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