This invention relates to a wet scrubbing system which is a combination of devices for removing micron sized paniculate matter from flue gases coming out of industrial boilers fired with bio-mass fuels such as bagasse, rise husk etc.
Background of the Invention
In existing industrial boilers, the yam^u-iaic matter containing flue gases are released in the air which later settle down in the vicinity also, causing air pollution and filth all around.
Attempts have been made to combat this problem to remove or catch the paniculate matter of the flue gases by use of multi-cyclone.dust collectors, high energy ventury and rotating disc type spray scrubbers, but the complex nature of this problem has made such attempts substantially ineffective.
Earlier, an attempts was made to use wet scrubbing, but this simple mechanism caused several problems, particularly those of corrosion at the fan which blows flue gases onto the chimney and blockages in the system, leading in turn to more severe problems such as high pressure drop across the scrubber.
Summary of the Invention ;
These problems have been overcome by the present invention which incorporates a specially designed wet scrubber having such sieve in the water column which has holes so designed in terms of pitch and aperture as to allow the particulate matter laden flue gases to bubble through efficiently and effectively, as well as in terms of total open area.
Additionally, the bubbling process through the water column has been made highly effective through especially designed holes in the sieve which cause atomization of the flue gases. Special attention has also been given in the wet scrubber development to the depth of the water column i.e. the water column on the sieve, which is typically kept at around 100 mm. As a result, there is also no falling down of the water from the sieve holes (weeping).
All these improvements have resulted in efficient wet scrubbing of the flue gases, lower pressure drop across the scrubber as well as practically zero carryover of water droplets with the scrubbed flue gases which would otherwise have caused corrosion in the system

upstream, particularly the fan. It is particularly notable that velocity of the flue gases is maintained at an optimum value of 2.75 M/Sec ± 10% in this invention which further helps in minimizing the carry over of water droplets and ensures scrubbing efficiency.
Present invention also incorporates a two stage mist separation system, consisting of a mist eliminator having a plurality of baffle plates and a centrifugal demister comprising of a set of curved blades so as to impart a whirling motion to the escaping water droplets if any, in the flue gases and ensuring their settling down instead of escaping.
The most significant part in this invention i.e. the sieve tray, is made of corrosion resistant material like stainless steel.
Present invention also incorporates a set of devices for treating the slurry produced out of the trapped particulate matter. These devices include a wedge wire type screen, a clarifier tank and a sand & graved sludge filter, which help recover water, clarify it and provide it for reuse at the wet scrubber.
Accordingly, the invention relates to a wet scrubbing system useful for removal of particulate matter from flue gases generated by industrial boilers fired with biomass fuels, comprising a wet scrubber containing a sieve tray having a plurality of holes, a two stage mist separation system and a combination of devices consisting of wedge wire type screen filter, a clarifier tank and a sand & gravel sludge filter for recovering water from slurry, recirculating it and removing sludge.
To assist with understanding the invention, reference will now be made to the accompanying drawing which shows one example of the invention.
Brief description of the drawing:
Fig. 1 shows one example of the wet scrubbing system including the essential devices which form part of this invention of wet scrubbing system.
Fig. 2 shows the sieve tray part of the wet scrubber. Fig. 3 shows the mist eliminator in the wet scrubber. Fig. 4 shows the centrifugal demister in the wet scrubber.
Fig. 5 shows the wedge type wire screen. Fig. 6 shows the sand & gravel sludge filter. Detailed description of the preferred embodiments
Referring to Fig. 1 it can be seen that the wet scrubbing system according to this invention comprises of a wet scrubber 2 of typically about 12 M height & 5 M dia in which flue gases 1 enter at the bottom and pass through holes in a sieve tray 3 (as shown in Fig. 2) where water column is present to allow bubbling of the flue gases and trapping of the paniculate matter. The slurry thus produced overflows through a weir plate (as shown in Fig. 2) down into the slurry tank 6.
The flue gases which are now relieved of the paniculate matter but laden with water particles now travel upwards to pass through a mist eliminator 4 (as shown in Fig. 3) and then through a centrifugal demister 5 (as shown in Fig. 4). The flue gases which are now free from paniculate matter as well as from water droplets are sucked by the fan 8 and thrown out through the flue gases chimney 9, in the form of a clear stack 10.
The other stream i.e. the slurry collecting in slurry tank 6 is pumped through a slurry pump 11 onto a wedge type screen filter 12 (shown in Fig. 5) from where dewatered sludge 17 is removed and clear water sent to clarifier tank 13 from where clear water is decanted down to clear water tank 15 and the enriched sludge is sent to a sand & gravel sludge filter 14 (as shown in Fig. 6). At this sand & gravel sludge filter 14, clear water is filtered down onto the clear water tank 15, pumped back to the sieve tray 3 with help of recirculation pump 16 and the dewatered sludge 17 removed from top.
Fig. 2 shows one example of the sieve tray(s) 3 which is 6 mm thick and of 4.5-5M dia, fixed inside the wet scrubber 2 having holes'18 of 6-8 mm dia numbering about 10,000 total where water is supplied by the clear water recirculation pump 16. Frothing slurry moves down to the slurry tank over the weir plate 19 which is of about 100mm height.
Fig. 3 shows one example of the mist eliminator 4 fixed inside of the wet scrubber 2 having a plurality of baffle plates 20 arranged in a disable stage.
Fig. 4 shows one example of the centrifugal demister 5 having curved blades 21 which render whirling motion to any of the escaping water droplets and ensuring their settling down instead of escaping.
Fig. 5 shows one example of the wedge wire screen filter 12 which has a screen 22 having wedge type holes of 0.75 mm opening on the receiving side and 1.25 mm opening on the discharge side from where dewatered sludge 17 is removed and the clear water taken to clarifier tank.
Fig. 6 show one example of the sand & gravel sludge filter 14 where sludge from the clarifier tank is received, first on layers of gravel 23 which are so layered that first layer of about 50mm thickness having gravels of 10-15mm size, second layer of about 50mm thickness having gravels of 20-30 mm size, third layer of about 100mm thickness having gravels of 50-63 mm thickness and a final layer of 100mm thickness having gravels of 100-150 mm size, supported on a perforated plate. Next is the layer of sand 24 which is about 1000 mm thick. While the dewatered sludge removal 17 is done at the top of the first layer of gravel 23, clear water from the sand 24 is taken to the clear water tank 15.

I claim:
1. A wet scrubbing system useful for removal of particulate matter from flue gases
generated by industrial boilers fired with biomass fuels, comprising a wet scrubber
containing a sieve tray having a plurality of holes, a two stage mist separation system
comprising a mist eliminator and a centrifugal demister and a combination of devices
consisting of wedge wire type screen filter, a clarifier tank and sand & gravel sludge
filter for recovering water from slurry, recirculating it and removing sludge.
2. A wet scrubbing system as claimed in claim 1 wherein the sieve tray(s) comprising of a
plurality of holes with such aperture and pitch that allow particulate matter in the flue
gases to get trapped in a water column in a process of bubbling.
3. A wet scrubbing system as claimed in claim 1 wherein the mist separation system
comprises of a mist eliminator and a centrifugal demister.
4. A wet scrubbing system as claimed in claim 3 wherein the mist eliminator comprises of
a plurality of baffle plates arranged in a single or double stage.
5. A wet scrubbing system as claimed in claim 3 wherein the centrifugal demister
comprises curved blades which render whirling motion to water droplets.
6. A wet scrubbing system substantially as herein described with reference to the
accompanying drawing.