Availability and Limitations of NOx Emission Control 

Resources for Natural Gas-Fired Reciprocating Engine 

Prime Movers Used in the Interstate Natural Gas 

Transmission Industry

Prepared for:

The INGAA Foundation, Inc.

By: 

Innovative Environmental Solutions, Inc. &

Optimized Technical Solutions

July 2014

INGAA Foundation Final

Report No. 2014.03

Innovative 

Environmental 

Solutions, Inc.
[ This Page Intentionally Blank ]
Resource Availability for NOx Control Projects for the Interstate Natural Gas Transmission Industry Page i

Table of Contents

Executive Summary.........................................................................................................................1

1.0 Introduction.............................................................................................................................3

2.0 Study Approach ......................................................................................................................5

3.0 Reciprocating Engine NOx Emission Reduction Technology Overview...............................7

3.1 Lean Burn Reciprocating Internal Combustion Engines – LEC......................................7

3.1.1 Enhanced Mixing...................................................................................................7

3.1.2 Mechanical Modifications .....................................................................................7

3.1.3 Operational Control – Ignition Timing..................................................................8

3.1.4 Operational Control – Air/Fuel Ratio ....................................................................8

3.1.5 Other (non-LEC) Technologies .............................................................................8

3.2 Rich Burn Reciprocating Internal Combustion Engines – NSCR...................................8

3.3 Selective Catalytic Reduction for Lean Burn Engines....................................................9

4.0 Reciprocating Engine Demographics (Population and Type) ..............................................10

4.1 Horizontal Engines........................................................................................................11

4.2 Low BMEP Four-stroke Cycle Naturally Aspirated Engines.......................................12

4.3 Opposed Piston Two-stroke Cycle Engines..................................................................14

4.4 Medium and Higher BMEP Four-stroke Cycle Turbocharged Engines .......................14

4.5 Low BMEP Two-stroke Cycle Engines........................................................................15

4.6 Medium and Higher BMEP Two-stroke Cycle Engines...............................................17

5.0 Analysis Assumptions and Methodology .............................................................................18

5.1 NOx Regulations – Background and Regulatory Risk..................................................18

5.2 Assumptions and Methodology for Reciprocating Engines (NOx) ..............................23

6.0 Cost and Schedule Estimates................................................................................................25

6.1 Capital Costs..................................................................................................................25

6.2 Schedule ........................................................................................................................27

7.0 Resource Analysis and Limitations by Project Phase...........................................................30

7.1 Permitting ......................................................................................................................30

7.2 Initial Design .................................................................................................................31

7.3 Procurement...................................................................................................................31

7.4 Cost Estimating and Scheduling....................................................................................32

7.5 Detailed Design .............................................................................................................32

7.6 Construction ..................................................................................................................32

7.7 Commissioning and Start-up.........................................................................................33

7.8 Operation and Maintenance...........................................................................................34
Resource Availability for NOx Control Projects for the Interstate Natural Gas Transmission Industry Page ii

8.0 Conclusions...........................................................................................................................34

9.0 Bibliography .........................................................................................................................36

Appendices

Appendix A: Acronyms................................................................................................................37

Appendix B: Survey questions for operator and service provider interviews..............................38

List of Tables

Table 4-1: Engine type by geographical risk category ................................................................10

Table 4-2: Engine counts by NOx emission levels......................................................................11

Table 4-3: Number of horizontal engines....................................................................................12

Table 4-4: Number of low BMEP four-stroke cycle engines......................................................13

Table 4-5: Number of opposed piston two-stroke cycle engines.................................................14

Table 4-6: Number of medium and higher BMEP four-stroke cycle turbocharged engines.......15

Table 4-7: Number of low BMEP two-stroke cycle engines.......................................................17

Table 4-8: Number of medium and high BMEP two-stroke cycle engines.................................18

Table 5-1: Regulatory Risk Ranking for States with Engines in the Database ...........................22

Table 6-1: Costs to achieve 3 g/hp-hr ($ in thousands) ...............................................................26

Table 6-2: Costs to achieve 1 g/hp-hr ($ in thousands) ...............................................................27

Table 6-3: Estimated number of engines that can be modified/replaced per year.......................28

Table 6-4: Time required to modify engines to achieve 3 g/hp-hr (in years)..............................29

Table 6-5: Time required to modify engines to achieve 1 g/hp-hr (in years)..............................29

List of Figures

Figure 4-1: Horizontal Engine....................................................................................................12

Figure 4-2: Ingersoll-Rand KVG ...............................................................................................13

Figure 4-3: Fairbanks-Morse MEP-8 .........................................................................................14

Figure 4-4: Ingersoll-Rand 616KVT..........................................................................................15

Figure 4-5: Clark HBA Low BMEP Engine ..............................................................................16

Figure 4-6: Cooper-Bessemer Z330...........................................................................................17

Figure 5-1: Projected ozone nonattainment areas (at 60 – 70 ppbv) and actual ozone 

nonattainment areas for the 75 ppbv ozone NAAQS..............................................20

Figure 5-2: Updated northeast U.S. ozone nonattainment projections from the Ozone 

Transport Commission based on 2010 – 2102 ozone data......................................20
Page 1

Executive Summary

New and proposed changes in air emission regulations are likely to impact the interstate natural 

gas transportation industry. Regulations that are expected to impact the industry include the 

ozone National Ambient Air Quality Standards (NAAQS) and the nitrogen dioxide (NO2) one 

hour NAAQS. Since nitrogen oxides (NOx) react in the atmosphere to form ozone, it is 

regulated as a precursor to ozone. As emission control rules are developed and permitting 

actions implemented in response to these federal standards, there will be ongoing pressure to 

reduce emissions of NOx from existing equipment that has not yet been impacted by NOx 

control rules, or has been minimally impacted. For example, EPA and states have not yet 

implemented permitting requirements related to a recent revision to the NO2 NAAQS. In 

addition, due to lawsuits and evolving EPA policy, states have been passive in recent years 

regarding the next set of rules to reduce NOx to address ozone NAAQS concerns. In response to 

a new ozone NAAQS planned for 2015, there is a potential for broad NOx controls in the eastern 

half of the U.S. to address NOx transport. Along with the revised ozone NAAQS and 

implementation of the NO2 NAAQS, upcoming court decisions, including a Supreme Court 

decision on EPA authority to institute regional NOx rules, will likely break the regulatory 

logjam. 

Based on these expected air regulation changes that result in new requirements to reduce NOx 

from existing equipment, a significant number of stationary engines that drive natural gas 

compressors in interstate natural gas transmission service will likely require modification to meet 

new federal or state NOx rules or permitting requirements. Based on current technical 

resources, the projected time to implement retrofit NOx control (or replacement) is far in 

excess of typical regulatory schedules.

The focus of the research for this report was to evaluate the resources required of the operating 

companies, emission reduction suppliers, engineering service providers, and contractors to 

implement NOx control regulations for low speed reciprocating engines used in the interstate 

natural gas transportation industry. The information gathered in developing this report included 

input solicited from both operating companies and vendors who provide emission reduction 

retrofit equipment. The assessment included an evaluation of an industry database to estimate 

the number and type of engines that would be impacted by changes to the emission regulations.

Resource and schedule requirements assessed in this report are based on the actual fleet (i.e., 

make, model, NOx control status) from that database. This study found:

• The special technical expertise to design, construct, and commission emission reduction 

projects is not widely available to the industry for the slow speed integral engines prevalent 

in natural gas transmission. Regulations that require installation of NOx control on a large 

number of reciprocating engines will require a significant lead time to train and develop

employees and service provider resources to implement emission reduction projects on a 

timely basis.

- Availability of this special technical expertise and building this capability is the primary 

resource constraint that will affect the ability to meet regulatory obligations, if those 

regulations affect a large percentage of the existing fleet. Based on current capabilities 

and a scenario where NOx regulations broadly affect the existing fleet of uncontrolled 

reciprocating engines, the estimated time to complete upgrades to over 2600 engines is 
Page 2

nearly forty years. Although strategies to comply may result in many engines (e.g., lower 

horsepower, lower use engines) being retired or replaced, scheduling implications due to 

the lack of available expertise would still extend far beyond the expected regulatory

timelines. In addition, even if a smaller percentage of the fleet requires control, the 

required time (e.g., 15 years for 1000 engines) would likely exceed any expected 

regulatory schedule. Building this technical expertise to address regulatory 

implementation timelines are significant concerns.

• Engine NOx control projects are generally much less costly than engine replacement. If the 

anticipated regulations are implemented, capital costs to modify currently uncontrolled 

reciprocating engines used in the interstate natural gas transmission industry are estimated at 

approximately $4 billion to achieve NOx emission rates of 3 grams per horsepower hour 

(g/hp-hr) and over $6 billion to achieve 1 g/hp-hr.

• The age of the impacted equipment (most of the engines are over 40 years old) requires 

additional time to engineer and construct reliable emission reduction modifications due to 

inaccurate or missing engineering records that reflect the current equipment configuration.

• Some engines with low specific power output will require extra time to properly design and 

construct emission reduction modifications and maintain the same power operating range.

• Based on previous experience, the timeline of obtaining air permits is a key parameter 

defining the overall schedule for completing a specific emission reduction project – i.e., 

permitting can slow the project timeline. 

• Equipment outages that last several weeks to over a month to implement emission reduction 

modifications may have significant impact on available pipeline capacity and could cause 

significant capacity disruption, especially if the schedule overlaps with implementation of other 

regulations such as pipeline integrity testing. 
Page 3

1.0 Introduction

As shown below, air emissions from a natural gas compressor station are regulated by a complex 

web of federal and state regulation. This report focuses on the potential availability of resources 

– equipment, people, and expenditures – needed to modify these compressor stations to meet 

possible NOx emission requirements. 

Air emissions continue to be a major regulatory issue, and air quality regulations pose a risk to 

normal operations of existing natural gas transmission infrastructure. NOx emissions are the 

primary air pollutant of interest from natural gas-fired combustion sources. NOx can react in the 

atmosphere to form ozone, so NOx is regulated as a precursor to ozone. NOx can also react to 

form nitrates, and ammonium nitrate is an aerosol that comprises a portion of atmospheric fine 

particulate (PM2.5). The U.S. Environmental Protection Agency (EPA) establishes NAAQS for 

six criteria pollutants, including ozone, NO2, and PM2.5. NOx emissions are comprised of NO 

and NO2, but all NOx emissions are generally considered NO2 under the NO2 NAAQS. 

Based on ambient air monitoring, if the air quality in a particular location does not meet (i.e., 

“attain”) the NAAQS, that geographical area will be designated as a “nonattainment” area. Then, 

state or local agencies are required to develop regulations that decrease emissions from existing 

sources to reduce ambient pollutant levels and attain the NAAQS. In addition, NOx emissions

may be evaluated by regulatory agencies using a model that simulates the behavior of the exhaust 

plume (i.e., a “dispersion model”) to assess whether NO2 impacts beyond the compressor station 

fence line exceed the NO2 NAAQS. If so, mitigation would likely be required. 

