The increased value being placed on responsible corporate citizenship and green initiatives has prompted cement manufacturing companies to look closely at how they will comply with the proposed National Emission Standards for Hazardous Air Pollutants (NESHAP) Particulate Matter (PM) limits. The effect of these potential new limits in the United States has had a ripple effect on the design, production and adoption of filtration technologies and best practices around the world.
This blog examines solutions to baghouse particulate removal efficiency to meet the proposed NESHAP PM limits and presents the results of two studies conducted on the effects of taped seams vs. standard felled seam construction on emissions reduction.
Is there a magic solution?
Even before the details of these new regulations were settled, many cement manufacturers were already in search of a special solution — a magic filter bag that would make even the most stringent emissions challenges a thing of the past. As much as a singular solution that meets this need is desired, no such option exists.
A filter bag is only one of the many components in a dust collector. The key to effective particulate removal efficiency depends on the successful operation of all parts of the system.
These companies must consider a host of components, operating conditions and other variables such as age and maintenance history in order to ensure their dust collection system will offer optimum performance and meet the emissions levels defined in the NESHAP.
Meeting the new standards
It is well known that polytetrafluoroethylene (ePTFE) filters are exceptionally efficient in capturing particulate on the collection surface of a filter bag without having to rely on a dust-cake or control layer. A properly designed and maintained dust collector installed with these filters can meet the emissions levels required by the new NESHAP rule.
Meeting the limits is achievable, but the question that continues to elude companies is: For how long? There is a lack of historical data available for continuous outlet emissions that shows the point in a filter’s life when emissions begin increasing.
Why do emissions occur?
Design, operation, age and process conditions are the variables that contribute to filter life. Where these can present dynamic conditions individually, their combination makes for an endlessly complex matrix of possibilities. To this end, the performance of a system with the same specifications can operate differently in every environment in which it is applied.
In order to mitigate these dynamics, companies are choosing to segment the challenges they face in order to contain the variables.
For example, understanding that increased emissions will result as ePTFE membranes age and develop fractures and fissures along flex lines, gives the plant operations managers and engineers something to evaluate and preempt through scheduled maintenance.
Can dust penetration occur at the needle holes on filter bag seams?
One hypothesis based on a previously published study, suggests that dust penetration/leakage can occur through the stitching on filter bag seams and taping them can have a positive impact on the reduction of particulate emissions.
Separate testing, commissioned by Parker Hannifin, that adhered to the US EPA’s Environmental Testing Verification (ETV) testing protocol yielded different results.
Why the discrepancy?
The independent testing commissioned by Parker put the filtration through the rigors that more closely resemble normal baghouse operation, in a laboratory setting. This testing offers a more accurate portrayal of durable filter life than the previously published study that only simulated the early seasoning phase. ETV protocol does not draw conclusions from results taken during the seasoning phase.
ETS, Inc, an independent laboratory, was commissioned by Parker to perform the testing. The tests were performed per the US EPA’s ETV program and in accordance with the ASTM Test Method D830-02 utilizing Parker 22 oz. woven fiberglass media with ePTFE membrane. The results showed:
- Tape sealing of the seams offers no significant additional benefit for reduction or prevention of emissions through filter bags.
- In actual application, seams that have been taped could actually reduce the total filtration area of the bag.
- Taped seams do not offer any significant additional benefits compared to stitched seams.
- Standard felled seam construction on filter bags is not a source of increased particulate matter emissions.
The truth is that there is no magic filter bag to be discovered, but there are practical lessons to be learned: Particulate removal efficiency is a function of the effective operation of all parts of the system. Properly installed and maintained ePTFE membrane filters are an effective solution for meeting the NESHAP emission limits.
This article was contributed by the Filtration technology team.