3.4 Summary of EPA Regulatory
Framework
As mentioned previously this article
looks mainly at large diesel engines
used
in
emergency
standby
application. Figure 3 shows the EPA
regulatory impacts for this type of
application. If the critical power facility
is large (ie has engines that exceed 500
HP), is located in a non-attainment area
and does full load testing it may
require some form of NOx (NO
2
)
mitigation.
A summary Table of EPA Regulations
associated with large stationary Diesel
engines
used
in
emergency
applications is shown in Table 1.
4.0 Technology to Deal With Air
Emissions from Diesel Engines
For large stationary diesel engines up
to and including Tier 3, engine
manufacturers have adopted many
Clean Air Act (Congress) innovative
technologies that typically focus on in-
cylinder
optimizations.
Looking
beyond Tier 3 much of the focus has
been on exhaust after-treatment
technologies. For diesel engines the
most
common
after-treatment
emission control technologies are:
n
Oxidation catalyst
to deal with CO
and unburned Hydrocarbons
n
Diesel particulate
filter to meet
Particulate Matter (PM) requirements
n
Selective Catalytic Reduction (SCR)
to meet NOx requirements
As mentioned previously, often NOx
(NO
2
) becomes the constraining
pollutant from a NAAQS standpoint.
All diesel engines will also require
some level of exhaust silencing. As a
result a common configuration for
large critical power facilities in non-
attainment areas is to use Tier 2 (for
engines > 752 HP) and Tier 3 (for
engines <752 HP) with SCR and
silencing.
4.1 Oxidation Catalysts and
Particulate Filters
For diesel engines, oxidation catalysts
are often combined with particulate
filters. This can be done by applying
the catalysts (which are usually
Platinum Group Metals) to a
particulate filter. Another common
approach is to have separate oxidation
catalysts upstream of the particulate
filters. The oxidation catalyst creates
heat
(by
oxidizing
unburned
hydrocarbons) and shifts nitrogen
oxide creating a favorable environment
for the particulate filters to regenerate.
4.2 Selective Catalytic Reduction
(SCR)
SCR works by injecting a reductant
(usually 32.5% concentration urea also
known as diesel exhaust fluid or DEF)
into the exhaust stream. The Urea is
converted into ammonia (NH
3
) in the
hot exhaust stream. The NH
3
combines, in the presence of a catalyst,
with the NOx in the exhaust to
produce harmless water vapor (H
2
O)
and Nitrogen (N
2
). Many SCR systems
can achieve NOx reductions of 95% or
higher. Some exhaust after-treatment
vendors offer multi-function systems
that combine SCR, silencing and slots
that can be filled, if required, with
oxidation catalysts and particulate
filters. This gives the Critical Power
Engineer a lot of flexibility – allowing
him to add catalysts and filters late in
the project cycle without impacting the
size of the emissions unit and the
surrounding piping should it be
required for the air permit. Figure 5
shows an SCR system which combines
silencing and other emissions functions
in a single cube mounted on an
enclosure housing a large standby
diesel genset.
35
Table 1
Summary of EPA Regulations for Large Stationary Diesel Engines Used in Emergency Applications
Figure 4
Example of Selective Catalytic Reduction
System with Integral Silencing
