Cleaning Gaseous Pollutants

An overview of several technologies is presented in this section. Each of these technologies are point source controls for handling gaseous pollutants. They are all well-established methods of gas cleaning, some based on many decades of industry practice. 

The technology is primarily applicable to the removal of inorganic fumes, vapors, and gases (e.g., chromic acid, hydrogen sulfide, ammonia, chlorides, fluorides, and SOj) volatile organic compounds (VOC) and particulate matter (PM), including PM less than or equal to 10 micrometers ( m) in aerodynamic diameter (PM,q), PM less than or equal to 2.5 m in aerodynamic diameter (PMj 5), and hazardous air pollutants (HAP) in particulate form (PM ap)- 

Absorption is widely used as a raw material and/or product recovery technique in separation and purification of gaseous streams containing high concentrations of VOC, especially water-soluble compounds such as methanol, ethanol, isopropanol, butanol, acetone, and formaldehyde. Hydrophobic VOC can be absorbed using an amphiphilic block copolymer dissolved in water. However, as an emission control 

Inorganic Gases Control device vendors estimate that removal efficiencies range from 95 to 99 %. 

VOC Removal efficiencies for gas absorbers vary for each pollutant-solvent system and with the type of absorber used. Most absorbers have removal efficiencies in excess of 90 percent, and packed-tower absorbers may achieve efficiencies greater than 99 percent for some pollutant-solvent systems. The typical collection efficiency range is from 70 to greater than 99%. 

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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. 

Wet scrubbers have found widespread use in cleaning contaminated gas streams because of their ability to effeetively remove both particulate and gaseous pollutants. Specifically, wet scrubbing describes the technique of bringing a contaminated gas stream into intimate contact with a liquid. The types most widely used for particulate control are spray towers, packed-bed units, ionizing wet scrubbers, and venturi scrubbers. 

Gaseous hydrogen peroxide is a key component and product of the earth s lower atmospheric photochemical reactions, in both clean and polluted atmospheres. Atmospheric hydrogen peroxide is believed to be generated exclusively by gas-phase photochemical reactions (lARC, 1985). Low concentrations of hydrogen peroxide have been measured in the gas-phase and in cloud water in the United States (United States National Library of Medicine, 1998). It has been found in rain and surface water, in human and plant tissues, in foods and beverages and in bacteria (lARC, 1985). 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. It might also be mentioned at this time that the exhaust (cleaned gas) from an absorption air pollution control system is usually released to the atmosphere through a stack. To prevent condensation in and around the stack, the temperature of this exhaust gas should be above its dew point. A general rule of thumb is to ensure that the exhaust gas stream temperature is approximately 50°F above its dew point. 

Robinson, E. and Moser, C. E 1971 Global gaseous pollutant emission and removal mechanism. Proc. 2nd Internal. Clean Air Cong., Washington, 1097 1101. 

The risks for environment alterations could be prevented and reduced by clean motors, with a drastic reduction of gaseous pollutants. The lead problem will be progressively resolved by the new European standards of lead addition to fuels but the lead already present in soils will remain a threat for some sensitive crops and forages. 

Like many original methods of mechanical process technology, an old technique for the successful removal of airborne dust was first observed in nature. The capacity of rain to clear the air has been used since ancient times to remove suspended dust particles by passing the contaminated gas through a water spray (wet scrubbers). The liquid droplets capture the solid (and some of the gaseous) pollutants and collect them in a sump. While this reduces air pollution it transfers part of the separation problem to a secondary cleaning process the removal of fine particulate solids from a liquid. 

Such a combustion system is characterized by relatively low emissions of gaseous pollutants (such as NOx or CO). Because fuel is fed in very small pieces, however, the particle content in the flue gas is relatively high. To meet particle emission regulations, cleaning of the flue-gas is necessary in most countries. Therefore such systems are often equipped at least with a cyclone or... 

Provide for supply of clean and unpolluted lubricants and propulsion fuels to propulsion machinery and other plant. Filters, coalescer-filters, etc., may be necessary to remove solid, liquid, or gaseous pollutants. 

The terms synthetic fuel and alternative fuel are closely related, but not truly synonymous. Synthetic fuel was coined as a term in the middle of the twentieth century to include gaseous, liquid and solid fuels produced from coal, oil shale, tar sands, and biomass. Alternative fuel appeared as a popular phrase during the latter years of the twentieth century which referred to fuels that were not totally based on petroleum and held potential as clean-burning, low-polluting, commercially viable fuel resources. 

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