Zero-discharge treatment process for wet desulfurization wastewater in coal-fired power plants

Among the mature processes used for flue gas desulfurization at home and abroad, the limestone-gypsum wet desulfurization (Wet Flue Gas Desulfurization, WFGD) process is the most widely used in coal-fired power plants.

This process accounts for more than 90% of the total industrial desulfurization in my country and has the advantages of mature technology, high desulfurization efficiency, reliable operation, wide load range, and good adaptability to coal types. However, the desulfurization wastewater produced by this process is the most difficult wastewater to be treated at the end of the power plant system, and its characteristics of high suspended solids, high salt content, and various heavy metals make it difficult to treat even after traditional processes.

This kind of wastewater still has high salt content and is corrosive, and it is difficult to directly discharge or merge into municipal sewage, which has become a new challenge for power plants to achieve “zero discharge” of desulfurization wastewater.

On the basis of analyzing the source, characteristics, traditional treatment process and existing problems of desulfurization wastewater, this paper focuses on summarizing and expounding several zero-discharge processes, and analyzes the possible problems in its application, and puts forward specific suggestions and measure.

Source and characteristics of desulfurization wastewater

1.1 Source of desulfurization wastewater

Judging from the actual operation of coal-fired power plants in my country, the desulfurization wastewater produced by boiler wet desulfurization mainly comes from the wastewater discharged from the desulfurization tower, and the amount of wastewater discharged is generally determined by controlling the concentration index of Cl in the slurry in the desulfurization tower.

While using FGD process to remove SO2 in flue gas, acid gases such as HCl and HF in flue gas will also be absorbed by desulfurizer and transferred to desulfurization slurry. Since the desulfurization slurry is recycled, even if the concentration of HCl, HF and other acid gases in the flue gas is much lower than that of SO2, with the operation of the desulfurization system, the concentration of Cl- and F- in the slurry will gradually increase.

The combination of aluminum and F- in the slurry has a shielding effect on the dissolution of limestone and reduces the desulfurization efficiency; Ca2+ and Cl- in the slurry form an ion pair CaCl2, which affects the dissolution of calcium hydroxide as an absorbent, and the increase in Cl- concentration will also lead to Desulfurization efficiency decreases and gypsum quality decreases.

At the same time, it is corrosive to pipelines and systems. In order to run the system stably and ensure the desulfurization efficiency and the product quality of gypsum, part of the slurry needs to be discharged, and the Cl concentration in the slurry is generally controlled to be less than 20kg/m3.

1.2 Characteristics of desulfurization wastewater

Factors such as limestone quality, desulfurization system design and operation, pollutant control equipment in front of the desulfurization tower, and coal quality affect the quality and quantity of desulfurization wastewater.

Among them, limestone is the source of some pollutants in desulfurization wastewater, including nickel and zinc in desulfurization wastewater and fine particles, aluminum and silicon contained in clay impurities.

The design and operation of the desulfurization system affect the quality of the desulfurization wastewater mainly in the use of additives, the degree of oxidation or the way of oxidation, and the construction materials of the desulfurization system.

The pollutant control equipment before the desulfurization tower refers to the dust removal and denitration equipment. The improvement of dust removal efficiency may reduce the concentration of total suspended particulate matter in desulfurization wastewater, but the fly ash of fine particles may also increase the content of volatile metals in desulfurization wastewater.

The denitration equipment can increase the ratio of Cr3+ to more toxic and more soluble Cr6+; the ammonia escaping from the denitration system will increase the ammonia nitrogen concentration in the desulfurization wastewater.

The quality of coal combustion is the main factor affecting desulfurization wastewater. High sulfur coal and high chlorine coal will increase the discharge of desulfurization wastewater.

Desulfurization wastewater has the following characteristics:

(1) The water quality is unstable. Affected by factors such as coal quality, limestone quality and the operation of the desulfurization system, the water quality of the same desulfurization equipment may vary greatly at different times.

