CA1127479A

CA1127479A - Waste liquor fluidized bed steam generator

Info

Publication number: CA1127479A
Application number: CA328,267A
Authority: CA
Inventors: William D. Stevens
Original assignee: Foster Wheeler Energy Corp
Current assignee: Foster Wheeler Energy Corp
Priority date: 1978-06-29
Filing date: 1979-05-24
Publication date: 1982-07-13
Also published as: GB2030688B; GB2030688A

Abstract

WASTE LIQUOR FLUIDIZED
BED STEAM GENERATOR

ABSTRACT OF THE INVENTION
A fluidized bed system for oxidizing waste liquors containing organic and inorganic materials and for generating steam. The waste liquors are oxidized in a fluidized bed furnace which has a number of water cooled tubes for placing water within the tubes in heat exchange relationship with the fluidized bed to generate steam.

Description

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B/~CK~;RO[.iND OF ri'~lE INVr;N'I'ION
__ _______ In the manufacture o~ pu]p, certain orc~anic compoullds are separated fram the cellulose components by cooking the raw materials in cooking liquors. The cooking liquors include inter alia inor~anic salts SUCil as sodium sulEide, sodium bisulfite, sodium hydroxide, sodium carbonate, calcium bisulfite, magnesium bisulfite, sodium sulfite and sodium bicarbonate. The cooking process produces a waste li~uor which is ~nown as "brown liquor" or "black liquor". The black liqùors eontain lignin and other organic compounds; and therefore, the black liquor has calorific value. The black liquor also contains other chemieals which are not completely consumed in the cooking operation. The organic compound and unconsumed chemieals 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 5-15~ by weight.
The inorganic salts are rather expensive; and there-fore, from a economic viewpoint, it is advantageous to reclaim ~0 them. In the past the black liquor has been concentrated and then smelted at temperatures of approximately 1700F 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 chemicals which can be used again in the pulping process. Under these conditions, hydrogen sulfite and sulfur dioxide are usually released and must be recovered to avoid air pollution. Since the temper-atures are above the fusion point of the inorganic chemicals, ,t a portion of these chemicals are usually vaporized and must be ~.~

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separated from the remainder of the exhaust gases in orderto be recvvered.
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 oE the problems set forth above have been overcome by treating the black liquor in a fluidized bed oxidation process which is shown and described in U.S. Patent No.
3,309,262 to George G. Copeland and John E. Hanway, Jr. The patent discloses a process in which the black liquor is sprayed over a fluidized bed which is made up of solid parti-cles of residual inorganic chemicals derived from the black liquor. An oxygen containing fluidizing gas is passed up-ward through the bed to maintain the particles in a fluidized state. under these conditions a portion of the black liquor will evaporate and another portion of the black liquor will depGsit on the particles. At least a portion of the fluidized bed is maintained at a temperature abovc the combustion 2n temperature of the concentrated black li~uor but below the fusion temperature of the particles. Such a temperature would probably be within the range of from approximately 1000F to 1800F and often below approximately 16oooF. Under these conditions the carbon content of the or~anic material is burned off and agglomerates of residual inorganic material are formed which are heavy enough to stay within the fluidized bed and not become entrained in the fluidizing gas to pass out of the bed.
The agglomerates are yrown to a size where they can be taken out o~ the fluidized bed and eventually recycled to the pulping ~0 operation.

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Such a process is not without its disadvantages. For one, the heat energy generated by the fluidized 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 sufficlent water available to keep the temperatures within the bed within the desired range.

SUMMARY_OF THE INVENTION

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 fluidized bed made up of particles comprising inorganic materials recovered from black liquor, a means for injecting the black liquor into the fluidized bed, means for flowing a fluidizing gas upward through the fluidized bed to maintain it in a fluidized state and water tubes which carry water in heat exchange relationship with the fluidized bed to generate steam and maintain the fluidized bed at a temperature below the fusion point of the inorganic materials.

More particularly, in one aspect the invention pertains to a steam generator adapted to contain a bed of fluidized particles of inorganic materials derived from waste liquor. The generator includes means for introducing waste liquor containing inorganic materials and having more than 45 percent by weight solids into the steam generator, means for flowing fluidized gas upward through the steam generator, and means for withdrawing particles from the steam generator. A
plurality of tubes are provided for flowing water in hea-t exchange relationship with the fluidized bed to extract heat therefrom, the tubes providing sufficient exchange surface to 7~79 maintain a portion of the bed at a temperclture of between 1000F
and 1800F and to evaporate the water.

Another aspect of the invention comprehellds a process for oxidizing black liquor from a paper pulp operation in a fluidized bed steam generator made up of solid particles, the particles being substantially entirely of the inorganic components of the black liquor. The process comprises the steps of providing black liquor so that it contains solids at a concentration of more than 45 percent by weight, dlrecting fluidizing gas upward through the bed, the gas containing enough oxygen to support oxidation of substantially all of each of the organic components of the liquor in the bed, directing the black liquor into the bed, indirectly exchanging heat from the bed to a cooling medium to maintain the bed at a temperature of between approximately 1000F to 1800F and below the temperature at which the inorganic components of the liquor fuse, whereby the inorganic components of the liquor form a number of the particles, and deposit on the previously formed of the particles.

