3.1. Coal Analysis
The results of the ultimate and proximate analyses of the coal samples are shown in Table 1
. It is shown that the ash yield in Ulankarma lignite is only 7.81%, while in Ulanqab lignite, it is 36.62%. Ulankarma lignite has more than a 3% sulfur content, which belongs to hyper-sulfur coal, while at the Ulanqab lignite, it is less than 0.5%.
The percentages of major element oxides, as well as loss on ignition, for the Ulanqab and Ulankarma lignites are listed in Table 2
The two lignites have substantially different ash components. The content of Fe2
in Ulankarma lignite is very high, as shown in Table 2
, which is a typical hyper-iron coal [36
] and has high sulfur content in this case. The total of SiO2
content in Ulanqab lignite is about 80%, while that in Ulankarma lignite is less than 40%. In the Ulankarma lignite, low content of SiO2
, as well as high content of Fe2
and CaO result in lower ash melting points. The analysis results of the coal ash fusibility in a weakly-reducing atmosphere are exhibited in Table 3
]. It can be seen from Table 3
that the softening temperature of Ulankarma lignite ash is 1060 °C, but it is 1210 °C for Ulanqab lignite ash. In general, iron could reduce the melting point of coal ash.
3.3. Minerals Transformation during Coal Pyrolysis
As shown in Figure 4
a, the main minerals in 800 °C pyrolysis of semi-coke consist of quartz, muscovite and sanidine. It could be found from the comparison of the mineral composition between raw coal and semi-coke that orthoclase was present in raw coal, but not present in semi-coke, while sanidine is formed. Although the quartz and muscovite do not change, the characteristic peaks of quartz and muscovite in the pyrolysis semi-coke are significantly enhanced, indicating that there are increased contents of quartz and muscovite [44
]. The decomposition of large amounts of volatile organic compounds during the pyrolysis process leads to the increases of the relative content of inorganic minerals. Furthermore, part of the aluminosilicate minerals are decomposed into SiO2
under pyrolysis. Orthoclase and sanidine are feldspar minerals in the K-feldspar subfamily of homogeneous multiple variants. With the increase of temperature, the order of crystal structure of orthoclase is destroyed, and it is gradually transformed into completely disordered sanidine. The formation of sanidine at such a high temperature is consistent with its formation in coal and coal-bearing sequences. For example, sanidine is a high-temperature mineral in altered volcanic ashes (tonsteins) and usually used as an indication of volcanic ash [45
As shown in Figure 4
b, the main minerals in the semi-coke of Ulankarma lignite (800 °C) consist of quartz, kaolinite and orthoclase. Kaolinite starts to lose crystal water and is converted to metakaolinite above 327 °C [9
], and then, the metakaolinite will decompose into γ-Al2
around 827 °C [47
]. The decomposition products of kaolinite can react with K2
O to produce orthoclase in the temperature range of 500–800 °C [48
]. Therefore, it can be concluded that kaolinite is gradually decomposed during the pyrolysis process and is partially converted to orthoclase, but a small amount of kaolinite could still be found in the semi-coke because the pyrolysis temperature is relatively low.
As shown in the SEM image in Figure 5
a, the material attached to the carbon particles is pyrite, which is consistent with the XRD results. After 800 °C pyrolysis, carbon content in the semi-coke rises because of the emission of the volatile organic matters. Fe and S elements are found on the carbon particles in the semi-coke (Figure 5
b), which is apparently present as pyrrhotite (Fe1−x
= 0–0.233) based on the analysis of the atomic ratio. The atomic ratio of Fe element to S element is 0.73:1 in Ulankarma semi-coke, which lies in the range of 0–0.233 [36
]. This suggests that FeS2
(in the coal) is gradually decomposed when the pyrolysis temperature is higher than 500 °C and S escapes in the form of S2
(g). When the pyrolysis temperature reaches 800 °C, S escapes at a higher rate, and pyrrhotite is eventually formed with a molecular formula of Fe0.73
3.4. Mineral Transformation during the Semi-Coke Reduction Process
Based on the results of XRD analysis in Figure 6
a, the iron-containing mineral is present in the form of magnetite (Fe3
) in the 900 °C reduction ash, which is converted from pyrrhotite. Sanidine is also present in the ash, but muscovite is not present in the pyrolysis semi-coke. The reason for the absence of the muscovite in the semi-coke is the gradual removal of the hydroxyl group under the action of high temperature. As the temperature rises above 900 °C, the demineralized muscovite reacts with CaO or FeO to produce orthoclase by the following reactions.
