Method and arrangement for generating steam at a digester plant of a chemical pulp mill

Abstract

The present invention relates to a method of producing steam at a digester plant of a chemical pulp mill. Black liquor is discharged from the digester at a first temperature and pressure. The black liquor is treated in a flash tank for generating flashed black liquor and flash steam at a second temperature and a second pressure, which are lower than the first temperature and pressure. The flashed black liquor is taken for further treatment to the evaporation plant. The flash steam is led to an indirect heat exchange contact with clean liquid in the reboiler for boiling the liquid and for generating steam. The steam is led from the reboiler into a fan or compressor for increasing the steam pressure to a third pressure, which is higher than the second pressure and for adjusting the pressure of the steam in the flash tank. Steam at a third pressure is used for pretreating comminuted cellulosic fiber material, such as chips, prior to cooking.

Claims

1. A method for producing steam at a digester plant of a chemical pulp mill, the method comprises: a) a suspension formed of pretreated comminuted cellulosic fiber material and liquid is fed into a digester for producing chemical pulp; b) the pretreated comminuted cellulosic fiber material is treated in the digester with a treatment chemical for producing the chemical pulp and black liquor; c) the black liquor is discharged from the digester at a first temperature and a first pressure; d) the black liquor is treated in a flash tank which produces flashed black liquor and flash steam at a second temperature and a second pressure, which are lower than the first temperature and the first pressure; e) the flashed black liquor is fed to an evaporation plant; f) the flash steam is fed to a reboiler in which the flash steam is in indirect heat exchange contact with a clean liquid in the reboiler which boils the clean liquid and generates; g) the steam formed in step f) is fed from the reboiler into a fan or compressor which increases a pressure of the steam to a third pressure, which is higher than the second pressure and regulates the second pressure of the steam in the flash tank in step d); and h) the steam at the third pressure is applied to heat the comminuted cellulosic fiber material before the pretreated comminuted cellulosic fiber material is fed to the digester.

2. The method according to claim 1, wherein in step g) the third pressure of the steam being discharged from the fan or compressor and the second pressure of the steam in the flash tank are adjusted according to a requirement for steam in the pretreatment of the comminuted cellulosic fiber material.

3. The method according to claim 1, wherein the digester produces high kappa pulp.

4. The method according to claim 1, wherein the temperature of the black liquor discharged from the digester is below 145 C.

5. An arrangement for producing steam at a digester plant of a chemical pulp mill, said arrangement comprising: a vessel configured to steam a cellulosic fiber material; a feed system in which a suspension of cellulosic fiber material and liquid is formed and the suspension is pressurized; a continuous digester configured to digest the suspension, which digester is connected to a black liquor conduit configured to discharge the black liquor stream from the digester; a flash tank connected to said black liquor conduit, and in which the pressure of the black liquor is decreased for producing flashed black liquor and flash steam, whereby the flash tank is provided with a conduit for discharging the flashed black liquor and feeding the flashed black liquor to an evaporation plant of the chemical pulp mill; a flash steam conduit for feeding the flash steam from the flash tank to a reboiler, in which clean steam is generated due to an indirect heat exchange relationship with the flash steam and a clean liquid; and a steam conduit for feeding the clean steam generated in the reboiler into the steaming vessel, whereby the steam conduit includes a fan or a compressor which increases a pressure of the clean steam generated in the reboiler.

6. The arrangement according to claim 5, wherein the fan or compressor is connected to a pressure indicator controller which receives a pressure signal from a pressure sensor located in the flash tank.

7. The arrangement according to claim 5, wherein at least one vessel for treating the suspension is arranged between the feed system and the digester.

8. A method to produce steam comprising: feeding a suspension of pretreated comminuted cellulosic fiber material and liquid into a digester vessel in a chemical pulp mill; adding a treatment chemical to the pretreated comminuted cellulosic fiber material in the digester vessel; converting the pretreated comminuted cellulosic fiber material to pulp in the digester vessel, wherein the conversion produces black liquor; discharging the black liquor form the digester vessel; flashing the discharged black liquor in a flash tank and discharging from the flash tank flash steam and flashed black liquor; feeding the flashed black liquor to an evaporation plant in the chemical pulp mill; feeding the flash steam to a heat exchanger and transferring heat energy from the flash steam to vaporize a clean liquid to produce clean steam; increasing the pressure of the clean steam to a pressure greater than a pressure of the flash steam when discharged from the flash tank, wherein the increase of the pressure of the clean steam is performed using a fan or centrifugal compressor and the amount of the increase in the pressure is based on a requirement for steam to pretreatment the comminuted cellulosic fiber material; and after increasing the pressure of the clean steam, pretreating comminuted cellulosic fiber material with the clean steam to produce the pretreated comminuted cellulosic fiber material fed to the digester vessel.

