WO2007073333A1

WO2007073333A1 - System and method for the generation of steam from hot black liquor in a digester plant

Info

Publication number: WO2007073333A1
Application number: PCT/SE2006/050583
Authority: WO
Inventors: Vidar Snekkenes; Krister Olsson; Lennart Gustavsson; Katarina Vallhagen; Finn Oulie
Original assignee: Metso Fiber Karlstad AB
Current assignee: Metso Fiber Karlstad AB
Priority date: 2005-12-21
Filing date: 2006-12-15
Publication date: 2007-06-28
Anticipated expiration: 2008-06-21
Also published as: SE0502851L; SE528574C2

Abstract

The invention concerns a system and a method for the generation of steam in a digester plant for the production of chemical cellulose pulp. The pressure of hot, pressurised black liquor from a digester (4) is reduced in a first step (11 a/11 b) for the formation of sufficient quantities of black liquor steam (BL-ST) that is used for the steam pre-treatment of the chips in a second pre-heating step (2). Pure steam (P-ST) for the steam pre-treatment of the chips in a first pre¬ heating step (1 ) is formed in sufficient quantities through re-heating (13) the black liquor whose pressure has been reduced before a final subsequent pressure reduction, where the increased quantity of black liquor steam is led to a steam converter (14) for the generation of pure steam. This system entails an efficient use of the heat content of the black liquor with simple process equipment, with less problems from clogging, and it generates sufficient quantities of pure and more contaminated steam that is used in a first steam pre-treatment step and a second steam pre-treatment step, respectively. At the same time, an increased dry solids content of the black liquor is achieved through re-heating and repeated reduction in pressure, which reduces the load on the recovery plant.

Description

SYSTEM AND METHOD FOR THE GENERATION

OF STEAM FROM HOT BLACK LIQUOR IN A

DIGESTER PLANT

The present invention concerns a system and a method for the generation of steam in a digester plant during the production of chemical cellulose pulp in accordance with the introductions to claims 1 and 4.

The Prior Art

In association with the production of chemical cellulose pulp from wood chips in digester plants, the chips are to be first pre-treated with steam, not only for the expulsion of air but also of the preheating of the chips before the cook. The expulsion of air is important to ensure that the chips are fully impregnated with cooking fluid. Steam-treatment of chips takes place conventionally in several steps, in which untreated chips are first admitted to a first pre-treatment vessel, typically a chip bin, and there steam pre-treated at a lower first pressure, typically atmospheric pressure up to an excess pressure of approximately 1 bar, after which the pre-heated chips are sluiced in to a second pre-treatment vessel in which the chips are steam pre-treated at a considerably higher pressure, typically an excess pressure of 1-3 bar.

Black liquor steam is used in both of these steam pre-treatment steps in older systems, but this involves an increased risk of the release of malodorous chemicals, since this black liquor steam contains gases known as TRS gases (totally reduced sulphur gases) from the cook. Different systems have therefore been tested for the generation of pure steam for use in the first steam pre-treatment step, which substantially reduces the risk of the release of malodorous gases.

A system is revealed in US 6.722.130 for the generation of this pure steam from black liquor in which the pressure of the black liquor is first reduced in order to produce black liquor at atmospheric pressure and black liquor vapour, where this black liquor steam is condensed in subsequent steps and form the pure steam from this condensate. A system was revealed long ago in US 2.029.360 in which a steam converter is used in order to heat a pure process fluid for the generation of pure steam in a steam converter in the form of a heat exchanger. A variant was also revealed here in which the quantity of expelled clean steam in the heated clean process fluid can be increased by injecting steam into this heated process fluid. The system has no effect, however, on the dry solids content of the black liquor whose pressure has been reduced.

A system is revealed in US 6.176.971 for the generation of pure steam to be used in the chip bin in which a steam converter for the black liquor is used in which the black liquor heats a pure process fluid, and where the quantity of clean steam produced is increased by placing under negative pressure the pure steam side of the steam converter in the steam converter with a steam- driven ejector. The reduced pressure of the pure steam side ensures that more heat can be withdrawn from the black liquor, which in itself gives a greater quantity of steam, while the supply of steam to the ejector also contributes to the delivery of greater quantities of steam. In this case, however, the steam vapour formed consists of a mixture of pure steam that has been expelled from the process fluid and steam that has been taken from the steam supply network of the mill for driving the ejector.

