Document ID: EPA-HQ-OAR-2002-0086-0014
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2008-04-02T04:00Z

MEMORANDUM

To:	Docket

From:	John Schaefer, EPA/OAQPS/SPPD

Date:	August 17, 2007

Subject:	Combined HAP Process Vent MACT Floor for Semiconductor MACT
Supplemental Proposal

INTRODUCTION

	This memorandum describes the methodology performed to determine the
maximum achievable control technology (MACT) floor for hazardous air
pollutant (HAP) emissions from existing and new major combined hazardous
air pollutant (HAP) process vents in the semiconductor manufacturing
source category.  A combined HAP process vent is a process vent
containing both organic and inorganic HAP.  A typical organic HAP used
in the semiconductor industry would be xylene, examples of inorganic HAP
used include methanol, hydrochloric acid and hydrogen fluoride.

	The Semiconductor manufacturing industry typically segregates all
process emissions either organic or inorganic HAP streams.  This is done
for several reasons, the primary environmental reason is that organic
process vent streams can easily be controlled by combustion devices
(incinerators), and inorganic streams can be readily controlled by wet
scrubbing technology.   A combined hap process vent stream, especially
one that combines process heat is much more difficult to control,
especially since the process heat must be removed or the stream must be
cooled in order to utilize wet scrubbers to control inorganic HAP. 
During the development of the Semiconductor Manufacturing NESHAP, we
developed separate control requirements for inorganic and organic HAP. 
At the time all of the manufacturing we examined exclusively separated
all emission streams into organic and inorganic constituents.

After promulgation of the NESHAP in May 2002, it was brought to our
attention that one plant of which we are now aware has a different
process vent approach system.  Specifically this plant combines
inorganic and organic vent streams into a single atmospheric process
vent.  At the time we developed the MACT standard, we were not aware of
any sources that combined their inorganic and organic vent streams, and
therefore we had no data on such sources.  Rather during the development
phase of the rule, we determined that since 1980 industry practice has
been to strictly separate process vent emissions into streams containing
either organic or inorganic HAP.    Therefore, we have developed a new
MACT floor based on combined HAP process vent information obtained after
the promulgation of the final rule.

	The MACT floor analysis is based on emission estimates for four process
vents.  These four process vents are the only combined HAP vents that we
are aware of in the semiconductor manufacturing industry.

METHODOLOGY FOR DETERMINING THE MACT FLOOR

	For standards established under section 112(d) of the CAA, the minimum
level of required control is defined as the “MACT floor.”  For new
sources, emission standards “shall not be less stringent than the
emission control that is achieved in practice by the best controlled
similar source.”  For existing sources, the emissions standards must
be at least as stringent as either “the average emission limitation
achieved by the best performing 12 percent of the existing sources,”
or “the average emission limitation achieved by the best performing
five sources” for categories or subcategories with less than 30
sources.  Since there are only four identified combined HAP process
vents in this industry the MACT floor is the average emission limit
achieved by the four vents.

A.  Definition of “Average”

	The minimum level of control defined under section 112(d) of the CAA
(“. . . the average emission limitation achieved by the best
performing 12 percent of the existing sources. . .” and for a source
category with fewer than 30 sources “. . . the average emission
limitation achieved by the best performing five sources. . .”) is
commonly referred to as the MACT floor.  The term “average” is not
defined in the CAA.  In a Federal Register notice published on June 6,
1994 (59 FR 29196), the EPA announced its conclusion that Congress
intended “average,” as used in section 112(d)(3), to mean a measure
of mean, median, mode, or some other measure of central tendency.  The
EPA concluded that it retains substantial discretion, within the
statutory framework, to set MACT floors at appropriate levels, and that
it construes the word “average” (as used in section 112(d)(3)) to
authorize the EPA to use any reasonable method, in a particular factual
context, of determining the central tendency of a data set.

	In addition, the EPA stated it has discretion to use “best
engineering judgement” in collecting and analyzing data relevant to a
MACT floor determination and in assessing the data’s
comprehensiveness, accuracy, and variability, in order to determine
which sources achieve the best emission reductions.  The EPA also has
discretion in determining the appropriate “average” in each source
category (see 59 FR 29199). 

B.  Process Vent Emissions

	In developing a limitation for the semiconductor manufacturing process
it is important to understand that a semiconductor manufacturing plant
does not achieve a pseudo steady state of operations like typical
manufacturing operations.  This is because the facility is a collection
of tools tied into a process vent.  An example of this might be that if
100 tools are tied into a vent, it might be possible to utilize a
maximum of 30 of the tools at any given time.  However, due to the
complex nature of semiconductor manufacturing operations the plant could
be at full operation and not utilizing any of the tools at a given
instant due to the complex nature of production scheduling.  

