GB2030688A - Fluidised bed incinerators - Google Patents

Abstract

The invention relates to the treatment of waste liquors and particularly to a steam generator and systems including same therefor. In known systems for treating such liquors to extract inorganic compounds therefrom, fluidised beds have been used but the heat generated has largely been wasted and can in some instances be so great as to generate temperatures in excess of the fusion temperature of the compounds. The present invention incorporates into such fluidised bed systems a means for utilising the generated heat and at the same time controlling the bed temperature. The means comprises a plurality of tubes (118, 146 Fig. 2) which pass through the fluidised bed (22) in heat exchange relationship therewith to extract heat therefrom. The tubes will carry a fluid, normally water which may be evaporated to steam by the heat in the bed. <IMAGE>

Description

SPECIFICATION Treatment of waste liquors The invention relates to steam generators incorporating a fluidised bed for the treatment of waste liquors such as are produced in the manufacture of pulp. The invention also relates to a treatment system for such liquors incorporating the steam generator.

In the manufacture of pulp, certain organic compounds are separated from the cellulose components by cooking the raw materials in cooking liquors. The cooking liquors include inter alia inorganic salts such as sodium sulfide, sodium bisulfate, sodium hydroxide, sodium carbonate, calcium bisulfite, magnesium bisulfite, sodium sulfite and sodium bicarbonate. The cooking process produces a waste liquor which is known as "brown liquor" or "black liquor". The black liquors contain lignin and other organic compounds; and therefore, the black liquor has calorific value. The black liquor also contains other chemicals which are not completely consumed in the cooking operation. The organic compound and unconsumed chemicals are suspended and dissolved in a solvent and medium which includes washing water added during the pulp processing.

Often, the suspended and dissolved matter are present in the black liquor at a solids content of 515% by weight. Sometimes the solids content is higher.

The inorganic salts are rather expensive; and therefore, from an economic viewpoint, it is advantageous to reclaim them. In the past the black liquor has been concentrated and then smelted at temperatures of approximately 1 7000 F to burn any combustible organic or other carbonatious fraction leaving the residual inorganics at a temperature above their fusion points. The inorganic chemicals are then subjected to reducing conditions to convert them into derived compounds which can be used again in the pulping process. Under these conditions, hydrogen suifide and other sulfurous compounds are usually released and must be recovered to avoid air pollution. Since the temperatures are above the fusion point of the inorganic chemicals, a portion of these chemicals are usually vaporised and must be separated from the remainder of the exhaust gases in order to be recovered.

The processes described above are complex requiring complicated and expensive equipment.

Further, the processes tend to be hard on equipment to thereby limit the useful life thereof; and consequently, add to the cost of practicing such processes. Many of the problems set forth above have been overcome by treating the black liquor in a fluidised bed oxidation process which is shown and described in U.S. Patent No: 3,309,262. This patent discloses a process in which the black liquor is sprayed over a fluidised bed which is made up of solid particles of residual inorganic chemicals derived from the black liquor.

An oxygen containing fluidising gas is passed upward through the bed to maintain the particles in a fluidised state. Under these conditions a portion of the black liquor will evaporate and another portion of the black liquor will deposit on the particles. At least a portion of the fluidised bed is maintained at a temperature above the combustion temperature of the concentrated black liquor but below the fusion temperature of the particles. Such a temperature would probably be within the range of approximately 1 0000Fto 1 8000F and often below 16000 F.Under these conditions the carbon content of the organic material is burned off and aggiomerates of residual inorganic material are formed which are heavy enough to stay within the fluidised bed and not become entrained in the fluidising gas to pass out of the bed. The agglomerates are grown to a size where they can be taken out of the fluidised bed and eventually recycled to the pulping operation.

Such a process is not without its disadvantages. For one, the heat energy generated by the fluidised bed reaction is not used with a high degree of efficiency. Further, it has been necessary to feed the black liquor into a fluidized bed at a fairly low concentration in order to have sufficient water available to keep the temperatures within the bed within the desired range.

It is an object of the present invention to overcome drawbacks found in the prior art such as those discussed above. Accordingly, a steam generator is provided with a fluidised bed made up of particles comprising inorganic material recovered from black liquor, a means for injecting the black liquor into the fluidised bed, means for flowing a fluidising gas upward through the fluidised bed to maintain it in a fluidised state; and tubes which carry a fluid in heat exchange relationship with the fluidised bed to evaporate the fluid. The tubes and the fluid in them can also serve to maintain the fluidised bed at a temperature below the fusion point of the inorganic materials.

