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With the rapid development of the power industry, and particularly the increasing demands for environmental protection, the construction of flue gas desulfurization facilities in thermal power plants has accelerated. By the end of 2004, flue gas desulfurization facilities with a capacity of around 20 million kilowatts were in operation or under construction across the country, while facilities with a capacity of about 30 million kilowatts were still under construction. To date, hundreds of flue gas desulfurization (FGD) technologies have been developed both domestically and internationally. For the desulfurization of flue gas from large thermal power plants, the wet limestone-gypsum FGD process is the predominant method, and it has become the preferred technique for flue gas desulfurization in coal-fired power plants in China. The question of whether it is necessary to heat the flue gas after wet flue gas desulfurization – that is, whether a flue gas heat exchanger needs to be installed for flue gas desulfurization – has long puzzled the desulfurization industry. This article discusses issues related to the installation of flue gas heat exchangers (GGHs) for reference by industry professionals as well as relevant decision-making bodies. I. Overview at Home and Abroad (1) Domestic Situation: The FGD units in power plants that were constructed in the early stages in China are currently in operation, and flue gas heat exchangers have been installed in all of these flue gas systems. Projects Phase 1 and Phase 2 of Luoshuang, which utilize Japanese Mitsubishi technology, adopted water-cooled tubular flue gas reheaters ； The FGD systems at Chongqing Power Plant, Zhejiang Banshan Power Plant, and Beijing No.1 Thermal Power Plant adopt technology from the German company Steinmuller, and are all equipped with gas-to-gas rotary flue gas heat exchangers. Subsequently, GGHs were also installed in the Shijingshan Thermal Power Plant, Phase II of Beijing No.1 Thermal Power Plant, Shandong Huangtai Power Plant, Jiangyin Xiagang Power Plant, and the FGD units of Zhejiang Qianqing Power Plant. In the desulfurization projects currently in use in China, it is common to install GGHs; however, there are also some projects that do not use such devices. In these cases, the flue gas that has been treated in the absorption tower is not heated before being released directly into the atmosphere through chimneys. Power plants such as the 600MW Changshu Power Plant, Ligang Power Plant, Huanghua Power Plant, Taishan Power Plant, Wangtan Power Plant, Tuoketuo Power Plant, Chaozhou Power Plant, Wushashan Power Plant, and Houshi Power Plant do not have flue gas temperature-raising heat exchangers installed after wet flue gas desulfurization. (II) Abroad 1. Germany. Germany carried out large-scale construction of FGD systems in the 1980s and 1990s. Due to regulatory requirements at that time, which stipulated that the exhaust gas temperature must not be lower than 72°C, all FGD systems installed during that period were equipped with GGHs, and these were mainly rotary GGHs. After years of operation, it was found that the GGH is the failure point of the entire FGD system, **affecting the system’s availability. According to the German company, almost 100% of GGH units experienced failures during operation. After Germany joined the EU, most EU member states did not have any regulatory requirements regarding the temperature of flue gas emissions. Therefore, starting in 2002, Germany adopted EU standards and removed the restrictions on flue gas emission temperatures. Therefore, in the FGD units recently installed in the former East German region, some power plants no longer equip GGHs. German desulfurization companies believe that not installing GGH is the trend in the future development of FGD. An increasing number of power plants in Germany are now discharging desulfurized flue gas through cooling towers; this allows them to avoid installing GGH units, eliminates the need for investment in wet chimneys, and **improves the dispersion capacity of pollutants in the flue gas. 2. United States. In the United States, environmental regulations do not set requirements regarding the exhaust gas temperature at the chimney outlet; as a result, desulfurization systems installed in the U.S. since the mid-1980s generally do not include GGHs, with only about 25% of desulfurization power plants equipped with such GGHs. Some power plants in the United States, considering that the absence of GGHs in flue gas desulfurization systems might have adverse effects on the surrounding environment due to excessively low flue gas temperatures, have installed burners that burn clean fuels at the bottom of the chimneys to temporarily heat the desulfurized flue gas when meteorological conditions are unfavorable for dispersion. This method requires low investment and low operating costs; at the same time, it also helps protect the environment. It is a practical solution that is worth considering. 