A. Fundamentals 1. Introduction 1.1 Principle mechanism Shaft kilns and cupola furnaces are used for the mass conversion and melting of granular and coarse materials. The material is transported through the vertical shaft by gravity. The hot gas flows in a counter current to the material. Typical processes are summarized in Table 1-1. For calcination processes mainly the name shaft kiln is used and for melting processes mainly the name cupola furnace. For the reduction of iron ore the name blast furnace is common. In the following for the general description the designation shaft kiln is used. Calcination of calcite Calcination of magnesite Calcination of dolomite Calcination of iron ore Reduction of iron ore Reduction of lead ore Sintering of fireclay Melting of cast iron scrap Melting of copper scrap and anodes Melting of zinc scrap and ingots Melting of lead scrap Melting of aluminium scrap and ingots Melting of rock for mineral wood Melting of solid waste Table 1-1: Typical processes in shaft kilns CaCO 3 CaO + CO 2 MgCO 3 MgO + CO 2 CaCO 3 MgCO 3 CaO MgO + 2 CO 2 FeCO 3 FeO + CO 2 FeO + CO Fe +CO 2 Table 1-2 summarizes typical characteristic values of cupola furnaces and blast furnaces and Table 1-3 presents those values of common shaft kilns. From the two tables it can be seen that blast furnaces are the biggest one in size and max. outflow. However, the outflow in relation to cross section is only a little bit larger than that of normal shaft kilns. Pure melting furnaces have the highest output flux. Here, the material has only to heat up and to supply with the melting enthalpy. In lime calcination and iron ore reduction the material needs a lot of reaction enthalpy for the mass conversion. This results in a higher energy consumption and a lower outflow flux. The energy supply is relatively high in the coke fired melting furnaces. The air has to be burnt with an excess air number lower than one that CO is produced to protect the iron from oxidation. The flue gas leaving the furnace contains a CO concentration of 20 25 %. This gas is after burnt to preheat the combustion air. Coke must be used as fuel because it builds a carrier framework for the material and the hot melt flow. Figure 1-1 shows schematically a shaft kiln as an example for the calcination of limestone. Limestone particles are filled in a container, weighed on a balance, lifted to the top of the kiln and then poured into the shaft. The material passes a sluice 101
before it falls on the packed bed. The gas has to be separated for cleaning. On the way down the particles are initially preheated from the hot gas and after reaching the reaction temperature calcinated. For the supply with energy fuel and air are injected with burners placed in the wall. The jet hits immediately after leaving the burner against particles and is converted into the vertical direction. The penetration depth of the burner jet in horizontal direction is therefore very low. As a consequence burners inside the bed are necessary. Characteristics Cupola furnaces Blast furnaces Process iron scrap mineral copper iron ore melting melting melting reduction Max. diameter in m 2-4 1.5-3 1-2 10-15 Cross sectional area in m 2 4-20 2-8 1-2 80-180 Shape cross section round rectangular round round round Height of packed bed in m 3-20 4-8 10-12 25-33 Max. working volume in m 3 5000 Size of particles in mm 20-150 scrap 50-250 scrap 20-40 irregular Max. output in t/d 2000 500 70 17000 Output flux in t/d/m 2 200 100 20 50-70 Production rate in t/d/m 3 1,5 1.5-2.5 Mean solid velocity in m/h 2-3 2-3 2-3 1.5-2 3 2 Air flux in mstp / m / s 3-4 1 1.5... 0.5-0.8 Gas pressure at top in bar 1 1 1 1-3.5 Gas press. at bottom in bar 1.3 1.2 1.2 2-6.5 Hot blast temperature in C 800-1000 800 200 1100-1300 Max. solid temp. in C 1000-1500 1400-1600 1060 1400-1500 Max. gas temp. in C 1800-2000 2000-2200 1150 2000-2200 Mean kind of fuel coke natural gas coke natural gas coke Fuel energy supply in kiln MJ/kg output 5-6 5-6 0.8 16-18 Table 1-2: Typical characteristics of Cupola and Blast furnaces 102
Characteristics Normal shaft Mixed-Feed Annular PFR Output capacity, t/d 150 300 100-200 200-600 200-800 Inner diameter, m 2.0 3.0 2.5 6 3.0 4.5 2.5 3.5 * Cross-sect. area, m 2 3 7 6 30 20-23 6-10 * Height of solid bed, m 10 15 15-20 15-25 15 20 Output flux, t/d/m 2 40 45 10-25 15-30 20-30 * Solid velocity, m/h 1.8 2.0 0.5 1.0 0.6 0.7 0.6 1.4 Air flux, m 3 STP/m 2 /s 0.6-0.7 0.1-0.12 0.6-0.8 0.8-1.1 Min. particle size, mm 30 20 30 20 Max. particle size, mm 150 200 250 160 Total press., drop, mbar 200 250 10-30 200-400 300-400 Mean kind of fuel Energy supply natural/lean gas lignite anthracite coke natural/lean gas coal/oil natural/lean gas lignite/pet coke MJ/kg lime 3.8-4.8 3.9-4.5 3.8-4.1 3.3-4.0 kcal/kg lime 910 1150 930-1080 910-980 790-950 Max. solid temp., C 1400 1500 1100-1300 1100-1200 1100-1200 Max. gas temp., C 1500 1600 1300-1400 1200-1300 1200-1300 Lime type hard-burnt hard/middle middle/soft soft-burnt reactivity low low/medium medium/high high * Data given for one shaft Table 1-3: Typical characteristics of common shaft kilns 103
