Every working day, millions of cubic feet of industrial air are cleansed of hundreds of tons of particulate and containment gases using wet scrubbers. These specially designed scrubbing systems prevent contaminants from attaining levels that may be injurious to people, plant & animal life, and the environment.

The contaminants that these systems would otherwise accumulate in our ecosystem, to be removed only at a far greater cost when dispersed than when concentrated at their source. A properly designed scrubbing system provides an important opportunity to control the contaminant effectively at a point of lowest cost and complexity.

Many fuels contain sulphur, which after combustion form hazardous SO₂. The low sulphur content fuels are expensive. The higher cost of low sulphur content (like HSD, LDO etc.) fuels is forcing the people to use cheaper fuels like various heavy oils. These contain large percentages of sulphur. The SO₂ produced after combustion is proportionately higher that endangers the environment. Hence for these applications, in addition to solid particulate removal, desulphurization of flue gases becomes an important aspect.

Desulphurization plants are installed to protect the environment. Industries consider it to be a liability and hence running cost should be kept as low as possible. The running cost of using heavy oils and then desulphurizing the flue gases is still lower than that of using light oils.

Various methods can be employed to carry out desulphurization of gases-

• The gas to be scrubbed is introduced in the venturi and scrubbed by alkaline liquid solution.
• The gas is accelerated in the venturi. The liquid is sprayed just above the throat of the venturi covering the entire gas path.
• The contaminants impact on the finely sprayed droplets and become heavier.
• The SO₂ reacts with the alkaline solution and gets consumed.
• The gas and liquid flow concurrently downwards into a cyclonic mist eliminator.
• The heavier contaminant particles and precipitated reacted particles are separated by centrifugal action in the cyclone.
• The clean gas leaves the cyclone from the top and is exhausted through I.D. Fan to chimney/stack.
• Back pressure control loop ensures constant pressure for smooth running of the equipment on which desulphurization system is installed.
• The I.D. Fan provides the necessary gas flow at designed pressure drop.
• The scrubbing liquid separated from cyclone is collected in the circulating tank.
• The pH of scrubbing liquid falls down as alkali is consumed.
• The pH control loop provided senses the pH continuously and intakes the fresh alkaline solution to maintain the pH.
• The water if evaporated by the hot gases is made up by tank level control loop.
• The liquid passes through filter where solids get separated and liquid is given to circulating pump.
• The circulating pump pumps the liquid to nozzle for atomization.

Venturi is the most important component of the system. The venturi has a converging, straight & diverging portion (refer figure 3). The gas is accelerated in the venturi. The liquid is sprayed just above the throat of the venturi covering the entire gas path. The contaminants impact on the finely sprayed droplets and become heavier.

The velocity of gas at venturi throat decides the pressure drop of the gas stream and the particle separation efficiency.

The venturi construction is simple to operate, easy to clean and has better particle separation efficiency at low pressure drop.

2. CYCLONE SEPARATOR :

The liquid & gas pass to cyclone type mist eliminator to separate the liquid from the gas. The liquid droplets along with solids and gas are introduced in the cyclone tangentially. The centrifugal forces on droplets throw them on cyclone wall and get separated. The clean gas pass out of the cyclone.

Advantages of cyclone are-

• Low pressure drop.
• Simple in construction.
• Easy to clean.
• High particle separating efficiency. 

3. I. D. FAN

The I. D. Fan sucks the gas through the system and leaves it to chimney. It takes care of the pressure drop in the system.

Salient Features:

• Designed for the required duty point.
• High efficiency – upto 80%.
• Tested for performance.
• Tested for vibrations as per IS 1940.
• Tested for rise in bearing temperature.
• The bearing block is single piece machined component – alignment & rigidity.
• Drain plug for condensate removal.
• Access door for inspection/cleaning.
• Damper at outlet to control airflow rate.
• Manometer tapping at inlet & outlet.
• Direct driven & belt driven construction.
• Guard for belt – safe installation.
• Rigid base frame and supports – vibration free performance.

