Sulfur oxides removal from flue gases

Technological interest during these 30 years has focused on automotive air pollution and its control, on sulfur oxide pollution and its control by sulfur oxide removal from flue gases and fuel desulfurization, and on control of nitrogen oxides produced in combustion processes. 

Ebara [Electron beam ammonia reaction] A dry process for removing sulfur and nitrogen oxides from flue-gas. A beam of high energy electrons is injected into the gas, to which a stoichiometric quantity of ammonia has been added. The product, a mixture of ammonium sulfate and ammonium nitrate, is collected downstream by an electrostatic precipitator or a bag filter. Developed by Ebara Corporation, Japan, and piloted in Indianapolis in 1986. 

The Shell flue gas desulfurization (SFGD) process described in 1971 [4] removes sulfur oxides from flue gas in a PPR using a regenerable solid adsorbent (acceptor) containing finely dispersed copper oxide. At a temperature of about 400°C, sulfur dioxide reacts with copper oxide to form copper sulfate according to the reaction. 

A modification of the SFGD process just described is the Shell flue gas treating process, which not only removes sulfur oxides from flue gas, but can also effect a substantial reduction of the nitrogen oxides content. This is based on the activity of copper, whether in the oxidic or sulfate form, to catalyze the reaction of nitrogen oxides with ammonia according to the reactions... 

For example, in bench-scale tests when hot synthetic flue gas containing 2000 ppm SO2 was forced past a single melt spray nozzle at 25 ft/sec, sulfur oxide removal efficiencies of 97-100% were obtained at molar gas-to-liquid ratios of 0.01-0.30 little (if any) melt in the exit gas could be foimd. The oxidation of M2SO8 (Equation 3) does not appear to be very rapid at the scrubber temperature. For example, when synthetic flue gas containing 1 vol % O2 and 0.1 vol % SO2 was bubbled through molten carbonate eutectic at 450°C, only 18, 31, and 44 wt % of the sulfite (formed from the absorbed sulfur dioxide) were oxidized to sulfate in 4, 7, and 14 hrs, respectively. When the synthetic flue gas contained 5 vol %. O2 and 0.3 vol % SO2, 42, 49, and 55 wt % of the absorbed sulfur oxide were oxidized to sulfate at similar times. Neither water nor fly ash had any appreciable effect on the oxidation rate however, it is anticipated that NO absorption will increase oxidation of sulfite. 

Kasaoka et al. [102] prepared vanadia catalysts supported with titania, activated carbon, and a mixture of carbon and titania, as supports for the simultaneous removal of SO2 and NOv at temperatures ranging from 400 to 425 K. The vanadia on titania catalyst was most appropriate. SO2 from flue gas is oxidized to SO3 and forms sulfuric acid. Ammonia reacts with sulfuric acid forming (NH4)2S04 and NH4HSO4. The catalysts were regenerated with water after treating the catalysts with gaseous ammonia to neutralize the acid sites on the catalyst. 

Electrostatic Mist Precipitators. The gas leaving the scrubbers is essentially free of halogens and dust but it still contains acid mist. The amount of acid mist depends primarily on conditions in the smelter. In gas from copper converters, the sulfur trioxide content may vary from 2 to 10% of the total sulfur oxides. The amount of sulfur trioxide formed depends largely on the temperature and time the gas contacts the iron oxide in the dust and the scale on the carbon steel flues. The sulfur trioxide combines with the moisture in the gas to form sulfuric acid vapor. When the gas is cooled in the scrubbers, most of this vapor condenses as a finely divided acid mist, although some of it is absorbed in the scrubber liquor. Sulfuric acid mist, which is generally considered to be particles less than 5 is very difficult to remove from a gas stream, so only a portion of the mist will be removed in the scrubber. If the remaining mist were allowed to enter the contact section of the acid plant it would corrode the carbon steel ducts and heat exchangers and the main blower. It must, therefore, be removed as completely as possible in the purification section of the plant. This is accomplished in the electrostatic mist precipitators. 

Each of the coal properties interacts in a significant way with generation technology to affect performance. For example, higher-sulfur content reduces efficiency of pulverized coal combustion (PCC) due to the added ena gy consumption and operating costs to remove sulfur oxides (SOx) from the flue gas. 

Tsuchiai. H.. et al.. Removal of sulfur dioxide from flue gas by the absorbent prepared from coal ash Effects of nitrogen oxide and water vapor. Ind. Eng. Chem. Res.. 35(3). 851-855 (1996). 

Recovery systems in which sulfur dioxide or elemental sulfur is removed from the spent sorbing material, which is recycled, are much more desirable from an environmental and sustainability viewpoint than are throwaway systems. Many kinds of recovery processes have been investigated, including those that involve scrubbing with magnesium oxide slurry, sodium sulfite solution, ammonia solution, or sodium citrate solution. One type of recovery system uses a solution of sodium sulfite to react with sulfur dioxide in the flue gas... 

The zinc oxide process was developed by Johnstone and Singh (1940) at the University of Illinois. Although development was utility sponsored, the process has not been used commercially. However, a considerable amount of pilot-plant work was conducted, and features of the process design were worked out in considerable detail. The process is illustrated in Figure 7-17. The flue gas is contacted with a solution of sodium sulfite and bisulfite and sulfur dioxide is absorbed, thus causing an increase in bisulfite content. The solution is next passed into a clarifier, in which particulate matter removed from the gas stream is separated, and finally into a mixer in which it is treated with zinc oxide. At this point, the original ratio of sulfite to bisulfite is restored, and zinc sulfite is precipitated in accordance with the following reactions ... 

Minor and potential new uses for ammonium thiosulfate include flue-gas desulfurization (76,77), removal of nitrogen oxides and sulfur dioxide from flue gases (78,79), converting sulfur ia hydrocarbons to a water-soluble form (80), and converting cellulose to hydrocarbons (81,82) (see Sulfur REMOVAL AND RECOVERY). 

Metal Oxide - Since metals are less electrophilic than silicon, metal oxide adsorbents show even stronger selectivity for polar molecules than do siliceous materials. The most commonly used metal oxide adsorbent is activated alumina, used primarily for gas drying. Occasionally, metal oxides find applications in specific chemisorption systems. For example, several processes are under development utilizing lime or limestone for removal of sulfur oxides from flue gases. Activated aluminas have surface areas in the range of 200 to 1,000 ftVft Average pore diameters range from about 30 to 80 A. 

Sulfur dioxide removal processes can be used to treat flue gas from industrial boilers, heaters, or other process gases where sulfur compounds are oxidized. These processes have generally been proven in utility applications. More recently, several industrial SO2 removal installations have been completed. 

Both lime and slaked limes are use to reduce sulfur emissions, which contribute to acid precipitation, from power plants, particularly coal-fired plants. By using lime, more than 95% of the sulfur can be eliminated from the emissions. Calcium oxide reacts with sulfur dioxide to produce calcium sulfite CaOfe) + S02( —> CaS03(). Sulfur dioxide is also removed by spraying limewater in the flue gas. Limewater, also called milk of lime, is a fine suspension of calcium hydroxide in water. Other pollutants removed with lime include sulfur trioxide, hydrofluoric acid, and hydrochloric acid. 

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