CA1263518A

CA1263518A - Use of waste solids from fluidized bed combustion processes for additional flue gas desulfurization

Info

Publication number: CA1263518A
Application number: CA000410060A
Authority: CA
Inventors: Robert D. Stewart; Robert L. Gamble
Original assignee: Foster Wheeler Energy Corp
Current assignee: Foster Wheeler Energy Corp
Priority date: 1981-09-23
Filing date: 1982-08-25
Publication date: 1989-12-05
Also published as: JPS5858134A; NL8203672A; GB2107207B; GB2107207A

Abstract

THE USE OF WASTE SOLIDS FROM FLUIDIZED
BED COMBUSTION PROCESSES FOR ADDITIONAL
FLUE GAS DESULFURIZATION.

ABSTRACT OF THE DISCLOSURE
A process for obtaining additional flue gas desulfurization using waste solids from a fluidized bed combustion system in which sulfurous fuels are burned in a bed of acceptor particles. The process comprises the following steps: (1) withdrawing coarse waste particles from the fluidized bed combustion system; (2) subjecting the coarse waste particles to mechanical grinding means for attriting and disintegrating whereby the waste particles are reduced to a finely divided chemically reactive state;
and, (3) injecting the finely divided waste particles into the fluidized bed combustion system whereby additional flue gas desulfurization is achieved.

Description

i3~ii~i!3 E~ACKGROUND OF THE INVENTION
-1) Field of the Invention The invention is in the field of fuel burning fluidized bed combustion systems. Plore particularl~, the invention relates to methods of using waste solids from A
fluidized bed combustion process for obtaining additional flue gas desulfurization.

2) Description o'f'the Prior Art Fluidized bed combustion systems in which sulfurous fuels are burned in a bed of acceptor (such as limestone, dolomite, etc.) for the purpose of reactin~ with or capturing sulfur oxides wi~h the acceptor's alkaline compounds ~CaO, Na2O, K2O) characteristically utilize the acceptor inefficiently, Furthermore, where solid fuels, with ashes containing considerable alkaline value, are burned in fluidized bed combustion systems, the process produces a flyash which is more chemically reactive than that produced by conventional firing techniques. Although such ashes may have considerable potential for sulfur capture, they are similarly utilized inefficiently. For example, where bituminous coal is fired with limestone acceptor, the theoretical alkaline value of the acceptor is only 25 to 50% utilized and that of the ash only 10 to 20~.
SUMMARY OF THF`INVENTI'ON
An object of the present invention is to improve the economic performance of the fluidized bed combustion process by more completely utilizing the alkaline chemical value in the usual wastes produced. The fluidized bed combustion process must f~rst act as a calciner for acceptors producin~ oxides from carbon'ates, hydrates, bicar~onates, etc.

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In most conventional dry or wet flue gas desulfurization processes, a premium must be paid for finely ground and/or pre-calcined acceptor. By means of the low cost improvements provided by the present invention, the fluidized bed combustion process is capable of producing finely ground and pre-calcined acceptor.
The invention is a flue gas desulfurization process utilizing the waste solids from a fluidized bed combustor, the waste solids having additional alkaline chemical value, to effect the capture of sulfur oxides in the combustion products. Calcined limestone ac~s as an alkaline acceptor in the fluidized bed combustor and is removed from the fluidized bed as fines :in the effluent or as waste solids from the tap of the fluidized bed combustor. The coarser waste solids are combined with the fines and the mixture reduced to a finely divided state before being re-introduced into the fluidized bed in a powder, slurry or solution form.
Alternatively, the mixture can be used as a gas scrubbing media in wet and/or dry scrubbers.
Thus, broadly, the invention contemplates a process for obtaining additional flue gas desulfurization using waste solids from a fluidized bed combustion system ~' .

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in which sulfurous fuels are burned in a bed of acceptor particles, cornprising the steps of withdrawing coarse waste particles from the fluidized bed combustion system, subjecting the coarse waste particles to means for attriting and disintegrating whereby the coarse waste particles are reduced to finely divided particles having increased available alkaline chemical value, and, injecting the finely divided waste particles into the fluidized bed combustion system whereby additional flue gas desulfurization is achieved, and wherein fine dust particles are collected from the fluidized bed combustion system and the fine dust particles are combined with the finely divided particles produced in the subjecting step and injected into the fluidized bed combustion system in that step.

~RIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic process diagram illustrating the - general principles of the invention.
Fig. 2 is a schematic process diagram illustrating the use of fluidized bed combustion wastes to improve the performance of the fluidized bed combustion (FBC) process acting as a "dry scrubber" and/or to scrub gases from other conventional combustion units. This embodiment of the invention uses a slaker/hydrator to produce a thick alkaline slurry for pressure injection at various points in the fluidized bed combustion system and in a dry scrubber.

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Fig. 3 is a schematic process diagram illustrating the use of fluldized bed combustion (FBC) wastes to improve the performance of the fluidized bed combustion process acting as a "dry scrubber" an~for to scrub gases from other conventional combustion units. This embodiment of the invention uses fluidized bed combustion wastes in pneumatically injected dry powder form.
Fig. 4 is a schematic process diagram illustrating the use of fluidized bed combustio~ (FBC) wastes to produce an alkaline scrubbing liquor for 1ue gas desul~urization utilizing final con~entional wet scrubbin~ processes.
DESCR~PTION OF THE PREFERRED EMBODIMENTS
In the present invention, processes are provided for reducing fluidized bed combustion waste solids to a finely divided chemically reactive state in a dry powder form, in a thick alkaline slurry form, or in a thi~ alkaline solution form for use as follows:
(a.) As an improvement of the basic fluidized bed combustion process by injecting activated wastes into: the bed;
the freeboard; the gas collection column tmultibed units);
before primary dust collection; and, before final dust collection tas with dry scrubbing~
~b.) For use as a scrubbing media for other conventional combustion systems producing sulfur oxide laden gases. This use may be in conjunction ~ith or independent from the fluidized bed combustion unit producing the media. Where ashes from conventional systems are reactive, they can be integrated into the process.
Thus, the fluidized bed combustion wastes, which are normally produced, can be activated and used to .,,~.~ ' '' i ~3~8 improve the sulfur removal efficiency and the acceptor utilization efficiency of the basic 1uidized bed combus-tion ~rocess.
Furthermore, the fluidized bed combustion wastes can be used to prepare effecti~e dry or wet scrubbing media for use in desulfurizing gases from conventional combustion systems in conventional scrubbing apparatus such as dry scrubbing apparatus (using proven spray dryer designs) and wet scrubbing apparatus (usin~ ventu~i designs or tray designs~.
Referring to Fig. l, the drawing is a schematic diagram illustrating the general principles of the inventive process using fluidized bed combustion wastes for flue gas desulfurization in conventional combustion systems and to improve the desulfurization performance of fluidized bed combustion systems.
In fluidized bed combustion (FBC) system lQ, fuel particles 14 (such as coall are burned using air 12 in a fluidized bed of acceptor particles 16 (such as limestone~
which normally have a particle size range of l/8" x 0~ FBC
system 10 acts as a base calciner for acceptor particles 16, thereby producing alkaline compounds (such as CaO, Na2O, K2O) which react with and capture sulfur oxides produced by the combustion process.
FBC system 10 produces alkaline wastes in two forms: first, as fine size dust particles ~flyash) 18 which are collected from the flue gas; and, second, as coarse size waste particles 20 which normally have a size ran~e of l/8"
x 0 which come out from the tap or drain of system 10. FB~
system 10 produces exhaust g~ses ll laden with SO2 and SO3 which, without the present invention, may exceed environmental L

