Calcium method, magnesium method, double-alkali method, ammonia method for desulfurization? Which one is good?

I believe that there is no definitive answer regarding desulfurization processes; the best option is simply the one that suits the specific situation. First, it’s necessary to consider the type of boiler in use, as well as whether there are any by-products or waste materials available that can be utilized in the desulfurization process. The ideal solution is one in which the desulfurization products, along with wastewater and waste residues, are easy to handle, require low costs, achieve good desulfurization results, ensure compliance with emission standards, and allow for stable operation

Selection and Application of Desulfurization Technologies in Small and Medium-sized Thermal Power Plants Abstract: Based on the experience gained in upgrading desulfurization technologies in medium and small thermal power plants, this paper introduces the desulfurization processes that are commonly used in such plants in China, along with their advantages, disadvantages, and application scopes, to assist these plants in making choices and implementing these technologies in practice. 　　1 Introduction 　With the rapid development of the socio-economy, energy conservation and environmental protection have come to attract increasing attention from society. The emission of pollutants by local thermal power plants has also drawn significant attention from local regulatory authorities. In the newly revised **Pollutant Emission Standards – Emission Standards for Air Pollutants from Thermal Power Plants 6 (GB13223-2003)**, while taking into account both power generation development and environmental protection objectives, the limits on air pollutant emissions from thermal power plants have been significantly increased. Since most small and medium-sized thermal power plants do not have desulfurization equipment, they all need to install such equipment in order to meet environmental regulations. Currently, there are a variety of desulfurization technologies available on the market. Determining the appropriate desulfurization technology by taking into account the specific conditions in each region is crucial for the survival of local small and medium-sized thermal power plants. Several options are proposed for reference on how medium and small thermal power plants should choose desulfurization technologies. 　　2 Introduction to Desulfurization Technologies For medium and small coal-fired boilers, due to constraints such as funding and space limitations, it is advisable to use desulfurization processes that feature a simple structure, low investment costs, minimal space requirements, high desulfurization efficiency, and low operating expenses. The desulfurization processes commonly used in domestic medium and small thermal power plants include: the simple wet lime-stone-gypsum rejection method, the double-alkali method, the magnesium oxide rejection method, the ammonia method, the simplified ammonia method, the semi-dry flue gas recirculation method, and the in-furnace calcium injection method. 　　2 1 Limestone – Gypsum wet flue gas desulfurization process: The limestone (lime) – gypsum wet flue gas desulfurization process uses inexpensive and readily available limestone or lime as the desulfurization absorbent. Limestone is crushed and ground into a powder, which is then mixed with water to form an absorbent slurry. When lime is used as the absorbent, the lime powder is digested and then mixed with water to form an absorbing slurry. Inside the absorption tower, the absorption slurry comes into contact with the flue gas; sulfur dioxide in the flue gas reacts with calcium carbonate in the slurry to form calcium sulfite. Calcium sulfite then reacts further with the oxygen-rich air introduced into the tower, and the final product of this reaction is gypsum. The flue gas after desulfurization has its fine liquid droplets removed by a demister; after being heated in a heat exchanger, it is discharged into the chimney or directly into the chimney (the chimney must be treated to prevent corrosion). The desulfurization gypsum slurry is dehydrated using a dehydration device, after which it is recovered or discarded. Since the absorption slurry can be recycled, the utilization rate of the desulfurization absorbent is very high. 　　The simplified limestone-gypsum disposal method is the most commonly used process for flue gas desulfurization in small and medium-sized boilers. It represents a simplified version of the traditional limestone (lime)–gypsum wet desulfurization process; by ensuring high desulfurization efficiency, it reduces investment and operating costs. However, most of the desulfurization by-products are discarded, resulting in secondary pollution. 　　Limestone (lime) – Wet flue gas desulfurization using gypsum is currently the most mature and widely used desulfurization technique in the world. According to statistical data, in the United States, Germany, and Japan, the capacity of power plants that use this technique accounts for over 93% of the total desulfurization capacity of such power plants. 　　2. 2 Double-alkali method The sodium-calcium double-alkali method is a desulfurization process in which sodium alkalis (sodium hydroxide or sodium carbonate) are used as absorbents during the absorption cycle in order to address issues such as wear, blockage, and scaling in desulfurization towers and circulation pipelines. Lime is employed to regenerate these sodium alkalis in order to reduce operating costs. First, a soluble alkaline solution is used as an absorbent to absorb SO2, and then lime milk or lime is used to regenerate the absorbent solution. Since different types of alkalis are employed in the absorption process and the treatment of the absorbent solution, this method is known as the double-alkali method. 　　