Analysis of Thermal Behavior of Crystalline Minerals in Bituminous Coal Samples under Air and Argon Atmospheres.

The thermal behavior of ash components in two bituminous coal samples [Upper Freeport (the "UF") and Illinois #6 (the "IL")] was investigated under air and argon atmospheres in the temperature range of 800-1200 °C using thermal gravimetric-differential thermal analysis, X-ray diffraction transmission electron microscopy (TEM), and TEM-energy dispersive X-ray spectrometry measurements. The UF treated under air formed the needle-like crystals which were assumed to be mullite-related substances formed by transformation of andalusite, because the crystals are mainly composed of SiO2 and Al2O3. In contrast to the UF, the IL did not generate such crystals; however, when the IL was treated under air after carbonization under Ar, crystals appeared. The composition of the UF with an Al/Si ratio higher than that of the IL favored the formation of mullite-related substances, while the presence of lime in the IL inhibited the formation of mullite-related substances. Oldhamite was formed by the reaction of lime with sulfur at the carbonization of the IL under Ar and remained even at the successive air treatment. As lime was consumed, the formation of mullite-related substances was ceased to be inhibited under air after Ar treatment.

Introduction

Coal is mainly used for coal-fired power generation fuel and iron-making coke, while the coal utilization brings about the generation of a large amount of ash. The coal fly ash can be used for (1) recovery of Na[1] and Al2O3,[2] (2) absorbents of SO2[3] and urea,[4] (3) structural materials,[5,6] (4) dielectric materials,[7] and so on. On the other hand, ash components in coal have problems of affecting the high-temperature corrosion of gas turbine blades in thermal power generation,[8−10] use of coal combustion byproducts,[11] and the production and property of coke used in iron making.[12−14] Coal samples of various mines in America,[12,15−18] China,[8,9,19−21] South Africa,[22,23] Indonesia,[24−26] and others[27,28] have been investigated to overcome the problems above.

The current major interest in the ash problems is the formation of fine ash particles during the coal conversions such as pyrolysis, liquefaction, gasification, and combustion. The presence of organometallic aluminosilicates may contribute to the formation of such fine ash particles, because such aluminosilicates are very reactive with alkali and alkali-earth metals to be fusible during the gasification and combustion, causing the slagging problems.[24] Control over the reaction atmosphere is very important, because several gases such as N2[20] and He[25] in pyrolysis processes of coal, H2O/He,[25] and N2/CO2[29] in gasification processes and N2/O2,[8] N2/air,[12] air/O2[23] air/CO, CO2[20] and air[12,20,29] in combustion processes were used, respectively. The coal sintering temperate under reducing conditions was lower than that under air[20] in combustion processes.

As the coal ash-based products are still subject to the actual research theme, it is very important to understand the thermal behavior of ash components, because the microscopic structures of such components and their quantification have not yet been clarified completely and would have affected the processes.

There are some relevant studies on coal samples using such methods as the X-ray diffraction (XRD) method,[12,17,30] transmission electron microscopy (TEM),[15,17,18,28,31] thermal gravimetric analysis (TGA),[11,28,32−34] differential thermal analysis (DTA),[8,9,11,34] solid-state nuclear magnetic resonance (NMR),[16,17,24,35] and emission.[23,27,36] In refs,[15,17,18] the structures of coal were investigated using TEM, but information on ash was not given. In refs,[28,31] details of the thermal behavior of ash components were not given.

Thus, it has not yet been clarified satisfactorily what chemicals are formed and how ash components change with varying atmosphere and treating temperature. In addition, the crystallization behavior of ash components and the morphology of ash-based products have not attracted such attention as to the problems of high-temperature corrosion of gas turbine blades. Therefore, in this reported study, changes in the behavior of ash components were investigated for the two bituminous coal samples [Upper Freeport (UF) and Illinois #6 (IL) differing each other in carbon content and ash component] of the Argonne Premium Coal Sample series[12,15−18] with varying atmospheres (air, argon, and argon to air) and treating temperatures (800–1200 °C) using XRD, TEM, TEM–energy-dispersive X-ray spectrometry (EDS), and TG–DTA.

