WO2012175810A1

WO2012175810A1 - Method and arrangement for improving energy efficiency of a drying section of a paper machine or the like

Info

Publication number: WO2012175810A1
Application number: PCT/FI2012/050647
Authority: WO
Inventors: Pertti Heikkilä; Henri Chydenius; Mikko Kemppinen
Original assignee: Metso Paper, Inc.
Priority date: 2011-06-21
Filing date: 2012-06-21
Publication date: 2012-12-27
Also published as: FI20115629A; FI20115629A0; EP2723941A1; CN103703185B; EP2723941B1; FI124076B; FI20115629L; CN103703185A

Abstract

The invention relates to a method and an arrangement for improving energy efficiency of a drying section of a paper machine or the like. In the method a paper web or the like is dried in an airborne web-dryer (1) by blowing heated air towards the web, and air is removed from the airborne web-dryer (1) as exhaust air, which air has been in contact with the web to be dried. Then the paper web or the like is led from the airborne web-dryer (1) to at least one subsequent dryer unit (11, 11'), where the web is dried by blowing air towards it. The exhaust air from the airborne web-dryer (1) is led to at least one subsequent dryer unit (11, 11') for use as drying air.

Description

METHOD AND ARRANGEMENT FOR IMPROVING ENERGY EFFICIENCY OF A DRYING SECTION OF A PAPER MACHINE OR THE LIKE

The invention relates to a method and an arrangement for improving the energy efficiency of a drying section of a paper machine or the like according to the preambles of the independent patent claims presented hereafter.

A web travelling in a drying section of a paper machine or the like is dried without contact by means of air blows or by bringing the web into contact with heated cylinder surfaces. Airborne web-dryers are commonly used in the drying section of a paper machine for drying a web. In the airborne web-dryer heated air is led towards the web to be dried on one side of the web or alternatively simultaneously on both sides of the web, which ensures an efficient heat transfer. When the hot air comes into contact with the moist web, the water in the web is evaporated into the air, and the web dries. At the same time the temperature of the air decreases and its percentage of moisture increases. Thereafter the air is removed from the airborne web-dryer, and a part of it is led as recirculation air back to the heating unit of the airborne web-dryer and a part of it is removed as exhaust air from the airborne web-dryer. The exhaust air from the airborne web-dryer may be recirculated through exhaust devices, in which thermal energy in the air is possibly recovered.

After the first airborne web-dryer the drying section of a coating machine is usually provided with 1-3 dryer units, in which the web is dried with air blows having a temperature which is normally lower than the temperature of the blow air used for drying the web in the first airborne web-dryer, for example 150-250 °C.

The temperature of the drying air used in the first airborne web-dryer after the coating station is typically about 200-450 °C. The temperature of the exhaust air from the airborne web-dryer is often 100-300 °C, even after a possible heat recovery. At the moment the exhaust air from the airborne web-dryer is usually led directly to the outdoor air or it is combined with other air flows of the paper factory and led to a common heat recovery unit. There has been a strong presupposition that the exhaust air from the airborne web-dryer contains too much exhaust gas formed in the heating of air and too much moisture transferred from the web for it to be possible, or even permissible, to use it in other processes. An object of the present invention is to reduce or even eliminate the above- mentioned problems appearing in prior art.

One object of the invention is to improve the energy efficiency of a drying section of a paper machine, and to reduce the amount of energy needed for the drying.

Another object of the invention is to provide a method and an arrangement, which may enable an increase in the drying temperature in the subsequent dryer units after the airborne web-dryer. In order to realise the above-mentioned objects the invention is characterised by what is presented in the characterising parts of the enclosed independent claims.

Some advantageous embodiments of the invention are presented in the dependent claims.

The embodiment examples and advantages mentioned in this text relate, where applicable, to both the method and the arrangement according to the invention, even if this is not always specifically mentioned. A typical method according to the invention for improving the energy efficiency of a drying section of a paper machine or the like comprises

- drying a paper web or the like in a first airborne web-dryer by blowing heated air towards the web,

- removing as exhaust air from the airborne web-dryer air which has been in contact with the web to be dried,

- leading the paper web or the like from the first drying airborne web-dryer to at least one subsequent dryer unit, where the web is dried by blowing drying air towards it, and - leading exhaust air from the airborne web-dryer to at least one subsequent dryer unit for use as drying air.

