Influence of coal thermoplastic properties on coking pressure generation: Part 2 - A study of binary coal blends and specific additives

A number of coal blends and pitch/coal blends were evaluated using rheometry, thermogravimetric analysis and microscopy to confirm and further elucidate the coking pressure mechanism previously proposed by Duffy et al. (2007) [1]. We confirm that blending a low rank, high fluidity, low coking pressure coal, with a high rank, low fluidity, high coking pressure coal can significantly reduce the coking pressure associated with the latter. Interestingly, blending does not necessarily result in a fluidity that is midway between that of the two coals; sometimes the fluidity of the blend is less than that of the low fluidity coal, especially when the coals are significantly different in rank. This occurs because the increase in complex viscosity (η*) through resolidification of the low rank, high fluidity coal counteracts the reduction in η* resulting from softening of the high rank, low fluidity coal. It has also been confirmed that the η* of the resultant blend can be estimated from the η* of each component coal using a logarithmic additivity rule commonly employed for polymer blends.Polarised light microscopy has indicated that the degree of mixing between coals of different rank is minimal, with fusion restricted to the particle surface. It is therefore inappropriate to think of such a coal blend in the same way as a single coal, since each component coal behaves relatively independently. This limited fusion is important for understanding the coking pressure mechanism for blends. It is proposed here that the lower rank coal, which softens at lower temperature, is able to expand into the interparticle voids between the high rank coal that is yet to soften, and these voids can create channels for volatiles to traverse. Then, and importantly, when the high rank coal begins to expand, the pore structure developed in the resolidified structures of the low rank coal can facilitate removal of volatiles, while the resolidified material may also act as a suitable sorbent for volatile matter. This is considered to be the primary mechanism by which coal blending is able to alleviate coking pressure, and applies to addition of inert material also.Addition of a coal tar pitch was found to increase fluidity but also to extend the thermoplastic range to lower temperatures. This caused an increase in the swelling range, which was accompanied by a long plateau in η*, a feature which has previously been observed for certain high fluidity, high pressure coals. Elasticity and η* at the onset of expansion were also higher for both the pitch impregnated coals and the high pressure blends, which supports previous findings for singly charged high pressure coals, and confirms the potential use of such criteria for identifying potentially dangerous coals/blends.     

Influence of porosity and fissuring on coking pressure generation

Nine bituminous coals of different rank and geographical origin were carbonized at pilot scale coke oven (300 kg) in order to study the pressure generated during coking. At the same time their contraction/expansion was assessed by means of the Koppers-INCAR test. Semicokes were carefully recovered from the test so that their structure could be studied. The semicokes were separated into two parts, i.e. one that had been heated to 575 °C and the other that had been heated to 700 °C. The true and apparent density of the semicokes was measured together with their pore size distribution by means of mercury porosimetry and the results were related to the dangerousness of coals. The structure of the semicokes from safe and dangerous coals is different especially in those obtained at lower temperature. In addition, the fissures of the semicokes were evaluated. The area of the fissures was found to be greater in the case of non-dangerous coals.     

Influence of additives of various origins on thermoplastic properties of coal

Two coking coals of different rank were chosen in order to assess the influence of various additives on their thermoplastic properties. Six additives of different origin and characteristics were selected: two non-coking coals, together with a commercial coal tar pitch, a residue from the bottom of the benzol distillation tower and two residues from the tyre recycling industry. The effect of the additives on coal thermoplastic properties was assessed by means of the Gieseler test. The additives were pyrolyzed to a final temperature of 550 °C and the tar characterized by means of Fourier transform infrared spectroscopy (FTIR) and gas chromatography (GC). The influence of the additives on coal thermoplasticity is related to the volatile matter content of the additive, its evolution profile with temperature and the composition of the tar obtained during the pyrolysis of the additives.     

Influence of the permeability of the coal plastic layer on coking pressure

Ten coals of different rank and coking pressure characteristics were chosen in order to study the time of occurrence of the phenomena that take place during the coking of a coal and the way they affect the generation of dangerous coking pressures. Parameters derived from thermoplastic, thermogravimetric and permeability tests were studied together with semicoke contraction and the coking pressure generated by the coals in a movable wall oven. It was found that for safe coals, the maximum evolution of volatile matter occurs near the temperature of maximum fluidity. The position of the maximum rate of volatile matter evolution with respect to the zone of low permeability varies depending on the coking pressure characteristics of the coals. In addition, the relationship between the period of low permeability to the resolidification temperature may serve to indicate the degree of dangerousness of a coal. The fissure pattern of the semicoke was found to be related to the coking pressure and semicoke contraction.     

