Experimental study on the reuse of industrial wastewater for desulfurization process water in power

In order to improve the utilization rate of water resources in the whole plant, a power plant in Hebei province compares the quality requirements of the treated industrial wastewater with that of desulfurization process water, and finds that the two are close; the treated industrial wastewater only accounts for 7% of the total desulfurization process water, which can be used as desulfurization process water. In order to further reduce the risk and formulate a reasonable industrial wastewater utilization scheme, the field test was carried out.

The results show that the desulfurization system works normally after the treated industrial wastewater is used as desulfurization process water; the desulfurization efficiency is greater than or equal to 95%; the density of desulfurization slurry is about 1130kg/m3; the moisture content of gypsum by-product of desulfurization is basically kept at about 10%, the mass fraction of CaCO3 is less than 3%, and the purity of CaSO4 · 2H2O is more than 90%, which meets the requirements of the market for gypsum quality. Therefore, the industrial wastewater treated by a power plant in Hebei can be reused for desulfurization system in the plant.

The limestone gypsum wet process is adopted for flue gas desulfurization of a power plant in Hebei Province. Under the rated power condition, the maximum process water consumption of desulfurization system is 140m3 / h, and the water consumption is large. The process water of desulfurization system in this power plant is fresh water obtained after desalination, which is expensive to produce water. The industrial wastewater of the power plant is about 10m3/h, and a small part is transported to the coal yard spray system after treatment. The rest is discharged directly, which not only causes low utilization rate of water in the whole plant, but also increases the discharge capacity of the whole plant.

In order to further reduce the cost of water, reduce the discharge of wastewater and improve the recovery and utilization of the whole plant wastewater, the plant plans to reuse the treated industrial wastewater to desulfurization system. Therefore, the feasibility of reuse of industrial wastewater after treatment and the influence of water quality change on desulfurization system should be studied. Through the analysis of the experiment, a reasonable industrial wastewater utilization scheme is developed, and the process is detailed in the following.

1、 Water quality of industrial wastewater and desulfurization process of power plant

The design output of the treatment system of industrial wastewater (hereinafter referred to as industrial wastewater) of power plant is 10m3 / h. According to the discharge cycle of industrial wastewater, it is divided into two types: regular wastewater and non recurrent wastewater. The regular wastewater mainly includes the regeneration drainage of water treatment and desalination equipment, filter flushing drainage, reverse osmosis drainage, condensate fine treatment device regeneration drainage, etc.; non recurrent wastewater mainly includes air preheater cleaning wastewater, dust remover flushing water, boiler chemical cleaning wastewater, main plant surface flushing water, etc.

For the regular wastewater, the treatment method is: it is collected into the wastewater storage tank of the wastewater centralized treatment station through their respective collection system, stirred and aerated by pump and compressed air, and then pumped to the final neutralization tank after mixing evenly, and then it will flow into the clean water tank after adjusting the pH value. The process flow is shown in Figure 1. The process flow of non recurring wastewater treatment system is shown in Figure 2. After adjusting pH value, the wastewater is condensed and flocculation reaction is completed by reaction tank and flocculation tank, and then enters inclined plate sedimentation tank to complete the separation of flocculation body and water. The water from the upper part of the sedimentation tank flows into the final neutralization tank for pH adjustment, and the final outlet water flows into the clean water tank, and the regular wastewater is summarized.

Figure 1 Schematic diagram of the process flow of regular wastewater treatment

Figure 2 Schematic diagram of non recurring wastewater treatment process

According to the chemical field operation regulations of the power plant (hereinafter referred to as operation regulations), the pH value of effluent of industrial wastewater treatment system is 6-9, suspended matter is less than 70mg/l, cod is less than or equal to 100mg / L. The main indicators of treated industrial wastewater (see Table 1) meet the requirements of operation regulations.

Table 1 water quality of industrial wastewater and desulfurization process after treatment

According to the water quality requirements of desulfurization process water of Bawei company and the research results in the literature [1], the water quality requirements for general desulfurization process are shown in Table 1.

Through comparative analysis, although cod, cl-, dissolved solids in the treated industrial wastewater slightly exceed the requirements of desulfurization process water, the water quality can meet the requirements of general desulfurization process water quality because the industrial wastewater accounts for a small proportion of the total water used in desulfurization process.

2、 Analysis of field test data

In order to further verify the feasibility of the treated industrial wastewater as desulfurization process water, the treated industrial water should be introduced to the desulfurization system, and the influence of industrial wastewater reuse on the desulfurization system should be observed.

1. test plan

According to the plan, temporary pipeline system installation shall be carried out, and the outlet main pipe of clean water pump of original industrial wastewater system will directly lead to the pit between 1 and 2 desulfurization tower. The treated industrial wastewater is transported to the process water point of desulfurization tower by the original pit pump of the power plant, and the electric regulating valve and electromagnetic flowmeter are added to the pipeline.

The industrial wastewater after the first time is added into the pit 120m3 for treatment, after the pit pump is opened to transport it to the desulfurization system, the desulfurization efficiency and gypsum quality of the desulfurizer are monitored; in the case of no change in desulfurization efficiency and gypsum quality of desulfurization tower, the treated industrial wastewater is gradually added, and the gradient of 10m3 / D gradually increases. If gypsum quality obviously decreases, increase monitoring frequency, analyze the reasons, and stop filling in industrial wastewater after treatment. After commissioning the relevant equipment and instruments, the test analysis is carried out, including the overall operation status of desulfurization system, desulfurization efficiency, gypsum quality, etc.

2. analysis of test data

The industrial test lasts for 18 days. When the treated industrial wastewater reuse amount is 10m3/h, that is, the maximum industrial wastewater quantity of the plant is reached, the whole test is finished. The test results are shown in Fig. 3-7.

Figure 3 curve of slurry density change during test

Figure 4 curve of desulfurization efficiency during test

Figure 5 curve of water content change of gypsum during the test

Fig. 6 wastewater treatment process of the curve of the change of the mass fraction of dehydrated gypsum calcium carbonate during the test

Figure 7 curve of change of caso4-2h2o mass fraction in gypsum during the test

It can be seen from Fig. 3-7 that, although the industrial waste water after mixing treatment increases continuously during the test, the slurry density of desulfurization system can be maintained at about 1130kg/m3, and the desulfurization efficiency is still greater than or equal to 95%; the quality of gypsum of desulfurization by-product has not changed significantly, the water content of gypsum is basically kept at 10%, the mass fraction of CaCO3 in gypsum is less than 3%, CaSO4 ᛽ 2H2O purity is more than 90%, Meet the quality requirements of commercial gypsum [2,3,4]. In addition, the pressure difference of demister is within the normal operation range during the test, and there is no abnormal phenomena such as blistering and blocking of absorption tower. The experimental results show that the treated industrial wastewater can be used as desulfurization process water.

3、 Conclusion

The results show that the water quality of industrial wastewater of a power plant in Hebei Province can meet the requirements of desulfurization process except cod, cl-, dissolved solid. Since the added industrial waste water is only 7% of the total water used in desulfurization process, the treated industrial wastewater is used as the process water of desulfurization system and the system operates normally. The experience of this paper is of great significance in water saving and environmental protection, and can be used for reference of related power plants.