A further variant for the generation of pure steam is revealed in US 6.306.252 for use in the chip bin, where the black liquor from the digester is led through a heat exchanger in which pure process water is heated, after which the pressure of the heated process water is reduced, such that pure steam is generated.

The different solutions show that considerable development has been invested into the production of new systems for the generation of pure steam from hot cooking fluids, which pure steam is to be used in at least the first steam pre-treatment of the chips. The systems have had the property that the quantities of steam generated, not only pure steam but also the black liquor steam that is used in a second steam pre-treatment operation, are not sufficient for the steam pre-treatment effect desired. A second disadvantage is that the systems do not reduce the dry solids content of the black liquor in association with the production of pure steam, and this means that the motivation to install these systems in older plants that have problems with foul odours, remains low, these plants often having capacity problems also on the evaporation side.

Aim and Purpose of the Invention

The present invention aims to offer an improved system and method for the generation of pure steam from black liquor, where sufficient quantities of both black liquor steam and pure steam can be obtained, which quantities of steam are required for a steam pre-treatment process in several steps where pure steam is initially used, followed by pre-treatment with black liquor steam. Direct generation of black liquor steam from the black liquor is the most efficient form of steam generation.

A second aim is to obtain an increased dry solids content of the black liquor when its pressure has been reduced, which dry solids content allows a reduced load in the recovery process. The evaporation capacity at older plants is often fully used and an increased production of cellulose pulp, which gives increased volume of black liquor that is sent to the evaporation process, cannot be considered unless the black liquor previous to the evaporation acquires a dry solids content that is greater than the previous content. The system according to the invention has the advantage that no new quantities of water are introduced in the process than those that are already present in the chips after the steam pre-treatment.

A further aim is that the quantity of pure fresh steam that is removed from the steam network of the mill and which is added to the chips, and is in this way contaminated, is reduced. The pure steam that is required for the pre- treatment of the chips is obtained instead through heating of process water and in close association with the digester. It is for this reason unnecessary to remove steam from the steam production in the soda boiler (the steam network of the mill). The fresh steam that is taken from the steam network of the mill for heating the black liquor is not mixed with the chips, with the resulting contamination, and instead a pure condensate is obtained that can be returned to the soda boiler for generation of steam in the soda boiler and return to the steam network of the mill. The requirements for purity for the condensate that is used during steam generation are very high with the aim of avoiding clogging and precipitation in the system.

The need for fresh steam from the steam network of the mill will be reduced, in total, since an increased evaporation effect is obtained for the black liquor already in the digester plant, and this entails a reduced requirement for steam in the evaporation process.

Description of Drawing

Figure 1 shows schematically a system according to the invention.

Detailed Description of Preferred Embodiments

Figure 1 shows a system for the generation of steam in a digester plant for the production of chemical cellulose pulp, according to the invention. The system comprises a digester vessel 4 in which chips are cooked in a pressurised hot cooking fluid. At least one withdrawal means is present in the digester vessel, in this case shown as a withdrawal strainer 10, for the withdrawal of hot, pressurised and spent cooking fluid, known as "black liquor" BL, from the digester vessel. The black liquor is led to a first pressure reduction means, here in the form of two pressure reduction tanks 11a, 11b coupled in series, where the pressure of the black liquor is reduced to a first lower pressure during the expulsion of a first quantity of black liquor steam BL-ST. Before the chips are fed to the digester vessel, the untreated chips CH are steam pre-treated in a first steam pre-treatment vessel 1 , typically a chip bin, in which the chips are pre-heated with a first steam at a first pre-treatment pressure, typically at a pressure that is equal to atmospheric pressure up to a pressure approximately 1 bar above atmospheric pressure. The pre-heated chips CHi are then fed by suitable sluice means 9 to a second steam pre- treatment vessel 2 in which pre-heated chips are pre-heated, before being cooked in the digester vessel, with a second steam at a second steam pre- treatment pressure, which second pre-treatment pressure is higher than the first pre-treatment pressure, typically between 1 - 3 bar. Formation of a slurry from the chips with cooking or impregnation fluid Liq follows in a conventional manner placing the chips mixture CS under pressure by a suitable feed arrangement 3 and feeding it to the top of the digester. The drawing shows a single-vessel digester plant, but the invention can be used on all forms of configurations of continuous digestion plants, for example, two-vessel digestion plants. The invention can, of course, also be used in a batch digestion plant, in which steam pre-treatment of the chips takes place in two steps, in which the first step uses pure steam for the steam pre-treatment. Only those components that are important for the invention are shown in the drawing, and other types of digester circulations can, of course, be present in the digester system. The feed arrangement 3 can be either of conventional type with sluice-feed with a high-pressure tap, with or without a top separator at the top of the digester. The feed arrangement may be also one with simple pumping technology, in which the mixture of chips CS flows through the pressurising pumps.

