Determination of sulfur dioxide removal efficiency in FGD units of thermal power plants

In this study, the efficiency of desulphurisation from flue gas was aimed to be determined with limestone solution in flue gas desulphurisation unit (FGD) at thermal power plants. First, the modeling studies on sulfur dioxide removal efficiency in the literature have been examined in detail and a model based on two film theory and mass transfer laws have been chosen. The total gas mass transfer coefficient, liquid-gas interface area, flue gas flow rate, solution flow rate, absorber volume and temperature are the parameters, which affect removal efficiency in the sulfur dioxide removal model. In addition, the validity of the sulfur dioxide removal model was evaluated by analyzing the operating data of the four FGD units of Plant I and Plant II thermal power plants located at Zonguldak. In FGD units, the mass transfer coefficient is mainly dependent on the fluid-gas contact area and flow dynamics, as well as the physical environment conditions. When the mass transfer improvement factor is 5, the closest model estimations were obtained to the operational removal efficiencies. This result shows that even though diffusion is the most important step in the transfer of sulfur dioxide gas to the limestone solution, the reaction rates formed in the solution are also effective. When the ratio of solution flow rate (L) to flue gas flow rate (G) is increased, the removal efficiency estimates obtained from the sulfur dioxide removal model also increase. The same conclusion was also observed in the analysis of the operational removal efficiencies obtained from FGD units. Depending on the L/G ratio, the highest difference between the model and operational removal efficiencies is 4.1%. In addition, the optimum L/G working ratios were obtained from the sulfur dioxide removal model as 15.5 and 13.3, respectively, in the FGD units in Plant I and Plant II. It was observed that these L/G ratios were applied in FGD units. These results show that the sulfur dioxide removal model is consistent with the operating data obtained from the four FGD units studied. The droplet diameter is an important parameter that determines the interface area between liquid and gas. Since the increase in droplet diameter will cause a decrease in the liquid-gas contact area, a decrease in sulfur dioxide removal efficiency is predicted. The sulfur dioxide removal model results shows approximately 3% decreases in removal efficiency as the droplet diameter increased from 1600 to 2000 µm. The analysis of the operating data of FGD units shows that increases in temperature and pH cause increases in sulfur dioxide removal efficiency. However, since the developed sulfur dioxide removal model is valid for constant temperature and pH in FGD unit, the effect of variations in these parameters on removal efficiency could not be determined.