Application of European version of mill in desulfurization limestone powder

1 Introduction
The limestone- gypsum flue gas desulfurization wet process (FGD) and the in-boiler calcium injection process are the most mature and widely used technologies for controlling sulfur dioxide emissions in the world. More than 90% of domestic thermal power plants use limestone. Gypsum desulfurization technology.

Whether it is calcium injection in boiler or limestone-gypsum method for desulfurization and purification of flue gas after combustion, the medium for absorbing sulfur dioxide is limestone powder. The particle size of limestone powder has a great influence on desulfurization efficiency. The smaller the particle size, the better the sulfur dioxide is. The absorption, the preparation of limestone powder has become a prerequisite for desulfurization of thermal power plants and steel plants, and to improve the efficiency of desulfurization. In addition, the preparation cost of limestone powder has a great influence on the desulfurization cost. The low-cost preparation of high-fine limestone powder technology has become an important part of environmentally friendly desulfurization technology.

2 Main factors affecting the efficiency of limestone desulfurization
(1 ) Limestone quality
Generally, the content of calcium carbonate in limestone should be higher than 85%. If the content is too low, some problems will be caused due to more impurities, resulting in an increase in absorbent consumption and transportation cost, and a decrease in gypsum purity.

The quality of limestone is determined by the content of CaO. The higher the purity of limestone, the better the desulfurization efficiency. However, in the process of formula design, it is necessary to calculate its chemical composition, but also its physical properties. The calcium oxide content of the limestone first grade product is 48% to 54%, but the limestone is not necessarily the higher the CaO content, the better. For example, limestone with CaO>54% is marbled due to high purity, is not easy to be ground, and has strong chemical stability, so it is not suitable for use as a desulfurizer.

(2 ) Limestone particle size (fineness)

The fineness of limestone powder is an important factor affecting the desulfurization efficiency. The reaction contact area largely determines the chemical reaction rate. The finer the particles of limestone powder, the larger the mass specific surface area, and the contact area of ​​chemical reaction per unit mass. The bigger. Since the limestone dissolving reaction is a solid-liquid two-phase reaction, the reaction rate is proportional to the specific surface area of ​​the limestone particles. Therefore, the finer limestone particles have good dissolving performance, and various related reaction rates are higher, and the speed can be faster. It reacts with HSO ₃ ^- in the slurry to absorb sulfur dioxide gas more quickly, and the desulfurization efficiency and limestone utilization rate are higher.

However, the smaller the particle size of limestone, the higher the energy consumption for crushing. To grind into a finer limestone powder, a larger mill is required, which consumes higher electric energy, which increases the cost. Therefore, it is necessary to weigh the pros and cons and consider it comprehensively. The limestone powder usually required to pass through a 250 mesh sieve or a 325 mesh sieve has a sieving rate of 90%. At the same time, the particle size of the limestone powder is related to the quality of the limestone. In order to ensure that the desulfurization efficiency and limestone utilization rate reach a certain level, when the content of impurities in the limestone is high, it should be ground finer.

(3 ) Effect of limestone reactivity on the performance of desulfurization system
The characteristics of limestone as an absorbent include not only its chemical composition, but also its reactivity. The alkali amount of the desulfurization system is provided by the dissolution of limestone powder. The activity of the absorbent affects the solubility and dissolution rate of the absorbent. The conversion properties in an acidic environment. The activity of the absorbent includes the type of absorbent, the physicochemical properties, and the acidic environment with which it reacts. The physicochemical properties of the absorbent include purity, crystal structure, impurity content, particle size distribution, total unit surface area including the inner surface (i.e., porosity), and particle density. The higher activity of limestone can achieve higher sulfur dioxide removal efficiency while maintaining the same limestone utilization rate. Limestone has high reactivity and high utilization rate of limestone. The excess CaCO ₃ content in gypsum is low, that is, the purity of gypsum is high.

3 limestone powder preparation technology and process
The limestone powder for desulfurization requires 250 mesh or 325 mesh (45 μm), and the 325 mesh limestone powder has better desulfurization effect. However, the preparation of such fine limestone powder, the current main grinding machine is Raymond mill (pendulum mill), suspension roller mill, ball mill or European mill. In the past, Raymond mills, suspension roller mills, and ball mills used more. In recent years, with the demand for fineness of limestone powder and the need for energy saving and emission reduction, more and more power plants have been desulfurizing to prepare limestone powder. Mill program. The process flow for preparing limestone powder from the European version of the mill is shown in Figure 1.

