Upload
doantuong
View
216
Download
2
Embed Size (px)
Citation preview
Reaction Engineering International
Improving Design of SCR Systems with CFD Modeling
B. Adams, C. Senior
Reaction Engineering International
DOE Environmental Controls ConferenceMay 16-18, 2006 Pittsburgh, PA
Reaction Engineering International
Project Objective
• Demonstrate value of CFD in SCR design and tuning– Improved insight of flue gas properties through
AIG, SCR catalyst, and ductwork – Impact of flue gas profiles and flow rates on SCR
performance– Guidance for matching AIG flows with flue gas
profiles and resulting impact of AIG tuning
Reaction Engineering International
Project Approach• Use CFD and 1-D reactor modeling to predict
flow properties and NO reduction in 550 MW coal-fired boiler SCR system– Flow patterns and velocity profiles– Overall pressure drop through duct work– Temperature distribution– NH3 distribution from ammonia injection grid (AIG)– NO reduction through SCR catalyst– Ammonia slip after catalyst– NO reduction sensitivities
Reaction Engineering International
Modeling Approach
• Three model components:– Duct flow model– Ammonia Injection Grid
(AIG) submodel– SCR catalyst submodel
Duct Flow Model
SCR Reactor
Duct Flow Model
Duct Flow Model
Detailed AIG Model
- ∆P- Flow distribution
- NH3 distribution
- SCR model inlet properties• NH3 distribution• NO distribution• Flow distribution
- NO reduction- NH3 slip
- ∆P- Flow field- NH3 slip distribution- NO distribution
Reaction Engineering International
Inlet conditions to each
catalyst cell
SCR Model – Implementation1-D plug flow model with heterogeneous kinetics for NO reduction by NH3 over vanadia/titania catalysts Solves gas transport and surface reactions along catalystNumber of “active sites” for reaction calibrated with design dataPredicts dependencies of NH3/NO ratio, temperature, residence time and flue gas composition
NH3
NO
> 200
< 100
ppm
> 230
< 150
ppm
Results at SCR Inlet
> 5
< 0
m/sVelocity
Surface Flux
NONH3
N2H2O
NONH3
N2H2O
1-D catalystchannelmodel
Outlet conditions from each catalyst
cell to CFD
Reaction Engineering International
SCR GeometriesUnit 3-1 Unit 3-2
Model inlet Model out
Measurement inlet
Measurement out
Flow Direction
Flow Direction
Flow Direction
Model inletModel out
Measurement inlet
Measurement out
Unit 3-2 has shorter run length between turn and AIG and shorter run length between AIG, turn, and guide vanes
Reaction Engineering International
Gas Velocity Profile
MCR
30% Load
MCR
Velocity profiles can suggest improvements to guide vane design
Unit 3-1
Reaction Engineering International
0
20
40
60
80
100
0 2 4 6 8 10 12 14
Location
Pres
sure
, mm
H2 O
Pressure Drop
0
2
4
6
8
10
12
0 2 4 6 8 10 12 14
Location
Pres
sure
, mm
H2 O
SCR Reactor
SCR ReactorMCR 30% Load
Unit 3-1
Full Load
Part Load
Profiles identify specific contributors as well as total system pressure drop
Reaction Engineering International
Flow ComparisonsUnit 3-1 Unit 3-2 Total
Flow, 103 kg/hr 1128.5 1128.5 2257.0MCR ∆P (mm H2O, SCR) 40.0 37.5 -
∆P (mm H2O, N1 to N3 or N4 to N6) 89.1 79.2 -Total ∆P (mm H2O) 94.0 84.0 -
Flow, 103 kg/hr - 694.2 -Model 30% Load ∆P (mm H2O, SCR) - 13.1 -
(July 17, 2004) ∆P (mm H2O, N1 to N3 or N4 to N6) - 27.7 -Total ∆P (mm H2O) - 29.0 -
Flow, 103 kg/hr 431.5 431.5 863.030% Load ∆P (mm H2O, SCR) 5.0 4.9 -(design) ∆P (mm H2O, N1 to N3 or N4 to N6) 11.7 10.4 -
Total ∆P (mm H2O) 12.0 10.8 -
Flow, 103 Nm3/hr 948.3 871.0 1819.2MCR Flow, 103 kg/hr 1260.8 1158.0 2418.9
