Method and apparatus for preparing green liquor

Abstract: The invention relates to a method and an arrangement for adjusting the operation of a dissolver of a recovery boiler. In the invention, green liquor generated in the dissolver (2) is applied to a separate vent gas scrubber cooler (4), to which vent gas generated in the dissolver (2) is simultaneously applied for washing. The vent gas scrubber cooler comprises a separate condenser (5), onto whose surface the green liquor is sprayed. Water is applied through the condensers (5) and it cools the green liquor and, when heated, is applied to further use. At least part of the cooled green liquor is returned to the green liquor tank (2).

Description: The invention relates to a method of adjusting the operation of a dissolver of a recovery boiler, in which method melt from the recovery boiler and a solvent, such as weak white liquor, are applied to the dissolver for producing green liquor from the melt and the solvent, vent gas separated from the dissolver is washed to separate sulphurous odour gases and solid dust-like particles from the vent gas, and the green liquor generated in the dissolver is cooled. The invention further relates to an arrangement for treating green liquor, the arrangement comprising a dissolver, wherein melt from a recovery boiler is applied for dissolution, means for applying a solvent to the dissolver, washing means for washing vent gas separated in the dissolver, and cooling means for cooling the green liquor generated in the dissolver. When black liquor is combusted, a char bed is generated on the bottom of the recovery boiler, melt being generated from the loaf at a high temperature, the melt being removed from the boiler as a continuous flow via melt chutes into a dissolver. The temperature of the melt flowing into the dissolver is about 750 to 850°C, and it is dispersed using vapour and/or green liquor sprays before it drops into the dissolver. The dispersant may naturally also be water and weak white liquor obtained from other process steps. To enhance the dissolution and to prevent the melt dropped into the dissolver from sedimenting, the dissolver usually comprises one or more high-capacity mixers and, in some embodiments, also a circulating system for circulating the solution. The concentration of the green liquor in the dissolver is typically between 100 and 150 g/l calculated as the content of Na 2 O. The concentration is adjusted by applying water and/or weak liquor to the dissolver so that the ratio of the melt introduced into the dissolver and the water or weak liquor applied keeps the concentrations in said order of magnitude. From the dissolver, green liquor is removed e.g. by using a liquid-level detector, which controls green liquor pumps that pump green liquor to the production of white liquor. Different washing solutions may naturally also be applied to the dissolver from the recovery boiler flue gas scrubber and/or a vent gas scrubber and cooler. Melt dispersion and the heat and dissolution heat in the melt cause strong evaporation of the water, which is usually called venting. The amount and composition of the vent gas produced vary quite much depending on e.g. the recovery boiler, its running situation and the solution used in melt dispersion. For one ton of dry matter of combusted black liquor, about 400 to 1600 m 3 of vent gas may be produced. The temperature of the vent gas may also vary, being on an average about 80 to 90°C. Similarly, the water content in the vent gas may vary, being typically about 0.33 to 1.94 kg H 2 O/kg dry air. Furthermore, because the melt contains different sulphur compounds, the vent gas also contains odorous sulphur compounds that mainly originate from the reducing zone of the recovery boiler furnace. Usual values of sulphur compounds are below 100 mg/m 3 , but extremely high contents have also been measured. Further, the vent gas may contain varying numbers of solid dust-like particles, part of which form as the compounds in the melt cool and solidify to a fixed form. On the other hand, part of the dust-like particles form as a result of the chemical reactions occurring in the dissolver. In known technology, the vent gas is washed in a separate scrubber, from where the washing solution is typically led back to the dissolver, the vent gas being led to further processing. The temperature of the green liquor run from the dissolver often exceeds 90°C. When white liquor is made by mixing green liquor with calcium oxide, much heat is generated in the reaction, which raises the temperature of the green liquor above its boiling point. This, in turn, causes extra evaporation and venting, which, in turn, causes chemical and energy losses and harmful extra emissions. The temperature of the green liquor discharging from the dissolver can be lowered by lowering the temperature of the green liquor in the dissolver. A known way is to apply more weak white liquor to the dissolver, but this results in a decrease in the concentration of the green liquor. It is also known to cool weak white liquor but this requires much water and, in practice, the result is often tepid cooling water that cannot be used in the pulp process, and thus the heat and water are wasted. The sediments caused by poorly soluble alkalis contained by the liquor on the surface of the heat exchanger also render the solution quite useless. It is also possible to cool green liquor with a separate cooler either in the dissolver or before lime slaking. However, since green liquor contains many different salts, which easily sediment on cool heat transfer surfaces and cause clogging therein, this solution is not either very usable. Finnish patent 102398 discloses a solution wherein green liquor is expanded in a vacuum tank, and the cooled green liquor is returned to the dissolver for adjusting the temperature therein. US-A-6 113 739 refers to a process for recovering chemicals and energy from black liquor, wherein the vent gas from the green liquor formation is washed in a separate scrubbing unit. Present technology has significantly many drawbacks, wherefore their use is difficult or expensive. A large number of separate devices render the solution complex and expensive, causing much extra maintenance and cleaning work. The resulting large amounts of water at low temperature are not useable, but are wasted. Furthermore, sediments and the resulting clogging in process devices cause extra maintenance work, disturbing the normal operation of the process. The object of the present invention is to provide a method and an arrangement for avoiding the problems of known solutions and providing an apparatus that is relatively simply and easily implemented and managed and also operates reliably. The method of the invention is defined in the present claim 1. The apparatus of the invention is is defined in claim 5. The essential idea of