Pressing Onward: A Dewatering Blog

What is Flue Gas Desulfurization and Where Do Filter Presses Fit In?

By Michael Stock

Sulfur dioxide (SO2). While it might sound like just another harmless chemical, it actually wreaks havoc in the environment causing acid rain and sulfuric acid mist. Not only is it harmful to the environment, it is also harmful to human health, and as a result is regulated by the Clean Air Act.

So how is this chemical being released into the atmosphere? By the burning of fossil fuels. When fossil fuels are burned, about 95 percent of the sulfur is converted to sulfur dioxide (SO2). One way to combat SO2 emissions from power plants is through flue gas desulfurization.


What is flue gas desulfurization?

Flue gas desulfurization (FGD) is the technology used for removing sulfur dioxide (SO2) from the exhaust (or flue gases) produced by coal-fired power plants. A majority of FGD systems have two stages: one for Fly Ash removal and the other for SO2 removal. The second step uses equipment like scrubbers and filter presses (more on this later).



History of FGD

The flue gas desulfurization process, in a more primative form has been around for more than 150 years. The process was first used in England in 1850's. However, large-scale FGD units were not used in commercial operations in the United States until the 1970's

The rise in the number of FGD units was probably impacted by the passage of the Clean Air Act in 1963. The Clean Air Act was the first law in the United States intended to control air pollution.

Since 1963 the Clean Air Act has been amended impose stricter and more specific regulations, one such ammendment is Title IV of the 1990 amendments. Title IV set a goal of reducing annual SO2 emissions by 10 million tons below 1980 levels.

The Acid Rain Program (ARP), was also established under Title IV. The program requires emission reductions of chemicals known to contribue to acid rain, like sulfur dioxide (SO2) and nitrogen oxides (NOx). The regulations regarding SO2 set a cap on the total amount that can be emitted by power plants.

One way of meeting the SO2 levels outlined in Title IV is using the flue gas desulfurization process.


What role do filter presses play in the process?

As previously mentioned, there are two steps in the FGD process:

  • Step 1 - remove remove fly ash
  • Step 2 - remove SO2

In the second step the flue gas passes through some type of scrubbing system, most likely a wet scrubbing system. Wet scrubbing systems use an alkaline slurry of limestone pumped into a large chamber where the calcium in the limestone reacts with (and removes) the SO2 in the flue gas.

Wet scrubbing systems are preferred when the sulfur content of the coal used by the power plant is greater than 2%. According to EPA 85% of FGD systems in the U.S. are wet scrubbing systems, probably because wet systems have a 90% removal efficiency.


As the chart above shows, the FGD process follows these steps:

  1. scrubbing
  2. clarification
  3. sludge holding
  4. dewatering

Although dewatering (and filter presses) are used at the end of the FGD process they play just as big of a role in eliminating SO2 from flue gases.


Flue gas desulfurization is just one of the many processes where a filter press can be used. The extremely versatile J-Press® filter press by JWI™ can also be used to meet almost any dewatering needs. Download our FREE guide to learn more!

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