According to the invention, the system comprises also a heating arrangement 13 for the black liquor after the pressure on the black liquor has been reduced in the first pressure-release arrangement 11a, 11b in which heating arrangement 13 the black liquor BL₂ the pressure of which has been reduced is heated to form reheated black liquor BL₂H- This heating arrangement is preferably constituted by a heat exchanger in which the black liquor is heated indirectly by steam St, and from which heating arrangement is obtained a pure steam condensate Cond. An indirect heat exchanger is to be preferred since in this case no dilution of the black liquor by steam condensate is experienced, something that would otherwise entail an increased load on the evaporation system of the recovery plant. The black liquor BL₂ at this position has an increased dry solids content, relative to the dry solids content of the black liquor BL that was withdrawn from the digester, and thus it has an increased tendency for clogging any heat exchangers or reboilers that are used. The reheated black liquor BL_2H is subsequently led to a second pressure-reduction arrangement 12 in which the pressure of the reheated black liquor BL_2H is reduced while a second quantity of black liquor steam is expelled. This second quantity of black liquor steam is subsequently led to a steam converter 14 arranged to receive the second quantity of black liquor steam, which heats pure process fluid P. LJq in the steam converter while pure steam is formed. The pure process fluid P. LJq can be recycled, as is shown in the drawing, over the heat-transfer surfaces of the steam converter through a pump 15 that withdraws heated process fluid from the bottom of the steam converter and leads this heated fluid to return in to the top of the steam converter, where the fluid is finely dispersed over the heat-transfer surfaces of the steam converter in a conventional manner. New pure process fluid P. LJq is added to the circulatory circuit through a level-controlled valve 16. The pure steam P-ST that is driven from the steam converter 14 is subsequently led to the first steam pre-treatment vessel 1 , while the first quantity of black liquor steam is led to the second steam pre-treatment vessel 2, each along the relevant steam lines.

The hot pressurised black liquor BL typically maintains a temperature in the interval 130-160 ⁰C, corresponding to the cooking temperature in the digester, where the pressure on this black liquor is reduced in such a manner that a first quantity of black liquor steam BL-ST is generated while the black liquor BL₂ the pressure of which has been reduced obtains a temperature that is at least 15 ⁰C lower than the cooking temperature.

The black liquor the pressure of which has been released is subsequently heated by at least 5 °C in order to form a re-heated black liquor BL₂H. It is preferable that the black liquor the pressure of which has been reduced is heated by at least 10-15 ⁰C in order to form the re-heated black liquor BL_2H. The pressure of the re-heated black liquor BL_2H is again reduced in order to generate a second quantity of black liquor steam and black liquor BL₃ the pressure of which has been reduced, which black liquor BL₃ from which the pressure has been reduced maintains a second lower temperature, which second lower temperature is at least 5 ⁰C lower than the first lower temperature, preferably at least 10-15 ⁰C lower than the first lower temperature. This black liquor BL₃ is then led to the recovery plant. It is preferable that the reduction in pressure is regulated such that the second quantity of black liquor steam that is led to the steam converter maintains a temperature of at least 105-115 ⁰C.

Sufficient quantities of both black liquor steam and pure steam are obtained by this technique, where the second quantity of black liquor steam formed is led to a steam converter in order to produce pure steam. The pure steam in then used for heating the chips in the first step 1 and where the first quantity of black liquor steam is used for pre-heating the chips in the second step 2.

