Semi-Dry Process

Spray dry absorber (SDA)

SDA is a semi-wet, sometimes called semi-dry, flue gas treatment process for removing the acidic gases HF, HCl, SO2 and SO3. SDA processes are installed in coal-fired power plants and municipal and special waste incineration plants, as well as in sinter bands of steel plants.

The process typically uses a suspension of Ca(OH)2 in water, known as lime slurry. This suspension is sprayed into the hot flue gas. The water evaporates, increasing humidity and reducing the temperature of the flue gas. Both effects facilitate the neutralizing reaction of the now dry Ca(OH)2 particles with the acidic gas pollutants. The SDA process generates a dry powder mainly consisting of fly ash and sorbent reaction products. These solid particles are collected downstream, usually in a bag filter.

Lhoist solution

In most cases the lime slurry is prepared on site from Quicklime, or by suspending Sorbacal® H hydrated lime in water. In some cases it is also delivered as ready-to-use slurry.

For good performance with a SDA, a high quality lime slurry with very fine Ca(OH)2 particles is mandatory.

Lhoist experts are able to optimize your current SDA process by selecting the most suitable lime for manufacturing high quality lime slurry in your existing preparation process. Moreover, we have developed specific lime slurry preparation and storage technologies to increase your SDA efficiency.


Combined with the injection of activated carbon, the SDA process can be adapted for simultaneous micropollutant removal.

Compared to wet scrubbers, water consumption in SDA is rather low. The end product of the SDA process is a dry powder. Since there is no waste water generated, water treatment is unnecessary.

A number of SDA units have been upgraded by combining them with an additional DSI or FSI. The DSI can be integrated either before or after the SDA process. Such a combined process offers much better peak control capabilities. It is most effective when a high-surface-area hydrate like Sorbacal® SP or Sorbacal® SPS is used in the DSI.

Discover the optimum solution for your process and facility by contacting our experts.