Lime: shaft furnaces or rotary furnaces?
Limestone calcining can be performed in shaft or rotary furnaces. In shaft furnaces only hard rock can be calcinated, and in rotary furnaces both hard rock and soft rock slimes, for example, chalk can be calcinated. Temperature rise accelerates calcium carbonate decomposition reaction, but excessively high calcinating temperature affects the product quality adversely, as "over-burning" phenomenon develops.
To determine the limestone calcinating furnace type which is most appropriate for use in a particular case, it is expedient to consider the particulars of the process with each furnace type: shaft and rotary.
Lime calcinating in rotary furnaces. Peculiarities of heat exchange in rotary furnaces as compared with shaft furnaces. Furnace design, advantages and disadvantages of their operation.
Rotary furnaces 30 to 100 m long, 2 to 4 m in diameter, 3 to 4° tilt angle and 0.5 to 1.2 rpm rotation speed. Their specific daily capacity is 500 to 700 kg/m3 on a per full volume of calcinating drum. As the length of furnaces grows, their production capacity increases, and the fuel consumption decreases.
To minimize the fuel consumed for lime calcination, different methods are used in rotary furnaces and for utilization of heat of the gases exiting furnaces at 750 to 800°С. In particular, heaters are installed downstream of furnaces. The lump material meant for calcination is fed to the heaters. From here it is fed at 500 to 8000С into a rotary furnace, and from there it is fed to the cooler. With such a furnace operation method, the heat consumed for calcination is decreased to 4,600 to 5,030 kJ/kg of lime.
Fuel-oil residue and gas are used as a fuel burnt as a flare in the furnace drum immediately. It is not recommended to use solid powdered fuel with increased ash content for calcination, as the ash sedimented on the surface of limestone pieces forms low-melting compounds at high temperatures resulting in formation of a skull in the form of strong annular structures impairing normal operation of the furnace.
As the limestone moves through the drum, it sequentially passes through zones of drying, heating to 1,123 to 1,153 К (850 to 880°С), calcination and preliminary cooling. When dense limestones are calcinated, the drying zone is absent in the furnace due to low humidity of the material. The preheating zone occupies 50 to 70% of the furnace length, the calcination zone occupies 25 to 30%. Its length is regulated by changing the burning fuel flare length. From calcination zone the lime is fed to preliminary cooling zone occupying about 5% of the furnace length. The final cooling is performed in a special cooler. The air heated in the cooler to 573 to 673 К (300 to 400°С) by the cooled lime is fed as secondary air to the furnace for fuel combustion. The primary air in the amount of 15 to 20% of the total air consumed for combustion is fed through the burner. To accelerate the heat exchange, the preheating zone is equipped with chain and metal cellular heat exchangers. Suspension type heat exchangers located behind the furnace and conveyor lattice exchangers can also be used.
The heat exchange in the rotary furnaces is effected by irradiation method, and that in shaft furnaces — by convection method. In rotary furnaces it is due to large heat exchange surface area and due to the fact that triatomic gasses (V2=VCO2+VH2O+VSO2} are capable of sufficient heat exchange by means of irradiation.
The length of calcinating rotary furnaces is 30 to 100 m at 1.8 to 3 m diameter, their capacity is 400 to 500 t/day, which is 2 to 4 times as much as the capacity of shaft furnaces. One of the most important technological advantages of calcination in rotary furnaces is short time of passage of the material from charge location to furnace outlet location, which ensures high speed of the process control. Rotary furnaces ensure small size flow process diagram, they enable automation of the process and reduction of capital expenditures on civil construction of shops. High-quality lime can be produced in rotary furnaces by calcination at medium and rather high temperatures. Due to short time of holding the material in the furnace, the risk of "over-burning" in them is minimum. In this case the lime is much more homogeneous in terms of its composition and contains less admixtures.
Advantages of rotating furnaces:
1) high-quality lime;
2) use of any raw materials;
3) use of any fuel type;
4) production of any lime type (building lime, metallurgical lime)
1) high specific amount of metal;
2) high capital expenditure;
3) significant fuel consumption (as compared with shaft furnaces);
4) significant electric power consumption (as compared with shaft furnaces).
Summary. Rotary furnaces enable producing soft-calcinated high quality lime from small-piece limestone and from soft carbonate rocks (chalk, tuff, shell limestone) which must not be calcinated in shaft furnaces due to tendency of such materials to "arch" in the shaft resulting in breakdown in the calcination process.
Calcination in gas-fuelled shaft furnaces. Requirements to raw material size range. Fuel burners and their arrangement in the furnace. Furnace design, advantages and disadvantages of their operation.
