1-Hour SO2 NAAQS Implementation Modeling

Dan P. Dix

23rd Virginia Environmental Symposium

April 11, 2012

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Agenda Summary of NAAQS NAAQS Implementation Updates Dispersion Modeling Basics NAAQS Modeling Demonstration

Approach Ambient SO2 Monitoring

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About ALL4 Environmental consulting firm Founded 2002 – currently 30+ employees Offices in Kimberton, PA and Columbus, GA Specialize in air quality consulting:

• Complex air permitting and strategy development

• Air dispersion modeling• Ambient air quality monitoring

Dispersion modeling as a company-wide initiative

www.all4inc.com

National Ambient Air Quality

Standards (NAAQS)

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NAAQS Background “Backdrop” of the Clean Air Act States design their SIPs and enforce

and implement their regulations to meet the NAAQS

Air quality construction permit programs are designed around NAAQS compliance• PSD: Maintaining NAAQS attainment• NNSR: Getting into NAAQS attainment

NAAQS reevaluated every 5 years

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NAAQS SummaryPollutant

Averaging Period

Primary/Secondary

Historic NAAQS (µg/m3)

Revised NAAQS (µg/m3)

CO1-Hour Primary 40,000 10,0008-Hour Primary 10,000 40,000

Ozone 8-HourPrimary/

Secondary75 ppb Withdrawn

Pb3-Month Rolling

Primary/Secondary

1.5 0.15

PM10 24-HourPrimary/

Secondary150 150

PM2.5

24-HourPrimary/

Secondary65 35

AnnualPrimary/

Secondary15 15

NO2

1-Hour Primary N/A 188

AnnualPrimary/

Secondary100 100

SO2

1-Hour Primary N/A 1963-Hour Secondary 1,300 1,300

24-hour Primary 365 Revoked

AnnualPrimary/

Secondary80 Revoked

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Attainment/Nonattainment Designations

U.S. EPA philosophy on the SO2 NAAQS implementation process:• Proposed NAAQS – designations based

on ambient monitoring data• Final NAAQS – designations based

primarily on air quality modeling data Shift to reliance on air quality

modeling will become a critical issue for individual facilities

NAAQS Implementation

Updates

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SO2 NAAQS Implementation NAAQS Implementation Schedule:

• June 2011: Initial state nonattainment recommendations to U.S. EPA (most counties were “unclassifiable”)

• June 2012: EPA to finalize attainment status (most states will still be “unclassifiable” or attainment)

• June 2013: Maintenance SIP submittals including individual facility modeling to achieve compliance with the NAAQS

• August 2017: Full NAAQS compliance in all areas

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Implementation Update Draft guidance for states to evaluate

designations using AERMOD was released on September 22, 2011

Most states are currently reviewing the U.S. EPA guidance and crafting their plans

States or facilities conducting modeling? U.S. EPA indicated at 10th Conference on Air

Quality Models that final guidance will not be released this year due to the scope of comments made.

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SO2 Maintenance SIP Submittals

U.S. EPA: Revising PSD/NNSR programs to include new NAAQS is not sufficient. Five components are required:• “Attainment Emission Inventory”• Maintenance Demonstration• Control Strategy• Contingency Plan• Verification of Continued Attainment

Maintenance SIP will list enforceable 1-hour emission limits (August 2017)

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SO2 NAAQS Implementation State SIPs will be based on AERMOD

dispersion modeling for the following individual facilities (by order of priority):• SO2 Actual Emissions > 100 tons per year

• SO2 PTE > 100 tons per year• Smaller facilities “with a potential to

cause or contribute” to a NAAQS violation States are considering other options based

on population

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SO2 NAAQS Implementation Legal challenges ongoing:

• Science behind NAAQS levels• Approach of using modeling

Under the current approach, if states don’t perform modeling, U.S. EPA will through Federal Implementation Plan (FIP)

Some states don’t have the resources to complete evaluations and don’t think U.S. EPA does either.

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SO2 NAAQS Implementation What are other States doing?

• Maine - >100TPY Actual, Protocols due June 30, 2012, and final analysis due December 31, 2012.

• Lake Michigan Air Directors Consortium (LADCO) has developed Protocol for states to follow.

Wisconsin, Michigan, Minnesota, Indiana, and Illinois included.

>100 TPY PTE, facilities given option to complete themselves or have state complete.

Michigan were due December 31, 2011. Minnesota completed March 12, 2012.

• Nebraska – Power plants have joined to conduct modeling themselves and conducting tracer study.

• Missouri – Facilities conducting modeling due by April 2012.• Connecticut - >15TPY PTE, conducted by State by July 2012.

