TECHNICAL ANALYSIS REPORT For - Alaska

ALASKA DEPARTMENT OF ENVIRONMENTAL CONSERVATION Air Permits Program

TECHNICAL ANALYSIS REPORT For

Air Quality Control Minor Permit AQ0856MSS01

Alaska Pipeline Company Gudenrath Compression Station

TURBINE REPLACEMENT Prepared By: Brittany Crutchfield Supervisor: Patrick Dunn Date: Preliminary – January 29, 2014

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Technical Analysis Report - Permit AQ0856MSS01 Preliminary – January 29, 2014 Alaska Pipeline Company –Gudenrath Compressor Station

ABBREVIATIONS/ACRONYMS

AAC ..............................Alaska Administrative Code APC ............................... Alaska Pipeline Company CTG ...............................Combustion Turbine Generator Department ....................Alaska Department of Environmental Conservation DLE ...............................Dry Low Emissions EF ..................................Emission Factor EU..................................Emission Units ID ...................................Identification Number of Emission Units MR&R ...........................Maintenance, Recording, and Recording N/A ................................Not Applicable ORL ...............................Owner Requested Limits PTE ................................Potential to Emit SIC .................................Standard Industrial Classification TAR ...............................Technical Analysis Report NTE ...............................Not to Exceed NG .................................Natural Gas

Units and Measures Btu .................................British Thermal Units hphr................................horsepower-hour hphr/yr ...........................horsepower-hours per year lb, lbs .............................pound, pounds lb/1,000 gal ....................pounds per thousand gallons burned lb/MMBtu ......................pounds per million British Thermal Unit input kW .................................kilowatts MMBtu ..........................million British Thermal Units MMBtu/hr ......................million British Thermal Units per hour TPY, tpy ........................tons per year ºF ...................................degrees Fahrenheit

Pollutants CO .................................Carbon Monoxide NOx ................................Oxides of Nitrogen PM-10 ............................Particulate Matter with an aerodynamic diameter less than 10 microns SO2 ................................Sulfur Dioxide VOC ..............................Volatile Organic Compound

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Table of Contents

1. Introduction ........................................................................................................................... 4 1.1 Stationary Source Description ............................................................................................ 4

2. Emissions Summary and Permit Applicability .................................................................. 4 2.1 Potential to Emit (PTE) ...................................................................................................... 4

2.2 Assessable Emissions .......................................................................................................... 5

2.3 Department Findings .......................................................................................................... 5

3. Permit Requirements ............................................................................................................ 6 3.1 Requirements for All Minor Permits ................................................................................... 6

3.2 Requirements for a Minor Permit issued under 18 AAC 50.502(c)(3) ............................... 6

3.3 State Emission Standards .................................................................................................... 6

3.3.1 Visible Emission Standard .............................................................................................. 6

3.3.2 Particulate Matter Standard ........................................................................................... 7

3.3.3 Sulfur Dioxide Standard ................................................................................................. 7

3.4 Requirements for Stationary Sources Not Subject to Title V Permitting Requirements. .... 7

4. Permit Administration.......................................................................................................... 7

Appendix A: Emissions Calculations .......................................................................................... 8

Appendix B: Modeling Memorandum ...................................................................................... 10

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1. Introduction This Technical Analysis Report (TAR) provides the Alaska Department of Environmental Conservation’s (Department’s) basis for issuing Air Quality Control Minor Permit AQ0856MSS01 to Alaska Pipeline Company (APC). This minor permit authorizes the installation and operation of two new 1,600 hp Solar Saturn 20 combustion turbines. This action requires a permit under 18 AAC 50.502(c)(3).

1.1 Stationary Source Description APC’s Gudenrath facility is an existing source approximately 8 km north of Sterling, AK. Gudenrath consists of four natural gas-fired combustion turbines, a diesel emergency generator, two boilers, a hot water heater, and a furnace.

The proposed project consists of installation of two new CAT natural gas Solar Saturn 20 (Solar 20) combustion turbines with ratings of 1,600 hp. The two new Solar 20s will be replacing two existing 1,200 hp Solar Saturn 10 natural gas-fired combustion turbines.

APC previously replaced two Solar-10 turbines with two Solar-20 turbines in 2011 triggering minor permitting under 18 AAC 50.502(c)(3) for existing NOx emissions greater than 40 tpy with a project increase of at least 10 tpy NOx. APC did not acquire a minor permit at that time.

In June 2012 APC approached the Department about the permitting requirements for replacing the remaining two Solar-10 turbines with Solar-20 turbines. The Department informed APC at that time that the initial replacement of the two Solar-10 turbines in 2011 as well as the proposed replacement of their remaining two Solar-10 turbines triggered minor permits under 18 AAC 502(c)(3) for NOx. The Department agreed with APC that the two projects are separate for permit applicability determinations.

