I. Definition of Wet Smoke Pellets
Chimney exhausts saturated wet flue gas, upon contact with cooler ambient air, undergoes cooling. During this process, the water vapor within the flue gas becomes supersaturated and condenses, forming droplets that refract and scatter light, resulting in the smoke plume appearing white or gray, known as "wet smoke plume" (commonly referred to as "white smoke"). Important note: The "wet smoke plume" refers to the feathers of a bird, not rain. It is not plaster rain! It is not chimney rain!
II. Mechanism of Wet Smoke羽 Formation
As shown in the figure, this is the saturation curve of moist air. Assuming the state of the moist flue gas at the chimney outlet is located at point F, the flue gas is in an unsaturated state when leaving the chimney. The mixing process of moist flue gas with ambient air begins along line AB, and upon reaching point B, the flue gas becomes saturated moist flue gas. Subsequently, the mixing of moist air with ambient air follows the curve BDE, with the excess water vapor condensing into liquid droplets, forming a moist plume. Based on the mechanism of moist plume formation and dissipation, existing treatments effective against moist plumes can be identified.
Technological categorization:
1. Flue Gas Heating Technology, including typical GGH (Gas-to-Gas Heat Exchanger), Tubular GGH, MGGH (Multi-Gas Heat Exchanger), Steam Heaters, etc.
2. Condensate烟气 technology, typical of new construction spray towers, water-cooled sources, air-cooled sources, and other artificial cooling sources.
3. Condensate Reheating Technology: By combining both techniques, considering economic factors, there are limitations to both direct heating and condensation methods. Heating is restricted by the original flue gas temperature, while condensation is limited by ambient air and water temperatures. Under these conditions, if the condensate reheating technology is adopted, combining heating and condensation, it can expand the system's range of adaptation to environmental temperature and humidity for wet smoke plume elimination.
Section 3: Main Technologies for Wet Cigarette Smoke Treatment
Most domestic boilers and kilns undergo wet flue gas desulfurization before emission, reducing the temperature to 45-55°C, while coking furnace desulfurization flue gas reaches 65°C. At this point, the flue gas is typically saturated with moisture. The flue gas is directly discharged through the chimney into a cooler environment, where the water vapor in the flue gas condenses to form wet flue gas plumes during the temperature drop.
Based on the mechanisms of formation and dissipation of wet smoke plumes, existing technologies with治理 effects on wet smoke plumes can be categorized into flue gas heating technology, flue gas condensation technology, and flue gas condensation reheating technology. Currently, flue gas condensation and flue gas condensation reheating technologies are in operation within the power industry, most of which are not primarily aimed at treating wet smoke plumes. Their main objectives are to reduce emissions, collect water, and save water. The technical specifications have not yet been formulated in conjunction with the elimination of wet smoke plumes, but they have objectively contributed to the treatment of wet smoke plumes.
Wet electrostatic precipitators, flue gas mist eliminators, acoustic mist eliminators, chimney water rings, and dehumidifiers used in some coal-fired power plants can remove condensate from flue gas. However, since the condensate in flue gas constitutes a limited portion of the water vapor content (less than 1‰), removing the condensate can only mitigate "wet plume," not effectively eliminate it, and this paper does not delve into it further. Additionally, using cooling tower emissions for wet plume control is applicable to new units but not suitable for retrofitting existing units, and this paper does not explore it in depth either.
Section 4: Flue Gas Heating Technology
The flue gas heating technology heats the wet saturated flue gas at the desulfurization outlet, causing the relative humidity of the flue gas to deviate from the saturation humidity curve. The mechanism of wet plume elimination is illustrated in Figure 2. The initial state of the wet flue gas is located at point A, and after heating, it warms up along AB, then mixes and cools along BC to the ambient state point C. The entire ABC process does not intersect with the saturation humidity curve, thus no wet plume is produced.
Heating technologies in service are categorized into two main types based on heat exchange methods: indirect heat exchange and direct heat exchange. The main technologies representing indirect heat exchange include: rotating GGH, tubular GGH, heat pipe GGH, MGGH, steam heaters, etc. The main technologies representing direct heat exchange include: secondary air mixing for heating, direct gas heating, and mixed hot air heating, etc. The main technical and economic comparisons of various heating technologies, with the same technical indicators for achieving wet smoke plume control, are shown in Table 1.
Direct heating technology, although with a lower initial investment, is too costly as a method for treating wet smoke plumes due to high operating costs and the fact that it does not utilize waste heat from flue gas. In practical applications, there are few examples. In indirect heating technology, both rotary GGH and tubular GGH have varying degrees of air leakage. Under the large-scale ultra-low emission environment of coal-fired power plants in China, their application as a method for treating wet smoke plumes is also limited. Large-scale heat pipe GGH poses difficulties in布置 blowdown nozzles and increases the land area required, and there is no application on large units. The steam heating method also has high energy consumption due to the heat source issue. Therefore, considering the current ultra-low emission of flue gas and energy-saving requirements, MGGH has a promising application prospect as one of the methods for treating wet smoke plumes.
Section 5: Flue Gas Condensation Technology
The smoke condensation technology cools the wet saturated flue gas at the desulfurization outlet, causing the flue gas to cool along the saturation humidity curve, with a significant reduction in moisture content during the cooling process. The mechanism of wet plume elimination is shown in Figure 3. The initial state of the wet flue gas is at point A, which undergoes condensation along AF after cooling, and then mixed and cooled along Fc to the ambient state point C. The Fc change process does not intersect with the saturation humidity curve, thus no wet plume is produced.
