Document

WPCA/DUKE SCR Seminar
Tuesday June 7, 2005
1:30PM-2:30PM
NH3 Injection/Gas Mixing and
the Effect on Reactor
Performance
Presented
by
Kevin Rogers
The Babcock & Wilcox Company
.1
Subject Breakdown
Two General Areas of Discussion:
(1) Influence Ammonia &
NOx Molar Ratio
Blending has on
Reactor Performance
Distribution Contour Plot
60
50
NOx conc.
(ppm)
40
30
20
A
B
C
10
1
2
D
3
Point
E
4
5
Port
F
6
(2) Ammonia Injection &
Gas Mixing to Achieve
Molar Ratio Distribution
Goals
.2
Distributions Descriptions
Distribution goals commonly stated as X% of the Data
to fall within Y% of the Mean (Arithmetic Average):
80% of points to be within 10% of the Mean
80% of points to be within 20% of the Mean
85% of points to be within 15% of the Mean
100% of points to be within 25% of the Mean

Often more than one is applied to single distribution goal
(i.e. – velocity or NH3/NOx molar ratio at a given location)
System blending and reactor performance can be more
easily correlated when the distributions are adequately
defined by a single parameter.
.3
Coefficient of Variance Provides a Single
Description of the Degree of Maldistribution
Coefficient of Variance (COV, Cv, CV):
Standard deviation as a percentage of the arithmetic average
Often called ‘the % RMS’
The Root-Mean-Square of the Deviations (σ ), expressed as a
percentage of the mean (x)
Cv =
σ =
n
σ
x
1
2
(
x
−
x
)
∑
i
( n − 1) i =1
100 %
1
x =
n
n
∑x
i =1
i
.4
Normal Distribution Relationship
Need to measure many points to capture ±3σ
50 pt or less profiles often have
Maximums and Minimums below 3σ
.5
Relative Effect of Distributions on Reactor
Efficiency @ 70% DeNOx
Average NOx Removal Efficiency
Velocity
Mole Ratio
70% NOx Removal
@ Uniform Conditions
Temperature
0
5
10
15
20
Coefficient of Variation (Cv), %
.6
Relative Effect of Distributions on Reactor
Efficiency @ 90% DeNOx
Average NOx Removal Efficiency
Velocity
90% NOx Removal
@ Uniform Conditions
Temperature
Mole Ratio
0
5
10
15
20
Coefficient of Variation (Cv), %
.7
Relative Catalyst Volume vs Inlet NH3/NOx Cv
and NOx Removal (@ 2 ppm Ammonia Slip)
1.35
90_500
1.30
Increasing Catalyst Volume
90_300
90_100
CVR
1.25
85_500
1.20
85_300
85_100
1.15
80_500
1.10
80_300
1.05
80_100
1.00
0
1
2
3
4
5
6
7
8
9 10
NH3/NOx Molar Ratio Cv, %
11
12
13
CVR Plt_KR040802_1
.8
Local Peak Ammonia Slip vs Molar Ratio
(NH3/NOx ) Cv
Increasing
Reactor
Outlet
Localized
Peak NH3
Slip Local
Peak Slip
90% DeNOx
500 ppm Inlet NOx
Out-of-Range above 8% Cv
Local Peaks Caused by High
Inlet NH3/NOx at 2 Standard
Deviations above Average
Constant Avg NH3 Slip = 2 ppm
90% DeNOx
100 ppm Inlet NOx
Effect of Inlet NOx
Greater as Efficiency
Increases
80% DeNOx
500 ppm Inlet NOx
80% DeNOx
100 ppm Inlet NOx
0
2
4
6
8
10
NH3/NOx Cv, %
.9
Relative Influence of Inlet Distributions on
Reactor Outlet NOx Cv
Reactor Outlet NOx Profile Cv, %
Increasing Reactor Outlet NOx Cv
60
Inlet NOx = 300 ppm
50
Slip = 2 ppm
Ammonia-to-NOx
Cv @ 5.0%
40
30
20
10
Velocity Cv @ 15.0%
Temperature Cv @ 2.6%
0
40
45
