Kinetics On Absorption Of SO₂ And CO₂ In Coal-fired Flue Gas

The air pollution caused by coal firing has attracted much attention in China. The power industry is the prominent source of coal fired pollutants, in which the pollution control becomes a focused issue. The pollutants in the coal fired flue gas include dust, sulfur dioxide (SO2), nitrogen oxide (NOx), heavy metals, and carbon dioxide (CO2) etc. As the precursor of haze and acid rain, SO2 in the flue gas is featured on low concentration and severe harmfulness. CO2 existing in low concentration in the flue gas is the crucial contributor for green house effect. These two pollutants, as polar molecular and acid gases, have the similar properties which could react with alkaline materials. Thus it is much important to develop novel technology about controlling the two acid gases for the power industry.To meet with the requirement of emission standard for SO2 in the flue gas, the catalyst for promoting the absorption rate during the desulfurization process has received wide attention. In this thesis, effects of ten catalysts on the absorption rate of SO2 were investigated under the condition close to the wet limestone scrubbing. The results indicated that most catalysts accelerate the SO2 absorption by CaCO3, in which the catalytic performance follows the order:sodium tetraborate> triethanolamine> sodium diacetate> sodium acetate> sodium humate> boric acid> adipic acid> sodium citrate> magnesium sulfate> sodium sulfate. The sodium tetraborate dominates the catalytic effect which improves the removal efficiency by 8.84%. The synergy effect of the combined catalysts was not observed by comparison with the individual catalyst. Moreover, the effects of the operating conditions in presence of sodium tetraborate were studied, and the overall reaction rate and mass transfer coefficient were achieved. The optimal operating conditions were determined as follows:concentration of sodium tetraborate 0.625 g/L, CaCO3 concentration 0.75g/L, 45 ℃. The kinetic model of SO2 absorption with catalyst was established according to the double-film theory, in which the rate controlling step of the overall reaction rate was deduced.Sulfite oxidation is another key process in flue gas desulfurization. The catalytic performance of five transition metals, including Co2+、Mn2+、Fe2+、Ni2+ and Cu2+, on promoting the sulfite oxidation were investigated. The results indicated that the transition metal ions have selective catalytic effect on the oxidation of specific sulfite. The ions of Co2+ and Mn2+ were found to exhibit the most active behavior. The oxidation rates of three sulfites, sodium sulfite, magnesium sulfite, and ammonium sulfite, were dramatically sensitive to Co2+ at the same catalyst concentration and were comparably insensitive to Mn2+. The reaction order of Co2+ in the four sulfite oxidation, magnesium sulfite, ammonium sulfite, sodium sulfite and calcium sulfite, were achieved to be 0.45,0.46,0.23 and 0.02, respectively. By contrast, the substantial promotion of the oxidation rate of calcium sulfite was caused by only Mn2+. The reaction order of Mn2+ in the four sulfite oxidation, magnesium sulfite, ammonium sulfite, sodium sulfite and calcium sulfite, were achieved to be 0.14,0.10,0.15 and 0.44, respectively. The enhancement factors of the overall reaction catalyzed by the transition metals for different sulfite oxidation were obtained, indicating that the mass transfer of oxygen is the rate controlling step in the sulfite oxidation. The transition metals accelerate the overall oxidation rate by promoting the intrinsic reaction rate. Thus the mechanism on sulfite oxidation catalyzed by transition metals was revealed.Moreover, the normal ionic liquid and functional ionic liquid were synthesized for CO2 absorption. The result showed that the absorption capacity of the normal ionic liquid, [Bmim]FP6, is limited because the absorption of CO2 a physical process. The functional ionic liquid, [NH2p-mim]Br, was prepared by one-step synthesis method. The bulk absorption loading of aqueous [NH2p-mim]Br for CO2 was measured under normal pressure condition, indicating that its absorption loading reached 0.41mol CO2/mol IL at the absorption equilibrium state. This functional ionic liquid exhibited good performance for CO2 absorption even after the regeneration process. The absorption loading of the combination of [NH2p-mim]Br and piperazine were also measured, which reached 0.82mol CO2/L IL at the absorption equilibrium state. It showed that piperazine is an effective promoter for facilitating the reaction of [NH2p-mim]Br into carbamate. By using ion exchange method, two amino acid ionic liquids were synthesized, including [Bmim][Gly] and [Bmim][Ala]. Effects of ionic liquid concentration, CO2 partial pressure and temperature, on the absorption rate of CO2 were studied, indicating that the absorption performance of these two amino acid ionic liquids are better than normal ionic liquids [Bmim]Br. It might because that [Bmim][Gly] and [Bmim][Ala] are featured with zwitter ionic properties, which could react with CO2. The kinetics of CO2 absorption by [Bmim][Gly] showed that CO2 partial pressure is the dominant factor. The reaction orders with respect of CO2 and [Bmim][Gly] were obtained as well as the apparent activation energy. Taking the kinetic model and such a low apparent activation energy into account, the overall absorption was inferred to be controlled by mass transfer of CO2. Under the same other conditions, [Bmim][Gly] exhibited better absorption performance than MDEA, but not as good as MEA. However, as a promising absorbent in application, [Bmim][Gly] have more prominent advantages than MEA because of its less volatility, corrosivity and energy consumption.