US3852039A - Furnace for the combustion of waste liquors or other solutions or suspensions containing combustible organic substance and various combinations with alkali - Google Patents

Abstract

Disclosed is a furnace for the combustion of waste liquors from chemical pulping processes or similar solutions containing organic substances and alkali. The furnace is cylindrical and is characterized by an upper annular collecting channel formed between the upper portion of the annular furnace walls and an outer circumferential wall having a vault thereover for forming the top wall of the furnace. The annular channel collects gases formed during combustion which rise to the top of the furnace and removes the gases from the furnace through a discharge passageway formed in the outer circumferential wall. The upper portion of the annular furnace wall is tapered so as to form a relatively larger distance between the furnace wall and vault on a side of the channel opposite the discharge passageway and a relatively smaller distance between the furnace wall and vault on a side of the channel near the discharge passageway.

Description

Unite States Hasselberg aen 1 r [75] Inventor: Sven Rudolf Gunnar Hasselberg,

Broby, Sweden [73] Assignee: Broby Industri er AB, Broby,

Sweden [22] Filed: Feb. 8, 1973 [21] Appl. No.: 330,508

Related US. Application Data [62] Division of Ser. No. 60,532, Aug. 3, 1970,

abandoned. l

[30] Foreign Application Priority Data Mullen 23/277 R X Dec. 3, 1974 2,261,995 1 H1941 Greenawalt 23/277 R X 2,806,768 9/1957 Bender et a1 23/277 R X FOREIGN PATENTS OR APPLICATIONS 162,004 0/1958 Sweden 227,154 0/1969 Sweden Primary Examiner-S. Leon Bashore' Assistant ExaminerAlfred DAndrea, Jr.

[5 7 ABSTRACT Disclosed is a furnace for the combustion of waste liquors from chemical pulping processes or similar solutions containing organic substances and alkali. The furnace is cylindrical and is characterized by an upper annular collecting channel formed between the upper portion of the annular furnace walls and an outer circumferential wall having a vault thereover for forming the top wall of the furnace. The annular channel collects gases formed during combustion which rise to the top of the furnace and removes the gases from the furnace through a discharge passageway formed in the outer circumferential wall. The upper portion of the annular furnace wall is tapered so as to form a relatively larger distance between the furnace wall and vault on a side of the channel opposite the discharge passageway and a relatively smaller distance between the furnace wall and vault on a side of the channel near the discharge passageway.

4 Claims, 3 Drawing Figures PATENTEB BEE 3 4 susa'ma FURNACE FOR THE COMBUSTION'OF WASTE LIQUORS OR OTHER SOLUTIONS OR SUSPENSIONS CONTAINING COMBUSTIBLE ORGANIC SUBSTANCE AND VARIOUS COMBINATIONS WITH ALKAIJI This is a (x) division, of application Serial No. 60,532, filed Aug. 3, 1970, now abandoned.

The present invention relates primarily to a method for the combustion of waste liquors or other solutions or suspensions containing combustible organic substance and various combinations with alkali, in which method the liquor evaporated to suitable concentration is burned continuously in a furnace into which the liquor is sprayed and in which the alkali included in the liquor is converted into a soda smelt which is discharged from the bottom of the furnace.

The term liquor as used herein is generally to be understood as waste liquors from chemical pulping processes of cellulosic raw material, containing various combinations with alkali in or without the presence of sulphur-containing substances. The liquors may be e.g., waste liquors from e.g. the preparation of sulphite pulp, neutral sulphite pulp or so-called NSSC pulp, soda pulp or sulphate pulp. Also other types of waste liquors containing dissolved or suspended organic combustible material and various combinations with alkali may come in question, e.g. solutions of sodium formate, etc.

It is known since long in the cellulose industry to burn evaporated waste liquors in so-called soda pans for generating steam for heat and power production and for converting sodium compounds included in the liquor into a soda smelt which depending on the cooking method may contain sodium carbonate (Na Co lor imixture of varying amounts of sodium carbonate and sodium sulphide (Na s).

Plants of this type are very expensive. The combustion chamber in the soda pan is, for instance, provided with cooled walls, and material and design must be chosen with great care in order to avoid e.g. corrosive attacks. The cost for small cellulose mills becomes so high that it may be doubtful whether they are economically profitable. Also, in many cases the content of combustible material in the liquor may be too small to make steam generation economical.

