Regular Article
Physical and Chemical Characteristics of Coal-binder Interface and Carbon Microstructure near Interface
2014 年 54 巻 11 号 p. 2470-2476
詳細
2014 年 54 巻 11 号 p. 2470-2476
In this study, investigations were made to understand the mechanism of increase in coke strength on binder addition by analyzing physical and chemical characteristics of coal-binder interface. Three binders, oil derived Asphalt pitch (ASP), coal derived HyperCoal (HPC) and Coal tar pitch (CTP) were used to understand the effect of binder. A new method to observe the coal-binder interface boundary at microscopic level by Scanning Electron Microscope (SEM) was developed. When base coal was a non-caking coal, coal-binder interface boundary was clearly observed by SEM for the first time. From observed images, it was found that ASP and HPC bound differently on the coal surface. When base coal is a caking coal, the interface could not be distinguished but sulfur mapping confirmed the presence of interface. Preliminary Laser Raman analysis suggests there may be some interactive effect of coal and binder on each other’s carbon structure development. Contribution of fraction of coal surface coated with binder towards coke strength is considered.
Coke plays an important role in steel making process by acting as reducing agent, spacer and also providing heat for the process. The strength of coke is the most important parameter that determines its usability in a blast furnace. Cokes produced from caking coals show excellent strength but are expensive.1,2) Most of the cokes used in blast furnace are produced by mixing different expensive caking coals and less expensive slightly or non-caking coals while identities of these coals are typically a trade secret. With increasing prices of caking coals and also growing call to reduce CO2 emissions, coke producers are considering to increase the fraction of low quality non-caking/caking coals. However, with higher fraction of non-caking coals, the strength of the produced coke is significantly reduced and the coke is not suitable for use in the blast furnace. To overcome the problem of decrease in coke strength, there has been recent interest in using oil/coal derived additives as binders so that the coke strength is maintained even with increased low quality non-caking coals addition.3,4,5,6) Takanohashi et al. explained this increase in coke strength by investigating the interaction between caking and slightly caking or non-caking coals using NMR analysis and H/C ratio and concluded that binders (HPC) enhance the thermoplasticity of the coal blend due to their high thermoplasticity and overlapping thermoplastic temperature range with that of base coals.4) Sakimoto et al. introduced carbon matrix connectivity index to estimate the coke drum index which directly correlates with the coke strength.5) They concluded that binders (HPC) increase the carbon matrix connectivity of the coke leading to increased coke strength.
Scientific understanding of how the coke strength increased on binder addition is very important and one of the primary aim of this study. The above studies used change in thermoplasticity properties and/or matrix properties using image analysis to understand and explain the binder effect on coke strength. However, there were no attempts to understand the coal-binder interaction at microscopic level i.e. coal-binder interface and carbon structure. It is imperative that knowledge of coal-binder interface will be necessary to better understand the binder effect on coke strength. The bonding of binder on coal surface is expected to be dependent on chemical composition and chemical structure of both the binder and coal. Physical and chemical analysis of the coal-binder interface and carbon around it will be a first step to understand the increase in coke strength on binder addition effect. In this research study, we report a simple approach developed to observe the coal-binder interface and first observation of coal-binder interface using scanning electron microscope (SEM) at microscopic level. Different type of binders and their binding characteristics with coal, biomass char were investigated. Laser Raman spot mapping was used to analyze the carbon structure around interface. The aim of this study is to observe the coal-binder interface at microscopic level and investigate the role of carbon structure of individual coal and binder on coal-binder interface. Attempts were also made to explain effect of binder type on tensile strength of coal-binder composite.