For example, NOx Reasonably Achievable Control Technology (RACT) regulations may be 

adopted by a state to address ozone nonattainment within the state. These rules may apply to 

limited in-state areas (e.g., urban nonattainment areas) or statewide. In addition, many urban and 

eastern U.S. areas have already reduced NOx emissions, but NOx transported from upwind states 

contributes to nonattainment. This phenomenon is referred to as “NOx transport,” and EPA may 

adopt multi-state regional rules to reduce NOx across the eastern U.S. and mitigate NOx 

transport. For natural gas-fired combustion sources, NOx is the primary pollutant that may be 

regulated by these rules. Emissions of other pollutants such as formaldehyde, which is regulated 

as a hazardous air pollutant (HAP), may also trigger emission controls. However, it is generally 

expected that existing catalyst control providers can meet demands to address new regulatory 

requirements. 

Thus, this report focuses on potential resource constraints associated with regulations that require 

retrofit installation of NOx emission control technologies. More specifically, the report focuses 

on combustion-based technologies – i.e., low emissions combustion (LEC) – for lean burn 

reciprocating engines commonly used in interstate transmission and most likely subject to retrofit 

NOx control in the future (i.e., this older equipment is typically “grandfathered” and many units 

have not been subject to NOx regulations to date). The reciprocating engines of interest are low 

speed “integral” engines uniquely used for gas compression, where the compressor and its driver 

– i.e., the reciprocating engine used to power the compressor – are integrated into a single piece 

of equipment with a common crankshaft. These legacy integral engines comprise a large portion 

of the horsepower capacity in interstate natural gas compression and can also be found in gas 

processing and, to a lesser extent, in gathering compression. Although these engines have been 
Page 4

in service for many years, because they were designed specifically to compress natural gas, 

natural gas-fired integral engines remain the most efficient option for gas compression.

LEC is the preferred approach to reduce lean burn engine NOx emissions, but EPA or states may 

consider additional controls such as selective catalytic reduction (SCR). Rich burn reciprocating 

engines may require non-selective catalytic reduction (NSCR) to reduce NOx and formaldehyde, 

but rich burn engines are a smaller percentage of interstate transmission prime movers and many 

rich burns already use NSCR emissions control in response to the federal HAP standard or state 

requirements.

The focus of this report is to assess the availability of resources to implement air emission 

modifications on the reciprocating engines used to transport natural gas on the interstate pipeline 

system. For example, there are currently a handful (or less) of service providers that provide 

LEC control for reciprocating engines. Thus, depending upon the breadth and timing of 

emission control requirements, there is a concern that resource constraints may impact timely 

compliance or the reliability of gas delivery. 

This report will identify potential constraints, including potential bottlenecks in the supply chain,

regarding availability of capital equipment and technology service providers to address retrofit 

NOx control of the existing fleet of equipment. Scheduling issues will be discussed, including

constraints that may point to the need for phasing regulatory compliance.

The regulatory context is based on an expectation that existing reciprocating engines that do not 

include NOx control technology will eventually be required to retrofit NOx control technology in 

response to recent or upcoming revisions to the federal ozone or NO2 NAAQS. NOx control 

resource availability for reciprocating engines is the most probable resource constraint due to the 

limited service providers and trained labor in this field, so this report focuses on potential 

resource limitations and the time required to control these engines, especially lean burn engines. 

In addition to this Introduction, the report includes:

• An Executive Summary. 

• Section 2 discusses the study approach. 

• Section 3 provides an overview of reciprocating engine NOx control technologies, 

• Section 4 discusses engine demographics (population and type) for interstate transmission

prime movers. 

• Section 5 presents NOx related air quality regulatory concerns and analysis assumptions and 

methodology. 

• Section 6 presents methodology and results related to costs and schedule.

• Section 7 presents the analysis and discussion of limitation by project phase, and Section 8 

provides conclusions. 

• Appendices provide a list of acronyms and the survey questions used for discussions with 

operators, equipment manufacturers, and service providers.
Page 5

2.0 Study Approach

The information in this report was gathered confidentially by Innovative Environmental 

Solutions (IES) and Optimized Technical Solutions (OTS) and the information has been 

consolidated in a manner that the individual responses cannot be broken out by the vendor or 

operator supplying the information. This is done to prevent business strategies or other 

competitive information from reaching competitors. All efforts were made to prevent the 

communication of information that could result in anticompetitive practices.

Interviews were performed with multiple emission reduction vendors and operating companies. 

In addition, a literature review was performed, and relevant information is cited. There is 

significant uncertainty regarding the requirements (e.g., required emission levels), geographical 

breadth, and timing of NOx regulations. Since there are many possible regulatory outcomes, 

specific dates and regulations are not a focus, but a general discussion of the NOx control 

regulatory threat is provided in Section 5. Instead of considering a specific regulation, the 

analysis in this report focuses on identifying potential resource constraints based on an 

assumption that many legacy prime movers without NOx control will eventually require upgrade 

(or replacement). To consider technology needs, costs, and scheduling, the analysis considers 

the engine types (i.e., model) based on an available database. Two NOx targets are considered 

(i.e., 3 g/bhp-hr or 1 g/bhp-hr) based on previous experience for likely regulatory endpoints and 

guidance from advisors, and a general ranking of regulatory risk for three geographical areas is 

used when assessing implications. States were categorized as high risk/stringency (where 

regulations may already be in place in some cases – e.g., northeast state where much of the fleet 

is already controlled), moderate risk (e.g., midwest and eastern states where compressor stations 

may be implicated as contributors to eastern U.S. NOx transport), and lower risk states, where 

regulation is less likely (e.g., northern plains states, where control is less likely unless EPA 

initiates broad national requirements for existing equipment). In addition, costs were considered 

under the premise that NOx control would be widely required for uncontrolled units either due to 

NOx regulations (e.g., in response to ozone nonattainment) or permitting actions to address 

conformance with the NO2 NAAQS. 

This effort began with interviews of multiple interstate gas pipeline operating companies that had

previous experience with NOx reduction projects. In addition, independent interviews were 

conducted with six different suppliers of equipment and services that modify reciprocating 

engines used in the interstate natural gas transportation industry. Interviews covered all aspects 

of the execution of NOx reduction projects from start to finish. Specific topics in project 

execution included:

• Permitting – Permitting is the process of obtaining air emission permits from the regulating 

agency – typically a state agency. This phase requires an initial engineering analysis to 

determine the modifications required and the expected air emissions for each emission source

at a facility for the permit application. Defined technologies must be settled upon prior to 

completing the permit.

• Initial design – This portion of the project assesses: (1) which emission reduction technologies 

are suitable for a facility and the associated suppliers for the emission reduction equipment, (2)

the reduction in air emissions that can be expected with the applicable emission reduction 

equipment, and (3) the initial design for the equipment changes. Input from the suppliers of 
Page 6

emission reduction equipment is usually required during this portion of the project. Initial 

design information is also necessary to provide information for the permitting process, but 

other, more detailed design considerations are also addressed under this topic.

• Cost estimating and scheduling – This part of the project establishes the expected 

timeframe and financial costs expected to modify the equipment for emissions control.

Personnel resource availability and allocations are also typically performed in this portion of 

the project execution.

• Contracting and procurement – In this portion of the project, detailed cost and schedule

information to implement an emission control project is solicited from suppliers and 

contractors. This requires the creation of design specifications, evaluation criteria, and 

contractual terms and conditions.

• Detailed design – This phase involves developing detailed designs based on specific 

emission control equipment selected. Detailed design includes but is not limited to the 

development of material lists, drawings, engine control system modifications, and 

commission plans.

• Construction – This portion of the emission control project is where physical modifications 

are made to the engine and ancillary equipment.

• Commissioning – This is the phase of the emission control project where calibration, 

configuration, and tuning of the engine is performed to ready the engine for continuous 

operation. Training of the operating and maintenance personnel is also often performed 

during this phase.

• Operation and maintenance – This is the post-conversion and commissioning portion of the 

project. Proper operation and maintenance of the engine and emission control equipment is 

required to ensure compliance with the emission limits.

This report discusses resource constraints for each of these phases for reciprocating engine NOx 

control projects, as described by the operators and suppliers of emission reduction equipment. 

For NSCR application to rich burn engines, a higher level review was conducted because it is 

likely that there are an adequate number of vendors to address market demand and fewer engines 

are likely to be affected. 

Cost and schedule estimates are based on the number and size of engines to be converted, 

information provided by pipeline operators and suppliers of emission reduction equipment, and 

reference publications. Factors that influence technology requirements and costs for 

reciprocating engines include:

• Combustion cycle (two-stroke, four-stroke),

• The engine make and model, 

• Brake mean effective pressure (BMEP) of the engine,

• Engine aspiration type (natural, turbocharged), and

• The number of power cylinders.

Based on engines identified in an industry database, this information was used to determine the 

most probable type of modifications needed to achieve the target NOx emission level. 
Page 7

3.0 Reciprocating Engine NOx Emission Reduction Technology Overview

NOx control technologies for reciprocating engines are discussed below. LEC technologies for 

lean burn engines are discussed in Section 3.1, and NSCR for rich burn engines is discussed in 

section 3.2. Section 3.3 discusses SCR. Although SCR has not been applied to existing integral 

engines, it could be considered by regulatory agencies, especially for lean burn engines where 

LEC is not available or cannot achieve the target NOx level. 

3.1 Reciprocating Internal Combustion Engines – LEC for Lean Burn Engines

An overview of LEC NOx control technologies for lean burn reciprocating engines is described 

in this section. [1] [2] [3]

NOx emissions from natural gas combustion are formed from nitrogen and oxygen in the 

combustion air, and NOx emissions increase significantly at higher combustion temperatures. 

LEC achieves lower NOx by providing sufficient excess air to reduce the maximum combustion 

temperature and minimize NOx formation. The use of LEC to control NOx is possible on fuelinjected two-stroke cycle engines and many four-stroke cycle engines. Since the excess air may 

hinder combustion or light-off of the air-fuel charge in the cylinder, LEC-equipped engines 

generally require a high energy ignition source. This is most commonly implemented by the use 

of pre-chamber combustion systems that ignite a smaller charge that then ignites the in-cylinder 

air-fuel mixture. The power operating range may decrease on two-stroke cycle engines when 

converted to LEC. 

Since peak combustion temperatures are influenced by the amount of excess air and air-fuel 

mixing, there are several methods that can be used to minimize peak temperatures and NOx 

formation during lean combustion. Regulatory agencies use the term “LEC” broadly and a 

number of technology approaches can be used depending on the engine and NOx emission limit. 

In many cases, multiple LEC related technologies may be required – e.g., additional air through 

new or upgraded turbocharging, higher energy ignition /precombustion chambers, and enhanced 

mixing. Several primary technologies are described below. For the engine types in the fleet, the 

analysis considered the proper technology approach to achieve target NOx emissions of 3 g/hp-hr 

or 1 g/hp-hr.

3.1.1 Enhanced Mixing

Pockets of rich fuel can exist in engines equipped with direct fuel injection. The resulting locally 

rich fuel mixtures result in localized higher combustion temperatures with an associated increase 

in NOx formation. Engine modifications to produce enhanced air/fuel mixing can counteract this 

effect. For example, “high pressure” fuel injection systems have been developed that take 

advantage of the higher pressure gas available from the pipeline to greatly improve air-fuel 

mixing. On some engines, enhanced mixing modifications may also require high energy ignition

modifications to prevent engine misfires. 

