Theoretical research and practice of mechanical atomization evaporation of desulfurization wastewater

Wang Tao1, XING Hao-Ruo2, LIU Dao-Kuan2, ZHANG Feng1, GUO Shao-yuan2, WU Kai2, MA Shuang-chen2 *

(1. National Energy Fexian Power Generation Co., LTD., Linyi 273425, China; 2. Department of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, China)

Abstract: Taking the desulfurization wastewater produced in the production process of coal-fired power plant as the research object, the treatment of desulfurization wastewater by mechanical atomization and evaporation technology was discussed. Through theoretical research, the main factors affecting evaporation are found out including droplet diameter, temperature, wind speed, etc. In view of the shortage of existing natural evaporation technology, the natural evaporation technology is combined with the intensified evaporation technology such as installing atomizing device, increasing the water heat and increasing the velocity of stroke in the evaporation process, thus forming the mechanical atomizing evaporation technology. At present, mechanical atomizing evaporation technology has been applied in desulfurization wastewater treatment of coal-fired power plants, coal chemical wastewater treatment and other fields, and has achieved certain results. However, attention should be paid to prevent the possible environmental pollution in the process of use. There are still some shortcomings in this technology. In the future, multi-energy coupling and other methods can be adopted to improve it, so that mechanical atomizing evaporation really becomes a low-cost zero-emission technology.

Key words: desulfurization wastewater; Mechanical atomizing evaporation; Natural evaporation; Forced evaporation; Multienergy coupling

0 Introduction

With the arrival of 2020, the "Action Plan for Water Pollution Prevention and Control" planned to be implemented in the "14th Five-Year Plan" has also entered a critical year, so the management and protection of water environment is still the top priority of the current work. Various industries are experimenting with innovative designs and technologies to improve the treatment of sewage, and industrial wastewater accounts for a large proportion of the sewage that needs to be treated.

Power generation enterprises are mainly in the form of thermal power generation. In order to respond to the basic national policy of environmental protection and control various environmental pollution problems brought by the operation of power plants, many thermal power plants have carried out beneficial exploration. Limestone-gypsum wet desulfurization technology is widely used because of its mature process, high removal efficiency and stable operation. However, the desulfurization wastewater is a part of the refractory wastewater in the production process of thermal power plant due to its high salt content, certain corrosion, easy scaling, high heavy metal content and complex composition. The existing desulphurization wastewater treatment technologies mainly focus on the process of pretreatment, concentration and reduction, terminal solidification, and achieve the ultimate goal of zero discharge of desulphurization wastewater by selecting different treatment processes.

Economic benefit and reliability are important criteria for evaluating technology selection [6]. For example, spraying desulfurization wastewater into coal yard evaporates desulfurization wastewater through boiler combustion, which is simple and economical. However, in the process of coal combustion, chlorine in desulfurization wastewater will be precipitated in the form of hydrogen chloride, which will not only increase the corrosion risk of boiler equipment, but also further enrich in desulfurization tower, resulting in continuous increase of chlorine ion content in desulfurization wastewater [7]. The commonly used membrane treatment technology also has the disadvantages of unstable operation and high operation cost, and there are also some problems in the use conditions of membrane treatment technology and the replacement and cleaning of membrane after use [8]. Evaporative crystallization technology has a good effect on zero emission, but there are some problems such as complex system, high energy consumption, easy scaling of equipment and difficult utilization of by-products. The traditional natural evaporation technology also has some problems, such as large standing area, low efficiency and possible environmental pollution. Therefore, this paper introduces a simple, economical and stable technology, namely mechanical atomization and evaporation technology.

In this paper, the influence factors of evaporation process are determined by analyzing the natural evaporation process of water body, so as to explore the advantages of mechanized evaporation technology and put forward a further improvement scheme.

1. Research status of natural evaporation technology at home and abroad

1. 1 Foreign research status

As the evaporation of fresh water is affected by temperature, humidity, natural rainfall, solar radiation, wind speed, atmospheric pressure and other factors, natural evaporation is a complicated process, which needs a lot of experimental data support.

In 1802, Dalton proposed a formula for calculating evaporation based on the principle of aerodynamics, and took into account the effects of wind, temperature and humidity on evaporation.

w is the evaporation rate of water surface; (Ew-e) is the saturation difference of air, ew is the saturated water vapor pressure at the water surface temperature, e is the actual water vapor pressure of air on the water surface. p is atmospheric pressure; C is the proportional coefficient related to wind speed. Thus, the rate of evaporation at the water surface is proportional to the difference between the saturated vapor pressure of the air above the water surface and the actual vapor pressure, inversely proportional to the air pressure at the water surface, and proportional to the wind speed at the water surface. This formula also provides basic ideas for subsequent formulas.

