The main component of limestone is CaCO3, which contains various impurities such as MgO, Fe2O3, Al2O3, SiO2, etc. These impurities are the main components of the suspended matter of desulfurization wastewater. Coal and limestone also contain a small amount of heavy metals, which has good solubility in weakly acidic desulfurization wastewater. The electrostatic precipitator of the power plant is difficult to remove fine particles smaller than 0.5 μm, causing many heavy metals to be washed in the absorption tower. It is enriched in the FGD slurry, and at the same time, selenium is one of the extremely volatile harmful trace elements in coal. It is almost completely volatile during the combustion process, and it exists in the form of +6 selenate in the desulfurization wastewater. toxicity. Therefore, the desulfurization wastewater has great significance.
As the water in the slurry of the desulfurization device is continuously circulated, it will be enriched with heavy metal elements, Cl- and fine particles, which will accelerate the corrosion of the desulfurization equipment, affect the desulfurization efficiency, and the other will affect the quality of gypsum. Therefore, the desulfurization device must discharge a certain amount of wastewater and enter the desulfurization wastewater treatment system. After neutralization, sedimentation, flocculation, sedimentation and dehydration treatment, http://euhowblog.com
reaches the standard and is discharged to the industrial wastewater adjustment tank. The raw wastewater treatment process system consists of neutralization, sedimentation, flocculation, sedimentation and dewatering systems as shown in Figure 1.
1.1 Neutralization reaction
First, the waste slurry from the absorption tower of the desulfurization system is collected in the wastewater buffer tank and pumped to the reaction tank neutralization tank of the wastewater treatment system. A certain amount of lime milk is added to the neutralization tank, and the pH value of the wastewater is adjusted to a range of 9 to 9.7 to reduce the corrosiveness of the wastewater. At the same time, most of the heavy metals in the water are precipitated as hydroxides, and the wastewater is in a dissolved state. Fluoride is removed as a precipitate of calcium fluoride. The calcium hydroxide solution itself can also function as a flocculant. After the pH of the wastewater is adjusted, the subsequent flocculation and clarification treatment effects can be improved, and the dosage of subsequent chemicals can be reduced.
1.2 Settling reaction
The purpose of the organic sulfide chemical solution treatment is to remove the heavy metal ions remaining in the wastewater and cannot be removed in the form of hydroxide precipitation. Generally, heavy metal ions in desulfurization wastewater exist in two different forms: one is in a free state and the other is in the form of a dissolved complex. Free heavy metal ions can generally be removed by precipitation with calcium hydroxide, but because the solubility of complex heavy metal dissolved matter is much lower than the solubility of its hydroxide, it cannot be removed by adding calcium hydroxide. For this reason, this kind of metal can only be removed by seeking a metal precipitate with a lower solubility than the complex heavy metal dissolved matter. Most of the heavy metal sulfide precipitation can meet this requirement. Some http://euhowblog.com
compounds formed by some heavy metals such as mercury and chloride in wastewater cannot be removed by adding calcium hydroxide, but the addition of sulfide can meet the requirements. Although most heavy metal ions can form metal sulfide precipitates, the sulfides (such as hydrogen sulfide and sodium sulfide) used to form metal sulfide precipitates are generally highly toxic, and the metal sulfide precipitates formed are often highly dispersed and extreme In some cases, a low collapsibility or sedimentation of natural colloids is observed. Therefore, organic sulfides are often used, and organic sulfides (such as TMT15 are non-toxic and environmentally friendly on the one hand, and the precipitates formed on the other hand have good sedimentation properties.
1.3 Flocculation reaction
In the flocculation system, the heavy metals in the water are further removed by increasing the pH value and adding polyiron and organic sulfur, and the Ca (OH) 2 is controlled by the pH value. The dosage of polyferric iron and organic sulfur is determined through debugging, and it is added in proportion according to the amount of wastewater. In the precipitation system, a coagulant is added to make the precipitated particles grow more easily and settle.
1.4 Sedimentation and dewatering system
After the suspended matter is separated from the clarification / concentration tank, part of the thin sludge is returned to the neutralization tank by the sludge circulation pump, and the other part of the clarified water is discharged into the fresh water tank for recovery. The sludge from the bottom of the clarification / concentration tank is transferred to the filter press, made into a cake, and transported to the ash yard by truck.
