Influence of thermoplastic properties on coking pressure generation: Part 1 - A study of single coals of various rank

In this study a number of high coking pressure coals with different fluidities were evaluated alongside a number of low pressure coals also with differing fluidities. This was to confirm findings from an earlier study using a limited selection of coals, and to establish rheological parameters within which a coal may be considered potentially dangerous with regards to coking pressure. The results have confirmed and elaborated on previous findings which show that parallel plate displacement (ΔL) and axial force profiles can be used to distinguish between high and low pressure coals, with peak values indicating cell rupture and subsequent pore network formation. This is thought to correspond with plastic layer compaction in the coke oven.For low pressure coals pore coalescence occurs quite early in the softening process when viscosity/elasticity are decreasing and consequently a large degree of contraction/collapse is observed. For higher pressure coals the process is delayed since pore development and consequently wall thinning progress at a slower rate. If or when a pore network is established, a lower degree of contraction/collapse is observed because the event occurs closer to resolidification, where viscosity and elasticity are increasing. For the higher fluidity, high coking pressure coals, a greater degree of swelling is observed prior to cell rupture, and this is considered to be the primary reason for the high coking pressure observed with these coals. An additional consequence of these events is that high pressure coals are likely to contain a higher proportion of closed cells both at and during resolidification, reducing permeability in both the semi-coke and high temperature plastic layers, respectively.Using a rheological mapping approach to follow viscoelastic changes during carbonisation it has been possible to identify specific regions associated with dangerous coals. These tend to be fusible coals where at the onset of expansion, δ (elasticity) < 54° and η* (complex viscosity) > 5 × 10⁵ Pa s, and where in most cases δ_MAX does not exceed 65°.     

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.     

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.     

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.     

Hydrothermal dewatering of lower rank coals. 3. High-concentration slurries from hydrothermally treated lower rank coals

A novel method of producing a product of low intra-particle porosity (<1 μm pore radius) from highly porous Latrobe Valley raw brown coals uses a combination of hydrothermal and evaporative drying. Low porosity coal was made in three different batch autoclave systems at 320 °C for residence times as low as 10 min. Higher temperatures (up to 350 °C) increased porosity slightly but the water vapour pressure and the loss of organic material were significantly increased. Although the low and high porosity products differed dramatically in appearance and hardness, other chemical and spectroscopic properties were similar with the exception of pyrolysis-gas chromatography-mass spectrometry patterns.The relationship between intra-particle porosity and the maximum wt% dry solids concentration of aqueous slurries (for a viscosity of less than 1000 mPa s), ?_max, established by earlier workers for hydrothermally treated brown coals was found to hold for the new products and was extended to a wider range of porosities and a range of mean particle sizes (mps) (20-100 μm). A range of surfactants (anionic, cationic and neutral), which led to an increase of up to 7% in ?_max for a bituminous, Blair Athol coal, increased ?_max for products of hydrothermal or the new treatment by only 2-4%. This small increase resulted, however, in the formation of slurries of the low porosity products with ?_max's of up to 64%, similar to those obtained with high rank coals, and considered to be of commercial interest.     

The effects of pressure on the isothermal fluid behavior of bituminous coals

An investigation into the effects of pressure (helium gas) on the isothermal fluid behavior includes: (1) the effect of pressure on the rate of melting and coking as evidenced by the rate constants k(melt) and k(coke); (2) the effect of pressure on the energies of activation of melting and coking; (3) the effects of pressure on the characteristic times; (4) the effects of pressure on the maximum isothermal fluidity. Results from the effects of pressure on k(melt) revealed that it was generally the high total sulfur, low nitrogen, low reactives/mineral matter ratio, medium rank coals which show the greatest increase in k(melt), whereas the highest rank coals show the least decrease in k(coke). The energies of activation of melting and coking were not significantly affected by pressure. The investigation also reveals increases or decreases in the respective times of softening, maximum fluidity, resolidification and total time of fluid behavior under isothermal pressurized conditions. There appears the possibility that these shifts may be rank dependent. Additionally, the lower rank coals show the largest relative increase in their fluidities when subjected to pressure. Empirical relationships were derived in order to quantitatively predict the maximum isothermal fluidity for most (fluid) coals at a given pressure.     

