Sequential Transformation Behavior of Iron-Bearing Minerals During Underground Coal Gasiﬁcation

minerals

Article Sequential Transformation Behavior of Iron-Bearing Minerals during Underground Coal Gasiﬁcation

Shuqin Liu *, Weiping Ma, Yixin Zhang, Yanjun Zhang and Kaili Qi School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China; [email protected] (W.M.); [email protected] (Y.Z.); [email protected] (Y.Z.); [email protected] (K.Q.) * Correspondence: [email protected]; Tel.: +86-10-6233-9156

Received: 31 December 2017; Accepted: 12 February 2018; Published: 28 February 2018

Abstract: Detailed mineralogical information from underground coal gasification (UCG) is essential to better understand the chemical reactions far below the surface. It is of great scientific significance to study the mineral transformation and identify the typical minerals in certain process conditions, because it may help to ensure the stable operation of gasification processes and improve the utilization efficiency of coal seams. The transformation of iron-bearing minerals has the typical characteristics during the UCG process and is expected to indicate the process parameters. In this paper, UCG progress was subdivided into pyrolysis, reduction and oxidation stages, and the progressive coal conversion products were prepared. Two types of lignite with different iron contents, Ulankarma and Ulanqab coals, were used in this study. The minerals in the coal transformation products were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation performed using the phase diagram of FactSage 7.1 was used to help to understand the transformation of minerals. The results indicate that the transformation behavior of iron-bearing minerals in the two lignites are similar during the pyrolysis process, in which pyrite (FeS2) in the raw coal is gradually converted into pyrrhotite (Fe1−xS). In the reduction stage, pyrrhotite is transformed into magnetite (Fe3O4) and then changes to FeO. The reaction of FeO and Al2O3 in the low iron coal produces hercynite above 1000 ◦C because of the difference in the contents of Si and Al, while in the high iron coal, FeO reacts ◦ with SiO2 to generate augite (Fe2Si2O6). When the temperature increases to 1400 C, both hercynite and augite are converted to the thermodynamically-stable sekaninaite.

Keywords: underground coal gasiﬁcation; hyper-iron coal; coal ash; mineralogy

1. Introduction During high-temperature coal combustion and gasification at high temperatures, the reactivity differences of organic matters in coal can be almost ignored, while the transformation behavior of minerals becomes important to the stability of the process [1]. The reactions of inorganic minerals during combustion and gasification include a series of complicated physical and chemical changes that eventually form ash and slag with complex compositions [2]. Hence, detailed information about the mineralogical properties of coal ash is not only essential to optimize the operation parameters during coal utilization, but also significant for improving coal utilization efficiency and identifying the impacts of solid wastes on the environment. Underground coal gasification (UCG) is the process of in situ conversion of coal directly into combustible gaseous products. A sketch of the UCG process is shown in Figure1[ 1]. The first step of UCG is to choose a proper location and then design and construct an underground reactor [3]. Boreholes are drilled from the surface to the coal seam, followed by a horizontal channel connecting the boreholes along the bottom of the coal bed. After a gasifier is prepared, the coal at one end of the

Minerals 2018, 8, 90; doi:10.3390/min8030090 www.mdpi.com/journal/minerals Minerals 2018, 8, x FOR PEER REVIEW 2 of 20

MineralsBoreholes2018 are, 8, 90drilled from the surface to the coal seam, followed by a horizontal channel connecting2 of 20 the boreholes along the bottom of the coal bed. After a gasifier is prepared, the coal at one end of the channel is ignited, and gasification agents such as air, oxygen and steam are injected into the channelreactor. Accompanied is ignited, and by gasiﬁcation a series of agents coal reaction such ass air,including oxygen pyrolysis, and steam reduction are injected and into oxidation, the reactor. the Accompaniedcombustible gas by ais series discharged of coal reactions from the including production pyrolysis, borehole reduction [4]. andUCG oxidation, is carried the combustibleout in the gasunderground is discharged coal from bed thewithout production physical borehole coal mining, [4]. UCG transportation is carried out or in coal the undergroundpreparation, which coal bed is withoutregarded physical as supplementary coal mining, to transportation the coal mining or coal method. preparation, The composition which is regarded and heat as supplementary value of the toproduct the coal gas mining depends method. on the The initial composition gas injected, and heatthe position value of for the gas product injection gas dependsand the ontemperature the initial gasprofile injected, of the the coal position bed. for gas injection and the temperature proﬁle of the coal bed.

Figure 1. Sketch of the underground coal gasification (UCG) process. Figure 1. Sketch of the underground coal gasification (UCG) process. Because UCG is always performed within the coal seam, several hundred meters beneath the surface,Because only UCG the isinjection always performedand production within pa therameters coal seam, (temperature, several hundred pressure, meters flow) beneath can thebe surface,determined. only theIt is injection particularly and production difficult to parameters determine (temperature, the actual reaction pressure, conditions, flow) can beespecially determined. the Ittemperature is particularly field difficultdistribution to determine and thermal the equilibriu actual reactionm of the conditions, underground especially gasifier. the Therefore, temperature it is fieldnecessary distribution to investigate and thermal the equilibriumrelationship of betw the undergroundeen gasification gasifier. technology Therefore, and it ismineralogical necessary to investigatecharacteristics the relationshipof ash and slag between through gasification UCG simulation technology experiments and mineralogical [5]. On characteristicsthe other hand, of ashthe andpotential slag throughfor groundwater UCG simulation pollution experiments from UCG-generated [5]. On the otherresidue hand, has thealso potential been a concern for groundwater in recent pollutionyears [6]. fromThe leaching UCG-generated behavior residue of toxic has elements also been from a concern solid residues in recent is years closely [6]. related The leaching to the behaviorcharacteristics of toxic of UCG elements ash and from slag. solid residues is closely related to the characteristics of UCG ash and slag.Numerous research papers have been published on the conversion of minerals during coal combustion,Numerous but researchonly a few papers papers have have been reported published on the on thetransformation conversion ofof mineralsminerals duringduring coalcoal combustion,gasification [7,8]. but onlyMost a fewof the papers study have of ash reported chemistry on the during transformation the high-t ofemperature minerals duringgasification coal gasificationprocess has [focused7,8]. Most on ofthe the investigation study of ash of chemistry ash depo duringsition and the high-temperatureslag formation and gasification on the difficulties process hasfound focused in industrial on the investigation gasifiers regarding of ash deposition fluidized andbedslag gasification formation and and entrained on the difficulties flow gasification found in industrial[9,10]. However, gasifiers regardingfew papers fluidized focus bedon gasificationthe formation and entrainedmechanism flow of gasificationash and slag [9,10 ].during However, the fewUCG papers process focus [11]. on the formation mechanism of ash and slag during the UCG process [11]. Mineral transformation occurs at elevated temperatures, includingincluding chemical reactions betweenbetween thethe clayclay minerals,minerals, carbonatecarbonate minerals,minerals, pyrite and quartzquartz inin thethe coalcoal [[12–14].12–14]. The reaction raterate andand degree werewere greatlygreatly affected affected by by temperature. temperature. Generally, Generally, the the softening softening temperature temperature of mineralsof minerals in the in gasificationthe gasification condition condition was foundwas found to be to lower be lower than thatthan in that the combustionin the combustion condition condition [15]. The [15]. fusible The minerals,fusible minerals, such as thesuch carbonate, as the carbonate, sulfate minerals sulfate andminerals feldspar, and tend feldspar, to become tend ato “solvent becomemineral” a “solvent at highmineral” temperatures, at high temperatures, which promotes which the meltingpromotes and th slagginge melting of and gasification slagging residues of gasification [16]. In additionresidues to[16]. temperature, In addition the to furnace temperature, type is anotherthe furnace important type is factor another for ashimportant formation factor behaviors for ash [17 formation]. behaviorsThere [17]. are few reports on the formation and properties of UCG slag [18]. In the USA, a series of residuesThere and are rocks few wasreports sampled on the from formation the UCG and test properti site neares of Centralia, UCG slag Washington. [18]. In the X-rayUSA, diffractiona series of (XRD)residues and and a scanning rocks was electron sampled microscope from coupledthe UCG to antest electron site near microprobe Centralia, (SEM-EDS) Washington. were used X-ray to analyzediffraction the mineralogical(XRD) and a characteristicsscanning electron of the samples,microscope and coupled a moderate to temperaturean electron reactionmicroprobe was confirmed(SEM-EDS) [ 19were]. Reduced used to ironanalyze reacted the withmineralogical clay minerals characteristics to form a solidof the solution samples, of and aluminum-rich a moderate hercynitetemperature (FeAl reaction2O4), which was confirmed then serves [19]. as the Reduced precursor iron and reacted reacts with with clay SiO 2mineralsat high temperaturesto form a solid to form sekaninaite (Fe2Al4Si5O18)[20–23].