To further illustrate this example, If the tools emit 20 parts per
million volume (ppmv) of HAP when all 30 tools are utilized the maximum
process vent emissions reached will be 20 ppmv.  However, at any given
moment none of the tools could be being utilized during the production
process so the process vent emission could be 0.  Therefore, for this
example at any given time all that can be truly known about process vent
emissions is that they are between 0 and 20 ppmv HAP.  For the purposes
of the MACT floor analysis we have decided to use the maximum
representative operating conditions to estimate the process vent
emissions.  We believe this is a reasonable assumption since any
controls utilized should be designed to control emissions at the maximum
representative operating conditions.

C.   MACT Floor

We obtained data for four combined HAP process vents operating at
maximum representative operating conditions.  This means the HAP
concentrations represent the maximum HAP emissions achieved by the
process vents under real world conditions.  The emission estimates
utilized are based on the highest emitting combination of tools run
during an actual production cycle.  The HAP emission concentrations for
organic, inorganic and total or combined HAP are presented in Table 1.

Since, none of these process vents are controlled, the emission limit is
not based on the concentration of HAP after control by the “best
performing” control device.  Rather it is based on the practical
emission limit achieved according plant operation.  For subcategories
with fewer than 30 sources the MACT floor is based on the average of the
best performing five sources.  Therefore, we based the MACT floor on the
average emission limitation achieved by the four combined HAP process
vents in our database.   As shown in table 1, three of the process vents
are lower than the average and one process vent is higher than the
average.

Table 1 Emission Point HAP concentrations (ppmv)1

EP #	Organic HAP	Inorganic HAP	Total HAP

1	7.15	2.03	9.18

2	11.06	24.52	35.58

3	0.06	0.02	0.08

4	8.47	3.57	12.04

Average	6.69	7.54	14.22

D.  Consideration of Regulatory Alternatives Above the MACT FLOOR

	Regulatory options considered above the floor would set an emission
limitation that would require the utilization of add on control devices
to meet the standard.  After conversations with industry representatives
we believe that MACT floor can be met by changes in facility operations
(e.g. permit limitations on equipment operation and production).  In
order to set a limitation that would require the use of add on control
devices a limit must be selected that could not be met by accepting
production limitations.  This is not easily done for an industry where
the variability in emissions is as great as it is for semiconductors.
However, we believe that applying the requirements for organic and
inorganic HAP streams to combined vent streams would require the
utilization of add on control devices.  

The promulgated MACT rule required process vents containing organic air
toxics to reduce emissions by 98 percent or to below 20 parts per
million by volume, while inorganic process vents are required to reduce
emissions by 95 percent or to below 0.42 parts per million by volume. 
Requiring these limits would require the control of inorganic HAP
emissions.  In practice this could be achieved by segregating the
organic and inorganic HAP constituents and segregating the process heat
from the inorganic HAP streams.  Segregation of process heat is
necessary since added process heat makes the utilization of wet scrubber
technology problematic.  After the necessary duct work and conveyance
system had been installed a wet scrubber could then be installed to
control inorganic HAP emissions.  

A 1999 engineering report, conducted on the Kodak Semiconductor fab in
Rochester, NY estimated that it would cost approximately $4,000,000 to
control all nine process vents at the facility we have identified with
the combined hap process vents.2  The estimated cost in the report was
based on a flow rate for all vents of 85,000 CFM, the maximum gas
flowrate that the system could produce.  In order to develop a new
estimate, we proportioned the cost based on the ratio of average gas
flowrates for the sources requiring control, 40,000 SCFM is the average
gas flowrate of the three emission sources identified in Table 2, while
the average flow rate for all vents is approximately 68,000 SCFM.3  The
actual design was based on maximum flowrates, which were about 25%
higher than the actual flow rates.  However, the ratio used for scaling
the estimate should be about the same whether maximum or average
flowrates are used.  So the costs were based on scaling the costs
associated with 68,000 SCFM down to 40,000 SCFM.  This means that a $4
million costs would scale to approximately $2.4 million.

Table 2 Sources Requiring Control if alternative MACT Floor Used3

EP #	Lb/yr Inorganic HAP	Air Flow (SCFM)

1	114	18,294

2	241	11,840

4	257	10,290

Total	612 (0.306 tpy)	40,424

Table 3 Sources Requiring Control if alternative MACT Floor Used

Cost	Cost ($ per ton)

Cost to control 3 sources listed in Table 23	$2.4

Cost Per Ton	> $7.5

  

1 Memorandum from Gregory A. Smith, Brian D. Sopko, Roy W. Wood, Eastman
Kodak Company, Rochester, NY. To John Schaefer, USEPA, Research Triangle
Park, North Carolina. Semiconductor MACT - Combined HAP Emissions. April
18, 2007.

2 Kodak Building 81 South, Upgrade Exhaust Systems Final Technical
Report. Erdmund Anthony, Consulting Engineers.  August 1999.

3 Memorandum from Kodak to John Schaefer, Combined HAP Emission
Calculations & Cost of Control Data Pursuant to NESHAP for Semiconductor
Manufacturing (40 CFR Part 63 Subpart BBBBB), August 28, 2007.