The effect of disintegrated iron-ore pellet dust on deposit formation in a pilot-scale pulverized coal combustion furnace. Part II: Thermochemical equilibrium calculations and viscosity estimations⋆
Introduction
Iron ore pellets primarily consist of iron oxides with small amounts of various additives and a specific binder all of which are intended to improve the mechanical and metallurgical properties of the pellets. Unlike most of the pelletizing plants that use hematite concentrates, the Swedish iron ore company LKAB (Luossavaara-Kiirunavaara Aktiebolag) uses magnetite concentrates for production of commercial iron ore pellets. Finely ground moist magnetite concentrate (delivered from the on-site concentration plant) is shaped into small spherical pellets (green balls) of 10–14 mm in diameter in rolling drums. The two most widely-used processes in pelletizing include the travelling-grate process [commonly used for hematite (Fe2O3) concentrates], and the grate-kiln process which is commonly used for magnetite (Fe3O4) concentrates. During the grate-kiln process (which is the focus here), the green balls are transported on a moving grate in a 22–25 cm layer (the bed) while subjected to drying/oxidation by means of preheating gas streams of increasing temperatures (up to approximately 1200 °C). Upon leaving the grate, the pellets enter a rotating kiln to reach the firing temperatures of over 1250 °C necessary for the sintering process. The pellets are then transferred into an annular cooler where cold air is blown through the sintered finished products. The total residence time of the pellets during the entire process (on the grate, in the kiln, and in the cooler) is approximately 60 min [[1], [2], [3]]. Estimated temperatures in the kiln are ∼1200 °C at the inlet, while in the burner zone, the temperature reaches ∼1350 °C and the flame temperature reaches up to 1700–1800 °C. To reach a sufficiently high process temperature in the kiln which is crucial for the sintering process, pulverized coal is combusted as the predominant fuel, however, fuel oil is also used when starting up the kiln and/or when problems with the coal supply arise [4,5]. Preheated air (∼1200 °C) from the first zone of the cooler is used as combustion air with an air/fuel ratio corresponding to an oxygen partial pressure of 0.16 atm in the flue gas. During the process, the microstructure and the phase composition of the pellets change drastically. The most important chemical reaction is the exothermic oxidation of magnetite (Fe3O4) to hematite (Fe2O3) which is the predominant phase in the finished products. A more detailed description of the grate-kiln setup is given in Part I of this study.
One of the important aspects during fuel conversion is the transformation of ash-forming matter through which the formation of molten ash material can result in deposition of slag (deposits) upon the furnace wall. Deposit formation or slag formation is a challenging issue in the iron-ore pelletizing process and cause numerous complications for the pelletizing plants. Coal ash particles together with disintegrated iron-ore pellet dust particles amass on the refractory walls, resulting in the buildup of deposits, most drastically in the hot areas (e.g., fireside slagging closer to the burner, inside the rotary kiln, and the beginning of the cooler) [5]. Prearranged maintenance at the LKAB's grate-kiln production plants is carries out annually to repair the moving grate and to mechanically remove the sintered deposits from the inner walls, a process that takes approximately 1 to 2 weeks of out-and-out shutdown. In addition to the aforementioned planned maintenance, unscheduled stoppages are encountered when the deposited layer disturbs the flow of gas and pellets and/or when a chunk of deposited matter falls from the inner walls. This causes production disturbances and severely affects the production capacity of the pelletizing plant. Furthermore, the formation of deposits in rotary kilns of iron-ore pelletizing plants, not only cause mechanical strains but also degrade the refractory lining over time due to high temperature corrosion [6].
The transformation of ash-forming elements in pulverized-coal combustion has been comprehensively studied [[7], [8], [9], [10], [11]], in contrast, ash deposition phenomena in iron-ore pelletizing rotary kilns have yet to be understood and there is very limited information regarding this in the literature [[12], [13], [14], [15], [16], [17], [18]]. The potential for coal firing in the grate-kiln system was previously investigated [19]. A certain type of coal that caused slagging in a grate-kiln setup was discussed in a previous study [20], the grading properties of the coal and burner operation were also explored in the same study. Deposit formation in a rotary-kiln (for oxidized pellets) burning natural gas was previously studied [13]. A study conducted by LKAB suggested that coal ash could hypothetically act as a binding bridge in between the iron-ore pellet particles [21].
In contrast to the limited understanding of ash transformation phenomena during the iron-ore pelletizing process, the mechanisms of fly ash formation in pulverized-coal combustion are extensively covered in the literature [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. Deposit formation mechanisms in pulverized-fuel combustion from fly ash particles are also broadly discussed in the literature [32,33]. Several noteworthy reviews and scientific papers addressing ash transformation phenomena in pulverized-fuel combustion were mentioned in Part I of this study [[34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49]].
