2.1 Process Introduction(1)

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Asiakirjan nimi - Name of Document Tunniste - Identification Rev.

PROCESS DESCRIPTIONS2.1 INTRODUCTION

CFBT-582-1 1

Osasto - Department Kohde - Subject Sivu - Page

Power GenerationSystems

RAPP PB3 PROJECTCFB MULTIFUEL BOILER

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1 2005 - 08 - 30 T. Anttonen

0 2005 - 02 - 11 T. Anttonen

Muut.Rev. PvmDate LaatinutPrepared by PvmDate TarkastanutReviewed by PvmDate HyvksynytApproved byKLY-1 1997-05 P:\582-RAPP-PB3\int\Kayttoonotto\koulutus\Training Manuals\Folder 1\2.1Process_introduction(1).doc

This document is the exclusive intellectual property of Kvaerner Power Oy and is furnished for the sole purpose of operating and maintaining of the specific project, and re-use of the document for any other project orpurpose is prohibited. The document or the information shall not be reproduced or copied, or disclosed to a third party without prior written consent of Kvaerner Power Oy.

Kvaerner Power Oy, Tampere, Finland 2005

TABLE OF CONTENTS

2. PROCESS DESCRIPTION.................................................................................... 2

2.1

Introduction............................................................................................................ 2

2.1.1 General.................................................................................................................. 22.1.2

Circulating Fluidized Bed Process ......................................................................... 3

2.1.3

Condensate and feedwater system ....................................................................... 5

2.1.4 Boiler water/steam system..................................................................................... 62.1.5

Vents, Drains and Blowdown............................................................................... 12

2.1.6 Chemical Dosing and Sampling........................................................................... 132.1.7

Air System ........................................................................................................... 15

2.1.8 Flue Gas System ................................................................................................. 182.1.9 Solid fuel handling system ................................................................................... 202.1.10

Limestone Feeding System.............................................................................. 22

2.1.11 Sand feeding system........................................................................................ 222.1.12

Coarse Material Removal System.................................................................... 23

2.1.13 Fly Ash Handling System ................................................................................. 242.1.14

Oil System........................................................................................................ 24

2.1.15

Sootblowing steam system............................................................................... 25

2.1.16 Cooling Water System ..................................................................................... 262.1.17

Medium and Low Pressure Steam System ...................................................... 26

2.1.18 Compressed Mill Air System ............................................................................ 262.1.19 Compressed Instrument Air System................................................................. 26


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2. PROCESS DESCRIPTION

2.1 Introduction

This section describes general operation principles and technical

data of a circulating fluid bed boiler with auxiliaries.

2.1.1 General

The unit will be designed to generate 650 t/h of high pressure steam,

at 142 bar(a) and 540C from feedwater at 130 C.

The main fuel for the boiler is bituminous coal. Also bark and

woodwaste can be fired. The design allows also use of smalleramount of sludge (reservation made for sludge silo and feeding

equipment). Boiler start-up burners are designed for heavy fuel oil

and diesel oil.

Kvaerner's circulating fluidized bed boiler is a single drum unit which

consists of a gas-tight, membrane construction furnace, three

parallel cyclones and a second pass where the primary and tertiary

superheaters and smooth economizer section are located. In thethird pass are located finned tube economizers and air preheaters.

Kvaerner Power CFB technology ensures low emission and high

efficiency combustion process. The boiler is furnished with limestone

injection to meet sulfur dioxide emission requirements.


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2.1.2 Circulating Fluidized Bed Process

A fluid bed contains a mass of particulate solids through which an

upwardly flowing fluid is passed at a velocity sufficient to cause the

particles to behave like a liquid. The bed material consists mainly of

fine sand particles, fuel ash and limestone which can be fed to the

furnace to reduce SO2emission. The fluidizing medium consists of

air and flue gas produced by the combustion of the fuel.

In the circulating fluidized bed (=CFB) boiler the velocity of fluidizing

medium is so high that part of the particles is carried with the gas.