This is the case in, for instance, the preparation of semichemical or other high-yield pulps in the yield range of 70 to 90 percent of included lignocellulosecontaining material.

The object of the present invention is to make an essential contribution to an economical solution of the problem of neutralizing waste liquors and recovering chemicals included therein also in smalll mills and particularly in semichemical pulp mills.

The invention will now be described in more detail with reference to the accompanying drawings in which:

section and a vertical longitudinal axis, a cyclone evaporator 3 connected to the outlet 2 of the furnace for direct evaporation of liquor, a suction fan 4 located after the evaporator 3 for sucking the gases from the evaporator 3 through a pipe 5 to an outlet 6, for instance in the form of a chimney, but through a branched pipe a portion of the gases is recirculated to the upper part of the furnace 1 or to the outlet 2 of the furnace. The means for this recirculation is not shown in FIG. 1 but the flow of gas is indicated by arrows, which are drawn in full lines to illustrate the gas flow from the furnace l to the cyclone evaporator 3, and in dashed lines to illustrate the gas flow from the cyclone evaporator 3 to the outlet 6, part of which is returned to the outlet of the furnace l (or the upper part of the furnace).

The cyclone evaporator 3 effects direct evaporation of thin liquor introduced into it, by means of the hot combustion gases and steam from the liquor combustion furnace 1. This gas is caused to pass countercurrently to the thin liquor in a known manner. The evaporated liquor is transferred from the cyclone evaporator 3 to the furnace l for combustion therein as will be described below. The gases leaving through the outlet 6 still contain a substantial amount of heat the heat content of which may be utilized in the pulp mill in a per se known manner, for instance for heat production or pre-evaporation.

The liquor combustion furnace l which, as has already been mentioned, is a brick furnace having a circular cross-section and a vertical longitudinal axis, is shown in detail in F168. 2 and 3. In the embodiment shown the furnace is internally of circular-cylindrical form but internally it may also be slightly circularconical, in which case the diameter is to increase in the downward direction so that the bottom area is larger than the top area.

The furnace 1 has a bottom 10 with an inclined flat bottom surface. Provided at the lowest point of the furnace bottom 10 is an outlet 12 in the furnace wall so that the smelt soda formed through the liquor combustion can discharge from the furnace through this outlet. The furnace has a wall 13 built of hard-burnt brick and is lined with an approximately 25 cm thick layer 14 of firebrick which is highly resistant to infiltration of alkali under the prevailing combustion conditions. The bottom section of the furnace is lined both in the bottom plane proper It] at 11 and up to a level of about 2 m above the bottom plane at 14' and also in an inner layer up to a level of about 1 m at I4" by means of alkali resistant, refractory brick, for instance of chrome brick type. The layer 11 of alkali resistant, refractory brick in the bottom plane 10 rests on a layer 10' of stamping mass or porable compound which is applied on the brick layer 10''. The stamping mass or porable compound may be of the same quality as the brick layer and is intended to prevent soda from penetrating down into the brickwork.

' The furnace has at its top a vault 15 of refractory brick of substantially the same quality as the upper part of the furnace wall. This vault 15 rests on a collecting channel I6 for combustion waste gases built of brick and extending around the cylindrical outer wall of the furnace. Also the collecting channel 16 may be built in substantially the same way as the furnace wall l3, 14. The vault 15 rests freely relative to the cylindrical furnace wall 14 on the collecting channel 16 so that the furnace wall 14 can move without having any influence on the vault 15. The collecting channel 16 for waste gases communicates with the furnace 1 through a gap between the vault l5 and the cylindrical furnace wall 14 and is provided with openings 17, 18 covered by doors (not shown) for inspection and cleaning purposes.

To make the furnace operate properly it is most important that the gas flow in the furnace shaft is as uniform as possible and for this reason the upper edge 19 of the furnace is inclined stepwise, as shown, so that the gap between the furnace wall 14 and the vault tapers successively towards a point (at 20) where the furnace outlet 2 (see also FIG. 1) adjoins the collecting channel 16.