Understanding the mechanisms behind coking pressure: Relationship to pore structure

During carbonisation coal undergoes both physical and chemical changes that result in the generation of gas and tar and the formation of an intermediate plastic state. This transformation is known to generate high internal gas pressures for some coals during carbonisation that translate to high pressures at the oven wall. In this study, three low volatile coals A, B and C with oven wall pressures of 100 kPa, 60 kPa and 20 kPa respectively were investigated using high-temperature rheometry, ¹H NMR, thermogravimetric analysis and SEM, with the primary aim to better understand the mechanisms behind the coking pressure phenomenon. Rheometer plate displacement measurements (ΔL) have shown differences in the expansion and contraction behaviour of the three coals, which seem to correlate with changes in rheological properties; while SEM images have shown that the expansion process coincides with development of pore structure. It is considered that the point of maximum plate height (ΔL_max) prior to contraction may be indicative of a cell opening or pore network forming process, based on analogies with other foam systems. Such a process may be considered important for coking pressure since it provides a potential mechanism for volatile escape, relieving internal gas pressure and inducing charge contraction. For coal C, which has the highest fluidity ΔL_max occurs quite early in the softening process and consequently a large degree of contraction is observed; while for the lower fluidity coal B, the process is delayed since pore development and consequently wall thinning progress at a slower rate. When ΔL_max is attained, a lower degree of contraction is observed because the event occurs closer to resolidification where the increasing viscosity/elasticity can stabilise the expanded pore structure. For coal A which is relatively high fluidity, but also high coking pressure, a greater degree of swelling is observed prior to cell rupture, which may be due to greater fluid elasticity during the expansion process. This excessive expansion is considered to be a potential reason for its high coking pressure.     

The mechanism of coking pressure generation II: Effect of high volatile matter coking coal, semi-anthracite and coke breeze on coking pressure and contraction

One of the most important aspects of the cokemaking process is to control and limit the coking pressure since excessive coking pressure can lead to operational problems and oven wall damage. Following on from a previous paper on plastic layer permeability we have studied the effect of contraction of semi-coke on coking pressure and the effect of organic additives on contraction. A link between contraction (or simulated contraction) outside the plastic layer and coking pressure was demonstrated. The interaction between this contraction, local bulk density around the plastic layer and the dependence of the permeability of the plastic layer on bulk density was discussed as possible mechanisms for the generation of coking pressure. The effect of blending either a high volatile matter coal or one of two semi-anthracites with low volatile matter, high coking pressure coals on the coking pressure of the binary blends has been explained using this mechanism.     

The effect of plastic size on coke quality and coking pressure in the co-carbonization of coal/plastic in coke oven

In the recycling process of waste plastics using coke ovens, coals and added plastics are carbonized and changed into coke, tar, oil and coke oven gas in a coke oven chamber. In this study, the effect of added plastic size on coke quality and the effect of plastic addition on coking pressure was investigated. In the case of a plastic addition rate of 2%, the coke strength reached a minimum at the particle size of 10 mm for polyethylene (PE) and 3 mm for polystyrene (PS). The mechanism was attributed to the weak coke structure formed on the interface between plastic and coal. The result indicates that large or small plastic particles are favorable in order to add waste plastics to blended coals for coke making without affecting coke strength . Furthermore, it was also shown that a 1% addition of large size agglomerated waste plastics to blended coals did not increase coking pressure. Based on this fundamental study, and considering the ease of handling plastics, we have determined that the size of waste plastic used in a commercial-scale recycling process of waste plastics using coke ovens is about 25 mm. Nippon Steel Corporation started to operate a waste plastic recycling process using coke ovens at Nagoya and Kimitsu works in 2000 and at Yawata and Muroran works in 2002. Now the total capacity is 120,000 tons per year as of 2003 and this process is operating smoothly.     

The mechanism of coking pressure generation I: Effect of high volatile matter coking coal, semi-anthracite and coke breeze on coking pressure and plastic coal layer permeability

One of the most important aspects of the cokemaking process is to control and restrain the coking pressure since excessive coking pressure tends to lead to operational problems and oven wall damage. Therefore, in order to understand the mechanism of coking pressure generation, the permeability of the plastic coal layer and the coking pressure for the same single coal and the same blended coal were measured and the relationship between them was investigated. Then the ‘inert’ (pressure modifier) effect of organic additives such as high volatile matter coking coal, semi-anthracite and coke breeze was studied. The coking pressure peak for box charging with more uniform bulk density distribution was higher than that for top charging. It was found that the coking pressure peaks measured at different institutions (NSC and BHPBilliton) by box charging are nearly the same. The addition of high volatile matter coking coal, semi-anthracite and coke breeze to a low volatile matter, high coking pressure coal greatly increased the plastic layer permeability in laboratory experiments and correspondingly decreased the coking pressure. It was found that, high volatile matter coking coal decreases the coking pressure more than semi-anthracite at the same plastic coal layer permeability, which indicates that the coking pressure depends not only on plastic coal layer permeability but also on other factors. Coking pressure is also affected by the contraction behavior of the coke layer near the oven walls and a large contraction decreases the coal bulk density in the oven center and hence the internal gas pressure in the plastic layer. The effect of contraction on coking pressure needs to be investigated further.     

