Abstract: In most coal-fired power plants in China, the saturated wet flue gas from the wet flue gas ...
Abstract: In most coal-fired power plants in China, the saturated wet flue gas from the wet flue gas desulfurization process is directly discharged, resulting in visible wet plumes. Currently, many environmental protection and energy-saving technologies have achieved significant effects in treating wet plumes, but the technical indicators have not been formulated in combination with the treatment of wet plumes. Wet plume treatment technologies can be classified into three major categories: heating type, condensation type, and condensation and reheating type. The characteristics of each type of technology and their adaptability in wet plume treatment have been studied, and the applicable ranges of these three types of technologies under different environmental temperatures and humidity conditions have been explored based on the formation and dissipation mechanism of wet plumes.
0 Introduction
At present, over 90% of the coal-fired power plants in China adopt the limestone-gypsum wet flue gas desulfurization process for their desulfurization facilities. With the expansion and acceleration of the national ultra-low emission of coal-fired flue gas, in order to meet the requirements of ultra-low emission limits, most of the desulfurization devices do not have a GGH (Gas-Phase Heat Exchanger). The saturated wet flue gas is discharged from the chimney and mixes with the cooler ambient air to cool down. Water vapor becomes oversaturated and condenses, causing refraction and scattering of light, resulting in the appearance of white or gray "wet plumes" (commonly known as "big white smoke"). The phenomenon of wet plumes weakens the "sense of gain" of the public regarding environmental protection efforts. Some local residents near coal-fired power plants have raised demands for the control of wet plumes, and certain local government departments have also put forward requirements for the control of wet plumes from coal-fired power plants.
1. Mechanism of wet plume formation
At present, in the vast majority of coal-fired power plants in China, the flue gas is treated with wet desulfurization before being discharged. The temperature is reduced to 45°C to 55°C. At this point, the flue gas is usually saturated wet flue gas, containing a large amount of water vapor. If the flue gas is directly discharged through the chimney and enters the cooler ambient air, due to the lower saturation specific humidity of the ambient air, during the process of the flue gas temperature decreasing, the water vapor in the flue gas will condense to form wet plumes. The formation mechanism of wet plumes is shown in Figure 1. The curve in Figure 1 represents the saturation curve of wet air. Suppose the state of the wet flue gas at the chimney outlet is at point A, while the state of the ambient air is at point F. When the flue gas leaves the chimney, it is in an unsaturated state. The process of the wet flue gas and the ambient air mixing starts along the line AB, and reaches point B after which the flue gas becomes saturated wet flue gas. After that, the mixture of wet air and ambient air changes along the curve BDE, and the excess water vapor will condense into liquid droplets, forming wet plumes.
2. Main treatment technologies for wet smoke plumes
Based on the mechanism of the formation and dissipation of wet plumes, the existing technologies with effective treatment effects for wet plumes can be classified as gas heating technology, gas condensation technology, and gas condensation and reheating technology. Currently, there are already operational gas condensation and gas condensation reheating technologies in the power industry. Most of them are not specifically designed for the treatment of wet plumes; their main purposes are emission reduction, water collection, and water conservation. The technical indicators have not been formulated in combination with the elimination of wet plumes, but objectively, they still have achieved the effect of treating wet plumes.
Some coal-fired power plants employ technologies such as wet electrostatic precipitators, flue gas dehumidifiers, sonic dehumidification, chimney water collection rings and dehumidifiers. Although these technologies can remove the condensate water from the flue gas, the proportion of water vapor in the flue gas condensate water is extremely limited (less than 1‰). Therefore, removing the condensate water from the flue gas can only alleviate the "wet plume" phenomenon, but cannot effectively eliminate the wet plume. This aspect will not be further studied in this article. In addition, there is a method of using cooling towers for emission to achieve wet plume control, but this is more suitable for new power plants and is not suitable for the retrofit of existing plants. This aspect will not be deeply explored in this article either.
The following elaborates on the technologies of flue gas heating, flue gas condensation, and flue gas condensation and reheating in accordance with their technical principles.
