The dust removal technology for coal-fired boiler flue gas has gone through the process of water film dust removal, bag dust removal, and electrostatic dust removal. With the improvement of national environmental protection requirements and the practical application of bag filters, there is now a trend of using bag filters again.

In the early application of boiler bag filters, many projects suffered the painful experience of failure. The main reason was that the quality of the supporting components at that time, especially the filter material, which was a key component of the dust collector, was not up to standard. Both domestically and internationally, there was a lack of materials that were reasonably priced, heat-resistant, and suitable for dust removal from coal-fired boiler flue gas. After the failure of the bag filter, the basic application of dust removal for coal-fired boiler flue gas is electrostatic precipitator. But with the increasing national environmental protection requirements and the widespread use of electrostatic precipitators, their weaknesses are gradually exposed.

Firstly, the dust removal efficiency of the electrostatic precipitator cannot be guaranteed. At the initial stage of operation, the dust removal efficiency of the electrostatic precipitator can basically reach 99%. However, due to changes in the type of coal used in the power plant and limitations in the structure and working principle of the electrostatic precipitator, as the operating time continues, the internal components of the electrostatic precipitator deform and accumulate dust, causing changes in the electric field and a continuous decline in dust removal efficiency. The emissions seriously exceed the standard, becoming a concern for the power plant. Secondly, the comprehensive operating cost of electrostatic precipitators is high. After one cycle of operation, some structures of the electrostatic precipitator deform and are severely damaged. In order to ensure normal operation, the power plant has to regularly repair the electrostatic precipitator. The human and material resources invested in maintenance, as well as the losses caused by maintenance shutdowns, are not insignificant for power plants. The power consumption of electrostatic precipitator itself is metaphorically referred to as an electric tiger.

At present, with the increasing improvement of production technology for supporting components such as bag filter media, the factors that caused the failure of bag filters in the application of coal-fired boiler flue gas dust removal have been solved. Especially the successful application of offline maintenance technology has provided strong support for the use of pulse bag filters in boiler dust removal technology. The national environmental protection policy has greatly increased the requirements for boiler smoke and dust emissions. The new "Emission Standards for Pollutants from Thermal Power Plants" (GB13223) has been released, which increases the concentration of smoke and dust emissions from coal-fired power plants to 50mg/m3. Some local environmental protection departments have put forward stricter requirements. Since March 1, 2002, Beijing has implemented the "Beijing Boiler Pollutant Comprehensive Emission Standard" (DB11/119), which allows for stricter emission concentrations of smoke and sulfur dioxide than many advanced national standards. Starting from November 1, 2005, the smoke and dust emission concentration of operating thermal power plant boilers is only allowed to be 30mg/Nm3. Currently, it is difficult to meet the requirements by changing the four electric field electrostatic precipitator to a five electric field or a six electric field. The "Regulations on the Collection and Use Management of Pollutant Discharge Fees" implemented on July 1, 2003, will impose pollutant discharge fees on enterprises and double the fees for excessive emissions. This will increase the operating costs for enterprises with low dust collector efficiency.

Due to the fact that the dust collector is a key equipment in the boiler dust removal system, its working condition directly affects the success or failure of the entire system. Therefore, every aspect of the design, manufacturing, installation, commissioning, and operation of the dust collector needs to be carefully arranged. After comprehensive consideration and comparison of the performance characteristics of atmospheric blowback bag filter, rotary pulse bag filter, and pipe spray pulse bag filter, in order to ensure the long-term, safe, and stable operation of the dust collector under various environmental conditions and reflect the best price performance ratio of the dust collector, it was decided to adopt a pulse bag filter that includes offline dust cleaning and offline maintenance functions. This form of dust collector has been applied in numerous engineering projects by many manufacturers.

The pulse bag filter has the characteristics of downward air intake, offline dust cleaning, offline maintenance, external filter dust removal, good dust cleaning effect, fully enclosed filtration area, easy operation for maintenance and repair outside the machine, low emission concentration, and low operating costs.

