United States 

Environmental Protection 

Agency 

Research and 

Development 

METHANE EMISSIONS FROM THE 

NATURAL GAS INDUSTRY 

Volume 11: Compressor Driver Exhaust 

Prepared for 

Energy Information Administration (U.S. DOE) 

Prepared by 

National Risk Management 

Research Laboratory 

Research Triangle Park, NC 27711 

GRI-94/ 0257.28 

EPA - 600/R-96 -080k 

June 1996 
FOREWORD 

The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's land. air. ami water resources. Under a mandate of national 

environmental laws. the Agency strives to formulate and implement actions leading to a compatible balance between human activities and the ability of natural 

systems to support and nurture life. To meet this mandate, EPA's research 

program is providing data and technical support for solving environmental problems today and building a science knowledge base necessary to manage our ecological resources wisely, understand how pollutants affect our health. and prevent or reduce environmental risks in the future. 

The National Risk Management Research Laboratory is the Agency's center for 

investigation of technological and management approaches for reducinr{ risks 

from threats to human health and the environment. The focus of the Laboratory's 

research program is on methods for the prevention and control of pollution to air, 

land~ water, and subsurface resources; protection of water quality in public water 

systems; remediation of contaminated sites and groundwater: and prevention and 

control of indoor air pollution. The goal of this research effort is to catalyze 

development and implementation of innovative. cost-effective environmental 

technologies; develop scientific and engineering information needed by EPA to 

support regulatory and policy decisions; and provide technical support and infOrmation transfer to ensure effective implementation of environmental regulations 

and strategies. 

This publication has been produced as part of the Laboratory's strategic longterm research plan. It is published and made aVailable by EPA 1 s Office of Research and Development to assist the user community and to link researchers 

with their clients. 

E. Timothy Oppelt, Director 

National Risk Management Research Laboratory 

EPA REVIEW NOTICE 

This report has been peer and administratively reviewed by the U.S. Environmental 

Protection Agency, and approved for publication. Mention of trade names or 

commercial products does not constitute endorsement or recommendation for use. 

This document is available to the public through the National T echnica! lnfonnation 

Service, Springfield, Virginia 22161. 
METHANE EMISSIONS FROM 

C!{'T- '!'I/o :zs7. 2.8 

EPA-600iR-S6-080k 

June 1896 

THE NATURAL GAS INDUSTRY, 

VOLUME 11: COMPRESSOR DRIVER EXHAUST 

FINAL REPORT 

Prepared by: 

Carole J. Stapper 

Radian International LLC 

8501 N. Mopac Blvd. 

P.O. Box 201088 

Austin, TX 78720-1088 

DCN: 95-263-081-07 

For 

GRI Project Manager: Robert A. Lott 

GAS RESEARCH INSTITUTE 

Contract No. 5091-251-2171 

8600 West Bryn Mawr Ave. 

Chicago, IL 60631 

and 

EPA Project Manager: David A. Kirchgessner 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

Contract No. 68-Dl-0031 

National Risk Management Research Laboratory 

Research Triangle Park, NC 277!1 
DISCLAIMER 

LEGAL NOTICE: This report was prepared by Radian International LLC as an account 

of work sponsored by Gas Research Institute (GRI) and the U.S. Environmental Protection 

Agency (EPA). Neither EPA, GRI, members of GRI, nor any person acting on behalf of 

either: 

a. 

b. 

Makes any warranty or representation, express or implied, with respect to the 

accuracy, completeness, or usefulness of the infonnation contained in this 

report, or that the use of any apparatus, method, or process disclosed in this 

report may not infringe privately owned rights~ or 

Assumes any liability with respect to the use of, or for damages resulting 

from the use of, any information, apparatus, method, or process disclosed in 

this report. 

NOTE: EPA's Office of Research and Development quality assurance/quality control 

(QA/QC) ~equirements are applicable to some of the cowtt data generated by this project. 

Emission data and additional count data are from industry or literature sources, and are not 

subject to EPA/ORD's QAIQC policies. In all cases, data and results were reviewed by the 

panel of experts listed in Appendix D of Volume 2. 

II 
Title 

Contractor 

Principal 

Investigator 

Report Period 

Objective 

Technical 

Perspective 

Results 

RESEARCH SUMMARY 

Methane Emissions from the Natural Gas Industry, 

Volume 11: Compressor Driver Exhaust 

Final Report 

Radian International LLC 

GRI Contract Number 5091-251-2171 

EPA Contract Number 68-DI-0031 

Carole J. Stapper 

March 1991 -June 1996 

Final Report 

This report describes a study to quantify the annual methane emissions 

from compressor driver exhaust, which is a significant source of methane 

emissions within the gas industry. 

The increased use of natural gas has been suggested as a strategy for 

reducing the potential for global wanning. During combustion, natural 

gas generates less carbon dioxide (C02) per llllit of energy produced than 

either coal or oil. On the basis of the amount of C02 ernined, the 

potential for global wanning could be reduced by substituting natural gas 

for coal or oil. However, since natural gas is primarily methane, a potent 

greenhouse gas, losses of natural gas during production, processing, 

transmission, and distribution could reduce the inherent advantage of its 

lower C02 emissions. 

To investigate this, Gas Research Institute (GRI) and the U.S. 

Environmental Protection Agency's Office of Research and Development 

(EPA/ORD) cofimded a major study to quantify methane emissions from 

U.S. natural gas operations for the 1992 base year. The results of this 

study can be used to construct global methane budgets .and to determine 

the relative impact on global warming of natural gas versus coal and oil. 

The national annual emissions for compressor drivers in each industry 

segment are as follows: t)roduction, 6.58 ± 200% Bscf; gas processing, 

6.84 ± 130% Bscf; transmission, 10.2 ± 17.1% Bscf; and storage, 1.19 ± 

27.2% Bscf. 

lli 
Technical 

Approach 

Project 

Implications 

Based on data from the entire program, methane emissions from natural 

gas operations are estimated to be 314 ± 105 Bscf for the 1992 base 

year. This is about I .4 ± 0.5% of gross natural gas production. The 

overall program also showed that the percentage of methane emitted for 

an incremental increase in natural gas sales would be significantly lower 

than the baseline case. 

The program reached its accuracy goal and provides an accurate estimate 

of methane emissions that can be used to construct U.S. methane 

inventories and analyze fuel switching strategies. 

The industry has two primary types of compressor drivers that fire 

natural gas: 1) reciprocating engines and 2) turbines. Methane 

emissions result from the incomplete combustion of natural gas in the 

driver, which allows methane to exit the driver in the exhaust stream. 

The techniques used to determine methane emissions were developed to 

be representative o·f annual emissions from the natural gas industry. 

However, it is impractical to measure every source continuously for a 

year. Therefore, annual emissions for compressor drivers were 

determined by extrapolating measured emissions using activity factors 

where the national emissions estimate is the product of the emission 

factor and the activity factor. 

Emissions test data for each driver type were collected by Southwest 

Research Institute (SwRI) for compressors in natural gas industry service. 

SwRI data for emissions, fuel use rates, and compressor model numbers 

were used with data in GRI's TRANSDAT compressor database to 

develop the emission factors. Equations relating the SwRI data and the 

distribution of compressor models and operating hours found in 

TRANSDAT were developed to calculate emission factors for each type 

of compressor driver. 

Activity factors for each industry segment were developed using site visit 

data, company surveys and databases, and data published in the 

American Gas Association's Gas Facts and the Oil & Gas Journal. The 

national annual emissions for each industry segment were then calculated 

as the product of the emission factor and activity factor for each 

compressor driver. 

For the 1992 base year the annual methane emissions estimate for the 

U.S. natural gas industry is 314 Bscf ± 105 Bscf (± 33%). This is 

equivalent to 1.4% ± 0.5% of gross natural gas production. Results from 

this program were used to compare greenhouse gas emissions from the 

fuel cycle for natural gas, oil, and coal using the global warming 

iv 
potentials (GWPs) recently published by the Intergovernmental Panel on 

Climate Change (IPCC). The analysis showed that natural gas 

contributes less to potential global warming than coal or oil, which 

supports the fuel switching strategy suggested by IPCC and others. 

In addition, results frorr. this study are being used by the natural gas 

industry to reduce operating costs while reducing emissions. Some 

companies are also participating in the Natural Gas-Star program, a 

voluntary program sponsored by EPA's Office of Air and Radiation m 

cooperation with the American Gas Association to implement costeffective emission reductions and to report reductions to the EPA. Since 

this program was begun after the 1992 baseline year, any reductions in 

methane emissions from this program are not reflected in this study's 

total emissions. 

Robert A. Lott 

Senior Project Manager, Environment and Safety 

v 
TABLE OF CONTENTS 

Page 

1.0 SUMMARY ................................................. I 

2.0 INTRODUCTION ............................................. 2 

3.0 DATA SOURCES 3 

3.1 Previous Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

3.2 TRANSDAT Database ..................................... 4 

3.3 Current Study ........................................... 5 

4.0 COMPRESSOR DRJVER EXHAUST EMISSION FACTORS .............. 7 

5.0 COMPRESSOR DRIVER ACTIVITY FACTORS . . . . . . . . . . . . . . . . . . . . . II 

5.1 Production Industry Segment ............................... 12 

5.2 Processing Industry Segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

5.3 Transmission Industry Segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

5.3 .1 Compressor Station Horsepower and Hours . . . . . . . . . . . . . . . . . 19 

5.3.2 Underground Storage Horsepower and Hours ................ 22 

5.3.3 Generator Driver Horsepower and Hours ................... 22 

5.4 Industry Activity Factors .................................. 27 

6.0 NATIONAL COMPRESSOR DRIVER EMISSION ESTIMATES .......... 29 

7.0 CONCLUSIONS ............................................. 30 

8.0 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

APPENDIX A - National Estimate of Methane Emissions from Compressors 

in the U.S. Natural Gas Industry . . . . . . . . . . . . . . . . . . . . . A-I 

APPENDIX B - Source Sheet P-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1 

APPENDIX C - Production Segment Site Visit Results . . . . . . . . . . . . . . . . . C-1 

APPENDIX D - Transmission Compressor Station Site Visit Results . . . . . . . . D·l 

APPENDIX E- Conversion Table ................................ E-1 

VI 
LIST OF FIGURES 

Page 

A-1 Effect of Horsepower and Speed on Total Hydrocarbon Emissions for 

a Cooper Bessemer 2 Cycle Engine ............................... A-5 

A-2 Relative Horsepower Contribution of Each Engine Model in the 

Emissions and Industry Databases ................................ A-14 

A-3 Relative Horsepower Contribution of Each Turbine Model in the 

Emissions and Industry Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-15 

VII 
LIST OF TABLES 

Page 

4-1 SwRI Field Test Emission Factor Statistics 9 

5-1 Compressor Driver Data for Production Segment . . . . . . . . . . . . . . . . . . . . . 12 

5-2 Regional Data for Production Segment Extrapolation . . . . . . . . . . . . . . . . . . . 13 

5-3 

5-4 

5-5 

5-6 

5-7 

5-8 

6-1 

7-1 

A-1 

C-1 

C-2 

Compressor Driver Data from Processing Segment Site Visits . . . . . . . . . . . . . 15 

Annual Operating Hours for Processing Segment . . . . . . . . . . . . . . . . . . . . . . 18 

Annual Operating Hours for Storage Segment . . . . . . . . . . . . . . . . . . . . . . . . . 23 

Generator Driver Data from Transmission Compressor Stations . . . . . . . . . . . . 24 

Generator Driver Data from Storage Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

Compressor Driver Activity Factors for Each Industry Segment . . . . . . . . . . . . 28 

Compressor Driver Emissions for the Natural Gas Industry by. Segment . . . . . . . 29 

Ranking of Compressor Driver Exhaust Emissions by Industry Segment . . . . . . 30 

Description of the Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-8 

Gathering Compressor Data from Production Segment Site Visits C-4 

Regional Data for Production Segment Horsepower Extrapolation C-5 

C-3 Annual Operating Hours for Production Segment . . . . . . . . . . . . . . . . . . . . . C-6 

D-1 Compressor Driver Data from Transmission Segment Site Visits . . . . . . . . . . . DA 

D-2 Annual Operating Hours for Transmission Segment . . . . . . . . . . . . . . . . . . . . D-6 

vm 
1.0 SUMMARY 

This report is one of several volumes that provide background information 

supporting the Gas Research Institute and U.S. Environmental Protection Agency Office of 

Research and Development (GRJ-EPA/ORD) methane emissions project. The objective of 

this comprehensive program is to quantify the methane emissions from the gas industry for 

the 1992 base year to within ± 0.5% of natural gas production starting at the wellhead and 

ending immediately downstream of the customer's meter. 

This report quantifies the amount of unburned methane released in 

compressor driver exhaust in natural gas production, gas processing, and transmission. 

Emissions from generator driver exhaust, while minor, are also included in this report. 

Emission estimates for each industry segment were based on data from one or more of the 

following sources: l) site visits, 2) company databases; and 3) published data. The factors 

that affect the quantity of methane emissions from compressor drivers are: type of driver, 

horsepower. and operating hours. 

Compressor driver exhaust is a significant source of methane emissions. It 

accounts for 24.8 Bscf of methane emissions, which is about 7.9% of methane emissions 

from the natural gas industry. 
2.0 INTRODUCTION 

In the natural gas industry there are two primary types of compressor drivers 

that fire natural gas: 1) reciprocating engines, and 2) gas-fired turbines. Methane 

emissions result from the incomplete combustion of the natural gas, which allows some of 

the methane ln the fuel to exit in the exhaust stream. Compressor driver exhaust emissions 

represent a significant source of methane emissions in all of the industry segments where 

these sources are present. Emissions from generator driver exhaust, while minor, are also 

included in this report. 

Annual emissions were calculated as the product of the emission factor and 

the activity factor. To develop the compressor driver emission factors, test data from 

Southwest Research Institute (SwRl) and GRI's TRANSDAT compressor database 1 were 

used. The activity factors were developed to characterize the compressor drivers in each 

industry segment. Data were gathered from site visits, company databases, and published 

data from the American Gas Association (A.G.A.) and the Oil and Gas Journal. 

This report describes how the emissions from compressor driver exhaust were 

determined. Section 3 discusses the data used to make the emission estimates. Section 4 

presents the development of the emission factors for engines and turbines. Section 5 

describes the development of the activity factors for each industry segment (production, 

processing, and transmission, including storage and generators). The annual emissions for 

each segment and the overall national emissions estimate are provided in Section 6. 

Conclusions are given in Section 7. This report is one of several documents prepared for 

the GRJ/EPA methane emissions project. 