3.1.2 Mechanical Modifications

Modifications that can be used to reduce NOx emissions include upgrading or conversion to 

turbocharging, adding additional turbocharger after-cooling, installing different cam shafts, 

installing different (e.g., electronic) fuel injection valves, etc. These are not discussed in detail 
Page 8

here as they usually result in minor reductions as an independent action, but are typical 

technologies that may be included as part of the emissions reduction project. Since dispersion 

modeling may be required to ensure that modeled offsite impacts do not exceed the NO2 NAAQS, 

stack height extensions are a modification that can reduce modeled ground level concentrations. 

In general, a stack height that is at least 1.5 times the height of the compressor building peak 

height is desired to avoid building-induced “downwash” that brings the plume to ground level 

sooner and results in higher modeled impacts. 

3.1.3 Operational Control – Ignition Timing

The peak combustion pressure (and thereby the peak combustion temperature) can be lowered by 

retarding (or delaying) the ignition timing of the engine. The lower combustion temperature 

results in lower NOx emissions. Only nominal NOx reductions are achieved when used in 

isolation and fuel consumption increases. This may also reduce the maximum power the engine 

can produce, and typically increases carbon monoxide (CO), volatile organic compounds 

(VOCs), and total unburned hydrocarbons (THC) emissions. VOCs are regulated as an ozone 

precursor and are a subset of THC (the methane and ethane components of THC are excluded 

from VOCs because these two hydrocarbons are minimally reactive in ozone producing 

atmospheric reactions). 

3.1.4 Operational Control – Air/Fuel Ratio

Altering engine control methods can reduce the formation of NOx and improve the reliability of 

maintaining emissions at the prescribed level. On engines that utilize air/fuel controls, some NOx

reductions may be possible by changing the control curve to operate the engine leaner without 

other engine modifications. This control change is limited by the amount of air available from the 

turbocharger and the ability of the ignition system to ignite the leaner mixture without misfires.

3.1.5 Other (non-LEC) Technologies

The technologies described above are not intended to be a comprehensive list. There are other 

control methods such as exhaust gas recirculation that are not covered here due to their limited 

effectiveness and limited use on the types of engines typically utilized in interstate natural gas 

transmission.

3.2 Rich Burn Reciprocating Internal Combustion Engines – Nonselective 

Catalytic Reduction

Rich burn engines are designed to operate with minimal excess air – i.e., with combustion air 

approximately equal to the amount required to completely combust the fuel with minimal excess

air. The “stoichiometric” amount of air is the exact amount required to burn the fuel with no 

excess. For reciprocating engines that operate near stoichiometric conditions (excess oxygen in 

the exhaust <0.5%), NSCR is the accepted emissions control option. This type of exhaust 

control technology is commonly used on automobiles and also referred to as a 3-way catalyst 

because emissions of NOx, CO, and VOCs can be reduced. For this control technology to work 

effectively for all three pollutants, an air to fuel ratio controller (AFRC) is required that 

maintains the AFR over a very narrow operating band. With too little air, NOx will be low, but 

CO and VOCs may significantly increase. With too much air (and thus too much oxygen 

available), CO and VOCs will be reduced (i.e., NSCR operates as an oxidation catalyst), but 
Page 9

NOx reduction across the catalyst will be impacted and post-catalyst NOx will significantly 

increase.

Catalytic elements used in NSCR can be contaminated by engine oil carryover and other 

operating factors. Cleaning and replacement is required on a periodic basis. Unburned 

hydrocarbons in the exhaust stream reduce the life of the catalyst. This is the common 

technology of choice for rich burn four-stroke cycle engines. 

3.3 Selective Catalytic Reduction for Lean Burn Engines 

For NOx control from lean burn engines, this report focuses on LEC. SCR is discussed here 

because it is often considered by regulators when assessing NOx controls for combustion 

equipment and future control rules may require SCR in some cases (e.g., if LEC is not effective 

on a particular engine model). SCR is an exhaust control for lean combustion (i.e., excess air is 

available) that reduces NOx by reaction with ammonia or urea over a catalyst. [4] [5] Ammonia 

or urea injection is required with precise reagent feedrate control based on the NOx 

concentration and the NO/NO2 ratio of the NOx. The reagent and NOx “selectively” react on the 

catalyst to (ideally) form water and molecular nitrogen (N2). To date, SCR application to U.S. 

gas transmission sources has been very limited, and SCR has not been applied to an existing 

integral engine. Technical concerns about the SCR performance for gas transmission engines 

include exhaust temperature requirements, reagent control (and sophistication of current 

systems), and treatment of potential variations in the reciprocating engine exhaust NO/NO2 ratio. 

SCR has been more commonly applied to larger utility scale turbines and boilers, with very 

limited gas transmission applications to date. Recently, some new 4-stroke cycle lean burn 

engines have been sited with SCR, but retrofit application to lean burn prime movers has not 

occurred. While NSCR for rich burn engines exploits the chemical processes available at 

stoichiometric combustion to reduce multiple pollutants, SCR “selectively” reduces NOx. The

use of SCR is most effective when operating in the exhaust temperature range of 480 to 800 °F. 

The minimum operating temperature of the catalyst is dependent on the composition of the 

exhaust gases and the type of catalyst materials used, and the typical exhaust temperature range 

for some lean burn engines may present challenges. Engines that have variable power loads 

require more sophisticated controls to inject the proper amount of reagent, and it is not evident 

that robust control schemes have been developed for transmission applications. The installation 

of a continuous emissions monitor could be required to effectively control the amount of reagent 

necessary to achieve the desired NOx emission rate.

SCR catalytic elements can be contaminated by byproducts of combustion (such as oil ash) and 

engine oil carryover. Cleaning and replacement is required on a periodic basis, and extra 

management is required to ensure adequate inventories of reagent are maintained.

Due to the issues described above and the desire to prevent NOx formation rather than 

controlling NOx in the exhaust, LEC is preferred over SCR for existing lean burn engines.
Page 10

4.0 Reciprocating Engine Demographics (Population and Type)

An INGAA engine database (originally developed by PRCI and GRI over 10 years ago) includes 

3665 low speed reciprocating engines used in the interstate natural gas industry. It should be 

noted that the number of engines in the tables below reflect large bore, low speed reciprocating 

engines. There are other engines used in the natural gas transportation industry. Most of the 

other engines are newer high-speed four-stroke cycle engines that are already equipped with low 

NOx emissions (typically LEC or NSCR) technology, such as best achievable control technology

(BACT) required during permitting of new units or technology to meet New Source Performance 

Standards (NSPS) and National Emission Standard for Hazardous Air Pollutants (NESHAP)

regulations. 

Although this database is not complete (e.g., it was estimated to include over 80% of interstate 

transmission facilities when it was compiled), and changes in the fleet have occurred since the 

database was last updated, the information from this source still provides a reasonable basis to 

assess counts of existing engines and technology requirements for NOx control. Of the 3665 low 

speed engines, 35 are known to have been abandoned or replaced leaving a net of 3630 engines. 

These engines were categorized by engine type and geographical region as described in Section 

5, where regulatory risk (i.e., likelihood and level of control) was used to categorize states into 

three regulatory risk categories depending on the state where the engine is located. Table 4-1

shows engine counts for each category for the three primary engine types. (The states in each 

risk category are identified in section 5.)

Table 4-1. Engine Type by Geographical Regulatory Risk Category.

Engine type Category 1 Category 2 Category 3 Total

2-stroke cycle 321 1975 344 2640

4-stroke cycle lean 82 320 51 453

4-stroke cycle (rich) 50 405 82 537

Total 453 2700 477 3630

The average engine size is 2145 brake horsepower with an average of 12 power cylinders per 

engine. To assess technology requirements and potential resource issues, these engines have 

been grouped or classified based on different engine types/subtypes and current emission control 

capabilities. Each group type requires a different approach to NOx reduction with an associated 

impact on costs and schedules. The two-stroke cycle and four-stroke cycle lean engines are 

candidates for LEC and the four-stroke cycle rich burn engines are candidates for NSCR NOx 

control technologies. For the purposes of this discussion, the “rich burn” engines include older, 

horizontal engines that do not include air to fuel ratio control and may operate near 

stoichiometric conditions (i.e., “rich”) or leaner (i.e., with excess air). The term “stoichiometric” 

refers to the condition where the amount of oxygen available from the combustion air is 

equivalent to the amount required to completely combust all of the fuel without any excess 

oxygen. Rich burn engines operate near stoichiometric conditions, while lean burn engines have 

excess air. Regulatory definitions, such as the federal NSPS and NESHAP standards, typically 
Page 11

use ten percent excess air (which is equivalent to two percent excess oxygen in the exhaust) as 

the threshold between rich and lean operation.

To estimate the number of engines that would require NOx emissions control, and assess 

associated capital costs and timeline, the INGAA database of engines was analyzed by current 

emission levels as shown in Table 4-2. Note that this table excludes 417 low BMEP four-stroke 

rich burn engines that are assumed to have NSCR catalysts as described in Section 4-2.

Table 4-2. Engine Counts by NOx Emission Levels.

Emission capability Category 1 Category 2 Category 3 Total

NOx < 1 g/hp-hr 64 0 0 64

1 < NOx < 3 g/hp-hr 339 162 36 537

NOx > 3 g/hp-hr 0 2238 374 2612

Total 403 2400 410 3213

The majority of reciprocating engines used in the interstate natural gas transportation industry 

were installed before 1960. Most have been exempt from clean air regulations issued after they 

were installed since existing units are “grandfathered” – i.e., exempt from regulations that affect 

new sources. A common exception is engines that have been controlled in response to NOx 

RACT or similar rules adopted to decrease the NOx inventory from existing sources as a strategy 

to address ozone nonattainment. A discussion follows on each of the primary engine categories, 

related engine counts, and NOx control technology available.

4.1 Horizontal engines

Horizontal engines are four-stroke cycle naturally aspirated engines with four double acting 

power pistons. These engines were manufactured by Cooper-Bessemer (Type 22 through Type 

26) and Worthington (24X36 and 26X36). These engines are the first type of reciprocating 

engines widely used to compress natural gas for transportation. They are a derivative of 

horizontal steam engines. The double acting power cylinders (combustion occurs on both sides 

of the piston) have very large cylinder bores (22” to 26”) and strokes (typically 36”). It is 

difficult to control the air/fuel mixture for these units – e.g., for use with post-combustion 

catalytic control. Likewise, they are not suited to LEC conversion. Per the INGAA database, 

there are 120 engines in this group, where a retrofit NOx control technology is not available,

with an average power rating of 1530 hp. 
Page 12

Figure 4-1. Horizontal Engine

Because these engines have higher operating and maintenance costs, they are generally operated 

on a last on, first off basis in response to demand. Therefore, their annual operating hours tend 

to be lower than newer engines operating at the same facility.

Since technology upgrades are not available, the assumed “NOx control” option for these 

engines is replacement. For cost estimating purposes, the assumed replacement equipment is 

3000-4000 hp gas turbines equipped with dry low NOx (lean premixed) combustors. Although 

larger gas turbines would reduce the capital replacement costs, smaller gas turbines are a better 

match (although still larger) to the engines being replaced. Larger engines do not have the 

necessary turndown capability to match the horizontal engines. Gas turbines were selected over 

other engine types due to the lower capital costs. Where minimizing fuel is a major 

consideration, high speed four-stroke cycle reciprocating engines would be selected as the 

replacement engine. All of these engines are located in geographical regulatory risk Categories 2 

and 3 as shown in Table 4-3. These engines are a subset of the four-stroke rich burn engines 

found in Table 4-1. All horizontal engines are assumed to have NOx emission rates greater than 

3 g/hp-hr.