(2) The water quality is weakly acidic. The pH value is generally 4-6.5.

(3) The content of suspended solids is high. Generally between 10000~150000mg/L, the main components include ash, inert substances and flocculation sediments.

(4) High salt content. Total dissolved solids (TDS) are generally between 25,000 and 60,000 mg/L, and the anions and cations with the highest content are Cl- and Mg2+, respectively. Other anions and cations include Ca2+, SO42-, F and the like. In addition, it also contains the first type of pollutants and the second type of pollutants stipulated in GB8978-1996 “Integrated Wastewater Discharge Standard”.

Desulfurization wastewater treatment process

2.1 Chemical precipitation process

At present, domestic treatment of desulfurization wastewater generally adopts conventional chemical precipitation technology, namely “neutralization-precipitation-flocculation” triple box technology. The process flow is as follows:

The desulfurization wastewater enters the neutralization tank, the sedimentation tank, and the flocculation tank in turn through the pipeline, and finally settles in the clarifier and adjusts the pH value in the outlet tank to neutrality before discharging.

Among them, the pH value is adjusted to about 9 by adding caustic soda or limestone in the neutralization box, and most of the heavy metal ions such as Fe3+, Zn2+, Cu2+ in the solution will form insoluble hydroxide precipitation and be separated from the solution. The main function of the precipitation box is to use the organic sulfur precipitant TMT-15 or Na2S to it, to separate the heavy metals such as Pb2+ and Hg2+ that cannot be precipitated in the form of hydroxides that are not removed by the neutralization box.

Since the desulfurization wastewater entering the wastewater system has been concentrated and separated in two stages by a wastewater cyclone and a gypsum cyclone, the suspended solids contained in it are small and the settling performance is poor. In order to improve the settling ability.

It is necessary to add a flocculant (ferric chloride, FeClSO4) to the flocculation box and add a coagulant (polyacrylamide) to the outlet pipeline of the flocculation box. After entering the clarifier, the active flocs generated by coagulation adsorb the fine metal oxides precipitated in the water to realize the separation of water and suspended solids.

Finally, the clean water enters the water outlet tank, and the pH value is adjusted by adding hydrochloric acid, and the discharge reaches the standard. A small part of the sludge at the bottom of the clarifier is returned to the neutralization tank as contact sludge, and most of the sludge will be sent to the plate and frame filter press through the sludge feed pump for dehydration into a mud cake and shipped out.

The wastewater treated by the triple box process can effectively remove suspended solids impurities and various heavy metal ions, and meet the comprehensive sewage discharge standard. However, this treatment process requires high process control, and the removal effect of Cl- and SO42- is very limited. Recycling of desulfurized wastewater after treatment.

2.2 Membrane concentration reduction technology

The idea of concentration reduction technology is to concentrate the pretreated desulfurization wastewater by a certain concentration process, reduce the amount of wastewater, and reduce the amount of subsequent evaporation and solidification treatment, thereby reducing the treatment cost. Among them, membrane concentration technology mainly includes nanofiltration (Nanofiltration, NF), reverse osmosis (Reverse Osmosis, RO), forward osmosis (Forward Osmosis, FO) and electrodialysis (Electro Dialysis, ED).

  (1)NF.

Nanofiltration membranes can not only retain small organic molecules, but also have a high rejection rate for divalent or high-valent ions, especially anions, but the rejection rate for monovalent ions is less than 90%.

  (2)RO.

Reverse osmosis is to apply a pressure greater than the natural osmotic pressure on the concentrated solution, so that the solvent passes through the semi-permeable membrane from the concentrated solution to the dilute solution. According to different processes, reverse osmosis can be divided into disc type reverse osmosis, high pressure reverse osmosis and special flow channel reverse osmosis.

The reverse osmosis technology is safe and reliable, the effluent is stable, the salt removal rate is high, and the energy consumption is low. The disadvantage is that it is easy to pollute and scale.

