What are the industrial desulfurization methods?

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Desulfurization is a technology used to remove sulfur dioxide (SO2) generated in processes such as coal combustion and industrial emissions. The development of desulfurization technologies is aimed at reducing sulfur dioxide emissions and protecting the environment and human health. Desulfurization methods can generally be classified into three categories: pre-combustion desulfurization, in-combustion desulfurization, and post-combustion desulfurization. Among them, post-combustion desulfurization, also known as flue gas desulfurization (FGD for short), can be divided into the following five methods based on the type of desulfurizing agent in FGD technology: the calcium method using CaCO3 (limestone) as a base, the magnesium method using MgO as a base, the sodium method using Na2SO3 as a base, the ammonia method using NH3 as a base, and the organic alkali method using organic alkalis as a base. The commercially used technology worldwide is the lime process, which accounts for over 90% of applications. Based on whether the absorbent and desulfurization products are in a dry or wet state during the desulfurization process, desulfurization technologies can be classified into wet, dry, and semi-dry (semi-wet) methods. The following mainly introduces two methods: dry and wet. Dry desulfurization: Compared with the conventional wet scrubbing process, it has the advantage of lower investment costs ； The desulfurization product is in a dry state and mixed with fly ash ； There is no need to install a demister or a reheater ； The equipment is resistant to corrosion, scaling, and clogging. Its disadvantage is that the utilization rate of the absorbent is lower than that in the wet flue gas desulfurization process ； It is uneconomical when used for high-sulfur coal ； The mixing of fly ash with desulfurization products may affect their comprehensive utilization ； High requirements are placed on the control of the drying process. ⑴　Spray-dry flue gas desulfurization process: In this process, atomized lime slurry comes into contact with flue gas in a spray dryer tower. After reacting with SO2, the lime slurry forms a dry solid reaction product, which is then collected by a dust collector together with fly ash. China conducted pilot tests on rotary spray dry flue gas desulfurization at the Baima Power Plant in Sichuan Province, gaining some experience that provided a basis for optimizing the parameters of this technology for use in 200–300MW power units. ⑵　Fly ash dry flue gas desulfurization technology: Its features include a desulfurization efficiency of over 60%, stable performance, and it reaches the performance level of conventional wet desulfurization methods ； The desulfurizer has low costs ； It requires less water, eliminates the need for drainage treatment and flue gas reheating, and the total cost of the equipment is 1/4 lower than that of wet desulfurization methods ； Coal ash desulfurization agents can be reused ； Without slurry, it is easy to maintain, and the equipment system is simple and reliable. Wet process: It mainly uses slurries such as limestone (CaCO3), lime (CaO), or sodium carbonate (Na2CO3) as wash agents to wash the flue gas in a reaction tower, thereby removing SO2 from the flue gas. The main chemical reaction mechanisms in the lime or limestone method are as follows: Lime method: SO2 + CaO + 1/2H2O → CaSO3·1/2H2O; Limestone method: SO2 + CaCO3 + 1/2H2O → CaSO3·1/2H2O + CO2. Its main advantages include the possibility of widespread commercial application, abundant availability of absorbents at low costs, and the fact that the waste residue can either be discarded or recovered as commercial gypsum. The lime/limestone method is the most widely used FGD process in the world; for high-sulfur coal, the desulfurization rate can exceed 90%, and for low-sulfur coal, it can be over 95%. The traditional lime/limestone process has its potential drawbacks, primarily in the form of equipment scaling, clogging, corrosion, and wear. To address these problems, second- and third-generation lime/limestone desulfurization process systems have been developed. Other relatively mature wet FGD processes include: the magnesium hydroxide method ； Sodium hydroxide method ； Davy Mckee Corporation’s Wellman-Lord FGD process in the United States ； Ammonia process, etc. In the wet process, the issue of flue gas reheating directly affects the investment in the entire FGD process. Since the flue gas after desulfurization using wet processes generally has a low temperature (45°C), usually below the dew point, if it is discharged directly into the chimney without reheating, acid mist is likely to form, which can corrode the chimney and also hinder the dispersion of the flue gas. Therefore, wet FGD units are generally equipped with a flue gas reheating system. The most commonly used are the technically mature regenerative (rotary) flue gas heat exchangers (GGH).