~0 BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a diagrammatic view showing an embodiment of the present invention;
Fig. 2 is a view partly in section showing the steam generator of Fig. l; and Fig. 3 is a top view in section taken substantially along the line 3 - 3 of Fig. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in Fig. l, the present invention incIudes a reactor in the form of a fluidized bed steam generator l0 which receives oxygen containing fluidizing gas from conduit 12 : ' :

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leadin~ from a pre-heater 14 which receives the fluidizing gas, which may be air, under pressure frorn blower 16~ The amount and velocity of the fluidizing gas can be controlled b~ means of a valve 18 which is placed in a conduit 20 which allows air to pass to the blower 16 from the atmosphere.
The fluidizing gas passes upward through a fluidized bed 22 to maintain the particles thereof in a fluidized state and supply oxygen to support combustion.
Exhaust gases containing the products o~ combus-tion and possibly water vapor along with some entrained solidfrom the fluidized bed 22 pass out of the steam generator 10 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 yent 32. The particulate matter removed from the exhaust gases passes down through conduit 34 to a screw conveyor 36 which conveys the particulate matter back into the fluidized bed 22 within the steam ;~
generator 10.
As will be explained presently, material is con-tinuously discharged from the fluidized bed 22. Makeup material is 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 ma~ter is carried in a medium o~ dilute black liquor which is introduced into the wet 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 conduic 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 concentrated liquor is available throu~h 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 fluidized bed 22. It can be introduced above the fluidized bed 22 or at a level below the top oE the fluidized bed 22. If it is desired to introduce it above the fluidized bed, a valve 78 in the 20 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 pressurizes 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 in]ect the liquor at a level below the top of the ~luldized 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 z~

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 fluidized 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. Similar-ily, the sprayer 96 receives water through a conduit 102 which is pressurized by means of compressed air supplied through a conduit 104 which connects with the conduit 102.
The particles grow in the fluidized bed 22 during combustion as the inorganic components from the black liquor ' deposit on the particles. Dry particles are continuously discharged from the fluidized bed steam generator 10 through a conduit 106 into a classifier 108. The rate o discharge is controlled by a valve 110 which is positioned in the conduit 106. Particles are classified according to size with the ~0 larger particles being discarded or treated chemically torecover the inorganics. The smaller particles are returned to the fluidized bed through a conduit 112 which connects with the conduit 38 so that the smaller partlcles will drop into the hopper 40 to be conveyed by the screw conveyor 36 back into the fluidized bed 22.
The system described above functlons ln a manner sub-~, stantially similar to the system described in U.S. Patent 3,309,262. -In the past where fluidized beds were used to recover the inorganic chemicals derived from the black liquor, such as

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in the sys~em shown in U.S. Patent No. 3,309,262, the liquorwas introduced at a concentration of approximately thirty-five percent (35~) solids, and additional water was made available to maintain the temperature within the fluidized 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 secondarily, i-t 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 Figs. 2 and 3 includes a series of tubes in which water is heated and evaporated into steam. The front and rear walls 114 and 116 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 122 is also made up of the tubes 118 with interconnecting fins 120 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 124. 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 118 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 , .

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rear walls 114 and 116 and to side walls 128 and 130 by a header 132 which surrounds the furnace. The header 132 receives water from a feeder tube 134 which is connected with a downcomer 136 extending downward from a water drum 138. The water drum 138 receivqs 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 146 which make up a partition wall 147 as shown best perhaps in Fig. 3. Only some of the water tubes 146 are shown in Fig.
2 for clarity, it being understood that in the preferred embodi-ment they extend over the width of the steam generator 10 be-tween the side walls 128 and 130. As shown in Fi~. 3 the water tubes 146 are spaced so that they each touch contiguous tubes.
This arrangement is due to the fact that the water tubes 146 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 152 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 steam drum 124 and flows down through the downcomer tubes 148 to the water drum 138. The water then flows downward through the downcomer 136 to enter the headers 132 and 144. The water from the header 132 then flows upward through the water tubes 118 and the water from the heac1er 144 flows upward through the water tubes 146.
The water in the tubes 118 and 1~6 is converted into steam or a steam-~later mixture and enters the steam drum 124. Steam is taken off through a steam pipe 154"and delivered to the point of use.
Water in the steam-water mixture in the steam drum 12~ 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 steam or a 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 150 between the duct plate 126 and the roof 122.~ Since heat is extracted by the tubes 150 before thè gases reach the tubes 148, the water in the tubes 148 will not be heated as much as water within the tubes 150. Thus, the tubes 148, in effect, serve as heated downcomers. The exhaust gases leave the steam ~enerator 10 through the duct 24.
The black liquor is sprayed in through the sprayer ~0 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 min1mized or completely eliminated because the fluidized bed 22 will be maintained at temperatures below the fusion point of the bed material by reason of the extraction of heat from the fluidized bed 122 by the tubes 146 and 118. ~
With the present invention, it is possible to recover the inorganics from the black liquor in a fluidized bed without continuously spraying water into the bed to limit the .

