BACKGROUND OF INVENTION

The present invention relates to a method for generating steam in a digester plant of a chemical pulp mill.

Conventionally, in older fiberline systems has a chip bin and a subsequent steaming vessel been used for steaming/heating of wood chips or other cellulose material. Initial steaming in chip bin has used fresh low pressure steam, reaching a temperature in the range 80-100 °C. The subsequent steaming in steaming vessel normally used flashed steam from black liquor flash tanks, reaching a temperature of 100-120 °C. The steamed chips are thereafter slurried in cooking or impregnation liquor. First in digester top has the chips or slurry been heated to full digester temperature, lying in the range of 130-180 °C, using expensive medium pressure steam from the steam net if the chips are to be heated to full cooking temperature in a steam phase digester. In case of a hydraulic digester, the heating is made using a heating circulation. In those systems has a volume of spent hot black liquor (typically at full cooking temperature of 130-180 °C.) been withdrawn from the cooking system. The heat value in the hot black liquor volume is used initially to generate steam for steaming vessel and heat white liquor, or alternative liquid volumes charged to the cooking systems. The spent hot black liquor is typically sent first to two or more flash tanks connected in series where steam fractions are produced at successively lower temperature and pressure as the liquor is flashed in stages to temperatures of approximately 95-1 10 °C. Finally the black liquor is sent to the evaporator system in the recovery island.

However, in recent state of the art digester systems has the digester system been improved and simplified considerably and in digester systems with Compact Cooking G2™, sold by Valmet, is a single first impregnation vessel, ImpBin™, replacing all previous chip bins, steaming vessel and impregnation vessel. In such ImpBin vessel are the chips steamed and impregnated with hot black liquor that is supplied from digester, and when added to ImpBin is allowed to flash off steam for steaming and using the residual black liquor as the impregnation liquor. Different concepts have been proposed where the heating to full digester temperature is replacing the addition of clean steam into digester to usage of flash steam from black liquor. Clean condensate will thus not be lost in digester, and cooking liquors will not be diluted with clean water that increase evaporation work in the recovery operations.

One of the first patents disclosing a reboiler producing dirty steam for heating in digester top is shown in Valmet AB ^' s SE 453,673. Here is dirty hot steam produced in a steam converter (reboiler) using the heat value in hot black liquor as heating medium in the steam converter.

Another system is disclosed in US 4,944,840, where the heat value in hot black liquor is recovered in 3 serially connected evaporation stages. Steam from the evaporation stages are directly introduced to the impregnation and cooking zones in a digester for heating the fibrous material.

Yet a system has been proposed in US 6,176,971 , but for the generation of pure steam to be used in the chip bin or chip tube. Here is the clean steam obtained from the reboiler pressurized before added to the chip bin. If an educator is used driven by fresh steam, could a lower pressure be established in the reboiler and more heat value be extracted from the hot black liquor.

In US 2007/0131363 is disclosed a complex system where the hot black liquor is sent to an evaporation stage II and vapor from evaporation stage III is returned to chip bin for heating. Some of the heat value in the black liquor is used in pre-heaters heating white liquors exclusively.

In US8512514 is disclosed a complex dual heat recovery system, using a first evaporator to generate foul steam for heating in digester top using a first extraction flow from digester, and a second heat exchanger for producing clean steam from a second extraction flow from digester.

US8691049 concerns a method using the CrossCirc™ concept promoted by Valmet AB. In the CrossCirc concept is hot black liquor added directly into the transfer system in order to reduce steam consumption in digester top for reaching full cooking temperature. In this patent is a reboiler also used to recover residual heat value in the return liquor withdrawn in the top separator, and this reboiler is producing clean steam for chip bin steaming. In US8808498 is yet another system disclosed where flash steam from black liquor heats white liquor, and the flashed black liquor is used in a second heat exchanger in order to heat white liquor.

The known solutions provide different systems for producing steam for heating needs in the digester plant and for improving the energy economy of the pulp mill. However, most system fails to take a common approach for heating all feed flows to digester, and includes complex systems with multiple heat exchange apparatuses.

