Optimization of a Dual-Fuel Low-NOx Combustion System for a - - PowerPoint PPT Presentation

Optimization of a Dual-Fuel Low-NOx Combustion System for a Tangentially-Fired Utility Boiler Operating at a High Elevation.

by

• F. McKenty, N. Brais, M. Mifuji, L. Gravel, and Y. Sirois

STAR Global Energy Forum – Houston, Tx 23-24 June, 2009

Brais Malouin & Associates Inc. 144 Barr Street, St-Laurent, Qc. Canada, H4T 1Y4 Tel: (514) 382-8866 www.bma.ca

Images courtesy

• f

Cerrey S.A. de C.V.

• Av. Republicana Mexicana 300

San Nicolas de Los Garza, N.L. Mexico C.P. 66450

Industrial Boiler Manufacturer

3

Overview

• Motivation
• Objectives
• Problem Description
• CFD Modeling
• Results & Analysis
• Summary
• Conclusion

4

Introduction Motivation

• Stricter pollutant emission regulations are presenting new challenges

to boiler and burner manufacturers in order to meet the new emission specifications.

• Develop new Boiler/Burner designs
• Retro-fit existing boilers with new combustion systems capable
• f meeting the emission specifications.
• Physical constraints limit the positioning of the new combustion system.
• Re-use existing components (fans, ducts etc..) as much possible to limit costs.
• Maintain the existing boiler’s operational characteristics:
• Wall heat transfer
• Heat transferred to Superheaters, Reheaters and Convection Banks
• Gas temperature at key locations

5

Introduction Objective

• Replace the existing 32 burner (16 Natural Gas and 16 Heavy Oil #6)

Tangentially Fired combustion system with a new Low NOx combustion system having 16 Dual-Fuel burners and 8 Over Fire Air (OFA ) ports.

• Use CFD to optimize the combustion system’s firing angles in order to maintain the

existing boiler’s operational characteristics

6

Problem description Tangentially fired boilers

Operating principal:

• Four or more burners located in the corners or on the boiler walls

are fired tangentially to a target circle located at the center of the boiler.

• Objective:
• Create a rotating flow pattern in the center of the furnace.
• Use the furnace as a mixing vessel.
• Create a fireball in the middle of the furnace instead of

several individual flames.

• This type of boiler design was originally developed for coal firing

in order to minimize the space required for large utility boilers.

7

• The size (diameter and height) of the fireball is highly dependant on the

diameter of the target circle.

• If the target circle diameter is too large; the diameter of the fireball could

increase until it reaches the furnace walls.

• If the target circle diameter is too small; the jets could impinge with one

another and the rotating motion of the flow is lost.

• The size of the fireball is dependant on burner jet penetration into the

furnace.

• Jet penetration is a function of jet momentum.
• Increased jet momentum means increased penetration and higher jet

velocities at the location of the target circle and vice versa.

Problem description Tangentially fired boilers

8

• Increasing or decreasing the momentum of the burner jets will change the

furnace aerodynamics.

• A target circle that was adequate for a given burner (jet momentum)

may yield an inappropriate shape of the fireball if the burners are replaced with burners having jets with more or less momentum than the original burners.

• Jet momentum is defined as:
• Density decreases with altitude.
• Velocity increases with altitude for a given flow rate.
• An increase in velocity will increase jet momentum.
• Consequence: A target circle diameter that is optimal at sea level may no longer

be adequate when the boiler is located at high altitude (2000 ft and more).

Problem description Tangentially fired boilers

) (N V A V V m G = ⋅ = =

→ → →

• ρ

A m V ρ

• →

=

9

• Example: Comparison of a natural gas flame with standard target circle diameter

for a single burner level T-fired boiler at sea level and at 5200ft altitude.

Problem description Tangentially fired boilers

Fireball at sea level Fireball at 5200 ft

10

• Example: Comparison of a natural gas flame with the target circle diameter
• ptimized for operation at sea level for a single level T-fired boiler

with the same design operating at 5200ft altitude.

