Field of invention

The present invention relates to a method and a device for increasing the drying capacity of a paper or board machine or pulp dryer and/or increase the power production from a mill's turbine. This method is characterised by low investment costs and high energy efficiency.

Background

In the drying section of a paper or board machine (PM or BM) the fiber web is passed over a number of drying cylinders which are heated by means of steam. The web is normally held against the drying cylinders by means of dryer fabrics. The drying section is usually splitted into a number of drying groups, where each drying group is made up of a number of drying cylinders and rolls and normally at least one fabric. Many PM and BM drying systems consist of more than one drying sections (for example pre- and after drying sections).

Another way of splitting the drying sections is to divide the drying cylinders into steam groups, in which each drying cylinder in a steam group and normally also all steam groups are supplied with steam from the same header pipe. The drying groups may coincide with the steam groups.

It should be noted that all indications of pressure hereinafter relate to overpressure, unless otherwise is stated.

Steam is supplied to the PM or BM from one or several main steam conduits under constant or some sliding pressure.

In most cases this steam, which originates from a steam boiler, has passed through one or more turbines, which have converted some of the energy content of the steam into electric or mechanical power. The steam produced by the boiler usually keeps a pressure of about 30-100 bar, but pressures of up to about 140 bar can be found. This steam is called high- pressure (HP) steam. A mill's steam system normally keeps one constant HP steam pressure level. However, there might be some mills with more than one HP steam pressure level in different sections of boilers and turbines. Higher pressure of HP steam normally means higher specific power production from the turbine as kWh/t steam.

From the turbine, steam has normally more than one exhaust, mainly to turbine end exhaust, also named back pressure steam exhaust, normally connected to low pressure (LP) steam net with a pressure range of about 2-6 bar. There are also turbines with back pressure exhaust connected to medium pressure (MP) steam net with a pressure level in the range of 10-18 bar. Lower exhaust pressure means specific more power production from turbine as kWh/t steam. This is always valid for the back pressure steam net, normally LP steam, and also in most cases also valid for steam extraction to MP steam net. There are also often one or more steam extractions from turbine to medium or intermediate (MP) steam net at a pressure level of about 10-25 bar. One ton of LP steam from turbine creates nearly twice as much power compared to one ton of MP steam. Typical values are about 180 respectively about 100 kWh power/ton of steam. The capacity of steam extraction from turbine to MP net is often limited due to turbine construction and this means that at high consumption of MP steam there is often to some extend a direct reduction of HP- to MP steam net and this means of course further decreased power production.

A mill's own power production is of a very high economical importance.

As the paper or board machines are very big steam consumers of the mill's total steam consumption, it is of outmost importance how the machine(s) uses LP/MP steam. It could very well mean that influence from one paper machine can create very big annual cost difference (cost differences of 5-10 MEuro is not unusual) when comparing bad and good steam solutions concerning a mills electricity costs.

Some, mills also have a low low pressure fresh (LLP) steam net in the pressure range of about 0,5-1 ,5 bar. Some steam consumers are then able to replace LP- with LLP-steam. The supply of steam to LLP might be further turbine expansion of LP in a turbine or upgrading waste heat through process integration. Most of the steam supplied to the drying cylinders of a PM or BM will condense during drying of the paper web. The condensate formed together with some of the supplied steam has to be removed from the drying cylinders. Each drying cylinder is equipped with one or more discharge pipes (siphons or spiral scoops). After removal through the discharge pipes the discharge is separated in a separation vessel (normally one vessel for each big steam group) to steam and condensate. The steam leaving the separation vessel is named blow through steam (bt-steam) and keeps, due to the pressure drop, a lower pressure compared to the steam pressure in the cylinders.

The steam pressure drop and the part of bt-steam will vary depending of equipment (stationary- or rotating siphones etc) and how they are operated. It is preferred to reuse the bt-steam that flows from the steam groups and replace expensive fresh steam in other positions (normally to other adjacent steam groups).

If the receiver steam group keeps a lower steam pressure, the steam will flow by its own and this is called cascade coupling. However, seeing to the total steam consumption of the receiver steam group, the supplied bt- steam is just a minor part and the main part is fresh steam. One the other hand, if the pressure difference isn't enough, the bt-steam pressure has to be increased (for example when returning the bt-steam to its own steam group). This is normally done by using a thermo compressor (TC), which normally uses MP-steam as motive steam. Usage of MP-steam (which replaces an equal amount of suction/ LP-steam) means a decrease of own valuable power production. It is thus of great importance to keep as high bt-steam pressure and low amounts as possible to keep the needed TC-operation as low as possible and promote cascade coupling.