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Iron-bearing minerals have been the focus of many studies [24]. The iron element can reduce the melting point of ash and will largely control the ash slagging property. Fe2+ also reduces the viscosity of the aluminosilicate melt and accelerates the sintering process of the ash deposits. During the combustion process, the iron-bearing mineral is particularly easy to react with sulfur to form ash and sediment [25–27]. In this study, based on the ﬁrst UCG ﬁeld test of lignite, laboratory UCG simulation tests were performed to prepare UCG semi-coke (800 ◦C) and ash in different atmospheres. XRD and SEM-EDS were used to identify the composition and microstructure of the typical minerals formed and existing in UCG ash and slag, which help to understand the transformation of iron-bearing minerals during the UCG process [28].

2. Experiment and Modeling

2.1. Coal Sampling and Analysis Ulanqab and Ulankarma lignites are used in this study. The Ulanqab lignite is from a neighboring coal mine of the UCG field test area, which is located in Gonggou coal field, Ulanqab, Inner Mongolia, China. The coal bed has an average depth of 280 m. The Ulankarma lignite is taken from Shengli coal field, Inner Mongolia, China. The coal samples are collected according to the collection standard and then are crushed, ground and sieved before analysis. Proximate analysis and ultimate analysis were carried out based on ASTM Standards D3173-11 (2011) [29], D3175-11 (2011) [30] and D3174-11 (2011) [31]. The total sulfur content was analyzed following ASTM Standard D3177-02 (2002) [32]. A scanning wave-length dispersive X-ray fluorescence spectrometer (XRF; Thermo ARL Advant’XP+, Xenemetrix, Round Rock, TX, USA) was used to determine the major-element oxides (SiO2, TiO2, Al2O3, Fe2O3, MgO, CaO, MnO, Na2O, K2O and P2O5) in high-temperature ashes (815 ◦C) of the samples, which were prepared as pressed powder mounts. The loss on ignition (LOI) for each sample was also determined at this temperature.

2.2. Experimental Installation The multifunctional pyrolysis/gasification experimental system (HWD-500, Beijing Henven Scientific Instrument Factory, Beijing, China) is used to determine the reaction extent and residence time for coal pyrolysis, semi-coke reduction and residual-coke oxidation during the UCG process. We use the apparatus to complete independent pyrolysis and gasification experiments, accomplish temperature control, agent control and mass metrology. The highest work temperature is up to 1300 ◦C; the range of heating rate is 0.1–20 ◦C/min; and the mass measurement is 500 g. The schematic diagram is shown in Figure2. CO 2 and N2 are supplied by a high-pressure gas cylinder whose pressure and flow are controlled using a needle valve; the air is supplied by the air compressor; the plunger pump automatically inhales distilled water and then pumps the steam generator to produce steam. The pressure gauge is used to indicate intake agent flow. N2 is a protective gas, and CO2/H2O (g) is the gasification gas. The oxidizing agent enters into thermobalance and reacts with coal/char to form pyrolysis gas/coal gas. Thereafter, the gas is purified by a cold water bath and secondary tar absorption device and then dried by the dryer; finally, flow is measured through a wet-type gas flowmeter. The sampling port is used to collect the gas to measure the gas composition at different times. Minerals 2018, 8, 90 4 of 20 Minerals 2018, 8, x FOR PEER REVIEW 4 of 20 Minerals 2018, 8, x FOR PEER REVIEW 4 of 20

Figure 2. Multifunctional pyrolysis/gasification experimental system. 1, Cylinders; 2, needle valve; Figure 2. Multifunctional pyrolysis/gasification experimental system. 1, Cylinders; 2, needle valve; 3,Figure pressure 2. Multifunctional gauge; 4, air pyrolysis/gasificationcompressor; 5, wet-ty experimentalpe gas flowmeter; system. 1,6, Cylinders;sampling 2,port; needle 7, dryer;valve; 3, pressure gauge; 4, air compressor; 5, wet-type gas flowmeter; 6, sampling port; 7, dryer; 8, secondary 8,3, secondarypressure gauge;tar absorption 4, air compressor; device; 9, cold5, wet-ty waterpe bath; gas flowmeter;10, thermobalanc 6, samplinge; 11, plungerport; 7, pump;dryer; tar absorption12,8, steamsecondary generator. device; tar absorption 9, cold water device; bath; 9, 10, cold thermobalance; water bath; 10, 11, thermobalanc plunger pump;e; 11, 12, plunger steam generator.pump; 12, steam generator. Char/slagChar/slag preparation preparation in in different different reactionreaction cond conditionsitions isis conducted conducted using using a high-temperature a high-temperature Char/slag preparation in different reaction conditions is conducted using a high-temperature pipepipe furnace furnace (SK-G08163, (SK-G08163, Tianjin Tianjin Zhonghuan Zhonghuan FurnaceFurnace Corp., Corp., Tianjin, Tianjin, China), China), which which is a isbatch-type a batch-type high-temperaturepipe furnace (SK-G08163, tubular resistanceTianjin Zhonghuan furnace. The Furnace heating Corp., element Tianjin, is a China),silicon-molybdenum which is a batch-type bar; the high-temperature tubular resistance furnace. The heating element is a silicon-molybdenum bar; furnacehigh-temperature adopts a TY/PMFtubular resistanceceramic fiberfurnace. sheet; The and heating the furnace element tube is a silicon-molybdenumis corundum material. bar; The the the furnacefurnaceadopts adopts a TY/PMF ceramic ceramic fiber fiber sheet; sheet; and andthe furnace the furnace tube istube corundum is corundum material. material.The working temperature in the furnace is up to 1600 °C,◦ and temperature control is programmed with The workinganworking accuracy temperaturetemperature of ±1 °C. The in in the therange furnac furnace of pressuree is is up up to is to 1600 − 16000.10~0.05 °C,C, and andMPa. temper temperature A schematicature control controlof the is programmedhigh-temperature is programmed with with ◦ an accuracytubean accuracy furnace of ± 1ofis shown±1C. °C. The Thein range Figure range of 3.of pressure COpressure2 and is Nis−2 −are0.10~0.05 supplied MPa. MPa. by Aa Ahigh-pressureschematic schematic of the ofgas thehigh-temperature cylinder, high-temperature whose tubepressure furnacetube furnace and is shown flowis shown are in controlled Figurein Figure3. COby3. CO a2 needleand2 and N Nvalv22 arearee; suppliedthe air is suppliedby by a ahigh-pressure high-pressure by an air compressor; gas gas cylinder, cylinder, H whose2O (g) whose pressure and flow are controlled by a needle valve; the air is supplied by an air compressor; H2O (g) pressureis produced and flow by are a steam controlled generator. by a N needle2, CO2/H valve;2O (g), the and air air is enter supplied into thermobalance by an air compressor; and then Hreact2O (g) is is produced by a steam generator. N2, CO2/H2O (g), and air enter into thermobalance and then react producedwith coal/char by a steam to form generator. pyrolysis N2 ,gas/coal CO2/H gas.2O (g),Thereafter, and air the enter gas into is purified thermobalance by a tar absorption and then react with coal/char to form pyrolysis gas/coal gas. Thereafter, the gas is purified by a tar absorption with coal/chardevice and toa formwash pyrolysisbottle. After gas/coal the furnace gas. Thereafter,reaches room the temperature, gas is purified char by/slag a tar is absorptiontaken out for device device and a wash bottle. After the furnace reaches room temperature, char/slag is taken out for and aanalysis. wash bottle. After the furnace reaches room temperature, char/slag is taken out for analysis. analysis.