Results and Discussion

Thermal Behavior of the UF and IL under Air

Figure shows TG–DTA curves of the UF and IL (Tables S2 and S3) up to 1200 °C under air or argon atmosphere and TG–DTA curves of the UF and IL up to 800 °C under air again after heat-treated at 800 °C under argon atmosphere. TG–DTA curves of the UF and IL heat-treated under air atmosphere showed similar tendency. The weight increase in the range 200–300 °C because of oxygen adsorption[37−40] and the large weight loss in the range 500–600 °C ended at 700 °C. The weight began to increase again from 700 °C slightly. Engin et al.[33] also reported that TG increases the peak in the 600–900 °C under O2 atmosphere for low-grade lignite coals. Although the increase may be attributed to the complete oxidation of ash, their details are unknown.

Figure 1

TG–DTA curves of the UF and IL up to 1200 °C under air or argon atmosphere and TG–DTA curves of the UF and IL up to 800 °C under air again after heat-treated at 800 °C under argon atmosphere.

TG curves of the UF and IL under argon atmosphere did not show such significant weight loss as observed under air atmosphere, and it was found that the weight gradually decreased from the beginning to the end of measurement: 30% and 65% weight losses were observed up to 1200 °C for the UF and IL measured under argon atmosphere, respectively. On the other hand, the weak peak of the DTA profile of the IL was observed around 400 °C. When TG–DTA of the UF and IL heat-treated under argon atmosphere was performed under air atmosphere again, the peaks of the DTA profile were observed at the temperatures higher than those of the coal samples heat-treated only under air atmosphere. In this analysis, large weight losses were also observed at 600–700 °C. Similar results were obtained for the UF and IL heat-treated down to the lower temperatures.

Figure gives XRD patterns of the UF and IL heat-treated under air atmosphere at each temperature. The raw UF mainly consists of quartz (SiO2), kaolinite [Al4Si4O10(OH)8], calcite (CaCO3), and pyrite (FeS2). The quartz and andalusite (Al2SiO5) existed under all heat-treatment conditions. The calcite was present in the UF heat-treated at 800 °C and disappeared at 1000 °C. The kaolinite, calcite, and pyrite were transformed to the andalusite, anhydrite (CaSO4), and hematite (Fe2O3), respectively, with increasing heat-treatment temperature. In the present case, mullite (Al6Si2O13) was not observed. Table S1 was used for the assignment of XRD data.

Figure 2

XRD patterns of the UF and IL heat-treated at various temperatures under air atmosphere.

The raw IL mainly consists of quartz, kaolinite, calcite, and pyrite, similar to the raw UF. The quartz and andalusite existed under all heat-treatment conditions. The calcite was present in the IL heat-treated at 800 °C and disappeared at 1000 °C. The kaolinite and pyrite were transformed to the andalusite and hematite, respectively, with increasing heat-treatment temperature. The calcite was transformed to anhydrite and lime (CaO). The difference between UF and IL is considered to be because of the difference in the amount of calcium contained in the ash, as seen from Table S3.

Figure shows TEM images of the UF and IL heat-treated at various temperatures under air atmosphere. From the TEM observation, needle-like crystals, which were not seen in the untreated coal samples, were observed after heat treatment of the UF under air atmosphere. From the TEM observation of the IL, needle-like crystals, which were observed above for the UF heat-treated under air atmosphere, were not observed.

Figure 3

TEM images of the UF and IL heat-treated under air atmosphere at temperatures 800, 1000, and 1200 °C.

Thermal Behavior of the UF and IL under Argon

Figure gives XRD patterns of the UF and IL heat-treated under argon atmosphere at each temperature. The quartz and andalusite existed under all heat-treatment conditions, and the kaolinite was transformed to the andalusite with increasing heat-treatment temperature, similar to the case under air atmosphere. Pyrite (FeS2) was transformed to troilite (FeS), and calcite disappeared unlike the case under air atmosphere.

Figure 4

XRD patterns of the UF and IL heat-treated at various temperatures under argon atmosphere.

In the case of the IL, the quartz and andalusite existed under all heat-treatment conditions, and the kaolinite and pyrite were transformed to the andalusite and troilite with increasing heat-treatment temperature, similar to the case of the UF. The calcite was transformed to oldhamite (CaS) as a point segregating the IL from the UF. The difference between UF and IL is considered to be because of the difference in the amount of calcium and sulfur contained in the untreated coal samples as seen from Tables S2 and S3. It can also be seen that the diffraction line of oldhamite becomes strong as the temperature rises, indicating that calcium oxide (lime) would react with the hydrogen sulfide formed from the reduction of pyrite to eventually produce calcium sulfide (oldhamite) under the reduction conditions.

