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MEMORANDUM 

TO: Docket ID No. EPA-HQ-OAR-2002-0058 

FROM: Jim Eddinger, U.S. Environmental Protection Agency, OAQPS 

DATE: January 3, 2011 

SUBJECT: Development of Alternate Equivalent Output-Based Emission Limits for Boilers 

and Process Heaters located at Major Source Facilities 

The purpose of this memorandum is to present the methodology used and results for the 

development of alternate output-based emission limits that are equivalent to the maximum 

achievable control technology (MACT) floor determinations for boilers and process heaters at 

major sources of hazardous air pollutants (HAP) reported in the memorandum “Revised MACT 

Floor Analysis (2011) for the Industrial, Commercial, and Institutional Boilers and Process 

Heaters National Emission Standards for Hazardous Air Pollutants – Major Source.”1 

The development of the alternate equivalent output-based emission limits followed a 

four-step process: First, a list of the best performing units for each subcategory was compiled. 

Next, the steam energy output for each best performing unit was determined. Third, for all units 

reporting steam energy output data, the boiler efficiencies were averaged and assigned as a 

subcategory-specific boiler efficiency factor. Finally, we then applied the average boiler 

efficiency factor to the final MACT limits that are in the current format of pound per million Btu 

heat input to develop the alternate output-based limits. 

We investigated ways to promote energy efficiency in boilers by changing the manner in 

which we regulate flue gas emissions. In an effort to promote energy efficiency in industrial, 

commercial, and institutional (ICI) steam generating facilities, we have developed alternate 
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output-based emission limits, which are equivalent to the final MACT Floor emission limits that 

are in either the heat-input format or concentration. 

Traditionally, boiler emissions have been controlled on the basis of boiler input energy 

(lb of pollutant/million Btu heat input). However, input-based limitations allow units with low 

operating efficiency to emit more emissions per output (steam or electricity) produced than more 

efficient units. Considering two units of equal capacity, under current regulations, the less 

efficient unit will emit more emissions because it uses more fuel to produce the same amount of 

output (steam or electricity). One way to regulate mass emissions and boiler efficiency is to 

express the emission standard in terms of output energy. Thus, output-based emission standards 

would provide a regulatory incentive to enhance unit operating efficiency and reduce emissions. 

The output-based format selected for the alternate standards is mass of pollutant emitted per 

boiler steam output (lb/million Btu steam output). 

The objective of an output-based standard is to establish an emission limit in a format 

that incorporates the effects of boiler efficiency. Additionally, the limit should be in a format 

that is practical to implement. Thus, the format selected must satisfy the following: (1) provide 

flexibility in promotion of boiler efficiency; (2) permit measurement of parameters related to 

stack emissions and boiler efficiency, on a continuous basis; and (3) be suitable for equitable 

application on a variety of facility configurations. The format of lb/million Btu steam output 

accounts meets all three criteria. A second output-based format option considered was pound per 

Megawatt-hour (lb/MWh) which accounts not only for boiler efficiency but also the turbine 

cycle efficiency. Although some ICI boilers generate, or cogenerate, electricity, the majority of 

ICI boilers produce steam only for process operation or heating. However, the boiler’s total 

energy output can include both electrical and thermal (process steam) outputs. Both of these 
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energy outputs are relatively easy to measure accurately, and currently are measured routinely in 

power plants. Based on this analysis, an emission limit format based on mass of pollutant 

emissions per steam energy output is selected for the alternate output-based standards. Emission 

limits based on this format can be applied equitably on a variety of facility configurations. 

Because steam output is the main energy output at all ICI facilities, it is desirable to use a format 

in “lb/million Btu steam.” 

Compliance with the alternate output-based emission limits may require continuous 

measurement of boiler operating parameters associated with the mass rate of emissions and 

energy outputs. Instrumentation already exists in facilities to conduct these measurements since 

the instrumentation is required to support normal facility operation. Consequently, compliance 

with the alternate output-based emission limits is not expected to require any additional 

instrumentation. However, additional signal input wiring and programming is expected to be 

required to convert the above measurements into the compliance format (lb/million Btu steam). 