2 
3.0 DATA SOURCES 

Compressor driver exhaust emissions had been identified in earlier studies as 

a significant source of methane emissions. However, these studies were based on limited 

data and did not calculate error bounds for the estimated emissions. This study co!Iected 

and. evaluated a large number of data for compressor drivers and performed an error 

analysis of the emissions estimate. This section provides background information on 

previous studies and an introduction to the data and the approach used for the current study. 

3.1 Previous Studies 

Previous estimates of methane emissions from compressor driver exhaust 

have ranged from about 114 Bscf (2.2 Tg/yr) to 11.5 Bscf (0.22 Tg/yr).2

•

3 The 114 Bscf 

estimate, from Pipeline Systems, Inc. in 1990, was based on an estimated emission factor 

established by the South Coast Air Quality Management District (1120 lb/MMscf fuel)-' 

This estimate used a "model" installation to describe typical compressor facilities for field 

production, transmission, storage, and gas processing; then used these "model" installations 

to extrapolate to a national estimate. The "model" installations were based on site visits to 

three production facilities, three pipeline systems, five injection/withdrawal plants, and two 

gas plants. Eight of the site visits were in California, two were in Texas, and the remaining 

site visits were in the Central Plains Region. The estimate from Pipeline Systems, Inc. was 

based on a limited nwnber of data and are probably biased due to the disproportionate use 

of California sites. 

The 11.5 Bscf estimate was a preliminary estimate developed in I 992 as part 

of the GRIIEPA methane emissions project. 3 (The preliminary emissions estimate is 

described in detail in the paper titled "National Estimate of Methane Emissions from 

Compressors in the U.S. Natural Gas Industry," which can be found in Appendix A.) This 

estimate was based on data published by A.G.A. and data contained in the GRI 

TRANSDAT compressor database. The GRI TRANSDAT database includes data from 

3 
A.G.A. and SwRl. AG.A. data are gathered from government agencies, such as U.S. 

Department of Energy (DOE) and Federal Energy Regulatory Commission (FERC), and 

from surveys of its member companies in transmission and distribution. SwRl data were 

generated during a field testing program on natural gas compressor driver emissions and 

operating characteristics. 

This preliminary study assumed that the GRJ TRANSDAT database included 

information on all compressors used in the natural gas industry when it only contained data 

collected from transmission companies. Therefore, this study underestimated methane 

emissions from compressor driver exhaust for the natural gas industry. 

3,2 TRANSDAT Database 

The GRJ TRANSDAT database is actually composed of three data subsets: 

Industry Database, Operating Database, and Test Database. The Industry Database lists 

8282 compressor drivers (engines and turbines) used in the gas industry in 1989. 

Horsepower in this database is given by compressor model number for each gas company 

and accounts for a total of !6.2 million horsepower (MMhp), installed. TRANSDAT data 

were collected from transmission companies, and it may include a limited amount of data 

on compressors used for production and processing. The Operating Database contains 

annual operating hours and operating horsepower for 1515 compressors (3.2 MMhp, total). 

The Test Database contains emissions data from field tests collected by SwRI and includes 

data on methane emissions, fuel use, fuel use rate, and horsepower for 24 I compressors. 

The Operating and Test Databases were combined to generate a fourth 

database, the Emissions Database, that was then used to develop weighted emission factors. 

This database combines compressor model number, methane emissions, fuel use rate, and 

annual operating hours for 775 reciprocating engines and 86 gas-fired turbines. Therefore, 

the emissions can be calculated for each of the 775 engines and 86 turbines as they were 

operated during the year using SwRl emissions data. 

4 
3.3 Current Study 

Data were gathered during this study to supplement the GRI TRANSDA T 

database to account for emissions from all industry segments. Data were collected during 

site visits and company surveys to determine the number of compressors and their 

horsepower and operating hours. Because it is not practical to visit every site or test every 

compressor driver, a method was developed using emission factors and activity factors to 

calculate total emissions from compressor drivers based on a limited data set. The emission 

and activity factors were defined such that their product would equal the total emiSSIOns 

from compressor driver exhaust for each segment of the industry. Emissions from 

reciprocating engines and turbines were evaluated separately. 

For most source categories in the gas industry, the emission factor is defined 

as the average emission rate determined from a large number of randomly selected sources. 

The activity factor is then the total number of sources within the source category, such as 

the total count of compressor drivers. However, for compressor driver exhaust. the 

emission factor was evaluated in terms of emissions per horsepower ·hour (scf/hp ·hr) since 

this has less variability than an emission factor per compressor unit. By calculating 

emissions on a per horsepower basis the variability in emissions due to compressor size can 

be eliminated. As a result, the activity factor is horsepower·hours per year. This means 

that: 

Total Emissions (set)= EF (scf/hp·hr) x AF (hp·hr) (1) 

The site visit and company survey data provided information on activity 

factors for the various segments of the industry, and SwRJ test data were used to develop 

the emission factors. However, the emission factor was not based on a simple average of 

the SwRI test data because the emission factor needed to be weighted to reflect the 

compressor population of the industry and because some compressor models are used as 

baseloaded compressors and operate a higher percentage of the time. For these reasons, 

5 
A.G.A 's Operating Database, which contains horsepower, operating hours, and model 

numbers for over 1500 compressors, was used to develop weighted industry emission 

factors for reciprocating engines and turbines. The emission factors for engines and 

turbines are 0.240 scf!hp·hr ± 5% and 0.0057 scf/hp·hr ± 30"/o, respectively. The method 

for calculating these weighted emission factors is described in the foiiowing section. 

6 
4.0 COMPRESSOR DRIVER EXHAUST EMISSION FACTORS 

The emission factors for reciprocating engines and gas-fired turbines were 

determined from compressor test data collected by SwRI and weighted to reflect both the 

population of compressor models and their operating hours in the natural gas industry. The 

information needed to develop the weighting procedure was available in GRI's 

TRANSDA T database. 

SwRI followed several procedures to ensure the quality of their compressor 

data. First, the emissions analyzers were calibrated us~ng gases certified by the National 

Institute of Standards and Testing. Second, an oxygen molar balance was performed 

between the reactants and the products of combustion to verify the results. Third, each of 

the data trends was inspected graphically in an effort to identify potential outliers. There 

were generally five data points for eac}:l test. and any of the points that did not foil ow the 

trend were marked as outliers. Data that were collected by outside sources and provided to 

SwRI for inclusion in their study were also scrutinized graphically to validate the results. 

Where technica11y justified, statistical outliers were discarded in the development of the 

emission factors for the GRI/EP A methane emissions project. 

An average emission rate was calculated for each model of compressor 

engine and turbine driver in the Emissions Database using the test data collected by SwRI 

for that model. Using the fuel use rate and average emission parameter for each 

compressor model, the emission rate was calculated as follows: 

ER (m) = EP (m) X FUR (m) (2) 

where: ER<mJ = average emission rate for model, m (scflhr) 

EP <ml =average emission.parameter for model, m (scf CH/scf fuel) 

FUR (m) = average fuel use rate for model, m ( scf fuel/hr) 

7 
The methane emissions for each compressor driver in the Emissions Database 

were calculated by multiplying the emission rate (Equation 2) by the annual operating hours 

for each engine or turbine. The total emissions for each model of compressor driver in the 

database were calculated as follows: 

where: E (m) 

HR, 

M 

M 

E(m) ~ L (ER (m) X HR ,) 

i~ I 

= total emissions for model, m(scf) 

= annual operating hours for compressor i, (hr/yr) 

= number of compressors of model, m 

The total emissions for all compressor drivers in the database were the sum 

of the total emissions calculated for each model. 

(3) 

(4) 

where: TE 

K 

= total emissions for database, ( scf) 

= number of unique compressor models 

These emissions were calculated based on data from 775 engines and 86 

turbines and weighted to reflect the distribution of compressor models found in the gas 

industry as well as the percentage of time these models are operated. Emissions can also 

be affected by the compressor load. However, the emissions are generally linear over a 

fairly large range of operating horsepower. To ensure that significant differences did not 

exist between the horsepower at which the compressor drivers are typically operated and 

the horsepower at which they were tested, the following comparison was made: 

N ? 

I hp (test); :::::: 

i=l 

N 

I hp {operating) i 

i=l 

8 
The two values agreed to within 2%.3 

The emission factors, for engines and turbines, were then calculated using the 

following equation. 

where: 

N N 

Emission Factor ~ TE I [I HP ; x (I HR , IN)] 

HP 

HR 

i=l i=l 

= average operating horsepower during HR, (hp) 

~ annual operating hours, (hrlyr) 

N = number of compressors 

The values obtained are: 

Engines: 1.165 x !o' scf = 0.240 scf/hp·hr (1.3 X Hf hp) X (3742 hr/yr) 

Turbines: o.oo5 x 1 o• scf = 0.0057 scf/hr·hr (0.4 X Jo' hp) X (2326 hr/yr) 

(5) 

Confidence limits were calculated for the emission factors based on the variation in 

SwRI's field test data. Table 4-1 shows the statistics used to determine the 90% confidence 

limits for each of the emission factors. The resulting uncertainty is ± 5% for reciprocating 

engines and ± 30% for gas-fired turbines (using a 90% confidence limit). 

TABLE 4-1. SwRI FIELD TEST EMISSION FACTOR STATISTICS 

Average Emission Standard 

Factor Deviation 

Driver Type No. Tests No. Models (lb Methanellb fuel) (lb MethaneJib fuel) 

Engines 902 229 0.0307 0.0279 

Turbines 105 12 0.000107 0.000187 

9 
Appendix B contains the source sheet for compressor driver exhaust emissions, which 

summarizes the approach taken to calculate the emission factors. The source sheet also 

summarizes the activity factors used to determine the overall national emissions estimate 

from compressor drivers. The next section describes the development of these activity 

factors. 

10 
5.0 COMPRESSOR DRIVER ACTIVITY FACTORS 

Information from site visits, company surveys, A.G.A. 's Gas Facts, and the 

Oil & Gas Journal was used to develop the activity factors. Data on horsepower and 

operating hours were obtained from site visits and company databases for compressor drivers 

located at 297 field gathering stations, 18 gas processing plants, 51 transmission compressor 

stations, and 11 underground storage stations. Data for electric generator drivers at 41 

transmission compressor stations and 3 underground storage fields were also obtained from 

the site visits and company databases. 

Compressors used for gas lift (oil production) were excluded from the 

following analysis, since this gas is not marketed and is not considered part of the natural gas 

industry. Some sites used electricaiiy driven compressors. These electrically driven 

compressors were also excluded in the development of the activity factors, since they have 

no exhaust and therefore have no methane emissions. 

For the production industry segment, horsepower· hour data were provided by 

one company for 516 engines located at 244 gathering stations. Production data for 53 

gathering stations (from site visits and company surveys) were analyzed separately and are 

presented in Appendix C for comparison. For each of the remaining industry segments, 

separate horsepower and operating hour estimates were made, which were multiplied together 

to get the horsepower· hour activity factors. 

Data were gathered for engines and turbines, allowing separate activity factors 

to be developed for the two driver types. Data were also gathered for installed versus active 

horsepower in each industry segment (based on operating hours). Finally, data were 

gathered that could be used to estimate national emissions from each of the natural gas 

industry segments (e.g .• site data for gas throughput). 

II 
The national estimates were extrapolated using 1992 data as the base year, 

except for the transmission segment estimate of compressor station horsepower that uses 

1989 data as the base year. A more detailed explanation of this variation appears in Section 

5.3.1. The following sections describe the development of the horsepower· hour estimate for 

nrntfu,..tinn !>""' th .. hA .. <- ........ ,,. ... _ ~-..t ---··-' -----~'- '-

Confidence limits were assigned to be ± 200% based upon an engineering 

analysis. Rigorous propagation of the error using the ratio method produced an estimate for 

the upper bound of ± 576% (185,630 MMhp·hr). The ratio method assumes that sites are 

randomly sampled, so that a large site is proportionately representative of a large section of 

the population (see Volume 4 on statistical methodology).6 However, an analysis of this 

bound showed it to be unreasonably high. A.G.A.'s Gas Facts reports that lease and plant 

fuel usage for 1992 was 1.2 trillion scf.7 Assuming all of this fuel was fired in compressor 

drivers, this would produce 150,080 MMhp-hr. In reality, much of this fuel is actually used 

in heater burners; therefore, the actual upper bound must be well below 150,080 MMhp·hr 

( ± 44 7%). A separate analysis of the company database produced an upper bound of only 

66,490 MMhp·hr (± 119%). This estimate was based on average horsepower·hours by 

business unit and applied a confidence limit analysis to the average of these data. Each 

alternative approach resulted in lower confidence limits than the calculated ± 576%; 

therefore, the confidence limits were set at ± 200%. 

5.2 Processin2 Industry Se2"'ent 

Horsepower 

Horsepower in gas processing could be extrapolated to estimate a national total 

by using either plant throughput or plant count. Horsepower is technically related to plant 

pressure (aP) and throughput. However, there are no national figures for average plant ilP. 

Without the ilP factor, plant horsepower is not related to throughput alone. Therefore, 

national gas plant horsepower was determined by averaging the values obtained when 

extrapolating by throughput and by plant count. 
1992 to calculate the processing segment horsepower. 8 This gives a total compressor driver 

horsepower of about 2.2 MMhp for engines and 2.8 MMhp for turbines in the processing 

segment. 

Site 

GPI 

GP2' 

GP3 

GP4 

GP5 

GP6 

GP7 

GP8 

GP9 

GPIO 

TOTAL' 

AVERAGE 

TABLE 5-3. COMPRESSOR DRIVER DATA FROM 

PROCESSING SEGMENT SITE VISITS 

llmalled Engine Itmalled Turbine 

hp. hp. 

8300 35000 

0 27000 

0 20000 

3700 0 

1!000 0 

6740 (20000) 0 

5925 (17490) 0 

6267 (18500) 0 

6267 (18500) 0 

36600 23000 

84799 105000 

8480 10500 

Total site horsepower (including gas lift for oil recovery) is shown in parentheses. 

b Estimated. 

Excludes gas lift. 

THROUGHPUT METHOD: 

Engine hp ·hp = Total Site hp x U.S. Throughput 

Total Site MMcfd 

= 84799 hp 

1775 MMcfd 

= 2.22 MMhp 

x 46510.7 MMcfd 

15 

MMcfd 

350 

750 

140 

60 

49 

56 

40 

130 

130 

70 

1775 

177.5 

(7) 
Turbine hp " 105000 hp 

1775 MMcfd 

"2.75 MMhp 

X 46510.7 MMcfd 

The second extrapolation method uses the average installed horsepower per site 

and the total number of gas plants in the United States in 1992 to calculate the processing 

segment horsepower. 7 This gives a total compressor driver horsepower of about 6.2 MMhp 

for engines and 7.6 MMhp for turbines in the processing segment. 

