MEMORANDUM 

DATE: January 21, 2009 

SUBJECT: Subcategorization and MACT Floor Determination for Stationary 

Reciprocating Internal Combustion Engines ≤500 HP at Major Sources 

FROM: Tanya Parise, Alpha-Gamma Technologies, Inc. 

TO: Jaime Pagán, EPA Energy Strategies Group 

THROUGH: Bradley Nelson, EC/R 

The purpose of this memorandum is to document the subcategorization and 

analysis of the maximum achievable control technology (MACT) floor for existing 

stationary reciprocating internal combustion engines (RICE) less than or equal 500 

horsepower (HP) located at major sources that are subject to National Emission 

Standards for Hazardous Air Pollutants (NESHAP). A “major source” means any 

stationary source or group of stationary sources located within a contiguous area and 

under common control that emits or has the potential to emit considering controls, in the 

aggregate, 10 tons per year or more of any hazardous air pollutant or 25 tons per year 

or more of any combination of hazardous air pollutants (HAP). 

Introduction

The EPA is developing a regulation affecting existing stationary RICE less than 

or equal to 500 HP located at major sources that will address toxic air emissions from 

these engines. The regulation will be developed following criteria set forth under section 

112(d) of the Clean Air Act (CAA). The MACT floor standards for existing sources must 

be no less stringent than the average emission limitation achieved by the best 

performing 12 percent of existing sources. The EPA has previously issued a similar 

regulation affecting existing and new stationary RICE greater than 500 HP at major 

sources, which was promulgated in 2004. 
Subcategorization 

Section 112 of the CAA allows the EPA to establish subcategories among a 

group of sources. These subcategories are based on technical criteria that differentiate 

sources in terms of technical feasibility or applicability of emissions control. The 

following subcategories are necessary in order to capture the differences which could 

affect the emissions of HAP from these engines: 

• <50 HP 

• Landfill/Digester Gas 50≥HP≤500 

• Emergency Compression Ignition (CI) 50≥HP≤500 

• Non-Emergency CI 50≥HP≤500 

• Emergency Spark Ignition (SI) 50≥HP≤500 

• Non-Emergency Two-Stroke Lean Burn (2SLB) 50≥HP≤500 

• Non-Emergency Four-Stroke Rich Burn (4SRB) 50≥HP≤500 

• Non-Emergency Four-Stroke Lean Burn (4SLB) 50≥HP≤249 

• Non-Emergency 4SLB 250≥HP≤500 HP 

 EPA has previously discussed the appropriateness of establishing the above 

subcategories and the rationale is not repeated in this memorandum. The rationale can 

be found in the memorandum titled Subcategorization of Stationary Reciprocating 

Internal Combustion Engines ≤500 HP, available from http://www.regulations.gov as 

Document ID No. EPA-HQ-OAR-2005-0030-0012. 

MACT Floor Determination 

General Approach

The MACT floor for existing stationary RICE must be no less stringent than the 

average emission limitation achieved by the best performing 12 percent of existing 

sources. For the MACT floor determination, EPA consulted the Population Database. 

The Population Database was originally developed for the NESHAP for stationary RICE 

greater than 500 HP located at major sources. Documentation on the Population 

Database can be found in the docket for that rulemaking (EPA-HQ-OAR-2002-0059). 

The EPA queried the Population Database to determine how many existing stationary 

RICE less than or equal to 500 HP have catalyst type controls. A similar analysis was 

conducted in 2006 with the results summarized in the memorandum titled “Analysis of 

the RICE NESHAP Population Database,” available from http://www.regulations.gov as 

Document ID No. EPA-HQ-OAR-2005-0030-0007. 

In order to establish the emission limit for each subcategory of stationary existing 

engines, EPA referred to the Emissions Database. The Emissions Database was 

originally developed in support of regulatory development for the original RICE NESHAP 

addressing existing and new stationary engines greater than 500 HP located at major 

sources. Documentation on the Emissions Database can be found in the docket for that 

rulemaking (EPA-HQ-OAR-2002-0059). The Emissions Database is an extensive 

compilation of criteria and HAP source test data obtained from various sources such as 

the California Air Resources Board, EPA’s Source Test Information Retrieval System, 
tests conducted by the Gas Research Institute, and testing conducted at Colorado State 

University. 

