Sulfur dioxide capture

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. 

Similar acid-base chemistry is involved in the American Smelting and Refining Company s (ASARCO s) sulfur dioxide capture process using aqueous dimethylaniline (Eq. 3.25). 

Sulfur dioxide capture and subsequent reduction to a sulfur product has also been tested. The American Smelting and Refining Company s process involves combustion of the cooled and cleaned smelter gases with methane (Fig. 13.10 Eq. 13.38). 

FIGURE 13.1 I The citrate process for sulfur dioxide capture and conversion to elemental sulfur developed by the U.S. Bureau of Mines. Details available from U.S. Bureau of Mines [40]. 

Fluidized-bed combustors can be either atmospheric or pressurized (Yeager and Preston, 1986). The atmospheric type operates at normal atmospheric pressure while the pressurized type operates at pressures 6-16 times higher than normal atmospheric pressure. The pressurized fluid-bed boiler offers a higher efficiency and less waste products than the atmospheric fluid-bed boiler. There is also a circulating (entrained) bed combustor which allows for finer coal feed, better fuel mixing, higher efficiency, as weU as an increased sulfur dioxide capture. 

Newton, G.H. Chen, S.L.. and Kramlich, J.C., Role of porosity loss in limiting sulfur dioxide capture by calcium-based sorbents, AIChE J., 35(6), 988-994 (1989). 

Ghosh-Dastidar, A., et al. Investigation of high-reactivity calcium carbonate sorbent for enhanced sulfur dioxide capture, Ind. Eng. Chem. Res., 35(2), 598-606 (1996). 

Formation of emissions from fluidised-bed combustion is considerably different from that associated with grate-fired systems. Flyash generation is a design parameter, and typically >90% of all soHds are removed from the system as flyash. SO2 and HCl are controlled by reactions with calcium in the bed, where the lime-stone fed to the bed first calcines to CaO and CO2, and then the lime reacts with sulfur dioxide and oxygen, or with hydrogen chloride, to form calcium sulfate and calcium chloride, respectively. SO2 and HCl capture rates of 70—90% are readily achieved with fluidi2ed beds. The limestone in the bed plus the very low combustion temperatures inhibit conversion of fuel N to NO. ... 

At atmospheric pressure, calcium carbonate almost completely calcines to free lime, and it is this that captures the sulfur dioxide. As the free lime is not completely sulfated, the resulting sorbent ash is veiy alkaline, consisting primarily of CaS04 and CaO, with small amounts of CaCO,3. 

Smelting releases sulfur dioxide gas from a metal sulfide ore. Before the twentieth century, most of the sulfur dioxide expelled in the smelting process poured out of the factory s smokestacks directly into the atmosphere. Sulfur dioxide in the atmosphere, however, is a powerful greenhouse gas. Today, most of the hot sulfur dioxide gas released in the smelting process is captured, cooled, cleaned, and converted into sulfur trioxide ... 

In the Wellman-Lord process, sodium sulfite is used to capture the sulfur dioxide. The sodium bisulfite thus formed is later heated to evolve sulfur dioxide and regenerate the sulfite scrubbing material. The sulfur dioxide-rich product stream can be compressed or liquefied and oxidized to sulfuric acid, or reduced to sulfur. 

High fly ash resistivity may be overcome by introducing polar chemicals which adsorb themselves on the fly ash surface and reduce surface resistivity. Howard ( .) had reported in 1918 that particulate capturability was excellent in those flue gases containing high concentrations of sulfur trioxide. This polar molecule was proven to be the species responsible for reducing surface resistivity. The unusual characteristic of western coal is its low sulfur content (approx. 0.5% S). When burned, sulfur is converted to sulfur dioxide and a portion of the dioxide is fully oxidized to the trioxide. 

The ability of lime to sorb AS4O6 increases as temperatures increase between 400 and 1000 °C (Li et al., 2007). The presence of either sulfur dioxide (SO2) or carbon dioxide (CO2) gases did not substantially interfere with the ability of lime to capture AS4O6. CaSCL is also capable of sorbing at least some AS4O6 (Li et al., 2007). 

ClausMaster A physical absorption system for capturing residual sulfur dioxide from the Claus process. Developed by Monsanto, now MECS (Monsanto Enviro-Chem Systems). 

At elevated pressure, the partial pressure of carbon dioxide inhibits calcination, and sulfur dioxide is captured by displacement of the carbonate radical. The overall effect is similar except, as no free lime is formed, the resulting sorbent ash is less alkaline, consisting solely of CaS04 and CaCOs. 

Another important use of lime is in pollution control. Many factories release harmful gases into the atmosphere through smokestacks. Lining a smokestack with lime allows some of these gases to be captured. The lime is known as a scrubber. Lime captures one harmful gas, sulfur dioxide (SO2), which is a contributor to acid rain (a form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes) ... 

Any sulfur trioxide forms sulfate which is involatile on acidification, unlike sulfite. Capture of the released sulfur dioxide in a fresh aqueous base, followed by titration with standard iodine solution, then gives the concentration of sulfur dioxide present in this second solution (Eq. 2.13, 2.14). 

The origins of two of the major sulfur dioxide discharges in Canada are combustion sources and sour gas plants, which produce waste gas streams which contain 0.15-0.50% sulfur dioxide. These are too low to be economically attractive for containment. Smelter sources produce roaster exhaust gas streams containing 2-5% sulfur dioxide, or with modern equipment up to about 15%, concentrations which are economically favorable for capture of the sulfur... 

Or the sulfur dioxide may be reduced catalytically with methane or other hydrocarbons to hydrogen sulfide. The hydrogen sulfide produced by this method is captured by amine scrubbing of the reduced gas stream (Eqs. 3.18-3.20). 

The Wellman-Lord process uses the effective sodium sulfite-sodium bisulfite equilibrium to capture sulfur dioxide from flue gases [40] (Eqs. 3.26 and 3.27). 

The citrate process, in which much development work has been invested by the U.S. Bureau of Mines and by Pfizer, Inc., uses an aqueous solution of citric acid to capture sulfur dioxide (Eqs. 3.28 and 3.29). 

Variables affecting this process have recently been reviewed [44]. With either reagent, however, a throwaway product is obtained. Land has to be allocated for lagoon disposal of the spent scrubber slurries, or other systems have to be set up to handle the waste solid. Recent variations of this approach are to employ a zeolite prepared from fly ash [45] or the alkalinity of fly ash itself in water slurry as means to capture sulfur dioxide. 

The alkaline lime, in a gas-solid phase reaction, reacts with sulfur dioxide in the combustion gases to form solid particles of calcium sulfite and calcium sulfate which are captured in electrostatic precipitators (Eqs. 3.39 and 3.40). 

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