Compared to electrostatic precipitators, pulse bag dust collectors have a smaller footprint, do not have the necessary high-voltage rectification equipment and control building for electrostatic precipitators, do not require dedicated dust collector power supply and transformation equipment, and have lower long-term operating costs.

From an investment perspective, the cost of current pulse bag filters is slightly higher than that of three field electrostatic precipitators, approaching or lower than that of four field electrostatic precipitators. With the gradual localization of main components such as filter media for bag filters, the cost of bag filters will be further reduced.

With the revision and improvement of national environmental regulations and the further enhancement of people's environmental awareness, offline dust cleaning low-pressure pulse bag filters will become the preferred choice for boiler flue gas dust removalequipment

1、 Comparison of economic and technical conditions between bag filter and electrostatic precipitator:
Performance comparison between bag filter and electrostatic precipitator
1.1 Advantages of Bag Dust Collector:

1.1.1 High dust removal efficiency can ensure that the dust emission concentration is below 50mg/m3.
1.1.2 Unit combination form, simple internal structure, few auxiliary equipment, low investment, and lower technical requirements compared to electrostatic precipitators, without the need for dedicated operators.
1.1.3 Can capture dust with high specific resistance, making it difficult for electrostatic precipitators to recover.
1.1.4 Bag filter has stable and reliable performance, good adaptability to load changes, easy operation and management, and is particularly suitable for capturing fine and dry dust. The collected dry dust is easy to handle and recycle.
1.1.5 Can achieve non-stop maintenance, i.e. offline maintenance.
1.1.6 The dust collector occupies a small area and can be specially designed according to the site requirements.
1.1.7 The automation level is high, and all equipment in the dust removal system is equipped with detection and alarm functions. The requirements for operators are low, and the labor intensity of operation and maintenance personnel is low.

1.2 Disadvantages of Bag Dust Collector

1.2.1 Bag filters have corresponding requirements for filter media when used to purify dust containing oil mist, water mist, and strong adhesion.
1.2.2 Explosion proof measures are required when purifying dusty gases with explosion hazards or sparks using bag filters.
1.2.3 When dealing with dusty gases with high relative humidity, insulation measures should be taken (especially in winter) to prevent "bag sticking" caused by condensation; When used for purifying corrosive gases, suitable corrosion-resistant filter media should be selected. Cooling measures should be taken to treat high-temperature flue gas, reducing the flue gas temperature to below the temperature that the filter bag can withstand for long-term operation, and high-temperature resistant filter media should be used as much as possible.

1.3 Advantages of electrostatic precipitator

1.3.1 The initial dust removal efficiency can reach 99%, which can capture fine dust below 1um. However, from an economic perspective, a reasonable dust removal efficiency is generally controlled.
1.3.2 It can handle a large amount of flue gas and can be used in high temperature (up to 500 ℃), high pressure, and high humidity (relative humidity up to 100%) situations. It can operate continuously and achieve automation.
1.2.3 has the characteristic of low resistance, and the pressure loss of the electrostatic precipitator is only 100-200Pa.

1.4 Disadvantages of electrostatic precipitators

1.4.1 The equipment is large and consumes a lot of steel, requiring high-voltage transformers and rectifiers. Typically, the peak output power of high-voltage power supply equipment is high

The voltage is 70-100KV, so the investment is high.
1.4.2 The technical level required for manufacturing, installation, and management is relatively high.
1.4.3 Dust removal efficiency is greatly affected by the specific resistance of dust, generally with a comparative resistance less than 104-105 Ω· cm or greater than 1010 Ω· cm
Dust of 1011 Ω· cm, if certain measures are not taken, the dust removal efficiency will be affected.
1.4.4 Pre treatment devices should be installed for dusty gases with an initial concentration greater than 30g/cm3.
1.4.5 does not have offline maintenance function. Once the equipment malfunctions, runs with problems, or can only be shut down for maintenance.

