POWER May 2012 - 64

AIR EMISSIONS
manufactured for the unique demands of each
plant, taking into account permit conditions
and operating demands as envisioned for future
years. SCR systems can achieve greater
than 95% NOx
reduction; however, when the
efficiency of the SCR is pushed beyond 85%
NOx
conversion, and/or if outlet emissions
are less than 5 ppm, it becomes much more
sensitive to a number of independent system
parameters.
These parameters include overall catalytic
potential, effective ammonia injection/
mixing into the flue gas stream, flue gas
characteristics for inlet NOx
, velocity, and
temperature distributions. High-efficiency
SCR catalyst system designs can successfully
address these concerns through system
modeling, flow correction devices, enhanced
catalyst volumes, and robust ammonia injection
grid design.
As plants upgrade, repair, or otherwise
modify plant equipment in the future, the
performance environment for reliable emission
control can be affected. Understanding
these potential impacts to the SCR system
is essential and should be carefully studied
before a modification is approved. For example,
components of the ammonia delivery
system may deteriorate over time or lack sufficient
functionality to meet the demands of
running with aging catalyst or tighter emission
criteria.
A properly executed baseline survey conducted
by a qualified catalyst management
provider will serve to fully assess the current
condition of each SCR unit in the fleet. These
SCR system surveys must be site-specific, as
each location will have its unique history and
permitting requirements. The survey should
begin with each site's air permit requirements
and goals (which can vary significantly, based
on the age of the unit, geographic location,
cost of ammonia, and more), identification of
the SCR system supplier and equipment, and
site operating history. Next, site maintenance
records, catalyst test reports, and control room
feedback should be assembled. Finally, a documented
physical inspection of the SCR catalyst
systems is recommended to help verify
the historical records and equipment status.
SCR Management Plan
Following data collection and physical inspections
at the plant, diagnostic laboratory performance
testing may be needed to determine
whether or not the catalyst condition is sufficient
to meet performance requirements in
view of field operating data and system requirements.
It is important to verify that the testing is
completed under conditions that closely match
actual SCR system operating conditions and not
under a set of theoretical or standardized design
conditions. The sample should represent a typical
cross section of the SCR, and the operating
history should be known. A convenient method
for sampling purposes is to incorporate an easily
removable sample tray within the catalyst
module (Figure 6).
The samples in the tray are quickly removed
when the unit is off-line or during
outages. This avoids rigorous drilling of
the catalyst to extract core samples that are
typically not required for units with homogeneous
honeycomb product but may be recommended
for units with alternate products and/
or those that have localized impacts such as a
tube leak. Tests are conducted in a controlled,
laboratory environment on custom-built, validated
SCR catalyst test equipment, allowing
accurate determination of performance and
comparisons of the sample catalytic potential
with that of previously tested elements.
The assessment of field operating data
determines the performance requirements
of the SCR, the SCR operating conditions,
the test conditions for the laboratory performance
test, and the performance threshold.
Changes in the field operating data relative to
previous evaluations may warrant changing
the test conditions or the performance threshold.
An analysis of the field operating data
CIRCLE 39 ON READER SERVICE CARD
64
www.powermag.com
POWER | May 2012
http://www.powermag.com
POWER May 2012

Table of Contents for the Digital Edition of POWER May 2012
