Optimization of a Dual -Fuel Low-NOx Combustion System for a …mdx2.plm.automation.siemens.com/sites/default/files/... · 2018-05-06 · Optimization of a Dual -Fuel Low-NOx Combustion

Optimization of a Dual-Fuel Low-NOx Combustion System for a Tangentially-Fired Utility Boiler

Operating at a High Elevation.

byF. 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 1Y4Tel: (514) 382-8866www.bma.ca

Images courtesyof

Cerrey S.A. de C.V.Av. Republicana Mexicana 300San 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

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Introduction

Motivation

• Stricter pollutant emission regulations are presenting new challengesto boiler and burner manufacturers in order to meet the newemission specifications.

Develop new Boiler/Burner designs

Retro-fit existing boilers with new combustion systems capableof 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

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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 theexisting boiler’s operational characteristics

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Problem description

Tangentially fired boilersOperating principal:

• Four or more burners located in the corners or on the boiler wallsare 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.

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• 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 couldincrease until it reaches the furnace walls.

• If the target circle diameter is too small; the jets could impinge with oneanother and the rotating motion of the flow is lost.

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

• Jet penetration is a function of jet momentum.

• Increased jet momentum means increased penetration and higher jetvelocities at the location of the target circle and vice versa.

Problem description

Tangentially fired boilers

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• Increasing or decreasing the momentum of the burner jets will change thefurnace 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 replacedwith burners having jets with more or less momentum than the originalburners.

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

Problem description


)(NVAVVmG =⋅==→→→•

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• Example: Comparison of a natural gas flame with standard target circle diameterfor a single burner level T-fired boiler at sea level and at 5200ftaltitude.

Problem description


Fireball at sea level Fireball at 5200 ft

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• Example: Comparison of a natural gas flame with the target circle diameteroptimized for operation at sea level for a single level T-fired boilerwith the same design operating at 5200ft altitude.

Problem description


Fireball at sea level Fireball at 5200 ft

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


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

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• Combustion of the fuel increases the gas temperature in the burner jet and causes the expansion of hot combustion gases.

Problem description


Temperature (K)

Dual-Fuel (54% Oil – 46% NG) Firing

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

Problem description



Velocity Magnitude (m/s)

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• The local acceleration is more pronounced and localized for heavy oil burners becauseall the fuel is concentrated in front of the oil gun.

Problem description




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• The jets from Oil flames usually have much higher momentum and penetrationthan 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 ongas firing does not vary proportionally to the effect on oil firing.

Problem description


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• 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 theoptimal firing angles for Natural Gas and the optimal firing angles for Heavy Oil.

Problem description


Natural Gas Firing

Burner Level 1 - Velocity Magnitude (m/s)

Oil #6 Firing

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• Even when the optimal firing angles for Natural Gas have been modified toaccommodate oil firing, the final firing angles for Natural Gas yield a muchsmaller vortex diameter in the center of the furnace.

• Reducing the vortex diameter helps keep the reacting regions away fromthe furnace walls.

Problem description


NG-Firing – Original sea level firing angles


NG-Firing – Optimized firing angles

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•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

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

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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 typeof fuel firing by comparing theoretical and CFD predictions of Furnace OutletPlane (FOP) gas temperatures.

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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

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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.

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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 recombiningthe products from each of the streams and transport equations.

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CFD Modeling

Discretization: Detailed Burner

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CFD Modeling

Discretization: Detailed Burner - 4 million cells

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CFD Modeling

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

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CFD Modeling

Discretization: Boiler & Burners - 8 million cells

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Results and Analysis

Optimization of the Firing System

• 25 different burner angles and OFA placement configurations weretested with CFD.

• The final firing system design allows for both Natural Gas and Oil #6fireball flames to develop properly and be contained inside thefurnace area.

• The final Firing System / OFA design allows for all combustionproducts to be properly mixed within the furnace area.

• The final firing system design was also verified to operate properly at50% turndown.

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Natural Gas Firing

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Natural Gas Firing


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Natural Gas Firing


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Natural Gas Firing

Burner Level 3- Velocity Magnitude (m/s)

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Natural Gas Firing


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Natural Gas Firing

OFA Level - Velocity Magnitude (m/s)

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Natural Gas Firing

Temperature (K)

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Natural Gas Firing

Furnace Centerline - Temperature (K)

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Natural Gas Firing

Platen SH3 - Gas Temperature (K)

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Natural Gas Firing

Furnace Wall Heat Flux (W/m2)

Right view Left view

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Heavy Oil #6 Firing

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CFD Analysis - Salamanca Unit 3

Oil #6 Firing

Droplets diameter (m)

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CFD Analysis - Salamanca Unit 3

Oil #6 Firing


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Oil #6 Firing


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Oil #6 Firing


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Oil #6 Firing

Burner Level 3- Velocity Magnitude (m/s)

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Oil #6 Firing


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Oil #6 Firing

OFA Level - Velocity Magnitude (m/s)

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Oil #6 Firing

Temperature (K)

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Oil #6 Firing


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Oil #6 Firing


50


Oil #6 Firing



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Temperature (K)

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

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Temperature Contours VS Type of Fuel Fired


Gas Oil #6 Dual Fuel

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Luminous Flame Contour VS Type of Fuel Fired

Gas Oil #6 Dual Fuel

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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 %

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Summary

• The STAR-CD simulations have enabled us to design a combustion systemwith the following characteristics:

•The optimized burner firing angles enable complete mixing of combustion productsand 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 notbe subject to excessive heating.

• The gas temperature contours in the furnace area are similar for all three firingmodes; the oil flames being slightly hotter.

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Summary

• The STAR-CD simulations have enabled us to design a combustion systemwith 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 becauseof the increased radiation from the Oil flame. The wall heat flux corresponding tothe dual-fuel fireball is between the two extremes as expected.

• The steady-state simulation results show that emissions and temperatureswill be within technical specifications.

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Conclusions

• The aerodynamics of the flow and the mass transfer inside the furnace areextremely complex.

• Altitude creates an additional constraint which must be taken into account.

• The combustion system design must be adjusted to accommodate the sizeand altitude of the boiler to be retro-fitted.

• CFD is an integral and critical part of BMA’s engineering process whendealing with the interaction of so many complex physical phenomena.

• With so many operational requirements, CFD analyses are necessary in orderto gain enough information about boiler operation under different conditionsin order to ensure that the design meets all of the project’s requirements.

Documents

Optimization of a Dual -Fuel Low-NOx Combustion System for a …mdx2.plm.automation.siemens.com/sites/default/files/... · 2018-05-06 · Optimization of a Dual -Fuel Low-NOx Combustion