Excess Emission CEMS vs Compliance CEMS in Subpart Db

For some reason, NOx CEMS in Subpart Db seems to be coming up a lot lately in my work. I have recently had several conversations with clients and regulatory agencies regarding how NOx CEMS are treated in Subpart Db. And there seems to be some confusion on the subject. So I thought a blog post would be appropriate. If your boiler has a heat input rate less than or equal to 250 MMBtu/hr, Subpart Db states that any NOx CEMS installed is an excess emission CEMS not a compliance CEMS. Given that, the installed excess emission CEMS is not subject to the requirements of Appendix F including quarterly CGAs and annual RATAs. Here’s how it works…

40 CFR 60 Subpart Db establishes two categories of Continuous Emission Monitoring Systems (CEMS) – Excess Emission CEMS and Compliance CEMS. These two categories differ in use of their output as well as their QA/QC requirements. 

60.48b(b) requires the installation of a NOx CEMS on any affected unit subject to the NOx standards in the subpart – EXCEPT that units with a heat input rate less than or equal to 250 MMbtu/hr can opt to predict NOx from process data.

Section 60.46b(e)(4) establishes the two categories of CEMS. It states, in part…

“During periods when performance tests are not requested, NOx emissions data collected [from units ≤250 MMBtu] are used to calculate a 30-day rolling average emission rate on a daily basis and used to prepare excess emission reports, but will not be used to determine compliance with the NOx emission standards.” (emphasis added)

This is not an option. This is a mandatory requirement that applies to all affected units with a heat input less than or equal to 250 MMBtu/hr subject to the NOx standard in this subpart. But what does this mean from a practical standpoint?

A CEMS used to determine compliance with the NOx emission standards (compliance CEMS), directly determines compliance with the NOx standard. If the compliance NOx CEMS records NOx readings above the limit for the appropriate averaging period, that constitutes a presumptive violation of the NOx standard in and of itself. No additional data is needed. Could the data be used as credible evidence of a violation? Yes. Most likely. But that’s another discussion.

However, if a CEMS used only to prepare excess emission reports (excess emission CEMS) records NOx readings above the limit, there is no presumptive violation. A stack test using the appropriate EPA reference method would have to be performed to determine the compliance status of the unit.

This may seem somewhat counter-intuitive at first. A compliance CEMS and an excess emission CEMS have exactly the same hardware. Why can one be used for compliance and the other not? It’s all about the QA/QC. A compliance CEMS is subject to the more stringent QA/QC found in 40 CFR 60 Appendix F. In addition to the daily zero and span checks, the compliance CEMS is also required to undergo quarterly Cylinder Gas Audits (CGA) and annual Relative Accuracy Test Audits (RATA). The excess emission CEMS is subject to the daily checks but not the quarterly CGAs or annual RATA.

This difference in QA/QC requirements can be easily seen in the applicable regulations. 40 CFR 60, Appendix F is where on-going QA/QC procedures such as CGAs and RATAs are found. There are exactly 12 references to “Appendix F” in Subpart Db of which only one refers to NOx CEMS and that is qualified. For reference, here is a list of each Appendix F reference found in Subpart Db.

1)            60.46b(j)(12)         This section deals with PM CEMS requirements

2)            60.47b(e)(2)          This section deals with SO₂ CEMS requirements

3)            60.47b(e)(4)(i)       This section deals with alternative SO₂ CEMS requirements

4-6)        60.47b(e)(4)(ii)      This section deals with alternative SO₂ CEMS requirements (3 citations)

7-8)        60.47b(e)(4)(iii)     This section deals with alternative SO₂ CEMS requirements (2 citations)

9)            60.48b(j)(4)(i)(D)   This section deals with CO CEMS requirements

10)          60.49b(g)(10)        This section deals with reporting requirements for NOx CEMS that are subject to Appendix F.

11)          60.49b(k)(10)        This section deals with reporting requirements for SO₂ CEMS that are subject to Appendix F.

12)          60.49b(l)(9)           This section deals with reporting requirements for SO₂ CEMS that are subject to Appendix F.

Citation #10 [60.49b(g)(10)] does refer to NOx CEMS and it states:

“(g) Except as provided under paragraph (p) of this section [low capacity factor units], the owner or operator of an affected facility subject to the NOx standards under §60.44b shall maintain records of the following information for each steam generating unit operating day:

(1)…
(10) Results of daily CEMS drift tests and quarterly accuracy assessments as required under appendix F, Procedure 1 of this part.” [emphasis added]

Note the words “as required under appendix F.” Appendix F specifically excludes non-compliance CEMS from applicability. The very first sentence of Appendix F states:

“1.1 Applicability. Procedure 1 is used to evaluate the effectiveness of quality control (QC) and quality assurance (QA) procedures and the quality of data produced by any continuous emission monitoring system (CEMS) that is used for determining compliance with the emission standards on a continuous basis as specified in the applicable regulation.”

Furthermore, §60.48b(e) of Subpart Db states:

“The procedures under §60.13 shall be followed for installation, evaluation and operation of the continuous monitoring systems.”

Section 60.13 also specifically excludes non-compliance CEMS from Appendix F applicability. It states [60.13(a)]…

“For the purposes of this section, all continuous monitoring systems required under applicable subparts shall be subject to the provisions of this section upon promulgation of performance specifications for continuous monitoring systems under appendix B to this part and, if the continuous monitoring system is used to demonstrate compliance with emission limits on a continuous basis, appendix F to this part, unless otherwise specified in an applicable subpart or by the Administrator.” [emphasis added]

So… once again… if your boiler has a heat input rate less than or equal to 250 MMBtu/hr, Subpart Db states that any NOx CEMS installed is an excess emission CEMS not a compliance CEMS. Given that, the installed excess emission CEMS is not subject to the requirements of Appendix F including quarterly CGAs and annual RATAs. The excess emission CEMS is subject to the requirements of §60.13 which include daily zero and span calibration checks among other QA/QC requirements.