SITE METHOD 

Total Site hp Engine hp·hp " :=-7:':='7=~::-- x U.S. Gas Plants Total Nwnber of Sites 

" 84799 hp 

10 Plants 

x 726 Plants 

"6.16 MMhp 

Turbine hp = 105000 hp 

10 Plants 

"7.62 MMhp 

x 726 Plants 

(8) 

An average of the results of the two extrapolation methods is used to 

determine the national estimate for compressor driver horsepower in the processing segment. 

The average engine horsepower is 4.19 MMhp, and the average turbine horsepower is 5.19 

MMhp. Both estimates are based on the total installed horsepower from the sites visited, 

excluding horsepower used for gas lift. 

Confidence limits were calculated for the average site visit hp/MMcfd and 

average site visit hp/plant using the ratio method. Confidence limits for the U.S. gas plant 

16 
throughput and the total U.S. number of gas plants were estimated to be± 5% and± 2%, 

respectively' The uncertainty for the first extrapolation method (based on throughput) is 

± 114% for engines and ± 120% for turbines. The uncertainty for the second extrapolation 

method (based on number of plants) is ± 71.4% for engines and ± 7.6% for turbines. For 

the average of the two methods, the uncertainty is ± 132% for engines and ± 99.4% for 

turbines (90% confidence limits). 

Operating Hours 

Table 5-4 shows the "operating hours" for the compressor drivers at the 10 

sites visited plus operating hour data from two company databases for an additional 18 gas 

plants. For the site visit data, actual operating hours were only available for two of the sites. 

For the remaining eight sites, compressors lhat were running during the site visit were 

assigned annual operating hours of 8760. Compressors that were idle were assigned annual 

operating hours of 0. One company database (17 plants) included operating hours of 8760 

for some units. These data may be maximum hours, not actual hours: However, no other 

data were available for these units, so the data were used as provided. The operating hours 

listed for each site in Table 5-4 are the average hours for all of the drivers at the site. 

Confidence limits were calculated for the operating hours estimate from the variation of the 

site data. The resulting uncertainty is ± 11.5% for engines and ± 48.4% for turbines (90% 

confidence limits). 

5.3 Transmission lndustnr Sea:rnent 

The activity factor estimates for the transmission segment include horsepower 

and operating hours for compressor stations and storage fields. These estimates have been 

calculated for compressor drivers and generator drivers at both types of facilities. Section 

5.3.1 discusses the compressor driver activity factor estimates for compressor stations. (Site 

visit data for transmission compressor stations are included in Appendix D for comparison.) 

17 
TABLE 5-4. ANNUAL OPERATING HOURS FOR PROCESSING SEGMENT 

Average No. of Average 

Site No. of Engines • Operating Hours Turbines • Operating Hours 

GPlb 7 6257 2 4380 

GP2 0 8423 

GP3b 0 8760 

GP4b 4 4380 0 

GP5b 7 5006 0 

GP6' 6 16 2920 0 

GP7" 4 12 6570 0 

GP8b 4 8760 0 

GP9b 4 8760 0 

GP10 15 5724 5 3816 

GPll 2 0 0 

GP12 5 7376 0 

GP!3 4 4617 0 

GP!4 5 8570 0 

GP15 9 7949 0 

GP16 8760 0 

GPI7 14 8760 0 

GP!8 !0 4043 0 

GP19 3 5840 0 

GP20 II 8760 0 

GP21 7 5325 0 

GP22 46 8760 0 

GP23 8 8760 0 

GP24 10 5011 0 

GP25 8 6487 0 

GP26 5 8176 0 

GP27 8760 0 

GP28 9 7949 0 

TOTAL 203 9 AVERAGE~ 6626 6345 

90%LIMIT' + 11.5% + 48.4% 

• Total number of drivers (including sas lift for oil recovery) are shown in parentheses. h Based on active (8760 hrs) versus i Je (0 hrs). c Based on site averages. 

18 
Section 5.3 .2 discusses the compressor driver activity factor estimates for underground 

storage fields. Activity factor estimates for generator drivers are discussed in Section 5.3.3. 

5.3.1 Compressor Station Horsepower and Hours 

Horsepower 

The transmission compressor station horsepower estimate is based on the GRI 

TRANSDAT database, which includes transmission, storage, and associated field 

compression for 1989. TRANSDAT has not been revised since the 1989 data were 

collected; therefore, for these activity factors, 1989 was used as the base year. It is not 

possible to separate the compressor data in GRI TRANSDAT by the specific application~ 

however, an adjustment for storage compressor horsepower can be made based on data from 

A.G.A. for !989. While the GRI TRANSDAT data could be adjusted to a 1992 basis using 

A.G.A. 's transmission horsepower data, this approach was rejected because £he annual data 

showed no trend and differences were attributed to changes in reporting protocols by 

A.G.A. 's member companies. 

GRI TRANSDAT's Industry Database contains information on 7489 

reciprocating engines and 793 gas turbines. .The total installed horsepower for these 

compressors is 11.2 MMhp for engines and 5.0 MMhp for turbines, which accounts for 

about 97% of the gas utility industry installed horsepower reported by A.G.A. for !989 

(16.7 MMhp). 7 A.G.A.'s data were gathered through a survey of their member companies 

and includes approximately 96% of total gas industry sales. The GRI TRANSDAT 

horsepower can be adjusted for the 1989 storage horsepower reported by A.G.A. (!,462,971 

hp) to give !0.2 MMhp for engines and 4.5 MMhp for turbines. GRI TRANSDAT'S 

engine/turbine horsepower split of 69.1% engines and 30.9% turbines was applied to the 

storage horsepower before adjusting the totals. 

19 
A.G.A. also separately reports that 13.9 MMhp is for transmission in 1992. 7 

This is based upon the annual A.G.A. survey issued to member companies. GRI 

TRANSDAT horsepower, less the storage segment, is about 6% higher than the A.G.A. 

number. This discrepancy probably results from differences in the field gathering 

horsepower reported by GRJ TRANSDAT and A.G.A .. Currently, there is no way to 

separate this type of horsepower from that used fOr mainline transmission. No other 

adjustments to the GRI TRANSDAT horsepower data were made during this study. 

Horsepower in the GRJ TRANSDAT Industry Database is reported as site 

totals. The national estimate for transmission compressqr stations is simply a sum of all of 

these site totals. As a result. the uncertainty cou1d not be calculated from the data as was 

done for the other industry segments. Therefore. confidence limits for compressor station 

horsepower were conservatively estimated by engineering judgement to be ± 10% for each 

type of compressor driver. 

Operating Hours 

The average annual compressor operating hours for the transmission segment 

was determined from information reported on FERC Form No. 2 for the year 1992. In the 

FERC database, transmission companies report the total number of operating hours per stage 

of compression for each compressor station. The total number of stages of compression 

operating on the date of peak demand is also reported by station. The value for operating 

hours was divided by the total number of stages to give the average annual operating hours 

for the station. The average annual operating hours at every station for all the companies in 

the FERC database were then summed and divided by the total number of stations to produce 

the nationwide activity factor. Transmission compressor station annual operating hours were 

3787 hours per year. For a few stations the information in the FERC database was not 

considered to be accurate (less than 1 % of the total), so the average operating hours for the 

rest of the company's stations was applied to the station with questionable data. Confidence 

limits for the FERC operating hours were calculated to be ± 4.0%. 

20 
Because the FERC database does not identify the type of driver, reciprocating 

engine or turbine, a procedure was developed to split the total average operating hours (3787 

hrlyr) between the two types. The FERC data were weighted by the total number of each 

type of driver using data from GRl TRANSDAT as shown in the following equation. 

where: 

793rumm~ (T,) + 7489...,, (R,) = 3787 hr/yr (FERC) 

(793rumm + 7489..., .. ) 

= Turbine operating hours, hr/yr 

= Reciprocating engine operating hours, hr/yr 

(9) 

The following relationship between T hr and Rm was based on data provided by 

a company for 107 transmission compressor stations, which included 89 turbines and 524 

reciprocating engines. 

T,. = 2718 hr/yr = 053 R,. 5086 hr/yr (10) 

Using these two equations to solve for T11r and Rm resulted in annual operating hours of 2118 

hrlyr for turbines and 3964 hrlyr for reciprocating engines. 

Confidence limits for the transmission compressor station operating hours were 

calculated to be ± 31.3% for turbines and ± 13.9% for reciprocating engines. The 

confidence limits were based on assumed uncertainties of ± 10% for the GRI TRANSDAT 

data and calculated confidence limits for both the FERC and company data. 

21 
5.3.2 Underground Storage Horsepower and Hours 

Horsepower 

Horsepower data from A.G.A. 's Gas Facts were used for this industry 

segment. A.G.A. reports that for 1992 the total installed compressor horsepower associated 

with underground storage was 1,920,441 horsepower (entire United States).7 The 

horsepower data were gathered by A.G.A. through a survey of their member companies, 

which includes approximately 96% of total gas industry sales. Since GRI TRANSDAT 

includes storage compressor horsepower, the horsepower split between engines and turbines 

was assumed to be the same as the split found in this database (69.1 %, engines and 30.9%, 

turbines). As a result, the horsepower_ attributed to engines is 1.33 MMhp and to turbines is 

0.59 MMhp. Confidence limits for storage horsepower (engines and turbines) were 

calculated based on an assumed ± 5% uncertainty for the A.G.A. horsepower and an 

assumed ± 10% uncertainty for the GRI TRANSDAT horsepower splits. 

Operating Hours 

Table 5-5 shows the operating hours of compressors at four sites surveyed and 

the operating hours provided by one company for their seven storage fields. The operating 

hours listed are the average hours for all of the drivers at the site. Confidence limits were 

calculated for the operating hour estimate from the variation of the site data. The resulting 

uncertainty is ± 23.1% for engines and ± 620% for turbines (90% confidence limits). 

5.3 .3 Generator Driver Horsepower and Hours 

The generator driver estimates are based on site visit data and data provided 

by one company for 34 transmission compressor stations and three storage fields. Site visit 

data were co11ected for generator drivers (for electricity generators) at seven of the 

transmission compressor stations surveyed. For compressor stations, an average generator 

22 
TABLE 5-S. ANNUAL OPERATING HOURS FOR STORAGE SEGMENT 

Average Average 

No. of Operating No. of Operating 

Site Engines Hours Turbines Hours 

US! 9 2400 0 

US2 !4 5122 4 52.5 

US3 2 5607 0 

US4 3 l !56 0 

US5·1 5 2700 0 

US5-2 3 38!3 0 

US5-3 3 5832 0 

USS-4 4 3727 0 

USS-5 3 3390 0 

US5-6 4 3327 0 

US5-7 0 2 5781 

TOTAL 50 6 

AVERAGE' 3707 2917 

90% LIMIT• ± 23.1% ± 620% 

• Based on site averages. 

horsepower per station was established to determine total generator horsepower. Similarly, 

for storage fields, an average generator horsepower per field was established to determine 

total generator horsepower. 

Data for generators used in transmission compressor stations are shown in 

Table 5-6. Only one turbine driver was instaiied at the 41 facilities surveyed; therefore, the 

data shown in Table 5-6 represent an engine/turbine horsepower split of 97%/3%. Using the 

average installed horsepower per station, the total number of transmission compressor 

stations in the United States for 1992 was used to extrapolate to a national estimate of 1.45 

MMhp for reciprocating engine generator drivers and 0.045 MMhp for turbine generator 

drivers. 9 The number of compressor stations is an estimate based on data from FERC Fonn 

No. 2 for 46 transmission companies and is described in Volume 5 on activity factors. 4 

23 
TABLE 5-6. GENERATOR DRIVER DATA FROM TRANSMISSION 

COMPRESSOR STATIONS 

Installed A.nrage Installed Average 

Site 

. 

No. Engine Operating No. Turbine Operating 

.,..,~ bp Ho= Turbines hp Ho= 

GSI 3 1542 3232 0 0 

GS2 I 514 0 0 0 

GS3 2 1028 1095 0 0 

GS4 0 0 0 0 

GS5 2 3SOO 88 0 0 

GS6 sao 88 0 0 

GS7 I sao 88 0 0 

GSS-1 3 I I JCo 9 0 0 

GSS-2 s ISO! 1688 0 0 

GSS-3 30 0 0 0 

0584 I 43S 2 0 0 

"GSS-5 4 2190 44SS 1080 474 

GSS-6 2 S90 4379 0 0 

GSS-7 43S 0 0 0 

GSS-8 4 1786 2279 0 0 

GSS-9 I l9S 89S 0 0 

GSS-10 22S 0 0 0 

GSS-1! 60 0 0 0 

GSS-12 227 12 0 0 

GSS-13 64(1 5217 0 0 

GSS-14 3 ll48 4756 0 0 

GSS-15 s 1974 S290 0 0 

GSS-16 43S 0 0 0 

058-17 2 940 4392 0 0 

GSS-18 3 1177 2060 0 0 

GSS-19 7 2135 3660 0 0 

GSS-20 2 816 33 0 0 

GSS-21 2 816 0 0 0 

GSS-22 2 290 0 0 0 

GSS-23 4 1556 27 0 0 

GSS-24 

'" 0 0 0 

GSS-25 2 370 129 0 0 

GSS-26 

'" 0 0 

GSS-27 lao 0 0 0 

GSS-28 I 19S 398 0 0 

GSS-29 2 784 4902 0 0 

GSS-30 436 6 0 0 

GSS-3! 7 2365 4370 0 0 

GSS-32 62S 2S4 0 0 

GSS-33 2 816 269 0 0 

GSS-34 I ISO 0 0 0 

TOTAL 87 3S006 1080 

AVERAGE' 

"' 1352 26 474 

90% LIMIT' ± 38.0% ± 620%" 

• Based on site averages. 

' Assumed equal ro compressor rurbine drivers a! storage fields. 

24 
Engine hp·hr = Total hp 

No. Stations 

x No. U.S. Stations 

35006 hp x 1700 Stations 

41 Stations 

= 1.45 MMhp 

Turbine hp = 1080 hp x 1700 Stations 

4!Stations 

= 0.045 MMhp 

Conf1dence limits were .calculated for the horsepower estimates from the 

variation of the data. The uncertainty calculated for the compressor station estimate is 

(11) 

± 10%. The resulting uncertainties of the national e~timates of generator driver horsepower 

are ± 23.3% for reciprocating engines and ± 166% for turbines (90% confidence limits). 

Table 5-6 also shows the annual operating hours for the generators at 

compressor stations. The operating hours listed are the average hours for all of the drivers 

at the station. The wide variation reflects the sites that use generators for continuous 

electricity generation versus the sites that use generators only for emergency backup. Since 

there was only one data point for turbine drivers, the uncenainty was assumed to be the same 

as that for turbine compressor drivers in underground storage at ± 620%. Confidence limits 

were calculated for the reciprocating engine operating hours estimate from the variation of 

the data. The resulting uncertainty is ± 38.0% (90% confidence limits). 

