Semi-dry scrubbing

By controlling the reaction, relatively coarse crystals can be grown, or, alternatively, the calcium sulfite can be oxidised to gypsum. In either case, the crystals can readily be de-watered for disposal or sale. 

This is an elegant variant of the dual alkali process, which exploits the fact that many magnesium salts are soluble. 

Quicklime, with about 7 % MgO, is slaked to produce a milk of lime. This is fed to the scrubber and removes oxides of sulfur (29.10, 29.11). The liquor that collects in the sump of the scrubber is a suspension of calcium sulfite and sulfate and a solution of magnesium hydrogen sulfite/sulfate. 

The suspension is then treated in an external reactor with fresh milk of lime containing further magnesium oxide. The magnesium hydrogen sulfite and sulfate react with calcium hydroxide, forming magnesium sulfite and sulfate, which are recycled, and precipitate calcium sulfite hemi-hydrate and gypsum (29.12,29.13). 

As in the gypsum process, the calcium sulfite may be oxidised by injecting compressed air into the reaction vessel. Also, by classifying the precipitate in hydroclones, fines can be recycled to the reactor and crystals can be grown to the required size for de-watering for disposal or sale. 

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Adsorptive Processes. The use of activated carbon, sprayed into a dry/semi dry scrubbing unit along with lime or less frequently packed in an adsorption unit positioned after the particulate removal device and prior to the stack, has become a standard component in gas cleaning trains as a means of PCDD/F control on all sizes of plant fed with MSW or clinical waste. Other adsorptive media such as zeolites are also being tested. The inclusion of an adsorptive device in combustion systems fired with wood and agricultural wastes is not normally contemplated, and as noted above, an interesting issue to be resolved is whether different waste types generate flyash of different activities relative to PCDD/F formation. 

Semi-dry scrubbing uses the principle of the spray drier. Milk of lime is atomised at the top of the spray drier chamber into relatively hot flue gases (e.g. 220 °C). The gases carry the spray towards the base of the chamber. The water in the milk of lime evaporates, cooling the gases and, at the same time, acid gases (SO2, SO3, together with any HCl/HF present) dissolve and react with the lime. 

The semi-dry scrubbing technique can be used to achieve the required rapid cooling from 450 °C, but at a cost, as this reduces the amount of recoverable heat (boiler flue gases are normally cooled to about 200 °C in a waste heat boiler). 

Mercury and cadmium are more volatile (boiling points 356 and 765 °C respectively) and are consequently more difficult to remove. They are reduced to about 0.2mg/Nm in total in the semi-dry scrubbing and pre-cooled dry injection processes and to about 0.1 mg/Nm in the wet scrubbing processes [29.10]. Where lower emission levels are required, the injection of activated carbon can reduce the level to 0.05 mg/Nm [29.10]. 

Semi-dry scrubbers involve a chemical reaction with a wet slurry, and a dry scrubbing of the gases with dry collection of co-products. Typically the process is used to treat acid gases in a spray dryer, coupled with a pulse-jet fabric filter. In the scheme shown in Figure 7.16, with a vertical downflow spray dryer, an alkahne slurry is injected to neutralize the adds present in the flue gas. The gas temperature is controlled by the flow rate of the dilution water. 

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