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Comprehensive Overview of Dust Removal Technologies for Ultra-Low Smoke Emission

Ultra-low smoke emission refers to the ultra-low emission of particulate matter in flue gas, which includes not only smoke particles but also secondary particulate matter generated during wet desulfurization. Therefore, achieving ultra-low smoke emission requires necessary dust removal treatment.

Dust removal technologies generally include the removal of smoke particles in flue gas after flue gas denitrification, referred to as primary dust removal technology. Mainstream technologies include electrostatic precipitator (ESP) technology, baghouse technology, and electrostatic-bag composite dust removal technology. The secondary removal of particulate matter in flue gas after desulfurization or the simultaneous removal of particulate matter during desulfurization is termed secondary dust removal or deep dust removal technology. Wet electrostatic precipitator technology is mainly used for the removal of particulate matter after desulfurization, and the composite tower desulfurization technology is used for the simultaneous removal of particulate matter during desulfurization. Efficient demisters or wet dust removal devices are also added inside wet desulfurization towers. The following provides a detailed introduction to these dust removal technologies.

Primary Dust Removal Technology
  1. Electrostatic Precipitation (ESP) Technology

ESP technology utilizes strong electric field corona discharge to generate a large number of free electrons and ions in the gas, which adsorb onto dust particles passing through the electric field. The charged dust particles, under the influence of the electric field Coulomb force, adhere to the collecting plates, fall into the ash hopper through vibration, and are discharged through the ash removal system, achieving dust collection.

Advantages: High dust removal efficiency, low pressure loss, easy to use with no secondary pollution, not highly sensitive to flue gas temperature and composition, relatively easy for equipment operation and maintenance, and good safety and reliability.

Limitations: Large footprint of the equipment, dust removal efficiency significantly affected by coal type and fly ash composition.

Depending on whether water is used for electrode surface ash removal, ESP can be divided into dry ESP and wet ESP. Dry ESP is commonly referred to as electrostatic precipitator, such as electrostatic dust removal technology and low-temperature electrostatic dust removal technology; wet ESP is commonly referred to as wet electrostatic precipitator and is only used for secondary dust removal after wet desulfurization.

  1. Electrostatic Dust Removal Technology

Electrostatic dust removal technology involves applying high-voltage direct current between the corona electrode and the dust collection electrode, creating a strong electric field that ionizes the gas and charges the dust particles. Positively and negatively charged dust particles move towards the corona electrode and dust collection electrode, respectively, depositing on the collecting plates.

Compared to other dust removal devices, electrostatic precipitators consume less energy, have high dust removal efficiency, and are suitable for removing dust particles in the range of 0.01 to 50 μm from flue gas. They are also suitable for high-temperature and high-pressure conditions. However, due to the charged dust collection mechanism, electrostatic precipitators are sensitive to fly ash properties (composition, particle size, density, resistivity, adhesiveness), making it challenging to collect high-resistivity and fine dust. Changes in operating conditions also have a significant impact on dust removal efficiency. Additionally, electrostatic precipitators cannot capture harmful gases and have higher requirements for manufacturing, installation, and operation.

  1. Low-Temperature Electrostatic Dust Removal Technology

Low-temperature electrostatic dust removal technology reduces the inlet flue gas temperature to below the acid dew point using a flue gas cooler.

The decrease in temperature in low-temperature electrostatic dust removal technology leads to a significant increase in the resistivity of the dust, an increase in breakdown voltage, a decrease in flue gas flow rate, and higher dust removal efficiency. Simultaneously, the condensation of gaseous SO3 into liquid sulfuric acid mist in flue gas results in the adsorption and chemical reaction with dust particles, removing most of the SO3 from the flue gas. Compared to conventional dry electrostatic precipitators, low-temperature electrostatic precipitators require fewer electric fields, smaller circulation areas, lower operating power consumption, and significant energy savings. However, the reduced resistivity of dust may weaken the electrostatic adhesion force of dust particles to the anode plate, leading to an increase in secondary dust emission.

  1. Baghouse Technology

Baghouse technology utilizes the principle of filtration, employing bag-type filter units made of woven fabrics to capture dust in dusty flue gas. The dust cake layer accumulated on the surface of the filter bag is detached and falls into the ash hopper under the action of reverse acceleration and reverse-penetrating airflow.

Advantages: Small footprint of the baghouse, high dust removal efficiency (usually ensuring an outlet emission concentration below 50 mg/m3), large range of gas treatment, not affected by coal type, fly ash composition, concentration, and resistivity, simple structure, flexible use, stable and reliable operation, and easy operation and maintenance.

Limitations: Limited adaptability in high-temperature, high-humidity, and highly corrosive gas environments due to the filter bag material. The operational resistance is relatively high, with an average operational resistance around 1500 Pa, and some baghouses may exceed 2500 Pa after a short period of operation. Additionally, filter bags are prone to damage and detachment, and old bags are difficult to recycle effectively.

