POWER October 2013 - 69

EMissions
2. Uneven conditions. This CFD profile of baseline velocity results in the SCR hood area
shows widely variable airflow and areas of recirculation behind the turning vanes, resulting in
ash dropout. Courtesy: Fuel Tech
achieve an even velocity distribution while
the straightening grid below straightened
the flow direction.
The turning vane system requires exact
spacing and angling of turning vanes during
SCR construction to ensure required flow
distributions are met. This solution is also extremely
sensitive to changes to the upstream
flow distribution, and any changes to the system
require remodeling and retuning of the
vanes to maintain the required distributions.
In contrast, the GSG combines the turn3.
Straight and even flow. The new design produced vertical, non-recirculating, uniform
velocity to the catalyst. Courtesy: Fuel Tech
ing vanes and straightening grid into a single
sloped grid. The GSG has been shown to be
a robust flow corrective solution in a number
of previous installations, as it is much less
sensitive to upstream flow distributions compared
to traditional solutions. This means
that the catalyst and catalyst performance are
protected even when the unit is not running at
optimum design conditions, including economizer
bypass operation.
The CFD model results showed that installation
of the GSG corrected the large
flow recirculation zones, and flow was
vertical out of the GSG. However, the flow
statistics were not within the project team's
target of +/-15% of arithmetic mean velocity.
The SCR hood truss/gussets and egg
crate structural steel were creating large
flow velocity variances before the first
catalyst layer.
The CFD model was reanalyzed, considerinstallation.
A planned boiler inspection outage
was extended to accommodate the work.
Design of the new flow distribution devices
for the SCR hood began in July 2012, with
the outage scheduled for October. With the
short time frame for design and fabrication,
the contracted services were secured for flow
modeling, structural engineering, fabrication,
and construction installation.
Black & Veatch was contracted to provide
design drawings and specifications, review
the flow modeling and proposed flow distribution
devices, and evaluate constructability.
The entire team participated in preliminary
flow model result review meetings to provide
immediate input to flow modeling and
direction to engineering. Probable flow distribution
devices were detailed by engineering
to obtain construction installation bids.
Catalyst installation was planned for Layers
2-3-4 to allow Layer 1 to be a work platform
for the SCR hood modifications. Flow
October 2013 | POWER
device fabrication was performed off-site at
fabrication shops to speed production. Fabrication
was scheduled to continue through
the start of the October outage because demolition
would occur before new materials
would be needed.
A CFD modeling study analyzed the removal
of the SCR hood turning vanes and
replacement with Fuel Tech's Graduated
Straightening Grid (GSG) technology. The
GSG device consists of parallel plates installed
in the SCR hood on the diagonal, to
turn the flue gas and fly ash vertically into the
first catalyst layer.
GSG technology improves on traditional
SCR airflow design in several ways. Until
recently, controlling the velocity distribution
and flow direction into the face of the
first catalyst layer was accomplished with
large turning vanes along with a straightening
grid placed immediately above the
catalyst. The turning vanes were tuned to
www.powermag.com
ing three scenarios: removal of the two trusses,
removal of the egg crate support steel, and
removal of both truss and egg crate. Removal
of all internal structures had the best results,
but removal of the egg crate steel had a nearly
identical result without the expense of replacing
the SCR hood truss system. However, the
flow statistics were still not close enough
to the recommended +/-15% of mean. The
SCR inlet flue contraction and truss/gussets
were both working to push flow to the SCR
inlet corner and away from the back half of
the SCR reactor casings at the division wall.
The CFD model run was used to tune a variable
perforated plate across the leading edge
of the GSG blades at several positions. This
variable perforated plate prevented excess
flue gas from making the quick turn into the
front of the reactors and pushed more flow
past the truss/gusset system to the back half
of the reactors.
The final modification arrangement included
demolition of the turning vanes in the
hood of the SCR reactors and demolition of
the original egg crate flow straightener, perforated
plate, and its support steel.
Installation
The conceptual design developed by Fuel
Tech in the flow model was then developed
69

http://www.powermag.com

POWER October 2013

Table of Contents for the Digital Edition of POWER October 2013