POWER May 2012 - 60

AIR EMISSIONS
Managing the Catalysts of a
Combustion Turbine Fleet
Natural gas-fired fleets comprising diverse turbine unit types are operating
their units more these days because of the historic low price of natural
gas. With increased operating hours, fleet owners are challenged to find
the best ways to manage their SCR catalyst systems.
By Terry McTernan, PE, Cormetech Inc.
T
he majority of gas-fired combustion
turbine (CT) fleets have made broad investments
in selective catalyst reduction
(SCR) systems in order to meet emerging air
emissions regulations. Low natural gas prices
have moved these plants to first in the dispatch
queue in many regions of the U.S., displacing
coal-fired plants. In fact, U.S. demand for
natural gas is projected to grow 2.5% per year
through 2035, effectively doubling the amount
of natural gas used for power generation.
Many utilities are responding to the predictions
that the relatively low natural gas prices
we are now experiencing will become a " new
normal " by either retiring selected coal-fired
plants, thereby avoiding billions of dollars in
environmental upgrades, or replacing those
same plants with natural gas-fired equipment.
Some utilities have negotiated with regulators
a " refueling " -the replacement of a coal plant
with a new gas-fired plant (a more efficient
plant with much lower emissions). Others have
" repowered " a plant, where the steam turbine
side of the old plant is retained but the boiler
island is removed and replaced with CTs and
heat recovery steam generators (HRSG). Other
utilities have not experienced load growth over
the past few years and are able to defer the " retire
or reuse " decision, for now.
One of the emerging operational decisions
utilities and merchant generators with a large
fleet of gas-fired combustion turbines must
face in an era when gas plants are running
baseload instead of cycling seasonally is determining
the most economic way to manage
NOx
reduction SCR catalyst systems. In this
article we discuss the process of economically
managing a fleet of SCR-equipped CTs.
Diversity of Units and SCR
CTs burning natural gas are able to achieve NOx
emissions and ammonia slip as low as 2 ppm
when using an SCR catalyst with ammonia injection.
In addition to baseload CTs used in the
combined cycle plants discussed above, CTs are
also used in simple cycle plants that are usually
only called on to operate during periods of high
60
1. Typical combined cycle SCR. The
SCR is a separate section located within the
HRSG assembly where gas temperatures are
optimal for SCR performance, typically 600F
to 800F. Courtesy: Cormetech Inc.
2. Typical simple cycle SCR. The SCR
used on a simple cycle combustion turbine
(CT) is located in an enclosure attached to the
CT exhaust. Courtesy: Cormetech Inc.
electrical demand. Both, when outfitted with an
SCR, present unique plant design challenges.
A more recent trend is to build combined cycle
plants with the capability to operate as either
a baseload or peaker unit, thus presenting new
challenges for both the steam generator design
and the NOx
emission control system.
In a combined cycle plant, the SCR modules
are housed within an inner section of the
HRSG at an optimized temperature location,
typically 600F to 800F. A typical SCR catalyst
bed housing appears as just another section
within an HRSG (Figure 1).
For simple cycle gas turbine applications, the
SCR reactor is located in an expanded outlet duct
immediately downstream of the turbine (Figure
2). The duct size is optimized to accommodate
the SCR catalyst reactor performance. The short
transition section from the turbine outlet to the
SCR inlet poses challenges with the system design.
The turbine exhaust flue gas temperature
is often too hot to be efficiently treated by the
SCR system. Many units rely on the injection
of tempering air to cool the flue gas down to
exhaust temperatures below 900F (±25F). An
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economic evaluation considering a number of
design and operational parameters is performed
to determine if tempering air or a high-temperature
catalyst is the best selection. The evaluation
must consider factors such as the capital and
operating costs, operating hour limits, volume
of catalyst, duct size and back pressure, purge
fan versus tempering air fan cost, cost of the air
distribution equipment, and so on.
For both systems, the SCR catalyst system
requires ammonia to be injected into and thoroughly
mixed throughout the flue gas stream
(Figure 3). To deliver a uniform supply of ammonia
into the flue gas stream, a piping network
or an ammonia injection grid (AIG) is installed
upstream of the catalyst. The rate of ammonia
flow is then regulated across the grid via a series
of control valves. It is critical that the ammonia
concentration within the exhaust gas be
homogenous as it enters the SCR catalyst bed to
prevent excessive slip of unreacted ammonia or,
inversely, areas starved of ammonia, resulting
in localized incomplete NOx
reduction. Based
on analyses of SCR operating data and catalyst
samples, AIG and duct modifications may be
POWER | May 2012
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POWER May 2012

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