I recently received a call from a small pellet mill operator inquiring about a visible stack emission problem at his plant. The pellet mill is operating a rotary dryer and is small enough that they are not required to have pollution controls on the process. However, they have started to receive complaints from neighbors about a visible blue haze being emitted from their stack.
The blue haze is a result of condensable organic compounds in the gas stream which condense in the duct and form very small, submicron particulate. This particulate is an aerosol mist. When the mist in the exhaust passes out the top of the stack, the light reflects through the mist and a blue haze appears.
This is a particulate control problem. In general, there are three options to consider for particulate control.
A bag-house is a fabric filter that is generally used for higher temperature applications. It would not be a suitable solution for this process because of the moisture content and the temperature of the exhaust gas. The bags would plug up and have maintenance issues. The other two options to consider are discussed below:
Venturi Scrubber - A Venturi scrubber is a lower cost option, however, it also is not suitable for this application. The efficiency of a Venturi is dependent upon the particle size distribution (PSD) and drops off rapidly for particles less than 1 micron. Because this gas stream has a large concentration of sub-micron particulate (as evidenced by the blue haze), a Venturi will do very little to resolve the problem.
Wet Electrostatic Precipitator (WESP) - A Wet Electrostatic Precipitator is the most suitable technology for this problem. A WESP has a higher capital cost than a Venturi system, but the removal efficiency is independent of the particle size. It is most often used in situations where there is a high concentration of sub-micron particulates and the particulate are beyond the removal capability of a Venturi.
Although a WESP will resolve the blue haze caused by the aerosol mist, it will not do anything to remove gas phase volatile organic compounds (VOC). Larger pellet mill operators that have permit limits for VOCs will likely need a WESP for particulate control, followed by a thermal oxidizer for VOC control.
To learn more about Envitech's wet electrostatic precipitators, download our brochure.