The wet flue gas desulfurization processes, forms, and mechanisms in various countries around the world are similar, mainly using slurries such as limestone (CaCO3), lime (CaO), or sodium carbonate (Na2CO3) as detergents to wash the flue gas in the reaction tower, thereby removing SO2 from the flue gas. This process has a history of 50 years. After continuous improvement and refinement, the technology is relatively mature and has the advantages of high desulfurization efficiency (90%~98%), large unit capacity, strong adaptability to coal types, low operating costs, and easy recovery of by-products. According to statistics from the US Environmental Protection Agency (EPA), among the wet desulfurization units used in thermal power plants across the United States, wet lime method accounts for 39.6%, limestone method accounts for 47.4%, and the two methods together account for 87%; The double alkali method accounts for 4.1%, and the sodium carbonate method accounts for 3.1%. More than 90% of large thermal power plants in countries around the world (such as Germany, Japan, etc.) use wet lime/limestone gypsum flue gas desulfurization process.

The main chemical reaction mechanism of lime or limestone method is:

石灰法： SO2+CaO+1/2H2O → CaSO3·1/2H2O

Limestone method: SO2+CaCO3+1/2H2O→CaSO3·1/2H2O+CO2

Its main advantages are that it can be widely commercialized, and its absorbent resources are abundant, with low cost. The waste residue can be discarded or recycled as commercial gypsum. At present, the lime/limestone method is the most widely used FGD process in the world, with a desulfurization rate of over 90% for high sulfur coal and over 95% for low sulfur coal.

The traditional lime/limestone process has potential defects, mainly manifested as equipment fouling, blockage, corrosion, and wear. To address these issues, various equipment manufacturers have adopted various methods and developed second-generation and third-generation lime/limestone desulfurization process systems.

The wet flue gas desulfurization process is also more mature, including magnesium hydroxide method; Sodium hydroxide method; Wellman Lord FGD process from Davy McKee Company in the United States; Ammonia method, etc.

In wet process technology, the reheating problem of flue gas directly affects the investment of the entire FGD process. Because the flue gas after wet flue gas desulfurization generally has a lower temperature (45 ℃), mostly below the dew point, if it is directly discharged into the chimney without reheating, it is easy to form acid mist, corrode the chimney, and is not conducive to the diffusion of flue gas. So wet flue gas desulfurization devices are generally equipped with flue gas reheating systems. At present, the most widely used technology is the mature regenerative (rotary) flue gas heat exchanger (GGH). GGH is relatively expensive and accounts for a high proportion of the overall investment in the FGD process. In recent years, Mitsubishi Corporation in Japan has developed a leak free GGH that can effectively solve the problem of smoke leakage, but the price is still relatively high. The former German company SHU has developed a new process that eliminates the need for GGH and chimney. It installs the entire FGD device in the cooling tower of the power plant and uses the waste heat of the circulating water to heat the flue gas. The operation is good and it is a very promising method.