Data for generators used in storage fields are shown in Table 4-7. The data 

were provided by one company for 9 storage fields, but only 3 fields had generators. The 

data shown represent an engine/turbine horsepower split of 60%140%. Using the average 

installed horsepower per field, the total number of storage fields in the United States for 

1992 was used to extrapolate to a national estimate of 0.085 MMhp for reciprocating engine 

25 
generator drivers and 0.057 MMhp for turbine generator drivers. The number of storage 

fields is from A.G.A. 's Gas Facts (475 storage fields).' Confidence limits were calculated 

for the horsepower estimates from the variation of the data. The resulting uncertainties of 

the national estimates of generator horsepower are ± 126% for reciprocating engines and 

± 184% for turbines (90% confidence limits). 

En gme . h ~- _T:..:oc::tal:::-='hp"- p w = ,., 

No. Fields 

x No. U.S. Fields 

Turbine hp 

= 1611 hp x 475 Fields 

9Fields 

= 0.085 MMhp 

= 1080 hp x 475Fields 

9 Fields 

= 0.057 MMhp 

(12) 

Table 5-7 also shows the annual operating hours for the generators at storage 

fields. The operating hours listed are the average hours for all of the drivers at the field. 

Since there was only one data point for turbine drivers, the uncertainty was assumed to be 

the same as that for turbine compressor drivers in underground storage at ± 620%. 

Confidence limits were calculated from the variation in the operating hours data. The 

resulting uncertainty is ± 377% (90% confidence limits). 

TABLE 5-7. GENERATOR DRIVER DATA FROM STORAGE FIELDS 

"""""" Anrage ...... .. Anrage No. 

- Site Ope!rating No. Turbine Openting 

........ hp u- Tw-- hp Houn 

GFJ I 1000 n 0 0 

GF2 0 0 0 0 

GF3 0 0 0 0 

GF4 0 0 0 0 

GF5 0 0 0 0 

GF6 0 0 1080 36 

GF7 0 0 0 0 

GF8 2 611 306 0 0 

GF9 0 0 0 0 

TOTAL 3 1611 AVERAGE• 

1080 

179 191 

90% LIMIT" 120 36 

± 377% ± 620%. 

• ase on Slte averages. 

• Assumed equal to compressor rurbine drivers a1 storage fields. 

26 
Generator drivers in production and processing were not included in this 

analysis since these sources are negligible. Production generators are rare. Processing 

generators are more common but rarely operate. The generator data reported by one 

production and processing company accounted for less than 3% of their total drivers and 

corresponded to less than 0.01 Bscf methane emissions. 

5.4 Industry Activity Factors 

The two main types of compressor and generator drivers in the natural gas 

industry are reciprocating gas engines and gas-fired turbines. Horsepower· hour data were 

available for the production industry segment activity factor calculation. Two pieces of 

information were needed to calculate the activity factors (hp·hr) for each type of driver in 

each of the remaining industry segments. These were the installed horsepower and the 

average annual operating hours. The final hp·hr activity factors were calculated using the 

·national estimates for compressor horsepower and average operating hours for these industry 

segments, as described in the preceding sections. Table 5-8 summarizes each of these 

estimates and shows the resulting activity factors for both engines and turbines by industry 

segment. The table also includes the 90% confidence limits calculated for each factor. 

Activity factor confidence limits were propagated from the confidence limits for the 

individual terms using a standard statistical approach. 

27 
TABLE 5-8. COMPRESSOR DRIVER ACTIVITY FACTORS FOR EACH INDUSTRY SEGMENT 

Installed Installed Annual Annual 

Engine Turbine Hours Hours Engine Turbine Industry Segment MMhp" MMhp' Engine Turbine MMHp ·hr J\.1MHp · hr 

Production NA NA NA NA 27,460 ± 200% 0 

Processing 4.19 ± 132%~ 5.19 ± 99.4%b 6626 ± 11.5% 6345 ± 48.4% 27,760 ± 133% 32,910 ± 121% 

Transmission 

Compressor Drivers 10.2 ± 10.0% 4.55 ± 10.0% 3964 ± 13.8% 2118 ± 31.3% 40,380 ± 17.1% %35 ± 33.0% 

Generator Drivers 1.45 ± 23.3% 0.045 ± 166% 1352 ± 38.0% 474 ± 620% 1962 ± 45.4% 2l.2 ± 1215% 

Storage 

Compressor Drivers 1.33 ± 13.5% 0.59 ± 13.5% 3707 ± 23.1% 2917 ± 620% 4922 ± 26.9% 1729 ± 626% 

N 

00 Generator Drivers 0.085 ± 126% 0.057 ± 184% 191 ± 377% 36 ± 620% 16.3 ± 621% 2.05 ± 1312% 

• Does not include horsepower associated with gas lift for oil recovery or with electric drivers . 

b Average of two estimation methods. 
6.0 NATIONAL COMPRESSOR DRIVER EMISSION ESTIMATES 

The annual emissions for compressor drivers were determined by multiplying 

the emission factors (Section 4) by the horsepower·hour activity factors (Section 5) for 

reciprocating engines and gas-fired turbines and adding these values for each industry 

segment. Table 6-1 shows the resulting emissions for each industry segment and the overall 

national emissions estimate for each type of compressor driver. Using the same approach for 

error propagation, the total annual emissions from compressor drivers and generator drivers 

for the entire industry are estimated to be 24.8 Bscf ± 64%. 

TABLE 6-1. COMPRESSOR DRIVER EMISSIONS FOR THE NATURAL GAS 

INDUSTRY BY SEGMENT 

Driver 

Engines, Bscf Turbines, Bscf 

Production 6.58 ± 200% 0.00 

Processing 6.65 ± 133% 0.186 ± 129% 

Transmission 9.68 ± 17.9% 0.0546 ± 45.7% 

Storage l.l8 ± 26.9% 0.00979 ± 654% 

Generators 0.474 ± 45.6% 0.000132 ± 1163% 

TOTAL 24.57 ± 65.1% 0.256 ± 97.8% 

Approximately 92% of the emissions from compressor driver exhaust are 

estimated to be from reciprocating engines used in the production, processing, and 

transmission segments; another 5% of the emissions are from engines in the storage segment. 

If generator drivers are included, the percentages for all drivers are 94% and 5%, 

respectively. All other categories are negligible in comparison; therefore, it is more 

important to accurately detennine the activity factors for reciprocating engines in production, 

processing, and transmission. 

29 
7.0 CONCLUSIONS 

As a result of the data generated in this study, the natural gas industry 

emissions from compressor driver exhaust were estimated to be 24.8 Bscf ± 64%. Nearly 

99% of the methane emissions are from reciprocating engines, and approximately 40% of the 

methane emissions are from engines in the transmission industry segment. The methane 

emissions from compressor driver exhaust represent about 7.9% of the methane emissions 

from the entire natural gas industry. 10 Table 7-1 shows how these emissions rank with 

respect to the other sources identified in each industry segment. 

TABLE 7-1. RANKING OF COMPRESSOR DRIVER EXHAUST EMISSIONS BY 

INDUSTRY SEGMENT 

Compressor Driver Segment Segment Emissions from 

Industry Emissionsb Emissions' Compressor Drivers 

Segment Bscf Bscf % 

Production 6.58 84.4 7.8 

Processing 6.84 36.4 18.9 

Storage 1.19 18.2 6.5 

Transmission 10.2 98.3 10.4 

Distribution 0 77.0 0 

TOTAL' 24.8 314.3 7.9 

Compressor driver totals are for reciprocating engines and gas-fired turbines combined. 

b Includes generator drivers. 

Compressor driver exhaust emissions represent a significant source of methane 

emissions in all of the industry segments where these sources are present. Since almost all 

of the emissions are due to reciprocating engines, any further work should focus on these 

types of drivers. The confidence limits on the engine emission factor were ± 5% while the 

error bound on the activity factor ranged from ± 17.1% to ± 621%. Therefore, improving 

the engine activity factors would have the greatest impact on the precision of the emissions 

estimate. 

30 
As part of the requirements for Title V operating permits, many facilities will 

implement recordkeeping practices that may benefit this type of data collection. If companies 

were to collect horsepower·hour data for each compressor driver, the emission factors 

developed as part of this project would allow a direct way to determine their methane 

emissions. Access to these databases would also provide the resource to improve the natural 

gas_ industry activity factors for compressor drivers. Using this kind of data would further 

improve the precision of the overall compressor driver exhaust emissions estimate. 

31 
8.0 REFERENCES 

L Biedennan, N. GRI TRANSDAT Database: Compressor Module. (prepared 

for Gas Research Institute). npb Associates with Tom Joyce and Associates, 

Chicago, IL, August 1991. 

2. Pipeline Systems, Incorporated. Annual Methane Emission Estimate of the 

Natural Gas Systems in the United States, Phase 2. Prepared for Radian 

Corporation, September !990. 

3. Jones, D.L., L.M. Campbell, C.E. Burldin, M. Gundappa. and R.A. Lon, 

"National Estimate of Methane Emissions from Compressors in the U.S. 

Waste 

Natural Gas Industry." Radian Corporation and Gas Research Institute, Air & 

Management Association Conference Proceedings, Paper# 92-142.02, 

Kansas City, MO, 1992. 

4. Stapper, B.E. Methane Emissions from the Natural Gas Industry, Volume 5: 

Activity Factors, Final Report, GRI-94/0257.22 and EPA-600/R-96-0BOe. 

Gas Research Institute and U.S. Environmental Protection Agency, June !996. 

5. U.S. Department of Energy/Energy !nfonnation Administration. Natural Gas 

Annual, 1992. DOE/EIA-0131 (92), Washington, DC, September 1992. 

6. Williamson, H.J., M.B. Hall, and M.R. Harrison. Methane Emissions from 

the Natural Gas Industry, Volume 4: Statistical Methodology. Final Report, 

GRI-9410257.21 and EPA-600/R-96-0BOd. Gas Research lnstirute and U.S. 

Environmental Protection Agency, June 1996. 

7. Gas Facts: 1992 Data, American Gas Association, Arlington, VA, 1993. 

8. Oil & Gas Journal. 1992 Worldwide Gas Processing Survey Database, 1993. 

9. 

10. 

Federal Energy Regulatory Commission (FERC) Fonn No.2: Annual Report 

of Major Narural Gas Companies, 1992 database. 

Harrison, M.R., L.M. Campbell, T.M. Shires, and R.M. Cowgill. Methane 

Emissions from the Natural Gas Industry, Volume 2: Technical Repon, Final 

Report, GRI-94/0257.1 and EPA-600/R-96-0SOb. Gas Research Instirute and 

U.S. Environmental Protection Agency, June 1996. 

32 
APPENDIX A 

National Estimate of Methane Emissions from 

Compressors in the U.S. Natural Gas Industry 

A-1 
92-142.05 

National Estimate of Methane Emissions from Compressors in the U.S. Natural 

Gas Industry 

Donna 

Radian 

Lee Jones, Lisa M. Campbell, Clint E. Burklin, and Mahesh Gundappa 

Corporation, Research Triangle Park, North Carolina 27709 

Robert A. Lott 

Gas Research Institute, Chicago, Illinois 60631 

INTRODUCTION 

energy 

The 

generated 

combustion of natural gas emits much less carbon dioxide per unit of 

has been 

than other fossil fuels. For this reason, one strategy that 

other 

suggested for reducing global warming is to encourage switching from 

be a more 

fossil 

potent 

fuels to natural gas. However, methane is currently thought to 

gas 

greenhouse gas than carbon dioxide; if so, leakage of natural 

advantage 

(which 

of 

is approximately 90 percent methane) could reduce or eliminate the 

using natural gas because of its lower carbon dioxide emissions. 

switching 

Two major issues must be addressed before the consequences of the fuel 

strategy can be evaluated. First, there is a need to better define 

the impact of methane relative t~ carbon dioxide on global warming. Second, 

Because 

it is important to better define methane emissions from the gas industry. 

Environmental 

of the latter issue, the Gas Research Institute (GRI) and the U.S. 

quantify methane 

Protection Agency (EPA) have developed a cooperative program to 

methane emissions 

emissions from U.S. gas operations. Currently, estimates of 

production. 

from the gas industry range from 0.5 to 4.0 percent of 

methane 

The GRI/EPA program is a CQmprehensive program to determine 

determine 

emissions from the wellhead to the customer's meter. The goal is to 

emissions to within approximately 3 billion cubic meters (m3

100 billion 

), or 

U.S. production, 

standard cubic feet (scf), which is approximately 0.5 percent of 

been established 

To achieve this overall accuracy, an accuracy target has 

for each source category in the natural gas industry. 

One source category is the exhaust from compressor engines that are used 

to move the gas through the system. Compressors are located in production 

fields, processing plants, gas storage facilities, and along transmission 

might 

lines. A preliminary study indicated that the exhaust from compressor engines 

account for more than 50 percent of the industry's methane emissions. 1 

Corporation 

Because the uncertainty in this early estimate was quite large, Radian 

conducted a study to determine methane emissions from both 

by 

reciprocating and turbine compressor engines. The accuracy target established 

within 

the GRI/EPA program was to determine the emissions for this source category 

30 billion scf. 

In this study, methane emissions were estimated by multiplying 

methane 

the 

emission rate of the unit by the activity of the unit. The emission 

rate expresses the amount of emissions per operating characteristic, 

independent of the other features of the unit. The activity expresses the 

A-2 
92-142.05 

operating characteristic of the unit which was, in this case, either a 

reciprocating engine or gas turbine. For example, when the operating 

characteristic is fuel usage, the activity is annual fuel used by compressors. 

When the operating characteristic is operating time, the activity is hours of 

operation (per year). 

The emission rates used in this study were in the form of methane 

emissions per hour. The activity of each unit was the annual operating hours 

for that unit. A national estimate of annual methane emissions was determined 

from emissions estimates using available data for compressors in the natural 

gas industry. 

APPROACHES 

The Ideal Emission Estimate 

Ideally, the annual methane emissions from compressors ih the natural 

gas industry would be determined from the sum of the annual emissions from 

each unit in the industry, where the emission rate may vary over time and from 

unit to unit, This relationship is expressed by the following equation: 

N 

Emissions - L J [ ER1 (t) x A1 (t) J dt 

i-1 

(1) 

where ER1 (t) is the instantaneous emission rate at time (t) for compressor1 , 

A1 (t) is the instantaneous activity at time (t) for compressor1 , and N is the 

total number of compressor units in the industry. To reflect annual 

emissions, the integral in Equation (l).is evaluated over the time interval t

to 1 

tz, where tz · t 1 is equal to one year. This approach produces no 

uncertainty in annual emissions. 

