Sulfur dioxide removal processes selection

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). 

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 control of both SO2 and NO, concentrations in flue gas by a single process has obvious advantages over the use of two separate processes, and considerable effoit has gone into attempts to develop an economical combined NO,/SO, control process. Unfortunately, no combined process has yet achieved widespread acceptance, although numerous approaches have been proposed and a few have reached commercial status. Combined NO,/SO, control processes that have been used commercially, or are believed to be in an advanced stage of development, are described in Chapter 7, Sulfur Dioxide Removal. To avoid repetition, this section is limited to comparative evaluations of available combined processes and very brief descriptions of selected technologies. 

Wine and beer industry Polyphenols can alter color and flavor of products such as wines. There are many aggressive ways of removing polyphenolic compounds, such as using polyvinylpolypyrrolidone (PVPP) or sulfur dioxide. However, polyphenol removal should be selective to avoid the undesirable alteration of the wine s organoleptic characteristics. For this reason, one option is to use laccases that polymerize the polyphenolic compounds during the wine-making process and then to remove these polymers by clarification (Morozova and others 2007). Several papers have reported that laccase is able to remove undesirable polyphenols and produce stable wines with a good flavor. 

UCAP A process for selectively removing residual sulfur dioxide from the tail gas from the Claus process. It had not been commercialized by 1983. 

It is also doubtful that the industry will be in a position for many years to come to undertake sulfur removal from residual fuels solely to improve product quality. A number of consumer industries demand low sulfur fuel oils, but these special requirements can at present be met more appropriately by selection of crude rather than by adoption of desulfurization processes. In general industrial use, it is corrosion and atmospheric pollution that are the main disadvantages of high sulfur content. But there is no sign yet of the development of a cheap desulfurization process, the cost of which can be substantially offset by the gain in efficiency resulting from permissible lower stack temperatures or by the elimination of flue gas scrubbing equipment previously necessary for reduction of sulfur dioxide content. 

On the industrial scene, the most prominent applications both in scale and number are seen in the petroleum industry. Liquid extraction is used here to separate petroleum fractions selectively and to purify or otherwise refine them. In the Edeleanu process, which is close to a century old, liquid sulfur dioxide is used to extract aromatics from various feedstocks. The removal of the ever-present sulfur compounds is accomplished by extraction with sodium hydroxide solutions. In addition, a wide range of organic solvents is used in the purification and refining of various lubricants. 

Activated Coke or Char. Processes based on active forms of carbon resemble the copper oxide processes in that they remove NO, by selective catalytic reduction with ammonia using the sorbent as a catalyst. However, the mechanism for SO2 removal is entirely different. Sulfur dioxide is adsorbed on the active carbon, which also acts as a catalyst for the oxidation of adsorbed SO2 to SO3. In the presence of moisture, the sulfur trioxide forms sulfuric acid on the char. Regeneration of the sulfuric acid-laden char is accomplished in a separate vessel where the sorbent is heated to about 400°C. At this temperature, the sulfuric acid reacts with a portion of the carbon, forming a gas phase containing sulfur dioxide, carbon dioxide, moisture, and various impurities. This gas stream is further processed to produce sulfuric acid or elemental sulfur, and the remaining char is recycled, with makeup, to the contactor. Several variations of the basic process are discussed in Chapter 7 under the broad heading Adsorption Processes. ... 

Sulfur Compounds. Various gas streams are treated by molecular sieves to remove sulfur contaminants. In the desulfurization of wellhead natural gas, the unit is designed to remove sulfur compounds selectively, but not carbon dioxide, which would occur in Hquid scmbbing processes. Molecular sieve treatment offers advantages over Hquid scmbbing processes in reduced equipment size because the acid gas load is smaller in production economics because there is no gas shrinkage (leaving CO2 in the residue gas) and in the fact that the gas is also fliUy dehydrated, alleviating the need for downstream dehydration. 

Gas purification processes fall into three categories the removal of gaseous impurities, the removal of particulate impurities, and ultrafine cleaning. The extra expense of the last process is only justified by the nature of the subsequent operations or the need to produce a pure gas stream. Because there are many variables in gas treating, several factors must be considered (/) the types and concentrations of contaminants in the gas (2) the degree of contaminant removal desired (J) the selectivity of acid gas removal required (4) the temperature, pressure, volume, and composition of the gas to be processed (5) the carbon dioxide-to-hydrogen sulfide ratio in the gas and (6) the desirabiUty of sulfur recovery on account of process economics or environmental issues. 

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