Monday, April 28, 2008

Emission Controls

Volatile organic compounds (VOC) and particulate matter (PM) can be controlled using two technologies: 1) Wet Electrostatic Precipitator (WESP) and 2) Regenerative Thermal Oxidizer. The WESP reduces the concentration of particulate matter in the gas stream to levls that are both acceptable for discharge to the atmosphere and suiable for treatment in downstream VOC control equipment. The RTO destroys VOCs with high-temperature combustion(1,500 degrees F).

With respect to air emission control, gases must be cleaned in order to meet local, state and federal environmental requirements. Treatment of these gases with the combined WESP/RTO system, right, results in a gas stream that exceeds all modern standards for the emission of particulate matter VOCs with a minimum of energy and maximum operational reliability

Operation of the wet device starts by spray quenching the gas stream to the wet-bulb temperature with recycled water. The quenching process serves two functions: first, the sprays serve to remove a large portion of the coarse particles that come from the dryer, and second, the resulting temperature reduction causes heavy organic vapors to condense into particles. After quenching, the gas stream passes upward into an array of discharge and collecting electrodes. Here, the electrostatic action takes over to charge, precipitate and remove over 90 percent of the remaining particles. Exiting the top, the clean gas is now suitable for treatment in the RTO.

The RTO system reduces the VOCs (including HAPs) by high-temperature combustion. Given the large volumes of gas to be treated, simply burning the gas stream at the required 1500˚F (as in a simple, direct-fired oxidizer) [is] economically unfeasible. Therefore, alternative energy-saving techniques [should be] employed. RTOs work by alternating the incoming gas stream through heat recovery beds prior to exposure to a 1500˚F combustion zone. These heat-recovery beds are filled with ceramic material that absorbs heat from the combustion chamber. Keeping the heat in these beds greatly minimizes the total energy expended. Typically, the temperatures rise from inlet to outlet of an RTO system is in the range of 75˚F, a far cry from the 1500˚F rise that would by required by a simple direct-fired oxidizer. Yet, the gas stream is exposed to the 1500˚F required for excellent VOC and HAP destruction.

(Source: Pollution Engineering magazine: "The Search for Economical Emission Controls")

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