FI118431B - Procedure for the treatment of black liquor - Google Patents

Description

118431

METHOD FOR HANDLING THE BLACK MALK

The present invention relates to a process for treating black liquor burned in a recovery boiler 5, in particular for adjusting the properties of the liquor and improving the operation of the boiler plant.

When waste liquor is incinerated in pulp processes, the object 10 is to separate the organic and inorganic parts of the dry matter of the waste liquor. The heat from the organic portion of the dry matter is recovered and this heat produces as much steam as possible. Process chemicals are collected from the inorganic portion of the dry matter in a form that can be converted at a later stage of the process into a suitable form for reuse in the cooking process.

The recovery boiler has so far proven to be superior to 20 in the recovery of heat and chemicals from waste liquor. ; The waste liquor is sprayed as droplets into the furnace furnace.

The sludge boiler is supplied with a stoichiometric amount of air equivalent to the waste liquor plus an excess air to ensure • · * * complete combustion. In a hot combustion chamber, water, the volatile and gaseous portions of the dry matter are evaporated from the droplets of lye. The remaining carbonaceous material burns mainly in the furnace of the V * recovery boiler. The combustible material brings heat to the heating surfaces inside the boiler, superheater and boiler cooking piping, and the flue gases are removed from the top 30 of the boiler. The ash from the waste liquor droplets, i.e. the inorganic * waste liquor from the waste liquor, collects on the bottom of the boiler in a so-called.

* · *, To be removed, and then taken back through the various stages of the process to the cooking process.

In the past, black liquor has been burned at a dry content of about 63-75%. It is advisable to remove as much water from the black liquor as possible before feeding it to the 2 1 1 8431 recovery boiler, since evaporation of the remaining water in the black liquor cools the furnace, slows down the combustion process and makes it more difficult to recover the heat transferred to the flue gases. At present, the aim is to concentrate black liquor to a significantly higher dry matter content (i.e. 75-90%), which in turn has significant advantages in burning black liquor. Liquid droplets of high dry matter dry faster and with less heat and burn faster. The reduced flue gas flow allows smaller and cheaper heat transfer surfaces, as well as the possibility to position the boiler residual heating surfaces so that more electricity can be produced. The liquor nozzles can be mounted at a steeper angle downwards and sprayed in larger volumes of 15 liquids into the furnace furnace, causing the liquor droplets to travel in an upward gas flow, so-called. carryover, decreases. Raising the dry matter also increases the temperature of the lower furnace, whereby the sulfur dioxide emissions from the recovery boiler are reduced and the reduction in the pulp increases.

«* •« · • · · • «« *, j. However, there are problems with the handling of high solids black liquor. The viscosity of the black liquor increases exponentially as the solids content increases. In order to overcome the resistance caused by viscosity, the black liquor must be evaporated and transferred to a power boiler at high temperature and without pressure under higher pressure.

It is an object of the present invention to provide a better method than known in the art for improving the operation of a pulp mill boiler when burning in a boiler. to a very high solids content, ie> 85% by weight, of concentrated black liquor. In particular, it is an object of the invention to provide a method for improving the final concentration of such black liquor and for improving the feed system and combustion control in a recovery boiler, whereby high solids cartilage liquor can be uniformly and regularly fed to the boiler furnace.

5

To achieve the above purposes, it is essential to the present invention that more than 75% by weight of concentrated black liquor is fed to a pressurized storage tank, 10 of the black liquor is fed to a final concentration step where its dry matter content is increased to by heating under pressure at a temperature above the boiling point of the lye atmosphere, and from the last 15 stages of concentration, the black liquor is fed to the recovery boiler.

It is central to the present invention that a concentrator 20 is connected after the pressurized liquor storage tank, in which the residence time is considerably shorter than that of the tank; you. Black liquor can most advantageously be fed directly from the concentrator to the recovery boiler. In this way, when the properties of the black liquor change, the properties of the liquor fed into the soda • • tjt space, such as viscosity and, more importantly, the flash tendency (pressure ♦ ^ shake), can be quickly adjusted, which appears to be sprayed. · · * · '' The speed of the lye droplets immediately after the syringe and thereby the droplet orbits in the boiler.The adjustment typically occurs by changing the temperature in the last concentration of the concentrate by changing the pressure of the secondary vapor.

Then the last evaporation unit is exactly where • ♦. The properties of the black liquor fed to the boiler are adjusted.

The process according to the present invention can control the supply of concentrated black liquor to a very high solids content in a recovery boiler. The dry matter content is then greater than 85, preferably 88 to 95, most preferably 90% by weight or more. This high solids content improves boiler efficiency. The amount of flue gases formed is lower than before due to the low water content of the black liquor. In addition, since the specific heat of water vapor is higher than that of the flue gases from the combustion of dry liquor, the thermal capacity flow of the flue gases of the recovery boiler significantly decreases with respect to the steam flow of the boiler, thereby improving the steam process for power generation. This is valuable for the mill while reducing the use of fossil fuels in power supply. The aforesaid steam process improvement can be more easily accomplished by connecting a high pressure preheater or preheaters to the feed water line 15 and raising the feed water temperature.

