1 Background of transformation

A cement company has twoFor the 5,000 t/d cement production line, the kiln tail is equipped with a high-temperature long-bag pulse dust collector, which was put into production and operation in April and May 2012 respectively. After a period of operation, according to the owner's reflection and through on-site investigation, the dust collector has the following problems:

（1) The resistance of the kiln tail dust collector system is high, the maximum is about 1 · 800 Pa, and the resistance is still high after upgrading. The high resistance increases the energy consumption of the system and restricts the output of the production line.

（2) The damage of the filter bag causes the emission of the equipment to exceed the standard, which cannot meet the emission standard requirements.

（3) The inspection door and upper box of the dust collector are seriously corroded. Although the anti-corrosion treatment is repeated, the effect is not ideal.

In summary, it is imperative to transform the kiln tail bag dust collector.

2 Modification scheme

comparison and selection of 2.1 renovation schemes

According to the actual production situation, in order to achieve less investment, low resistance, low emission concentration for the purpose, for the kiln tail bag dust collector transformation ideas have two programs.

1) Scheme I

Retain the air inlet and non-standard pipes of the dust collector, and use the original bag filter shell to transform and increase the filter area of the filter bag. At the same time, the inertial dust removal device is added inside the dust collector, and the new type of dust removal device is adopted.The walk-in structure can reduce the condensation caused by the air leakage of the equipment; on the other hand, the dust concentration is reduced by the early capture to reduce the load of the dust collector and fundamentally reduce the dust emission of the equipment. In addition, after the equipment modification, the resistance of the dust collector will be reduced by 400~500 Pa, which can reduce the energy consumption of the fan. The number of pulse valves is the same before and after the transformation, and the consumption of compressed air is basically unchanged.

（1) Keep the column, ash hopper and corresponding ash conveying system of the original bag dust collector, and remove the original top clean air chamber, lifting valve box, lifting valve and canopy.

（2) Increase a certain shell height, install newly designed blowing unit, clean air chamber box, air outlet pipe, valve, stair railing, etc. The size of the filter bag was changed from 160 × 6 000 to 160 × 7 000, increasing the filter area.

（3) In order to reduce the damage of the filter bag, a diversion device is added at the air inlet of the ash hopper.

（4) Keep the lower shell of the original bag filter, and carry out local reinforcement on the seriously corroded column and the pressure plate on both sides.

Advantages of Scheme 1: (1) Reduce the air leakage rate of the system and reduce the phenomenon of "paste bag" of filter bags;(2) dust emission value ≤ 10 mg/Nm3, strive for ≤ 5 mg/Nm3;(3) The structural flow field is smoother, the import and export resistance is greatly reduced, and the required energy consumption is obviously reduced;(4) The air flow uniform distribution device makes the internal flow field of the equipment more uniform;(5) After the transformation, the overall load of the newly added dust collector and the retained original dust collector is less than the original civil foundation design load after verification, and the overall design is safe and reliable, so no additional civil construction is required. Disadvantages:(1) The investment is relatively large, and the total weight of the newly added part of the equipment is about 250 t (including local reinforcement of the internal corrosion part);(2) The demolition workload is large, and the installation workload is also large.

2) Scheme II

Make full use of the original equipment components, keep the existing equipment still, remove the air outlet flange and its side end plate, and add new one-span equipment. New steel columns, ash hoppers, bottom beams, shells, air cleaning chambers and other components need to be installed from the ground to connect the original non-standard pipelines and ash conveying system (with feeder and gate valve to lock the air), which is consistent with the original equipment.At the same time, the air leakage part of the original dust collector needs to be repaired.

Advantages of Scheme 2: (1) The investment is relatively small, and the total weight of the newly added part of the equipment is about 160 t (including local reinforcement of the internal corrosion part);(2) The workload of dismantling and installation is small;(3) The import and export resistance has been significantly reduced to meet the requirements;(4) The dust emission value has been significantly reduced to meet the requirements. Disadvantages:(1) Due to the local transformation of the equipment, the dust emission value and resistance and other equipment performance are easily affected by the original equipment;(2) After the local transformation, the air leakage rate of the original equipment cannot be guaranteed, and the system resistance value is reduced slightly;(3) Involving some civil engineering, the construction is relatively cumbersome.

The above two schemes have advantages and disadvantages, but the improvement in dust emission and resistance reduction is obvious. The first scheme has a slightly larger investment but the effect is obvious after transformation, and various performances are guaranteed. The performance of some parts of the scheme 2 transformation is guaranteed. Due to the connection with the original equipment, there are uncertain factors, and some parts of the original equipment are seriously corroded, so the safety and reliability of operation are not as good as that of scheme 1. Comprehensive consideration of comparison, the adoption of a transformation. The parameters of the original bag dust collector and the modified dust collector are shown in the table.1.