Thermogravimetric investigations in prediction of coking behaviour and coke properties derived from inertinite rich coals

The coal quality, temperature, pressure, heating rate, various processes and reactor type affect coking behaviour of coal and resulting coke properties. Several petrographic and chemical methods were proposed earlier for prediction of coking behaviour of coals. Inertinite rich coal samples (I^mmf>30 vol%) having different petrographic compositions were selected for thermogravimetric investigations (DTA, DTG and TGA) and their coking behaviour was studied. The petrographic build up, micro-structural properties (porosity and cell wall thickness) and mechanical strength of the resulted cokes were also investigated. ΔH and ΔH_max (the main endothermic area of heat absorption and fast absorbing main endothermic area, respectively) were distinguished in DTA curves. ΔA and ΔA_max (the main decomposition area and fast disintegrating main decomposition area) under DTG curves were identified. ΔH_max/ΔA_max shows good correlation with Roga's indices (RI, caking properties) as well as with petrographic caking ratio (PCR). The coarse mosaic content of cokes seem to depend on LΔT_max/ΔT_max (ratio of weight loss during fast decomposing main reaction to temperature difference) under DTG. L^mΔT/ΔT (ratio of weight loss during main decomposing reaction to temperature difference) under DTG exhibits correlation with p₁ (mean pore size) and t₁ (mean cell wall thickness) of cokes. ΔA_max/(L^mΔT_max) also indicates good relationship with mechanical strength of cokes. (L^mΔT_A−T_B)/(L^mΔT) (i.e. ratio of weight loss during main endothermic reaction under DTA to weight loss during main decomposing reaction) appears to have relationship with DD (compactness) of cokes. The course of main endothermic reaction/main decomposition reaction under DTA, DTG and TGA seems to govern coking behaviour and the resulting coke strength, which in turn is controlled by microlithotypes.     

The possible role of fissure formation in the prevention of coking pressure generation

Fissure patterns have been studied for both high-volatile, low-coking pressure coals and low-volatile, high-coking pressure coals. The high-volatile coals form an extensive pattern of interconnected fissures, which seem to form early on and extend further toward the plastic region than the low-volatile coals, which form only a few fissures that do not extend very far into the charge. It is proposed that the combination of high-fluidity and extensive fissure network present for high-volatile coals may assist in allowing continual release of volatiles throughout the coking process and play a part in preventing the generation of high gas pressures. It was also found that a higher proportion of the volatiles for the low-volatile, high-coking pressure coals is released after the coal has been converted to semi-coke, which may play a role in preventing the effective release of gas.     

Comparison of the associative structure of two different types of rich coals and their coking properties

Solvent extractions of two different types of Chinese rich coals i.e. Aiweiergou coal (AG) and Zaozhuang coal (ZZ) using the mixed solvent of carbon disulfide/N-methyl-2-pyrrolidinone (CS₂/NMP) with different mixing ratios were carried out and the caking indexes of the extracted residues were measured. It was found that the extracted residues from the two types of coals showed different changing tendencies of the caking indexes with the extraction yield. When the extraction yield attained about 50% for ZZ coal, the extracted residue had no caking property. However for AG coal, when the extraction yield reached the maximum of 63.5%, the corresponding extracted residue still had considerable caking property with the caking index of 25. This difference indicated the different associative structure of the two coals although they are of the same coalification. Hydro-thermal treatment of the two rich coals gave different extract fractionation distributions for the treated coals compared to those of raw coals respectively. The coking property evaluations of the two coals and their hydro-thermally treated ones were carried out in a crucible coking determination. The results showed that the hydro-thermal treatment could greatly improve the micro-strengths of the resulting coke from the two coals, and the improvement was more significant for the more aggregated AG coal. The reactivities of hydro-thermally treated AG coal blends were almost the same as those of raw coal blends. The higher coke reactivities of AG raw coal and its hydro-thermally treated ones than those of ZZ coal might be attributed to its special ash composition.     