the invention is to wash the vent gases in the dissolver and to cool the green liquor in the dissolver in one vent gas scrubber cooler, into which the vent gases and green liquor are applied from the dissolver. Furthermore, it is essential that the vent gas scrubber cooler comprise a condenser for condensating the vent gas applied to the vent gas scrubber cooler, and that the green liquor flowing along the condensation surface of the condenser be cooled at the same time as the cooling water flowing through the condenser is heated to a temperature usable in view of the pulp process. According to a second preferred embodiment of the invention, a surface condenser of falling film type is used, onto the upper part of whose condensing surface the green liquor is sprayed. A still further essential idea of the invention is that the solution cooled is returned from the vent gas scrubber cooler to the dissolver to lower its temperature, and the vent gases are led to further processing into the recovery boiler or the dissolver, for example. According to a preferred embodiment of the invention, weak white liquor and a possibly required additional solution may be applied to the vent gas scrubber cooler to achieve the desired washing result. An advantage of the invention is that only one device is required for washing the vent gases and cooling the green liquor, wherein both the washing and the cooling take place simultaneously. At the same time, the cooling water flowing through the condenser in the device is discharged sufficiently warm to be able to be utilized in the process, and thus recover the heat obtained in cooling the green liquor, which improves the energy economy of the entire process. In the solution of the invention, particles and sulphurous compounds are washed from the vent gas, resulting is substantially clean vent gas. Furthermore, the downward flowing liquid simultaneously washes the heat transfer surface of the condenser preventing it from being clogged. In addition, the operating costs and maintenance of the vent gas scrubber cooler of the invention are minimal. The invention is described in detail in the attached drawing, which schematically shows an arrangement according to the invention. The figure shows a recovery boiler 1, to which a dissolver 2 is connected in a known manner. From the recovery boiler 1, melt is applied in a manner known per se, as schematically shown by line 3, into the dissolver 2, and, when being applied, the melt is dispersed by means of vapour and/or a solvent applied to the dissolver. The arrangement further comprises a vent gas scrubber cooler 4, wherein a condenser 5 is arranged. Through the condenser 5, cooling water is applied on the counterflow principle to its lower end in accordance with line 6, and discharged from the upper end of the condenser in accordance with line 7. Vent gas generated in the dispersion of melt, for example, is applied from the dissolver via a channel denoted by double line 8 to the upper end of the vent gas scrubber cooler 4. Similarly, warm green liquor is applied from the dissolver to the upper end of the vent gas scrubber cooler 4 as shown by line 9. The green liquor is sprayed with suitable nozzles onto the surface of the condenser 5 at its upper end, whereby the green liquor simultaneously washes the vent gases flowing downward, and the flow caused by the condensing vent gas is directed from a gaseous mixture onto the surface of a heat exchanger, whereby both particle and sulphur emissions are washed into the washing solution. From the lower end of the vent gas scrubber cooler the washing solution is preferably at least partly returned to the dissolver as shown by line 10, whereby the cooled washing solution cools the temperature of the green liquor in the dissolver 2. The washing solution may be returned to the dissolver either . entirely, or, if the cooling of the green liquor does not require such an amount, partly directly to further processing, such as e.g. to causticizing as schematically shown by arrow 11. In the vent gas scrubber cooler, when flowing through the condenser, the cooling water is sufficiently heated, allowing it to be used later in suitable places throughout the entire pulp production process. This allows the heat obtained from cooling the green liquor to be utilized, thus reducing the energy consumption of the entire process. The vent gases discharged from the lower end of the vent gas scrubber cooler 4 can, in turn, be led along a channel 12 e.g. either to the recovery boiler 1 or the dissolver 2, as need be. The heat delivery surface of the condenser can be provided with nodules or other elements to detach the flowing film from the heat delivery surface at given intervals. This improves heat transfer and washing performance. To dissolve melt, weak white liquor is applied to the dissolver 2 as shown by line 13. However, part of the weak white liquor can also be applied to the upper end of the vent gas scrubber cooler as shown by line 14 or to some point on the condensing surface of the condenser 5 as shown by line 15. Furthermore, in addition, oxidized white liquor and/or water and/or NaOH solution can also be applied to either the green liquor nozzles or via separate nozzles, as shown by line 16, should this be necessary because of the concentration of the green liquor solution, for example. Line 17 in the figure shows a flue gas channel extending from the recovery boiler 1, along which channel the flue gases are led to an electric filter 18. From the electric filter 18, the flue gases are led in accordance with line 19 to an optional flue gas scrubber 20, and, from there, to the atmosphere as shown by arrow 21. If desired, part of the flue gases can be led to the dissolver 2 e.g. by mixing them with the washed vent gases to be returned to the dissolver as shown by line 22. Similarly, part or all of the vent gases discharging from the vent gas scrubber cooler 4 along channel 12 can be applied to the flue gas scrubber 20 or to the atmosphere as shown by arrow 23. In some cases it is advantageous to provide the condensing surface of the condenser with nodules or other elements for detaching the film flowing along the condensing surface of the condenser at given intervals. This increases the heat transfer contact between the gas and the liquid, thus improving the washing performance. In the above description and the drawings the invention was described by way of example and is in no way restricted thereto. It is essential that the vent gases exiting the dissolver are washed at least with the green liquor taken from the dissolver in a vent gas scrubber cooler provided with a condenser, so that the vent gases are washed and condensed simultaneously as the green liquor is cooled so that impurities in the vent gases can be removed and the temperature of the green liquor in the dissolver can be adjusted suitable.

