US2893333A - Method of firing steam generators with evaporated sulphite waste liquor - Google Patents

Description

July 7, 1959 B. T. BRUN 5 METHOD OF FIRING STEAM GENERATORS WITH EVAPORATED SULPHITE WASTE LIQUOR Filed 001;. 29, 1957 WASTE SULF/TE LIQUOR AND FLUX STEA/W AND FLUE GASES GENERATOR MOUL TEN ASH 8 2b FLUE GASES INVENTOR.

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United States Patent METHOD OF FIRING STEAM GENERATORS WITH EVAPORATED SULPHITE WASTE LIQUOR Bengt Torsten Brunes, Djnrsholm, Sweden Application October 29, 1957, Serial No. 693,193

1 Claim. (Cl. 110-1) The main object of the present invention is to provide a method of firing steam generators by burning evaporated sulphite waste liquor wholly or partially containing calcium as cation. In doing so, and if no special steps are taken against it, flue gases will be obtained which are intermingled with very large quantities of extremely light ashes which involve considerable difliculties in the designing and operation of the steam generator. For instance, the latter must be oversized considering the appreciable impairment in the absorption of heat caused by ash deposits in the hearth and super heater etc. In order to reduce the risk of shutdowns, the steam generator must also be designed so as to obviate such deposits as far as possible and, in cases where they must inevitably arise they should be removed readily and efiiciently, which substantially increases the cost of the plant. In addition, and

, almost without exception, an ash separator of a very efficient design must be installed after the steam generator for separating the ashes murder to avoid thereby sanitary inconveniences in the surroundings.

It has been suggested to provide a method of eliminating said inconveniences by mixing the liquor with a flux agent before the combustion, whereby the ashes of the liquor will precipitate or deposit in a molten, liquid state and will be separated from the flue gases before these are utilized in the steam generator.

A complete combustion then takes place while the total water content of the liquor will be evaporated simultaneously. For this reason the contents of dry substance of the liquor must be comparatively high, at least about 60 percent by weight if, when firing with liquor exclusively and with a reasonable addition of flux agent, such an elevated combustion temperature, up to 1450-1500 C., is to be achieved that the ashes will melt into a state which is free-flowing enough for enabling separation. However, in most cases, evaporation of the liquor to such a high degree of concentration is not economically or operatively suitable, as far as the process of evaporation itself is concerned. An optimum usually accepted is about 50 percent, at which concentration the combustion per se does not offer any unsolved problems. At this concentration, however, free-flowing ashes cannot be obtained, even if the liquor is unrestrictedly mixed with flux agent, at the combustion temperature occurring, but the firing must be carried out with an admixture of a more valuable auxiliary fuel, which in many cases is practically unadvisable.

One object of the invention is to provide a method of performing the firing in such a way that the ashes will be obtained from the combustion process in a molten, liquid state and separated from the flue gases even on the assumption that liquor alone is burnt. Said liquor has not been evaporated further than to the highest degree of concentration hitherto considered normal and/or with the addition of an amount of flux agent which is less than that hitherto required at a given concentration of the liquor supplied.

The invention is chiefly characterized in that the com- 2,893,333 Patented July 7, 1959 bustion is performed in two consecutive stages of combustion, in each of which the liquor ashes formed being separated from the flue gases before these are utilized in the steam generator, the liquor being burnt incompletely in the first stage at a comparatively low temperature, and the total or at least substantially the total water contents of the liquor being incorporated with the flue gases so that combustion residue will be formed which contains liquor ashes and a considerable amount of unburnt liquor in a solid state. This combustion residue is burnt in the second stage substantially without the steam containing flue gases formed in the first stage being present, and the second stage combustion taking place at a considerably higher temperature than the first stage combustion. The liquor is mixed with such an amount of flux agent that the liquor ashes will assume a molten, liquid state during the combustionat said higher temperature.

An example of an arrangement for performing the invention is illustrated diagrammatically in the accompanying drawing in a sectional elevation.

By means of outlets 2a and 2b, respectively, for flue gases, and by means of a furnace front 3, two turbulence or cyclone furnaces 1a and 1b are connected to the hearth 4 of a steam boiler, not shown in the drawing. A number of tangential intakes 5 are provided for supplying combustion air into the respective furnaces. In addition, the furnace 1a is provided with an injection device 6 for liquor and a bottom outlet 7 for combustion residues, and the furnace 1b is provided with an additional tangential inlet 5a and a bottom outlet 8 for molten ashes. The furnace 1b is also provided with cooling coils 9 in the masonry. A conduit 10 interconnects outlet 7 and inlet 5a.