On December 6, 2012 the Department requested APC to submit a minor permit application for the two proposed Solar-20 turbines in response to APC’s permit applicability assistance request in June 2012. The Department also requested APC to submit an ambient analysis for NO2 for all four Solar-20 turbines with the minor permit application to avoid a possible compliance action for the replacement of the Solar-10 turbines in 2011.

2. Emissions Summary and Permit Applicability

2.1 Potential to Emit (PTE) All emissions are calculated based on unrestricted operations. Table 1 presents the potential emissions for the stationary source. Detailed emission calculations are shown in Appendix A.

Table 1: Stationary Source Potential to Emit (TPY) Parameter CO NOx PM-10 PM-2.5 SO2 VOC

Previous PTE 51.1 59.4 0.7 0.7 0.3 1.7 New PTE 26.1 75.6 2.1 2.1 1.1 0.7

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Total PTE 105.2 Change in PTE -25.0 16.2 1.4 1.4 0.8 -1.0 Permit Threshold

[18 AAC 50.502(c)(3)] N/A 10 N/A N/A N/A N/A Triggered N/A Yes N/A N/A N/A N/A

Title V Permit Threshold [18 AAC 50.306] 100 100 100 100 100 100

Triggered No No No No No No

2.2 Assessable Emissions Emission fee requirements are required for each minor permit issued under 18 AAC 50.542, as described in 18 AAC 50.544(a). Since this permit is classified under 18 AAC 50.502(c)(3), the Department is establishing the assessable emissions requirement in this permit. Table 2 shows the assessable emissions are 102 tpy.

Table 2: New Assessable Emissions (TPY) Description of Emissions CO NOx PM-101 SO2

1 VOC1 Total PTE 26.1 75.6 2.1 1.1 0.7 106

Assessable Emissions 26.1 75.6 0.0 0.0 0.0 102 Table Notes: 1PM-10, SO2, and VOC emissions are below 10 tpy and do not contribute to assessable emissions.

2.3 Department Findings The Department has made the following findings regarding APC’s application:

1. Gudenrath Compressor Station (GCS) is an existing stationary source with no existing Title I or Title V permits.

2. The project is classified under 18 AAC 50.502(c)(3), for existing NOx emissions greater than 40 tpy with a project increase of at least 10 tpy NOx.

3. APC submitted an ambient analysis as required under 18 AAC 540(c)(2)(A).

4. The ambient analysis submitted by APC shows that stack height requirements are needed on the four Solar-20 turbines to protect the annual NO2 Alaska ambient air quality standard (AAAQS). The Department summarized these requirements in the modeling review memorandum (See Appendix B) and included them in the minor permit per 18 AAC 50.544(c)(1). Because two of the Solar-20 turbines have already been installed the Department is requiring APC to verify the stack height requirement in the first operating report so that APC will have time to modify the stack heights if necessary.

5. In addition to the requirements specified in the modeling review memorandum to protect the annual NO2 AAAQS the Department is also requiring APC to remove the existing turbines from service before either of the new turbines become fully operational. The Department is including this requirement because APC’s ambient analysis does not include the existing turbines operating.

6. APC is not required to obtain a Title V permit as all potential emission are under the 100 tpy Title V permit threshold.

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3. Permit Requirements

3.1 Requirements for All Minor Permits As required by 18 AAC 50.544(a), this minor permit issued under 18 AAC 50.542 has included the following:

1. the name and description of the stationary source, the project, the Permittee, contact information in the cover page of the permit;

2. the requirement to pay fees in Section 2 of the permit;

3. no specific conditions established under 18 AAC 50.201; and

4. the applicable standard permit conditions required in 18 AAC 50.544 contained in 18 AAC 50.345 are listed in Sections 5, 6 and 7 of the permit.

3.2 Requirements for a Minor Permit issued under 18 AAC 50.502(c)(3) As required under 18 AAC 50.544(c), this minor permit classified under 18 AAC 50.502(c) contains:

1. Terms and conditions as necessary to ensure that the source will not cause or contribute to a violation of an ambient standard.

2. Performance tests for state emission limits. The Department is not requiring any performance tests as discussed in Section 3.3 of this TAR.

3. Maintenance requirements according to the manufacturer’s or operator’s maintenance procedures. Maintenance requirements are in Section 1 of the permit.

4. Procedures for recordkeeping requirements under 18 AAC 50.544(c)(1)(D).

5. Excess emission and permit deviation requirements are specifically required under 18 AAC 50.240. Operating reports as stipulated under 18 AAC 50.544(c)(1)(E).