  (3)FO.

Forward osmosis relies on the huge osmotic pressure driving force generated by the extract, so that the water molecules on the high brine side diffuse spontaneously and selectively into the extract side. The extraction solution is an ammonium carbonate solution formed by dissolving ammonia and carbon dioxide in water according to the specific substance ratio.

Forward osmosis has low energy consumption, high effluent quality, and light fouling, but there are complex extraction solutions, complex systems, and high investment costs.

  (4)ED.

The working principle of electrodialysis is that when the raw water is alternately arranged in the compartment between the cathode and anode through the cationic and anion membranes connected to direct current, the charged ions migrate directionally under the action of the electric field force. Due to the selective permeability of the cationic and anion exchange membranes, Part of the water is concentrated and part of the water is desalinated to form alternately arranged thickening chambers and thinning chambers to separate and purify wastewater.

Electrodialysis has the advantages of low energy consumption, less environmental pollution, and less drug dosage, but it has low tolerance to calcium and magnesium scale, and it is difficult to remove substances that are difficult to dissociate. .

2.3 Evaporative curing technology

Evaporation and solidification unit is the key unit to achieve zero discharge of desulfurization wastewater. It uses heat source to evaporate desulfurization wastewater, and the liquid after evaporation and condensation is reused, and the solid content crystallized is used as a resource.

The main technologies include steam evaporation crystallization technology and flue gas evaporation crystallization technology. Steam evaporation crystallization technology includes multiple effect evaporation crystallization (Multiple Effect Distillation, MED) and mechanical compression evaporation crystallization (Mechanical Vapor Re-compression, MVR); flue gas evaporation crystallization mainly includes low temperature flue gas direct evaporation crystallization and high temperature flue gas bypass evaporation crystallization.

2.3.1 Vapor evaporation crystallization technology

(1) MED technology

MED is an evaporation technology developed on the basis of single-effect evaporation. This technology reduces operating costs by reusing the thermal energy of steam multiple times to reduce consumption of thermal energy.

The MED system generally includes multiple evaporators (i.e., multiple effects). The waste water and fresh steam entering the evaporator are heat exchanged as the first effect, and both the secondary steam and the concentrated liquid are generated into the second effect evaporator to continue the evaporation and exchange. hot. That is, the evaporation heat source of the latter effect comes from the secondary steam generated by the former effect, and the concentrate produced by the former effect will continue to be concentrated in the latter effect.

In order to ensure the heat transfer power of each effect and realize multiple thermal energy utilization between effects, the operating pressure of each effect must be reduced step by step, so that the secondary steam pressure and steam boiling point of each effect are sequentially reduced. Finally, the high-salt wastewater is gradually evaporated and concentrated under the action of heating steam of various effects, enters the crystallizer to produce crystalline salt, and realizes solid-liquid separation through the separator. Since the temperature of the heating steam decreases gradually, the evaporation effect after the four effects is poor, so the general multi-effect evaporator only achieves four effects.

(2) MVR technology

MVR technology is to send waste water and steam into the heater for heat exchange, and the waste water vaporizes to generate secondary steam. The secondary steam after gas-liquid separation is sent to the compressor to be compressed to increase the enthalpy, and then returned to the heater to heat the waste water. The secondary steam generated will enter the compressor again, and so on.

The waste water reaches a supersaturated state with the continuous increase of the concentration until the salt is analyzed, and finally the salt and water are recovered and reused after solid-liquid separation.

The heat energy required for evaporating wastewater in the MVR process mainly comes from the heat energy released by the condensation of steam, and external steam is required when it is first started. After normal operation, only the electrical energy required for the control system, steam compressor and driving the steam, waste water, condensate flow and circulating water pump in the evaporator is consumed.

This process can achieve zero discharge of desulfurization wastewater, but the disadvantage is that the system is complex, the investment and operation costs are high, and the water quality requirements are relatively high.

