Removal sulfur dioxide

It is generally unacceptable to emit sulfur dioxide, thus the scmbber effluent must be treated for sulfur dioxide removal. If the plant aheady possesses faciUties for the production of sulfuric acid, this rather concentrated sulfur dioxide stream can be easily fed into the wet gas cleaning circuit and disposed of in the sulfuric acid plant. The quantity is so small that it does not put any additional burden on the sulfuric acid plant. Because no tellurium is carried over with the selenium dioxide during roasting, it is possible to produce a selenium product which can be purified to commercial grade (99.5-99.7%). 

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. 

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. 

Figure 19. Variable venturi scrubber for flyash and sulfur dioxide removal.

While the use of low-sulfur fuels is one mechanism to reduce sulfur dioxide emission, alternatively most approaches focus on scrubbing or ridding the emissions in smoke stacks of sulfur dioxide gas. A number of different types of scrubbers, i.e., sulfur dioxide removal systems, are available for industry. One system sprays the flue gas into a liquid solution of sodium hydroxide. The hydroxide combines with SO2 and O2 to form the corresponding sulfate which can be removed from the aqueous solution ... 

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. 

Find the fraction of sulfur dioxide removed from a gas stream under the following conditions ... 

The presence of any wet scrubbers in the sulfur dioxide removal systems. 

Scrubbing Efficiency. The effect of the stoichiometric ratio of total alkali fed to the scrubber [Ca(0H)2 plus NaOH] to total acid gas (SO2 plus HC1) is shown in Figure 2. The theoretical limit for reaction of the alkali with the acid gases is indicated in the figure. (Actually, some removal of HC1 can be expected with no alkali present.) Sulfur dioxide removal efficiencies were found to exceed 99 percent when alkali/acid stoichiometric ratios were greater than about 1.9. HC1 removal efficiencies generally exceed SO2 removal efficiencies at any given alkali/ acid stoichiometric ratio. 

Fabric filters are the preferred collection equipment as additional sulfur dioxide removal takes place in the baghouse Cl,2) Typically, under conditions such that 80% of total inlet SO2 is removed, 60 to 70% of the removal takes place in the spray dryer with 10 to 20% additional removal taking place in the bag filters... 

The combustion of sulfur-rich char is accompanied by the production of an undesirable reaction product, viz., sulfur dioxide. However, most of the sulfur dioxide should be removed from the combustion gases before they leave the combustor. This may be accomplished by the introduction into the combustor of suitable additives which can absorb sulfur dioxide. Limestone is such an additive. The limestone reacts with sulfur dioxide in the presence of oxygen to form calcium sulfate, which is a solid product and can be easily removed from the reactor. In this work, a model is proposed for the prediction of sulfur dioxide removal from the combustion gases, based on knowledge of gas-solid reactions taking place on a single pellet. 

Figure 1 shows the three possible locations of the SCR unit inunediately after the boiler (high-dust arrangement, HD), after the particulate removal (low-dust arrangement, LD), and after the sulfur dioxide removal (tail-end arrangement, TE) [20]. 

Table 2 Sulfur Dioxide Removal Performance of SFGD plant at Showa Yokkaichi Sekiyu Refinery...

Hydrogen sulhde and sulfur dioxide removal were studied using HFLM technique for coal gasification apphcations. References of the works carried out in the field of gas separations are presented in Table 13.10. 

Green, G., Installation and Initial Operation of Sulfur Dioxide Removal... 

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

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

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. 

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