Figure 3. High-temperature tube furnace. 1, Cylinders; 2, needle valve; 3, pressure gauge; 4, air Figurecompressor;Figure 3. High-temperature3. High-temperature 5, water steam generator;tubetube furnace. 6, plunger 1, Cylinders; 1, Cylinders;pump; 2,7, needlehigh 2, temperature needle valve; 3, valve; pressure tube 3, furnace; pressuregauge; 8,4, tarair gauge; compressor; 5, water steam generator; 6, plunger pump; 7, high temperature tube furnace; 8, tar 4, airabsorber; compressor 9, gas; 5, bottle. water steam generator; 6, plunger pump; 7, high temperature tube furnace; absorber; 9, gas bottle. 8, tar absorber; 9, gas bottle.

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2.3. Preparation of Sequential Transformation Products

2.3.1. Determination of Reaction Conditions The purpose of this test is to conﬁrm the reaction conditions for the preparation of semi-coke, ash under reduced atmosphere and ash under oxidized atmosphere. Speciﬁcally, the aim of the part is to clarify the reaction time and reaction extent of pyrolysis, reduction and the oxidation process. According to the mechanism of UCG and practical reaction condition, assumptions were made as follows:

(1) Methane is derived from the coal pyrolysis process. (2) The final gas consists of pyrolysis gas and gasification gas. (3) The highest temperature of the pyrolysis reaction was set as 800 ◦C. (4) The temperature of the oxidation zone was 200 ◦C higher than that of corresponding gasification zone.

Based on the above assumptions, the multifunctional pyrolysis/gasiﬁcation experimental system was used for determining the reaction condition (mainly reaction time) of pyrolysis, reduction and the oxidation process.

(1) Coal samples were placed in a multifunctional pyrolysis/gasiﬁcation experimental system with ◦ N2 (2 L/min) protective atmosphere from room temperature to 800 C at a heating rate of 5 ◦C/min and constant temperature for 30 min. The release of pyrolysis products ﬁnished, and the thermobalance did not continue to lose weight.

(2) With the reduction agent of H2O (g) (5 g/min) and CO2 (2 L/min), the completion times (t2) of the reduction process from 900–1300 ◦C were 55 min, 40 min, 25 min, 15 min and 20 min, respectively. Among these, the completion time of the 1300 ◦C gasiﬁcation process was 20 min, which was longer than that of the 1200 ◦C gasiﬁcation process. The reason was that the melting degree of slag at 1300 ◦C was stronger than that at 1200 ◦C, and the melting slag partially coated unburned carbon and then hindered the reaction of the reduction agent with carbon.

(3) With the oxidation agent of air, the completion time (t3) of the oxidation process from 1100–1500 ◦C was about 60 min. Due to the low oxygen concentration of air, the oxidation process at different temperatures had little difference. Thus, the reaction time of each oxidation ﬁnal temperature (t3) was 60 min.

2.3.2. Preparation of Semi-Coke/Ash/Slag According to the atmosphere and ﬂow rate in the conversion conditions, as well as the residence time corresponding to different ﬁnal temperature conditions, we used the high-temperature tube furnace to prepare semi-coke (800 ◦C), ash under reduced atmosphere (900–1300 ◦C) and ash under oxidized atmosphere (1100–1500 ◦C). The product should be cooled to room temperature for analysis and characterization.

2.4. Sample Analysis

2.4.1. XRD Analysis The ash samples were crushed, ground and sieved. Samples below 75 µm were selected for XRD analysis, which was performed on a powder diffractometer (D/max-2500/pc XRD, Rigaku, Japan) with Ni-ﬁltered Cu-Kα radiation and a scintillation detector. The XRD pattern was recorded over a 2θ range of 2◦–90◦ at a scan rate of 8◦/min and a step size of 0.02◦. Jade 5.0 (MDI, Livermore, CA, USA) software was used to analyze the XRD curve for qualitative analysis [33]. Minerals 2018, 8, 90 6 of 20

2.4.2. SEM-EDS Analysis The raw coal and char and slag samples were ﬁrstly crushed, ground and sieved. The particles below 75 µm were collected for SEM-EDS analysis. A ﬁeld emission scanning electron microscope (FE-SEM, MERLIN Compact, Zeiss, Jena, Germany) and an energy dispersive X-ray spectrometer (EDS, INCA, Oxford, UK) were used to study the morphology of the minerals and to determine the distribution of minerals [34]. The samples were prepared under low-vacuum SEM conditions. The analytical conditions were as follows: working distance 9.2~12.4 mm, beam voltage 15.0 kV. The images were captured via a secondary electron detector.