The pipe 7 for evaporated liquor from the cyclone evaporator extends into the liquor combustion furnace 1 through the center of the vault 15 (see also FIG. 1) and ends inside the furnace in a sprayer 21 located immediately below the upper edge of the inner furnace wall 14 at a point on the vertical central axis of the furnace. The sprayer is so designed and adjusted that the liquor is evenly distributed practically throughout the cross-section of the furnace but without coming into contact with the furnace wall until it comes down at the bottom or bottom section of the furnace, as indicated by the cone of dispersion 22 in FIG. 2. The droplet size is adjusted so that the liquor is evaporated to dryness, as the droplets fall onto the furnace bottom, to such a degree that the desired combustion reaction in the bottom section 6 is obtained. On the other hand, the droplet size should not be too small so that the liquor droplets would accompany the waste gases leaving the furnace. A droplet size-of between l and 2 mm has been found suitable for e.g., NSSC liquor which has a dry solids content of about 50 percent organic material and about 50 percent sodium salts.

Air for the combustion is supplied to the furnace, on one hand, as primary air in the lower section and, on the other hand, as secondary air in one or more (two in the case shown) sections higher up in the furnace. The primary air as well as the secondary air is supplied to the furnace through nozzles which are arranged in groups in annular rows with the inlets of the nozzles of each such row located in the same plane and evenly distributed around the furnace. In FIG. 2, the primary air nozzles are designated by 23 and the two rows of secondary air nozzles by 24 and 24' respectively. The row of primary nozzles may comprise a large number of nozzles, say, about twenty, as indicated by crosses. These may extend radially while the secondary air nozzles 24, 24' may be tangentially directed to produce turbulence in the gas flow in the furnace. The fact that many primary nozzles 23 are used around the furnace gives uniform reducing conditions in the lower part of the furnace.

In its lower part the furnace has one or more oil burners 25 which may be operated to start or, if desired, to sustain liquor combustion. In the lower part the furnace also has an inlet port 26 and, in-the part of the furnace wall 14 situated inside the collecting channel 16, a row of apertures 27 for recirculating liquor or smelt to the furnace from the channel 16.

The minimum height of the furnace depends substantially on the time required for dry evaporation of the falling droplets and on the time required for total combustion of the fume gases formed in the bottom section of the furnace, where their path is indicated by arrows in FIG. 2. A furnace height of about 9 m is suitable for combustion of waste liquor from a NSSC mill. In that case, the primary air inlets 23 should be placed about 065m above the furnace bottom, and the two secondary air inlets about 2.6 and 3.6 m, respectively, above said furnace bottom level.

The cross-section of the furnace is dependent on the heat load on the bottom which usually should not exceed about 1,200 Mcal per m and n.

The air combustion temperature is preferably adjusted so as not essentially to exceed about l,O00l,l00C, which temperature can be obtained in the shown furnace by combustion of waste liquor having a heat value of about 1,300 eff. kcal/kg liquor. The heat value of waste liquors, evaporated to about 50 percent by weight, from NSSC pulp production is generally about l,200-l,400 eff. kcal/kg, half of the dry solids content being organic substance. If the waste liquor has a higher content of organic material in the dry substance, the liquor concentration must be done correspondingly lower. If the waste liquors have a lower heat value, i.e., a lower content of organic material in the dry substance, then the temperature required for the combustion of the liquor and the preparation of the smelt may be maintained by injection and burning of oil by means of the oil burners 25 placed in the furnace wall. The combustion temperature may be moderated as desired by controlled returning of a certain amount of the waste gas from the cyclone evaporator 3 to the inlet 28 in the outlet 2.

The ratio of primary air to secondary air depends on the kind of liquor to be burned. Thus, a pure soda waste liquor may be burned with essentially all air as primary air. However, in burning NSSC liquor, the conditions in the lower zone of the furnace should be reducing, if sulphur occurring therein is to be present as sulphide in the smelt. In that case, about 40 percent of the air is supplied as primary air. In this connection, however, there is always a risk of hydrogen sulphide (11 8) being present in the combustion gases. To counteract this, an excess of air should be used, for instance so that the waste gases contain about 5 percent 0,.

