Prevention of Significant Deterioration Permit Pursuant to 40 CFR 52.21 and 

Minor NSR Permit in Indian Country Pursuant to 40 CFR 49.151-161 

U.S. ENVIRONMENTAL PROTECTION AGENCY, REGION 9 

PSD Permit Number:

Tribal NSR Permit:

AZ 08-01A 

T-0004-NN 

Permittee: Salt River Project Agricultural Improvement and 

Power District 

Mail Station PAB352 

P.O. Box 52025 

Phoenix, AZ 85072 

Facility Name: Navajo Generating Station 

Facility Location: On the Navajo Nation Indian Reservation, 

approximately five miles east of Page off State 

Route 98, in Coconino County, Arizona. 

Pursuant to the provisions of the Clean Air Act (CAA), Section 110(a) and 301(d), and the Code 

of Federal Regulations (CFR) Title 40, Sections 49.151-161, the United States Environmental 

Protection Agency Region 9 (EPA) is issuing a Minor New Source Review (NSR) permit in 

Indian County to Salt River Project for Navajo Generating Station (NGS). The Prevention of 

Significant Deterioration (PSD) air quality permit conditions pursuant to 40 CFR Section 52.21 

in AZ 08-01A last issued on February 6, 2012, and included in this combined PSD and Minor 

NSR permit (“Permit”), remain unchanged. The PSD-specific conditions are in Section IX and 

the Minor NSR-specific conditions are in Section X. This permit revision authorizes construction 

of the Refined Coal Treatment System (RCTS) at NGS. 

SRP is authorized to construct and operate the NGS as described herein, in accordance with the 

Permit, the federal PSD regulations at 40 CFR 52.21, the Minor NSR regulations at 40 CFR 

49.151-161, and other terms and conditions set forth in this Permit. Failure to comply with any 

condition or term set forth in this Permit is subject to enforcement action pursuant to Section 113 

of the Clean Air Act. This Permit does not relieve NGS from the obligation to comply with 

applicable federal, state, and Navajo Nation Environmental Protection Agency air pollution 

control rules and regulations. 

Per 40 CFR 49.159(a) Tthe minor NSR portion of this permit will become effective 30 days 

upon after service of notice of this final permit decision unless review is requested on the being 

finally issued and signed permit pursuant to 40 CFR 49.159(df)(2). The PSD portion of this 

permit remains effective and is unchanged. 

 

Deborah Jordan Date 

Director, Air Division 
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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Navajo Generating Station 

PSD Permit No. AZ 08-01A: Low NOX Burner Project 

Project Description 

The Navajo Generating Station (NGS) has a combined power generating capacity of 2,250 net 

megawatts and consists of three existing coal-fired steam generating units (Boiler Units 1, 2, and 

3), associated air pollution control devices, and auxiliary equipment. The facility was constructed 

in 1970 and is located on the Navajo Nation Indian Reservation approximately five miles east of 

Page, Arizona. Units 1, 2, and 3 are operated by the Salt River Project Agricultural and Power 

District (SRP) and co-owned by the following six entities: U.S. Bureau of Reclamation, SRP, 

Los Angeles Department of Water and Power, Arizona Public Service Company, Nevada Power, 

and Tucson Electric Power Company. The Permittee receives coal with a maximum sulfur 

content of 1.5% by weight from the Peabody Western Coal Company’s Kayenta Mine. 

SRP decided to implement voluntary emission reduction projects at the NGS by retrofitting all 

three boilers with Low-NOx Burners (LNBs) and separated over-fire air (SOFA) systems. All 

three boilers have been retrofitted since PSD Permit AZ 08-01 was issued on November 20, 

2008. 

Although the LNB/SOFA Project significantly reduces emissions of NOx at NGS, it also results 

in a significant increase in CO emissions. This significant increase in CO emission triggered a 

Best Available Control Technology (BACT) review for emissions of CO as part of the issuance 

for PSD Permit AZ 08-01. The boilers were retrofitted and not originally designed for optimum 

combustion as with new units. Thus, it was difficult to appropriately assess the required CO 

emissions limitation. 