The most commonly used method in the double-alkali process is to absorb SO2 using caustic soda (NaOH), soda ash (NaCO3), or sodium sulfite (Na2SO3), and then the absorption solution is regenerated with lime. The regenerated sodium-alkali solution is returned to the washing system as an absorbent liquid, while the calcium sulfite or calcium sulfate formed during regeneration is disposed of or recycled after treatment. The sodium-calcium double-alkali method is one of the more commonly used desulfurization methods. It has been successfully implemented on a large scale abroad in countries such as Japan and the United States. There are at least dozens of double-alkali desulfurization units in Japan and the United States, which are used successfully in power plants and industrial boilers. 　　This process combines the advantages of the lime method and the sodium-alkali method; it solves the problem of easy scaling inside the tower that occurs with the lime method, while also benefiting from the high absorption efficiency of the sodium-alkali method. 　　2. 3 Magnesium oxide disposal method The magnesium oxide disposal method is a simplified version of the magnesium oxide regeneration method and magnesium sulfate recovery method, as it eliminates the regeneration and recovery systems for desulfurization products. This process features high desulfurization efficiency, a simple structure, lower investment costs, and reduced space requirements. Additionally, it does not generate large amounts of solid waste, thus eliminating the need for storage areas. The final product of desulfurization is a magnesium sulfate (commonly known as Epsom salt) solution, which is discharged along with the wastewater. The magnesium oxide method developed in China currently uses magnesium oxide (with a purity of 80–85%) as a desulfurization agent. This agent is mixed with water to form a magnesium hydroxide solution, which is then sent to a desulfurization tower where it reacts with SO2 in the flue gas to produce magnesium sulfite. Subsequently, this mixture is sent to an aeration tank where it is oxidized, converting magnesium sulfite into magnesium sulfate, before being discharged after treatment. 　　Due to magnesium’s higher reactivity than calcium, the desulfurization efficiency using magnesium is higher, and there is no issue of slag formation. However, the cost of the desulfurizing agent is higher. To reduce operating costs, the magnesium oxide method is mainly used in areas with abundant magnesite reserves, such as Dandong in Liaoning and Laizhou in Shandong. 　　2. 4 Simple ammonia method The ammonia-based desulfurization process uses ammonia water as the absorbent, with sulfuric acid as a by-product used as fertilizer. The flue gas emitted from the boiler is cooled to 90–100°C in a flue gas heat exchanger. It then enters a pre-washer, where HCl and HF are removed through washing. After washing, the flue gas passes through a droplet separator to have any water droplets removed before entering the desulfurization tower. Ammonia solution with a concentration of 5% is sprayed from the top of the tower to wash the flue gas; the SO2 in the flue gas comes into full contact with the ammonia solution during this washing process. The washed flue gas is discharged, and a demister is used to remove any remaining water droplets. It is then heated in a flue gas heat exchanger before being released through a chimney, or it may be directly discharged into the chimney (the chimney must be treated to prevent corrosion). The sulfuric acid solution with a concentration of about 25% produced in the washing process is discharged from the washing tower; it can be sent to a fertilizer plant for further processing or sold directly as liquid nitrogen fertilizer. Alternatively, this solution can be further concentrated, evaporated, and dried to produce granular, crystalline, or solid fertilizer for sale. 　　The simple ammonia method is a simple desulfurization technique that eliminates the need for ammonium sulfate recovery; a Venturi tower is commonly used as the absorption tower, with waste ammonia water serving as the desulfurizing agent. Although this method has a simple structure and low investment costs, the disposal of the desulfurization product, ammonium sulfide, causes secondary pollution. Moreover, if there is no waste ammonia water available, the operating costs of this method are also high; as a result, it has gradually been phased out. 　　2 5 Flue gas circulation semi-dry process: The untreated flue gas emitted from the boiler enters at the bottom of the absorption tower (i.e., the fluidized bed). The bottom of the absorption tower is equipped with a Venturi device; as the flue gas passes through this venturi tube, its velocity increases. Here, it mixes with very fine absorbent powder, and intense friction occurs between the particles as well as between the gas and the particles, thereby creating a fluidized bed. With the addition of a uniform mist to lower the temperature of the flue gas, the absorbent reacts with sulfur dioxide in the flue gas to form CaSO3 and CaSO4. The flue gas, containing a large amount of solid particles after desulfurization, is discharged from the top of the absorption tower and enters the recirculation dust collector. The separated particles are returned to the absorption tower via an intermediate ash bin. Since these solid particles are recycled hundreds of times, the utilization efficiency of the absorbent is high. Dry slaked lime powder is generally used as an absorbent; other dry powders or slurries that have the ability to absorb sulfur dioxide can also be used as absorbents. 　　The flue gas circulation semi-dry method is a semi-dry desulfurization process, which consists of components such as absorbent preparation, absorption towers, recirculation of desulfurization ash, dust collectors, and a control system. This method features low investment and minimal land use. The 65 MW units at Shandong Qilu Petrochemical utilize the semi-dry circulating fluidized bed technology developed by Tsinghua Tongfang, achieving a desulfurization efficiency of over 85%. 　　2. 6 Calcium injection into the furnace Calcium injection into the furnace is a simple dry desulfurization process; however, it was once abandoned due to its low desulfurization efficiency. But with the widespread use of circulating fluidized bed boilers, it has regained relevance. In circulating fluidized bed boilers, CaO circulates in the furnace along with the fuel, **which increases the reaction time, provides an appropriate reaction temperature, and enhances the desulfurization efficiency.