This kind of TC-operation can normally be used for steam economy improvements and it is an old and well known application and it means no capacity increase as increased steam pressure for main steam group.

In the prior art there are occasions when the paper or board machine is production limited by it's drying capacity, i.e. the drying sections restricts the speed of which the web can be run. These situations may, for instance, occur when it is desirable to increase the speed of the machine or obtain higher grammage, if the press dryness are reduced or if the output dry solid content of the drying section is to be increased. Often only one or a few steam groups limit the maximum production or drying capacity, e.g. the maximum speed. A limited drying capacity can be avoided, for instance, by lengthening the drying section or by increasing the saturation temperature of the supplied steam, which is usually performed by increasing the steam pressure. To lengthen the drying section means a great investment, a great loss of production due to the stoppage time (which may last several weeks) during reconstruction and results in a machine having a drying section which might in many cases be over dimensioned. Often this solution is not justified from economic point of views. Even if there might be similar other changes of the machine, the drying section extension will determine the total stoppage time and often extend the stoppage time normally needed.

In order to achieve an optimal steam usage in a paper or board mill, it is important to keep down the steam pressures needed, in for example a drying section in a paper or board machine and the heat transfer from the steam in a cylinder to a paper web should also be as high as possible. Unnecessary pressure drops should be avoided in the steam distribution system as well as within the drying sections. Also, some changes of cylinders division into steam groups may benefit drying capacity through higher average steam pressure for the drying section.

There are today different methods used in order to increase steam pressures of steam groups concerning investments and increased annual operating costs (mainly losses in own power production). The magnitude of the pressure increase needed and the length of the operating time with the increased pressure varies with different steam groups. The optimal would be to reduce the magnitude and time but still maintain the desired production capacity. The pressure increase should be as selective as possible and only be made for the actual group(s) and the time when a pressure increase is necessary.

The drying cylinders must of course be classified for higher steam working pressure or possible be upgraded to the desired allowed increased pressure.

To increase steam pressure for the entire steam system of a paper or board machine or pulp dryer or even a whole paper mill should mean high investments and increase of operating costs and should therefore be avoided as long as possible. The drying system of a paper or board machine consists of at least one drying section but often two or more. Each drying section normally has 1-2 main steam groups. For one machine there is normally at least one big main group that contains several drying cylinders with high steam consumption and a high steam pressure increase demand when increasing the drying capacity or production.

The technical most simple method to increase the drying capacity or production is probably to change the steam supply to a steam group from LP- to MP-steam when needed, this is possible if MP-steam is available or could be to a reasonable low investment. Every operating hour with MP-steam means a high increase in cost due to the reduced own power production. It can however, be acceptable if it only concerns a few annual operating hours.

Another way to increase steam pressure is to use the prior art TC- system technology (see WO 01/27388). The TC normally uses MP-steam as motive steam and the higher pressure increase needed, the more motive steam (MP) needs to be used and corresponding less suction steam (LP) is used. As an example, at rather small pressure increase of suction LP-steam (typical up to 0.5 bar) up to about 70 % of MP steam is replaced by LP steam (compared to full MP steam supply). With increased TC discharge steam pressure the part of motive MP steam increases and at some pressure level the steam supplied from the TC consists in practice only of MP steam (only motive MP and no suction LP steam). Such operation conditions would create very big power production losses. The TC technology has a low energy efficiency but it might be a total cost effective method if it is used for rather small pressure increases, short operating times and with selective steam use for the actual steam groups.

Steam is also used when drying pulp. There exists a number of different kind of pulp dryers. The most common type of pulp dryer is the airborne dryer. The pulp web moves along about 20 storeys from top to bottom (each storey is at least 50 m long) and the web is carried by air circulation. The heating consist of fresh steam heated circulation air batteries. The air batteries are collected in one or more steam groups and it is in principal similar to drying cylinder steam groups. The higher the steam pressure/steam saturation temperature the higher is the drying capacity.

Other pulp dryers are pulp cylinder dryers and they are very similar to corresponding paper dryers. Also flash dryers and back pressure dryers can be used.

