General Provisions: MACT Determinations - Further Methods to Compute the MACT Floor

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The purpose of this section is to provide details on three different methods and a technology approach that can be used to determine the MACT floor (minimum level of control) for an applicable source.  As future MACT standards are proposed or promulgated for the various source categories, more methods for determining the MACT floor could be developed.

Method 1 - Computing the MACT Floor Using Existing State and Local Regulations

The steps for computing a MACT floor using this method are as follows:

Step A: Conduct a geographical survey.
Determine the total number of existing similar emission units in the source category or subcategory, and conduct a survey to determine the geographical location of these similar emission units. Group the emission units according to the State or locality in which they are located.

Step B: Review State or local air pollution regulations.

Review the different State or local air pollution control regulations that are applicable to the emission unit in each State or locality where an emission unit is located.

Step C: Rank the State or local air pollution regulations.

For the State and local regulations identified in Step B, rank the regulations in order of stringency. The regulations that require the greatest level of control should be listed first.

Step D: Rank emission units.
Determine the total number of emission units and the number of emission units complying with each stringency level. Based on the level of regulation stringency, rank the emission units in order from most stringent to least stringent.

Step E: Make a MACT floor finding.
Based on the distribution of sources in the various States and the stringency of the respective State requirements, it may be possible to construct a database that would support a MACT floor determination. Note that a determination must also be made that sources in the States actually achieve the required control levels.

Method 2 - Computing the MACT Floor Using Control Efficiency Ratings

To use this method to calculate the MACT floor, the permitting authority will evaluate emission units that use add-on control devices or other methods whose HAP control efficiencies have been clearly demonstrated in practice. The MACT floor and MACT emission limitation can be computed as follows:

Step A: Determine HAP emission reduction efficiency for each control device.

For each emission unit in the source category or subcategory, the ability of each control technology to reduce HAP emissions should be determined as a percentage of reduction efficiency. Acceptable methods for determining the efficiency
rating are:

1. Federal and State enforceable permits limits on operation of the control technology, where compliance has been demonstrated;
2. Actual reported efficiencies. 

In addition vendor data of demonstrated performance achieved in similar service may be used in conjunction with good engineering judgment.

Step B: Calculate the MACT floor using the methodology on page 4 of this section.

Method 3 - Computing the MACT Floor Using Emissions Reduction Ratios

The emission reduction ratio is a fraction of uncontrolled emissions to controlled emissions. The MACT floor is computed using the emission reduction ratios. To compute the emission reduction ratio for each emission unit, the permitting authority
must review emissions data or other information to determine uncontrolled and controlled emissions levels for these units. The step-by-step process is detailed below.

Step A: Compute an uncontrolled emission level for each emission unit.
The uncontrolled emission level for an emission unit is the maximum amount of HAP that could be emitted from the emission unit using current design specifications at full capacity utilization in the absence of controls.

Step B: Compute a controlled emission level for each emission unit.
The controlled emission level is the maximum amount of HAP that could be emitted from the emission unit under the source's current design specification and at
full capacity utilization taking into consideration the application of federally enforceable controls. Ideally, a controlled emission level should be computed for all emission units, even when a single uncontrolled emission level is used. However, if only general information is known about the types of control technologies that are being used in practice, a controlled emission level could be estimated for each control scenario. Then a controlled emission level for each emission unit would be assigned based on the types of controls that major sources use.

Step C: Compute the emission reduction ratio for each emission unit.

The emission reduction ratio for each emission unit can be computed using the following formula:

                        Uncontrolled Emission Limit - Controlled Emission Limit
                                             Uncontrolled Emission Limit

Step D: Determine the MACT floor using the methodology on page 4 of this section.

Technology Approach

The technology approach is used when insufficient emissions data are available to determine an average emission limitation. Under this approach, EPA determines which technology is being used by the average of the best performing 12 percent of sources in the category, and then determines the average emission limit that this technology is capable of achieving in practice on a continuous basis. Available emissions data are used to assign a performance value for each emission control identified (percent removal, outlet grain loading, etc.). The MACT floor calculation is performed based on these performance values. Typically, a median is used rather than the arithmetic average since an arithmetic average generally would not correspond to any given control. The following example illustrates this approach.

A source category emitting metal HAP is comprised of 500 sources.  A survey of the sources finds that 300 facilities use  cyclones to control HAP emissions, 150 facilities use wet scrubbers, and 50 facilities use fabric filters. Based on available emissions data, it is determined that cyclones are 25-percent efficient at removing HAP emissions, wet scrubbers are 75-percent efficient, and fabric filters are 99-percent efficient. The best controlled 12-percent of sources would include 10 sources with wet scrubbers and 50 sources with fabric filters. The median corresponds to fabric filters. Therefore, fabric filters would be identified as the MACT floor technology, and an emission limitation would be set based on the available performance data for fabric filters.