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temperatures. Further, the concentrations of the black liquor can be higher th~n in the fluidized beds of the prior ar-t.
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 fuel into the fluidized bed 22. For example, natural gas can be injected with the fluidizing gas and burned. Other fuels such as fuel oil or coal can be burned within the fluidized bed 22 to supply heat.
The foregoing describes but one preferred embodiment of the present invention. Other embodiments being possible without exceeding the scope of the present invention as defined in the following claims:

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Claims

WHAT IS CLAIMED IS:

1. A steam generator adapted to contain a bed of fluidized particles of inorganic materials derived from waste liquor comprising:
means for introducing waste liquor containing in-organic materials and having more than 45 percent by weight solids into said steam generator;
means for flowing fluidized gas upward through said steam generator;
means for withdrawing particles from said steam generator;
and a plurality of tubes for flowing water in heat exchange relationship with said fluidized bed to extract heat therefrom said tubes providing sufficient exchange sur-face to maintain a portion of said bed at a temperature of between 1000°F and 1800°F and to evaporate said water.

2. The steam generator defined in claim 1 wherein said means for introducing waste liquor is a sprayer positioned above said fluidized bed.

3. The steam generator defined in claim 1 wherein said means for introducing waste liquor is positioned below the top of said fluidized bed.

4. The steam generator defined in claim 1 wherein said steam generator has four vertical walls to define an enclosure in which is contained said fluidized bed, said walls comprising a plurality of said tubes for flowing water in heat exchange relationship with said fluidized bed.

5. The steam generator defined in claim 4 wherein said tubes comprising said walls are spaced apart with each tube being joined to adjacent tubes by vertically extending fins.

6. The steam generator defined in Claim 4 further comprising a division wall extending vertically within said enclosure, said division wall comprising a plurality of vertically extending tubes, said tubes being dimensioned so that the lower portions thereof will extend through said fluidized bed.

7. The steam generator defined in Claim 6 wherein the majority of said tubes of said division walls contact adjacent tubes on either side thereof.

8. A system for treating waste liquor and recovering inorganic materials therefrom, comprising:
a steam generator adapted to contain a fluidized bed of particles of inorganic materials derived from said waste liquor, said steam generator having means for introduc-ing waste liquor containing inorganic materials and having more than 45 percent by weight solids, means for flowing fluidized gas upward through said steam generator; means for withdrawing particles from said steam generator; a plurality of tubes for placing water in indirect heat exchange relationship with said fluidized bed and providing sufficient heat exchange surface to remove enough heat from said fluidized bed to maintain at least a portion of said bed at a temperature of between 1000°F and 1800°F and generate steam;
a collector connected with said steam generator and adapted to receive from said steam generator exhaust gases and entrained solids and to separate said solids from said exhaust gases; and means to return said separated solids to said fluidized bed.

9. The system defined in Claim 8 wherein said means for returning solids to said fluidized bed comprises a screw conveyor.

10. The system defined in Claim 8 further comprising a wet scrubbing system, said wet scrubbing system being con-nected with said collector to receive therefrom the exhaust gases from said collector and remove particulate matter before exhausting said exhaust gases to the atmosphere.

11. The system defined in Claim 8 further comprising a classifier for receiving particles from said fluidized bed and separating the larger from the smaller particles;
and means to convey said smaller particles to said means to return said separated solids to said fluidized bed.

12. The system defined in Claim 11 further comprising a preheater fox preheating fluidizing gas upward through said steam generator and maintain said fluidized bed in a fluidized state.

13.A process for oxidizing black liquor from a paper pulp operation in a fluidized bed steam generator made up of solid particles said particles being substantially entirely of the inorganic components of said black liquor, comprising the steps of:
a) providing black liquor so that it contains solids at a concentration of more than 45 percent by weight;
b) directing fluidizing gas upward through said bed, said gas containing enough oxygen to support oxidation of substantially all of each of the organic components of said liquor in said bed;

c) directing said black liquor into said bed;
d) indirectly exchanging heat from said bed to a cooling medium to maintain said bed at a temperature of approximately 1000°F to 1800°F and below the temperature at which the inorganic components of said liquor fuse;
whereby the inorganic components of said liquor form a number of said particles, and deposit on the previously formed of said particles.

14. The process defined in Claim 13 wherein said cooling medium is aqueous whereby steam is produced.

15. The process defined in Claim 14 wherein said liquor is introduced into said bed by being sprayed over the top of said bed.

16. The process defined in Claim 14 wherein said liquor is introduced into said bed by being flowed into said bed at a level below the top thereof.