ANALYSIS OF DEVICES FOR CLEANING DUST GASES GENERATED IN DRY CEMENT PRODUCTION PROCESSES

ABSTRACT

The article presents an analysis of dust-cleaning methods and devices for cleaning dusty gases generated during cement production processes. In order to choose the optimal construction for the process, a four-stage structural analysis was conducted based on the MatLab program, and the parameters of each stage were studied. Light filters were recommended as the optimal design for the process. In addition, it is determined that the total contact surfaces of the phases should not exceed 82% of the total working volume, the non-working zones should not exceed 16%, the aerodynamic resistance should not exceed 4 kPa, and the dust gas temperature should not exceed 450-500 ℃.

АННОТАЦИЯ

В статье представлен анализ методов пылеочистки и устройств для очистки запыленных газов, образующихся в процессах производства цемента. С целью выбора оптимальной конструкции для процесса был проведен четырехэтапный структурный анализ на основе программы MatLab и изучены параметры каждого этапа. Светофильтры были рекомендованы как оптимальная конструкция для данного процесса. Кроме того, определено, что суммарные поверхности контакта фаз не должны превышать 82 % от общего рабочего объёма, нерабочие зоны не должны превышать 16 %, аэродинамическое сопротивление не должно превышать 4 кПа, температура пылегазов не должна превышать 450-500 ℃.

Keywords: cement, rotary kiln, systematic analysis, hierarchical level, stage, filter, scrubber, cyclone, electro filter.

Ключевые слова: цемент, вращающаяся печь, систематический анализ, иерархический уровень, ступень, фильтр, скруббер, циклон, электрофильтр.

Introduction. In the realm of dry cement production processes, the effective management of dust gases has become a paramount concern for both environmental sustainability and operational efficiency. The diverse array of devices designed for cleaning dust gases presents a critical facet of this management strategy. These devices exhibit variations in construction and cleaning methods, categorizing them into distinct groups based on mechanical and electrical methodologies. The mechanical methods, a primary focus of this analysis, further branch into dry and wet devices, each offering unique approaches to tackle the challenges posed by dust gases in cement production. Additionally, within the realm of electrical methods, a dichotomy emerges in the form of one-zone and two-zone device types, each tailored to address specific nuances in the dust gas cleaning process. This paper embarks on an exploratory journey into the intricacies of these devices, delving into their structural diversity and cleaning mechanisms. By understanding the distinctions within these device categories, industries engaged in dry cement production can make informed decisions to optimize their operations, enhance environmental stewardship, and align with evolving regulatory standards. Figure 1 provides a visual roadmap, illustrating the classification of dust gas cleaning devices, setting the stage for a comprehensive analysis of their functionalities and implications within the context of modern cement production processes.

Figure 1. Classification of dust cleaning devices

It is important to correctly select the appropriate dust cleaning schemes and devices to ensure that the dust content does not exceed 6 mg/m³ on site and 0.1 g/m³ in the atmosphere within the sanitary standards (SN 245-71).

There are widely used methods of capturing cement dust under the influence of gravity, inertial force, centrifugal force, wet method, and high voltage electric discharge.

These dust capture technologies are selected depending on the temperature, humidity, dust concentration dust particle size, density, abrasiveness and electrical resistance of the gas being cleaned. Studies have shown that up to 10-15% of the product may come out as dust. If the technological regime is violated, the speed and temperature increase, the dust emission can increase even more. At present, to reduce the emission of dust, a ring-shaped chain is hung in the furnace's glow zone. This action reduces the dust emission from the furnace by 3-5 times.

The use of natural gas and liquid fuel in rotary kilns reduces dust emissions by 30-50%.

Methods such as spraying water on a dry mill, maintaining a stable technological mode, hermetizing grinding, transporting, and pouring processes, and reducing the speed and height of material fall also reduce dust emission.

Of course, these methods reduce the loss of raw materials, the consumption of fuel and cement, and reduce the emission of dust into the atmosphere. However, these methods cannot ensure the level of sanitary standards. That is why special dust-cleaning equipment is installed for cement production enterprises.