In 1926, Bowen proposed the Bowen ratio method based on the surface energy balance equation, which optimized the calculation process and reduced some errors. In practical application, its accuracy is also relatively high. In 1948, Penman established the joint evapotranspiration equation of energy balance and aerodynamics. Part of the parameters in the calculation method were established according to specific climatic conditions, which has good application effect in the UK, but poor universality [12]. In 1974, Ryan Harlenman proposed the R-H model after considering the flat plate heat transfer analogy and the evaporation formula of natural water temperature. In 1990, Adams improved the R-H model by using the method of adding natural and forced convection vectors. The accuracy of the improved model has been improved.

In 1985, Shuttle-worth and Wallance proposed a series of double-layer evapotranspiration models (i.e., dual-source model or S-W model), which have higher accuracy than previous single-layer evapotranspiration models.

Because evaporation process is affected by climate, region and other conditions, and is limited by measurement methods and instruments, no general calculation model of surface evaporation has been formed, so each region needs to choose a suitable model according to its climate conditions.

1. 2 Domestic research status

There are also a lot of researches on the calculation of surface evaporation in China, most of which consist of empirical formulas or semi-empirical formulas by analyzing the observation data of local observation stations. Therefore, most of the formulas have strong local characteristics and are difficult to be popularized. In these published studies, three formulas for calculating surface evaporation have been widely used.

1.2. 1 Li Wanyi formula

E = [0.1 + 0.24 (1-2) 0.5] (e0 - e150) v [v0.8 + 52 v

Where: E is evaporation on water surface, mm; φ is the relative humidity, in small number; e0 is water vapor pressure, hPa; e150 is the water vapor pressure in the air at 1.5 m above the water surface, hPa; v is the wind speed at 1.5 m above the water surface, m/s.

This formula was proposed by Li Wanyi from Bayangaole Evaporation Experimental Station of Yellow River Water Resources Commission of the Ministry of Water Resources [16]. He analyzed the factors affecting surface evaporation and made some assumptions about the physical process of surface evaporation. Compared with the general model, the model has some improvement, but the model structure has some contradictions in the relationship between the evaporation mechanism of water surface and the mathematical principle, and the model parameters are determined only by using the data of a single station.

1.2. 2 General Formula A

E = [+ 01.0 + 0.027 + 0.015 6 v2 02 alpha 054 | Δ Δ tt2 |

Where, v is the wind speed at 1.5 m above the water surface, m/s; Δt is the temperature difference of water vapor, and α04=0 when Δt≥0, α04=0 when Δt < 0; es is water vapor pressure, hPa; ea is the water vapor pressure in the air 1.5m above the water surface, hPa.

This formula was proposed by Pu Peimin et al. [17-18] from Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. The formula integrated the influence factors of water and steam temperature, relative, humidity, air pressure and wind speed, and determined the correlation coefficient by combining the test data of water surface evaporation in different regions. However, in the actual application process, it is difficult to obtain some data, so it is not conducive to popularize and use.

1.2. 3 General Formula B

E = 6 v2 [0.027 + 0.015 + 0.025 | t | Δ] 1 2 x (es - ea).

 Based on the experimental data of surface evaporation obtained from a low-speed backflow wind tunnel system with controllable environmental parameters, a calculation model of surface evaporation coefficient including two factors of wind speed and water vapor temperature difference was established. The coefficient of this model is obtained from the analysis of laboratory data and has been tested by many evaporation laboratories at home and abroad. The formula includes the influence of wind and water vapor temperature difference on surface evaporation respectively, so it can show the combined action of free convection and forced convection in the process of surface evaporation [15]. However, its disadvantages are also obvious. Because the coefficient is determined according to laboratory data, there will be A large error in the actual application of climate change, and its applicability is slightly worse than that of formula A.

In addition to the above formulas, Shi Chengxi and Hong Jialian also revised foreign formulas according to the actual situation to adapt to specific environmental requirements [21]. However, there are too many factors to be considered in the natural evaporation process, so a unified formula has not been formed.

1.3 Calculation of evaporation capacity of natural evaporation

The relative humidity of a place is 0.5, the water vapor pressure difference (e0-e150) is 12 hPa, and the variable is the wind speed at 1.5 m above the water surface. E0.5 = 3.283 3 mm, E1. 0= 3.694 1 mm, E1. 5= 4.281 5 mm were calculated by substituting wind speed 0. 5, 1. 0, 1. 5 m/s into Li Wanyi formula respectively. A surname

The result shows that the natural evaporation capacity can hardly reach the treatment rate required by industry. The natural evaporation technology is sensitive to climate, and the high humidity of air and the low wind speed will affect the processing efficiency. At the same time, the evaporation pond area required by natural evaporation is also large: all these factors restrict the application of natural evaporation technology.

Therefore, it is necessary to introduce mechanical forced evaporation method, which makes the air contact with the wastewater to be treated more fully by adding mechanical atomizing equipment. At the same time, the equipment also increases the air velocity and speeds up the evaporation process [22]. The introduction of this method can greatly reduce the construction area of evaporation pond, improve evaporation efficiency and save construction cost [23].

