Comparison of desulfurization processes

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Desulfurization processes are all based on the fundamental mechanism by which ammonia reacts with SO2 and water to produce desulfurization products; the main methods include the wet ammonia method, electron beam ammonia method, pulsed corona ammonia method, and simple ammonia method. 1. Electron beam ammonia method (EBA method) and pulsed corona ammonia method (PPCP method): The electron beam ammonia method and the pulsed corona ammonia method involve using electron beams or pulsed corona discharges to treat flue gas that has been cooled to around 70°C and into which water and ammonia have been injected. Under the influence of a strong electric field, some molecules in the flue gas get ionized to form high-energy electrons; these high-energy electrons then activate, break down, and ionize other flue gas molecules, resulting in the formation of various active particles and free radicals such as OH, O, and HO2. In the reactor, SO2 and NO in the flue gas are oxidized by active particles and free radicals into higher-order oxides, SO3 and NO2. When these react with H2O present in the flue gas, H2SO4 and HNO3 are formed. In the presence of NH3 or other neutralizing agents, aerosols of (NH4)2SO4/NH4NO3 are generated, which are then collected by a dust collector. The electric field of the pulsed corona discharge flue gas desulfurization and denitration reactor itself also has a dust removal function. The energy consumption and efficiency of these two ammonia-based methods still need improvement, and the key equipment such as high-power electron beam accelerators and pulsed corona generators is still in the development stage. 2. Simple ammonia method: Commercially available processes based on the simple ammonia method include the TS and PS ammonia-based desulfurization processes. These processes make use of simple reaction devices designed for the rapid reaction between H2O, NH3, and SO2 under gas-phase conditions. Strictly speaking, the simple ammonia method is a type of ammonia-based process in which ammonia is not recovered; most of the desulfurization products are unstable immonium salts in aerosol form, making recovery very difficult. As a result, the economic advantages of this method cannot be realized ； Furthermore, some of the desulfurization products decompose to release SO2 again after being discharged with the flue gas, resulting in secondary pollution. Therefore, this process can only be used in installations with low environmental requirements, a source of waste ammonia, and no need for long-term operation. 3. Wet ammonia method: The wet ammonia method is a relatively mature and industrially established ammonia-based desulfurization process, and it enables both desulfurization and denitrification. The wet ammonia process generally consists of three main steps: desulfurization absorption, intermediate product treatment, and by-product production. Depending on the process and by-products, the wet ammonia method can be further divided into the ammonia-ammonium sulfate fertilizer method, ammonia-ammonium phosphate fertilizer method, ammonia-acid method, ammonia-sulfurous acid method, etc. (1) Absorption process: The desulfurization absorption process is the core of the ammonia-based flue gas desulfurization technology; it is based on the reaction between SO2 and NH3 in an aqueous solution: SO2 + H2O + xNH3 = (NH4)xH2·XSO3 (1), resulting in the intermediate product sulfurous acid. Wherein, x=1.2－1.4. Producing the product directly from immonium is the AN method for sulfurous acid. (2) Treatment of intermediate products: The treatment of intermediate products mainly falls into two categories: direct oxidation and acidolysis. a) Direct oxidation – ammonia-ammonium sulfite method: In a multi-functional desulfurization tower, air is blown in to oxidize ammonium sulfite into ammonium sulfate; the reaction is as follows: (NH4)XH2-XSO3 + 1/2O2 + (2-x)NH3 = (NH4)2SO4. b) Acidolysis – amino acid method: Acids such as sulfuric acid, phosphoric acid, and nitric acid are used to acidolyze the desulfurization product, ammonium sulfite, thereby producing the corresponding ammonium salts and the gas sulfur dioxide. The reactions are as follows: (NH4)XH2-XSO3 + x/2H2SO4 = x/2(NH4)2SO4 + SO2 + H2O (3) (NH4)XH2-XSO3 + xHNO3 = xNH4NO3 + SO2 + H2O (4) (NH4)XH2-XSO3 + x/2H3PO4 = x/2(NH4)2HPO4 + SO2 + H2O (5) In reaction (3), by-products are formed; the intermediate products, after treatment, yield ammonium salts and the gas sulfur dioxide. Ammonium salts are sent to fertilizer production equipment to be converted into finished nitrogen fertilizers or compound fertilizers ； Gaseous sulfur dioxide can be used to produce liquid sulfur dioxide as well as fed into sulfuric acid production plants to manufacture sulfuric acid. The sulfuric acid produced can then be used to manufacture phosphoric acid, phosphate fertilizers, and other products. 4. Denitrification by the wet ammonia method: The wet ammonia method can achieve a certain degree of denitrification while simultaneously removing sulfur compounds. The reaction equations are: 2NO + O2 = 2NO2; 2NO2 + H2O = HNO3 + HNO2; NH3 + HNO3 = HN4NO3 + H2O; NH3 + HNO2 = HN4NO2 + H2O; 4(HN4)2SO3 + 2NO2 = N2 + 4(HN4)2SO4b