Method and device for cleaning flue gas

Abstract

Method for cleaning flue gas, in particular with a pulp production plant, wherein thick lye, exclusively or together with another fuel, is burned in a combustion chamber of a boiler (11), wherein flue gas produced during combustion is fed in a flue gas stream (41) to a mass force separator and dust (36) which predominantly contains magnesium oxide is separated in the mass force separator, wherein the flue gas is subsequently subjected to further treatment, characterized in that at least a partial stream of the dust separated in the mass force separator is returned to the combustion chamber.

Description

[0001] The invention relates to a method and a device for cleaning flue gas, in particular with a pulp production plant, wherein thick lye, exclusively or together with another fuel, is burned in a combustion chamber of the boiler, wherein the flue gas produced during combustion is fed in a flue gas stream to a mass air separator and dust, which mainly contains magnesium oxide, is separated in the mass air separator, wherein the flue gas is subsequently subjected to further treatment.

[0002] Such processes and devices are well known from the prior art and are regularly used in pulp production. As a starting material for pulp, wood chips, for example from hardwood, are impregnated with a cooking liquor containing magnesium bisulfite and boiled, whereby lignin is dissolved from the wood, leaving pulp behind. In addition, a number of other processes for pulp production are known, the processes that use magnesium bisulfite in the cooking liquor are also referred to as sulfite processes or magnifite processes. After the pulp is removed from the cooking liquor, the liquor is evaporated to a solids content of approximately 55 to 60% by mass, resulting in the so-called thick liquor. The thick liquor is then combusted in a liquor boiler or kettle, whereby the magnesium salts contained in the thick liquor are calcined to magnesium oxide, which then becomes the main component of fly ash.of dust contained in flue gas. The flue gas produced during combustion in the boiler is fed into a mass force separator, in which a large portion of the dust contained in the flue gas is separated.

[0003] During the combustion of the thick lye solution in the boiler, it is atomized and burned within a combustion chamber using, for example, burner lances and nozzles. This combustion in the combustion chamber can be carried out without the addition of any further fuel. However, it may also be possible to assist the combustion by adding an additional fuel, such as heavy fuel oil. This can be done via a dedicated burner. A boiler, or lye boiler, can therefore have multiple lye burners and heavy fuel oil burners.

[0004] The boiler's flue gas contains, in addition to fly ash (primarily magnesium oxide) and exhaust gases such as carbon dioxide, nitrogen, and oxygen, gaseous sulfur dioxide, which passes through the mass airflow separator. Following the mass airflow separator, the flue gas is then fed to a further treatment process. During this treatment, the sulfur dioxide is removed from the flue gas. After separation, the magnesium oxide and sulfur dioxide are reused in the production of cooking liquor and magnesium hydrogen sulfide, respectively.

[0005] The formation of magnesium oxide and sulfur dioxide in the combustion chamber, as well as their removal by flue gas cleaning, depends on numerous system parameters, such as the droplet distribution of the thick lye solution in the combustion chamber, the combustion chamber temperature, and the pH value during further flue gas treatment. When flue gas is discharged from the plant into the environment or atmosphere, legally prescribed emission limits, including those for dust, must be observed. Dust emissions also depend, among other things, on the operating parameters and the suitability of the flue gas cleaning system.

[0006] The present invention therefore aims to propose a method and a device for cleaning flue gas, which enables improved cleaning of flue gas in pulp production in a cost-effective manner.

[0007] This problem is solved by a method having the features of claim 1, a device having the features of claim 17 and a system having the features of claim 18.

[0008] In the inventive process for cleaning flue gas, in particular with a pulp production plant, thick lye is burned in a boiler, either exclusively or together with another fuel, in a combustion chamber of the boiler, wherein the flue gas produced during combustion is fed in a flue gas stream to a mass force separator and dust, which predominantly contains magnesium oxide, is separated in the mass force separator, wherein the flue gas is subsequently subjected to further treatment, wherein at least a partial stream of the dust separated in the mass force separator is returned to the combustion chamber.