Problem description Tangentially fired boilers

Fireball at sea level Fireball at 5200 ft

11

• Additional problems are encountered when trying to optimize the firing configuration

for both Natural Gas and Heavy Oil firing:

• The optimal target circle diameter for natural gas firing is most often too small for

Oil firing because of the difference in the distribution of momentum in the flames.

• The core of the central vortex for oil flames can become unstable.

Problem description Tangentially fired boilers

1st burner level - NG 1st burner level – Oil #6

12

• Combustion of the fuel increases the gas temperature in the burner jet and causes

the expansion of hot combustion gases.

Problem description Tangentially fired boilers

Temperature (K)

Dual-Fuel (54% Oil – 46% NG) Firing

13

• The expansion of the gases causes local acceleration in the jet in the ratio of about 5/1.

Problem description Tangentially fired boilers

Dual-Fuel (54% Oil – 46% NG) Firing

Velocity Magnitude (m/s)

14

• The local acceleration is more pronounced and localized for heavy oil burners because

all the fuel is concentrated in front of the oil gun.

Problem description Tangentially fired boilers

Dual-Fuel (54% Oil – 46% NG) Firing

Velocity Magnitude (m/s)

15

• The jets from Oil flames usually have much higher momentum and penetration

than Natural Gas flames.

• It is obvious that the optimal target circle diameter will be different depending on the fuel.
• The problem is compounded when operating at high altitudes because the effect on

gas firing does not vary proportionally to the effect on oil firing.

Problem description Tangentially fired boilers

16

• The firing angles should be determined according to the burner design,

fuels to be fired and the altitude at which the unit will operate.

• The target circle for dual fuel burners must be a compromise between the
• ptimal firing angles for Natural Gas and the optimal firing angles for Heavy Oil.

Problem description Tangentially fired boilers

Natural Gas Firing

Burner Level 1 - Velocity Magnitude (m/s)

Oil #6 Firing

17

• Even when the optimal firing angles for Natural Gas have been modified to

accommodate oil firing, the final firing angles for Natural Gas yield a much smaller vortex diameter in the center of the furnace.

• Reducing the vortex diameter helps keep the reacting regions away from

the furnace walls.

Problem description Tangentially fired boilers

NG-Firing – Original sea level firing angles

Burner Level 1 - Velocity Magnitude (m/s)

NG-Firing – Optimized firing angles

18

• Boiler Characteristics:
• 300 MW Utility Boiler
• Gross Heat Input

: 800MW (2725 MMBTU/hr)

• Steam Generation

: 907,000 kg/hr (2,000,000 lbs/hr)

• Altitude

: 1722 m (5649 ft)

• Project:
• Replace the existing combustion system (32 burners : 16 NG, 16 Oil)

with a new 16 Dual-Fuel tilt-burner Low NOx firing system.

• New Combustion System characteristics:
• 16 tilting burners
• 8 OFA ports
• The firing system must be able to operate with:
• Natural Gas
• Heavy Oil #6
• Dual Fuel Firing (NG and Oil)
• 25% FGR
• 12% Excess Air

Problem description Tangentially Fired 300MW Utility Boiler

19

Problem description

Tangentially Fired 300MW Utility Boiler: New Combustion System :

• Most of the NOx produced in this furnace are the result of thermal NOx formation.
• Thermal NOx formation requires:
• High temperatures (T>1800K).
• The presence of sufficient quantities of oxygen and nitrogen.
• Limiting Thermal NOx formation in this furnace is achieved by:
• Fuel Staging at the burner level
• Fuel and air are injected in such a way as to minimize the presence
• f high temperature zones and high concentrations of oxygen at the same place.
• Furnace Staging
• The overall Fuel/Air mixture at the burner level is maintained fuel-rich

in order to minimize NOx formation by denying the reaction the necessary oxygen.

• The remainder of the combustion air is injected at the OFA level once the

temperature of the combustion products has decreased. Secondary combustion, in excess air, at the OFA level of the CO remaining in the combustion products therefore takes place at a lower temperature and produce less thermal NOx.

