Walt
Stevenson/
RTP/
USEPA/
US
04/
18/
2006
09:
00
AM
To
edmond_
toy@
omb.
eop.
gov
cc
Rick
Vetter/
RTP/
USEPA/
US@
EPA,
RobertJ
Wayland/
RTP/
USEPA/
US@
EPA,
Cathy
Coats/
RTP/
USEPA/
US@
EPA
Subje
ct
UCL
for
CEMS
vs
stack
test
­­­­­
Edmond
Below
is
the
new
preamble
text.
Let
me
know
what
you
think.

Walt
H.
What
emission
variability
factor
is
appropriate?

Although
most
commenters
and
EPA
used
similar
statistical
methodology,
differences
were
identified
in
assumptions
used
to
develop
emission
variability
factors
(
in
particular,
the
upper
confidence
limit
(
UCL)).
The
UCL
addresses
how
often,
on
average,
one
would
estimate
that
an
emissions
level
may
exceed
a
certain
value
(
the
standard).
For
analysis
of
CEMS
data,

such
as
sulfur
dioxide
or
nitrogen
oxides,
where
365
tests
(
24­

hr
CEMS
average)
are
conducted
per
year,
EPA
and
commenters
agreed
the
emission
limit
should
be
set
at
a
level
that
would
be
expected
to
be
exceeded
only
once
per
year
at
a
well
operated
MWC
plant.
Once
per
year
translates
into
a
99.7
percent
UCL
(
364/
365
=
0.997
or
99.7
percent).
A
number
of
commenters
suggested
the
use
of
a
99.7
percent
UCL
for
development
of
limits
using
both
CEMS
data
(
sulfur
dioxide
and
nitrogen
oxide)
and
stack
test
data
(
cadmium,
lead,
mercury,

particulate
matter,
dioxin,
and
hydrogen
chloride).
This
is
one
area
where
EPA
disagrees
with
these
commenters.
EPA
concludes
a
different
assumption
is
appropriate.
For
stack
test
emission
limits,
EPA
used
a
different
and
lower
UCL.
This
is
the
same
approach
EPA
used
at
proposal.
Analysis
of
data
to
estimate
emission
limits
to
be
enforced
by
stack
test
methods
must
be
done
using
a
different
approach
than
where
enforcement
is
to
be
based
on
CEMS.
Historically,
for
stack
test
data,
EPA
used
its
judgement
to
select
appropriate
emission
limits
in
consideration
of
emissions
variability
over
a
wide
range
of
operating
conditions,
and
consideration
of
the
limitations
of
compliance
determination
by
infrequent
stack
testing.
For
this
rulemaking,
EPA
moved
a
step
forward
using
statistical
methods
to
aid
in
estimating
appropriate
emission
levels
for
stack
test
compliance.
The
UCL
for
estimating
emission
limits
enforced
by
infrequent
stack
testing
must
also
reflect
a
reasonable
consideration
of
emissions
variability
and
compliance
limitations
of
stack
testing.
Based
on
EPA's
experience,
EPA
concluded
an
UCL
of
99
percent
was
appropriate
to
estimate
achievable
emission
levels
for
emission
limits
enforced
by
stack
testing.
Therefore,
just
as
done
in
the
December
19,
2005
proposal,
EPA
continues
to
use
a
99
percent
UCL
for
estimating
emission
limits
to
be
enforced
by
stack
testing
and
99.7
percent
UCL
for
estimating
emission
limits
to
be
enforced
by
CEMS.
The
commenters
did
not
provide
any
persuasive
information
for
the
use
of
a
99.7
percent
UCL
for
both
CEMS
and
stack
test
compliance
methods.