The figure shows a central burner with axial supply. A lot more designs exist to improve the horizontal mixing in the cross section. The basic constructions are discussed later in a separate chapter. The homogenization of the temperature and concentration in the cross section is a main problem in shaft kilns. Above the burners the calcination takes place. Below the burners the lime has to be cooled down. Therefore, a part of the combustion air flows from the bottom in counter current through the packed bed. The output of the material is managed in different ways, e. g. by moving plates. The gas leaving the shaft contains a lot of dust which is formed by the friction between the particles, by cracking of particles and from the ash of coal firings. Therefore a filter system is necessary. All openings at the top of the kiln for charging stone and the bottom of the shaft for discharging lime are sealed by hydraulically operated traps. Figure 1-2 shows for a normal shaft kiln as an example the typical profile of the mean temperature of the solid and of the gas. For the explanation of the process it makes sense to divide the kiln into zones. After inserting the particles with ambient temperature these are heated up by the hot combustion gas in counter current. The decomposition of the limestone according CaCO 3 CaO + CO 2 can start after reaching temperatures of 810 C 840 C depending on the CO 2 -concentration of the gas because of equilibrium conditions. This is the end of the preheating zone and the beginning of the reaction zone. The end of this zone has to be reached before the injection level of the fuel. Behind this injection lies the cooling zone. Here the particles have to be cooled down to temperatures of about 50 C 80 C. The ambient air in counter current serves as a cooling agent. Above the level of the fuel injection the temperature of the gas increases rapidly. After exceeding the particle temperature heat can be transferred for the endothermic reaction. The mass flow of the gas and of the solid changes along the kiln as it is depicted principally in the below part of Figure 1-2. The input flow of the solid (limestone) is about 1.7 times higher than the output flow (lime) because of the CO 2 separation. The total mass flow of the air for combustion is up to 1.4 to 1.8 times higher as the mass flow of the lime depending on the kind of fuel. Necessary for the cooling in counter current is a mass flow ratio of almost one because the specific heat capacities of lime and air are similar. So a part at the air can also be injected into the kiln together with the fuel which has advantage for the cross sectional homogenisation of the gas. Beginning at the fuel injection level the gas flow increases due to the CO 2 separation and due to the fuel conversion if the fuel is a solid. To calculate the energy consumption for the process energy balances have to be conducted for every zone. This is necessary to ensure that the temperature of the gas at the transition between preheating and reaction zone is always higher than the temperature of the solid. If only the total kiln would be balanced a flue gas temperature could be the result belonging to the dotted line shown in the figure. But this profile is impossible due to the second law of thermodynamics. Such critical positions are named in chemical process engineering as pinch points. 104
The process shown in Figure 1-2 is totally in counter current. Therefore, the lime reaches at the end of the calcination a relativity high temperature which gives a so called hard burnt lime with a low reactivity. If a so called soft burnt lime with a high reactively has to be produced calcination in co current is necessary. In this case the cooling air is sucked off at the end of the cooling zone and injected again in the kiln together with the fuel at the beginning of the calcination zone. At the end of this zone the hot combustion gas is sucked off and injected into the preheating zone. The different calcination processes will be discussed later in separate chapters in more detail. Figure 1-3 depicts schematically a cupola furnace for the melting of cast iron. Scrap, coke and lime are weighed, transported to the top and fall in the furnace passing again a sluice. Near the bottom hot air is injected through several nozzles, which are distributed on the circumference. The distribution to the single nozzles occurs by a big annular tube. With the air a little bit of an additional fuel can be injected. This fuel can be coal powder, heavy oil or residue derived fuel. The air burns with the coke mainly to CO 2. On the way up CO 2 reacts with coke to CO according to the Boudouard reaction C + CO 2 2CO. The produced CO prevents the oxidizing of the iron. That is the reason that coke has to be used as fuel. In the case of the blast furnace the CO reduces the iron ore according FeO + CO Fe + CO 2. The CO 2 reacts again with the coke to give CO. Therefore the top gas contains about 20 25 Vol. % of CO. The top gas is cleaned and