3.3 State Emission Standards Currently, APC does not have a Title V permit for the Gudenrath Compressor Station and is not subject to Title V permitting as a result of this project. Therefore the Gudenrath Compressor Station will not be subject to any ongoing monitoring, recordkeeping and reporting (MR&R) under Title V permitting for state emission standards.

3.3.1 Visible Emission Standard EUs 1through 11 are fuel-burning equipment subject to the state standard for visible emissions in 18 AAC 50.055(a)(1). EUs 1 through 6 and 8 through 12 consist of natural gas fired turbines, small natural gas fired boilers, a small natural gas fired heater and a small natural gas fired furnace. The Department did not include initial or ongoing MR&R because these units burn natural gas as fuel and will most likely not exceed the visible emission standard.

EU 7 is a small diesel fired emergency generator. The Department did not include any initial or ongoing MR&R for this unit because it is unlikely this small diesel fired unit will exceed the visible emission standard.

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3.3.2 Particulate Matter Standard EUs 1, through 11 are fuel-burning equipment subject to the state standard for PM emissions of 0.05 grains per dry standard cubic foot of exhaust gas (gr./dscf) in 18 AAC 50.055(b)(1). The Department did not include any initial or ongoing MR&R for EUs 1 through 6 or 8 through 12 because these units burn natural gas as fuel and will most likely not exceed the particulate matter standard.

The Department did not include any initial or ongoing MR&R for EU 7 because this small diesel fired unit is unlikely to exceed the particulate matter standard.

3.3.3 Sulfur Dioxide Standard EUs 1, through 11 are fuel-burning equipment subject to state standards for SO2 under 18 AAC 50.055(c). Sulfur-compound emissions from fuel-burning equipment shall not exceed 500 ppm averaged over any three hour period. The Department did not include any initial or ongoing MR&R for EUs 1 through 6 or 8 through 11 because these units burn natural gas as fuel and will most likely not exceed the SO2 standard.

The Department did not include any initial or ongoing MR&R for EU 7 because this small diesel fired unit is unlikely to exceed the SO2 standard due to the sulfur content of diesel fuel readily available.

3.4 Requirements for Stationary Sources Not Subject to Title V Permitting Requirements.

As described in 18 AAC 50.544(d), for a stationary source that is not subject to Title V permitting under 18 AAC 50.326, the Department will include in the minor permit requirement to periodically affirm whether the stationary source is still accurately described by the application and minor permit, and whether the owner or operator has made changes that would trigger a requirement for a new permit under 18 AAC 50. This requirement is included in the minor permit under Affirmation of Title-V Avoidance in Section 5 of the permit.

4. Permit Administration The Permittee may operate under minor permit AQ0856MSS01 upon issuance.

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Appendix A: Emissions Calculations

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Technical Analysis Report - Permit AQ0856MSS01 Preliminary – January 29, 2014 Alaska Pipeline Company – Gudenrath Compressor Station

EU Description Yearly

Operation (hr/yr)