A typical arrangement according to the invention for improving the energy efficiency of a drying section of a paper machine or the like comprises

- a first airborne web-dryer for drying a paper web or the like,

- one or several subsequent dryer units, which are arranged after the first airborne web-dryer in the travel direction of the web, and which are arranged to dry the web with the aid of an air blow, whereby

the arrangement comprises a connection, with which exhaust air from the first airborne web-dryer is arranged to be led to an air circulation of at least one subsequent dryer unit for use as drying air.

Now it has surprisingly been realised that exhaust air from the first airborne web- dryer may be used as drying air for one or several subsequent dryer units without the possible impurities and/or moisture in the exhaust air causing a major drawback to the drying process of the subsequent dryer unit. The use of the exhaust air from the airborne web-dryer at least as a part of the drying air of the subsequent dryer unit enables significant savings in energy, which significantly improves the total energy economy of the process. The use of the exhaust air from the airborne web-dryer may also enable the use of a higher drying air temperature in the dryer unit following the airborne web-dryer. For example with steam-heated dryer units the steam pressure has limited the drying air temperature to a maximal level of 175-180 °C. Now it is possible to feed exhaust air from the first, advantageously gas-heated, airborne web-dryer to the subsequent dryer unit, the temperature of which exhaust air clearly exceeds this temperature, whereby the drying temperature may correspondingly be increased if desired. Alternatively the drying temperature of the subsequent dryer unit may be kept at the conventional level, whereby the energy used by the dryer unit decreases. The invention thus provides significant advantages with reasonable investment costs.

In the solution according to the invention the moisture and/or impurities do not need to be removed from the exhaust air of the airborne web-dryer to be used before it is led to the air circulation of the subsequent dryer unit as drying air. The exhaust air may thus be led directly to the air circulation of the subsequent dryer, without it passing through cleaning and/or drying devices. In this application an airborne web-dryer means a drying device located in the drying section of a paper machine or the like. The airborne web-dryer comprises one drying box or alternatively two oppositely arranged drying boxes or the like. The drying air is heated to a high temperature of 200-450 °C by burning gas and led to blow nozzles of the drying box of the airborne web-dryer. From the blow nozzles the heated drying air is blown with high speed from both sides of the web to be dried towards the web surface. The drying thus takes place in a contact-free manner in the airborne web-dryer.

In this application the subsequent dryer unit means a contact-free dryer, where the paper web is dried by means of air blows. The temperature of the drying air of the subsequent dryer unit is typically lower than the temperature of the drying air of the airborne web-dryer. In one embodiment the temperature of the drying air of the subsequent dryer unit is typically 130-200 °C, even more typically 50-180 °C. In one embodiment of the invention the drying air of the subsequent dryer unit is mainly comprised of exhaust air from the airborne web-dryer, and optionally also of recirculation air of the dryer unit, and drying air of the subsequent dryer unit is led towards the web to be dried without a separate heating step. This embodiment of the present invention thus enables that the dryer unit or units following the airborne web-dryer do not have at all their own heating device, i.e. the drying energy needed by the drying air of the subsequent dryer unit is obtained entirely from the exhaust air from the airborne web-dryer. The subsequent dryer unit is thus free from a heating device. The subsequent dryer unit's own recirculation air or clean replacement air may however be mixed with or added to the exhaust air from the airborne web-dryer to be fed into the subsequent dryer unit, which exhaust air is fed to the subsequent dryer unit for use as drying air, in order to adjust the volume and/or temperature of the drying air, if the temperature of the exhaust air obtained from the airborne web-dryer is too high and/or the volume of exhaust air is too small. In that case the drying air of the subsequent dryer unit comprises untreated exhaust air from the airborne web-dryer and unheated replacement air or recirculation air of the subsequent dryer unit. In another embodiment of the invention the subsequent dryer unit following the airborne web-dryer comprises its own heating device, which may be used to heat the recirculation air of the dryer unit or the replacement air to a desired temperature. The subsequent heating device is gas-, electrically, oil- or steam- heated, preferably steam-heated. The heating device may for example be electrically heated or oil-heated, most typically the heating device uses steam for heating the drying air. In one preferred embodiment of the invention the subsequent dryer unit following the airborne web-dryer is steam-heated. In one embodiment of the invention the subsequent dryer unit following the airborne web- dryer is gas-heated, i.e. its heating device comprises a gas burner.