Methodology. As mentioned above, dust cleaning devices are selected taking into account the characteristics of dust particles, and gas conditions (temperature, dew point). At the same time, the capabilities of the dust capture device are also taken into account. Because the dust is separated from the gas flow under the influence of such forces this force should be able to force the dust particle to move separately concerning the gas flow, i.e. to move in a perpendicular direction, or to move in a direction other than the direction of the flow, in a direction that does not correspond to it. Dust in a gas stream is separated from the gas stream under the influence of forces that force it to move in a different direction relative to this stream. According to the nature of this driving force, cement dust capture devices are divided into the following types:

1. Dust settling chambers - in these devices, cement dust settles under the influence of gravity. The device is simple, low energy consumption, but it can be used effectively only for the deposition of dust with a size of more than 50 μm. Also, such devices work well in laminar mode with a low gas flow rate. For this reason, dust deposition chambers are mainly used as a primary capture device for large dust particles.

2. Aspiration-coagulation mines - in these devices, dusty air moves from bottom to top at a speed of 1-1.5 m/s. it is mainly used to catch the dust of dry mills. If the dusty air supplied to it enters with a dust content of 300 ÷ 500 g/m3, after the apartment, this indicator drops to 25-60 g/m3. That is, the dust is reduced by 5-10 times. In aspiration coagulation mines, in addition to the forces of gravity, coarsening (coagulation) occurs during the upward movement of dust.

3. Cyclones - in these devices, cement is deposited under the influence of centrifugal forces. The gas flow speed should not be less than 18-25 m/s so that the centrifugal force settles the dust, that is, it hits the wall of the cyclone apparatus and settles.

In cement production, cyclones are widely used mainly for primary dust capture. Cyclones are used as a group of cyclones that connect several cyclones to increase performance. Compared to dust deposition chambers, cyclones are more efficient. They can also clean high-velocity gases with a size range greater than 5 µm.

4. Electric filters - these devices are now widely used in almost all cement enterprises.

The working principle of the device is based on the ionization of gases under the influence of high-voltage electric current. There are settling and shaking mechanisms inside the apparatus body. Dusty gas is introduced between the electrodes at a speed of 0.5-2m/s. The performance of the device is high. However, the structure of this high-performance multi-field apparatus is very complex, it has many moving mechanisms, and parts, high energy consumption, takes up a lot of space (large dimensions), and has a high mass. At the same time, it is difficult to dispose of the dust caught in it. Because, no matter where the ultra-fine dust is returned from the technological unit, it does not allow to lose the opportunity to leave the rotary furnace again together with the combustion products in a large flow.

5. Centrifugal scrubber (or water hydrocyclones) is the simplest device working in the wet method, its body is cylindrical, and the lower part consists of a short body.

Dusty gas is introduced from the lower part of the device body in a tangential direction at a speed of 15-23 m/s. The dusty gas begins to rise the casing along the spiral line. On top of the cylindrical body, circular nozzles are installed, in which water is sent to the body in the pilot's direction. This water flows down and forms a film on the inner wall of the apparatus.

The dust particles contained in the dusty gas rising from below hit the wall under the influence of centrifugal forces, get wet in the water film and flow into the cone part of the apparatus in the form of a pulp. These wet machines have a high level of cleaning, but these high-efficiency machines also capture non-hardening dust and provide high-quality cleaning of the smallest particles. The process of wet dust capture is based on the contact of a dusty gas stream with a liquid, in which the liquid wets the suspended particles and removes them from the apparatus in the form of a slurry. Wet dust capture is a very simple and effective method.

6. Filters - it is known that the method and equipment for cleaning gases from small particles are selected based on the size of aerosols (dispersed content).

The most effective way to clean gases with a particle size of 0.05÷100 µm is their filtration, in which the gas stream to be cleaned is passed through a porous material of different densities and thicknesses. In this case, the main part of the particles contained in the gas flow is caught on the surface of this porous material (filter).

A filter filled with coke, sand, gravel, and nozzles of various shapes and sizes is used to catch larger dust. Paper felt or material made of fibres of different thicknesses and densities is used to catch very small dust.