The primary representative technologies for flue gas condensation in coal-fired power plants include phase change condensers, condensate separators, zero补水 desulfurization systems, and integrated flue gas waste heat recovery and emission reduction systems, as shown in Table 2. From the names of these technologies, it can be seen that their main functions are focused on emission reduction, water collection, and energy saving. In principle, all these technologies cool down the clean flue gas after desulfurization, in line with the mechanism shown in Figure 3. These technologies have already achieved the effect of wet plume control in practical applications.
Condensation technology is primarily divided into two main categories based on heat exchange methods: indirect heat exchange and direct heat exchange. Direct heat exchange mainly utilizes newly constructed spray towers as heat exchange equipment, requiring a certain amount of space. The refrigerant comes into direct contact with the clean flue gas, resulting in high heat exchange efficiency. However, it necessitates supplementary dosing control of the refrigerant water system's pH value, making the system more complex. Indirect heat exchange predominantly employs tubular heat exchangers as the heat exchange equipment, where the refrigerant does not come into direct contact with the clean flue gas, resulting in a simpler system.
According to different cold sources, condensation technologies are further categorized into: water-cooled, air-cooled, and other artificial cold sources. The water-cooled system is simple, consisting only of a pump and circulating piping, typically an open-loop system. It has low operating costs and requires minimal space. Systems using air-cooled sources usually require a cooling tower in the circulating water system, making them more complex and space-consuming than water-cooled systems. The newly added cooling tower can become a new source of white smoke near the ground. Other artificial cold sources, such as heat pumps, occupy more space and have higher energy consumption (for example, a steam lithium bromide heat pump requires 0.7MJ of steam to exchange 1MJ of heat). Systems using ambient air, rivers, or seawater as cold sources are significantly affected by seasonal changes in the quality of the cold source. For instance, in East China, the temperature difference between winter and summer can be 20-30 degrees Celsius, resulting in considerable differences in condensation efficiency between seasons for the same system.
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, capturing fine particles and various pollutants such as SO during this process. Therefore, as a method for wet flue gas treatment, flue gas condensation technology not only effectively eliminates white smoke but also achieves the joint removal of multiple pollutants from the flue gas, with the condensed water being suitable for desulfurization补水.
Section 6: Flue Gas Condensation Reheating Technology
The烟气condensation and reheating technology combines the two aforementioned methods. Its mechanism for eliminating wet smoke plumes is shown in Figure 4. The initial state of the wet flue gas is at point A, which undergoes cooling to condense along AD, then reheats along DE, and finally mixes and cools along EC to reach the ambient state point C. The EC change process does not intersect with the saturation humidity curve, thus no wet smoke plume is produced. The dispersion mechanism of wet smoke plumes indicates that ambient humidity and temperature have a significant impact on their formation and scale. Theoretically, under given ambient temperature and humidity conditions, if cost is not considered, both heating and condensation technologies can achieve the elimination of wet smoke plumes (with sufficient heating temperature and low enough condensation temperature). However, based on the actual conditions of coal-fired power plants, from an economic standpoint, both pure heating and condensation methods have their respective limitations. Heating is restricted by the original flue gas temperature conditions, while condensation is restricted by ambient air and water temperatures. Under these conditions, if the condensation and reheating technology is adopted, combining heating and condensation, it can expand the system's adaptability range for ambient temperature and humidity in eliminating wet smoke plumes.
For example, the saturated wet flue gas temperature after wet desulfurization is 50%. Considering factors such as the selection of cold and hot sources, heat exchange temperature difference, etc., the heating method should not exceed a temperature increase of 30°C, and the condensation method should not exceed a temperature decrease of 25°C. Under these conditions, the adaptability of the three types of technologies in treating wet flue gas plumes to environmental conditions is shown in Figure 5.
The upper boundary of the applicable range is the wet smoke plume elimination zone for various technologies. Clearly, the applicable range of cooling and reheating technology is much greater than that of simple heating and condensation technologies. When the relative humidity is 80%, heating technology can eliminate wet smoke plumes when the ambient temperature is greater than 15°C; cooling technology can eliminate wet smoke plumes when the ambient temperature is greater than 9°C; and condensation reheating technology can eliminate wet smoke plumes when the ambient temperature is greater than -6.5°C.
Several typical applications
Boilers and industrial kilns
The desulfurization flue gas outlet temperature ranges from 45-55°C, with low flue gas temperature and humidity. Direct heating or direct condensation technology can be used, as well as circulating water cooling or air cooling. Then, utilize the flue gas waste heat through GGH or MGGH heat exchange to achieve flue gas de-bleaching. The condensate water is used as a supplement for the desulfurization process water, without disrupting the desulfurization balance.
2. Sintering Machines and Vertical Kilns for Steel Mills
The desulfurization flue gas outlet temperature is between 55-60°C, with relatively high humidity in the flue gas, and a large volume of flue gas. A circulating water cooling system is used to reduce the temperature and remove condensation, lowering the flue gas temperature to 40-45°C. Then, MGGH is employed to utilize the waste heat of the flue gas for desulfurization. The condensed water is used as a supplement for the desulfurization process water, ensuring the desulfurization balance is not disrupted.
3. Coke Oven
The desulfurization flue gas outlet temperature reaches up to 65°C, with a high moisture content in the flue gas. A circulating water cooling system is used to reduce the temperature to around 45°C, followed by utilizing GGH or MGGH to heat the wet flue gas with excess heat, with steam as a supplement. The flue gas can be directly discharged or returned to the original chimney for heat reserve; a large amount of condensate water is produced, which is simply treated and recycled.
Typical Process Route:
Usage:
Shougang Group's X Coke Chemical Company, in conjunction with low-temperature denitrification and ammonia-based desulfurization, uses GGH technology to utilize waste heat from flue gas for desulfurization and achieve white smoke removal. It was put into operation in April 2017 and has yielded good results.
