www.all4inc.com | Philadelphia | Atlanta | Houston | Washington DC

Techniques for SO2 Designations

Mark Wenclawiak, CCM | mwenclawiak@all4inc.com | 678-460-0324 June 15, 2015

The Importance of Variable Emissions and Ambient Air

2 Your environmental compliance is clearly our business.

Agenda

Key Distinctions from U.S. EPA SO2 Modeling TAD to Regulatory Modeling Requirements

Comparison of Model Results for Regulatory Permitting Analysis Versus Designation Analysis

Case Study Findings and Resulting Implications of Upcoming Modeling and Regulatory Policies

Comparison of SO2 Modeling TAD to

Regulatory Modeling Requirements

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SO2 Modeling TAD

Actual emission rates from past 3 years

Characteristics can be varied on an hourly basis

Potential for 26,280 hours of different emission characteristics for each emission source

Use of current stack heights

Stack Characteristics

Regulatory Modeling

Potential to emit (PTE)

Can vary emissions rates – but typically not varied hourly (and could result in permit limits)

Potential to model stack heights as GEP, not current stack height

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Varied emissions data is intensive and time consuming

Intermittent sources (e.g., emergency generators) only evaluated if operated enough to contribute to 99th percentile

Actual emissions can be an improvement over allowable emissions

Considerations for TAD Modeled Emission Rates

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SO2 Modeling TAD Distance between receptors

can vary depending on state guidance

Cartesian grid with dense receptor spacing closer to the Facility

Remove receptors from areas where ambient monitors could not be installed (e.g., water, cliff, etc.)

Ambient Air Receptor Placement

Regulatory Modeling

Distance between receptors can vary depending on state guidance

Receptors cover all area surrounding a facility regardless of terrain

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Traditional PSD Receptor Grid

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DRR Receptor Grid

Simplified Case Study Comparison

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Single stack for a coal fired boiler

• 230 foot stack height

• Typical mill building downwash (including a nearby recovery furnace building)

• Standard receptor grid spacing – no terrain

• 140,000 acfm flowrate

• PTE Scenario: 565 lb/hr emission limit

• Actual emissions scenario: Varied coal firing rates, 0.6% sulfur content, fixed flowrate (varied flowrates would impact results but not drastically since flow is boiler loading and flow is fairly constant)

Case Study

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Simple example to highlight:

• Differences due to actual versus allowable emission rates

• Significant concentration gradients and differences between fence line and exposure receptor impacts

2 scenarios

• Constant PTE emission rate (NAAQS)

• Variable emission rate (actual rates from CEMS data)

Case Study

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Use of actual emissions is an improvement over allowable emissions

Emission Rate Considerations

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Case Study Results

When using varying hourly emission rates

• 4th high concentrations are 33% lower than PTE

• 1-hour maximum is 21% lower than PTE

Implications of Upcoming Policies

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Constant maximum emission rate overly conservative

• Compounded by combining with observed monitoring concentrations as background

Higher concentrations during periods of SSM

• May 22, 2015 SIP call rule

Emission Variability Processor (EMVAP)

• Incorporate varying emissions with Monte Carlo statistical technique

• 50th percentile monitored concentration as background

Importance of Variable Emissions

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What 1-hour background concentration do you add to modeled concentration?

• Highest hourly concentration

• 3-year average of 99th percentile of maximum daily (p=0.0001)

• EMVAP and 50th percentile background concentration

Probability of exceedance 0.005; equivalent to 99.5th percentile, compared to 99th percentile form of the standard

Selection of Ambient Air

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Projected final DRR in September 2015

Steps to take:

• Identify the applicability of your facility

• Conduct air dispersion modeling using the three year actual emissions approach outlined in the TAD

• Evaluate options for compliance with dispersion modeling

The idea is to mirror the steps that the agency and U.S. EPA will take

Planning Ahead

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Questions & Open Discussions