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.     

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.     

Solvolytic liquefaction of coals with a series of solvents

Solvolytic liquefaction of coals of different rank was studied with a variety of solvents at 370–390 °C under nitrogen in order to elucidate the role of solvent in coal liquefaction of this kind and to find a suitable solvent for the highest yields of liquefaction. The yield was found to depend strongly upon the nature of the coal as well as the solvent under these conditions. Pyrene and a SRC-BS pitch were excellent solvents for Miike coal, which was fusible with high fluidity at these temperatures. However, the former was less efficient for Itmann and Taiheiyō coals which were fusible at a higher temperature and non-fusible, respectively. The mechanism of solvolytic liquefaction is discussed, including nature of coal and solvent at reaction temperatures, in order to understand the properties required for high yields with non-fusible coals in solvolytic liquefaction. It is found that for liquefaction with a high yield if the coal is non-fusible, solvolytic reaction should take place between solvent and coal, so giving a liquid phase of low viscosity at the reaction temperature. The solvolytic reaction may be one of hydrogen transfer when SRC-BS is used as the solvent.     

Ignition characteristics of coal blends in an entrained flow furnace

Ignition tests were carried out on blends of three coals of different rank - subbituminous, high volatile and low volatile bituminous - in two entrained flow reactors. The ignition temperatures were determined from the gas evolution profiles (CO, CO₂, NO, O₂), while the mechanism of ignition was elucidated from these profiles and corroborated by high-speed video recording. Under the experimental conditions of high carbon loading, clear interactive effects were observed for all the blends. Ignition of the lower rank coals (subbituminous, high volatile bituminous) enhanced the ignition of the higher rank coal (low volatile bituminous) in the blends. The ignition temperatures of the blends of the low rank coals (subbituminous-high volatile bituminous) were additive. However, for the rest of the blends the ignition temperatures were always closer to the lower rank coal in the blend.     

The effect of coal blend fluidity on the properties of coke

The effect of maximum fluidity of coal blends on coke quality was investigated using high fluidity coals and pitch to increase the fluidity of the blends. The results show that high fluidity of the blends could improve the growth of anisotropic carbon in coke. It also improves the coke M₃₀, but this is restricted when using high fluidity coals as additives. There is no significant improvement of the M₁₀ by increasing the fluidity. The reactivity, however, could be reduced to lower than the expected value by increasing the coal blend fluidity.     

Optical properties of carbonized sporinite in vitrinite/sporinite blends

A study is made of the morphological and optical properties of a blend of concentrated sporinite and vitrinite macerals (equal parts) from the same coal carbonized between 400–900 °C at 5 °C/min under nitrogen. Two coals of different rank were used and the results compared with properties of unblended carbonized sporinites. Coke from a blend of low-rank vitrinite and sporinite produced a two-phase optical texture, whereas coke from the medium-rank vitrinite and sporinite mixture produced a three-phase optical texture. A ‘Transitional Zone’ of optical texture was observed at interfaces between carbonized vitrinite and sporinite, this zone being wider for the coal of higher rank. The increase of bireflectance in oil with carbonization temperature of the sporinite in the blends was lower than for the carbonized unblended sporinite, caused by a reduction in fluidity of sporinite with blending. The decrease of refractive index for sporinite carbonized > 600 °C in the blends was lower than that of the unblended sporinite.     

Char characterization and DTF assays as tools to predict burnout of coal blends in power plants

