Density Functional Theory Study Of SO₂ Oxidation In Flue Gas

Sulfur element of pulverized coal mainly exists in the form of SO2, followed by a small amount of SO3 in boiler combustion process. SO3 generation influenced by factors such as temperature, gas residence time, SO2 concentration, O2 concentration, NO2 concentration, ash composition and SCR catalyst in boiler operation. SO3 generation amount is small, but harmful for boiler operation safe. It may cause high temperature corrosion of waterwall, low temperature corrosion of flue tail heating surface, congestion of SCR catalyst and air preheater, reduce removal efficiency of mercury in flue gas, and increase the opacity of exhaust smoke. Therefore, the study of formation mechanism of SO3 in coal-fired boilers has important significance.In order to study formation mechanism of SO3, density functional theory respectively under homogeneous gas conditions and heterogeneous solids catalyzed conditions are researched. The results show that SO3 generation is divided into three stages under homogeneous gas conditions, H2S is oxidized to SH under reductive atmosphere in furnace, oxidation of SH to SO2 and oxidation of SO2 to SO3. The most important primitive reaction is H2S+O=SH+OH in first stage, which activation energy is 176.92 kJ/mol; The second stage has three reaction pathways, the first pathway is reaction of SH and H, OH to generate S, S is oxidized by oxygen in the flue gas to SO, and then reacted with OH and oxygen to regenerate SO2, the second way is SH directly oxidized by O and O2 to SO, SO then reacts with OH or O2 to SO2, the last approach is SH oxidized by O2 to intermediate HSO, then HSO oxidized again to SO2, which is the main reaction of SO2 generation,111.589 kJ/mol for activation energy; The main reactio of third stage is SO2 and OH reacts to intermediate HOSO2, which is oxidized by O2 to SO3, the activation energy of process is 23.84 kJ/mol.In heterogeneous solids catalyzed conditions, SO2 catalytic oxidation by Fe2O3 in fly ash has three transition states and seven intermediates, imaginary frequency for transition states is present and unique respectively, ferrous oxide is further reduced to Fe2O needs three transition states and three intermediates, far more difficult than the former. Therefore, SO2 catalytic oxidation by Fe2O3 is control step of the reaction, high valence state of iron in favor of SO2 oxidation. SO2 catalyzed oxidation of AI2O3 in fly ash is relatively simple,which contains two intermediates and two transition states,more difficult than SO2 catalytic oxidation by Fe2O3. SO2 catalyzed oxidation by V2O5 in SCR is relatively simple, which flexible controlled in order to avoid the formation of sulfate materials to clog SCR device.