2.4.3. Thermodynamic Modeling FactSage was introduced in 2001 as the fusion of two well-known software packages in the field of computational thermochemistry: F*A*C*T/FACT-Win and ChemSage. The thermochemistry models can be used to analyze equilibrium conditions for reactions occurring between inorganic and/or organic materials, as well as providing insight into the mineral formation speciation. The database can assist in understanding, as well as predicting what can and will happen with specific coal and mineral sources inside the gasification process [35]. In this study, thermodynamic equilibrium modeling was conducted using the “Equilib” and “Phase diagram” module in FactSage 7.1, which is the Gibbs energy minimization workhorse of FactSage. It calculates the concentrations of chemical species when specific elements or compounds reacting or partially reacting to reach a state of chemical equilibrium. FactSage is used to calculate the phase diagram of Al2O3–SiO2–FeO and Al2O3–SiO2–Fe2O3 for analyzing the effect of ash composition on final mineral composition and transformation. For the calculations, the equilibrium module was employed together with the databases Ftoxid and FactPs. Additionally, the solution phases of Ftoxid-SLAG and Ftoxid-oPyr (databases) were selected to simulate the gasification. During the reduction stage, the temperature was changed from 900–1300 ◦C in 50 ◦C increments; the total pressure is 1 atm; and the partial pressure of (CO, H2) is 0.1 atm. The iron oxide is input in the form of FeO. In the oxidation process, the temperature varies from 1100–1500 ◦C in 50 ◦C increments; the total pressure is 1 atm; and the partial pressure of O2 is 0.21 atm. The iron oxide is input in the form of Fe2O3.

3. Results and Discussion

3.1. Coal Analysis The results of the ultimate and proximate analyses of the coal samples are shown in Table1. 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%.

Table 1. Proximate and ultimate analysis of coals (%).

Proximate Analysis/% Ultimate Analysis/% Samples Mad Aad Vad FCad Cad Had Oad Nad St,ad Ulanqab 9.22 32.62 34.00 33.38 45.22 3.16 8.70 0.64 0.44 Ulankarma 17.90 7.81 28.60 45.69 54.33 2.42 14.06 0.70 3.39 M, moisture; A, ash; V, volatile matter; FC, ﬁxed carbon; C, carbon; H, hydrogen; O, oxygen; N, nitrogen; St, total sulfur; ad, air dry basis.

The percentages of major element oxides, as well as loss on ignition, for the Ulanqab and Ulankarma lignites are listed in Table2. Minerals 2018, 8, 90 7 of 20

Table 2. Percentages of major element oxides (%), and loss on ignition (LOI, %), and base to acid ratio (B/A).

Samples SiO2 Al2O3 Fe2O3 CaO TiO2 MgO K2O Na2O MnO2 SO3 P2O5 LOI B/A Ulanqab 61.55 16.58 5.10 5.59 0.75 2.68 2.50 0.85 0.07 2.46 0.11 1.72 0.22 Ulankarma 30.91 8.98 38.43 6.58 0.53 1.57 0.53 0.50 0.04 6.75 0.05 5.48 1.18

The two lignites have substantially different ash components. The content of Fe2O3 in Ulankarma lignite is very high, as shown in Table2, which is a typical hyper-iron coal [ 36] and has high sulfur content in this case. The total of SiO2 and Al2O3 content in Ulanqab lignite is about 80%, while that in Ulankarma lignite is less than 40%. In the Ulankarma lignite, low content of SiO2 and Al2O3, as well as high content of Fe2O3 and CaO result in lower ash melting points. The analysis results of the coal ash fusibility in a weakly-reducing atmosphere are exhibited in Table3[ 37]. It can be seen from Table3 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.

Table 3. Coal ash fusibility (◦C) in a weakly-reducing atmosphere.

Ash Melting Point Ulanqab Ulankarma Deformation Temperature (DT) 1160 1060 Softening Temperature (ST) 1210 1060 Hemispherical Temperature (HT) 1230 1070 Fluid Temperature (FT) 1260 1080

3.2. Mineral Composition in Raw Coals X-ray diffraction analysis was used to identify the typical minerals and mineral transformation behavior that occurs during the UCG process. The XRD analysis showed that the major minerals found in the Ulanqab lignite consist of quartz (SiO2), muscovite (KAl2Si3AlO10(OH)2) and orthoclase (KAlSi3O8). Quartz is the most common mineral in coal [38,39]. Clay minerals are the most important components in coal, and in many cases, they are intimately associated with organic matter in coal [40,41]. The main minerals in Ulanqab lignite are in good agreement with the content of SiO2 and Al2O3 in the ash. The main minerals found in the Ulankarma lignite are quartz, kaolinite (Al2Si2O5(OH)4), gypsum (CaSO4·2H2O) and pyrite (FeS2). Kaolinite is also the main component of the clay mineral. Gypsum and pyrite are consistent with the high content of CaO and Fe2O3, respectively, in the Ulankarma lignite. High contents of gypsum and pyrite have also been observed in the lignites in the adjacent coal deposit, Shengli, in Inner Mongolia [42,43].

3.3. Minerals Transformation during Coal Pyrolysis As shown in Figure4a, 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. Minerals 2018, 8, 90 8 of 20

For example, sanidine is a high-temperature mineral in altered volcanic ashes (tonsteins) and usually used as an indication of volcanic ash [45,46]. Minerals 2018, 8, x FOR PEER REVIEW 8 of 20

Q Pyrolysis semi-coke M M S M Q

Q Intensity

Q Coal M M O Q

0 1020304050607080 2θ (°)

(a)

pyrolysis semi-coke Q K Q O Q

Intensity Q

Q Q Coal G K P P

0 20406080 2θ (°)

(b)

Figure 4. XRD patterns of coal and semi-coke (800 °C). (a) Ulanqab lignite; (b) Ulankarma lignite. Figure 4. XRD patterns of coal and semi-coke (800 ◦C). (a) Ulanqab lignite; (b) Ulankarma lignite. Q—quartz; M—muscovite; S—sanidine; O—orthoclase; K—kaolinite; G—gypsum; P—pyrite. Q—quartz; M—muscovite; S—sanidine; O—orthoclase; K—kaolinite; G—gypsum; P—pyrite. As shown in Figure 4b, the main minerals in the semi-coke of Ulankarma lignite (800 °C) consistAs shown of quartz, in Figure kaolinite4b, the and main orthoclase. minerals Kaolinite in the semi-coke starts to lose of Ulankarma crystal water lignite and is (800 converted◦C) consist to of quartz,metakaolinite kaolinite and above orthoclase. 327 °C [9], Kaolinite and then starts, the metakaolinite to lose crystal will water decompose and is converted into γ-Al2O to3 and metakaolinite SiO2 around◦ 827 °C [47]. The decomposition products of kaolinite can react with K2O to produce◦ above 327 C[9], and then, the metakaolinite will decompose into γ-Al2O3 and SiO2 around 827 C[47]. orthoclase in the temperature range of 500–800 °C [48]. Therefore, it can be concluded that kaolinite The decomposition products of kaolinite can react with K2O to produce orthoclase in the temperature is gradually decomposed◦ during the pyrolysis process and is partially converted to orthoclase, but a rangesmall of 500–800 amount ofC[ kaolinite48]. Therefore, could still it canbe found be concluded in the semi-coke that kaolinite because is the gradually pyrolysis decomposed temperature is during the pyrolysisrelatively processlow. and is partially converted to orthoclase, but a small amount of kaolinite could still be foundAs shown in the in semi-coke the SEM image because in Figure the pyrolysis 5a, the materi temperatureal attached is to relatively the carbon low. particles is pyrite, whichAs shown is consistent in the SEM with imagethe XRD in results. Figure 5Aftera, the 800 material °C pyrolysis, attached carbon to the content carbon in particlesthe semi-coke is pyrite, whichrises is consistentbecause of withthe emission the XRD of results. the volatile After organic 800 ◦C matters. pyrolysis, Fe carbonand S elements content inare the found semi-coke on the rises becausecarbon of theparticles emission in the ofsemi-coke the volatile (Figure organic 5b), which matters. is apparently Fe and Spresent elements as pyrrhotite are found (Fe on1−xS) the (x carbon=