NOX CO SO2 VOC PM-10 PM-2.5 Emission

Factor PTE TPY

Emission Factor

PTE TPY

Emission Factor

PTE TPY

Emission Factor

PTE TPY

Emission Factor

PTE TPY

Emission Factor

PTE TPY

1 Solar Saturn 20 8,760 4.4 lb/hr1 19.27 1.47

lb/hr2 6.44 3.40E-03 lb/MMBtu 0.26 2.10E-03

lb/MMBtu3 0.16 6.60E-03 lb/MMBtu3 0.50 6.60E-03

lb/MMBtu3 0.50


lb/hr2 6.44 3.40E-03 lb/MMBtu 0.26 2.10E-03


lb/MMBtu3 0.50


lb/hr2 6.44 3.40E-03 lb/MMBtu 0.26 2.10E-03


lb/MMBtu3 0.50

6 Solar Saturn 20 8,760 3.93

lb/hr1 17.21 1.47 lb/hr2 6.44 3.40E-03

lb/MMBtu 0.26 2.10E-03 lb/MMBtu3 0.16 6.60E-03

lb/MMBtu3 0.50 6.60E-03 lb/MMBtu3 0.50

74 Emergency Generator 500 0.031

lb/hp-hr 0.36 6.68E-03 lb/hp-hr 0.08 2.05E-03

lb/hp-hr 0.02 2.47E-03 lb/hp-hr 0.03 2.20E-03

lb/hp-hr 0.03 7.21E-04 lb/hp-hr 0.03

85,6 Boiler A 8,760 0.018 lb/hr 0.08 0.023

lb/hr 0.10 1.41E-04 lb/hr 6.18E-04 1.29E-03

lb/hr 5.67E-03 1.79E-03 lb/hr 7.83E-03 1.79E-03

lb/hr 7.83E-03

95,6 Boiler B 8,760 0.013 lb/hr 0.07 0.019

lb/hr 0.08 1.18E-04 lb/hr 5.15E-04 1.08E-03

lb/hr 4.72E-03 1.49E-03 lb/hr 6.53E-03 1.49E-03

lb/hr 6.53E-03

105,6 Hot Water Heater 8,760 0.003

lb/hr 0.02 0.004 lb/hr 0.02 2.71E-05

lb/hr 1.19E-04 2.48E-04 lb/hr 1.09E-03 3.43E-04

lb/hr 1.50E-03 3.43E-04 lb/hr 1.50E-03

115,6 Furnace 8,760 0.007 lb/hr 0.03 0.009

lb/hr 0.04 5.29E-05 lb/hr 2.32E-04 4.85E-04

lb/hr 2.13E-03 6.71E-04 lb/hr 2.94E-03 6.71E-04

lb/hr 2.94E-03

Total 75.58 26.08 1.06 0.67 2.05 2.05

Table Notes: 1 Source Test from August 31, 2012 2 Source Test from October 15, 2012 3 AP-42 Table 3.1-2a 4 AP-42 Table 3.3-1 5 AP-42 Table 1.4-1. 6 AP-42 Table 1.4-2

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Appendix B: Modeling Memorandum

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MEMORANDUM State of Alaska Department of Environmental Conservation

Division of Air Quality

TO: File DATE: 17 December, 2013

THRU: Alan Schuler, PE FILE NO: AQ0856MSS01 Engineer, DEC Air Permits Program PHONE: (907) 465-5324 FAX: (907) 465-5129

FROM: James Julian Renovatio, EIT SUBJECT: Review of Alaska Pipeline Company’s Engineering Assistant, DEC Ambient Analysis for the Gudenrath Air Permits Program Turbine Replacement Project

INTRODUCTION

This memorandum summarizes the Alaska Department of Environmental Conservation’s (Department’s) findings regarding the ambient assessment submitted by the Alaska Pipeline Company (APC) for their application to replace two turbines at the Gudenrath Compressor Station (Gudenrath). APC submitted this assessment in support of their 2 April, 2013 minor permit application. The pollutant subject to review for this application is oxides of nitrogen (NOx). The Department finds APC’s ambient analysis adequately demonstrates that operating the Gudenrath emissions units (EUs) within the restrictions listed in this memorandum will not cause or contribute to a violation of the annual nitrogen dioxide (NO2) Alaska Ambient Air Quality Standards (AAAQS) as provided in 18 AAC 50.010. BACKGROUND

APC presently operates Gudenrath as an existing stationary source without an Air Quality Control permit. Their minor permit application proposes the installation of two refurbished 1,600 horsepower (hp) natural gas-fired turbines to replace two existing 1,200 hp natural gas-fired turbines. The replacement turbines will continue to be used for both peaking and backup when base-load sources are unavailable. Gudenrath is located approximately eight kilometers (km) north of Sterling, Alaska and consists of four natural gas-fired turbines, a diesel-fired emergency generator, two boilers, a hot water heater, and a furnace.

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Review of Gudenrath Compressor Station 17 December, 2013 Ambient Assessment Minor Permit AQ0230MSS02 Application Submittal

APC submitted a minor permit application on 2 April, 2013; they included an ambient assessment with their application. The Department requested additional information from APC on 31 May, 2013, with regard to their ambient assessment. APC responded with a revised ambient assessment as a part of their 23 September, 2013. APC’s consultant, the Arctic Slope Regional Corporation (ASRC) prepared the minor permit application and ambient assessment on behalf of APC.

Project Classification APC’s minor permit application is classified under 18 AAC 50.502(c)(3) for NOx. The case-specific details for this permit classification are described in the Technical Analysis Report. Applications submitted under this classification must include an ambient analysis of each triggered pollutant in accordance with the application information requirements of 18 AAC 50.540(c)(2)(A). APC submitted an ambient AAAQS analysis of their annual NO2 impacts. Minor permit applicants are not required to demonstrate compliance with the one-hour NO2 AAAQS in accordance with 18 AAC 50.540(l).

SOURCE IMPACT ANALYSIS

Approach APC used computer analysis (modeling) to evaluate their annual NO2 ambient air impacts. They used a screening model, as discussed in the subsequent section, so no meteorological data was required. Also, the emissions from Gudenrath were modeled using a single stack due to the limitations of the screening model. APC provided multiple model runs to simulate their emissions impacts throughout the modeling domain and by season. Succinctly, they modeled 12 domain-specific sectors centered about the modeled stack, equally spaced at 30-degrees, and across 12 months of seasonally changing surface characteristics for each sector. The Department performed a composite sensitivity analysis based on their aforementioned approach of 144 discrete model runs, which is described in the Results and Discussion section. While the sensitivity analysis, as discussed in subsequent sections, indicates that APC’s multiple-run model is acceptable, the Department finds that the complexity of the approach was difficult to review. The following discussion regards APC’s ambient NO2 demonstration.