Comparison of flame propagation properties of petroleum coke and coals of different rank

Ignition and flame propagation characteristics of 18 kinds of coal and a petroleum coke were investigated through a laser ignition experiment. Flame stability was strongly influenced by amount of volatile matter and pyrolysis rate. Lean limit of flame propagation was strongly influenced by amount of volatile matter. Flame propagation was observed when pyrolized volatile matter was mixed with surrounding air or oxygen, until the concentration of pyrolized volatile matter reached a constant value. Flame propagation velocity was strongly influenced by pyrolysis rate. As the pyrolysis rate increased, the flame propagation velocity increased. The flame propagation velocity of petroleum coke was higher than that of coal with the same volatile content. The flame propagation of petroleum coke was superior to what was expected based on the volatile content, primarily because the high pyrolysis rate caused a shorter ignition delay than what would be expected given the volatile content. A database for the lean limit of flame propagation was used to develop a flame stability model to estimate lean flammability of a large-scale burner. The model could predict the effect of the coal rank, the particle diameter distribution for lean flammability limit. The estimated lean flammability limit of petroleum coke (volatile content 11.5%) was equal to that of lv bituminous coal with volatile content of about 15%.     

Influence of air oxidation on the pressure exerted by coking coals during carbonization

The effects of air oxidation of three Spanish coals on the pressure exerted during carbonization have been studied. Coals were oxidized by air in an oven at 120 and 140 °C. The extent of oxidation was assessed by the Audibert-Arnu dilatometer test. Coal samples oxidized at different levels were subjected to the Koppers-lncar laboratory coking pressure test. The results obtained indicated that for coals classified as ‘dangerous’ the ‘danger’ increased and reached a maximum at a certain level of oxidation, after which it decreased sharply. This behaviour did not occur with coals classified as safe.     

AP–TPR study of sulphur in coals subjected to mild oxidation. Part 1. Demineralised coals

Five samples of coal characterised by different degree of coalification and different content of sulphur have been subjected to oxidation in the dry phase and in aqueous media in order to determine the effect of oxidation on the sulphur groups. The transformations were studied by the classical chemical methods and by atmospheric pressure–temperature programmed reduction. It has been shown that the greatest changes in sulphur groups occurs in the case of the oxidation with HNO₃ and peroxyacetic acid, while the air oxidation and the oxidation in the O₂/Na₂CO₃ system is much less effective. The oxidation leads to formation of sulphoxides, sulphones and sulphonic acids. The non-thiophene groups have been found much more susceptible to oxidation than the thiophene ones.     

Macerals in bituminous coals and the coking process: Part 1: Effect of basic coal properties on the process of thermal degradation

Bituminous coals produced in the Ostrava—Karviná coal basin show considerable variation in their maceral composition and rank. In order to explain the significance of the petrographic composition, structure and chemical parameters in coal degradation processes, a large number of coal samples was investigated. From the relationships obtained, the behaviour of macerals during these processes is discussed. It is found that isometamorphic vitrinites are characterized by a distinctive behaviour affected to a first approximation by the atomic ratios C/H and C/O. Inertinite, despite its occasional significantly higher volatile matter, does not yield an appreciable amount of fused material.     

Speciation and thermal transformation of sulfur forms in high-sulfur coal and its utilization in coal-blending coking process:A review

The utilization of high-sulfur coal is becoming more urgent due to the excessive utilization of low-sulfur,high-quality coal resources, and sulfur removal from high-sulfur coal is the most important issue. This paper reviews the speciation, forms and distribution of sulfur in coal, the sulfur removal from raw coal,the thermal transformation of sulfur during coal pyrolysis, and the sulfur regulation during coal-blending coking of high organic-sulfur coals. It was suggested that the proper characterization of sulfur in coal cannot be obtained only by either chemical method or instrumental characterization, which raises the need of a combination of current or newly adopted characterization methods. Different from the removal of inorganic sulfur from coal, the organic sulfur can only be partly removed by chemical technologies;and the coal structure and property, particularly high-sulfur coking coals which have caking ability,may be altered and affected by the pretreatment processes. Based on the interactions among the sulfur radicals, sulfur-containing and hydrogen-containing fragments during coal pyrolysis and the reactions with minerals or nascent char, regulating the sulfur transformation behavior in the process of thermal conversion is the most effective way to utilize high organic-sulfur coals in the coke-making industry.An in-situ regulation approach of sulfur transformation during coal-blending coking has been suggested.That is, the high volatile coals with an appropriate releasing temperature range of CH_4 overlapping well with that of H2 S from high organic-sulfur coals is blended with high organic-sulfur coals, and the C–S/C–C bonds in some sulfur forms are catalytically broken and immediately hydrogenated by the hydrogencontaining radicals generated from high volatile coals. Wherein, the effect of mass transfer on sulfur regulation during the coking process should be considered for the larger-scale coking tests through optimizing the ratios of different coals in the coal blend.     