3. Japan. Since Japan is a small, elongated island country. To reduce pollution in mainland Japan, high flue gas temperatures have been used to enhance the dispersion of the exhaust gases. Therefore, GGHs are installed in almost all FGD units of Japanese power plants. II. Functions of GGH (1) Raising the flue gas temperature and elevation. In wet flue gas desulfurization systems, the temperature of the clean flue gas at the outlet of the absorption tower is generally 47–51°C. By installing a GGH, the purified, saturated wet flue gas can be heated to around 80°C before being emitted, thereby increasing the height to which the flue gas rises when discharged through the chimney. Based on actual calculations from a power plant, for two 300MW units sharing one chimney with a height of 210m, there is a significant difference in the flue gas rise height when a GGH is installed compared to when it is not, under conditions where the environmental humidity is not saturated. However, from the perspective of environmental quality, the main focus is on the contribution of the major pollutants (sulfur dioxide, dust, and nitrogen oxides) to ground-level concentrations when a GGH is installed versus when it is not. In the same case, the following table is obtained through calculation. Pollutant: Sulfur dioxide – Secondary standard limit (0.15 mg/Nm3); Dust – Secondary standard limit (0.15 mg/Nm3); Nitrogen oxides – Secondary standard limit (0.12 mg/Nm3). With GGH: 1.13%; Without GGH: 2.57%; With GGH: 1.99%; Without GGH: 4.51%; With GGH: 4.30%; Without GGH: 9.74%. As can be seen from these calculation results, since the emission levels of sulfur dioxide and dust are reduced after dust removal and desulfurization, their contribution constitutes only a very small portion of the allowable environmental limits, regardless of whether a GGH is installed or not. Since FGD cannot effectively remove nitrogen oxides, the source intensity of these pollutants does not decrease; therefore, the installation of a GGH has a significant impact on the levels of nitrogen oxides. However, as shown in the table above, their concentration remains at only 10% of the allowable levels set by environmental regulations, so their impact on the environment is not significant. In fact, the fundamental measure to reduce the environmental impact of nitrogen oxides is to install denitration devices to lower the concentration levels in the atmosphere through diffusion; this is merely a temporary solution that can only alleviate local environmental pollution, not overall environmental pollution. However, if the environment around the power plant is saturated, the rise of the wet flue gas differs significantly from that when the environment is unsaturated; in this case, whether a GGH is installed alongside the FGD or not has little impact on the height to which the flue gas rises, and it does not lead to an increase in ground-level pollution levels. (II) Reducing the problem of white smoke emanating from chimneys after wet flue gas desulfurization. Since the flue gas discharged from the chimney after the FGD system is saturated, condensed water vapor forms white plumes at lower ambient temperatures. In the southern cities of our country, such smoke plumes generally appear only in winter ； It occurs more frequently in areas in the north with lower environmental temperatures. Generally speaking, white smoke after FGD is difficult to eliminate completely; to fully remove it, the flue gas must be heated to above 100°C. After installing the GGH, the flue gas temperature is around 80°C; therefore, it is only possible to prevent condensation of the flue gas near the chimney outlet, allowing white smoke to form at a farther distance. The issue of white smoke is not an environmental problem; in other words, it has no impact on environmental quality. It is rather a matter of public perception, and moreover, compared to cooling towers, chimneys produce much less white smoke. Therefore, it is necessary to strengthen publicity and gain a scientific understanding of the white smoke issue. (III) GGH cannot reduce the corrosion of the tail flue ducts and chimneys. In the 1980s and 1990s, since the performance of the FGD process was not yet fully understood, it was believed that by heating the desulfurized flue gas with a GGH, raising its temperature to 80°C would reduce the corrosive effect of this gas on downstream equipment. However, practice has shown that after being heated by the GGH, the flue gas temperature remains below its acid dew point, resulting in new acid condensation in the tail flue ducts and chimney. Therefore, the idea that the use of GGH will not cause corrosion to the tail flue and chimney is incorrect. The main reasons are as follows: First, although the SO2 content in the flue gas after desulfurization is reduced, very little SO3 is removed, and the corrosive components in the flue gas have changed significantly, including CL-, SO32-, SO42-, F-, etc. The