Rather than setting the CO emission rates equal to the lowest proposed BACT limitation for new 

units, EPA incorporated a proposed emission rate of 0.42 lb/MMBtu, based on a rolling 30-day 

average into the previous permit along with language that would allow the limit to be ratcheted 

down following the first 18 months of operation of the LNB/SOFA systems. In accordance with 

this permit requirement, SRP submitted CO and NOx emissions data from Unit 3, the first boiler 

unit outfitted with the LNB/SOFA system. This data was used to determine the appropriate CO 

emissions limit for all three boilers. Although this permit revision establishes the final CO 

emissions limitation for Units 1, 2 and 3 at NGS, Demonstration Period reporting requirements 

are still required for Units 1 and 2. 

Minor NSR Permit T-0004-NN 

Administrative Amendment (August 2015) 

NGS has added a new mercury control system consisting of a calcium bromide application 

system and a new powdered activated carbon (PAC) injection system for Units 1, 2, and 3. The 

addition of this equipment also increases truck traffic at NGS for the routine delivery of calcium 

bromide and PAC. This project results in emissions increases of particulate matter (PM, PM10, 

and PM2.5) from the PAC system’s storage silos and fugitive truck traffic emissions for deliveries 

of calcium bromide and PAC. 
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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Refined Coal Treatment System (Proposed Project) 

NGS is adding a new Refined Coal Treatment System (RCTS) which adds calcium bromide and 

cement kiln dust to the coal prior to being pulverized for combustion in Units 1, 2, and 3. The 

project consists of: 

 Installation of two coal feed belt conveyors (BC-11A and BC-11B); 

 Installation of two mixing pugmills where cement kiln dust and calcium bromide will be 

applied to the coal; 

 Installation of two refined coal product belt conveyors (BC-12A and BC-12B); 

 Installation of two dust collection and filtering systems, DC-12 and DC-13, for the new 

coal handling operations; 

 Installation of three 150 ton cement kiln dust storage silos, each equipped with 

baghouses (DC-14, DC-15, and DC-16); 

 Installation of two 20 ton day bins for cement kiln dust, each equipped with baghouses 

(DC-17 and DC-18); 

 Installation of a 8,700 gallon storage tank and two 405 gallon day tanks for calcium 

bromide storage; 

 Increased truck traffic on unpaved roads for delivery of cement kiln dust and calcium 

bromide; 

This project results in emissions increases of particulate matter (PM, PM10, and PM2.5) from the 

RCTS’s cement kiln dust storage silos and day bins, belt conveyors, and fugitive truck traffic 

emissions for deliveries of cement kiln dust and calcium bromide. The project will also result in 

minimal increased emissions of particulate matter (PM, PM10, and PM2.5) from boiler Units 1, 2 

and 3 due to the increased ash content of the coal, but the emissions limits for those Units will 

remain unchanged. 
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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EQUIPMENT DESCRIPTION 

Unit ID / 

Stack ID Unit Description Maximum Capacity Controls 

Existing Equipment

Boiler 1- 

U1/ 

Stack- S1 

One (1) pulverized coal-fired 

boiler, using No. 2 fuel oil for 

ignition fuel. Stack S1 equipped 

with SO2, CO, and NOx CEMS, 

and a COMS. 

7,725 

MMBtu/hr; 

750 Net MW 

FGD system, SCBR1 (1999); 

ESP1, LNB/SOFA system 

(2011) 

Boiler 2- 

U2/ 

Stack- S2 

One (1) pulverized coal-fired 

boiler, using No. 2 fuel oil for 

ignition fuel. Stack S1 equipped 

with SO2, CO, and NOx CEMS, 

and a COMS. 

7,725 

MMBtu/hr; 750 

Net MW 

FGD system, SCBR2 (1998); 

ESP2, LNB/SOFA system 

(2010) 

Boiler 3- 

U3/ 

Stack- S3 

One (1) pulverized coal-fired 

boiler, using No. 2 fuel oil for 

ignition fuel. Stack S1 equipped 

with SO2, CO, and NOx CEMS, 

and a COMS. 

