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High Pressure Cogeneration for Sugar Sector in Pakistan

Services to Technology

Providers Training and Capacity building activities

with HP boiler manufacturers

Session 1: the water and steam system Trainer: Frans Baltussen

Date: first half 2015

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EUROPEAN UNION


Sugar processing flow chart

See next sheet

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Sugar processing flow chart

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The co-generation plant in sugar mill

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The co-generation plant in existing Sugar mill configuration with 25 bara system

Boiler

Mill house

shredder

Crushing rolls

Power house Sugar

process

house

De-superheater

2,2 bara 130 C

Drying steam 4 bara,170 C

Steam bypass

PT

TT

25

bar

a,

35

0 C

PT

Bagasse

E

VSD

Boiler

PT

Bagasse

E

VSD

deaerator

E

Condensate vessel

Clean condensate 85% of input steam, 95 C

E

Contaminated condensate and steam

RO and

softening

plant

1,2 bara 105 C

E

Sugar mill own consumption

Surplus power in public grid

PT

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The boiler is used for generating steam at 25 bara and 350 C. Efficiency of boiler is low with approx. 72% on LHV 2. The steam is used in mill house to drive the rollers and shredder. 3. The rest of the steam is used for generating electric power. A part will be used in sugar mill and surplus will be supplied in public grid. 4. Steam from all back pressure steam turbines is fed to the evaporators in the sugar process house. This steam should be slightly overheated. However not too much. 5. The sugar cane has a certain amount of cellulose. It is called bagasse. Can be used in boilers to generate the steam required.

This part will be changed into a HP cogeneration system

Bagasse in carrier

The co-generation plant in existing Sugar mill configuration with 25 bara system

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1. TCD: This is the installed capacity of the sugar mill for crushing cane specified in Tons Cane per Day ( TCD) 2. Duration season: The total season of crushing is depending on the area. It is 110 till 150 days. 3. Utility factor: How much period of the total season the sugar mill have been in operation. 4. Bagasse amount: the cellulose in the sugar cane which will be used for firing in the boilers. This is mostly approx. 29 till 30,5% of the crushed cane weight. 5. Process steam demand. This is the process steam amount necessary in the process house for making the sugar. This amount depends on area and system used. The process steam amount in Sindh area is in example higher then in other parts of Pakistan. It varies from 40 to 55% of crushed cane. 6. Own electricity demand mill house plus sugar process: The drives of boiler are mostly electric, in mill house it is mostly all with steam turbine. When all are electric drive then the electric consumption is 30 till 35 KW per crushed ton of cane.

Indicators use in sugar industry:

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Boiler used in existing sugar mills for producing 25 bara steam at 350 C

To ID fan and stack 230 C

Flue gas air preheater

Space in which sometimes a small economizer is placed

Steam drum

water drum

Bi- drum design

furnace

Dumping grate

Air supply

Bagasse spreader

superheater

Steam outlet

Bagasse supply from feeder

Fly ash arrestor

Efficiency approx. 72% on LCV

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HP bagasse fired boiler 140 tph 65,7 barg 485 C, single drum and self supporting

Economiser 3

evaporator

HP superheater single-drum

Flue gas Air preheater plus steam air preheater

cyclones

Economiser 1

Economiser 2

In HP cogeneration system are nowadays 2 systems used in sugar mills: • Bi-drum suspended design • Single drum self supporting design

To ID fan and stack 150 till 160 C

Desuperheating station

furnace

Dumping grate

Bagasse spreader

Efficiency approx. 87% on LCV

Over firing air

22

50

0

Boiler types – self supporting

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HP bagasse fired boiler 140 tph 65,7 barg 485 C, bi-drum drum and suspended

Boiler types - suspended

Bi-drum HP superheaters

Economiser

Flue gas Air preheater

cyclones

furnace

Dumping grate

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Steam is favored for: • Power production • Heat transfer It is a unique combination of: • High thermal capacity • High critical temperature • Wide availability • Non toxic nature It has a high thermal capacity and high expansion capabilities which makes equipment small and is a clean product. The properties of water and steam. 1. Enthalpy in KJ/kg: the internally stored energy as per unit mass of flowing stream 2. Specific entropy in KJ/kgK: is a measure of thermodynamic potential of a system in the units of energy

per unit mass and absolute pressure 3. Specific volume in m3/kg: is the volume per unit of mass

