SPECIAL SECTION/BRICK & CLAY RECORD: Optimized Emission Control

August 1, 2007
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At approximately 10% of the cost of continuous opacity monitors, triboelectric bag leak detectors can achieve and maintain cost-effective dust collector performance conditions.



Triboelectric bag leak detectors (BLDs) are increasingly being specified for the effective early warning of dust collector filter failure, providing the means for manufacturers to respond quickly before non-compliance conditions occur. The application of this technology not only prevents permit violations, it can also optimize the performance of baghouse emission control systems. At approximately 10% of the cost of continuous opacity monitors (COMs), triboelectric BLDs can achieve and maintain cost-effective dust collector performance conditions.

Principle of Operation

The principle of the triboelectric BLD is based on the ability to measure the minute electrical current generated when particles in the flow stream impact an insulated probe connected to an earth-grounded measuring circuit. The particulate present in the gas stream strikes the probe, and the force of impact results in electron transfer to the probe, which is measured as current. Measurement of the relative changes of the current level provides a means to monitor relative particulate flow. It also provides an early indication of filter failure or fabric material deterioration, which would eventually lead to a failure event.

The small changes in the electric field associated with the particle impacts are measured in picoamps. Monitoring picoamp variation provides the basis for interpreting the relative flow of the particles in the gas stream and the performance of the filter system. The triboelectric signal can be an analog or logarithmic output, and is displayed as a percent of scale or as picoamps. The impact of no particles is measured as 0%, with the relative increase of particle strikes measured up to 100% of the scale, or directly as picoamps. Triboelectric BLDs can detect particles as small as 2 microns in diameter (which would be invisible to a COM).

Continuous Opacity Monitors

In contrast, COMs are employed to measure the loss of light associated with the passage of a beam of light (opacity) through an exhaust plume. A light beam is directed across a stack and is reflected back to the source from a mirror on the opposite side of the stack. By measuring the reduction of the amount of light returning to the measuring sensor, the light loss can be calculated to measure relative opacity.

The particles in the gas stream affect the amount of light by absorbance, reflection or refraction. The amount of light absorbed is then associated with the size, shape and structure of the particle, as well as the quantity of particles in the gas stream. However, a COM cannot correlate the mass amount of particles in the flow stream with the gas flow.

Opacity monitoring is a qualitative measurement and is generally limited to opacity levels above 10%. When lower opacity standards are followed, triboelectric BLD systems (and other particle monitoring techniques) should be employed. In higher-opacity applications where filter failures occur, COMs respond only when sufficient opacity can be detected and after significant emissions have occurred.

Figure 1. General layout of a negative pressure baghouse.

Put to the Test

A demonstration project1 compared the performance of COM and BLD technologies on a negative pressure baghouse (generally configured as shown in Figure 1). The triboelectric BLDs were installed strategically in four quadrants of the two-section baghouse. The COM system was installed in the stack located a few yards away.

Figure 2. Abnormal cleaning pulses at the triboelectric BLD location.

Figure 2 displays individual abnormal cleaning pulses, providing the means to locate the section with abnormally high cleaning pulses and an indication of the sector in which a filter failure may be developing. The negative pressure baghouse signal represents on-line cleaning. Notice how the spikes develop immediately and drop off slowly, representing residual dust release from the filter following each cleaning pulse. Warning and alarm set points can be established based on the amplitude of the signal that results after each cleaning pulse.

Figure 3. Comparison of triboelectric BLDs and COMs.

COMs do not have the sensitivity to monitor cleaning pulses and are not useful as early warning filter media failure detectors. At high emissions levels and at applicable COM operating ranges, the overall correlation between the two technologies was demonstrated (see Figure 3).

Triboelectric BLDs can be used as an alternative to COMs to provide early warning and location of impending fabric filter failures. Since triboelectric BLDs have been demonstrated to be effective-and costs are approximately 10% of the cost of an individual COM device-a significant economic and operational advantage exists for users to select triboelectric technology for bag leak monitoring applications.

For additional information regarding triboelectric bag leak detectors, contact Auburn Systems, LLC, 8 Electronics Ave., Danvers, MA 01923; (800) 255-5008 or (978) 777-2460; fax (978) 777-8820; e-mail sales@auburnsys.com; or visit www.auburnsys.com.

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