Mechanism Research On The Absorption, Oxidation And Crystallization In The Process Of Limestone-Gypsum WFGD

In recent years, the emission standards of sulfur dioxide have been increasingly stricted.In carrying out to control the emissions of sulfur dioxide, limestone-gypsum wet FGD has been introduced and applied to the large-scale coal-fired power plant.Due to the prevalence of coal instability and the shaving peak for the national power plant, the local problem has occurred in application of wet FGD technology, such as the decreased operation hours and the removal efficiency.As a result of limestone-gypsum wet FGD technology involved in mass transfer and chemical reaction mechanism is very complex, especially the mass transfer mechanism in the absorption of sulfur dioxide by the falling alkaline droplets and the distribution of sulfur dioxide gas concentration field in the tower, the process of the oxidation of sulfite and the crystallization of sulfate were directly bearing on the operation of the system stability and desulfurization performance. These factors are very important for designing the reaction devices, reducing initial investment and operation cost, operating stably and maintaining easily and it is worth of research in depth.Both the physical performance of the spray liquid in the scrubber and the involved chemical reactions are analyzed, and a model describing the absorption process of SO2 into limestone slurry with a spray scrubber is presented, in the model. The calculations show that the enhancement factor exponentially grows with an increasing value of pH and logarithmically decays with an increasing value of the resistance force. To verify the accuracy of the model, experiments were also carried out, and the results suggest that the model, after combining the physical performance of the spray and the enhancement factor, can more precisely describe SO2 absorption in a spray scrubber. Furthermore, including the effects of the concentration of inlet SO2, flue gas velocity and spry on the fileld distribution and desulfurization performance, several simulations were launched which describe and clarify the effects of variables on SO2 absorption. The results of numerical simulation can provide a basis for further design and optimization of the scrubber.The process of oxygen transfer in a vessel was presented by CFD simulations and PIV (Particle Image Velocity). The logarithmical expression has been found which successfully describe the relation between the concentration of oxygen and the dissolution time. The disc impeller which can bring the larger turbulent kinetic energy dissipation than the narrow impeller and the dissolution concentration of oxygen grows faster with an increasing value of impeller speed.To further look through the oxidation mechanism of sulfite, mathematical models of the general oxidation of calcium sulfite were presented,which included the dissolution of calcium sulfite from solid to liquid,diffusion of oxygen from gas to liquid,and chemical reaction in liquid.Based on the experimental results of characterized kinetics,the rate control step of macroscopical kinetics was inferred. The effects of manganese and ferric ions on the rate of the sulfite oxidation was studied which is similar to the conditions of WFGD. It is shown that the combined effect of both manganese and ferric ions has 10-30 times acceleration.The influence of the presence of magnesium and ferric ions at different ratios on the rate of gypsum crystallization was explored which is similar to the conditions of WFGD. The results show that addition of both Mg2+ and Fe3+ increased induction time and decreased the growth efficiency compared with the baseline(without impurity) depending on the concentration and type of impurity. The effects of the presence of Mg2+ and Fe3+ on the surface energy and the rate of nucleation are estimated by employing the classical nucleation theory. Furthermore, an edge detection program was developed to quantify the effects of impurity on the filtration rate of gypsum product. The presence of Mg2+ and Fe3+, have a much greater reduction on the growth rate of the special faces of gypsum crystal, which leads to the formation of needle crystals compared to the baseline which favors the formation of plate or flakes.