Emission _Estimate Using Test Data 

The ideal approach was considered impractical because it would require a 

massive data-gathering effort by the industry. Fortunately, a substantial 

amount of data was available for both the emission rates and activity factors 

of compressor engines. For the emission rates, data for reciprocating engines 

and gas turbines were available for a number of models that were tested during 

relatively short time periods. Values for the annual activity of compressors, 

in terms of operating hours, were also available for some models. If data for 

all compressors in the industry were available, the following equation could 

be used to estimate methane emissions: 

N 

Emissions- L (ER1 !At-test X ~(t) /At•ly•ul 

i-1 

A-3 

(2) 
92-142.05 

where ER1 is the emission rate of compressor1 (in grams of methane per hour) 

evaluated over the time period of the test; where the activity, Ai, of 

compressor 1 corresponds to the annual operating hours of compressor1 ; and 

where N is the total number of compressor units in the industry. This 

equation assumes that the emission rate of the compressors during the test 

period represents the emission rate of the compressors during the year, on the 

average. 

manufacturer 

Methane emissions from compressors will vary from manufacturer to 

maintained, 

and model to model. Assuming that the compressors are properly 

will 

differences in methane emissions, even for the same model, also 

be caused by operating the compressors at different horsepower levels or 

speeds, or for longer or shorter periods of time in order to satisfy the 

operational needs of the system. 

Figure 1 illustrates the relationship between operating load and 

emissions for a Cooper Bessemer two-cycle engine. 2 The figure shows the 

total hydrocarbon emission rate at varying loads, where speed is used to lower 

horsepower. 

methane 

Total hydrocarbon emissions can be considered a surrogate for 

emissions, since metharie is expected to comprise a large portion (over 

90 percent) of total hydrocarbon emissions from compressors. The data show 

with 

that the emission rate for a Cooper Bessemer engine increased at lower loads, 

a 40 percent increase in emissions at SO percent of rated horsepower 

(full load), and a possible 100 percent increase if loads close to 30 percent 

of full load are used. A similar increase in methane emissions can be 

expected for other engine and turbine models, although the magnitude of the 

increase for other models is not known at this time. 

The emission rate of any compressor, then, is a function of the rated 

horsepower and operating horsepower. The equation below summarizes this 

relationship: 

(3) 

where ER1 is the operating emission rate of compressor1 ; ER1° is the emission 

rate of compressor1 at the rated horsepower, Hp1°; and Hp1 is the operating 

horsepower. The data from the Cooper Bessemer illustrate that a linear 

relationship can be used to represent emissions for this engine as a function 

of horsepower, within 3 percent accuracy, over the range of horsepower from 

SO to 100 percent of rated horsepower. In this situation, the average 

horsepower at which the engine operates over the year (as in Equation (2)) 

could be used to evaluate the integral in Equation (1) without any loss of 

accuracy. 

If the information needed to develop Equation (3) was known for all 

engines and the relationship between horsepower and emissions for each engine 

was linear, the following equation could be used to more accurately estimate 

annual methane emissions from compressor engines from test data: 

A-4 
• 

h 

l 

12 

11 

1 

• 

• 

4 

3 

2 

20 

"' ~ -~-1!] ~~ ·-. ··~ 

30 40 50 60 70 80 

fWcent Raled 1-torMpowet 

·H · 231 RPM ·D· 264RPM 

. . 

00 100 110 120 130 

-f- 196 RPM -Q 297 RPM -o JJO RPM 

L---~-----------------------------------

Figure 1. Effect of horsepower and speed on total hydrocarbon emissions for a 

Cooper Bessemer 2 cycle engine• 

~ 

N 

. 

'' 

u• 
92-142.05 

Annual Emissions 

(grams) 

N 

L [ ER (Hp) 1 x Annual Hours (Hp) 1 ] 

i-1 

(4) 

where ER (Hp) 1 is the emission rate of compressor 1 as a function of horsepower 

(in grams of methane per hour); Annual Hours (Hp) 1 are the annual operating 

hours of compressori at a specified horsepower, and N is the total number of 

compressor units in the industry. 

Scaled-up Emission Estimate Using Test Data 

Although emission and activity data for all compressors in the industry 

were not known, an estimate of methane emissions for the industry was made by 

proportioning the emissions for a subset of the industry (with known emission 

and ac-tivity data) according to the proportion of horsepower contributed by 

this subset. This approach assumes that the emission rates for the test 

engines during the test period and the operating hours estimated for the 

subset represent the average operation of compressors in the industry. A more 

detailed description of this approach is found in the Method section of this 

paper. 

It was found that for the emission test data, on the average, the 

compressor emission rates were measured at virtually the same horsepower as 

the operating horsepower that was recorded for the compressors with the 

activity data (where the average horsepower of the compressors for the test 

data was equal to 0.985 times the average of the horsepower for the activity 

data). Therefore, knowledge of the relationship between horsepower and 

emissions was not needed for the analysis presented here. 

An estimate of methane emissions using test data for a subset of the 

industry may underestimate actual industry emissions if many compressors are 

operated at lower loads for a significant period of time, since methane 

emissions are likely to increase when compressors are operated at less than 

full load. Future work in this area should include an analysis of the 

relationship between horsepower and methane emissions, so that individual 

compressor emission rates can be estimated with more accuracy. 

DATABASES 

Description of the Databases 

A number of databases were obtained for this study, that provided 

information on compressor emissions and operations. The databases were part 

of GRI's TRANSDAT compressor module. 3 Two databases in the compressor module 

contain information about the distribution of compressors in the natural gas 

industry. One of _the databases is an almost complete listing of engines and 

turbines in the gas industry, accounting for 16.2 million out of the 

16.7 million total horsepower that was reported by the American Gas 

Association (AGA) for this industry in 1989. 4 This nindustry Databaseft 

A-6 
92-142.05 

contains information concerning 8,282 compressors in the natural gas industry. 

However, individual compressor horsepower is not reported in the database; 

only the total horsepower for all compressors of a specific type is reported 

for each gas company. 

A second smaller database in the compressor module (the "Operating 

Databasen) is a subset of the Industry Database (with 1,515 units) 

corresponding to 3.2 million total horsepower. The Operating Database is the 

only database in TRANSDAT that contains information on annual operating hours 

for each unit. The operating horsepower of each compressor is also recorded 

in the database. The data in the Operating Database were obtained from an AGA 

survey of 112 companies in ozone nonattainment areas, where NOx emissions were 

minimized. Because emissions of NOx and hydrocarbons are inversely related, 

the operation of compressors to minimize NOx will likely increase emissions of 

hydrocarbons and, hence, of methane. Consequently, the data in the Operating 

Database, such as the operating horsepower, may represent maximum methane 

emission conditions. The use of these data, then, presents a conservative 

estimate of industry emissions, if the data is representative of the industry 

as a whole. 

Data from emissions tests performed by Southwest Research Institute are 

contained in a third database in TRANSDAT (the "Test Database"). During the 

emission tests, the compressors were operated at close to full load (rated 

horsepower). Methane emissions, fuel use, fuel use rate, and horsepower were 

recorded for each emissions test for 241 models of engines and turbines. 

Since there was some variation in horsepower in the multiple emissions tests 

for the same compressor, it may be possible to define the relationship between 

horsepower and methane emission rate for the 241 models of compressors in this 

database. With this information, it would be possible to extrapolate from the 

test conditions to actual operating conditions at lower horsepower levels. 

Because of the limitations of this study, an analysis of this type was not 

performed. 

Because the Test Database contains only emission data and the other 

databases contain only operating data, a fourth database was developed that 

contains data for compressors for which information was found in both the 

Operating Database and Test Database. This fourth database was called the 

"Emissions Database." A total of 775 reciprocating engines and 86 gas 

turbines (out of 1,515 units in the Operating Database) fit the criteria and 

were included. 

Table 1 describes the contents of each of the four databases. 

Model-Matching Hierarchy 

A model-matching schema was designed to max~m1ze the amount of 

correlation between the Test Database data and the Industry and Operations 

Databases. Originally, when the compressors were matched according to exact 

model names, the Test Database accounted for only 38 percent of the units 

A-7 
T~BLE I, DESCRIPTION OF THE O~TAB~SES 

Industry Database 

Data 

Operating Database Test Database Ealsslons Database 

Engines Turbines Total Engines Turbines Total Engines Turbines Total Engines Turbines Total 

Total Units 7,489 19] 8 ,282 l' 38') lJO 1,515 219 

" 241 II> 

" Hanuf/Hodet• 

861 

922 144 1,0&6 Jta 31 )49 219 

" Total 

241 120 7 127 

Hp (106 ) 11.2 

Total 

'.0 16.2 2. 4 0.8 3.2 NA NA NA I ) 0. 4 I .7 

Hours (106 ) NA' NA NA 4.6 0.) 4.9 NA NA NA 2. 9 0 2 ) I 

•Nuaber of unique co~ressor models in the database. 

bNot applicable. 
92-142.05 

in the Operating Database and only 7 percent of the Industry Database. 

Through consultations with experts in the field of compressor emissions, a 

three-step hierarchy was designed to match more data in the Industry and 

Operating Databases to the Test Database. 

The first step, therefore, in the hierarchy was based on an exact match. 

For instance, an Ajax DPC-360 engine in the Industry or Operating Database was 

identified with Test Database data for an Ajax DPC-360 in Step (1) of the 

hierarchy. 

The second step in the matching procedure matched compressors by 

substring name, where the horsepower of both compressors was within 

± 20 percent. An example of Step (2) in the hierarchy was a match of a Clark 

BA8T engine with a Clark HBAST engine (common substring of BAST), where the 

horsepower 

was 

for the Clark HBA8T engine in the Emissions Database was 1,911, and 

2,050 for the Clark BAST engine in the Industry Database. The third step 

in the hierarchy consisted of matching the rated horsepower per cylinder 

(± 20 percent) in compressors with similar initial names (manufacturer) but 

with varying substrings. An example of Step (3) is a match of a Clark BAS 

engine, with 248 horsepower per cylinder (total horsepower- 1,242) and a 

Clark BAS engine, with 211 horsepower per cylinder {total horsepower- 1,000). 

Following the execution of the three steps in the model-matching 

hierarchy, 37 percent of the models and 57 percent of the units in the 

Operating Database (and 23 and 55 percent, respectively, in the Industry 

Database) were matched to emissions data. The execution of the three-step 

hierarchy also increased the amount of data in the Emissions Database to a 

total of 1.7 million horsepower, from the previous total of 1.3 million 

horsepower after Step (1) only, and accounting for over one-half of the 

horsepower in the Operating Database and Industry Database. 

METHOD TO ESTIMATE METHANE EMISSIONS 

The Emissions Database was used to 

compressors in the natural gas industry. 

emissions is described below. 

Equations 

estimate.methane emissions from 

The method used to calculate 

An average emission rate was obtained for each model of compressor 

engine and turbine in the Emissions Database from the average of all methane 

which 

emission tests in the Test Database for that model. Since the time period in 

each test was conducted was not given, the emission rates, in units of 

grams of methane per hour, were calculated from the reported methane emissions 

per unit of fuel (in grams per m3 ) and the reported fuel use rate (in m3 per 

hour) for each compressor, as in the equation below: 

f\-9 
92-142.05 

Emission Rate Average Emission Rate1 '- X (g/hr) (g/m') 

where FUR1 was the average fuel use rate for compressor1 . 

The methane emissions for each compressor were then calculated as in 

Equation (6) below: 

Em iss ions1 l - ER1 x Annua Operating Hours (grams) 1 

where ER1 was the methane emission rate of compressor1 calculated with 

Equation (5), and Annual Operating Hours 1 were obtained from the data for 

compressor1 in the Emissions Database. 

To determine the total methane emissions for the compressors in the 

Emissions Database, the following equation was used: 

Annual Emissions 

(grams) 

N 

L [ ER1 x Annual Operating Hours1 J 

i-l 

(5) 

(6) 

(7) 

where the emissions from each unit are calculated as in Equation (6), and N 

was the total number of compressors in the database. 

Estimate of National Emissions 

If data for all compressors in the industry were available, a national 

estimate could be calculated using Equation (7). Since the compressors in the 

Emissions Database were only a subset of the compressors in the natural gas 

industry, a procedure was necessary to relate the methane emissions calculated 

with Equation (7) to a national estimate. 

The ratio of the total horsepower from compressors in the industry 

(16.7 million horsepower) to the total horsepower of the compressors in the 

Emissions Database (1.7 million horsepower) was used to seale up the methane 

emissions calculated in Equation (7) by a factor of (16.7/1.7), or 9.8, to 

estimate national emissions. This relationship is shown in the equation 

below: 

National Methane 

Emissions 

Methane Emissions for 

Emissions Database X 

Scaling Factor 

Hp, 

HpED 

(8) 

where Hp1 is the total horsepower in the industry4 and Hpm is the total 

horsepower in the Emissions Database, which produce a scaling factor of 9.8. 

A-10 
92-142.05 

RESULTS 

National Emission Estimate 

An estimate of national methane emissions from compressors using the 

approach discussed above was 0.22 teragrams (Tg) of methane. Over 99 percent 

of the emissions were estimated to be from reciprocating engines and less than 

1 percent from gas turbines. This estimate was based on the assumption that 

the compressors in the Emission Database represent compressors used in the 

natural gas industry, on the average. If the industry compressors are 

operated at horsepower levels much less than the rated horsepower, the methane 

emissions estimated here could underestimate the industry emissions, since 

methane emissions are thought to vary inversely with horsepower. 

Field, Plant, and Pipeline Compressor Emissions 

Compressors are used in field and plant operations as well as in 

transmission activities. The emission estimate above does not apportion 

methane emissions among the segments of the natural gas industry that use 

compressors. The U.S. Department of Energy (DOE) provides estimates of the 

amount of fuel used in field (lease), plant, and pipeline applications, and 

these estimates were used to apportion methane emission estimates among the 

sources. 5 The methane emission estimates were based on the assumption that 

all the fuel reported for field and plant purposes was used by compressors. 

This assumption is likely to be an overestimate, because fuel is known to be 

used for field and plant purposes by equipment other than compressors. 

Although the portion of lease and plant fuel used for other purposes was not 

known, estimates of the fuel used by these sources served as rough estimates 

of fuel used by compressors in field, p~ant, and pipeline operations. 