SUMMARY OF INVENTION

The inventive method is developed for generating steam in a digester plant of a chemical pulp mill comprising:

extracting a volume of hot black liquor (HBL) at full cooking temperature from the digester (D);

adding a charge of alkaline cooking liquor (WL) to the hot black liquor forming a mixed liquor (WL+HBL);

generating vapor by evaporating the mixed liquor by heating the mixed liquor indirectly with fresh steam (ST P), wherein an evaporated mixed liquor flow (X) is separated from the vapor;

heating cellulosic feed material in the digester with the generated vapor (STBL) separated from the evaporated mixed liquor; and

adding the heated evaporated mixed liquor to the digester plant (XALTI or XALT2 or XALT3) for heating the cellulosic feed material in or before the digester (D).

In a preferred embodiment is the evaporated mixed liquor used as impregnation liquid in early chip treatment systems, i.e. black liquor impregnation, which is advantageous from an emission point as the evaporated mixed liquor has a lower content of malodorous gases than the content of malodorous gases in the volume of hot black liquor when extracted from digester. The inventive method may also further involve that the evaporation of the mixed liquor takes place in a reboiler (Reb) and the charge of alkaline cooking liquor (WL) is introduced into the hot black liquor in or before the reboiler where adequate mixing takes place during the boiling conditions prevailing in the reboiler.

The inventive method may also further involve that said evaporated mixed liquor obtaining a lower dry matter content than the hot black liquor and a lower concentration of alkali than the fresh alkaline cooking liquor. The inventive method may also further involve that a clean steam condensate is obtained from the indirect heating in the reboiler and this clean condensate is used as heating medium in a preheater, preheating the alkaline cooking liquor or the mixed liquor before being fed to the reboiler. The inventive method may also further involve that the heated evaporated mixed liquor flow (X) is heated to a temperature above the prevailing full cooking temperature, and charged at least in part to the top of the digester (DTOP) where the heated evaporated mixed liquor generates steam as well as hot cooking liquor. The inventive method may also further involve that the heated evaporated mixed liquor is heated to a temperature above the prevailing full cooking temperature, and charged at least in part to a preceding impregnation vessel (IB), preferably at end of the impregnation vessel and preferably directly ahead of transfer of the cellulosic feed material to the digester (D).

The inventive method may also further involve that the heated evaporated mixed liquor flow (X) is heated to a temperature above the prevailing full cooking temperature, and charged at least in part to a preceding impregnation vessel (IB), preferably at start of the impregnation vessel and more preferably directly into a chip volume established over a liquid level (Liq in said impregnation vessel, such that steam released from the heated evaporated mixed liquor will steam the cellulosic feed material before impregnation. The advantages with these alternatives will be explained in more detail in connection with the detailed description following below.

SUMMARY OF THE DRAWINGS

Fig. 1 shows schematically a state of the art digester system;

Fig. 2 shows a modification of the digester system in Fig 1 where the inventive steam generation system is installed;

Fig. 3 shows a detail view of the steam generating system, with identified process parameters disclosed.

DETAILED DESCRIPTION

Fig. 1 illustrates a state of the art digester system of the Compact Cooking G2™ concept, sold by Valmet. The Compact Cooking G2™ concept includes a single first atmospheric impregnation vessel IB, ImpBin™, replacing all previous chip bins, steaming vessel and impregnation vessel. In such ImpBin vessel is the chips CHIN steamed and impregnated with hot black liquor TSRET that is supplied from digester D, and when added to IB/ ImpBin is allowed to flash off steam for steaming and using the residual black liquor as the impregnation liquor forming a liquor level Liqi. below the steam flash off position. The system includes the CrossCirc™ concept promoted by Valmet AB, where hot black liquor HBL withdrawn from digester is added directly into the transfer system in order to reduce steam consumption in digester top for elevating the temperature. Final heating is obtained by an additional heating with medium pressure steam ST P reaching full cooking temperature in digester top. A first heating of the chips towards full cooking temperature then takes place during the transfer to the top separator TS, and the excess transport liquor TSRET is withdrawn in the top separator and used as the flashing impregnation liquor. In most installations in warmer climates is the steam flashed off from the hot black liquor in the atmospheric ImpBin vessel enough for full steaming of chips and purging of air bound in the chips. Need for additional low pressure steam for steaming chips may be totally avoided, but for extreme conditions during winter time when chips may be deep frozen. As indicated, the extraction position for the hot black liquor at full cooking temperature may be located anywhere in the digester, i.e. within the indicated zone A. As shown here is a dedicated heater used for the white liquor WL charge, and the addition to digester top is conventionally made using a white liquor header distributor (not shown) that sprays the white liquor into the top. However, handling concentrated white liquor at full cooking temperature involves increased corrosion exposure, and the heaters may be subjected to alkali corrosion even if the heater is made in stainless steel needing even better and more expensive material in the heater. Also the spraying of concentrated hot white liquor into radially directed nozzles towards inside if digester shell may expose the digester shell to alkali corrosion, which has been seen in digesters made in carbon steel material. Addition of hot alkaline liquor into digester top may not be ideal for a perfect mixing into the chips, as the only mixing effect is from the free fall of chips passing the alkaline sprays.

