An experimental study was performed on a countercurrent pilot-scale packed scrubber for wet flue gas desulfurization (FGD). The flow rate of the treated flue gas was around 300 Nm3/h, so the pilot-plant capacity is one of the largest with respect to other published studies on a pilot-plant wet FGD. The tests were carried out at an SO2 inlet concentration of 2000 ppm by changing the recycle slurry pH to around 4.8 and the L/G ratio to between 7.5 and 15. Three types of limestone were tested, obtaining desulfurization efficiencies from 59 to 99%. We show the importance of choosing an appropriate limestone in order to get a better performance from the FGD plant. Thus, it is important to know the reactivity (on a laboratory scale) and the sorbent utilization (on a pilot-plant scale) in order to identify if a limestone is reactive enough and to compare it with another type. In addition, by using the transfer-unit concept, a function has been obtained for the desulfurization efficiency, using the L/G ratio and the recycle slurry pH as independent variables. The Ca/S molar ratio is related to these and to the SO2 removal efficiency. This function, together with a simplified function of the operation variable cost, allows us to determine the pair (L/G ratio and pH) to achieve the desired SO2 removal with the minimum operation cost. Finally, the variable operation costs between packed towers and spray scrubbers have been compared, using as a basis the pilot packed tower and the industrial spray column at the Compostilla Power Station's FGD plant (in León, Spain).

Presently, decentralized feedback control is the only control strategy used in wet limestone flue gas desulfurization (WLFGD) plants. Proper tuning of this control strategy is becoming an important issue in WLFGD plants because more stringent SO 2 regulations have come into force recently. 3 Controllability analysis is a highly valuable tool for proper design of control systems, but it has not been applied to WLFGD plants so far. In this paper a decentralized control strategy is designed and applied to a WLFGD pilot plant taking into account the conclusions of a controllability analysis. The results reveal that good SO 2 control in WLFGD plants can be achieved mainly because the main disturbance of the process is well-aligned with the plant and interactions between control loops are beneficial to SO 2 control.

No abstract

In order to meet more stringent SO 2 regulations that have come into force recently, the operating conditions of wet limestone flue gas desulfurization (WLFGD) plants must be changed from design conditions to others where a higher SO 2 removal is obtained; this results in higher operating costs. Furthermore, because of process disturbances, the SO 2 removal target is usually set somewhat higher than necessary to provide a safety margin for meeting SO 2 emission limits. Thus, the larger the safety margin, the higher the operating costs. How much larger the safety margin needs to be depends on the proper tuning of the control strategy and dynamic properties of the WLFGD plant to be controlled. Both aspects can only be studied if a dynamic model of the plant is available. However, dynamic modeling of WLFGD plants for control purposes has not been addressed in the literature so far. To deal with with this issue, in this paper we propose an identification methodology for control purposes applicable to full-scale plants. The identification methodology is put into practice in a WLFGD pilot plant, and the empirical dynamic model obtained is suitable for predicting the dynamic behavior of the pilot plant in a wide range of operating conditions. The dynamic behavior of the pilot plant is further analyzed from the perspective of the physical-chemical phenomena of the WLFGD process.

In previous articles by the authors on seawater S(IV) oxidation kinetics, a significant catalytic effect was demonstrated by means of a commercially available activated carbon. The aims of this study carried out at pilot plant scale were to assess the use of high-efficiency structured packing and to validate the positive results obtained previously in laboratory studies. A comparison between a packed tower and a spray column was made by maintaining the same desulfurization efficiency. A 47% reduction in seawater flow can be obtained with a packed tower. This option seems to be more economical, with a reduction in operation costs of least of 33%. With the appropriate activated carbon, it is possible to reach a greater oxidation rate at a low pH level than by operating conventionally at a high pH level without a catalyst. A preliminary technical and financial comparison between the advanced seawater desulfurization process (equipped with a packed tower and a catalytic oxidation plant) and the conventional process (spray tower and noncatalytic oxidation) was carried out.