Experimental and modelling analysis of seawater scrubbers for sulphur dioxide removal from flue-gas

Scrubbing with seawater is a reliable technology for flue-gas desulfurization in coal-fired power plants or in marine applications. The introduction of new regulations for emission control in Europe and the due increment of operational efficiency are pushing forward the optimization of scrubber's design, for which packing columns may be a better solution compared to spray columns. The design of a seawater scrubber mainly relies on a correct assessment of the equilibrium conditions and of the mass transfer coefficients in the absorber. In this work, experiments are performed on a model flue-gas (32 m3·h−1) having a SO2 concentration in the range 500-2000 ppmv, treated in a packed column filled with a structured packing (Mellapak 250X®) at 1 atm and 25 °C, with liquid-to-gas ratio between 1.06 and 3.44 kg·kg−1. Three different absorbing solutions are investigated: (a) a seawater; (b) a basic solution obtained by adding 200 mg·L−1 NaOH solution to the seawater and (c) distilled water, used as benchmark. In order to analyze the packed column tests, SO2 equilibrium absorption tests at low concentrations (100-2000 ppmv) are carried out in a feed-batch reactor, using the same absorbing solutions tested in column tests. The experimental tests in packed column indicate that, for SO2 concentration as low as 500 ppm, an absorption efficiency above 98% can be achieved by using liquid-to-gas mass ratio about to 2.91 kg·kg−1. In this condition, few differences appear among solutions (a) and (b). For the highest SO2 concentration (i.e. 2000 ppmv), a maximum of 85% efficiency is observed for liquid-to-gas mass ratio of 3.44 kg·kg−1. Modeling of both the equilibrium and the dynamic data are implemented in Aspen Plus® V 8.6. However, while modeling assures a very good prediction of the equilibrium data, it is not able to properly describe the dynamic test results, with a systematic underestimation for tests having a removal efficiency higher than 85%.