It can be seen from Fig. 1 that the equipment used in the milling system is: crusher , hoist, electromagnetic vibrating feeder, and European version of the mill. Since the European version of the mill has a powder selection device inside, there is no need for an external classifier, so the process is simple. The process flow is briefly described as follows: After the large-sized limestone is crushed to the required particle size by the jaw crusher , it is sent to the storage hopper by the hoist, and then uniformly and quantitatively fed into the European version of the millmeal by the vibrating feeder. The machine is ground in the grinding chamber, and the limestone powder after grinding is blown by the wind and classified by a classifier.

The fine-grained limestone powder enters the new isolated cyclone dust collector with the airflow and is separated and collected. The collected limestone powder product is sent to the stone powder bin through the discharge port and used for desulfurization. The unqualified powder is separated by the powder separator and then smashed toward the wall of the tube, and falls back along the wall of the tube and then returned to the mill for grinding. The whole system is operated under negative pressure, and the system dust does not overflow, thus ensuring on-site cleaning.

Since the limestone entering the mill sometimes has a certain water content, the heat generated during grinding will cause it to become water vapor, increasing the total amount of gas in the system, and at the same time there will be air entering the system with the limestone due to the pipe joint. This will increase the total amount of gas in the system. The frictional heat during grinding also causes the gas to heat up and expand in volume. At this time, in order to maintain the negative pressure running state of the system, the valve between the fan and the main engine can be turned on, so that the excess gas is automatically introduced into the pulse bag filter and filtered out to ensure the environment is up to standard.

4 European version of the mill structure and working principle
The whole structure of the European version of the mill consists of a main machine, a classifier, a pipe device, a centrifugal draft fan, an isolated cyclone dust collector, and a dust collector.

(1) Working principle of the host
The motor drives the transverse drive shaft through the V-belt, and the transverse drive shaft drives the main shaft through the bevel gear at the other end. The upper end of the main shaft is connected with a grinding roller hanger, and the grinding roller member is suspended on the grinding roller hanger through the cross-arm shaft, and the entire grinding roller member can swing around the cross-arm shaft. When the main shaft rotates, the grinding roller member rolls around the inner circumference of the grinding ring under the action of the centrifugal force while rotating around the central axis. The lower end of the roller hanger is equipped with a blade holder, the blade holder is equipped with a blade holder, and the blade holder is provided with a blade. The blade edge shovel the material in the mill and send it to the grinding ring and the grinding roller for rolling.

(2) Working process of the classifier
The variable frequency speed regulating motor drives the leaf disc to rotate, and the blade on the leaf disc rotates to form a rotating air flow. The centrifugal force of the large granular material is large, and is thrown back to the mill wall after being thrown to the cylinder wall, and the qualified material passes through the classifier. The rotation speed of the motor should be adjusted according to the particle size of the finished product: if you want to obtain finer-grained dust, you should increase the rotation speed of the blade. If you want to obtain coarser-grained dust, you should reduce the rotation speed of the blade. The rotation speed should be carefully adjusted according to the mesh number of the finished product, the specific gravity of the material, the local temperature and humidity, the pressure and the specific characteristics of the particles, and the state of use of the mill. ,

Technical characteristics of limestone powder prepared by 5 European version of mill
MTW European version of the mill uses a number of patented technologies such as bevel gear integral transmission, internal thin oil lubrication system, curved air duct, etc. The performance is stable, easy to operate, low energy consumption, adjustable and controllable finished product size grinding materials are widely used. With special dust collectors, the dust emission concentration can fully meet the national environmental protection regulations, and has been widely used in many power plant desulfurization limestone milling projects under the powers of Guodian, Huadian, Huaneng and Datang.
The main features of the technology are:

(1) the entire drive bevel gears, energy loss, high efficiency: the traditional pendulum mill, a roll mill for an otherwise suspended reducer, the drive shaft through the coupling, difficulty in installation of large, easy to produce noise, reduction Efficiency, MTW European version of the mill is the overall drive of the bevel gear, the structure is more compact, the installation adjustment is more convenient and efficient, and the efficiency is greatly increased.