Table 4-3. Number of horizontal engines.

Engine Type Category 1 Category 2 Category 3 Total

4-stroke – horizontal 0 105 15 120

4.2 Low BMEP four-stroke cycle naturally aspirated engines

Like the horizontal engines, the low BMEP four-stroke cycle engines (see Figure 4-2) used in 

natural gas transmission sometimes operate near stoichiometric conditions and other times 

operate lean. These engines differ from horizontal engines in that they can generally be fitted 
Page 13

with air-fuel controls to enable operation with NSCR. However, depending on the units, 

operation with NSCR sometimes requires higher exhaust temperatures than the unit was initially 

designed. Alternatives technologies include converting to LEC (with the addition of a 

turbocharger) [6] or installing SCR. SCR has had very limited application to existing 

reciprocating engines, and installation of SCR may limit the ability of the engine to operate at 

rated power.

Figure 4-2. Ingersoll-Rand KVG

The analysis in this report assumes that NOx reductions would be achieved with installation of 

NSCR, with engine modifications completed as necessary to accommodate higher exhaust 

temperatures. However, existing rich burn engines at “major sources” (i.e., larger facilities such 

as compressor stations with multiple reciprocating engines) were required to install NSCR to 

comply with an EPA regulation, with compliance required by 2007 for the 2004 RICE NESHAP 

rule. Therefore, resource limitation were not assessed and costs to modify these engines are not 

estimated here because any incremental costs (e.g., to better tune the air to fuel ratio controller 

(AFRC) for NOx reduction) should be minimal. It is likely that some of these engines will 

require control or additional modifications to achieve required NOx reductions. The number of 

these engines in each geographical regulatory risk category can be found in Table 4-4. This is a 

subset of the four-stroke cycle rich burn engines in Table 4-1.

Table 4-4. Number of low BMEP four-stroke cycle engines.

Engine Type Category 1 Category 2 Category 3 Total

4-stroke cycle low BMEP 50 300 67 417
Page 14

4.3 Opposed piston two-stroke cycle engines

The gas pipeline transportation industry employs about 14 two-stroke cycle opposed piston 

engines such as the Fairbanks-Morse engine shown in Figure 4-3. These engines typically use 

superchargers to provide scavenging air. 

The engines utilize two power pistons per power cylinder with intake and exhaust ports at 

opposite ends of the cylinder liner. The two pistons are connected to separate crankshafts which 

are synchronized through a vertical driveshaft and bevel gears. Because of the higher number of 

moving parts on these engines, they tend to have higher maintenance costs than other engines 

used in this industry. 

Figure 4-3. Fairbanks-Morse MEP-8

The engines can be converted to lean burn combustion or controlled using post-combustion 

catalytic reduction. NOx emission rates below 3 g/hp-hr are difficult to achieve at full load and 

speed on these engines through lean combustion alone. Based on the operating cost of these 

engines and emission characteristics, the analysis in this report assumes emission reduction is 

achieved with lean burn conversion and an emission rate of 3 g/hp-hr. Replacement is assumed 

if lower emission rates are required. The location of these engines by geographical regulatory 

risk category is in Table 4-5. This is a subset of the two-stroke cycle engines in Table 4-1.

Table 4-5. Number of opposed piston two-stroke cycle engines.

Engine type Category 1 Category 2 Category 3 Total

2-stroke cycle opposed piston 1 13 0 14

4.4 Medium and higher BMEP four-stroke cycle turbocharged engines

Medium and higher BMEP four-stroke cycle turbocharged engines (see example in Figure 4-4) 

operate too lean to utilize NSCR for NOx reduction, and these engines are generally suited to 
Page 15

NOx control through lean burn combustion and enhanced mixing methods – i.e., LEC 

technologies. 

Figure 4-4. Ingersoll-Rand 616KVT

For cost estimating and resource availability assessment purposes in this study, reducing 

emissions to 3 g/hp-hr is assumed to be achieved through lean combustion. To lower NOx 

emissions to 1 g/hp-hr requires the addition of enhanced fuel mixing. The assumed emissions 

and location of these engines by geographical regulatory risk category can be found in Table 4-6. 

This table is a subset of four-stroke cycle rich and four-stroke cycle lean engines in Table 4-1.

Table 4-6. Number of medium and higher BMEP four-stroke cycle turbocharged engines.

Emission type Category 1 Category 2 Category 3 Total

NOx < 1 g/hp-hr 36 0 0 36

1 < NOx < 3 g/hp-hr 46 26 5 77

NOx > 3 g/hp-hr 0 308 46 354

Total 82 334 51 467

4.5 Low BMEP two-stroke cycle engines

Low BMEP two-stroke cycle engines are generally older engines (i.e., the “first generation” of 

pipeline integral 2-stroke engines) that utilize low volume pumps or blowers to provide engine 

scavenging (Figure 4-5). To reduce NOx emissions, LEC is applied through the removal of the 

mechanically driven scavenging and installation of turbochargers with after-cooling and 

precombustion chambers. Because of the low BMEP of these engines, there is typically less 

exhaust energy to support turbocharger operation. As a result, the power turndown range of 

converted units is limited. The addition of enhanced fuel mixing technologies would extend the 

power operating range for these engines.
Page 16

Figure 4-5. Clark HBA Low BMEP Engine

Therefore, conversion to 3 g/hp-hr of NOx is assumed to require both conversion to leaner

combustion and the installation of enhanced fuel mixing. With these modifications, the engines 

are capable of achieving 1 g/hp-hr of NOx in a narrower range near rated speed and power. 

Adding a turbocharger sometimes allow these engines to operate at a higher rated power. This 

can increase the available turndown range of these engines as more heat energy is available to 

drive the turbocharger. Power increases of 5-40% are possible. The ability to increase the rated 

power of the engines is dependent on the design of the mechanical, structural, and auxiliary 

components (crankshaft, connecting rods, pistons, foundation, jacket water cooling, etc.) to 

handle the higher loads on the engine. The gas compressor cylinders may also require 

modifications to be able to utilize the additional power. This is usually achieved by installing 

larger compressor pistons and additional unloaders.

An increase of the engine power rating usually requires modifications to the air permit and filing 

with the Federal Energy Regulatory Commission. In some cases, the update of several engines 

could allow the abandonment of another engine thereby eliminating the need (and associated 

costs) to control emissions for that engine. 

On a per horsepower basis, the costs of NOx control conversion for these units are generally 

more than the costs for higher BMEP two-stroke cycle engines (discussed below). This is due to 

additional intake/exhaust manifold modifications and the installation (versus an upgrade) of a 

turbocharger/after-cooler system. The assumed emission limits and location of these engines by 

geographical regulatory risk category is shown in Table 4-7. This is a subset of the two-stroke 

cycle engines in Table 4-1.
Page 17

Table 4-7. Number of low BMEP two-stroke cycle engines.

Emission type Category 1 Category 2 Category 3 Total

NOx < 1 g/hp-hr 5 0 0 5

1 < NOx < 3 g/hp-hr 72 0 0 72

NOx > 3 g/hp-hr 0 461 44 505

Total 77 461 44 582

4.6 Medium and higher BMEP two-stroke cycle engines

Medium and high BMEP two-stroke cycle engines are engines that utilize centrifugal blowers 

and/or turbochargers to provide engine scavenging (Figure 6). These engines are generally 

larger than the first generation low BMEP 2-cycle engines discussed above. They are well suited 

to lean burn combustion to reduce NOx formation. A reduction in the power operating range can 

be expected on some of these engines in order to maintain very low NOx emissions. The 

addition of enhanced fuel mixing technologies extends the operating range of these engines 

and/or further reduces NOx emissions.

Figure 4-6. Cooper-Bessemer Z330

For the cost estimating purposes of this study, reducing emissions to 3 g/hp-hr is assumed to be 

achieved through lean combustion. This emission level is consistent with NOx limits for retrofit 

control of lean burn engines in federal and status rules, and is an expected upper bound target in 

upcoming rules. However, it is possible that lower NOx emission levels, such as 1 g/hp-hr 

limits, will be pursued in some jurisdictions. Lower emission limits also require the addition of 

enhanced fuel mixing. The assumed emission limits and location of these engines by 

geographical regulatory risk category is shown in Table 4-8. This is a subset of the two-stroke 

cycle engines in Table 4-1. Note that this group includes many engines, and often these engines 

are more highly utilized than smaller, older low BMEP engines discussed in the previous section. 
Page 18

Thus, the medium to high BMEP two-stroke cycle engines are a prime candidate for regulation 

when states or EPA consider NOx control regulations. 

Table 4-8. Number of medium and high BMEP two-stroke cycle engines.

Emission type Category 1 Category 2 Category 3 Total

NOx < 1 g/hp-hr 23 0 0 23

1 < NOx < 3 g/hp-hr 221 134 31 386

NOx > 3 g/hp-hr 0 1372 269 1641

Total 244 1506 300 2050

5.0 Regulatory Background and Analysis Assumptions and Methodology

NOx emission regulations continue to evolve and are not fully defined at this time. However, as 

EPA and the states implement regulations in response to ozone NAAQS nonattainment and the 

NO2 NAAQS, it is expected that new regulations and permitting actions over the next 5 to 15

years will impact the status of many engines that are currently uncontrolled. The following 

section provides an overview of NOx regulations and regulatory risk, and section 5.2 discusses 

assumptions used to assess implications.

5.1 NOx Regulations – Background and Regulatory Risk

For existing equipment, NOx regulations are generally in response to concerns with ozone 

NAAQS nonattainment because NOx is an ozone precursor – i.e., it reacts in the atmosphere to 

form ozone. In addition, 2010 revisions to the NO2 NAAQS increased the stringency of that 

rule. If offsite impacts (i.e., beyond the facility fence line) of NO2 determined using a dispersion 

model exceed the NAAQS, then mitigation may be required. At this time, the most likely 

triggers for retrofit NOx control for natural gas transmission prime movers are:

• NOx RACT Rules: Revisions to the ozone NAAQS planned for 2014 – 2015 will likely 

increase the number of nonattainment areas, and result in new state-level NOx RACT rules 

later this decade. Such rules are highly likely to occur unless EPA provides a different 

regional solution, and state rules could result in different requirements and applicability (e.g., 

statewide or county-specific rule) from state to state. States tend to rely on rules developed 

by other states, so states that are early actors in the next few years could provide “model 

rules” that would be broadly implemented. 

• Regional Rule: EPA may implement a broad (e.g., eastern half of U.S.) regional rule similar 

to the 2004 NOx SIP Call Phase 2 Rule that required NOx reductions from large sources. 

Over 200 natural gas-fired prime movers were controlled in response to the 2004 SIP Call 

Rule. A similar new regional rule could supplant or supplement state NOx RACT Rules (see 

previous bullet), could quicken the schedule, and would likely affect many eastern U.S. 

engines. EPA may be hesitant to pursue this (and instead defer to the states) due to legal 

issues that remain to be resolved with a regional rule for electric utilities. 

