Equipping the cement industry with dust systems at that time was a significant cost that many industry owners could not afford.

With the change in the structure and upgrading of dust systems and the advances of the present age, economic issues and their associated costs have been affected.

The main sources of pollution and dust emissions in the cement industry are cement kilns, their fuel consumption, raw materials and cement mills, other equipment such as transmission and feeding systems, silos and packing machines are also emitted in dust emissions.

 Characteristics of particles

 Particle size

Particles are divided into four groups: dust (Dust), mist (Mist), fume (Fume) and phage (Fog).

Due to the variety of particle sizes in different parts of the cement production line, gas purification equipment in different parts must have the properties related to those materials.

 Relative particle density

Density of particles has a great effect on separation efficiency, so that separation of particles with low density is difficult because they tend to be suspended in the gas.

Depending on the technology used, two main categories of dusting equipment are usually used in the cement industry, which include electrofilters and bag filters.

 Electrofilters

Until the late 1990s, electrofilters were used to dust combustion gases in cement kilns. Electrofilters work by using an electric field to separate dust particles suspended in the gas. Under the influence of the electric field, the dust particles are absorbed by the collecting electrode. The rate of particle adsorption by the collecting electrode depends on the potential difference between the collecting electrodes and the discharge electrodes (Discharge). The particles adsorbed by the collecting electrode are separated from the electrodes to a lesser extent by the discharge electrode by mechanical equipment (impact hammers) and are driven out by transmission systems such as conveyors and helical systems installed at the bottom of the electro-filter.

In recent designs, the discharge electrode is made of a rigid strip and the collecting electrode is made of a corrugated steel plate. One of the advantages of this method is that the discharge electrode does not twist towards the collecting electrode (due to its rigid structure) and creates a uniform corona along the entire length of the discharge electrode. One of the basic characteristics of electrofilters is the potential difference between their electrodes.

 One of the factors affecting the efficiency of electrofilters is the impact system, which includes the following:

– The time cycle of hammer blows.

– Intensity of hammer blows

Usually the time cycle of the hammers is set in the period and start-up for the maximum input to the electrofilter. Studies have shown that the adjustment of the impact cycle of impactors according to the amount of input dust has a positive effect on increasing the efficiency of electrofilters. Excessive increase of the time cycle prevents the formation of a layer of dust on the plates, so it intensifies the intensity of sparking (Sparking) and this phenomenon, in addition to reducing the voltage and reducing the dusting efficiency, also reduces the life of the electrodes. At the same time, we have not taken the opportunity to stick the materials, which have the property of adhesion, to prevent the particles from returning to the gas stream.

The short cycle time of the impactors increases the thickness of the bed plate. If the bed thickness of the material exceeds 10 mm, it will cause the following problems:

Excessive increase of the material layer on the plates reduces the distance between the electrodes, which causes a spark in the electro-filter. In response to such conditions, the control system reduces the voltage and consequently the current, which will reduce the particle charge rate as well as the force applied to the particles moving towards the collector plate, and given that the electrofilter efficiency It directly depends on the particle adsorption rate, so it reduces the efficiency of the electrofilter.

Excessive increase of the material layer leads to a decrease in the useful volume of the gas flow and an increase in the gas flow velocity, which leads to the return of the material to the gas flow (Rc-entrainment). The accumulation of material on the discharge electrodes increases the voltage for a given working current. The effect of material accumulation is equivalent to the effect of increasing the diameter of the discharge electrode, and since the corona voltage is a function of the diameter of the discharge wire, this voltage is increased and the curve on the plates . Therefore, by considering factors such as dust concentration in the gas, inlet gas flow rate, dust absorption intensity in each field and the thickness of the dust layer on the positive electrodes, the impact cycle time of the drums is adjusted.

In view of the above, we conclude that in order to eliminate the negative effects of increasing or decreasing the impact timer cycle too much on the efficiency of electro-filters, it is necessary to take the impact timer cycle out of the fixed state and operate it under different conditions.