There are two distinctive phases of solids in the combustion

chamber, the dense phase and the dilute phase. The dense phase,

or bed, consists of fuel ash, sand, a small percentage of unburned

fuel and possibly of limestone. The fluidization gas elutriates fine

particles into the dilute phase. The dilute phase density has a direct

effect on the heat transfer rate in the furnace. Most of the solids

elutriated from the combustion chamber are separated from the flue

gas stream in the cyclone separators and cleaned flue gas flows to

the second pass. The separated solids fall down to the bottom of the

cyclones where so called loop seals are located. The loop seals are

small fluid beds, which act as a seal preventing flue gas to flow from

lower part of the furnace to the cyclones. From the loop seals solidparticles are returned back to the furnace.

Normally the bed temperature varies between 750 940 C and due

to large mass of the circulating material the temperature is almost

constant throughout the furnace. The bed temperature is controlled

by variating the air distribution, using flue gas recirculation and

changing excess air level.


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The fuel properties can vary within given range without adversely

effecting the combustion. The rate of combustion is so high that the

steam output can be increased/decreased rapidly by changing the

fuel flow according to the load demand. The lower part of furnace

walls and other areas where risk for erosion exists are covered by

refractory to minimize wearing in these sections. Fluidizing velocity is

designed to be a moderate to ensure low erosion impact into

refractory in dense bed area.

The welds are grinded in furnace walls and the number of openings

is minimized to avoid erosion. The boiler is furnished with the

functionally needed maintenance doors and sight glasses.

Moderate inlet velocity of flue gas is used in cyclones. The round

form of cyclone is used, which ensures high separation efficiency.

The cyclones, located between the furnace and the second pass,

are totally water cooled operating in the parallel natural circulation

loops to the furnace. The gas tight walls of the cyclones are of

membrane construction. From the flue gas side cyclones are

completely protected against erosions by refractory.

The thermal expansion of the cyclones and furnace is the same due

to same cooling media in water circulation. Therefore there is noneed for expansion joints between the cyclones and the furnace.


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2.1.3 Condensate and feedwater system

Condensate water from the mill processes and turbine is introduced

to the condensate collecting tank (not in scope). From the

condensate tank water is pumped to the deaerator. Make-up water

from the demineralization plant and the condensated water from the

primary air steam preheater are also led to the deaerator. In the

dearator most of oxygen dissolved in condensate and make-up

water is thermally removed by heating make-up water with the same

low pressure steam which is used to feedwater tank pressure

control. Feedwater tank serves as feedwater storage having about

15 minutes storing capacity when boiler is operated at MCR load.

The deaerator is integrated to the feed water tank so that the water

is first sprayed in a part of the steam space (pre-deaeration) and

then in the second stage the steam is injected to the water (final

deaeration). During start-up water is heated to the operating

temperature by leading low pressure steam below the water level of

the feedwater tank through a perforated pipe and by distributing the

steam evenly over the whole length of the feedwater tank.

From the feedwater tank water is pumped by direct drive feedwater

pumps (not in scope) to the boiler. Normally three parallel feedwaterpumps are in operation while the fourth one is stand-by.


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2.1.4 Boi ler water/steam system

Feedwater is introduced to the CFB boiler at the inlet of an

economizer. After the economizer sections water enters the steam

drum.

From the steam drum boiler water is fed to the furnace walls and

floor and to the cyclone walls via downcomers, supply pipes and

distribution headers. The furnace and the cyclones have separate

downcomers and circulation pipes and thus they form separate

circulation loops.

In boilers with natural circulation the circulation of water and

steam/water mixture is based on the change of specific density, i.e.

the heavier steamless water column in the downcomers presses thelighter steam/water mixture from the steam generating surfaces to

the drum.

From the steam drum the saturated steam flows to the superheaters.

The superheated steam temperature is controlled in two-stage

steam attemperators by spraying feedwater to steam flow.

High pressure parts of the boiler are protected by safety valve

system consisting of two safety valves located in the drum and two

located in main steam line before the main steam stop valve.


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2.1.4.1 Economizers

The feedwater is introduced at the both ends of the distribution

header of the first economizer section.

In the three stage economizer sections feedwater temperature is

raised by flue gas before entering the steam drum.

The flue gas flows downwards, across horizontally arranged

economizer sections. The feedwater rises counter/cross flow to the

flue gas. Two first two economizer packages are of finned tube

construction and the last, the third one is constructed of smooth

tubes. The advantage of the finned tube construction is the effective

heat transfer ability and space utilization.

