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Centre of Competence Paper and Board

Kenniscentrum Papier en Karton (KCPK)

Energy optimisation in drying: ventilation and heat recovery

Jobien Laurijssen

Energy use in paper production

Stock preparation

Wire and press

Drying

Other treatmens

Utilities

Energy use in paper drying

The energy needed for paper drying is a function of:

amount of water evaporation (ton water / ton paper)

X

efficiency of water evaporation (GJ / ton water)

-

energy recovery (GJ / ton paper)

Amount of water evaporation

Removal of water

Wire (mechanical)Press (mechanical)Drying section (thermical)

Large differences in

– Amount of water removal– Energy use of water removal

Water removal in wet end : 0.01 GJ / ton waterWater removal in dry end : 3.8 GJ / ton water

Reduce water removal in dryer

• Increase dry matter content after press section

• Reduce water additions in between drying sections

• Avoid excessive drying

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Energy use after-drying section

0

500

1000

1500

2000

2500

8 16 24 32 40 48 56 64

kJ /

kg p

aper

(bo

ne d

ry)

consistency starch solution [%]

Heat of sorption

Heat of sorption

Extra energy that is needed for the evaporation of bound water

Measures to reduce evaporation

• Increase dry matter content after press section

� 1% increase is 4% steam reduction

• Avoid over-drying

� Bound water has highest energy demand!

• Reduce water additions in size press/coater

� High consistency additives

Efficiency of water evaporation

Heat Consumption: exhaust air - supply air = 90%(example) paper out - paper in = 2%

heat losses = 8%steam - condensate = 100%

Paper web in

Paper web out

Exhaust air / kg H2O

Supply air / kg H2OCondensate / kg H2O

Steam / kg H2O

HoodEvaporation 1 kg H2O

Heat losses

Dryer energy balance Defining energy use

Energy content of exhaust air (kJ / kg air)

Energy content of supply air (kJ / kg air)

Energy input (kJ / kg air) = Energy content (exhaust air –supply air)

Air use (kg drying air / kg water )

Energy use (kJ / kg water) = Air use x Energy input

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Drying air parameters

Dew point (°C) is the temperature at which air at decreasing

temperatures will condensate

Relative humidity (%) indicates the amount of moisture that the air

contains as compared to amount that it can maximally contain at the

same temperature

Absolute moisture content (g/kg) is the absolute amount (g) of

moisture present in a kilogram of air

Relationship between parameters

• Absolute moisture content is independ on air temperature

• The ability of air to hold water increases with temperature

• Relative humidity decreases with increasing air temperature

TemperatureDewpoint

Absolute moisture

Relative humidity

100%

Psychrometric (mollier) chart

Shows by means of continuous lines the relation between the following parameters (all expressed per kg dry air):

- Absolute humidity g vapour /kg dry air

- Relative humidty %- Dew point temperature °C- Partial vapour pressure kPa

- Specific volume m3 / kg dry air- Dry bulb temperature °C

- Wet bulb temperature °C- Enthalpy kJ /kg dry air

Explanation ‘mollier chart’1.2 19.9 partial vapour press. in kPa

D23.0

492°C

450°C

85 °C

wb=

63,162 °C

60 °C

HEATING

HRC

10 °C

29 J/g

8 150abs. humidity in g/kg drying air)

168

????

HRC

430 °C i4

°CU1

wb= 60°C59°C

10°C i1

6i1=

80

u2= 144

Exhaust air (U1):Temp = 80°CRel. hum. = 40%

Supply air (i1):Temp = 10°CRel. hum. = 80%

Calculating energy use

Energy input (kJ/ kg) =

Air use (kg dry air / kg PWE) =

Energy use (kJ / kg water) = Air use x Energy input

supplyexhaust

1000

ww

g

−

supplyexhaust h– h

4

430 °C i4

°CU1

wb= 60°C59°C

10°C i1

6i1=

80

u2=144

463 – 26 = 437 kJ/kg air

supplyexhaust h– h

supplyexhaust

1000

ww

g

−

1000 / (144 - 6) = 7,25 kg air/ kg water

Energy use = 437 * 7,25 =

3167 kJ/ kg water

Exhaust air (U1):Dewpoint = 59°CRel. hum. = 40%

787 °C i4

°C U1

wb= 71°C70°C

10°C i1

6i1=

93

u2=277

835 – 26 = 809 kJ/kg air

supplyexhaust h– h

supplyexhaust

1000

ww

g

−

1000 / (277 - 6) = 3,69 kg air/ kg water

Energy use = 809 * 3,69 =

2985 kJ/ kg water

Exhaust air (U1):Dewpoint = 70°CRel. hum. = 40%

Condition drying air in exhaust pipe

Two basic conditions:

- Process requirement: max RH

- Construction requirement: max. dew point hood

At higher temperatures, energy

consumption decreases !