4. CIRCULATION TANK

• It holds the liquid to be circulated for scrubbing and receives the liquid from the cyclone.
• The tank size has to be adequate to avoid frequent change of liquid.
• The solids separated from gas are collected in the tank that increases its concentration in the liquid. Increase in solids concentration beyond design concentration adversely affects the spray characteristics.
• If the gas to be scrubbed is at higher temperature, part of scrubbing liquid will evaporate and lost. This should be compensated by topping up the circulation tank intermittently.
• Drain is provided to clean the tank.
• The circulation tank has automatic liquid level control. Level is continuously monitored and controlled through a solenoid valve fitted on incoming fresh liquid line. Isolation valves provided facilitate maintenance of solenoid valve while system is running. Liquid level indicator is also provided separately.
• Over flow system is incorporated to prevent the spillage of the liquid in case of exigency. It also removes the suspended matter or foam from the circulating tank.

5. SPRAY SYSTEM

This consists of inlet filters, circulation pump, valves, pipeline, pressure gauges & switches, orifice type flow meter, and nozzle assembly.

SELF CLEANING FILTERS:

• These are provided to separate the solids from liquid to avoid chocking of the pump.
• A stand-by filter with isolating valves is provided to give uninterrupted operation during cleaning.
• The filter element is made of stainless steel mesh with back up perforated plate that can be replaced easily if damaged.
• Filters are provided with self-cleaning arrangement. Figure 4 shows the self-cleaning arrangement. When one filter is being used the other is being cleaned by keeping appropriate valves open or close.
• The self-cleaning action can be made automatic by providing motorized valves from A to H as shown in figure 2.

CIRCULATING PUMP:

• This is a centrifugal pump developing required pressure & flow for proper atomization.
• Standby pump is provided with pressure gauge & switch at the outlet. Pressure indicator is provided at the inlet that indicates chocking of the filter.
• Pressure gauge is provided at the outlet of the pump.
• Isolation valves are provided for both the pumps & filters. Needle valves are provided for pressure gauges and switches.

ORIFICE TYPE FLOW METER:

• This monitors the liquid flow-rate continuously. If the flow-rate drops below the set level pressure switch connected across the orifice plate gives an alarm. This avoids dry running of the pump.
• Pressure switch across orifice plate gives alarm if the flow rate falls below designed flow rate. 

NOZZLE ASSEMBLY:

• This is a narrow angle, solid cone spray, and single fluid nozzle. It is used to form fine spray at the throat of venturi.
• The nozzle position is kept adjustable to select the suitable position of nozzle covering the entire throat.
• The nozzle is made of stainless steel.

6. pH CONTROL LOOP :

• The pH of the circulating alkaline solution falls down as the SO₂ in the gas reacts and get consumed. To maintain the reactivity of the solution, the pH of solution has to be maintained.
• The pH of the solution in the circulating tank is continuously sensed. If the pH drops down, fresh alkaline solution is taken in.
• The alkali solution is transferred to the circulating tank by pump. To maintain the correct pH metered quantity of solution should be pumped. The pH control loop through variable speed drive alters the speed of pump thereby metering the pumping solution.

7. BACK PRESSURE CONTROL LOOP :

• For smooth working of any equipment, back pressure cannot be exceeded beyond a particular limit. Hence continuos pressure monitoring system is provided at inlet of the desulphurization plant.
• Pressure transmitter senses the pressure continuously and gives signal to motorized damper valve. The proportionate movement of damper maintains the pressure constant.

8. ALKALI SERVICE TANK :

• This acts as a day tank for alkali solution. The alkali solution can be prepared by dissolving the alkali in solid form in water. The tank is provided with a stirrer to facilitate the solution preparation.
• The tank is provided with top up facility for water or ready solution.

9. SIEVE TANK :