7,725 

MMBtu/hr; 750 

Net MW 

FGD system, SCBR3 (1997); 

ESP3, LNB/SOFA system 

(2009) 

PAC Silo 

A 

Storage silo for powdered 

activated carbon (PAC) 

1,200 scfm Baghouse (integral to system)

PAC Silo 

B 

Storage silo for PAC 1,200 scfm Baghouse (integral to system)

FugitivePAC Truck traffic on unpaved roads for 

PAC delivery 

30 VMT/yr Water spray, chemical 

stabilization, or gravel

FugitiveCaBr2 Truck traffic on unpaved roads for 

calcium bromide delivery 

365 VMT/yr Water spray, chemical 

stabilization, or gravel

New Equipment 

DC-12 Coal handling dust collection 

system 

16,500 scfm High-efficiency dust collector

DC-13 Coal handling dust collection 

system 

6,000 scfm High-efficiency dust collector

DC-14 Cement kiln dust silo 1 450 scfm Baghouse (integral to system)

DC-15 Cement kiln dust silo 2 450 scfm Baghouse (integral to system)

DC-16 Cement kiln dust silo 3 450 scfm Baghouse (integral to system)

DC-17 Cement kiln dust day bin 1 1,200 scfm Baghouse (integral to system)

DC-18 Cement kiln dust day bin 2 1,200 scfm Baghouse (integral to system)

FugitiveCement 

Kiln Dust 

Truck traffic on unpaved Roads 

for cement kiln dust delivery 

720 VMT/yr Water spray, chemical 

stabilization, or gravel

FugitiveCaBr2 Truck traffic on unpaved roads for 

calcium bromide delivery 

70 VMT/yr Water spray, chemical 

stabilization, or gravel
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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PERMIT CONDITIONS 

I. PERMIT EXPIRATION

As provided in 40 CFR 52.21(r), this PSD Permit shall become invalid if construction: 

A. is not commenced (as defined in 40 CFR 52.21(b)(9)) within 18 months after the 

approval takes effect; or 

B. is discontinued for a period of 18 months or more; or 

C. is not completed within a reasonable time. 

II. PERMIT NOTIFICATION REQUIREMENTS

Permittee shall notify EPA Region 9 in writing or by electronic mail of the: 

A. date construction is commenced, postmarked within 30 days of such date. 

B. actual date of initial startup, as defined in 40 CFR 60.2, postmarked within 15 

days of such date. 

C. date upon which initial performance tests will commence, in accordance with the 

provision of Section IX.F, postmarked not less than 30 days prior to such date. 

Notification may be provided with the submittal of the performance test protocol 

required pursuant to Section IX.F. 

D. date upon which initial performance evaluation of the CEMS will commence in 

accordance with 40 CFR 60.13(c), postmarked not less than 30 days prior to such 

date. Notification may be provided with the submittal of the CEMS performance 

test protocol required pursuant to Condition IX.E.5. 

III. FACILITY OPERATION 

At all times, including periods of startup, shutdown and malfunction, Permittee shall, to 

the extent practicable, maintain and operate the facility including associated air pollution 

control equipment in a manner consistent with good air pollution control practices for 

minimizing emissions. Determination of whether acceptable operating and maintenance 

procedures are being used will be based on information available to the EPA which may 

include, but is not limited to, monitoring results, opacity observations, review of 

operating maintenance procedures and inspection of the source. 

IV. MALFUNCTION REPORTING
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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A. Permittee shall notify EPA by facsimile or electronic mail at R9.AEO@epa.gov 

within two (2) working days following the discovery of any failure of air 

pollution control equipment, process equipment, or of a process to operate in a 

normal manner, which results in an increase in emissions above the allowable 

emission limit stated in Condition IX.B of this permit. 

B. In addition, Permittee shall notify EPA in writing or electronic mail within fifteen 

(15) days of any such failure described under Condition IV.A. This notification 

shall include a description of the malfunctioning equipment or abnormal 

operation, the date of the initial malfunction, the period of time over which 

emissions were increased due to the failure, the cause of the failure, the estimated 

resultant emissions in excess of those allowed in Condition IX.B, and the methods 

utilized to mitigate emissions and restore normal operations. 

C. Compliance with this malfunction notification provision shall not excuse or 

otherwise constitute a defense to any violation of this permit or any law or 

regulation such malfunction may cause. 