** The process of calcium injection into the furnace combined with humidity enhancement and desulfurization in the flue gas at the rear of the furnace is an improvement on the calcium injection desulfurization process; it involves adding a humidity enhancement section at the back of the boiler to improve the desulfurization efficiency. This process uses limestone powder as an absorbent; the limestone powder is sprayed into the furnace at a temperature range of 850 to 900°C. There, the limestone decomposes upon heating into calcium oxide and carbon dioxide, and calcium oxide reacts with sulfur dioxide in the flue gas to form calcium sulfite. Since the reaction takes place between the gas and solid phases, it is affected by the mass transfer process, resulting in a slow reaction rate and a low utilization efficiency of the absorbent. In the tail humidification activation reactor, humidifying water is sprayed in the form of mist and comes into contact with unreacted calcium oxide to form calcium hydroxide, which then reacts with sulfur dioxide in the flue gas. When the calcium-sulfur ratio is controlled at 2.0–2.5, the desulfurization efficiency of the system can reach 65–80%. Due to the reduction in flue gas temperature caused by the addition of humidifying water, the outlet flue gas temperature is generally kept 10–15°C above the dew point. The humidifying water evaporates rapidly as it is heated by the flue gas temperature; the unreacted absorbent and reaction products exit with the flue gas in a dry state and are collected by the dust collector. 　　This desulfurization process is used in Finland, the United States, Canada, France, and other countries. 　　3 Selection of Desulfurization Technologies The choice of desulfurization methods should primarily take into account the following five factors: 3.1 Desulfurization efficiency factor In small and medium-sized thermal power plants, the source of coal is relatively unstable and susceptible to external influences; the sulfur content in the coal fed into the plant can vary from time to time, resulting in unstable levels of SO2 emissions. To meet emission requirements, the desulfurization method should be selected based on its efficiency at the highest possible emission levels. When the sulfur content in coal exceeds 2, the SO2 concentration in the flue gas is around 4000 mg/Nm; to meet the emission standard of 400 mg/Nm, the desulfurization efficiency needs to be over 90%. Currently, the efficiency can be maintained above 90%, with wet flue gas desulfurization being the only method used. Especially under the condition of total pollutant discharge control, a high desulfurization efficiency is a key factor, whether from an environmental or economic perspective. 　　3.2 Site factors 　　The small and medium-sized thermal power plants in operation today are primarily used for heating purposes, and most of them are part of renovation projects. In the early stages of plant construction, due to historical reasons, the issue of desulfurization was not taken into consideration, and no space was reserved for desulfurization equipment. During renovations, it is necessary to take into account the available space on site and arrange the desulfurization equipment properly without disrupting normal operations. Among various desulfurization methods, semi-dry desulfurization requires less space and can be arranged flexibly; therefore, it should be given priority in locations with limited space. 　　3.3 Desulfurization agent factors When the desulfurization efficiency can meet the requirements and there are no site constraints, operational costs need to be considered, primarily with regard to the consumption of desulfurization agents. Currently, there are a wide variety of desulfurization agents available on the market, including limestone, ammonia, magnesium oxide, sodium hydroxide, etc. Local resources that can be utilized should be given priority, such as limestone deposits and waste liquids from chemical plants. The ideal approach is to recycle waste materials, but it is necessary to take into account the quantity and sustainability of such waste, so as to avoid a situation where no waste is available after a few years. When no waste materials are available, it is preferable to choose desulfurizing agents that are abundant in supply and inexpensive, in order to reduce operating costs. 　　3.4 Factors related to desulfurization by-products The purpose of desulfurization is to reduce the emission of pollutants; therefore, the treatment of all desulfurization by-products is also a crucial step. If the wrong method is chosen, it can lead to secondary pollution, which goes against the goal of desulfurization. At present, the products of semi-dry desulfurization are difficult to utilize due to their complex composition. 　　3.5 Investment factors 　　Since the investment in desulfurization equipment yields social and environmental benefits with almost no economic returns, under the influence of economic factors, the total investment in such equipment is an important factor that must be taken into consideration. It is necessary to consider the company’s capacity to bear the costs, act according to its capabilities, and achieve results with minimal investment. 　　4 Conclusion Medium and small thermal power plants play a vital role in providing centralized heating in local areas, and they will remain widespread in the short term. It is an undeniable fact that power plants consume a lot of energy and cause significant pollution. At present, what we urgently need to address is the conflict between environmental protection and economic considerations; we must make full use of local resources to carry out flue gas desulfurization in order to prevent secondary pollution. To address this issue properly, making the right choice and applying desulfurization technologies is a crucial first step that deserves great attention.

I have also worked in the environmental protection industry for a few years; I have handled many projects involving ammonia-based desulfurization. Considering overall costs and waste disposal, I recommend using the ammonia method, as the by-products of this process can be utilized in a wider range of applications