WASTE DISPOSAL BY FLUID BED INCINERATION AND ENERGY RECOVERY MODES

HENRY S. KWON Dorr-O liver Incorporated

Stamford, Connecticut

ABSTRACT

Fluid bed incineration is a unique and econom­ical process for disposing of wastes in an environ­mentally acceptable manner. Heat recovery methods within the process may reduce energy requirements and operating costs. Energy may also be produced for use outside the boundaries of the incineration system.

FLUID BED

A fluid bed is a dense, uniform suspension of solids maintained in a turbulent motion by an upward moving air stream. The turbulent mixture of air and solids behaves as if it were a fluid and possesses characteristics of a boiling liquid. The violent mixing action of the solids and gases results in uniform distributions of temperature, composition and particles within the bed for optimum contact between solids and oxygen, and provides a large surface area for rapid heat transfer.

COMBUSTION CHARACTERISTICS

OF FLUID BEDS

When the bed becomes fluidized, all the particles are suspended, with each individual particle fully exposed to the gas stream. This phenomenon makes available an extremely large surface area upon which reactions can take place. Combustible solids are dispersed rapidly through­out the bed, making intimate contact with

oxygen. This enables extremely rapid, complete combustion of solids and greatly reduces the need for long residence time. A fluid bed characteris­tically holds the combustible solids long enough to achieve an extremely high combustion efficiency.

Another important feature in fluid bed opera­tion is the enormous heat reservoir capacity of the bed. An incinerator of 15 ft (4.6 m) freeboard diameter, for example, contains about 8.5 x 106

Btu (8.96 X 103 MJ) in the bed during normal operation. In order to lower the bed temperature by 100 F (55.5 C) heat transfer of 0.6 x 106 Btu (0.63 X 103 MJ) will be required. The huge quantity of heat contained in the bed allows almost instantaneous start-up, usually with no auxiliary fuel, after the unit has been shut down for a short period.

FLUID BED INCINERATOR

The fluid bed incinerator normally consists of the windbox, the fluid bed, and the freeboard. Fluidizing air is introduced into the windbox and distributed uniformly to the bed area by tuyeres or orifices in the constriction plate or refractory dome. The bed becomes fluidized and oxygen is available for combustion.

Feed material to be incinerated normally is injected into the fluid bed and combustion takes place on the surface of individual particles. Heat resulting from combustion is absorbed by the bed material and, in turn, released to evaporate moisture from the reed and volatilize the organic

fractions. The highly turbulent motion of the bed provides an ideal environment fOf rapid and intimate contact of combustibles with the bed material at a high temperature. A typical bed is made up of inert material such as sand, sized from 20 to 80 mesh.

Inert ash and some bed material are elutriated from the bed by combustion gases. In the free­board, most solids disengage from the upward moving combustion gas stream and fall back to the bed. However, the off-gas stream always entrains some solids and the bed material is continuously depleted. In some cases ashes are generated during combustion, which could replenish the bed. Otherwise, a small amount of sand may have to be fed into the bed, from time to time, as make-up.

SLUDGE FEED

SLUDGE

EFFLUENT WATER DEWATERING

FLUIDIZING

AIR BLOWER

FREEBOARD

SEWAGE SLUDGE INC INE RAT ION SYSTEM

A conventional system for the incineration of sewage sludge involves auxiliary fuel burning, the quantity of fuel depending on the heating value of the sewage sludge and the amount of water in the sludge. When auxiliary fuel consumption is not acceptable, the conventional system has to be modified so that incineration can be autogenous with sludge solids only.

Conventional Sewage Sludge Incineration System

Conventional incineration systems are shown in Figs. 1 and 2. The system with a cold windbox (Fig. 1) has been used for the incineration of