(July 16, 2004) Design SCR ∆P (mm H2O) 39.8 37.3 -Measured ∆P (mm H2O, N1 to N3 or N4 to N6) 98.4 73.7 -
Flow, 103 Nm3/hr 576.2 522.1 1098.330% Load Flow, 103 kg/hr 766.1 694.2 1460.3
(July 17, 2004) ∆P (mm H2O, N1 to N3 or N4 to N6) 44.1 34.1 -
• Predictions consistent with the measured data
• Differences likely due to:– Inlet flow profile– Different flow
rates– Measurement
locations/grid– Model accuracy
Reaction Engineering International
NO Reduction Dependencies• NO reduction depends on:
– NH3/NO ratio – insufficient ammonia (ratio<1.0) will not reduce all available NO (NO remaining)
– Residence time – insufficient residence time will not allow NH3 enough time to react with NO (NO and NH3 remaining)
• NH3 slip depends on:– NH3/NO ratio – too much ammonia (ratio>1.0) will reduce all
NO (NH3 remaining)– Residence time – insufficient residence time will not allow
NH3 enough time to react with NO (NO and NH3 remaining)
Reaction Engineering International
NO Concentration ProfileUnit 3-1 vs 3-2 at MCR
Flue Gas
Flue Gas
Both cases show biased NOx profiles
Reaction Engineering International
NH3 Concentration ProfileUnit 3-1 vs 3-2 at MCR
Flue Gas
Flue GasBoth cases show biased NH3 profiles
Reaction Engineering International
SCR Model Predictions INLET OUTLET INLET OUTLET
NH3, ppm @ 6%O2 Dry 171 4.0 171 4.2MCR, Fully Open AIG NO, ppm @ 6%O2 Dry 210 42.7 210 43.0
NO Reduction, % - 79.6 - 79.5
NH3, ppm @ 6%O2 Dry 171 0.0 171 0.230% load, Fully Open AIG NO, ppm @ 6%O2 Dry 210 38.8 210 38.9
NO Reduction, % - 81.4 - 81.4
30% load with higher O2 NH3, ppm @ 6%O2 Dry - - 171 1.5Fully Open AIG NO, ppm @ 6%O2 Dry - - 210 40.2
NO Reduction, % - - - 80.8
Unit 3-1 Unit 3-2
• Ammonia slip likely due to higher than design velocities in catalyst (lack of residence time)
• Limited residence time also reduces NO reduction• Will more uniform NH3/NO profiles improve performance?
Reaction Engineering International
Outlet NO Profile at MCR
Unit 3-1: measured
Unit 3-1: predicted
Unit 3-2: measured
Unit 3-2: predicted
A1 A81
5
B8 B1
4
3
2
1
5
4
3
2
A1 A81
5
B8 B1
4
3
2
1
5
4
3
2
STD = 16.54%Average = 54.2 ppm
STD = 7.83%Average = 43.3 ppm
STD = 33.52%Average = 45.4 ppm
STD = 25.72%Average = 49.0 ppm
Measurement taken with fully open AIG nozzles (NO ppm, dry)
Reaction Engineering International
NO Exit Profile ResultsComparison of predicted and measured NO profiles
MCR(outlet), measured on 07/14/04
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
Ratio of m easured NO to average inle t NO
Rat
io o
f pre
dict
ed N
O to
ave
rage
inle
t NO
Unit 3-1
Unit 3-2
Measurement taken with AIG nozzles fully open
Reaction Engineering International
AIG Tuning• Background: Modeling results with fully open AIG
nozzles indicate a bias in the flue gas flow in the AIG region, leading to NH3 biasing at the catalyst inlet
• Objective: Use CFD modeling to simulate AIG tuning and show how CFD can be used to guide AIG in the SCR duct (Unit 3-2)
• Approach: Use zonal AIG biasing to achieve more uniform NH3/NO ratio (0.814)
Reaction Engineering International
Using CFD For Tuning
• CFD can show profiles through system
• CFD can be used to trace source of profile
• Streamlines correlate local NO at outlet with corresponding NH3/NO entering the catalyst, and ultimately with the corresponding AIG nozzles
Reaction Engineering International
NH3 Concentration ProfileUnit 3-2 at MCR
Flue Gas In
Flue Gas Out
Flue Gas In
Flue Gas Out
