Spray Dryer Absorption (SDA) is a dry scrubbing process that is generally used for low-sulfur coal. SDA FGD systems are typically located after the air preheaters, and the waste products are collected either in a baghouse or electrostatic precipitator. However, to achieve sulfur dioxide (SO2) reduction above 80% with good reagent use, the dry scrubber is generally followed by a baghouse.
Flue gas is treated in an absorber by mixing the gas stream concurrently with atomized lime slurry droplets. The lime slurry is atomized through rotary cup spray atomizers or through dual fluid nozzles. Some of the water in the spray droplets evaporates, cooling the gas at the inlet from 300°F or higher to 160°F to 180°F, depending on the relationship between approach to saturation and removal efficiency. The droplets absorb SO2 from the gas and reacts the SO2 with the lime in the slurry. The desulfurized flue gas, along with reaction products, unreacted lime, and the fly ash passes out of the dry scrubber to the bag-house.
The SO2 absorbed in the atomized slurry reacts with lime in the slurry to form calcium sulfite (CaSO3) in the following reaction:
SO2 + CaO + 1/2 H2O => CaSO3• 1/2 H2O
A part of the CaSO3 reacts with oxygen in the flue gas to form calcium sulfate (CaSO4): CaSO3 + ½O2 + 2H2O => CaSO4•2H2O
Circulating Dry Scrubber (CDS)
There have been tremendous improvements in dry scrubbing technologies as their performance is now equal to wet FGD in most cases. Both of the new technologies Circulating Dry Scrubbers (CDS) and Novel Integrated Desulfurization (NID) are similar in nature and performance. They can achieve 98% SO2 removal on 2.5% sulfur coal. They are able to do this by recycling each particle 50 to 100 times back to a fluidized bed before discharging.
The flue gas flows through a reactor from the bottom to the top and then enters a downstream particulate control device, which can be either a fabric filter or an electrostatic precipitator. Fly ash from incineration and fresh hydrate is dosed into the reactor, while a large part of the solid material from the reactor is fed back to the fluidized bed as recirculate. Water is also injected to lower the flue gas temperature and achieve higher separation performance. CDS systems use a separate conventional hydrator to prepare the quicklime. Several companies offer this technology in the US.
Novel Integrated Desulfurization (NID) by Alstom
Alstom's design is the same for the reaction chamber but they add quicklime and water directly into the particles being recycled in a patented device they call their "mixer/hydrator." Of the two NID systems currently installed in the US Carmeuse supplies quicklime to both units.