To cope with this problem, caking additive method which improves fluidity of coal has been developed and commercialized. However, since tight supply of high fluidity coal is anticipated in the future, it is of great importance to develop more effective caking additive. Therefore, in this study, we investigated effect of 11 kinds of polyaromatic hydrocarbons which include oxygen, sulfur and nitrogen containing compounds on coal fluidity in order to search for more effective chemical substances. The additives were added to low fluidity coal, and fluidity analyses were carried out according to the Gieseler plastometer method. Addition of sulfur and oxygen containing compounds lowered fluidity of coal, whereas addition of aromatic amines enhanced fluidity of coal. Coal fluidity ameliorated with increasing the molecular weight of aromatic amine, and N,N’-di-2-naphthyl-1,4-phenylenediamine (DNPD) was the most effective aromatic amine in this study. Carbonization tests in an electric furnace were conducted to investigate an effect of DNPD on coke strength. As a result of adding only 1 wt% DNPD, fluidity of blended coal and coke strength (Drum Index) were highly improved.
In the present study, we prepare several types of specimens from non-caking coal — including specimens 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, as well as specimens consisting of physical blends with HPC — and examine the influence of heating conditions and types of caking agents on the production of high-strength coke using a SUS tube. We also investigate the influence of heating conditions and types of caking agents on the strength of coke from pelleted specimens and determine the optimal conditions for producing high-strength coke from non-caking coal. HPC with a wide range of thermoplastic properties is more effective as caking agents than additives containing only low- molecular-weight or high-molecular-weight components. In addition, the strength of the produced coke depends on the amount of the additive, and optimal values of the additive amount are present. It was found that the following heating schedule is effective for producing high-strength coke from non-caking coal with added caking agents: First, high-speed heating (20°C/min) to an intermediate temperature in the range 400–600°C, recognized as the thermoplastic temperature range for typical caking coal; then, low-speed heating (3°C/min) to the temperature range of 900–1000°C. Moreover, we demonstrate that, by increasing the rate of heating in the thermoplastic temperature range, it is possible to reduce the amount of caking agent added.
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.
Hypercoal (HPC) is examined as a caking additive to the mixture of strongly coking coal and non-slightly coking coal. Samples were coked, then their strength and crystallinity of the carbon structure by Raman spectra were measured. Hypercoal addition increased strength of coke, and a good correlation was observed between the mechanical strength of coke and the Raman parameter defined by the ratio of D3-band to G-band. Results suggest that mutual melting between blended coal and HPC brought strong carbonaceous structure with highly crystallized. The Raman parameter can predict the coke strength to some extent.
In continuation of the present authors’ studies on production of high strength coke from lignite by sequential binderless hot briquetting and carbonization, this study has been carried out aiming at proposing methods to produce high strength coke from non-/slightly caking coals of subbituminous to bituminous rank. This paper firstly demonstrates preparation of cokes with cold tensile strengths above 10 MPa from two single non-caking coals (particle size; < 106 µm) by applying briquetting at temperature and mechanical pressure of over 200°C and 100 MPa, respectively. Such strength of coke is obtained over a wide range of heating rate, 3–30°C/min, during carbonization with final temperature of 1000°C. Then, a simple pretreatment, fine pulverization of coal to particle sizes smaller than 10 or 5 µm, is examined. This pretreatment enables to prepare coke with tensile strength even over 25 MPa, by decreasing porosity of resulting coke and more extensively the size of macropores simultaneously. The coke strength changes with carbonization temperature having a particular feature; significant development of strength at 600–1000°C, i.e., after completion of tar evolution, in which macropores and non-porous (dense) part of coke shrink in volume, inducing bonding and coalescence of particles and thereby arising the strength.
Sequential coal briquetting and carbonization was applied to preparation of cokes from 9 non- or slightly caking coals with carbon contents (fC) 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 fC 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.
In our previous study, it was revealed that high MF coal having longer “maximum permeation distance”, which was developed as a unique thermoplasticity index, forms lower roundness pores and thinner pore-wall structures in coke and that coke strength deteriorated when the coal blend included the longer maximum permeation distance coal. Therefore, techniques for reducing the adverse effects of long maximum permeation distance coal on coke strength are essential so as to utilize the coal more efficiently. Some practical techniques of design and control regarding coal grain size were developed for ameliorating coking property of long maximum permeation distance coal in our previous paper. The techniques are based on the facts that the coke strength deterioration caused by long maximum permeation distance coal in coal blend was suppressed with decreasing the coal size.
In this paper, influence of weathering, which is mild oxidation with air atmosphere, on permeation distance and coke strength were researched in order to clarify possibilities of controlling maximum permeation distance in another way of the techniques of coal size adjustment. As a result, it was found that the measured maximum permeation distance and coke strength deterioration caused by long permeation distance coal was reduced by weathering processing although the fluidity was impaired. Accordingly, some control techniques of maximum permeation distance by weathering were proposed for more effective utilization of long maximum permeation distance coal.
