The Outotec traveling grate sintering process begins with the preparation of a raw mix of iron ores, fluxes, in-plant dust and spillage fines, solid fuel, and return fines. Water is added to the raw mix to obtain optimum permeability for lower electricity consumption, maintained by conveying the raw mix carefully onto the sinter machine. Its surface is then ignited, air is induced through the ignited layer and sintering proceeds in a vertical direction in the sinter strand's material bed. The sinter is then cooled, usually in a separate sinter cooler located at the sinter machine's discharge end.
The cooled sinter is crushed down to a predetermined particle size. Undersized sinter that is not suitable for the blast furnace is recycled in the return fines bin. Product sinter from Outotec sinter plants complies with all modern requirements for blast furnaces such as grain size, basicity, and slag composition.
Outotec sinter plants are designed to meet the most stringent environmental regulations. For effective dust collection, electrostatic precipitators and/or bag filters dedust the sinter plant waste gas and air from the plant dedusting system. We provide processes for limiting dioxin, SOx, and NOx emissions, while incorporating noise attenuation equipment to meet local regulations.
In the EOS process, CO and pollutants such as SOx and NOx, dust and dioxins/furans are passed through the sinter layer together with the recirculated gas. While CO gas is post-combusted, substantially saving on solid fuel, the pollutants are partially retained in the sinter layer and/or thermally decomposed. Outotec Travelling Grate Sinter Plants can easily be converted into EOS plants. with minimum production disruption.
All raw materials are dosed automatically based on the required composition of the overall mix. Water and additional binders are added to achieve the optimal bed permeability. The hearth layer bin is equipped with an adjustable gate that ensures the correct height of the hearth layer. The feed hopper outlet has adjustable gates and variable roll feeder speed. Ultrasonic sensors control the bed height for optimum productivity.
The Outotec ignition furnace consists of standardized segments with straight, refractory-lined vertical side walls and a laterally arranged special combustion chamber on each side. The furnace design ensures even temperature distribution and uniform ignition.
Our pallet car design has proven itself in hundreds of plant operations around the world for decades, and we are continuously working to improve it even further. The pallet cars are designed to suffer minimum wear, giving them a long operational lifespan. Grate bars can be easily removed for maintenance purposes.
Our state-of-the-art process control and automation solutions help to ensure stable and continuous production, high availability and reliability, and excellent end-product quality with minimum intervention from operators.
The sinter plant operated by the Cherepovets Metallurgical Combine Severstal is using cantilevered grates to crush the hot sinter. The use of such grates solves the problem of improving the durability of the crushing machines and results in a substantial monetary savings.
Iron ore sintering is a material preparation process employed worldwide in the production of iron and steel. According to statistical data on pollution, sintering plants rank second in terms of toxic emissions, after the incineration of municipal solid waste (Menad et al., 2006; Remus et al., 2013). Of the eight CORINAIR (Core Inventory of Air Emissions (environment)) standard gaseous compounds, all except ammonia are known to be emitted by sinter plants. As described in Chapter 14 on iron ore sintering, sintering involves the combustion of fossil fuels like coal and coke breeze to generate the heat required for sintering reactions. Therefore, emissions from the sintering process arise primarily from the combustion reactions in the sintering bed on the traveling sinter strand. The off-gas from the combustion reactions contains dust entrained directly from the strand along with combustion products such as CO, CO2, SOx, NOx, and particulate matter. The concentration of these substances varies with the quality of the fuel and raw materials used and the combustion conditions. Emissions also include volatile organic compounds (VOCs) formed from volatile material in the coke breeze and oily mill scale, and dioxins/furans formed in the presence of carbon, chlorine and metal catalysts, such as Cu, under certain operating conditions. While the majority of VOCs and dioxins/furans are vaporized, some of them may recondense and be trapped in the sinter bed. Metals are also volatilized from the raw materials used, and other acid vapors and high resistivity dusts are formed from the halides present in the raw materials. In addition, emissions also arise from material-handling operations, such as sinter discharge, crushing and screening, which result in airborne dust.
Original purpose of the iron-ore sintering process is to agglomerate fine ores into lump burdens for blast furnace (BF). Since sintering conditions, e.g., kinds of ores and used fluxing materials, and pregranulation processes, strongly affect the metallurgical properties of the produced sinter, many researches and developments on the sintering process have been made in order to achieve a stable and efficient BF operation. Major iron ores used in Asian and Oceanian countries are of Australia, Brazil, and India and they have relatively wide particle size ranges less than several millimeters. Some domestic ores are also used in China together with the imported ones, from which 45% of steel in the world is produced.
In the sintering process, iron ores are usually blended and mixed with fluxing materials, e.g., limestone and burnt lime, and fuels (so-called agglomeration agents), e.g., coke, anthracite, and some recycle dusts. Then, it is sent to a pregranulation process in which the mixture of raw materials is granulated with addition of water to have a size distribution less than about 10mm. The granulated materials are charged to a sintering machine to form the packed bed of granules with the care to give suitable segregations of granule size and coke content in the vertical direction. The granulation and charging are important operations, since they govern the coke combustion rate and temperature profile of the sintering bed through the influence on the permeability of the sintering bed.