9. The method of claim 8 wherein the heat exchanger is included with a reboiler and the reboiler transfers heat to contribute to the vaporization of the clean liquid.

10. The method of claim 9 wherein the reboiler is a falling film tube reboiler.

11. The method of claim 8 wherein the steps of the method are performed continuously.

12. The method of claim 8 further comprising applying suction from a fan to a clean steam discharge port of the heat exchanger to draw the clean steam from the heat exchanger.

13. The method of claim 8 wherein the digester vessel is a continuous digester vessel.

14. The method of claim 8 further comprising regulating the pressure of the flashed steam in the flash tank.

15. The method of claim 8 further comprising controlling the fan or centrifugal compressor based on information indicating a pressure of the flashed steam in the flash tank, wherein the information indicating the pressure is provided by a pressure indicator controller which receives a signal from a pressure sensor located in the flash tank.

Description

DESCRIPTION OF THE DRAWING

(1) FIG. 1 illustrates schematically a system according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(2) FIG. 1 illustrates a system for producing steam at a digester plant and for treating black liquor such that the heat economy of the cooking process is improved. A fiber line system comprises a chip bin 10, wherein wood chips or other cellulosic material introduced via line 11 are steamed with steam obtained from line 12. Live steam can be introduced via line 12, if needed. Exhaust gases from the chip bin in line 50 are treated in a separation device 49 for removing debris, such as slivers and saw dust. Via line 13 (white liquor) and line 14 (return circulation from the digester) liquid is added for forming a suspension, after which the suspension is pressurized with a pump 15 (this zone is also referred to as feed system). Then follows a continuous digester 17 (this zone is also referred to as cooking system). Prior to the digester the suspension can alternatively be fed for treatment into a treatment vessel or vessels (can be an impregnation vessel, pre-hydrolysis vessel or other vessels, not shown). The suspension of chips and cooking liquid is fed via line 16 to the top of the digester 17, whereto also steam 31 is added. The FIGURE illustrates the components only that are important in view of the invention, and the cooking system can naturally comprise also other types of chip steaming or feed circulation or digester circulations. The digester 17 comprises two or more screen arrangements 18, 19 and 20. The digester comprises a circulation line 21 in connection with a screen 19. Black liquor is discharged from the screen 19 to circulation line 21, whereto cooking liquor is added via line 22 and liquor via line 23. Line 21 also comprises a heater 24. Filtrate from pulp washing is led via line 25 and further via lines 26 and 27 into the digester. Filtrate can also be added into the white liquor via line 28 and into pulp being discharged from the digester in line 30 via line 29.

(3) Black liquor discharged from the digester and having a temperature of typically 120-160 C. and a solids content of 12-17% is discharged via screens 18 and 20 and led via lines 32 and 33 into a flash tank 34. In the flash tank the pressure of the black liquor is decreased for producing flash steam and for producing flashed black liquor, the solids content of which increases in the flashing. The flashed black liquor is led via line 35 to the evaporation plant of the mill, where the black liquor is concentrated to a high solids content prior to combustion in a recovery boiler. The discharge line can have a heat exchanger 36 and a fiber filter 37.

(4) The flash steam is led via line 38 into a reboiler 39, wherein the energy contained therein is recovered. The steam is set in an indirect heat exchange contact with clean liquid free of volatile compounds for heating the liquid to a temperature exceeding the boiling temperature for producing clean steam. The reboiler is preferably a falling film tube reboiler. Flash steam is condensed inside the tubes, while clean liquid boils on the outer surface of the tubes for producing clean steam. The amount of produced steam is increased when the side of the clean steam is set to under-pressure by means of a fan. The decreased pressure in the clean steam side ensures that more heat can be obtained from the black liquor, which heat itself releases a greater amount of steam.

(5) The steam is discharged via line 45/12, which according to a substantial feature of the invention is provided with a pressure increasing device 41, which is a fan or a compressor. Steam is introduced into the fan from the reboiler, the pressure of the steam is increased so that the pressure and the temperature are adequate for heating the chips in the bin 10. By means of the pressure increasing device, such as a fan, the amount of steam produced in the flash tank can be increased.