V. RIGHT OF ENTRY

The EPA Regional Administrator, and/or an authorized representative, upon the 

presentation of credentials, shall be permitted: 

A. to enter the premises where the source is located or where any records are 

required to be kept under the terms and conditions of this PSD Permit; and 

B. during normal business hours, to have access to and to copy any records required 

to be kept under the terms and conditions of this PSD Permit; and 

C. to inspect any equipment, operation, or method subject to requirements in this 

PSD Permit; and 

D. to sample materials and emissions from the source(s). 

VI. TRANSFER OF OWNERSHIP

In the event of any changes in control or ownership of the facilities to be constructed, this 

PSD Permit shall be binding on all subsequent owners and operators. Permittee shall 

notify the succeeding owner and operator of the existence of this PSD Permit and its 

conditions by letter, a copy of which shall be forwarded to EPA Region 9 at the address 

shown in Section X. 

VII. SEVERABILITY
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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The provisions of this PSD Permit are severable, and, if any provision of the PSD Permit 

is held invalid, the remainder of this PSD Permit shall not be affected. 

VIII. OTHER APPLICABLE REGULATIONS

Permittee shall construct and operate this project in compliance with this PSD Permit, the 

application on which this Permit is based, and all other applicable federal, state, and local 

air quality regulations. This PSD Permit does not release the Permittee from any liability 

for compliance with other applicable federal, state and local environmental laws and 

regulations, including the Clean Air Act. 

IX. SPECIAL CONDITIONS PURSUANT TO 40 CFR 52.21 - PSD 

 Low-NOx Burner (LNB) & Separated Over-fire Air (SOFA) Requirements 

A. Prior to commencement of installation, the Permittee shall submit the following 

information to EPA: 

1. Design specifications of the LNB/SOFA system to be installed. 

2. At least one month prior to the date of initial start-up, an LNB/SOFA system 

operating plan which sets forth measures that will be taken to maintain and 

operate the system in a manner to ensure compliance with the emission 

limits specified in Condition IX.B. 

B. Emission Limits 

1. Carbon monoxide (CO) emissions from each unit shall not exceed the 

following: 

a. 0.23 lb/MMBtu based on a 30-day rolling average, and 

b. 0.15 lb/MMBtu based on a 12-month rolling average 

2. Nitrogen oxide emissions (NOx) from each unit shall not exceed 0.24 

lb/MMBtu based on a 30-day rolling average. 

C. Demonstration Period Requirements 

1. Demonstration Period is defined as the first 18 months of operation after 

installation of the LNB/SOFA system. 

2. After the Demonstration Period for each of the U1 and U2 LNB/SOFA 

systems, the Permittee shall submit to EPA a written report together with 

CO CEMS data showing actual CO emissions while maintaining NOx

emission levels at or below 0.24 lb/MMbtu on a 30-day rolling average. The 
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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report shall provide all supporting documentation identifying the 

combustion characteristics that impact CO emissions. This report shall also 

evaluate the ten highest occurrences for a one-hour average and an 8-hour 

average for pounds per hour CO. If these averages are inconsistent (higher) 

with the modeling submittal, either a new modeling analysis will be required 

to assure maintenance of the CO NAAQS or a short term limit will be 

established for the permit. 

D. At all times, including periods of startup and shutdown, the Permittee shall, to the 

extent practicable, maintain and operate the LNB/SOFA system in a manner 

consistent with good combustion practices to minimize emissions. 

E. Continuous Emission Monitoring Systems 

1. Within 60 days of completion of installation of each LNB/SOFA system, the 

Permittee shall install, and thereafter operate, maintain, certify, and qualityassure a continuous emission monitoring system (CEMS) for each boiler 

which measures stack gas CO concentrations in lb/MMbtu. 

2. The CO CEMS shall meet the applicable requirements of 40 CFR Part 60 

Appendix B, Performance Specifications 3 and 4A, and 40 CFR Part 60 

Appendix F, Procedure 1. The diluent monitor (O2 or CO2) must meet the 

requirements of 40 CFR Part 75. 

3. Permittee shall operate, maintain, and quality-assure according to the 

requirements of 40 CFR Part 75, a CEMS for each boiler which measures 

stack gas NOx concentrations in lb/MMbtu. The NOx CEMs must meet the 

requirements of 40 CFR Part 75. 

