A Study on Mechanism of Evaporation Reduction of Desulfurization Wastewater in Air Cooling Tower

It’s a new technology for the concentration and reduction of desulfurization wastewater by the indirect air cooling tower. In this paper, indoor experiments are conducted to explore the evaporation characteristics of the evaporation tower and the evaporation law of desulfurization wastewater in the filler and analyze the factors affecting the evaporation characteristics, and a power plant adopting this technology is tested to verify the conclusion of the indoor experiments. The results show that the evaporation rate of the evaporation tower is positively correlated with the cross-sectional wind speed, water spray density, packing height and other parameters, and the influence of the interrupted wind speed on the evaporation rate is the most significant. The difference of vapor pressure has a positive linear relationship with that of mass fraction of the air in inlet and outlet, and the results of model test conform to those of actual one basically. On the condition of ventilation rate of 817.56m3/s, the evaporation efficiency of a mechanical ventilation cooling tower with a drenching area of about 400m2 can reach 16.2-18.4t/h, which basically meets the engineering needs of desulfurization wastewater concentration and reduction in millions of thermal power plants.


Introduction
On January 16, 2023, the National Energy Board released the 2022 national electricity industry statistics, showing that by the end of December, the cumulative national installed capacity of power generation was 2.56 billion kilowatts, of which the thermal power generation accounted for about 52%, although the growth rate of thermal power generation is slowing down, it still occupies a dominant position in the energy structure in China.In order to effectively control the emission of pollutants from coal-fired power plants, China has issued laws, regulations and policies, such as the "Technical Guide for the Pollution Prevention and Control in Thermal Power Plants", "Water Pollution Control Action Plan".The former states that zero-discharge of desulfurization wastewater is the key to achieving nearzero wastewater discharge [1].More than 85% of coal-fired power units in China adopt limestonegypsum wet desulfurization technology.While efficiently removing sulfur dioxide from flue gas, the continuous circulation of slurry will cause Cl -enrichment.In order to prevent excessive chloride ions from inhibiting limestone dissolution, reducing desulfurization efficiency and corroding equipment, the system needs to discharge a portion of wastewater regularly to control the Cl -content within a certain range.This part of wastewater discharged regularly is the desulfurization wastewater [2].The desulfurization wastewater has poor water quality; low pH; strong corrosiveness; high concentration of operate, with high system stability and low operation and maintenance costs; 4) low pre-treatment requirements, low water quality requirements; 5) small floor area.
During the operation of the evaporation tower, the temperature of the circulating desulfurization wastewater is lower than the air and it is opposite with the general mechanical ventilation cooling tower.There is little research on the evaporation characteristics of water under the process technology of evaporation reduction of desulfurization wastewater into air cooling tower.In order to obtain the evaporation and heat dissipation pattern of desulfurization wastewater, this study built a small evaporation tower model indoors, studied the evaporation characteristics of the evaporation tower and the evaporation pattern of desulfurization wastewater in the filler, analyzed the factors affecting the evaporation rate of the evaporation tower, and conducted a real tower test in the power pant using this technology to verify the conclusion of the indoor test, and the research results can provide a basis for the design and promotion of evaporation tower.

Test Set Up Design
The design of the test device is shown in figure 2, the tower test section is 3.0m high, the drench section is 0.6m×0.6m, the drench density adjustment range is 4.0~20.0(m 3/(m 2 h)), and the wind speed adjustment range of the water spray section is 0.5~3.5m/s.The test system consists of water circulation system, air circulation system and measurement system.Since the salt content of the desulfurization wastewater mainly affects the end of evaporation, and the evaporation trends of droplets with different salt contents are basically the same at the beginning of evaporation [20,21], the ordinary water is used instead of desulfurization wastewater in the water circulation system to carry out the indoor tests.Water from the reservoir through the pump to the heating tank, heated and sent to the water distribution device, the distribution device is made up of three pressure water pipes with a diameter of 25mm, each pipe is equidistantly installed on the three shower nozzles, circulating water sprays from the shower nozzles uniformly, after electromagnetic flowmeter and the flow rate is measured by the electromagnetic flowmeter and returned to the reservoir for recycling.Air is drawn in through the air inlet by a centrifugal fan at the end of the test rig and heated by the electric heater to control the dry bulb and wet bulb temperature of the inlet to achieve the parameters required to be controlled by the test.After the air enters the test section of the tower and exchanges heat with water, it is discharged from the air duct, partially discharged outdoors, and partially returned to the air inlet to adjust the wet bulb temperature of the inlet air.The inlet tower air volume is controlled by changing the fan speed through a frequency converter.The measurement system mainly includes air parameters measurement system and circulating water parameters measurement system.Air parameters measurement system includes: inlet air dry and wet bulb temperature, atmospheric pressure, wind speed, outlet air dry and wet bulb temperature; circulating water parameters measurement system includes: import and export water temperature and circulating water volume.