Model Selection There are a number of air dispersion models available to applicants and regulators. The U.S. Environmental Protection Agency (EPA) lists these models in their Guideline on Air Quality Models (Guideline), which the Department has adopted by reference in 18 AAC 50.040(f). APC used the screening option of EPA’s AERMOD Modeling System (AERMOD) for their ambient NO2 analysis by way of the EPA’s AERSCREEN interface. AERSCREEN can only estimate the pollutant concentrations from a single EU. However, it generates a “worst-case” meteorological data set for the given EU characterization, which allows it to be used when actual meteorological data does not exist. APC conducted multiple AERMOD screening runs using the current version of AERMOD, version 12345. The screening option of AERMOD is an appropriate modeling system for this application since there are no representative meteorological data for Sterling, Alaska.

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Review of Gudenrath Compressor Station 17 December, 2013 Ambient Assessment Minor Permit AQ0230MSS02

Meteorological Data

AERMOD typically uses hourly meteorological data to estimate plume dispersion. The MAKMET meteorological pre-processor is available to estimate worst-case hourly meteorological conditions when there are no representative meteorological data. APC ran MAKEMET through the AERSCREEN interface to develop their meteorological data. MAKEMET requires the specification of minimum and maximum ambient temperatures, average wind speed, and an anemometer height. Additionally, the area surrounding the modeled EU must be characterized with regard to the following three surface parameters: noon-time albedo, Bowen ratio, and surface roughness length. EPA has provided additional guidance regarding the selection and processing of these surface characteristics in their AERMOD Implementation Guide. The Department finds that APC correctly used the MAKEMET meteorological pre-processor for Gudenrath. Additional details regarding the development of case-specific input surface parameters for MAKEMET are provided as follows. Parameter Specification

APC used minimum and maximum temperatures of 239 K and 305 K, respectively, a minimum wind speed of 0.5 meters-per-second, and an anemometer height of 10 meters. These are reasonable temperatures for Soldotna based on a review of the last 10 years of data collected at its airport. The minimum wind speed is consistent with EPA guidance. The assumed anemometer height is reasonable for the stack heights at Gudenrath.

Surface Characteristics

APC derived domain-specific values for albedo and Bowen ratio, as revised in their 23 September, 2013 supplemental submission, by observing the land use classification for each square kilometer (km) in a 10 km-by-10 km grid. The land use classifications observed included: coniferous forest; swamp; cultivated land, grassland; and water. APC assigned individual values for these surface characteristics in order to adjust them by season and sector. EPA and Department guidance prescribes the weighting of individual monthly values within the aforementioned grid domain in order to derive composite monthly values. The Department finds APC’s approach and results are inconsistent with the aforementioned guidance. The Department, however, notes that, while albedo and Bowen ratio have a broader-scale impact than the surface roughness, they are not major factors of influence when considering relatively short stacks. Therefore, the impact of changing these parameters by-sector is inconsequential for Gudenrath. APC’s resultant composite values for albedo and Bowen ratio are illustrated Tables 1 and 2. APC evaluated the surface roughness length within a circular area of 1 km radius about the modeled emissions source. They segregated this circular area into 12 equal 30-degree sectors to determine a surface roughness length by sector. Each sector contains one or more of the previously described land use classifications. APC weighted the

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surface roughness length associated with each classification according to Department guidance in order to derive a composite value for each sector. They did this for each month in order to accommodate the seasonal variance in the surface roughness length. The Department finds APC‘s approach and results in comport with the aforementioned guidance and acceptable for the Gudenrath Project. The resultant composite values for surface roughness, as revised in their 23 September, 2013 supplemental submission, are illustrated Table 3.