Since the June 4, 2010 proposal, we conducted an effort to obtain steam data (flow, 

temperature, and pressure) for the test period corresponding to the reported emission data from 

the best performing units comprising the proposed MACT floor pool. In determining alternate 

equivalent output-based emission limits, we first determined for each of the best performing 

units the Btu output of the steam and then calculated the boiler efficiency for each of the boilers 

having available steam/heat input data. Boiler efficiency is defined as steam Btu output divided 

by fuel Btu input. Next, we determined the average boiler efficiency factor for each subcategory 

from the best performing units in that subcategory. We then applied the average boiler 

efficiency factor to the final MACT Floor (lb/million Btu heat input) limits to develop the 

alternate output-based limits. The efficiency factor approach was selected because the 
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alternative of converting all the reported data in the database to an output-basis would require 

extensive data gathering and analyses. Applying an average boiler efficiency factor, based on 

the individual boiler efficiency of the best performing units, would essentially convert the heat 

input-based limits to output-based emission limits. 

We have concluded that including alternate output-based emission limits in the final rule 

would not lessen the stringency of the MACT floor limits and would provide flexibility in 

compliance and cost and energy savings to owners and operators. We also recognize that we 

must ensure that the alternate emission limits can be implemented and enforced, will be clear to 

sources, and most importantly, will be no less stringent than implementation of the MACT floor 

limits. 

The first step of the boiler efficiency factor development process involved developing a 

list of the best performing units reported in the MACT Floor Memo, prepared for the proposed 

rule, and determining the steam energy output for each of the best performing units. Appendix A 

presents the list of the best performing units along with the available steam output information. 

As part of the Information Collection Request (ICR), both the questionnaire (ICR phase 

1) and the testing phase (ICR phase 2) requested as part of the emission data submittal both heat 

input and steam flow during the test. However, in order to determine steam energy output (BTU 

per hour), the steam conditions (temperature and pressure) are needed in addition to steam flow 

information (1000 pounds per hour). We first reviewed the available test reports for steam data 

information. However, very few reported the steam temperature and pressure. Therefore, we 

sent an email, under our ICR authority, requesting steam data (flow, temperature, and pressure) 

from the best performing units that corresponded to the test period of the submitted emission 

data. From the obtained steam data (flow, temperature, and pressure), we calculated the steam 
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energy output (million Btu of steam per hour) using Steam Tables2 to determine the specific 

enthalpy (Btu per pound) at the reported steam temperature and pressure. Appendix A presents 

the steam conditions, steam flow, steam energy, and the calculated boiler efficiency for each of 

the best performing units for which we obtained steam information. 

 Boiler efficiency factors are specific to a certain subcategory. Ten different 

subcategories were targeted in this analysis, all of which are listed in Table 1. Although we 

requested and obtained steam data for the best performing units identified for the proposed rule, 

the average boiler efficiency factors in Table 1 were developed based only on those units that 

were also identified as best performing units for the final rule. 

Table 1 – Subcategories and Assigned Boiler Efficiency Factors 

Subcategory Average Boiler Efficiency 

Factor (%) 

Solid fuel 97.4 

Pulverized Coal/Fossil 

Solid 102.7 

Coal/Fossil Solid Stoker 106.3 

Coal/Fossil Solid Fluidized 

Bed 112.3 

Biomass/Bio-based Solid 

Stoker 91.1 

Biomass/Bio-based Solid 

Fluidized Bed 80.6 

Biomass/Bio-based Solid 

Dutch Oven 121.7 

Biomass/Bio-based Solid 

Fuel Cell 61.0 

Biomass/Bio-based Solid 

Suspension Burner 72.4 

Liquid Fuel Units1 94.0 

Gas 2 60.1 

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 Efficiency of all liquid units was estimated to be the same, regardless 
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of location. 

To calculate an average boiler efficiency factor for each subcategory, all available best 

performing unit-specific boiler efficiency listed in Appendix A were averaged except for the 

high and low values. This approach was taken because some subcategories had 

uncharacteristically high or low values which were considered outliers. Therefore, for 

consistency, the high and low boiler efficiency values were excluded from the determination of 

the average boiler efficiency factor for each subcategory. 