Fully Open AIG Nozzles (Uniform NH3 injection)
AIG Tuning (Biased NH3 injection)
Reaction Engineering International
NH3/NO Ratio ProfileUnit 3-2 at MCR
Flue Gas In
Flue Gas Out
Flue Gas In
Flue Gas out
Fully Open AIG Nozzles (Uniform NH3 injection)
AIG Tuning (Biased NH3 injection)
Reaction Engineering International
NO Concentration ProfileUnit 3-2 at MCR
Flue Gas In
Flue Gas OutFlue Gas Out
Flue Gas In
Fully Open AIG Nozzles (Uniform NH3 injection)
AIG Tuning (Biased NH3 injection)
Reaction Engineering International
Predicted Effects of AIG Tuning
NH3/NO RatioSTD = 8.1%
Average = 0.814
NH3/NO RatioSTD = 1.3%
Average = 0.814
NOSTD = 25.7%
Average = 43.0 ppm
NOSTD = 4.7%
Average = 42.5 ppm
UniformNH3+Air Flow
BiasedNH3+Air Flow
Catalyst Inlet
MeasurementOutlet
Dry, 6% O2
Reaction Engineering International
SCR Model Predictions
• Why didn’t the NO reduction improve with a more uniform NH3/NO ratio at the SCR reactor inlet?– More uniform NH3/NO ratio improved reduction in low ratio regions,
but decreased reduction in high ratio regions– Average residence time unchanged, still too short to allow full
conversion to 81.4%– Both results were close to the optimal reduction specified in the
catalyst design for this load, NH3/NO ratio, and catalyst size (80%)
AIG ConditionsINLET OUTLET INLET OUTLET
NH3, ppm @ 6%O2 Dry 171 4.2 171 3.8NO, ppm @ 6%O2 Dry 210 43.0 210 42.5
SCR Inlet NH3/NO Mole Ratio 0.814 0.814
NO Red, % 79.50 79.71NH3 Slip, ppm @ 6%O2 Dry 4.2 3.8
Unit 3-2: MCRUniform Tuning
Reaction Engineering International
SCR Model: Effect of Gas Velocity on NO Reduction
Velocities plotted are from actual duct velocities
Average vel: 4.56 m/s
All non-blue colors represent velocities
that could lead to suboptimal NO
reduction
Velocity profile at SCR catalyst inlet
76
77
78
79
80
81
82
0 2 4 6 8
Catalyst Channel Gas Velocity, m/s
NO
Red
uctio
n, %
Suboptimal NO reduction
Average velocity: 4.56 m/s
2.2 m/s
81.4%
> 6
< 2.2
m/s
At MCR, catalyst design does not allow sufficient residence time to react all NH3 with NO, even with optimal NH3/NO ratio
Reaction Engineering International
Predictions at Higher NH3/NO Ratios
75
80
85
90
95
0.80 0.85 0.90 0.95 1.00 1.05
NH3/NO Ratio
NO R
educ
tion,
%
Fully Open AIGTuned AIGDesign Data
0
5
10
15
20
25
0.80 0.85 0.90 0.95 1.00 1.05
NH3/NO Ratio
NH
3 Sl
ip, p
pm (d
ry, 6
% O
2)
Fully Open AIGTuned AIGDesign Data
NH3/NO Ratio vs NO Reduction NH3/NO Ratio vs NH3 Slip
MCR LoadMCR Load
Reaction Engineering International
0
5
10
15
20
25
0.00 0.50 1.00 1.50 2.00 2.50
Residence Time, s
NH
3 S
lip, p
pm (d
ry 6
% O
2) Fully Open AIGTuned AIG
Predictions at Higher Residence Times
Residence Time vs NH3 Slip
MCR
90
91
92
93
94
95
96
97
98
99
100
0.00 0.50 1.00 1.50 2.00 2.50
Residence Time, s
NO
Red
uctio
n, %
Fully Open AIGTuned AIG
Residence Time vs NO Reduction
NH3/NO = 1.0
NH3/NO = 1.0
MCR
Reaction Engineering International
What Can Model Do• Predict performance and identify potential design
problems before design/installation– Predict NO reduction and ammonia slip (measurements verify
accuracy for the same NH3/NO ratio, load, catalyst design)– Guide how much NH3 needed to reach NO reduction goal– Guide amount of catalyst needed for NO reduction goal– Suggest duct designs to provide more uniform flow
• Aid tuning and troubleshooting– Illustrate tuning impacts
• Guidance on how to tune AIG injection • Determine if tuning worthwhile for a particular design
– Identify measurement locations for testing and/or analyzer