The result of using DOE estimates of fuel use was that 39 percent of 

total compressor fuel was attributed to field compressors, 26 percent to plant 

compressors, and 35 percent to pipeline. It is likely, however, that a higher 

percentage of total compressor fuel is used for pipeline, ano that lower 

portions are used for field and plant. The AGA estimated that 84 percent of 

total compressor horsepower was used for the pipeline 4

; if fuel use can be 

assumed to be proportional to horsepower, the percent of fuel used by pipeline 

compressors could be over- twice as high as the estimate based on the DOE 

information. 

Since methane emissions were assumed to be proportional to fuel use, the 

DOE breakdown in fuel among the three sources of compressor emissions in the 

natural gas industry was used to apportion the national estimate of 0.22 Tg of 

methane emissions between the three sources. The results were that, of the 

total 220 megagrams (Mg) of annual methane emissions estimated for compressors 

in the industry, 86 Mg of methane was estimated to be emitted from field 

compressors, 57 Mg from plant compressors, and 77 Mg from pipeline 

compressors. As discussed above for apportioning fuel use between these three 

sources, the emissions from pipeline compressors were probably underestimated, 

and the portions for the other two sources were overestimated. Another 

A-ll 
92-142.05 

possible error in the breakdown of emissions was the assumption that the 

emission rate and operating characteristics of compressors in field, plant, 

and pipeline applications were similar. 

UNCERTAINTY IN EMISSION ESTIMATES 

Since the true value of methane emissions from compressors in the 

natural gas industry is not known, the accuracy of the estimate of 0.22 Tg of 

methane emissions cannot be assessed. However, the data can be used to 

produce an approximation of the uncertainty in the emission estimate. Like 

confidence intervals, the uncertainty estimated for a value can describe, with 

a high degree of confidence, the range in which the true value lies, 

The target uncertainty for the total estimate of methane emissions from 

all sources in the natural gas industry was close to 3 billion m3 (100 billion 

scf). Because there are many segments in the natural gas industry that emit 

methane that are part of the overall methane emission estimate in the GRI/EPA 

project, the goal for each segment was to produce a methane estimate with as 

little uncertainty as possible. The target uncertainty for compressors was 

approximately 800 million m3 (30 billion scf). 

The following explains the procedure used to estimate the uncertainty in 

the annual methane emission estimate from compressors and the results of this 

estimate. 

Basis of Uncertainty in the Compressor Emission Estimate 

The only sources of uncertainty in the approach described above that 

could be estimated were the analytical error associated with the test data 

used to calculate the methane emission rates and the scale-up to national 

emissions based on the ratio of the horsepower in the Emissions Database to 

the estimate of compressor horsepower in the industry. 

The uncertainty in the hydrocarbon analysis was estimated to be 

± 10 percent, based on the expected gas chromatograph (with a flame ionization 

detector) capabilities. Likewise, the uncertainty associated with fuel flow 

measurements was estimated to be ± 2.5 percent. The uncertainty associated 

with the scale-up between the Emissions Database and total industry horsepower 

was more difficult to quantify. Two indirect assessments of this uncertainty 

are discussed below, using comparisons between the Emissions Database and the 

Industry Database, which was taken to be a fairly good representation of the 

industry as a whole. The uncertainty for the scale-up to nationwide emissions 

was estimated to be approximately 1 percent, based on estimated significant 

figures in total horsepower. For an estimated industry horsepower of 

16.7 million, the uncertainty was estimated to be 0.17 million horsepower; for 

the Emissions Database, with 1.7 million horsepower, the uncertainty was 

estimated to be 0.017 million horsepower. 

~12 
92-142.05 

Procedure to Calculate Uncertainty 

The software "@Risk" 6 was used to estimate the uncertainty in the 

estimate of methane emissions from compressors. The variables of uncertainty 

were entered into the program along with the values discussed above. The 

"@Risk" uncertainty estimating procedure recalculated the total methane 

emission estimate using Latin Hypercube sampling from normally distributed 

intervals along the range of each variable, defined by the expected value and 

the (estimated) standard deviation of that value. In the case where the 

standard deviation was estimated from significant figures, a uniform interval 

(as opposed to a normally distributed interval) was used for the Latin 

Hypercube sampling. 

In the "@Risk" program, the recalculation was performed 500 times, each 

time using a different value in the interval of each variable in the emission 

equations. The results of the 500 calculations, excluding values below zero, 

were analyzed for variation about the mean. The uncertainty was reported as 

the coefficient of variation, or the standard deviation divided by the mean, 

as a percent. 

Estimates of Uncertainty 

The uncertainty for the estimate of methane emissions from compressor 

engines and turbines, at 0.22 Tg (11.5 billion scf), was estimated to be 

approximately 1 percent, or 4.3 million m3 (0.15 billion scf). This value is 

well below the 800 million m3 target for compressor engines and turbines. 

Comparison of the Database 

One component of uncertainty in the methane emission estimate that could 

not be quantified for the uncertainty analysis was the degree to which the 

compressors in the Emissions Database represented the compressors in the 

industry. The Emissions Database was compared with the Industry Database in 

two procedures that were designed to assess the representativeness of the 

Emissions Database for compressors in the industry and, indirectly, the 

uncertainty associated with the scaling factor. 

Figures 2 and 3 are plots for engines and turbines, respectively, 

showing the total horsepower for each model in the Industry Database versus 

the total horsepower of that model in the Emissions Database. The line formed 

from the relationship between the total horsepower for each database was 

plotted on each graph. For engines, a line through 11.2 million and 

1.3 million, the total horsepower for engines in the Industry and Emissions 

Databases, respectively, constituted the ideal relationship between the total 

horsepower for each type of engine in the two databases. Similarly, for 

turbines, a line through S million and 0.4 million produced the ideal 

relationship for the total horsepower for each type of turbine in the two 

databases. The figures show that the horsepower contribution of the 

compressors in the Emissions Database was higher than the horsepower 

A-13 
• 

• 

" 

, 

"' • 

c 

0 

... .!! 'D E c w: 

. , 

> c 0 0~ 

' st. -... " 

·c 

" 

0 

0 

.. 

:r 

00,--y-----------------------------------------------, 

80 

70 

60 

50 

40 

30 

20 

• 

• 

• 

Ideal 

- • • • 

• • • 

... • II • • =-'· 

••• 

• 

• 

0 too 200 300 400 

(Thousands) 

Horsepower Conlribttlon tn the kldustry Oal:abase 

Figure 2. Relative horsepower contribution of each engine model in the 

Emissions and lnduslrv Datah,.~P~ 
220 

210 

200 

100 

180 

170 

160 

.. 1:;<1 Jl 

, 

"' 1.a 

c 

• 

0 130 ·: . . E i 120 

w. 110 . , :> 

~ ' ~~ 100 

v. ~ 

c 00 £ 

8 80 .. I 10 

60 

50 

.., 

30 

20 

10 

0 

r 

r 

r 

r 

-

r 

tr 

• 

• • 

• 

I 

0 200 

(Thousands) 

HP ConltibuUon - Industry Oalabasa 

• 

400 

• 

I 

600 

Figure 3. Relative horsepower contribution of each turbine model in the 

Emissions and Industry Databases 
92-142.05 

contribution 

positive direction 

in the Industry Database. However, the trend in general is in a 

for both databases, showing that compressors 

contribute 

that 

a large amount of horsepower 

contribute 

to the total in one database also 

significantly to the other. Conversely, compressors 

contributors 

that are small 

in one database are of similar ranking 

representativeness 

in the other. This comparison presents a qualitative assessment of the 

of the Emissions Database for the industry's compressors, 

A more quantitative assessment of how well the Emissions Database 

represents 

emission 

industry compressors was performed using a comparison between the 

Database, 

factors derived from the Emissions Database and the Industry 

compressors 

with the Industry Database used as a close approximation of 

for 

in the natural gas industry. To determine the emission factors 

for engines 

the Emissions Database, an overall average emission factor was calculated 

only 

and turbines, separately, after Step 1 in the hierarchy, where 

the data that were exact matches with th~ compressors 

Database were 

in the Test 

engines and 

included. Overall average emission factors were calculated for 

engine and 

turbines, weighting the model-specific emission factors for each 

in 

turbine, respectively, by the horsepower contribution of that model 

the Emissions Database. 

specific 

To determine the emission factors for the Industry Database, modeloverall average 

emission factors were d_eveloped after Step III in the hierarchy, and 

weighted 

emission factors were calculated for engines and turbines, 

respectively, 

by the horsepower contribution of the engine and turbine models, 

average 

in the Industry Database. The results were that the overall 

of methane 

emission factors calculated for the Emissions Database were 18 grams 

emitted per m3 

emitted per m3 

of fuel for engines, and 0.16 grams of methane 

calculated 

of fuel for turbines. The overall average emisSion factors 

for the Industry Database were 20 grams of methane emitted per m3 

of fuel for engines, and 0.17 grams of methane emitted per m3 of fuel for 

turbines. This analysis showed quantitatively 

a fairly good 

that the Emissions Database was 

representation of the Industry Database and, therefore, 

industry. 

the 

CONCLUSIONS AND RECOMMENDATIONS 

The annual methane emissions estimate of 0.22 Tg for compressors 

natural gas 

in the 

of 10. Although 

industry was less than the previous estimate of 2.2 Tg by a power 

emissions, 

compressors are still a significant source of methane 

this estimate reduces the importance of compressors 

assessment 

in the 

of the natural gas industry's contribution to global warming. 

The uncertainty for the estimate, at 4.3 million m3 of methane 

emissions 

(0.15 billion scf), was much less than the portion of total industry methane 

uncertainty 

contributed by compressors, and was 0.15 percent of the target 

(100 billion scf) for the industry. 

compressor 

Future 

operation, 

work in this area should include an assessment of the effect of 

in terms of horsepower, on the methane emission rate. 

A-16 
92-142.05 

On the basis of the data on this subject, the annual methane emissions could 

be higher than the current estimate by a factor of 2, if compressor operation 

is at horsepower levels significantly lower than compressors' rated 

horsepower. 

REFERENCES 

1. Annual Methane Emission Estimates of the National Gas Systems in the 

United States, Phase 2 by B. Tilkicioglu, Pipeline Systems, Inc. under 

subcontract to Radian Corporation, Research Triangle Park, North 

Carolina. U.S. Environmental Protection Agency Contract No. 68-02-4288. 

Research Triangle Park, North Carolina. September 1990. 

2. 

Gillette. 

Two Cycle Clean Burn (Integral) Gas Engines. D. Blizzard, and A. D. 

Presented at the S~all Prime Movers NOx Control Workshop, San 

Diego, CA. June 1991. 

3. TRANSDAT: CompreSsor Module. Tom Joyce and Associates, Washington, 

D.C., for the Gas Research Institute, Washington, D.C. August 1991. 

4. 1990 Gas Facts. The American Gas Association, Arlington, Virginia. 

1990. Page 65. 

5. Natural Gas Annual 1989. Department of Energy, Energy Information 

Administration, Washington, D. C. Volume I. 1989. 

6. @Risk, Risk Analysis and Simulation Add·in for Lotus 1-2·3. 

Version 1.5. Palisade Corporation, Newfield, NY. March 1989. 

ACKNOWLEDGMENTS 

This project was cofunded by Bob Lott, of the Gas Research Institute, 

Development, 

Chicago, Illinois, and David Kirchgessner, of the Office of Research and 

U.S. Environmental Protection Agency, Research Triangle Park, 

North Carolina. Radian gratefully acknowledges the contribution made by the 

sponsors. 

A-17 

APPENDIX B 

Source Sheet P-1 

B-1 
SOURCES: 

OPERATING MODE: 

EMISSION TYPE: 

ANNUAL EMISSIONS: 

BACKGROUND: 

P-1 

AL~SEGMENTSOURCESHEET 

Compressors, Generators 

Nonnal Operation 

Unsteady, Combusted (Compressor Driver Exhaust) 

24.4 Bscf ± 64% 

Compressors are used to move gas through the system. They are located in production fields, processing 

plants, gas storage fields, and along transmission lines. Methane emissions are found in compressor driver 

exhaust (reciprocating engines and gas turbmes) because of the incomplete combustion of the natural gas burned 

as fuel. 

EMISSION FACTOR: (0.240 ± 5% scf/hp.flr, engines and 0.0057 ± 30% scf/hp·hr, turbines) 

An average emission rate was calculated for each model of compressor engine and rurbine in the GRI 

TRANSDAT Emissions Database (1), which is based on compressor tests conducted by Southwest Research 

Institute (SwRI). The emission rates were calculated from the reported methane emissions per unit of fuel and 

the reponed fuel use rate (FUR) for each oo~pressor model, as follows: 

where: E:Rxm) = average emission rate for model, m (scf/hr) 

EPcmJ = average emission parameter for model, m (scf CHJscf fuel) 

~m) = average fuel use rate for model, m (scf fuel/hr) 

The following equation was used to determine the total emissions for the 86 turbines and 775 reciprocating 

engines in the Emissions Database. 

K 

TE• L 

m" [ 1 (ER<•> x HR, ) l. 

where: TE = total emissions for database, (scf) 

HR; = annual operating hours for compressor i, QJ.r/yr) 

K = number of unique compressor mcxiels 

M = number of compressors of model, m 

B-2 

(I) 

(2) 
The emission factors, for engines and turbines, were then calculated using the following equation. 

Emission Factor = TE I [ E 

i = I 

where: HP = average operating horsepower during HR., (hp) 

HR. = annual operating hours, {br{yr) 

N = number of compressors 

(3) 

This equation considers that some models could be operated at a higher percentage of the time because they are 

base loaded compressors. The average emission factors for the compressor drivers in the Emissions Database 

are 0.240 scf/hp·hr for reciprocating engines and 0.0057 scflhp-b.r for turbines. 

EF DATA SOURCES: 

I . ·National Estimate of Methane Emissions from Compressors in the U.S. Natural Gas 

Industry" (2). 

EF ACCURACY: ± 5%, engines and± 30%, turbines 

Basis: 

The accuracy for the EF is estimated based on propagation of error from the spread of samples in the 

database. However, engineering judgement was used to assign accuracy for two of the individual terms 

in the equation, as follows: 

I. Hydrocarbon analysis was estimated to be ± 10%, based on the generally accepted accuracy 

of gas chromatographs (flame ionization detector). 

2. Likewise, fuel flow measurements were estimated to be ± 2.5%. 

ACTIVITI' FACTORS: (horsepower-boor) 

Horsepower· hour data were available for the production industry segment activity factor calculation. Two 

pieces of infonnation are needed to calculate the activity factor, which is expressed as horsepower·hours (hp·hr) 

for each type of driver in each of the remaining industry segments. These are the installed horsepower and the 

average operating hours. The following table presents these parameters and the resulting activity factors for 

both engines and turbines in each segment of the industry. The sources and methods for calculating all the 

values presented in the table below are given in the next section: AF Data Sources. 

