POWER

NIDTM FLUE GAS 

DESULPHURIZATION SYSTEM 

FOR THE POWER INDUSTRY
NIDTM Flue Gas Desulphurization System

The NIDTM system is a Dry Flue Gas Desulphurization (DFGD) process that is based on the reaction between 

SO2 and Ca(OH)2 in humid conditions. The humidified mixture of hydrated lime and reaction product is 

injected into the NID system absorber and cools the inlet flue gas by evaporation. The cooled flue gas then 

flows to the dust collector, preferably a Fabric Filter (FF) or an Electrostatic Precipitator (ESP), where the 

particles in the flue gas are removed and recycled back through the NID FGD system. 

Alstom has over 100 NID system installations covering applications in power generation, iron and steel and 

waste-to-energy. The NID system is the innovative result of Alstom’s many years of experience in dry and 

semi-dry FGD systems and other dry flue gas cleaning processes.

Fabric Filter

The use of a fabric filter offers a distinct advantage in dry FGD systems. 

The cooled, humid flue gas provides excellent filtration and reaction 

conditions, resulting in the following advantages:

•	 very low particulate emissions; and

•	 additional gas absorption (SO2

, HCl, SO3, HF, Hg) in the dust cake.

Absorber

The moistened fine powder recycle provides instantaneous flue gas cooling 

and humidification. The NID system absorber is integrated into the fabric filter 

inlet duct, minimizing the system footprint. The reaction and drying times 

within the NID FGD system are less than two seconds.

DUST

H2O

REAGENT

The heart of the NID system is the patented mixer/humidifier. The equilibrium 

moisture content in the ash received from the fabric filter is increased a few 

percent by the addition of water. The mixer uniformly distributes the water 

and lime into the entire recycle product stream from the fabric filter prior to 

injection into the flue gas. 

The humidified solids in the mixer continue to behave as a free-flowing 

powder, without clumping, enabling even distribution of the moist powder 

into the flue gas for SO2

 absorption. The blending of the lime, water, and 

recycle product is done externally of the flue gas. This ensures a homogeneous 

mixture prior to injection back into the flue gas stream. 

Mixer/Humidifier

MIXER SCHEMATIC
Compact Design/Small Footprint

The combined size of the NID system mixer and absorber is less than 20% of a 

conventional spray dryer or fluidized bed reactor. This allows the complete FGD process 

to fit within the footprint of the fabric filter. The NID FGD system is the most suitable 

technology for retrofit purposes. Many existing precipitator support structures have been 

converted into NID systems.

•	 Low investment cost

•	 Low maintenance due to few components

•	 Low power consumption

•	 Compact reactor design housed under the

particle collector 

•	 Low reactive reagent can be used

•	 High SO2

 removal efficiency

•	 Water, fly ash, reagent are blended outside

the flue gas.

•	 Not limited by coal sulphur content

•	 High efficiencies for HAP’s and SO3

 over 

conventional WFGD.

•	 High SO2

 efficiency, 90 to 98% with lime 

stoichiometries on par with conventional 

spray dryer absorber (SDA) DFGD.

ADVANTAGES OF THE NID SYSTEM

RECIRCULATION CONCEPT

TC

SO2

WATER

END

PRODUCT

FLUE GAS

HYDRATOR

HUMIDIFIER

REAGENT

Reagent/Recirculation Concept FABRIC FILTER

Hydrated Lime (Ca(OH)2), quick lime (CaO) or alkaline ashes can be used

as reagents. CaO is dry slaked to Ca(OH)2

 in the system, which is the real 

reaction partner to SO2

. Since the recirculated material has a solid content of 

95% or greater, very high recirculation rates are attained. Recirculation rates 

of 30 to 50 times or higher, ensure high lime utilization.

•	 lightweight aggregate;

• 	 acid mine drainage control;

•	 mine backfilling;

•	 disposal cell sealing layers; or

•	 road beds

End Product

The typical end product is a dry powder consisting of a pozzolanic mixture 

of fly ash, calcium sulfite/sulfate, -hydroxide, -carbonate, -chloride, etc. 

These products may be disposed of in a landfill, but may also be used in the 

following applications:

End Product Handling

A mechanical transport system or conventional dense-phase pneumatic

conveying system can be utilized, due to the low water content in the 

end product. The end product is stored in a silo similar to the storage of 

conventional semi-dry end products.

Fabric Filter

Absorber

NIDTM SYSTEM 3D ILLUSTRATION ABSORBER

RECIRCULATION
PWS/BPROB/NIDFGD08/eng/ECS/07.08/US/6583 ©Alstom 2008. Alstom, the Alstom logo and any alternative version thereof are trademarks and service marks of Alstom. The other names mentioned, registered or not, are the property of their respective companies.

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