2.1.4.2 Steam Drum

The purpose of the drum is to operate as a separator in the water-

steam circulation between the steam generator and the superheater

as well as boiler short-term water storage.

The feedwater coming from economizer enters the drum at the both

ends through a perforated distribution pipe. From the drum, water is

supplied to the steam generating surfaces, furnace and cyclones viadowncomers. From the steam generating surfaces the steam/water

mixture is returned back to the drum through cyclone type steam

separators. After cyclone separators steam is furthermore dried in

chevron type secondary scrubbers before leaving steam drum.


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2.1.4.3 Furnace

The size of the furnace is determined by the design fuels and the

required evaporation capability. The heat released in the furnace is

transferred effectively by the means of radiation heat transfer and

particle convection to the furnace wall tubes.

Proper water circulation in the furnace walls is ensured by using a

sufficient amount of downcomers, circulating pipes and supply and

collection headers. The water circulation in the furnace walls is

carried out by natural circulation.

By downcomers (5 pcs) originating from the drum, the water is

introduced to the level of the lower section of the furnace.

Distribution headers of the furnace are fed through supply pipesoriginating from lower part of the downcomers. The furnace bottom

forms a fluidized combustion grate which is also fully water cooled

since it is used to provide water to the rear wall distribution header.

In the furnace wall tubes part of the boiler water is evaporated.

Furnace walls are made of gas tight membrane construction.

From the collection headers of each wall section the water/steam

mixture is introduced by the upper circulation pipes (raiser pipes)

back to the drum.

The furnace is equipped with tight inspection openings (sight

glasses) in order to monitor the burners, grate and furnace. For the

maintenance and repair of the furnace and heat transfer surfaces the

boiler is equipped with tightly closed access doors.


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2.1.4.4 Cyclone separators and loop seals

The size of the cyclones is determined by produced flue gas amount

and the required dust separation efficiency. The heat released in the

cyclones and loop seals is transferred by particle convection to the

refractory layer protecting wall tubes. Gas tight cyclone and loop

seal walls are same type of membrane construction as furnace

walls.

By downcomers (2 pcs) originating from the drum, the water is

introduced through supply pipes to distribution headers of the loop

seals. After flowing through loop seal tubes water/steam mixture

enters to wall tubes of the cyclones. In upper parts of the cyclones

water/steam mixture is collected to round shaped collection headers

and then led through upper circulating pipes (raisers) back to the

steam drum.

Since cyclones and loop seals are totally refractory covered only

little heat is transferred to those evaporation surfaces. Most of

evaporation takes place in the furnace wall tubes.

2.1.4.5 Superheaters

The purpose of the superheater section is to achieve a steam

temperature of 540 C after the superheaters.


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The superheater has been divided into three stages. The first stage

(primary superheater) is a convective type superheater (located in

the 2nd pass), the second stage (secondary superheater) is a

radiant type superheater (located inside the furnace) and the third

stage (tertiary superheater) again a convective type superheater

(located in upper part of the 2nd pass).

From the drum the saturated steam is introduced by connection

pipes (saturated steam pipes) to the 2nd pass front wall distribution

header. Second pass walls, roof and load carrying tubes (supporting

tubes for heat transfer bundles in the 2nd pass) are the first part of

the primary superheater. Steam flows first downwards in front wall

tubes and is then divided to side wall tubes and to first row of load

carrying tubes. Steam which has flown upwards through side walls is

collected to upper headers of the side walls and then led through

connection pipes to 2nd pass roof distribution header. Steam then

flows downwards through second pass roof and rear wall tubes to

rear wall collection header. To this same header is collected steam

from load carrying tubes after it has flown the second row of carrying

tubes. From the second pass rear wall collection header steam flows

through connection pipes to the actual primary superheater sections.

The actual primary superheater sections (2 pcs) are located in themiddle part of the 2nd pass. In the superheater tubes the steam

flows counterflow to the flue gas. The primary superheater sections

are supported from the above mentioned load carrying tubes.


