Add-On Environmental Control Methods

      There are four general processes used for emission control: (i) adsorption, (ii) absorption, (iii) catalytic oxidation, and (iv) thermal oxidation.

      Adsorption is a physico-chemical phenomenon in which the gas is concentrated on the surface of a solid or liquid. Subsequently, the captured gas can be desorbed with hot air or steam either for recovery or for thermal destruction. Usually, activated carbon is the adsorbing medium, which can be regenerated upon desorption. Adsorbers are widely used to concentrate a low gas concentration prior to incineration unless the gas concentration is high in the inlet airstream. Adsorption also is employed to reduce odors from gases which have potential odor problems. The only major limitation for an adsorption system is the requirement for minimization of particulate matter and/or condensation of liquids (e.g., water vapor) that could mask the adsorption surface and drastically reduce its efficiency.

      Absorption differs from adsorption in that it is not a physico-chemical surface phenomenon, but an approach in which the absorbed gas is ultimately distributed throughout the absorbent (liquid). The process depends only on physical solubility and may include chemical reactions in the liquid phase (chemisorption). Common absorbing media used are water, caustic, sodium carbonate, and nonvolatile hydrocarbon oils, depending on the type of gas to be absorbed. Usually, gas-liquid contactor designs which are employed are plate columns or packed beds.

      Catalytic oxidation is used predominantly for destruction of volatile organic compounds (VOCs) and carbon monoxide, these systems operate in temperature regime of 205 to 595°C (400 to 1100°F) in the presence of a catalyst. Without the catalyst the system would require much higher temperatures to operate. Typically, the catalysts used are a combination of noble metals deposited on a ceramic base in a variety of configurations (e.g., honey-comb-shaped) to enhance good surface contact. Catalytic systems are usually classified based on bed types such as fixed-bed (or packed-bed) and fluid-bed. These systems generally have high destruction efficiencies for most volatile organic compounds, resulting in the formation of carbon dioxide, water, and varying amounts of hydrogen chloride (from halogenated hydrocarbons).

      Thermal oxidation systems without the use of catalysts, operate at temperatures in excess of 815°C (1,500°F). These operating temperatures are 220-610°C (395-1100°F) higher than catalytic systems.

      Particulate matter control (often referred to as dust control) has been one of the primary concerns of industries, since the emission of particulate matter is readily observed through the deposition of fly ash and soot as well as in impairment of visibility. Differing ranges of control can be achieved by use of various types of equipment. Upon proper characterization of the particulate matter emitted by a specific process, the appropriate piece of equipment can be selected, sized, installed, and performance tested. The general classes of particulate matter control devices are: (i) cyclones: wet, dry, axial flow, multi-cyclones, (ii) fabric filters, (iii) wet scrubbers, and (iv) electrostatic precipitators.

Other than settling chambers for large particles, cyclone-type collectors are the most common of the inertial collector class. The particle-laden gas stream enters an upper cylindrical section tangentially and proceeds downward through a conical section. Particles migrate by centrifugal force to the wall and are removed through a seal at the apex of the inverted cone. A reverse-direction vortex moves upward through the cyclone and discharges through a top center opening. Cyclones are often used as primary collectors because of their relatively low efficiency (50-90% is usual). Some small-diameter high-efficiency cyclones are utilized. Particulate matter is removed by centrifugal forces generated by providing a path for the carrier gas to be subjected to a vortex-like spin. Cyclones are effective in removing coarser fractions of particulate matter. The equipment can be arranged in either parallel or series to both increase efficiency and decrease pressure drop.

      Fabric filters are typically designed with non-disposable filter bags. As the dusty emissions flow through the filter media (typically cotton, polypropylene, Teflon, or fiberglass), particulate matter is collected on the bag surface as a dust cake. Fabric filters are generally classified based on the filter bag cleaning mechanism employed and operate with collection efficiencies above 99%.

      Wet scrubbers are devices in which a counter-current spray liquid is used to remove particles from an airstream. Device configurations include plate scrubbers, packed beds, orifice scrubbers, venturi scrubbers, and spray towers, individually or in various combinations. Wet scrubbers can achieve high collection efficiencies at the expense of prohibitive pressure drops. These units for particulates operate by contacting the particles in the gas stream with a liquid. In principle, the particles are incorporated in a liquid bath or in liquid particles which are much larger and therefore more easily collected. Other techniques include high-energy input venturi scrubbers, electrostatic scrubbers where particles or water droplets are charged, and flux force/condensation scrubbers where a hot humid gas is