Investigation On The Formation And Decomposition Mechanism Of Ammonium Sulfate And Ammonium Bisulfate Fine Particles During SCR Process Of Coal-Fired Flue Gas

Selective catalytic reduction (SCR) of NOx with NH3 is recognized to be the main and effective technology for NOx reduction. De-NOx can form fine particles for coal-fired power plants. Previous study have shown that main ingredients of sub micron fine particles generated during De-NOx is ammonium bisulfate and ammonium sulfate. However, most of works focus on the investigation of macro phenomenon, but pay less attention to the deep discussion and study of the mechanism between the formation of fine particles and SCR De-NOx. This work mainly study catalytic oxidation of SO2, the mechanism of the formation and degradation of ammonium bisulfate, the mechanism of the decomposition of ammonium sulfates over commercial V2O5-WO3/Ti02 catalyst based on In situ FTIR spectroscopy, TG, DTG, TG-FTIR, X-ray diffraction (XRD), providing test base for controlling the formation of ammonium sulfates and emission of fine particles. The main results were summerised as follows:The process and mechanism of catalytic oxidation of SO2:the absorption of SO2 on V2O5 sites occupying its O atom in the form of SO32-; reaction between absorbed SO2 and 0 atom in V5+-OH to form an VOSO4-like structure; O2 reoxidate V4+ to V5+ which is posed by oxygen vacancies resulted from occupying its O atom by SO2, promoting the transformation from VOSO4-like intermediate to SO3 and V2O5. NH3 and NO competitively absorb with SO2, suppressing its oxidation behavior.O2 promote the absorption of SO2 on catalyst.During the SCR de-NOx, the formation of ammonium bisulfates mainly occurs in two ways:the reaction between absorbed coordinated NH3 on Lewis acid sites of V=O bonds and SO2 with O2; the reaction between absorbed mental sulfates VOSO4-like intermediate and NH3. Both of two ways occurs accompanying the reaction of SCR de-NOx. And the reaction behavior is closely related with NH3/NO. The reaction do not need H2O to participate in. During the SCR de-NOx, the formation temperature of ammonium bisulfates occurs about 250℃, and the generating ammonium bisulfates can decompose around 400℃, generating sulphuric acid species which absorbs on catalyst. The ammonium ions in ABS reacting continuously with NO lower the temperature window of its decomposition behavior,which means the mutual inhibition between the removal of NO and the formation of ABS, and the generating sulphuric acid species can absorb on catalyst. NH3 and ammonium bisulfate competitively react with SO2, occurs at the same time, which means the mutual inhibition between two reaction behaviors. The competitive reaction between NH3 and NO weakens the ability of reaction between NO and ammonium bisulfate. Ammonium bisulfate deposited on catalyst havet the behavior of volatilization in the temperature window of SCR De-NOx. And the loading of ammonium bisulfate deposited on catalyst also affect its behavior of decomposition and volatilization:the less loading of ammonium bisulfate, the harder it decomposes and volatiles.Ammonium bisulfate decomposes to H2O, NH3 and SO2 in one step. The ammonium bisulfate loaded on commercial catalyst decomposes in two steps:① The removal of ammonia from ammonium bisulfate;② Sulphuric acid species which absorbs on catalyst releases SO2 start. The ammonium bisulfate loaded on commercial catalyst have obvious decomposition behavior step by step:the removal of ammonia and the release of SO2 occurs in two different temperature range, and the commercial catalyst delays the total decomposition behavior of ammonium bisulfate, mainly resulting from the effect of ammonium bisulfate on delayed release of SO2. But the commercial catalyst does promote the removal of ammonia, leading to the advance of its temperature. Ammonium sulfate decomposes in three steps. The ammonium sulfate loaded on commercial catalyst decomposes in four steps, ammonium sulfate can transform to ammonium bisulfate through the removal of ammonia. The commercial catalyst leads to the removal of ammonia and the release of SO2 occurring in two different temperature range, promoting the removal of ammonia and delay the release of SO2, raising the temperature window of total decomposition behavior of ammonium sulfate, broadening the temperature range of the decomposition of ammonium bisulfate.