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limits. Adjacent to FBC system 10, there may be a con~entional combustion system 22, in ~hich fuel particles 26 are burned using air 24, thereby producing conventional ash particles 28 which also may have high alkaline scrubbing value.
Coarse waste particles 20 and conventional ash particles 28, if ha~ing high alkaline value, are fed into mechanical grinding means 30 fQr attriting and disintegrating the waste particles and the ash particles to produce finely divided particles 33. Grinding means 30 may be, for example, a ball mill or a roll mill. Finely divided particles 33 would preferably have a particle size ran~e where 70 to 90%
by weight pass through a two hundred mesh sieve. Alternatively, Fig. 1 shows that coarse waste particles 20 and ash particles 28 may be ~ed into a treatment vessel where they are subjected first to water/steam treatment 32 ahead of mechanical grinding means 30. Water/steam treatment 32 explodes hot waste particles 20 and ash particles 28 in order to assist in producing finely divided particle~ 33.
In one embodiment of the invention, Fig. 1 shows ~ that dry finely divided particles 33, now having increased al~aline value, are then combined with fine dust particle.s 18 and injected through feed line 40 into fluidized bed combustion system 10 to enhance desulfurization in system 10. Dry finely divided particles 33 and fine dust particles 18 may also be injected through ~eed line 41 into dry scrubber 42 which removes sulfur oxides from exhaust gases 11. The exhaust gases 11 then pass to final dust ~emoval means 52 where dry waste solids 56 are produced and removed and where clean gases 54 exit to the atmosphere.

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In another embodiment of the inven~ion, Fig. 1 shows that dry finely divided ~articles 33 and fine dust particles 18 can be combined and fed into slaker/hydrator 34 where they are mixed with water 36 to produce thick alkaline slurry 38, containing calcium hydroxide. Thick alkaline slurry 38 is injected through feed line 40 into fluidized bed combustion system 10 and throu~h feed line 44 into dry scrubber 42 to react with and capture sulfur oxides in exhaust gases 11.
The injection of dry finely divided alkaline waste particles 33 or the injection of thick alkaline slurry 38 into fluidized bed combustion system 10 may improve the direct desul~urization performance enough to obviate the necessity for final gas scrubbing in most cases.
In another embodiment of the invention, Fig. 1 shows that dry finely div:ided particles 33 and fine dust particles 1~ can be combirled and fed into ~laker/hydrator 34 where they are mixed with water 36 to produce thin alkaline scrubbing liquor 44, containing calcium hydroxide. Thin alkaline scrubbing liquor 44 is injected into wet scrubber 46 to remove sulfur oxides from exhaust gases 11 to produce clean gases 50 and spent sludge 48 is removed.
Re~erring to Fig. 2, the drawing shows in more detail one embodiment of the inventive process tshown generally in Fig. 1) using a slaker~hydrator to produce a thick alkaline slurry for pressure injection at several points in fluidized bed combustion system 60 having one or more main cells which have freeboard zone 62 above fluidized bed 64 composed of fuel particles 7Q and acceptor particles 30 72 which are fluidized by air 68 flowing up through grid 66.

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~ xhaust gas 81 passes out through gas column 74 and then to dust collec~r 76 where fi~e dust particles (flyas~ 77 having alkaline value are remo~ed and reinjected into fluidized bed 64. A~ternatively, some of fine dust particles 77 may be con~eyed to spent material ~oldin~
vessel 98. After leaving dust collector 76, exhaust gas 81 then passes through air heater 100 and into gas/solids contactor dry scrubber 102. Exhaust gas 104, at a temperature above the dew point, then passes through bag house lOB where dry waste solids 112 are remo~ed and clean gases exlt to the atmosphere through stack 110.
As shown in Fig. 2, the plant may include a carbon burnup cell ~CBC) 78 ha~ing freeboard zone 80 above fluidized bed 82 which is fluidized by air 86 flowing up through grid 84. Some of fine dust particles 77 may also be injected into fluidized bed 82 to enhance desulfurization in that bed.
Coarse waste particles 61 ~rom main cell 60 are removed through the tap/drain of the cell and are conveyed to attritor/disintegrator cooler 90 where hot waste particles 61 (typically at a temperature of 1000 to 1500F.) are cooled by air flow 92. Coarse waste particles 83 from carbon burnup cell 78 are similarly removed through the tap/drain of that cell and conveyed to attritor/disintegrator cooler gO. Reactive flyash 88 from any adjacent conventional combustion systems may also be fed into the top of attritor/
disintegra~or cooler 90.
The combined coarse wast~ particles flow down through attritor/disintegrator 90 and are subjected to mechanical grinding means 94 whereby the particles are reduced to a particle size where preferably 70 to 90~ by weight will pass through a two hundred mesh sie~e. The finely divided particles are then pneumatically conveyed by air flow 96 back into the top of attritor~disintegrator ~0 where they are combined with ~inely divided reactive flyash 88. The combined finely di~ided particles ~7 are then pneumatically conveyed to spent material holding vessel 98.
The finely divided particles then pass out the bottom of holding vessel 9~ and are fed into slaker~hydrator 114 where they are mixed with water to produce thick alkaline slurry 118 containing calcium hydroxide. If required, the particles may be subjected to a wet grinding process by grinding means 116 and pumped back into hydrator~slaker 114~
Thick alkaline slurry 118 is then pumped through feed line 117 to gas/solids contactor dry scrubber 102 where the slurry is fed in by ~ny suitable conventional injection means such as a system oE nozzles. Simultaneously, thick alkaline slurry 118 is pumped through feed line 115 to fluidized bed combustion system 60 where the slurry is inject~d at several points: at the level of bed 64, at the level of freeboard zone 62t into gas column 74, and into dust collector 76. Any suitable conventional injection means, such as nozzles, may again be used for this purpose.
The thick alkaline slurry, which quickly dries to a fine powder, reacts with and captures sulfur oxides, thereby directly enhancing the performance of the fluidized bed combustion system (acting as a "~ry scrubber"~.
Referring to Fig. 3, the drawing shows in more detail another embodiment o~ the inventi~e process (shown generally in Fig. 1~ wherein fluidized bed combustion wastes are reduced to a finely divided state and then pneumatically 5~