Test Principle
The test method is slice simulation, taking a small control unit body in the cooling tower as the test object, the section wind speed and water density in the test are close to the actual project.According to the conservation of mass, the amount of water lost by evaporation per unit of time is equal to the increase of moisture content in the air, so the evaporation rate of circulating water can be calculated accurately by calculating the moisture content of air into the tower and air out of the tower [22].The evaporation rate is calculated according to equation (1): where w1 q is the evaporation rate of evaporation tower.dry ρ is the density of the dry air leaving the evaporation tower.
The moisture content of the inlet air of the evaporation tower is calculated according to equation (2): where a1 P is atmospheric pressure in indirect air-cooled towers, near evaporation towers.vθin p  is the saturated vapor pressure corresponding to the dry bulb temperature of the inlet air to the evaporation tower. in  is relative humidity of evaporation tower inlet.out Q is the outlet air flow of the evaporation tower, which is dimensionless.The mass fraction of air inlet to the evaporation tower is calculated according to equation (3): where in ω is evaporation tower inlet air mass fraction.
The moisture content of the outlet air and the mass fraction of air outlet are calculated in the same way.
According to the theory of mass transfer kinetics, the evaporation of water depends on the difference between the number of molecules escaping from the water surface and reabsorbed by the water, after the heat exchange between water and air into equilibrium, the amount of molecules escaping and being absorbed is equal, proportional to water vapor pressure.The driving force of water evaporation is the difference between the saturation vapor pressure corresponding to the water surface temperature and the partial pressure of water vapor in the air (hereinafter referred to as the vapor pressure difference).Therefore, the evaporation rate per unit of time through the water surface can be calculated according to the equation ( 4): The vapor pressure difference is calculated according to equation ( 5): (5)

Test Working Conditions
The test was divided into 0.6m filling height group and 0.3m filling height group, and the atmospheric pressure in the test was 101.6kPa.The test conditions were set as follows: The first group of tests used S-wave water-drenching filler, and the height of the filler was 0.6m, the inlet tower dry bulb temperature was divided into 10C, 20C, 30C.A total of 63 conditions were carried out, the waterspraying density of each inlet dry bulb temperature was 7t/(hm 2 ), 10t/(hm 2 ), 13t/(hm 2 ), and the cross-section wind speed gradually increased form 1.0m/s to 2.5m/s.The second group still used Swave water-drenching filler, and the height of the filler was 0.3m.According to the temperature rise range of the inlet air, the experiment was divided into three groups, respectively 10C, 20C, 30C.

Analysis of Experimental Results
The first group of the test that filling height is 0.6m, each inlet dry bulb temperature conditions, the evaporation rate of different drench density and relationship between the section wind speed as shown in figure 3, figure 5, figure 7, with the section wind speed increases, evaporation rate increases, the two are basically linear relationship.Comparison of figure 3, figure 5, figure 7, found only in figure 3, before the wind speed increased to 2m/s, the evaporation rate decreases when the drench water density increases.Combined with the inlet air heating test results, the reason for the low evaporation rate when the drench density is higher is the decrease in the decrease of inlet water temperature during the test.
According to figure 4, the test began with a higher water temperature of 8.1C, when the evaporation rate was high.As the water is recycled in the test, the water temperature gradually decreases under the cooling of the air, which reduces the evaporation rate of water.In figure 6, and figure 8, after a period of circulation of spray water, although the water temperature is still decreasing, it is relatively stable.At this time, the variation of the temperature of spray water is minor, therefore when the inlet tower dry bulb temperature is 20C; 30C, figure 5; figure 7; are in line with the law that the evaporation rate increases with the increase of spray water density.This is also shows that the change of spray water density has little effect on the evaporation rate, and more critical factors are the inlet air temperature, the inlet water temperature, the relative humidity of the inlet air, and the wind speed of the filling section.In the second group of experiments which the filling height is 0.3m, the evaporation, inlet water temperature, and the inlet air temperature under each working condition are compared with the experiments of 0.6m filling height, which verifies the correctness of the test conclusion in the first group of experiments.Comparing the evaporation rate of the evaporation tower with the two packing heights under the same other conditions, found that the packing height has little effect on the evaporation rate.
According to the equation 1 ~ equation 5, it can be seen that the evaporation capacity is related to the difference of air mass fraction between the inlet and outlet of the evaporation tower (∆), and the difference of vapor pressure (∆).It is inferred that there is a correlation between the difference of air mass fraction between the ∆ and ∆.The figure 11 shows the relationship between ∆ and ∆ in different filling height, the greater the difference in vapor pressure, the greater the difference in mass fraction, and the two are basically linear.Fitting the steam pressure difference between 0.3m packing height and 0.6m packing height, the following expressions can be obtained respectively: ∆ could be calculated by equation (8): By comparing equation ( 6) and equation ( 7), it can be seen that there is a linear relationship ∆ and ∆, and the slope is relatively close.