Dispersion Modeling Basics

and Inputs

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AERMOD Process

Hourly Wind Speed

Hourly Wind Direction

Hourly Ambient Temperature

Land Use PatternsTopography

Building DimensionsStack DimensionsExhaust Velocity

Exhaust Temperature

Emission Rates

Predicted Ground Level Ambient Concentrations (µg/m3) for all

averaging times

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Air Quality Modeling Steps

1. Emission Inventory2. Meteorological Data

(AERMET/AERSURFACE)3. Terrain Data (AERMAP)4. Building Downwash (BPIPPRM)

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Emission Inventories Short-term (1-hour) emission rates Potential to be used as permit limits Intermittent emission units (e.g., emergency

generators, intermittent emission scenarios such as startup/shutdown operations or alternative fuels)• Latest guidance indicates following form of

standard as guideline for what to include (i.e., 99th percentile (4th highest))

Stack characteristics (height, temperature, velocity, diameter, location)

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Meteorological Data

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Meteorological Data

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Meteorological Data 5 years of National Weather Service data Minimum of 1 year of onsite data Surface characteristics and topography

surrounding the facility should be similar to (representative of) those surrounding the meteorological station

If no representative meteorological data are available, SO2 implementation guidance suggests possibility of using AERSCREEN (with agency approval)

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Terrain Data “Ambient Air”

“that portion of the atmosphere, external to buildings, to which the general public has access” or “the air everywhere outside of a contiguous plant property to which public access is precluded by a fence or other effective physical barrier”

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Building Downwash

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Building Downwash

NAAQS Modeling Demonstration

Approach

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Full NAAQS Evaluation Includes facility and other local

facilities Any modeled emission rates should

be acceptable as a 1-hour permit limit with the appropriate margin for compliance

Considerations for accounting for emissions during startup and shutdown

Emergency unit considerations

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Local Sources NAAQS evaluation must include

sources that result in a “significant concentration gradient” in the vicinity of the facility

Same emission rate considerations apply for local sources (although permit limit concerns wouldn’t apply)

State agency typically dictates which local sources to include in evaluation

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NAAQS Modeling Strategy Start with an evaluation of each

individual emission source Each source will have different

factors that drive resulting ambient concentrations

The cumulative ambient concentration from all sources (plus background) will be evaluated against the NAAQS

Evaluate each source against the NAAQS as a first step

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NAAQS Modeling Strategy Big picture factors that will drive

ambient concentrations for individual sources:• Elevated emission rates• Stack velocity (orientation of release

and flowrate)• Stack temperature (plume buoyancy)• Stack height versus surrounding terrain• Surrounding buildings and structures

(i.e., building downwash)

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Hypothetical Modeling Examples

Modeling of a hypothetical facility with the following SO2 emission sources:• Process SO2 source

• Fuel oil combustion SO2 source• Backup engine source

NAAQS modeling evaluation is based on SO2 potential-to-emit

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Hypothetical Facility Terrain

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“Process” SO2 Source SO2 Emission Rate: 240 lb/hr (CEMS) Stack Height: 290 feet Stack Diameter: 16.5 feet Exhaust Temp: 350 °F Exhaust Flow: 230,000 acfm Elevated emission rate, buoyant

source, tall stack (taller than the tallest buildings at the facility)

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Process SO2 Source Impacts

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Process SO2 Source Impacts Highest impacts in complex terrain far

from facility Wind speed doesn’t match location of

elevated concentrations Impacts occur during periods of

atmospheric stability and low mixing heights (typically early morning, low wind speed conditions)

High concentrations due partially to the limitations of the AERMOD dispersion model

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Combustion SO2 Source SO2 Emission Rate: 20 lb/hr (AP-42) Stack Height: 60 feet Stack Diameter: 2 feet Exhaust Temp: 225 °F Exhaust Flow: 16,000 acfm Buoyant source, short stack (shorter

than the tallest buildings at the facility)

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Combustion SO2 Source Impacts

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Combustion SO2 Source Impacts

Elevated concentrations are closer to the facility

Building downwash effects have a noticeable impact on ambient concentrations

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Engine SO2 Source SO2 Emission Rate: 3 lb/hr (Vendor) Stack Height: 10 feet Stack Diameter: 1.3 feet Exhaust Temp: 935 °F Exhaust Flow: Horizontal Discharge Horizontal discharge, short stack

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Engine SO2 Source Impacts

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Engine SO2 Source Impacts Elevated ambient concentrations at

the facility fenceline for two reasons:• Low stack height (10 feet)• No plume buoyancy due to horizontal

discharge Ambient air considerations become

very important (i.e., public access)

