Selective Catalytic Reduction (SCR) for the removal of NO x (NO and NO 2 ) This primer includes: • SCR and Catalyst Basics • SCR Design Considerations • Catalyst Management Trade names and companies mentioned are for illustration and clarification purposes but not for endorsement. 10 Commerce Drive Pelham, AL 35124 Phone(205)453-0236 Facsmile(205)453-0239 www.innovativecombustion.com 1
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Selective Catalytic Reduction (SCR)
for the removal of NOx (NO and NO2)
This primer includes:
• SCR and Catalyst Basics
• SCR Design Considerations
• Catalyst Management
Trade names and companies mentioned are for illustration and clarification purposes but not for endorsement.
10 Commerce Drive Pelham, AL 35124
Phone(205)453-0236 Facsmile(205)453-0239
www.innovativecombustion.com
1
SCR and Catalyst Basics
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Catalysis: a modification and especially increase in the rate of a chemical reaction induced by material unchanged chemically at the end of the reaction.
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Basic chemical reactions:
4NO + 4NH3 + O2 4N2 + 6H2O
2NO2 + 4NH3 + O2 3N2 + 6H2O
NO + NO2 + 2NH3 2N2 + 3H2O
The key reductant is ammonia, the ammonia molecules need to be thoroughly mixed with NOx in the
flue gas, this is essential for high NOx removal efficiency for all SCR systems. Thus, the mixing
system and ammonia injection grid design is closely related to removal efficiency. In general cold flow
modeling and Computational Fluid Dynamics (CFD) modeling are conducted to ensure that the system
has the least amount of maldistribution and ash dropout for a range of conditions. Due to
maldistribution in NH3 and NOx, a minute amount of NH3 in single digit ppm will exit the catalyst
layers, this is usually referred to as ammonia slip or slip.
CHEMISTRY
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SCR catalyst Economizer gas outlet with NOX
Ammonia (NH3) mixed with NOX
Exit consists of N2 + H2O.
Minor unreacted ammonia
/slip
Ammonia (NH3)
Ammonia Injection Grid (AIG)
NH3 reacts
with NO on
an active site
Typical SCR Process and Schematic
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Most commercial SCR catalysts are referred to as titania-vanadia base catalyst (since the 1970’s). Basic
material is similar to ceramic; the main component is titanium oxide (TiO2) with minor components such as
tungsten. As for the SCR reaction, the active sites are vanadium oxides (V2O5, V2O3).
There are many physical forms: homogeneous (extruded honeycombs), and heterogeneous (plates, corrugated).
In addition there are different pitches and/or cell openings for different applications (cleaner low dust flue gas
vs. high dust solid fuel boilers.)
The catalysts are packaged at different depths in the direction of gas flow. Some applications require multiple
layers of catalyst to meet the removal efficiency and expected operating life. Catalysts are sold by volumes in
cubic meters (m3). However, the effective specific geometric surface area is used for catalyst design, expressed
in m2 per m3. The total surface area, Acat in m2 is derived from specific area (m2 per m3) times total catalyst
volume (m3). This Acat is also used to derive the Area Velocity, AV (m/hr), a key design parameter and for
activity calculation. AV is defined as Flue Gas Flow rate in standard condition divided by Acat. Note: Internal surface area (pore volume) and pore distribution varies for different manufacturers, the
standard measurement is the BET surface area, m2/gm of material. [Brunauer–Emmett–Teller
theory/model]. BET surface area is used for quality control as well as for determining the exposed
catalyst sample condition.
SCR Catalysts
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In addition to physical differences, there are ranges in activity which is reflected by the bulk concentration of
vanadium in the formulation. High V concentrations will have higher activity and NOX removal efficiency for a
given catalyst volume, however, due to the concomitant oxidation reaction, a small amount SO2 is converted to
SO3. In general the ammonia slip will react with SO3 and form a sticky salt, ammonia bisulfate (ABS) due to its
high melting point. This is especially troublesome for downstream heat exchange surfaces (lower bulk flue
gas/metal temperature) such as air heater baskets and finned tubes.
Note: ABS reaction – NH
3 + SO
3 + H
2O (NH
4)HSO
4 (ABS) ; usually under [NH
3] << [SO
3]
SCR Catalysts, contd.
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SCR Catalyst Types and Physical Configurations
HETEROGENEOUS HOMOGENEOUS
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Plate
(Hitachi)
Rolled
Coated
Honeycomb (extruded)
(Cormetech, Ceram)
Extruded
Coated
Corrugated
(Haldor Topsoe)
Composite
Hybrid
SCR Catalyst Types and Physical Configurations
HETEROGENEOUS HOMOGENEOUS
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Plate-type structure
Flexible plates
Rectangular opening
Wall thickness: 0.6 to 0.8 mm
Pitch: 5 to 7 mm
Plate Pitch - center line to center line from one plate or wall to the next.