Water and steam as medium in co-generation plants

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The properties of water and steam are tabulated in steam tables by International Formulation Committee (IFC) which has representative of following countries: • Czech • France • Japan • Russia • UK • USA

Pressure In bara

temperature In C

Specific volume saturated water in m3/kg

Specific volume saturated steam in m3/kg

Density of saturated steam kg/m3

Enthalpy saturated water in KJ/kg

Enthalpy saturated steam in KJ/kg

Evaporation heat of water into steam in KJ/kg

Entropy of saturated water in KJ/kgK

Entropy of saturated steam in KJ/kgK

The properties of water and steam

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Steam tables with indication for particulate pressure for each temperature level the: • Specific volume • Enthalpy • entropy

Saturation line

The properties of water and steam

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At what temperature water boils?

How steam volume decreases with increased pressure? Note: Steam is not an ideal gas so the formula P*V/T = C does not fully apply. There will be deviation of 20% higher then the actual value. i.e.: at 1 bar v” is 1,694 m3/kg and Tsat is 100 C. At 40 bara Tsat is 250,3 C and with PV/T the v” is 0,059 m3/kg. The real figure is 0,050. means approx 18% too high.

What is impact of pressure on steam condition?

Means at higher pressure the steam volume flow is much less and pipe sizes can be less.

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E Temperature versus Enthalpy ( T – H diagram)

Enthalpy is the total energy in each kg of water and/or steam

The Enthalpy:

economizer

evaporator superheater

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More details given in T-H diagram

80 bar

374 C

318 C

282 C

221,2 bara

110 bara

66 bara

225 C 25 bara

1 bara

At 110 bara r=1258,7 KJ/kg

Comparison of evaporation heat

At 25 bara r=1839 KJ/kg

46% more then at 110 bara

Dryness of steam

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Heat load heating surfaces at different

pressures

962 1247 1345 1451

1838 1531 1406 1255

328 613 690 759

25 BARA, 350 C 66 BARA, 490 C 86 BARA, 520 C 110 BARA, 540 C

HEAT LOAD HEATING SURFACES

eco evap sup

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To review the heat absorbed by the heating surfaces we show the total heat versus the temperature in water/steam side and also flue gas side

Evaporator 47 ,2 MW

Evaporator 14,7 MW

Heat absorbed by heating surface Flue gas air-preheater:

13,8 MW

Flue gas air-preheater: 6 MW

25 bara, 350 C 66 bara, 485 C

The QT diagram

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318 C 110 bara

225 C 25 bara

1 bara

h’= 417,51 KJ/kg h’= 2675,4 KJ/kg

x dryness fraction

h= 800 KJ/kg

Dryness fraction steam: =weight of steam/total weight Or =heat supplied from boiler heating surface into water/total required evaporation heat (all per kg of water) X = (800-417,51)/2675,4 – 417,51) = 0,1694 Means 16,9 wt% is saturated steam and 83,1 wt% is water In case the dryness is 10% the specific volumes of the steam water mixture are: v = 0,9 * v’ + 0,1 * v”

Dryness of steam

Boiler pressure bara 10 25 110

Specific volume saturated water : v’

M3/kg 0,00011 0,00120 0,00149

Specific volume saturated steam: v”

M3/kg 0,1943 0,07991 0,01601

Specific volume water steam mixture with 10% dryness fraction: v

M3/kg 0,0195 0,0097 0,0029

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Dryness fraction steam: void fraction correlation

With low pressure large volume of steam at already low dryness fraction steam

How higher the pressure how lower the volume

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Entropy is a measure of the degree of disorder within a system. The greater the disorder the higher the temperature. You cannot measure it. It is difficult to imagine this figure. It can be only calculated out of measured data. The heat input per kg and then divided by the average temperature change. In small temperature change steps In steam turbines it is used to indicate the efficiency. A perfect expansion process as in a steam turbine is with a constant entropy. See line A – B in This is a isentropic process (ideal) However this is not possible as all kind of losses will occur like leakages between wheels and friction losses. The isotropic efficiency is the hT = 100 * (hA – hF) / (hA – hB) % The power supplied to the shaft is m * (hA – hF) KW m= mass of steam per kg The power at generator terminal is minus the losses in gearbox and generator

h1

h2 h3 h4

A

B

h1= friction losses in diffusor, heat into steam h2= friction losses in wheels, heat into steam h3= friction losses at outlet, heat into steam H4= losses through seals of wheels