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Modeling Refinements “Process” SO2 Emission Source:

• Stack height increase is technically and economically infeasible

• Raw materials are fixed due to product and consumer demand

• Upgrades to the scrubber could achieve control: ~30% more control (~170 lb/hr)

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Process SO2 Source Impacts (Before)

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Process SO2 Source Impacts (After)

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Modeling Refinements Combustion SO2 Emission Source:

• Stack height increase is technically and economically infeasible

• Fuel oil firing is desirable due to cost savings considerations

• Raw materials to the source bring inherent scrubbing capacity: 50 to 65% based on previous studies

• 50% inherent scrubbing brings emission rate to 10 lb/hr (justify through testing)

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Combustion SO2 Source Impacts (Before)

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Combustion SO2 Source Impacts (After)

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Modeling Refinements Engine SO2 Emission Source:

• Simplest fix is to change the stack discharge orientation from horizontal to vertical

• No changes to the vendor-guaranteed emission rate of the engine

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Engine SO2 Source Impacts (Before)

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Engine SO2 Source Impacts (After)

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Cumulative Concentrations The facility must cumulatively

comply with the NAAQS Addressing each individual source

helps as a first cut This scenario still exceeds the 1-hour

NAAQS for SO2 when the sources are taken cumulatively

Haven’t even considered ambient background concentrations

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Modeling Strategies Emissions Strategies

• Actual Distribution of Emissions• Evaluate adequacy of emission limits• Evaluate emissions control options• Evaluate alternate fuels and fuel

specifications• Evaluate alternate raw material

Facility Fence Line Strategies

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Modeling Strategies Stack/Exhaust Strategies:

• Combined source exhausts• Co-located exhaust points to

increase buoyancy• Turn horizontal stacks vertical• Increase stack heights

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Modeling Strategies Temporal pairing approach Plume transport time Surrounding surface

characteristics Wind speed monitor thresholds Mechanical mixing height

considerations Alternative models (e.g.,

CALPUFF)

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Modeling Strategies Use of PTE emissions and AERMOD

can over estimate concentrations Know issues with certain terrain and

meteorological conditions Consider Ambient SO2 Monitoring to

compare to AERMOD results

Ambient SO2 Monitoring

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Ambient SO2 Monitoring What’s involved in conducting an

ambient SO2 Monitoring program?• Who should consider?• Equipment• Sighting Considerations• Pros/Cons• Cost

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Ambient SO2 Monitoring Who Should Consider?

• Facilities that have conducted modeling with unfavorable results, however:

Recommend conducting exploratory monitoring to assess conditions first.

If favorable work with state to develop a approved monitoring plan.

Who Should Not Consider?• Facilities that have conducted modeling

with favorable results.

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Ambient SO2 Monitoring Equipment

Thermo 43i – Pulsed Fluorescence SO2 Analyzer

Thermo 146i – Multigas Calibrator Thermo 111 – Zero Air Supply Air Compressor SO2 Calibration Cylinder Gas Climate Controlled Shelter Co-located Meteorological Tower

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Ambient SO2 Monitoring

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Ambient SO2 Monitoring How do you decide where to site an

ambient SO2 monitor?• Typically sighted using air

dispersion modeling (i.e., AERMOD).

• Should Consider multiple monitors if possible.

Up-wind, down-wind, and other “hot zones” (i.e., building downwash)

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Ambient SO2 Monitoring Pros

Pros• Collection of monitoring data below the

SO2 NAAQS.• Monitoring data could be used to

discount air quality modeling results.• Potentially avoid need for permit limits,

pollution controls, fuel restrictions, or shutting down operations.

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Ambient SO2 Monitoring Cons

Cons• Collection of monitoring data above the

SO2 NAAQS.

• Potential changes to SO2 NAAQS SIP maintenance process.

• Time involved.• Cost

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Ambient SO2 Monitoring Engineering

• $8K – $15K Equipment Cost

• $75K – $100K Installation Cost

• $25K – $50K Operational Cost (Quarterly assurance, data

collection and review)• $25K – $75K

Potential partnering opportunities with “neighbors” to split cost.

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Final Thoughts States developing their modeling plans now States will reach out to request information and/or

modeling Be involved with the SIP process:

• Provide states with good information• Conduct your own modeling (either for the state or

in parallel with the state) Avoid surprises (new limits) at the end of the SIP

process Consider collection of ambient SO2 monitoring data

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Questions?

Dan Dixddix@all4inc.com

(610) 933-5246 x182393 Kimberton Road

PO Box 299Kimberton, PA 19442

All4 Inc.www.all4inc.com

www.enviroreview.com