Shaft furnaces are subdivided into mixed, semi-producer type and gas lime kilns. Mixed and semi-producer type furnaces are now built at small enterprises with low production capacity. Designs of such furnaces are outdated, and quality of lime produced in such furnaces leaves much to be desired. These types of calcinating furnaces are reaching back, so we will not consider them in this article.
When calcination is performed in gas-fuelled shaft furnaces, the lime quality is enhanced, the furnace production capacity increases, and working conditions improve. When transferring the existing mixed and semi-producer shaft furnaces to gas fuel, it is especially important to create conditions for uniform distribution of gas over the cross section of the shaft. In furnaces of less than 1.8 m diameter gas is fed to the furnace using burners inserted into special openings in furnace walls. If diameter is large, peripheral and central gas feeding is performed, and in case of slotted shaft cross-section only peripheral feeding on at least two levels is performed. The central gas feeding is performed using a vertical core or diametrical water-cooled metal beams, with under-beam burners. It allows introducing additional gaseous fuel into the shaft centre. Water-cooled beams complicate furnace operation and cause losses of heat with cooling water. To replace the damaged beam, it is necessary to shutdown and discharge the furnace. Therefore, these beams are replaced with air-cooled cantilevering tuyere burners which can be used without any furnace shutdown.
Three zones are separated in working space of the shaft furnace. Heating, calcination and cooling are executed there. The gas furnace preheating zone is 35% of useful height of the shaft, which allows decreasing off-gas temperature to 300 to 3500С (without considering of the cold air inflow), and heat lump material at the end of preheating zone to 9000С. Furnace gases exhausted from the preheating zone contain 24 to 26% of carbon dioxide (СО2) and 3 to 4% of oxygen (О2) in the expedient calcination mode. The calcination zone occupies 40% of the useful shaft height. Natural gas burning calcination and limestone dissociation are executed in calcination zone. The average temperature of gases in the calcination zone shall be maintained at 1,100 to 1,200°С.
Uniformity of gas flow temperature distribution over the shaft cross-section in the calcination zone depends on uniformity of gas and air distribution over the shaft cross-section, and on air/gas mixing conditions. Since air/gas mixing conditions in the furnace are not ideal, some excess air has to be fed into the furnace (flow rate is higher than stachyometric flow rate by 20 to 30%), i.e. the air excess factor is maintained within 1.2 to 1.3.
The cooling zone occupies one fourth part of the useful shaft height. It is designed for cooling lime to 80 to 1200С before the lime is fed to the discharge mechanism. A 2 to 3 m3 lime cooling bin is installed in the lower part of the furnace under the discharge device. It allows additional cooling of the material to 50 to 800С and heating the air fed from below to the shaft furnace.
The main goal during the furnace operation is uniform delivery of gaseous fuel and prevention of local overheating of the material which often results in formation of sinters ("sows", lumps) which impair gas dynamics in the furnace dramatically and may result in the furnace shut-down. To distribute the gaseous fuel (natural gas) uniformly, it is fed to the shaft on three levels: to axial zone — via the central (bottom) burner in the cooling zone at the bottom of the furnace, on the first tier over the cooling zone via eight cantilevering tuyere burners, on the second tier via eight cantilevering tuyere burners located in the calcination zone. In addition, using readings of heat sensors installed in the furnace lining in the calcination area, the process engineer can change gas delivery to individual burners and prevent overheating of the material and formation of sinters in the furnace.
Advantages of shaft furnaces:
1) low specific amount of metal;
2) moderate capital expenditure;
3) lower fuel consumption (as compared with rotary furnaces);
3) low electric power consumption (as compared with rotary furnaces).
1) insufficient degree of raw material decarbonization (as a rule, it is no more than 93 to 97%);
2) non-uniformity of calcination which increases as diameter increases and furnace height decreases;
3) rather high requirements to raw material uniformity in terms of quality and grain size, as well as the degree of the raw material contamination with argillaceous admixtures;
4) limited production capacity (100 t/day production capacity is critical for shaft furnaces due to the risk of producing a non-calcinated central zone).
Summary.The use of shaft furnaces is recommended when the high-quality and actually ideal uniformity and/or consistent supplies (availability) of raw material is ensured. Such furnaces allow saving electric power and fuel used for calcination. But to produce lime in rotary furnaces, these expenditures can be compensated by higher quality of lime and its final price. Of course, the latter consideration can be used at the enterprises where marketing plans and activities are traced closely and fulfilled completely.
Samara plant Strommashina manufactures equipment and installed its equipment on calcination and lime production systems. We can offer ready systems for calcination both in rotary kilns, and in shaft furnaces. To determine the type of furnace which is more appropriate and advantageous for limestone calcination to produce lime in your particular case (whether it is building or metallurgical lime), we recommend to contact our managers using the contact details from "Contacts" web site section. We will help you by all means.