Petrophysical characterization of coals by low-field nuclear magnetic resonance (NMR)

Nuclear magnetic resonance (NMR) has been widely used in petrophysical characterization of sandstones and carbonates, but little attention has been paid in the use of this technique to study petrophysical properties of coals, which is essential for evaluating coalbed methane reservoir. In this study, two sets of NMR experiments were designed to study the pore types, pore structures, porosity and permeability of coals. Results show that NMR transverse relaxation (T₂) distributions strongly relate to the coal pore structure and coal rank. Three T₂ spectrum peaks identified by the relaxation time at 0.5-2.5 ms, 20-50 ms and >100 ms correspond to pores of <0.1 μm, >0.1 μm and cleats, respectively, which is consistent with results from computed tomography scan and mercury intrusion porosimetry. Based on calculated producible and irreducible porosities through a T₂ cutoff time method, we propose a new NMR-based permeability model that better estimates the permeability of coals. In combination with mercury intrusion porosimetry, we also propose a NMR-based pore structure model that efficiently estimates the pore size distribution of coals. The new experiments and modeling prove the applicability of NMR in petrophysical characterization of intact coal samples, which has potential applications for NMR well logging in coalbed methane exploration.     

Co-carbonization of hard coals and coal extracts 1. The co-carbonization of low rank coals with extracts

Studies on the influence of an additive derived from coal on the coking properties of lower-rank coals and on the structure of cokes obtained from blends have been undertaken in our laboratory since 1978. The two coal extracts from flame coal (Int. Class. 900) and gas-coking coal (Int. Class. 632) were used as additives. The results indicate that the blends prepared from low-rank coals — flame coal (Int. Class. 900), gas-flame coal (Int. Class. 721) and the extracts possess better coking properties in comparison to the parent coals. The optical texture and the degree of structure ordering of the cokes obtained from blends is related to the amount of extract in the blend. With increasing extract content in the blend, increases were observed in the amount of optically anisotropic areas in cokes from low-rank coal/extract blends and the crystallite height (L_c) of cokes from the blends. The isotropic optical texture of cokes from low-rank coals can be modified by coal extracts to an anisotropic optical texture. The non-fusible coal is the most difficult to modify. An explanation of the observed phenomena is given.     

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.     

Plastic wastes, lube oils and carbochemical products as secondary feedstocks for blast-furnace coke production

Two plastic wastes (polyolefin-enriched and multicomponent), two lube oils (paraffinic and synthetic) and one coal-tar were assessed as individual and combined additives to coal blends for the production of blast furnace coke. The effects of adding 2 wt.% of these additives or their mixtures (50:50 w/w) on the coking capacity of coal, coking pressure and coke quality parameters were investigated. It was found that the two plastic wastes reduce fluidity, whereas the addition of oils and tar helps to partially restore the fluidity of the coal-plastic blend. From the co-carbonization of the coking blend with the different wastes in a movable wall oven of over 15 kg capacity, it was deduced that polyolefins have a detrimental effect on coking pressure. The addition of oils and tar to the coal-plastic blend has different modifying effects. Whereas paraffinic oil eliminated the high coking pressure caused by the polyolefins, polyol-ester oil had a weak reducing effect unlike coal-tar which had a strong enhancing effect. The compatibility of the oils/tar with plastics and coal and the beneficial influence of these combinations on coking pressure is discussed in relation to the miscibility of the plastic and the oily and bituminous additives, and the amount and composition of the volatile matter evolved from each additive during pyrolysis as evaluated by thermal analysis. Furthermore, it was found that coke reactivity towards CO₂ (CRI) and coke strength after reaction with CO₂ (CSR) are heavily dependent on the composition of the plastic waste, with polystyrene (PS) and polyethylene terephthalate (PET) having a clear negative effect. The porosity of the cokes obtained from blends containing plastic wastes is always higher, but the pores are smaller in size.     