In one embodiment exhaust air from the airborne web-dryer may be led to the subsequent dryer unit after the dryer unit's own heating device, whereby the drying air of the subsequent dryer unit is comprised of untreated and unheated exhaust air from the airborne web-dryer and of recirculation air and/or replacement air of the subsequent dryer unit, heated in its own heating device. When the temperature of the exhaust air from the airborne web-dryer is sufficiently high, preferably at least as high as, preferably higher than, the temperature of the drying air of the subsequent dryer unit, the exhaust air may be fed into the air circulation of the subsequent dryer unit after the heating device. The subsequent dryer unit thus comprises its own heating device, and the connection from the airborne web-dryer is thus connected to an air circulation network of the dryer unit after the heating device. Thus heating of air taking place in the heating device of the subsequent dryer unit may be reduced, which significantly saves energy. Alternatively exhaust air from the airborne web-dryer may be led to the subsequent dryer unit before the dryer unit's own heating device, whereby exhaust air, and optionally also recirculation air of the dryer unit, is heated in the heating device of the dryer unit before it is led as drying air towards the web to be dried. The subsequent dryer unit thus comprises its own heating device, and the connection from the airborne web-dryer is thus connected to an air circulation network of the dryer unit before the heating device. Thus the temperature of the exhaust air from the airborne web-dryer is preferably higher than the temperature of the circulation air of the subsequent dryer unit. In this way the amount of energy needed by the heating device of the subsequent dryer unit may be reduced, when the initial temperature of the air to be heated in the heating device is higher than when using only replacement air or recirculation air of the subsequent dryer unit. In one embodiment the temperature of the exhaust air from the airborne web-dryer is > 130 °C, preferably > 200 °C, more preferably > 250 °C.

Typically 5-20 volume-%, more typically 0-20 volume-%, most typically 10-15 volume-% of the drying air of the subsequent dryer unit is exhaust air from the preceding airborne web-dryer. In one embodiment of the invention over 50 volume-%, possibly even 60 volume-% or even 75 volume-% of the drying air of the subsequent dryer unit is exhaust air from the preceding airborne web-dryer.

According to one embodiment of the invention during start-up of the airborne web- dryer exhaust air from the airborne web-dryer is led to a by-pass system of the subsequent dryer unit, the volume of exhaust air of the subsequent dryer unit is reduced and recirculation air of the dryer unit is mainly used as drying air of the dryer unit. When starting up the gas burner of a gas-heated airborne web-dryer a pre-rinsing is usually performed, during which the exhaust air is preferably led directly to an exhaust. The temperature of the exhaust air from the airborne web- dryer may also be lower than in a normal stabilised running situation. In that case it is preferable to lead, instead of the subsequent dryer unit, the exhaust air from the airborne web-dryer to an alternative bypass system of the subsequent dryer unit, which may comprise for example heat recovery means and/or means for gas cleaning. Exhaust air from the airborne web-dryer is thus during start-up of the airborne web-dryer led to a system bypassing the subsequent dryer unit, which system comprises heat recovery means and/or means for gas cleaning. The exhaust air is led via these heat recovery means and/or cleaning means out of the drying process. At the same time the volumes of exhaust air and recirculation air of the subsequent dryer unit may be adjusted for the duration of the start-up of the airborne web-dryer. For example the volume of exhaust air of the subsequent dryer unit(s) is preferably to be reduced, so that their air balance remains constant also during start-up. The arrangement thus preferably comprises a by-pass system for the subsequent dryer unit and adjustment means for adjusting the volumes of exhaust air and recirculation air of the subsequent dryer unit. As soon as possible after the airborne web-dryer has reached a stable running situation, whereby the pre-rinsing has been performed and the temperature of the exhaust air from the airborne web-dryer has exceeded a predetermined limit value, the feeding of exhaust air from the airborne web-dryer to at least one subsequent dryer unit is started, and the volumes of recirculation air and/or replacement air of the subsequent dryer unit is reduced in a corresponding manner. In one embodiment of the invention exhaust air from the airborne web-dryer is led through a heat recovery unit before it is led to at least one subsequent dryer unit, i.e. the arrangement comprises a heat recovery unit, which is arranged between the first airborne web-dryer and the subsequent dryer unit. When the temperature of the exhaust air from the airborne web-dryer is sufficiently high, a part of the heat contained in the exhaust air may be recovered before the exhaust air is led to the subsequent dryer unit. Thus the energy efficiency of a drying section of a paper machine or the like may be further intensified. Even after the heat recovery unit the temperature of the exhaust air is typically 200-250 °C, i.e. it is well suited to be led as drying air or part thereof of the subsequent dryer unit.

