You have a choice when it comes to particulate matter (PM 2.5) emissions monitoring. For many this may mean employing a continuous opacity monitoring system (COMS) as dictated by their air permit. Many other plants simply choose to continue using whatever method they have always had in their CAM plan (Compliance Assurance Monitoring) regardless of the advances made in dust collector monitoring technology in recent decades. Some feel that there is little to be gained by revising their CAM plans to use newer monitoring technology. But what are the differences between the various monitoring methods in use today?d
But how does it compare to other common methods?
EPA Method 9 and 22 are a set of guidelines for conducting visual observations to monitor and evaluate particulate matter emissions (PM 10 and PM 2.5). Method 9 attempts to quantify and measure opacity, or what percentage of light is blocked by the particulate matter. (The more PM in the air the more light it blocks and the greater the opacity.)
Method 9 requires a trained and certified individual to make a planned observation of the stack for a set period of time per the guidelines set by the EPA and then make an assessment of the amount of visual opacity. The guidelines (or method) set by the EPA attempt improve the observer’s ability to assign opacity values accurately. Method 22 is conducted in a very similar way, but is intended only to be used as a qualitative test unlike method 9 which is a quantitative test. Method 22 is only used to determine the presence of opacity (indicating a failure of the emissions controls) and is therefore much less rigorous than method 9 and does not require a trained and certified observer to conduct the test.
Visual observations provide a quick method of confirming whether to not a failure of the emissions control system has occurred. However, the accuracy of opacity observations from a trained observer varies greatly based on a number of factors including weather, the type of emissions, the observer’s ability among others. Beyond this, the ability of the human eye to detect opacity changes is limited compared to technological means available today. Finally, the cost of training and certifying individuals annually as well as the costs associated with keeping a certified tester on site and available at all times for testing greatly increases the cost of using this monitoring method compared to triboelectric monitoring.
Triboelectric monitoring systems can be fully automated and remote, eliminating the need for manual readings by personnel. Triboelectric systems also have detection capabilities far superior to human eye.
Differential pressure (also known as pressure drop, or Delta P) refers to difference in pressure (in most emissions applications vacuum pressure or suction) between two sides of the emissions control device (e.g. fabric filter dust collector, cyclone, electrostatic precipitator, wet scrubber, etc.). Facilities with fabric filter dust collectors (the most common type of emissions control device) must measure differential pressure continuously for operations and performance monitoring of their systems. Differential pressure on fabric filters rises over time as dust builds up on the surface of the filter, thus increasing the resistance to flow it generates. When cleaned, the differential pressure drops and the resistance goes down. Eventually, the dust particles imbed themselves in the depth of the filter and lowering the differential pressure through cleaning no longer is possible and the filters must be replaced.
Since collection efficiency on fabric filter collectors peaks within a certain DP range and high DP indicates problems with filter condition, regulatory agencies view DP as an acceptable surrogate for monitoring emissions from these systems. Today, a number of facilities still retain air permits that require regular differential pressure monitoring and for plants to take corrective action should it rise above specified levels.
Differential pressure readings provide an invaluable resource for operators and maintenance personnel regarding the performance and condition of a dust collector. These readings are absolutely essential for making any operational, maintenance or output decisions involving these units. However, DP as an PM emissions surrogate has several key drawbacks. DP levels do not always prove a reliable indicator of emissions levels. Many additional factors affect a dust collector’s collection efficiency besides DP. Additionally, systems for monitoring DP foul easily and do not provide the level of reliability required for emissions monitoring. Finally, when problems do occur that increase emissions suddenly (e.g. leak in a filter) DP will not rise fast enough (or at all) to alert operators of the problem before an excursion has taken place and emissions limits have been exceeded.
Opacity meters measure opacity by sending a focused light beam between two mirrors on opposite sides of a duct or exhaust outlet. Any particles in the gas stream obscure the light beam as it goes and returns enabling the meter to measure opacity. Opacity meters were once cutting edge technology, providing significant benefits over older monitoring methods (see above), specifically the ability provide remote monitoring of sources. For this reason, opacity meters became the preferred monitoring method for most EPA regulations starting in the 1970s eventually becoming a key part of CEMS (Continuous Emissions Monitoring Systems) and COMS (Continuous Opacity Monitoring Systems) required in many industrial facilities.
However, since the 1970s the EPA has significantly tightened PM emissions standards across all industries. Whereas in the 1970s many air permits required plants to stay under 20% opacity. With the introduction of MACT (Maximum Achievable Control Technology) standards many industries now have to meet opacity limits of 10% or even lower! (FOOTNOTE http://auburnsys.com/sites/default/files/papers/Apr53-64.pdf). This means that opacity meters struggle to keep up as their have great difficulty detecting anything below 10% opacity. Despite failing to replace opacity meters as the standard for many existing regulations, the EPA nonetheless wrote in one proposed MACT standard that "opacity is not a good indicator of performance at the low, controlled levels characteristic of these [MACT-required] sources."
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. This means they have trouble effectively monitoring applications with emissions limits lower than 10% opacity. This reduced sensitivity also means they cannot effectively detect filter failures until they have intensified to the point of catastrophic failure (i.e. so severe that they cause the entire system to exceed its maximum allowable emissions limits). So unlike triboelectric detectors, opacity meters cannot detect the beginnings of filter failures, nor can they help pinpoint leaking filters to a specific section of the unit, compartment or row of bags.
Another key consideration is cost. Most opacity detection systems cost $25,000 or more for a basic system. Further components for a true COMS may push that number even higher. However, this does not include the costs associated with the careful installation and periodic calibrations required for a facility to achieve reliable results. Initial calibration usually involves physical installation of the delicate mirror elements and electronics and testing the output against a gravimetric test to verify accuracy. Thereafter, facilities must calibrate and clean the optical components every 30-60 days along with a limited recalibration using the filter and filter holder while connected to a computer for processing. Further, many jurisdictions require a full calibration every 12-24 months, often similar to the initial setup that requires a full cleaning and calibrations against a gravimetric test. Finally, most meters require a full rebuild ever 3-5 years, which includes replacing the circuits in the transmitter and the receiver along with the transceiver unit along with new mirrors. These rebuilds can cost up to several thousand dollars depending on the extent.