Mago- FGD Operation cost is less than Half than Other convention Single stage NaOH Scrubber……..
* Capability to meet low emission limits below 5 ppmv and greater than 99% removal efficiency.
* Ability to minimize caustic consumption.
*  Flexibility to use available alkali waste water as a potential scrubbing solution to reduce operating cost.
*  Reduced risk of fouling and the need for softened water related to calcium scaling.In addition to the main benefits of a two
    stage system, there are several other considerations for designing an SO2 scrubber system.
* Particulate Tolerance
* Ability to suppress a visible steam plume.
* Maintaining 24/7/365 operation and maximizing system uptime.
* Minimizing down-time during start-up and commissioning.
* Installation into limited space while providing adequate maintenance access for operators.
* Maintaining close cooperation with plant personnel during system design, installation,and start-up.
* These can include the possibility of tighter emission limits in the future and local incentives to reduce emissions beyond permit




Figure  shows a schematic of a 2-stage scrubber system. Hot gas from the thermal oxidizer is first cooled in an evaporative quencher using re-circulated water. Water that has not evaporatedflows from the quencher into the packed bed absorber.
The gas passes through the first stage packed bed to remove the majority of SO2 in the gas stream. The scrubbing water is collected in the sump and is re-circulated back to the top of the first stage packed bed and to the quencher. A dilute solution of sodium hydroxide is metered into the scrubber recirculation line to neutralize acid gases in the gas stream. The addition rate of
sodium hydroxide is controlled by the pH of the liquid in the absorber sump.
After the first stage, the gas passes though a second stage. The second stage acts as a polishing step to achieve the low outlet SO2concentration limit. The first and second stages are separated by a trap-out tray. A dilute solution of plant-supplied sodium hydroxide is metered into the second stage re-circulation line and is controlled based on the pH in the 2ndstage sump.
A blowdown stream is taken from the re-circulation line to purge the system of absorption products. Water is added to the system to make up for blowdown and evaporative losses. After passing through the packed bed the gas passes
through a mist eliminator at the top of the scrubber vessel. The entrainment separator collects any water droplets that
were entrained in the gas stream during scrubbing. The gas then passes through an interconnect duct, fan, and stack
before being discharged into the atmosphere.
An option is available to sub-cool the gas using a  Vaccume Heat Pipe heat exchanger and  Vaccume water. Sub-cooling condenses the moisture in the gas stream to supply a clean source of water in the scrubber and to suppress the steam plume under most meteorological conditions.
In Vaccume Heat Pipe, Cooling and reheating of gases is possible without adding extra energy.
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