The aim of this study is to predict efficiency deviations in the combustion of coal blends in power plants. Combustion of blends, as compared to its single coals, shows that for some blends the behavior is non-additive in nature. Samples of coal feed and fly ashes from combustion of blends at two power plants, plus chars of the parent coals generated in a drop-tube furnace (DTF) at temperatures and heating rates similar to those found in the industrial boilers were used. Intrinsic kinetic parameters, burning profiles and petrographic characteristics of these chars correlated well with the burnout in power plants and DTF experiments. The blend combustion in a DTF reproduces both positive and negative burnout deviations from the expected weighted average. These burnout deviations have been previously attributed to parallel or parallel-series pathways of competition for oxygen. No deviations were found for blends of low rank coals of similar characteristics yielding chars close in morphology, optical texture and reactivity. Negative deviations were found for blends of coals differing moderately in rank and were interpreted as associated with long periods of competition. In this case, fly-ashes were enriched in material derived from the least reactive char, but also unburnt material attributed to the most reactive char was identified. Improved burnout compared to the weighted average was observed for blends of coals very different in rank, and interpreted as the result of a short interaction period, followed by a period where the less reactive char burns under conditions that are more favorable to its combustion. In this case, only unburned material from the least reactive char was identified in the fly-ashes.     

Carbonization of coal blends: mesophase formation and coke properties

Laboratory investigations of strength of cokes from blends of coals incorporating pitch were supported by 7 kg trials. The stronger cokes showed a greater interaction between coal and pitch to produce an interface component of anisotropic mozaics which is relatively resistant to crack propagation. The process whereby coal is transformed into coke includes the formation of a fluid zone in which develop nematic liquid crystals and anisotropic carbon which is an essential component of metallurgical coke. Strength, thermal and oxidation resistance of coke can be discussed in terms of the size and shape of the anisotropic carbon which constitutes the optical texture of pore-wall material of coke. Coals of different rank form cokes with different optical textures. Blending procedures of non-caking, caking and coking coals involve the interactions of components of the blend to form mesophase and optical texture. Petroleum pitches used as additives are effective in modifying the carbonization process because of an ability to participate in hydrogen transfer reactions.     

Investigation into the fate of mercury in bituminous coal during mild pyrolysis

Two high-volatile bituminous coal (Lower Freeport #6A and Pittsburgh #8), used primarily for electricity production, were tested to determine the fate of their mercury content during mild pyrolysis. Mono-sized samples of the well characterized coals were tested under nitrogen in a horizontal tube furnace at different residence times at different temperatures throughout the range 275–600°C. The resulting char was analyzed for mercury, and compared to the original parent coal concentration. The percent Hg removal was found to be a function of both residence time and temperature. The data for both coals have shown two distinct regimes; a low temperature chemical evolution mechanism which follows an Arrhenius form (apparent activation energies for the Lower Freeport #6A and the Pittsburgh #8 coals are 25.6±1.5 and 21.7±1.9 kcal/mol respectively), and a higher temperature regime where the Hg evolution dramatically decreases. This can be attributed to the changing structure of the coal at these higher temperatures. The results of bomb calorimetry analysis performed on the Lower Freeport #6A coal samples verify that the overall heating value of the coal is essentially unaffected by mild pyrolysis at temperatures lower than 400°C.     

煤与生物质共热解的TGA-FTIR研究

利用热重分析仪和傅里叶红外光谱仪对煤与木屑混合物在惰性气氛中进行了共热解研究,考察煤阶及煤与生物质掺混比例对热解过程的影响.结果表明,煤与木屑共热解特性并不是单独煤和单独木屑热解特性的简单叠加;高阶煤与生物质共热解更有利于协同反应的发生.通过对红外吸收光谱的分析发现,木屑与不同煤化程度煤共热解析出气体的成分和含量也不同,说明煤阶对煤与生物质共热解的气态产物有明显影响,也从侧面揭示了混合物热解过程中煤与木屑之间发生了相互作用.     

Thermogravimetric study of interactions in the pyrolysis of blends of coal with radiata pine sawdust

The co-pyrolysis of coal-biomass blends were studied by using thermogravimetric analysis to look for thermal events indicating interactions that could cause synergic or inhibitory effects during the first stage in the co-combustion of these materials. Two coals from different rank were selected for the study and combined with radiata pine sawdust, the selected biomass compound. Pyrolysis assays were carried out on the individual components and the binary coal-sawdust blends (50% p/p) at different heating rates (10, 30, 50 °C/min) until reaching a maximum temperature of 1200 °C. The individual components behaved as expected and as is widely described in the specialized literature. Interactions detected in the blends resulted in greater-than-expected volatile yield values. These interactions were produced at pyrolysis temperatures over 400 °C, when most of the components in the blend are devolatilized, and are attributed to secondary reactions that inhibit the formation of char.     

