Roasting is an important process in the production process of carbon products, the roasting of raw products is in the roaster with coke powder as a protective medium, under the condition of isolated air, according to the technical requirements of the product, according to a certain heating rate to the maximum roasting temperature (1200 1300). Carbon products use asphalt as a binder, and the roasting furnace flue gas dominated by asphalt smoke is discharged during the roasting process, and the asphalt flue gas is a mixture composed of a large number of liquid and a small amount of gaseous polycyclic aromatic hydrocarbons, usually containing more than 80 kinds of polycyclic aromatic hydrocarbons such as 3, 4 benzopyrene, benzanthracene, carbazole, etc., and the particle size is mostly 01～1.0 m, the smallest is only 001 m, the largest is about 100 m, and most of them are carcinogenic and strong carcinogens, and it has been proved that the b(a)p contained in the flue gas has a strong carcinogenic effect on ** and the respiratory system. The roaster is the most polluting carbon plant, with a large amount of flue gas and a high concentration of pollutants. Different scale enterprises and different types of roasting furnaces have great differences in flue gas temperature and composition, so it is very necessary to choose the appropriate asphalt fume purification technology according to the flue gas characteristics of the roaster.
First, the basic situation of the carbon plant.
1.1. Production process.
A carbon factory in city A uses coke powder and medium-temperature coal asphalt as raw materials, and the two are mixed in a certain proportion and then pressed into green blocks. The fuel is raw coal, and the roaster type is a ring roaster structure. The pressure formed green block is loaded into 8 roasting furnaces, and the green block is filled with coke powder as a protective medium, between 1200 and 1300, and after a certain period of high temperature roasting, it becomes a carbon product. Since the main raw materials of the green block are coke powder and asphalt, a large amount of asphalt smoke and soot will escape during the roasting process.
1.2. Roasting process contaminants**.
The roasting furnace uses raw coal as fuel, and in the roasting process, in addition to the smoke and SO2 produced by the combustion of raw coal, the asphalt in the carbon green block can emit a large amount of asphalt smoke due to high temperature pyrolysis. In the heating and heating stage at the beginning of roasting, the main pollutants are soot and SO2;The middle stage of heating is the volatilization period of asphalt, and the main pollutant is asphalt smoke (containing benzopyrene, benzoanthracene, carbazole, etc.).The main pollutants in the later stage of roasting are soot, SO2 and a small amount of asphalt smoke. Pollutants such as soot, SO2 and asphalt smoke generated during the roasting process are collected by the collecting flue and discharged from the chimney at high altitude.
Second, the selection of flue gas purification process.
The composition of the flue gas produced by the carbon roaster is complex, there are a variety of pollutants such as asphalt smoke, soot, SO2, etc., and the flue gas temperature is extremely unstable, usually between 80 and 300, and this flue gas working condition has different degrees of influence on various purification methods of asphalt smoke.
At present, the asphalt smoke purification methods that are being studied and applied at home and abroad are: combustion method, absorption method, adsorption method and electrocapture method, in which the combustion method requires flue gas temperature of 700, and the concentration of asphalt smoke is higherThe absorption method mostly uses organic liquids such as gasoline and diesel as absorbents, and the purification efficiency is low, and there is a secondary pollution problemThe adsorption method uses porous substances (activated carbon, coke powder, alumina, etc.) with large specific surface area as adsorbents to physically adsorb asphalt smoke, but the system resistance is large, and it is troublesome to replace the filter material.
According to the actual production situation of the carbon plant, the above three purification methods are not applicable, and the atomized water (pretreatment) and electric tar precipitator are used as the comprehensive treatment scheme of asphalt flue gas of carbon roaster, which has the advantages of low investment, low system resistance, high purification efficiency, no secondary pollution problem, and various pollutants can be discharged up to the standard after treatment.
2.1. Process flow.
The high-temperature flue gas discharged from the carbon roasting furnace is collected from the underground flue to the main flue on the ground under the negative pressure of the fan, and enters the tubular cooler through the air volume regulating valve, which is used to atomize and humidify, and the atomized water is in contact with the flue gas, which can remove the dust and SO2 with larger particles in the flue gas on the one hand, and achieve the pretreatment effectOn the other hand, the flue gas temperature can be reduced, so that the flue gas temperature is kept at 80 90, the specific resistance of the flue gas is reduced, and the particulate matter and asphalt smoke are charged in the electric fieldThe pretreated flue gas enters the electric tar precipitator, and the flue gas particles are charged and collected in the trap, and the purified flue gas is discharged from the chimney at high altitude under the action of the induced draft fan. The soot particles and tar bitumen trapped into the dust collection plate flow along the plate into the ash collection chamber at the lower part of the dust collector.
Roaster - Wet pretreatment - Electrostatic precipitator - Fan - Chimney.