Minerals 2018, 8, 90 9 of 20

particlesMinerals in the2018,semi-coke 8, x FOR PEER (Figure REVIEW5 b), which is apparently present as pyrrhotite (Fe 1−xS) (x =9 of 0–0.233) 20 based on the analysis of the atomic ratio. The atomic ratio of Fe element to S element is 0.73:1 in 0–0.233) based on the analysis of the atomic ratio. The atomic ratio of Fe element to S element is Ulankarma semi-coke, which lies in the range of 0–0.233 [36]. This suggests that FeS2 (in the coal) is gradually0.73:1decomposed in Ulankarma when semi-coke, the pyrolysis which lies temperature in the range isof higher0–0.233 than[36]. This 500 ◦ suggestsC and S that escapes FeS2 in(inthe the form coal) is gradually decomposed when the pyrolysis temperature◦ is higher than 500 °C and S escapes of S2 (g). When the pyrolysis temperature reaches 800 C, S escapes at a higher rate, and pyrrhotite is in the form of S2 (g). When the pyrolysis temperature reaches 800 °C, S escapes at a higher rate, and eventually formed with a molecular formula of Fe0.73S[8]. pyrrhotite is eventually formed with a molecular formula of Fe0.73S [8].

(a)

(b)

Figure 5. Iron-bearing minerals in Ulankarma lignite and its semi-coke (SEM-EDS secondary Figure 5. Iron-bearing minerals in Ulankarma lignite and its semi-coke (SEM-EDS secondary electron images). (a) Pyrite; (b) pyrrhotite. electron images). (a) Pyrite; (b) pyrrhotite. 3.4. Mineral Transformation during the Semi-Coke Reduction Process 3.4. Mineral Transformation during the Semi-Coke Reduction Process Based on the results of XRD analysis in Figure 6a, the iron-containing mineral is present in the Basedform of on magnetite the results (Fe3O of4) XRDin the analysis 900 °C reduction in Figure ash,6a, which the iron-containing is converted from mineral pyrrhotite. is present Sanidine in the ◦ formis of also magnetite present in (Fe the3O ash,4) in but the muscovite 900 C reduction is not present ash, in which the pyrolysis is converted semi-coke. from The pyrrhotite. reason for Sanidine the is alsoabsence present of inthe the muscovite ash, but in muscovite the semi-coke is not is the present gradual in theremoval pyrolysis of the semi-coke.hydroxyl group The reasonunder the for the absenceaction of theof high muscovite temperature. in the As semi-coke the temperature is the gradualrises above removal 900 °C, of the the demineralized hydroxyl group muscovite under the reacts with CaO or FeO to produce orthoclase by the following reactions. action of high temperature. As the temperature rises above 900 ◦C, the demineralized muscovite reacts with CaO or FeO to produce orthoclaseKAl3Si3O by10 + theCaO following → KalSi3O reactions.8 + CaO·Al2O3

Kal3Si3O10 + FeO → KalSi3O8 + FeO·Al2O3 KAl3Si3O10 + CaO → KalSi3O8 + CaO·Al2O3 Magnetite is altered, while hercynite is formed in great quantities in the reduction ash at 1000 °C. It can be concludedKal3Si that3O10 the+ FeOmagnetite→ KalSi in 3theO8 +reduction FeO·Al 2ashO3 of Ulanqab is gradually transformed to FeO at a higher temperature and stronger reducing atmosphere. Then, FeO reacts ◦ Magnetitewith Al2O3 isto altered, produce while hercynite hercynite at the is 1000 formed °C ingasification great quantities conditions. in the When reduction the temperature ash at 1000 C. It canreaches be concluded 1100 °C, the that diffraction the magnetite peak intensity in the of reduction hercynite ashis further of Ulanqab enhanced, is graduallywhich indicates transformed that to FeOits atcontent a higher increases temperature with temperature. and stronger Thereafter, reducing at 1200 atmosphere. °C, the diffraction Then, peak FeO intensity reacts with of the Al 2O3 to producehercynite hercynite declines, at indicating the 1000 that◦C gasiﬁcationthe content begins conditions. to decrease. When Until the 1300 temperature °C, hercynite reaches minerals1100 ◦C, the diffractiondecrease inpeak abundance intensity ofand hercynite are accompanied is further enhanced, by the whichemergence indicates of sekaninaite. that its content As increasesthe withthermodynamic temperature. Thereafter,properties of at sekaninaite 1200 ◦C, theare diffractionmore stable, peak hercynite intensity is converted of thehercynite to sekaninaite 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 Minerals 2018, 8, 90 10 of 20 are moreMinerals stable, 2018, 8, x hercynite FOR PEER REVIEW is converted to sekaninaite above 1300 ◦C. The result is completely10 of 20 in accordance with the report by McCarthy et al. (1988) [19], which proposes the medium temperature above 1300 °C. The result is completely in accordance with the report by McCarthy et al. (1988) [19], reaction in the study of slag from the UCG ﬁeld test. which proposes the medium temperature reaction in the study of slag from the UCG field test.

Q S Ma Q 900 ℃ Q

Q An H Q 1000 ℃ Q

Q Intensity An H H Q 1100 ℃ Q

Q Mu AnH Mu H Q 1200 ℃ Q Q An SeMu Q 1300 ℃

0 20406080 2θ(°)

(a) Q

Q Ma Ma 900 ℃ Q

Q Ma Ma 1000 ℃

Q Si Ma Ma 1100 ℃ Intensity Q

Q Si Au 1200 ℃

Q Se Se Mu Se Mu Mu 1300 ℃

0 20406080 2θ(°)

(b)

Figure 6. XRD patterns of the ash under reduced atmosphere (900–1300 °C). (a) Ulanqab lignite; Figure 6. XRD patterns of the ash under reduced atmosphere (900–1300 ◦C). (a) Ulanqab lignite; (b) Ulankarma lignite. Q—quartz; Se—sekaninaite; Ma—magnetite; H—hercynite; An—anorthite; (b) UlankarmaMu—mullite; lignite. S—sanidine. Q—quartz; Si—sillimanite; Se—sekaninaite; Au—augite; Ma—magnetite; Se—sekaninaite. H—hercynite; An—anorthite; Mu—mullite; S—sanidine. Si—sillimanite; Au—augite; Se—sekaninaite.

Anorthite (CaAl2Si2O8) 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. Minerals 2018, 8, 90 11 of 20

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 Al2O3 and SiO2 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 [8,20,37,42].

