FI107058B - Method for burning of waste liquor containing silicate - Google Patents

Description

107058

Method for incineration of silicate-containing waste liquor

The invention relates to a method for burning silicate-containing waste liquor in a recovery boiler by means of preheated combustion air 5.

Recovery and regeneration of chemicals in a soda boiler is an integral part of the cellulose production process. The concentrate waste liquor obtained from cellulose digestion is injected into the recovery boiler, where it is burned to form ash containing cooking chemicals which, due to the high temperature in the boiler, melts and drains to the bottom of the boiler. At the bottom of the boiler, the melt is led to a solvent, where the melt dissolves to form so-called. green liquor 15, from which various processes can be used to make cooking chemicals for re-cooking.

In addition to wood, pulp and paper are made from other fibrous materials. These 20 are e.g. straw, bagasse, bamboo and reed. What is common to the above-mentioned raw materials is that they contain a lot of silicate, up to ten times more than the wood that the silicate in the pulping process gets into the pulp-separated waste liquor and thus into the recovery boiler.

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In addition to the high silicate content, the waste liquor resulting from the cooking of the above materials has a high hemicellulose content. The combination of these two factors results in a high viscosity of the waste liquor, which makes it difficult to evaporate the 30 waste liquors. High silicate content • also causes evaporation blockage problems. Thus, it has been found that with normal evaporation plant arrangements, silicate-containing waste liquors can be evaporated to about 50% solids. By way of comparison, 35 waste wood liquors can easily reach a dry matter content of 65%.

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Due to the low solids content, the silicate-containing waste liquor has a low calorific value and, when injected into the furnace, dries and burns slowly. In addition, low solids content causes the furnace to cool 5 and thus impedes or prevents the melt from flowing into the solvent at the bottom of the furnace. When burning silicate-containing waste liquor, the silicate also gets into the melt, which makes it difficult to drain the melt because the silicate raises the melting point of the melt. Further, the high melting point of the ash from the silicate-containing waste liquor will result in fouling of the furnace walls of the recovery boiler and also cause clogging of the air and liquor openings of the furnace.

A solution to the problems caused by the silicate has been proposed to remove the silicates from the waste liquor before the recovery boiler, for example by precipitating the silicate with lime or carbon dioxide. However, this is a difficult and economically expensive process that requires large reaction tanks and expensive reagents.

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Another way of solving the problem of silicates in liquor burning is to increase the temperature of the furnace. The decrease in furnace temperature has been compensated for by burning support material such as gas or oil in the furnace in the vicinity of the fuse. This is to keep the fuse running. The problem with back burner heating, however, is that since heat is transferred from the back burner flame to the fuse size mainly by irradiation, areas with locally high temperatures are formed on the fuse blast surface. In addition to this, it is difficult to control the effective back burner heating because the suitable temperature for the window surface temperature is low, only about 100 ° C. Namely, it has been found that the surface temperature of the nettle should be in the range of 950 to 1050 ° C.

The reason for this is that the melt temperature must be kept sufficiently high, about 950 ° C, in order to remain fluid and drain out of the recovery boiler. At temperatures above 1000 ° C * again, the heap begins to evaporate with sodium, which results in the heap surface being enriched with more difficult to decompose and evaporate sodium silicate, which results in the leaching of the sodium on the surface of the melt. As a result, the warming action of the support burners is inhibited and the melt begins to solidify, which necessitates additional support combustion. This, in turn, results in higher temperatures, increased evaporation of sodium, widening and strengthening of the peeling on the surface of the heap, and, eventually, melting of the melt from the boiler.

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Alternatives to combustion of the support fuel or to enhance its effect have been proposed to preheat the combustion air to 150-250 ° C instead of the normal 150 ° C. In order to bring the temperature above 150 ° C, heating 15 has been carried out using fresh steam from a pulp mill. However, this amount of preheating the combustion air is not sufficient to maintain a steady combustion and a temperature sufficient for the melt when burning the silicate-containing waste liquor.

It is an object of the present invention to provide a more economical and better controlled combustion process for the incineration of silica-containing waste liquor.

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The process according to the invention is characterized in that the preheating of the combustion air is carried out by means of an air preheater using a support fuel so that the temperature of the combustion air obtained from the air preheater is at least 250 ° C.

The essential idea of the invention is that the temperature of the heap, and thereby the flow of the molten liquid, is controlled within the narrow limits required by the properties of the silica slurry by raising the temperature of the combustion air, thereby avoiding the disadvantages of feeding the support fuel. In addition, it is not necessary to remove the silicates from the residue before it is discharged for incineration; When the temperature of the combustion air is raised, no peeling occurs in the fuse blade, the melt remains very fluid and is easily conducted to the solvent. In addition, the combustion process of the entire furnace is accelerated 5 and combustion is more complete because preheated combustion air results in faster drying and more complete combustion of the lye droplets. As a result, the amount of steam produced by the boiler is also increased.