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From the collection header of the primary superheater the steam is

introduced to two parallel steam attemperators. Attemperator is

divided to parallel units because then it is easier to control

temperature across the wide superheater. The purpose of the first

stage steam attemperators is to keep steam temperature after the

secondary superheater on the required level.

The steam attemperators are so-called spraying attemperator where

the feedwater is injected into the steam. While being vaporized and

superheated it binds the heat whereupon the temperature of the

steam leaving the attemperators falls. The spraying attemperator

consists of a spraying chamber, a spraying nozzle and an internal

venturi tube.

The vertical secondary superheater is located in the upper part of

the furnace, near the furnace front wall. It is a radiant type

superheater that operates on a so called parallel-flow principle,

where steam is led to superheater lower part header and steam then

flows upwards to collection headers of the superheater elements

located above the furnace roof.

From the collection header of the secondary superheater the steam

is introduced to the second attemperator stage. Also thisattemperator is divided into two parallel spray chambers located on

both sides of the boiler and in these units steam temperature is

controlled to correspond required temperature after the tertiary

superheater section.


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From the second stage attemperators steam flows to the tertiary

superheater sections located in the upper part of the second pass.

Tertiary superheater consists of two sections and is convective type

superheater where steam flows counterflow to flue gas flow. From

the final superheater collection header steam is lead to the steam

turbine.

2.1.5 Vents, Drains and Blowdown

Vents are supplied for the purpose of venting the boiler during start-

up and shutdown. High pressure part vents are piped to the common

collection funnel to where vent valves are also gathered.

Air and vent steam are then led to the blowdown tank vent pipe. Also

water coming to funnel with vents is drained to that same vent pipe.

Drains are used for the drainage of the boiler during start-up and

shutdown. The drains from the economizer, furnace, cyclones,

superheaters and sootblowing system are piped to a common

drainage header and further to the blowdown tank. Drains from low

pressure parts are introduced to the blowdown tank through own

drainage header.

One furnace drain, which can be used for limiting drum high water

level, and feedwater tank overflow pipe are connected directly to

blowdown tank.

In the blowdown tank hot drains are cooled down and then led to the

sewer.


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All high pressure part vent and drain lines are equipped with two

sequential shut off valves.

Since small amounts of unvolatile matters (salts) enter the boiler

continuously with feedwater, these are enriched in water and the

quality of boiler water and steam becomes poor. To eliminate this

problem, continuous blowdown from the drum is taken out all the

time. The blowdown water is introduced to the continuous blowdown

tank where part of water evaporates due to lower pressure. Flash

steam (expansion steam) generated in continuous blowdown tank is

led to feedwater tank and condensate flows to blowdown tank where

it is cooled and finally led to sewer.

The flashed water vapor from the blowout tank is vented to

atmosphere via vent steam pipe to roof and water is lead to sewer.

2.1.6 Chemical Dosing and Sampling

Chemical dosing stations includes own tanks for:

oxygen scavenger

pH control chemical (amine) phosphate

The dosing tanks are used for preparation of chemical solutions. In

preparation of dosing solutions make-up water is used. System is

equipped with necessary ejectors for solution preparation and

venting fan, which removes harmful vent gases to safe location.


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Each tank is furnished with two parallel dosing pumps of which one

is in operation and the other ones are stand-by. Pumps used for

oxygen scavenger and pH control chemical dosing are diaphragm

type pumps and pumps for phosphate dosing piston type pumps.

The sampling system consists of:

sample probes

sample piping with valves

sample coolers

Sample tapping probes are installed in the condensate pipe before

deaerator, feedwater tank, feedwater pipe, continuous blow-down

pipe, downcomer, saturated and superheated steam pipes. The

water or steam is led into the sample cooler, where it is condensed

and cooled. Condensated and cooled sample is then analyzed in

laboratory or in on-line analyzers.

With continuous pH-monitoring are equipped the following samples:

condensate water

feedwater

boiler water


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With continuous conductivity monitoring are equipped the following

samples:

condensate water

feedwater from the feedwater tank and from feedwater line

boiler water saturated steam

superheated steam

In addition to those continuous O2-monitoring is provided for

feedwater line sample and continuous silica monitoring for boiler

water sample.