In one embodiment of the invention the arrangement comprises a gas-heated airborne web-dryer for drying a paper web or the like, and one or several subsequent steam-heated dryer units. The drying air of the airborne web-dryer is thus heated by burning gas and the heat needed for heating the drying air in the subsequent dryer unit is obtained from recirculation air steam. The drying air of the gas-heated airborne web-dryer is heated to a temperature of 200-450 °C, and the drying air in the subsequent dryer unit is heated to a temperature of 130-200 °C, preferably 130-185 °C. In the airborne web-dryer the temperature of the drying air is thus usually higher than in the subsequent dryer units, i.e. the paper web is dried in the airborne web-dryer by blowing air towards it, the temperature of which air is about 200-450 °C, and in the subsequent dryer units the paper web is dried by blowing air towards it, the temperature of which air is about 130-200 °C, preferably 130-185 °C.

In a second embodiment of the invention the arrangement comprises a gas-heated airborne web-dryer for drying a paper web or the like, and one or several subsequent gas-heated dryer units. Also in this case the temperature of the drying air in the airborne web-dryer is higher than in the subsequent dryer units, in which the drying temperature is substantially lower than the drying temperature of the airborne web-dryer. The temperature difference between the drying temperatures of the airborne web-dryer and the subsequent dryer unit is arranged to be at least 50 °C, preferably at least 100 °C, sometimes at least 150 °C, typically 100-200 °C.

The invention is described in more detail below with reference to the enclosed schematic drawing, in which

Figure 1 shows an arrangement according to prior art, and

Figure 2 shows an arrangement according to one embodiment of the present invention.

Figure 1 shows an arrangement according to prior art. The arrangement comprises a gas-heated airborne web-dryer 1. Air is removed from the airborne web-dryer 1 through an exhaust connection 2. A part of the removed air is fed as recirculation air back to the airborne web-dryer 1 , so that the recirculation air passes through a gas-heated heating device 3 to an air supply 4 of the airborne web-dryer. Fresh replacement air may also be fed to the heating device 3 of the airborne web-dryer 1 through a replacement air connection 5.

A part of the exhaust air from the airborne web-dryer 1 is led through a heat recovery unit 6 to an outlet connection 7, from which the exhaust air from the airborne web-dryer 1 is led directly out into the outdoor air or into an air circulation system of the mill. The heat recovered from the exhaust air led through the heat recovery unit 6 may be used to heat fresh air coming through a connection 8, which fresh air is led to the heating device 3 of the airborne web-dryer 1 as combustion air for the gas burner.

After the airborne web-dryer 1 has been arranged two successive steam-heated dryer units 11 , 11 '. The paper web to be dried, which is typically coated, is led from the gas-heated airborne web-dryer unit 1 to the first steam-heated dryer unit 11 , and therefrom further to the subsequent second steam-heated dryer unit 11 '.

Air is removed from the dryer units 11 , 11' through exhaust connections 12, 12'. A part of the removed air is fed as recirculation air back to the dryer units 11 , 11', so that the recirculation air from both dryer units 11 , 11' passes to the air supply 14, 14' of the dryer units 11 , 11' through the steam-heated heating devices 13, 13'. Fresh replacement air may also be fed to the heating devices 13, 3' of the dryer units 11 , 11 ' through replacement air connections 15, 15'. Steam is fed to the heating devices through steam supply connections 18, 18'. Figure 2 shows an arrangement according to one embodiment of the present invention. The reference numbers in Figure 2 correspond to the reference numbers in Figure 1.