[0009] Consequently, in a boiler or lye boiler of a pulp production plant, thick lye or evaporated cooking liquor is combusted in a combustion chamber. During this combustion, the thick lye can be burned alone or together with another fuel. The flue gas produced during combustion is fed to the air separator as a flue gas stream, for example, via a suitable flue gas duct. The air separator is designed as a dust filter, so that dust is separated within it. The dust contained in the flue gas consists primarily of magnesium oxide, which is formed during the combustion of the thick lye in the boiler's combustion chamber. The flue gas is then discharged from the air separator, for example, via another flue gas duct connected to the air separator, and sent for further treatment.The flue gas can pass through several different treatment stations or components of the flue gas cleaning system to ensure that, before being released into the atmosphere, it does not exceed a specified emission limit, including for dust. The magnesium oxide separated in the mass air separator can be used in a process for producing crude acid for pulp production.

[0010] According to the invention, the dust separated in the mass air separator is at least partially fed back into the combustion chamber of the boiler. The dust is thus returned from the mass air separator to the combustion chamber. This can be done, for example, via a duct through which the dust is blown into the combustion chamber. Surprisingly, it has been found that the return of the dust, or rather the predominantly magnesium oxide it contains, can reduce dust emissions. The emission level can therefore be lowered simply and cost-effectively, without the need for additional equipment such as filters or similar devices for treating the flue gas. Overall, this allows for improved flue gas cleaning at a comparatively low cost.

[0011] It may be planned that only a portion of the dust separated in the mass air separator is fed into the combustion chamber. Another portion of the dust can then be used in a process for the production of crude acid. Since the combustion of the thick liquor continuously produces flue gas and thus dust, it is necessary anyway to remove a portion of the dust separated in the mass air separator and prevent it from being returned to the boiler's combustion chamber.

[0012] The dust can be advantageously blown into the boiler's combustion chamber by means of an airflow. This can be achieved via a closed duct leading from the mass air separator to the boiler. For example, the flue gas flow or a dedicated blower can be used to generate the airflow. The dust can be metered into the airflow so that it is distributed or swirled as homogeneously as possible within the combustion chamber. This ensures particularly good mixing of the dust blown into the combustion chamber and the flue gas generated there.

[0013] The dust can be composed of particles and be in powder form. During the combustion of the thick lye in the boiler's combustion chamber, flue gas is produced, which contains, among other things, fly ash. The dust is therefore also formed from fly ash, i.e., solid particles contained in the flue gas and carried along with the flue gas stream. The dust separated in the inertial separator can consequently be in powder form.

[0014] During combustion in the combustion chamber, an aerosol can be produced. At relatively high temperatures in the combustion chamber, alkalis can evaporate and later condense into aerosols. If predominantly or exclusively hardwood is used for pulp production, the alkali content of the lye can be particularly high. The alkalis produced during combustion then exist as aerosols and leave the boiler's combustion chamber in the flue gas stream. Surprisingly, this can be counteracted by recirculating the dust from the inertial separator back into the combustion chamber.

[0015] The dust introduced into the combustion chamber can be colder than the aerosol and / or dust already present there. After the flue gas has been dedusted in the inertial separator, the dust temperature can drop further. This dust is then subsequently blown into the combustion chamber and may have a comparatively lower temperature than the dust already present. The cold dust introduced into the combustion chamber can promote the condensation of alkalis and / or aerosols on the dust particles. Condensation can be ensured if the dust is blown into the combustion chamber at a suitable vapor pressure. In particular, agglomeration and coagulation of the particles in the combustion chamber can occur. This coagulation can lead to the binding of the particles through a chemical or physical process.As it turns out, particle growth is also determined by the frequency of collisions between existing particles, i.e., by the particle concentration in the combustion chamber.

