How Flue Gas Desulfurization (FGD) Works

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Learn how flue gas desulfurization (FGD) works! We use an interactive 3D model to show you all of a flue gas desulfurizer’s main components and how it works.
This video focuses on the wet (not dry) method of desulphurisation.
Note that desulfurization (American English) may also be spelt desulphurisation (British English), but its meaning is the same!
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▶️Introduction
Fossil fuel power stations and other industrial plants that burn fossil fuels, require some form of flue gas cleaning in order to comply with ever more stringent environmental legislation. There are several parts to a flue gas cleaning system, these may include electrostatic precipitators (ESP), bag houses, forced draft fans, selective catalytic reduction (SCR) equipment and flue gas desulphurisation equipment.
Flue gas desulphurisation (FGD), is a process that removes sulphur oxides (SOx) from an exhaust gas stream. The most common sulphur oxides are sulphur dioxides (SO2) (note that sulphur is also spelt sulfur).
Chemical reactions between an alkaline reagent (lime, limestone, ammonia etc.) and the flue gas, convert the sulphur dioxide gas into calcium sulphite. Calcium sulphite can then be oxidised to form calcium sulphate.
How it Works
An alkaline reagent slurry is pumped from the base of the absorber tower to spray headers. The slurry is sprayed into the absorber tower through spray nozzles, and falls due to gravity down to the tower basin (effluent holding tank area).
Flue gas enters the tower and flows upwards. As it flows upwards, it comes into direct contact with the alkaline reagent slurry. The slurry converts the sulphur dioxide gas into a calcium based solid waste (calcium sulphite or calcium sulphate).
Calcium sulphite is represented by the chemical notation of CaSO3; calcium sulphate is CaSO4.
Both calcium sulphite and sulphate can be sold as by-products. It is possible to oxidise the calcium sulphite in the slurry using compressed air, this then yields calcium sulphate. Another name for calcium sulphate is ‘gypsum’, and it can be used for many applications, but its use in wallboard is the most common.
Gypsum is extracted and sold, usually by passing through a thickener (similar to a primary clarifier design), hydro cyclone, vacuum filters, or a combination of all these components.
A mist eliminator is used to reduce the amount of carry over from the absorber tower to the exhaust stack.
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