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Flue Gas Desulfurization

Dateline: 03/22/99

By Alan Bruzel

Commercial production of energy without concomitant injury to the environment remains an unrealized dream. Presently, pollution-minimizing strategies are implemented during the selection of the raw materials ("Do I use uranium, fossil fuels, or falling water?"), while others are at work after extraction of the energy ("How do I reduce the environmental impact of my process?").

One "end-of-pipe" methodology, known as flue gas desulfurization (FGD), is available to coal- or oil-fired power plants, and allows conversion of sulfur oxides (formerly emitted from smokestacks) into marketable commodities useful to industry. This process begins with either electrostatic precipitation or fabric filters removing fly ash from the combustion gases. The fly ash is then carted away. The flue gases are forced into a slurry of lime and water (also known as slaked lime, or calcium hydroxide, Ca(OH)₂) under oxidizing conditions provided by compressed air. The following reactions take place:

SO₂ + H₂OH⁺ + HSO₃^–

H⁺ + HSO₃^– + 1/2O₂2H⁺ + SO₄^2–

2H⁺ + SO₄^2– + Ca(OH)₂CaSO₄^.2H₂O

The acid rain-causing sulfur dioxide (SO₂) goes in, the construction product calcium sulfate dihydrate, CaSO₄^.2H₂O (also known as gypsum, plasterboard, or wallboard) – at about 98% purity – comes out.

This is a successful process, quite in step with the requirements mandated by the US Clean Air Act of 1990, as it results in a 95% reduction of SO₂ from the combustion waste gases. However, this is also an expensive process. FGD at Northern Indiana Public Service Company's Bailly Generating Station cost about $152,000,000. FGD is also an energy-requiring process, requiring up to 4% of an energy generator's capacity. (That power plant next door is convenient.) Because a FGD facility is able to generate hundreds of thousands of tons of gypsum in one year, gypsum production currently exceeds demand. As a result, many energy utilities are opting to use lower sulfur content fuels or natural gas rather than FGD. More imaginative uses of gypsum, such as in remediation of acid mine drainage or in reduction of aluminum in soils of strip-mined areas, may act as a spur toward further implementation of this innovative pollution management technique.

Recommended Web resources for additional information:

Ashes to Ashes, Dust to Building Material
Going from flue gas to wallboard. From Carla Helfferich, University of Alaska at Fairbanks.

Battelle Report: China Needs Energy Technology
Summary of economies realized with cleaner electrical production.

Fueling Reform: Energy Technologies for the Former East Bloc
A brief from the Office of Technology Assessment, US Congress.

Lime: Calcium Oxide -- CaO
Chemical of the Week from Bassam Z. Shakhashiri, University of Wisconsin at Madison.

Macroscopic to Microscopic Studies of Flue Gas Desulfurization By-Products for Acid Mine Drainage Mitigation
Neutralization potential of FGD solids. From US Geological Survey.

Outline of Flue Gas Desulfurization Technology
Overview from the New Energy and Industrial Technology Development Organization.

Sustainable Development & Electricity Generation: Comparing Impacts of Waste Disposal
Disposal problems of power plants resulting from mining, production, and cleanup operations. Article by Roger Seitz, International Atomic Energy Agency.

Waste Products Used to Treat Aluminum-Rich Soils
Gypsum-magnesium treatment reduces aluminum toxicity to plants. Written by Kelli Whitlock, Ohio State University.

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