[0016] Accordingly, the aerosol in the combustion chamber can condense on the blown-in dust, thereby increasing the mass of the dust particles in the combustion chamber. An increase in the mass and / or density of the particles or aerosols can be further promoted by the longer retention of the particles and aerosols in the combustion chamber. Comparatively larger particle masses can then be separated from the flue gas more effectively in the mass airflow separator due to the resulting increase in particle or dust concentration, compared to smaller particle masses. Since the thick lye contains a proportion of alkalis, such as potassium, depending on the type of hardwood used, alkalis can be released in the boiler at high temperatures, which can then condense into an aerosol. As it turns out, potassium oxide or other potassium compounds such as potassium sulfate can significantly influence dust emissions. The dust blown into the combustion chamber...Porous magnesium oxide particles can act as condensation nuclei for these alkalis or potassium compounds, allowing some of the potassium compounds responsible for dust emissions to bind to the magnesium oxide particles within the combustion chamber. The adsorption of potassium sulfate or potassium oxide to the porous magnesium oxide particles can increase the particle mass and, to a lesser extent, their size.

[0017] Consequently, the dust concentration in the inertial separator can be increased by the dust fed into the combustion chamber. Inertial separators are particularly effective at separating relatively large or heavy particles. It can be designed so that lighter particles separated in the inertial separator are returned to the combustion chamber, as lighter particles can accumulate a greater mass of potassium compounds compared to heavier particles. The inertial separator can be configured to separate the dust according to particle mass or size.

[0018] The dust separated in the mass air separator can contain at least 85% magnesium oxide by mass. The proportion of magnesium oxide can result from factors such as the type of wood used for pulp production, and thus the chemical composition of the thick liquor, as well as the combustion temperature in the combustion chamber.

[0019] The dust separated in the mass air separator may contain further proportions of alkalis, preferably potassium sulfate and / or potassium oxide. The amount of these proportions can result from the potassium content of the wood used for pulp production. For example, the dust may contain approximately 2.5% potassium sulfate by mass.

[0020] Accordingly, particles of the dust can consist of magnesium oxide with adsorbed alkalis, preferably potassium sulfate and / or potassium oxide.

[0021] The potassium oxide or potassium sulfate can be present in the form of an aerosol.

[0022] The dust separated in the mass force separator can be treated to reduce the potassium oxide and / or potassium sulfate concentration in the dust, thus preventing an increase in potassium compounds, especially when the dust is recycled back into the cooking process.

[0023] The thick liquor can be combusted with a solids content of 55 to 60% by mass. Combustion of thick liquor with this solids content can be used to generate electrical energy, for example, via steam turbines. Furthermore, process steam for paper and pulp production can be obtained. In addition, cleaning the flue gas from the combusted thick liquor can remove magnesium oxide and sulfur dioxide, which can be used to produce cooking liquor for pulp production.

[0024] The thick lye solution can be combusted in the boiler at a temperature of 1130 °C to 1260 °C. This temperature is necessary to achieve the desired decomposition into magnesium oxide and sulfur dioxide, which can then be used to produce new cooking lye. The thick lye solution can be introduced into the boiler using burner lances and atomized in the combustion chamber.

[0025] A cyclone, particularly a multicyclone, can be used as a mass force separator. In principle, dust removal using fabric filters or electrostatic precipitators would also be possible. However, due to the physical properties of the dust contained in the flue gas, these filters present process engineering challenges. As it turns out, a cyclone functions largely independently of the dust's physical properties, such as dust resistance, flow behavior, or porosity. A multicyclone consists of several individual cyclones and features an axial inlet for the flue gas stream. With an axial inlet, baffles arranged within the multicyclone can impart a rotary motion to the flue gas stream. This rotary motion intensifies with decreasing cyclone radius, leading to the formation of a so-called primary vortex.Centrifugal forces act on the dust particles, causing them to rotate outwards. The particles settle on a wall of the cyclone and are carried downwards by gravity. After the flow direction reverses at the bottom of the cyclone, the exhaust gas stream is forced into a tighter circular path in the center and then exits the cyclone as cleaned flue gas through a dip tube. Particle separation within the cyclone can occur at the cyclone wall or within the vortex, with the separation efficiency varying depending on the dust concentration in the flue gas. In particular, the turbulent flow regions within the cyclone can promote the agglomeration of alkali salts contained in the flue gas with porous magnesium oxide particles.