The pure steam that is produced in the steam converter is formed preferably from pure process water, preferably condensate from the pulp mill, which process water already maintains a relatively high temperature of 70-85 ⁰C. Alternatively, pure mains water can be used. It is not required, however, that this water satisfy the same high requirements for purity as the water that is used in the steam network of the mill. The condensate from the heat exchangers 13, 13b is, however, pure steam condensate and it can be returned to the steam network without any form of purification process. The process water can also, as is shown in Figure 1 , first be heated in a heat exchanger 20 with contaminated condensate, i.e. the contaminated condensate that is formed from the second quantity of black liquor steam in the steam converter 14.

When using the invention in a digester plant in Nordic conditions where the withdrawn black liquor maintains a temperature of 155 ⁰C, then the pressure of the black liquor is reduced in pressure release tanks (of type 11a, 11b) to give a temperature of 123 ⁰C, i.e. a reduction in temperature of 32 ⁰C. The quantity of black liquor steam that was then obtained is more than sufficient for the steam pre-treatment in a second step. In the subsequent heating of this black liquor, the pressure of which had been reduced, the temperature was raised from approximately 123 ⁰C to 137 ⁰C, i.e. a rise in temperature of approximately 14°C. This rise in temperature increased the quantity of pure steam that can be obtained from the steam converter in the system under consideration by more than 100%, from 3 kg/sec to just over 7 kg/sec. The second quantity of black liquor steam could be maintained at 110 ⁰C, and with a flow of just over 7.3 kg/sec, where the steam converter had a heat transfer surface of approximately 1 ,600 m², a k-value of 2,000 W/(m² * ⁰C) and a deltaT of 5.0 ⁰C. The condensate that had been added was heated by heater 20 to a temperature of just over 95 ⁰C and a quantity of steam corresponding in quantity to that expelled from the condensate (just over 7 kg/sec).

This quantity of pure steam is that which was required in order to pre-treat in a satisfactory manner the chips in the first step to a temperature at the chips of approximately 100 ⁰C.

At the same time, the rise in temperature contributed to the dry solids content of the black liquor, BL₃, increasing by 2-3 units of percent, which severely reduces the quantities of water that are to be expelled in the subsequent recycling operation in the evaporation. This can be exchanged for the possibility of increasing production in digester plants where the evaporation equipment is operated at the limit of its capacity, something that often is the case in older plants where the system has been pressed to higher production as time has passed.

Alternatively, the re-heating may take place through the addition of a quantity of heated cooking liquor to the black liquor, the pressure of which was reduced in the first step and which maintains approximately 123 ⁰C. Figure 1 shows such a variant in which spent cooking liquor, which typically maintains a temperature of approximately 110⁰C, is withdrawn in a lower withdrawal strainer 10b. This spent cooking liquor is subsequently led to a heat exchanger 13b where this spent cooking liquor is heated with fresh steam to a temperature in the interval 145-165 ⁰C. This heated spent cooking liquor is then mixed with the black liquor BL_2H whose pressure has been reduced in the first operation and that maintains a temperature of approximately 123 ⁰C, in such a manner and with such a quantity that the mixture reaches a temperature of approximately 137 ⁰C. The invention can be modified in a number of ways within the scope of the attached patent claims. For example, the first reduction of pressure, which generates the black liquor steam, may take place in a single step, or in more steps than two. The heating of the steam BL₂ from which the pressure has been removed can take place also through heat exchange with other hot process fluids than steam, or in combination with a subsequent heating using steam. The system according to Figure 1 has the advantage that no new quantities of water are introduced into the process than those that are already present with the chips after the steam pre-treatment, and then only those quantities of water that have been added in the first steam pre-treatment operation. The second steam pre-treatment operation is carried out fully and completely with regenerated steam, without the addition of any new quantities of fluid.