Figure shows TEM images of the UF and IL heat-treated at various temperatures under argon atmosphere. From the TEM observation, scale-like and honeycomb-like crystals were observed by heat treatment of the UF under argon atmosphere, whereas needle-like crystals were observed by heat treatment of the UF under air atmosphere. The scale-like and honeycomb-like crystals may be considered to be carbonaceous substances. In the case of the IL, the scale-like and honeycomb-like crystals may be considered to be carbonaceous substances, similar to the case of the UF. From the TEM observation, needle-like crystals, which were observed for the UF heat-treated under air atmosphere, were not observed.

Figure 5

TEM images of the UF heat-treated under argon atmosphere at temperatures 800, 1000, and 1200 °C.

Thermal Behavior of the UF and IL under Air after Argon

Figure gives XRD patterns of the UF and IL heat-treated at 800 °C under air atmosphere again after heat-treated under argon atmosphere. The quartz and andalusite existed under all heat-treatment conditions. Because the samples were heat-treated at 800 °C under air atmosphere again after heat-treated under argon atmosphere, calcite and anhydride was observed, similar to the case where they were heat-treated at 800 °C under air atmosphere. The troilite, which was formed under argon atmosphere, was transformed to hematite.

Figure 6

XRD patterns of the UF and IL heat-treated under air atmosphere again after heat-treated under argon atmosphere.

In the case of the IL, the quartz and andalusite existed under all heat-treatment conditions. Because the samples were heat-treated at 800 °C under air atmosphere again after heat-treated under argon atmosphere, calcite and anhydride were observed, similar to the case where they were heat-treated at 800 °C under air atmosphere. The troilite, which was formed under argon atmosphere, was transformed to hematite. The oldhamite existed only in the IL samples under all heat-treatment conditions, similar to the case under argon atmosphere. The precipitation of the oldhamite in such coal samples as heat-treated under air atmosphere were reported in some papers.[41−43] Liang et al.[41] reported that oldhamite exists in the coal samples heat-treated at 600 °C under air atmosphere. The data of Lu et al.[42] suggested that oldhamite seems to exist in the coal samples heat-treated at 850 °C under air atmosphere, although weak diffraction lines around 2θ = 45° and 65°, which may be due to oldhamite, were assigned to unknown peaks. Skhonde et al.[43] reported that oldhamite exists even if high ash bituminous coal blend was undergone by combustion after gasification under SO3 flow.

Table summarizes crystal phases precipitated in the UF and IL heat-treated under various atmospheres. The quartz (SiO2) and andalusite (Al2SiO5) existed under all heat-treatment conditions. Anhydrite (CaSO4), calcite (CaCO3), and hematite (Fe2O3) were observed in the UF and IL heat-treated under air atmosphere, while troilite (FeS) in closed square (■) was formed in the UF and IL heat-treated under Ar atmosphere.

Table 1

Crystal Phases Precipitated in the UF and IL Heat-Treated under Various Atmospheres

	UF									IL
	air			Ar			air after Ar			air			Ar			air after Ar
	La	Ma	Ha	L	M	H	L	M	H	L	M	H	L	M	H	L	M	H
lime (CaO)	●	●	●							●	●	●
calcite (CaCO3)	○						○	○	○	○						○	○	○
anhydrite (CaSO4)	○	○	○				○	○	○	○	○	○				○	○	○
oldhamite (CaS)													▲	▲	▲	▲	▲	▲
andalusite (Al2SiO5)	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○
quartz (SiO2)	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○	○
hematite (Fe2O3)	○	○	○				○	○	○	○	○	○				○	○	○
troilite (FeS)				■	■	■							■	■	■

L: 800 °C, M: 1000 °C, H: 1200 °C.

Any calcium compounds did not appear in the UF heat-treated under argon, probably because they would be dissolved in an amorphous phase. The lime shown in closed circle (●) was observed in the UF and IL heat-treated under air, while oldhamite (▲) was observed in the IL heat-treated both under argon and under air atmosphere again after heat-treated under argon atmosphere. It seems that this would be attributable to the fact that the contents of Ca and S in the IL are about twice those in the UF. Once the CaS was formed under argon heat treatment, it is considered to be relatively stable under air atmosphere heat treatment. From analogy with the IL heat-treated under argon, oldhamite may be formed in the UF heat-treated under argon. However, it seems that the amounts of Ca and S in the UF would be too small for the diffraction line of oldhamite to appear in XRD.