moisture content in the exhaust gas is high; SO3 dissolves completely in water, causing condensation to occur on the inner walls of the tail flue and chimney, which increases corrosion of the chimney. Secondly, after the installation of the GGH, fly ash in the flue gas accumulates on the heat exchange elements of the GGH. The heavy metals present in the fly ash act as catalysts, converting some of the SO2 in the flue gas into SO3. Although the amount is small, it still has an impact on raising the acid dew point of the flue gas. Tests show that the level of SO3 increases after the installation of the GGH ； Thirdly, tests have shown that the acid dew point of the flue gas after FGD is in the range of 90–120°C, while the temperature of the flue gas after reheating is around 80°C; as a result, new acidic condensates still form on the surfaces of the equipment located downstream of the FGD unit ； Fourthly, the temperature of the flue gas after being heated by the GGH is higher than the dew point of the flue gas, which prevents new condensation from forming. However, flue gas at 80°C cannot quickly evaporate the water that has already condensed on the surfaces of the flues or chimneys, nor can it rapidly dry the slurry that passes through the demister; instead, these liquid droplets gradually concentrate and dry out. This process transforms the originally weakly acidic droplets into highly corrosive acids, causing pitting corrosion on the flues and chimneys. Therefore, the belief that installing a GGH can reduce the corrosion of desulfurization flue gas on downstream equipment is a misconception. Furthermore, whether a GGH is installed or not, the chimney of wet FGD must be equipped with anti-corrosion measures and designed as a wet chimney, a fact that has been proven by decades of practice abroad. III. Problems arising from the installation of GGHs Since most FGD systems currently use rotary GGHs, the following discussion is primarily focused on this type of GGH; however, the conclusions are also applicable to other types of GGHs, such as water-cooled types and steam heat exchangers. (1) Increased investment and operating costs. Preliminary estimates suggest that over 80% of domestic thermal power plants using FGD systems currently employ GGHs. Assuming an additional 30,000 MW of FGD capacity is added each year, the direct equipment cost for installing GGHs amounts to around 1.1 billion yuan. When taking into account factors such as the increased pressure required by the booster fans due to the installation of GGHs, the additional control points needed in the control systems, the increased length of the flue ducts, as well as the costs associated with GGH supports and related construction work, these expenses together account for approximately 20% of the total investment in FGD systems. The pressure drop of the GGH unit on the flue gas is approximately 1000 Pa; taking into account the pressure drop in the flue caused by the installation of the GGH, the total pressure loss is around 1200 Pa. To overcome these obstacles, it is necessary to increase the head pressure of the booster fan, which **increases** the operating costs of the FGD system. (II) Reduce desulfurization efficiency. Leakage from the raw flue gas side to the clean flue gas side of a GGH reduces the system’s desulfurization efficiency. Although leakage between the raw flue gas side and the clean flue gas side of a rotary GGH can be kept below 1.0%, it remains an unnecessary loss. (III) An increase in operational failures of the desulfurization system. The raw flue gas is cooled in the GGH from around 130°C to 80°C, below the acid dew point; as a result, a large amount of viscous concentrated acid liquid is generated on the hot side of the GGH. These acids not only have a strong corrosive effect on the heat exchange elements and casing of the GGH, but also cause a large amount of fly ash from the flue gas to adhere to them. Furthermore, the tiny slurry droplets that pass through the demister and evaporate on the surface of the heat exchange elements also form solid scale. The aforementioned solids will block the channels of the heat exchange elements, further increasing the pressure drop in the GGH. There have been cases in domestic power plants where severe fouling of GGHs led to excessive vibration in the booster fans. (IV) Increased corresponding energy and water consumption. During operation and after shutdown, GGHs need to be flushed with compressed air, steam, and high-pressure water in order to remove dust accumulation and acid deposits from the heat exchange elements, which consequently increases energy consumption and water usage. The wastewater after GGH flushing is highly corrosive and must be specially treated before it can be discharged. IV. Problems arising from not installing a GGH: First, since the degree of cooling of the original flue gas increases, the water consumption in the FGD system is about 30-40% higher compared to when a GGH is installed ； Secondly, due