Ask the Expert, Q&A, Special Topics in Environmental Management

Boiler MACT

Q. Will a source having four large gas fired boilers with fuel oil back up trigger the Boiler Maximum Achievable Control Technology Standards (MACT) if it burns oil to avoid high natural gas prices?

A. Per 40 CFR 63.11193 and 40 CFR 63.11194, 40 CFR 63, Subpart JJJJJJ–National Emission Standards for Hazardous Air Pollutants (NESHAP) for Industrial, Commercial, And Institutional Boilers Area Sources applies to industrial boilers within one of the following subcategories and located at an area source of hazardous air pollutants (HAP):

  • Coal.
  • Biomass.
  • Oil.
  • Seasonal boilers.
  • Oil-fired boilers with heat input capacity of equal to or less than 5 million British thermal units (Btu) per hour.
  • Boilers with an oxygen trim system that maintains an optimum air-to-fuel ratio that would otherwise be subject to a biennial tune-up.
  • Limited-use boilers.

The question submitted indicates the boilers fire both natural gas and oil (back-up). Natural gas is not among the aforementioned subcategories, but oil is.

As stated in 40 CFR 63.11195, gas-fired boilers are not subject to this NESHAP. A gas-fired boiler is defined as follows:

Gas-fired boiler includes any boiler that burns gaseous fuels not combined with any solid fuels and burns liquid fuel only during periods of gas curtailment, gas supply interruption, startups, or periodic testing on liquid fuel. Periodic testing of liquid fuel shall not exceed a combined total of 48 hours during any calendar year.

To determine if the boilers in question qualify for this exemption we must evaluate how EPA defines a “period of gas curtailment.” The question submitted states that “they burn fuel oil during times of gas curtailment that does not include gas interruption. They burn fuel oil to not incur high gas pricing.” Under 40 CFR 63.11237, EPA defines “Period of gas curtailment or supply interruption” as follows:

“Period of gas curtailment or supply interruption means a period of time during which the supply of gaseous fuel to an affected boiler is restricted or halted for reasons beyond the control of the facility. The act of entering into a contractual agreement with a supplier of natural gas established for curtailment purposes does not constitute a reason that is under the control of a facility for the purposes of this definition. An increase in the cost or unit price of natural gas due to normal market fluctuations not during periods of supplier delivery restriction does not constitute a period of natural gas curtailment or supply interruption. On-site gaseous fuel system emergencies or equipment failures qualify as periods of supply interruption when the emergency or failure is beyond the control of the facility.”

As stated in the aforementioned definition, avoiding higher gas prices does not constitute a “period of curtailment.” Therefore, the boilers as described in the question do not qualify for the gas-fired boiler exemption.

The next step is to determine if the boilers would be part of the “oil subcategory” and be subject to the NESHAP. Under 40 CFR 63.11237, EPA defines the “oil subcategory” as follows:

“Oil subcategory includes any boiler that burns any liquid fuel and is not in either the biomass or coal subcategories. Gas-fired boilers that burn liquid fuel only during periods of gas curtailment, gas supply interruptions, startups, or for periodic testing are not included in this definition. Periodic testing on liquid fuel shall not exceed a combined total of 48 hours during any calendar year.”

Based on this definition, and the fact that we have already determined that the manner in which these boilers are operated does not meet EPA’s definition of “period of curtailment,” the boilers as described in the question would be part of the “oil subcategory” and be subject to the NESHAP.