The Combination of Simulation and Heat Pump Technology for the Treatment and Reconstruction of “Wet Plume” in fossil-fuel power station

The saturated wet flue gas discharged from the power plant chimney after wet desulfurization often forms a thick “white plume” at the chimney’s mouth. This “wet plume” phenomenon has a very bad impact on the lives of local people, and it will also cause corrosion damage to surrounding equipment while polluting the environment. This paper simulates the “wet plume” phenomenon caused by flue gas emissions under different flue gas parameters and environmental parameters by using the CFD simulation method and the new technology of “smoke tower integration” of power plant emissions. Through the simulation data, the “wet plume” pollution change is studied, which provides a reliable theoretical basis for the design of new power plants and the reconstruction of old power plants. At the same time, through the simulation data, the combination of flue gas indirect condensation technology and heat pump technology is selected to realize the recovery and reuse of flue gas waste heat and realize the dual embodiment of social value and practical value.


Introduction
In recent years, with the increasing improvement of people's living standards, their yearning for a better life, and the increasing emphasis on environmental protection and atmospheric protection by the country, many regions have also introduced corresponding implementation measures for smoke emissions.
In the "Technical Requirements for Testing Gypsum Rain and Colored Plumes of Coal-fired Power Plants in Shanghai (Trial)" issued by the Shanghai Environmental Protection Bureau in 2017, there is a definition of colored plumes [1] .During the process of discharging flue gas from coal-fired power plants into the atmosphere at the chimney mouth, due to temperature reduction, some vapor water and pollutants in the flue gas will condense, forming a mist-like water vapor at the chimney mouth.The mist-like water vapor will be formed due to the background color of the sky and the illumination of the sky.The subtle changes in color, due to observation angles and other reasons [2] , result in the formation of "colored smoke plumes", usually in colors such as white, gray white, or blue, which is the origin of "wet smoke plumes".According to the requirements of the "Air Pollutant Emission Standard DB12/810-2018 for Thermal Power Plants" released by Tianjin, coal-fired boilers should adopt corresponding technologies to reduce the temperature of flue gas emissions and reduce the emissions of soluble salts and condensable particles by collecting water droplets in the flue gas and saturated water vapor [3] .

The impact of wet plumes
The saturated wet flue gas emitted after wet desulfurization forms a thick "white plume" at the chimney mouth, which exists in the sky for a long time and causes serious visual pollution.In cold weather, it also forms a serious "chimney rain" phenomenon, causing droplets to fall near the chimney.Due to the pollutants and a large amount of salt inside, it can cause pollution and damage to the surrounding environment, At the same time, there is a huge safety hazard of corrupting surrounding equipment.

Main "wet smoke plume" treatment technologies
The treatment and elimination process of wet smoke plumes is a comprehensive treatment process, and when considering the design of treatment plans, the following aspects should be considered: (1)  How to reduce the absolute and relative humidity content of smoke should be considered; (2) The content of sulfur dioxide in flue gas should be effectively controlled, the generation and emission of nitrogen oxides should be controlled, and ammonia should be sprayed reasonably; (3) We remove small particles such as dust and acid mist inside, and try to avoid or reduce the generation of acidic aerosols [4] .Based on the generation principle of the "wet smoke plume phenomenon" and combined with production practice, the treatment methods can be roughly divided into the following three categories: flue gas heating technology, flue gas condensation technology, and flue gas condensation reheating technology [5] .

The "smoke tower integration" technology
The combination of flue gas and tower is a new type of flue gas emission in the coal-fired fossil-fuel power station, which eliminates the conventional chimney.After dust removal and desulfurization, the boiler flue gas is sent to the upper part of the distribution layer in the circulating water cooling tower through a flue and then is lifted by the thermal power of a large amount of hot and humid air in the cooling tower.Under the effect of hot air lifting, the emission height of flue gas is higher than that through the chimney in most cases.After adopting this technology, not only can the chimney be canceled, but also the flue gas heat exchanger GGH can be canceled, greatly simplifying the flue gas system of thermal power plants and reducing the operation and maintenance costs of the desulfurization system [6] .

Simulation method
It is assumed that the saturated wet flue gas after desulfurization and denitrification will be discharged through the 130 m high hyperbola counter flow natural draft smoke exhaust cooling tower.The saturated wet flue gas will enter the cooling tower from the flue to mix with hot air, and then rise to the cooling tower outlet after mixing, and then mix with the ambient air to produce a "wet plume" phenomenon.Using the CFD method, we simulate the formation and dissipation of wet smoke plumes during the mixing process of flue gas emissions with ambient air.Modeling steps: (1) We establish 3D geometric modeling; (2) We divide the grid; (3) We set up basic models, energy equations, and turbulence models; (4) We set boundary conditions, algorithms, and we perform solution calculations until convergence; (5) Result processing is to output calculation results [7] .

Geometric modeling
SCDM modeling software is used to establish 3D geometric modeling.The data used is a 5000 m 2 hyperbola counter flow natural draft smoke exhaust cooling tower, with a tower height of 130 m, a cooling tower outlet diameter of 52.62 m, an air inlet height of 7.6 m, a cooling bottom diameter of 87.0 m, and a flue diameter of 5.40 m.A simulation space with a mixed flow field is of 300 * 100 *

Mesh
The 3D geometric modeling is built using fluent masking.The minimum size of the face mesh is 0.3 m, the maximum size is 7.8 m, and the growth rate is 1.2.We add 3 layers of the boundary layer.The type of mesh division for the volume grid adopts "polyhedra", with a maximum mesh size of 10.8 m.