We determined the alternate output-based emission limits to each MACT floor emission 

limit by applying the average boiler efficiency for the subcategory to the values reported in the 

MACT Floor Memo. The alternate output-based emission limits are summarized for each 

subcategory in Table 2 (existing sources) and Table 3 (new sources). The data analyzed for the 

average boiler efficiency factor are shown in Appendix A. 

Table 2: Summary of Calculated Output-Based (MACT Floor) Limits for Existing Boilers 

and Process Heaters at Major Sources of HAP 

Subcategory Pollutant Output-based emission limits 

1. Units designed to burn solid 

fuel a. Particulate Matter 0.038 lb per MMBtu of steam output. 

 b. Hydrogen Chloride 0.04 lb per MMBtu of steam output. 

 c. Mercury 4.5E-06 lb per MMBtu of steam output.

 2. Pulverized coal/fossil solid a. CO 0.14 lb per MMBtu of steam output. 

 b. Dioxin/Furan 3.7E-12 lb per MMBtu (TEQ) of steam 

output. 

 3. Stokers designed to burn 

coal/fossil solid a. CO 0.25 lb per MMBtu of steam output. 

 b. Dioxin/Furan 2.8E-12 lb per MMBtu (TEQ) of steam 

output. 

4. Fluidized bed units designed 

to burn coal/fossil solid a. CO 0.08 lb per MMBtu of steam output. 

 b. Dioxin/Furan 1.8E-12 lb per MMBtu (TEQ) of steam 
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output. 

5. Stokers designed to burn 

biomass/bio-based solid a. CO 0.35 lb per MMBtu of steam output. 

 b. Dioxin/Furan 4.4E-12 lb per MMBtu (TEQ) of steam 

output. 

6. Fluidized bed units designed 

to burn biomass/bio-based solid a. CO 0.28 lb per MMBtu of steam output. 

 b. Dioxin/Furan 1.8E-11 lb per MMBtu (TEQ) of steam 

output. 

7. Dutch Ovens designed to 

burn biomass/bio-based solid a. CO 0.45 lb per MMBtu of steam output. 

 b. Dioxin/Furan 1.8E-10 lb per MMBtu (TEQ) of steam 

output. 

8. Fuel cells designed to burn 

biomass/bio-based solid a. CO 0.34 lb per MMBtu of steam output. 

 b. Dioxin/Furan 3.5E-9 lb per MMBtu (TEQ) of steam 

output. 

9. Suspension/Grate designed to 

burn biomass/bio-based solid a. CO 2.0 lb per MMBtu of steam output. 

 b. Dioxin/Furan 1.8E-10 lb per MMBtu (TEQ) of steam 

output. 

10. Units designed to burn 

liquid fuel a. Particulate Matter 0.0073 lb per MMBtu of steam output. 

 b. Hydrogen Chloride 0.0003 lb per MMBtu of steam output. 

 c. Mercury 3.3E-06 lb per MMBtu of steam output.

 d. CO 0.0083 lb per MMBtu steam output. 

 e. Dioxin/Furan 9.2E-09 lb per MMBtu (TEQ) of steam 

output. 

11. Units designed to burn other 

gases 

a. Particulate Matter 0.026 lb per MMBtu of steam output. 

 b. Hydrogen Chloride 0.001 lb per MMBtu of steam output. 

 c. Mercury 7.8E-06 lb per MMBtu of steam output.

 d. C0 0.005 lb per MMBtu steam output. 

 e. Dioxin/Furan 3.9E-11 lb per MMBtu (TEQ) of steam 

output. 

12. Units designed to burn liquid 

fuel located in non-continental 

States and territories

a. Particulate Matter 0.0073 lb per MMBtu of steam output. 

 b. Hydrogen Chloride 0.0003 lb per MMBtu of steam output. 

 c. Mercury 8.0E-07 lb per MMBtu of steam output.
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 d. CO 0.13 lb per MMBtu steam output. 

 e. Dioxin/Furan 9.2E-09 lb per MMBtu (TEQ) of steam 

output. 