• NO2 NAAQS mitigation: To date, there have been minimal regulatory actions in response to 

the 2010 NO2 NAAQS revision. However, example modeling has shown that the relatively 

short stacks common for reciprocating engines and conservatism in AERMOD (the regulatory 
Page 19

dispersion model) may result in offsite impacts that exceed the 100 ppbv 1-hour NO2 NAAQS 

for uncontrolled reciprocating engines. Although there has been limited action to date, this 

could result in state or federal requirements to mitigate these impacts, and agency actions could 

be prompted by litigation or pressure from third parties. For existing facilities, EPA or the 

state could require modeling at any time, such as when the operating permit undergoes its five 

year renewal. Third party challenges could also force states or EPA to act regardless of the 

permit renewal schedule. This issue has the potential to impact many existing, uncontrolled 

reciprocating engine prime movers. Mitigation may require multiple measures, including NOx 

control, increasing stack height, and/or increasing the property buffer (e.g., land purchase, 

moving the fence to the property line).

Regarding the ozone NAAQS, EPA planned to revise the NAAQS in 2011 in response to legal 

challenges. This was delayed and it was decided to complete the next revision under the 

schedule that requires NAAQS review every 5 years. That schedule currently plans for a 

proposed rule in the spring of 2014 and a final rule by mid-2015. That rulemaking will likely 

result in more nonattainment areas than under the current 75 ppbv NAAQS. This is illustrated in 

Figure 5-1 and Figure 5-2. 

Nonattainment is determined by assessing whether the 3-year average value (from ozone 

monitoring) exceeds the NAAQS. Projected nonattainment areas from EPA maps in 2010 for an 

ozone NAAQS of 60, 65 or 70 ppbv are shown in the left map in Figure 5-1. Those ozone levels 

were being considered for the 2011 rule that was deferred. The right map shows actual ozone 

nonattainment areas determined in 2012 for the current 75 ppbv standard. The projections for 60 

to 70 ppbv were based on 2006 – 2008 data. The nonattainment areas defined in 2012 are based 

on a 3-year average from 2008 – 2010 or 2009 – 2011. The eastern U.S. had a cool and wet 

summer in 2009, and hot, sunny weather is more conducive to ozone formation. Thus, states and 

third parties are challenging EPA and would like to plan for broader nonattainment and address 

NOx transport from upwind states. Figure 5-2 shows an analysis completed by the Ozone 

Transport Commission, a group of 12 northeastern states that collaboratively assess air quality in 

that area. If 2009 data is excluded and the 3-year average is based on 2010 – 2012 data, the 

figure shows significantly more areas above 75 ppbv and also shows areas that would not 

achieve a lower, 70 ppbv standard. 

Thus, if upcoming summers are not cool (like 2009), and EPA fulfills expectations to lower the 

ozone NAAQQS to a level between 60 and 70 ppbv in 2015, nonattainment areas will more 

likely resemble the 60 – 70 ppbv map on the left in Figure 5-1; or, for the northeast, the map in 

Figure 5-2. This would result in either a broad regional NOx control rule or requirements for 

many states to develop or update NOx RACT regulations for existing sources.
Page 20

Projected Ozone Nonattainment Areas Actual Ozone Nonattainment Areas

for 60, 65 and 70 ppbv Ozone NAAQS for Current 75 ppbv Ozone NAAQS 

 

Figure 5-1. Projected ozone nonattainment areas (left map) for ozone NAAQS of 60 ppbv 

(light blue), 65 ppbv (blue) or 70 ppbv (dark blue) and actual ozone 

nonattainment areas (right map) for the 75 ppbv ozone NAAQS (dark blue –

whole county; light blue – partial county; green – unclassifiable).

Figure 5-2. Updated northeast U.S. ozone air quality projections from the Ozone 

Transport Commission based on 2010 – 2012 ozone data (3-year average).

Regarding the NO2 NAAQS, a committee of state and federal modelers has published example

modeling results that show “typical” compression facilities with offsite impacts well above the 

NAAQS. [7] In Pennsylvania, a third party has requested that the state require modeling for all 
Page 21

compressor facilities based on an independent report presenting modeling results with impacts 

above the NAAQS. [8] To date, the state has provided analysis and data to justify not requiring 

such modeling, but additional third party challenges are likely. Thus, although requirements in 

response to the 2010 NO2 NAAQS have not yet been common, the 2010 NO2 NAAQS provides 

a platform for requirements that could broadly affect reciprocating engines that are not equipped 

with low NOx technology.

Thus, although schedules and mandates are highly uncertain, implementation of current and 

pending regulations will likely require retrofit NOx control (or replacement) for many existing 

natural gas-fired reciprocating engines. For this report, the specific timing and count of affected 

units was not defined. The analysis conducted, based on the actual engines in the fleet discussed 

in Section 4, focused on available resources and the cost and schedule to install controls based on 

NOx endpoints of 3 g/bhp-hr or 1 g/hp-hr. These emission levels are based on anticipated 

emission limits for typical NOx rules, with the lower limit based on potential requirements in 

more aggressive states.

In judging the likelihood of control and target NOx level (i.e., 3 or 1 g/bhp-hr), states were 

divided into three categories based on a judgment of the risk of NOx control and stringency over 

the next 5 to 15 years. This assessment was based on experiences with different states, proximity 

to areas that have previously failed to attain the ozone NAAQS or are expected to not attain if the 

ozone NAAQS is lowered in 2015, or within an area that has been targeted by eastern U.S. 

nonattainment areas as a “NOx transport” area. When considering the time required to install 

NOx controls, engine location and this ranking were considered (e.g., likelihood that a NOx level 

of 1 g/hp-hr NOx would be required). For assessing the likelihood of control and the NOx 

endpoint (3 g/hp-hr or 1 g/hp-hr), states were placed into one of three regulatory risk categories:

• Category 1 (high risk and stringency) – geographical region (i.e., states) where stringent NOx 

emissions limits are already in place (e.g., northeast states, California, Colorado); if NOx 

control is not in place, there is a high probability engine modifications will be required;

• Category 2 – geographical region where there is a moderate probability that lower NOx 

emission limits will be required (e.g., Midwest and southeast states implicated as contributors 

to NOx transport); and,

• Category 3 – geographical region with a lower probability that lower NOx emission limits

will be required.

The INGAA database includes engines in forty-one states, and the list of states and their ranking 

is shown in Table 5-1. 
Page 22

Table 5-1. Regulatory Risk Category for States with Engines in the Database.

State Risk Category

Alabama 2

Arizona 2

Arkansas 2

California 1

Colorado 1

Connecticut 1

Florida 2

Georgia 2

Idaho 3

Illinois 2

Indiana 2

Iowa 3

Kansas 2

Kentucky 3

Louisiana 2

Maryland 1

Michigan 2

Minnesota 3

Mississippi 2

Missouri 3

Montana 3

Nebraska 3

New Jersey 1

New Mexico 2

New York 1

North Carolina 2

Ohio 2

Oklahoma 2

Oregon 3

Pennsylvania 1

Rhode Island 1

South Carolina 3

Tennessee 2

Texas 2

Utah 3

Virginia 2

Washington 3

West Virginia 2

Wisconsin 3

Wyoming 3
Page 23

5.2 Assumptions and Methodology for Reciprocating Internal Combustion Engines (NOx)

The following assumptions and methods were used when assessing technology and resource 

requirements, schedules, and costs.

• Many rich burn engines employed in interstate natural gas transmission have been modified 

to reduce HAP emissions under the 2004 RICE NESHAP. Typically these four-stroke cycle 

rich burn engines added NSCR technology, including the associated engine air-to-fuel ratio 

control (AFRC). This technology also reduces NOx emissions. As such, four-stroke cycle 

rich burn engines were excluded from this analysis. It is believed that this is a reasonable 

assumption with the following caveats:

- Some of the smaller four-stroke cycle rich burn engines or engines at small facilities may 

not have installed NSCR.

- Some engines may require additional modifications or more attention to AFRC to meet 

NOx limits. This may be achieved by installing a larger catalyst or an upgraded AFRC.

• Some engines (i.e., horizontals) do not have suitable control technologies to reduce NOx

emissions as they sometimes operate near stoichiometric conditions (rich) at full load and 

lean conditions at reduced load. These engines lack the ability to control the air to fuel ratio 

across the load range. In addition, the engines are of older designs that have limited parts 

availability and higher operating costs. For the purposes of this study, it is assumed these 

engines are replaced with new engines and the associated compressor and ancillary 

equipment. Specific engine types include:

- Cooper-Bessemer four-stroke cycle horizontal engines, and

- Worthington four-stroke cycle horizontal engines.

• Some of the engines in the INGAA database have been removed from service since the 

database was last updated. Those units were removed from the analysis where information 

was provided by the operator or the author had personal knowledge regarding those units.

• For cost estimating purposes, the following scope of work was assumed for NOx control:

- 3 g/hp-hr NOx emission rate limit:

 NOx control through low emissions combustion,

 High energy ignition system is assumed to be precombustion chamber, either:

o Screw-in, or

o Head replacement;

 Turbocharger modifications are required for lean combustion,

 Turbocharger after-cooling enhancements are required, and

 May or may not require (depending on engine model):

o Fin fan after-cooler upgrades,

o Coolant surge tank modifications,

o Auxiliary generator upgrades,

o Air inlet piping and filter modifications, and/or

o Exhaust manifold, silencer, and exhaust stack modifications.
Page 24

 Alternate or additive approaches (depending on the engine and emission target) include:

o Enhanced mixing with or without high energy ignition system modifications, and

o Post-combustion treatment utilizing SCR in select cases.

- 1.0 g/hp-hr NOx emission rate limit:

 Modifications for 3 g/hp-hr NOx as outlined above plus enhanced mixing and/or postcombustion treatment utilizing SCR.

• To identify those engines already controlled, existing emission capabilities were assumed 

based on descriptions and attributes in the INGAA engine database. Specifically, engines 

were assumed to be controlled to:

- 3 g/hp-hr NOx or lower if:

 The engine was identified as having low NOx emission controls,

 The engine was identified as having post-combustion catalytic treatment (i.e., this 

implies that permitting closely scrutinized emissions), 

 The engine make and model number is known to be a low NOx emissions unit, 

 The engine is located in a state where control is already required for reasonably 

available control technology (RACT), or

 The operating company and/or emission reduction vendor identified specific engines 

that have been modified to achieve low NOx emissions.

- 1 g/hp-hr NOx or lower if the operating company and/or emission reduction vendor 

identified specific engines that were modified to be capable of this emission level.

• As discussed in Section 5-1 and shown in Table 5-1, U.S. locations were divided into three

areas based on a judgment of the risk of NOx control requirements and stringency over the 

next 5 to 15 years. 

• New air emission regulations or permitting requirements are expected to eliminate the 

grandfather provision in many cases and institute NOx control requirements for existing 

engines that currently are not controlled. The timing that the States (or other agencies) will 

implement NOx reduction rules and the level of NOx reduction cannot be defined at this 

time. For assessing the likelihood (and stringency) of control, States were categorized into 

three geographical regulatory risk categories as discussed above and shown in Table 5-1.

• To understand worst case scenarios in regards to the number of affected engines and a 

conservative cost projection, it was generally assumed that some level of control would be 

required for uncontrolled units. For example, engines in northern plains states that may not 

be affected by a regulation to address nonattainment could still require control to address 

NO2 modeled impacts. In general, with the exception of the rich burn engines in Table 4-4, 

engine counts without controls (e.g., horizontal engines; engines >3 g/hp-hr) from Section 4 

tables were included in the cost estimate and scheduling assessment, and additional engines 

(i.e., units currently controlled but with emissions between 1 and 3 g/hp-hr) are considered 

for the 1 g/hp-hr NOx limit, depending on location. These total over 2600 uncontrolled 

engines and over 500 engines that may require additional control if NOx requirements are 

reduced to 1 g/hp-hr. At this time, it is not expected that the lower NOx level would be 
Page 25

broadly required. So, while costs are estimated for the 1 g/hp-hr endpoint to provide that 

estimate in the report, the schedule to complete all retrofits judges the engine location when 

assessing the likelihood of a 1 g/hp-hr NOx requirement.

