4_Combustion Air System

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Combustion Air systemTommy Lundin



© Metso

Requirements on a modern air system

• Good boiler control, allowing a wide range of load

and black liquor quality variations

Minimized emissions

High availability of boiler

Good and safe working environment

Flexibility for integration of NCG system

Good bed control with stable smelt flow

High reduction rate and low amount of unburned carbonin smelt



© Metso

Combustion Air System

• Supply air to each air level at the correct:

• Temperature

• Pressure

• Flow

• Pattern

• In order to:

• Transport air to combustibles

• Mix air with combustibles

Page 3



© Metso

Combustion Air System

•Affects:• Furnace temperature

• Reduction efficiency

• Carry-over

• Boiler deposits

• Running time

Page 4



© Metso

Multilevel Air SystemTM combustion zones

Stable, undisturbed

liquor spraying zone

with even droplet

distribution

Final combustion

zone, completes

combustion and

ensure an even flow

pattern for optimal

superheaterperformance

Optimum drying,

pyrolysis and heat to

char bed reactions

Char bed control,reduced atmosphere

and smooth smelt

removal



© Metso

Interlaced Air Jet Technology

Tertiary air High secondary air

Secondary air Primary air



© Metso

Arrangements of air system, example

Primary air

Secondary air

High Secondary air

Tertiary air

Load burners

The Multilevel air system

offers a compact ducting

and air port design

resulting in a good andsafe working environment



© Metso

POWER DIVISION

Multilevel air distribution system



© Metso

Aire de Combustion – Valdivia RB



© Metso

Celulosa Arauco y Constitución, Valdivia, Chile

Capacity 3100 t ds/24hSteam 131 kg/s

86 bar

485 °C

Start-up 2004

Air port layout:• Tertiary (4+3), 1000 cm²

• High Secondary on two walls *)

• Secondary on two walls *)

• Primary 132 pcs, 120 cm²

* 4 + 2 big ports 1000 cm²+ CNCG burner

* 3 + 4 small ports 240 cm²

•2 start-up burners/ each wall

•3 load burners on front wall



© Metso 2010-08-09, Nueva Aldea, RB upgrade - Training material11

The next pages introduce the combustion air

system.

The pages thereafter give advices regarding

the combustion.

Introduction

The modern Multilevel Air System™ supplied by Metso has

been developed to enable a maximum burning rate of blackliquor for a given boiler size. At the same time, the availability is

maximized while the emissions from the boiler are minimized.

The combustion air system



© Metso

The combustion

2010-08-09, Nueva Aldea, RB upgrade - Training material

INTERNAL

12

A stable combustion – air and liquor

The air contributes to the combustion of black liquor. The secondary air contributing to

the combustion at the front wall is coming from the back wall and vice versa.

When black liquor load and air distribution changes, the velocity dampers should be

changed equivalently to keep the optimal air pressure in the port. Thus, the pressure

should remain while the flow in the ports changes with the load change.

The composition of chemicals in black liquor may vary, which affects the viscosity.

Viscosity is controlled by the temperature of the black liquor, and variations in the

liquor heat value requires changes in combustion air amount and distribution.

At hardwood liquor firing, the intensity is often higher in the furnace. In thesesituations, the secondary air flow should be reduced and the overfire air flow

increased. When the combustion is performed over the whole furnace cross section,

the result is better reduction ratio and lower emissions.



© Metso

The combustion

• Proper air/liquor and fuel ratio, air distribution and a hot bed give the best result to

maintain a stabile combustion. One and often the best way to control the oxygen

content in the flue gas is to adjust the secondary and overfire air.

• Adjust the combustion so that the oxygen content (O2) in the flue gas after theeconomiser is about 2.8 % (dry, and depending on the load) and at the same time

check that there is a low carbon monoxide (CO) content in the flue gases, about

100 – 300 ppm.

• Adjustments of the combustion air amounts should be done very carefully andslowly. If a load increase is needed, add first air and after that add fuel. If a load

decrease is needed, reduce fuel first and after that reduce air.


INTERNAL

13

A stable combustion – air and liquor



© Metso 2010-08-09, Nueva Aldea, RB upgrade - Training material

INTERNAL

14

The combustion A stable combustion – optimization of the air system

To optimize the combustion and minimize the carry-over, it is important to

decrease the incoming air below the liquor guns as far as possible.

As to begin with, reduce the primary air flow to the benefit of the tertiary air flow,

which accordingly should be correspondingly increased. When it comes to a point

when the primary air flow cannot be further reduced, according to the shape of thechar bed and the amount of unburned smelt in the smelt spouts, reduce the

secondary air flow to the benefit of tertiary air instead.