Calculation example

Same relative moisture content

Higher relative moisture content

IN OUT OUT OUT

Temp 10 89 95.2 89 ˚C

Rel. moist. Cont. 80 23 23 29.5 %

Dewpoint 6.7 55 60 60 ˚C

Enthalpy 25 396 504 495 kJ/kg

Absolute moisture 6 115 152 152 g/kg

Amount of air 9.18 6.83 6.83 kg/kg

Enthalpy difference 370 478 470 kJ/kg

Energy use 3399 3265 3211 MJ/ton H20

Energy cost reduction per year

€385,000 €542,000

* For a mill with a production of 200.000 ton per year

Supply air energy use and heat recovery

Less supply air needed:

- Less heating required (↓ Steam)

- Less fan power (↓ Electricity)

- More heat recovery options (↑ HRC)

- Smaller dimensions

Benefits of high dew point

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Measures to increase drying efficiency

- Increase dew point

- Reduce air flow (recirculate air if needed)

- Strive for a maximal relative moisture content

- Avoid too high supply air temperatures

- Install frequency driven ventilation

- Increase heat recovery

Hood types

• Drying energy is combination of steam and gas use

• Boiling type drying instead of convective type drying in MC

• Exhaust air temp. is much higher than in MC (300°C vs 90°C)

• Dewpoint is also higher (75°C vs 62°C)

• Energy use calculation is same as for MC(exhaust-supply air) but be careful with Mollier chart: combustion process adds moisture!

Yankee drying hood

• Total amount of intake air is sum of infiltration air, balance air

(make-up air) and combustion air . In principle this quantity of air

should be as low as possible:

• balance air valve closed

• infiltration air minimized (air knife)

• combustion air is necessity: from energy point of view, an air

factor of 1 would be best; but in this case the combustion

temperature will be too high (1800 °C). (In practic e, the air

factor applied in Yankee-hood burners is at least 1,5)

• Reduced drying air will increase the moisture content per kg dry air

and subsequently the dew point will go up .

Air flows Yankee drying hood

Yankee drying hood (high dew point)

Conventional dew point ~70-75°CX = 0,3-0,5 kg H2O/kg dry air

Ultra high dew point ~90°CX = 1,8 kg H2O/kg dry air

Air use , conventional dew point2-3 kg dry air / kg H2O

Air use , ultra high dew point0,6 kg dry air / kg H2O

• Increase dew point :• Reduce air factor burner• Reduce infiltration air

• Increase relative humidity

• Reduce hood height for better heat transfer

• Insulate hood

• Preheat burning air

• Avoid excessive drying (control moisture profile)

• Heat recovery!

Yankee hood energy optimization

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Practical experiences in reducing drying energy by optimizing air flows

Approach

- Measuring current process conditions

- Define energy balances

- Modelling of potential savings

- Create awareness by educating operators

- Practical tests to confirm saving potential and to identify risks

- Cost/benefit analysis

- Implementation

Measurements inside drying hood Measurements in supply and exhaust ductsCreate openings (>60) Measurement devices

8 full days of measurements in two teams of two

Modelling

• General drying model was developed for the Dutch industry

• Customizing model for paper mill X

• Input measurements in flowcharts

• Control of balances , correct deviations

• Input of verified measurements in drying model

• Define saving potential at various locations

Modeling results: € 800.000 euro/yr saving possible at:

Optimized air conditions en HRC until 60°C

Creating awareness• Awareness energy use and energy costs with operators

• Insight in possible saving potential and their role in reaching it

• Instructions to increase dewpoint

• Emphasis on limitations and risks related to drying hood

60°C

50°C

Dewpoint temperature1st operator session 2nd operator session

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Savings realised:

Dewpoint increased from 55°C to 63°C - Steamflow � Saving 1,7 ton/hour- Electricity air fans � Saving 100 kW

- Temp press water � 49,8 oC to 53,8 oC

Cost� € 0,-- (no out-of-pocket costs, but many hours were spend)

Saving � +/- € 400.000 per year

Large savings are possible

• Mill estimates saving potential of +/- 1million €/yr on 1

machine

• Preconditions: find economically feasible applications

for internal use of recovered heat

• All starts with awareness, attention, the right settings

and a clean machine

Conclusion

Heat recovery

Heat in paper- and board industry

Paper productionFuel H

Electricity Energy

conversion100%

13% 35%

6%

Waste heat drying sections

Waste heat via waste water

50%

Paper industry is large user and supplier of heat

Waste heat energy conversion

Approach in heat optimization

• Reduce heat demand

• Increase internal heat recovery

– Direct

– After upgrading, conversion or separation

• External waste heat delivery

• Sustainable production of heat

Prevention

Re-use

Sustainableproduction

Reduce heat demand (I)

� Reduce evaporation� Increase dry matter content after press section (vacuum, press,

increased dewatering (chem+temp))

� Avoid excessive drying(sensors, APC)� Less water additions (high consistency sizing / coating)

� Increase drying efficiency� Increase dewpunt

� Avoid heat losses

� Use lower value heat sources (waste heat) when possible

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VACUUM

PRESS

WIRE

STEAM

Space heatingOther users

(high consistency sizing )

increasedewpoint

Increase proces water temperature

sensors

Heat scan

HRC HRC

Reduce heat demand (II)

• Preheating supply air (air/air heat exchanger)

• Heating of process water (air/water heat exchanger)

• Heating of fresh water (air/water heat exchanger)

• Heating of circulation water for machine room ventilation

(air/water heat ex.)

Internal heat recovery options

Supply air heating

- Preheating supply air with exhaust air

- Efficiency depends on heat transfer efficiency

- Applied in most paper machines

- Supply air should only be around 20⁰C above dew point

Air / air heat exchanger

- Plate or tube design, - Cross flow- Heat transfer partly by condensation partly by convection

- Increased process water temperature reduces water viscosity:

- 10°C increase in temp � 1% higher dry matter content

- Increased process water temperature increases wire evaporation

- Heating of process water with steam often does not pay-off

Process water heating

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-Tube design-Cross flow-Heat transfer primarily by condensation on the exhaust side

Air / water heat exchanger Scrubber

787 °C i4

°C U1

wb= 71°C70°C

10°C i1

6i1=

93

u2=277

835 – 26 – 38 = 771 kJ/kg air

supplyexhaust h– h

supplyexhaust

1000

ww

g

−

1000 / (277 - 6) = 3,69 kg air/ kg water

Energy use = 771 * 3,69 =

2845 kJ/ kg water

Exhaust air (U1):Temp = 93°CRel. hum. = 40%

U3

47°C

430 °C i4

°CU1

wb= 60°C59°C

10°C i1

6i1=

80

u2=144

463 – 26 = 437 kJ/kg air

supplyexhaust h– h

supplyexhaust

1000

ww

g

−

1000 / (144 - 6) = 7,25 kg air/ kg water

Energy use = 437 * 7,25 =

3167 kJ/ kg water

HRC potential depends on temperature of application

Current 60°C 50°C 40°C 30°C 20°Csituation

HRC potential increases with increased dew point temperatures

Optimised 60°C 50°C 40°C 30°C 20°Csituation

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Internal heat recovery (II)

� Upgrading� Heat pump (closed system)

� Mechanical Vapour Recompression (open system)

� Conversion� Electricity

� Thermo Accoustic Power� Organic Rankine Cycle

� Cooling

� Separation of water and airstreams� Membrane technology� Chemical sorption

VACUUM

PRESS

WIRE

STEAM

HRC HRC

Heat pump/MVR

ORCTAP

Membrane/ Sorption

Space heatingOther users

Internal heat recovery (III)

Measures to improve heat recovery

- Avoid uncontrolled heat losses

- Reduce amount of waste heat Increase dew point

- Increase quality of waste heat

- Re-use internally:

- Supply air heating

- Process water heating

- Space heating

- Upgrade (heat pump, ORC etc.)

- Export