50
55
60
65
70
75
80
85
90
95
Operating DeNOx Efficiency, %
.10
Consider the following Example
DeNOx Efficiency:
Inlet NOx:
Slip:
90 %
270 ppm
1 ppm
Average Mole Ratio
= 0.90 + 1/270
= 0.9037 NH3/NOx IN
Excess Reagent
= (0.9037-0.90)/0.90
= 0.0041 NH3/NOx REMOVED
At an Inlet NH3/NOx Cv ≅ 5%
Maximums could easily range between +2σ and +3σ
Or 2 x 5 = +10%
to 3 x 5 = +15%
Or
1.1 x 0.9037 = 0.994 NH3/NOx to 1.15 x 0.9037 = 1.039 NH3/NOx
Need Much More Catalyst to
approach 100% @ 99.4% efficiency
not easily achieved – thus higher
local slip
An Infinite Amount of Catalyst will Not
Allow Excess Ammonia > 1.0 to be
Consumed (NOx has been Depleted)
.11
Assume a Typical Reactor Gas Flow Profile
Flow Cv ≅ 4%
10.00
8.00
% Deviation from Mean
6.00
4.00
2.00
0.00
-2.00
-4.00
-6.00
-8.00
-10.00
A
B
C
D
4
E
5
6
7
3
F
2
G
1
.12
Then Consider Two Possible Inlet NH3/NOx
Distributions
30.00
One @ Cv ≅ 5%
Avg Slip ≅ 1.0 PPM
% Deviation from Mean
20.00
10.00
0.00
-10.00
-20.00
-30.00
A
B
C
D
4
E
7
6
5
3
F
2
G
1
Another @ Cv ≅ 8.5%
Avg Slip Increases to ≅ 2.4 PPM
to maintain 90% removal
% Deviation from Mean
30.00
20.00
10.00
0.00
-10.00
-20.00
-30.00
A
B
C
D
4
E
5
6
7
3
F
2
G
1
.13
Resultant Outlet NOx Distributions
80.0
70.0
Avg Slip ≅ 1.0 PPM
Cv ≅ 46%
50.0
PPM
When Inlet NH3/NOx
Cv ≅ 5%
60.0
40.0
30.0
20.0
10.0
0.0
A
B
C
4
D
5
6
7
3
E
2
F
1
G
80.0
Avg Slip Increases to ≅ 2.4 PPM
to maintain 90% removal
60.0
50.0
PPM
When Inlet NH3/NOx
Cv ≅ 8.5%
70.0
Cv ≅ 69%
40.0
30.0
20.0
10.0
0.0
A
B
C
D
4
E
5
6
7
3
F
2
G
1
.14
Resultant Outlet NH3 Slip Distributions
40.0
35.0
Avg Slip ≅ 1.0 PPM
Cv ≅ 120%
25.0
20.0
PPM
When Inlet NH3/NOx
Cv ≅ 5%
30.0
15.0
10.0
5.0
0.0
A
B
C
D
4
E
5
6
7
3
F
2
G
1
40.0
Avg Slip Increases to ≅ 2.4 PPM
to maintain 90% removal
30.0
Cv ≅ 220%
25.0
PPM
When Inlet NH3/NOx
Cv ≅ 8.5%
35.0
20.0
15.0
10.0
5.0
0.0
A
B
C
D
4
E
5
6
7
3
F
2
G
1
.15
Distributions other than NH3/NOx
Velocity Distributions
- Less Critical to Performance
- Typical Concern - Deposition & Erosion
- Cv ≤ 15% For Catalyst Approach Generally Acceptable
Temperature Distributions
- Typically Less Critical to Performance
- Sintering & BiSulfate Formation Concerns
- Min/Max ± 20F to ± 50F Generally Acceptable
.16
NH3/NOx Molar Ratio Distributions
Less Critical Below 70% DeNOx Efficiency
Becoming Rapidly More Important Above 80%
90% Removal of 300 ppm Inlet NOx @ 2 ppm Slip
(Not Practical/Possible with NH3/NOx Cv = 10%)
Critical for 90% DeNOx and Above
Cv < 3% for 95% DeNOx
Excess Reagent Reduces Sensitivity [slip/inlet NOx]
(Gas Fired - High Slip Æ Coal Fired - Low Slip)
.17
High Efficiency (≥ 90%) = More Stringent Requirements
Increased AIG Flow Uniformity
Increased Dispersion @ AIG
Increased Mixing Energy
Interrelated
Increased Flue Length
Sacrifice of Velocity Profiles to Improve NH3/NOx
Profile
Blending of Peak Slip Downstream of Reactor
Tolerance to Mixing Error Rapidly Reduced for High
DeNOx Removal Efficiency (Cv drift 5% Æ 8% is a problem)
.18
SCR Ammonia Injection & Gas Mixing
Technology
•Why Mix ?