2 Principle and process of zero discharge of desulfurization wastewater
Desulfurization wastewater zero discharge system
first concentrates wastewater by extracting high-temperature flue gas from the inlet flue of the absorption tower, then quenches and separates the concentrated wastewater, and finally sends the separated wastewater to a drying bed for further heating and drying The hot secondary air is used as a drying medium to concentrate and dry the slurry into dust-containing gas and enter the flue before electrostatic dedusting, which is collected together with fly ash. The wastewater zero-discharge process (see Figure 2) system consists of a flue gas system, a concentration system, a concentrated quenching and tempering, a separation system, and a slurry drying system.
2.1 Flue gas system
By extracting the high-temperature flue gas in the flue in front of the desulfurization tower as the evaporation medium (the flue gas temperature is about 110 ¡ã C), the high-temperature flue gas enters the concentration tower and serves as the evaporation medium. After the wastewater is cooled and sprayed, it returns to the flue in front of the desulfurization tower. In order to overcome the system equipment and flue resistance of the concentration tower device, two centrifugal fans are set on the original flue gas side upstream of the concentration tower. After passing through the booster fan, the flue gas enters the absorption tower, after cooling and spraying to about 50 ¡ã C, it returns to the front flue of the desulfurization tower and enters the absorption tower together with the original flue gas. 2.2 Concentration system The upper part of the slurry circulation pump in the tower is sprayed to carry out countercurrent contact with the wastewater, and the tower is evaporated to realize the concentration of the wastewater. The chloride ion, sulfate ion and magnesium ion in the wastewater are continuously enriched. The low-temperature saturated flue gas after being washed by the concentration tower is removed from the upper side of the concentration tower after the mist droplets are removed by the demister, and then returned to the flue before the desulfurization tower. 2.3 Concentrated wastewater is quenched and tempered, and the concentrated wastewater of the separation system is adjusted by adjusting the pH value after slaked lime, and then enters the clarification tank for treatment. The upper supernatant liquid is sent to the drying system after overflow, and the bottom slurry is pumped to the existing wastewater workshop for concentration. The clarification tank is finally discharged through a filter press, and the solids after the filter press are transported to the coal yard for blending to achieve zero discharge of wastewater.
2.4 concentrated slurry drying system
The hot secondary air is extracted from the secondary air recirculation pipe as a drying medium, and after being pressurized by the hot air fan, it enters the inert carrier drying bed. During operation, ensure that the inert carrier particles in the bed are in a fluidized state, and spray the slurry on the surface of the inert particles. It exchanges heat and mass with high-temperature hot air. After drying, the slurry is ground through the collision between inert particles, then falls off the surface of the inert carrier, is carried by the gas, leaves the drying bed, and enters the flue before the electrostatic precipitator. Fly ash is mixed with electrostatic precipitator to realize zero discharge of wastewater.
(1) The cost of zero discharge of desulfurized wastewater is high, the equipment system is complex, and the subsequent maintenance costs are much higher than the conventional wastewater treatment system.
(2) Zero discharge of waste water saves energy and achieves efficient resource utilization in waste water discharge.
(3) Zero discharge of high-salinity desulfurization wastewater is realized, and there is no sewage discharge at all, and the unit is clean and efficient.
(4) Desulfurization wastewater is evaporated by bypassing the flue to meet the requirements of zero wastewater discharge. High-temperature flue gas is used for evaporation, no additional heat source is needed, low energy consumption for operation, and the requirements of energy saving and environmental protection are realized.
(5) Zero discharge of wastewater has realized partial treatment of the flue gas at the inlet flue of the absorption tower, reducing the content of dust in the flue gas entering the absorption tower, and optimizing the entire desulfurization system.
(6) Due to the evaporation technology of the flue gas, the adding of waste water is reduced and the cost is saved.
With the country's increasing emphasis on environmental protection, coal-fired power plants should pay more attention to the use of desulfurization wastewater. The technology of zero wastewater discharge not only ensures zero wastewater discharge, but also reduces the amount of chemicals used and reduces the cost of wastewater treatment. Through theory and practice, it has provided enterprises with more economical and feasible technologies and reached the country's "zero emission" goal.