  1. Electrostatic-Bag Composite Dust Removal Technology

Electrostatic-bag composite dust removal technology combines electrostatic dust removal technology with baghouse technology. The technology utilizes the primary electric field to collect most of the dust and charge the remaining dust. The subsequent baghouse area filters and intercepts the remaining dust, achieving flue gas purification. The charged dust that is not captured in the primary electric zone causes fine particles to polarize or coalesce into coarse particles. Due to the repulsion effect of like charges, the dust layer deposited on the surface of the filter bag is arranged orderly, with a loose structure resembling cotton wool. This results in low dust layer resistance, easy ash removal, and enhanced filtration performance due to the increased electrostatically charged dust, reducing operating resistance and extending the life of the filter bag.

Electrostatic-bag composite dust collectors can be classified into integrated electrostatic-bag composite dust collectors, split-type electrostatic-bag composite dust collectors, and embedded electrostatic-bag composite dust collectors based on their structural forms. Among them, integrated electrostatic-bag composite dust collectors are the most mature and widely used technology.

Advantages: Strong applicability to changes in coal type and dust resistivity compared to electrostatic precipitators, lower operating resistance than pure fabric bag dust collectors, longer filter bag life than fabric bag dust collectors, and lower energy consumption than electrostatic precipitators.

Limitations: Due to the combination of electrostatic dust removal and fabric bag dust removal units, the operation and maintenance of electrostatic-bag composite dust collectors are more complex.

Secondary Dust Removal Technologies
  1. Wet Electrostatic Precipitation Technology

Wet electrostatic precipitation technology involves washing off the dust adsorbed on the electrodes with water. Wet electrostatic precipitators are classified into plate-type, honeycomb-type, and tube-type based on the shape of the anode plate, with plate-type and honeycomb-type being more commonly used. Installed after desulfurization equipment, wet electrostatic precipitators effectively remove dust and secondary particulate matter generated during wet desulfurization, and can also synergistically remove SO3, mercury, and their compounds.

The main factors affecting the performance of wet electrostatic precipitators include the structural type, inlet concentration, particle size distribution, airflow distribution, dust removal system condition, and the amount of flushing water.

Advantages: Strong adaptability to dust, high dust removal efficiency, suitable for handling high-temperature and high-humidity flue gas; no secondary dust emission; absence of easily damaged components like hammering devices, ensuring high reliability; effective removal of sub-micron particles, SO3 aerosols, and gypsum micro-droplets, contributing to the effective control of PM2.5, blue haze, and gypsum rain.

Limitations: The flue gas temperature must be lower than the adiabatic saturation temperature of the flushing liquid; challenging to use in high-dust concentration and high SO2 concentration conditions; requires effective corrosion protection measures; while the flushing water in wet electrostatic precipitators uses a closed-loop system, it needs to be balanced with the desulfurization water system.

  1. Compound Tower Desulfurization Technology

In compound tower desulfurization, the flue gas is induced into the desulfurization tower by an induced draft fan. The first-level spray device in the radially inward air inlet pipe of the desulfurization tower performs pre-cooling and pre-desulfurization on the flue gas. The cooled and pre-desulfurized flue gas uniformly rises from the lower part of the desulfurization tower, sequentially passing through the high-density spray washing reaction zone and the absorption reaction zone formed by three-level spray devices. The desulfurization liquid, generated by spiral nozzles, produces extremely fine mist droplets, providing a large contact area for the full physical and chemical reaction of mass and heat transfer between the gas and liquid. This achieves the efficient removal of SO2. The desulfurization tower is equipped with two-stage dehydrating demisting devices. After desulfurization, the flue gas continues to rise, passing through two layers of baffle demisting devices. Multiple collisions of mist and small liquid droplets on the baffles form larger droplets. After separating from the flue gas, the dehydrated flue gas is discharged through the chimney.

When choosing from the various dust removal technologies mentioned above, electrostatic precipitators are suitable when dust removal difficulty for coal types is “relatively easy.” When coal type dust removal difficulty is “relatively difficult,” preference can be given to electrostatic-bag composite dust removal technology. Baghouse technology is also applicable for units of 300MW and below. For primary dust removal requiring smoke concentration below 10mg/m3 or 5mg/m3 without relying on secondary dust removal for achieving ultra-low emissions, the preference can be given to the ultra-clean electrostatic-bag composite dust removal technology. In other cases, considering factors such as the effectiveness of secondary dust removal technology, fluctuation in coal quality, site conditions, and investment and operating costs, a comprehensive decision can be made.

Additionally, the following principles can be followed: When the outlet smoke concentration of the primary dust collector is between 30mg/m3 and 50mg/m3, it is advisable to use wet electrostatic precipitators for secondary dust removal. When the outlet smoke concentration of the primary dust collector is less than 30mg/m3, wet electrostatic precipitators can also be used for secondary dust removal to achieve lower particle emission concentrations, adapting better to changes in factors such as the coal market, and investment and operating costs may increase appropriately. When the outlet smoke concentration of the primary dust collector is between 10mg/m3 and 30mg/m3, it is advisable to use compound tower desulfurization technology for synergistic dust removal, ensuring the effective performance of mist and dust removal in the compound tower.

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