3. Flue Gas Heating Technology
The flue gas heating technology involves heating the wet saturated flue gas at the desulfurization outlet, so that the relative humidity of the flue gas deviates from the saturation humidity curve. The mechanism of eliminating wet plumes is shown in Figure 2. The initial state of the wet flue gas is at point A. After heating, it rises along line AB, then mixes along line BC, and cools down to the environmental state point C. The entire ABC change process does not intersect with the saturation humidity curve, thus no wet plumes are generated.
The currently in-service heating technologies are classified into two major categories based on the heat exchange method: indirect heat exchange and direct heat exchange. The main representative technologies of indirect heat exchange include: rotary GGH, tubular GGH, heat pipe GGH, MGGH, steam heater, etc. The main representative technologies of direct heat exchange include: secondary air heating with heat, direct gas heating, heat air mixture heating, etc. If each heating technology is compared based on the same technical indicators for treating wet plumes, the main technical and economic comparison results are shown in Table 1.
The direct heating technology, although having a lower initial investment, has excessively high operating costs because its heat source does not utilize the residual heat from flue gas. As a means of treating wet smoke plumes, it is too costly and rarely used in practical applications. In the indirect heating technology, both the rotary GGH and the tubular GGH have varying degrees of air leakage. Under the current environment of ultra-low emission in China's coal-fired power plants, its application as a means of treating wet smoke plumes is also restricted. After the large-scale development of the heat pipe GGH, the layout of the soot blowers will become more difficult, and the occupied area will increase. At present, it has not been applied in large units. The steam heating method also has excessively high energy consumption due to the heat source issue. Therefore, in light of the current requirements for ultra-low emission of flue gas and energy conservation, if MGGH is used as one of the means for treating wet smoke plumes, it has the broadest application prospects.
4. Flue Gas Condensation Technology
The flue gas condensation technology involves cooling the wet saturated flue gas at the sulfur dioxide removal outlet, causing the flue gas to cool down along the saturation humidity curve. During this cooling process, the moisture content significantly decreases. The mechanism of eliminating the wet plume is shown in Figure 3. The initial state of the wet flue gas is at point A. After cooling, it undergoes AF condensation, then mixes with FC and is cooled to the environmental state point C. The FC change process does not intersect the saturation humidity curve, thus no wet plume is generated.
The main representative technologies for flue gas condensation in coal-fired power plants currently include: phase change condensers, condensate water separators, zero water replenishment systems for desulfurization, integrated systems for waste heat recovery and emission reduction of flue gas, etc. Their characteristics are shown in Table 2. From the names of these technologies, it can be seen that their main functions are mainly focused on emission reduction, water collection, and energy conservation. In principle, all these technologies are designed to cool the clean flue gas after desulfurization, which conforms to the mechanism shown in Figure 3. These technologies have achieved the effect of controlling wet smoke plumes in practical applications.
Condensation technology is mainly divided into two categories based on the heat exchange method: indirect heat exchange and direct heat exchange. Direct heat exchange mainly uses newly built spray towers as the heat exchange equipment, which requires certain land area. The refrigerant and clean flue gas directly come into contact, resulting in high heat exchange efficiency. However, it is necessary to supplement the refrigerant water system and control the pH value, making the system more complex. Indirect heat exchange mostly uses tubular heat exchangers as the heat exchange equipment, where the refrigerant and clean flue gas do not directly come into contact. The system is relatively simple.
According to the different cold sources, the condensation technologies can be classified as: water cooling source, air cooling source and other artificial cooling sources. Among them, the circulating water system formed by the water cooling source is the simplest, only equipped with pumps and circulation pipelines, and it is usually an open circulation. The operating cost is the lowest and the land occupation is small. The system using air cooling source usually needs to configure a cooling tower in the circulating water system, and the system is more complex and occupies more land than the water cooling system. The newly added cooling tower will become a new source of white smoke near the ground. Other artificial cooling sources, such as heat pumps, have a large land occupation and high energy consumption (for example, in the case of steam lithium bromide heat pump, 0.7 MJ of steam is consumed for every 1 MJ of heat exchange). For systems using ambient air or river seawater as the cold source, the quality of the cold source is more significantly affected by the season. Taking the eastern region as an example, the temperature difference between winter and summer is 20-30 degrees, so the condensation effect of the same system will vary greatly in different seasons.
