06 Secondary Air Upgrades

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Secondary air system upgrades

Foster Wheeler Service Thailand boiler days

30-31 October 2014

Sami Marjeta, Technical Manager, FWST


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Typical air systems in FW CFB

Fuel quality and effect of high volatile fuel

Scope of SA modification

Case examples

3D combustion modeling capabilities

Content

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Not all Foster Wheeler CFB have upper secondary air

Background variation in air system configuration

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1 2 3 4

PA X X X X

UPA X - X -Lower SA X X X X

Upper SA - X X X

Tertiary air - - - X

1 Low volatile fuel CFB

2 Medium or high volatile fuel

CFB

3 Large variation in CFB fuel

volatile content

4 BFB


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Typical trend in many cases is that volatile content of the fuel mixture

increases from original design

Possible reasons:

Bio fuel burning starts or increases

Coal quality decreases i.e. gets younger Biomass quality development

Fuel quality development

3


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What was good 20 years ago, is not good anymore

More and more strict emission regulations demand optimized

combustion

Load variation demands

Base load plant can have to face load control requirements from

electrical or process network (example USA plants due to shahle gas)

Lower fuel heating value coupled with higher volatile content

increases furnace velocities

All sums up to higher elevation combustion zone in furnace

CO corrosion

Increased requirements for air feeding

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Typical scope

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Process assesment (what is the current process situation)

Process measurements

CFD modeling (standard air nozzle or 3D combustion modeling)

Engineering

Conceptual

Sizing of ducts, nozzles, fans

Layout

Detail

Modification

Final commissioning and tuning Underlined items are mandatory, but without detailed information, their

results may wary

Recommend to have all (if you save in modeling, you lose in

commissioing and tuning)

Scope

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Depending on how difficult the demand is


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Case examples

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Case E.ON Coal Fired CFB Boiler, Finland

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Problem

Erosion in furnace and in backpass

potential reason high flue gas velocities and understochiometric

conditions with local high temperatures

erosive ash and bed quality

Goals

Balancing horizontal O2-, CO- and temperature profiles in lower furnace

by secondary air nozzle modifications, location and type defined by CFD

modeling

Balancing horizontal O2-, CO- and temperature profiles by gas profile

measurements for minimizing afterburning in upper furnace

Decrease of fly ash unburned carbon

Decrease of erosion in backpass by Vortex killer fins in vortex finder

and flue gas guidance plates

Case E.ON Coal Fired CFB Boiler, Finland

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Number of problems, number of solutions...


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Makes an air vortex

Typically used above fuel feeding to

provide a shroud of air between CO

gas and membrane wall

Lower penetration than standardnozzle

Secondary Air Nozzle Type: Vortex Nozzle

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Old nozzles, air veloc ities > 100 m/s Vortex nozzles, air veloc ities< 60 m/s

Air nozzle Velocities


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Furnace Velocities

Old nozzles, air veloci ties > 100 m/s Vortex nozzles, air veloci ties< 60 m/s


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Results

Results after gas profile measurements and secondary air

improvements:

during winter 2004-2005 no tube leakages, earlier ~3 stops/winter

fly ash UBC decreased from 30 % to 12 %

back pass erosions was decreased


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Completed SA modification at NPS units 7-8

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Secondary air modification made to NPS PB#7 & 8:

Gas profile measurement showed CO rich zero O2-zones (=reductive

conditions causing corrosion-erosion phenomena) near side walls

Reductive conditions removed by secondary air nozzle relocations and

by adding new secondary air level

NPS secondary air modification

Introduction


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Fuel quality development

No antracite anymore

Younger bituminous coal

Biomass share increase

Original flue gas amount was less than today Higher furnace velocity


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Background


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The scope of changes (which nozzles blocked, where to put new

nozzles etc.) was selected based on operation experience, wall

thickness and gas profile measurement and experience.

No 3D combustion modeling was done

When the changes were implemented, gas profiles were measured

again and dampers adjusted


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Scope


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3D combustion and Glow units 1-2

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Based on 1 directional (vertical) furnace model

The model has been applied to hundreds of

different cases and the code has been

developed based on the validation tests

Typical model construction time 1 week

Modeling 1-2 weeks, depending on amount of

cases

1 calculation run, i.e. how about if we move this

SA nozzle a little bit?takes 2-3 hours

3D model details

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Furnace dimensions 13.2 m x 6.6 m x 32.8 m.

Mesh size 74 400 calculation cells.


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3D combustion model

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What can it do?

Can tell what is predicted flue gas component (SO2, CO, Nox, O2) in

each furnace location (X, Y, Z direction)


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3D combustion model

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It will tell you where to add air and how much to get rid of CO close

to walls

It can help in lowering emissions, limestone or NH3 consumption

Model is validated and fine tuned with profile measurements

Profile measurement

point i.e. Validation

point


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Unique FW capability

Field gas profile measuments coupled to highly developed

combustion computer model

Each gas component profile

3D Combustion modeling Glow 1-2

existing air

supplyproposed air

supply


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During the boiler lifetime, fuel variety can be significant

CFB design can be altered to better match new fuel properties

Grid changes

Air system changes

Fuel feeding changes

If there is a clear trend in fuel quality development, boiler capabilities

can be modified

There is a CFB for every fuel, but not all CFBs can burn any fuel!

Conclusion


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06 Secondary Air Upgrades