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  • 1.
    Jonsson, Carrie
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Deposit formation in the grate-kiln process2013Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Deposit formation in the grate-kiln process is a challenge for the iron ore pellet production industry. The deposits cause disturbances that affect the production capacity of the pelletizing plant. To prevent or mitigate these occurrences, it is important to understand the deposit formation mechanism during the process, which is the overall goal of this work. The results from this work can be used to enhance the understanding of deposit formation in the iron ore pelletizing industry.In this work, particle and deposit formations were studied both in a full-scale grate-kiln plant (40 MW) and in a pilot-scale pulverised coal-fired furnace (400 kW). The sampled particles and deposits were characterized with scanning electron microscopy equipped with energy dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), laser diffraction (CILAS) and X-ray fluorescence (XRF).In the first part of this work, the initiating step in deposit formation— i.e. particle formation mechanisms— was investigated. Particles were sampled from the transfer chute in a full-scale grate-kiln production plant during combustion of oil and coal in separate firings. The results showed that particles in the flue gas consisted principally of fragments from iron ore pellets and minor ashes from heavy fuel oil and pulverised coal combustion. Three categories of particle modes were identified: (1) a submicron mode consisting of condensed products from vaporized species that had relatively high concentrations of Na and K for both combustion cases, with high concentrations of Cl and S during heavy fuel oil combustion, and high concentrations of Si, Fe and minor P, Ca and Al during coal combustion (2) a first fragmentation mode consisting of both iron ore pellet fines and fly ash particles with a significant amount of Fe (>65 wt %) for both combustion cases, with high concentrations of Ca and Si during heavy fuel oil combustion and high concentrations of Si and Al during coal combustion (3) a second fragmentation mode consisting almost entirely of coarse iron ore pellet fines, predominantly Fe (~90 wt %). The particles in the flue gas were dominated by iron ore fines within the second fragmentation mode, which contributed >96 wt % of the total mass of collected particles.In the second part of this work, short-term deposits were collected at the same location in the grate-kiln as the collection of particles. They were characterized by their chemical composition and microstructure in order to obtain information about the deposit formation. Deposit sampling was carried out during separate combustion firings of oil and coal. A significant difference in the deposition behaviour was observed: deposition during oil firing was negligible compared with coal firing. The deposits from coal firing were mainly fine-grained iron oxide particles embedded in a molten (bonding) phase that comprised mainly of Si, Al, Fe, Ca and O. Moreover, it was found that the prevailing flue gas direction determines the formation of the deposits on the probe and that inertial impaction controls the deposition rate. However, this rate can also be affected significantly by the amount of entrained particles that were present in the kiln.In the third part of this work, two different coals were combusted both in a full-scale grate-kiln plant and in a pilot-scale pulverized coal-fired furnace (ECF). The ECF is designed as a scaled-down grate-kiln for combustion testing. Particle and short-term deposit samplings were carried out in both appliances. Dust originating from iron ore pellets was only present in the grate-kiln as there was no flow of iron ore pellets in the ECF. The results showed that Na, K and Cl contents in submicron mode were higher in the grate-kiln than in the ECF, due to alkali circulation in the grate-kiln. The coarse mode particles (2.6-4.2 μm) sampled from the grate-kiln contained significantly more Fe, which originated from the iron ore pellets. The presence of coarse particles (>6 μm) was substantial (>96 wt % of the total particle mass) in the grate-kiln but insignificant in the ECF. The short-term deposits from the grate-kiln consisted of a variety of particles from both iron ore pellets and coal ash particles embedded in an iron-rich silicate molten phase. Short-term deposits from the grate-kiln were harder and denser compared to the shortterm deposits from the ECF. Short-term deposits from the ECF were porous and consisted of coal ash particles embedded in a silicate molten phase. The molten phase in short-term deposits from the gratekiln had a higher Fe content and a higher CaO/(SiO2+Al2O3) ratio than the molten phase from the ECF short-term deposits. Thermochemical calculations showed that the molten phase in the short-term deposits from the grate-kiln had a lower viscosity compared to the molten phase in short-term deposits from the ECF.