In addition to the timing cycle of the impactor, the intensity of the impact also affects the efficiency of the electrofilter. If the shocks are more than the designed design of the system, it causes the material to be completely drained from the surface of the electrodes, thus increasing the possibility of direct electrons hitting the plates, which due to electrical discharge, voltage and milliamperes are reduced and filter efficiency is reduced. In addition, inflicting stronger shocks increases the possibility of particles returning to the gas stream.

If the insulator and the plate holder system are not provided for stronger shocks, blows can cause cracks in the insulator that will lead to electrical leakage and ultimately reduced current.

Factors such as wear and tear of the anvils, derailment of the anvils cause the hammers to deviate from the adjustment position and affect the intensity of their direct blows.

The effect of gas distribution or in other words gas velocity profiles in electrostatic fields on the electrofilter efficiency in the famous Deutsch Anderson relation, which represents the electrofilter efficiency, is as follows.

-AW / V

n= I-e

n = Electrofilter dusting efficiency in%.

A = surface of dust absorbing electrodes (collecting electrode)

W = Speed ​​of particle adsorption by plates

V = gas flow

The effective surface area of ​​the adsorbent plates is one of the most important variables in the electrofilter efficiency according to the Deutsch relation. Therefore, one of the ways to increase the efficiency of electrofilters of working units is to increase the effective level of plate absorption. Which is done by increasing the electrostatic fields. Another parameter affecting the efficiency of the electrofilter is the change in the grain size distribution, which is done electrostatically due to changes in the process and its parameters by the following two mechanisms:

Pregnancy through electric field (Field charging

Diffusion charging

Particles larger than 0.5 m are charged with the first mechanism and particles smaller than 0.2 m are charged with the second mechanism. Therefore, intrinsic pregnancy, which is between 0.2m and 0.5m, is difficult. Therefore, the efficiency of the electrofilter to absorb particles in this range is greatly reduced. In other words, the particle size of the chimneys is in this range.

For particles in the above range, the dusting efficiency is equal to 80% and for particles larger and smaller than this range is much higher. About 90% of particles above 10m are separated by 20% of the initial length of the electrofilter. But by the end of it, only 80% of the particles are separated between 0.3m and 0.6m. Therefore, increasing the particle size increases their migration speed to the adsorbent plates, which also increases the filter efficiency.

Another parameter affecting the efficiency of the electrofilter is the temperature and humidity of the gas. So that its performance is optimal in a narrow range of gas temperature and humidity. Increasing the gas temperature directly involves increasing the gas velocity in the electrostatic field. Increasing the gas velocity reduces the rate of particle migration to the plates and reduces the particle retention time in the electrostatic field.

Increasing the temperature and decreasing the gas humidity increases the particle resistance. Because the performance of the electrofilter is optimal in a narrow range of the electrical resistance of the particle.

Lack of adjustment of electrode distance is one of the most effective variables in reducing the efficiency of electro-filters. Manufacturers of electrofilters with a positive electrode distance of 30 cm. A permissible tolerance of 1.5 mm and a maximum of 2% is recommended. One of the effects of not adjusting the distance between the electrodes is to reduce the voltage and current. In electrostatic fields, the maximum current and voltage between the electrodes are adjusted based on the shortest path with the least resistance. Therefore, current and voltage control will be performed at the point that has the shortest distance. Therefore, the spark phenomenon occurs at the point where the distance between the electrodes is small. Therefore, the electrical energy taken from the grid is directed to the spark point, which is enough energy to melt a small amount of the electrode at the spark point, which eventually leads to the rupture of the electrode. However, mechanical erosion, crimping, and corrosion, or a combination of these, all play a role in thinning the electrodes and eventually rupturing them. Usually, interfering with the standard distance of the electrodes plays an important role in this.

The most common method, in order to identify and track the amount of deviation of the electrode distance, is to perform an air load test in the electrostatic field, in which by measuring the life voltage of the field and plotting the current voltage characteristic curve and comparing it with the initial standard conditions, the value The deviation of the electrodes is determined.