A low rank coal, Adaro from Indonesia and an Anthracite and a biomass char were selected as representative of base coal and biomass samples. The two coals selected have different elemental composition and chemical structure and show different degree of development of carbon structure on heat treatment. Both coals are non-caking coals and do not show any thermoplastic properties. Biomass char used in this study was Indonesian origin and carbonized at the supplier’s place. Detailed information about parent biomass and carbonization conditions were not disclosed by the supplier because of trade secret. However, elemental analysis of the Biomass char was made at our laboratory and the properties of biomass char and coals are listed in Table 1. Three binders, an oil derived Asphalt pitch (ASP), a solvent extracted coal fraction HyperCoal (HPC) and a by-product of coke making process Coal Tar Pitch (CTP) were used. ASP was supplied as a common binder sample in the project while HPC was produced at AIST laboratory from Mount Owen coal by solvent extraction method described elsewhere in details.3) CTP was obtained from a source on non-identity disclosure condition. Properties of binders are also shown in Table 1. About 5 g of sample is prepared by mixing 75% coal and 25% ASP or HPC or CTP by weight percent on dry base. The size of coal particles was between 0.5–1 mm while ASP, HPC and CTP particles were <150 micrometer. The sample was loaded into a 20 mm steel pipe and a load of 200 g was put on the top to simulate the pressure conditions of a real coke oven. The holder was then carbonized in a muffle furnace from room temperature to 1273 K at 3 K/min heating rate under N2 flow. Once temperature reached 1273 K, sample was held for 30 min and then cooled down to room temperature in N2 flow. Figure 1 shows the schematic of coke sample preparation method. Individual carbonized samples of Adaro, Anthracite, ASP, and HPC were also prepared under similar conditions for comparison purpose. Samples were named as Adaro+ ASP1000, Adaro+HPC1000, Adaro+CTP1000, Anth+ ASP1000, Anth+HPC1000, Adaro1000, Anth1000, ASP1000, and HPC1000. A scanning electron microscope, Hitachi SU1510 equipped with an elemental analyzer, Horiba EMAX was used to observe the microstructure of coal-binder interface. For SEM-EDS observation, samples were fixed on an aluminum sample holder using a carbon tape and observed with 20 kV accelerating potential for most of the samples for which EDS was done. For low magnification observations, accelerating potential of 2 kV was used. EDS probe was an EMAX X-act series with probe area 10 mm2 and resolution of 135 eV. Element identification and collection time were all set to auto mode. The crystalline structure of carbon of coal and binder around the interface was observed by a Jobin-Yvon HR-300 Laser Raman spot analyzer. Ar-ion Laser with wavelength 514.5 nm and output power 6.7 mW (above sample surface) was used. Spectrum was obtained at following conditions: 20x lens, collection time was 10 s, 10 times while for 100x, it was 2 s, 10 times. Tensile strength of the samples was measured by Shimadzu’s Auto analyzer with an indirect tensile test.
| Moisture content, (%) | Elemental analysis, (wt%, daf) | Ash (db, wt%) | Thermal plasticity | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| C | H | N | S | O (diff) | ST | MFT | RT | MFD | |||
| K | K | K | ddpm | ||||||||
| Anthracite | 1.8 | 89.8 | 3.96 | 1.38 | 0.53 | 4.26 | 4.5 | – | – | – | – |
| Adaro | 19.0 | 73.4 | 5.36 | 1.16 | – | 20.0 | 2.9 | – | – | – | – |
| Biochar | – | 81.47 | 1.43 | 0.19 | – | 16.91 | – | – | – | – | – |
| Asphalt Pitch (ASP) | – | 86.0 | 5.62 | 1.41 | 5.72 | 1.21 | 0.4 | 447 | 575–718- | 782 | >60000 |
| Mount Owen HyperCoal (HPC) | – | 85.7 | 5.48 | 1.9 | 0.64 | 6.21 | 0.04 | 517 | 648–714 | 762 | >60000 |
| Coal Tar Pitch (CTP) | – | 91.59 | 4.02 | 1.36 | 0.16 | 2.87 | – | – | – | – | – |
| Mount Owen (MO) | 5.9 | 75.2 | 5.0 | 1.8 | 0.5 | 17.5 | 10.5 | 685 | 707 | 727 | 6 |
| White Haven (WH) | – | 82.0 | 5.6 | 1.9 | 0.3 | 10.3 | 6.8 | 676 | 704 | 728 | 11 |
| Goonyella (GON) | – | 87.8 | 5.2 | 1.79 | 0.56 | 4.6 | 8.8 | 670 | 729 | 771 | 977 |
Schematic diagram of carbonization method.
