As we considered in our article Different Methods for Measuring PM Emissions and Filter Performance, plants face a difficult task comparing data derived from various methods of measuring PM emissions. Often fabric filter manufacturers, dust collection system OEMs and other product manufacturers use units that are different than those used by regulatory authorities for emissions standards and for specific requirements of air permits.
In the last article we considered methodologies used by control equipment manufacturers and their purpose such as grains per standard dry cubic foot, or parts per million. Now we will consider those used by regulatory agencies, their reasons for doing so, and how we can convert back and forth between them when needed.
Your plant’s air permit could have several different methods of measuring particulate emissions. Some make use of indirect indicators of emissions such as opacity or differential pressure. Differential pressure was once widely used as a substitute for direct measurement of particulate emissions due to the fact that differential pressure was(and is) a key parameter in proper dust collection operation. This is because a baghouse is only at its most efficient when the DP is within a set range - higher could mean the filters are blocked or worn out and lower could indicate a loose seal or other problem exists. Although DP remains a helpful indicator, today its use as a primary method for emissions monitoring is only found on legacy applications, with so called “Grandfather Clauses” in their air permits as newer regulations require much stricter limits on PM emissions.
In a similar way, opacity was/is used as an indicator of total PM emissions. However, opacity is a subjective measurement and does not always correspond linearly to total PM emissions. For many years it was the only viable option for measuring dust collector emissions. With the advent of commercial triboelectric dust detection technology by Auburn Systems in the early 1970s that changed. In fact, some EPA air regulations no longer allow for the monitoring of opacity alone as the sole metric for emissions compliance. For example, the MACT standard for secondary lead smelting. Most new MACT standards now have triboelectric detection as a viable option for monitoring emissions compliance for those applications.
Plant Personnel Usually Looking for Pounds Per Hour lb/hr
But newer, more stringent permits likely call for a maximum pounds per hour or lb/hr of PM 2.5 and PM 10. This may be for each particular unit/source or divided between the entire plant.
This method measures the hourly mass emission rate by multiplying the concentration of air pollutant by the stack gas flow rate. As such, this method is well suited for plant wide use as it takes into consideration production changes and other factors. For example, while a dust collector may be operating at maximum design efficiency, increasing output will result in greater total emissions even if the collection efficiency stays the same. This means a plant with a limit stated in lb/hr will not be able to increase airflow to the dust collector without first making modifications to its baghouse system.
Use Caution When Using Test Data to Calculate Emissions Figures
Caution is required to avoid using just the emissions test data from the fabric filter manufacturer as it only describes what the fabric itself is capable of collecting under a certain set of conditions. However, additional components besides filter media play a role in the emissions generated from a production line or process. The filters are only one part of a complete dust collection system. If the dust collection system is undersized, and too much air is pushed through too few fabric filters, the collection efficiency of the system will drop quickly. While this is often a design flaw, it can also occur if operators decide to increase airflow past the operating parameters recommended by the manufacturer (such as if they want to increase production but do not want to upgrade the collection system).
How to Convert from Grains Per Dry Standard Cubic Foot (gr/dscf) to Pounds Per Hour (lb/hr)
The principles in these two articles could find applications, including the choice of dust collector media, for emissions. When deciding on which filter media to use in their dust collector, compliance engineers should carefully consider all requirements in their air permits. If their permit has a limit expressed in lb/hr, they would likely need to convert the fabric filter test data (likely in gr/dscf) into lb/hr to see whether it will be efficient enough for their application. This is done using the following equation courtesy of Michigan Department of Environmental Quality.
CONVERTING GRAINS/DSCF TO LBS/HR
C grains/dscf * 1 lb/7000 grains * Q dscfm * 60 min/hr = lbs/hr
___________________________________________________________ = pounds per hour
grains* lb * ft3 * min=lbs/hr ft3 * grains * min * hr
C = Pollutant concentration Q = Airflow
Once calculated, multiplying the concentration of a specific air pollutant (gr/dscf emissions from the fabric under test conditions) in the stack gas and stack gas flow rate (in this case, the CFM of the dust collector) together will equal a mass emission rate. The resulting figure in lb/hr then accurately describes the amount of pollutants emitted from that source over time.
As a final reminder, avoid relying on sales personnel, outside vendors, equipment manufacturers or others for final responsibility for calculating these values. While their input on certain products and strategies can prove useful during the selection process, ultimately the responsibility of meeting compliance falls upon the end user. Any data used for the decision making process should be free from potential bias and be thoroughly checked for accuracy by experienced compliance engineers.
As time goes on industrial facilities often must meet increasingly stringent and complex emissions standards from regulatory authorities. While the responsibility for compliance falls primarily on compliance and environmental personnel, maintenance, operations and management personnel do well to review the basics of these requirements and familiarize themselves with its terminology and methods. It can be quite complicated when equipment suppliers provide testing data in one form and air permits call for a complex mix of mechanical and operational parameters or direct emissions measurements in different forms. In such instances, seeking out educational sources and qualified advice from experienced professionals can go a long way to making sure your plant meets it emissions standards and can keep its focus on conducting business.