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

Baseline Coal Particle Trajectories265 MW Wall Fired Unit

Baseline Model Predictions265 MW Wall Fired Unit

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

CO (ppm, wet)

Baseline OFA Case 1 OFA Case 2

NOx (ppm, wet)

Baseline OFA Case 1 OFA Case 2

Overall Predictions

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

Acknowledgements

This work was supported by American Electric Power (AEP)