CaO + Al2O3 + 2SiO2 → CaAl2Si2O8 As shown in the XRD image in Figure6b. 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 (Fe2Si2O6) 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 Al2O3 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 Al2O3 and SiO2, which are respectively decomposition products of orthoclase and kaolinite in the semi-coke, n to produce sillimanite. As the temperature increases, sillimanite is converted to mullite, n-stable mineral [51]. The reaction process is as follows:

Al2O3 + SiO2 → Al2SiO5

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 (Figure7a). Therefore, augite and hercynite ◦ coexist in the reduction ash at 1000 C, but augite content is low. The content of Al2O3 and SiO2 in the ash of Ulanqab lignite is high, but the FeO from the reduction of magnetite tends to react with Al2O3 to form hercynite. Only a small amount of FeO reacts with SiO2 to generate augite. However, the Al2O3 in the Ulankarma ash is very low, and it is involved in the formation of the sillimanite and mullite, resulting in the Al2O3 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 Figure7b are present in the ash reduction of Ulanqab lignite at 1300 ◦C. As shown in the SEM image in Figure8, the augite mineral phase appeared at 1200 ◦C in the Ulankarma reduction ash, and the sekaninaite phase is present at 1300 ◦C. Minerals 2018, 8, 90 12 of 20

Minerals 2018, 8, x FOR PEER REVIEW 12 of 20 Minerals 2018, 8, x FOR PEER REVIEW 12 of 20

(a) (a)

(b)

Figure 7. Minerals in reduction ash of Ulanqab(b )lignite (SEM-EDS, secondary electron images). Figure 7. Minerals in reduction ash of Ulanqab lignite (SEM-EDS, secondary electron images). Figure(a) Augite 7. Minerals(1000 °C); in(b )reduction sekaninaite ash (1300 of Ulanqab°C). lignite (SEM-EDS, secondary electron images). (a) Augite (1000 ◦C); (b) sekaninaite (1300 ◦C). (a) Augite (1000 °C); (b) sekaninaite (1300 °C).

(a) (a)

(b)

Figure 8. Minerals in reduction ash of Ulankarma(b) lignite (SEM-EDS, secondary electron images). Figure(a) Augite 8. Minerals (1000 °C); in (b reduction) sekaninaite ash (1300 of Ulankarma °C). lignite (SEM-EDS, secondary electron images). Figure 8. Minerals in reduction ash of Ulankarma lignite (SEM-EDS, secondary electron images). (a) Augite (1000 °C); (b) sekaninaite (1300 °C). ( a) Augite (1000 ◦C); (b) sekaninaite (1300 ◦C).

Minerals 2018, 8, 90 13 of 20

3.5. Mineral Transformation during Residual-Coke Oxidation Process Minerals 2018, 8, x FOR PEER REVIEW 13 of 20 The XRD results of the oxidation ash of Ulanqab lignite are similar to the reduction ash in the mineral3.5. compositionMineral Transformation (Figure 9duringa). Hercynite Residual-Coke and Oxidation hematite Process are present in the 1100 ◦C oxidation ash at ◦ ◦ the sameThe time. XRD Hercynite results of decreases the oxidation at 1200 ash ofC, Ulanqab and then, lignite trace are amounts similar areto the observed reduction at ash 1300 in theC when massivemineral hematite composition is formed (Figure in the 9a). oxidation Hercynite ash.and Ahematite possible are reason present for in thisthe 1100 change °C oxidation could be ash explained at as follows:the same Fe 2+time.existing Hercynite in hercynite decreases reacts at 1200 with °C, theandoxidizing then, trace agent, amounts generating are observed Fe3+ atthat 1300 is present°C in thewhen form massive of hematite hematite at 1100is formed◦C, thenin the the oxidation Fe2+ is ash. continuously A possible convertedreason for this to hematitechange could and be finally 2+ 3+ completelyexplained transformed as follows: intoFe existing hematite in athercynite 1300 ◦C. reacts When with the the temperature oxidizing agent, reaches generating1400 ◦ CFe, the that hematite is present in the form of hematite at 1100 °C, then the Fe2+ is continuously converted to hematite and is still present. Hematite is eventually transformed into thermodynamically-stable sekaninaite at finally completely transformed into hematite at 1300 °C. When the temperature reaches 1400 °C, the highhematite temperatures. is still present. Hematite is eventually transformed into thermodynamically-stable ◦ Thesekaninaite content at ofhigh hematite temperatures. is reduced when the temperature reaches 1200 C, although it should continueThe to rise content theoretically of hematite with is reduced the transformation when the temperature of hercynite. reaches On 1200 the °C, other although hand, it anorthiteshould is generatedcontinue at1100 to rise◦C, theoretically but the anorthite with the content transformati beginson to of decrease hercynite. at 1200On the◦C. other Finally, hand, anorthite anorthite is is absent at 1300generated◦C. Therefore, at 1100 °C, it isbut inferred the anorthite that duringcontent thebegins oxidation to decrease of Ulanqabat 1200 °C. lignite, Finally, low-temperature anorthite is mixingabsent occurs at between1300 °C. the Therefore, iron-bearing it is minerals inferred andthat anorthite during whenthe oxidation the temperature of Ulanqab reaches lignite, 1200 ◦C, whichlow-temperature leads to the decrease mixingof occurs ironcontent between and the the ir absenceon-bearing of anorthite.minerals and anorthite when the Thetemperature content ofreaches quartz 1200 decreases °C, which attemperatures leads to the decrease ranging fromof iron 1100–1300 content and◦C, sincethe absence it participates of anorthite. in the formation of minerals such as mullite and anorthite. When the temperature reaches 1400 ◦C, The content of quartz decreases at temperatures ranging from 1100–1300 °C, since it the quartzparticipatescontent in the is furtherformation reduced, of minerals while such the as cristobalite mullite and is anorthite. formed in When great the quantities. temperature This is becausereaches the quartz1400 °C, could the changequartz content to form is cristobalite further redu atced, high while temperatures the cristobalite [52]. The is formed chemical in propertiesgreat ◦ of thequantities. cristobalite This whose is because melting the quartz point could is 1610 changeC are to form extremely cristobalite stable, at high so it temperatures is still present [52]. in the oxidationThe chemical ash at 1500 properties◦C. of the cristobalite whose melting point is 1610 °C are extremely stable, so it Theis still intensity present in of the the oxidation XRD diffraction ash at 1500 peak °C. of the oxidation ash is weak at 1400 ◦C, indicating that the contentThe of intensity the crystal of the mineral XRD diffraction is very low. peak Especially of the oxidation at 1500 ◦ashC, theis weak crystal at 1400 minerals °C, indicating in the ash are almostthat completely the content converted of the crystal to themineral vitreous is very material, low. Especially which isat called1500 °C, vitrification. the crystal minerals This phenomenon in the ash are almost completely converted to the vitreous material, which is called vitrification. This has also occurred in many ash-related studies. The vitrified ash is very stable, has no risk of leaching phenomenon has also occurred in many ash-related studies. The vitrified ash is very stable, has no contamination and can be safely landfilled or used as a building material [53,54]. risk of leaching contamination and can be safely landfilled or used as a building material [53,54].

Q 1100 ℃ An H Q H He Q

Q 1200 ℃ Mu An HMuHe H Q

Intensity Q

Q Mu He Mu Mu Q 1300 ℃ Q C Mu He Mu Mu 1400 ℃ Se C Mu Mu 1500 ℃

0 20406080 θ(°) 2 (a)

Figure 9. Cont.

