“The Good, the Bad and the Ugly” – Three challenging scenarios for cement plants to prove compliance under the new NESHAP regulations
Since September 2015, the new NESHAP (National Emission Standards for Hazardous Air Pollutants) regulations are in place. Many of the cement plants in the United States face the burden of proving their compliance with these new standards
Overview of EPA’s new standards
- MACT (Maximum Achievable Control Technology) approach for PM (Particulate Matter):
- EPA gathered stack test data from cement plants who will be affected by the rule.
- Results would be sorted and analyzed from the best performing (lowest emitting plant) to the worse performing (highest emitting plant)
- Limit was set at the average of the best performing 12% of existing plants (0.07 pounds / ton of clinker)
- New or significantly modified plants equal to the best performing data point: for PM, the NSPS (New Source Performance Standards) level is 0.02 pounds / ton of clinker.
- This data collection would exclude cement plants falling under the CISWI (Commercial/Industrial Solid Waste Incinerators) and HWC (Hazardous Waste Combustor) rules.
- CPMS (Continuous Parametric Monitoring System)
- The new regulation requires plants to prove compliance on an ongoing basis.
Challenges of the Continuous Parametric Monitoring System (CPMS)
Annually, plants will have to prove their compliance with the PM standard by performing a method 5 stack test on the kiln stack:
- three tests with the mill on
- three tests with the mill off
While performing the method 5 stack tests, correlations will have to be performed equating the results of the stack tests in pounds per hour to another signal (typically an electronic signal in milliamps) from stack monitoring equipment. The output signal from the stack monitoring equipment will now be the plant’s ongoing CPMS signal.
This signal on a 30 day rolling average is what the plants will be most interested in since if the site specific CPMS is exceeded, the plant has 48 hours to perform repairs and also must pay for and perform another method 5 stack test to prove the plant is actually compliant.
Example 1. “The Good” - Extremely low Method 5 stack test
- Brand new membrane bags have just been installed
- New cages in the baghouse
- All welds have been visually inspected for potential leaks
- Tubesheets have been thoroughly cleaned
- Outlet dampers, main outlet manifold, replaced non operating solenoids and diaphragm valves.
Another possible scenario: brand new baghouse which has stainless steel tubesheets and clean air plenums with brand new membrane bags.
On the stack test day, the plant is prepared well and gets the following very good results. See figure 1.
Because the stack test results were less than 75% of the 0.07 pounds per ton of clinker limit, the regulation states the plant’s site specific CPMS operating limit is scaled to the 75% level or 0.052 pounds per ton of clinker. See figure 2.
This affords the plant a good deal of margin to operate before any required remedial action is necessary. Daily spikes above the plant’s specific CPMS operating limit of 0.052 pounds per ton of clinker will not require any action on the plants part. Even if a daily spike happened to be above the 0.07 pounds per ton of clinker level, the plant would not be required to do anything. However, it may not be a bad idea to investigate what is causing the daily excursion in either of these cases. See figure 3.
If the 30 day rolling average hits the plant’s site specific CPMS operating limit of 0.052 pounds per ton of clinker, it is now required by law to make remedial actions within 48 hours and perform a new method 5 stack test within 45 days to prove the plant is compliant. See figure 4.
- Unbudgeted expense
- Typical stack testing company used in the past may be unavailable; new unfamiliar company to come on site to perform the testing.
- New CPMS limit is established based on the results of this new stack test
As long at the stack test confirms that the plant is below the 0.07 pounds per ton of clinker, there is no violation, although the plant has incurred the unanticipated cost of a stack test. Each time it exceeds the site specific CPMS, remedial action and a method % stack test is required.
Example 2. “The Bad” – A stack test right at the 75% limit of 0.052 pounds / ton of clinker
- Filter bags are around 18 months old or
- Excessive corrosion in the baghouse or
- Clean air compartment doors or lid do not seal optimally; baghouse continually sucking in potentially dusty air from outside
Results of method 5 stack test:
Precisely at the 75% of the limit level or 0.052 pounds per ton of clinker.