KAl3Si3O10 + CaO → KalSi3O8 + CaO·Al2O3
Kal3Si3O10 + FeO → KalSi3O8 + FeO·Al2O3
Magnetite is altered, while hercynite is formed in great quantities in the reduction ash at 1000 °C. It can be concluded that the magnetite in the reduction ash of Ulanqab is gradually transformed to FeO at a higher temperature and stronger reducing atmosphere. Then, FeO reacts with Al2
to produce hercynite at the 1000 °C gasification conditions. When the temperature reaches 1100 °C, the diffraction peak intensity of hercynite is further enhanced, which indicates that its content increases with temperature. Thereafter, at 1200 °C, the diffraction peak intensity of the hercynite declines, indicating that the content begins to decrease. Until 1300 °C, hercynite minerals decrease in abundance and are accompanied by the emergence of sekaninaite. As the thermodynamic properties of sekaninaite are more stable, hercynite is converted to sekaninaite above 1300 °C. The result is completely in accordance with the report by McCarthy et al. (1988) [19
], which proposes the medium temperature reaction in the study of slag from the UCG field test.
) is present in the reduction ash of Ulanqab lignite. Anorthite has also been detected in some volcanic ash influenced coals [49
] and is an indication of a high-temperature mineral. Anorthite is detected at 1000 °C and formed in great quantities at 1100 °C, and then, its content gradually decreases at 1200 °C. With the appearance of anorthite and the increase in content, quartz continues to decrease. Therefore, it can be concluded that quartz is involved as a reactant in the formation of anorthite. It is also inferred that Al2
from the decomposition of orthoclase and anorthoclase react with CaO from the decomposition of augite to form anorthite, and the specific reaction is as follows [8
CaO + Al2O3 + 2SiO2 → CaAl2Si2O8
As shown in the XRD image in Figure 6
b. Magnetite is found in the reduction ash of Ulankarma at 900 °C, which is in agreement with Ulanqab lignite, and the magnetite is also present in the temperature range of 1000–1100 °C.
Magnetite is present at the same time as augite (Fe2
) in the reduction ash at 1200 °C. The reaction process is similar to that of the hercynite, and the magnetite is reduced to FeO at a higher temperature, then it reacts with the decomposition products of the orthoclase and kaolinite. Hercynite is present in the reduction ash of Ulanqab in the temperature range of 1000–1200 °C [50
]. While hercynite is not present in the reduction ash of Ulankarma at 1200 °C, augite is formed in great quantities. A possible reason for this phenomenon could be explained as follows: The hercynite is not conducive to forming due to the low content of Al2
in the ash of Ulankarma. The SiO2
content is relatively high, and the augite is conditionally formed. Augite is transformed to sekaninaite, the thermodynamically-stable mineral, at a high temperature of 1300 °C.
The orthoclase and kaolinite in the semi-coke are completely decomposed at a high temperature of 900 °C. Sillimanite has formed at 1100 °C and is produced in large quantities at 1200 °C. After 1300 °C, the sillimanite is no longer present, but mullite has started to form. The Al2
, which are respectively decomposition products of orthoclase and kaolinite in the semi-coke, can react to produce sillimanite. As the temperature increases, sillimanite is converted to mullite, the thermodynamically-stable mineral [51
]. The reaction process is as follows:
Al2SiO5 + 2Al2O3 + SiO2 → Al6Si2O13
Compared to the Ulanqab lignite, anorthite is not present in the reduction ash of the Ulankarma lignite. This is because of the low content of Al2O3 in the Ulankarma ash, resulting in the absence of anorthite. The diffraction intensity of the mineral in the reduction ash at 1300 °C is weak, indicating that the crystal minerals have disappeared and the ash has been melted, which is very close to the ash melting point of the Ulankarma lignite.
In the XRD analysis, hercynite is proven to exist in the reduction ash of Ulanqab at 1000 °C, and a small amount of augite minerals is also found in the SEM (Figure 7
a). Therefore, augite and hercynite coexist in the reduction ash at 1000 °C, but augite content is low. The content of Al2
in the ash of Ulanqab lignite is high, but the FeO from the reduction of magnetite tends to react with Al2
to form hercynite. Only a small amount of FeO reacts with SiO2
to generate augite. However, the Al2
in the Ulankarma ash is very low, and it is involved in the formation of the sillimanite and mullite, resulting in the Al2
that can react with FeO to almost disappear; thus, the reaction produces augite, rather than hercynite during the reduction process. Consistent with the results of XRD analysis, sekaninaite minerals as shown in Figure 7
b are present in the ash reduction of Ulanqab lignite at 1300 °C.
As shown in the SEM image in Figure 8
, the augite mineral phase appeared at 1200 °C in the Ulankarma reduction ash, and the sekaninaite phase is present at 1300 °C.