Figure 2(a) shows a light microscope image of carbonized Adaro coal particles. It can be seen that carbonized coal particles are loose and not bonded to each other. Figure 2(b) shows a light microscope image of Adaro+HPC1000. It can be seen that otherwise loose Adaro coal particles are bound together by binder which coated and filled the in-between void space. It can be seen that binder has bonded the coal particle same as the cement binds sand and gravel to form concrete. It is imperative that the bond strength of binder and coal particle at the interface will also contribute to the strength of the resultant coke/semi-coke in addition to the strength of the binder and coal itself. Therefore, observation and analysis of coal-binder interface will lead to enhanced understanding of the coal-binder interaction mechanism and increase in coke strength on binder addition. As the bonding of binder on coal particles surface is expected to be dependent on carbon microstructure, chemical composition and chemical structure of both the binder and coal, we investigated the coal-binder interface at microscopic level by SEM-EDS analysis and Laser Raman spectroscopy.
A light microscope image of (a) carbonized Adaro coal particles, (b) Adaro+HPC.
Figure 3(a) shows the Adaro+ASP1000 sample obtained after carbonization. When this sample is observed under SEM, it can be seen that ASP was completely coated on the Adaro particles and coal-binder interface or boundary could not be observed as shown in Fig. 3(b). To observe the interface boundary, the surface of sample in Fig. 3(a) was slightly polished by a grinding paper so that the ASP or HPC coating could be removed and boundary becomes visible. The polished surface can be seen as reflecting surface in Fig. 3(c). The polished surface was re-examined under the SEM and shown in Fig. 3(d). This approach for the first time allowed to observe and image coal-binder interface at submicron level. Other samples were also polished similarly and SEM images of the coal-binder interface are shown in Figs. 4(a)–4(f). All images show the coal-binder interface.
Coke samples (a) before and (c) after polishing, SEM images (b) before and (d) after polishing.
SEM images of (a) Adaro+ASP1000, (b) Adaro+HPC1000, (c) Anth+ASP1000, (d) Anth+HPC1000, (e) camera image of Adaro+CTP1000, (f) SEM image of Adaro+CTP1000.
From Fig. 4, it can be seen that interface of ASP and HPC with Adaro and Anthracite is different. The interface of ASP with Adaro and Anthracite always show a clear boundary and a fine crack running all along the interface can be seen. The interface of HPC with Adaro and Anthracite can be differentiated but the boundary does not show any fine crack like ASP. Especially, Anthracite-HPC interface is very difficult to differentiate. This suggests that binding characteristics of ASP and HPC are different which may be because of origin of binder; ASP is oil derived while HPC is coal derived. One should also consider other possibilities before concluding that the fine crack observed as interface of Adaro-ASP and Anthracite-ASP is actually a crack. It is possible that it is only an apparent crack and not a real crack. ASP and HPC develops a highly porous structure on heat treatment. One can expect that bonding/coating of ASP and/or HPC on coal particle surface will not be uniform. At some places binder is bonded with coal particle while at other places there could be a pore between particle and binder. When the surface is polished by a sand paper, these pores will be exposed and a series of these unconnected pores appear like a fine crack running all along the interface. In such a case it is understandable that the interface appears as a fine crack, although in reality binder is actually bonded to the coal particle at several places. For Adaro-CTP, the interface is not visible in SEM image but can be seen clearly in a polarized light microscope image. The observation of CTP interface in polarized light microscope image suggests different carbon structure development of CTP and Adaro.