10 In order to prevent peeling and to ensure adequate fusion temperature and melt flow, the combustion air must be heated to at least 250 ° C but, if necessary, the air can be heated up to 400 ° C. Heating of the combustion air takes place by burning the support fuel in an air preheater through which the combustion air is led to the boiler. In this way, the temperature of the combustion air can be easily raised up to 400 ° C.

The lower furnace temperature is monitored during the lye combustion and if required, for example, by changes in the lye composition, the temperature is controlled by changing the droplet size of the lye injection and / or the combustion air ratio of the lower furnace. The air heating temperature can also be adjusted, but its effect is only visible over a longer period of time and thus cannot be used for immediate temperature control of the lower part of the furnace.

The invention will be described in more detail in the accompanying drawings, in which Figure 1 is a schematic representation of an embodiment of the method of the invention and Figure 2 is a schematic representation of another embodiment of the invention.

Figure 1 shows a recovery boiler 1 fed with evaporated silicate-containing waste liquor 2: via a liquor syringe 3. In the boiler, the droplets of waste liquor dry out and burn, forming ash, which melts and drains to the bottom of the boiler. At the bottom of the boiler is formed a fuse box 4, from which the melt is led out of the boiler by means of a funnel 5.

5 The combustion air required for combustion is introduced into the air preheater 9 where the air is heated to the desired temperature. The heating is done by burning fuel in the air preheater which may be gas, oil or other suitable fuel. The hot combustion air 10 from the air preheater is supplied to the boiler as primary air 6, secondary air 7 and tertiary air 8. The air distribution levels of the boiler and the supply of air thereto are not known per se and are therefore not further described.

15 The air heater 12 can operate in a so-called. duct combustion principle, wherein the fuel is burned directly in the heater by duct burners and the air thus heated is supplied to the air nozzles. The air preheater may also include a heat exchanger or exchanger which is heated by the combustion gases formed in the combustion of the support fuel 20. The heat exchanger or exchanger, in turn, heats the combustion air supplied to the boiler.

Figure 2 shows another practical embodiment of the invention 25. The secondary air 11 and the tertiary air 10 are fed directly into the recovery boiler so that they pass the air pre-heater 9. Alternatively, both the primary and 30 secondary air can to have heated air in the air preheater. The secondary air 7 'and the primary air 6 are then fed to the recovery boiler 1 at the high temperature to which they are heated in the air preheater 9. The tertiary air 10 is fed directly into the recovery boiler, past the air pre-35 gauge 9.

t '10 107058 provides the best result in preventing all peeling of the fuse, if all the combustion air, both primary and tertiary, is heated to said high temperature. However, the effect of tertiary air supplied to the boiler from a very high level is less pronounced 5 and in practice it has been found that the heating of the primary and secondary air is sufficient. Most importantly, at least the primary air is preheated. If desired, the air supplied to the air vent heater 9 may be operated from conventional plant back pressure or tap 10 steam heated to about 150 ° C, which according to the invention is heated to a high temperature of 150 to 250 ° C before being fed to the recovery boiler. Similarly, in the embodiment of Figure 2, both the air introduced into the air preheater and the secondary and tertiary air can be conventionally heated air of up to 150 ° C.

The invention disclosed above in the specification and drawing is described by way of example only and is not intended to be limiting thereof, but is intended to cover the various modifications and corresponding solutions within the scope of the appended claims.

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Claims (8)

Method for combustion of silicate-containing waste in a soda boiler by means of preheated combustion air, characterized in that the preheating of the combustion air is carried out with an air preheater which uses auxiliary fuel in such a way that the temperature of the combustion air supplied from the air preheater is at least 250 ° C. 2. A method according to claim 1, characterized in that the temperature of the combustion air supplied from the air preheater is 250-400 ° C. Method according to claim 1, characterized in that a duct burner is used as an air preheater. 15 4. A method according to claim 1, characterized in that as an air preheater a heat exchanger is used which is heated together with the support fuel. Process according to claims 1, 3 and 4, characterized in that gas or gas is used as a support fuel. 6. A method according to claim 1, wherein the combustion air heated in the air preheater is measured in the boiler as primary air. 7. A process according to claim 1, characterized in that the combustion air heated in the air preheater is measured in the boiler as primary and secondary air. 8. A process according to claim 1, characterized in that the combustion air heated in the air preheater is measured in the boiler as primary, secondary and tertiary air.