Comparison of ESP and Baghouse in Treating Cement Kiln Flue Gas

One. Effects of flue gas performance and flue gas conditions on dust collector performance

1.the influence of electrostatic precipitator. 

Electrostatic precipitator (ESP) uses the principle that dust particles are charged in electric field and move towards the dust collector pole under the action of electric field force to purify flue gas.

(1) the specific resistance of dust. 

The specific resistance of dust refers to the resistance value of the unit area and the thickness of the ash layer on the unit thickness. The specific resistance of dust is an important factor affecting the efficiency of ESP. According to the difficulty degree of ESP for dust collection, it can be divided into 3 kinds of specific resistance types: high, medium and low. Dust with low and high specific resistance is regarded as difficult to collect by ESP, but the influence mechanism is different. Dust particles with low specific resistance have good electrical conductivity. Once they reach the surface of the dust collecting plate, the negative charge they carry is released immediately. Due to electrostatic induction, a positive charge with the same polarity as the dust collector plate is obtained. If the repulsive force of the positive charge is greater than the adhesion force of the dust particles themselves, the precipitated dust will escape from the dust collector plate and return to the air stream. When the dust particles returning to the air stream collide with the anions again in space, the secondary charge moves to the dust collector again. The particles move back and forth, forming a jump on the dust collector plate, and may eventually be carried out of the ESP by airflow. Due to the high specific resistance of dust, the discharge process of dust particles deposited on the dust collector is very slow, and the voltage drop in the dust layer is very large. When the voltage drop reaches a certain level, abnormal phenomena begin to occur, resulting in partial breakdown of the dust layer and spark discharge, the so-called back corona phenomenon, in the dust collector and the material layer. A large number of cations were formed to neutralize the anions coming in, which increased the power consumption, reduced the dust removal efficiency, and even could not work properly. After the dust particles with negative specific resistance reach the precipitation plate, the neutralization proceeds at a proper speed, and the dust collection efficiency is high. Medium specific resistance is the most ideal region for the operation of the dust collector, in which the dust collection efficiency is not affected by the specific resistance value.

(2)the influence of dust properties.

The influence of dust chemical composition: the content of silicon, aluminum and calcium in the dust increases, the specific resistance increases; the content of alkali metal such as potassium and sodium, iron (Fe3O4) increases, the specific resistance decreases.

Dust adhesion: dust adhesion is high, vibration cleaning difficult, adhesion is too small, loose ash collection, easy to cause secondary dust.

The bigger the particle size, the higher the gravity of the particles, the bigger the settling velocity, the smaller the adhesion and the higher the dust collection efficiency.

Dust density: the greater the dust density, the higher the particle gravity, the greater the settling speed, the higher the dust collection efficiency.

The dust concentration of flue gas is too high to produce corona sealing, which affects the performance of ESP. The dust concentration of flue gas is too low to collect, the dust concentration is too high or too low, and the dust collection efficiency of electric field is reduced. The high concentration ESP at the end of the kiln collects 40%-50% dust by pre-collection measures. Before entering the electric field.

(3) The influence of flue gas conditions.

The influence of flue gas humidity: the flue gas humidity mainly affects the dust removal efficiency by affecting the dust specific resistance and the spark discharge, voltage and volt-ampere characteristics. The water content in the flue gas rises, the dew point rises, the specific resistance of the dust decreases, and the efficiency of the dust collector rises. Under the same corona voltage, the corona current increases with the smoke. The moisture content of the gas increases and decreases.

The influence of flue gas temperature: the specific resistance of dust is the biggest when the temperature is about 180 C, and the specific resistance of dust will decrease when the temperature is too high or too low. The specific resistance will drop sharply when the temperature is higher than 250 C, and the specific resistance will drop sharply when the temperature is below 100 C. The efficiency of dust collector decreases, the temperature of flue gas rises, the viscosity of flue gas increases, the flow resistance of dust from flue gas to dust collector increases, and the driving speed decreases.

The influence of flue gas composition: the content of SO2, O2, H2O and CO2 in flue gas is high, which has favorable influence on the corona of discharge electrode.

2.the influence of baghouse. 

Baghouse uses dust-containing gas through the filter bag for gas and dust separation, dust is blocked in the filter bag outside (inside) surface, clean gas is collected and discharged to complete the filtering process of dust-containing gas.

(1) the specific resistance of dust.

The dust specific resistance has no effect on the dust collecting efficiency of baghouse.

(2) the influence of dust properties.

The influence of dust chemical composition: the specific resistance caused by different dust chemical composition has no effect on dust collection efficiency.

Dust adhesion: Dust adhesion is high, filter bag cleaning is difficult, must increase the kinetic energy of cleaning, affect the life of filter bag, adhesion is too small, loose ash collection, easy to secondary dust.

