POWER July 2015 - 54

ENVIRONMENTAL ISSUES
Before You Use an Alternative Emissions Factor
There are several points to consider
in developing and using an alternative
emission factor.
In the early stages of permit application
development for combustion units,
applicants should carefully evaluate available
particulate matter (PM) stack test
results for similar combustion units under
similar duties, as well as combustion unit
vendor-provided data, for potential use
in developing representative alternate
emission factors. Such an evaluation will
be particularly important to help demonstrate
that a proposed project is either
below the PM2.5 major source emission
threshold related to permit applicability
or, where an air quality impact analysis
is required, in compliance with the PM2.5
National Ambient Air Quality Standards.
For a source located in a PM2.5 nonattainment
area, the PM2.5 emission factor
EPA developed and made available in 2010 a
comprehensive spreadsheet of revised PM10/
PM2.5 emission factors for the various combustion
units firing natural gas. EPA's alternate
emission factors were 89% to 94% lower
than the AP-42 emission factors.
Shortly on the heels of EPA publication of
the revised NEI emission rates, the Western
Regional Air Partnership (WRAP) incorporated
the EPA-adjusted emission factors in
its Federal Regional Haze Rule evaluation
analyses. The WRAP, formed in 1997, is a
voluntary organization of western states, native
tribes, and federal agencies. The WRAP
Stationary Sources Joint Forum used the
EPA-adjusted emission factors in the development
of baseline and future (2018) PM10
and PM2.5 emissions inventories for assessing
impacts and evaluating emission control
strategies pertaining to regional haze within
its extensive geographic domain. These assessments
were used by WRAP states in
support of their regional haze State Implementation
Plans submitted to the EPA.
Use of Agency-Approved Alternate
PM Emission Factors by Permit Applicants.
At least three permit applicants have
successfully developed and applied alternate
PM emission factors for the PM2.5 emissions
calculations for gas-fired combustion
units. In each case, the NYSERDA/API studies
results were cited as the technical basis
for the development of the alternate emission
factors. These projects are located in New
Mexico, North Dakota, and Texas. Two of the
projects, involving the permitting of combus54
chosen
will affect the overall cost of the
project because it establishes the amount
of emission offsets required with the associated
cost to obtain those offsets. At
the same time, applicants should evaluate
the emission rate to be sure that the required
demonstration of compliance with
this limit using EPA reference test methods
would be reasonably achievable.
Finally, for any required compliance
stack test for total PM2.5 emissions from
a gas-fired combustion source, it is important
that the test be conducted very
carefully, avoiding as best as possible,
contamination in sample collection, processing,
and analysis. Such a performance
test should be conducted using extended
(multi-hour) sampling periods to help
ensure collection of an amount of particulate
that can be measured with better
precision.
tion turbines, are discussed below. (See the
sidebar for issues to consider before taking
this path.)
In 2010, a permit application was submitted
to the New Mexico Environment Department
Air Quality Bureau (NMED-AQB)
for the construction of a natural gas-fired
simple cycle combustion turbine equipped
with selective catalytic reduction at an existing
power plant in Doña Ana County. The
applicant developed/proposed an alternate
PM emission factor for the turbine based on
an analysis of available test data for other,
similar (aeroderivative class) combustion turbines.
The available test data were carefully
screened, with the focus placed on tests with
extended sampling run times where no contamination
issues were noted. The filterable
and condensable components of the alternate
emission factor were 0.0012 and 0.0028 lb/
MMBtu, respectively, for a total PM emission
factor value of 0.0040 lb/MMBtu. The
alternate emission factor was used to demonstrate
that the net emissions increase for the
project did not trigger PSD review for PM2.5.
In June 2011, the NMED-AQB issued the
construction permit for the project.
In November 2012, a permit application
was submitted to the North Dakota Department
of Health (NDDH) for a proposed
combustion turbine project in Morton
County. The applicant developed/proposed
an alternate PM2.5 emission factor for the
combustion turbine based on the EPA's
rationale for deriving adjusted PM2.5
emission factors for the 2002 NEI. The
www.powermag.com
applicant focused on the emission factor
for the condensable PM component, taking
into account the EPA-adjusted factor,
while allowing for the uncertainty/variation
typical of stack test results based on
EPA reference test methods. On this basis,
the applicant proposed a condensable PM
emission factor equal to 50% of the AP-42
emission factor (0.00235 lb/MMBtu). For
filterable PM emissions, the applicant relied
on the manufacturer's guarantee provided
for the turbine. In February 2013,
the NDDH issued the construction permit
for the project with a PM2.5 emission limit
equivalent to the applicant's proposed total
PM2.5 emission rate.
What's Next
AP-42 emission factors significantly overestimate
PM emissions from gas-combustion
units because the historically relied-upon instack
PM testing methodologies were prone
to inaccuracies, yielding results biased high.
The EPA's December 2010 revisions to the
PM reference test methods and its April 2014
interim guidance for the processing of Method
202 measurements were positive steps
towards improving the accuracy of condensable
PM emission estimates.
However, even with these improvements
in the methodology, accurate measurement
of stack PM emissions using EPA reference
test methods will continue to be a challenge
for gas-fired combustion units. Additional
guidance should be available soon,
as the EPA intends to follow up its interim
guidance with issuance of a Best Practices
Handbook for conducting stack testing
for PM emissions in mid-2015. Proposed
modifications to Method 202 could follow
shortly thereafter.
As noted, over the past decade, both regulators
and permit applicants have relied on
the NYSERDA/API studies results to justify
the use of alternate PM2.5 emission factors
for gas-fired combustion units. The use of
alternate PM emission factors is expected to
become more common, driven by the need
for more accurate emissions estimates for
NAAQS compliance demonstrations, state
and federal regulatory planning, and other
regulatory assessments. The improvements
in the EPA's PM measurement methodologies
mentioned in this article should yield
more accurate PM2.5 emission rates and ultimately
contribute to the development of better
alternate emission factors. ■
-Louis Corio (lcorio@zephyrenv.com)
is a senior technical specialist at Zephyr
Environmental Corp., Columbia, Md., and
Karen Olson (kolson@zephyrenv.com) is
a principal at Zephyr Environmental Corp.,
Austin, Texas.
POWER | July 2015
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POWER July 2015

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