Boundary conditions and solution results
The environmental wind speed and smoke emission speed are both boundary conditions based on the velocity inlet; The outlet is bounded by a pressure outlet and set to ambient atmospheric pressure.
We simulate under different working conditions.Firstly, when the flue gas emission temperature is 51℃, the relative humidity of saturated wet flue gas is 100%, the ambient temperature is 0℃, and the relative humidity is 70%.We set the ambient wind speed to 5 m/s and the flue gas emission speed to 20 m/s.The simulated images are shown in Figure 2.

Simulation conclusion
From the trace plot and relative humidity XY plot, it can be seen that the wet flue gas exhausts from the tower at high speed, causing the surrounding air to curl up and form vortices.The flue gas velocity decreases and migrates back with the wind speed.When the flue gas temperature is 51℃ and the ambient temperature is 0℃, the length of the wet smoke plume with a relative humidity above 100% is about 50-80 m; When the flue gas temperature is 51℃ and the ambient temperature is -10℃, the length of the wet smoke plume with a relative humidity of over 100% has exceeded 300 m; When the ambient temperature decreases, the diffusion length of wet smoke plumes is relatively prolonged.If the temperature of the flue gas decreases, it is also saturated wet flue gas [8] .If the ambient temperature remains unchanged, the moisture content of the flue gas decreases, and the length of the wet flue gas plume decreases.
In winter environments, the length of "wet plumes" in traditional chimneys can even reach 2000 m.Compared with traditional chimneys, the integration technology of smoke towers can not only save GGH but also reduce chimney emissions, resulting in a significant reduction in the phenomenon of "wet plumes".From the simulation results, it can be seen that using direct and indirect condensation to reduce the flue gas temperature from 51℃ to 45℃ can greatly reduce the impact of "wet smoke plumes" under the average temperature of -10℃ in northern winter.At the same time, most of the wet flue gas can be condensed in the cooling tower, and the toxic and particulate matter carried can also be left in the cooling tower with the condensed water, reducing the diffusion of particles and toxic substances.
In the research process of "wet smoke plume", the traditional method of determining the formation conditions of a wet smoke plume based on the tangent between the mixing curve and the saturation curve is not fully sufficient, and the results are not intuitive.It is necessary to further consider the influence of diffusion factors in conjunction with CFD simulation.
During the simulation process, only turbulence models, energy equations, and component transport models were used to simulate the condensation and convective heat transfer processes of mixed gases.The numerical simulation results were consistent with the relevant experimental results, and the error was within a controllable range, which basically verified the correctness and reliability of the numerical model.
At the same time, the relative humidity of the simulated environment is set to a constant value, which has certain limitations.In subsequent research, we can continue to increase the changes in the simulated values to obtain more detailed and accurate results.

Research on the Transformation of a "Wet Smoke Plume" and the Design of Waste Heat Recovery and Utilization
Based on the above simulation data, the temperature of the wet flue gas is reduced from 51℃ to 45℃.Even under low winter temperatures in the north, the phenomenon of "wet smoke plumes" has been greatly reduced.Therefore, most northern power plants can choose treatment plans for renovation based on this data.Among the three major types of governance measures, due to the need for flue gas cooling, flue gas reheating methods can be directly abandoned.The economic feasibility of reheating after flue gas condensation is not strong, and it can also be excluded from the treatment plan.Only flue gas condensation technology can reduce flue gas temperature and combine it with heat pump technology for waste heat recovery and utilization, which can be said to kill two birds with one stone.Below is a research description of specific technologies and applications.
In flue gas condensation technology, indirect condensation heat exchange technology is chosen to prevent direct contact between the flue gas and cooling water.By installing a flue gas condensation heat exchanger in the flue gas after desulfurization (or in the wet removal device), the flue gas temperature is reduced through flue gas heat exchange, achieving a reduction in the moisture content of the flue gas after desulfurization.We simultaneously use closed-cycle cooling water to exchange heat with a water source heat pump.After the cooling water passes through the flue gas condensation heat exchanger for heat exchange and is heated by the heat exchanger, it is cooled down through a water source heat pump cycle and then re-circulated into the flue gas condensation.Demineralized water is generally used for cooling water.This method is basically applicable to any power plant [9] .
Compared to a 300 MW power plant, after investigation, the volume flow of flue gas at the outlet of the desulfurization system can reach 1500000 m 3 /h.In theory, the sensible heat enthalpy of 51℃ flue gas is Q1=852.18KJ/m 3 , and the sensible heat enthalpy of 45℃ flue gas is Q2=757.06KJ/m 3 [10] .The theoretical total heat recovery is Q=(Q1-Q2) V=102180000 KJ/h.The winter heating load is calculated based on 80 W/m 2 , and selecting a suitable heat pump unit can provide approximately 354514 m 2 of building heating.

Conclusions
Through simulation data and cloud maps, it can be seen that reducing the temperature of the flue gas can effectively reduce the length of the "wet smoke plume" in the tower.At the same time, most of the wet flue gas can condense in the cooling tower, and the toxic and particulate matter carried can also remain in the cooling tower with the condensed water, reducing the diffusion of particles and toxic substances.
After the renovation of flue gas cooling, the use of heat pump technology to recover and utilize the waste heat of flue gas not only responds to the national policy of "energy conservation and emission reduction", but also improves the thermal efficiency of the entire plant, while increasing the economic benefits of the factory.It is a versatile renovation measure, suitable for the renovation of old power plants and the installation of new power plants, and has practical promotion value.

3. 4 .
Solve the model We simulate fluids using energy equations, turbulence models, and component transport.The turbulence model adopts the k-epsilon standard model and standard wall function.Component transport establishes wet air parameters for mixed materials.The solution model is shown in Figure 1.

Figure 1 .
Figure 1.Solve the model after grid division