There are a lot of pulp dryers with the same aspects/problems as paper or board machine dryers, either there is a need to increase the drying capacity/production or they have operating conditions with very low efficiency concerning power production.

Yet another usage of steam within the paper or board production is during calendering. Practical all paper or board machines uses a calender. The purpose of calendering a paper or board is to create a smoother surface of the product. The calendering equipment might differ from one nip on-line calenders to very big off-line super calenders with several rolls and nips. Many paper or board qualities need a high web temperature during the calendering and it is normally created by internal oil heating in heating rolls which are in contact with the web. MP-steam often keeps a too low saturation temperature and can't be used for the heating of the oil and the oil heating then have to be made by electrical or natural gas combustion in heat exchangers. This is very expensive. Some mills have HP-steam available for high temperature calendering, but pressure/saturation temperature is not always enough.

A great drawback with the usage of pressurized steam during the production of paper or board is that it is does not have enough capacity and thus not economically beneficial. There is thus a need for a new system which uses the available steam in a more cost efficient way.

Summary of Invention

One object of the present invention is to define a solution to the above- described problems.

According to a first aspect of the invention, this object is achieved by a method for increasing the drying capacity of a steam group in a section of a paper or board machine or in a pulp dryer characterised in that a main steam flow having a first steam pressure is conducted to at least one mechanical vapour recompressor system (MVR), which is driven by a driving device, such as a motor or a turbine, that the output steam of said at least one mechanical vapour recompressor system is fed to the steam group and that the steam pressure of the steam that is fed to the steam group is being higher than the first steam pressure.

According to a second aspect of the invention, the above-mentioned object is achieved by a device for increasing the drying capacity of a steam group in a section of a paper or board machine or a pulp dryer, comprising a main steam pipe for supplying a main steam flow to the steam group, characterised in that the device further comprises at least one mechanical vapour recompressor system having an inlet for steam with a first steam pressure and an outlet for output steam which outlet is connected to the steam group, a branch between the main steam pipe of said inlet of said at least one mechanical vapour recompressor system for steam with a first steam pressure for conduction of the main steam flow to said at least one mechanical vapour recompressor system and a driving device such as a motor or turbine which is connected to the mechanical vapour recompressor system and drives the at least one mechanical vapour recompressor system.

Additional characteristic features and advantages of the invention are stated in the depended claims and in the description below.

Since the main steam flow is conducted to a mechanical vapour recompressor system, which is driven by a driving device, the steam group can be fed with steam having a pressure that exceeds the pressure of the main steam flow, which means that the drying capacity of the steam group is increased.

The method can preferably be used for increasing the drying capacity in the drying section of a paper or board machine. It can also be used to increase the capacity of a pulp dryer.

The output steam may also be fed to a thermocompressor before the steam is fed to the steam group. The main steam flow may also be conducted to a thermocompressor to increase the pressure before the steam is conducted to the mechanical vapour recompressor system. In this way it is possible to further increase the pressure and in some cases it is more economically beneficial to use both a thermocompressor and a mechanical vapour recompressor system.

It is also possible to feed one part of the main steam flow to a thermocompressor and the other part of the main steam flow to the mechanical vapour recompressor system. In this way, it is possible to receive steam with the most appropriate pressure in the most efficient way seeing to the total power production of the mill and to what is economically most beneficial.

Also, more than one mechanical vapour recompressor systems can be used. This can be used when the needed increase of the pressure is very high and this still makes it possible to only use low pressure steam.

The pressure of the steam fed to the mechanical vapour recomressor system is preferably between 2-6 bar and the pressure fed to the steam cylinders is preferably between 3-12 bar.

Brief description of the drawing

The invention will now be described in more detail with reference to the accompanying schematic drawings which for the purpose of exemplification show a currently preferred embodiment of the invention.

Fig. 1 Show how steam with increased pressure is supplied to a steam group.

Detailed description

As seen in Fig. 1 , a main steam flow, preferably low-pressure steam, is fed through the conduit (1) to a mechanical vapour recompressor system (10) where the pressure of the steam is increased. A driving device, such as an electric engine or a turbine (11) drives the mechanical vapour recompressor system (10). Steam with increased pressure is fed from the mechanical vapour recompressor system (10) through conduit (2) to a steam group (12). In some cases it might be necessary to cool the produced steam and "cooling" can be applied through conduit (3), in order to avoid undesired superheating of the steam.