to the low temperature of the exhaust gases, when the moisture in the ambient air is near saturation and the conditions for meteorological dispersion are poor, condensation droplets form as the gases leave the chimney outlet, resulting in what is known as \"chimney rain\"; on the ground around the chimney, it feels like light rain is falling ； Thirdly, since the FGD system cannot effectively remove nitrogen oxides, it is necessary to calculate the ground concentration of nitrogen oxides as well as the distance from the chimney to the point where the maximum ground concentration is reached ； Fourth, without installing a GGH, sufficient attention must be paid to the corrosion issues of the tail flue and chimney caused by the flue gas after desulfurization. Therefore, the flue gas does not heat up after wet flue gas desulfurization, and direct emission of the wet flue gas may pose two potential problems: a reduced elevation height, which could lead to increased ground-level pollution levels, as well as corrosion of the tail ducts and chimneys. However, the decrease in the rise height of flue gas after desulfurization can be compensated for by the reduction of pollutants in the flue gas after desulfurization, thus avoiding an increase in environmental pollution ； Corrosion of the tail flue and chimney can be addressed through anti-corrosion measures. V. Summary (1) The installation of GGH in FGD systems was an idea that emerged abroad during the 1980s and 1990s. Long-term practice has shown that GGH plays little role in FGD systems, but it does cause significant negative effects. (II) Chimneys from which clean flue gas is emitted after wet FGD must be equipped with anti-corrosion measures, regardless of whether a GGH is installed or not. Therefore, the idea that anti-corrosion measures need not be taken for the chimney after installing a GGH in order to save on the costs associated with such measures is not only incorrect but also dangerous. (III) The investment and operating costs of GGHs are extremely high. The total cost of installing GGH systems for two 300MW units amounts to over 30 million yuan, accounting for more than 20% of the total investment in FGD systems. The annual operating cost is around 5.3 million yuan (note: the cost of constructing a wet chimney with good corrosion resistance that can be used for a long time is approximately 12–20 million yuan). (IV) Based on the current operating conditions of GGH units in use domestically, the performance of most GGH units is poor. Due to the short operating time, corrosion issues have not yet become fully apparent; the main problem at present is the blockage of heat exchange elements, which causes the FGD system to stop operating. (5) Regarding the issue of increased contributions of nitrogen oxide emissions to ground-level concentrations due to the absence of GGHs, this should primarily be addressed by installing flue gas denitration equipment, and the decision on whether such equipment is necessary should be based on emission standards. Companies should not be forced to install GGHs, which bring more disadvantages than advantages, in order to avoid nitrogen oxide emissions; GGHs can only modestly improve the nitrogen oxide levels in the local environment, but not the overall environmental quality. From an economic perspective, the savings in investment and operating costs resulting from the elimination of GGHs could potentially offset the investment costs associated with installing flue gas denitration systems, and it would also be possible to reduce the overall emissions of nitrogen oxides, thereby addressing the issue at its root. Therefore, from the perspective of pollutant emission control alone, the decision to install a GGH should be left to the owner, with environmental protection authorities merely setting the requirements for pollutant control. (VI) Installing a GGH can help alleviate the problems of chimney rain and white smoke in the area around the chimney. However, white smoke does not affect environmental quality; it cannot be considered an environmental factor, let alone used as a standard or basis for installing GGHs. Especially since there are no standards regarding exhaust gas temperature in our country, companies cannot be forced to install GGHs. Of course, it is understandable for some urban power plants to install GGHs in order to improve the visual appearance. (7) The heating of flue gas after wet flue gas desulfurization in thermal power plants mainly aims to increase the flue gas lift height and effective source height under certain conditions and to a certain extent, thereby improving the flue gas dispersion conditions; it has no effect on the emission concentration and volume of pollutants. (8) In areas with environmental capacity, such as rural areas and certain coal-fired power plants located near the sea, GGHs may not be installed provided that the requirements regarding emission standards, total emission controls, and environmental functions are met.