In the furnace 1a the liquor is dried and ignited and partly burnt while being caught by the air of combustion and driven spirally towards and along the periphery of the furnace to the bottom 7 where it arrives in the shape of a combustion residue of ashes and unburnt solid particles separated from the steam containing flue gases escaping through outlet 2a. Through the conduit 10 this combustion residue is conducted to the furnace 1b where it is completely burnt while turbulently passing through this furnace. In this course a small quantity of combustion gas from the furnace 1a may happen to flow along to the furnace 117 but this quantity is so small that the combustion there in any case will take place virtually without such gas being present, so that the elevated combustion temperature may arise at which the ashes will melt into a liquid state in the presence of the flux agent added. The molten ashes may be drained off through outlet 8, while separated from the flue gases escaping through outlet 2b. In this case it is presupposed that the liquor is supplied to the furnace 1a mixed with a suflicient amount of flux agent, but it is also possible to add flux agent at a later point before the final combustion, for instance in the conduit 10 or in the furnace 1b.

The following numerical example of a process of combustion in the device illustrated may give some information about the possibilities of the invention.

If a liquor having a dry substance proportion of about 47 percent by weight is incompletely burnt in the furnace 1a at a temperature. of about 800 C., a combustion temperature of 1650 C. may be attained at the subsequent combustion in the furnace 1b, provided that about 10 percent of the heat supplied is lost by radiation, said combustion temperature being higher than the temperature which may be expected in practice when firing a 60 percent liquor in one step in a furnace not cooled.

At this temperature, 1650 C., the ashes achieves a molten state sufliciently free-flowing for enabling the withdrawal if the liquor is mixed with flux agent in a quantity corresponding to about 2 percent Na O in the ashes at the most.

the other hand,ifthe 47 percent liquor isburnt in one step it is hardly possible to reach a higher combustion temperature than 1250 C. .An admixture of flux agent correspondnig to as much as up to 6 or 8 percent Na O in the'ashes willpgive-themelting point 12 C.,whereas a-temperature 250 C. higher, that is 1450 -C., has to be reached before the ashes will be sufliciently -free flowing for being drained off, that is to say that even if about four times as much flux agent is added as inthe combustion according to the present invention a one step combustion process will not result in a sufliciently high temperature to enable the ashes to be separated in a molten, liquid state.

As amatter-of course the arrangements for the combustion may be modified in many ways within the scope of the invention. -As regards the furnace 1a it is likely that no better embodiment is available for the time being than the one illustrated. The furnace 1b on the other hand may be designedzaccording to many various .models available in the field of melt-combustion of dry fuels, such as coal. Neither is it necessary .to arrange the furnace one above the other in order that the combustion residue .from the furnace 1a should be able to be transferred merely by gravityaction to the furnace 11;, but it is also conceivable that such a transfer may be accomplished by means of some positive conveyor device, such as a screw conveyor, injector or the like.

It is also possible, by maintaining a higher pressure in the furnace 1a than in furnace 1b, to cause a small part of the flue gas formed in the furnace 1a to escape through the conduit .10 as a gas conveyor for the combustion residue.

As anexample of the flux agent I may mention alkali salts, for instance sodium or potassium sulphate or carbonate, magnesium salts, such as sulphate carbonate, sand etc. An addition of 2% by weight (calculated as Na,0 in the ashes) of said flux agent results in a melting point of 1310' C. and an addition of 4% by weight (calculated as N830 in the ashes) results in a melting point of 1250 C.

What I claim is:

A method of firing steam generators with a firing sys tem comprising two combustion chambers by burning dry substance of sulphite waste liquor at least partly containing calcium as cation and supplied with a flux agent to convert the ashes formed thereby into a molten liquid state, comprising the steps of burning incompletely in a first or said combustion chambers such liquor in a concentrated state thereby evaporating at least substantially the total water content of the liquor to produce a combustion residue in a solid powdered 'state, which contains ashes and a considerable amount of nonburned dry substance from the liquor, passing from said first combustion chamber the flue gases and the vapor formed therein as heating medium to the steam generator and passing the combustion residue to asecond of said combustion chambers, supplying additional combustion air'to said second combustion chamber so as to burn at least substantially completelythe nonburned dry substance of said residue, and supplying the flux agent in an amount sufiicient to cause the residue of'ashes passed from the first combustion chamber and ashes newly formed in the second combustion chamber to assume a molten liquid state in that second combustion chamber at the combustion temperature-obtained therein, and passing from the second combustion chamber the flue gases formed therein as heating medium tothe steamgenerator andpassing the residue of molten ashes out of the firing system.

Colliau Feb. 1, 1887 Soderlund et a1. May 18, 1954

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