Self-exothermic reaction of coal is initiated by the reaction of coal in a pile with oxygen in the air to be oxidized. Then, the heat generated by the oxidation promotes further oxidation, resulting in ignition. In order to prevent this phenomenon, it is necessary to understand the initial stage of oxidation of coal in the condition of heat-accumulation. Conventionally, there are very few researches to understand the early stage of coal oxidation. In this study, we aimed at to elucidate the earlier oxidation stage of coals by employing low temperature oxidation of a mass of coals and several instrumental analysis techniques. The oxidation we used were heating 50 g of coal charged in a stainless steel closed container at 80°C for 24 h by flowing hot air to simulate the self-exothermic reaction condition. XRD and Raman spectroscopic measurements showed that the carbon skeleton structure of coal did not change by the oxidation treatment, while FTIR, 1H NMR and 13C NMR measurements showed a decrease in aliphatic side chains and an increase in hydroxyl groups in coal. The information obtained here will help to understand the whole process of self-exothermic reaction of coal to prevent burning.
The effect of heating rate on the swelling ratio of coke during heating of coal was investigated using numerical simulations. The mathematical model in our previous study was modified to observe the effect of the heating rate, and the equation for the reaction rate of coal pyrolysis and gas formation was changed to a first-order temperature-dependent equation. The original and viscosity-dependent classical bubble nucleation equations were compared, and the results showed that the swelling ratio in the numerical simulation results reduced with an increase in the mass transfer rate of gas from coal. At low mass transfer rate (1 × 10−13 m3/s), the dependence of the swelling ratio on the heating rate was not straightforward. The swelling ratio increased in the order of the following heating rates: 10, 30, and 3°C/min. At high mass transfer rate (1 × 10−12 m3/s), the swelling ratio increased with increasing heating rate. Experimental results of varying the heating rate agreed well with the results of the large mass transfer case. The numerical simulation results indicated two issues—the gas escaping from coal is a key factor affecting the swelling of coal during heating, and the swelling ratio increased with increasing heating rate because of the short gas release time.
In this study, effect of carbonization heating rate and solvent removal method on the tensile strength of cokes prepared from chemical upgraded coal samples was investigated. A low rank coal, L coal, was solvent treated at 420°C, 2 h with 1-methylnapthlene (1-MN) solvent. Upgraded coal samples were obtained by removing 1-MN from the solvent treated samples. Two different approaches were applied to remove 1-MN from the treated samples; washing with tetrahydrofuran (THF) and that with n-hexane (HEX). All upgraded samples were first pelletized under a mechanical pressure and time of 40 MPa and 10 min, respectively, and then carbonized at heating rate of 3, 10 or 25°C/min. Carbonized coke samples strength was measured as tensile strength (MPa). It was found that lower heating rate gave higher tensile strength irrespective of solvent removal method. This may be due to that lower heating rates allowed the coal upon heating to undergo softening, melting and re-solidification for longer time. n-HEX-washed upgraded samples showed higher strength than THF-washed ones when the same heating rates were applied. Surfaces of coke samples were observed by scanning electron microscopy (SEM) and it was found that pore development structures of cokes n-HEX-washed samples and those from THF-washed samples were different from each other. This difference may be responsible for that in the strength.
It is known that mixing the particle of inert material such as coke breeze to blending coal increases the mean size of lump coke. The size of lump coke is changed by the fissure formation, and it is assumed that the shape of lump coke is also changed by this mechanism. Therefore, the fissure formaiton of coke was investigated using the Gaudin-Meloy-Harris size distribution function. The coke blended with the coke breeze had larger grain size compared to the coke without mix of the coke breeze. In addition, the α value, which is the characteristic value of the Gaudin-Meloy-Harris size distribution function, reduced, suggesting that the shape of lump coke after breaking may be distorted. Therefore, as a result of analyzing coke fissures, it became clear that a change occurred in the fissure formation of the coke. The simulation results showed that the voidage of the coke packed layer was increased by increasing the coke grain size or distorting the shape of lump coke. It suggests that the coke produced from coal with coke breeze may increase the voids in the blast furnace.
The effect of inert components in coke on the shrinkage ratio of coke was numerically investigated. The carbonization process of semi-coke was simulated by using the finite element method. Coke models with a coke matrix, pores, cracks, and/or inert components were used. The numerical results using a coke model composed of a coke matrix and pores indicated that pores did not affect the shrinkage behavior of semi-coke. In addition, cracks did not affect the shrinkage behavior. On the other hand, from a numerical simulation using a coke model with inert components, the addition of inert components decreased the shrinkage ratio of coke. When the inert components were added, the elastic modulus of the inert components, viscosity of the matrix of semi-coke, and size of the inert components affected the shrinkage ratio. Furthermore, cracks extending from the inert component drastically decreased the shrinkage ratio of coke because the thermal stress around the interface between the matrix and inert components opened the crack.
Development of coke production technology which can maintain high coke strength with use of certain amount of high rank coal is needed. Accordingly, it is important to investigate influence of coke pore structure on coke strength and dominant factors of coke strength. In this study, results of three-dimensional analysis such as components of coke structure and Mises stress of two types of high strength coke will be presented. Three-dimensional composition of coke was successfully visualized and quantified by three-valuation method. Moreover, three-dimensional stress analysis was conducted to investigate the relationship between coke structure and coke strength.
Briquette blending carbonization process is one of the effective cokemaking technologies to increase the blending ratio of semi-soft coking coals. The effect of coal briquette size on coke quality and bulk density in coke oven was studied. It was revealed that coke strength DI15015 is dependent on the briquette size. DI15015 shows a maximum, a minimum or monotonic increase with increasing briquette size, which depends on the blend composition of the briquette and the powder coal. The mean size of coke decreases and coal charge bulk density increases with increasing the briquette size. Choice of suitable briquette size is important from the viewpoint of coke quality, productivity and coke oven operation.