In the arrangement according to Figure 2 air removed from the gas-heated airborne web-dryer 1 is after the heat recovery unit 6 led to a connection 9, by means of which the exhaust air from the gas-heated airborne web-dryer 1 is fed to the air circulation of the subsequent steam-heated dryer units 11 , '. Exhaust air from the airborne web-dryer 1 may be fed into the air circulation network of the dryer units 11 , 11' before the heating device 13, 13' or after the heating device 13, 13' of both dryer units 11 , 11' through connections 10, 10', 10", 10"'. Depending on the temperature of the exhaust air from the airborne web-dryer 1 , the exhaust air may be fed through the connections 10, 10" directly to the air supply connection 14, 14' of the dryer units 11 , 11 ' or the exhaust air may be fed through the connections 10', 10"' before the heating devices 13, 13' of the dryer units 11 , 11'. In the latter case the exhaust air from the airborne web-dryer 1 is heated to the drying temperature used by the dryer unit 11 , 11' in the heating device 13, 13'. The exhaust air from the drying units 11 , 11 ' is removed through an exhaust 5 connection 17 directly out into the outdoor air or into the air circulation system of the mill.

During the start-up of the airborne web-dryer 1 the connection 9 between the airborne web-dryer 1 and the subsequent dryer units 11 , 1 ' may be shut using an 10 regulating member 19, whereby the exhaust air from the airborne web-dryer is directed through the exhaust connection 7 directly out into the outdoor air or into the air circulation system of the mill. The arrangement according to the invention may also comprise other regulating or shutter members 16, by means of which the air circulation of the arrangement may be controlled and regulated.

Ί5

The invention is not meant to be limited to the embodiments shown as examples above, but the aim is to interpret it extensively within the scope of protection defined by the claims presented hereafter.

Claims

1. A method for improving energy efficiency of a drying section of a paper machine or the like, comprising

- drying a paper web or the like in an airborne web-dryer by blowing heated air towards the web,

- leading the paper web or the like from the airborne web-dryer to at least one subsequent dryer unit, where the web is dried by blowing drying air towards it, characterised in

leading exhaust air from the airborne web-dryer to at least one subsequent dryer unit for use as drying air.

2. The method according to claim 1 , characterised in the drying air of the subsequent dryer unit being mainly comprised of exhaust air from the airborne web-dryer and optionally also of recirculation air of the dryer unit, and in leading the drying air of the subsequent dryer unit towards the web to be dried without a separate heating step.

3. The method according to claim 2, characterised in obtaining drying energy of the drying air of the subsequent dryer unit entirely from exhaust air of the airborne web-dryer.

4. The method according to claim 2, characterised in mixing or adding recirculation air of the subsequent dryer unit or clean replacement air to the exhaust air from the airborne web-dryer, which exhaust air is supplied to the subsequent dryer unit for use as drying air, in order to adjust the volume and/or temperature of the drying air.

5. The method according to claim 1 , characterised in leading exhaust air from the airborne web-dryer to the subsequent dryer unit after the dryer unit's own heating device, whereby the drying air of the subsequent dryer unit is comprised of the exhaust air from the airborne web-dryer and of in its own heating device heated recirculation air of the dryer unit and/or replacement air.

6. The method according to any of the claims 1-5, characterised in the temperature of the exhaust air from the airborne web-dryer being at least as high as, preferably higher than, the temperature of the drying air of the subsequent dryer unit.

7. The method according to any of the preceding claims 1-6, characterised in the temperature of the drying air of the subsequent dryer unit being 130-200 °C, more typically 150-180 °C.

8. The method according to claim 1 , characterised in leading exhaust air from the airborne web-dryer to the subsequent dryer unit before the dryer unit's own heating device, whereby exhaust air, and optionally also recirculation air of the dryer unit, is heated in the heating device of the dryer unit before it is led as drying air towards the web to be dried.

9. The method according to claim 8, characterised in the temperature of the exhaust air of the airborne web-dryer being higher than the temperature of the recirculation air of the subsequent dryer unit.

10. The method according to any of the preceding claims 1-8, characterised in the temperature of the exhaust air from the airborne web-dryer being > 130 °C, preferably > 200 °C, more preferably > 250 °C.

