The Impact of Air Quality Regulations on Distributed ... - NREL

2.5 NOx Control Technologies for
DG
As noted above, NOx emissions are the
primary emission control issue for DG
technologies and there are two primary
approaches to this issue:
• Low-NOx combustion technologies
(inherently low-emitting combustion
and process design)
• Post-combustion NOx control
<strong>The</strong> primary focus <strong>of</strong> emission control for DG
technologies has been inherently low emitting
low NOx combustion technology. This is the
preferred approach to emissions reduction for
the small DG (less than 1MW). <strong>The</strong> cost <strong>of</strong>
post-combustion NOx control technologies
developed for large power plants does not
scale linearly with size and can be
prohibitively expensive for small DG.
However post-combustion control
technologies must be considered, especially
for larger DG
2.5.1 Low-NOx Combustion
Technologies
Most DG technologies apply some form <strong>of</strong>
low-NOx combustion. <strong>The</strong> formation <strong>of</strong> NOx
is primarily linked to a high-temperature
reaction between oxygen and nitrogen in the
combustion air. Low-NOx combustion designs
try to minimize this reaction by limiting the
peak combustion temperature and by
controlling the availability <strong>of</strong> oxygen at times
when NOx could be formed. In many cases,
extra combustion air is used to cool the flame.
This is referred to as “lean” combustion
because there is more combustion air than is
needed to burn the available fuel. In
reciprocating engines these techniques are
referred to as “lean-burn combustion.” Gas
turbine versions <strong>of</strong> this approach include “dry
low-NOx” or “DLN” combustion and “lean
pre-mix” combustion. <strong>The</strong>se techniques
typically include very sophisticated
management <strong>of</strong> fuel and combustion air to
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maintain low emissions and good
performance over a wide operating range.
Another, less developed low-NOx technology
is “catalytic combustion.” This approach uses
a catalyst to flamelessly oxidize the fuel and
release its heat at a temperature below the
NOx formation temperature. Catalytic
combustion has been demonstrated for
combustion turbines and has achieved very
low-NOx levels. <strong>The</strong> cost and durability have
yet to be demonstrated, but some companies
are starting to <strong>of</strong>fer turbines using this
technology.
Though highly sophisticated and complex in
their design, low-NOx combustion systems
are simpler and easier for users to operate and
maintain than post-combustion technologies.
This makes them well suited to DG
applications in which the goal is to minimize
user requirements.
2.5.2 Post-Combustion NOx Control
Technologies
<strong>The</strong>re are several post-combustion
technologies that can be applied to DG units
to reduce NOx. Most <strong>of</strong> these were developed
for larger combustion systems and have
significant operational or cost drawbacks in
DG applications. Nevertheless, they are <strong>of</strong>ten
considered favorably by regulators and need
to be understood in evaluating regulation <strong>of</strong>
DG projects. <strong>The</strong>se NOx control technologies
include the 3-way catalyst (for rich-burn gas
engines) and selective catalytic reduction
(SCR) and SCONOx TM (for lean-burn gas
engines, diesel engines, and combustion
turbines).
<strong>The</strong> 3-way catalyst is the same technology
used to control emissions from automobiles. It
gets its name because it reduces emissions <strong>of</strong>
NOx, CO and unburned hydrocarbons (also
called volatile organic compounds or VOC).
It is a passive technology that is efficient and
relatively simple. However, it can only be