FI102979B - Method of combustion of mesa in a cellulose process - Google Patents

Description

! 102979

A method for burning lime slurry in a cellulosic process

The present invention relates to a process for burning lime mud in a cellulose process, wherein the black liquor is burned in a soda water boiler, the green liquor from the sludge in the recovery boiler is causticized with calcium oxide to form cooking liquor, and

10 In the cellulose process, it is now imperative to recover and recycle as many chemicals as possible without adding new chemicals, in order to avoid environmentally harmful emissions while making the plant more economically viable. 15 The recovery process of a modern pulp mill includes a shale digester, a recovery boiler, a caustic sludge and a lime kiln that processes separated chemicals. The process of recovering existing pulp mills and associated equipment is a very large part of the mill's investment and their operating costs are very significant. Further, in old small mills where the recovery process is not yet in use, eg the release of cooking chemicals, it is expensive to invest in a new process and may not be economically feasible for the operation of the mill.

In the evaporator, the black liquor is dewatered to maximize its dry solids content, thereby enabling its combustion and the recovery of the thermal energy it contains by incineration in a recovery boiler. 30 The melt resulting from the combustion of black liquor in a soda boiler, when dissolved in water, forms green liquor which, in turn, is causticized with calcium oxide in the caustic soda, thereby converting the soda or sodium carbonate in the green liquor into sodium hydroxide. In this reaction, calcium 35 oxide, in turn, forms calcium carbonate, or 2 102979 lime. In order to reuse honey, it must be burned into calcium oxide, which is what happens today in a lime kiln. A typical lime kiln is a rotary drum oven, where the lime slurry is fed to one end of the furnace and flows out of calcium oxide-5 binder at one end. Such a drum furnace can have a length of 40 to 120 meters and a diameter of 2 to 4 meters, and is therefore a very expensive investment. Alternatively, fluidized bed furnaces presented in place of a rotary drum furnace are also expensive investments and, in addition, their use and emission problems have in practice prevented their application.

WO 94/29518 discloses a solution in which slurry is fed in slurry form with black liquor to a boiler for incineration. According to the publication, the purpose of the process is to improve heat recovery at the bottom of the recovery boiler and further promote desulphurisation in the process.

U.S. Patent 3,366,535, in turn, discloses a solution in which calcium carbonate is mixed with black liquor prior to incineration and fed therewith into a recovery boiler for combustion. It is stated in the publication that in the recovery boiler the black liquor can be simultaneously burned and the calcium carbonate converted into calcium oxide.

The solutions of the publications require that calcium carbonate be fed into the recovery boiler mixed with black liquor, which complicates the handling of the black liquor and reduces the combustion conditions at the bottom of the recovery boiler. A further problem is that the conversion of calcium carbonate to calcium oxide occurs only partially, which diminishes the function and unnecessarily burdens the soda lime. The disadvantage of this only partial calcination is that the separation of calcium oxide from the calcium carbonate is difficult, which further complicates the process.

It is an object of the present invention to provide such a method which does not require a lime kiln and yet provides an effective calcination. The process according to the invention is characterized in that the calcium carbonate is fed in a dusty form to the recovery boiler at a point where it is substantially completely burned with calcium oxide, and the calcium oxide formed is subsequently separated from the flue gases and recycled to causticization.

The essential idea of the invention is that the lime sludge from the caustic soda is first dried by the flue gases from the recovery boiler, after which the dry lime sludge is fed into a 10 dusty recovery boiler at , where it is separated from the flue gases and recycled to causticisation.

An advantage of the invention is that a separate, expensive lime sludge kiln is no longer needed in the pulp process. This also removes dust emissions from the lime kiln nowadays. In addition, the pulp mill's recovery process is simplified. However, the most significant benefit comes from the savings in investment costs of eliminating the lime kiln. In this case, it is also profitable for small pulp mills to recover cooking chemicals, which results in significant reductions in environmental emissions, both water and air. It is also an advantage that the calcium carbonate feed can be optimally executed so that the calcination is as complete as possible. Further, the addition of lime to compensate for the loss of lime is easy by feeding excess, cheap calcium carbonate into a soo-30 dacti, like lime. Particularly preferably, the process of the invention is applicable to a soda pulp process wherein the cellulose is boiled in NaOH so that no sulphate-containing fly ash is produced.

The invention will be explained in more detail in the accompanying drawings, in which Fig. 102979 shows the implementation of the process of the invention in the soda cellulose process and Figure 2 shows the implementation of the process of the invention in the sulfate cellulose process.