The degree of reduction in the combustion of sulphur-containing waste liquors should be high and lie about percent. The thiosulphate content is low, about 0.5 percent in the smelt, like the sulphate content which is about 3 percent. As it is impossible completely to prevent some liquid droplets and ash from accompanying the waste gases out of the furnace, a suitable direct evaporator is used for the evaporation, which in the embodiment shown in FIG. 1 is a cyclone evaporator 3, as mentioned, since cyclone evaporators have been found especially suitable to free as far as possible the effluent waste gases, which are saturated with or rich in steam, from liquid droplets, and of course the waste gases are at the same time freed from the major portion of entrained ash which is returned to the furnace. The evaporator 3 is fed with waste liquor (thin liquor) of a concentration of, for instance, about 20-25 percent. At a gas temperature of about l,050C in the furnace l, the liquor may be evaporated to a concentration suitable for combustion, which may be about 50 percent but depends on the kind of liquor. By recirculating to the outlet 2 of the furnace 1 a portion of the cooled waste gases enriched with steam, the hot waste gases may be cooled to a temperature suitable for direct evaporation, e.g. 350C.

The effluent combustion gases enriched with steam, which may maintain a temperature of about 100C, still contain the major portion of the heat content of the waste gases. Also liquor and ash particles in wet and solid form, respectively, occur in a smaller amount in the waste gases and may under certain circumstances cause inconveniences in the form of mist if they are released directly into the air. Under certain conditions, e.g. when burning sulphur-containing waste liquors, also small amounts of as well S as H 5 may occur in the effluent waste gases. Therefore, before the waste gases are released into atmosphere or used for the production of hot water or for heating purposes, it is advisable first to freethe gases from entrained particles and possibly accompanying acid constituents, such as $0 and H 8, by Washing the gases in a scrubber of known type with circulating wash liquid at the dew point, whereby condensation of steam out of the gases is avoided and an enriched wash liquid can be obtained. This liquid can be recirculated to the liquor for evaporation or be used otherwise in the process. The purified gas is then cooled down in a scrubber by direct contact with water, which may be done either by supplying fresh water to the scrubber and directly utilizing discharged hot water for various purposes, or by feeding the scrubber with circulating water which is indirectly cooled.

In the combustion of waste liquors leaving smelt soda as a residue during combustion, it has thus been made possible to obtain a satisfactory reliability in operation as a result of the improved furnace construction according to this invention, suitable refractory lining, control of heat load and effective heat value of included waste liquor and especially the method of introducing the liquor into the furnace.

Regarding thefurnace, it should be mentioned that brick construction walls certainly have been tried before in the pulp industry for combustion of waste liquors but due to unsatisfactory durability and reliability in operation one has proceeded to employing the system of cooled furnace walls which now is generally used. This invention, however, provides a vertical shaft furnace of brick construction which operates excellently without any cooling whatsoever of walls or bottom section and it has been established on the basis of carefully checked test running conditions that this furnace can be used several years under corresponding conditions without requiring any rebuilding. The furnace of this invention thus represents a great step forward in settling the problem of neutralizing waste liquors and recovering included chemicals in small mills and especially semichemical pulp mills.

Of course certain modifications of the furnace according to the invention can be made within the scope of the inventive idea. Among the objects of possible modifications there may first be mentioned the measurements stated and the location of the air nozzles. Another possible modification contributing to the desired uniform gas flow in the upper part of the furnace and out through the outlet 2 is to arrange the collecting channel 16 eccentrically relative to the furnace so that it tapers towards the outlet 2.

What I claim to secure by Letters Patent is:

1. In a furnace having a substantially circular cross section and a vertical axis for the continuous combustion of waste liquors containing combustible organic substances combined with alkali and having means for spraying said waste liquors into the top of the furnace, in the upper part of which moisture is evaporated from the waste liquor to concentrate the liquor and in the lower part of which the concentrated liquor is subject to combustion, the gaseous products of combustion rising in said furnace to effect the evaporation of moisture from the waste liquors being sprayed thereinto, the improvement comprising a circular furnace wall means extending circumferentially around the outside of the top of said furnace wall for forming therewith an annular collecting channel, a vault means mounted over the top of the furnace and supported circumferentially on the outer circumference of said annular channel means, said vault means being spaced from the upper end of said furnace wall to provide an annular, tapered gap therebetween, and an outlet means communicating with said annular channel means for removal therefrom of combustion products and evaporated moisture, the annular, tapered gap being widest at that point diametrically opposite said outlet means and being narrowest at the point adjacent said outlet means, whereby the combustion products and evaporated moisture will flow upwardly in the furnace substantially uniformly, through the annular tapered gap into said annular channel means and finally through said outlet means.

