Removal of Sulfur-Containing Gases

Since the time when English kings recognized that the burning of coal could produce noxious fumes (Chapters 14 and 15), there has been a series of attempts (not always continuous) to mitigate the amounts of noxious gases entering the atmosphere, the least of which have been attempts to reduce the amount of sulfur oxide(s) (particularly sulfur dioxide) released to the environment. 

Historically, the first method for removing sulfur dioxide from flue gases consisted of simple water scrubbing of the flue gas to absorb sulfur dioxide into solution (Plumley, 1971), and the method was first used in London in during the 1930s. Since then, various regulatory organizations in many countries have set standards for sulfur dioxide anissions, which must be met immediately, or in the very near future. 

Sulfur dioxide represents a high percentage of the sulfur oxide pollutants generated in combustion. The removal of the sulfur dioxide from the combustion gases before they are released to the stack is essential, and a considerable number of procedures exist for flue gas desulfurization (FGD). These procedures may be classified as wet or dry (Mokhatab et al., 2006 Speight, 2007, 2008) depending on whether a water mixture is used to absorb the sulfur dioxide or whether the acceptor is dry. 

The majority of the stack gas scrubbing processes are designed to remove sulfur dioxide from the gas streams some processes show the potential for the removal of NOx. However, there is the current line of thinking that pursues the options that enable SOx and NOx to be controlled, at least as far as possible, by modification of the combustion process. Sulfur (as already noted) can be removed by injecting limestone, with the coal into a boiler while modifications of the combustion chamber, as well as methods of flame temperature regulation and techniques that lower combustion temperatures, such as injection of steam into the combustion region are claimed to reduce emissions of NOx. 

In a wet limestone process, the flue gas contacts a limestone slurry in a scrubber  

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