[0026] A wet scrubber, particularly a Venturi scrubber, can be used as a cleaning stage downstream of the mass air separator. It is also possible to use multiple cleaning stages, each with its own wet scrubber. A quench stage can be installed upstream of the wet scrubber to cool the flue gas stream. In this stage of the exhaust gas treatment, the moisture content of the flue gas can increase by 5 to 6% by volume. Particles and dust are also removed here. In the subsequent wet scrubber, or in the multi-stage Venturi scrubbers and / or counterflow scrubbers, sulfur dioxide is absorbed from the flue gas and neutralized with magnesium hydroxide. Specific pH values ​​must be maintained in the wet scrubber. This can be achieved by feeding a suspension into the wet scrubber. A droplet separator can follow the wet scrubber, and the flue gas stream can then be released into the atmosphere via a stack.an environment will be provided.

[0027] In the device according to the invention for cleaning flue gas, particularly for pulp production plants, the device comprises a force separator for separating dust, which predominantly consists of magnesium oxide, from the flue gas. The flue gas is generated during the combustion of thick liquor, either exclusively or together with another fuel, in a combustion chamber of a boiler of the plant and can be fed to the force separator in a flue gas stream. The device allows the flue gas to be fed to the force separator for further treatment, and at least a partial stream of the dust separated in the force separator can be fed to the combustion chamber. For the advantages of the device according to the invention, reference is made to the description of the advantages of the method according to the invention.

[0028] Further advantageous embodiments of the device result from the feature descriptions of the method claim. 1 related subclaims.

[0029] The plant for pulp production according to the invention comprises a device according to the invention for cleaning flue gas.

[0030] A preferred embodiment of the invention is explained in more detail below with reference to the accompanying drawings.

[0031] They show: Fig. 1 a schematic representation of a device for cleaning flue gas according to the state of the art; Fig. 2 a schematic representation of another device for cleaning flue gas; Fig. 3 A schematic sectional view of an embodiment of a multicyclone.

[0032] The Fig. Figure 1 shows a device known from the prior art 10for cleaning flue gas within a pulp mill. In a boiler 11 the device 10 Thick lye is burned as a waste product of pulp production, with the flue gas produced during the combustion of the thick lye passing through a cyclone. 12 Dust is supplied for dust separation. The dust is discharged via an outlet. 13 from the cyclone 12 The flue gas is diverted by the cyclone. 12 continue to a quench 14 The air is then directed through a process where any remaining dust is separated from the flue gas. The dust is collected in a sedimentation container. 15 The flue gas is collected and the washing liquid is neutralized in a neutralization tank. The flue gas is then removed by the quench. 14 using a fan 17 for further treatment of a cascade of washers 18 , 19 , 20supplied. Here, sulfur dioxide is separated from the flue gas, using a container. 21 The sulfur dioxide captured from the flue gas is used to strengthen cooking acid. The cooking acid is stored in a tank. 22 clarified and stored. By means of another fan or blower. 23 The flue gas is then subjected to a scrubber. 24 , with which any remaining sulfur dioxide is separated from the flue gas. The scrubber 24 is therefore connected to a sulfide tank 25 connected. The sulfide tank can be used to adjust the pH value. 25 with fresh water 26 and vapor condensate 27 supplied from pulp production. Finally, the cleaned flue gas is discharged via a chimney. 28 into an environment 29 expelled.

[0033] The Fig. 2 shows another device 30for cleaning flue gas within a pulp production plant, which is essentially the same as the one described in the Fig. The device shown in 1 corresponds to this. In contrast to the device shown in Fig. 1 is in the device 30 a line 31 provided for, which are connected to the outlet 13 of the cyclone 12 is connected. Via the line 31 can in the cyclone 12 The collected dust is returned to a combustion chamber of the boiler. 11 to be recycled. The plan is to only recycle a portion of the dust that accumulates in the cyclone. 12 separated from the flue gas, into the boiler 11 to be attributed to the dust. 11 blown in and enables agglomeration and / or coagulation of aerosol and fly ash or dust particles produced during the combustion of the thick lye via the pipe 31Dust particles are blown into the combustion chamber. This makes it possible to increase the mass of the dust particles in the flue gas and thus separate the particles from the flue gas in the cyclone. 12 to make it easier.