The adjustments to the continuous blow-down flow and to dosing

chemical feeding will be done based on the indications from the

samples. There are no actual control loops and the adjustments will

be done by the operator.

2.1.7 Air System

The combustion air is divided into primary (fluidizing) air, secondary

air and loop seal air. The combustion air for the start-up burners is

supplied from the secondary air system.

Combustion air distribution to different locations depends mainly on

fuel (coal / woodwaste) and boiler load. Boiler control system takes

care of air distribution according to preset values but in some

operation situations fine tuning may be required by the operators.


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Along with the low combustion temperatures, the combustion air is

staged to obtain low NOx -emission levels by utilizing fluidizing air

and secondary air. The fluidizing air is introduced into the furnace

through air nozzles penetrating the furnace floor. The secondary air,

which completes the combustion, is introduced through air ports

located on the furnace walls at two separate levels. The fluidization

results in an expanded combustion zone with high turbulence,

intimate solids to-gas contact that results in a high heat transfer rate

within the bed.

The bed temperature depends on the quality and amount of the fuel

in the bed and bed material circulation rate (fluidizing velocity).

2.1.7.1 Primary Air System

The primary air (fluidizing air) is introduced into the furnace by two

parallel high pressure primary air fans (2 x 60 %), located at ground

floor. The primary air pressure is controlled by inlet vanes and air

flows to two separately controlled windbox sections are controlled by

dampers.

Primary air is preheated by low pressure steam in steam coil

preheater, when needed, before entering tubular flue gas air

preheater.

Flue gas air heater bundles are made of vertically arranged plain

tubes where air flows inside the tubes and flue gas outside them.

The flue gas air heater sections are located in the lower part of the

third pass and they are the last heat transfer surfaces of the boiler.


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The purpose of flue gas air preheater is to increase combustion air

temperature, which helps the combustion of wet fuels. At the same

time it decreases flue gas outlet temperature and thus increases

boiler thermal efficiency.

The fluidizing air is introduced into the furnace from the windbox,

located underneath the furnace, through evenly spaced fluidizing air

nozzles penetrating the furnace floor.

2.1.7.2 Secondary Air System

The secondary air is fed into the furnace by two parallel high

pressure secondary air fans (2 x 60 %), located at ground floor. The

secondary air pressure is controlled by variable frequency drive of

the fan and air flows to separate sections are controlled by dampers.

Secondary air is not preheated.

The secondary air is introduced into the furnace through secondary

air ports on the furnace walls. The secondary air ports are located at

two elevations. The first level is located directly above the bed and

the second is above the fuel feeding openings few meters above the

bottom.

2.1.7.3 Loop seal air system

Fluidizing air for loop seals is produced by two parallel roots type

high pressure blowers. During the cold start-up both blowers are

running parallel but when normal operation temperature is reached

only one blower is needed and the other is stand-by.


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Air pressure is adjusted by rotation speed of the blowers and air flow

for each loop seal section is controlled by dampers.

2.1.8 Flue Gas System

2.1.8.1 Furnace and Cyclones

The boiler furnace is a combustion chamber where the thermal

energy bound in the fuel is released. The flue gas amount and

properties generated in the combustion process depend on the fuels

used.

In three parallel cyclones most of particles conveyed with flue gas

from the combustion chamber are separated and returned back tothe furnace.

2.1.8.2 Second and third pass

The walls in the 2nd pass are cooled by superheated steam flowing

in the wall tubes. After plain tube economizer flue gas is led to third

pass where walls are constructed with a plate casing.

In the second and third pass flue gas cools down when it flows

across superheater, economizer and air preheater sections.


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2.1.8.3 Electrostatic precipitator (not in scope)

Fly ash is removed from the flue gas in electrostatic precipitator (not

in scope), which comprises two parallel chambers and three fields.

Both ESP chambers are designed for 60% boiler capacity. Flue gas

ducts before the ESP chambers are equipped with guillotine

dampers.

Dust-laden flue gas from the boiler is sucked by means of a flue gas

fan into the inlet funnel of the electrostatic precipitator and further to

the precipitator casing where dust collection takes place. The

cleaned gas is led from the precipitator casing through the outlet

funnel via the flue gas fan and the flue gas ducts to the stack.

