Article pubs.acs.org/EF
Deposit Formation in a Grate-Kiln Plant for Iron-Ore Pellet Production. Part 1: Characterization of Process Gas Particles Carrie Y. C. Jonsson,*,† Jesper Stjernberg,‡,§ Henrik Wiinikka,†,∥ Bo Lindblom,†,§ Dan Boström,⊥ and Marcus Ö hman† †
Energy Engineering, Division of Energy Science, and ‡Engineering Materials, Division of Materials Science, Luleå University of Technology, S-971 87 Luleå, Sweden § Luossavaara-Kiirunavaara Aktiebolag (LKAB), S-971 28 Luleå, Sweden ∥ Energy Technology Centre (ETC), Box 726, S-941 28 Piteå, Sweden ⊥ Energy Technology and Thermal Process Chemistry (ETPC), Umeå University, S-901 87 Umeå, Sweden ABSTRACT: 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.
1. INTRODUCTION Iron ore is one of our most important natural resources, with ∼1600 Mt mined in 2010.1 Almost all (98%) of the mined iron ore is used in steelmaking.2 Mined iron ore can be used directly as lump ore or converted to, e.g., pellets, to be reduced either by direct reduction or in a blast furnace. In 2010, 25% of the mined iron ore was converted into pellets.1 During the pelletizing process, the iron ore is first crushed into a powder, mixed with additives and a binder, and balled into green pellets (9−15 mm in diameter) that are thereafter sintered to pellets in an induration furnace.3 The two most common processes used for pelletizing today are the traveling grate [most often used for hematite (Fe2O3) ores] and the grate-kiln process [most often used for magnetite (Fe3O4) ores]. The traveling grate process uses a stationary bed of pellets, which are transported through the entire process, including zones of drying, oxidation, sintering, and cooling. The grate-kiln process (which is the focus in this work) uses a shorter grate, with part of the oxidation and sintering taking place in the kiln, a rotating furnace that achieves a more homogeneous induration of the pellets. The total residence time of the pellets in a grate-kiln furnace is around 30 © XXXX American Chemical Society
min. The residence time can vary from plant to plant because of different dimensions among grate-kiln plants. Rotary kilns were developed for production of Portland cement in the late 19th century. Besides in cement production, rotary kilns are used for drying or sintering in many different applications, e.g., lime regeneration or refinement and processing of raw minerals, such as iron ore. Kiln designs vary with their applications.4 A burner is located at the outlet of the kiln, and the primary fuel is coal powder. However, fuel oil is used as a start-up fuel and when problems with the coal supply arise. During production of the iron-ore pellets, deposit formation on the walls of the induration machine frequently occurs, especially in the hot part (the end of the grate, rotary kiln, and beginning of the cooler). Accumulation of deposits causes disturbance in the production of the iron-ore pellets, because the gas and pellet flow is affected by the deposit layer and, therefore, Received: May 23, 2013 Revised: August 21, 2013
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dx.doi.org/10.1021/ef400973w | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Figure 1. Grate-kiln process schematic drawing (modified from the source with permission from Metso Minerals). The transfer chute was the sampling location where particles were collected with the particle-sampling system.
The initial work was divided into two papers: In the first paper (this paper), the focus was on characterizing the particles in the flue gas during combustion of oil and coal to obtain information of the particle formation mechanisms in a grate-kiln furnace. This was performed because particle formation is the first step in the formation mechanism of the deposits. Particle samples from the transfer chute between the kiln and the grate were withdrawn with a dilution probe and analyzed with respect to morphology and chemical composition. In the second paper of this series (10.1021/ef4009746), the focus is on characterizing the shortterm deposit in a grate-kiln furnace to gain an understanding of the deposition formation mechanisms in a grate-kiln furnace.
extended maintenance stops may occur. It has been observed that buildups of deposits in rotary kilns for iron-ore pellet production cause not only mechanical strains but also degrade the liners over time by corrosion.5 Deposition of particles (fly ash) on walls and boiler tubes (called slagging if the material is deposited in the furnace and fouling if the material is deposited in the convection path) is a well-known phenomenon during pulverized coal combustion in power plants.6 The fly ash from coal combustion is produced by two mechanisms,7−15 resulting in a bimodal particle size distribution (PSD). The bulk of the mineral matter is transformed by fusion on the surface of the burning char particles into large ash particles, 0.5−20 μm in diameter. Smaller particles,