then used as lean fuel gas in other processes. For example it can be burnt in combustion chambers to generate steam or it is burnt in regenerators to preheat the air. In modern processes the air can be preheated up to 1200 C. In the range of the nozzles the scrap melts and flows down above a bed of pure coke. The liquid iron flows out through one or two horizontal orifices. Above the iron melt the slag, which has a lower density than the metal, flows out. To lower the melting temperature of the slag lime is added to the crap and the coke. Figure 1.4 shows typical profiles of the temperature and the mass flow of gas and solid. The solids are preheated from the gas again in counter current. When the coke comes in contact with oxygen it begins to burn and its temperature exceeds that of the scrap. In the melting zone the scrap remains almost at melting temperature. The combustion air is preheated mostly to 1100 C 1300 C. It burns immediately after injection with the coke. At the high temperatures the reaction products are CO 2 and CO. Until down to temperatures of about 900 C the CO 2 reacts with the coke to give 2CO (Boudouard-reaction). That is the limit for the pure preheating zone of the solids. The melting process in cupola furnaces will later be considered more intensively as well. In the following of this chapter an overview will be given about specialities and problems which are mentioned generally. These will be discussed later in separated chapters in detail. 105
1.2 Gas flow Pressure drop The gas flow through the packed bed causes a high pressure drop. Therefore, a classifying of the solid is necessary to keep the pressure drop as low as possible. The pressure drop is influenced by the reciprocal value of the void fraction with the power of three and by the reciprocal particle size. The void fraction is the fraction of the gas volume to the volume of the kiln. In a packed bed with particles of different size the small particles fall into the gap between the large particles and reduce the void fraction. A packed bed with particles of equal size has the lowest pressure drop. As a consequence, the particles have to be sieved and classified before inserting in kilns. The ratio between the diameters of the largest and the smallest particle in a kiln should be lower than two. In lime calcinating, for example, some shaft kilns were operated parallel, each kiln with another stone size. For the production of pig iron in blast furnaces, the powdered and concentrated iron ore has to be sintered in a sintering machine at temperatures of about 1200 C. The sinter is then broken and classified. In another way fine iron ore is granulized to pellets. The different materials used, sinter, pellets, coke and lime, are given as layers in the furnace. Packed beds with layers of materials with different size have a lower pressure drop than mixtures. The lower limit of the particle size for an economic pressure drop is about 30 mm. In lime production, where the lower particles from the grinding cannot be enlarged by sintering, these small particles have to be calcinated in rotary kilns. Particles greater than about 150 mm are not used because their calcinating time would be too long. Radial homogenisation The penetration depth of radial injected gas jets is relatively low, as already mentioned before. From industrial experience it is known that the penetration depth is some what in the range of 1 to 1.5 m. Therefore, the diameter of cupola furnaces, in which only radial injection is possible, is limited to 3 or 3.5 m. If higher throughputs are wanted, furnaces with rectangular cross sections are built. The width of this furnace is than limited to 3 or 3.5 m. The problem of rectangular shapes is the strength of the wall due to the thermal expansion. Cylindrical cross sections possess principally a higher strength than other shapes. In blast furnaces with the greatest diameters up to 14 m an un-reacted cone forms around the axis because of the limited penetration depth. In lime shaft kilns internal burners are common to improve the fuel distribution. Lances can be arranged from the top or form the bottom into the bed. Another design is burner beams which are arranged in horizontal direction. Normally they have to be cooled to ensure a sufficient strength. Some lime kilns have an annular cross section. That is for an internal recirculation of gas. In cupola furnaces no internal beams are possible because of the reactions between melt and refractory. 106
