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Major Source Boiler MACT Compliance with Oxygen Trim Systems

Posted: March 29th, 2013

Author: All4 Staff 

The National Emission Standards for Hazardous Air Pollutants (NESHAP) for Major Sources: Industrial, Commercial, and Institutional Boilers and Process Heaters promulgated at 40 CFR Part 63, Subpart DDDDD, commonly referred to as the Major Source Boiler MACT (MSBM) standards, provide multiple continuous compliance options that require monitoring.  Among these options is the use of a continuous oxygen (O2) trim system for continuous compliance with the carbon monoxide (CO) emission standard.  This article will discuss the following questions about O2 trim systems:

  • What are O2 trim systems?
  • Does my boiler have an O2 trim system?
  • Can I put an O2 trim system on my existing boiler?
  • How are O2 trim systems use for continuous compliance with MSBM?
  • Are there any advantages/disadvantages to O2 trim systems?

What are O2 trim systems?

The MSBM defines an O2 trim system at 40 CFR §63.7575 as:

“Oxygen trim system means a system of monitors that is used to maintain excess air at the desired level in a combustion device. A typical system consists of a flue gas oxygen and/or CO monitor that automatically provides a feedback signal to the combustion air controller.”

In other words, an O2 trim system is designed to continuously measure and maintain an optimum air-to-fuel ratio in the boiler combustion zone.  The amount of O2 present in the exhaust gas of a boiler indicates how much excess air was present in the gas mixture.  An O2 trim system measures the amount of O2 in the exhaust gas stream and provides feedback to automatically position the air damper to the proper position to maintain a set point of excess air.  For the most part, boilers that operate with minimal excess air are more efficient.

Does my boiler have an O2 trim system?

Since O2 trim systems make boilers operate more efficiently, thereby using less fuel, you may be surprised to learn that your boiler already has an O2 trim system.  Plant engineers and/or operators will know if your boiler uses an O2 trim system.  When asking around, be sure to inquire about several specific items of the O2 trim system, such as:

  • Can the automatic nature of the O2 trim system be disabled?
  • If the O2 trim system can be disabled, do boiler operators operate the boiler in “manual” mode?
  • Can the boiler be operated with the O2 trim system in “automatic” mode at all times?
  • How is the O2 trim system used during startup, shutdown, fuel switching, etc.?
  • How is the O2 trim system maintained and quality-assured?
  • What parameters of the O2 trim system are recorded in a data historian?

Remember that if an O2 trim system is used as a compliance option for the MSBM, it is now an environmental instrument used to demonstrate continuous compliance.  As a result, the O2 trim system must be operated at all times, potentially requiring revisions the boiler’s standard operating procedures.  The O2 trim system must also be installed, calibrated, maintained, and operated in accordance with the manufacturer’s recommendations.  The data measured from the O2 trim system must be quality-assured to assess its accuracy even if the manufacturer’s recommendations are limited.  The data measured from the O2 trim system must also be recorded for compliance purposes.

Can I put an O2 trim system on my existing boiler?

O2 trim systems most likely can be installed your existing boilers.  However, there may not be a cost effective impact on boiler efficiency.  O2 trim systems are primarily designed to increase the efficiency of boilers.  If your boiler is already designed to operate at lower excess air levels, any improved efficiency that is gained from an O2 trim system may not be cost effective in comparison to the O2 trim system design, installation, and operating costs.  However, now that an O2 trim system can be part of a compliance option for the MSBM, its cost effectiveness may increase.

How are O2 trim systems use for continuous compliance with MSBM?

An O2 trim system is a continuous compliance option for certain MSBM subcategory boilers that are subject to a CO emission standard.  O2 trim systems can also extend the boiler tune-up requirements for other MSBM subcategory boilers.  For continuous compliance with MSBM, a minimum O2 trim system set point is established from the lowest hourly average O2 concentration measured during the most recent CO performance test.  Continuous compliance is demonstrated by properly maintaining and quality-assuring the O2 trim system and the allowing the O2 trim system to automatically control the amount of excess air to the established set point.

Are there any advantages/disadvantages to O2 trim systems?

There are advantages to using an O2 trim system as a compliance option for the MSBM. The O2 trim system may already exist, making the installation cost minimal. It is also a “set-point” compliance demonstration, meaning that the O2 trim system is set to a concentration that was established during performance testing and compliance is determined by the O2 trim system adjusting the excess air to that set-point. The O2 measurements from the O2 trim system are not used for direct compliance.

An O2 trim system may also have compliance advantages during startup and shutdown. The MSBM has defined the periods “when startup ends” and “when shutdown begins.” At this point, certain subcategory boilers will be subject to the numerical emission standards for CO rather than the work practice standards. Many boilers may struggle with meeting a numerical CO emission standard by measuring CO emissions directly by a continuous emission monitoring system (CEMS). An O2 trim system, on the other hand, does not provide a direct measurement against the emission standard and could provide an alternative means for compliance during these periods.

The disadvantages of O2 trim systems include the costs associated with installation (if applicable) and the cost associated with maintaining the O2 trim system as an “environmental instrument.” However these costs may be small in comparison to the other MSBM compliance options for CO. Other disadvantages may include the impact on boiler operations, in that there may be operational limitations when using the O2 trim systems during startup, shutdown, fuel switching, etc.

Determining your MSBM compliance strategy can be the best and worst part of the implementation process. Evaluating the available compliance options for cost, operational impacts, and long-term facility plans can be exciting, but it can also be overwhelming. Educate yourself on the MSBM, the multiple types of control and monitoring technologies/ approaches, and compare to what others are doing. The decisions made today can greatly impact your facility’s future success.

What CO compliance options are you considering for your facility? Contact Eric Swisher at 610.933.5246 x117 or eswisher@all4inc.com to discuss the impacts that an O2 trim system may have for your boiler.


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