6.0 Cost and Schedule Estimates 

The primary purpose of this report is to assess resource constraints associated with retrofit NOx 

control. In assessing technology requirements and resource constraints, costs were also 

determined. Cost and schedules are discussed in this section, and Section 7 discusses resource 

availability (i.e., skill sets and staffing) for each project phase. Information is tabulated for:

• Capital cost for NOx control to achieve NOx emissions of either 3 or 1 g/hp-hr (see 

Tables 6-1 and 6-2). Separate costs are shows for each emission level, and these costs are 

not incremental.

• Annual engine retrofits (or replacements) that can be completed based on current staffing 

levels for key technology and engineering service providers (see Table 6-3).

• Time required to complete retrofits (or replacements) for the projected engine counts, 

based on an emission level of 3 or 1 g/hp-hr and current staffing levels. 

− Table 6-2 shows the number of years required to control engines to 3 g/hp-hr. 

− Marginally longer time is required to achieve 1 g/hp-hr for some engine types. 

Table 6-5 shows the incremental time required to achieve the lower NOx limit. 

• The regulatory risk “categories” shown in the tables are based geographical assignment 

of regulatory risk by state as discussed in Section 5.1 and shown in Table 5-1.

6.1 Capital costs

The costs to modify or replace engines to reduce NOx emissions were gathered through 

interviews with operating companies, other published sources, and the author’s experience.

As described above, unit (engine and compressor) replacement was assumed for horizontal and 

in some cases opposed piston two-stroke cycle engines. Replacement units were assumed to be 

dry low NOx gas turbines. Replacement costs were estimated at a fixed cost per unit 

($1,800,000) plus a variable cost proportional to engine size ($895/hp). In the case of horizontal 

and opposed piston two-stroke cycle units, one new turbine unit was assumed for every two units 

that were replaced.

In select cases where SCR was considered, costs were estimated at a fixed cost per unit 

($250,000) plus a variable cost proportional to engine size ($375/hp). Lean burn combustion 

modifications were estimated at a fixed cost per unit ($152,000) plus a variable cost proportional 

to engine size ($725/hp). Because of additional turbocharger and after-cooling requirements for 

two-stroke cycle engines, multipliers for the variable costs (for lean combustion conversion only) 

are 1.22 and 1.08 for low BMEP and medium/high BMEP two-stroke cycle engines respectively.

Enhanced mixing modifications were estimated at a fixed cost per unit ($235,000) plus a variable 

cost proportional to engine size ($85/hp). When both lean combustion and enhanced mixing 

modifications are implemented at the same time (e.g., converting from > 3 g/hp-hr to <1 g/hp-hr) 
Page 26

costs were estimated at a fixed cost per unit ($427,000) plus a variable cost proportional to 

engine size ($785/hp).

Costs were assessed for affected engines to reduce emissions to 3 g/hp-hr. For reducing to less 

than 1 g/hp-hr, the additional costs were determined for further reducing emissions from these 

engines. In addition, costs were determined for additional engines that are already controlled to 

3g/hp-hr that require additional reductions to achieve 1 g/hp-hr. Note that the costs to modify 

engines for NOx control are significantly lower than the replacement costs. This is due in large 

part to the fact that both the engine and the gas compressor must be replaced if the engine is 

replaced. As a general rule, the replacement equipment has a lower overall unit efficiency1and 

therefore uses more fuel (and by correlation has higher CO2 emissions) than the units they are 

replacing.

The total estimated NOx control capital costs to modify reciprocating engines used in the natural 

gas transportation industry are summarized below. Table 6-1 represents the costs to reduce 

emission rates to less than 3 g/hp-hr, and Table 6-2 represents the costs to reduce emission rates 

to less than 1 g/hp-hr. For 3 g/hp-hr, total costs are nearly $4 billion and “average” costs per unit 

for the different engine types were about $1 to $1.5 million for 2-stroke engines and $2 million 

for 4-stroke engines. Replacement costs exceeded $2 million per unit. 

Note that these two tables are exclusive, i.e., the tables present costs to achieve either 3 or 1 

g/hp-hr and are not to be added together. In addition, while many units may need to achieve 

NOx limits on the order of 3 g/hp-hr, it is currently not expected that a large percentage of the 

fleet will need to achieve 1 g/hp-hr. For example, engines in higher risk areas (Category 1) are 

more likely to face the more stringent emission limit, while engines in lower risk Category 3 

would be far less likely to require a 1g/hp-hr emission limit. Thus, costs to achieve 1 g/hp-hr 

NOx are a conservative upper bound (i.e., broad NOx regulations are more stringent than 

expected) for the engine fleet in the INGAA database. 

Table 6-1. Costs to achieve 3 g/hp-hr ($ in thousands).

Emission type Category 1 Category 2 Category 3 Total

Horizontal engines $0 $236,308 $36,054 $272,362

Opposed piston $756 $14,311 $0 $15,067

Med & high BMEP 4-stroke $0 $660,355 $89,798 $750,153

Low BMEP 2-stroke $0 $685,751 $65,811 $751,562

Med & high BMEP 2-stroke $0 $1,770,389 $330,588 $2,100,977

Total $ 756 $3,367,114 $522,251 $3,890,121

 

1 Generally reciprocating engines drive reciprocating compressors while gas turbine engines drive centrifugal 

compressors. As a general rule, reciprocating engines are more fuel efficient than gas turbines. Likewise, 

reciprocating compressors are generally more efficient than the centrifugal compressors. There are exceptions where 

the gas turbine unit is the more efficient option. An example would be a high speed, low BMEP reciprocating engine 

driving a reciprocating compressor with small valve flow area replaced with a gas turbine/centrifugal compressor.
Page 27

Table 6-2. Costs to achieve 1 g/hp-hr ($ in thousands).

Emission type Category 1 Category 2 Category 3 Total

Horizontal engines $0 $236,308 $36,054 $272,362

Opposed piston $2,108 $38,100 $0 $40,208

Med & high BMEP 4-stroke $18,235 $807,287 $111,779 $937,301

Low BMEP 2-stroke $24,290 $629,647 $60,415 $714,352

Med & high BMEP 2-stroke $94,840 $3,645,435 $680,204 $4,420,479

Total $ 139,473 $5,356,777 $888,452 $6,384,702

6.2 Schedule

Starting in 1999 as part of the NOx State Implementation Plan (SIP) Call (an eastern U.S. 

regional NOx rule for large existing sources), approximately 200 natural gas transmission engines 

were converted to LEC. From interviews with the operators and the emission reduction 

equipment suppliers, the conversion process took six years in total to fully implement. Additional 

engines were controlled in the late-1990’s through about 2009 in response to state RACT rules. 

Based on interviews with pipeline operations and emission reduction equipment suppliers having 

experience with previous conversion projects, NOx control for each engine requires between 1 

and 2 1⁄2 years to complete (from inception to completion of commissioning). The longer 

duration projects are those that require more infrastructure modifications (such as cooling 

equipment and auxiliary generators), on-engine modifications (after-coolers, turbochargers, 

lubrication systems, cooling systems, fuel systems, pre-chambers, intake/exhaust systems, and 

controls), and engine overhaul. Generally, older engines require more time to design the 

conversion. This is due to inaccurate or missing engineering records on the current configuration 

of and ancillary equipment used on the engine.

Taking into account both the lead time and conversion time and based on currently available 

resources (i.e., trained personnel), the average number of units that can be modified to lean 

combustion on a sustained basis is approximately 75 engines per year. This is the key resource 

constraint that will affect NOx control requirements that affect these engines, even if only a 

subset of engines (e.g., a quarter of the current total capacity) is affected by new NOx control 

rules. Note that this projection is somewhat higher than the approximately 50 units per year 

converted under the NOx SIP Call even though additional technical resources are not in place at 

this time. The higher number of units converted per year assumes some efficiency of scale will 

be achieved based on processes and standards developed from previous efforts. A relatively 

optimistic projection for the annual number of conversions was selected so that the analysis 

reflects an “optimistic” scenario without considering how staffing and training will respond to 

market demand.

The estimated number of engines that can be retrofit per year is based on current resource 

availability. While a dramatic increase in market demand would likely result in hiring and 

training of additional resources, the special skills associated with this niche market would require 

time to build that resource. In addition, until new regulations are adopted the market and timing 

is not clear, thus this resource base will not grow until the market is clear. 
Page 28

Based on the engine counts in Section 4 and the regional location of the engines, an estimate of 

the number of engines that can be modified per year is shown in Table 6-3, and the resulting 

number of years necessary to address NOx controls for the reciprocating engines used in natural 

gas transmission is presented in Tables 6-4 and 6-5. 

The number of engines that can be modified per year depends on differences in scope related to 

the engine type (e.g., some engines are configured for lean combustion while others are not).

The estimated number of engines that can be modified per year by engine type and emission 

level is shown in Table 6-3. The count varies for different engine types and emission level due 

to the complexity of the retrofit (e.g., need for additional equipment such as turbochargers, need 

to upgrade ancillary support such as cooling, etc.). It is not anticipated that the number of annual 

engine conversions can be significantly improved upon within a short timeframe (e.g., if demand 

for NOx control significantly and quickly increases) because specific skills and specialized 

training are necessary to serve this market.

Table 6-3. Estimated number of engines that can be modified/replaced per year.

Modified to achieve NOx emission levels of:

Engine type & initial emissions < 3 g/hp-hr < 1g/hp-hr

Horizontal engines 40 40

Opposed piston 40 40

Med & high BMEP 4-stroke > 3 g/hp-hr 75 64

Med & high BMEP 4-stroke < 3 g/hp-hr N/A 82

Low BMEP 2-stroke > 3 g/hp-hr 60 50

Low BMEP 2-stroke < 3 g/hp-hr N/A 75

Med & high BMEP 2-stroke > 3 g/hp-hr 75 64

Med & high BMEP 2-stroke < 3 g/hp-hr N/A 80

Based on the number of engines to be controlled (engine counts from the INGAA database as 

shown in Section 4) and the estimated number of engines that can be modified per year, the 

estimated time required to implement modifications to achieve 3 g/hp-hr is shown in Table 6-4.

2

 

For NOx levels of 1 g/hp-hr, far fewer engines will be affected. To provide an upper bound on 

capital costs for control, , costs in Table 6-2 were estimated assuming that all of the affected units 

would need to achieve the low NOx level. The schedule to achieve the lower NOx level and 

timing presented in Table 6-5 considers a much smaller subset of engines, such as engines in a 

high risk location such as the northeast (Category 1). While Table 6-2 shows the costs to control 

all engines to 1 g/hp-hr, Table 6-5 shows the nominal incremental time required for this smaller

subset of “high regulatory risk” engines to achieve 1 g/hp-hr. 