Modification of sub-bituminous coal by steam treatment: Caking and coking properties

A Chinese sub-bituminous Shenfu (SF) coal was steam treated under atmospheric pressure and the caking and coking properties of the treated coals were evaluated by caking indexes (G_RI) and crucible coking characterizations. The results show that steam treatment can obviously increase the G_RI of SF coal. When the steam treated coals were used in the coal blends instead of SF raw coal, the micro-strength index (MSI) and particle coke strength after reaction (PSR) of the coke increased, and particle coke reactivity index (PRI) decreased, which are beneficial for metallurgical coke to increase the gas permeability in blast furnace. The quality of the coke obtained from 8% of 200 °C steam treated SF coal in coal blends gets to that of the coke obtained from the standard coal blends, in which there was no SF coal addition in the coal blends. The removal of oxygen groups, especially hydroxyl group thus favoring the breakage of the coal macromolecules and allowing the treated coal formation of much more amount of hydrocarbons, may be responsible for the modified results. The mechanism of the steam treatment was proposed based on the elemental analysis, thermo gravimetric (TG) and FTIR spectrometer characterizations of the steam treated coal.     

An experimental study of fragmentation of coals during fast pyrolysis at high temperature and pressure

An experimental apparatus has been developed in order to perform tests of primary fragmentation of solid fuels under severe heating conditions (up to 2200 K and 12 bar). Particles are laid on the strip and pyrolyzed under inert conditions, fragments are recovered and analyzed by a laser granulometer to assess the fragmentation propensity of the fuel.Experiments have been carried out at temperatures between 1400 K and 1900 K, heating rate of 5000 K/s, pressure in the range 1-12 bar. Four different coals have been studied: Gracem, Venezuelan, Omsky, and Kleincopje, classified respectively as anthracite, high and medium volatile bituminous coals.Results show that primary fragmentation at high heating rate and high temperature may result in the formation of relatively coarse fragments and sometimes in a multitude of fines. The probability of fragmentation and the propensity to form coarse versus small fragments varies from coal to coal. For a given coal fragmentation increases monotonously with temperature, whereas the effect of pressure is nonmonotonous.The role of different chemico-physical properties of coals on the pattern and the extent of primary fragmentation is discussed, in particular volatile matter content, ash melting point, rigidity and porosity of the carbon structure and swelling index.     

Effect of inorganic corrosion inhibitors on the caking properties of coking coals

The caking properties of two Western Canadian coking coals were affected by treatment with some inorganic corrosion inhibitors in high concentrations at room temperature. Leaching the coals with water resulted in removal of adsorbed inhibitor and hence, restoration of caking properties. Treatment of coals at room temperature with inhibitors in ‘normal’ concentrations and at a pressure of 10 350 kPa did not affect the caking properties to any significant extent. Treatment of the coals with low concentrations of inhibitors at 10350 kPa and 96 °C resulted in marked changes in the caking properties which were irreversible on leaching with water. In this case, a definite reaction took place between the coals and the inhibitors. Microscopic examination of the original coals and samples treated with inhibitors showed a greater number of fine particles and partial oxidation for coals tested at 10350 kPa and 96 °C.     

Part 1: A study on the properties of synthetic hydroxyledgrewite

In this work, for the first time chemical and physical properties of hydroxyledgrewite synthesized under hydrothermal conditions were examined. Hydroxyledgrewite was synthesized in the primary mixture consisted of CaO and amorphous SiO₂·nH₂O, when the molar ratio of CaO/SiO₂ was equal to 2.25. The synthesis has been carried out in unstirred suspensions under saturated steam pressure at 200°C temperature for 48 hours. It was proved that synthetic hydroxyledgrewite is stable till 675°C and at higher temperature recrystallized to γ‐C₂S, ‐C₂S, while upon subsequent cooling transformed into β‐C₂S. In addition to this, it was also determined that the density and specific heat capacity at 25°C are equal to 2.668 ± 5 g/cm³ and 0.928 J/(g·K), respectively. Synthetic hydroxyledgrewite showed disordered aggregates of plate‐like particles, while calculated S_BET value is equal to 13.961 m²/g. According gas adsorption results, it was obtained that hydroxyledgrewite is a mesoporous material. Also, it was obtained that after 72 hours of activated hydroxyledgrewite hydration, the amount of released heat was equal to 74.34 J/g. The product of synthesis and calcination were characterized by XRD, STA, DSC, SEM, TEM, FT‐IR analyses, and BET method.