A system according to the invention for treating waste liquor and recovering inorganic materials therefrom, comprises a steam generator having a chamber for a fluidised bed of particles of inorganic material derived from such waste liquor, and a plurality of tubes for placing a fluid in heat exchange relationship with a said fluidised bed to remove heat from a said fluidised bed and evaporate a said fluid; a collector connected to said steam generator and adapted to receive from the chamber exhaust gases and entrained solids, and separate such solids from such exhaust gases; and means for returning such separated solids to the chamber.

The invention will now be described by way of example and with reference to the accompanying drawings wherein: Figure 1 is a diagrammatic view showing an embodiment of the present invention; Figure 2 is a view partly in section showing the steam generator of Figure 1; and Figure 3 is a top view in section taken substantially along the line 3-3 of Figure 2.

As shown in Figure 1, the present invention includes a reactor in the form of a fluidised bed steam generator 10 which receives oxygen containing fiuidising gas from conduit 12 leading from pre-heater 1 4 which receives the fluidising gas, which may be air, under pressure from blower 16. The amount and velocity of the fiuidising gas can be controlled by means of a valve 1 8 which is placed in a conduit 20 which allows air to pass to the blower 1 6 from the atmosphere.

The fluidising gas passes upward through a fluidised bed 22 to maintain the particles thereof in a fluidised state and supply oxygen to support combustion.

Exhaust gases containing the products of combustion and possibly water vapour along with some entrained solid from the fluidised bed 22 pass out of the steam generator 1 0 and into a duct 24 which passes the exhaust gases into a cyclone collector 26. The gases then pass through a conduit 28 to a wet scrubbing system 30 where the entrained particles are removed from the exhaust gases. The exhaust gases are then vented to the atmosphere at the vent 32. The particulate matter removed from the exhaust gases in the cycline collector 26 passes down through conduit 34 to a screw conveyor 36 which conveys the particulate matter back into the fluidised bed 22 within the steam generator 1 0.

As will be explained presently, material is continuously discharged from the fluidised bed 22.

Makeup particulate material can be introduced through a conduit 38 which can be metered by means of a hopper 40 and a feeder valve 42 which regulates the passage of the makeup material into the screw conveyor 36.

Fine particulate matter, which is recovered in the wet scrubbing system 30, passes through conduit 44 and down through an entrainment separator 46 and a conduit 48 which leads to a sump 50. In passing from the wet scrubbing system 30 to the sump~50, the particulate matter is carried in a medium of dilute black liquor which is introduced into the web scrubbing system 30 through a conduit 52 under action of a pump 54 which receives the liquor through a supply conduit 56. The fines can be taken out at the sump or alternately can be moved by a pump 60 through a conduit 62 and conduit 64 to the conduit 56 to be recycled back to the wet scrubber 30 under control of a valve 66 in the conduit 64.

The supply conduit 56 contains-relatively dilute liquor. More concehtrated liquor is available through the supply conduit 68. Black liquor which has been pre-concentrated by any conventional evaporation process flows through the supply conduit 68 to a reservoir 70 and then flows through a conduit 72 and through a pump 74 which forces the liquor through a conduit 76.

The liquor must be introduced into the fluidised bed 22. It can be introduced above the fluidised bed 22 or at a level below the top of the fluidised bed 22. If it is desired to introduce it above the fluidised bed a valve 78 in the conduit 76 is closed so that the liquor will flow through a conduit 80. A valve 82 in the conduit 80 is opened as is a valve 84 in a conduit 86 which conveys air under pressure. The compressed air mixes with liquor supplied by the conduit 80 and pressurises the liquor so that it can be sprayed through a sprayer 88 at the end of the conduit 86 within the steam generator 10.

If it is desired to inject the liquor at a level below the top of the fluidised bed, the valve 78 is opened as is a valve 90 in a line 92 which connects the conduit 86 with the conduit 76. The valves 82 and 84 are closed so that compressed air and liquor will pass down through the conduit 76 to the liquor inlet 93 in the steam generator 10.

