1-hour so2 naaqs implementation modeling
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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Secondary75 ppb Withdrawn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
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