During production stops or paper breaks it might be necessary to recirculate the steam by conduit (4). In this way the production of high pressure steam will continue. The steam can also be blown out or conducted to other groups or steam consumers through valve (5).

The steam conducted to the mechanical vapour recompressor system (10) preferably has a steam pressure of about 2-6 bar and the steam that has passed the mechanical vapour recompressor system (10) and which is fed to the steam group (12) preferably has a steam pressure of about 3-12 bar.

In some special cases, for example during start up, during long sheet break situations or when there is no need for pressure increase (for ex at very low grammages) it is also possible to feed the main steam flow directly to the steam group (12) through conduit (7).

In conventional paper or board machines, the drying cylinders are dimensioned to stand a pressure of up to about 7 bar and usually a pressure of at least about 0.5 bar is required to obtain satisfactory heating. The maximum permissible pressure can, however, vary considerably depending on the type of paper or board machine and the age of the drying cylinders. In a board machine, the maximum permissible pressure can be 12 bar, in a new fine paper machine 5-7 bar and in older fine paper or newsprint machines as low as 2 bar. The steam can be removed from the system through the safety valve (6) in order to make sure that the steam fed to the drying cylinders (12) does not exceed the maximum allowable steam cylinder pressure.

The steam fed to the mechanical vapour recompressor system is not recycled residual (bt-)steam but fresh steam normally from the turbine. In some applications a small share of recycled bt-steam may be used.

In general but especial when it comes to higher magnitudes of pressure increase of steam and longer operating times, there is a need to find much more energy efficient solutions for increasing drying capacity within the paper or board production, concerning both investment- and operating costs. One solution is described herein and it has surprisingly been found that the use of a Mechanical Vapour Recompression system (MVR) which is a general gas compression made by a compressor with moving parts (centrifugal-, screw-, reciprocating compressors etc or general named turbo compressors) will effectively increase the steam pressure in a very cost and technical efficient way.

It is possible to increase the steam pressure of for example a drying section of a PM or BM by installing a mechanical vapour recompressor system (MVR) which increases the pressure of the steam flow that passes the compressor, by increasing the pressure of the steam, normally LP steam, to desired level. A MVR system keeps a high energy efficiency. A one stage compressor is able to double the steam pressure if needed. The benefit of a MVR system for the use of steam pressure increase of steam supplied to a steam group, compared to other solutions, is to achieve a higher pressure increase and thus making it possible to increase the net power production from the mill's turbine. The benefit will increase with larger pressure increases, higher steam flow to steam group and during longer operating times.

If lower steam pressure and steam consumption is needed, for example when the paper produced has low grammage, the MVR-unit could be down loaded to partial capacity in a very efficient way by controlling the driving speed.

For a MVR it is often sufficient to use only LP steam from the turbine and still increase the capacity of for example a drying section to desired levels. Even though the MVR needs some driving energy the net power production will increase much due to the fact that low or no amounts of MP steam need to be used in the steam groups in a PM or BM or pulp dryer and instead expand more steam through turbine to LP pressure level and thereby increase power production.

If more than doubled pressure is needed another MVR system or another compressor could be coupled in serial. In this way it is not necessary to use any intermediate-pressure steam which instead can be used in the power production which means a higher power production. It is thus possible to use more than one MVR in serial coupling depending on the pressure needed. Also, a combination of one or more TCs and one or more MVR can be used. This might for example be suitable when the needed pressure increase is not that high or if maximum steam flow design capacity of the MVR is a bit low for short operating times and the use of small amounts of complementary steam flow are needed. Sometimes, if the steam flow demand is very high during a short amount of time a small amount of MP steam can alternatively be used, and thus a connection between the MP steam net and steam group after the MVR is necessary. For example, if the MVR capacity design is 60 t/h and there is a need for about 65 t/h during few annual hours it might be useful with a complementary TC or MP supply of 5 t/h.

A MVR system can also be used for improving the power production for the machines where there is no need for increasing the drying capacity. For example, many BM and kraft PM ^' s use MP (about 12 bar steam net) steam for the large steam groups but most of the time they need much lower steam pressure, often about 6 bar. It would in these cases be much more efficient to use LP steam (about 3.5 bar) and compress to 6 bar and thus expand more steam through turbine to LP pressure level and increase power production. The use of the MVR system according to the invention can thus be used in order to compress LP steam instead of using pressure reduced MP steam during production of a paper or board product.