Real Tower Test
In order to further verify the test results, a real tower test study was carried out.The test object is the desulfurization wastewater evaporation tower in the indirect air-cooling tower of unit3 in a millionunit thermal power plant in Ningdong.As shown in figure 12, The main tower is 35m high, with a waste liquid steam discharge tower cylinder above and a square tower with a side length of 20m.The tower cylinder is 25m high and 10.4m in diameter, which is composed of five hyperbolic small tower cylinders.There is a desulfurization wastewater storage tank, the depth of the tank is 2m, the area is about 2600m2.The filler is S-wave with a height of 1.25m; the drenching area of the evaporation tower is about 400m2, and the height of the air inlet is 5.0m.Test method reference "Industrial Cooling Tower Test Procedures" (DL/T 1027-2006), test parameters include: air pressure in the air-cooling tower, inlet and outlet temperature of evaporation tower, dry and wet bulb temperature of the inlet and outlet air of evaporation tower, wind speed of the evaporation tower, circulating water quantity of evaporation tower.The above test parameters were obtained by arranging the measurement points, and the arrangement of the measurement points and the accuracy of the measuring instruments during the test are shown in figure 13 and table 1.

Analysis of Real Tower Test Results
The test season was winter, with an average ambient wind speed of 2.1m/s and an average ambient temperature of 3.7℃.The atmospheric pressure in the air-cooling tower (i.e., the average value of measurement results at point 1) is 860.65 hPa.Since the water temperature in the pool is relatively uniform, the analysis suggests that the water surface temperature is approximated as the discharge water temperature.The measurement results of evaporation tower inlet water temperature, outlet water temperature, circulating water volume (measurement point 2, measurement point 3, and measurement point 33); and the saturated vapor pressure corresponding to the water surface temperature are shown in table 2. Analysis of table 2 shows that during the operation of the evaporation tower, the water temperature in the reservoir remains basically unchanged (the change is less than 0.5℃).The results of the dry and wet bulb temperature measurements of the air inlet to the evaporation tower (measurement point 4 to measurement point 7) are shown in table 3.  4. According to the measurement results, it can be calculated that the average ventilation volume of evaporation tower is 817.56 m 3 /s, the average dry bulb temperature of evaporation tower outlet air is 9.0℃, and the average wet bulb temperature of evaporation tower outlet air is 8.7℃.Compare with table 2 and table 4, we can find that the water surface temperature and evaporation tower outlet wet bulb temperature are nearer, and the evaporation tower outlet air is close to saturation.Table 4. Measurement results of wind speed and dry and wet bulb temperature of outgoing tower air at each measuring point on the throat section of the first floor of tower barrel; And evaporation tower ventilation volume, evaporation tower average outlet air dry and wet bulb temperature calculation.Combine with equation (1) to equation ( 5), the evaporation rate of the evaporation tower, the difference in vapor pressure and the difference in air mass fraction between import and export can be calculated by different inlet air dry and wet bulb temperatures and different circulating water volumes, the calculation results are shown in table 5. Compare the evaporation rate of evaporation tower before and after 10:10 time point in table 5, the average value of circulating water is about 2350m 3 /h and 1910m 3 /h respectively, and found that the evaporation rate did not decrease with the reduction of circulating water, which verifies the conclusion obtained in the model test: the change of drench water density has less influence on the evaporation rate, and the more critical factors are the inlet tower air temperature, inlet water temperature, inlet tower air relative humidity, packing section wind speed, etc.As the two circulating water conditions in table 5, respectively, corresponding to the dry and wet bulb temperature of the incoming tower air are not significantly higher, the incoming water temperature, filling section wind speed in the calculation process to take a fixed value, it can be further inferred that at this time the evaporation tower evaporation rate changes are caused by the relative humidity of the incoming tower air.The evaporation rate of the evaporation tower is fitted with the relative humidity of the incoming air, as shown in figure 14, and it was found that the two were negatively linearly correlated, which was consistent with equation (1) and equation (2).The vapor pressure difference in table 5 was fitted to the difference between the inlet and outlet air mass fractions, as shown in figure 14, and the two were linearly related with the following fitting equation.The slope was close to the fitting equation of the model test, verifying the reliability of the model test.