3.5. Mineral Transformation during Residual-Coke Oxidation Process
The XRD results of the oxidation ash of Ulanqab lignite are similar to the reduction ash in the mineral composition (Figure 9
a). Hercynite and hematite are present in the 1100 °C oxidation ash at the same time. Hercynite decreases at 1200 °C, and then, trace amounts are observed at 1300 °C when massive hematite is formed in the oxidation ash. A possible reason for this change could be explained as follows: Fe2+
existing in hercynite reacts with the oxidizing agent, generating Fe3+
that is present in the form of hematite at 1100 °C, then the Fe2+
is continuously converted to hematite and finally completely transformed into hematite at 1300 °C. When the temperature reaches 1400 °C, the hematite is still present. Hematite is eventually transformed into thermodynamically-stable sekaninaite at high temperatures.
The content of hematite is reduced when the temperature reaches 1200 °C, although it should continue to rise theoretically with the transformation of hercynite. On the other hand, anorthite is generated at 1100 °C, but the anorthite content begins to decrease at 1200 °C. Finally, anorthite is absent at 1300 °C. Therefore, it is inferred that during the oxidation of Ulanqab lignite, low-temperature mixing occurs between the iron-bearing minerals and anorthite when the temperature reaches 1200 °C, which leads to the decrease of iron content and the absence of anorthite.
The content of quartz decreases at temperatures ranging from 1100–1300 °C, since it participates in the formation of minerals such as mullite and anorthite. When the temperature reaches 1400 °C, the quartz content is further reduced, while the cristobalite is formed in great quantities. This is because the quartz could change to form cristobalite at high temperatures [52
]. The chemical properties of the cristobalite whose melting point is 1610 °C are extremely stable, so it is still present in the oxidation ash at 1500 °C.
The intensity of the XRD diffraction peak of the oxidation ash is weak at 1400 °C, indicating that the content of the crystal mineral is very low. Especially at 1500 °C, the crystal minerals in the ash are almost completely converted to the vitreous material, which is called vitrification. This phenomenon has also occurred in many ash-related studies. The vitrified ash is very stable, has no risk of leaching contamination and can be safely landfilled or used as a building material [53
The XRD results of the oxidation ash of Ulankarma lignite (Figure 9
b) show that hematite could be found at the temperatures ranging from 1100–1200 °C. The magnetite is oxidized to hematite in an oxidizing atmosphere, and then, hematite is transformed to sekaninaite when the temperature is over 1300 °C. As the temperature reaches above 1400 °C, the content of the sekaninaite is still high. This is due to the high melting point of sekaninaite.
SEM images of 1100 °C ash and 1300 °C ash are shown in Figure 10
a, b, respectively. The iron-bearing mineral is present in the form of hematite at 1100 °C oxidation ash, which is in agreement with the XRD analysis. The EDS analysis results of the mineral in Figure 10
b show that the contained elements are complex, including Si, Al, Fe, Ca, K and Mg, and the content of each element is high. Thus, we can speculate that the mineral is ash. When the temperature reaches 1300 °C, the crystal minerals begin to melt largely due to the ash melting point of Ulankarma being 1060 °C.
As shown in the SEM image in Figure 11
a, the hercynite appears in 1100 °C oxidation ash of Ulanqab lignite, which is consistent with the XRD analysis. Figure 11
b shows that the low-temperature co-melt of iron-bearing minerals and calcium is formed when the temperature is below the ash melting point, which in turn leads to the decrease of crystal minerals in 1200 °C oxidation ash of Ulanqab lignite. Sekaninaite is stably present in 1400 °C since calcium is not present in oxidation ash of Ulankarma lignite.
The sequential transformations of iron-bearing minerals with increasing temperature are displayed in Figure 12
. Using XRD and SEM analyses, we can summarize that the composition of ash leads to the formation of typical iron-bearing minerals and affects the transformation of the iron-bearing minerals. The transformation behavior of iron-bearing minerals in the two lignites is similar during the pyrolysis process, in which pyrite (FeS2
) in the raw coal is gradually desulfurized into pyrrhotite (Fe1−x
S). In the reduction stage, pyrrhotite is transformed into magnetite (Fe3
) at 900 °C and then changes to FeO. Due to the difference in the content of Si and Al, the reaction of FeO and Al2
in the Ulanqab lignite produces hercynite above 1000 °C, while in the Ulankarma lignite, FeO reacts with SiO2
to generate augite (Fe2
). When the temperature rises to 1400 °C, both hercynite and augite are converted to the thermodynamically-stable mineral sekaninaite.