Scrubbing processes sulfur emissions

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. 

J. D. Brady, Flue Gas Scrubbing Process for Sulfur Dioxide and Particulate Emissions Preceding CO2 Absorption, Enviromental Progress, Vol. 6, No. 1, Feh. 1987. 

Particulate emissions from zinc processing are collected in baghouses or ESPs. SO2 in high concentrations is passed directly to an acid plant for production of sulfuric acid by the contact process. Low-concentration SO2 streams are scrubbed with an aqueous ammonia solution. The resulting ammonium sulfate is processed to the crystalline form and marketed as fertilizer. 

Ultimately, pollution can only be avoided by complete removal of SO2 from the effluent gases, but this council of perfection is both technologically and economically unattainable. Many processes are available to reduce the SO2 concentration to very low figures, but the vast scale of power generation and domestic heating by coal and oil still results in substantial emission. SO2 can be removed by scrubbing with a slurry of milk of lime , CafOH) . Alternatively, partial reduction to H2S using natural gas (CH4), naphtlia or coal, followed by catalytic conversion to elemental sulfur by the Claus process can be used ... 

The detrimental effects of acid rain are a major reason why legislation such as the Clean Air Act places strict limitations on sulfur and nitrogen emissions. It is also a reason why low sulfur coal is preferred over high sulfur coal. To reduce sulfur dioxide emissions, industry also uses a technique call scrubbing. Industrial scrubbers employ a variety of physical and chemical processes to remove sulfur dioxide from emissions. Another technique used to combat acidification of lakes is to treat these systems with lime. The lime acts to neutralize the acid, but such techniques are usually costly and are only a temporary remedy for combating the problem. 

The sulfate process has traditionally used batch ore digestion, in which concentrated sulfuric acid is reacted with ilmenite. This reaction is very violent and causes the entrainment of sulfur oxides (SOA) and sulfuric acid in large amounts of water vapor. In an effort to reduce the particulate emissions, scrubbers have been installed at most plants, but these, in turn, have necessitated the treatment of large quantities of scrubbing liquid before discharge. Other waste-disposal problem products are spent sulfuric acid and copperas (FeS04-7H20). 

Rare earths in SOx and NOx abatement additives for FCC. Sulfur oxides can be formed in the regenerator due to the oxidation of the sulfur compounds contained in the coke. There are four options to abate SOx emissions in FCC units Processing low-sulfur feeds (S < 0.3%), hydrodesulfurization of the feed, flue gas scrubbing, and the use of SOx additives. 

In the direct neutralization process, ammonia is distributed using a perforated pipe (sparger) positioned beneath the bed of material in the granulator. If sulfuric acid is used, it too is usually distributed through a sparger beneath the bed of material whereas the phosphoric acid, if used, is most often sprayed or dribbled on top of the bed of material. When sulfuric acid is used, precautions should be taken to ensure that the acid is added at a particular location with respect to the ammonia to ensure quick and uniform neutralization and thus minimize the unwanted reaction of sulfuric acid with muriate of potash (MOP) that is usually present in most NPK formulations. This acid/MOP reaction causes the formation of very corrosive hydrochloric acid, which reacts rapidly with ammonia to form a dense fume of ammonium chloride that is very difficult and costly to collect in the plant s emission control (scrubbing) system. 

To prevent the emission of SO2 into the atmosphere, industrial processes scrub the gas to remove the SO2. The exhaust gas flows through stacks lined with calcium carbonate that, when heated, capture sulfur oxides in the form of calcium sulfate dust. 

---