2. 2
NOx and SCR Basics
SCR Design
Common Problems
Ammonia Control
Flow Modeling
Table of Contents
Presentation Outline
3. 3
NOx Basics
NOx Basics
NOx Introduction
• Nitrous Oxides are emitted during combustion processes due to the availability of oxygen and
nitrogen in the air and coal
• NOx in the atmosphere reacts in the presence of sunlight to form ozone (O3), which
contributes to greenhouse gases
NOx Forms
• Fuel NOx
• Major source of NOx emissions from nitrogen bearing
fuels
• 20 to 30 % of N in the fuel is converted to NOx
• Contributes 80% of total NOx emissions from boiler
• Thermal NOx
• NOx formed through high temperature oxidation of
nitrogen in combustion air
• Formation rate decreases with decreased temperature
and residence time
• Usually formed above 1200C
4. 4
SCR Basics
Selective Catalytic Reduction (SCR) Basics
SCR Introduction
• SCR uses a catalyst to convert NOx into separate nitrogen and
oxygen molecules
• Reactions occur at temperatures ranging from 340 to 400C
• Higher NOx reduction efficiency (~98%) than SNCR (~20-40%)
SCR Process
• Ammonia reagent is injected in the flue gas through
an ammonia injection grid
• Reagent is usually diluted with compressed air or
steam to aid in injection
• Reagent and flue gas mix and enter the reactor
chamber
• Flue gas and ammonia diffuse through the catalyst
and contact active sites to reduce NOx to nitrogen
and water
• Heat of the flue gas provides energy for the reaction
5. 5
Design Requirements
Design Requirements
Design Parameters
• Required NOx removal efficiency
• Usually reduced to 100 mg/Nm^3
based on government standards
• Maximum amount of unreacted
ammonia in the flue gas (ammonia slip)
• Commonly 2-3 ppm/Nm^3
• Maximum oxidation rate of SO2 to SO3
• Desired lifetime of the catalyst
• Design lifetime is usually 16,000 or
24,000 hours
6. 6
Chemical Reaction
Chemical Reaction
Ammonia
• Aqueous ammonia is a diluted form of ammonia (20-30%)
• Ammonia is vaporized before injection by a vaporizer
• 1 mole of ammonia is required to remove one mole of NOx
• Catalyst lowers activation energy and increases reaction rate
Urea
• Urea has an ammonia equivalence of approximately
53% by weight
• Solid urea is mixed with deionized water and
decomposed to ammonia, CO2, and water
• Cheaper to transport than ammonia due to solid
phase
7. 7
Reactor Location
Reactor Location
Upstream (Hot Side)
• SCR reactor placed before the air preheater and
ash hopper
• Does not require reheating of flue gas due to
sufficiently high temperatures
• Contains greater concentrations of particulate
which must be taken into account
Downstream (Cold Side)
• SCR reactor placed after the air preheater and ash
hopper
• Requires reheating of flue gas using redirected hot
flue gas, which is relatively costly
• Flue gas is virtually free of particulate, and catalyst
degradation is decreased
8. 8
Reactor Layout
Reactor Layout
Catalyst Grouping
• Catalysts are first cut to size and placed into individual blocks
• Blocks are grouped together into easy to transport modules
• Several modules (20-100) form a layer in the reactor
• A reactor typically holds 1 or 2 layers of catalyst
• Larger units include two reactors
9. 9
Catalyst Types
Catalyst Structure Types
Block geometry
• Geometries are meant to maximize surface area and minimize pressure drop
• Plate type catalyst are created by paste pressing catalyst onto rolls of stainless steel, plates are
then assembled into blocks
• Honeycomb catalysts are formed through an extrusion process with an associated cell pitch
• Corrugated catalyst consists of glass fiber coated with titanium dioxide and impregnated with
active components, catalyst is more porous and lighter than plate and honeycomb catalysts
10. 10
Catalyst Sizing
Catalyst Sizing
Cell Pitch and Wall Thickness
• Choice of cell pitch and wall thickness is determined mainly by the dust content of the fuel
• High dust gases (coal fired boilers) require a cell pitch of 7-10 mm and wall thickness of 0.8 –
1.0 mm
• Low dust gases (gas fired boilers) require a cell pitch of 4-5 mm and wall thickness of 0.4 mm
11. 11
Pore Sizing
Pore Sizing 1
Catalyst Composition
• Based on a porous titanium dioxide carrier material on which the active components,
vanadium pentaoxide and tungsten trioxide, are dispersed
Pore Creation
•Micro and Meso pores arise from titanium dioxide carrier
•Macro pores are created during a controlled drying and calcination process
Size Distribution
•Micro pores provide a high surface area for catalyst activity
•Meso pores provide branching to micro pores and can handle some poisoning
•Macro pores enhance gas diffusion into the catalyst and ensure access to the interior active sites
13. 13
Catalyst Degredation
Catalyst Degradation
Causes for Degradation
• Chemical absorption of ash components (sodium,
potassium) onto catalyst active sites
• Fouling of catalyst surface by fine ash particles and other
fouling agents (arsenic, chromium, calcium, etc.)