It is estimated that about 94% of the emissions tn con.pressor and generator driver exhaust are from 

reciprocating engines used in production, processing, and transmission, with about 5% attributable to 

reciprocating engines used in storage. All other categories are negligible in comparison. Therefore, it is more 

important to accurately determine the activity factors for reciprocating engines in production, processing, and 

transmission. 

B-3 
COMPRESSOR DRIVER ACTIVITY FACTORS FOR EACH INDUSTRY SEGMENT 

lmtallod 

""""" Ann~! Ann~! 

Industry Engino Turbine H•= Ho= E- ,.,.,. Segment MMhp' MMhp' Engine TorbiDe MMHp ·hr MMHpiuProduction NA NA NA NA 27.460 ± 200% 0 

Processing 4.19 ± 132%' 5. !9 ± 99.4%' 6626 ± 11.5% 6345 ± 48.4% 27,760 ± !33% 32.910 ± 121% 

Transmission 

Compressor Drivers 10.2 ± 10.0% 4.55 ± 10.0% 3964 ± 13.8% 2Il8 ± 31.3% 40.380 ± 17.1% 9635 ± 33.0% 

Generator Drivers 1.45 ± 23.3% 0.045 ± !66% 1352 ± 38.0% 474 ± 620% l%2 ± 45.4% 21.2 ± 1215% 

Storage 

Compressor Drivers 1.33 ± 13.5% 0.59 ± 13.5% 3707 ± 23.!% 2917 ± 620% 4922 ± 26.9% !729 ± 626% 

Generator Drivers 0.085 ± 126% 0.057 ± !84% 191 ± 377% 36 ± 620% 16.3 ± 621% 2_05 ± 1312% 

• Does not include horsepower associated with gas lift for oil recovery or with electric drivers. 

'Average of two esttmation methods. 

AF DATA SOURCESo 

I. The production segment horsepower is based on the total installed horsepower· hours for data 

provided by one company for 516 compressor drivers (all reciprocating engines). The 

horsepower· hours for the company was divided by their production before scaling to a national 

estimate. National horsepower· hour was calculated using the 1992 Jrulrketed production for the 

U.S. {Natural Gas Annual 1992, (3)]. 

2. The processing segment horsepower was determined by taking the average of two methods. 

Each of the methods uses site data for the lO gas plants visited. The first method scales to a 

national estimate by multiplying the total U.S. gas plant throughput as of January I, 1993 

{46,510. 7 MMcfd, Oil & Cas Journal (4)] by the total site visit horsepower per throughput 

(47 .8 hp/MMcfd, engines and 59.2 hp/MMcfd, turbines). The second method scales to a 

national estimate by multiplying the total number of gas plants in the U.S. {726, Oil & Gas 

JourTUll (4)] by the total site visit horsepower per number of gas plants visited (10), which is a 

scale-up ratio of about 73. The annual operating hours are based on the 10 sites plus data 

from two companies for an additional18 gas plants. An average of the average operating 

hours per site was calculated to get the processing segment operating hours (203 engines and 9 

turbines). 

3. The transmission segment compressor station horsepower for each compressor driver type is 

based on the GRI TRANSDAT database. Installed horsepower was taken from the Industry 

Database module of GRI TRANSDA T. The annual operating hours are based on information 

reponed on FERC Form No.2. FERC data does not distinguish between driver type. The 

FERC data were split between engines and turbines based on data in GRI TRANSDAT and 

data provided by one transmission company (524 engines and 89 turbines). 

4. The storage segment horsepower came from Gas Facts (5) data for 1992 (1,920,441 hp). The 

split between engines and turbines was assumed to be the same as the engine and turbine splits 

found in GRI TRANSDAT (69.1 %, engines and 30.9%, turbines). The annual operating 

hours are based on 11 storage stations (50 engines and 6 turbines). An average of the average 

operating hours per station was calculated to get the storage segment operating hours. 

B-4 
5. The generator driver horsepower (compressor stations) is based on the total installed 

horsepower for 7 of the transntission sites visited and company data for 34 transmission 

compressor stations. To scale to a national estimate, the total horsepower per station was 

multiplied by the total number of transmission compressor stations [l700, FERC Fonn No.2 

(6)J in the U.S. The annual operating hours are also based on data from the site visits and 

company data. An average of the average generator operating hours per station was calculated 

to get generator operating hours (87 engines and 1 turbine). 

6. The generator driver horsepower (storage fields) is based on the total installed horsepower for 

9 storage fields (one company). To scale to a national estimate, the total horsepower per field 

was multiplied by the total number of storage fields [475, Gas Facrs (5)J in the United States. 

The annual operating hours are also based on the company data. An average of the average 

generator operating hours per field was calculated to get generator operating hours (3 engines 

and l turbine). 

AF ACCURACY' 

Basis: 

Errors were propagated from each of the following terms: 

1. Production Hp·hr: The production Hp·hr accuracy is based upon an engineering analysis and 

set at ± 200%. 

2. Transmission Hp·hr: The transmission Hp·hr accuracy is based upon an assigned estimated 

error of± 10% for the horsepower data in the GRI TRANSDAT database and error 

propagation from the FERC operating hours. 

3. Other segment Hp-br: The accuracy of the site visit data for horsepower and operating hours 

was also propagated using the spread of the data, but from much smaller data sets. The 

accuracy of the horsepower-hour activity factors for each industry segment are calculated 

statistically using the individual terms for horsepower and operating hours. 

ANNUAL EMISSIONS: (24.57 ± 65.1% Bscf, engines + 0.256 ± 97.8% Bscf, turbines) 

The annual emissions were determined by multiplying an emlssion factor by the horsepower· hour activity factor 

for reciprocating engines and turbines and summing these values for each segment. The following table shows 

the resulting emissions for each industry segment and the overall national estimate. 

ANNUAL COMPRESSOR EMISSIONS FOR THE NATURAL GAS INDUSTRY BY SEGMENT 

Compnssor Production 

""""'"" Transmission Storage Generators TOTAL 

Engines, Bscf 6.58±200% 6.65±133% 9.68±17.9% l.l8±26.9% 0.474±45.6% 24.57±65.1% 

Turbines, Bscf 0.00 0.186±129% 0.0546±45.7% 0.00979±654% 0.000132± 1163% 0.256±97.8% 

REFERENCES 

J. Biederman, N. GRl TRANSDAT Database: Compressor Module. (prepared for Gas Research 

Institute). npb Associates with Tom Joyce and Associates, Chicago, IL, August 1991. 

2. Jones, 

Methane 

D.L., L.M. Campbell, C.E. Burklin, M. Gundappa, and R.A. Lott, "National Estimate of 

Gas 

Emissions from Compressors in the U.S. Natural Gas Industry." Radian Corporation and 

Research Institute, Air & Waste Management Association Conference Proceedings, Paper # 92· 

142.02, Kansas City, MO, 1992. 

B-5 
3. U.S. 

DOE/EIA-0131 

Department of Energy/Energy Information Administration. Natural Gas Annual, 1992. 

(92), Washington, DC, September 1992. 

4. Oil & Gas Journal. 1992 Worldwide Gas Processing Survey Database, 1993. 

5. Gas Facts: 1992 Data, American Gas Association, Arlington, VA, 1993. 

5. 

Companies, 

Federal Energy Regulatory Commission (FERC) Fonn No. 2: Annual Report of Major Natural Gas 

1992 database. 

B-6 
APPENDIX C 

Production Segment Site Visit Results 

C-1 
Table C-1 shows the production segment data collected for each of the 28 sites visited 

and the data provided by one c9mpany for 25 gathering stations. The data have been 

grouped into four geographic areas so that a regional extrapolation can be conducted for this 

industry segment. The following four regions were selected based on differences in 

production rates and equipment populations; Pacific Mountain. Gulf Coast, Central Plains, 

and Atlantic & Great Lakes. Differences in onshore versus offshore production were not 

considered because of the lack of available site data. (This approach is described further in 

the Methane Emissions from the Natural Gas Industry, Volume 5: Activity Factors). 1 

No turbines were installed at any of the production facilities surveyed; therefore, the 

data shown in Table C-1 represent an engine/turbine horsepower split of 100%/0% for this 

segment. Excluding compressors used for gas lift and electrically driven compressors, the 

average horsepower per MMcfd was calculated for each region. The United States marketed 

natural gas production, by region, for 1992 was used to extrapolate from the regional ratios 

(hp/MMcfd) to a national estimate of compressor engine driver horsepower in the production 

segment of 3.61 MMhp (0 MMhp for turbine drivers)' The following equation describes the 

national extrapolation using the regional approach. 

Engine hp = region 4 ( h ) L A vg P x MMcfd~,ro.. , 

region 1 M:Mcfd ~gion x 

This estimate for the production segment is based on the total installed horsepower 

from the sites surveyed. Table C-2 shows the regional data used to extrapolate to the 

national horsepower estimate. The table also shows the 90% confidence intervals for· each of 

the values used in the calculation. Confidence limits were calculated using the ratio method 

for each region's average hp/MMcfd and the total horsepower from the variation of the site 

data. The resulting uncertainty is ± 63.2%. 

Table C-3 shows the "operating hours" for the compressors at the sites surveyed. For 

the production segment, annual operating hours were not available for most of the sites 

visited; therefore, the compressors that were running during the site visit were assigned 

annual operating hours of 8760. Compressors that were idle were assigned annual operating 

hours of 0. This approach was used for 9 of the sites visited. Actual operating hours were 

available for two of the 2 sites visited and for the 25 gathering stations surveyed (one 

company). Operating hours for the remaining sites were not included in this analysis since 

The 

compressors were either used for gas lift, had electric drivers, or were not found at the site. 

operating hours listed for these sites are the average hours for all of the drivers at the 

site. This table also reflects the fact that no turbines were surveyed in the production 

industry segment. Confidence limits were calculated for the operating hours estimate from 

the variation of the data. The resulting uncertainty is± 9.7% (90% confidence limits). 

Activity factors based on the site data presented here are 23,820 MMhp·hr for 

engines and 0 MMhp·hr for turbines. Confidence limits were calculated for the engine 

driver activity factor to be ± 64.3%. This activity factor is within the error bounds of the 

C-2 
activity factor calculated using the company database for horsepower·hours (Section 4.1). 

This database provided one data .Point for turbine drivers in the production segment, thus 

affirming their presence in production. However, the number of these drivers is assumed to 

be very small and not significant for this estimate. 

REFERENCE 

1. Stapper, B.E. Methane Emissions from the Natural Gas Industry, Volume 5: Activity 

Factors, Final Report, GRI-94/0257.22 and EPA-600/R-96-0SOe. Gas Research 

Institute and U.S. Environmental Protection Agency, June 1996. 

2. U.S. Department of Energy/Energy Information Administration. Natural Gas 

Annual, 1992. DOE/EIA-0131 (92), Washington, DC, September 1992. 

C-3 
TABLE C-1. GATHERING COMPRESSOR DATA FROM 

PRODUCTION SEGMENT SITE VISITS 

Installed Avg 

Site hp. MMcfd hp/MMcfd 

Pacific Mountain 

PM! 37941 (112000) 104 

PM2 1650 4.5 

PMJ 0 (200 "J 12 

Total ' 39591 120.5 328.6 

Gulf Coast 

GCI 0 (NA ') 4.5 

GC2 7467 d 23 

GCJ 4950 25.5 

GC4 700 d 112 

GC5 0' 12 

GC6 8878 54 

GC7 0 18 

GC8 625 250 

GC9 0 (NA ') 0.50 

Total r 22620 499.5 45.3 

Central Plains 

CPI 6000 180 

CP2 4650 42.7 

CP3 4480 24.0 

CP4 ~ 72192 542 

Total 1 87322 1004.7 86.9 

Atlantic & Great Lakes 

AGI 0 24 

AG2 0 0.18 

AGJ 0 0.18 

AG4 0 0.17 

AG5 0 0.39 

AG6 0 0.37 

Continued 

C-4 
TABLE C-1. (CONTINUED) 

Installed 

Site hp. MMcfd 

AG7 50' 0.37 

AG& 0 0.18 

AG9. 0 0.23 

AGIO 0 0.30 

AG!l 0 0.35 

AGI2 0 0.!3 

AG!3 0 0.35 

Total r 50 27.2 

Total site horsepower (including gas lift for oil recovery) is shown in parentheses. 

b Horsepower associated with electric drivers. 

Not available. 

d Estimated. 

Total for 25 gathering stations; peak production rate. 

r Excludes gas lift and electric drivers. 

TABLE C-2. REGIONAL DATA FOR PRODUCTION 

SEGMENT HORSEPOWER EXTRAPOLATION 

Site Avg National Marketed 

Region hp/MM<::fd 90% Limit Production 

MMcfd 

Pacific Mountain 328.6 !: 48.2% 2696 

Gulf Coast 45.3 ± 132% 31544 

Central Plains 86.9 ± 94.6% 14860 

Atlantic & Great Lakes !.84 ± 244% 2166 

Total U.S. 51265 

90% Limit 

C-5 

Avg 

hp/MMcfd 

! 84 

Engine 

MMhp 

0.886 

!.43 

1.29 

0.00398 

3.61 

± 63.2% 
TABLE C-3. ANNUAL OPERATING HOURS FOR PRODUCTION SEGMENT 

Site No. of Engines • Average Operating Hours 

PMI b 19 (56) 8760 

PM2 17 7884 

PM3 0 (9)'·' -- GCI 0 (14) -- GC2 b 12 8030 

GC3 b 4 8760 

GC4 b I 8760 

GCS b 0 (!)' -- GC6 b 37 8523 

GC7 0 --

GC8 25. 4380 

GC9 0 (I) -- CPI b 50' . 8760 

CP2 • 31 8195 

CP3 b 64 8760 

CP4-I I 5924 

CP4-2 I 8364 

CP4-3 2 4753 

CP5-4 I 0 

CP4-5 I 6884 

CP4-6 18 5193 

CP4-7 I 7631 

CP4-8 I 7309 

CP4-9 3 6953 

CP4-IO 2 5833 

CP4-II 5 6386 

CP4-12 I 5505 

CP4-13 I 7235 

CP4-14 I 7333 

Continued 

C-6 
TABLE C-3. (CONTINUED) 

Site No. of Engines • Average Operating Hours 

CP4-15 2 8031 

CP4-16 1 6841 

CP4-17 1 7607 

CP4-18 2 6303 

CP4-19 1 6946 

CP4-20 2 6340 

CP4-21 7 748 

CP4-22 1 7384 

CP4-23 2 6908 

CP4-24 2 7176 

CP4-25 1 579 

AG1 1 -- AG2 0 -- AG3 0 --

AG4 0 -- AG5 0 -- AG6 0 --

AG7 2 . NA' 

AG8 0 -- AG9 0 -- AGIO 0 -- AGll 0 --

AG12 0 -- AG13 0 --

TOTAL AVERAGE r 315 6599 90% LIMIT r ± 9.7% 

• Total number of compressors (including gas lift for oil recovery) is shown in parentheses. 

b Based on active (8760 hrs) versus idle (0 hrs). 

c Electric drivers. 

d Estimated. 