C D

E F

H in KJ /kg

S in KJ /kgK

The Entropy:

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The enthalpy versus the entropy (H- S diagram)

Also called Mollier diagram

Extraction point in sugar mill at 2,5 bara Steam temperature should be slightly superheated 5 C. Other wise the heating surface in evaporators are not working properly.

110 bara 540 C 64 bara ,

480 C

25 bara , 350 C

Isotropic efficiencies different pressure levels: At 110 bara: Isotropic eff= (3464 – 2350)/(3464 – 2090) * 100%= 80,2% Power per kg/s of steam: 1114 KW At 62 bara: Isotropic eff= (3370 – 2360)/(3370 – 2150) * 100%= 82,7% Power per kg/s of steam: 1020 KW

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110 bara 540 C 64 bara ,

480 C

25 bara , 350 C

Isotropic Efficiency existing steam turbine with 66% is lower then the 80 to 83% for the HP steam turbine. Means high losses

Why the existing steam turbine in sugar mill for driving the shredder and crushing rollers are

replaced by electric drives??

The flows of 15 to 30% are too high for extracting from High pressure steam turbine. Will reduce the isotropic efficiency. The steam temperature is too high at extraction point which requires de-superheating. This results in less steam over HP steam turbine.

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There are 2 types of turbines: The impulse type (curtis wheel) is used in the higher pressure zones in order to minimize the leakage around the wheels The reaction turbine (parson wheel) is used in the lower pressure zones as in condensors.

Steam turbine types in co-generation plants

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Why the process steam should be only slightly superheated in ?

2. To minimize the condensate in steam lines which causes high energy and water losses

1. Poor heat transfer when it is superheated

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What happen in case of throttling the steam ?

In case we extract steam from steam drum for burner or sootblowers or deaerator. The enthalpy will remain at inlet and outlet approx. the same. The valve needs to have special internals to avoid corrosion due to wet steam at smallest opening in valve

With 40 bar to 15 bar dry steam

With 110 bar to 15 bar wet steam

With 64 bar to 15 bar wet steam

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The Rankine cycle is used to predict the efficiency of steam turbine systems. We take the T-S diagram for reviewing the complete cycle in steam and water system of power plant

Limit the wet steam amount in wheel

Efficiency rankine cycle is: (Pturbine – Ppump)/ Qin * 100%

Ppump

P turbi

ne

Rankine Cycle

Rejected heat

TS diagram of a rankine cycle operating between pressure of 0,06 bar and 50 bar

Entropy in KJ/kgK

Heat input

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System with one boiler and one

condensing /extraction steam turbine

Process steam 2,2 bara, 135 C

E

Condensate return 85% at 98 C Demi water

E

Advantage Disadvantage

It is a simple Condensing wheel has low efficiency during season

Having low investment The steam turbine will have low efficiency as it will run

at 60 to70% of the steam load during off season.

When one main equipment trip then sugar mill will

shut down

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E

Condensate return 85% at 98 C

Demi water

E

E

System with two parallel systems each 50%

capacity and consisting each of one boiler and

one condensing/extraction steam turbine


High redundancy in case same quality will be taken

for equipment as of ‘option. a’ Condensing wheel has low efficiency during season

When one unit trips then other unit can run at 50%

load

High investment costs of approx. 24% of total

investment cost because of additional boiler and

steam turbine.

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E


E

E


In case back pressure trips then

condensing/extraction steam turbine can run and

plant has an output of 70% load

2nd steam turbine which result in higher investment

cost

High efficiency during season and off season High storage of bagasse

The size of boiler and steam turbine can be 12 to

14% smaller

System with one boiler and one back pressure

steam turbine for season operation and one

condensing steam turbine for off season

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E


E

E E

System with two parallel systems each 50% capacity and

consisting each of one boiler and one back pressure

steam turbine for operation during season, and one

condensing/extraction steam turbine for off season


In case back pressure steam turbine trips the plant

can run still at 100% capacity.