Compared to pulverized coal fired boilers, there are similarities as well as differences regarding ash deposition phenomena in the rotary kilns of iron ore pelletizing plants. In contrast to pulverized coal fired boilers, a grate-kiln process is characterized by a highly oxidizing atmosphere, a longer residence time, and the presence of recirculating alkalis and disintegrated iron-ore pellet dust in the flue gas. The interaction between fly ash particles from coal combustion with the intrinsic entrained Fe-rich particles (arising from the disintegration of iron-ore pellets) makes ash deposition - in a grate-kiln process - a much more complex phenomenon compared to that in ordinary pulverized coal-fired boilers. Moreover, ash deposition in a grate-kiln process is affected by several factors including the process conditions, the chemical properties of iron-ore pellets and coal-ash and their potential interactions with one another and with the refractory walls.
Considering the foregoing, this study was initiated with the objective of investigating the effect of disintegrated pellet dust particles on melt formation (liquid slag) and, consequently, on deposit formation in a grate-kiln process. This study was inspired by our investigations and findings from our previously conducted pilot-scale [5,50] and full-scale measurement campaigns [14,15] which spotted a considerable difference in the rate of deposition and the properties of the resulting deposits when iron ore pellet dust was present. Accordingly, several experiments were carried out in a pilot-scale (0.4 MW) Experimental Combustion Furnace (ECF) with the objective of investigating the effect of disintegrated iron-ore pellet dust on deposit formation in a pilot-scale pulverized coal combustion furnace to be able to generalize the research findings to a wider range of situations and address deposit formation in the full-scale rotary-kiln process.
Characterization of fly ash particles and deposits was presented in Part I of this study whereas thermochemical equilibrium calculations and viscosity estimations are discussed here (part II of this work).
As previously explained in Part I, the major inorganic elemental distributions comprising the bulk deposit material were different from the original fuel ash. The most prominent difference was the enrichment of Si and Al against most of the other inorganic elements. S and Cl were depleted in the collected deposits suggesting that these elements along with part of Na and K were likely to have formed non-oxide compounds, most of which are volatile under the global conditions of pulverized-fuel firing. The significant enrichment of K and Na - especially the particles captured in stage 1 of the impactor (Part I)-, with respect to their original content in the fuel, supports the abovementioned suggestion. This stems from the chemical association of these elements in the fuel along with the high degree of fragmentation of fuel particles (which is characteristic of high temperature pulverized-fuel firing) implying the likelihood of volatilization of smaller ash particles. This is in fact fractionation of ash forming elements during high temperature pulverized firing and constitutes a potential for melt formation. Considering this, the composition of the sampled slags is used as inputs in the thermochemical equilibrium calculations of this study instead of using the original fuel ash composition. In light of the findings in Part I which emphasized the significant role of the coal ash particles in deposit formation, a sequential ash transformation scheme is proposed on the basis of the most abundant inorganic elements in the sampled deposits (slags). This was meant to study the effect of change in deposit composition (associated with pellet dust addition) on phase equilibrium conditions under the global conditions in the ECF.
Section snippets
Description of the experimental combustion furnace (ECF)
The 0.4 MW pilot-scale pulverized coal fired furnace, owned by the iron ore pelletizing company LKAB in Sweden, is designed to simulate a downscaled grate-kiln plant. This pilot-scale furnace is referred to as ECF (Experimental combustion furnace) in this work. The ECF is a horizontal furnace 14 m long with the outer- and inner diameter of 1200 mm and 800 mm respectively. It is lined with 200 mm-thick refractory bricks and the outer steel mantle has a thickness of 10 mm. Heated secondary air of
Results and discussion
The average flue gas temperature measured with the temperature probe during coal combustion (with and without the addition of pellet dust; see Table 1) and natural gas combustion (where only pellet-dust was added to the gas stream with the same thermal throughput as the coal-fired runs i.e. 0.4 MW), at positions 1, 2, and 3, were approximately 1350 °C, 1250 °C, and 1150 °C, respectively. However, due to the deposition and coverage of ash upon the alumina tube (alumina shield), the average slag
Conclusions
In this work, a sequential ash transformation scheme (valid for high-rank coals and based on the pilot-scale experimental observations and TECs) is proposed on the basis of the most abundant inorganic elements in the sampled deposits (slags). This was done through investigating the effect of change in slag composition (associated with pellet dust addition) on phase equilibrium conditions under the global conditions in a 0.4 MW pilot-scale experimental combustion furnace (ECF). The most
Acknowledgments
LKAB (Luossavaara-Kiirunavaara Aktiebolag) and Luleå University of Technology are acknowledged for their financial support of this study (Dnr 93_2014). Many thanks to the supportive personnel at RISE-ETC (Piteå, Sweden) and Swerea MEFOS (Luleå, Sweden) for their efforts and dedication to the project.
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2019, Fuel Processing TechnologyCitation Excerpt :Preset maintenance periods, which take approximately two weeks of total process halt, are carried out annually to mechanically remove the deposited ash layer from the inner walls of the furnace and transfer-chute. However, besides these planned stoppage periods, unplanned shutdowns may be encountered when massive growth of deposit chunks disturb the flow of gas and pellets [6,7]. This severely affects the production capacity of the pelletizing plants.
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Note that this manuscript is a companion to [Fuel Processing Technology, 177 (2018) 283-298] in this issue.