A three-dimensional numerical study of the combustion of coal blends in blast furnace

The practice of blending coals for pulverized coal combustion is widely used in ironmaking blast furnace. It is desirable to characterize the combustion behaviour of coal blends and their component coals. A three-dimensional numerical model is described to simulate the flow and combustion of binary coal blends under simplified blast furnace conditions. The model is validated against the experimental results from a pilot-scale combustion test rig for a range of conditions, which features an inclined co-axial lance. The overall performance of coal blend and the individual behaviours of their component coals are analysed, with special reference to the influences of particle size and coal type. The synergistic effect of coal blending on overall burnout is examined. The results show that the interactions between component coals, in terms of particle temperature and volatile content, are responsible for the synergistic effect. Such synergistic effect can be optimized by adjusting the blending fraction. The model provides an effective tool for the design of coal blends.     

Ash deposition characteristics of Moolarben coal and its blends during coal combustion

We report a systematic and comprehensive laboratory investigation of the ash deposition behavior of Moolarben (MO) coal, which has recently begun to be imported into Korea. Ash deposition experiments were conducted in a drop tube reactor, and a water-cooled ash deposit probe was inserted into the reactor to affix the ash. The tests were conducted using five types of single coals (two bituminous and three sub-bituminous, including MO coal) and blended coals (bituminous coal blended with sub-bituminous coal). Two indices represent ash deposition behavior: capture efficiency and energy-based growth rate. A thermomechanical analysis evaluated the melting behavior of the resulting ash deposits. The MO coal had the least ash deposition of the single coals due to its high melting temperature, indicated by high ash silica content. Indonesian sub-bituminous coals formed larger ash deposits and were sticky at low temperatures due to relatively high alkali content. However, blends with MO coal had greater ash deposition than blends with other bituminous coals. This non-additive behavior of MO coal blends is likely due to interactions between ash particles. Coals with higher silica content more effectively retain alkali species, resulting in lower melting temperatures and larger ash deposits. Therefore, we recommend that when blending in a boiler, silica-rich coals (SiO₂>80%, SiO₂/Al₂O₃> 5) should be blended with relatively low-alkali coals (Na₂O+K₂O<3%), and the blending ratio of the silica-rich coals indicates less than 10%, which can safely operate the boiler.     

Study on the ash fusion temperatures of coal and sewage sludge mixtures

The coal, sewage sludge, water and chemical additives are milled to produce coal-sludge slurry as a substitute for coal-water slurry in entrained-flow gasification, co-gasification of coal and sewages sludge can be achieved. The ash fusion temperature is an important factor on the entrained-flow gasifier operation. In this study, the ash fusion temperatures (DT, ST, HT and FT) of three kinds of coals (A, B and C), two kinds of sewage sludges (W1 and W2) and series of coal-sewage blends were determined, and the mineral composition during the ash melting process was analyzed by X-ray diffraction (XRD). The results showed that the ash fusion temperatures of most coal-sewage blends are lower than those of the coals and sewage sludges. The ashes have different mineral composition at different temperature during the heating process. It was found that the mineral composition of AW1 blend ash is located in the low-temperature eutectic region of the ternary phase diagram of SiO₂-Al₂O₃-CaO. The minerals found in BW1 blend ash are almost the same as those in B coal ash. Kyanite is detected in CW1 blend ash, which results in the ash fusion temperatures of CW1 blend ash higher than those of C coal. We found that sodium mineral matters are formed because of NaOH added to W2, which can reduce the ash fusion temperature of coal-sewage blends.     