In order to moderate the temperatures within the steam generator 10 so that they are maintained at a level lower than the fusion temperatures of the inorganics making up the fluidised bed, water can be injected through sprayers 94 and 96. The sprayer 94 is at the end of a conduit 98 which supplies water. Compressed air is supplied through a conduit 100 which connects with the conduit 98. Similarly, the sprayer 96 receives water through a conduit 1 02 which is pressurised by means of compressed air supplied through a conduit 104 which connects with the conduit 102.

The particles grow in the fluidised bed 22 during combustion as the inorganic components from the black liquor deposit on the particles. Dry particles are continuously discharged from the fluidised bed steam generator 10 through a conduit 106 into a classifier 108. The rate of discharge is controlled by a valve 110 which is positioned in the conduit 1 06. Particles are classified according to size with the larger particles being discarded ortreated chemically to recover the inorganics. The smaller particles are returned to the fluidised bed through a conduit 112 which connects with the conduit 38 so that the smaller particles will drop into the hopper 40 to be conveyed by the screw conveyor 36 back into the fluidised bed 22.

The system described above functions in a manner substantially similar to the system described in U.S. Patent 3,309,262.

In the past where fluidised beds were used to recover the inorganic chemicals derived from the black liquor, such as in the system shown in U.S.

Patent No: 3,309,262, the liquor was introduced at a concentration of approximately thirty-five percent (35%) solids, and additional water was made available to maintain the temperature within the fluidised bed below the eutectic point of the bed material. Most of the available water was evaporated in the process in order to maintain the low temperatures. This had several disadvantages.

For one, it limited the concentration of black liquor which could be treated and consequently it was inefficient from an energy viewpoint. Available heat energy was used to evaporate water rather than being made available to perform useful work.

In the present invention the steam generator 10 as shown in Figures 2 and 3 includes a series of tube in which water is heated and evaporated into steam. The front and rear walls 116 and 114 are made up of a plurality of vertically extending tubes 118 which are connected by a plurality of fins 120. The tubes 118 of the front wall 116 extend to a roof 122 and pass at a slight angle with the horizontal to a header 123 which passes the steam or steam and water mixture to a steam drum 124. The roof 1 22 is also made up of the tubes 11 8 with interconnecting fins 1 20 as are the walls 114 and 116. This is not shown in detail for the sake of clarity.

The tubes 118 of the front wall 114 extend to a header 125 which passes the steam or steam and water mixture to the steam drum 1 24. Fins between the tubes extend from the bottom of the front wall 114 to a duct plate 126. Above the duct plate 126, the tubes 1 8 are not joined by fins so that the flue gases can pass through the tubes 118, above the duct plate 126 and below the steam drum 124.

Water is made available to the tubes of the front and rear walls 116 and 114 and to side walls 128 and 130 by a header 132 which surrounds the furnace. The header 132 receives water from a feeder tube 1 34 which is connected with a downcomer 136 extending downward from a water drum 138. The water drum 138 receives feedwater from a feedwater supply tube 140.

A feeder tube 142 is also connected to the downcomer 136 and supplies water to a header 144 which in turn supplies the water to a plurality of vertically extending water tubes 1 46 which make up a partition wall 148 as shown best perhaps in Figure 3. Only some of the water tubes 146 are shown in Figure 2 for clarity, it being understood that in the preferred embodiment they extend over the width of the steam generator 10 between the side walls 128 and 1 30. As shown in Figure 3 the watertubes 146 are spaced so that they each touch contiguous tubes. This arrangement is due to the fact that the water tubes 1 46 are receiving heat from two sides; and therefore; more heat is transferred through each tube 146 than each of the tubes 118 which receive heat from only one side.

A collection header 1 52 receives steam from the tubes 146 and conveys the steam into the steam drum 124. A number of downcomer tubes 148 extend down from the steam drum 124 to the water drum 138 and a number of evaporation tubes 150 extend from the water drum 138 up to the steam drum 124. In practice, feedwater enters the steam generator 10 through the feedwater supply tube 140 and enters the water drum 1 38 and flows down through downcomer tubes 148 to the water drum 1 38. The water then flows downward through the downcomer 1 36 to enter the headers 132 and 144. The water from the header 132 then flows upward through the water tubes 1 8 and the water from the header 144 flows upward through the water tubes 146.The water in the tubes 1 18 and 146 is converted into steam or a steam-water mixture and enters the steam drum 124. Steam is taken off through a steam pipe 1 54 and delivered to the point of use.