injected in dry powder form. Elements in Fig. 3 which are the same as in Fig. 2 bear the same reference numerals.
Fluidized bed combustion system 60 has one or more main cells which have freeboard zone 62 above fluidized bed 64 composea o~ uel particles 70 and acceptor particles 72 which are fluidized hv air 68 flowing up through grid 65.
Exhaust gas 81 passes out through ~as column 7~ then to dust collector 76 where fine dust particles (flyash) 77 havlng alkaline value are removed and reinjected into fluidized bed 64. Exhaust gas 81 then pas~es through air heater 100 and into gas/solids contactor dry scrubber 122. Exhaust gas 104, at a temperature above the dew point, then passes through bag house 108 where dry waste solids 112 are removed and clean gas e~its to the atmosphere.
As shown in Fig. 3, the plant may include a carbon burnup cell (CBC) 78 having freeboard zone 80 above fluidized bed 82 which is fluidized by air 86 flowing up through grid 84. Some of fine dust particles 77 may also be injected into fl~idized bed 82 to enhance desulfurization in that bed.
Coarse waste particles 61 from main cell 60 are removed through the tap/drain of the cell and are conveyed to attritor/disintegrator cooler 90 where hot waste particles 61 are cooled by air 1OW 92. Coarse waste particles 83 are similarly removed from the tap/drain of carbon burnup cell 78 and conveyed to attritor~disintegrator 90. Reactive flyash 88 from any adjacent conventional combustion systems may also be fed into the top of attritor~disintegrator 90.
The com~ined coarse waste particles flow down through àttritor/disintegrator 90 and are subjected to mechanical grinding means 94 whereby the particles a~e reduced to a particle size range where preferably 70 to ~Q
percent by weight will pass through a two hundred mesh sieve. Alternatively, the coarse waste particles may be subjected to high velocity steam/water injection treatment 120 in attritor/disintegrator cooler 90. The steam/water injection treatment performs an in situ/remote slaker~hydrator function to make a finely divided lime product for injection purposes. It should also be understood that attritor/dis-integrator cooler 90 shown as an external apparatus in Fig.3 may be constructed to be integral with the fluidized bed combustion system.
The finely divided particles are then pneumatically conveyed by air flow 96 back into the top of attritor/
disintegrator 90 where they combine with reactive 1yash 88.
The combined finely divided particles are then pneumatically conveyed to and injected into gas~solids contactor dry scrubber 122 through any suitable injection means. Slmul-taneously, finely di~ided dry particles 124 are pneumatically injected into fluidized bed combustion system 60 at the level of bed 64, at the level of ~reeboard zone 62, into gas column 74, and into dust collector 76. Any suitable conven-tional injection means may again be used for this purpose.
The finely divided dry particles react with and capture sulfur oxides, thereby directly enhancing the performace of the fluidized bed combustion system acting as a "dry scrubber".
Referring to Fig. 4, the drawing sho~s in more detail another embodiment of the inventive process (shown generally in Fig. 1~ wherein ~luidized bed combustion wastes are used to produce a thin alkaline scrubbing liquor o~
solution for flue gas desulfurization utilizing conventional wet scrubbing processes.
It should be understood that certain elements are not shown in Fig. 4 for the sake of clarity. Thus, Fig. 4 omits the following common elements shown in Figs. 2 and 3:
fluidized bed combustion system 60, having one or more main cells ~0 which have freeboard zone 62 above fluidized bed 64 composed of fuel particles 70 and acceptor particles 72 which are fluidized by air 68 flo~ing up through grid 66;
exhaust gas 81 which passes through gas column 74 and then to dust collector 76 where fine dust paxticles (flyash) 77 having alkaline value are removed and reinjected into fluidized bed 64; the plant may also include carbon burnup cell 78 having freeboard zone 80 above fluidized bed 82 which is fluidiz,ed by air 86 flowing up through grid 84;
some of fine dust part~cles 77 may be injected into fluidized bed 82; coarse waste particles 61 are removed through the tap/drain of main cell 60 and con~eyed to alkaline material holding vessel 130 shown in Fig. 4; coarse waste particles 83 are similarly removed through the tap~drain of carbon burnup cell 78 and conveyed to alkaline material holding vessel 130; and, fine dust particles 77 and reactive flyash 88 from any adjacent conventional combustion system are also fed into alkaline material holding vessel 130.
It should also be understood in connection with the embodiment shown in Fig. 4 that the sources of the coarse fluidized bed combustion waste particles, the fine dust particles, and the reactive flyash may be at a remote location. Therefore, the aforementioned material may be 5~