To draw the current-voltage curve while the control panel is in manual control mode, the current and voltage measurement is started at zero and gradually increases the voltage, at the starting point of the corona, a sudden change is observed. The secondary current is then increased in steps such as 50 mA by 100 mA and the amount of current voltage is recorded in a table. This increase continues until the spark conditions are reached.

The reasons for not adjusting the distance between the electrodes are:

 Bending of the plates during installation or construction.

 Wrapping or twisting of the discharge electrode retaining frames during fabrication.

 Loosen the discharge electrodes (stiffness of the electrodes should be 15kgf).

 The percussion system is not adjusted.

 Filling hoppers Accumulation and discharge of material due to clogging and accumulation of material under the electrode holder

 Lack of adjustment of ceiling hangers and sandals holding the electrodes.

Electrofilters have many advantages. For this reason, they have found a special place and many applications in the cement industry. The main advantages of using electrofilters are the following:

 Low resistance to gas flow, approximately 100 to 200 Pascals.

 Ability to work at high temperatures (even more than 400 degrees Celsius).

Has a relatively high efficiency. Electrofilters are able to absorb a wide range of dust particle sizes.

 Has a very low dust output, about 10 to 50 mg per cubic meter.

 Easy access to spare parts for periodic repairs.

Among the above, the first two advantages are the most important advantages of the extensive use of electro-filters to de-dust the gases caused by the combustion of cement furnaces.

 The high cost of buying, installing and operating them.

 High cost of using the system in terms of energy consumption

 The need to take the system out of the circuit due to issues such as rising CO in the production process, which stops the dusting operation when the voltage is cut off.

 Has a low efficiency compared to the amount of energy consumed and maintenance costs.

The last two require a search for new solutions. Also, the new environmental laws, which have been developed according to the standards of European countries, increase the need to pay attention to this issue.

 Bagfilter

Another equipment that is widely used to de-dust the air in the cement industry are bag filters. These types of filters have a high dust efficiency, which according to the type of dust is up to 99.9% efficiency. A factor that limits its use. Is the temperature of the inlet gas to it. With new advances in the production of bag fibers, the scope of their use has become wider. With the use of new textile products, the bags can withstand temperatures above 200 degrees Celsius. The following factors play a key role in the application of bag filters:

 Material of bags used

 Dust removal area

 The structure and shape of the bags

 Type of bag dusting system

The latter leads us to pulse-based dust systems. In this type of bag filters, pulsation of bags is done by injecting a pulse of compressed air in a short time (approximately 1-0 seconds). These types of filters are widely used in various parts of the cement industry.

 The pulse jet filter mainly has six main parts. These parts are:

 Inlet duct and dust inlet chamber with filter.

 Filter fiber bags

 Mesh (bag and shelf holder)

 Pulse valves

 Blower tube to bags (pulse jet nozzle tube)

Exhaust (refinery gas chamber and duct).

The dust-carrying gas enters the filter and is returned by a shield (baffle) under the bag to the material outlet hoppers.

The inlet is responsible for reducing the velocity of the gas, allowing large particles to fall freely into the hoppers under the filter, and distributing the gas carrying the fine dust particles evenly between the bags. The gas distribution ensures that the gas flow on the bags does not cause excessive wear. The dust carrier gas goes to the bags and from there enters the cleaning air chamber and then is sent to the chimney by the suction fan. At the same time as the above operation is performed continuously, the pulse air is blown into the bags periodically through the pulse valves and nozzles by the electronically programmed panel, causing the material cake to separate from the outer surface of the bags. The separated material enters the hoppers and from there is directed to the outside of the filter by transmission systems.

Compressed air pulse that cleans the bags is not depreciated due to the volume of relevant equipment such as gear motors, hangers, positive and negative anvils compared to the impact hammers of positive and negative electrodes in electrofilters and does not withstand mechanical repairs and adjustments. .