Recent increase in the world steel production has made the deterioration of iron ore property apparent. In Australia, low-phosphorous Blockman ores, which are of hard hematite group, are being depleted and the ratio of goethite ores, which contains a larger amount of combined water, is increasing. There are mainly two types of goethite ores, i.e., Pisolite and Marra Mamba, and a part of latter has tended to export by blending with low-phosphorous Blockman ores . In addition, a new type of ore called high-phosphorous Blockman ore will start to be exported in future. It also contains a significant amount of combined water, but has the possibility to be a major ore in future, since it has a large amount of deposit. A problem will be that preliminary removal of phosphorous component from this type of ores will not be easy, because it tends to coexist with goethite phase . When using a larger amount of such ores, therefore, development of an efficient dephosphorous technology will be necessary during steel refining process.
The major agglomeration agent used for iron-ore sintering process is coke fine, which is undersize of coke charged to a BF. In the sintering process, coke combustion is one of the most important reactions, which affects temperature profile and structural change of the sintering bed and therefore governs strength and yield, productivity and metallurgical properties of the produced sinter. Besides coke, other heat sources are anthracite usually used for the reduction of NOx emissions, carbon contained in BF dust, and metallic iron and lower oxides of iron, i.e., FeO and Fe3O4, in mill scales. The sintering process is based on the reaction heats of the above solid fuels and the generated heats tend to accumulate in the lower bed with progress of the process. Therefore, the maximum temperature of the lower bed tends to be higher than that of the upper one. For more flexible control of the bed temperature profile, an injection technology of hydrocarbon gas from the top of the sintering bed has been developed . On the other hand, utilization of biomass char has been also attempted as a carbon neutral fuel . In the sintering process, selection of agglomeration agents and control of their reaction are important keys to reduce the CO2 emission.
Like other sintering processes, iron ore sintering converts iron ore fines of often 8mm sizing into larger agglomerates, namely, sinter, between 5 and 50mm particle size, which possess the physical and metallurgical characteristics and gas permeability required for efficient blast furnace operation. As shown in Figure 14.1, iron ore sintering is carried out in three stages: raw material preparation, ignition and firing, as well as cooling.
The sintering process begins with the preparation of a sinter mixture consisting of iron ore fines, fluxes, solid fuel (called bonding agents in Japan) such as coke breeze, and return fines from the sinter plant and blast furnace as well as recycled ferruginous materials from downstream iron and steelmaking processes. After being mixed in a rotating drum, water is then added to the mixture. Granulation is carried out in the same or a different rotating drum by controlling the moisture content and particle motion of the sinter mixture, sometimes with the help of binders, to form agglomerates of the sinter mixture or granules (also called micropellets, quasiparticles, or pseudoparticles). These granules are much coarser compared with the original sinter mixture and assist in obtaining optimum permeability of the sinter mixture during the sintering process.
The moistened granules of the sinter mixture are then loaded to a depth of typically 0.51m on a sinter strand, which is a continuous grate moving continuously at typically 23m/min. The sinter strand is normally about 46m wide with an effective sintering area of up to 600m2 and is covered generally with a layer of sized sinter screened out from the sinter product as a bedding material for the protection of the grates. After the granules are loaded and leveled on the sinter machine, the sinter bed passes a series of gas or oil burners, which heat the granules and ignite the coke particles at the surface of sinter bed. The heat generated from combustion of the coke particles continues to raise the temperature of successive layers of the sinter bed to generate a melt phase first from adhering fines and then by assimilation of coarse nucleus particles, which on cooling solidifies into a sinter matrix that bonds the initially loose iron ore particles into lumps of clinker-like material. The peak temperature of the burning coke layer (also called the flame front) reaches approximately 13001375C. The downdraft suction applied to the sinter bed helps to preheat the air sucked in from the top, to cool the sintered bed, and to heat and ignite the coke particles in the layer below the flame front. This allows the sintering of the iron ore granules on the grate to move downward with the flame front, while the grate proceeds horizontally toward the discharge point of the strand. The sinter strand speed and gas flow are so controlled that burn through (i.e., the point at which the flame front reaches the base of the strand) occurs just prior to the hot sinter being discharged.
At the end of the strand, the sintered product in the form of cake falls off the grate into a hot sinter breaker (primary crusher) where the hot sinter cake is crushed to a predetermined top particle size of typically 150200mm. The hot crushed sinter is sometimes screened to remove hot return fines and then discharged onto a straight or annular cooler, which cools the sinter down to about 150C. After the cooler, coarse sinter particles of larger than 5075mm are usually crushed by a secondary crusher and conveyed to the screening station where the product sinter, hearth material, and return fines are separated. The return fines, which are too fine and not suitable for use in blast furnaces (generally 5mm or under in particle size), are conveyed back to a bin for recycling in the sintering process.
The off-gas from selected zones of the sinter machine is sometimes mixed with cooler off-gas and/or ambient air and recirculated to the sinter machine by a sintering flue gas recirculation system or Emissions Optimized Sintering (EOS) system. This process not only allows savings in bonding agents due to the contribution of CO postcombustion and recirculated heat but also reduces the volume of waste gas and emissions from a sinter plant. For detailed information on EOS and sintering flue gas recirculation systems, please refer to Chapter 18 on sintering emissions and their mitigation technologies. The off-gas from other zones of the sinter machine is treated by a series of treatment steps after the primary dedusting step (e.g., an electrostatic precipitator or multi-cyclones) to reduce dust, acid gases, as well as harmful metallic and organic components. Selective catalytic reduction, where V2O5 is used as a catalyst to reduce NOx into N2, is often applied to remove NOx in the off-gas. SOx removal is achieved via installation of sintering flue gas desulfurization equipment. In addition to the conventional wet-type systems using limestone/lime, Mg(OH)2, or ammonia as absorbent, dry-type desulfurizing systems utilizing activated coke adsorption are now used. These systems not only are effective for desulfurization but also are effective for the removal of NOx and dioxins.