Carbon foams from coals. A preliminary study

Carbon foams were obtained from a bituminous coal with good plasticity properties by a two-stage thermal process under different pressure and temperature conditions. The first stage was a controlled carbonisation treatment under pressure at 450 and 500 °C. In the second stage the carbonisation product was baked at 1100 °C. The foams produced display a macroporous texture with pressure and temperature determining the mean pore size and the amount of pores. The pressure increase reduces the pore size, while the increasing temperature increases the pore volume.     

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.     

Geology, geochemistry and biomarker evaluation of Lafia-Obi coal, Benue Trough, Nigeria

Lafia-Obi coals from four boreholes were investigated in this study to ascertain their sedimentological and geochemical characteristics. This data enables the proper evaluation of their oil source rock potential and industrial utilization. The Lafia-Obi area is underlain by rhythmic sequences of shale, sandstone and limestone with varying thicknesses of interbedded coal, inferring deposition under shallow marine conditions. The ash and moisture contents of the coal are high suggesting good potential for use in the steel industry. The relatively low volatiles and high vitrinite/inertinite contents indicate that the coal has an appreciable coking quality. Although the total organic carbon content exceeds the kerogen threshold of 0.5 wt% for generation of crude oil, the high vitrinite reflectance values (R₀>1.0%) and several geochemical maturity indices indicate mature to overmature facies. The bitumen compositions also reflect full maturity. The geological and geochemical characteristics of the Lafia-Obi coals are compared and contrasted with those of Enugu coals, with emphasis on the possible economic applications of the former.     

Nuclear magnetic proton relaxation studies of oxidized coals

Coals of NCB rank 301 a (coking), 502 (caking) and 802 (very weakly caking) are oxidized in air at 373 K or 383 K for up to 42 days. Spin-lattice and spin-spin relaxation times, T₁ and T₂ respectively, of oxidized coals are measured using a Bruker SXP 4–100 and FT spectrometer. Free radical concentrations in the coals are obtained using a JES PE e.s.r. spectrometer. Infrared spectra of oxidized coals are obtained and optical textures of cokes from fresh and oxidized coals are assessed by optical microscopy. For two coking coals, decreasing values of T₁, and increasing concentration of free radicals occurred with oxidation at 383 K to 16 and 28 days. Thereupon values of T₁, increased and free radical concentrations decreased with further progressive oxidation. At the point of inflexion in properties, resultant cokes from the coals ceased to shown any anisotropy in their optical textures and became isotropic resembling cokes from low-rank coals. For the caking coals, T₁ increased at all stages of oxidation to 42 days with decreasing concentrations of free radicals. Two values of T₂ were found in each coal corresponding to a rigid and mobile component ((T₂)_r < (T₂)_m). The rigid component (T₂)_r was not affected by oxidation but values of (T₂)_m decreased with increasing duration of oxidation. It is considered that coking and caking coals exhibit different effects of oxidation with perhaps phenols and quinones in caking coals acting as inhibitors to the growth of stable free radicals. Oxidized coking coal may behave like fresh caking coal.     

CO2 gasification rate analysis of coal char in entrained flow coal gasifier

Coal chars of four coal types were gasified with carbon dioxide using a PDTF or TGA at high temperature and pressure. Test conditions of temperature and partial pressure of the gasifying agent were determined to simulate the conditions in air-blown or oxygen-blown entrained flow coal gasifiers. Coal chars were produced by rapid pyrolysis of pulverized bituminous coals using a DTF with a nitrogen gas flow at 1670 K. In gasification tests with the PDTF, gasification temperatures were 1670 K or below and partial pressures of carbon dioxide were 0.7 MPa or below. Carbon monoxide of 0.6 MPa or below was supplied for the gasification tests with the TGA.As a result, coal types showed a large difference in the char gasification rate with carbon dioxide, and this difference remained large without decreasing even in the high-temperature area when the gasification rate was controlled by pore diffusion the same as in entrained flow gasifiers. Inhibition of the gasification reaction by carbon monoxide was also observed. Reaction rate equations of both the nth order and Langmuir-Hinshelwood type were applied to the char gasification reaction with the random pore model and the effectiveness factor, and the applicability of these rate equations to air-blown and oxygen-blown entrained flow gasifiers evaluated. Gasification rate equations and kinetic parameters applicable to a pore diffusion zone at high temperature were obtained for each coal.     