The collected dust falls down to the precipitator bottom cones and isled via ash conveyors to the ash silo.

2.1.8.4 Flue gas fans

The two (2 x 60 %) parallel operated radial-type induced draft (ID)

fans, located after the electrostatic precipitator, provide the draft

required to keep flue gas pressure in upper part of the furnace at

about -0,1 ... -0,2 kPa. The draft is controlled by the variable

frequency drives of the fans. The ID fans also discharge flue gas into

the stack.


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2.1.8.5 Recirculating gas system

When needed, part of the flue gas can be recirculated after the ID

fans through recirculation flue gas fan to the lower part of the

furnace. Flue gas recirculation system is used to lower combustion

temperature in the furnace and it can also be used to increase full

superheating range. Recirculating flue gas pressure in controlled by

inlet vanes of the fan and flow to separate sections is adjusted by

dampers. Recirculating flue gas duct before the fan is equipped with

guillotine damper.

2.1.9 Solid fuel handling system

2.1.9.1 Coal feeding system

From the coal storage (not in scope) crushed coal is fed by belt

conveyor (not in scope) to dividing drag conveyor (not in scope)

which transfer fuel to two coal silos located by the furnace left side

wall.

The coal is conveyed from the silos to the furnace with three

independent feeding lines.

Coal is discharged from the feeding silos by variable frequency

controlled chain dischargers, two pieces on each silo. Dischargers

carry coal to the mixing screws where it is mixed to the fuel coming

from biofuel silos. After mixing screws fuel (or fuel mixture) goes to

drag chain type feeding conveyors.


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On the furnace front wall fuel is divided to two parallel drag

conveyors from which the fuel is divided to the six feeding points, so

that each drag conveyor is feeding three separate feeding points. On

the rear wall side there is only one drag conveyor. Each of the three

conveyors are equipped with variable frequency drives.

The even distribution of fuel to all feeding points is controlled by

metering screws on each feeding points. Also these screws are

variable frequency controlled.

Each fuel feeding chute is equipped with rotary lock feeders in order

to prevent any backflows from the furnace. The lower part of each

fuel feed chutes on the front wall is equipped with constant speed

feeding screw. These screws are equipped with a feed/purge air

system that improves the fuel injection and distribution while also

cooling the feeding system. On the rear wall fuel is fed from the

rotary feeders directly to loop seals.

2.1.9.2 Bark feeding system

Biofuel is fed from the bark yard (not in scope) by a belt conveyor

(not in scope) to dividing drag conveyor (not in scope) located above

the biofuel feeding silos.

Round shaped bark silos are equipped with slewing screw

dischagers. Dischargers drop biofuel to drag conveyors, which

furthermore take biofuel to mixing screws, where biofuel and coal

are mixed. After that biofuel is fed to the combustion chamber

through same conveyor system and feeding points as coal.


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Bark flow is controlled by variable frequency controlled electric motor

drives of the silo discharger, drag conveyor and metering screw.

2.1.10 Limestone Feeding System

Limestone system is used to reduce flue gas SO2emission.

Big and moist limestone particles are first crushed and dryed in a

separate limestone prehandling factory consisting of limestone

receiving hopper, intake belt with magnet, mill feeding hopper with

feeder, integrated mill and separator, cyclone, baghouse filter, diesel

oil fired burner, fans and all necessary connections in the drying and

crushing loop. Dried limestone is then conveyed to limestone

storage (or feeding) silo by bucket conveyor.

From the storage silo limestone is discharged by variable frequency

controlled dosing screws. After screws there is a rotary lock feeder,

which drops the limestone to the pneumatic feeding lines. Pneumatic

feeding is divided in six feeding points at furnace front wall. Feeding

openings are located close to the fuel feeding points. The pneumatic

conveying air comes from separate high pressure blower.

2.1.11 Sand feeding system

Sand handling system is used to produce suitable bed material for

fluid bed combustion.


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Sand handling system includes sand receiving hopper with

discharger screw, elevator type of conveyor, rotary feeder and

piping.