20

Problem description

Tangentially Fired 300MW Utility Boiler: New Combustion System : CFD investigation of the new combustion system:

• Determine the optimal tangential burner firing angles to:
• Ensure stable fireball aerodynamics for all fuels fired at the given altitude.
• Ensure adequate mixing of Combustion Products and OFA for complete combustion.
• Avoid flame impingement on the superheaters and the furnace walls.
• Ensure that the furnace heat transfer characteristics are similar to those of

Cerrey’s performance run predictions (idealized design cases) for each type

• f fuel firing by comparing theoretical and CFD predictions of Furnace Outlet

Plane (FOP) gas temperatures.

21

CFD Modeling

Numerical Models

• Code: STAR-CD
• Fluid Flow
• Steady-State (SIMPLE)
• Oil Droplets
• Lagrangian Particle Tracking Model - Coupled
• Turbulence
• High-Reynolds k-ε Model
• Radiation
• Discrete Ordinate Model – Participating Media Radiation Model
• Superheaters, Re-Heaters and Economizer
• Volumetric heat-sink corresponding to empirical measurements => sorent.f
• Source term for dP according to empirical measurements

=> sormom.f

22

CFD Modeling

Numerical Models

• Combustion
• In-house 5 fluid stream combustion model implemented with
• STAR-CD User Subroutines sorsca.f and scalfn.f
• The 5 fluid streams represent:
• Heavy Oil #6
• Natural Gas
• FGR Gases
• Dry Combustion Air
• Water Vapor
• NOTE: - Because of the varying levels of humidity

(0% in Natural Gas to 100% in the oil atomizing steam) streams 1-4 are considered dry.

• The humidity content of the different streams is

modeled using the 5th scalar for water vapor.

23

CFD Modeling

Numerical Models

• Combustion
• Primary Reaction Model
• Each fuel stream is considered to react independently of the others.
• Combustion air within a given cell is distributed proportionally between

the different fuel streams present.

• Chemical Equilibrium solution (Gibbs Free Energy Minimization) for the

reaction of each fuel stream with its portion of the combustion air.

• Transport equation for CO
• CO concentrations obtained from the chemical equilibrium model in the Reacting zones

are used as the source of CO production for the kinetically controlled transport equation

• Transport equation for NOx
• Reaction rate for thermal NOx production based on the Zeldovich mechanism
• Final combustion product concentrations are obtained by recombining

the products from each of the streams and transport equations.

24

CFD Modeling

Discretization: Detailed Burner

25

CFD Modeling

Discretization: Detailed Burner - 4 million cells

26

CFD Modeling

Inlet Boundary Conditions for the Boiler Simulations are obtained from the Detailed Burner Simulations

27

CFD Modeling

Discretization: Boiler & Burners - 8 million cells

28

Results and Analysis

Optimization of the Firing System

• 25 different burner angles and OFA placement configurations were

tested with CFD.

• The final firing system design allows for both Natural Gas and Oil #6

fireball flames to develop properly and be contained inside the furnace area.

• The final Firing System / OFA design allows for all combustion

products to be properly mixed within the furnace area.

• The final firing system design was also verified to operate properly at

50% turndown.

29

Results and Analysis

Natural Gas Firing

30

Results and Analysis

Natural Gas Firing Burner Level 1 - Velocity Magnitude (m/s)

31

Results and Analysis

Natural Gas Firing Burner Level 2 - Velocity Magnitude (m/s)

32

Results and Analysis

Natural Gas Firing Burner Level 3- Velocity Magnitude (m/s)

33

Results and Analysis

Natural Gas Firing Burner Level 4 - Velocity Magnitude (m/s)

34

Results and Analysis

Natural Gas Firing OFA Level - Velocity Magnitude (m/s)

35

Results and Analysis

Natural Gas Firing Temperature (K)

36

Results and Analysis

Natural Gas Firing Furnace Centerline - Temperature (K)

37

Results and Analysis

Natural Gas Firing Platen SH3 - Gas Temperature (K)

38

Results and Analysis

Natural Gas Firing Furnace Wall Heat Flux (W/m2) Right view Left view

39

Results and Analysis

Heavy Oil #6 Firing

40

CFD Analysis - Salamanca Unit 3

Oil #6 Firing Droplets diameter (m)

