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PADEP’s CSMM Rev. 8 Frequently Asked Questions Document Updated!

Posted: November 18th, 2019

Authors: Matt C. 

Did you know that the Pennsylvania Department of Environmental Protection (PADEP or Department) Continuous Emissions Monitoring (CEM) Section within the Division of Air Quality released an updated Frequently Asked Questions (FAQs) document to support Continuous Source Monitoring Manual Revision No. 8 (CSMM Rev. 8) on October 8, 2019?  Well if not, don’t worry, because now you do!  The updated document provides a reference point for certain gray areas within CSMM Rev. 8 and continuous monitoring in the Commonwealth of Pennsylvania.  This article will be the first of several that detail PADEP’s most recent responses to questions related to CSMM Rev. 8.

The first question and answer (Q&A) we’ll discuss is related to the use of a diluent cap for diluent carbon dioxide (CO2) and oxygen (O2) monitors.  What is a diluent cap?  A diluent cap is utilized for instances in which the heat input of a boiler or stationary gas turbine approaches zero (i.e., whenever the CO2 or O2 concentration is at or near ambient air levels, such as during unit startup and shutdown).  During these operating scenarios, calculated pollutant emission results often spike to non-representative levels that result in violations of an existing emission standard.  The non-representative pollutant emission results are typically caused by extremely high or low diluent CO2 or O2 concentrations measured by the CO2 or O2 analyzer and subsequently used to calculate pollutant emissions rate (lb/MMBtu).  When diluent CO2 or O2 concentrations reach such levels, the calculated pollutant emissions that utilize these values become unrealistic and not representative of operations. So how do we handle these scenarios?  Well, prior to this updated FAQ document being published by PADEP, 40 CFR Part 75, Appendix F was often referred to for guidance addressing these scenarios because 40 CFR Part 75 provides an option for the calculation of nitrogen oxide (NOX) emissions (lb/MMBtu) with the use of a “diluent cap”.  The diluent cap provision allows for the substitution of more reasonable CO2 (minimum 5.0% CO2) or O2 (maximum 14.0% O2) concentrations during instances in which the hourly average CO2 concentration is <5.0% CO2 or the hourly average O2 concentration is >14.0% O2.  Thus, the diluent cap functions to correct these unrealistic emissions that are calculated during shutdown/startup scenarios.

We know what a diluent cap is, what the purpose is, and where it comes from.  However, if you aren’t a source continuously monitoring for NOX subject to the requirements of 40 CFR Part 75 this is only guidance that can be referenced as supporting evidence in your petition to PADEP to utilize a diluent cap.  And prior to this updated FAQ document, the mention of a diluent cap in CSMM Rev. 8 (in the “Quality Assurance” section), states that if sources are subject to Federal requirements for diluent caps, they must petition the Department for use of such substituted data for PADEP purposes.  With this recently released FAQ document from the Department, we now have specific guidance from PADEP applicable to boilers or stationary gas turbines required to monitor emissions on a lb/MMbtu basis.  So, what does PADEP say about diluent caps?

  • For boilers, the diluent cap requirements are identical to what 40 CFR Part 75 details for NOX emissions rate (lb/MMBtu) – a minimum concentration of 5.0% CO2, or a maximum concentration of 14.0% O2 can be substituted for the measured diluent gas concentration value for any operating hour in which the hourly average CO2 concentration is <5.0% CO2 or the hourly average O2 concentration is >14.0% O2.
  • For stationary gas turbines, the requirements change – a minimum concentration of 1.0% CO2 or a maximum concentration of 19.0% O2 may be substituted for measured diluent gas concentration values for any operating hours in which the hourly average CO2 concentration is less than 1.0% CO2 or the hourly average O­2 concentration is greater than 19.0% O2.

PADEP also notes that substitution of the diluent concentration must occur at the hourly level – you should not substitute data at the minute level and include any “normal” diluent concentrations in the hourly average.  Additionally, for quarterly reporting purposes, for hours in which the substituted diluent data is utilized, a Monitoring Code (MC) 99 and Method of Determination Code (MODC) 14 should be utilized, unless otherwise stated by the Department in the petition approval letter.  We can’t forget about that last part – if you want to utilize either of the previously described diluent cap provisions mentioned in the FAQ document, you still must petition the Department for use of a diluent cap.  And in addition to petitioning the Department on the use of a diluent cap, you will also have to work with your data acquisition and handling system (DAHS) vendor to ensure the MC and MODC stipulated by PADEP for these scenarios are successfully incorporated into your quarterly electronic data reports (EDR).

If you have any questions about diluent caps, or any other general continuous monitoring system (CMS) questions, please reach out to me at (610) 933-5246 extension 139, or at mcarideo@all4inc.com.  Lastly, don’t forget to keep an eye out for our next article that goes over PADEP’s response to the question “How is a 3-hour average, rolling by one hour calculated?”  That’s right, there is more than one way to calculate a 3-hour rolling average – let’s get clear on how it is done in Pennsylvania.


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