(6) The black liquor has typically been pressurized in the digester to a pressure of 5-15 bar (abs) and its temperature corresponds to the temperature of cooking treatment (approximately 110-180 C.). Especially preferably the invention is applied for cases where the temperature of the liquor discharged from the digester is below 145 C. The structure of the flashing tank 34 is substantially conventional, and it is provided with a feed opening 42 for hot, pressurized black liquor, a discharge opening 44 for flash-evaporated steam and a discharge opening 43 for cooled black liquor having a lower pressure. Flash tanks are typically operated so that the pressure prevailing in them is lower than the pressure of black liquor led thereto, typically approximately 1-4 bar (abs). Flash tanks are containers designed for this purpose, which facilitate the pressure decrease of hot, pressurized black liquor, whereby liquid, typically water, is vaporized from the black liquor very quickly, into steam, whereby also the concentration of waste cooking chemical and the products of a cooking reaction (dissolved solids) is increased. The steam produced in this quick evaporation is discharged via a steam discharge opening 44 into line 38 at a pressure prevailing in the tank 34, e.g. 1-4 bar (abs) and at a saturation temperature corresponding to the prevailing pressure, e.g. 100-140 C. After flashing evaporation the remaining liquid, which typically also is at a temperature of approximately j 100-140 C. settles onto the bottom of the tank 34, wherefrom it is discharged from a discharge opening 43 via line 35. Typically the remaining liquid is led into the evaporation system.

(7) Steam in line 12 is used for steaming introduced wood chips. The pressure required for this steam is typically determined based on the pressure requirements of the final object of application in chip steaming. According to the present invention, however, increasing the pressure of steam in line 45 by means of a fan 41 to a desired pressure level (in line 12) facilitates the production of steam in line 45 at a lower pressure Thereby, the flash tank 34 and reboiler 39 can be operated at a lower temperature, whereby in accordance with the invention more steam can be produced in the flash tank and the reboiler, at a lower steam temperature. Thereby, also the temperature of the black liquor being discharged from the flash tank into line 35 is lower and the solids content higher.

(8) The pressure increasing device, such as a fan 41, is typically provided with rotation speed control or guide vane control. The fan is controlled with a pressure indication control 52, which receives the control signal from a pressure sensor 54 located in the flash tank 34. The pressure of the steam in the flash tank 34 and the reboiler 39 is typically the same as the pressure of the steam fed into the fan, although a minor pressure loss typically takes place between the flash tank and the line 45, caused by the equipment and piping. When the need for steam in the chip bin increases or decreases, the rotation speed of the fan is increased or decreased correspondingly. Correspondingly, also the temperature and solids content of black liquor going to the evaporation plant can be adjusted, especially the solids content can be increased.

(9) The clean liquid is led via line 40 into the reboiler 39. It can typically comprise condensate from an evaporator, demineralized water, boiler feed water or adequately clean water fraction, such as e.g. hot water of the mill. The clean steam produced in the reboiler contains a substantially smaller amount of non-condensable gases than steam produced by direct flashing of black liquor. Clean steam in line 45/12 is preferably used for wood chips preheating by means of steam, e.g. in the chip bin 10. When this steam is used for treating chips, a less amount of volatile compounds are generated for collection and treatment by the NCG-system of the mill, since said steam does not bring volatile compounds into the pre-steaming process.

(10) Flash steam from the black liquor flash tank contains volatile compounds, such a sulfur compounds. These compounds enter the foul condensate and into the stream of concentrated non-condensable gases, CNCG, generated in the reboiler. The foul condensate is sent from the reboiler via line 46 to the evaporation plant where it is treated in a manner known per se. Cleaner foul condensate is led via line 46, e.g. to pulp washing. The CNCG stream is led via line 47 to the condenser 48.

EXAMPLE

(11) The starting point is that the temperature of black liquor from the digester entering the flash tank is 136 C. The pressure in the flash tank is 100 kPa (g). Then the temperature of the liquor exiting the tank is 121 C. and the solids content 14.7%. 225 kg low pressure steam/Adt pulp has to be introduced into the chip bin.

(12) By installing a fan or compressor downstream of the reboiler, the pressure in the flash tank can be reduced to 20 kPa (g). The temperature of the exiting black liquor is then 106 C. and the solids content 15.2%. The reboiler produces an adequate amount of steam into the chip bin and no low pressure steam is required there.

(13) The present invention provides at least the following advantages: Reduced need for live steam for chip heating. Additionally, the amount of steam is easy to adjust by means of a fan/compressor in accordance with the amount of steam required. The overall heat economy of the mill is improved, since the solids content of discharged black liquor being led to the evaporation plant is increased. The steam consumption and capacity requirement of the evaporation plant are decreased. The investment costs of the evaporation plant are decreased due to decreased capacity requirement. Expensive so-called boiler water is saved. Reduced detrimental odors (compared to the use of flash steam). Reduced explosion risks (compared to the use of flash steam).