Typical   Multi-Pollutant   Control System

The DAP System (controls mercury, HCl, Condensibles, Particulate)

Flue gas and ash (from the boiler), enters and pass upward through an evaporative cooling/conditioning section into a venturi section for mixing and acceleration. As gases are cooled and condensibles (SO3) become particles, mercury is oxidized/adsorbed on ash where acids have previously condensed.

The sorbent material from the sorbent silo is mixed and added dry to the DAP reactor above the venturi. Most HCl and mercury and some SO₂ are sorbed onto particles in the DAP Reactor.

The gas then passes out of the reactor to the baghouse where the sorbent is captured forming a cake on the bag surface.  The particulate and Condensibles are also captured in the cake and, on cleaning, sent to the final silo for disposal.  The now cleaned gas exits the baghouse to the fan and stack. 

Upgrade - When the mixer and silo are properly sized, DAP can be upgraded for full SO₂ control by adding fresh sorbent slurry at the mixer to partially reacted solids recycled from the baghouse silo.

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 Typical Multi-Pollutant System

The first step in adding air pollution controls for an existing boiler is to add a fabric filter or retrofit a precipitator with a pulse baghouse as part of a Multi-Pollutant Control system.

Semi-Dry scrubbing has developed from wheel type spray dryers to rotating reactors. Our paper, Advances in Semi-dry Absorption for Multi-Pollutant Control given at the MEGA Conference – 8/22/01 details technology changes. Fluid Bed Reactors and Rotary Atomized Spray Dryer are discussed in the paper but not in this discussion.

The operation of the RAP flash drying process is accomplished by adding lime slurry outside of the reactor.  It is added as a slurry, which introduces Ca(OH)_2" as small particles that immediately are mixed and combined as a thin coating on recycle material which is then introduced into the reactor by gravity.  The resulting material is 85 to 95% solids, dry and flows well.

 The recycle material returned from the baghouse is quite fine with a mean size of 46 microns and 42% below 50 microns and 96% below 200 microns.  This gives us 30 to 60 % more surface area than previous reactor type semi-dry scrubbers (recycling off a cyclone) and orders of magnitude increase in surface area than old style semi-dry type scrubbers.  The reason is that complete drying occurs, creating less agglomeration and some de-agglomeration, all of the recycle material is returned from the baghouse increasing the percentage of fines and finally the mechanical recycling breaks down the material as it moves through the mechanical conveying operations.

Flash drying is more efficient and versatile than previous spray droplet or circulating bed drying:

·       It requires ~1 second to dry wetted solids compared to 3-5 sec for fluid bed and 8-14 sec for nozzle and rotary disc atomization and drying;

·       hence smaller reactor and smaller footprint; wetted and flash dried solids have more surface area;

·       hence higher mass transfer rates of SO2 from gas to liquid to solid;

·       resulting in higher removal efficiencies at any given exit temperature, or higher exit temperatures for any targeted SO2 removal efficiency.

This may translate into less or no corrosion and reduced or no needs for existing stack modifications; cooling can be staged to optimize mercury oxidation and capture, additional NO oxidation and capture without compromising SO2 removal (see 2004 Mega Symposium paper).