The Battle Between Triboelectic Dust Detection and Opacity Meters

Posted by Earl Parker on Jan 31, 2017 1:30:00 PM

Regulating authorities require nearly all industrial facilities to measure dust particulate emissions in some fashion. Opacity meters were, for decades, the standard required by the EPA and under many State and Local air permits.

However, in recent years, triboelectric particulate monitors have been promoted by some as superior to opacity meters, and have begun replacing opacity meters in many applications across all industries. So why the change? Is there a reason to switch to triboelectric monitoring devices for dust/particulate monitoring?

Can Opacity Meters Keep Up With Changing Regulations? 

Since opacity is the degree to which light is able to pass through something (air for example), industrial applications are able to use opacity as a measurement of dust pollution being emitted from a stack or other source. More dust and particles in the air means less visibility and more emissions. 

Opacity meters measure emissions by sending a beam of light between two mirrors on either side of a duct or exhaust outlet. A sensor is then used to measure the percentage of light being blocked by the particles in the air stream. It is important to note that using opacity as a measurement of particle concentration is not 100% consistent across all applications. Readings can be affected by misalignment and clouding of the lenses, atmospheric conditions, the color of the particulate, fumes in the gas streams, and other factors. As an example, even if two different facilities have the same opacity levels of 10 percent, there is no guarantee that their actual emissions levels are the same. 

Most opacity detection systems cost $25,000 or more. This does not include the costs associated with the careful installation and periodic calibrations required for a facility to achieve reliable results. One important limitation to opacity meters is that they are unable to detect particle concentrations of 10% opacity or less. For many of the reasons listed above, opacity meters are prone to inaccuracies when measuring concentrations of 10% or lower.  

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Opacity meters measure the degree to which visibility is obstructed by dust and other particulate

While it used to be fine for EPA regulated industries to keep their emission below 20% opacity, the recent tightening of emission standards from the EPA (such as MACT) is seeing many organizations being subject to emissions limits much lower than 10% opacity. This has caused opacity meters to become ineffective in many EPA regulated applications. By the time they're able to measure an increase in emissions, an organization may already be experiencing a reportable event and subject to regulatory fines and forced shutdowns. For this reason the EPA noted in one MACT standard, "opacity is not a good indicator of performance at the low, controlled levels characteristic of these [MACT-required] sources."

Is There an Advantage to Triboelectric Detectors and How Do They Work? 

By using an electrically isolated metallic probe placed into the ductwork, triboelectric detectors are able to identify passing particles using the triboelectric effect. Essentially, when particles impact the probe a DC signal is created, and when particles pass near the probe (but do not contact it) an AC signal is created. These signals are then transmitted to a controller unit where signal data is visible on a display or computer nearby. 

While in the past, triboelectric detectors could use only the DC or AC signal, the latest generation is able to use both signals, allowing for enhanced sensitivity and improved operation. Dust concentrations are now able to be detected down to 0.00002 g/dscf (or 0.005 mg/m3). This means they are roughly 20 times more sensitive than the best opacity detectors, while also being able detect much smaller particles (down to less than 1.0 micron in size) and having a faster response time - taking only one second from detection to visual output. Particulates are able to be monitored and baselines can be established even when the particles are virtually invisible to the eye. 

Combined with the increased sensitivity of triboelectric detectors, tightening air permit requirements have seen many facilities decide to replace their opacity-based monitoring systems and move to triboelectric monitoring. In addition to meeting compliance requirements, these facilities are now benefiting from predictive monitoring capabilities that they were not possible with less sensitive opacity-based systems. Triboelectric monitoring systems allow plant personnel to monitor emissions in real time, seeing emissions problems before they become serious. Being able to schedule preventative maintenance (such as filter bag replacement) allows facilities to reduce system downtime, prevent production stoppages, and even prevent emissions issues from escalating to reportable events.

For these reasons and more, many companies have decided that switching to triboelectric monitoring is a wise investment. If you would like to know more details about how triboelectric dust monitoring systems can benefit your facility, download our free eBook on "Intelligent Dust Monitoring". 

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Topics: MACT Reporting, Particulate Monitoring, Dust Detection, Environmental Monitoring