Figure shows TEM images of the UF and IL heat-treated under air atmosphere again after heat-treated under argon atmosphere. From the TEM observation in the UF, needle-like crystals were observed, similar to the case under air atmosphere. From the TEM observation in the IL, needle-like crystals, which were not observed for the IL heat-treated under air atmosphere, appeared. Thus, the needle-like crystals were observed in UF (air), UF (air after argon), and IL (air after argon).

Figure 7

TEM images of the UF and IL heat-treated under air atmosphere again after heat-treated under argon atmosphere at temperatures 800, 1000, and 1200 °C.

TEM–EDS images were measured, as shown in Figure , for (a) UF heat-treated at 1000 °C under air, (b) IL heat-treated at 1000 °C under air, and (c) IL heat-treated under air atmosphere again after heat-treated under argon atmosphere at 1000 °C. The chemical composition determined by TEM–EDS measurement for UF is shown in Table S3. The results of TEM–EDS measurement seem to roughly reflect overall chemical composition of coal. Therefore, we semiquantitively discussed chemical composition using EDS mapping data. The oldhamite seen only in the IL heat-treated under air atmosphere again after heat-treated under argon atmosphere at 1000 °C was identified from the Ca and S composition mapping in Figure c. It was found from TEM–EDS observation that needle-like crystals mainly consist of Si and Al (Figure a,c). In this case, andalusite (Al2SiO5) is assumed from the XRD data available to be one candidate for needle-like crystals. While this crystalline phase existed in both UF and IL under all heat-treatment conditions, it is doubtful whether it would be related to needle-like shape.

Figure 8

TEM–EDS images of (a) UF heat-treated at 1000 °C under air, (b) IL heat-treated at 1000 °C under air, and (c) IL heat-treated under air atmosphere again after heat-treated under argon atmosphere at 1000 °C.

There are some reports on needle-like crystals mullite (3Al2O3·2SiO2: Al6Si2O13)[34,44−47] and anorthite (CaAl2Si2O8)[45] including Si and Al as coal ash. Moreover, it is known that andalusite transforms to mullite at high temperatures.[48] Because needle-like crystals were not seen in the IL heat-treated under air atmosphere, such reported results may be arisen from the difference in ash composition of the untreated coal samples taken. The UF with more Al and less Ca relative to the IL may produce mullite rather than anorthite. Because the composition of mullite has a higher Al/Si ratio compared to andalusite, the andalusite precipitated from the UF with an Al/Si ratio higher than the IL may tend to transform to needle-like mullite-related substance. Therefore, the needle-like crystals seen in UF (air), UF (air after argon), and IL (air after argon) are assumed to be mullite-related substances, which was partially transformed from andalusite but not detectable in XRD patterns. Moreover, needle-like crystals for UF (air, 1200 °C) vanished, as shown in Figure . Mullite reacts with CaO to produce anorthite by heat-treating at temperatures higher than 1130[20] and 800 °C.[21] It is consistent with our assumptions that needle-like crystals are mullite-related substances.

The needle-like crystals were not seen in UF (Ar) and IL(Ar). In addition, Ding et al.[49] reported that transformation of andalusite to mullite was depressed under the carbon-embedded conditions (reduction conditions), compared to air conditions. These are not contradicting with our results.

While the needle-like crystals were not formed in the presence of lime as in IL (Air) with an Al/Si ratio lower than UF, these crystals were formed in the presence of oldhamite instead of lime as in IL (Air after argon). Because most of the carbon is burnt at 600 °C under air atmosphere, and lime is in contact with other species of ash such as mullite-related substance before neutralization by the acid components (sulfuric acid, etc.), it was easy for needle-like crystals to be formed for the UF-treated under air atmosphere, as the UF contains an amount of lime lower than IL. On the other hand, C suppresses the reaction between species of ash under Ar atmosphere and H2 reacts with pyrite to generate H2S and troilite. The lime is neutralized by H2S to form oldhamite, which would lead to the loss of the ash-dissolving ability. The sulfur in FeS2 was also migrated to Ca compounds (CaS or CaSO4) by heat-treating.[42] This was also supported by the fact that, in EDS measurement, the distributions of Ca and S were almost the same as each other. It is therefore assumed that the crystallization of the Al2O3–SiO2 system would progress by heat-treating under air atmosphere again after heat-treating under argon. Such is more likely to happen, because the content of sulfur in the IL was much higher than that in the UF.