 EPA extracted tests from the Emissions Database containing formaldehyde and 

carbon monoxide (CO) test results. Formaldehyde is a hazardous air pollutant that is 

emitted by stationary engines. The tests were divided into the subcategories listed on 

page 1 of this memorandum and the formaldehyde and CO emissions test data were 

analyzed. In some cases, the dataset for each subcategory was limited. For example, 

EPA only has one test for engines less than 50 HP. Therefore, the data may not be 

considered representative of that particular subcategory. Consequently, it would be 

more appropriate to use test results from all size engines within each subcategory or 

family of engines, where possible. This approach takes into account more variability 

and would be more representative of actual emissions assuming a reasonable number 

of tests are available. Further, for each subcategory, EPA selected the best performing 

12 percent of engines in terms of formaldehyde. The formaldehyde emissions from the 

best performing 12 percent were then averaged together to obtain the average emission 

limitation achieved by these engines. Where possible, EPA selected the corresponding 

CO test results and averaged those tests to determine the CO limit. The MACT floor 

determination for each subcategory is described below and is summarized in Table 1. 

EPA has previously conducted an analysis for the RICE NESHAP for engines 

greater than 500 HP at major sources, which investigated whether CO could be used as 

a surrogate for HAP from stationary engines. The conclusion of the analysis was that 

CO is a reasonable surrogate for HAP for SI lean burn engines and CI engines. 

However, CO was not determined to be a reasonable surrogate for HAP for 4SRB 

engines. It is beneficial for industry and EPA to use CO as a surrogate for HAP if that is 

determined to be appropriate as it is much easier to test for one pollutant instead of all 

HAP. Testing for CO is easier and more economical than testing for several HAP, and 

can be done either through source testing or continuous emission monitoring systems, 

making it more flexible than testing for several HAP. 
4

<50 HP

According to information in the Population Database there are no existing 

stationary engines less than 50 HP using catalyst type controls. Therefore, EPA 

determined that the MACT floor is the emission level that is achievable by existing 

engines of this size operating without add-on controls. 

In order to determine the level being achieved by engines less than 50 HP, EPA 

referred to the Emissions Database. Only one test was available for engines less than 

50 HP. This test was for a 4SRB engine operating on propane. EPA is not expecting 

any stationary CI engines less than 50 HP since they are typically considered nonroad 

mobile engines. Also, EPA does not expect any lean burn engines in this subcategory 

as lean burn engines tend to be found in larger engine size segments. Therefore, it is 

assumed that engines less than 50 HP would be 4SRB engines. Subsequently, EPA 

reviewed emissions from 13 tests of 4SRB engines between 50 and 500 HP plus the 

one test available for the less than 50 HP engine. EPA selected the best performing 12 

percent (or 2 tests) of 4SRB engines less than 500 HP without add-on controls and 

averaged the formaldehyde emissions to determine the MACT floor limit for engines 

below 50 HP. As a result, the MACT floor for engines below 50 HP is 2 parts per million 

by volume, dry basis (ppmvd) of formaldehyde at 15 percent oxygen (O2). 

Landfill/Digester Gas ≥50 HP

According to information in the Population Database there are no existing landfill 

or digester gas engines greater than or equal to 50 HP using catalyst type controls. 

Therefore, EPA determined that the MACT floor for this subcategory is the level 

achievable by existing landfill and digester gas engines operating without add-on 

controls. 

EPA consulted several sources, including the Emissions Database, in order to 

determine the level being achieved by landfill and digester gas engines. Based on 

reviewing recently obtained test reports for landfill and digester gas engines, EPA 

concluded that the latest information obtained on the current levels being achieved by 

landfill gas engines is the most appropriate and representative information and was 

therefore used to determine the MACT floor limit. EPA analyzed the CO emissions from 

landfill and digester gas test reports. EPA selected the best performing 12 percent and 

averaged those 12 percent to determine the MACT floor. Therefore, the MACT floor for 

landfill and digester gas stationary RICE greater than or equal to 50 HP is 177 ppmvd of 

CO at 15 percent O2. 
5

Emergency CI 50≥HP≤500

No existing CI emergency stationary RICE greater than or equal to 50 HP and 

less than or equal to 500 HP were found to have catalyst type controls in the Population 

Database. For that reason, the MACT floor for this subcategory is the level achievable 

by existing CI engines operating without add-on controls. 