The plant is not in violation, however this means it now has to operate on a rolling average below this level all the time. There is no operating buffer between the stack test level and the CPMS site specific operating level as in the first example. This is actually the same scenario as any stack test between 0.052 and 0.07 pounds per ton of clinker.
- Any time the rolling average eclipses stack test level, the plant must make remedial actions within 48 hours.
- New method 5 stack test has to be performed within 45 days to prove the plant is compliant.
Based on the results of this new stack test, a new CPMS limit is established. See figure 5.
If the stack test confirms that the plant is below the 0.07 pounds per ton of clinker, there is no violation and it must reset the CPMS to this new stack test level. If the stack test confirms that the plant is above the 0.07 pounds per ton of clinker level, it is now out of compliance and subject to a violation.
Example 3. “The Ugly” – Multiple excursions above your CPMS limit
- Older baghouse in need of repairs or
- Filter bags are not performing as well as hoped or
- Some residual dust was left in the clean air plenum after hopper evacuation system malfunction with improper cleaning after replacement
The plant is likely to eclipse its specific CPMS limit. One of the more surprising parts of the regulation to many plant managers is the fact that if the plant eclipses its site specific CPMS limit four times during a year period it is assumed to be in violation. This could be the case even if the CPMS never goes above the scales limit of 0.07 pounds per ton of clinker. There could also be the case even if the four resulting method 5 stack tests also all confirm that the plant is below the 0.07 pounds per ton of clinker. The sheer fact that it eclipses the site specific CPMS four times, can be subject to a violation.
See figure 6.
The limit is low if the plant is falling into the existing plant category and extremely low if the plant falls into the NSPS category. The addition of a CPMS requirement will have a dramatic effect on the ability to remain consistently in compliance. Attention to all aspects of the baghouse, the filter bags, the cages, the cleaning system, the operation of the baghouse and preventative maintenance will all be essential.
Filter bag life and baghouse rehabilitation
The current thinking from many cement plants is that the result of these new regulations will have a net effect of an overall shorter effective bag life. Plants which have been used to getting 5 years life are hoping to get 3 to 4 years ideally without having to schedule and pay for unexpected method 5 stack tests. Plants that currently get only 3 to 4 years bag life have even bigger concerns and challenges ahead of them. To best counteract this, some plants have invested in new baghouses or significant baghouse rehabilitations.
- Preventing rust anywhere on the clean side of the tubesheet
- Installing door seal gaskets, lid gaskets, etc. to ideally eliminate any false air in leakage will help prevent corrosion and the influx of potentially
dust laden air.
- Replacing all cages with new membrane filter bags to ensure the most optimal fit possible
- Prevent dust emission through stitch holes
Prevent dust emission through stitch holes
Testing using Filter Sense® leak detection equipment has shown that membrane filter bags emit dust through their stitches with each pulse. These spikes can easily be 50 times higher than the baseline emissions between pulses. See figure 7 which shows emissions of membrane bags over a 1 hour period.
- Dust emission through stitch holes may contribute to a site specific exceedance if one or more other sources of emissions are also present
- This may be especially the case later in bag life when pulse frequency may have the likelihood to be increased.
Gore has developed and launched GORE® Low Emission Filters for strict environmental compliance which include a patented seam tape technology effectively covering up every stitch hole on the filter bags. They eliminate the emission spikes corresponding with pulsing of the filter bags.
New regulations are going to require a new holistic approach to the baghouse, its performance and filter bag selection:
- Adopting improved baghouse maintenance programs
- Monitoring key parameters of baghouse performance
- Installing high performance membrane filter bags and new cages
- Keeping the stitches leak-free during pulsing
All together best prepare the cement plant to start from a position of the lowest possible emissions. By doing this, this affords the plant the maximum amount of operating room prior to hitting the site specific operating limit.
The requirement of having to prove compliance with the use of a CPMS increases the chances of unexpected costs and worst case, potential down time. Although the total net effect on bag life on many plants will be a decrease, plants which prepare and implement sounds measures as well as selecting robust and proven filter media, will minimize the impact on the plant.
National Emission Standards for Hazardous Air Pollutants (NESHAP):
United States Environmental Protection Agency (EPA): http://www3.epa.gov
Gore is a member of the Portland Cement Association: http://www.cement.org
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