Adaro and Anthracite show no thermoplastic properties and therefore do not mix with binders on heating. The interface with ASP and HPC is also very clear in both cases. However, as reported in the first section, binders are used together with caking coals that show thermoplastic properties to increase or restore the coke strength, thus it is also important to investigate the interface when both coal and binder show melting characteristics. For this, we prepared carbonized coke samples by mixing White Haven coal, a slightly caking coal with ASP binder. ASP was chosen as binder because it contains high amount of sulfur which can be used as identification marker during SEM-EDS analysis. Figure 5(a) shows a SEM image of polished WH-ASP1000 sample. Unlike Adaro-ASP, no coal particles are visible. This is because WH is slightly caking coal and undergoes melting on heat treatment just like ASP. Observation of WH-ASP1000 will provide understanding of how two melting phases interact and whether or not an interface similar to Adaro-ASP will be formed. Figure 5(b) shows a SEM image where no interface can be seen. From this image alone it is not possible to say if this is melted WH coal or ASP binder. However, sulfur mapping of above spot shown in Fig. 5(c) shows presence of sulfur in the lower-left portion suggesting the existence of an interface. This observation is very unique as this clearly indicates that an interface exists even for two melting phases although it can’t be distinguished by SEM image.
(a) SEM image of WH+ASP1000, (b) SEM image showing interface of WH-inert and ASP, (c) Sulfur mapping of spot (b) showing clearly the ASP interface.
To investigate the extent of penetration of binder into the coal, we used single coal and biomass char particle of about 10 mm size. Samples were prepared with only ASP binder as presence of sulfur which can be identified by EDS make it possible to distinguish the binder from base coal or biomass char. Adaro-ASP, and Biomass char-ASP samples were prepared. Figure 6(a) shows a camera image of Adaro-ASP single particle. The surface is completely coated with ASP. A part of this particle was polished so that Adaro-ASP surface can be visible. Figure 6(b) shows low magnification light microscope image showing very clearly the Adaro-ASP interface. When same spot is observed at high magnification under SEM-EDS as shown in Fig. 6(c), the interface was clearly distinguishable. However, above image does not provide information whether the binder penetrated the coal or is just coated on the surface. ASP contains high amount of sulfur and thus examining the sulfur distribution can provide the information about the extent of binder penetration. For this purpose sulfur mapping was carried out using SEM-EDS technique of a spot that showed Adaro coal on both sides so that penetration can be confirmed. Figure 6(c) show the sulfur line mapping. It can be seen that in addition to strong sulfur signals observed where the ASP can be confirmed by SEM image, some sulfur signals were also present in close vicinity to the interface. However, observing the magnified SEM images, it is more likely that only sulfur has penetrated and not the pitch itself. Sulfur at high temperature changes into gaseous state and diffuses inside. It is more likely that ASP is only present on the surface of the Adaro coal particle.
(a) Single Adaro coal particle coated with ASP, (b) low magnification SEM image and (c) Sulfur line mapping of a high magnification SEM image.
Figure 7(a) shows the single Biomass char particle coated with ASP. The particle was cut in the middle and SEM image of the cross-section is shown in Fig. 7(b). Interface of Biomass char and ASP is not clear but still distinguishable. To identify the interface, sulfur mapping was done and shown in Fig. 7(c). This image confirms the Biomass char-ASP interface. Here also, ASP appears to be coated only on the surface, even if there is some penetration of binder, it is not very deep inside the biomass char.
(a) Single Biomass char particle coated with ASP, (b) SEM image and (c) Sulfur mapping.
The SEM and light microscope analysis showed the coal-binder interface clearly, however these do not provide the information about the carbon microstructure of coal, binder and coal-binder interface. To investigate the carbon microstructure, Laser Raman spot analysis was carried out. Figure 8 shows the Raman spectrum from a single spot analysis of Adaro+ASP1000 when the beam was focused on the Adaro. Similar Raman spectra were obtained from a single spot analysis of Adaro+HPC1000, Anth+ASP1000, Anth+ HPC1000, Adaro1000, Anth1000, ASP1000, and HPC1000. Several approaches have been used to characterize carbon structure using Raman spectroscopy.7) The most common are the comparison of ratio of intensity of D band and G band, ID/IG and ratio of intensity of minimum point between two peaks and intensity of G band, IV/IG. In Fig. 9, IV/IG values were plotted against ID/IG. For Adaro-ASP and Adaro-HPC as shown in Fig. 9(a), the IV/IG values are shifted towards left and ID/IG towards higher side than for individual Adaro, ASP and HPC values. For Anthracite-ASP and Anthracite-HPC as shown in Fig. 9(b), values are shifted reversely. Although the shift for Adaro and Anthracite as base coals are different, the shift is definitely there. The results suggest that there appears to be an interactive effect between base coals and binders on each other’s carbon structure.