transported from the remote location and then be fed into alkaline material holding vessel 130.
As shown in ~ig. 4, pre-ground material 132 of a sufficien-tlv small particle size is conveyed directly Erom alkaline material holdin~ vessel 130 to slaker~hydrator agita-tor 140. Coarse particles of a larger size are ~ed from holding vessel 130 into attritorfdisintegrator 134.
The coarse waste particles flow down through mechanical grinding means 138 whereby they are reduced to a particle size range where preferably 70 to 9Q ~ by weight will pass through a two hundred mesh sieve. The finely divided particles are then pneumatically conveyed by air flow 136 into the top of attritor/disintegrator 134. Finely divided particles 135 are then pneumatically conveyed to holding vessel 139. A
portion of finely dividecl particles 135 are conveyed out the top of holding vessel 139 and combined with exhaust gas 81.
The remaining portion of finely divided particles 135 pass out the bottom of holding vessel 139 into slaker/hydrator agitator 140. In slakerfhydrator 140, the finely divided particles are mixed with water and subjected to wet grinder 142 as required in order to form alkaline scrubbing liquor 144 containing calcium hydroxide.
Alkaline scrubbing liquor 144 is then pumped to final conventional wet scrubber 146 where the scrubbing liquor is ed into sump 147 and into the scrubbing chamber by any suitable conventional injection means, such as spray nozzle 150, whereby the scrubbin~ liquor is contacted with sulfur oxide laden exhaust gas 81 and the sulfur oxides are removed. The scrubbing liquor from the bottom of sump 147 is recycled into the scrubbing chamber through spray nozzle 148. The spent scrubbing liquor is pumped to separator 156 where sludge waste 160 i5 removed and reclaimed water 158 is recycled to slakexthydrator 14~. The clean gas passes out through demister 151, which ls periodically washed down with water 152, and then the clean gas passes through. reheater means 154 and exits to the atmosphere.
The above-described embodiments are intended to be illustrative, not restrictive. The full scope of the inVention is defined by the claims, and any and all equivalents are intended to be embraced.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for obtaining additional flue gas desulfurization using waste solids from a fluidized bed combustion system in which sulfurous fuels are burned in a bed of acceptor particles, comprising the steps of:
(1) withdrawing coarse waste particles from said fluidized bed combustion system;
(2) subjecting said coarse waste particles to means for attriting and disintegrating whereby said coarse waste particles are reduced to finely divided particles having increased available alkaline chemical value;
(3) injecting said finely divided waste particles into said fluidized bed combustion system whereby additional flue gas desulfurization is achieved; and (4) wherein fine dust particles are collected from said fluidized bed combustion system and said fine dust particles are combined with said finely divided particles produced in step (2) and injected into said fluidized bed combustion system in step (3).