Claims

1. A system for the generation of steam in a digestion plant for the production of chemical cellulose pulp, which comprises - a digester vessel (4) in which chips are cooked in a pressurised hot digester fluid

- a withdrawal arrangement (10) for the withdrawal of hot, pressurised, spent cooking fluid, known as black liquor (BL), from the digester vessel - at least a first pressure reduction arrangement (11 a, 11b) in which the pressure of the black liquor is reduced to a first lower pressure while a first quantity of black liquor steam (BL-ST) is expelled

- a first steam pre-treatment vessel (1) in which untreated chips are pre-heated with a first steam at a first pre-treatment pressure before they are cooked in the digester vessel

- a second steam pre-treatment vessel (2) in which pre-heated chips are pre-heated with a second steam at a second pre-treatment pressure before they are cooked in the digester vessel, which second pre-treatment pressure is higher than the first pre-treatment pressure c h a r a c t e r i s e d in that the system also comprises

- a heating arrangement (13) for the black liquor (BL₂) after the pressure of the black liquor (BL) has been reduced in the first pressure reduction arrangement (11a, 11b) in which the black liquor from which the pressure has been released is heated in order to form a re-heated black liquor (BL₂H)

- a second pressure reduction arrangement (12) in which the pressure of re-heated black liquor (BL_2H) is reduced while a second quantity of black liquor steam is expelled

- a steam converter (14) arranged to receive the second quantity of black liquor steam that heats pure process fluid in the steam converter in order to form pure steam

- means for leading the pure steam (P-ST) to the first steam pre- treatment vessel, and - means for leading the first quantity of black liquor steam (BL- ST) to the second steam pre-treatment vessel.

2. The system according to claim 1 , c h a r a c t e r i s e d in that the heating arrangement (13) for the black liquor (BL₂) is constituted by a heat exchanger in which the black liquor is indirectly heated with the aid of steam (St).

3. The system according to claim 2, c h a r a c t e r i s e d in that the pressure reduction arrangement (11a, 11b) is constituted by two pressure reduction vessels (11a, 11b) connected in series, in which the pressure of the black liquor from the digester is reduced in two steps to the first lower pressure while a first quantity of black liquor steam (BL-ST) is expelled from both of these pressure reduction vessels.

4. A method for the generation of steam in a digester plant for the production of chemical cellulose pulp in which untreated chips are preheated in a first step with a first steam at a first pre-treatment pressure before the cook, followed by - the pre-heated chips are pre-heated in a second step with a second steam at a second steam pre-treatment pressure, where the second steam pre-treatment pressure is higher than the first steam pre-treatment pressure, and

- where the chips that have been pre-treated with steam are subsequently cooked in a cooking fluid at high pressure and high cooking temperature,

- where hot, pressurised, spent cooking fluid, known as "black liquor", is withdrawn at a cooking temperature of 130-160 ⁰C from the cook, and its pressure is reduced for the generation of a first quantity of black liquor steam, and black liquor whose pressure has been reduced and that maintains a lower pressure and a first reduced temperature, which first reduced temperature is at least 15 ⁰C lower than the cooking temperature c h a r a c t e r i s e d in that the black liquor that has had its pressure released is heated by at least 5⁰C in order to form a re-heated black liquor, after which the pressure of the re-heated black liquor is again reduced in order to generate a second quantity of black liquor steam and black liquor the pressure of which has been reduced that maintains a pressure and a second lower temperature, which second lower temperature is at least 5⁰C lower than the first lower temperature, where the second quantity of black liquor steam is led to a steam converter in which the second quantity of black liquor steam is used to produce pure steam, where the pure steam is used for the pre-heating of the chips in the first step and where the first quantity of black liquor steam is used for pre-heating of the chips in the second step.

5. The method according to claim 4, characterised in that the first reduced temperature of the black liquor, the pressure of which has been reduced, is at least 25⁰C lower than the cooking temperature.

6. The method according to claim 5, characterised in that the black liquor whose pressure has been reduced is heated by at least 10-15⁰C for the formation of a re-heated black liquor.

7. The method according to claim 4, characterised in that the second quantity of black liquor steam that is led to the steam converter maintains a temperature of at least 105-115⁰C.

8. The method according to claim 4, characterised in that the pure steam that is produced in the steam converter is formed from pure process water, preferably condensate from the pulp mill.

9. The method according to claim 8, characterised in that the process water is first heated with contaminated condensate, i.e. with contaminated condensate that is formed from the second quantity of black liquor steam in the steam converter.

10. The method according to any one of the preceding claims, characterised in that the black liquor is heated sufficiently to ensure that the second quantity of black liquor steam generated increases at least 100%, calculated as a quantity of steam in kg/sec, relative to what would have been generated if the black liquor had not been heated prior to the final reduction in pressure.