Sorption
Removal of SOx by wet scrubbing
Limestone based wet flue gas desulphurization (FGD) is a highly efficient absorption process and best suited for larger quantities of flue gas coming from cement and power plants.
Pollutants such as high contaminant loads of sulphur compounds (SO2, SO3, H2S) and further acid components such as halogens (F2, Cl2, Br2, I2) and their hydrogenated derivatives are removed. The limestone scrubbers have proved to be the best solution and produces gypsum which can used as dewatered sludge with a residual moisture of abt. 10 %. The scrubber itself is a type of spray tower without internals and is operated in counter current flow Limestone scrubbers can be made of stainless steel, FRP or rubber-lined / coated carbon steel.
This absorption technology is increasingly applied in cement mills, where the raw meal is used as absorbent.
Nowadays, even small off-gas volumes with high SO2 concentrations such as from metal smelters are cleaned in limestone scrubbers, if there is no downstream sulfuric acid plant.
The most common sorbents are hydrated lime (Ca[OH]2) , limestone (CaCO3) and caustic soda (NaOH).
The reaction taking place in wet scrubbing using a limestone/hydrated lime slurry produces calcium sulfite (CaSO3) which may further react with oxygen to produce gypsum (CaSO4) as a final reaction product.
Caustic soda is limited to smaller installations due to its higher cost in comparison with lime, but it has the advantage of forming a solution rather than a slurry.
Main types of scrubbers used for wet FGD process
Venturi-scrubbers
Venturi Scrubbers can be used for simultaneous removal of SO2 and fly ash. In fact, many of the industrial sodium-based throwaway systems are venturi scrubbers originally designed to remove particulate matter. These units were slightly modified to inject a sodium-based scrubbing liquor. Although removal of both particles and SO2 in one vessel can be economic, the problems of high pressure drops and finding a scrubbing medium to remove heavy loadings of fly ash must be considered. However, in cases where the particle concentration is low, such as from oil-fired units, it can be more effective to remove particulate and SO2 simultaneously.
Key features:
- Excellent SOx removal efficiency.
- High reliability.
- Insensitive to high-volume dust loads in the raw gas.
- Adjustable throat allows large variation in flow rates
Limestone Scrubbers
The flue gas is led through a scrubber where limestone slurry is added via spraying. The limestone reacts with the SOx in the flue gas and its products are removed from the gas. A following demister stage removes water droplets. This scrubber is a type of spray tower without internals and is operated in counter current. The pollutant concentration of the gas to be treated determines the number of nozzle levels accommodated in the scrubber. The limestone scrubber produces gypsum which can be used in cement plants or to fabricate plasterboards.
Key features:
- Excellent SOx removal efficiency.
- Creating a product used in building materials industry
- High reliability.
- Insensitive to high-volume dust loads in the raw gas.
- Reduced risk of solid build-ups.
Caustic soda scrubber
Caustic soda (NaOH) is injected in counter-current scrubbing zones into the flue gas to achieve removal of SOx.
The liquid droplets fall down and the remaining solution is collected on the bottom while fine droplets carried with the gas flow are removed by a wet vertical ESP section.
Key features:
- Very High SO2 removal with Na2CO3 or NaOH
- Combination of an Electrostatic Precipitator and a Spray-Absorber in one casing →
EP-Absorber - Low pressure drop
- Stack on top — compact design
Packed column scrubbers
A packed column scrubber is applicable when a caustic soda solution is used as absorbent since there is no danger of build-up as there would be with solid calcium compounds.
In a packed column scrubber, the gas is treated passing through one or two stages of packing and SO2 reacts with the caustic soda to form sodium sulfate. The Absorption efficiency depends on the intensity of the contact between gas and scrubbing liquid, which is ensured by structured or piled polypropylene packings.
Key features:
- Excellent SOx removal efficiency.
- High reliability.
- Insensitive to high-volume dust loads in the raw gas.
- Reduced risk of solid build-ups.
- Typically, Packed Towers operate at much lower pressure drops than Venturi scrubbers and are therefore less costly to operate. They also typically offer a higher SO2 removal efficiency.
Working Principle
Working Principle: Removal of SO2 by wet scrubbingFor the wet desulphurization of flue gas, a scrubbing liquid containing limestone, hydrated lime or caustic substances is recirculating and injected to the exhaust where SO2 is absorbed in the liquid and finally reacts e. g. to Gypsum.
Simultaneously with the absorption, the flue gas is saturated with water vapor.
The reagent is fed to the sump of the scrubber unit and intermittent dewatering is used to drain the gypsum.
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