2. The process of Claim 1 comprising the additional step of injecting said finely divided particles into a final gas/solids contacting device.

3. The process of Claim 1 comprising the additional step of injecting said combined fine particles into a final gas/solids contacting device.

4. The process of Claim 1 wherein said coarse waste particles are reduced in step (2) to finely divided particles having a particle size range where 70 to 90% by weight will pass through a two hundred mesh sieve.

5. The process of Claim 1 comprising the additional steps of collecting ash particles from a conventional combustion system and subjecting said ash particles to said means for attriting and disintegrating whereby said ash particles are reduced to finely divided particles having increased available alkaline chemical value.

6. The process of Claim 1 comprising the additional step of subjecting said coarse waste particles to a high-velocity steam/water injection treatment to assist in producing said finely divided particles.

7. A process for obtaining additional flue gas desulfurization using waste solids from a fluidized bed combustion system in which sulfurous fuels are burned in a bed of acceptor particles, comprising the steps of:
(1) withdrawing coarse waste particles from said fluidized bed combustion system;
(2) subjecting said coarse waste particles to means for attriting and disintegrating whereby said coarse waste particles are reduced to finely divided particles having increased available alkaline chemical value;
(3) feeding said finely divided particles into a slaker/hydrator wherein said particles are mixed with water to form an alkaline slurry; and, (4) injecting said alkaline slurry into said fluidized bed combustion system whereby additional flue gas desulfurization is achieved.

8. The process of Claim 7 wherein fine dust particles are collected from said fluidized bed combustion system and said fine dust particles are combined with said finely divided particles produced in step (2) and fed into said slaker/hydrator in step (3).

9. The process of Claim 7 comprising the additional step of injecting said alkaline slurry into a final gas/solids contacting device.

10. The process of Claim 7 wherein said coarse waste particles are reduced in step (2) to finely divided particles having a particle size range where 70 to 90% by weight will pass through a two hundred mesh sieve.

11. The process of Claim 7 comprising the additional steps of collecting ash particles from a conventional combustion system, combining said ash particles with said coarse waste particles, and subjecting the combined ash particles and coarse waste particles to said means for attriting and disintegrating in step (2) whereby said combined ash particles and coarse waste particles are reduced to finely divided particles having increased alkaline chemical value.

12. The process of Claim 7 comprising the additional step of subjecting said alkaline slurry formed in step (3) to a wet grinding means to reduce the size of the particles in said slurry.

13. A process of Claim 7 comprising the additional steps of feeding some of said finely divided particles into a slaker/hydrator wherein said particles are mixed with water to form an alkaline scrubbing liquor; and, injecting said alkaline scrubbing liquor into a final conventional wet scrubbing apparatus whereby flue gas desulfurization is achieved.

14. The process of Claim 13 wherein fine dust particles are collected from said fluidized bed combustion system and said fine dust particles are combined with said finely divided particles produced in step (2) and fed into said slaker/hydrator in step (3).

15. The process of Claim 13 wherein said coarse waste particles are reduced in step (2) to finely divided particles having a particle size range where 70 to 90% by weight will pass through a two hundred mesh sieve.

16. The process of Claim 13 comprising the additional steps of collecting ash particles from a conventional combustion system, combining said ash particles with said coarse waste particles, and subjecting the combined ash particles and coarse waste particles to said means for attriting and disintegrating in step (2) whereby said combined ash particles and coarse waste particles are reduced to finely divided particles having increased available alkaline chemical value.

17. The process of Claim 13 comprising the additional step of subjecting said alkaline scrubbing liquor formed in step (3) to a wet grinding means to reduce the size of the particles in said liquor.