Conclusions

In this study, changes in the behavior of ash components in the two bituminous coal samples (UF and IL) with varying the treating atmosphere and temperature were investigated using TG–DTA, XRD and TEM measurements. The quartz (SiO2) and andalusite (Al2SiO5) existed under all heat-treatment conditions. Anhydrite (CaSO4), calcite (CaCO3), and hematite (Fe2O3) were observed in the UF and IL heat-treated under air atmosphere, while troilite (FeS) was formed in the UF and IL heat-treated under argon atmosphere. All the crystalline phases in heat-treated UF were observed in heat-treated IL. The lime was observed in the UF and IL heat-treated under air, while oldhamite was observed only in the IL heat-treated under argon and air atmosphere again after heat-treated under argon atmosphere, probably because contents of Ca and S in the IL were larger than those in the UF. Once CaS was formed under argon heat treatment, it would be maintained even under air atmosphere heat treatment, because of its high thermal stability.

From the TEM observation, needle-like crystals, which were not seen in the untreated samples, were observed in the UF heat-treated under air atmosphere, while such crystals were not seen in the IL heat-treated under air atmosphere. The needle-like crystals seen in UF (Air), UF (Air after argon), and IL (Air after argon) were found from TEM–EDS and XRD observation to mainly consist of SiO2 and Al2O3, suggesting that they would be mullite-related substances. The composition of mullite-related substances has a higher Al/Si ratio, compared to andalusite. Therefore, it is likely that the andalusite phase formed from the UF would transform to needle-like mullite-related substance because the UF has an Al/Si ratio higher than the IL. On the other hand, the needle-like crystals were not formed for the IL heat-treating under air probably because of the presence of lime and the IL having a lower Al/Si ratio relative to the UF. However, the needle-like crystals were found for the IL heat-treated under air after Ar treatment, suggesting that lime was neutralized by the hydrogen sulfide formed under argon treatment to eventually form oldhamite, which promoted the formation of the needle-like crystals.

Experimental Section

Coal Samples and Heat Treatment

Bituminous coal samples, UF and IL, were purchased from Argonne National Laboratory. Ultimate and proximate analysis of these coal samples are shown in Table S2. Ash contents of the UF and IL are 13.03 wt % and 14.25 wt %, respectively.

Ash components of these coal samples are shown in Table S2. Roughly, the UF has a higher carbon content (86%), a higher Al2O3 content, a lower CaO content, and a lower SO3 content, compared to the IL. Heat treatment of the UF and IL was performed at 800–1200 °C under air, argon, or argon to air atmosphere and was cooled in the furnace.

Characterization of Coal Samples Using TG–DTA, XRD, TEM, and TEM–EDS

The original as-received (AR) and pulverized coal samples were subjected to the following measurements: Thermal gravimetric (TG) and differential thermal analyses (DTA) measurement of coal samples using Shimadzu DTG-60AH was performed under air or argon atmosphere with the following conditions: flow rate of 100 mL/min, temperatures of 800–1200 °C at a heating rate of 10 °C/min, without retention at target temperature, and by furnace cooling. Further, TG–DTA measurement of the UF and IL heat-treated up to 800–1200 °C under argon atmosphere was also performed at 800 °C at a heating rate of 10 °C/min, without retention at the target temperature and by furnace cooling under air atmosphere to eventually investigate the oxidation process.

For the sample XRD measurement, 0.7 g each of the coal samples was heat-treated in 100 mL/min under air or argon atmosphere up to 800–1200 °C at a heating rate of 10 °C/min, without retention at target temperature and by furnace cooling. The UF and IL samples heat-treated under argon atmosphere was again heat-treated at 800 °C at a heating rate of 10 °C/min, without retention at target temperature and followed by furnace cooling under air atmosphere. XRD patterns were measured in order to examine the crystal structures of ash in the coal samples heat-treated under air or argon atmosphere. The XRD measurement system (Ultima IV, Rigaku) with the use of Ni-filtered Cu Kα radiation (λ = 0.15405 nm) was used for coal samples (sample amount 0.10 g) held on the glass plates under the following conditions: 2θ = 10–70°, sampling width of 0.02°, scan speed of 4°/min, voltage of 40 kV, current of 20 mA, radiation split of 2/3°, radiation column limitation slit of 10.00 nm, scattering slit of 2/3°, detecting slit of 0.45 nm, and offset angle of 0°.

Transmission electron microscopy (TEM) measurement of the samples heat-treated by TG–DTA measurement under air, argon, or argon to air atmosphere was performed using transmission electron microscope JEM-1011 (JEOL Co. Ltd.). Transmission electron microscopy equipped with TEM-EDS for the selected samples was performed using field emission electron microscope JEM-2100F (JEOL Co. Ltd.).