Fig. 2 illustrates an improvement of the Compact Cooking G2™ concept as shown in Fig. 1 , and includes a steam generation system operating according to the inventive method. In the inventive concept is at least a part of the hot black liquor HBL, holding full cooking temperature which in this example is 141 °C, instead sent to a reboiler Reb for further heating. As shown may also the hot black liquor HBL be heated in preheater HE before being charged to the reboiler, and this alternative will be more described below with reference to Fig. 3. Also, a charge of fresh alkaline cooking liquor, typically white liquor but any kind of alkaline cooking liquor may be used, is charged into the hot black liquor and preferably before the preheater HE. Typically the alkaline cooking liquor holds a temperature below 1 00 °C and stored in atmospheric tanks, and hence the mixed liquor WL+HBL will reach a lower temperature at about 1 30 °C. If a preheater HE is used this temperature may be increased to about 1 34 °C before charged to the reboiler Reb. The reboiler holds of volume of mixed liquor WL+HBL and a coil or piping for heating media is installed in this liquor volume, in this case the heating media is medium pressure steam STMP. Hence, the heating of the mixed liquor will be done indirectly, and clean steam condensate will not be lost in the mixed liquor and dilute it. The coil is supplied with medium pressure steam STMP and a condensate STc is extracted from the heating coil. The heating in the reboiler results in heavy boiling and the added alkaline liquor will be perfectly mixed with the hot black liquor, and a black liquor steam STBL is boiled off from the mixed liquor. The black liquor steam STBL boiled off is led to the top of the digester, and this volume of steam may totally replace the conventional charge of clean medium pressure steam to digester top. Several positive effects of this inventive heat treatment of the mixed liquor are obtained.

Firstly, the heated hot black liquor is elevated to temperatures above full cooking temperature, and the heat value in this hot black liquor increases, and that without dilution of clean steam condensate.

Secondly, both the addition of alkali and heating have positive effects on the content of dissolved hemicellulose in the black liquor, that need time, temperature and high alkali concentration in order to increase cleavage rate on end groups of the hemicellulose. When the cleavage effect has reached a critical order could the hemicellulose start to precipitate onto cellulose and increase yield from the cook. The additional retention time in the reboiler, the heating and the alkali addition all contribute synergistically in increased cleavage rate.

Thirdly, the common heating of both black liquor and alkaline cooking liquor in a liquor mixture in one single heating apparatus avoids both lignin precipitation risks in a reboiler fed with black liquor with high solids content and low alkali level close to lignin precipitation level as well as avoids alkali corrosion risks in dedicated heaters for concentrated cooking liquor.

Fourthly, the content of malodorous gases in the black liquor will be reduced, such as sulfur and methylmercaptans, as they will boil off to a large extent with the black liquor steam as they have a lower boiling point. Most of these malodorous gases, especially methylmercaptans, has well known delignification effects improving the delignification rate in the digester.

The heated hot black liquor WL+HBL may reach a temperature well over the full cooking temperature, and this flow X may be used in several alternative routes, either for exclusive use or in combinations.

First option given priority to reduce need for medium pressure steam addition to digester top involves routing the flow X to digester top at the indicated position XALTL The overheated mixed liquor will then flash off additional steam as well as supply of both alkali and black liquor to cook at full cooking temperature.

Second option given priority to reduce need for medium pressure steam addition to digester top by heating the cellulosic material already in transfer circulation involves routing the flow X at end of impregnation vessel IB and preferably directly ahead of the transfer system at the indicated position XALT2. The overheated mixed liquor will heat the slurry with cellulosic material when mixed into this slurry. Third option given priority to increase steaming effect in top of impregnation vessel involves routing the flow X at top of impregnation vessel IB, and preferably into the return flow TSRET from the top separator at the indicated position XALT3. The overheated mixed liquor will heat the return liquor when mixed into it and more steam will be flashed off when finally dumped into the volume of cold chips lying above the liquor level Liqi_. This option may be preferred in mills operating in cold climate and fed with deep frozen chips.