2. In a furnace as claimed in claim 1 the further improvement in which the upper end of the furnace wall lies in a plane inclined at an angle to the vertical axis of the furnace, said plane being lower at the point diametrically opposite to said outlet means and said vault means lies in a substantially horizontal plane above the wall of the furnace thereby forming the annular, tapered gap.

3. In a furnace as claimed in claim 1 the further improvement in which said furnace wall, said vault means and said annular channel means are formed at least in part of refractory brick.

4. In a furnace as claimed in claim 1 wherein the annular gap between the top of said furnace wall and said vault means is sufficient at all points to permit free expansion of said furnace wall. I

Claims (4)

1. IN A FURNACE HAVING A SUBSTANTIALLY CIRCULAR CROSS SECTION AND A VERTICAL AXIS FOR THE CONTINUOUS COMBUSTION OF WASTE LIQUORS CONTAINING COMBUSTIBLE ORGANIC SUBSTANCES COMBINED WITH ALKALI AND HAVING MEANS FOR SPRAYING SAID WASTE LIQUORS INTO THE TOP OF THE FURNACE, IN THE UPPER PART OF WHICH MOISTURE IS EVAPORATED FROM THE WASTE LIQUOR TO CONCENTRATE THE LIQUOR AND IN THE LOWER PART OF WHICH THE CONCENTRATED LIQUOR IS SUBJECT TO COMBUSTION, THE GASEOUS PRODUCTS OF COMBUSTION RISING IN SAID FURNACE TO EFFECT THE EVAPORATION OF MOISTURE FROM THE WASTE LIQUORS BEING SPRAYED THEREINTO, THE IMPROVEMENT COMPRISING A CIRCULAR FURNACE WALL MEANS EXTENDING CIRCUMFERENTIALLY AROUND THE OUTSIDE OF THE TOP OF SAID FURNACE WALL FOR FORMING THEREWITH AN ANNULALR COLLECTING CHANNEL, A VAULT MEANS MOUNTED OVER THE TOP OF THE FURNACE AND SUPPORTED CIRCUMFERENTIALLY ON THE OUTER CIRCUMFERENCE OF SAID ANNULAR CHANNEL MEANS, SAID VAULT MEANS BEING SPACED FROM THE UPPER END OF SAID FURNACE WALL TO PROVIDE AN ANNULAR, TAPERED GAP THEREBETWEEN, AND AN OUTLET MEANS COMMUNICATING THE SAID ANNULAR CHANNEL MEANS FOR REMOVAL THEREFROM OF COMBUSTION PORIDCTS AND EVAPORATED MOISTURE, THE ANNULAR, TAPERED GAP BEING WIDEST AT THAT POINT DIAMETRICALLY OPPOSITE SAID OUTLET MEANS AND BEING NARROWEST AT THE POINT ADJACENT SAID OUTLET MEANS, WHEREBY THE COMBUSTION PRODUCTS AND EVAPORATED MOISTURE WILL FLOW UPWARDLY IN THE FURNACE SUBSTANTIALLY UNIFORMLY,THROUGHT THE ANNULAR TAPERED GAP INTO SAID ANNULAR CHANNEL MEANS AND FINALLY THROUGH SAID OUTLET MEANS.

2. In a furnace as claimed in claim 1 the further improvement in which the upper end of the furnace wall lies in a plane inclined at an angle to the vertical axis of the furnace, said plane being lower at the point diametrically opposite to said outlet means and said vault means lies in a substantially horizontal plane above the wall of the furnace thereby forming the annular, tapered gap.

3. In a furnace as claimed in claim 1 the further improvement in which said furnace wall, said vault means and said annular channel means are formed at least in part of refractory brick.

4. In a furnace as claimed in claim 1 wherein the annular gap between the top of said furnace wall and said vault means is sufficient at all points to permit free expansion of said furnace wall.

Images