3rd steam turbine and 2nd boiler which result in

higher investment cost.

High efficiency during season and off season High storage of bagasse

In case one boiler trips the plant can run still at 50%

capacity

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More process steam required in relation to electric power result in higher plant efficiency

season Off-season

Minimum steam flow over condensing wheel into condensor

High losses in condensor

What is efficiency during season and off season

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What is effect of higher pressure system

for sugar mill with its specific demands

Description Unit 66 bar (a), 485oC 110 bar (a), 540oC

Season/Off season - Season Off season Season Off season

Steam production boiler tph 149.7 103 154 108.5

Bagasse consumption tph 63.4 43.4 65.5 46.1

Bagasse consumption ton 205,416 40,387 212,220 36,167

Total hours power generation hours 3,240 982 3,240 785

Power output generator kW 26,139 24,569 30,021 28,325

Power consumption of the sugar mill,

shredder with electric drive kW 7,574 - 7,574 -

Power plant own consumption kW 2,091 2,580 2,425 2,718

Power supply to the grid kW 16,474 21,989 20,022 25,607

Total power generation MWh 53,376 21,593 64,871 20,101

Total power supply to the grid annually MWh 74,969 84,972

Annual Revenues (Based on 10,500 PKR

per MWh)

PKR

million 787.2 892.2

Power plant size MW 26.1 30

13,3 % more

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Boiler


E


Demi water

E

FG

eco stack

Steam air preheater

air

To water storage

For additional steam air pre heaters

System with one boiler and one condensing

/extraction steam turbine and air steam

preheater

Prevents corrosion in flue gas air preheater and generate more power over full season for fixed bagasse amount Approx. 5,2% in 66 and 110 bara system

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The HP co-generation plant with steam air preheaters 66 bara

More steam over steam turbine and extracted to heat up the air. No increased losses in condenser while more energy generated

2,5 barg

9 barg

Extraction pressure control

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What is effect on output of additional air

preheating with 1 additional steam air

preheater. For 66 bara

Description Unit 66 bar (a), 485oC at 45% steam demand

Steam Air Preheater

With steam preheaters

from 2.2 and 9.0 bar (a)

extraction

With steam preheaters

from 2.2 bar (a) extraction

only

Season/Off season - Season Off Season Season Off Season

Steam production boiler tph 149.7 103 147 101.5



Total hours power generation hours 3,240 982 3,240 992


Power consumption mill with electric drive

shredder kW 7,574 - 7,574 -

Power plant own consumption kW 2,091 2,580 2,080 2,542

Power supply to the grid kW 16,474 21,989 16,201 21,916


Total power supply to the grid annually MWh 74,969 74,232

Annual revenue (Based on 105,00 PKR per

MWh)

PKR

million 787.2 779.4

1% more

7,8 million PKR more

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The HP co-generation plant with steam air preheaters 110 bara

Flue gas air preheater eliminated

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Description Unit 110 bar (a), 540oC at 45% steam demand

Steam air preheaters

With steam preheaters from 2.2

and 9 bar (a) and 21 bar (a)

extractions

With steam preheaters from 2.2

bar (a) extraction only

Season/off season - Season Off Season Season Off Season

Steam production boiler tph 154 108.5 146 102



Total hours power

generation hours 3,240 785 3,240 857


Power consumption mill

with electric drive shredder kW 7,574 - 7,574 -

Power plant own

consumption kW 2,425 2,718 2,310 2,555

Power supply into grid kW 20,022 25,603 19,148 24,651


Total power supply into

grid annually MWh 84,972 83,166

Annual revenue (Based on

10,500 PKR per MWh) PKR million 892.2 873.2

What is effect on output of additional air

preheating with 2 additional steam air

preheater. Total 3. For 110 bara

2,2% more

19 million PKR more

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Extraction pressure control

The HP co-generation plant with BFW preheaters for 66 and 110 bara

More steam over steam turbine and extracted to heat up the air. No increased losses in condenser while more energy generated

Boiler


E


Demi water

E

FG

eco stack

air

BFW preheaters

Take care for corrosion at cold end

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Thank You for Your Kind Attention

funded by the european union services to technology...

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