Limit of gas amount The limit of the gas amount in the kiln is given by the velocity at which particles on the top of the bed are begin to be fluidised and are no more fixed with the bed. This velocity is determined by the smallest particle. 1.3 Solid flow Strength of particles A high strength of the particles is necessary that they do not burst under the big weight of the high packed bed. If particles would crack the small pieces lower the void fraction and disturb the gas flow. Also dust is produced during cracking which leaves the kiln with the gas flow. Above the packed bed the velocity of the gas is much lower as in the bed because the cross section area for the gas flow is reduced in the packed bed. As a result bigger dust particles can not be transported any more and fall down on the packed bed. An enrichment of dust particles follows. The dust layer disturbs considerably the gas flow. The particles lose partly mass because of the reaction, e.g. limestone CaCO 3 CaO + CO 2, iron ore FeO + CO Fe + CO 2, coke C + CO 2 2 CO. Thus, also the reacted particles must have a sufficient strength. Gluing and sticking of particles At high temperatures particles can become soft on their surfaces. They can cling together and form thus heaps. If there are bridges or burner beams in the kiln these heaps can not pass through and block the kiln. Particles can stick to the wall so that the slipping is handicapped. This gluing reduces the cross section. As a consequence the velocity in the middle of the shaft is increased and more dust is contained in the top gas. Because of abrasion and crushing the bed in the lower part of the furnace can be compressed. Then the gas flow is disturbed and the pressure drop increases. Below the hangings zone gaps are formed. 1.4 Control The controlling of such kilns is very difficult, because of the limited possibilities of measuring. In the kiln it is nearly impossible to measure temperatures and concentrations. Through the wall no thermocouples can be stuck into the bed, because of the moving bed they would be cut off. For transient measuring of temperatures small pipes with thermocouples inside could be set on the top of the packed bed and transported between the particles through the kiln. But in the hot region of the kiln, where the temperatures are of greatest interest, the pipes mostly crush. The thermocouple measures only a mixture of the gas and solid temperature. Measurements of the temperature and the concentration of the gas are possible behind the sluice and before injection in the kiln. The problem is to get representative 107
values of the cross section. The distribution above the bed cannot be always assumed as homogeneous. An error of the measurements occurs because of false air and leakages. If the material comes out in the solid state, e.g. in lime calcinations, it is difficult to measure the particles temperature. The particles have no uniform temperature. The core is hotter than the surface. The temperatures depend on size and shape. A controlling of the kilns is therewith possible only with input and output data. Another problem for the controlling is the time lag of the kiln. The way through the kiln needs more than one day. The thick wall of the kiln needs some days to reach the steady state. So, the kiln needs a few days to reach new stationary conditions after changing parameters. For a better control mathematical simulations of the processes are required. 1.5 Refractory Lining Requirements for refractory are: - high temperature resistance, no melting or softening at high temperatures - no reaction with the solid and gas - high abrasion resistance for the moving bed - low conductivity to reduce heat loss through the wall. Because one refractory material cannot fulfil all requirements the kiln wall consists of linings and of zones of different materials. Figure 1-5 illustrates a lining of a wall. The inside wear lining has a high strength and abrasion resistance. The outer lining has a high insulation effect. Zones with high temperature need a better quality than zones of low temperature. Especially liquid metals act a high erosion effect on the refractory material. Therefore, in cupola furnaces the refractory in the melting zone has to be replaced after some days of operating. Walls of blast furnaces consist in the melting zone of cooled graphite and corundum stones. All kilns operate under pressure. Therefore an outer steel shell is necessary which must be sealed air tight. Lime shaft kilns have a wear lining in the calcinating zone made of high quality magnetite bricks and backed secondary insulating lining made from light fireclay bricks and calcium silicate boards. The refractory material has to be selected very sensitively for every temperature range and process. 108
Fuel Figure 1-1: Scheme of a lime shaft kiln. 109
Preheating Combustion Calcination Reaction Cooling Temperature T gas T solid T eq T solid T gas T Lime T e Furnace height Fuel and secondary air T air Preheating zone Reaction zone Cooling zone Mass flow solid gas solid Furnace height gas Fuel and secondary air Cooling air Figure 1-2: Typical temperature and mass flow profiles of solid and gas in a normal shaft kiln. 110
Lime Top gas Hot air (wind) 4%C Figure 1-3: Principle of a copula furnace 111
Preheating zone Preheating and coke reaction zone Melting zone gas Temperature coke scrap Wind injection gas Mass flow coke scrap Furnace height Figure 1-4: Principle of temperature and mass flow profiles in a cupola furnace 112
Preheating zone wear line working line insulation Burning zone Cooling zone Figure 1-5: Lining of a wall of a lime shaft kiln 113