Emissions, scrubbing

Optimized modern dry scrubbing systems for incinerator gas cleaning are much more effective (and expensive) than their counterparts used so far for utility boiler flue gas cleaning. Brinckman and Maresca [ASME Med. Waste Symp. (1992)] describe the use of dry hydrated lime or sodium bicarbonate injection followed by membrane filtration as preferred treatment technology for control of acid gas and particulate matter emissions from modular medical waste incinerators, which have especially high dioxin emissions. 

Particulate emissions from zinc processing are collected in baghouses or ESPs. SO2 in high concentrations is passed directly to an acid plant for production of sulfuric acid by the contact process. Low-concentration SO2 streams are scrubbed with an aqueous ammonia solution. The resulting ammonium sulfate is processed to the crystalline form and marketed as fertilizer. 

Wet scrubbers rely on a liquid spray to remove dust particles from a gas stream. They are primarily used to remove gaseous emissions, with particulate control a secondary function. The major types are venturi scrubbers, jet (fume) scrubbers, and spray towers or chambers. Venturi scrubbers consume large quantities of scrubbing liquid (such as water) and electric power and incur high pressure drops. Jet or fume scrubbers rely on the kinetic energy of the liquid stream. The typical removal efficiency of a jet or fume scrubber (for particles 10 g. or less) is lower than that of a venturi scrubber. Spray towers can handle larger gas flows with minimal pressure drop and are therefore often used as precoolers. Because wet scrubbers may contribute to corrosion, removal of water from the effluent gas of the scrubbers may be necessary. 

Contaminated solid wastes are generally incinerated, and the flue gases are scrubbed. The emissions levels cited in Table 4 are those recommended by the World Bank Organization that should be achieved. 

Figure 12. Air emissions from sludge water generated from scrubbing blast furnace off-gases.

Environmental impacts. Discharges to atmosphere (particulates and other toxic or noxious emissions), surface waters (scrubbing water), and land (furnace residues) may require extensive treatment to assure protection of the environment. 

A gaseous emission is to be treated for the removal of ammonia. Table 9.4 provides the stream data. Two scrubbing agents are considered for the removal of ammonia, water, Si, and an inm anic solvent, S2. The absorption of ammonia in water is coupled with the following chemical reaction ... 

Ultimately, pollution can only be avoided by complete removal of SO2 from the effluent gases, but this council of perfection is both technologically and economically unattainable. Many processes are available to reduce the SO2 concentration to very low figures, but the vast scale of power generation and domestic heating by coal and oil still results in substantial emission. SO2 can be removed by scrubbing with a slurry of milk of lime , CafOH) . Alternatively, partial reduction to H2S using natural gas (CH4), naphtlia or coal, followed by catalytic conversion to elemental sulfur by the Claus process can be used ... 

Cunic, J. D., Diener, R, and Ellis, E. G., Exxon Research and Engineering, Scrubbing—Best Demonstrated Technology for FCC Emission Control. presented at NPRA Annual Meeting. San Antonio, Texas, 1990. 

Sulfur dioxide emissions from power plants can be reduced by spraying a water solution of calcium hydroxide directly into the smokestack. This "scrubbing" operation brings about the reaction... 

Waste treatment prior to disposal may introduce phase changes which result in quite different pollution control considerations. For example, the gases generated by incineration of a solid waste can be scrubbed with liquid in order to meet an acceptable discharge criterion hence, in addition to ash for disposal, a liquid effluent stream is produced and requires treatment. Other waste treatment processes may result in the liberation of flammable or toxic gaseous emissions as exemplified in Table 16.5. 

In the blast furnace, the reaction of the nitrogen in the blast with coke leads to the formation of poisonous chemicals such as hydrogen cyanide and cyanogens, and each cubic meter of the blast furnace gas contains from 200 to 2000 mg of these compounds. The blast furnace gas is scrubbed with water in the dust collection system the cyanide compounds dissolve in the water, which is then discharged after the compounds have been destroyed. Another poisonous emission in blast furnace operations is hydrogen sulfide. The sulfur present in the coke is converted into calcium sulfide in the slag, the water-quenching of... 

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 ... 

Dispersion, Flaring, Scrubbing, and Containment An example of an overpressure protection system designed to reduce emissions to the atmosphere and at the same time provide adequate protection to the equipment has been described [234]. The equipment indicated is used for the manufacture of ethylene-vinyl acetate-vinyl chloride polymer emulsions. The design pressures are up to 100 bar. 

Ventilation Local exhaust Mandatory. Must be filtered or scrubbed to limit exit concentrations to <0.00001 mg/m3. Air emissions will meet local, state, and federal regulations. 

Vent gases containing EDC vapour, not only from the storage and transport of EDC, but also from the EDC unit itself, must be controlled because of EDC toxicity and other reasons. Conventionally these vent emissions are controlled using thermal oxidation, where the EDC vapour is converted to carbon dioxide, water, and hydrogen chloride. The latter is then scrubbed from the flue gases, resulting in a byproduct hydrochloric acid stream. Consideration is required as to how this stream can be used or disposed of. 

The chlorine dioxide emission totals following this scrubbing procedure are very low, and result from the use of low concentrations of hydrogen peroxide. The advantages are that the peak loads of chlorine can be controlled more easily and the amounts of hydrogen peroxide needed (and therefore its cost) are relatively small. 

Present theories of the origin of acid rain indicate that we can limit acid rain by reducing sulfur dioxide emissions and moving to low-sulfur fuels but, only about 20% of the world s petroleum reserves are low in sulfur. Switching U.S. midwestern power plants to low-sulfur coal could cause economic problems since much of the coal from the Midwest and Appalachia has a high sulfur content. Most of the electric power generated in the Midwest uses high-sulfur coal and it would cost tens of billions of dollars to scrub the sulfur out of coal. 

Reducing the amount of C02 in the atmosphere could involve prescrubbing to take the carbon out of fuels before combustion, leaving only hydrogen to be burned. Another approach is postcombustion scrubbing which removes C02 from the emissions stream after burning. 

The background for the development of VK69 was a need for reduction of S02 emissions from double-absorption plants by installing a more active catalyst at low temperature downstream from the intermediate absorption tower. Clearly, the catalytic solution should be more competitive than the alternatives, e.g. tail gas scrubbing or triple-absorption layout, in terms of capital and operating costs. In the following, the required technical performance of the catalyst with respect to S02 oxidation activity, mechanical strength and pressure drop is discussed, and input from the literature and from practical experience in the field is presented. Reviews of the extensive literature published on sulphuric acid catalysts can be found in [2-5],... 

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