•Technology
Fundamentals
•AIG & Mixer Design
•Field Performance
.19
Representation of Blending
Decreasing Scale
of Segregation
Decreasing
Intensity of
Segregation
Decreasing
Intensity of
Segregation
.20
AIG & Mixing Fundamentals
Mixing is Dominated by Three Primary Factors
1. Original Distribution – How well ammonia is deposited
over the space or profile of an initial NOx distribution (Scale of
Segregation)
2. Stretching and Folding
- How larger regions of
varying concentrations are being thinned and spread across
the area via macroscopic turbulence (Scale of Segregation)
3. Molecular Diffusion – Occurring simultaneously
throughout and necessary for the final an approach to
complete homogeneity – influenced & facilitated by the
increased surface area brought about by 1 & 2 (Intensity of
Segregation)
.21
AIG & Mixing Fundamentals
Aspects of Initial Ammonia Dosing
1. Flow Dosing – The relative matching of regional or
local ammonia flow to the regional or local flue gas flow (Fairly stable flow profile contours through the plane of
ammonia injection typically allow for stable flow dosing)
2. NOx Dosing
- The relative matching of regional or local
ammonia flow to the regional or local NOx Concentration (NOx profile contours are often not as stable with regard to
the positioning of high a low concentration regions and can
thus be difficult to adjust to).
.22
AIG & Mixing Fundamentals
Aspects of Initial Ammonia Dosing – Cont’d
3. Spatial Dosing – Defined by the spacing of the
injection points and thus the injection point
quantity: (Initial Scale of Ammonia Segregation)
1. Increasing the number improves the final blend to a
point of diminishing return.
2. Too few reduces system effectiveness and efficiency.
3. Too few can over-sensitize placement of injection
points relative to mixer vane elements.
.23
B&W Mixing Approach
Efficient Use of Duct Length with Rapid Shear,
Controlled Turbulence and Duct Macro Flow
Patterns
Ra
Di pid
sp Fu
ers ll
ion Duc
AI t
G
.24
Mixer Development
Test Stands
• From Simple to Complicated – Size, Cost, Testing & Run Times
• More Complicated for Analysis of Arrangement Performance
(the performance of devices in complex arrangements)
Predictive Formula
• Structured Array of Test Programs
• Initial Application & Optimization
Field Performance
.25
Mixing Effectiveness
CO Tracer Concentration COV, %
40
35
ξ 1' = 1 −
30
27
≅ 0.23
35
25
No Mixer
ξ 3' = 1 −
20
12
≅ 0.64
33
ξ 7' = 1 −
15
8
≅ 0.72
29
No Mixer 37.5TL
10
Forced Mixing
4x6PVM40S 37.5TL
5
0
-1
0
1
2
3
4
5
6
7
8
L/D
.26
NOx COV, %
Influence of Bend Design
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
NLTS_004_A
ξ’a
ξ’b
No Mixer
ξ’c
Mixer_Std Bend
Mixer_Modified Bend
0
10
20
30
40
50
60
70
Equivalent Centerline Distance, ft
AIG
Mixer
Bend
Hood
Catalyst
.27
Influence of Injection Point Quantity
Mixer
Inlet
Location
200
180
NLTS_004_A
160
NH3 COV, %
140
Na = 1.9 pts/m2
120
100
80
Na = 7.5 pts/m2
6.7%
Na = 15.0 pts/m2
4.8%
3.1%
60
40
20
0
0
10
20
30
40
50
60
70
Equivalent Centerline Distance, ft
AIG
Mixer
Bend
Hood
Catalyst
.28
Operating Unit Molar Ratio Drift (w/o Static Mixing)
Inlet NH3/NOx Molar Ratio COV, %
12
87%
10
8
87%
DRIFT
83%
6
4
85%
90%
85%
2
TUNING
0
0
10
20
30
40
50
60
70
Reactor Outlet NOx COV, %
.29
Operating Unit Molar Ratio Drift (with Static Mixing)
Inlet NH3/NOx Molar Ratio COV, %
5
92%
4
DRIFT
93%
90%
3
90%
2
TUNING
92%
1
0
0
10
20
30
40
50
Reactor Outlet NOx COV, %
60
70
.30
CFD Based Sensitivity Analysis
25
NH3/NOx COV, %
20
No Forced Mixing
15
With Forced Mixing
10
5
System Configuration: 1.5L+2L
Mixer Type:
4TV
0
0
5
10
15
20
25
30
35
Inlet NOx COV, %
.31
Design Considerations
Forced Mixing for Stable Uniformity
Stability
• W/O Forced Mixing: Molar Ratio COV 2% Æ 8% Æ 15%
• With Forced Mixing: Molar Ratio COV 2% Æ 5%
Important Optimization Parameters
• Initial Dosing (AIG Design, NOx profile, Flow Profile)
• Injection Nozzle Quantity
• Directivity of Mixing Effectiveness
• System Length vs Energy Efficiency
• Arrangement Design (Dampers, Exp/Con, Bends, Hoods,
etc.)