Table 3: Summary of Calculated Output-Based (MACT Floor) Limits for New Boilers and 

Process Heaters at Major Sources of HAP 

Subcategory Pollutant Output-based emission limits 

1. Units designed to burn 

solid fuel a. Particulate Matter 0.0011 lb per MMBtu of steam output. 

 b. Hydrogen Chloride 0.0021 lb per MMBtu of steam output. 

 c. Mercury 3.4E-06 lb per MMBtu of steam output. 

 2. Pulverized coal/fossil 

solid a. CO 0.01 lb per MMBtu of steam output. 

 b. Dioxin/Furan 2.8E-12 lb per MMBtu (TEQ) of steam 

output. 

 3. Stokers designed to burn 

coal/fossil solid a. CO 0.005 lb per MMBtu of steam output. 

 b. Dioxin/Furan 2.8E-12 lb per MMBtu (TEQ) of steam 

output. 

4. Fluidized bed units 

designed to burn coal/fossil 

solid 

a. CO 0.02 lb per MMBtu of steam output. 

 b. Dioxin/Furan 1.8E-12 lb per MMBtu (TEQ) of steam 

output. 

5. Stokers designed to burn 

biomass/bio-based solid a. CO 0.13 lb per MMBtu of steam output. 

 b. Dioxin/Furan 4.4E-12 lb per MMBtu (TEQ) of steam 

output. 

6. Fluidized bed units 

designed to burn 

biomass/bio-based solid 

a. CO 0.18 lb per MMBtu of steam output. 

 b. Dioxin/Furan 1.8E-11 lb per MMBtu (TEQ) of steam 

output. 

7. Dutch Ovens designed to 

burn biomass/bio-based 

solid 

a. CO 0.45 lb per MMBtu of steam output. 

 b. Dioxin/Furan 1.8E-10 lb per MMBtu (TEQ) of steam 

output. 

8. Fuel cells designed to 

burn biomass/bio-based 

solid 

a. CO 0.23 lb per MMBtu of steam output. 
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 b. Dioxin/Furan 2.86-12 lb per MMBtu (TEQ) of steam 

output. 

8. Suspension burners 

designed to burn 

biomass/bio-based solid 

a. CO 0.84 lb per MMBtu of steam output. 

 b. Dioxin/Furan 1.8E-10 lb per MMBtu (TEQ) of steam 

output. 

9. Units designed to burn 

liquid fuel a. Particulate Matter 0.001 lb per MMBtu of steam output. 

 b. Hydrogen Chloride 0.003 lb per MMBtu of steam output. 

 c. Mercury 0.2E-06 lb per MMBtu of steam output. 

 d. CO 0.0026 lb per MMBtu of steam output. 

 e. Dioxin/Furan 4.6E-12 lb per MMBtu (TEQ) of steam 

output. 

10. Units designed to burn 

other gases a. Particulate Matter 0.004 lb per MMBtu of steam output. 

 b. Hydrogen Chloride 0.003 lb per MMBtu of steam output. 

 c. Mercury 2.0E-07 lb per MMBtu of steam output. 

 d. CO 0.002 lb per MMBtu of steam output. 

 e. Dioxin/Furan 4.1E-12 lb per MMBtu (TEQ) of steam 

output. 

11. Units designed to burn 

liquid fuel located in noncontinental States and 

territories

a. Particulate Matter 0.001 lb per MMBtu of steam output. 

 b. Hydrogen Chloride 0.003 lb per MMBtu of steam output. 

 c. Mercury 8.0E-07 lb per MMBtu of steam output. 

 d. CO 0.043 lb per MMBtu steam output. 

 e. Dioxin/Furan 4.6E-12 lb per MMBtu (TEQ) of steam 

output. 

REFERENCES 

1. A. Singleton, ERG. Revised MACT Floor Analysis (2011) for the Industrial, 

Commercial, and Institutional Boilers and Process Heaters National Emission Standards 

for Hazardous Air Pollutants – Major Source. January 2011. 

2. J.H. Keenan, F.G. Keyes, P.G. Hill, and J.G. Moore, Steam Tables:Thermodynamic 

Properties of Water Including Vapor, Liquid, and Solid Phases – English Units, 1969.
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APPENDIX A 

BEST PERFORMING UNITS 

(See Excel spreadsheet)