4. The CO CEMS shall complete a minimum of one cycle of operations 

(sampling, analyzing and data recording) for each successive 15-minute 

period. 

5. Permittee shall submit a CO CEMS performance test protocol to the EPA no 

later than 30 days prior to the test date to allow review of the test plan and to 

arrange for an observer to be present at the test. The performance test shall 

be conducted in accordance with the submitted protocol, and any changes 

required by EPA. 

6. Permittee shall furnish the EPA a written report of the results of 

performance tests within 60 days of completion. 

7. The CO CEMS shall be tested annually and quarterly in accordance with the 

requirements of 40 CFR 60 Appendix F, Procedure 1. The NOx CEMS shall 

meet the quality assurance requirement found in 40 CFR Part 75. 

F. Performance Test 

A thirty day initial performance test for CO and NOx shall be conducted with the 

CEMS starting the day after successful completion of the performance testing for 
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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the CO CEMS. A report of the NOx and CO hourly emissions during this initial 

test shall be submitted to EPA within 30 days of completion of the test. 

G. Recordkeeping and Reporting Requirements 

1. Permittee shall maintain records of the hours of operation for U1, U2 and 

U3 on a monthly basis. 

2. Permittee shall maintain records of the amount of fuel used in U1, U2 and 

U3 on a monthly basis. 

3. Permittee shall maintain all records on site of actual operating data and 

emissions calculations for emissions limits required in Condition IX.B. 

4. Permittee shall maintain CEMS records that contain the following: the 

occurrence and duration of any startup, shutdown or malfunction, 

performance testing, evaluations, calibrations, checks, adjustments, 

maintenance, duration of any periods during which a continuous monitoring 

system or monitoring device is inoperative, and emission measurements. 

 

5. Permittee shall maintain records and submit a written report of all excess 

emissions to EPA semi-annually. The report is due on the 30th day 

following the end of the calendar quarter and shall include the following: 

a. Time intervals, data and magnitude of the excess emissions, the 

nature and cause (if known), corrective actions taken and preventive 

measures adopted; 

b. Applicable time and date of each period during which the CEMS 

was inoperative (monitor down time), except for zero and span 

checks, and the nature of system repairs or adjustments; and 

c. A negative declaration when no excess emissions occurred or when 

the CEMS has not been inoperative, repaired, or adjusted. 

6. Excess emissions shall be defined as any operating day in which the 30-day 

rolling average CO and NOx concentration, as measured by the CEMS, 

exceeds the maximum emission limits set forth in Condition IX.B. 

7. A period of monitor down time shall be any unit operating hour in which 

sufficient data are not obtained to validate the hour for CO, NOx, or O2. 

8. Excess emissions indicated by the CEMS shall be considered violations of 

the applicable emission limit for the purpose of this permit. 

9. All records required by this PSD Permit shall be retained for five years 

following the date of such measurements, maintenance, and reports. 
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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X. SPECIAL CONDITIONS PURSUANT TO 40 CFR 49.151-161 – MINOR NSR IN 

INDIAN COUNTRY – Units IDs: PAC Silo A, PAC Silo B, Dust Collectors DC-12 

and DC-13, Cement Kiln Dust Storage Silos DC-14 through DC-16, Cement Kiln 

Dust Day Bins DC-17 and DC-18, Fugitive-PAC, Fugitive-CKD, Fugitive-CaBr2 

A. Operating Requirements 

1. Vehicle miles traveled (VMT) for truck traffic associated with the delivery 

of powdered activated carbon (Fugitive-PAC) shall not exceed 30 VMT per 

12-month period. 

2. VMT for truck traffic associated with the delivery of calcium bromide 

(Fugitive-CaBr2) shall not exceed 435 VMT per 12-month period. 

3. VMT for truck traffic associated with the delivery of cement kiln dust 

(Fugitive-CKD) shall not exceed 720 VMT per 12-month period. 

4. The dust on the site roadways shall be controlled by applications of water 

such that visible fugitive dust emissions do not exceed 20 percent opacity. 

The Permittee may use applications of water, chemical dust suppressants, or 

gravel to control dust from site roadways.Roadways shall be swept as 

needed between applications of water.

