Sulfur dioxide removal processes

As worldwide attention has been focused on the dangers of acid rain, the demand to reduce sulfur dioxide [7446-09-5] emissions has risen. Several processes have been developed to remove and recover sulfur dioxide. Sulfur can be recovered from sulfur dioxide as Hquid sulfur dioxide, sulfuric acid, or elemental sulfur. As for the case of hydrogen sulfide, sulfur dioxide removal processes are categorized as adsorption, absorption, or conversion processes. 

An additional benefit of adsorption-based sulfur dioxide removal processes is that nitrogen oxides, NO, are also removed by the sorbent. Nitrogen oxides desorb when the sorbent is heated using hot air. 

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. 

Process Alternatives. Sulfur dioxide removal processes can be categorized as throwaway or recovery. Throwaway processes produce a liquid or solid waste that requires disposal. Recovery processes convert the sulfur dioxide to elemental sulfur or sulfuric acid. Throwaway processes have been used in most utility applications, but there could be greater incentives for using the recovery processes in industry. 

Westvaco has, over the past six years, developed a sulfur dioxide removal process using activated carbon that operates after the usual low... 

Tihe aqueous carbonate process (ACP) has been under development at Atomics International for the last 4% yr. The program aims to establish a technology which eliminates or minimizes the major problems encountered in operating most other sulfur dioxide removal processes. That technology includes the use of sodium carbonate as the scrubbant in the modified spray dryer and the complete regeneration of the sulfur dioxide removal products to recover elemental sulfur and produce sodium carbonate for reuse in the spray dryer-scrubber. 

In conclusion, this second-generation sulfur dioxide removal process is now ready for full-scale installation. It promises to solve many of the... 

T he idea to use the various forms of coal to remove sulfur dioxide is not new and was described in an English patent as early as 1879 (I). However, massive research and development programs to develop commercially viable sulfur dioxide removal processes were not initiated until 80 years later, when ecological considerations forced public concern. 

Environmental regulations are the driving force behind the need for and selection of FGD systems and dictate many design criteria. For example, they limit the amounts of the pollutants which can be discharged to the atmosphere and to any waterway. They also place limits on the concentration of toxic metals and other chemicals in landfilled byproduct, which can significantly affect the FGD process selection. Landfill material characteristics, such as leachate composition, permeability, and con >ressive strength and the availability of a suitable landfill site can also be important. Expected future regulations on traces of toxic substances and fine particulate may also affect the selection of a sulfur dioxide removal process. 

The indusion or exclusion of denitrification capability does not alter the sulfur dioxide removal process significantly, although the equipment configuration must be different to accommodate the denitrification equipment. The primary difference is the addition of a selective catalytic reactor (SCR) using ammonia to reduce the NOx ahead of the SO2 catalyst (Collins etal., 1991). 

The catalytic oxidation of sulfur compounds in gas streams is the basis for numerous gas purification proce,s.ses, most of which are covered in other sections of this book. The oxidation of SO2 to SOj as part of a sulfur dioxide removal process is discussed in Chapter 7. The oxidation of HiS to elemental sulfur by the Claus process and related technologies is covered in Chapter 8. The liquid-phase oxidation of H2S to elemental sulfur is described in Chapter 9, and the oxidation of H S and other sulfur compounds in dry boxes and in other sulfur scavenging processes is included in Chapter 16. An area of technology that is not covered... 

Regenerable absorption processes have also been developed. In these processes, the solvent releases the sulfur dioxide in a regenerator and then is reused in the absorber. The WelLman-Lord process is typical of a regenerable process. Figure 11 illustrates the process flow scheme. Sulfur dioxide removal efficiency is from 95—98%. The gas is prescmbbed with water, then contacts a sodium sulfite solution in an absorber. The sulfur dioxide is absorbed into solution by the following reaction ... 

Liade AG offers the Clintox process for sulfur dioxide removal. This process uses a physical solvent to absorb the sulfur dioxide. A concentrated sulfur dioxide stream is produced by regeneration. The Clintox process can be iategrated with the Claus process by recovering sulfur dioxide from the iaciaerated tail gases and recycling the sulfur dioxide to the front of the Claus unit. 