Results. Selection of the analyzed methods and devices based on the physicochemical parameters of dusty gases, technological regulations and permissible norms of generated dusty gases makes it possible to save energy spent on the cleaning process. In addition, it makes it possible to maximally prevent the emission of dusty gases into the atmosphere, which is considered one of the urgent issues of today. Based on the above, the process of cleaning dusty gases generated in rotary kilns during the cement production process was selected as a research object, and a systematic analysis was carried out to choose a suitable dust-cleaning device for the process. Systematic analysis was carried out in four stages based on MatLab software.

The scheme of determining rational values in the form of systematic analysis in four steps is given in Fig. 2.

The information given in Figure 2 allows you to find the types of dust gas cleaning devices that can be used in the process, their operating parameters, and optimal modes. Each column of the four-step steps is important. Based on the task defined above, to determine which structure is appropriate to use as an aspirator in cement production processes, a four-factor systematic analysis method was used to fully compare the parameters of existing structures and reduce the error of the results, as well as to spend minimal energy on the process [1-4].

Cyclone, scrubber, electrostatic precipitator, dust settling chamber and light filter constructions were selected as the objects of systematic analysis. Systematic analysis was carried out in the following sequence:

Analytical analyses have shown that electrofilters and light filters are the most optimal constructions for cleaning dusty gases in high-temperature environments. In terms of the amount of energy spent on the process, the lightest filters were chosen [5].

In the second step, phase interaction and contact surfaces were considered. The input and output parameters of each auxiliary system are defined [6].

According to the results of the analysis, the collision surfaces of the filter and dust particles in narrow filters make up 82% of the total working volume. In addition, the fact that the fan is installed on the suction pipe significantly reduces aerodynamic resistance. It was determined that airflow filters are an option for the cement dust cleaning process, as the dust level and temperature of the air stream do not lose their performance even in high environments.

In this case, the level of dust removal is higher than 95%, and the aerodynamic resistance of the device must be 2-4 kPa.

In the first initial step, during the mineral dust cleaning process, the hot air and its transfer elements, the dust cleaning zone and the elements for releasing the cleaned air were considered in the devices. System input and output parameters are defined.

Figure 2. Optimization scheme in the form of systematic analysis in four stages

The analysis showed that electrofilters and light filters are considered the most optimal constructions for cleaning dusty gases in high-temperature environments. In terms of the amount of energy spent on the process, the lightest filters were chosen [7,8].

In the second step, phase interaction and contact surfaces were considered. The input and output parameters of each auxiliary system are defined [9,10].

According to the results of the analysis, the collision surfaces of the filter and dust particles in narrow filters make up 82% of the total working volume. In addition, the fact that the fan is installed on the suction pipe significantly reduces aerodynamic resistance. It was determined that airflow filters are an option for the cement dust cleaning process, as the dust level and temperature of the air stream do not lose their performance even in high environments. In this case, the level of dust removal is higher than 95%, and the aerodynamic resistance of the device must be 2-4 kPa.

In the third step, the size of the device, filled with working details, was seen. Input and output parameters are determined [11].

In the interaction of the phases, taking into account that the total working volume of the device is 500 m³ corresponding to the volume of dusty airflow and the aerodynamic resistance is in the range of 2-4 kPa, it was determined that the volume filled with working details is in the range of 400-450%. In the fourth step, the technical economic efficiency of the devices, energy consumption and the permissible dust gas temperature in the devices were seen. Input and output parameters are defined.

In addition, it was determined that the total contact surfaces of the phases should not exceed 82% of the total working volume, the non-working zones should not exceed 16%, the aerodynamic resistance should not exceed 4 kPa, and the dust gas temperature should not exceed 450-500 ℃.

Conclusions. From the results of the above analysis, it can be seen that the most optimal option for the cement dust cleaning process is narrow filter devices, and the use of this type of device brings technical and economic benefits. Currently, there is a tendency to use mainly fabric or mesh filters in the cleaning of dusty gases in industrial enterprises, including cement production processes. Devices of this type belong to the group of independent devices that work in the dry method. But the parameters in the process, i.e. the temperature of the dusty gas and the high degree of dustiness of the air, mean that there is a need to improve this type of device. In this case, it is desirable to study and create new types of existing filters made of fibre, cloth and seed materials.

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