These three options may be combined with the ordinary withdrawal of hot black liquor HBL as shown in Fig.1 , hatch-marked flow in Fig.2, and alternative or complementing withdrawals of hot black liquor HBL lower down in the digester, also shown as hatch- marked flow in Fig.2.

Options 2 and 3 for the X flow are specifically adapted for a Compact Cooking™ system with an ImpBin, but option 1 may be used in any digester system operating with conventional pressurized impregnation vessels as well as using other chip pretreatment systems with dedicated chip bins and steaming vessels. Option 2 may also be used in conventional 2-vessel digester systems where heating is to be obtained in first vessel. In Fig. 3 is disclosed an embodiment of the invention in a Compact Cooking™ system with an ImpBin implemented according to the first option.

The basic parameters for the digester system are as follows:

• Digester capacity: 4000 adt/d Steam to digester: 350 kg/adt, and 16,2 kg/s Steam pressure to digester top: 4.5 bar(g)

MP steam pressure after control valve: 10.5 bar(g)

MP steam temperature: 186.9 °C

BL+WL flow: 4.0 m ³ /adt

BL+WL temperature: 130 °C

Full cooking temperature: 141 °C

The basic parameters for the Reboiler (Reb) are as follows:

Effective heat exchange area: 754 m ²

k-value: 1835 W/(m ^2■ °C)

Condensate temp, decrease: 5.0 °C

The basic parameters for the Preheater (HE) are as follows:

Effective heat exchange area: 63 m ²

k-value: 1835 W/(m ^2■ °C)

ΔΤ: 28.2 °C

As seen in this embodiment are both a reboiler Reb and a preheater HE installed, as the condensate STc from the reboiler still has a considerable heat value that could be used in a preheater. But the invention do not require the preheater for obtaining the bulk volume of heated black liquor steam STBL. Practically the installation of a preheater may be motivated if there is a shortage in medium pressure steam, and/or the investment costs for the preheater motivates installation compared to savings in operating costs (i.e. cost for medium pressure steam). It is clearly seen in this example that the black liquor steam STBL is heated to 155.1 °C and at a heat value of 2753 kJ/kg. Also the overheated mixed liquor WL+HBL from the reboiler holds same temperature of 155.1 °C. Once the overheated mixed liquor is added to the digester top is an additional amount of steam released at a rate of 4.4 kg/s, which is almost 40 % of the black liquor steam STBL rate from the reboiler, while the cooking liquor WL+HBL is charged at full cooking temperature.

While the invention has been described in connection with installation in a Compact Cooking™ system, it is to be understood that the invention is not to be limited to the disclosed embodiment, but the inventive method is intended to cover various implementations in different digester systems such as conventional cooking, operated with ITC, EAPC, Lo-Solids or similar cooking liquor routing. The basic principles are that both the alkaline cooking liquor and black liquor as a mixed liquor is heated in one common indirect heat exchanger, that could avoid condensation of clean steam condensate in the mixed liquor, but still obtains a dilution effect of the black liquor by alkali addition. The increased flow rate of liquor passing the indirect heat exchanger increase heating efficiency. Black liquor typically has a dry matter content exceeding that of fresh cooking liquor by at least 5-20 %, so the dilution by fresh cooking liquor improves flow characteristics trough the indirect heat exchanger and reduce scaling tendencies, that often are seen in evaporation stages as a tar like deposition in the evaporation stages, where similar increase of dry matter content of undiluted black liquor is obtained. In a cost reduction alternative may the entire heating of the mixed liquor, i.e. black liquor and white liquor, by obtained in the indirect heater before charging to digester, which mean that the white liquor charge to digester top needs no dedicated white liquor heater, which save investment costs for such heater, and enable downscaling of any additional white liquor heater for other charges to the process. Indirect heaters for concentrated white liquor are due to working conditions, heat and alkali concentration, most expensive as they need to be made in high alloy corrosive resistant steel.

The total charge of alkaline cooking liquor needed for the entire cook may be charged to the reboiler, or at least 20-50 % of the total necessary charge.