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Control of Sulfur Oxides Emissions
Published in Jeff Kuo, Air Pollution Control Engineering for Environmental Engineers, 2018
The spent scrubbing liquid is then thickened, centrifuged to obtain the magnesium sulfite solids which are dried and calcined to generate magnesium oxide (MgO) and concentrated SO2 (Figure 9.5). The produced MgO can be slaked with water to produce Mg(OH)2 to be returned to the scrubber. The concentrated SO2 can be liquefied, sold as an intermediate chemical or for production of H2SO4 or elemental S (EPA, 2003). Reactions for regeneration and slacking are: () MgSO3+SO2→MgO () MgO+H2O→Mg(OH)2
Precombustion/Postcombustion Desulfurization
Published in S. Komar Kawatra, Advanced Coal Preparation and Beyond, 2020
In addition to lime and limestone, a number of other absorbents have been used to improve the efficiency of sulfur removal or to recover the sulfur in a marketable form while regenerating the absorbent. Next-generation scrubbers are therefore under development to improve the efficiency and reduce the quantity of unmarketable waste products (Feeney, 1995; Anonymous, 1992a,b). Several of the scrubber technologies that use other absorbents are as follows: Dual-alkali process: In this process, the absorption of sulfur dioxide is first carried out using a solution of a sodium alkali, such as NaOH, Na2CO3, or Na2SO3. Since these are all very soluble in water, they can absorb the sulfur dioxide very rapidly and completely and can be easily oxidized afterwards. Also, the absorbent is a clear liquid rather than a slurry, and so the problems with scaling and plugging of the scrubber are much reduced (Valencia, 1982). The oxidized sulfur-bearing alkali is then circulated to a vessel, where it is reacted with lime or limestone, which precipitates the sulfur as calcium sulfate and regenerates the sodium alkali. A flow diagram of the process is shown in Figure 3.3. The dual-alkali process is reported to be stable and resistant to disturbances, and to be capable of removing more than 99% of the sulfur dioxide from flue gases (Valencia, 1982; Hodges et al., 1992). Wellman-Lord process: This is a regenerable-sorbent process, producing SO2 gas, which can be sold for industrial uses. It uses a solution of sodium sulfite (Na2SO3), which absorbs SO2 and becomes a sodium bisulfite solution (NaHSO3). The sodium bisulfite is then decomposed in a forced circulation evaporator, releasing the SO2 at sufficiently high concentration to be compressed and sold as SO2 gas or used for producing elemental sulfur or sulfuric acid (Couch, 1995).Magnesium oxide process: The magnesium oxide slurry is used to collect SO2, and the resulting magnesium sulfite is thermally treated to release the SO2 and regenerate the absorbent, as shown in Figure 3.4. Like the Wellman–Lord process, this process is relatively complex and has a capital cost about 14% higher than limestone scrubbers (Burnett and Wells, 1982). It is therefore only economically viable when there is a reliable market for the by-products (Vernon and Jones, 1993).
Study on jet aeration oxidation of magnesium sulfite from magnesium-based exhaust gas cleaning system
Published in Environmental Technology, 2018
Lin Guo, Xiaojia Tang, Hui Wang, Tie Li, Weifeng Liu, Quan Liu, Yimin Zhu
Exhaust gas cleaning technology is one of the accepted methods to reduce sulfur dioxide emission from ships. A magnesium-based exhaust gas cleaning system (Mg-EGCS) has advantages of high efficiency, small footprint, and stable performance [1]. Magnesium sulfite is the most important by-product formed in the washing water of magnesium desulfurization. This insoluble material seriously affects the COD and turbidity of the washing water. Therefore, the treatment of magnesium sulfite is essential for Mg-EGCS. Forced oxidation is applicable to the treatment of magnesium sulfite on ships, and the oxidation of magnesium sulfite to soluble magnesium sulfate is carried out by air aeration in the waste water. At present, most forced oxidation systems use blowers or air compressors as air sources. In order to increase the contact area of the gas and the liquid and prevent the deposition, an additional mechanical stirring device is usually required [2,3]. These methods have many shortcomings, such as complicated devices, complicated operation, low oxygen utilization efficiency, high energy consumption, and high running cost, which hinder its application in EGCS. Therefore, it is necessary to develop a simple and efficient method of forced oxidation of magnesium sulfite.