Flue-gas scrubbing is a process for removing oxides of sulfur and nitrogen from the waste gases emitted by various industrial processes. Since the oxides of sulfur and nitrogen have been implicated in a number of health and environmental problems, controlling them is an important issue. The basic principle of scrubbing is that flue gases are forced through a system of baffles within a smokestack. The baffles contain some chemical or chemicals that remove pollutants from these gases.
A number of scrubbing processes are available, all of which depend on the reaction between the oxide and some other chemical to produce a harmless compound that can then be removed from the smokestack. For example, currently the most common scrubbing reaction involves the reaction between sulfur dioxide and lime. In the first step of this process, limestone is heated to produce lime. The lime then reacts with sulfur dioxide in flue gases to form calcium sulfite, which can be removed with electrostatic precipitation .
Many other scrubbing reactions have been investigated. For example, magnesium oxide can be used in place of calcium oxide in the scrubber. The advantage of this reaction is that the magnesium sulfite that is formed decomposes readily when heated. The magnesium oxide that is regenerated can then be reused in the scrubber while the sulfur dioxide can be used to make sulfuric acid . In yet another process, a mixture of sodium citrate and citric acid is used in the scrubber. When sulfur dioxide is absorbed by the mixture, a reaction occurs in which elemental sulfur is precipitated out.
Although the limestone/lime process is by far the most popular scrubbing reaction, it has one serious disadvantage. The end product, calcium sulfite, is a solid that must be disposed of in some way. Solid waste disposal is already a serious problem in many areas, so adding to that problem is not desirable. For that reason, reactions such as those involving magnesium oxide, sodium citrate and citric acid have been carefully studied. The products of these reactions, sulfuric acid and elemental sulfur, are valuable raw materials that can be sold and used. In spite of that fact, the limestone/lime scrubbing process, or some variation of it, remains the most popular method of extracting sulfur dioxide from flue gases today.
Scrubbing to remove nitrogen oxides is much less effective. In principle, reactions like those used with sulfur dioxide are possible. For example, experiments have been conducted in which ammonia or ozone is used in the scrubber to react with and remove oxides of nitrogen. But such methods have had relatively little success and are rarely used by industry.
Flue gas scrubbing has long met with resistance from utilities and industries. For one thing, they are not convinced that oxides of sulfur and nitrogen are as dangerous as environmentalists sometimes claim. In addition, they argue that the cost of installing scrubbers is often too great to justify their use.
[David E. Newton ]
American Chemical Society. Cleaning Our Environment: A Chemical Perspective. 2nd edition. Washington, DC: American Chemical Society, 1978.
Bretz, E. A. "Efficient Scrubbing Begins With Proper Lime Prep, Handling." Electrical World 205 (March 1991): 21–22.
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"Scrubbers, Low-Sulfur Coal, of Plant Retirements?" Electrical World 204 (June 1990): 18.