(2 ) Internal thin oil lubrication system, advanced and reliable : the traditional mill lubrication form is grease lubrication, lubrication resistance is large, temperature rises, bearing life is short, MTW European version of the mill uses an internal oil pump, no additional oil pump or lubrication station The lubrication of the spindle bearing and the tapered gear shaft bearing can be realized, which greatly improves the service life of the device.

(3) The mill air ducts in the traditional mill are all straight-type air ducts. In this structure, the airflow directly impacts the air duct plates to generate resistance. The energy loss between the air molecules colliding with each other is large, and the vortex is easily generated to cause the air passages. Disadvantages such as blockage. The curved air passage has small air loss and good material flow . The air duct of the MTW European version of the mill is a curved air duct. The tangential airflow inlet is smooth and the resistance is small. The internal outlet direction is conducive to the dispersion of materials and is not easy to block the material. The contrast between the curved air duct and the straight type air duct is shown in Figure 2.

(4 ) The special grinding roller and grinding ring structure is designed as a curved surface replaceable blade (see Figure 3 ), which improves the grinding efficiency and reduces the use cost . The traditional mill blade blade wears faster, the blade is a whole blade, and after the blade is worn, the blade needs to be replaced as a whole, waste material, and the stop replacement time is increased. MTW European version of the mill blade, the blade material of high wear-resistant alloy, long life, simply replace the replacement blade section, improved material utilization. In addition, the traditional flat blade, the material is piled up on one level after shovel, which makes the middle of the grinding roller wear seriously. The curved blade can guide the material to the façade, so that the grinding roller rings are upper, middle and lower. It can be ground to make it wear evenly, and it also increases the effective working area, thus increasing the output.

(5 ) The finished product has adjustable particle size and controllability, and the powder selection efficiency is high : the speed of the powder selection machine is controlled by frequency conversion, the rotation speed is more accurate, and the fine material sorting effect is better; the isolated cyclone powder collector and the powder selection efficiency are greatly improved, the inner cylinder The isolation structure is adopted between the mixed gas powder flow and the powder selection efficiency and the precision of the powder selection. Isolated cyclone powder collector and conventional powder collector

(6 ) Non-resistance air inlet volute (small observation door without eddy current) : the inlet volute of the traditional mill observes the inner surface of the door inside the door, and the inner surface of the inlet volute is not on a plane, so it is easy to produce The eddy current effect increases system energy consumption. The MTW European version of the mill uses the inner surface of the inner door plate and the inner surface of the inlet volute on the same surface, so that the eddy current effect can be effectively avoided.

(7 ) User-friendly design and easy operation : The European version of the mill not only adopts many advanced structures inside, but also adopts a beautiful curved structure design, which makes the whole machine more humanized and reduces unnecessary damage during the operation of workers. The possibility.

(8 ) Pay attention to the concept of environmental protection : the equipment has low noise and is equipped with advanced dust collector. The dust emission concentration is in full compliance with relevant national environmental protection standards. The airflow is sucked into the centrifugal induced draft fan by the upper return air pipe of the large cyclone collector. The entire airflow system of the induced draft fan is closed and circulating, and is circulated under positive and negative pressure conditions. The unique air self-circulation system greatly reduces the outside. Exhaust air volume, reduce dust emission concentration and avoid environmental pollution.

Selection of 6 European version of mill in environmentally friendly desulfurized limestone powder preparation system
The environmentally-friendly desulfurization limestone powder preparation system can be operated for 300 days per year, and the effective running time per day is 20 hours. According to the amount of limestone powder required for environmental desulfurization for one year, the required single-stage output is calculated, and then the single-stage production time of the single machine is selected. Model and specifications. The specifications of the European version of the mill for limestone powder preparation are shown in the table below.

7 Conclusion
In summary, the environmentally-friendly desulfurization limestone powder preparation technology adopting the European version of the milling mechanism powder technology, the fineness and particle gradation of the product are uniform, which can well meet the limestone powder required for flue gas desulfurization and improve the desulfurization efficiency. and the production line process smooth, investment and start fast, easy production management, comprehensive energy consumption than traditional Raymond, suspension mill, ball mill and other processes by 20% to 30%, a substantial increase in product quality, thermal power, iron and steel The company's energy saving and emission reduction provides reliable technical and equipment support.

Author: Houxian Yong, Xiao Wei (Henan Liming Heavy Industry Science and Technology Co., Ltd., Zhengzhou 450001)