Minerals 2018, 8, 90 14 of 20 Minerals 2018, 8, x FOR PEER REVIEW 14 of 20

Q Minerals 2018, 8, x FOR PEER REVIEW 14 of 20 He Q He Si He 1100 ℃ Q Si He He SiQ HeHe He 1100 ℃ Q He Si C Si Q Si 1200 ℃ Q HeHe He CC SiQ Si 1200 ℃ Intensity Se QMu Se 1300 ℃

C C QQ Intensity MuSe Mu 14001300 ℃℃ Mu SeSe

C SeQ MuMu CMuMu 1400 ℃ Se 1500 ℃

Se Mu CMu 0 20406080Se 1500 ℃ 2θ(°)

0 20406080(b)

Figure 9. XRD patterns of the ash under oxidized atmosphere2θ(°) (1100–1500 °C). (a◦) Ulanqab lignite; (b) Figure 9. XRD patterns of the ash under oxidized atmosphere (1100–1500 C). (a) Ulanqab lignite; Ulankarma lignite. Q—quartz; An—anorthite; He—hematite; H—hercynite; Se—sekaninaite; (b) Ulankarma lignite. Q—quartz; An—anorthite;(b) He—hematite; H—hercynite; Se—sekaninaite; C—cristobalite; Si—sillimanite; Mu—mullite. C—cristobalite;Figure 9. XRD Si—sillimanite; patterns of the Mu—mullite. ash under oxidized atmosphere (1100–1500 °C). (a) Ulanqab lignite; (b) TheUlankarma XRD results lignite. of Q—quartz;the oxidation An—anorthite; ash of Ulankarma He—hematite; lignite H—hercynite;(Figure 9b) showSe—sekaninaite; that hematite couldThe XRDC—cristobalite; be found results at of Si—sillimanite;the the temperatures oxidation Mu—mullite. ash ranging of Ulankarma from 1100–1200 lignite (Figure°C. The9 b)magnetite show that is oxidized hematite to could be foundhematite at the in temperaturesan oxidizing atmosphere, ranging from and 1100–1200 then, hematite◦C. The is transformed magnetite isto oxidizedsekaninaite to hematitewhen the in an The XRD results of the oxidation ash of Ulankarma lignite (Figure 9b) show that hematite oxidizingtemperature atmosphere, is over and 1300 then, °C. hematite As the temperature is transformed reaches to sekaninaite above 1400 when °C, the temperaturecontent of the is over sekaninaitecould be found is still athigh. the This temperatures is due to the ranging high melting from 1100–1200point of sekaninaite. °C. The magnetite is oxidized to 1300 ◦C. As the temperature reaches above 1400 ◦C, the content of the sekaninaite is still high. This is hematiteSEM inimages an oxidizing of 1100 atmosphere,°C ash and 1300and then,°C ash he arematite shown is transformed in Figure 10a, to sekaninaiteb, respectively. when The the due totemperature the high melting is over point 1300 of°C. sekaninaite. As the temperature reaches above 1400 °C, the content of the iron-bearing mineral is present◦ in the form of◦ hematite at 1100 °C oxidation ash, which is in agreementSEMsekaninaite images with is still ofthe high. 1100XRD Thisanalysis.C is ash due The and to EDSthe 1300 high analysis meltingC re ashsults point are of the shownof sekaninaite. mineral in in Figure Fi gure 10b10a, show b, respectively.that the ◦ The iron-bearingcontainedSEM elements imagesmineral ofare 1100 complex, is present °C ash including inand the 1300 Si, form Al,°C Feash of, Ca, hematiteare K shownand Mg, at in1100 and Figure theC content10a, oxidation b, ofrespectively. each ash, element which The is is in agreementhigh.iron-bearing Thus, with we themineral can XRD speculate is analysis. present that Thein the the EDSmineral form analysis ofis ash.hematite resultsWhen at the of1100 thetemperature °C mineral oxidation inreaches Figure ash, 1300 which 10 b°C, show isthe in that the containedcrystalagreement minerals elements with begin the XRD areto melt analysis. complex, largely The due including EDS to theanalysis ash Si, melting re Al,sults Fe, pointof Ca, the of Kmineral Ulankarma and Mg,in Figure andbeing 10b the 1060 show content °C. that ofthe each elementcontained is high. elements Thus, we are can complex, speculate including that theSi, Al, mineral Fe, Ca,is K ash.and Mg, When and the the temperature content of each reaches element 1300 is ◦C, high. Thus, we can speculate that the mineral is ash. When the temperature reaches 1300 °C, the the crystal minerals begin to melt largely due to the ash melting point of Ulankarma being 1060 ◦C. crystal minerals begin to melt largely due to the ash melting point of Ulankarma being 1060 °C.

(a)

Figure 10. Cont.

Minerals 2018, 8, 90 15 of 20

Minerals 2018, 8, x FOR PEER REVIEW 15 of 20

(b)

Figure 10. Minerals in oxidation ash of Ulankarma coal (SEM-EDS, secondary electron images). Figure 10. Minerals in oxidation ash of Ulankarma coal (SEM-EDS, secondary electron images). (a) 1100 °C; (b) 1300 °C. (b) (a) 1100 ◦C; (b) 1300 ◦C. Figure 10. Minerals in oxidation ash of Ulankarma coal (SEM-EDS, secondary electron images). As(a) 1100shown °C; in(b )the 1300 SEM °C. image in Figure 11a, the hercynite appears in 1100 °C oxidation ash of UlanqabAs shown lignite, in the which SEM imageis consistent in Figure with 11 a,th thee XRD hercynite analysis. appears Figure in 11b 1100 shows◦C oxidation that the ash of Ulanqablow-temperature lignite,As shown which in co-melt the is consistentSEM of imageiron-bearing with in Figure the minerals XRD 11a, analysis.the and hercynite calcium Figure appearsis formed11b shows in when1100 that °Cthe theoxidation temperature low-temperature ash isof co-meltbelowUlanqab of iron-bearingthe lignite,ash melting which minerals point, is andconsistentwhich calcium in turnwith is leads formedthe toXRD whenthe decreaseanalysis. the temperature ofFigure crystal 11b minerals is belowshows in the that1200 ash the°C melting point,oxidationlow-temperature which inash turn of co-meltUlanqab leads to of lignite.the iron-bearing decrease Sekaninaite minerals of crystal is stablyand minerals calcium present inis informed 1200 1400◦ C°Cwhen oxidation since the calciumtemperature ash ofis Ulanqabnot is presentbelow thein oxidation ash melting ash ofpoint, Ulankarma which inlignite. turn leads to the decrease of crystal minerals in 1200 °C lignite. Sekaninaite is stably present in 1400 ◦C since calcium is not present in oxidation ash of oxidation ash of Ulanqab lignite. Sekaninaite is stably present in 1400 °C since calcium is not Ulankarma lignite. present in oxidation ash of Ulankarma lignite.