SCRUBBING

WATER

VENTURI

SCRUBBER

IF REQUIRED

TRAY COOLING WATER

70°F

HOT WATER 150°F

SEPARATOR

AND TRAY COOLING

TO ASH

�:J-_______ DISPOSAL

ASH SLURRY

PUMP

F IG. 1. FLUID BED INC I NERATOR WITH CONVENT IONAL DEWAT ERING USING A COLD W INDBOX

SLubGE FEED

AIR

FLUIDIZING AIR BLOWER

SCRUBBING WATER

VENTURI SCRUBBER

TRAY COOLING WATER 70°F

HOT WATER 150°F

SEPARATOR AND

TO ASH �}----- DISPOSAL

ASH SLURRY PUMP

F IG. 2. FLUID BED INCINERATOR W ITH CONVENT IONAL DEWATERING USI NG A HOT WINDBOX

2

sludges with high heating value. With lower heat­

ing value sludges the auxiliary fuel consumption

becomes too high and a system with a hot (or

sometimes warm) windbox can be applied to

eliminate the need for auxiliary fuel burning or, at least, to minimize it. In this system, the sensible

heat of hot combustion gases is recovered and used to preheat the fluidizing air to 1000 F (538 C). With air preheating the system has an improved

thermal efficiency, thus requiring less system heat

input to evaporate water in the sludge. The system

heat input required to evaporate 1 lb of water is

2500 to 2600 Btu (5815 to 6048 kJ per kg water)

with the hot windbox compared with 3700 to

4100 Btu (8606 to 9536 kJ/kg water) with the

cold windbox. In the selection of the proper incineration

system for sludges, the dominant factor probably

is the capability of the system for dewatering the

feed sludge. The conventional sewage sludge

dewatering system uses a vacuum filter or a

centrifuge as the main solid-liquid separator.

Performance usually varies, depending on the type

of sewage sludges to be processed. With primary

sludge only, the dewatering system should produce

a cake of 25 to 30 percent solids. When waste

activated sludge is dewatered, the cake may contain only 10 to 15 percent solids. Handling

primary sludge mixed with waste activated sludge,

the solids content of the cake would range from 16 to 20 percent. When used with primary sludge

alone, a hot windbox system could provide autoge­

nous combustion when the heating value of the dry

FERRIC OILORIDE

HYDRATED LIME

solid is 6000 Btu/lb of dry solid (13,960 kJ/kg)

or higher. When waste activated sludge alone or in

combination with primary sludge is to be inciner­

ated, a hot windbox system will reduce the

auxiliary fuel requirements.

Modified Sewage Sludge I ncineration System As the need for energy saving becomes more

emphasized, there is an inevitable tendency to ban

the use of auxiliary fuel in sewage sludge inciner­

ators. Recently the State of Michigan took such an

action and other states seem to be heading for the

same direction. The conventional dewatering

systems may still be used for the disposal of

primary sludge if the incinerator is equipped with

a hot windbox. For the disposal of other types of

sewage sludge, however, it is apparent that the

conventional dewatering equipment will not

produce a cake with a high enough solids content

to support autogenous combustion.

The filter press has been employed in Europe as

a dewatering device for many years. Handling

sludges preconditioned with lime and ferric

chloride, the filter press can produce a cake with a

solids content of up to 50 percent. This is, in most cases, high enough to ensure autogenous combus­tion. In this regard, the filter press seems to offer

an attractive alternative to the conventional

vacuum filter or centrifuge. Fig. 3 shows a flow

sheet of the incineration system equipped with a filter press.

An important feature of the filter press system

, •

FREEBOARD

HEAT EXCHANGER

[FILTER I PRESS M,.. ____ r' \ / TO SCRUBBER

FEED

--

--

MIXING SLUDGE TOWER HOLDING

TANK

}-rro' L...

FEED PUMP

FLUID DRAG CONVEYOR �t[lJClBECl D:::a:rj

IEFF'LUEINT SCREW

FEEDER

/ '1....-1 \,. ./

FLUIDIZING AIR BLOWER

FIG.3. FLUID BED INCINERATOR WITH FILTER PRESS USING A HOT WINDBOX

3

is the addition of FeCh and Ca(OH)2 as sludge conditioning chemicals to improve dewatering. Depending on sludge characteristics, the amount of chemical dosage will vary. Primary sludge may need up to 3 kg of FeCh and 12 kg of Ca(OHh per 100 kg of dry solids feed.

To handle waste activated sludge, a higher dosage of chemicals may be re.quired for sufficient sludge conditioning, i.e., up to 10 kg of FeCl3 and 45 kg of Ca(OH)2 per 100 kg of dry solids feed. The conditioned sludge is fed into the filter press and then subjected to high pressure that forces water out through the ftlter cloth leaving behind the solids. Solids contents up to 50 percent in the cake are common and the need for auxiliary fuel consumption may be eliminated. For filter cake solids with high heating value, there is sufficient heating value to evaporate the water. When the heating value of the dry solids is relatively low, additional heat can be supplied by preheating the fluidizing air with sensible heat extracted from the hot off-gases, recovering otherwise wasted energy.