A laboratory scale assessment of the utilization potentials of sub-bituminous Nigerian coals as components of coking blends

This paper examines the ability of A240 Petroleum pitch to improve the coking characteristics of sub-bituminous Lafia and Enugu coals of Nigeria. Also examined is the compatibility of Enugu and Lafia coals with a prime coking Ogmore coal in a blend for coke making. The exercise is motivated by the desire to include non-coking Nigerian coals in coal blends for making blast furnace coke.The coking characteristics of Lafia and Enugu coals are highly susceptible to improvement by A240 petroleum pitch. The pitch also interacts with the improves the coking characteristics of a blend of Enugu and Lafia coals.No interaction occurs between Enugu and Ogmore coals. Little interaction occurs between Lafia and Enugu coals. A strong interaction occurs between Lafia and Ogmore coals. Ogmore coking coal tremendously improves the coking characteristics of a blend of Lafia Enugu coals. Optical microscopy, microstrength and reactivity tests reveal that high volatile coking Lafia coal act as a bridging coal between Enugu coal and a prime coking coal in a ternary coking blend.     

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.     

Fouling propensities of blended coals in pulverized coal-fired power station boilers

This paper presents the experimental results on the fouling propensity of five pairs of blended coals (19 coals and blends) tested in the Australian Coal Industry Research Laboratory (ACIRL) furnace. The results showed that the coal D has the highest fouling propensity among tested coals and blends. A parameter, growth rate (mm/h), is used to numerically rank the fouling propensity based on photos of fouling deposits taken over a period of test time. The growth rate correlates the fouling propensity better than the build up rate or the fouling coefficient. Five empirical fouling indices are examined against the fouling propensities of the above 19 coals and blends, and another 10 coals and blends previously tested in the Energy and Environmental Research Cooperation (EER) furnace. The linear correlation between the flue gas exit temperature/initial deformation temperature (FGET/IDT(ox.)), a measure of the overall heat transfer in the furnace, and the Na₂O, g/GJ, is proved to be a good tool for predicting the fouling propensity of coals. There is also a relationship between the FGET and the growth rate.     

Reactivity study of combustion for coals and their chars in relation to volatile content

To determine the effect of volatile matter on combustion reactivity, the pyrolysis and combustion behavior of a set of four (R, C, M and K coals) coals and their chars has been investigated in a TGA (SDT Q600). The maximum reaction temperatures and maximum reaction rates of the coals and their chars with different heating rates (5–20 °C/min) were analyzed and compared as well as their weight loss rates. The volatile matter had influence on decreasing the maximum reactivity temperature of low and medium rank coals (R, C and M coals), which have relatively high volatiles (9.5–43.0%), but for high rank coal (K coal) the maximum reactivity temperature was affected by reaction surface area rather than by its volatiles (3.9%). When the maximum reaction rates of a set of four coals were compared with those of their chars, the slopes of the maximum reaction rates for the medium rank coals (C and M coals) changed largely rather than those for the high and low rank coals (R and K coals) with increasing heating rates. This means that the fluidity of C and M coals was larger than that of their chars during combustion reaction. Consequently, for C and M coals, the activation energies are lower (24.5–28.1 kcal/mol) than their chars (29.3–35.9 kcal/mol), while the activation energies of R and K coals are higher (25.0-29.4 kcal/mol) than those of their chars (24.1–28.9 kcal/mol).     

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.     

Studies on the combustion behaviour of blends of Indian coals by TGA and Drop Tube Furnace

Combustion behaviour of blends of two Indian coals of same rank with wide variation in mineral matter content were studied using Thermogravimetric Analyzer (TGA) and Drop Tube Furnace (DTF). The characteristic TGA parameters determined from the burning profiles showed both additive and non-additive behaviour. The burnout temperature and peak temperature showed a linearly decreasing trend with the increasing proportion of the high ash coal. Deviation from the linear trend was observed in the case of the reactivity parameter. The high ash coal showed better TGA reactivity than the low ash coal. The higher TGA reactivity could arise from the combined effect of mineral matter and the nature and distribution of the macerals, particularly those of the inertinite group.The burnout in DTF showed a nonlinear effect. The burnout behaviour of the coals and their blend observed in DTF was not similar to that reflected in TGA. Coal blends with less than 50% of high ash coal showed better burnout than the individual coals.