Figure 1 illustrates the application of the process of the invention to the soda pulp process. In the soda pulp process, the wood is boiled in the cooking plant 1, from where the pulp is led to the wash 2 and the pulp washed therefrom further. From the scrubber, the black liquor is passed through evaporator 9 to a soot-10 dacetter 3, where the melt formed is dissolved in solvent 4 in aqueous solution, and the solution is further directed to the causticization 5. Since the cooking chemical sulphides. From the sump boiler 3, the flue gases are led to an electric filter 6, whereby a dusty solid mainly containing calcium oxide is separated from the flue gases which are conveyed to the flue gas dryer 7. The calcium oxide dries due to the heat of the flue gases and is transferred as a dry dusty material along with the flue gases to a second electrostatic precipitator 8 where it is separated and returned to be fed to the recovery boiler 1 at a temperature of about 1000 ° C. Feeding can be simplified pneumatically and when calcium carbonate is fed to such conditions in a recovery boiler it is very efficiently converted to calcium oxide. Since some lime is lost in the process, the loss can be compensated by feeding the imported calcium carbonate like lime into a recovery boiler at a temperature of about 1000 ° C where it is converted to fresh calcium oxide used in the 35 process.

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Fig. 2 shows a sulphate pulp process similar to Fig. 1, with a cooking plant 1, from which the pulp is led to the washing plant 2 and from there to further processing. From the scrubber 2, the black liquor is passed through the evaporator-5 slot 9 to the recovery boiler 3 for incineration. The melt formed in the sludge boiler 3 is dissolved in the solvent 4, from which the green liquor formed is led to causticization 5. The flue gases formed in the sludge boiler and the solid dust materials accompanying them are passed to an electric filter 10 6 where they are separated from the flue gases. In this case, the separated dusty solid is passed to a separate screen 10, where the calcium oxide is separated from the sodium sulfate which is returned to the recovery boiler 3 with the black liquor. . If only 7 is a flue gas dryer in which calcium carbonate is introduced into contact with the flue gases for drying. The dry dusty calcium carbonate carried with the flue gases is separated by a second electric filter 8 from the flue gases which are further introduced into the open air. The separated calcium carbonate, in turn, is returned to be fed to the recovery boiler 3 at its upper part at a temperature of about 1000 ° C. The calcium carbonate is then calcined into calcium oxide and the process cycle continues as described above. Any additional heat needed for the causticization in the recovery boiler can be easily produced by feeding into the recovery boiler a suitable additional fuel such as 30 oils or the like.

In the foregoing description and drawings, the invention is presented by way of example only and is not in any way limited thereto.

Claims (7)

1. A process for mesa burning in a cellulose process, in which the cellulose process black liquor is burned in a soda boiler (3), green liquor produced from the melt formed in the soda boiler (3) is causticized with calcium oxide to form boiling liquor, and the resulting calcium carbonate is burnt to the calcium carbonate. for ether use, characterized in that the calcium carbonate is fed in powder form at such a location in the soda boiler (3) that it is substantially completely combusted into calcium oxide, and that the resulting calcium oxide is then separated from the flue gases and refluxed. 2. A process according to claim 1, characterized in that the calcium oxide is separated from the flue gases by an electrophilter (6), the flue gases are led from the electrophilter (6) to a flue gas dryer (7), to which the calcium carbonate from the causticization (5). ) is measured so that it comes into contact with the flue gases and is mixed with the flue gases and that the flue gases and the calcium carbonate together are led to a second electro filter (8) where the dry calcium carbonate in powder form is separated from the flue gases to be returned to the soda boiler (3). Process according to Claim 1 or 2, characterized in that additional calcium carbonate is fed into the soda boiler (3) in such a way that it is substantially completely combusted into calcium oxide to compensate the limescale. Process according to any of the preceding claims, characterized in that the cellulose process is a sulfate cellulose process in which the liquor consists mainly of sodium hydroxide, sodium carbonate, sodium sulfide and sodium sulfate. 102979 Process according to any of claims 1-3, characterized in that the cellulose process is a soda cellulose process in which the boiling liquor consists mainly of sodium hydroxide. 5 Process according to any one of claims 1 to 3, characterized in that the cellulose process is a sulfate cellulose process, that the dry powdery material separated by the first electrofilter (6) is led to a screen (10) where the calcium carbonate is separated from the sodium compounds which together with the black liquor is returned to the soda boiler (3) and the calcium oxide obtained from the screen (10) is led to the caustic coating (5). Process according to any of the preceding claims, characterized in that the calcium carbonate obtained from the caustic is dried with the heat from the flue gases of the soda boiler (3).