Claims: A method of adjusting the operation of a dissolver of a recovery boiler, in which method melt from the recovery boiler and a solvent, such as weak white liquor, are applied to the dissolver for producing green liquor from the melt and the solvent, vent gas separated from the dissolver is washed to separate sulphurous odour gases and solid dust-like particles from the vent gas, and the green liquor generated in the dissolver is cooled, characterized by applying the green liquor from the dissolver and the vent gas generated in the dissolver simultaneously to a vent gas scrubber cooler, wherein the vent gas is washed with the green liquor from the dissolver to remove sulphurous odour gases and dust-like solid matter from the vent gas, simultaneously cooling the applied green liquor and vent gas in the vent gas scrubber cooler with a separate condenser that is arranged inside the vent gas scrubber cooler and the outer surface the separate condenser being in contact with the green liquor and the vent gas and the cooling water being arranged to flow inside of the condenser, and by applying at least part of the cooled green liquor from the lower end of the vent gas scrubber cooler back to the dissolver to lower the temperature of the green liquor therein whereby the green liquor from the dissolver is sprayed onto the outer surface of the condenser to its upper end so that the vent gas applied to the upper end of the vent gas scrubber cooler has to flow downward through green liquor sprays A method as claimed in claim 1, characterized by using a counterflow condenser as the condenser, to whose lower end the cooling water is applied and from whose upper end the heated cooling water is discharged. A method as claimed in claim 1 or 2, characterized by applying at least part of the washed vent gas from the lower end of the vent gas scrubber cooler back to the dissolver. A method as claimed in any one of the preceding claims, characterized by also applying, if need be, to the vent gas scrubber cooler, weak white liquor intended for dissolution onto the surface of the condenser to its upper part and/or to some other point on the surface of the condenser in the vertical direction. An apparatus for treating green liquor, the apparatus comprising a dissolver, wherein melt from a recovery boiler is applied for dissolution, means for applying a solvent to the dissolver, washing means for washing vent gas separated in the dissolver, and cooling means for cooling the green liquor generated in the dissolver, characterized in that the means for washing the vent gas generated in the dissolver and for cooling the green liquor generated in the dissolver comprise a separate vent gas scrubber cooler to whose upper part the green liquor and the vent gas are applied and from whose lower part at least part of the green liquor is returned to the dissolver, and the vent gas is discharged, that the vent gas scrubber cooler comprises a separate condenser onto whose outer surface the green liquor is applied and inside of which the cooling water is arranged to flow, which condenser condensates the vent gas flowing thereby and the green liquor flowing on its surface at the same time as the green liquor washes sulphurous odour gases and dust-like solid matter particles from the vent gas. An apparatus as claimed in claim 5, characterized in that the condenser is a counterflow condenser, to whose lower end cooling water is applied and from whose upper end the cooling water is discharged. An apparatus as claimed in claim 5 or 6, characterized in that nozzles for applying the green liquor are arranged to apply the green liquor to the upper part of the condensing surface of the condenser of the vent gas scrubber cooler such that it flows down along the condensing surface of the condenser. An apparatus as claimed in claim 5, characterized in that the condensing surface of the condenser is provided with nodules or other elements to detach the flowing film at given intervals from the surface to increase the heat transfer contact between the gas and the liquid and to improve the washing performance.

Inventors: AHTILA PEKKA, RUOHOLA TUOMO, TAMMINEN ARI, AHTILA, PEKKA, RUOHOLA, TUOMO, TAMMINEN, ARI
Applicants: KVAERNER POWER OY
Application year: 2001
Main classification: D21C11
IPC codes: D21C11