Filter Emission Control Systems
Background Information
Filter emission control systems, also known as baghouses or Air Pollution Control Equipment (APC), use different mechanisms to capture, separate and filter dust. Baghouses come in variable sizes and are used in many industries throughout the world. The three basic types of filter emission control systems are outlined below.
The role of quick-checks and filter emission control systems »
Why quick-checks are an important part of your Compliance Assurance Monitoring (CAM)
Mechanical Shaker
A mechanical shaker is used to shake the bag as one would shake out or beat a rug to remove the accumulated dirt. A timer or an increase in pressure drop across the baghouse are two typical mechanisms that initiate a cleaning cycle. Systems with low flow and pressure would not utilize the pressure drop mechanism.
The emission is typically accumulated on the filter media and when shaken off, the emission falls into a hopper, or if sufficiently light, is lifted back into the filter media. The cleaning cycle does not significantly alter emission levels even in the event of failure of the media or supporting structure.
No matter how frequent the shaking when the accumulated emission is removed, the accumulated emission is not generally noticeable because the dirty side is not much greater in airborne emission than during normal operation. When utilizing Method 9 or Method 22, because the sun is at the back of the observer the emission levels must increase to a substantial level to be visually noticed. That is where quick-checks become important.
By Goran tek-en [CC BY-SA 3.0], via Wikimedia Commons
By Goran tek-en (Own workderivative of Reverse-Air Baghouse.png) [CC BY-SA 3.0], via Wikimedia Commons
Reverse Air
In a reverse air system, like in the mechanical shaker system, emission levels on the dirty side never are excessive so even during the cleaning cycle the level of emissions would not be typically noticeable by Method 9 or by Method 22. If there is a bag leak or mechanical break in the supporting superstructure of the filter media, the emission levels will increase slowly over time as the break worsens. The increase in emissions are undetectable by Method 9 or Method 22 until there the system is sufficiently broken such as to let a visible emission through the system under normal operating conditions.
Failure of a reverse air system may take YEARS between a detectable quick-check and a detectable Method 9 or Method 22.
Baghouse leak detection systems were developed to address this issue because the sensitivity can be tuned to ignore a normal level of emission and detect slight increases.
Pulse (Pulse Jet)
A pulse system uses a sudden release of compressed air to intensely knock all the accumulated emissions off filter media. This pulse creates an intense cloud on the dirty side of the system, which if there is a tear in a bag or crack in a tube sheet, the emission cloud will escape creating a puff of emission. This puff may last less than one second. For this reason, when evaluating a pulse jet system, a Method 22 should be specified with either a duration sufficient to catch a cleaning cycle, or such that by design the observation period includes a cleaning cycle. Such a puff of a second in duration has a 1 in 15 (0.066667, or less than 7%) chance of being detected by Method 9 during each pulse.
For a baghouse with a pulse every 6 minutes and a Method 9 observation lasting 6 minutes there is a 1 in 15 (0.06667) chance of detecting the need for a repair. If the pulse cycle is determined by pressure drop (although a lack of sufficient pressure drop may be sufficient as a mechanism to detect a failure) this may be greatly reduced since it is the frequency of the pulse which affects the probability.
If the pulse cycle is once an hour (typical in many applications) the probability drops to 1/150 or 0.6 %: less than 1% chance it will be detected on any given observation. Thus, a problem could last more than a year and be undetected by Method 9, Visual Opacity Observation.
A quick-check can often determine that the base level (level of emission during normal operation, not cleaning cycle) of emission has changed when using the enhancement provided by the forward scattering effect of sunlight.
By Goran tek-en (Own workderivative of Reverse-Jet Baghouse.png) [CC BY-SA 3.0], via Wikimedia Commons
Visible Emissions Quick-Checks*
Because of the tendency to have slow and gradual failure modes, it is difficult to ascertain early-stage baghouse problems with standard Method 9 and Method 22 procedures. This is one reason to perform quick-checks.
Depending on your EPA Title V Compliance Assurance Monitoring (CAM) plan, you may have a daily quick-check built into your plan. A quick-check may be defined as: examine the facility or a list of affected sources daily using a quick-check method: look daily, if you see a visible emission, initiate a Method 9 or Method 22.
If a quick-check finds detectable emissions above the baseline in your Title V, then action is necessary and the action is determined by your CAM. You may be required to do a Method 9 or 22, or you may be required to do a quick-check with the sun at your back to determine that required emission levels are not exceeded. Regardless of the required action, the quick check becomes a valuable early detection system for your baghouse operations.
*Quick-check terminology: If you are required to look at your emission source daily to determine if there are any visible emissions present, but a Method has not been specified, we use the term quick-check.