Table 1. Surface parameters for the Gudenrath site: albedo Albedo by Sector and Month 1 2 3 4 5 6 7 8 9 10 11 12

January 0.753 0.737 0.716 0.712 0.712 0.705 0.837 0.787 0.771 0.837 0.737 0.799 February 0.753 0.737 0.716 0.712 0.712 0.705 0.837 0.787 0.771 0.837 0.737 0.799

March 0.753 0.737 0.716 0.712 0.712 0.705 0.837 0.787 0.771 0.837 0.737 0.799 April 0.639 0.512 0.606 0.498 0.499 0.495 0.633 0.606 0.603 0.629 0.555 0.550 May 0.639 0.512 0.606 0.498 0.499 0.495 0.633 0.606 0.603 0.629 0.555 0.550 June 0.587 0.511 0.500 0.498 0.498 0.509 0.630 0.606 0.602 0.629 0.512 0.546 July 0.587 0.511 0.500 0.498 0.498 0.509 0.630 0.606 0.602 0.629 0.512 0.546

August 0.587 0.511 0.500 0.498 0.498 0.509 0.630 0.606 0.602 0.629 0.512 0.546 September 0.628 0.516 0.504 0.499 0.501 0.495 0.643 0.611 0.603 0.640 0.520 0.621

October 0.628 0.516 0.504 0.499 0.501 0.495 0.643 0.611 0.603 0.640 0.520 0.621 November 0.753 0.737 0.716 0.712 0.712 0.705 0.837 0.787 0.771 0.837 0.737 0.799 December 0.753 0.737 0.716 0.712 0.712 0.705 0.837 0.787 0.771 0.837 0.737 0.799

Table Notes: Winter is defined as January-March. Spring is defined as April-May. Summer is defined as June-August. Fall is defined as September-October.

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Table 2. Surface parameters for the Gudenrath site: Bowen Ratio

Bowen Ratio by Sector and Month 1 2 3 4 5 6 7 8 9 10 11 12






Table 3. Surface parameters for the Gudenrath site: surface roughness

Surface Roughness Length (m) by Sector and Month 1 2 3 4 5 6 7 8 9 10 11 12






Averaging Periods

AERSCREEN uses the computer-generated meteorological parameters to estimate the worst-case one-hour pollutant concentrations. It can then apply scaling factors to estimate the three-hour, eight-hour, 24-hour, or annual average concentrations. The period-specific

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scaling factors are listed in the AERSCREEN User’s Guide. APC compiled the results from multiple AERSCREEN runs and then used the correct scaling factor of 0.1 for estimating the annual average NO2 impacts.

Coordinate System Air quality models need to know the relative location of all elements, such as EUs, structures (if applicable), and receptors, in order to properly estimate ambient pollutant concentrations. Therefore, applicants must use a consistent coordinate system in order to obtain the correct distances between all elements. APC used a site-specific coordinate system centered on the modeled emissions source. This is an acceptable coordinate system for this analysis.

Terrain Terrain features are typically identified through the use of topographic maps or digital elevation data. APC did not include terrain data in their modeling analysis because flat terrain was assumed. The Department finds this approach and assumption is appropriate for Gudenrath.

EU Release Parameters The assumed emission parameters have significant roles in an ambient demonstration. Therefore, the Department reviews these parameters very carefully. Emission Inventory and Rates

APC modeled the four natural gas-fired turbines, EUs 1 through 4, as a single point source with a combined, i.e. linearly additive, emission rate that is based on the unlimited annual operation of each turbine. APC did not include the emissions from the diesel-fired emergency generator, two boilers, hot water heater, or furnace in this combined rate. The Department found that excluding the emissions from the aforementioned ancillary equipment is acceptable for this project given their small size. The Department generally found the combined turbine emission rate, as modeled, to be consistent with the emissions information provided throughout the application. This approach is appropriate for an AERMOD screening model, which evaluates the impacts from a single emissions source.

Point Source Parameters

Applicants must provide the stack height, diameter, location, and base elevation in addition to the pollutant emission rates, exhaust plume exit velocity, and exhaust temperature for each modeled point source. APC’s modeled point source used a stack height, diameter, exhaust velocity, and exhaust temperature that is based on a single Gudenrath turbine stack, all of which are proposed to share a homogeneous design. The modeled stack location was positioned within the to-be-built compressor building, which represents the closest turbine-containing structure to the ambient air boundary as discussed in subsequent sections. The Department generally found the modeled stack parameters to be consistent with the

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vendor information or expectations for similarly sized turbine EUs. However, APC’s complex and non-comprehensive use of the AERMOD screening model lends uncertainty to the model results as discussed in the Results and Discussion section. Moreover, their analysis provides results that are modeled using a stack height of 10 meters instead of the 6.1 as indicated in their application. Therefore, the Department is including a minimum stack height requirement, as modeled at 10 meters, as an ambient condition to protect ambient air quality.