Interactions between coking coals and plastics during co-pyrolysis☆

Blends of three Australian coking coals and polypropylene, polystyrene, polyacrylonitrile and polyphenylene sulfide were prepared and the extent to which the blends fused on heating was monitored using proton magnetic resonance thermal analysis in order to identify interactions between them that could affect their fluidity. Different plastics had different effects. Polystyrene strongly reduced the fluidity of all of the coals, confirming previous findings. Polypropylene did not affect the fluidity of the two coking coals of lower rank. Polyphenylene sulfide reduced the fluidity of the coals at temperatures near the solidification temperature of the coals, and polyacrylonitrile appeared to increase the fluidity of the coals at temperatures near the softening temperature of the coals. The very different effects different plastics have on coal fluidity show that the interaction between plastics and coals must be carefully examined before plastics are added to coking coal blends.     

Application of a laboratory test to resolve the problem of coking a dangerous coal

A laboratory test developed to measure expansion and contraction was used to resolve the problem of coking a dangerous Spanish coal. The indications of this test were used to determine the minimum amounts of three different high volatile coals that it was necessary to add, to reduce the risk of coking this dangerous coal. Laboratory results were backed by tests carried out on a semi-industrial scale, without losing sight of certain safety limits.     

Fractal characteristic of three Chinese coals

Experimental and theoretical investigation about coal/char structure is presented. Surface structures of parent coal and char with different burn-off ratios were analyzed. We introduced the fractal theory into Scanning Electron Microscopy image analysis and utilize the particle surface fractal dimension (D_ps) to quantitatively describe the surface character of coal/char particles. D_ps of three Chinese coals reach their maximum in the 35-45 wt% char burn-off interval and then decrease with increasing carbon burn-off ratio. The inner-pore information of coal/char particles was determined by N₂ isotherm adsorption/desorption. Using fractal BET model, internal surface fractal dimension (D_s) of coal/char particles was calculated. The D_s change trend of three Chinese coals is similar to their S_BET development. It means the D_s can quantitatively describe the inner pore structure character of coal/char particles.     

The spontaneous combustion behavior of some low rank coals and a range of dried products

This paper compares the spontaneous combustion of two Victorian brown coals and a Pakistan lignite with that of products of the same coals which have been dried, mainly by the mechanical thermal expression technique (MTE) in which aqueous slurries of the materials are heated (125 or 200 °C) and pressed (5-15 MPa). Samples of briquettes obtained by steam heating and pressing from a similar Victorian coal have also been studied. The amount of residual moisture in the dried samples had the largest effect on T_crit values. The inorganic content of the samples varied and may also have contributed to the changes. T_crit increased with increasing particle size for the dried coals and MTE samples. There was no obvious correlation between T_crit and pore volume.     

A physicochemical study of carbonization phases,Part II: quenching experiments at the pilot scale

Previous experiments carried out on several carbonization phases showed a relationship between tar migration and the coking pressure. In the present study, tests analogous in principle to the previous ones were conducted at a pilot scale with 400 kg coal charges. Two coals were used: C28, which leads to no pressure (“safe”), and C19, which induces pressure (“dangerous”). N-methyl-2-pyrrolidinone (NMP) extractions from the carbonization phases of the two coals confirm that tar migration is dependent on the nature of the parent coal, i.e. whether “safe” or “dangerous”. In the case of dangerous coal, the impregnation of non-coked coal by tars has been evidenced. In relation to this phenomenon, increased coking pressure is likely to develop due to the enrichment of non-transformed coal by volatile matter, as well as to the drop in permeability of this phase. It is also suggested that heavy tars clog up the pores of the semi-coke of dangerous coals.