Honeycomb structure
Rigid
Square openings
Wall thickness: 0.4 to 0.9 mm
Pitch: 2 to 9.2 mm
Hybrid plate-type structure
Rigid
Corrugated openings
Wall thickness: 0.4 to 1.1 mm
Pitch: 2 to 12 mm
pitch pitch
SCR Catalyst Pitch
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Dust Load
gr/dscf
Plate Pitch Honeycomb Pitch Corrugated Pitch
< 2 5.0 mm 6.7 mm (22 cell) 5.8 mm
2 to 6 5.5 mm 7.4 mm (20 cell) 7.2 mm
6 to 10 6.0 mm 8.2 mm (18 cell) 8.3 mm
10 to 12 6.2 mm 9.2 mm (16 cell) 9.3 mm
> 12 6.5 mm NA > 9.3 mm
12 to 16 6.5 mm NA 10.3 mm
16 to 20 6.5 mm NA 12 mm
SCR Catalyst Pitch and Dust Loading
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• Catalyst elements arranged and packed in steel frames. o Plate – 2 levels of 8 element boxes
o Honeycomb – 72 monoliths
o Corrugated – 2 to 3 levels of 8 element boxes.
• Standardized cross-section module o Possible to interchange corrugated and
plate element boxes in most modules.
• Possible to interchange catalyst types within reactor
• Module height varies with the catalyst monolith heights, different for different catalyst suppliers.
• Most applications have a top grid/mesh designed onto the modules.
SCR Catalyst Blocks and Modules
SCR Design Considerations
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SCR Design Parameters
Both new and retrofit applications have the same basic design criteria and considerations.
However, some retrofits may have more stringent and available height, width and depth as well
as access to the reactor.
Projects are structured in many fashions, e.g., SCR system suppliers, EPC contractor, A/E firm as
sole owner engineer, A/E firm as partner, catalyst supplier and manufacturer as process design
engineer and catalyst supplier, catalyst manfacturer as supplier only with pass through guarantees
by A/E firm.
Most SCR guarantees are for a specific NOX removal efficiency (ETA, η) and/or Stack NOX
emissions for a certain cumulative operating hours. In general this is called ‘life’, even though
the catalyst proper is still capable of reducing NOX but not at a high level at the design
• Reactor size – layers, catalyst depth, modules per layer
• Optimize effective cross-section to mitigate erosion potential for high erosional ash, most applications are in the 15 to 17 actual feet per second range.
• Plant configuration – high or low dust, AIG only, AIG/Mixers
• Governing - flue gas ammonia to NOX distribution entering first layer, a reasonably low maldistributionnote (5%) is required for high removal efficiency.
• Sealing for intra- and inter-module contact surface as well as side-walls and grating and floor.
Note: The actual local ammonia to NOx mole ratio (stoichiometric ratio, SR) at
the catalyst inlet is a key design flue gas parameter for all SCR systems. It is
usually normalized to 1.0, thus, NSR, and is expressed in percent, in RMS or Std
Deviation divided by the mean value or Co-variance.
SCR Design Considerations
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• Governing - flue gas ammonia to NOX distribution entering first layer and its effect on slip and removal efficiency.
SCR Design Considerations
0
2
4
6
8
10
12
0
20
40
60
80
100
120
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
amm
on
ia s
lip
, pp
m
rem
ov
al
stoichiometry
Effect of maldistribution on removal and slip
higher maldistrbution will shift
removal to a lower value for the
same stoichiometry (and catalyst
volume.)
higher maldistrbution will shift
the ammonia slip to a higher value
for the same stoichiometry (and
Catalyst volume.)
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Catalyst Management
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SCR Catalyst Management
All applications are aimed at providing the design performance with the installed catalyst layer(s) for a
period beyond the guaranteed/expected operating life. Should there be a need to replace a layer, the
plan should yield the best option and fit in with the outage schedule. The users need to have a
comprehensive catalyst management plan (CCMP) in addition to a catalyst replacement plan from the
suppliers. Within the industry, the latter is also called ‘Catalyst Management Plan (CMP)’.
A CCMP has many essential components:
• Actively study SCR performance trending and evaluation of key SCR indicators.
• Conduct periodic full load SCR performance test for removal and ammonia slip under design
(guaranteed) SCR conditions.
• Perform SCR reactor outage inspection with documentation of the system: reactor flow devices,
AIG, and catalyst (appearance and deposition, seals/bypasses, modules and sidewalls) mapping.
• Follow the SCR shutdown procedures and SCR catalyst outage protection.
• Perform periodic sample extraction analysis (either built-in sample log or coring of sample):
1. Catalyst Activity Analysis (deactivation leads to low activity, K)
2. Physical and Chemical Analyses (root causes for deactivation)
• Review the supplier’s ‘CMP’ with data from the first three items.