Denmark and the UK were the first European countries to introduce an Emission Trading System in 2000 and 2002, respectively. Now the European Directive 2003/87/CE has been approved introducing an E.U. Emission Trading System. According to this, since January 1, 2005, installations involved in activities listed in Annex I (see Appendix 1) must have a greenhouse gas emissions permit (the ability to measure and report emissions). Application can be made to the competent authority.
For each period (2005-2007, 2008-2012), member states develop a national plan to allocate the total quantity of allowances (Assigned Amount of UnitsAAUs). The National Plan must be approved by the Commission of the European Communities. At least 95% of the allocation will be free of charge in the 2005-2007 period and at least 90% in the 2008-2012 period. Allowances can be traded within the European Community. By April 30 of each year, starting from 2006, the owner of each installation will surrender a number of allowances equal to its emission in the previous year. For those who do not comply with the obligation, a penalty is applicable (40 euros per tonne in the 2005-2007 period, 100 euros per tonne in the 2008-2012 period). The member states will organize a registry for allowances issued, traded, and cancelled. The Commission shall designate a central administrator to maintain an independent transaction log recording the issue, transfer, and cancellation of allowances. The Directive 2003/87/CE does not allow participants to comply with obligations delivering other credits obtained through JI [Joint Implementation] and CDM [Clean Development Mechanism] projects. A Linking Directive, that amends the 2003/87 in order to make credits coming from Emission Reduction Units (ERUs) and credits relative to Certified Emission Reductions projects (CERs) valid for complying with the E.U. ETS obligation, has been recently approved.
This Black Certificate Market will join the GC and EEC markets in Italy, even though it will be at a European level while the latter two markets will be national. For some participants, Green and Black markets (i.e., electricity producers) will overlap, and it can be useful to have the same market platform to trade their certificates. In this sense, GME is going to organize an emission rights market where both Italian and other European operators can buy or sell their black certificates, providing a complete offer of environmental markets.
Production and Processing of Ferrous MetalsMetal ore (including sulphide ore) roasting or sintering installationsInstallations for the production of pig iron or steel (primary or secondary fusion) including continuous casting, with a capacity exceeding 2.5 tonnes per hour
Mineral IndustryInstallations for the production of cement clinker inrotary kilns with production capacity exceeding 500 tonnes per day, orlime in rotary kilns with production capacity exceeding 50 tonnes per day, orother furnaces with production capacity exceeding 50 tonnes per dayInstallations for the manufacture of glass including glass fiber with a melting capacity exceeding 20 tonnes per dayInstallations for the manufacture of ceramic products by firing (in particular, roofing tiles, bricks, refractory bricks, tiles, stoneware or porcelain) withproduction capacity exceeding 75 tonnes per day, and/orkiln capacity exceeding 4 m3 and setting density per kiln exceeding 300 kg/m3.
Installations for the production of cement clinker inrotary kilns with production capacity exceeding 500 tonnes per day, orlime in rotary kilns with production capacity exceeding 50 tonnes per day, orother furnaces with production capacity exceeding 50 tonnes per day
Installations for the manufacture of ceramic products by firing (in particular, roofing tiles, bricks, refractory bricks, tiles, stoneware or porcelain) withproduction capacity exceeding 75 tonnes per day, and/orkiln capacity exceeding 4 m3 and setting density per kiln exceeding 300 kg/m3.
The ore group iron ore textural classification scheme (Table 2.7 and Figures 2.7 and 2.8) has been developed to link ore texture to downstream processing performance including lump/fines ratio, beneficiation, blast furnace lump physical and metallurgical properties, or fine ore sintering quality (Clout, 2002). The Ore Group scheme defines textural groupings on the basis of similarities in mineralogy, ore texture, porosity, mineral associations, and hardness. Groups are also distinguished by what mineral forms the matrix and what is interstitial. Ore groups are the basic building blocks of iron ores. Textural characteristics are visible in hand specimen or under the petrographic microscope. Textures are often liberated from each other in both lump ore (40mm) and fine ore (6.3 to 0.06mm). The scheme follows a logical classification tree in the decision-making process (Figure 2.8) that can be defined numerically, measured objectively, and classified using automated optical image analysis (Donskoi et al., 2013). It groups together similar, but not perfectly, identical examples, with distinct processing characteristics. The scheme is easily adaptable to the introduction of new categories through the decision tree process and coding using three- or four-letter codes (Table 2.8).
Figure 2.7. Simplified iron ore classification scheme matrix. Note that hematite here can refer to either martite or microplaty hematite or both. Goethite may be the typical brown, hard or the yellow, ochreous variety.
Figure 2.8. The Iron Ore Group classification tree showing dominant mineral type, hardness, and texture steps. H, hematite; G, goethite; GB, brown goethite; OG, ochreous yellow goethite; GV, vitreous goethite; D, dense; P, porous; DH, dehydrated hematite.
