Gaseous pollutants removal systems

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

One of the methods of controlling air pollution mentioned in the previous chapter was pollution removal. For pollution removal to be accomplished, the polluted carrier gas must pass through a control device or system, which collects or destroys the pollutant and releases the cleaned carrier gas to the atmosphere. The control device or system selected must be specific for the pollutant of concern. If the pollutant is an aerosol, the device used will, in most cases, be different from the one used for a gaseous pollutant. If the aerosol is a dry solid, a different device must be used than for liquid droplets. 

Gaseous pollutants may be easier or harder to remove from the carrying gas stream than aerosols, depending on the individual situation. The gases may be reactive to other chemicals, and this property can be used to collect them. Of course, any separation system relying on differences in inertial... 

Scrubbers and absorbers Wet systems used for the removal of aerosols and other gaseous pollutants from an airstream. 

One way to control gaseous pollutants like SO2 and SO3 is to remove the gases from fuel exhaust systems by absorption into a liquid solution or by adsorption onto a solid material. Absorption involves dissolving the gas in a liquid while adsorption is a surface phenomenon. In each case, a subsequent chemical reaction can occur to further trap the pollutant. Lime and limestone are two solid materials that effectively attract sulfur dioxide gas to their surfaces. The ensuing chemical reaction converts the gaseous pollutant to a solid nontoxic substance that can be collected and disposed or used in another industry. 

Figure 14 Incineration system flow diagram. Waste is incinerated in the presence of air and supplemental fuel the incinerator can be multiple hearth, fluidized bed, liquid injection, rotary kiln, or other types caustic or lime scrubbers are used to remove gaseous pollutants from exhaust gases (from Ref. 11).

Diesel particulate filters (DPF), which remove particulate from the exhaust stream. They can also be coated with catalyst, e.g. DOC, LNT or SCR, to enable removal of gaseous pollutants in the same system component. 

Such a combustion system is characterized by relatively low emissions of gaseous pollutants. Because fuel is fed in very small pieces, however, the particle content in the flue gas is relatively high. To meet particle emission regulations in most countries, the use of a flue-gas cleaning is necessary. Therefore such systems are often equipped at least with a cyclone, but often this is not sufficient. In some cases, therefore, a fabric or bag house filter or even an electrostatic dust removal system is also used. Such combustion units are characterized by fully automatic operation and low emissions at thermal capacities of several MW up to some 10 and more MW and are mainly located at places where wood dust is produced as a residue, such as the furniture industry. 

Gaseous pollutants form part of all natural waters. They include not only inert or noble gases, nitrogen above all, which do not create problems in industrial applications, but also considerable quantities of dissolved oxygen and carbon dioxide which have to be removed in order to forestall soious damage to equipment and piping systems. 

The removal of particulate matter from gas streams is the most widely practiced means of air pollution control and is very important in sustaining the quality of the atmosphere. A number of devices that differ widely in effectiveness, complexity, and cost have been developed for this purpose. The selection of a particle removal system for a gaseous waste stream depends on the particle loading, nature of particles (size distribution), and type of gas scrubbing system used. 

The dry product is primarily collected in cyclone collectors (a few bag houses still remain), sieved, and finally packaged in moisture barrier containers. The exit air from the dryer often has to be treated to meet local pollution control laws. While many of the older dryers use gas incineration, as energy costs have increased these incineration systems have become quite costly to operate. New dryer installations use scrubbing systems (e.g., aqueous/chemical sprays) to remove entrained solids and gaseous volatile flavors. 

However, activated carbons are the most extensively applied industrial adsorbents for the removal of pollutants from gaseous and aqueous and nonaqueous streams, because of their exceptionally powerful adsorption properties and their readily modifiable surface chemistry [217,218], Carbon is the primarily applied adsorbent in the case of liquid-solid adsorption systems. 

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