• Thermal degradation of pores due to vanadium
vaporization into VOCl3
Erosion
• Erosion by abrasive components in the fly ash (quartz)
• Erosion is proportional to gas velocity to the 3rd power
• Gas velocity should be kept around 4-5 m/s for high
particulate gases (~40 g/Nm^3)
• Minimized by keeping flow angle vertical
14. 14
SO3 Formation
SO3 Formation
Formation of SO3
• Vanadium pentaoxide, an active component of the catalyst,
contributes to the oxidation of SO2 to SO3
• SO3 will react with residual ammonia to form ammonia
bisulphate (ABS)
• ABS can cause fouling in the catalyst, reducing reduction
efficiency
• ABS also causes fouling and corrosion in the air preheater
Rate of formation
• Oxidation rate increases with temperature and pressure
• Oxidation is not limited by diffusion, and can diffuse into
the titania clusters
• Lowering vanadium decreases oxidation rate but also
decreases NOx efficiency
• High ratio between active surface area and catalyst bulk
density minimizes oxidation rate
15. 15
Ammonia Mizing
Ammonia Mixing
Ammonia Distribution
•Uneven ammonia distribution can create ammonia rich and
ammonia lean zones in the gas
•Ammonia rich zones cause greater ammonia slip
•Ammonia lean zones create areas of decreased NOx
reduction efficiency
Static Mixers
•Static mixers are often placed after the ammonia injection
grid to increase ammonia-flue gas mixing at short distances
•Mixers should minimize disc area in order to have low
pressure loss
•Mixing efficiency calculated through CFD models
•Increasing number of nozzles and changing nozzle angle also
increases mixing
16. 16
Ammonia Flow Control
Ammonia Flow Control
Rapid Load Change
• Rapid load chances can cause non uniform
outlet NOx concentrations and ammonia slip
• SCR control system should quickly regulate
ammonia injection
Analyzing System
• Two NOx analyzers are placed ahead of the
ammonia injection grid and at the exit of the
reactor
• NOx output can be controlled by setting a
constant outlet NOx concentration (i.e. 100
mg/Nm3) or constant NOx reduction (i.e. 80%)
• Ammonia concentration is calculated
proportionally to the flue gas flow rate, inlet and
outlet concentrations, and NOx reduction point
17. 17
Computer Simulation
Flow Modeling 1
CFD Models
• Ensure uniform velocity in the flue gas throughout the process
• Verify proper mixing of ammonia into flue gas
• Optimize layout of ducts and decrease dust deposits
Splitter Plates
• Splitter plates should be installed to achieve uniform velocity distribution
• Plates should be installed at turns, especially before the ammonia injection grid and catalyst
layers to minimize ammonia slip
18. 18
Flow Modeling
Flow Modeling 2
Popcorn Ash
• Large particle ash (LPA) can cause plugging in catalyst layers and reduce NOx reduction
• Hoppers should be installed to capture a portion of the ash
• A wire mesh should be installed before the reactor along with soot blowers to separate large
particles
SCR Bypass
• Possible to conduct maintenance on the reactor while keeping the boiler under operation
• Installed if NOx reduction is only required during certain times of year
• Increased duct work increases pressure drop and overall costs
• Ash deposits can accumulate at dampers at the entrance of the bypass, thus requiring
additional soot blowers or ash hoppers
19. 19
NOx Basics
References
• “Implementation of SCR DeNOx technology on coal-fired boilers”, Hans Jensen-Holm, Haldor
Topsoe, Published at 12th SO2 and NOx Pullution Control Technology International Seminar,
Chongqing, P.R. China, 2008
• “Implementation of SCR techology on coal-fired boilers in P.R. China”, Hans Jensen-Holm,
Nan-Yu Topsøe and Jim Jianhua Cui, Haldor Topsoe, Presented at PowerGen Asia/China
Power, Hong Kong, P.R. China, 2006
• “SCR Design Issues in Thermal Power Plants, Hans Jensen-Holm”, Peter Lindenhoff, Sergey
Safronov, Haldor Topsoe, Presented at Russia Power, Moscow, Russia, 2008
• “Steam, It’s Generation and Use”, Babcock and Wilcox Company, 41st Edition