EPA analyzed formaldehyde emissions from 34 tests for engines of all sizes and 

5 tests for engines between 50 and 500 HP without add-on controls. Due to the limited 

number of tests in the smaller size group, EPA believes that the average of the top 12 

percent of engines between 50 and 500 HP may not be representative of what engines 

in this size group are achieving. Therefore, EPA believes that it is more appropriate to 

use the dataset containing a higher number of tests, which takes into account more 

variability and would be more representative of actual emissions. EPA averaged the top 

12 percent (or 4 tests) for formaldehyde. CO test results of the best performing 12 

percent was not available. Therefore, EPA reviewed emissions from all CO tests (10 

tests) and selected the best performing 12 percent (or 1 test). As a result, the MACT 

floor for CI emergency stationary RICE greater than or equal to 50 HP and less than or 

equal to 500 HP is 4 ppbvd of formaldehyde at 15 percent O2 or 40 ppmvd of CO at 15 

percent O2. 

Non-Emergency CI 50≥HP≤500

No existing CI non-emergency stationary RICE greater than or equal to 50 HP 

and less than or equal to 500 HP were found to have catalyst type controls, according 

the Population Database. For that reason, the MACT floor for this subcategory is the 

level achievable by existing CI engines operating without add-on controls. 

In terms of non-cumulative emissions, there is no difference between emissions 

from non-emergency and emergency CI engines. Therefore, the MACT floor for nonemergency CI engines between 50 and 500 HP engines is equivalent to the MACT floor 

for emergency CI engines between 50 and 500 HP and EPA analyzed the same data in 

order to determine the MACT floor limit. As a result, the MACT floor for CI nonemergency stationary RICE greater than or equal to 50 HP and less than or equal to 

500 HP is 4 ppbvd of formaldehyde at 15 percent O2 or 40 ppmvd of CO at 15 percent 

O2. 

Emergency SI 50≥HP≤500

 According to the Population Database there are no SI emergency stationary 

RICE greater than or equal to 50 HP and less than or equal to 500 HP using catalyst 

type controls. Therefore, EPA determined that the MACT floor for this subcategory is 

the level achievable by existing emergency SI engines operating without add-on 

controls. 
6

 As noted above, there is no difference between emissions from emergency and 

non-emergency engines on a non-cumulative basis. Therefore, emissions from 

emergency SI engines would be the same as from non-emergency engines. Based on 

industry estimates, the majority of emergency SI engines are 4SRB engines. Therefore, 

the MACT floor for emergency SI engines is equivalent to the level achieved by 4SRB 

engines operating without add-on controls. Consequently, the MACT floor for SI 

emergency stationary RICE greater than or equal to 50 HP and less than or equal to 

500 HP is 2 ppmvd of formaldehyde at 15 percent O2. 

Non-Emergency 2SLB 50≥HP≤500

According to the Population Database, no existing SI non-emergency 2SLB 

stationary RICE greater than or equal to 50 HP and less than or equal to 500 HP were 

found to have catalyst type controls. Therefore, the MACT floor for this subcategory is 

the level achieved by 2SLB engines operating without add-on controls. 

EPA analyzed formaldehyde emissions from 16 2SLB tests for engines between 

50 and 500 HP without add-on controls. Then, EPA took the average of the best 

performing 12 percent (or 2 tests) of engines, found the corresponding CO tests and 

averaged those tests as well. As a result, the MACT floor for non-emergency 2SLB 

stationary RICE greater than or equal to 50 HP and less than or equal to 500 HP is 16 

ppmvd of formaldehyde at 15 percent O2 or 85 ppmvd of CO at 15 percent O2. 

Non-Emergency 4SLB 50≥HP≤249

According to the Population Database, there are no non-emergency 4SLB 

stationary RICE greater than or equal to 50 HP and less than or equal to 249 HP using 

catalyst type controls. Therefore, the MACT floor for this subcategory is the level 

achieved by 4SLB engines 50 to 249 HP operating without add-on controls. 