 

2 While the elapsed time for an engine retrofit or replacement from inception to completion may take 1-2 calendar 

years, the work can be performed by multiple engineering firms and construction companies resulting in an average 

resource time of less than one year per project. 
Page 29

Based on current technical resources, the projected time to implement retrofit NOx control 

(or replacement) is far in excess of typical regulatory schedules. Primarily due to the large 

number of engines in Category 2 (units primarily in southeast and midwest states), Table 6-4 

shows that it would take decades to address NOx controls for a large number of engines, even if 

the annual rate of retrofit conversions is doubled. Since the retrofit market is currently very 

limited (i.e., rules are not currently requiring installation of retrofit NOx controls), there is no 

incentive to increase capacity at this time. So, this limitation will not be addressed until 

indicated by market demand and confirmed regulations. Section 7 includes additional discussion 

of manpower and other resource constraints on a project level basis.

Table 6-4. Time required to modify engines to achieve 3 g/hp-hr (in years) 3

Emission type Category 1 Category 2 Category 3 Total

Horizontal engines 0.00 2.6 0.4 3.0

Opposed piston 0.03 0.3 0.00 0.3

Med & high BMEP 4-stroke 0.00 4.1 0.6 4.7

Low BMEP 2-stroke 0.00 7.7 0.7 8.4

Med & high BMEP 2-stroke 0.00 18.2 3.6 21.8

Total 0.03 32.9 5.3 38.2

A small number of engines (e.g., in Category 1) may require control to the more stringent 1 g/hphr NOx level. Table 6-5 shows the incremental time required to address that more stringent 

mandate and shows minimal impact on schedule to achieve the lower NOx level. However, 

considering capital costs in Tables 6-1 and 6-2, a lower NOx limit will have a significant cost 

impact, with engine-specific costs on average about 65% higher to achieve 1 g/hp-hr. 

Table 6-5. Incremental time required to modify a subset of high risk engines to 1 g/hp-hr 

rather than 3 g/hp-hr (in years).

Emission type Category 1 Category 2 Category 3 Total

Horizontal engines 0.00 0.07 0.01 0.08

Opposed piston 0.00 0.01 0.00 0.01

Med & high BMEP 4-stroke 0.00 0.06 0.01 0.07

Low BMEP 2-stroke 0.00 0.15 0.01 0.16

Med & high BMEP 2-stroke 0.00 0.28 0.06 0.34

Total 0.00 0.57 0.09 0.66

 

3 Unless resource constraints are addressed via staffing and training, the overall timeline cannot be shortened by 

executing the projects for multiple regions/engine types simultaneously. For example, replacing all of the horizontal 

engines in Categories 2 and 3 will take about 3 years even if the work for both categories is performed at the same 

time. This is because executing projects in Category 3 states concurrently with projects in Category 2 states will 

require the reallocation of resources away from Category 2 states. 
Page 30

7.0 Resource Analysis and Limitations by Project Phase

This section discusses resource requirements and constraints chronologically by project phase. 

The resource considered in this section is available manpower – i.e., staffing and expertise by 

topical area. All phases of project planning and execution are discussed even though several 

have minimal impact on schedule or cost. The information gathered identified several 

constraints and concerns associated with the available resources to implement NOx emission 

reduction projects. Constraints are due to the availability of skilled or expert staff necessary to 

complete key project tasks, and the requirements and limitations are discussed qualitatively by 

project task. 

Interview subjects included multiple operating companies and six different suppliers of 

equipment and services to modify natural gas transmission reciprocating engines. 

7.1 Permitting

The permitting portion of the project typically involves evaluating many different cases. Design 

iterations are made to determine technology requirements to reduce emissions to required levels 

within the required timeframe. Secondary factors include the impact the proposed retrofit NOx 

control technology has on equipment efficiency and operational turndown. Due to the workload 

of the permitting agencies, the permitting process typically takes a long time to complete (months 

to years in some cases). Because of the long lead time involved, air quality permit applications 

are often filed before design optimization for the NOx control equipment can be completed. 

Complicating factors may include other regulatory limits. For example, implementing 

significant modifications to a piece of equipment to reduce air emissions may require 

modifications to reduce noise emissions to conform to local noise regulations. In addition, 

special care must be made to ensure retrofitting NOx control technology does not adversely 

impact emissions of other pollutants such as THC and CO, or additional controls (i.e., oxidation 

catalyst for CO or hydrocarbons) are considered to address this situation. 

The operators expressed concern that regulations to reduce NOx emissions will result in 

constraints at the state regulating agencies (e.g., timing to process permits and potential backlog 

due to state budget and staffing issues). The result could include:

• Slow response time in issuing air permits. This can result in delays to the project execution 

schedule, and can make it difficult to comply with new regulations with a hard deadline for 

implementation.

• Limited flexibility in approving new emission reduction technologies (i.e., time constraints 

due to agency staffing).

• Limited ability to modify the design for optimization after the permit application has been 

submitted without causing further delays.

These concerns could be mitigated somewhat if longer or phased implementation schedules are 

allowed. States generally propose a short compliance schedule (e.g., 18 months to two years 

from the rule date). It is incumbent upon the affected industry to try to convince the state during 

rule development to allow longer schedules, including phased schedules. Thus, operators need to 
Page 31

be proactive in working with states (or EPA) to achieve “reasonable” schedules. However, it is 

highly unlikely that a decades long implementation scenario indicated by Table 6-4 will be 

achievable. 

7.2 Initial design

Based on the expected number of engines to be modified, pipeline operating companies will 

require additional engineering staff to perform initial design of NOx reduction modifications. 

Some of this will be achieved through the hiring of additional staff. However, based on the 

current trends and practices, most of this resource will be provided by firms that provide contract 

engineering services or turnkey solutions from LEC technology providers. Examples include: 

Black & Veatch, E-N Engineering, Mustang Engineering, Cameron, Dresser Rand, Hoerbiger,

GE Oil and Gas, and many others. The experience and knowledge of these engineering firms for 

retrofit NOx control projects is varied and in most cases is limited. This is due in part to high 

turnover that occurs in these firms and the wide variability of equipment to be modified. The use 

of engineering staff additions and/or engineering services will require a significant learning 

curve and training. This will require more time to execute initial engine modification designs. 

As experience improves over time, the project execution duration should be reduced. 

During this phase of project execution, providers of emission reduction equipment such as

Cameron, Dresser Rand, and Hoerbiger will receive numerous requests for potential options for 

emission reduction technologies. This is especially likely where:

• Very low emission rates are required,

• Few engines of the make/model have previously been modified for low NOx technology,

• The engine has a unique design and configuration that requires specialized engineering 

analysis, and/or

• The engine is required to operate in special modes (e.g., requires a wide power range).

This demand for quotes is a potential resource constraint for the emission reduction equipment 

providers, especially when demand initially increases. In general, the unique attributes 

associated with legacy integral reciprocating engines provide a common theme regarding 

personnel / expertise based constraints. If regulatory timelines can be extended for projects that 

are unique in design or configuration, this can be mitigated.

Some emission reduction equipment providers will provide turnkey services (design, procure, 

construct, commission). However, there are currently only a handful of companies that provide 

this service for NOx control of legacy integral units. The number of engines modified using 

turnkey services will be limited to what emission reduction equipment providers can provide 

with moderate increases in qualified technical staff through using contract employees, part time 

staff, subcontractors, and retirees. 

7.3 Procurement

Provided adequate personnel resources and experience exist in the initial design phase to produce 

procurement specifications, the actual procurement process should have minimal resource 

constraints. Qualified procurement personnel are available on a contract basis. In many cases,
Page 32

the pipeline operating company will use the same contract engineering services to perform the 

procurement process.

Discussions with NOx emission control equipment suppliers indicated that manufacturing 

production can be increased to provide the equipment within a reasonable time period (assumed 

to be at least 3 years). As such, the procurement of the hardware required for NOx emission 

reduction projects is not expected to be a constraint as long as reasonable regulatory schedules are 

negotiated, trends for increased demand are understood, and production responds accordingly. 

7.4 Cost estimating and scheduling

The cost estimating and scheduling phase typically use the same resources used in the initial 

design. This phase is not expected to have a significant resource constraint.

7.5 Detailed design

Detailed design for emission reduction projects will encounter much of the same resource 

constraints as described under initial design. The difference in this phase is more resources will 

be required to produce engineering drawings and project specification documents. Like the 

initial design phase, this will be achieved through a mix of additional employees for the pipeline 

operating company, contract services, and turnkey providers, with turnkey LEC technology 

companies the predominant resource – unless rapid growth in market demand prompts an 

alternative in the future.

At this phase of the project, the pipeline operating company will require additional resources to 

provide detailed project management oversight for the life of the project. Again, this will likely 

be achieved through the use of a mix of new employees and contract services with an associated 

training and learning curve period.

7.6 Construction

Pipeline operating companies and turnkey emission reduction vendors typically use contract 

construction services to implement the physical modifications to engines. These resources are 

generally available and should not be a constraint unless there are a number of major pipeline 

construction projects that would overlap with the timing of the engine emission reduction 

projects. Review of the potential for competing infrastructure projects is beyond the scope of 

this report, but other studies are available, such as the INGAA Foundation Report on midstream 

infrastructure projections through 2035. [9]

There is one exception; there is currently a shortage of welders qualified for natural gas piping.

Modifying a large number of engines to reduce NOx emissions (e.g., 25%, 50%, or possibly the 

vast majority of over 2600 engines identified in Section 4) would further constrain this resource. 

Increasing the timeline available to implement engine modifications and avoiding an overlap in 

the execution of those projects with major pipeline expansion projects would help mitigate this 

constraint.

During the construction period, the equipment is not available for the transportation of natural 

gas. To minimize the impact on pipeline capacity and associated disruption for shippers, outages 

are optimized by:
Page 33

• Scheduling the construction during periods of low system demands (such as during the 

summer for pipeline systems that have peak demands during the winter heating season),4

• Scheduling outages such that only one or two engines are out of service at any one station 

during any period of time, and

• Scheduling outages such that outages at compressor stations are staggered (e.g., start with 

outages at Stations A, C, E, & G, and when the modifications at those stations are completed, 

then Stations B, D, F, & H)

Future PHMSA regulations [10] may require pipeline operators to complete hydrostatic testing 

of pipelines (e.g., those installed prior to 1970). If the schedule to install NOx controls overlaps 

with other pipeline regulations, a reduction in available pipeline capacity may occur. This is 

exasperated by the overlapping vintage of the pipe that may be affected by PHMSA rules and the 

compressor engines likely to require NOx control. The result could result in shortages of natural 

gas in some areas of the country. To mitigate this possibility, it is important to understand not 

only air quality (or safety) regulatory schedules, but possible implications from different 

compliance activities across all regulations that occur in the same timeframe. Possible 

implications from overlapping requirements need to be communicated to regulators to try to 

avoid impacts on pipeline capacity and natural gas delivery.

7.7 Commissioning and Start-up

Proper commissioning of NOx reduction equipment is critical to assure the modified engine 

reliably meets air emission limits. The process of commissioning and safely placing into service 

an engine with new NOx control technology requires many different skills and specialized 

equipment, including:

• Engine mechanical specialist to tune the operation of the engine and equipment to measure 

the developed engine power,

• Control technicians to calibrate additional measurement points, verify pump rotations, etc.,

• Control system specialists to modify control software (to accommodate additional 

measurement points and control algorithms) and tune control loops,

• Qualified operating personnel to safely introduce fluids and energize systems, 

• Subsystem specialists with specific expertise in control valves, gas measurement, and

communications,

• Air emissions testing equipment and analysts, and

• Specialists to train the equipment operators on the operation and maintenance of the new 

equipment.