Measureme
Through the actual tower test, when the wind volume is 817.56m 3 /s, a mechanical ventilation cooling tower with 400m 2 shower area can evaporate 16.218.4twastewater per hour, cooling tower unit area evaporation rate is about 0.040-0.046t/(hm 2 ), which can meet the desulfurization wastewater concentration and reduction of engineering needs.

Conclusion
In this paper, for the evaporation mechanism of evaporation tower applied in desulfurization wastewater concentration and reduction technology, an indoor simulation test platform was built to experimentally study the evaporation characteristics of evaporation tower and obtain the influence law of parameters such as section wind speed; and the reliability of the indoor test results was analyzed and verified through the field test of the real tower, and the feasibility of the technology was also verified.The results of the study can provide reference for the design optimization of similar evaporation towers in the future.The main conclusions are as follows: (1) Through the indoor simulation test, it can be seen that the evaporation rate of evaporation tower is influenced by the cross-sectional wind speed, drench density and packing height, which are positively correlated with all three, among which the cross-sectional wind speed has the most significant influence on the evaporation rate; (2) It can be seen through the actual tower test, in the evaporation tower operation process, the temperature change of desulfurization wastewater in the evaporation tank is less than 0.5C and the wet bulb temperature of the air out of the tower is relatively close to the air out of the tower is close to saturation; (3) The vapor pressure difference and the difference between the inlet and outlet air mass fraction are positively linear, and the indoor simulation test gives a fitted relationship between the two, which is in good agreement with the test results of the real tower; (4) When the ventilation volume is 817.56m 3 /s, the evaporation efficiency of a mechanical ventilation cooling tower with a drenching area of about 400m 2 can reach 16.218.4tper hour, which can basically meet the actual engineering demand of desulfurization wastewater concentration and reduction in thermal power plants with one million units.

Figure 1 .
Figure 1.Mechanical ventilation evaporation tower for wastewater concentration and reduction process system.

Figure 2 .
Figure 2. Process of test system.

Figure 3 .
Figure 3.The relationship between cross-sectional wind speed and evaporation capacity under different water densities (packing height densities 0.6m, incoming tower air temperature10C).

Figure 4 .
Figure 4. Changes in inlet water temperature and incoming tower air temperature (packing height densities 0.6m, incoming tower air temperature10C).

Figure 5 .
Figure 5.The relationship between cross-sectional wind speed and evaporation capacity under different water densities (packing height densities 0.6m, incoming tower air temperature 20C).

Figure 6 .
Figure 6.Changes in inlet water temperature and incoming tower air temperature (packing height densities 0.6m, incoming tower air temperature20C).

Figure 7 .
Figure 7.The relationship between cross-sectional wind speed and evaporation capacity under different water densities (packing height densities 0.6m, incoming tower air temperature30C).

Figure 8 .
Figure 8. Changes in inlet water temperature and incoming tower air temperature (packing height densities 0.6m, incoming tower air temperature30C).

Figure 9 .
Figure 9.The evaporation amount of each working condition the packing height is 0.6m.

Figure 10 .
Figure 10.The evaporation amount of each working condition when the packing height is 0.3m.

Figure 11 .
Figure 11.The relationship between the mass fraction difference *1000 and the vapor pressure difference of the packing height (packing height 0.6m; packing height 0.3m).

Figure 12 .
Figure 12.Structure diagram of the evaporation tower.

Figure 13 .
Figure 13.Schematic diagram of the measurement point layout plan.

Figure 14 .
Figure 14.Relation between mass fraction difference *1000 and vapor pressure difference under real tower test.

Table 1 .
Measuring point layout and instrument accuracy.

Table 2 .
Measurement results of circulating water and temperature and calculation of related parameters.

Table 3 .
Evaporation tower inlet air dry and wet bulb temperature measurement results.Evaporation tower wind speed, evaporation tower outlet air dry and wet bulb temperature measurement results (measurement point 8 to measurement point 32) are shown in table

Table 5 .
Evaporation tower evaporation rate and steam pressure difference corresponding to dry and wet bulb temperature of different inlet air and circulating water temperature; And the difference between the inlet and outlet air quality scores.