What is Spray Dry Absorber (SDA) semi-wet process?

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 milk of lime (MoL) in Europe and lime slurry in the US. 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. 


Combined with the injection of activated carbon, lignite coke or dry sorbents, 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. No waste water is generated during this process, meaning 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. 

Lhoist product range dedicated to Semi-Wet process

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

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

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

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Semi-wet process