Raman spectrum of Adaro+ASP1000.
IV/IG vs. ID/IG plots showing interactive effect of coal and binder on each other’s carbon structure.
Measurement of strength of coal-binder interface is a highly desired parameter but very difficult to measure. Figure 10 shows the overall tensile/crushing strength of the coal-binder samples. For reference, tensile strength of an industrial coke strength is also shown. It can be seen that all the samples show low strength. This is expected result as these samples do not contain any caking coal fraction which provides the strength. The strength of cokes produced by non-caking coal Adaro and binders (ASP/HPC/CTP) can be assumed to be the sum of strength of coal particle, bonding strength of coal and binder and binder itself. It will be reasonable to assume that the overall strength will be primarily determined by the fraction of the coal particle surface that is bonded/wetted with the binder and the bonding strength of binder-coal particle interface. Using same base coal for different binders, contribution of strength of coal itself can be removed. With this assumption one can indirectly compare the effect of bonding strength of coal-binder on coke strength. From Fig. 10, the strengths are in order of Adaro-CTP<Adaro-HPC< Adaro-ASP and can be correlated with apparent density. This may be explained considering the extent of wetting or fraction of base coal surface coated with the binder. The fraction of coal particle surface wetted with binder will be dependent on the amount of binder available for bonding with coal particle which is reflected in apparent density because of same base coal. In addition, the amount of binder will also contribute to the strength. The difference of amount of binder will come from volatile matter content (VM) of the binder. Higher the VM, less amount of binder will actually be available for bonding with coal particle. The VM of HPC and ASP were nearly same around 41% and 40%, respectively and CTP was about 70% from TGA weight loss data. ASP and HPC may have different coke yields when they were co-carbonized with coal. From the results, the amount of binder or extend of wetting is considered a major factor for coke strength, suggesting the importance of the difference of the interface strength between coal and the each binder (ASP, HPC and CTP) but could not be distinguished with the present test.
Tensile strength of the coal-binder composites.
The lower strength of Biomass char-ASP than Adaro-ASP can primarily be attributed to the weak strength of biomass char itself in comparison to Adaro coal char although the contribution of binder interface cannot be neglected. The direct measurement of bonding strength of coal and binder interface is a major challenge and is a topic for further study.
Effect of binder addition to increase the coke strength was investigated by analyzing physical and chemical characteristics of coal-binder interface and carbon structure near interface. Effect of binder type and base coal rank was investigated by using an oil derived Asphalt pitch (ASP), a coal derived solvent extracted coal fraction HPC, and Coal Tar Pitch (CTP) as binder. Non-caking coal Adaro, Anthracite and a Biomass char were used as base coal and biomass. A new simple approach to observe the coal-binder interface at micron level using SEM has been reported. Following conclusions were drawn.
(1) Coal-binder interface was clearly observed by SEM for the first time. Binder was coated on the surface of the coal and also filled the void space between the particles leading to a solid structure.
(2) Bonding of binder on coal particle surface was found to be dependent on the type of binder. From observed images, it was found that ASP and HPC bound differently on coal. ASP interface was clearly distinguishable while HPC and CTP interface were not distinguishable. This could be due to the origin of the binder; ASP is oil derived while HPC and CTP were coal derived.
(3) Sulfur mapping showed penetration of sulfur inside coal particle, however, it is more likely that only sulfur in gaseous form penetrated the particle and not the pitch itself. From SEM-EDS analysis, it can be said that binder only coated on the coal particle surface at the microscopic level.
(4) Preliminary Raman analysis suggests that there may be some interactive effect of coal and binder on one other’s carbon structure development.
(5) Wetting or fraction of coal surface coated by binder may be the main factor contributing to the tensile strength of coal-binder composite.
This work has been done in Development of Cokemaking Technology from Low-grade and Nonconventional Carbon Resources, Division of High-Temperature Processes, Academic Society, the Iron and Steel Institute of Japan. The authors would like to acknowledge the research group members gratefully.
