Flue-gas desulfurization
Since stringent environmental regulations limiting SO2 emissions have been enacted in many countries, SO2 is being removed from flue gases by a variety of methods.
Common methods used: For a typical coal-fired power station, flue-gas desulfurization (FGD) may remove 90 per cent or more of the SO2 in the flue gases.
With the construction of large-scale power plants in England in the 1920s, the problems associated with large volumes of SO2 from a single site began to concern the public.
These three early large-scale FGD installations were suspended during World War II, because the characteristic white vapour plumes would have aided location finding by enemy aircraft.
This revised standard (PTC 40-2017) covers Dry and Regenerable FGD systems and provides a more detailed Uncertainty Analysis section.
If a scrubber system is not functioning properly (and the IMO procedures for such malfunctions are not adhered to), port States can sanction the ship.
Sulfuric acid mist is often the cause of the blue haze that often appears as the flue gas plume dissipates.
In wet scrubbing systems, the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a baghouse, and then into the SO2-absorber.
Another important design consideration associated with wet FGD systems is that the flue gas exiting the absorber is saturated with water and still contains some SO2.
The reaction taking place in wet scrubbing using a CaCO3 (limestone) slurry produces calcium sulfite (CaSO3) and may be expressed in the simplified dry form as: Wet scrubbing can be conducted with a Ca(OH)2 (hydrated lime) and Mg(OH)2: To partially offset the cost of the FGD installation, some designs, particularly dry sorbent injection systems, further oxidize the CaSO3 (calcium sulfite) to produce marketable CaSO4·2H2O (gypsum) that can be of high enough quality to use in wallboard and other products.
The process by which this synthetic gypsum is created is also known as forced oxidation: A natural alkaline usable to absorb SO2 is seawater.
The surplus of H+ is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO2 gas: In industry caustic soda (NaOH) is often used to scrub SO2, producing sodium sulfite:[13] To promote maximum gas–liquid surface area and residence time, a number of wet scrubber designs have been used, including spray towers, venturis, plate towers, and mobile packed beds.
Because of scale buildup, plugging, or erosion, which affect FGD dependability and absorber efficiency, the trend is to use simple scrubbers such as spray towers instead of more complicated ones.
The configuration of the tower may be vertical or horizontal, and flue gas can flow concurrently, countercurrently, or crosscurrently with respect to the liquid.
FGD scrubbers produce a scaling wastewater that requires treatment to meet U.S. federal discharge regulations.
[14] However, technological advancements in ion-exchange membranes and electrodialysis systems has enabled high-efficiency treatment of FGD wastewater to meet EPA discharge limits.
In fact, many of the industrial sodium-based throwaway systems are venturi scrubbers originally designed to remove particulate matter.
The high speed of a venturi would cause erosion problems, while a packed tower would plug up if it tried to circulate a slurry.
Counter-current packed towers are infrequently used because they have a tendency to become plugged by collected particles or to scale when lime or limestone scrubbing slurries are used.
Lime is typically used on large coal- or oil-fired boilers as found in power plants, as it is very much less expensive than caustic soda.
Fortunately, calcium sulfite can be oxidized to produce by-product gypsum (CaSO4·2H2O) which is marketable for use in the building products industry.
Caustic soda is limited to smaller combustion units because it is more expensive than lime, but it has the advantage that it forms a solution rather than a slurry.
The Chendu power plant in China started up such a flue gas desulfurization unit on a 100 MW scale in 1998.
FGD has been fitted by RWE npower at Aberthaw Power Station in south Wales using the seawater process and works successfully on the 1,580 MW plant.
Safety is one of the greatest benefits of this method, as the whole process takes place at atmospheric pressure and ambient temperature.