Due to the specialized skills required during this phase and expectations that demand will exceed 

the currently available resources, these are personnel resources that are likely to be constrained

 

4 With the increase in the number of natural gas fueled power plants installed over the last decade, many interstate 

pipelines that historically had peak loads during the winter heating season now also have peak demand periods in the 

summer. Off peak periods for these pipelines are now during the spring and fall. 
Page 34

for NOx control projects. Mitigation measures include utilizing available temporary resources 

(such as qualified contractors and retired employees), hiring and training new staff, and 

extending the timeline for modifications. 

7.8 Operation and Maintenance

Within the first year after commissioning an engine, additional adjustments are commonly 

required after break-in and to address changes to seasonal ambient conditions. This typically 

requires the same technical personnel and equipment used to commission the engine as discussed 

above. The same mitigation measures described above would apply to operation and 

maintenance of the equipment.

If adequate operating spares for the newly installed equipment are purchased as part of the 

project, material resources should not be a constraint.

8.0 Conclusions

There is a great deal of uncertainty about the breadth (i.e., how geographically broad), depth (i.e., 

stringency of the rule), and schedule for new NOx regulations that will affect existing natural 

gas-fired reciprocating engines. Litigation and scheduling delays have slowed the regulatory 

process. Although the timing and breadth of new NOx control regulations remains uncertain, it 

is likely that many of the natural gas-fired reciprocating engines in the existing prime mover fleet 

will require NOx control by 2025. There are several thousand legacy natural gas-fired 

reciprocating engines driving compressors in interstate transmission, and their location (i.e., in 

rural areas where emission rules are more rare) and history (i.e., operating prior to rulemakings 

that affect new equipment) make these engines candidates for future regulation. 

Based on previous projects and interviews with industry experts, modifying these engines will 

require significant time and capital. The capital required to address the fleet of engines without 

low NOx technology is significant. In addition, the lack of available expertise is likely to be a

primary constraint in addressing broad, new regulations on a timely basis. 

Based on interviews with operators, equipment vendors, and service providers, the primary 

conclusions from this study include:

• The special technical expertise to design, construct, and commission emission reduction 

projects for the low speed integral engines prevalent in natural gas transmission is available 

from a handful of companies, and with few NOx rules being adopted in recent years, this 

expertise may be migrating to other markets. Regulations that require installation of NOx 

control on a large number of reciprocating engines will require a significant lead time to train 

and develop resources to implement emission reduction projects. For example, if upcoming 

NOx regulations impact 25% of the current fleet of legacy reciprocating engines, the number 

of units requiring NOx control (over 600) would exceed the total number of units that have 

been controlled over the last 20 years. A higher percentage could be impacted (e.g., 50% of 

units affected is over 1300 units; even high impacts are possible), and market demand could 

significantly exceed the available resource base of skilled professionals.

• Availability of this special technical expertise and building this resource is the primary 

resource constraint that will affect the ability to meet regulatory obligations that affect 
Page 35

a large percentage of the fleet. Based on current capabilities and a scenario where NOx 

regulations broadly affect the existing fleet of uncontrolled reciprocating engines, the 

estimated time to complete upgrades to over 2600 engines is nearly forty years. 

- Although regulations may affect a smaller subset of engines or compliance strategies may 

result in many engines (e.g., lower horsepower, lower use engines) being retired or 

replaced, schedule implications due to the lack of available expertise will likely extend 

far beyond the required regulatory timeline (e.g., 1000 engines would require 15 years).

- There is no incentive to begin to build this resource base until it is clear that a market will 

exist – i.e., the regulations and associated schedule are in place. Since lawsuits and EPA 

priorities have slowed the next cycle of NOx regulations, it is not possible to project the 

timing, stringency, number of affected units, or schedule for implementing new NOx 

rules.

- In addition to regulatory risk associated with NOx rules in response to nonattainment 

with the ozone NAAQS, revisions that increased the stringency of the NO2 NAAQS in 

2010 could trigger new NOx control requirements for existing facilities through the 

permitting process – e.g., during permit renewal. This issue has the potential to trigger 

NOx control requirements sooner than the current ozone NAAQS timeline.

• Due to this resource constraint, it is imperative that companies engage state and federal 

regulators when NOx rulemakings commence. Emission regulations typically allow only one 

to two years to implement controls, and additional time and phased implementation will be 

needed to provide a more reasonable schedule. However, once the next round of NOx rules 

are initiated, it is unlikely that a decade(s) long schedule will be allowed. The schedule 

conflicts associated with regulatory timelines, market demand, current supply (i.e., available 

resources), and service provider growth will likely present a significant challenge over the 

next 5 to 10 years.

• Engine NOx control projects are generally much less costly than engine replacement. Capital 

costs to modify the fleet of currently uncontrolled reciprocating engines used in the interstate 

natural gas transmission industry are estimated at $3,890 million and $6,385 million to 

achieve NOx emission rates of 3 g/hp-hr and 1 g/hp-hr respectively.

• The age of the impacted equipment (most of the engines are over 40 years old) requires 

additional time to engineer and construct reliable emission reduction modifications due to 

inaccurate or missing engineering records that reflect the current equipment configuration.

• Some engines with low specific power output will require extra time to properly design and 

construct emission reduction modifications and maintain the same power operating range.

• Based on previous experience, the timeline to obtain air permits is a key parameter defining 

the overall schedule for completing a specific emission reduction project – i.e., permitting 

can slow the project timeline. 

• Equipment outages to implement emission reduction modifications may have a significant 

impact on available pipeline capacity, especially if the timeline overlaps with implementation 

of other regulations, such as pipeline integrity assessments that may be required by PHMSA.
Page 36

9.0 Bibliography

1. Ely, Charles G. II, Holden, Jay and Novacek, Loran. Solutions Reduce Engine Emissions.

The American Oil & Gas Reporter. May, 2010.

2. Olsen, Dan, et al. ERLE Cost Study of the Retrofit Legacy Pipeline Engines to Satisfy 1⁄2 

g/BHP-HR NOx. s.l. : Pipeline Research Council International, 2009.

3. Edgerton, Stephen, Lee-Greco, Judy and Walsh, Stephanie. Stationary Reciprocating 

Internal Combustion Engines: Updated Information on NOx Emissions and Control 

Techniques. s.l. : U. S. Environmental Protection Agency, 2000. 68-D98-026.

4. EPA-CICA Fact Sheet - SCR. s.l. : U. S. Environmental Protection Agency, 2003. EPA452/F-03-032.

5. Mussatti, Daniel C. EPA Air Pollution Control Cost manual - Sixth Edition, Section 4, 

Chapter 2, Selective Catalytic Reduction. s.l. : U. S. Environmental Protection Agency, 2003. 

EPA/452/B-02-001.

6. Non-Catalyst Emissions Reduction Package For The Ingersoll-Rand KVG-103. Potter, 

Randy, Glover, Earl and Williams, Carl O. s.l. : Gas Machinery Conference, 1999.

7. AERMOD Implementation Workgroup NO2 and SO2 Modeling, Snyder, Erik and Thurman, 

James. Presentation at EPA 10th Modeling Conference, March 2012.

8. Tran, Khanh T. AERMOD Modeling of NO2 Impacts of the Barto Compressor Station. Final 

Report to the Clean Air Council. January, 2013. 

9. North American Midstream Infrastructure through 2035: Capitalizing on Our Energy 

Abundance. ICF International, INGAA Foundation Report. March 2014.

10. Department of Transportation. Pipeline Safety: Safety of Gas Transmission Pipelines. 

Advanced Notice of Proposed Rulemaking. Federal Register. August 25, 2011, Vol. 76, 165.

11. Choquette, Gary and Choquette, Lisa. Preparing for 2013, Lessons Learned from NOx RACT 

and SIP Call. s.l. : Pipeline Research Council International, Inc., 2012.
Page 37

Appendix A: Acronyms

AFRC Air to Fuel Ratio Controller

BACT Best available control technology

BHP Brake horsepower

BMEP Brake mean effective pressure

CO Carbon monoxide

EPA Environmental Protection Agency

g Grams

hp Horsepower

Hr Hour

INGAA Interstate Natural Gas Association of America

LEC Low emission combustion

MACT Maximum achievable control technology

NAAQS National Ambient Air Quality Standards

NESHAP National Emission Standard for Hazardous Air Pollutants

NO2 Nitrogen dioxide

NOx Nitrogen oxides

NSCR Nonselective catalytic reduction

NSPS New Source Performance Standard

O2 Oxygen

PHMSA U.S. Dept. of Transportation Pipeline and Hazardous Materials Safety Administration

RACT Reasonably available control technology

RICE Reciprocating internal combustion engine

SCR Selective catalytic reduction

SIP State implementation plan (Dresser-Rand and others also use “SIP” for screw-in prechamber.)

THC Total unburned hydrocarbons

VOC Volatile organic compounds (For gas-fired combustion, VOCs are THC excluding 

methane and ethane, which are the two most prevalent exhaust hydrocarbons). 

Formaldehyde is also a VOC but is excluded from some regulations (e.g., it is more 

difficult to measure than THC and may not be included in data used to establish a 

VOC standard or emission limit).
Page 38

Appendix B: Survey Questions for Operator and Service Provider Interviews

1. Over the last three years, approximately how many engines has your company modified (as 

prime contractor) to meet emission levels of 3 g/hp-hr or less?

2. How many of those modifications were achieved through lean emission reduction?

3. Over the last three years, approximately how many engines has your company supplied parts, 

equipment, material, start-up support, or training to operating companies so they could 

modify their engines to meet emission levels of 3 g/hp-hr or less?

4. Over the last three years, approximately how many engines has your company supplied parts, 

equipment, material, start-up support, or training to other vendors or contractors so they 

could modify their engines to meet emission levels of 3 g/hp-hr or less (acting as a 

subcontractor)?

5. Over the last three years, approximately how many engines has your company modified (as 

prime contractor) to meet emission levels of 1 g/hp-hr or less (this should be a subset of (1) 

above)? 

6. How many of those modifications were achieved through lean emission reduction (this 

should be a subset of (2) above)?

7. Over the last three years, approximately how many engines has your company supplied parts, 

equipment, material, start-up support, or training to operating companies so they could 

modify their engines to meet emission levels of 1 g/hp-hr or less (this should be a subset of 

(3) above)?

8. Over the last three years, approximately how many engines has your company supplied parts, 

equipment, material, start-up support, or training to other vendors or contractors so they 

could modify their engines to meet emission levels of 1 g/hp-hr or less (acting as a 

subcontractor) (this should be a subset of (4) above)?

9. Estimate average number of employees and subcontract staff (i.e. full time equivalent 

positions) utilized to support the emissions reduction projects over the last three years.

10. Over the last three years, the qualified staffing to support emission reduction projects has:

a. Increased significantly

b. Increased some

c. Stayed about the same

d. Decreased some

e. Decreased significantly
Page 39

11. Based on current staffing, estimate the number of projects per year your company can 

support acting as the prime contractor.

12. Based on current staffing, estimate the number of projects per year your company can 

support acting as a subcontractor/parts provider.

13. Based on the technical skills required, estimate the time required to hire and fully train new 

staff to support emission reductions projects:

a. Up to six months

b. Six months to one year

c. One to two years

d. More than two years

14. Please describe any issues or concerns associated with implementing a large number of 

emission reduction modifications over a three year time span.