The main ore texture groups (Table 2.7) include dense hematite (Figure 2.5a), dense martite-hematite (Figure 2.5b), microplaty hematite (Figures 2.3i and 2.5c and d), microplaty hematite-goethite (Figures 2.3j and 2.5e), martite-goethite (Figure 2.3g), goethite-martite (Figures 2.3f and 2.5g), and goethite-rich (Figures 2.3h and 2.5h). Different types of hematite are subdivided into martite, microplaty, specular, TextureX, and undifferentiated. TextureX is nanometer-sized platy hematite, typically has a deep red color, and is powdery (Trudu et al., 2004). The distinction between martite-goethite and goethite-martite textures is on the basis that the first mineral listed forms the matrix or supporting structure, while the latter is interstitial. Each group can be further subdivided into physically hard to softer subcategories.
The ore textural groups can be divided into those associated with primary replacement of BIF (groups 14; Figures 2.3el and 2.5ae) and secondary textures interpreted to have resulted from more recent modification by near-surface hydration or goethitization (groups 610; Figure 2.5g and h) or dehydration (group 5) and surface hard-capping (Figure 2.3d) processes (group 11) (Table 2.7).
Analogous ore textural groups have been documented from iron ore deposits in Australia, Brazil, and Africa (Clout, 2002, 2005). The principles of the ore group classification scheme formed the basis for the material-type classification used by Rio Tinto Iron Ore for logging of drill holes and geologic mapping (Box et al., 2002; Clout, 2002). In addition, recent automated optical microscopy techniques have enabled far more detailed, objective ore group abundance and porosity information to be collected than is possible with visual logging or SEM-based analysis (Donskoi et al., 2013).
The total energy use distribution  by various units of the steel plant (coke ovens, sinter plant, blast furnaces, steel shop, rolling mills, and power plant) is shown in Figure 4.2.8. It can be noted that the major share (72%) of energy is required during iron making which includes coke making (12%), iron ore sintering (6%), and blast furnaces (54%). The steel making needs very little share (4%) as the process is exothermic in nature. The thermal energy used in power generation and hot rolling account for 8% and 11%, respectively. The cold rolling needs 5% energy mainly as electricity.
The total energy input is partly (52%) used to meet chemical, thermal, and process needs of the plant and the rest (48%) is wasted. This energy used and wasted is illustrated  in Figure 4.2.9. The efforts are made to recover the waste heat to improve the efficiency of the process. The amount of sensible heat available  in various units is indicated in Figure 4.2.10.
This group of compounds, polychlorinated dibenzo-p-dioxins (PCDD), and polychlorinated dibenzofurans (PCDF), is known collectively as dioxins. Their molecular structure consists of two benzene rings joined at their meta positions by oxygen links, and the molecules may contain chlorine atoms substituted in positions 19. The extent of chlorination ranges from one to eight atoms, so that there are 75 isomers of PCDD and 135 of PCDF. Dioxins and furans are concentrated by the food chain and accumulate in dairy food, meat, fish and human fat.
Dioxins are found in low concentrations in nature, but now mostly find their way into the environment via combustion systems containing chlorine, for example, incinerators, iron ore sintering, metal processing and recovery, and even diesel motors. They were also formed as a by-product from the preparation of 2,4,5-T, a herbicide once used for crop spraying.
Only those molecules with four or more Cl atoms are found to be toxic, that is, 17 in total, with the most toxic being the symmetrical tetrachloro dioxin (TCDD). When toxicities are reported, TCDD is given a value of 1 and the other congeners are rated in proportion to TCDD. The total concentration is thus reported as a TCDD equivalent, or TEQ. They are extremely toxic to some animals and their long-term effects on humans are still being investigated. Cancer and interference with the immune system are likely effects.
There are dioxin-like chlorinated compounds such as co-planar polychlorinated biphenyls (PCB) which are also allocated toxic equivalents. These are formed in flames along with dioxins, but generally contribute far less to the TEQ values of power station emissions and ambient air. There are over 100 such compounds, but only seven are considered of environmental significance (Cleverly et al., 2007).
PCDD and PCDF can be formed in flames when carbon, oxygen and chlorine are present. They form from organic precursors in the gas phase at temperatures between 550C and 900C, and on ash surfaces at temperatures between 200C and 400C when a suitable metal catalyst, particularly copper, is available. The carbon source for formation on the ash is generally elemental carbon and the formation is called de novo. In some circumstances when the chlorophenol and chlorobenzene concentrations in the gas phase are high, they can adsorb onto the ash and form PCDD/F. This route is known as precursor formation.
Since only some members of the two groups of compounds PCDD and PCDF are toxic, their concentrations are generally reported as toxic equivalents or I-TEQ of 2,3,7,8-tetrachloro-dibenzo-p-dioxin, which is the most toxic (ngI-TEQ g1 for ash and ngI-TEQ Nm3 for gas). For wood ash, the ratio of total mass of PCDD/F to the mass of I-TEQ is about 50, similar to the value for MSW ash.
The reactions which produce PCDD/F are complex, and the extent of I-TEQ release in flue gases is determined by a number of factors. Regarding the feed material, the amounts of precursors, chlorine and copper (or to a lesser extent iron) are important. The PCDD/F products in the flyash are concentrated on the finer particles. The combustion conditions which favour formation are high ash loadings (particularly of finer particles) and long residence times in the appropriate temperature zones. Temperatures above 900C will quickly destroy PCDD/F in the gas phase, and temperatures above 400C will destroy it on the solid phase.