When this is performed, it is important to have strict control over the optimization

and to follow the changes and affects in the boiler, to make sure that the amount

of air below the liquor guns is sufficient.

Control the content of sludge regularly, and check if there is much of unburned

smelt in the smelt spouts.



© Metso

The furnace bed

The furnace bed must be observed regularly from the beginning of liquor firing. Someconditions can lead to unburned liquor in the furnace bed.

High turbulence while large bed

The new air system increases the possibility to fire a larger amount of liquor in the boiler

with good results in form of small amount of carry-over and high reduction efficiency.

At the same time, the powerful air jets and the turbulent mixing causes that the

combustion in the lower part of the furnace is sensitive for adjustment of air distribution

and air pressure. If there is a large bed in the boiler at the same time as the firing

intensity increases, the smelt flow from the bed will also increase drastically. That is why

it is important to have a controlled operation of the boiler.


INTERNAL

15



© Metso

The furnace bed

Liquor distribution

It is very important to achieve a good distribution of the liquor, to prevent development of

ridges in the outer edge of the bed with pools of smelt behind. The bed should also be

prevented from becoming unstable and collapse and cause surges. This can spurt out

through the smelt spout openings.

During the cold start-up, liquor may enter the furnace bottom unburned (wet) and

this can result in plugging in the primary air nozzles or in the spout. This can alsohappen if the furnace is too cold, the liquor spraying pressure is too low or the aiming ofthe liquor guns is incorrect.

In these cases, increase the load of the burners, then clean the air ports and increasethe combustion air by opening the dampers for a short period of time on both sides ofdark (wet) area so that the wet area starts to burn again. Also check the liquor guns and

clean them if needed. If there is mechanical failure in a nozzle, replace it.

If this does not help, stop liquor firing and burn off the wet bed with the burners. Whenthe situation is back to normal, start liquor firing again.


INTERNAL

16



© Metso

The furnace bed

Note!

It is very important to prevent a sudden opening of plugged smelt spouts since this willresult in large, uncontrolled smelt discharge through the open spouts which might causeexplosions or splashes in the smelt dissolving tanks.

To avoid the described problems, monitor the bed and the combustion all the time.

Do not let unburned liquor flow through the smelt spouts to the dissolving tank.


INTERNAL

17



© Metso

POWER DIVISION

Primary air port design

Benefits

• Cast iron ports

• Proven design

• Low maintenance



© Metso

POWER DIVISION

Secondary and High Secondary air level

Benefits• Non welded sleeve

design

• Easy to change

sleeve

• Pressure control

possibilities

Valdivia, Chile Billerud,Gruvön, Sweden



© Metso

POWER DIVISION

Tertiary air level

Valdivia, Chile

Benefits• Non welded sleeve

design

• Easy to change

sleeve

• Pressure control

possibilities

Billerud,Gruvön, Sweden



© Metso

POWER DIVISION

• DNCG, CNCG, vent- and mix tank gas can be

integrated optimally with Multilevel Air System to form

Multifuel Combustion System.

NCG and Vent Gas

CNCG at secondary air level DNCG and vent gas at tertiary air level



© Metso

POWER DIVISION

Summary – Benefits of Multilevel Air SystemTM

• Each air level is located anddesigned for optimal boiler

control , using 2-wall interlaced

and 4-wall primary air patterns

• Ensures vigorous mixing and

efficient final combustion withlow CO, TRS and VOC

emissions

• High availability through a well-

proven design and arrangement

0721



© Metso

POWER DIVISIONTypical air elevations



© Metso

POWER DIVISIONWide operation range





Air curves monocrome



© Metso

POWER DIVISION

Forced draft fans

Primary air fan: Net Test block• Air flow nm3 /s 28.6 37.8

• Total pressure Pa 2380 3100

Secondary air fan: Net Test block

• Air flow nm3 /s 52.7 75.1


Tertiary air fan: Net Test block

•Air flow nm3 /s 33.2 51


Flow control type: Inlet vane, with pneumatic or electric

actuator



© Metso

POWER DIVISIONFlue gas system



© Metso

POWER DIVISION

Induced draft fans

• Number pcs 3• Drive type variable-frequency drive

• Induced draft fan 33.3 % MCR Test block

55 % MCR

• Flue gas flow / fan nm3 /s 55.0 91.0


• Temperature °C 180 180

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4_Combustion Air System

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