The bigger the particle size, the higher the gravity of the particles, the bigger the settling velocity, the smaller the adhesion, the higher the dust collection efficiency, and the tiny particles can easily pass through the gap between the filter bag and the filter bag and the ceiling.

The influence of the true density of dust: the greater the true density of dust, the higher the particle gravity, the greater the settling speed, the higher the dust collection efficiency.

The influence of dust concentration in flue gas: the dust concentration of flue gas has no obvious influence on the dust collection efficiency of bag filter. The baghouse reduces the flue gas concentration by pre-dust collection, which can reduce the number of dust cleaning bags, reduce the resistance of dust collector and prolong the life of filter bags.

(3) the influence of flue gas conditions.

The influence of humidity on flue gas: the humidity changes the dew point of flue gas, and the high dew point easily causes dew condensation and bagging, which increases resistance.

The influence of flue gas temperature: the influence of flue gas temperature depends on the temperature resistance of the filter bag.

Effect of flue gas composition: flue gas composition has no effect on filtration performance.

Two. Comparison of flue gas pretreatment measures

1.the use of ESP requires deep quenched and tempered of the flue gas. The resistance is not in the proper range, and the flue gas should be quenched and tempered. Spraying water mist into the tail gas of cement kiln is the most common and practical method to reduce the temperature of flue gas and avoid the high specific resistance temperature zone. The kiln flue gas should be reduced from 350 to 110 -150 C(the suitable temperature for ESP operation ).

2.the use of baghouse only needs to reduce the temperature of the flue gas. 

Although water spray can also be used in baghouse, the purpose is only to cool down without considering specific resistance. The glass fiber material filter bag can be kept at a temperature of 260 degrees for a long time, and the cooling water consumption is less than half of that of the ESP. Cooling methods include air heat exchanger cooling, cold air cooling, water heat exchanger cooling and so on.

Three. Comparison of operational resistance

1.the operation resistance of the ESP is low and stable.

The operation resistance of the ESP is <300Pa, and the resistance is stable for a long time.

2.the resistance of the baghouse is high.

The development of pulse spraying dust cleaning technology and the popularization and application of high performance and high temperature resistant filter materials have greatly reduced the resistance of baghouse. The resistance of baghouse at the end of pulse spraying kiln is less than 1000Pa, and the effect of dust cleaning affects the resistance.

Four. Realizing the difficulty of "zero emission".

1.ESP is difficult to achieve zero emission.

In an electric field, the finer the particles, the lower the probability of being trapped due to turbulent gas disturbance, and it is easy to escape from the ESP. The fine dust particles in the kiln tail smoke are smaller than 10 micron m, accounting for about 60%. Although ESP is a kind of high efficiency dust collecting equipment, its dust collecting efficiency is exponential curve with the area of dust collecting electrode plate. For a three-field ESP, the first electric field has a dust collection efficiency of 80%-90%, while the second and third electric fields only collect 10%-20% of the dust content. In order to achieve a low emission concentration below 30mg/Nm3, it is necessary to add a 45th electric field or to increase the plate area in the electric field.

2.baghouse is easy to achieve zero emission.

Baghouse can improve dust collection efficiency by choosing high performance filter media, reducing filter air speed and improving processing precision. Although there are a lot of fine particles in the flue gas of kiln tail, the use of pulse spray bag filter can completely control the emission concentration at < 10mg/Nm3, close to "zero discharge".

Five. Accident prevention and protection measures

The influence of excessive CO content in flue gas:

(1)ESP needs to be shut down for protection.

CO sampling and analysis system should be installed at the entrance of ESP to detect the flue gas. If the CO content in the flue gas of kiln tail exceeds 2%, an alarm should be issued and the initiating technicians should be notified to take measures in time. When the CO content exceeds 2.5%, the high voltage silicon rectifier should be closed to prevent the excessive CO content flue gas from spark explosion in ESP. In a rotary kiln, when the fuel is not completely burned, the CO content in the flue gas of the kiln tail exceeds the standard within a few hours of ignition and feeding, and the power supply to the electric field is often stopped. This is an accident discharge.

(2) No need to protect baghouse.

CO content exceeding the standard will not affect the baghouse, no need to install CO sampling and analysis system.

(3) Failure of power supply or high voltage line leads to the discharge of ESP.

When the power supply of the ESP fails to power the electric field or when the insulator in the high-voltage line trips due to condensation causing "creep" and other reasons, the electric field without high-voltage power is only a channel, also belongs to the accident discharge.

(4) Baghouse causes excessive discharge due to broken bags.

Due to aging, mechanical damage, etc. will cause broken bags, resulting in excessive discharge, baghouse can be installed by means of differential pressure gauge on-line detection of broken filter bags in the bag chamber.