During de novo synthesis, if chlorine is present only at low concentration as in coal, it will probably be incorporated into PCDD/F via metal chlorides in the solid (ash) phase. Copper is particularly active in catalysing the reaction. The copper can also act as a shuttle for chlorine between the gas and solid phases. It has been found that sulphur- and nitrogen-containing compounds can inhibit the formation of PCDD/F during combustion, presumably by poisoning the catalytic sites. For instance, the addition of coal to the solid feed of MSW incinerators leads to a significant fall in PCDD/F emissions. A correlation between PCDD/F formation in combustion systems and the (copper + chlorine) and sulphur contents of the fuel has been produced by Thomas and McCreight (2008).
The emission of PCDD/F from a combustion system is the sum of two parts the gas phase material and the solid phase material present on the ash. The latter is determined by the product of the ash loading of the gas (g Nm3) and the PCCD/F concentration on the ash (ng g1). Ash concentrations of PCDD/F have frequently been measured, but the ash loading in the stack depends on the type of combustion system, the fuel and the efficiency of any particulate removal system. Because of the cost of the sampling/analysis procedures, most reports of PCDD/F content do not separate the gas and ash results, but combine the two extracts together for analysis. Efficient gas cleaning to remove particulates is important for low emission values of PCDD/F.
The legislated emission limit for much of the world is 0.1ngI-TEQ Nm3 of dry fluegas. The individual measurements reported in the literature span a wide range, from 0.41pgITEQ per Nm3 (Fernndez-Martnez, 2004) to 120pgI-TEQ Nm3 of dry fluegas (Lin et al., 2007). From a study of a range of power stations, in the year 2000 the US EPA produced an emission factor for utility power generation of 0.079ngITEQ per kg of fuel combusted (Thomas and McCreight, 2008), or about 7pgITEQ Nm3. For a 25MJ kg1 coal burned to produce electricity at 40% overall efficiency, this amounts to 28.5 gITEQ per GWh produced. An Australian Report gives 7% of emissions to air from this source.
The formation of PAHs, including dioxins, in power station boilers is low because of the high temperature of the flame. If there is any change resulting from the move to USC boilers, it most likely to be in a negative direction.
First, let us take a solid material, such as a desk or a bookshelf, and imagine the stress situations in the material by arbitrarily selecting a virtual cube inside the material and by checking both normal and tangential stresses acting on its six planes. In some parts of solid materials, the tangential stress can be non-zero and/or normal stress can be negative, i.e., tensile stress, to make them stand upright keeping their form as they are. In contrast, a fluid is a state of material in which tangential stresses are absent at rest and in which normal stresses are always pressure, i.e., not tensile stress (cf. Imai, 1974).
Particulate matter can also be either solid-like or fluid-like. In nature, some mountains, cliffs and particularly sandy beaches are made of solid-like particulate matter. It is possible to stand and walk on a sandy beach, which indicates that the mass of sand particles that it is made up of are macroscopically in the solid-like condition due to gravity and some surface forces. However, this situation can be changed to a fluid-like state by the application of counteracting forces. Suppose air or water is introduced flowing upward far below the surface of the beach. The gravitational force acting downward on the sand particles can be counter-balanced at a certain velocity by the upward fluid drag force. Then, the local particle assemblies are broken (which are rather particle shape dependent), followed by the breakage of particle-to-particle contact bridges (liquid or solid bridges), if they exist. When all static forces between the contacting particles disappear, the bed of sand particles start behaving like a fluid, at which point we could even enjoy a dusty swim. In this fluidized condition, i.e., fluid-like condition, we can put a bar or a stick into the bed of solids with little resistance and stir the solids with it. If the bar or stick is made of a material of density lighter than that of the bed, it can float upon it.
Thus, a bed of particles in such a fluid-like condition is called a fluidized bed. If not in this condition, it is called a fixed bed. If all the particles are suspended and carried by the fluid, we call the group of particles an entrained bed by convention, even though there no longer exists any bedlike behaviour of the particles. For fine, light, dusty and sometimes fibrous particles, say less than 10m in diameter in an air atmosphere, such clear phase changes between fluidized bed and entrained bed modes as noted above do not exist, since their weight is so light that they can be suspended and float with only small turbulence or convective flow in the fluid.
Fluidization can be said to be the most powerful method to handle a variety of solid particulate materials in industry. For decades fluidization has been a key technology in fluid catalytic cracking (FCC) to make gasoline in the petroleum industry; in catalytic processes such as partial oxidation of ammonia to acrylonitrile to prepare acrylic resin; in gas phase polymerization processes of polyethylene and polypropylene; in the chlorination process of metals such as silicon for purification in the semiconductor industry; in the granulation process for the pharmaceutical industry; in fluidized bed combustion (FBC) of solid fuels (coal, wastes and biomass) to generate steam for boilers; in waste incineration of solids and sludge; and in other simpler operations including drying, dip powder coating, thermal treatment of metals by hot or cold sands, and even a bed of seriously burnt patients in hospitals. By the 1950s fluidization had become a technical principle of a domain of technology, using the terminology of W.B. Arthur (2009), that can be applied to any technological field.
