Article pubs.acs.org/EF
Deposit Formation in a Grate−Kiln Plant for Iron-Ore Pellet Production. Part 2: Characterization of Deposits Jesper Stjernberg,*,†,‡ Carrie Y. C. Jonsson,§ Henrik Wiinikka,§,∥ Bo Lindblom,§,‡ Dan Boström,⊥ and Marcus Ö hman§ †
Division of Materials Science, Luleå University of Technology, S-971 87 Luleå, Sweden Luossavaara-Kiirunavaara Aktiebolag (LKAB), S-971 28 Luleå, Sweden § Energy Engineering, Division of Energy Science, Luleå University of Technology, S-971 87 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: 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.
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. During 2010, 25% of mined iron ore was converted into pellets.1 During the pelletizing process, the iron ore is first crushed into a powder and, in a moist condition, mixed with additives and a binder. Thereafter, the concentrate is filtered to around 9% humidity and balled into green pellets, 9−15 mm in diameter. These are subsequently sintered to pellets in an induration furnace.3,4 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 is transported through the entire process, including zones of drying, oxidation, sintering, and cooling. The grate−kiln process (which is the focus of this work) uses a shorter grate, with part of the oxidation and sintering taking place in the kiln. The kiln is a rotating furnace that achieves a more homogeneous induration of the pellets.5 The total residence time of the pellets in a grate−kiln furnace is around 30 min; it takes another 40 min to travel through the cooler. Times vary from plant to plant, as the dimensions and rates differ among plants. Rotary kilns were originally developed for the production of Portland cement during the late 19th century. Besides 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 design varies with application,6 and they can be classified in groups, such as (1) wet kilns that are usually fed with slurry materials, which have © XXXX American Chemical Society
lengths on the order of 150−180 m and are commonly used as lime mud kilns, (2) long dry kilns, which have lengths on the order of 150−180 m and are commonly used in the cement industry, and (3) short dry kilns, which have lengths on the order of 15−75 m, are usually accompanied with an external preheater (e.g., a roaster grate), and are commonly used in the refrainment of metal ores. Within pelletizing of iron ore, the refractory linings in the kilns most often take the form of bricks. A burner is located at the outlet of the kiln, and the primary fuel is usually coal. However, fuel oil is used as a startup fuel and when problems with coal supply arise. During production of iron-ore pellets, deposits frequently form on the walls of the induration machine, especially in the hot part (in the end of the grate, in the transfer chute, in the kiln, and in the beginning of the cooler). Because the deposits may disturb the flow of both gas and pellets through the kiln, operational problems that reduce the production rate frequently occur in grate−kiln systems. Accumulations of deposits cause disturbance in the production of the iron-ore pellets because the flows are affected by the deposit layer, occasionally causing unscheduled plant shutdowns. Furthermore, it has been observed that buildups of deposits in rotary kilns for iron-ore pellet production not only cause mechanical strains but degrade the refractory liners over time by corrosion.7 Deposition of particles (fly ash) on walls (called slagging if the formation of material is fused or sintered on linings and equipment subjected to radiant heat exchange and fouling if the material is in the non-radiant convective heat-transfer portion at Received: May 23, 2013 Revised: July 15, 2013
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dx.doi.org/10.1021/ef4009746 | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
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temperatures below the melting temperature of the bulk coal ash8) is well-known during pulverized coal combustion in power plants.9 Fly ash from coal combustion is produced by two mechanisms, resulting in a bimodal particle size distribution.10−15 The bulk of the mineral matter is transformed into large ash particles, 0.5−20 μm in diameter, produced by fusion of mineral matter on the surface of the burning char particles. Smaller particles,