(a)

(b)

Figure 11. Minerals in oxidation ash of Ulanqab (SEM-EDS, secondary electron images). (a) Hercynite (1100 °C); (b) iron-bearing minerals and(b calcium) feldspar co-melt at 1200 °C. Figure 11. Minerals in oxidation ash of Ulanqab (SEM-EDS, secondary electron images). (a) FigureThe 11. sequentialMinerals intran oxidationsformations ash of of Ulanqab iron-bearing (SEM-EDS, minerals secondary with electronincreasing images). temperature (a) Hercynite are Hercynite◦ (1100 °C); (b) iron-bearing minerals and calcium feldspar co-melt◦ at 1200 °C. displayed(1100 C); (inb )Figure iron-bearing 12. Using minerals XRD andand calciumSEM analyses, feldspar we co-melt can summarize at 1200 C. that the composition of ash leadsThe sequentialto the formation transformations of typical iron-bearingof iron-bearing minerals minerals and withaffects increasing the transformation temperature of theare Thedisplayed sequential in Figure transformations 12. Using XRD and of SEM iron-bearing analyses, we minerals can summarize with increasing that the composition temperature of are displayedash leads in Figureto the formation12. Using of XRDtypical and iron-bearing SEM analyses, minerals we and can affects summarize the transformation that the composition of the of ash leads to the formation of typical iron-bearing minerals and affects the transformation of the Minerals 2018, 8, 90 16 of 20

Minerals 2018, 8, x FOR PEER REVIEW 16 of 20 iron-bearing minerals. The transformation behavior of iron-bearing minerals in the two lignites is similariron-bearing during the minerals. pyrolysis The process, transformation in which behavior pyrite (FeS of iron-bearing2) in the raw minerals coal is in gradually the two lignites desulfurized is into pyrrhotitesimilar during (Fe 1the−x S).pyrolysis In the process, reduction in which stage, pyrite pyrrhotite (FeS2) in is the transformed raw coal is intogradually magnetite desulfurized (Fe3O 4) at ◦ 900 Cintoand pyrrhotite then changes (Fe1−xS). to In FeO. the reduction Due to the stage, difference pyrrhotite in is the transformed content of into Si andmagnetite Al, the (Fe reaction3O4) at of 900 °C and then changes to FeO. Due to the difference in the content of Si and◦ Al, the reaction of FeO FeO and Al2O3 in the Ulanqab lignite produces hercynite above 1000 C, while in the Ulankarma and Al2O3 in the Ulanqab lignite produces hercynite above 1000 °C, while in the Ulankarma lignite, ◦ lignite, FeO reacts with SiO2 to generate augite (Fe2Si2O6). When the temperature rises to 1400 C, FeO reacts with SiO2 to generate augite (Fe2Si2O6). When the temperature rises to 1400 °C, both both hercynite and augite are converted to the thermodynamically-stable mineral sekaninaite. hercynite and augite are converted to the thermodynamically-stable mineral sekaninaite.

(a) (b)

Figure 12. Sequential transformation of iron-bearing minerals in UCG process. (a) Ulankarma lignite; Figure 12. Sequential transformation of iron-bearing minerals in UCG process. (a) Ulankarma lignite; (b) Ulanqab lignite. (b) Ulanqab lignite. 3.6. Thermodynamic Simulation of the Transformation of Iron-Bearing Minerals during Underground Coal 3.6. ThermodynamicGasification Simulation of the Transformation of Iron-Bearing Minerals during Underground Coal Gasiﬁcation As shown in Figure 13, when the total of SiO2 and Al2O3 content is about 0.8, it is easy to form Asaluminosilicate shown in Figure such as 13 mullite., when Quartz the total is formed of SiO in2 greatand Alquantities,2O3 content while is the about cristobalite 0.8, it is is less. easy to formThe aluminosilicate formation temperature such as mullite. of cristobalite Quartz is is higher formed than in great1400 quantities,°C. When the while total the of cristobaliteiron and is less. Thealuminum formation content temperature is about 0.7, of hercynite cristobalite is readil is highery formed. than Augite 1400 is◦C. observed When thewhen total the oftotal iron of and aluminumiron and content silicon is content about is 0.7, above hercynite 0.6. As isthe readily temperature formed. increases, Augite both is observed hercynite whenand augite the totalare of converted to sekaninaite, which is more thermodynamically stable. This is consistent with the iron and silicon content is above 0.6. As the temperature increases, both hercynite and augite are previous experimental results. converted to sekaninaite, which is more thermodynamically stable. This is consistent with the previous As shown in Figure 14, mullite could be observed when the total of SiO2 and Al2O3 content is experimentalabout 0.6. results. The ash melting point is substantially higher when there is a high content of Si and Al. AsQuartz shown is formed in Figure in great14, mullite quantities could at temper be observedatures ranging when the from total 1350–1450 of SiO2 °C.and Iron Al 2aluminateO3 content is aboutappears 0.6. The when ash the melting iron content point is is high, substantially which is based higher on the when content there of Si is and a high Al in content raw coal. of Si and Al. Quartz is formed in great quantities at temperatures ranging from 1350–1450 ◦C. Iron aluminate appears when the iron content is high, which is based on the content of Si and Al in raw coal. Minerals 2018, 8, 90 17 of 20 Minerals 2018, 8, x FOR PEER REVIEW 17 of 20 Minerals 2018, 8, x FOR PEER REVIEW 17 of 20

Figure 13. Phase diagram of Al2O3–SiO2–FeO during reduction. FigureFigure 13. 13.Phase Phase diagram diagram ofof AlAl22O3–SiO–SiO22–FeO–FeO during during reduction. reduction.

Figure 14. Phase diagram of Al2O3-SiO2-Fe2O3 during oxidation. FigureFigure 14. 14.Phase Phase diagram diagram of of Al Al22OO33-SiO22-Fe-Fe2O2O3 3duringduring oxidation. oxidation.

Minerals 2018, 8, 90 18 of 20

4. Conclusions (1) During the pyrolysis process, the iron-bearing minerals in the coal are transformed from pyrite (FeS2) to pyrrhotite (Fe1−xS). (2) The typical iron-bearing minerals found in the ash under reduced atmosphere in the temperature ◦ range from 900–1300 C involve magnetite (Fe3O4), ferrous oxide (FeO), hercynite (FeAl2O4) and augite (FeSi2O6). Magnetite changes to FeO, and then, FeO reacts with Al2O3 in the low-iron coal ◦ to produce hercynite at 1000 C, while FeO reacts with SiO2 in the hyper-iron coal to produce augite (FeSi2O6). (3) The typical iron-bearing minerals during the oxidation process of residual-coke include hematite (Fe2O3), hercynite (FeAl2O4), augite (FeSi2O6) and sekaninaite (Fe2Al4Si5O18). Hematite in hyper-iron coal ash reacts with SiO2 to produce augite, while in low-iron coal, it reacts with Al2O3 to form hercynite. (4) Hercynite (FeAl2O4) and augite (FeSi2O6) are transformed into sekaninaite (Fe2Al4Si5O18) above 1400 ◦C.

Acknowledgments: The authors are grateful for the financial support provided by the Natural Science Foundation of China (No. 51476185). The authors wish to thank Shifeng Dai for his great support of the X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. Author Contributions: Shuqin Liu conceived of, designed and performed the experiments and contributed reagents. Weiping Ma and Yixin Zhang performed the experiments, performed the X-ray diffraction (XRD) determination and thermodynamic simulation, observed the slag using SEM-EDS and analyzed the data. Yanjun Zhang and Kaili Qi analyzed the data. All authors participated in writing the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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