The most important feature of gas-solid fluidized beds in industrial processes is their temperature uniformity, which is generated as a result of frequent particle collisions microscopically and of good solid mixing macroscopically by bubble motion and/or solid circulation. Temperature uniformity is a critical demand of exothermic catalytic reactions to avoid dangerous chain reactions or to avoid melting of product polymer particles in polymerization. With this temperature uniformity, ash melting and clinker formation can be avoided in fluidized bed combustion and gasification. In fluidized bed combustion, the burning fuel particles are individually surrounded by non-combustible solid particles (bed materials) and the temperature differences between them are 100200C (Ross and Davidson, 1981). Heat generated by combustion is taken out continuously through the repeated contact of the bed materials onto the heat exchanger surfaces either immersed in the bed or placed vertically over the combustor wall. Since gaseous combustibles derived from solid or liquid fuels are always surrounded by hot particles, they can continue stable burning as long as oxygen is present. However, since the residence time of gas is much shorter than solids in a fluidized bed, the mixing of gaseous reactants, i.e., gaseous combustibles and oxygen in the case of combustion, is crucial in fluidized bed reactions.
In contrast, in fixed bed combustion, a sharp temperature non-uniformity ranging from the room temperature up to 1000C over less than 1/2min can easily be generated as in the case of iron ore sintering. There the heat generated is transferred to the flowing gas, which then gives the heat to the still cold solids that are waiting for ignition. In such situations no direct heat recovery from solids is possible but only through the flowing gas as the heat carrier. However, even in a fluidized bed combustor, some particles are defluidized due to insufficiency of fluidizing air supply, which provides a condition for fixed bed combustion, causing agglomeration and clinkering troubles. In entrained bed combustion, typical in pulverized coal combustion, individual particles burn while being separately entrained by the gas with no particle-to-particle collisions and with poor cooling by the surroundings. This is why fly ash particles from a pulverized coal furnace have a spherical shape, indicating that their temperature went up above ash melting temperature.
Another aspect of fixed bed combustion is the tar issue. With the sharp temperature gradient in fixed bed combustion, the combustible tar gases derived from pyrolysis of solid fuels are easily cooled down and removed from the combustion zone. This is the way people can enjoy an unburned flavour from smoking cigarettes with the problem of inhaling toxic tar. This is also one of the reasons for using coke instead of coal in a blast furnace for iron making, to avoid tar production and softening of the bed. Coke is able to support tons of iron ore without being crushed. Raw coal would fall to pieces, be softened by heat and in any case block the air flow.
One of the most troubling but interesting aspects of fluidization engineering is the management of particle properties and their changes in the course of physical collisions, thermochemical reactions and/or agglomeration. Also in some reactors very fine particles are formed through attrition, fragmentation, condensation or deposition. In such a mixed particle bed, the coarser and heavier fraction of particles may tend to settle on the column bottom while fine particles are flowing through the bed of coarser particles, entrained and carried out or elutriated from the reactor. However, it should also be noted here that there exists a non-uniformity of gas velocity distribution in fluidized beds, particularly around the distributor, where solid dead zone is formed due to the local gas velocity being insufficient for fluidization. In dead zones, some weak gas flow still exists and this ensures localized fixed bed combustion and resulting clinker formation troubles.
Fluidization is essentially a natural phenomenon that can be seen anywhere. For instance, every day we may encounter a sort of fluidization in settling sugar particles in a teacup. When particles settle in liquid or in gas, their velocity reaches an equilibrium or steady velocity. The steady settling velocity is determined by Stokes law in the viscous regime if the particle volume fraction is very low, say below 0.001, where particles are almost completely isolated from each other. We call this velocity the terminal velocity. The terminal velocity is a function of both particle and fluid properties.
With increased particle loading, the particle volume fraction increases so that the same fluid drag force can be achieved at a settling velocity much lower than the terminal velocity. This is because of the increased interstitial gas velocity due to the decreased area for fluid to flow around the particles. Among settling particles there is no static remnant stress. They are in a fluidized condition. Conversely, any particles, dusts, mists, etc., blown by air are fluidized. This is why a cloud flows in a fluid-like manner, although in the majority it consists of solids, i.e., ice particles. More can be found on clouds in Houze (1993). The violent convective flow in a cloud, particularly in a large cumulonimbus cloud that reaches as high as 10,000m, can be so strong that ice particles can take sufficient time to circulate in the cloud to collide with each other, to agglomerate and grow to become hail particles several millimeters in diameter. Precipitation takes place once the balance between the draft strength and particle gravity is lost. A hailstorm occurs if there is no sufficient heat supply.
Other fluidization phenomena observed in nature include avalanche and pyroclastic flows (for the latter, see Salatino, 2005). They can give us violent disastrous effects because of their fluid-like nature, i.e., flowability. In the case of an avalanche, its sliding speed is so rapid that air is taken into it from its front nose. The air is then distributed inside the bed of snow and ice particles and fluidizes them. The same mechanism takes place in the sliding period of pyrocrastic flow, i.e., the flow of very hot rock fragments followed by the eruption column collapse or explosion of a lava dome of a volcano. However, these deadly hot rocks can keep flowing even over long stretches of flat ground, since they can continuously self-supply the required up-drafting fluidizing gases through the flash evaporation of surface water with their own heat.