Ambient NO2 Modeling The modeling of ambient NO2 concentrations can sometimes be refined through the use of ambient air data or assumptions. Section 5.2.4 of the Guideline describes several approaches that may be considered in modeling the annual average NO2 impacts. APC used the Plume Volume Molar Ratio Method (PVMRM) to refine the estimated ambient NO2 concentrations. The use of the PVMRM is appropriate, but warrants discussion. EPA and Department Approval

PVMRM is a non-Guideline method and, therefore, requires EPA and Department approval in accordance with 18 AAC 50.215(c)(2). APC used PVMRM in both their initial and revised modeling analyses. EPA Region 10 granted APC permission to use PVMRM for Gudenrath on 25 November, 2013. The Air Permits Program Manager also gave his approval on this date.1

Public Comment

The use of a non-Guideline model is subject to public comment. Therefore, the Department is soliciting public comment regarding APC’s use of PVMRM in the public notice for the preliminary permit decision.

In-Stack NO2-to-NOx Ratio

The NOx emissions created during combustion are partly nitric oxide (NO) and partly NO2. After the combustion gas exits the stack, additional NO2 is created as the exhaust mixes with atmospheric ozone. The assumed NO2-to-NOx in-stack ratio is a variable that must be set for each EU that generates NOx emissions. Source-specific data should be used to define this ratio when available. When source-specific data is not available, an in-stack ratio of 0.5 may be used without justification for the purposes of modeling the one-hour NO2 impacts. In accordance with EPA guidance, this value represents a reasonable upper bound based on available in-stack data. EPA has not provided a similar “default” ratio for the purposes of modeling the annual average NO2 impacts.

1 The Commissioner delegated his authority regarding the use of non-guideline models to the Air Permits Program Manager on 3 June, 2008.

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APC used an in-stack NO2-to-NOx ratio of 0.75 in their 2 April, 2013 modeling analysis. They changed this ratio to 0.5 in their revised analysis of 23 September, 2013. The Department finds the latter value acceptable for Gudenrath.

Ozone Data

PVMRM requires ambient ozone data in order to determine how much of the NO is converted to NO2. A fixed background concentration is used when running the AERMOD screening model. APC used an ozone background concentration of 47 parts-per-billion (ppb) in their 2 April, 2013 modeling analysis. They changed this concentration to 55 ppb in their revised analysis based on comments provided by the Department2.

Downwash

Downwash refers to conditions where nearby structures influence plume dispersion. Downwash can occur when a stack height is less than a height derived by a procedure called “Good Engineering Practice,” which is defined in 18 AAC 50.990(42). The modeling of downwash-related impacts requires the inclusion of dimensions from nearby buildings. EPA has established specific algorithms for determining which buildings must be included in the analysis and for determining the profile dimensions that would influence the exhaust plume from a given stack. EPA has incorporated these algorithms into the “Building Profile Input Program” (BPIP) computer program. APC used EPA’s PRIME version of BPIP (BPIPPRM, version 04274) to determine the building profiles needed by AERMOD. The Department finds that this is an appropriate version of BPIP, but notes that APC transposed several of their building coordinates that were processed with BPIPPRIM and incorrectly defined the height of the compressor building within which the point source was modeled. The Department corrected these coordinates and height during a “spot-check” of APC’s various modeling runs and determined the net impacts of these incorrectly defined buildings to be inconsequential.

Ambient Air Boundary For the purposes of air quality modeling, ambient air means outside air to which the public has access. Ambient air typically excludes that portion of the atmosphere within a stationary source’s boundary. The shortest EU-to-boundary distance is used in an AERSCREEN run. APC assumed an ambient boundary distance of 100 meters in their 2 April, 2013 modeling analysis, but did not provide sufficient justification to use this value. They changed the ambient boundary distance to 14.6 meters in their revised analysis submitted on 23 September, 2013. APC selected the latter value as half of the minimum distance from the compressor building that will house the modeled point source, Plant C, to the south fence-line of the property. APC used this ambient boundary distance in each of its multiple AERMOD screening runs, which evaluated the impacts along different cardinal directions. The Department finds this distance an acceptable ambient air boundary for the Gudenrath AERMOD screening model.

2 26 August, 2013 e-mail chain, Background ozone concentration Sterling AK, between the Department and APC.

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Receptor Grid

AERSCREEN evaluates the ambient air impacts along a single downwind direction using a series of receptors that are generated using user-provided minimum and maximum distances. APC’s 2 April, 2013 modeling analysis, which consisted of multiple runs to account for directional and seasonal variability, was designed to evaluate the ambient air impacts to a distance of 5 km in each run. Their revised analysis submitted on 23 September, 2013, which also consisted of multiple runs to account for directional and seasonal variability, was designed to evaluate the ambient air impacts to a distance of 100 meters. The Department finds that APC’s revised receptor distance provides sufficient information to determine the maximum impacts for Gudenrath.

