Update on 40 CFR Part 63, Subpart MM

The anticipated proposed amendments to 40 CFR Part 63, Subpart MM (National Emission Standards for Hazardous Air Pollutants for Chemical Recovery Combustion Sources at Kraft, Soda, Sulfite, and Stand-Alone Semichemical Pulp Mills) were published in the Federal Register on October 31, 2019.  Related amendments to 40 CFR Part 60, Subpart BBa (Standards of Performance for Kraft Pulp Mill Affected Sources for Which Construction, Reconstruction, or Modification Commenced After May 23, 2013) were also proposed.  The remainder of this article focuses on the proposed amendments to 40 CFR Part 63, Subpart MM.

As expected, the proposed amendments to 40 CFR Part 63, Subpart MM are intended to accomplish the following:

1. Add the method for setting the minimum operating limit for scrubber liquor flowrate, which was omitted in the October 11, 2017 final rule.

2. Clarify that an operating parameter limit for Automatic Voltage Control (AVC) is not required.

3. Address the method for setting the minimum operating limit for fan amperage.

 
The first two amendments are fairly straightforward.  The method for setting the minimum scrubber liquor flowrate is consistent with the method for setting the minimum pressure drop (i.e., the lowest of the 1-hour average values associated with each test run during the performance test demonstrating compliance with the applicable emission limit).  The AVC changes consist of corrections to regulatory references throughout the rule that had previously suggested an operating parameter limit was required.  The more significant proposed amendments pertain to the smelt dissolving tank scrubber fan amperage, which is discussed further below.

Scrubber Fan Amperage

Recall that the October 11, 2017 final rule added fan amperage as an alternative to monitoring pressure drop for certain types of smelt dissolving tank scrubbers.  However, many mills found the method for establishing the minimum operating limit for fan amperage to be problematic.  In particular,  the lowest 1-hour average fan amperage value associated with each test run of the performance test may not provide enough flexibility to account for decrease in fan amperage due to changes in atmospheric pressure, that do not affect scrubber performance.  Due to this challenge, it was fairly well-known that U.S. EPA would propose amendments to the rule to change how the minimum operating limit is established to address those concerns.

In addition to changing how the minimum fan amperage operating limit is established, the October 31, 2019 proposed amendments would also establish two new alternatives to pressure drop.  The resulting three options are as follows:

1. The minimum fan amperage operating limit must be set as the midpoint between the lowest of the 1-hour average fan amperage values associated with each test run demonstrating compliance with the applicable emission limit in §63.862 and the no-load amperage value. The no-load amperage value must be determined using manufacturers specifications, or by performing a no-load test of the fan motor for each smelt dissolving tank scrubber. It must be verified that the scrubber fan is operating within five percent above or below the design RPM during the emissions performance test; or
   

2. The minimum percent full load amperage (PFLA) to the fan motor must be set as the percent of full load amps under no-load, plus 10 percent. The PFLA is calculated by dividing the no-load amperage value by the highest of the 1-hour average fan amperage values associated with each test run demonstrating compliance with the applicable emission limit in §63.862 multiplied by 100. The no-load amperage value must be determined using manufacturers specifications, or by performing a no-load test of the fan motor for each smelt dissolving tank scrubber. It must be verified that the scrubber fan is operating within five percent above or below the design RPM during the emissions performance test; or
   

3. The minimum RPM must be set as 5 percent lower than the design RPM.

 
As you can see, the new “midpoint” method for setting the minimum fan amperage operating limit (option 1) was not quite as simple as expected, and resulted in a proposed new definition for “no-load fan amperage,” which is proposed to be defined as “…the amperage pulled by the fan motor when the fan is operating under no-load, specifically the amperage value the motor would use if the fan belt was removed.”

You may have also noticed that all three options include provisions to operate the scrubber fan within 5% of the design revolutions per minute (RPM).  Due to the age of these scrubbers, many mills may not know the design RPM of the fan or have the capability to measure RPMs during the performance test.  This leads to uncertainty for mills that were planning to utilize the fan amperage option and that may conduct their performance test before the amendments are finalized.

What’s Next?

Comments on the proposed amendments were due by December 30, 2019.  As you can see, the proposed changes have the potential to impact performance testing, which is due by October 13, 2020.  More on performance testing considerations can be found in the previous article, Preparing for Subpart MM 2020 Performance Testing.