Water in the steam-water mixture in the steam drum 124 will settle on the bottom and flow downward through the downcomer tubes 148 to the water drum 138. Water will be heated in the evaporation tubes 150 and rise as a steam ora steam-water mixture into the steam drum 124.

The direction of flow within the tubes 148 and 150 is due to the direction of flow of the exhaust gases over the tubes 148 and 1 50 between the duct plate 1 26 and the roof 1 22. Since heat is extracted by the tubes 1 50 before the gases reach the tubes 148, the water in the tubes 148 will not be heated as much as water within the tubes 1 50.

Thus, the tubes 148, in effect, serve as heated downcomers. The exhaust gases leave the steam generator 10 through the duct 24.

The black liquor is sprayed in through the sprayer 88 or flowed in through conduit 76. Water can be supplied through either or both the sprayers 94 and 96. The water may be made available at critical times such as when the generator is started up but the use of the water can be minimised or completely eliminated because the fluidised bed 22 will be maintained at temperatures below the fusion point of the bed material by reason of the extraction of heat from the fluidised bed 22 by the tubes 146 and 118.

With the present invention, it is possible to recover the inorganics from the black liquor in a fluidised bed without continuously spraying water into the space above the bed to limit the temperatures. Further, the concentrations of the black liquor can be higher than in the fluidised beds of the prior art. This is advantageous because the higher concentration makes for a black liquor of higher heating content and consequently more available energy.

At certain times during operation it may be necessary to supply heat to start or continue the combustion within the steam generator 10. Heat is supplied by adding a suitable fuel to be bumed within the fluidised bed 22 to supply heat.

Claims (14)

1. A steam generator comprising a chamberfor a bed of fluidised particles of inorganic materials derived from waste liquor; means for introducing waste liquor containing inorganic materials into the chamber; means for flowing fluidising gas upward through the chamber; means for withdrawing particles from the chamber; and a plurality of tubes for carrying a fluid in heat exchange relationship with a fluidised bed in the chamber to extract heat therefrom and to evaporate said fluid.

2. A steam generator according to Claim 1 wherein the waste liquor introducing means is a sprayer positioned in the chamber at a location above that of a said fluidised bed contained therein.

3. A steam generator according to Claim 1 wherein the waste liquor introducing means is positioned in the chamber at a location below the top of a said fluidised bed contained therein.

4. A steam generator according to any preceding Claim wherein the chamber is defined by four vertical walls comprising a plurality of said tubes for flowing fluid in heat exchange relationship with said fluidised bed.

5. A steam generator according to Claim 4 wherein the tubes comprising said walls are spaced apart with each tube being joined to adjacent tubes by vertically extending fins.

6. A steam generator according to Claim 4 or Claim 5 including a division wall extending vertically within the chamber, which division wall comprises a plurality of vertically extending tubes, said tubes being dimensioned so that the lower portions thereof will extend through a said fluidised bed contained in the chamber.

7. A steam generator accqrding to Claim 6 wherein the majority of the tubes of the division wall contact adjacent tubes on either side thereof.

8. A steam generator substantially as described herein with reference to and as illustrated by Figures 2 and 3 of the accompanying drawings.

9. A system for treating waste liquor and recovering inorganic materials therefrom, comprising a steam generator having a chamber for a fluidised bed of particles of inorganic material derived from such waste liquor, and a plurality of tubes for placing a fluid in heat exchange relationship with a said fluidised bed to remove heat from a said fluidised bed and evaporate a said fluid; a collector connected to said steam generator and adapted to receive from the chamber exhaust gases and entrained solids, and separate such solids from such exhaust gases; and means for returning such separated solids to the chamber.

1 0. A system according to Claim 9 wherein the returning means comprises a screw conveyor.

11. A system according to Claim 9 or Claim 10 including a wet scrubbing system connected with the collector to receive therefrom said exhaust gases and remove particulate matter before exhausting said exhaust gases.

12. A system according to any of Claims 9 to 11 including a classifierforreceiving particles from the chamber and separating the largerfrom the smaller particles; and means for conveying smaller particles to the retuming means.

13. A system according to any of Claims 9 to 12 including a preheater for preheating fluidising gas and meansforforcing such fluidising gas upwards through the chamber to maintain a said bed fluidised.

14. Asystem fortreating waste liquor substantially as described herein with reference to and as illustrated by the accompanving drawings.