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As part of its risk assessment the EPA
allows emergency engines to meet
somewhat lower standards than non-
emergency ones because of the lower
annual operating hours. The definition
of emergency can be relatively
complex. Clearly a utility outage is an
emergency condition. There are
specific definitions for voltage and
frequency variations for electricity
reliability that can also constitute an
emergency situation. In general a total
of 100 hours per year is allocated to
emergency
generators
for
maintenance and testing. Of these 100
hours the EPA currently allows up to 50
hours to be used for demand response
programs in some jurisdictions – but
this aspect is currently under review
and may be removed. There are no
restrictions on the number of run hours
for the engine when it is being used
under emergency conditions.
The overall EPA regulatory framework
that can impact stationary diesel
gensets is shown in Figure 2.
3.1 EPA Regulatory Framework
NAAQS
Maintaining a National Ambient Air
Quality Standard (NAAQS) is a
fundamental concept of the Clean Air
Act. NAAQS are based on limits that
are designed to ensure healthy air
quality of all citizens regardless of
where in the country they live. As part
of NAAQS the EPA defines 6 “criteria”
pollutants. The modern lean burn
diesel
engine
has
improved
dramatically in recent years but can still
contribute significantly to 3 of the
criteria pollutants – NO
2
, PM and CO.
NO
2
is one of the constituents of NOx.
NOx formation is largely a function of
combustion temperature. Typically
higher combustion temperature results
in a higher level of NOx formation.
Particulate Matter (PM) is also a
function of combustion temperature.
Typically
higher
combustion
temperature results in less PM
formation. As a result undesirable NOx
and PM formation act in opposing
directions when engine designers are
looking at combustion temperature.
Engine efficiency also typically
improves at higher combustion
temperatures which is another
important consideration for engine
designers. Carbon Monoxide (CO) is
often a reflection of incomplete
oxidation of fuel in the combustion
chamber. Most major diesel engine
manufacturers have optimized their
combustion processes to such an
extent that often CO regulatory
requirements are not a constraint.
The required targets and the timetable
for NAAQS implementation are always
changing and apply to each of the
criteria pollutants. The EPA goes
through a public consultation process
to establish the required NAAQS levels
for each criteria pollutant. The US is
divided into a set of “areas” and the
EPA performs measurements of the
criteria pollutants in each area. Areas
which do not meet the NAAQS targets
for criteria pollutants are deemed
“non-attainment” areas. For each non-
attainment area the affected State is
required to prepare a State
Implementation Plan (SIP) to resolve
the issue and achieve attainment. A
special case is the US northeast (Maine
to Northern Virginia). Because this air
shed is highly populated, it has more
stringent air quality standards. The EPA
calls this area the Ozone Transport
Region (OTR).
When seeking an air permit for a new
diesel emergency generator, if there is
a NAAQS issue it will most likely relate
to NO
2
. In 2010 the EPA proposed
limits which are based on an hourly
worst case of 100 ppb (parts per
billion). It is not uncommon, during
certain times, for background
concentrations in non-attainment areas
to be high enough that very little NO
2
needs to be added to make an
installation exceed the limit. Prior to
2010 the NO
2
limit was based on a
yearly average.
By mid-2013 each State was to have
submitted a SIP for its non-attainment
areas with respect to NO
2
. When a
major data center, hospital or other
installation installs significant capacity
of new diesel standby generators the
typical hourly worst case occurs during
the full load test of the units. Modeling
is done of the site, typically using the
EPA’s
AERMOD
(Atmospheric
Dispersion
Modeling)
System.
AERMOD is a mathematical simulation
of how pollutants will disperse into the
atmosphere. The modeling takes into
account the topography of the site, its
major emissions sources, prevailing
wind conditions and other factors
which could lead to worst case
conditions.
Figure 2
Overall Regulatory Framework