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  • 2.
    Jonsson, Carrie
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Stjernberg, Jesper
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Wiinikka, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Lindblom, Bo
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Boström, Dan
    Umeå universitet.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Deposit formation in a grate-kiln plant for iron-ore pellet production: Part 1: Characterization of process gas particles2013In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 27, no 10, p. 6159-6170Article in journal (Refereed)
    Abstract [en]

    Slag formation in the grate-kiln process is a major problem for iron-ore pellet producers. It is therefore important to understand the slag formation mechanism in the grate-kiln production plant. This study initiated the investigation by in situ sampling and identifying particles in the flue gas from a full-scale 40 MW grate-kiln production plant for iron-ore pelletizing. Particles were sampled from two cases of combustion with pulverized coal and heavy fuel oil. The sampling location was at the transfer chute that was situated between the traveling grate and the rotary kiln. The particle-sampling system was set up with a water-cooled particle probe equipped with nitrogen gas dilution, cyclone, and low-pressure impactor. Sub-micrometer and fine particles were size-segregated in the impactor, while coarse particles (>6 μm) were separated with a cyclone before the impactor. Characterization of these particles was carried out with environmental scanning electron microscopy (ESEM), and the morphology of sub-micrometer particles was studied with transmission electron microscopy (TEM). The results showed that particles in the flue gas consisted principally of fragments from iron-ore pellets and secondarily of ashes from pulverized coal and heavy fuel oil combustions. Three categories of particle modes were identified: (1) sub-micrometer mode, (2) first fragmentation mode, and (3) second fragmentation mode. The sub-micrometer mode consisted of vaporized and condensed species; relatively high concentrations of Na and K were observed for both combustion cases, with higher concentrations of Cl and S from heavy fuel oil combustion but higher concentrations of Si and Fe and minor P, Ca, and Al from coal combustion. The first fragmentation mode consisted of both iron-ore pellet fines and fly ash particles; a significant increment of Fe (>65 wt %) was observed, with higher concentrations of Ca and Si during heavy fuel oil combustion but higher concentrations of Si and Al during coal combustion. The second fragmentation mode consisted almost entirely of coarse iron-ore pellet fines, predominantly of Fe (90 wt %). The particles in the flue gas were dominantly iron-ore fines because the second fragmentation mode contributed >96 wt % of the total mass of collected particles.

  • 3.
    Jonsson, Carrie
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Wiinikka, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Lindblom, Bo
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Stjernberg, Jesper
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Comparison of particle and deposit formation between a full-scale grate-kiln plant (40 MW) and a pilot-scale pulverised coal-fired furnace (400 kW)2013Conference paper (Refereed)
    Abstract [en]

    The iron ore pelletizing industry utilizes the grate-kilnprocess to dry and sinter the pellets into finished product.The grate-kiln process has a known deposit formation issuethat needs to be further understood. Combustion ofpulverised coal in the rotary kiln generates fly ash particles;in addition to that, particles generated from disintegratediron ore pellets are also entrained in the process gas stream.The combined effect of both sources of particles cantherefore contribute to the deposit formation in the process.In this work, particle- and deposit formation were studiedboth from a full-scale grate-kiln plant (40 MW) and from apilot-scale pulverised coal fired furnace (400 kW). Particleswere collected with a water-cooled probe with nitrogen gasas dilution medium at the tip of the probe. The particleswere separated simultaneously with a pre-cyclone and a 13stages low-pressure impactor during samplings. Depositswere collected with a refractory plate which was attachedat the tip of a water-cooled probe, exposed to the hightemperature (>1100 °C) process gas stream. Particles anddeposits were characterized with an environmentalscanning electron microscope and a scanning electronmicroscope that equipped with energy dispersivespectroscopy detector. A comparison of particle and depositcharacteristics between the grate-kiln plant and the pilotscale pulverised coal fired furnace is presented in this paper,with focus on the potential influence of disintegrated ironore pellets on the particle- and deposit formation process.

  • 4.
    Stjernberg, Jesper
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Jonsson, Carrie
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Wiinikka, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Lindblom, Bo
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Boström, Dan
    Umeå universitet.
    Öhman, Marcus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Deposit formation in a grate-kiln plant for iron-ore pellet production: Part 2: Characterization of deposits2013In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 27, no 10, p. 6171-6184Article in journal (Refereed)
    Abstract [en]

    Buildup of deposit material in chunks on refractory linings caused by combustion of various fuels is a well-known problem. This study characterizes the short-term deposits on refractory material in a grate–kiln process, carried out through in situ measurements using a water-cooled probe with a part of a refractory brick mounted in its end. Sampling was carried out during combustion of both oil and coal. A significant difference in deposition rates was observed; deposition during oil firing was negligible compared to coal firing. The deposits are mainly hematite particles embedded in bonding phase, mainly comprising Si, Al, Fe, Ca, and O. Moreover, it was found that the prevailing flue-gas direction determines the formation of the deposits on the probe and that inertial impaction controls the deposition rate. However, this rate can also be affected by the amount of air-borne particles present in the kiln.

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