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U.S. EPA Finalizes Major Source Boiler MACT Reconsiderations

Posted: November 19th, 2015

Author: All4 Staff 

On November 5, 2015, U.S. EPA finalized the proposed reconsiderations to the Major Source Boiler MACT1 rule. (As of the date of this article, the proposed reconsiderations to the Area Source Boiler MACT and CISWI rules have not yet been finalized.) The final Major Source Boiler MACT rule comes nearly one year after the reconsiderations were proposed on December 1, 2014, and just under three months before the final January 31, 2016 compliance date. While industry is generally happy with how the final rule addressed certain concerns, the final rule also introduced some new nuances to understand in advance of the compliance date.

While not comprehensive, this post highlights some of the more impactful changes to the rule that you need to know about.

Startup and Shutdown

U.S. EPA finalized a proposed alternate definition of “startup,” which extends startup for an additional 4-hour window after an affected source supplies “useful thermal energy” such as heat or steam for heating, cooling, or process purposes, or, generates electricity, whichever is earlier. Defining when startup ends is important because it determines when an affected source must transition from complying with work practice standards to complying with emissions limits and operating limits. The 4-hour window in the alternate definition of “startup” allows boiler and process heater gases to reach the parameters (e.g., temperature, pressure, flow, etc.) necessary for the safe operation of air pollution control equipment. However, there is a catch – owners and operators who choose to utilize the alternate definition must (1) develop, maintain, and implement a written startup and shutdown plan (SSP), and (2) maintain additional records associated with the 4-hour period (i.e., time that clean fuel combustion begins, time that non-clean fuel feeding begins, time when useful thermal energy is first supplied, time when PM controls are engaged, hourly steam temperature/pressure/flow, and all hourly average CMS data).

Clean Fuels

In the final rule, U.S. EPA added the definition of “clean dry biomass” and designated it as a clean fuel. Once “non-clean” fuels are combusted, certain types of air pollution control equipment must be engaged, even during startup according to its definition. In the final rule, owners and operators who choose to utilize the alternate definition of “startup” must engage all of the applicable control devices within four hours of the start of supplying useful thermal energy. Particulate controls must be engaged and operated within one hour of first feeding non-clean fuels. Owners and operators can request a variance from their permitting authority if they have extenuating circumstances that would impact the ability to engage particulate control devices within one hour of non-clean fuel firing.

Affirmative Defense

As expected, U.S. EPA removed the provisions for affirmative defense for violation of emission standards during malfunction. U.S. EPA explained in the proposed reconsideration that the basis for this removal was the 2014 Natural Resources Defense Council (NRDC) court decision vacating the affirmative defense provisions in the Portland Cement (PC) MACT.


U.S. EPA added language around the initial and ongoing reporting periods and associated deadlines. However, there is still some uncertainty around time period and deadline for the initial report. Fortunately, language was also added to clarify that sources with a Title V permit can submit the required Boiler MACT compliance reports according to the schedule established by the permitting authority. U.S. EPA also made changes to the electronic reporting requirements to clarify how to submit different types of data [e.g., performance test results, relative accuracy test audit (RATA) results, etc.] depending on what is supported by U.S. EPA’s Electronic Reporting Tool (ERT).

pH Monitoring

U.S. EPA added language to the rule to clarify that wet scrubbers utilized solely for particulate control and not HCl (i.e., water scrubbers vs. alkaline slurry/solution scrubbers) are not subject to pH monitoring as previously implied in certain parts of the rule. The 2-point performance evaluation frequency for pH monitors was reduced from quarterly to annually, and the daily performance evaluation frequency was replaced with a daily requirement to clean the pH probe and to document that the pH monitor calibration frequency is sufficient to maintain its specified accuracy.

Averaging Periods

U.S. EPA updated the definition of “30-day rolling average” to clarify that the 720-hour basis applies to CO CEMS; all other parameters can be calculated either using the 720-hour basis or a new option of using all valid hours of data from 30 consecutive operating days. Additionally, U.S. EPA added the definition of “rolling average” to explain how averages should be calculated for CO CEMS and other parameters.

Hybrid Suspension Grate Subcategory

The definition of “hybrid suspension grate (HSG) boiler” now states that the minimum 40% moisture content of the biomass fired must be demonstrated by monthly fuel analysis. However, the rule does not specify what constitutes monthly fuel analysis. Additionally, the CO emissions limit for HSG boilers was increased from 2,800 to 3,500 ppmvd @ 3% O2 as demonstrated by stack testing. The 30-day rolling average CO CEMS emission limit of 900 ppmvd @ 3% O2 did not change. An option to request an alternative test method was also added for use of CO2 as a diluent correction instead of O2.


U.S. EPA added a provision to the opacity operating limit for boilers and process heaters with a fabric filter and not using a PM continuous parametric monitoring system (CPMS), such that affected sources can comply with either a 10% limit as a daily block average or the highest hourly average opacity reading measured during the most recent performance test.

The November 20, 2015 Federal Register publication of the final rule can be found here.

What provisions of the final rule are you most concerned about? Let me know in the comments or reach out to me directly to discuss at lkroos@all4inc.com or 610.933.5246 x122.

Related content:

140 CFR Part 63, Subpart DDDDD (National Emissions Standards for Hazardous Air Pollutants for Major Sources: Industrial, Commercial, and Institutional Boilers and Process Heaters) establishes Maximum Achievable Control Technology (MACT) standards for boilers and process heaters at major sources of hazardous air pollutants (HAPs).


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