As described, fluidization is quite a fundamental phenomenon but related to a wide variety of scientific and engineering areas. Forces related to it include gravitational forces, fluid mechanical forces, elastic/plastic collision forces, electromagnetic forces, surface forces, yield forces of materials, etc. To understand fluidization, in reality we need to have knowledge of almost all the areas mentioned above. Accordingly, fluidization science has been a platform where scientists and engineers from different fields can meet, exchange ideas and in many cases change even their subjects or professions. Indeed, the fluidization community has been a place of gathering for people who know particles, fluids, mechanics, heat transfer, reaction kinetics, simulation and a variety of phenomena, engineering processes and even society.
Fluidized bed combustion and gasification share most of the technological advantages of fluidization with other applications. In this respect, this overview intends not to limit the subject within the realm of combustion and gasification but to provide an understanding that can further lead the coming decades of scientific progress and technological and social innovations by interpreting them from historical and philosophical viewpoints.
Ferrous metallurgy offers one of the best examples of how a traditional iconic polluter, particularly as far as the atmospheric emissions were concerned, can clean up its act, and do so to such an extent that it ceases to rank among todays most egregious offenders. But environmental impacts of iron- and steelmaking go far beyond the release of airborne pollutants, and I will also review the most worrisome consequences in terms of waste disposal, demand for water, and water pollution. And while iron and steel mills are relatively compact industrial enterprises that do not claim unusually large areas of flat land (many of them, particularly in Japan, are located on reclaimed land), extraction of iron ores has major local and regional land use impacts in areas with large-scale extraction, above all in Western Australia and in Par and Minas Gerais in Brazil.
All early cokemaking, iron smelting, and steelmaking operations could be easily detected from afar due to their often voluminous releases of air pollutants whose emissions were emblematic of the industrial era: particulate matter (both relatively coarse with diameter of at least 10m, as well as fine particles with diameter of less than 2.5m that can easily penetrate into lungs), sulfur dioxide (SO2), nitrogen oxides (NOx, including NO and NO2), carbon monoxide (CO) from incomplete combustion, and volatile organic compounds. Where these uncontrolled emissions were confined by valley locations with reduced natural ventilation, the result was a chronically excessive local and regional air pollution: Pittsburgh and its surrounding areas were perhaps the best American illustration of this phenomenon.
Recent Chinese rates and totals illustrate both the significant contribution of the sector to national pollution flows and the opportunities for effective controls. Guo and Xu (2010) estimated that the sector accounted for about 15% of total atmospheric emissions, 14% of all wastewater and waste gas, and 6% of solid waste, and they put the nationwide emission averages in the year 2000 (all per tonne of steel) at 5.56kg SO2, 5.1kg of dust, 1.7kg of smoke, and 1kg of chemical oxygen demand (COD). But just 5 years later spreading air and water pollution controls and higher conversion efficiencies reduced the emissions of SO2 by 44%, those of smoke and COD by 58%, and those of dust by 70%.
Particulates are released at many stages of integrated steelmaking, during ore sintering, in all phases of integrated steelmaking as well as from EAFs and from DRI processes, but efficient controls (filters, scrubbers, baghouses, electrostatic precipitators, cyclones) can reduce these releases to small fractions of the uncontrolled rates (USEPA, 2008). Sintering of ores emits up to about 5kg/t of finished sinter, but after appropriate abatement maximum EU values in sinter strand waste gas are only about 750g of dust per tonne of sinter, and minima are only around 100g/t, but there are also small quantities of heavy metals, with maxima less than 1g/t of sinter and minima of less than 1mg/t (Remus et al., 2013). In the United States, modern agglomeration processes (sintering and pelletizing) emit just 125 and up to 250g of particulates per tonne of enriched ore (USEPA, 2008). Similarly, air pollution controls in modern coking batteries limit the dust releases to less than 300g/t of coke and SOx emissions (after desulfurization) to less than 900g/t, and even to less than 100g/t.
Smelting in BFs releases up to 18kg of top gas dust per tonne of pig iron, but the gas is recovered and treated. Smelting in BOFs and EAFs can generate up to 1520kg of dust per tonne of liquid steel, but modern controls keep the actual emissions from BOFs to less than 150g/t or even to less than 15g/t, and from EAFs to less than 300g/t (Remus et al., 2013). Long-term Swedish data show average specific dust emissions from the countrys steel plants falling from nearly 3kg/t of crude steel in 1975 to 1kg/t by 1985 and to only about 200g/t by 2005 (Jernkontoret, 2014).
But there is another class of air pollutants that is worrisome not because of its overall emitted mass but because of its toxicity. Hazardous air pollutants originate in coke ovens, BFs, and EAFs. Hot coke gas is cooled to separate liquid condensate (to be processed into commercial by-products, including tar, ammonia, naphthalene, and light oil) and gas (containing nearly 30% H2 and 13% CH4) to be used or sold as fuel. Coking is a source of particulates, volatile organic compounds, and polynuclear aromatic hydrocarbons: uncontrolled emissions per tonne of coke are up to 7kg of particulate matter, up to 6kg of sulfur oxides, around 1kg of nitrogen oxides, and 3kg of volatile organics. Ammonia is the largest toxic pollutant emitted from cokemaking, and relatively large volumes of hydrochloric acid (HCl) originate in pickling of steel, when the acid is used to remove oxide and scale from the surface of finished metal. Manganese, essential in ferrous metallurgy due to its ability to fix sulfur, deoxidize, and help in alloying, has the highest toxicity among the released metallic particulates, with chromium, nickel, and zinc being much less worrisome.