Table1 Kiln tail bag dust collector transformation design parameters

2.2 shell modification

When removing the original bag filter, it is found that the shell column of the original filter and the shell plate near it are seriously corroded (see figure1). In order to ensure the safe use of the equipment, the corroded shell plate is pasted and strengthened, and the corroded and deformed original shell column is reinforced. However, if the corrosion is reinforced inside the shell, the newly added column is too close to the filter bag, which increases the risk of damage to the filter bag, and the internal construction of the dust collector is difficult, which will inevitably affect the construction period of the project. Considering various factors, it is decided to adopt the scheme of reinforcing the outside of the shell, remove the insulation near the original shell column, add profiles at the original column, and form a combined section with the original shell column to improve its design strength. At the same time, the shell plate with serious corrosion at the connection with the column is reinforced. The external reinforcement can be carried out when the dust collector is insulated as a whole after ignition, which can shorten the construction period.

Figure1 Original dust collector shell corrosion

Equipment Adoptionwalk-in structure (bag change in upper box), a square double-layer sealed access door is set on the upper outer plate of each upper box to avoid the risk of air leakage from the original top bag change structure, and the air leakage is minimized when checking the dust collector body and dust collection system.

The renovation project adopts modular installation. The upper box body and the blowing box are pre-assembled before hoisting, and then the whole hoisted to the original equipment shell, which greatly reduces the hoisting times of the crane and the welding workload of the workers working at high altitude, and saves the installation cost and installation period.

Selection and comparison of 2.3 filter bags

The temperature of the flue gas at the end of the kiln is high, and the temperature of the exhaust gas at the inlet of the dust collector is130~180 ℃, the maximum temperature can reach 260 ℃, the flue gas dust concentration is high, at 100g/Nm3, and the flue gas composition is complex, containing S, Cl, NH3 and other corrosive components. Therefore, the filter material should not only adapt to the efficient collection of dust, but also adapt to the high temperature environment. Therefore, the filter bag made of glass fiber film material is selected, which has the following characteristics:

（1) High temperature resistance, long-term use under working conditions up to 260 ℃, and good dimensional stability. The bag cage will expand when heated, reduce the friction between the filter bag and the bag cage steel wire, and play a role in protecting the filter bag.

（2) High filtration efficiency. Membrane filter material is mainly through the microporous polytetrafluoroethylene membrane surface filtration. The micron-sized pore size has extremely high filtration efficiency.

（3) good corrosion resistance. Glass fiber coating material is made of composite puffed microporous polytetrafluoroethylene film on glass fiber base cloth with special surface treatment. Its surface is smooth, hydrophobic and breathable, has good chemical stability and high corrosion resistance.

（4) Due to the self-cleaning and hydrophobic characteristics of the surface film PTFE, the film-coated filter material is easy to remove dust, and the filter material is a surface filter dust that will not penetrate into the inside of the filter material.

（5) Pre-coating shall be sprayed on the filter bag before commissioning of the equipment. In order to avoid the phenomenon of condensation paste bag on the surface of filter bag caused by condensed water during ignition.

Reconstruction of 2.4 air inlet pipe

At present, the equipment adopts the mode of air inlet from the lower part of the ash hopper, and the dust is divided from the air inlet pipe of the dust collector to the bag chambers on both sides. Considering the problem of dust accumulation at the inlet, the included angle between the air duct at the inlet of each bag chamber and the ash hopper isMore than 50 degrees, and the inlet wind speed is about 10 m/s or even higher. Such a large wind speed directly rushes into the bag chamber along the air inlet pipe, and a "turbulent flow" surface will appear inside the ash hopper, and the wind direction will also appear irregular movement state. In this case, "secondary dust" will appear inside the ash hopper and the bag chamber, and at the same time, local filter bags will also appear scouring phenomenon, the dust cannot settle normally, which makes the resistance of the equipment high. Due to the uneven load for a long time, the filter bag will be "hairy" and finally damaged.

ByCFD flow field analysis, combined with the specific situation of the actual site, the original ash hopper air inlet valve was removed, and a reasonable air flow uniform distribution device was added in the ash hopper air inlet and air duct to improve the air flow distribution in the ash hopper (see Figure 2). In fig. 2, the cross-sectional velocity of the ash hopper inlet reaches 10 m/s, and after passing through the newly added diversion device at the ash hopper inlet, the velocity obviously drops to about 5 m/s. At the same time, it can be seen that the airflow distribution is improved after passing through the guide device, and the whole enters the bag chamber more evenly, which protects the filter bag in the bag chamber.

Figure2 Airflow velocity streamline distribution in ash hopper

3 Effects

After the transformation of the dust collector has been in stable operation, the indicators are in line with the requirements. From Figure3. As can be seen from the data in fig. 4, the operating resistance of the dust collector after the transformation is about 700~800 Pa, and the dust emission concentration is about 8 mg/Nm3, which is better than the original assessment target. The comparison of the transformation data in Table 1 also effectively verifies the reliability and superiority of the design scheme. The successful operation of this project provides a technical transformation plan for the bag dust collector that was put into production before, and completes the secondary transformation at the lowest cost to adapt to increasingly stringent environmental protection regulations.

Figure3 After the transformation of dust collector resistance

Figure4 Outlet emission concentration of dust collector after transformation