When using a filter to dust the gases from the combustion and cooking of clinker, an important issue to consider is the measure of the resistance of the filter bags to high temperature gases. With the change of clinker production technology, the use of one of the filters has changed, the last of which is lowering the temperature of the exhaust gases to a temperature of less than 200 degrees Celsius.

Using one of the filters de-dusts the gases up to 10 mg / m3.

 The most important advantages of using jet pulse filters are:

– High user efficiency

– Low user costs

– Resistance to flammable gases, explosive dust particles and ..

– easy usage

– High durability and durability of bags

 Jet pulse filters have the following disadvantages:

– High resistance to gas flow (2000 to 3000 Pascals).

– Low efficiency against wet gases

– Limiting the temperature of the inlet gases due to the material of the bags

Proper use and high efficiency of a filter depends on the correct choice of filter and observance of the correct principles of installation, equipment and use.

 Poor performance of filters is usually due to the following factors:

– Improper installation of equipment (incorrect configuration of the suction system, problems in the dusting system of bags, incorrect installation of bags, etc.)

– Wrong filter selection (low filter capacity compared to the dusting area).

– Using bags with inappropriate material

– High humidity of the inlet gas

An important factor that should be considered when determining the area of ​​dust removal is to determine the appropriate speed of the dusting operation, which should take into account the following:

– If the existing dust density is more than 100g / m3, the dust collection rate should be Im3 / m2 / mm.

– If the dust density is between 50g / m3 to 100g / m3, the appropriate dusting speed is 12-1.4m3 / m2 / mm.

– If the dust density is less than 50 / g / m3, the suitable speed for dust removal is 1.5 m3 / m2 / mm.

Due to the advantages of jet pulse jet filters, their use in different parts of the cement production process is increasing.

 Baghouse

Another dust collection system is a house filter. In this type of filters, bags are also used for dusting operations. In this system, there are different methods and methods for dusting bags, which are:

– The method of injecting compressed air pulse in some devices, unlike bag filters, where the pulse is applied according to a specific ground program, the air pulse is applied intelligently and taking into account the pressure difference in the filter.

– Physical shaking of the bags by mechanical shaking systems to separate the layer (cake) of dust that is sitting on them.

– Reverse air system, in this method, the dust cake layer is cut mechanically by applying compressed air on the bags and separated from them.

Some systems use a combination of the above three methods.

 Hybrid composite filters

Although electrofilters have high mechanical strength and temperature resistance, they can be very expensive when the output dust level is less than 10 mg / m3.

Electrofilters have a lower pressure drop than bag filters in different volumes of gas. Also, although the filter bag can separate more dust from the electrofilter and work independently of gas and dust conditions, it is temperature sensitive.

If increasing the efficiency of the filter is intended by increasing the efficiency of the cement production line, the electrofilter will be very expensive. Especially when a house is added to the filter with a call filter, the cost is very high.

Converting an electrofilter to a bag filter requires a new and more powerful fan than the previous electrofilter fan. Therefore, in order to increase the efficiency of dust collection systems and use the advantages of electrofilters and bagfilters simultaneously, the idea of ​​combining the above two systems led to the birth of a hybrid filter combined in the family of dust collectors. The interaction of two electrofilter and bag filter systems in a single system has increased the efficiency of the system in absorbing very fine dust particles and the formation of an integrated, stable and economical system.

 The advantages of using hybrid systems include the following:

– Excellent efficiency in particle adsorption more than 99.99% (for different particle sizes)

– Bags in this system have a longer useful life and higher efficiency due to protection against coagulation of large particles (due to passing through the electro-filter stage).

– Shrinking equipment size due to the use of 65 to 75% of the equipment of a conventional bag filter and the use of a percentage of conventional electrofilter parts.

– Optimal energy consumption

– Easier repair, maintenance and upgrade of the system due to the lack of need to add many control parameters to it

With the widespread use of dust collection systems in the cement industry, this branch of industry has also joined the ranks of environmentalists.