[0034] The Fig. Figure 3 shows a multicyclone 32 in a schematic representation. The multicyclone 32 can be advantageously used in the device described above. The multicyclone 32 consists of a plurality of individual cyclones 33 within a shell 34 with a funnel 35 for collecting dust 36 The dust 36 can be accessed via an outlet 37 from the multicyclone 32 be diverted. Via a feeding tube 38 Flue gas is fed into the multicyclone 32 initiated and via a collector 39 diverted. In every cyclone 33 are guide vanes 40arranged in a ring shape, which create a flue gas stream 41 within the cyclone 33 set in rotation. This causes particles of the dust to be separated. 36 on a wall 42 of the cyclone 33 or in a vortex 43 , which is within the cyclone 33 is formed. The flue gas passes through a dip tube. 44 at the cyclone 33 in the collector 39 and the dust 36 falls down into the funnel. 35 The dust 36 can now at least partially be fed into a boiler for the combustion of thick lye.

Claims

[1] A method for cleaning flue gas, in particular with a plant for pulp production, wherein thick liquor is combusted in a boiler (11), exclusively or together with another fuel, in a combustion chamber of the boiler, wherein flue gas from the boiler produced during combustion is fed in a flue gas stream (41) to a mass force separator and dust (36) which predominantly comprises magnesium oxide is separated in the mass force separator, wherein the flue gas is subsequently fed to a further treatment, characterized by that at least a partial flow of the dust separated in the inertia separator is returned to the combustion chamber. [2] Method according to claim 1, characterized by that only a part of the dust (36) separated in the inertia separator is fed into the combustion chamber. [3] Method according to claim 1 or 2, characterized by that the dust (36) is blown into the combustion chamber with an air stream. [4] Method according to one of the preceding claims, characterized by that the dust (36) is formed from particles and is powdery. [5] Method according to claim 4, characterized by that an aerosol is created during combustion in the combustion chamber. [6] Method according to claim 5, characterized by that the dust (36) supplied to the combustion chamber is colder than the aerosol and / or dust (36) already present in the combustion chamber. [7] Method according to claim 5 or 6, characterized by that the aerosol precipitates in the combustion chamber on the blown-in dust (36), whereby a mass of the particles of the dust in the combustion chamber increases. [8] Method according to one of the preceding claims, characterized by that a dust concentration in the mass force separator is increased by the dust (36) supplied to the combustion chamber. [9] Method according to one of the preceding claims, characterized bythat the dust (36) separated in the inertia separator contains at least 85 mass percent magnesium oxide. [10] Method according to claim 9, characterized by that the dust (36) separated in the mass force separator contains proportions of alkalis, preferably potassium sulfate and / or potassium oxide. [11] Method according to claim 10, characterized by that particles of the dust (36) consist of magnesium oxide with deposited alkalis, preferably potassium sulfate and / or potassium oxide. [12] Method according to claim 10 or 11, characterized by that the dust (36) separated in the inertia separator is subjected to a treatment in which a potassium oxide concentration and / or potassium sulfate concentration in the dust is reduced, whereby an increase in the concentration of potassium compounds is avoided. [13] Method according to one of the preceding claims, characterized by that the thick liquor is burned with a solids content of 55 to 60 mass percent. [14] Method according to one of the preceding claims, characterized by that the thick liquor is burned in the boiler (11) at a temperature of 1130 C° to 1260 C°. [15] Method according to one of the preceding claims, characterized by that a cyclone (33), in particular a multi-cyclone (32), is used as the mass force separator. [16] Method according to one of the preceding claims, characterized by that a wet scrubber (18, 19, 20, 24), in particular a Venturi scrubber, is used as a cleaning stage downstream of the inertia separator. [17] Device (30) for cleaning flue gas, in particular for plants for pulp production, comprising a mass force separator for separating dust (36), which predominantly comprises magnesium oxide, from flue gas, wherein the flue gas is produced during combustion of thick liquor, exclusively or together with another fuel, in a combustion chamber of a boiler (11) of the plant and can be fed to the mass force separator in a flue gas stream (41), wherein by means of the device the flue gas can subsequently be fed to the mass force separator for further treatment with the device, characterized by that at least a partial flow of the dust separated in the mass force separator can be fed to the combustion chamber by means of the device. [18] Plant for pulp production with a device (30) according to claim 17.