2. Mechanical atomization and evaporation theory

There are many factors affecting the mechanical atomization evaporation process, so it is necessary to establish a reasonable model system to analyze it, the first is to establish the droplet evaporation process model study.

2. 1 Droplet evaporation theory

Taking pure water as the research object, the droplet will be affected by the air flow around it during evaporation. After the formula of droplet evaporation rate is established, the formula is quantitatively analyzed and the influencing factors are qualitatively tested. Then the evaporation rate can be controlled within a certain range. At the same time, it can be applied to the evaporation and concentration process of wastewater by strengthening some factors which have great influence, so as to achieve the purpose of reducing the concentration of wastewater.

By integrating the droplet evaporation rate expression formula dw/dt =

KX A(xw-x), the expression of mass transfer of spherical droplets in still air Sh = KX D/Dv, and the expression of the total transfer coefficient of spherical droplets Sh = 2 + k1 (Re)x (Sc)y (usually k1 is 0.60, x is 0.50, When y is 0. 33), the evaporation rate formula dw/dt is evaporation rate. Dv is the diffusion coefficient of solution; A is mass transfer area; D is the droplet diameter; uR is the relative velocity of the droplet and the air medium; ρa, μa is the density and viscosity of air; x is the moisture content of air on the droplet surface; xw is the saturation moisture content of air; xw-x is the driving force of mass transfer expressed by the difference of moisture content. Re is Reynolds number; Sc is Schmidt number.

By analyzing the data required by the formula, the specific influencing factors can be deduced.

2.2 Influencing factors of droplet evaporation

Assume that the droplet shape is ideal spherical and its volume is fixed

V, the relation between particle size and evaporation surface area S can be expressed as the following equation

As 6V / π is a constant value, it can be inferred that the size of evaporation surface area S is inversely proportional to droplet particle size d, and the object with larger evaporation surface area has higher evaporation rate. Therefore, when the droplet particle size is as small as possible within a certain range, the evaporation rate will also increase.

In the evaporation rate formula, x-x can be expressed by the calculation related to atmospheric pressure, partial pressure of water vapor and air temperature. Therefore, temperature and atmospheric pressure have certain influence on evaporation. Wet air density ρa is related to the relative humidity, so the relative humidity is also one of the influencing factors; The diffusion coefficient Dv increases with the increase of temperature, so the effect of temperature on evaporation is more obvious.

2.3 Other Factors

In terms of environmental factors, Dalton's evaporation law was extended and the droplet was considered as an evaporating whole after enlargement. When the saturated water vapor pressure, actual water vapor pressure and air pressure were constant, the wind speed also had an important influence on the droplet evaporation. By analogy, the liquid in the actual evaporation process is also affected by solar radiation. The calculation formula of radiation and evaporation on water surface is E = CE (w* Rs), where: Rs is the solar radiation amount calculated by equivalent evaporation, mm; CE is the correction coefficient dependent on average humidity and average wind speed; w* is the weight coefficient dependent on average humidity and air pressure [30].

From the above formula, it can be analyzed that radiation is close to the proportional relationship with evaporation, so radiation is also a factor affecting evaporation.

Based on the above analysis, it can be determined that the factors affecting the droplet evaporation process include temperature, wind speed, atmospheric pressure, relative humidity, droplet diameter and solar radiation. These influencing factors can also be applied to the subsequent study of forced evaporation.

3. Forced evaporation technology

Through the above analysis, some methods can be applied to the actual production and life, so as to strengthen the evaporation process. The evaporation method of wastewater treatment is generally fixed location, so the change of atmospheric pressure, relative humidity is small, can be temporarily not considered. (1) Influence of droplet diameter. The smaller the droplet diameter is, the larger the liquid evaporation surface area is, and the better the evaporation effect is. Therefore, the water in the evaporation pond can be atomized and ejected, and the purpose of enhancing evaporation can be achieved by increasing the evaporation area. The optimal atomized droplet diameter needs to be simulated in the actual test. The selection of atomizing device needs to consider the total volume of the pond water, the surface area of the pond, the flow of water to be evaporated and the physical and chemical characteristics of the water body, which directly affect the operating efficiency of the atomizing device and the final evaporation effect.

(2) The influence of temperature. The effect of temperature during forced evaporation is not only the effect of atmospheric temperature, but also the effect of additional heating. The excess steam generated during the operation of the factory can be used to indirectly heat the evaporation pond, raise the water temperature and increase the evaporation [31]. The electric energy and heat generated by installing solar panels or solar collectors on the top of the evaporation pond can also continue to be used in the forced evaporation process.

(3) The influence of wind speed. The increase of wind speed within a certain range can strengthen the evaporation process, so the increase of wind speed helps to strengthen the evaporation process [32]. But the excessive wind speed