Study On The Effects Of Limestone Reactivity And Flow Field On Wet Flue Gas Desulfurization Efficiency

With the development of economy and society, the environment is badly polluted by SO₂ and NOx from coal-fired boilers. At Present limestone-gypsum wet flue gas desulfurization technology is being widely applied all over the world because of its advantages: sophisticated technology and reliable operation, high desulfurization efficiency, widespread and cheap absorbent. In a WFGD system, the reactivity of limestone and the flow field distribution in the scrubber are of great effect on SO₂ absorption. According to the problems during the operation of foreign desulfurization process in China, in order to improve the SO₂ removal efficiency and operation economical efficiency of WFGD system and simultaneous removal SO₂ and NO_x through wet flue gas desulfurization process, the reactivity of limestone, optimization of the gas-liquid mass transfer in the scrubber and the absorption mechanism NO₂ by limestone slurry are studied in this dissertation.The impact of pH value, temperature, particle size distribution, gypsum, fly ash, Al³⁺, F^- and NO₃^- on the dissolution of limestone was investigated by pH-stat method. It seems that lower pH value, higher reaction temperature and smaller particle size are favorable to limestone dissolution. Gypsum or fly ash has a little inhibition effect on limestone dissolution and the aluminum/fluoride complexes (AlFx) inhibit the dissolution of limestone seriously.To study the relation between limestone chemical components and its desulfurization capacity, the desulfurization capacity of 42 kinds of limestone from different sources was tested. The correlation between limestone desulfurization capacity and its chemical components, including CaO, MgO, Al₂O₃, Fe₂O₃, SiO₂ and ignition loss was systemically analyzed to identify the key component which influenced the desulfurization capacity. The results showed that the content of CaO and MgO was the key factor influenced the desulfurization capacity of limestone. And the content of SiO₂ and Fe₂O₃ had some effect on limestone desulfurization capacity.In addition, as is shown in the XRD results, the desulfurization capacity of limestone was closely related to the crystallinity of calcite and the existence of dolomite would inhibit the dissolution seriously.In order to investigate the effects of limestone quality on SO₂ removal, the effects of limestone type and particle size distribution on SO₂ removal were studied by numerical model based on the two film theory of gas absorption and experimental method.The dissolution surface reaction rate constant variations accounted for differences between limestone types. As for the same kind of limestone, smaller particles had larger specific area and dissolved at a greater rate, thus could absorb more SO₂.By installing the gas-liquid mass transfer strengthening component in the spray scrubber for wet flue gas desulfurization, the mass transfer in it was studied by experimental and CFD simulation method. As can be seen from the results, flue gas can be prevented from escaping along scrubber wall by the component, flue gas flow field is rectified and the mixture of gas and liquid phase are strengthened too, they are all favorable to the absorption of SO₂. In addition, at a definite flow-through section, the pressure drop and SO₂ removal efficiency of the scrubber increase with the increase of the angle between the component and horizontal plane; and at a definite angle between the component and horizontal plane, the pressure drop and SO₂ removal efficiency drop with the increase of flow-through section.An experimental study on the absorption of low concentration nitrogen dioxide into limestone slurries in a stirred tank reactor with a plane gas-liquid interface has been performed. The absorption rates under various conditions such as limestone slurry concentration, reaction temperature, limestone particle size distribution; stirred speed, NO₂ inlet concentration and O₂ content in the simulated flue gas were measured and the absorption processes had been discussed. As can be seen from the results, limestone dissolution has a great effect on nitrogen dioxide absorption.NO₂ absorption rate increases with the increase of stirred speed, NO₂ inlet concentration and O₂ content in the flue gas, and lower reaction temperature is favorable to NO2 absorption. And NO₂ removal efficiency increases with increasing flue gas residence time in spray scrubber.