2.3.2 Flue gas evaporation crystallization technology

The flue evaporation process is to use the waste heat of exhaust flue gas to rapidly evaporate the pretreated desulfurization wastewater sprayed into the flue at the tail of the boiler, and the generated salt crystals and other impurities enter the dust collector together with the flue gas and are captured and transported outside the coal ash. The steam enters the absorption tower for recycling.

According to the selected evaporative flue position, the flue evaporation process can be divided into low temperature flue evaporation technology and high temperature flue bypass evaporation technology.

(1) Low temperature flue evaporation technology

When the low-temperature flue evaporation process is adopted, the desulfurization wastewater is sprayed into the flue generally in the flue between the air preheater and the dust collector. This process makes full use of the waste heat energy of the flue gas discharged from the power plant, and can achieve the purpose of zero discharge of desulfurization wastewater. Etc.

The disadvantage is that the flue gas temperature in this section of the flue is relatively low, and the flue gas exhaust temperature needs to be controlled above the dew point temperature. When the load unit load is low or the fluctuation is large, the wastewater evaporation effect is poor, and the residual wastewater will enter the dust collector with the flue, causing smoke Corrosion, fouling, clogging.

Therefore, this technology generally has more applications in the renovation of old units, and is less used for newly built units with ultra-clean emission requirements, and has certain safety hazards for long-term low-load operation units.

(2) Bypass flue gas waste heat evaporation crystallization technology

The bypass flue gas waste heat evaporation crystallization technology is to set up a flue bypass in parallel with the air preheater in the system, and introduce high-temperature steam from the front end of the air preheater as a heat source for wastewater evaporation to quickly evaporate the atomized desulfurization wastewater. The crystals and solid impurities produced are captured and removed together with the fly ash in the main flue after the bypass flue gas enters the air preheater.

The process adopts an independent operation mechanism, which can carry out independent maintenance and repair, and can realize zero discharge of desulfurization wastewater. It is suitable for units with low flue gas temperature, low load, or low temperature economizer technology.

However, using this process will reduce the temperature of the furnace inlet air to a certain extent and reduce the efficiency of the boiler, thereby increasing the coal consumption of the unit.

Conclusion

To sum up, with the increasingly strict national environmental protection standards, zero discharge of desulfurization wastewater from coal-fired power plants will become a trend. According to the characteristics of desulfurization wastewater and the advantages and disadvantages of each desulfurization wastewater treatment process, it is necessary to reasonably choose the pretreatment, chemical precipitation, concentration reduction, evaporation and solidification, flue gas evaporation and crystallization and other processes to optimize the treatment route to achieve the optimal treatment cost and effect.

The pretreatment of desulfurization wastewater is the basis for the reliable and economical operation of the subsequent treatment process. To achieve zero discharge of desulfurization wastewater, the following work needs to be done:

(1) The chemical precipitation process can remove most heavy metals and suspended solids well, and various indicators can meet the comprehensive sewage discharge standards. In order to cope with the fact that suspended solids (SS) and chemical oxygen demand (COD) often cannot meet the standard discharge, it is necessary to precisely control the amount of dosing, dosing methods, and equipment operation management to improve the effect of wastewater treatment.

(2) The evaporation and crystallization process has high energy consumption, equipment is easy to scale, and investment and operating costs are high. The softening treatment in the pretreatment process can reduce the content of Ca2+ and Mg2+, prevent the evaporator from scaling, and pass through membranes such as reverse osmosis. The concentration and weight reduction process concentrates the desulfurization wastewater to reduce the treatment cost of the evaporative crystallization process.

(3) The flue evaporation process system is simple, and the investment and operation costs are low. In the application, a reasonable flue evaporation process should be selected according to the actual situation of the unit to reduce the operation risk and operation cost. It is also necessary to strengthen the control and optimization of the desulfurization wastewater pretreatment process to avoid blockage of the atomization injection system.

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