But, again, modern controls can make a substantial difference: USEPAs evaluations show that the sectors toxicity score (normalized by annual production of iron and steel) declined by almost half between 1996 and 2005 and that the mass of all toxic chemicals was reduced by 66% (USEPA, 2008). And these improvements have continued since that time. Water used in coke production and for cooling furnaces is largely recycled, and wastewater volumes that have to be treated are relatively small, typically just 0.10.5m3/t of coke and 0.36m3/t of BOF steel. Wastewater from BOF gas treatment is processed by electrical flocculation while mill scale and oil and grease have to be removed from wastewater from continuous casting. EAFs produce only small amounts of dusts and sludges, usually less than 13kg/t of steel (WSA, 2014a). Dust and sludge removed from escaping gases have high iron content and can be reused by the plant, while zinc oxides captured during EAF operation can be resold.
But solid waste mass generated by iron smelting in BFs is an order of magnitude larger, typically about 275kg/t of steel (extremes of 250345kg/t), and steelmaking in BOFs adds another 125kg/t (85165kg/t). The BF/BOF route thus leaves behind about 400kg of slag per tonne of metal, and the global steelmaking now generates about 450Mt of slag a yearand yet this large mass poses hardly any disposal problems. Concentrated and predictably constant production of the material and its physical and chemical qualities, that make it suitable for industrial and agricultural uses, mean that slag is not just another bothersome waste stream but a commercially useful by-product.
The material is marketed in several different forms which find specific uses (NSA, 2015; WSA, 2014b). Granulated slag is produced by rapid water cooling; it is a sand-like material whose principal use is incorporation into standard (Portland) cement. Air-cooled slag is hard, dense, and chunky material that is crushed and screened to produce desirable sizes used as aggregates in precast and ready-mixed concrete, in asphalt mixtures or as a railroad ballast and permeable fill for road bases, in septic fields, and for pipe beds. Pelletized (expanded) slag resembles a volcanic rock, and its lightness and (when ground) excellent cementitious properties make it a perfect aggregate to make cement or to be added to masonry. Expanded slag is now widely used in the construction industry, and Lei (2011) reported that in 2010 Chinas cement industry used all available metallurgical slag (about 223Mt in that year). Brazilian figures for 2011 show 60% of slag used in cement production, 16% put into road bases, and 13% used for land leveling (CNI, 2012).
High content of free lime prevents the use of some slag in construction, but after separation both materials become usable, with lime best used as fertilizer. Because of its high content of basic compounds (typically about 38% CaO and 12% MgO), ordinary slag is an excellent fertilizer used to control soil pH in field cropping as well as in nurseries and parks and for lawn maintenance and land recultivation; slag also contains several important plant micronutrients, including copper, zinc, boron, and molybdenum.
Iron ore sintering process is an important sector for iron and steel industry as well as a major pollution emission source of PCDD/Fs. The PCDD/Fs emission of sintering process has not been properly controlled because the flue gas presents the following characteristics, including large amount, remarkable flow fluctuations and lower concentration. The generation mechanism of PCDD/Fs in the iron ore sintering was discussed systematically and the new developments and technologies of PCDD/Fs emission reduction were also summarized from the source, process and the end treatments. Commonly, the PCDD/Fs formed in drying and preheating zone is transferred to the lower part of the material layer which is a chemical transfer process in the iron ore sintering. Finally, the potential future development of PCDD/Fs emission reduction in the iron ore sintering was also pointed out.
Failure analysis of a sinter screen in a sinter plant has been presented. The sinter screen is used for removing fines (<5mm size) form the crushed hot sinter (temperature around 1000C) thereby facilitating the cooling of sinter as well as smooth running of blast furnaces. The screens are cracking prematurely within 15days. The material of the failed screens is found to be ASTM A297 HK 40 which is basically a heat resistant cast austenitic stainless steel alloy. Based on the failure analysis, a new material HK 40 (Nb modified) has been recommended which increases the service life of the component by 6times. Samples of both HK 40 and HP 40 steels from unused and used screens are collected for analysis. The analysis consists of plant visit and visual observation, chemical analysis, fractography, microstructural examination using optical and scanning electron microscopes, characterization with EDS analysis, and measurement of hardness, impact toughness and wear properties. The analysis reveals that HK 40 screens are cracking because of material degradation at elevated service temperature due to formation of sigma phase along with chromium carbide network leading to intergranular corrosion. Further, HK 40 screen exhibit significant increase in hardness and decrease in impact toughness after service compared to that of HP 40 due to material degradation. Microstructural examination of HP 40 steel does not exhibit any sigma phase formation and intergranular corrosion. A comparative study of HK 40 and HP 40 steels before and after service as hot sinter screen has been carried out.
Cracking of screens made of heat resistant cast HK 40 alloy steel has been analyzed. Analysis shows sigma phase formation along with carbide network in HK 40 steel. Sigma phase and carbide network in HK 40 steel lead to intergranular corrosion. Sigma phase formation increases the hardness and decreases toughness in HK 40 steel. Recommended material increases the service life of hot sinter screens by 6times.