• Not available. 

f Based on site averages. 1 Includes vapor recovery. 

C-7 

APPENDIX D 

Transmission Compressor Station Site Visit Results 

D-1 
Data were collected during site visits and company surveys for 51 transmission 

compressor stations. The compressor horsepower is related to the gas throughput and the 

pressure. Throughput is available for most of the sites surveyed, but a national figure is not 

available. Pressure data are not available for any of the sites surveyed or nationally. As a 

result, the only extrapolation method currently available for this industry segment is based on 

the number of compressor stations. 

Table D~1 shows the number of compressors, installed horsepower, and gas 

throughput for each of the sites surveyed in the transmission segment. The engine/turbine 

horsepower split for this segment is 63.8% engines and 36.2% turbines (compared to 69.1% 

for engines and 30.9% for turbines in TRANSDAT). Using the average site horsepower per 

station and the total number of transmission compressor stations in the United States for 1992 

from the FERC Form No. 2, the site data were scaled up to give a national estimate of 

compressor horsepower in transmission compressor stations of about 11.1 MMhp for engines 

and 6.3 MMhp for turbines. 

Engine hp = Total hp x No. U.S. Stations No. Stations 

= 334240 hp x 1700 Stations 51 Stations 

= 11.1 MMhp 

Turbine hp = 190030 hp x 1700 Stations 51 Stations 

= 6.33 MMhp 

Confidence limits were calculated for the horsepower from the variation of the data. 

The uncertainty calculated for the compressor station estimate used to scale up the site visit 

horsepower is ± I 0%. The resulting uncertainties for the national horsepower estimates for 

this industry segment are ± 22.0% for engines and ± 44.9% for turbines. 

Table D-2 shows the operating hours for the compressors at the 9 sites surveyed and 

for one company that provided data for 39 transmission compressor stations. The operating 

hours listed are the average hours for all of the drivers at each of the sites. Confidence 

limits were calculated for the operating hours estimate from the variation of the average site 

data. The resulting uncertainty is ± 25.0% for engines and ± 48.1% for turbines (90% 

confidence limits). 

Activity factors based on site visit data alone are 26,100 MMhp·hr for engines and 

13,460 MMhp·hr for turbines. The uncertainties for these activity factors are ± 33.7% for 

D-2 
engines and ± 69.2% for turbines. These activity factors are within the error bounds of the 

activity factors calculated using TRANSDAT (horsepower) and FERC (operating hours). 

However, the TRANSDAT database and the FERC database are much more representative of 

the transmission industry segment and were used in the overall emissions estimates. 

D-3 
Site 

TSI 

TS2 

TS3 

TS4 

TS5 

TS6 

TS7 

TS8 

TS9 

TSIO-I 

TS!0-2 

TSI0-3 

TSI0-4 

TS!0-5 

TSI0-6 

TSI0-7 

TS!0-8 

TSI0-9 

TSI0-10 

TSIO-II 

TSIO-I2 

TSIO-I3 

TSIO-I4 

TSIO-I5 

TSIO-I6 

TS!O-I7 

TSI0-I8 

TSIO-I9 

TSI0-20 

TSI0-2I 

TABLE D-1. COMPRESSOR DRIVER DATA 

FROM TRANSMISSION SEGMENT SITE VISITS 

No. of Installed No. of Installed Engines Engine hp Turbines Turbine hp 

I3 32650 0 ---

0 0 2 6900 

0 0 2 6900 

2 800 0 ---

10 I4560 2 20090 

I2 17570 I I2090 

0 0 I 40000 

4 I4000 2 26500 

7 I4000 3 37500 

4 6900 0 --

4 8000 0 --

5 I340 0 ---

3 8I5 0 --

4 8400 0 ---

2 I200 0 -- I 2400 0 --

2 4000 0 --

2 I320 0 --

3 2475 0 - 3 4050 0 ---

0 - I IOOO 

10 13000 0 ---

7 11200 0 --

2 <800 0 --

8 I0600 0 -

2 I200 0 ---

I 2400 0 ---

I9 32800 0 --

0 --- I 9300 

5 850 0 ---

D-4 

MMcfd 

843 

200 

37.5 

50 

NA 

NA 

NA 

4.9 

4.9 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

NA 

Continued 
No. of 

Site Engines 

TSJ0-22 I 

TSI0-23 I 

TSI0-24 3 

TSI0-25 I 

TSI0-26 I 

TSI0-27 7 

TSI0-28 2 

TSI0-29 0 

TSI0-30 2 

TSI0-31 2 

TSI0-32 I 

TSI0-33 3 

TSI0-34 7 

TSI0-35 2 

TSI0-36 I 

TSI0-37 I 

TSI0-38 II 

TSI0-39 I 

TSII 6 

TSI2 7 

TSI3 13 

TOTAL 208 

AVERAGE' 

NA = Not Available 

a Based on site averages. 

TABLE D-1-

(CONTINUED) 

Installed No. of 

Engine hp Turbines 

2400 0 

2400 0 

6000 0 

2400 0 

2400 0 

7000 2 

2700 0 . 

-- I 

4800 0 

3350 I 

2400 0 

6000 0 

7510 0 

4000 0 

2400 0 

270 0 

12380 3 

2400 0 

16900 0 

10400 0 

24800 0 

334240 22 

6554 

D-5 

Installed 

Turbine hp MMcfd 

--- NA 

--- NA 

--- NA 

--- NA 

--- NA 

8500 NA 

--- NA 

4250 NA 

--- NA 

4250 NA 

-- NA 

-- NA 

- NA 

--- NA 

--- NA 

- NA 

12750 NA 

-- NA 

-- NA 

0 140 

0 NA 

190030 1409.5 

3726 
Site 

TS1 

TS2 

TS3 

TS4 

TS5 

TS6 

TS7 

TS8 

TS9 

TS10·1 

TS10·2 

TSl0-3 

TSI04 

TS10·5 

TS!0-6 

TS10-7 

TS!0-8 

TS!0-9 

TSI0-10 

TSI0-11 

TSI0-12 

TSI0-13 

TSI0-14 

TS!0-15 

TS10·16 

TS10·17 

TSI0-18 

TSI0-19 

TS10-20 

TSI0-21 

TS!0-22 

TABLE D-2. ANNUAL OPERATING HOURS FOR 

TRANSMISSION SEGMENT 

No. of Average No. of 

En2ines Operating -Hours Turbines 

13 3303 0 

0 .. 2 

0 .. 2 

2 7884 0 

10 4066 2 

12 2082 1 

0 .. 1 

4 6366 2 

7 6813 3 

4 5019 0 

4 3020 0 

5 2343 0 

3 0 0 

4 4310 0 

2 0 0 

1 5914 0 

2 3150 0 

2 35 0 

3 1574 0 

3 2307 0 

0 .. 1 

10 3653 0 

7 52 0 

2 158 0 

8 2179 0 

2 1351 0 

I 4576 0 

19 6117 0 

0 -· 1 

5 470 0 

1 5695 0 

D-6 

Average Operating ·Hours 

.. 

3066 

1139 

.. 

3420 

6314 

0 

4517 

4698 

.. 

.. 

.. 

.. 

.. 

.. 

.. 

.. 

.. 

.. 

.. 

0 

.. 

.. 

.. 

.. 

.. 

.. 

--

579 

--

--

Continued) 
No. of 

Site Engines 

TSJ0-23 I 

TSl0-24 3 

TSI0-25 I 

TSI0-26 I 

TSI0-27 7 

TSI0-28 2 

TS!0-29 0 

TSI0-30 2 

TSI0-31 2 

TSI0-32 I 

TSI0-33 3 

TSI0-34 7 

TS!0-35 2 

TSI0-36 I 

TSJ0-37 I 

TSI0-38 !I 

TSI0-39 I 

TOTAL 182 

AVERAGE" 

90% LIMIT • 

• Based on site averages. 

TABLE D-2. 

(CONTINUED) 

Average 

O!"_ratinJiHours 

2747 

570.5 

502.9 

688.5 

557 

99 

-· 

615 

1015 

497 

4769 

509 

736 

474.5 

360 

1240 

586.9 

2343 

± 25.0% 

D-7 

No. of Average Turbines Operating Hours 

0 --

0 --

0 --

0 --

2 557 

0 --

I I823 

0 --

I 1015 

0 --

0 ·-

0 --

0 --

0 --

0 -· 

3 497 

0 --

22 

2125 

+ 48.1% 

APPENDIX E 

Conversion Table 

E-1 
Unit Conversion Table 

English to Metric Conversions 

I scf methane ~ 19.23 g methane 

I Bscf methane ~ 0.01923 Tg methane 

1 Bscf methane ~ 19,230 metric tonnes methane 

I Bscf ~ 28.32 million standard cubic meters 

I short ton (ton) ~ 907.2 kg 

I lb ~ 0.4536 kg 

I ft3 ~ 0.02832 m3 

I ft3 ~ 28.32 liters 

I gallon ~ 3. 785 liters 

I barrel (bbl) ~ 158.97 liters 

I inch ~ 2.540 em 

I ft ~ 0.3048 m 

1 mile ~ 1.609 km 

I hp ~ 0.7457 kW 

I hp-hr ~ 0.7457 kW-hr 

I Btu ~ 1055 joules 

I MMBtu ~ 293 kW-hr 

I lb/MMBtu ~ 430 g/GJ 

T CFJ ~ 1.8 T CC) + 32 

I psi ~ 51.71 mm Hg 

Global Warming Conversions 

Calculating carbon equivalents of any gas: 

MMTCE • (MMT of gas) x (MW, carbon) x (GWP) 1v!W, gas 

E-2 
Calculating C02 equivalents for methane: 

MMT of C02 equiv. " (MMT CH4) x x (GWP) (

MW, co,J 

MW, CH4 

where MW (molecular weight) of C02 ~ 44, MW carbon~ 12, and MW CH, ~ 16. 

Notes 

scf ~ Standard cubic feet. Standard conditions are at 14.73 psia and 60"F. 

Bscf ~ Billion standard cubic feet (109 scf). 

MMscf ~ Million standard cubic feet. 

Mscf ~ Thousand standard cubic feet. 

Tg ~ Teragram (1012 g). 

Giga (G) ~ Same as billion (I 09

). 

Metric tonnes ~ 1000 kg. 

pstg Gauge pressure. 

ps1a ~ Absolute pressure (note psia = psig + atmospheric pressure). 

GWP ~ Global Warming Potential of a particular greenhouse gas for a given 

time period. 

MMT ~ Million metric tonnes of a gas. 

MMTCE ~ Million metric tonnes, carbon equivalent. 

MMT of C02 eq. ~ Million metric tonnes, carbon dioxide equivalent. 

E-3 
TECHNICAL REPORT OAT A (PI~~ Te4d l~W~Uctiom on tM «rUU ~jon compl~ting} 

1. REPORT NO. 

I' EPA-600/R-96-080k 3. RECIPIENT'S ACCESSION NO. 

4. TITLE AND SUBTITLE Methane 5. REPORT DATE Emissions from the Natural Gas Industry, 

Volumes June 1996 1-15 (Volume 11: Compressor Driver Exhaust 6. PERFORMING ORGANIZATION CODE 

'· AuTHOR(sl L. Caml&bell, M. Cam/belL M. Cow~l, D. Ep- 8. PERFORMING ORGANIZATION REPORT NO. 

rrerson, 

• Shires, 

M. 

B. 

Hall, 

Stapper, 

• Harrison, C. 

• Hummel, D. yers, DCN 96-263-081-17 Stapper, J. Wessels, and * . PERFORMING ORGANIZATION NAME AND ADDRESS Radian 10. PROGRAM ELEMENT NO . International LLC 

P.O. Box 201088 11. CONTRACT~RANT NO. 

Austin, Texas 78720-1088 5091-251-2171 (GRl) 

68-Dl-0031 (EPA) 

12. SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF iPORT /NO PERIOD COVERED 

EPA, Office of Research and Development Final; 3 91-4 96 

.Air Pollution Prevention 14. SPONSORING AGENCY CODE and Control Division 

Research Triangle Park, NC 27711 EPA/600/!3 

1s.suPPLEMENTARY NoTes EPA project officer is D. A. Kirchgessner. MD-63. 919/541-4021. Cosponsor GRI ' 

project officer is R. A. Lott. Gas Research Institute, 8600 West Bryn I 

Mawr Ave., Chicago, ' IL 60631. (*)H. Williamson (Block 7). 1' ABSTRACTThe 15-volume report summarizes the results of a comprehensive program 

o quantify methane {CH4) emissions from the U.S. natural gas industry for the base 

lvear. The objective was to determine CH4 emissions from the wellhead and ending 

~ownstream at the customer's meter; The accuracy goal was to determine these 

~~issions within +/-0. 5% of natural gas production for a 90% confidence interval. Fm 

e 1992 base year, total CH4 emissions for the U.S. natural gas industry was 314 

+/-105 Bscf (6.04 +/- 2.01 Tg). This is equivalent to 1.4 +/- 0.5% of gross natural 

gas production, and reflects neither emissions reductions {per the voluntary AmeriGas Association/EPA Star Program) nor incremental increases (dUe to increased 

gas usage) since 1992. Results from this program were used to compare greenhouse 

as emissions from the fuel cycle for natural ga~. oil. and coal using the global warming potentials (GWPs) recently published by the Intergovernmental Panel on Climate 

Change (IPCC). The analysis showed that natural gas contributes less to potential 

global warming than coal or oil, which supports the fuel switching strategy suggested 

by the IPCC and others. In addition. study results are being used by the riatural gas 

industry to reduce operating costs while reducing emissions. 

7. KEY WORDS AND DOCUMENT ANALYSIS 

.. DESCRIPTORS b.IOENTIFIERSIOPEN ENDED TERMS e. COSATI Field/Group 

Pollution IPollution Prevention 13B 

Emission Stationary Sources l4G 

Greenhouse Effect Global Warming 04A Natural Gas 21D 

Gas Pipelines 15E 

Methane )o7c 

18. OISTR18UTION STATEMENT 19. SECURITY CLASS [This RepCHJ 21. NO. OF PAGES 

Unclassified 81 

Release to Public 20. SECURITY CLASS (Thi:& page) 22.PRICE 

Unclassified 

EPA Form 2220-1 (9·73) E-4 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

Office of Research and Development National Risk Management Research Laboratory 

Technology Transfer and Support Division 

Cincinnati, Ohio 45268 

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