SCR Systems
Compliance
with the stringent emissions limits commonly requires the use of
SCR systems on large fossil-fired plants. For boilers and combined
cycle combustion turbines, an SCR system can be installed in existing
boiler ducting (duct-SCR) or in an external reactor. The duct-SCR
is substantially more cost-effective, and thus is favored in most
applications. A typical application of a duct-SCR and
a typical application of an external SCR reactor is shown below. Both photos show
units retrofitted by AUS on a turnkey basis.
Duct-SCR
Installation on 180 MW Boiler
External
SCR Reactor on 480 MW Boiler
SCR
Reactor
The SCR reactor is typically a steel fabricated assembly with structural
supports, ladders and platforms, catalyst removal doors, and catalyst
removal hardware. Depending upon the size of the system, the reactor
can be provided in one-piece, multiple sections, or flat panels
for field erection.
The
reactor typically includes an internal framework to support the
catalyst modules. Often the framework can be made to accommodate
additional catalyst modules to provide a future additional increase
in NOx removal capabilities.
For
high-temperature applications, such as simple cycle combustion turbine
exhausts, the reactor and associated transition ducts are fabricated
from carbon steel and supplied with internal insulation and stainless
steel liners. In these cases, stainless steel or high alloy steel
is used for the catalyst support framework.
Typical
Catalyst Modules
SCR
Reactor Being Installed in a HRSG
SCR
Process Description
AUS can supply SCR systems using aqueous NH3, anhydrous
NH3, or the urea-to-NH3 process to suit the
client’s preference and local regulatory requirements. This
section describes the SCR process based on using aqueous NH3
in a typical boiler application.
Two
(2) 100 percent capacity high temperature fans, one operating and one
spare, extract hot flue gas or preheated air from the boiler ducts
and boost the pressure so that this gas can be used for the evaporation
and injection of the NH3. The fans are sized to provide sufficient
flow to ensure that the vaporizer outlet temperature remains high
enough to prevent localized condensation.
Hot
flue gas or air from the operating fan flows through a common plenum
into a vaporizer. The flow of NH3 solution (normally
19 or 29 wt. % NH3) into the vaporizer is regulated in
proportion to the amount of NOx to be removed by the
SCR. NH3 solution is injected through a set of air-atomized
nozzles into the vaporizer. The small diameter droplets produced
by these nozzles quickly vaporize and mix with hot diluent gas before
the gas stream exits the vaporizer.
The
mixture of NH3 and diluent gas is mixed with flue gas using an injection
grid at the economizer outlet and flows through the SCR reactor.
The gas from the SCR system then flows to the boiler air heater
to be discharged to the stack.
SCR
System NH3 Vaporizing and Metering Skid
Installed
NH3 Vaporizing and Metering Skid
The
injection grid is generally divided into multiple horizontal and
vertical regions. Individual valves for each region permits NH3/diluent
gas flow rate to be adjusted during initial commissioning to optimize
NOx reduction and to account for duct top-to-bottom and
side-to-side NOx stratification.
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