1.5 Comparison Table

Comparison of operational reliability between bag filter and electrostatic precipitator

2.1 Bag filter
2.1.1 Can ensure a long-term dust emission concentration of<50mg/m3. Not affected by inlet dust concentration and specific resistance.
2.1.2 Main accessories - The service life of the filter material is over 30000 hours.
2.1.3 Main accessories - The service life of the electromagnetic pulse valve is over 1 million times.
2.1.4 All operating equipment shall be equipped with detection and alarm devices, which can detect faults and alarm in the first time.
2.1.5 Main maintenance work - filter bag replacement can be performed by only two people.
2.1.6 Utilize offline functions for maintenance and repair, without affecting the normal operation of the boiler.
Under the severe cold conditions in the north, strict heating and insulation measures have been taken for the compressed air injection system and body of the dust collector to avoid the possibility of condensation.

2.2 Electrostatic precipitator
2.2.1 During the initial operation, it can maintain normal operation and achieve the expected dust removal efficiency. But it is affected by the condition of the inlet flue gas. Due to the influence of ash and sulfur content in fuel, the dust removal efficiency of electrostatic precipitators will be severely affected.
After running for a period of time, the electrode may deform, causing changes in the electric field and resulting in a decrease in dust removal efficiency.
2.2.3 Maintenance and overhaul can only be achieved after shutdown of the furnace.

Comparison of maintenance convenience between bag filter and electrostatic precipitator

3.1 Bag filter

Once a malfunction occurs, timely alarms and indications can be obtained from the control system.
3.1.2 The faulty compartment can be maintained and repaired offline (with the boiler running normally).
3.1.3 Fault maintenance is carried out outside the machine, without the need to enter the interior of the dust collector.
During daily maintenance, measures should be taken to seal damaged filter bags (when the bag damage rate is below 5%) in order to further reduce daily workload.

3.2 Electrostatic precipitator
3.2.1 Due to the lack of offline maintenance function for electrostatic precipitators, once a malfunction occurs, the boiler can only be shut down for maintenance, otherwise it can only operate with defects.
3.2.2 During maintenance, employees need to enter the interior of the dust collector, as the working environment is harsh.
3.2.3 The degree and location of damage to the internal devices of the dust collector rely on manual inspection, which requires high labor intensity for maintenance.

Comparison of operating costs between bag filter and electrostatic precipitator

4.1 Human Resources:
Due to the high degree of self-control of the pulse bag filter, the operation status of all dust removal system equipment, including all operating parameters during dust removal, is collected by PLC and sent to the main control cabinet. Therefore, personnel in the boiler control room can monitor the operation of the dust removal system. No dedicated operators are appointed.
Two or more operation inspection personnel shall be appointed for the electrostatic precipitator.

4.2 Operating costs:
Power consumption of pulse bag filter equipment

Gas source consumption table

Power Consumption Table (Unit: KW)

4.3 Spare parts cost:
4.3.1 The service life of the electromagnetic pulse valve is over 1 million times, therefore, its cost can be disregarded.
4.3.2 The service life of the filter material is 30000 hours. Calculated based on an annual operating time of 8000 hours, it needs to be replaced after about 4 years. According to relevant foreign experience, there is a record of using filter media for 7 years without replacement. The 4.4 pulse bag filter has the function of offline maintenance. During the operation of the boiler, the filter can be maintained without stopping the machine, avoiding losses caused by shutdown maintenance.

Given that alkaline powder needs to be sprayed into circulating fluidized bed boilers for in furnace desulfurization or other furnace types for dry/semi dry desulfurization outside the furnace, which has a significant impact on the specific resistance of dust and increases the dust content in flue gas, the performance characteristics of bag filters that are not affected by dust specific resistance and load are particularly prominent. In addition, the advantages in operating costs, ease of operation and maintenance, land area, and initial investment provide strong guarantee for meeting long-term and gradually increasing environmental protection requirements.

At present, the application of boiler bag filters in central cities and tourist cities is gradually being promoted and showing a significant development trend.

The above comparative data shows that bag filters have the advantages of electrostatic precipitators in various aspects. Moreover, the use of bag filters conforms to the development of national environmental protection level and achieves one-time investmentBeneficial, avoiding the need for secondary renovation and investment in electrostatic precipitators in the future.