The present invention will be described in more detail with reference to the accompanying figure, which illustrates one embodiment of a preferred embodiment of the invention.

i «· ··· · • I *

In a multi-stage evaporator, concentrated black liquor * * *: * mixed with ash and a possible substitute chemical, • · · · ···· 25 from a strong liquor tank (not shown) along line 2 ·! "to auxiliary hardener 1. The liquor is heated along line 4 by means of intermediate steam (· 11 V bar) (eg 11-12 bar) and the condensate formed is removed from line 5. The upper part of the concentrator produces steam 3 which can be utilized in black -: ***: 30 in the earlier stages of lye evaporation Concentrator *. operates at a higher pressure than pressure Concentrated * · t, 1 produces black liquor with a solids content of more than 75%, preferably about 80-83% Black liquor is pumped: Y: further in line 6 to the expansion tank 7, with a liquor pressure of · · 35 mouths to the temperature of the reservoir 9. The pressure must be maintained throughout the processing of the black liquor at values corresponding to the viscosity of the black liquor so low 118431 5 that the liquor remains pumped. the temperature is typically about 130-160 ° C and the pressure corresponding thereto.

5

After the pressure vessel 9, the black liquor can preferably be heat treated. Such a heat treatment is described in FI patent 75615 / wherein the temperature of the black liquor is raised above the cooking temperature, preferably above 170 ° C.

10 In this way, the viscosity of the black liquor can be irreversibly reduced. Sulfur-containing gases also separate from the lye, and this can reduce the sulfur content of the black liquor entering the recovery boiler. The black liquor typically has a dry solids content of about 80%, whereby its viscosity must be reduced to a pressure higher than the pressure to allow the black liquor to be pumped. The heat treatment can further advantageously affect the pumpability of the lye.

Heat treatment is carried out on line 10 for black liquor:. with reactor 11 being preferably in line, whereby the temperature of the black liquor is typically raised above 185 ° C, e.g. to about 190 ° C, for example by direct steam injection. The heat treatment time is typically about 1-60 ”· * 25 min. , preferably 1-30 min. A shorter time may also be sufficient.

After the heat treatment, the black liquor 13 is finally concentrated to a desired solids level for combustion, ie. over 85%,! 30 preferably 88-95%, most preferably 90% or more. The end curing unit 14 operates at a lower temperature than the heat sink. a processing reactor, whereupon the black liquor 13 expands • *. lighter 14 and sulfur-containing secondary vapor may be removed from the tip of the final concentrator from line 15.

The concentrator 14 is heated by high pressure steam (typically above 15 bar, e.g. 16-20 bar) from line 16 and the condensate formed is removed through line 17.

118431 6 The heat treatment allows the final concentrator to be kept cleaner, as the heat treatment has reduced the viscosity and adhesion of the lye. The heat treatment may also be carried out prior to the concentrator 1, whereupon the reactor 11 may be subjected to further heat treatment.

The black liquor from the final concentrator 14 in a very high solids, e.g. 90% by weight, is pumped directly along line 10 18, 20 to the recovery boiler 21, whereby the properties of the black liquor fed into the furnace can be controlled by changing the temperature in the final concentrator.

It is also possible, although not advantageous, to alternatively pump black liquor in very high solids, greater than 85%, along line 18 into the expansion vessel 19 where the temperature of the black liquor can be adjusted to a suitable level for feeding the recovery boiler 21 by applying lye to pressure. corresponding to the desired temperature: This also increases the dry solids content of black liquor * · «· by a further 1-2%, and also at this stage the liquor-containing gases emit ··, ·. From the expansion tank 19 black liquor t · is pumped along line 20 to the recovery boiler for combustion. The present invention allows the black liquor to be evaporated to a very high dry solids content, and the temperature of the liquor can be adjusted quickly and accurately, so that the characteristics of the feed liquor can be adjusted. no-: 1ί: 3 0 soon to be optimized for feeding into the recovery boiler.

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Claims (8)

Method for treating black liquor burned in a soda boiler, especially for controlling the properties of the liquor and improving the operation of the boiler plant, characterized in that black liquor which is concentrated to a dry matter content of more than 75% by weight is measured in a pressurized storage container, the black liquor measuring vessel from to the final concentration step, where its dry matter content is raised to over 85% by heating it under pressure at a higher temperature than the boiling temperature of the atmosphere at atmospheric pressure, and the black liquor is measured from the last concentration step to the soda boiler, whereby the black liquor after the last concentration step is measured directly or via an expansion tank to the soda boiler. •; * · 20 Process according to claim 1, characterized in that the dry matter content of the liquor in the last concentration step M ·· · * · *: is increased to above 85, preferably 88 to 95% by weight. • · • •• t j * · .. Process according to claim 2, characterized in that the dry matter content of the liquor in the last concentration step is increased to 90% by weight or more. ·· • · ♦ ·· ··· Method according to claim 1, characterized in that the black liquor removed from the storage container 30 is heated for heat treatment of the liquor to a temperature ···. which is higher than in the last concentration step, and • ·· • ·. ·. ; the liquor is expanded to the final concentration step for separating sulfur-containing gases from the liquor. Process according to claim 1, characterized in that the temperature of the liquor in the storage container is 140 - 160 ° C. 6. A process according to claim 4, characterized in that the temperature of the heat treatment is above 185 ° C. 10 Process according to claim 1, characterized in that the viscosity and flashing of the black liquor in the boiler nozzles are controlled by changing the temperature of the last concentration step. 15 Process according to claim 1, characterized in that the black liquor after the last concentration step is allowed to expand to a temperature lower than the temperature of the last concentration step, but the temperature and pressure are nevertheless kept at a higher temperature than the boiling temperature at atmospheric pressure and at an atmospheric pressure. higher than an atmospheric pressure, and the tili boiler fed «<·· black liquor viscosity and flashing in the liquor nozzles is controlled by controlling the expansion after the last step of concentration. «N • · · • ·« «· • · • ♦♦ • · • ·« ·· • ♦ ♦ ·· • · • · ··· ♦ ♦ · · ♦ ·· • ♦ ♦ · • ♦ · • ·· • ·