EPA analyzed formaldehyde emissions from 34 4SLB tests for engines of all 

sizes without add-on controls. All 34 tests were for 4SLB engines above 500 HP. 

Therefore, the entire data set was analyzed to determine the MACT floor. EPA took the 

average of the best performing 12 percent of engines (or 4 tests) for formaldehyde and 

identified the corresponding CO values from the top 12 tests for formaldehyde. The 

corresponding CO values were then averaged. As a result, the MACT floor for 4SLB 

stationary RICE greater than or equal to 50 HP and less than or equal to 249 HP is 2 

ppmvd of formaldehyde at 15 percent O2 or 95 ppmvd of CO at 15 percent O2. 

Non-Emergency 4SLB 250≥HP≤500

For non-emergency 4SLB engines between 250 and 500 HP, EPA found that 31 

out of 538 engines are using catalyst type controls. This means that 5.7 percent of the 

non-emergency population of 4SLB engines between 250 and 500 HP are controlled 

with aftertreatment that yields HAP reductions. This percentage is below the criteria for 
7

a MACT floor that would require add-on controls for non-emergency 4SLB engines in 

this size range. Therefore, the MACT floor for this subcategory is the level achieved by 

4SLB engines 250 to 500 HP operating without add-on controls. 

EPA analyzed formaldehyde emissions from 34 4SLB tests for engines of all 

sizes without add-on controls. All 34 tests were for 4SLB engines above 500 HP. 

Therefore, the entire data set was analyzed to determine the MACT floor. EPA took the 

average of the best performing 12 percent of engines (or 4 tests) for formaldehyde and 

identified the corresponding CO values from the top 12 tests. The corresponding CO 

values were then averaged. As a result, the MACT floor for 4SLB stationary RICE 

greater than or equal to 250 HP and less than or equal to 500 HP is 2 ppmvd of 

formaldehyde at 15 percent O2 or 95 ppmvd of CO at 15 percent O2. 

Non-Emergency 4SRB 50≥HP≤500

For SI non-emergency stationary 4SRB engines greater than or equal to 50 HP 

and less than or equal to 500 HP, EPA found that 197 out of 3,533 are using catalyst 

type controls, according to the Population Database. This equates to a percentage of 

5.6 percent, which is below the criteria for a MACT floor based on add-on control. 

Therefore, the MACT floor for this subcategory is the level achieved by 4SRB engines 

50 to 500 HP operating without add-on controls. 

EPA analyzed formaldehyde emissions from 21 tests for engines of all sizes and 

13 tests for engines between 50 and 500 HP. EPA believes the number of tests 

available between 50 and 500 HP is a reasonable amount and would be representative 

of 4SRB engines in this size category. Therefore, EPA used the dataset between 50 

and 500 HP. EPA averaged the formaldehyde emissions from the best performing 12 

percent of engines (or 2 tests) without add-on controls. As a result, the MACT floor for 

4SRB stationary RICE greater than or equal to 50 HP and less than or equal to 500 HP 

is 2 ppmvd of formaldehyde at 15 percent O2. 
8

Table 1. Summary of MACT Floor Determination for Existing Stationary RICE 

Subcategory MACT Floor 

<50 HP 2 ppmvd CH2O @15% O2 

Landfill/Digester Gas 50≥HP≤500 177 ppmvd CO @15% O2

Emergency CI 50≥HP≤500 

4 ppbvd CH2O @15% O2

or 

40 ppmvd CO @15% O2

Non-Emergency CI 50≥HP≤500 

4 ppbvd CH2O @15% O2

or 

40 ppmvd CO @15% O2

Emergency SI 50≥HP≤500 2 ppmvd CH2O @15% O2

Non-Emergency 2SLB 50≥HP≤500 

16 ppmvd CH2O @15% O2

or 

85 ppmvd CO @ 15% O2

Non-Emergency 4SLB 50≥HP≤249 

2 ppmvd CH2O @15% O2

or 

95 ppmvd CO @15% O2

Non-Emergency 4SLB 250≥HP≤500 

2 ppmvd CH2O @15% O2

or 

95 ppmvd CO @15% O2

Non-Emergency 4SRB 50≥HP≤500 2 ppmvd CH2O @15% O2