5. The Permittee shall load vehicles to prevent their contents from dropping, 

leaking, blowing, or otherwise escaping. This shall be accomplished by 

loading so that no part of the load shall come in contact within six (6) inches 

of the top of any side board, side panel, or tail gate; otherwise the truck shall 

be tarped. Any material spillage on roads shall be cleaned up immediately. 

6. The Permittee shall post a speed limit sign of 15 miles-per-hour or lower on 

site so that it is visible to truck traffic. 

B. Monitoring and Recordkeeping Requirements 

1. The Permittee shall monitor and maintain records on a calendar month basis 

of each PAC delivery, the VMT of each delivery, and determine the 12-

month rolling total. 

2. The Permittee shall monitor and maintain records on a calendar month basis 

of each calcium bromide delivery, the VMT of each delivery, and determine 

the 12-month rolling total. 

3. The Permittee shall monitor and maintain records on a calendar month basis 

of each cement kiln dust delivery, the VMT of each delivery, and determine 

the 12-month rolling total. 
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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4. At least once during each calendar week the Permittee shall perform a 

visible emissions survey for each PAC Silo (Silos A and B), Dust Collectors 

DC-12 and DC-13, Cement Kiln Dust Storage Silos (DC-14 through DC16), and Cement Kiln Dust Day Bins (DC-17 and DC-18). The survey shall 

be performed during daylight hours by an individual trained in EPA Method 

22 while the equipment is in operation. If visible emissions are detected 

during the survey, the permittee shall perform a 6-minute EPA Method 9 

observation. If visible emissions during the 6-minute EPA Method 9 

observation exceed 7 percent opacity, the Permittee shall take corrective 

action so that within 24 hours no visible emissions are detected. 

5. Baghouse or dust collector filters for PAC Silo (Silos A and B), Dust 

Collectors DC-12 and DC-13, Cement Kiln Dust Storage Silos (DC-14 

through DC-16), and Cement Kiln Dust Day Bins (DC-17 and DC-18) shall 

be inspected and/or replaced by the Permittee as often as necessary to ensure 

proper operation or more frequently as indicated by pressure differential 

readings or other indication of filter failure. 

6. The Permittee shall maintain an on-site inventory of spare bags or filters of 

each type used for PAC Silo (Silos A and B), Dust Collectors DC-12 and 

DC-13, Cement Kiln Dust Storage Silos (DC-14 through DC-16), and 

Cement Kiln Dust Day Bins (DC-17 and DC-18) to ensure rapid 

replacement in the event of bag or filter failure. 

7. The Permittee shall maintain records of the date and time of any road 

watering or application of chemical dust suppressants or gravel performed. 

8. At least once during each calendar month, the Permittee shall inspect to 

check that water is flowing to discharge spray nozzles in the road watering 

system or water spraying vehicles. The owner or operator must initiate 

corrective action within 24 hours and complete corrective action as 

expediently as possible. The dates and results of the monitoring performed 

on the road watering system or water spraying vehicles, any corrective 

action taken as a result of each survey, and the result of any corrective 

action shall be recorded. 

XI. AGENCY NOTIFICATIONS 

All correspondence as required by this permit must be forwarded to: 

Director, Air Division (Attn: AIR-3) 

EPA Region 9 

75 Hawthorne Street 

San Francisco, CA 94105-3901 

e-mail: R9AirPermits@epa.gov and AEO_R9@epa.gov
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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Environmental Department Director 

Navajo Nation EPA 

P.O. Box 339 

 Window Rock, AZ 86515
Navajo Generating Station Draft Minor NSR Permit 

February 2016 

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Attachment A: Abbreviations and Acronyms 

acfm actual cubic feet per minute 

CAA or the Act Federal Clean Air Act 

CFR Code of Federal Regulations 

CKD cement kiln dust 

EPA United States Environmental Protection Agency, Region 9 

EU emissions unit 

hr hour 

lb/hr pounds per hour 

NAAQS National ambient air quality standards 

NSR new source review 

PM particulate matter 

PSD prevention of significant deterioration 

RCTS refined coal treatment system 

scfm standard cubic feet per minute 

VMT vehicle miles traveled