.32
Design Considerations – Cont’d
• NOx removal duty (Inlet loading,
% removal, allowable slip)
• NOx & Temperature Profiles
• Allowable draft loss
• Straight flue lengths available
• Bends & overall flue geometry
• AIG inlet gas flow profile
• Quantity of injection points
• Mixer Placement
• Flow correction prior to catalyst
.33
Poor Arrangements for High Removal
Simple Close-Coupled
Arrangements Will Not Work
for High NOx Removal SCR
Projects
Insufficient distance to
inject and blend for very
low and stable molar
ratio maldistributions
Short opportunity to
blend peak local slip
concentrations prior
to AH
.34
Arrangements for Promotion of Mixing
All mixing
considered
complete prior
to flow
straightening
into the catalyst
Permanent
sample grid
Location
Greater opportunity
for blending of
peak local slip
concentrations
prior to the AH
Good mixing distance
prior to bend
Mixer Location
AIG Location
Flow
correction ‘as
required’ prior to
the AIG (Often
perforated
plate and/or
vanes)
.35
AIG Field Tuning
Decrease ammonia flow in region of
lower gas flow through the AIG for
improved initial ammonia dosing
Increase ammonia flow in region of
higher gas flow through the AIG for
improved initial ammonia dosing
When units are designed with static mixers, this simple initial
tuning trial, based on model study results for the flow profile
through the AIG, is often sufficient to obtain an acceptable blend.
.36
AIG Field Tuning
Reactor Outlet Distribution
275 MW
275 MW
350 lb/hr
90.0%
Initial Load
Final Load
NH3 Flow
% Reduction
port/point
1
A
B
C
D
E
F
2
28
19
16
19
22
21
3
15
13
14
20
22
23
4
28
29
21
21
24
22
5
6
40
37
36
28
29
21
36
38
33
29
29
25
26
29
30
41
47
38
Std. Dev.
8.27
%RMS
30.72
Average (ppm)
26.92
Distribution Contour Plot
60
50
NOx conc.
(ppm)
40
30
20
A
B
C
10
1
2
D
3
Point
E
4
5
Port
F
6
.37
AIG Field Tuning
Given:
270 ppm Inlet NOx
0.50 ppm ammonia slip
Md = 0.90 + 0.50/270 = 0.9019
Outlet NOx Cv Target ≅ 44% - 46% @ inlet NH3/NOx Cv ≅ 5%
NOx Cv is 30.7% Achieved Æ Solving for the inlet NH3/NOx
Cv @ outlet NOx Cv is 30.7% yields an inlet NH3/NOx Cv ≅ 3%
.38
Field Uniformity Performance
Field Site Æ
A
B
C
D
E
Mixer Design
Other
Other
B&W
B&W
B&W
Removal Efficiency, %
87%
86%
92%
93%
89%
SCR Outlet NOx COV
18%
23%
16%
10%
24%
SCR Inlet NH3/NOx COV
2.5%
3.8%
1.9%
1.2%
3.2%
Relative Mix Length
1.5
3.3
1.0
2.7
2.8
Relative Shock Loss
2.0
1.0
2.7
3.3
1.3
.39
.40

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