11. The method according to any of the preceding claims 1-10, characterised in that during start-up of the airborne web-dryer

- exhaust air from the airborne web-dryer is led to a system bypassing the subsequent dryer unit,

- the volume of exhaust air from the subsequent dryer unit is reduced, and

- recirculation air of the dryer unit is mainly used as drying air in the subsequent dryer unit.

12. The method according to claim 11 , characterised in leading exhaust air from the airborne web-dryer during start-up of the airborne web-dryer to a system bypassing the subsequent dryer unit, which system comprises heat recovery means and/or means for gas cleaning.

13. The method according to claim 11 or 12, characterised in adjusting the volumes of exhaust air and recirculation air of the subsequent dryer unit for the duration of the start-up of the airborne web-dryer.

14. The method according to any of the preceding claims 1-10, characterised in leading exhaust air from the airborne web-dryer through a heat recovery unit before leading it to at least one subsequent dryer unit.

15. The method according to claim 14, characterised in the temperature of the exhaust air after the heat recovery unit being 200-250 °C.

16. The method according to any of the preceding claims 1-15, characterised in heating the drying air of the airborne web-dryer by burning gas and obtaining the heat needed for heating the drying air in the subsequent dryer units from recirculation air steam.

17. The method according to claim 16, characterised in heating the drying air of the gas-heated airborne web-dryer to a temperature of 200^450 °C, and heating the drying air in the subsequent dryer unit to a temperature of 130-200 °C, preferably 130-185 °C.

18. The method according to any of the claims 1-17, characterised in over 50 volume-%, possibly even 60 volume-% or even 75 volume-% of the drying air of the subsequent dryer unit being exhaust air from the preceding airborne web- dryer.

19. The method according to any of the claims 1-17, characterised in 5-20 volume-%, more typically 10-20 volume-%, most typically 10-15 volume-% of the drying air of the subsequent dryer unit being exhaust air from the preceding first airborne web-dryer.

20. An arrangement for improving energy efficiency of a drying section of a paper machine or the like, which arrangement comprises

- a first airborne web-dryer for drying a paper web or the like,

- one or several subsequent dryer units, which are arranged after the first airborne web-dryer in the travel direction of the web, and which are arranged to dry the web with the aid of an air blow,

characterised in that the arrangement comprises a connection, with which exhaust air from the airborne web-dryer is arranged to be led to an air circulation of at least one subsequent dryer unit for use as drying air.

21. The arrangement according to claim 20, characterised in that the subsequent dryer unit is free from a heating device.

22. The arrangement according to claim 20, characterised in that the subsequent dryer unit comprises its own heating device, and the connection from the airborne web-dryer is connected to an air circulation network of the dryer unit after the heating device.

23. The arrangement according to claim 22, characterised in that the subsequent heating device is gas-, electrically, oil- or steam-heated, preferably steam-heated.

24. The arrangement according to claim 20, characterised in that the subsequent dryer unit comprises its own heating device, and the connection from the airborne web-dryer is connected to an air circulation network of the dryer unit before the heating device.

25. The arrangement according to any of the preceding claims 20-24, characterised in that it comprises a heat recovery unit, which is arranged between the first airborne web-dryer and the subsequent dryer unit.

26. The arrangement according to any of the preceding claims 20-25, characterised in that the arrangement comprises a by-pass system for the subsequent dryer unit and adjustment means for adjusting the volumes of exhaust air and recirculation air in the subsequent dryer unit.

27. The arrangement according to claim 26, characterised in that the bypass system of the subsequent dryer unit comprises heat recovery means and/or means for gas cleaning.

28. The arrangement according to any of the preceding claims 20-27, characterised in that it comprises a gas-heated airborne web-dryer for drying a paper web or the like, and one or several subsequent steam-heated dryer units.

29. The arrangement according to claim 20, characterised in that the arrangement comprises a gas-heated airborne web-dryer, and one or several gas- heated subsequent dryer units, whereby the temperature difference of the drying temperatures between the airborne web-dryer and the subsequent dryer unit is arranged to be at least 50 °C, preferably at least 100 °C, sometimes at least 150 °C, typically 100-200 °C.