Sand is received to the hopper (volume 20 m3), which also serves

as a small storage. To this hopper comes also sieved ash from the

bottom ash screening. Sand is discharged from the hopper via screw

into the elevator which lifts sand up enough to be able to gravitation

fed it via rotary feeder and piping to fuel feeding screw located on

the front wall of the furnace.

2.1.12 Coarse Material Removal System

The bottom ash, including coarse particles, is removed from the

furnace through twelve chutes, which penetrate the furnace floor and

the primary air windbox. The coarse material chutes are equipped

with pneumatic valves, which are opened only when bottom ash is

discharged. The bottom ash from the chutes falls into the water

cooled bottom ash screws (3 pcs).

The cooling of these bottom ash screws is done with closed cooling

water system, which includes unpressurized cooling water tank, twocirculating pumps (with the other pump as a reserve), heat

exchanger and all the necessary piping needed. The circulating

water is cooled via heat exchanger, which is using cooling water of

temperature 32 C.


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Bottom ash is transferred from the screws by bottom ash conveyor

to bottom ash screening. From the screen coarse material is led via

drag conveyor and bucket elevator to ash silo and fine particles are

returned to sand hopper. When it is not necessary to return fine

material to the process, screening system can be bypassed, and all

bottom ash material is conveyed to ash silo.

2.1.13 Fly Ash Handl ing System

Fly ash consists of the ash in the fuel, unburned carbon, fine sand

fractions and limestone. The fly ash is mainly collected from the flue

gas in the electrostatic precipitator. The fly ash is collected from the

electrostatic precipitator hoppers to the drag conveyors and

transferred mechanically to ash silo.

Fly ash retained in the 2nd and 3rd pass hoppers is collected and

mechanically transferred to the ash silo. Fly ash silo is equipped with

moisturizing screw discharger.

2.1.14 Oil System

Heavy fuel oil and diesel oil are used as a boiler start-up fuels in six

start-up burners. Diesel oil is used also for limestone drying. Both

oils are delivered from pumping units, designed for 2 x 100 % burner

capacity of power boiler including all necessary equipment.


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The start-up burners are located on furnace walls, two on each side

wall and two on rear wall. The combustion air is supplied from the

secondary air ducting. The start-up burners are utilized to heat the

bed (sand or ash) to the temperature required to begin the firing of

solid fuel.

The combustion air to the burners is measured with aerofoil type

flow measuring devices and controlled with a control damper.

Burners utilize a gas electric igniter system. The propane gas

igniters offer maintenance free igniters, which allow in a very smooth

burner start-up operation.

2.1.15 Sootblowing steam system

The boiler is furnished with sootblowers for the removal of ash

deposits from the heat transfer surfaces.

Due to wide second and third pass width sootblowers are located on

the both sides of the boiler. The tertiary superheater packages are

equipped with totally retractable sootblowers. The multi-nozzle type

sootblowers with motion (partial stroke) are utilized to clean the

primary superheaters, plain tube economizer and air preheaters.Finned tube economizers are equipped with rake type sootblowers.

Sootblowing steam is taken after the primary superheater. The

sootblowing steam pressure is reduced by pressure reduction valve

prior to the sootblowers and system is protected against

overpressure by safety valve.


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2.1.16 Cooling Water System

Closed circuit cooling water is furnished to the heat exchanger of the

closed circulation of the water-cooled bottom ash screws, feedwater

pumps, sample coolers and to five lubrication oil units of the fans.

2.1.17 Medium and Low Pressure Steam System

Medium pressure steam is used for heavy fuel oil preheating and oil

atomizing in the burners.

Low pressure steam is used for feedwater tank pressure control and

combustion air preheating (primary air) and as extinction steam for

fuel feeding equipment.

2.1.18 Compressed Mill Air System

Compressed mill air is taken from the existing compress air system.

The biggest mill air consumers are the start-up burners, fuel feeding

rotary feeders cleaning and limestone storage silo fluidizing. Some

mill air is also used in ash, coal and limestone silo filter cleaning and

for bag filter cleaning in the limestone prehandling.

2.1.19 Compressed Instrument Air System

Instrument air is primarily used for burner flame detector cooling and

for pneumatic actuators of control valves and dampers.

2.1 Process Introduction(1)

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Transcript of 2.1 Process Introduction(1)