Solvent dewaxing removes wax from lubricating oil stocks, promoting crystallization of the wax. Solvents include furfural, phenol, cresylic acid-propane (DuoSol), liquid sulfur dioxide (Eleleanu process), B,B-dichloroethyl ether, methyl ethyl ketone, nitrobenzene, and sulfur-benzene. The process yields de-oiled waxes, wax-free lubricating oils, aromatics, and recovered solvents. 

In an effort to avoid discharge of scrubber effluent into the Thames River, Imperial Chemical Industries, Ltd., British Power Authority consultants, and the Howden Construction Company together developed the ICI Howden process, a closed-loop, lime-based system for the removal of sulfur dioxide. This process was installed at the Swansea Power Plant in 1935 and at the Fulham Power Plant in 1937. Both systems operated successfully until their shutdown during World War II. 

Process Experience of the RC/Bahco Sulfur Dioxide Removal System... 

A third emission reduction choice is to remain with the existing front end process, which continues to produce a sulfur dioxide-containing waste gas stream, and move to some system which can effectively remove the sulfur dioxide from this waste gas before it is discharged. Many methods are available, each with features which may make one more attractive than the others for the specific sulfur dioxide removal requirements (Table 3.8). Some of the selection factors to be considered are the waste gas volumes and sulfur dioxide concentrations which have to be treated and the degree of sulfur dioxide removal required. It should be remembered that the trend is toward a continued decrease in allowable discharges. The type of sulfur dioxide capture product which is produced by the process and the overall cost are also factors. Any by-product credit which may be available to offset process costs could also influence the decision. Finally, the type of treated gas discharge required for the operation (i.e., warm or ambient temperature, moist or dry, etc.), also has to be taken into account. Chemical details of the processes of Table 3.8 are outlined below. 

The sulfone is a versatile functional group comparable to the carbonyl functionality in its ability to activate molecules for further bond construction, the main difference between these two groups being that the sulfone is usually removed once the synthetic objective is achieved. The removal most commonly involves a reductive desulfonylation process with either replacement of the sulfone by hydrogen (Eq. 1), or a process that results in the formation of a carbon-carbon multiple bond when a P-functionalized sulfone, for example a (3-hydroxy or (3-alkoxy sulfone, is employed (Eq. 2). These types of reactions are the Julia-Lythgoe or Julia-Paris-Kocienski olefination processes. Alkylative desulfonylation (substitution of the sulfone by an alkyl group, Eq. 3), oxidative desulfonylation (Eq. 4), and substitution of the sulfone by a nucleophile (nucleophilic displacement, Eq. 5) are also known. Finally, p-eliminations (Eq. 6) or sulfur dioxide extrusion processes (Eqs. 7, 8 and 9) have become very popular for the... 

Research-Cottrell realized that the industrial and utility markets required different types of sulfur dioxide removal equipment. Therefore, it contracted with Ab Bahco Ventilation of Sweden to market their sulfur dioxide removal technology in the U.S. and Canada. Bahco technology is particularly applicable to industrial boiler and process applications (7, 8, 9). The first U.S. Bahco installation will handle seven stoker-type boilers at the Rickenbacker Air Force Base in Columbus, Ohio. 

Feeding solutions from the absorber system and the regeneration system through surge tanks enables the entire recovery process to operate smoothly and reliably despite frequent gas flow and concentration fluctuations. In addition, the surge tanks allow the regeneration section to be shut down for up to 3 days without interfering with the sulfur dioxide removal in the absorption section. This is possible because the absorber is the only part of the system that contacts the flue gas and removes the sulfur dioxide. 

Regenerative Aqueous Carbonate Process for Utility and Industrial Sulfur Dioxide Removal... 

It is the most common FGD process—the flue gas is treated with limestone slurry for the sulfur dioxide removal. Approximately 95% of the SOj from the flue gas can be eliminated. The method can be used for medium-to high-sulfur coals. A slurry waste or a saleable slurry by-product is obtained. 

---