Vorsana

Flue Gas Desulfurization (FGD)

Coal contains some sulfur, which becomes sulfur dioxide (SO₂) and sulfur trioxide (SO₃) when the coal is burned to produce electric power. Collectively, these sulfur compounds are known as SOx. About 95% of SOx is sulfur dioxide.

Although sulfur dioxide is only a tiny fraction of the volume of flue gas (very much less than 1%), flue gas desulfurization by scrubbing is necessary because SOx combines with water in the atmosphere to form sulfuric acid, which falls as acid rain. Acid rain has devastated forests and oceans, leading to very strict emission controls (in developed, responsible countries).

Scrubbers remove sulfur dioxide by contacting the dispersed SO₂ molecules with a sorbent (lime or limestone). A chemical reaction captures the sulfur in the form of a harmless solid, which becomes the filler in wallboard, called gypsum. Wet scrubbing injects a sorbent solution; dry scrubbing injects a sorbent powder. Mixing to cause contact of the sorbent with the target molecules is typically done by spraying, flow through a packed bed, or blasting the sorbent into the gas through a nozzle (e.g. Venturi). See the excellent Wikipedia article on flue gas desulfurization.

Removal efficiency for wet SOx scrubbing ranges from 50 - 98%. The price paid for high removal efficiency is a large volume of dilute wastewater containing sulfuric acid and requiring storage and treatment with a consequent waste of space and resources. A lagoon of toxic wastewater is not a welcome neighbor anywhere. Dry SOx scrubbers have a lower removal efficiency, <80%, but do not create a wastewater problem. Low concentration of SOx in a high volume of nitrogen means that a large amount of scrubbing solution must be injected in order to contact enough of the sparse scrubbing targets to achieve satisfactory removal efficiency.

Good mixing (high turbulence) improves collection efficiency and reduces the amount of sorbent that needs to be injected. Extracting the carbon dioxide, which is a reaction product and might cause a backward reaction, keeps the sorbent reaction going forward. Extraction of the nitrogen in flue gas (which is approximately 75% of the volume) would greatly improve collection efficiency by improving the contact of sorbent with sulfur dioxide, but there is presently no known way to extract the nitrogen ballast.

Sulfur trioxide becomes sulfuric acid mist in the smokestack and in the wastewater from wet scrubbing of flue gas. Removing sulfur trioxide from flue gas before this can happen is very difficult.

At Vorsana, we have a way to extract the nitrogen ballast so as to concentrate the scrubbing targets and reduce the volume of scrubbing wastewater. In the same device, very highly turbulent scrubbing improves contact, and carbon dioxide is extracted from the reaction zone continuously, driving the forward sorbent reaction. Suspended solids are dynamically thickened into a sludge.