41

CFD Analysis - Salamanca Unit 3

Oil #6 Firing Velocity Magnitude (m/s)

42

Results and Analysis

Oil #6 Firing Burner Level 1 - Velocity Magnitude (m/s)

43

Results and Analysis

Oil #6 Firing Burner Level 2 - Velocity Magnitude (m/s)

44

Results and Analysis

Oil #6 Firing Burner Level 3- Velocity Magnitude (m/s)

45

Results and Analysis

Oil #6 Firing Burner Level 4 - Velocity Magnitude (m/s)

46

Results and Analysis

Oil #6 Firing OFA Level - Velocity Magnitude (m/s)

47

Results and Analysis

Oil #6 Firing Temperature (K)

48

Results and Analysis

Oil #6 Firing Furnace Centerline - Temperature (K)

49

Results and Analysis

Oil #6 Firing Platen SH3 - Gas Temperature (K)

50

Results and Analysis

Oil #6 Firing Furnace Wall Heat Flux (W/m2) Right view Left view

51

Results and Analysis

Dual-Fuel (54% Oil – 46% NG) Firing

52

Results and Analysis

Dual-Fuel (54% Oil – 46% NG) Firing Velocity Magnitude (m/s)

53

Results and Analysis

Dual-Fuel (54% Oil – 46% NG) Firing Temperature (K)

54

Results and Analysis

Dual-Fuel (54% Oil – 46% NG) Firing Furnace Centerline - Temperature (K)

55

Results and Analysis

Dual-Fuel (54% Oil – 46% NG) Firing Platen SH3 - Gas Temperature (K)

56

Results and Analysis

Dual-Fuel (54% Oil – 46% NG) Firing Furnace Wall Heat Flux (W/m2) Right view Left view

57

Results and Analysis

Comparisons of Natural Gas, Oil#6 and Dual Fuel Firing

58

Results and Analysis

Temperature Contours VS Type of Fuel Fired Furnace Centerline - Temperature (K) Gas Oil #6 Dual Fuel

59

Results and Analysis

Luminous Flame Contour VS Type of Fuel Fired Gas Oil #6 Dual Fuel

60

Results and Analysis

OPTIMIZED FIRING SYSTEM RESULTS

Case CFD T(K) FOP Cerrey T(K) FOP relative difference of FOP Temperatures Natural Gas 1606 1578 1.8 % Oil #6 1490 1469 1.4 % Dual Fuel (54% Oil – 46% NG) 1544 1480 4.3 %

61

Summary

• The STAR-CD simulations have enabled us to design a combustion system

with the following characteristics:

• The optimized burner firing angles enable complete mixing of combustion products

and Over Fire Air before leaving the furnace for all three firing modes.

• The temperature of the combustion products at the FOP (Furnace Outlet Plane)

closely match with Cerrey’s design predictions.

• The temperatures at the FOP correspond to the inlet temperatures of Platen SH3,

and these temperatures closely match design values so the Platen SH3 should not be subject to excessive heating.

• The gas temperature contours in the furnace area are similar for all three firing

modes; the oil flames being slightly hotter.

62

Summary

• The STAR-CD simulations have enabled us to design a combustion system

with the following characteristics.

• The Luminous Flame Contours are similar for all three fuels.
• The Furnace Wall Heat Fluxes are within normal values for natural gas and

heavy oil flames. Wall Heat Fluxes are normally higher for Oil than NG because

• f the increased radiation from the Oil flame. The wall heat flux corresponding to

the dual-fuel fireball is between the two extremes as expected.

• The steady-state simulation results show that emissions and temperatures

will be within technical specifications.

63

Conclusions

• The aerodynamics of the flow and the mass transfer inside the furnace are

extremely complex.

• Altitude creates an additional constraint which must be taken into account.
• The combustion system design must be adjusted to accommodate the size

and altitude of the boiler to be retro-fitted.

• CFD is an integral and critical part of BMA’s engineering process when

dealing with the interaction of so many complex physical phenomena.

• With so many operational requirements, CFD analyses are necessary in order

to gain enough information about boiler operation under different conditions in order to ensure that the design meets all of the project’s requirements.