Flue Gas Desulfurization FGD systems were originally designed to remove sulfur dioxide from the flue gas generated by the boiler.  In the early days of FGD, the particulate-laden flue gas from the boiler was treated in the venturi, prior to treatment in the FGD tower.  Over time, the venturi was replaced with the ESP, and now baghouse for particulate control, and FGD has evolved to absorber tower alone. 

These large mass-transfer devices were designed to transfer the mass of sulfur dioxide from the flue gas to the liquid phase, and subsequently provide aqueous bisulfite/sulfite ions.  On the other side of the mass transfer, generally alkaline-earth metals such as calcium, magnesium and/or the alkali sodium provide the cation for sulfite/bisulfite precipitation.  

Many of the newest FGD units no longer create a sludge to be de-watered and treated, but a gypsum by-product used by wallboard companies, cement manufacturers, and even for agricultural use.

Typical WFGD Absorber Module, with large, flared slurry recirculation tank on the bottom of the
tower, and multiple spray headers to achieve the very-high sulfur dioxide removal efficiency that is typical of FGD systems today.  Nominal 500MW Module – Side-View

With the passage of the MATS rule, the WFGD was suddenly drawn into the role, at many plants, for the removal of mercury from the coal-generated flue gas.  Not only does the FGD have to remain effective at controlling sulfur dioxide, it needs to take on the added task of mercury suppression.  Add to this the burden of cycling load impact, which is a common denominator among many US plants, if not most, of the coal-fired plants still in operation today, and it has become a difficult assignment at many plants. 

Please see our mercury page to learn more.

To learn more about the WFGD absorber and how cycling load effects mass transfer, please click here.

Dry FGD

Dry FGD takes on many forms. This is a typical industrial arrangement for SO2/Particluate control with a dry scrubber followed by a fabric filter.


Dry FGD (dry scrubbers) provide some distinct advantages over wet scrubbers, not the least of which is the use of less water and a dry product for disposal. Years ago, Steve Feeney led the start-up of a 450MW dry FGD system at the Craig Station in Craig, Colorado.

Many features of dry scrubbers have changed since those early days, and the use of circulating fluid bed dry systems, and Dry Sorbent Injection DSI have become more prevalent.

We have experience with the different types of dry systems, both semi-wet and fully dry, and can provide our expertise to investigate your system, and provide our engineering evaluation for improvements in operation and performance.