The flue gas condensation technology cools the wet flue gas after desulfurization, causing a large amount of gaseous water in the flue gas to condense into droplets. During this process, various pollutants such as fine particles, SO2, etc. can be captured. Therefore, as a means of treating wet flue gas, the flue gas condensation technology not only has a good effect on eliminating white smoke, but also can achieve combined removal of multiple pollutants in the flue gas. The condensed water can be used as makeup water for desulfurization.
5. Flue Gas Condensation and Reheating Technology
The flue gas condensation and reheating technology combines the above two methods. The mechanism for eliminating the wet plume is shown in Figure 4. The initial state of the wet gas is at point A. After cooling, it condenses along the line AD, then heats along the line DE, and finally mixes along the line EC and cools down to the environmental state point C. The change process of EC does not intersect the saturation humidity curve, so no wet plume is generated. The mechanism for the dissipation of the wet plume indicates that the environmental humidity and temperature have a significant impact on the formation and size of the wet plume. Theoretically, under given environmental temperature and humidity conditions, if the cost is not considered, both the heating technology and the condensation technology can achieve the elimination of the wet plume (if the heating temperature is high enough and the condensation temperature is low enough). However, based on the actual situation of coal-fired power plants, from an economic perspective, the simple heating and condensation methods have their own limitations. Heating is limited by the original flue gas temperature conditions, and condensation is limited by the temperature of the ambient air and water. Under these conditions, if the condensation and reheating technology is adopted, combining heating and condensation can be used, thereby expanding the adaptability range of the system for the environmental temperature and humidity.
For example, the saturated wet flue gas temperature after wet desulfurization is 50%. Considering factors such as the selection conditions of cold and hot sources and the heat exchange temperature difference, the heating method limits the temperature increase of the flue gas to no more than 30%, while the condensation method limits the temperature decrease of the flue gas to no more than 25%. Under these conditions, the adaptability of the wet flue gas treatment effects of these three technologies to environmental conditions is shown in Figure 5.
The upper part of the applicable range boundary is the area for eliminating wet smoke plumes of various technologies. Clearly, the cooling and reheating technology has a much wider applicable range for environmental conditions than the simple heating technology and condensation technology. When the relative humidity of the environment is 80%, the heating technology can eliminate wet smoke plumes when the environmental temperature is greater than 15℃; the cooling technology can eliminate wet smoke plumes when the environmental temperature is greater than 9°C; and the condensation and reheating technology can eliminate wet smoke plumes when the environmental temperature is greater than -6.5°C.
6 Conclusion
(1) The existing technologies that can effectively treat wet flue gas in coal-fired power plants were elaborated, summarized, and initially compared. The applicability of various technologies in treating wet flue gas in coal-fired power plants was also studied.
(2) The flue gas heating technology is the most widely used technology at present. In light of the current requirements for ultra-low emission and energy conservation of flue gas, the MGGH technology has the broadest application prospects.
(3) The flue gas condensation technology also has a significant effect on the treatment of wet smoke plumes, and it can achieve combined removal of multiple pollutants. Currently, most of the applications of this technology in the industry are not specifically aimed at the treatment of wet smoke plumes. The main purposes are emission reduction, water collection, and water conservation. The technical indicators have not been formulated based on the elimination of wet smoke plumes, but objectively, they have played a role in the treatment of wet smoke plumes.
(4) Condensation re-heating technology is a combination of flue gas heating and flue gas condensation technologies. It integrates the characteristics of heating and condensation technologies, and has a wider application scope for the treatment of wet smoke plumes.
(5) Technologies such as wet electrostatic precipitators, demisters, acoustic demisters, and chimney water collection rings can effectively remove the condensate water from the flue gas. However, since the proportion of water vapor in the flue gas's water content is very limited, these technologies are difficult to be regarded as the mainstream methods for controlling wet plumes and cannot effectively eliminate the wet plumes.