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CFD Modeling for Design of NOxReduction in Utility Boilers
Seventeenth Annual ACERC Conference Salt Lake City, UT
February 20-21, 2003
S. VierstraJ.J. Letcavits M.A. Cremer, B. R. Adams, J.R. Valentine
SAVvyEngineeringSAVvy
Engineering
Why Use CFD Modeling?
Stringent NOX emissions limits imposed on utilities
SCR can be used to achieve limits, but is expensive
Other less expensive NOX reduction options such as OFA are available
CFD is a cost effective approach to evaluate options
REI Utility Boiler Modeling
0
2
4
6
8
10
12
14
16
18
<200 MW 201-400 MW >400 MW
Unit Size (MW)
# o
f U
nit
s
Cyclone T-Fired Wall-Fired
Summary of Reaction Engineering International (REI) Boiler Modeling
NOx REDUCTION STRATEGIES
» Staging, OFA, ROFA» Reburning, FLGR, SNCR, RRI» Co-firing, Fuel Blending &
Switching» Advanced Concept LNBs &
Proof-of-Concept Furnaces
OPERATIONAL IMPACTS
» CO Oxidation» Unburned Carbon-in-Ash (LOI)» Waterwall Wastage» Heat Rate
over 100 Utility Boilers Modeled ~37,000 MW Capacity Cyclone, Turbo, Wall, & T-fired Firing Coal, Oil, Gas, Biomass,
Petcoke, Tires, Blends
Project Objectives
Design and evaluate NOx reduction due to OFA in two units
» 265 MW wall-fired PC unit
» 530 MW cyclone-fired unit
Wall Fired PC Unit
265 MW B&W Opposed Wall Fired
18 Babcock Borsig Power CCV low NOx Burners
Subcritical
Eastern Kentucky bituminous coal (1% Sulfur)
Baseline NOx emissions 0.6 lb/MMBtu 12 FW Burners
Partial Division Wall
Superheater Pendants
6 RW Burners
Baseline Model265 MW Wall Fired Unit
One-half furnace modeled – symmetry plane
Cartesian grid
650,000 computational nodes
Vertical model exit downstream of secondary superheater
Predicted CO Distribution265 MW Wall Fired Unit
Proposed OFA Elevation
Fro
nt W
all
> 20,000 ppm
0 ppm
Fro
nt W
all
Fro
nt W
all
OFA Design Driven by Location of High CO and High Mass Flow
Front Wall
Rear Wall
Sym
met
ry P
lane
Sid
e W
all
Front Wall
Rear Wall
Sym
met
ry P
lane
Sid
e W
all
CO (ppm)
0
>20,000
Axial MassFlux (kg/m2/s)
< 0
> 4.5
Proposed OFA Port Layout265 MW Wall Fired Unit
7 rear wall ports
4 front wall ports
Interlaced rather than opposed
Elevation 10 ft above top burners
OFA jet velocity 170 ft/sec
Two configurations at different furnace staging levels
Burner modifications to maintain primary to secondary burner velocity ratio
Rear Wall
Symmetry Plane
Side Wall
6 ft 6 ft 3 ft 6 ft
12 ft 3 ft 6 ft
6 ft 6 ft 3 ft 6 ft
12 ft 3 ft 6 ft
Predicted Results – OFA265 MW Wall Fired Unit
Furnace Staging
Burner Modifications
Predicted NOx
Predicted Carbon in Fly
Ash
Predicted Furnace Exit
CO
Baseline none none0.58
lb/MMBtu8% 85 ppm
Initial OFA 0.90primary &secondary
0.39 lb/MMBtu
20% 801 ppm
Revised OFA 0.95 primary only0.38
lb/MMBtu13% 1000 ppm
Post Retrofit Test Data265 MW Wall Fired Unit
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 100 200 300
Unit Load, MWg
NO
x, l
b/M
Btu
Pre Retrofit
'Post Retrofit, notoptimized
Model - Pre Retrofit
Model - Post Retrofit
Cyclone Fired Unit
B&W 530 MW, supercritical
Opposed wall-fired» 2 over 3 on front wall» 3 over 3 on rear wall
60% PRB/40% Eastern bituminous fuel
Barrel water injection for NO2 plume control
Baseline NOx emissions 1.8 – 1.9 lb/MMBtu
Cyclone Barrel Model
Cyclone Barrel Model» 10’ diameter
» Radial Burner
» 350,000 computational cells
» Unstaged (SR=1.15) and staged (SR=0.90) were simulated
Coal Chute
Tertiary & Primary Air Inlets
Secondary Air Inlet
Reentrant Throat & Exit to FurnaceCoal Chute
Tertiary & Primary Air Inlets
Secondary Air Inlet
Reentrant Throat & Exit to Furnace
Baseline particle trajectories
Furnace Model
Barrel exit results were interpolated into 750,000 computational cell furnace furnace model
Simulations were performed for the unstaged baseline condition and staged OFA configurations
Baseline results:
» Predicted furnace exit gas temperature consistent with observations
» Furnace exit CO predicted to be less than 100 ppm
» Predicted NOx emissions of 1.96 lb/MMBtu consistent with CEMs
Initial OFA Port Layout
CO (ppm, wet)
5 frontwall ports
6 rearwall ports
Ports placed above barrel centerlines
Baseline results suggested that front/rear port distribution should be reversed – structural limitations did not allow this
Staggered ports allow for deeper penetration/improved mixing
300 ft/s jet velocities
Predictions show high CO pockets in corners, average exit CO 2093 ppm (vs. 85 ppm baseline)
Modified OFA Port Layout
Prediction of high furnace exit CO led to modified design
Addition of 4 low velocity (100 fps) auxiliary ports in the corners
16% of total OFA flow to auxiliary ports
Main port jet velocity proportionately reduced
Predicted average exit CO 413 ppm (vs. 2093 ppm initial OFA layout)
CO (ppm, wet)
4 low velocityauxiliary ports
Overall Predictions
Predicted Furnace Exit Temperature
Predicted CO (dry)
Predicted NOx
Baseline 2438 F. 217 ppm1.96
lb/MMBtu
Initial OFA (case 1)
2299 F. 2093 ppm0.35
lb/MMBtu
Revised OFA (case 2)
2340 F. 413 ppm0.37
lb/MMBtu
OFA Case 3 2364 F. 324 ppm0.47
lb/MMBtu
Post Retrofit Test Data530 MW Cyclone Fired Unit
0
0.5
1
1.5
2
2.5
0 100 200 300 400 500 600
Unit Load, MWg
NO
x, lb
/MM
Btu
Pre Retrofit Post Retrofit Model - Pre Retrofit Model - Post Retrofit
Summary
CFD modeling is an effective tool in design and evaluation of NOx reduction technologies in utility boilers
CFD utilized to develop conceptual design of OFA system in 265 MW wall fired furnace fitted with low NOx burners» Model predictions indicated NOx reductions over 30% could be
achieved with limited increase in stack CO and carbon in fly ash» Subsequent installation of the OFA system has confirmed
predictions
CFD based OFA design developed for 530 MW cyclone fired furnace» Model predictions indicated 80% NOx reduction with small increase
in furnace exit CO» Subsequent installation confirmed predictions of 0.37 lb/MMBtu