2.2 How it works.
When the high-temperature flue gas is collected from the underground collection flue to the main flue on the ground, the flue gas temperature is above 300, a tubular cooler is installed in the main flue, and the atomized water is sprayed into the cooler at high pressure along the flue gas flow direction, and the atomized water injected can automatically adjust the water volume according to the temperature of the flue gas. The atomized water absorbs the heat of the flue gas, reduces the temperature of the flue gas to 80 90, and at the same time can remove the smoke and SO2 gas with larger particles in the flue gas, and the wastewater generated enters the sedimentation tank, and the quicklime (CAO) is added to the sedimentation tank, and after neutralization and precipitation treatment, it is recycled to achieve no discharge of wastewater.
The flue gas temperature after passing through the tubular cooler is about 80 to 90, which is higher than the softening point of the medium temperature coal asphalt, and the specific resistance of the flue gas is also reduced to 2Below 0 1010 ·cm, it is conducive to the capture of asphalt smoke by electric tar precipitator. The flue gas enters the high-voltage electric field of the electric tar precipitator through the airflow distribution plate, and the asphalt flue gas, polymer organic matter oil mist and dust particles are driven to the anode by the electron charge released by the corona wire, and are driven to the anode under the action of electric field force, and on the anode plate, the charge is neutralized, and the particles are attached to the surface of the anode plate, and the particles are gathered in the form of liquid paste, which flows down along the surface of the dust collector by its own weight, and flows into the ash collection chamber at the lower end of the dust collector, and the ash is manually removed regularly, so as to achieve the purpose of asphalt flue gas purification of the roaster.
2.3. Structural characteristics of electric tar precipitator.
The electric tar precipitator is a vertical multi-tube electrostatic precipitator, and the whole precipitator is divided into three sections: upper, middle and lower, the upper and middle parts are high-voltage electrostatic fields, the lower part is the ash collection chamber, and the shell is a cube-shaped structure. The flue gas of the roaster enters from the side of the lower end, and the purified clean gas is discharged from the top of the upper end, and the ash collection chamber at the bottom of the dust collector is made of brick-concrete structure, which is reasonable in structure, saves investment, and is convenient for maintenance. The lower end of the air inlet is a variable pipe, and the air inlet is equipped with 2 layers of perforated plate type airflow distribution plate, so that the gas is evenly distributed in the electric field, and the middle part is a high-voltage electrostatic field with a height of 4m, and its cross-sectional structure is 49 squares of 400 400mm.
Inside the dust collector, the anode plate of each cube and its 1 corona wire in the middle form an electric field, in order to ensure that the asphalt and dust collected by the anode are in a flowing state and will not be stranded on the surface of the anode, a steam insulation interlayer is provided on the outer surface of the dust removal equipment. Water steam** in a boiler room of our factory, the temperature control device is used to automatically adjust the steam temperature, and the steam temperature is easy to control at about 90. The tar and asphalt collected on the anode plate exist in a fluid state and automatically flow into the ash collection chamber by gravity. If the temperature of the steam insulation interlayer is too high (more than 100), the specific resistance of dust in the electric field decreases, and the dust removal efficiency decreasesIf the temperature is too low (below 80), the asphalt smoke is easy to condense into a paste, which affects the normal operation of the system. It can be seen that controlling the temperature in the electric field is the key to the stable, reliable and efficient operation of the dust collector.
2.4. System operation control.
The high-voltage silicon rectifier device of the electric tar precipitator adopts PLC constant current source automatic control, according to the principle of PLC resonance, the DC current output of the constant current power supply does not change with the change of load, if the corona of the electric field is converted into breakdown, the body current remains constant, and the electric field can be recovered quickly, without blocking the electric field for a long timeIf the dust resistivity increases, the output voltage will increase automatically, which can ensure the dust removal effect and have strong adaptabilityIf there is undervoltage or overvoltage, the control part can automatically complete the voltage adjustment through the feedback voltage, so that the dust collector can continue to work stably in the best power supply state of the critical spark voltage. If the adjustment is invalid, the system can automatically cut off the power supply of the main circuit and give an alarm.
3. Effect of flue gas purification treatment.
Since the official operation of the asphalt flue gas treatment system of the roaster of a carbon plant in a city in March 2005, the purification effect is good, and after monitoring, the concentration of all pollutants in the purified flue gas has reached the requirements of the national emission standards (smoke concentration 200mg m3, asphalt smoke concentration 50mg m3, benzo(a)pyrene concentration 0.).30×10-3mg/m3)。
During the operation of the dust removal system, a small amount of asphalt has been condensed on the anode plate, which affects the purification efficiency, and finally the scraper is used to scrape it manually. Through the automatic control system, the temperature of the steam insulation layer of the electric tar precipitator is adjusted, and the problem of asphalt coagulation has been solved to a certain extent.
IV. Conclusions. The use of atomization spray and electric tar precipitator to purify the asphalt flue gas of the carbon roaster has created a new process for the treatment of low-concentration asphalt smoke, and its purification effect on asphalt smoke, especially p-benzo(a)pyrene, is much higher than that of the combustion method. The main problem with this process is that the asphalt collected on the anode plate is easy to coagulate and is not easy to remove. How to solve the problem of asphalt adhesion and coagulation is a problem worth considering in the future.
*: Liu Zhangxian Xiangzhang shares environmental protection technology.