Examples:

In order to describe how different methods for pressure increase acts, a typical calculation example is described below.

For a big fine grade PM (about 7m width and main grammage 80 g/m2) is planned a large increased production capacity from machine speeds about 1050 to 1 400 meter per minute without extending the drying sections. The steam pressure of the pre- drying section main group will during most of the operating time be increased from about 3.5 bar (LP) to about 6.5 bar and steam flow will be increased from 50 t/h (LP) to 60 - 65 t/h for the main group. By changing from LP- to MP steam it will cause a very big decrease for the mills power production for several reasons like, lower specific power output, direct reduction of HP- to MP net, much lower efficiency operating point for turbine etc which leads to an average decrease of at least 10 MW or on annual base 5-10 MEuro.

With a TC system 50 t/h LP will be changed to about 50-55 MP and 5-

10 t/h LP. Average power production will thus decrease about at least 9 MW.

On the other hand by a MVR-system 60 - 65 t/h LP outtake from turbine will instead increase power production about 2 MW or more and that covers very well the driving effect for the compressor of about 1.5 - 2 MW. So the net result is that the MVR application doesn't cause any power production losses but a slight net increase. By increasing LP steam flow through turbine, the whole turbine system will normally operate with higher efficiency which means further improved power production.

Typical investment costs for installation of steam pressure increasing device to supply a big main group could be estimated to about 1 ,5 respective 1 MEuro for MVR- respective TC-system. (Compared to a drying section extension at about 20 MEuro and a long shut down time for rebuild).

A MVR system can also be used in other applications, for example by recovering steam from TMP- and CTMP refiners. A big part of all electrical power input to drive the refiners will be converted to heat that is possible to recover as steam in steam reformers. This recovered steam often keeps a too low pressure to be supplied to the ordinary steam net (normally LP net) in the mill. So the steam has to be compressed and the common method is by TC but the MVR technology is a competitive alternative. By using an efficient steam compression system, such as a MVR, it is possible to recover the refiner's heat as steam to a bigger extent and also compress steam fractions with lower pressure to a useful level as LP or LLP steam net.

A MVR-system can also be used in the mills which desire even higher steam pressure for some of their steam consumers (for example drying/- production increase or energy improvements) which could not be achieved with full MP steam pressure. Typical applications are hot circulation air dryers for coating paper drying or build in to a paper machine drying cylinder section, some drying cylinders/steam groups for some kraft PM ^' s, Yankee drying cylinders and calendering. With a MVR system MP steam could be compressed to desired level. To make this with a TC system (with HP steam as motive steam) might be theoretical possible but complicated, very costly and with a very low energy efficiency. A MVR system may be able to compress MP steam from about 10-15 bar to about 20-25 bar or more if needed. Preferably, the MP steam should have passed the mills turbine.

In system using oil heating, such as calenders, it is very beneficial to increase the pressure of the used MP steam with a MVR system according to the invention. It is thus possible to decrease the net consumption of electrical power previously used in order to achieve the required oil temperature.

Theoretical HP-steam with sufficient pressure/saturation temperature can be used, but in many applications it is a more complicated and less efficient method, since you often need to reduce the pressure of the HP-steam. Consequently, the best way to achieve high oil circulation temperatures is to use a MVR system.

It is also possible to increase HP steam pressure when needed by a MVR, for example from 35 bar out from steam boiler to 60 bar desired for calendering oil heating.

A mechanical vapour recompression system can also be designed to be fed with MP- or HP-steam for further pressure increase to a mills higher pressure demand steam groups or other steam consumers.

There is a similar opportunity to improve a mills power production if there is a chemical kraft pulp mill. The recovery boiler as well as the bark boiler has to use a lot of soot blowing to keep the boilers heating tubes clean. At some pulp mills several percentage of the HP steam production need to be used for soot blowing. Normally HP-steam (60-100 bar) is directly reduced to some extent cooled to about 20 bar in order to make it suitable for steam sooting. Not many turbines have extraction of steam with a pressure suitable for steam sooting. To compress MP steam (extracted from turbine) of about 12 bar to 20 bar with a MVR system would be much more efficient and create a higher net power production.