Background Concentrations The impact from neighboring (off-site) sources must be accounted for in a cumulative impact assessment. In accordance with Section 8.2.3 of the Guideline, “…all sources expected to cause a significant concentration gradient in the vicinity of the [applicant’s source] should be explicitly modeled.” In the case of an AERMOD screening model, the impact from other sources must be accounted for through ambient monitoring data, i.e. background concentrations. The background concentration must be evaluated on a case-specific basis for each of the modeled pollutants. The data used to represent the background concentration must represent the non-modeled sources such as nearby off-site, natural, area and long-range transport. Once the background concentration is determined, it is added to the modeled concentration to estimate the total ambient concentration. APC did not propose or include an annual NO2 background concentration in both their initial and revised modeling analyses. Moreover, they assert that a background concentration is not required in Appendix 1 of their permit application. The Department does not have a regulatory or factual basis to exclude the impacts from the aforementioned non-modeled sources. Therefore, the Department added an annual NO2 background concentration of 28.2 µg/m3 to APC’s maximum modeled result to represent the impact from off-site sources. This concentration represents the high concentration observed during the 2011 through 2012 monitoring effort at the Chugach River International Station in Anchorage. The Department notes that this value is likely conservative, but finds that it is better to over-estimate impacts than under-estimate them in an ambient demonstration.

RESULTS AND DISCUSSION

The maximum modeled NO2 impact from APC’s ambient standard demonstration, as revised by the Department, is presented in Table 4. The background concentration, total impact, and ambient standard are also presented for comparison. The total modeled impact is less than the AAAQS. Therefore, the proposed emissions will not cause or contribute to a violation of the annual average NO2 AAAQS.

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Review of Gudenrath Compressor Station 17 December, 2013 Ambient Assessment Minor Permit AQ0230MSS02 Table 4. Maximum impact compared to the ambient standard

Pollutant Avg. Period Max. Modeled Concentration

(µg/m3)

Background Concentration

(µg/m3)

Total Impact (µg/m3)

Ambient Standard (µg/m3)

NO2 Annual 46.7 28.2 74.9 100 Table Notes: The maximum modeled concentration for the annual average is the product of the maximum 1-hour concentration and a scaling factor of 0.1. APC provided a modeled annual concentration of 1.73 µg/m3 which represents the high downwind concentration at 270° with a 10 m stack. The Department notes that APC’s results, as presented in their 23 September, 2013 revised analysis, differ from the tabular results presented in this memorandum. This difference is attributable to several factors:

• APC did not include an annual NO2 background concentration in their modeled results; • APC did not correctly pre-process their building parameters, which may have affected the

downwash impacts; and • APC did not correctly identify the highest modeled impacts in all wind directions.

The Department also notes that APC’s revised modeling analysis provides results that are modeled using a stack height of 10 meters instead of the 6.1 as indicated in their application. Their revised analysis was designed for 144 discrete AERMOD screening runs, though several inputs, outputs, and meteorological files were missing. Each screening run was intended to account for the ambient air impacts along 12 sector directions, equally spaced at 30-degrees, and across 12 months of seasonally changing surface parameters for each sector. 144 surface and profile meteorological files, developed using MAKEMET, were intended to be used with each AERMOD screening run. While APC’s screening approach has merit, their application did not include the maximum modeled impacts. The failure to include the maximum modeled impacts is because some of the discrete model runs were not reporting the high concentrations that were not captured along a single direction of receptor impacts with changing surface parameters and rotating building profiles. The aforementioned omission can be demonstrated in the results of a non-screening AERMOD run. The Department performed a composite sensitivity run, from which it calculated the concentration value in Table 4, using APC’s month- and sector-specific surface roughness values; the albedo and Bowen Ratio were fixed in this run since they demonstrate a low impact on the modeled response. A wind sector graph of the Department’s sensitivity run results is presented in Figure 1.

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Figure 1. Department Modeled 1-hour NO2 concentrations by wind sector with composite imputs The Department recommends that APC more thoroughly develop their modeling assumptions in future submittals and encourages the optional submission of a modeling protocol. CONCLUSION

The Department reviewed APC’s modeling analysis for the Gudenrath Project and concluded the following:

1. The annual NO2 emissions associated with operating the proposed EUs will not cause or contribute to a violation of the AAAQS listed in 18 AAC 50.010.

2. APC’s modeling analysis, as revised by the Department, fully complies with the showing requirements of 18 AAC 50.540(c)(2).

3. APC conducted their modeling analysis in a manner consistent with the Guideline as required under 18 AAC 50.215(b)(1).

The Department developed conditions in Minor Permit AQ0856MSS01 to ensure APC complies with the AAAQS. These conditions are summarized as follows:

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1. APC shall maintain minimum exhaust stack heights of no less than 10 m on the natural gas-fired turbines, EUs 1 through 4, to protect the annual NO2 AAAQS.

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TECHNICAL ANALYSIS REPORT For - Alaska