Managing Boiler MACT 2025 Alternative Limits Expiration: Engineering Testing Before Your Compliance Test
Sources subject to October 6, 2022 40 CFR Part 63, Subpart DDDDD revisions were required to comply with a number of revised emission limits. The revisions affected both new and existing boilers and process heaters, changing 34 emission limits — most only modestly, but some significantly enough to require control-equipment modifications or upgrades. The most dramatic change was the new-source HCl limit for solid fuel-fired units, which dropped from 2.2E-02 lb/MMBtu to 2.1E-04 lb/MMBtu, a reduction of two orders of magnitude.
Because some sources needed more time to meet the revised limits, the EPA built optional alternative limits into the rule — contained in Tables 14 and 15 for new and existing sources, respectively — as a grace period. Those alternative limits sunset on October 6, 2025, meaning affected sources are now held to the limits in Tables 1 and 2. The rule doesn’t specify additional compliance-demonstration requirements tied to that sunset, so many facilities are heading into their regularly scheduled performance tests without realizing their applicable limits have changed.
Some facilities conducting their first performance test under the Table 1 and Table 2 revised emission limits are finding that their units do not meet the applicable limits. Alliance Technical Group’s testing teams have encountered this challenge on-site, underscoring the importance of evaluating current performance before the required test.
Engineering and investigative testing can help facilities assess that uncertainty earlier. Alliance develops targeted testing programs to evaluate emissions, investigate performance concerns, and inform adjustments before the required compliance performance test.
Which Boiler MACT Limits Changed in 2022?
Of the 34 revised limits, 28 became more stringent and six became less stringent. For existing sources, the relevant changes:
Subcategory | Pollutant | Former Table 15 limit | Current Table 2 limit | Change |
Solid Fuel | HCl | 2.2E-02 lb/MMBtu | 2.0E-02 lb/MMBtu | ~9% lower |
Solid Fuel | Mercury | 5.7E-06 lb/MMBtu | 5.4E-06 lb/MMBtu | ~5% lower |
Liquid Fuel | HCl | 1.1E-03 lb/MMBtu | 1.1E-03 lb/MMBtu | No change |
Liquid Fuel | Mercury | 2.0E-06 lb/MMBtu | 7.3E-07 lb/MMBtu | ~64% lower |
Gas 2 (other) | HCl / Mercury | 1.7E-03 / 7.9E-06 | 1.7E-03 / 7.9E-06 | No change |
Gas 2 (other) | PM | 6.7E-03 lb/MMBtu | 7.3E-03 lb/MMBtu | ~9% higher |
Source: EPA's October 6, 2022, final rule and the current Table 2 and Table 15 to Subpart DDDDD of Part 63. Facilities should confirm the applicable limit for their specific unit, fuel subcategory, pollutant, and compliance method.
The HCl limit decreased for solid-fuel units. The mercury limit decreased for both solid-fuel and liquid-fuel units, with the liquid-fuel reduction representing the largest numerical change in the table. Liquid-fuel and Gas 2 HCl limits did not change in this amendment.
These existing-source figures aren’t the full picture. New-source limits tightened as well — as noted above, the solid fuel new-source HCl limit fell by two orders of magnitude — and fluidized bed biomass-fueled units saw sharp particulate matter reductions on both sides: the new-source PM limit dropped from 9.8E-03 to 4.1E-03 lb/MMBtu, and the existing-source PM limit dropped from 1.1E-01 to 7.4E-03 lb/MMBtu.
How Fuel and Operating Conditions Affect HCl Compliance Margin
HCl emissions correlate with fuel chloride content as well as control-device performance. A facility without add-on acid gas controls can see HCl levels shift with fuel variability alone, independent of control-equipment condition.
This affects testing frequency as well. Under §63.7515, a facility demonstrating emissions at or below 75% of the applicable limit for two consecutive years may qualify for reduced-frequency testing, once every three years instead of annually. A lower emission limit also lowers the numerical value of that 75% threshold, reducing the available margin even without a change in actual performance.
The same consideration applies to compliance demonstration via fuel analysis: compliance via fuel analysis for HCl requires monthly sampling by default, and may be reduced to quarterly only after 12 consecutive months at or below 75% of the compliance level. A single result above that threshold returns the facility to monthly analysis.
Managing Mercury Compliance Under the Revised Boiler MACT Limits
Mercury compliance is subject to the same reduced-frequency testing mechanics as HCl under §63.7515. A facility qualifying for reduced-frequency mercury testing before the 2025 transition may no longer qualify under the current, lower limit, depending on recent test results.
Mercury compliance via fuel analysis follows the same monthly-to-quarterly structure described above. Facilities relying on fuel analysis for mercury should confirm that recent results still support their current testing frequency under the applicable Table 1 or Table 2 limit.
When to Conduct Engineering Testing Before Your Compliance Test
Engineering and investigative testing may be valuable before your next required performance test if:
Your facility is preparing for its first performance test under the revised limits and has not yet evaluated whether current emissions meet the applicable standards
Your previous results were close to the now-applicable HCl, mercury, or particulate matter limit
Your facility has changed fuels, operating loads, reagent feed rates, or control-system settings
Your dry scrubber, sorbent injection system, baghouse, or electrostatic precipitator has undergone maintenance or modifications
Your team has observed changes in control-device performance or emissions trends
You want to evaluate performance under new representative operating conditions before your required compliance performance test.
How Alliance Helps You Prepare
Alliance’s testing professionals can develop a targeted measurement program around your facility’s specific pollutant, equipment, and operating concerns. The resulting data can help your team:
Understand its current compliance margin
Identify factors contributing to performance variability
Evaluate emissions across different operating scenarios
Make informed adjustments before the required test
Approach test day with fewer unknowns and greater confidence
Backed by the nation’s largest stack testing network, Alliance Technical Group brings the technical expertise and national resources needed to investigate potential concerns and help your facility prepare.
If your facility has not yet tested under the newly-applicable Boiler MACT limits, now is the time to assess current performance. Alliance can help you identify potential concerns and determine what adjustments may be needed before your required compliance test.
Boiler MACT Frequently Asked Questions
Which pollutants are regulated under Boiler MACT?
Subpart DDDDD covers particulate matter, HCl, mercury, and carbon monoxide, with limits and work-practice requirements that vary by unit design, fuel type, and source classification.
When did existing sources have to comply with the revised limits?
The alternative limits in the rule gave new and existing sources a voluntary grace period to come into compliance after the EPA finalized the revised limits in October 2022. That grace period sunset on October 6, 2025, so sources that had been relying on the alternative limits are now required to meet the current limits in Tables 1 and 2.
What changed for HCl versus mercury in 2025?
The HCl limit decreased for solid-fuel units only. The mercury limit decreased for both solid-fuel and liquid-fuel units, with liquid-fuel mercury reflecting the largest percentage reduction of any pollutant addressed in the rule change.
What happens if a performance test result is above 75% of the applicable limit?
It may affect eligibility for reduced-frequency testing under §63.7515. This is a separate consideration from exceeding the emission limit itself.
What happens if a performance test exceeds the applicable emission limit?
It may trigger reporting, corrective-action, and other compliance obligations, depending on the unit, pollutant, and permit conditions.
How is an engineering test different from a compliance test?
A compliance performance test is the formal demonstration submitted to regulators. An engineering or investigative test uses applicable reference methods to evaluate performance beforehand, so risk can be identified and addressed before the required compliance performance test.