Figure 4 shows one example of a system opera­tion using a ftlter press. Although the incinerator temperature is maintained at around 1500 F (816 C), temperature variations during the opera­tion have to be anticipated due to changes in sludge characteristics. Temperatures between a minimum of 1400 F (760 C) and a maximum of 1650 F

(899 C) are considered acceptable. Suppose a ftlter cake of 5000 Btu/1b dry solids (11,630 kJ/kg) is burned in an incinerator. If the fllter press pro­duces a sludge cake with 45 percent solids, the operation would be autogenous with a cold wind­box at an operating temperature of 1500 F (816 C). If the fllter press produces a sludge cake with 42.2 percent solids, the combustion still can be auto­genous with a cold windbox, but the gas temper­ature would be 1400 F (760 C). If the same sludge (42.2 percent solids) is handled in an incinerator equipped with a warm windbox, the temperature of the autogenous combustion would rise to 1650 F (899 C). If the sludge contains only 36.5 percent solids and the incinerator is equipped with a warm windbox, the operation can be autogenous but the temperature would decrease to 1400 F (760 C). Equipped with a hot windbox, the incinerator temperature would rise to 1650 F (899 C).

Since the combustion involves a large amount of iron and lime, the possibility of scale formation in the duct work and heat exchanger should be given careful consideration. When phosphate is present in the sewage, ferric chloride sometimes is added to remove phosphate as precipitated iron phosphate. The iron phosphate then goes to the chemical conditioning stage with the sludge and more ferric chloride is added. At this' stage, the amount of iron in the sludge may become excessive. After the

OPERATING TEMPERATURE: CHEMICAL DOSAGE: FeCl, 5%

>< o '" Cl z: '"

V> Cl • --' Cl o W.Ll

o --'0 -0 o� ....

>- --' ex « ::;<0 --' >< => «

V> ex . <oJ Cl I­« .Ll 3:� ::1:0 wo z: 0 w� => .... o� '" '"

30

20

10

32

10

20

34

Min. 1400°F (760°C) Nor. 1500°F (815°C) Max. 1650°F (89goC)

7000 6500 6000

Ca(OH), 15%

HEATING VALUE % V.S. 4500 Btu/lb 47.5 5000 Btu/lb 52.6 5500 Btu/l b 57. g 6000 Btu/lb 63.2 6500 Btull b 68.4 7000 Btull b 73.7

(1000 Btu/lb = 2326 kJ/kg)

FIG. 4. INCINERATOR OPERATION WITH FILTER PRESS

4

REFUSE

RECYCLE

WATER PULPER

r""'-TI----- RECOVERED

METAL

JUNK

REMOVER WASHER

CLASSIFIER-

TO SCRUBBER

....-----1 CLEANER 1-__ RECOVERED

PACKAGE FIBER

FREEBOARD

LIQUID

CYCLONE

GLASS,

ALUMINUM EFFLUENT WATER SEWAGE

SLUDGE

AIR

COMPRESSOR

FLUIDIZING AIR BLOWER

F IG. 5. DOMESTIC REFUSE INCINERAT ION SYSTEM AS APPL IED TO HYDRASPOSAL SYSTEM (BLACK-CLAWSON)

reaction is completed in the bed, iron compounds could become a major cause of scaling due to softening of the compounds at high temperature. A way to prevent this undesirable phenomenon must be devised.

Domestic Refuse Incineration System

A fluid bed incinerator can also be applied to the disposal of domestic refuse. The first system to incinerate pulped refuse or pulped refuse mixed with sewage sludge was installed at the Solid Waste Recycling Plant in Franklin, Ohio (Fig. 5)

The operation of the front end of the plant is as follows:

Refuse is delivered to the plant by private contractors and individual citizens. The refuse is fed into a pulper by a conveyor. The pulper converts all pulpable and friable materials into a water slurry and separates metal from the slurry. Metal is recovered after washing in the junk remover-washer. The slurry is then fed into a liquid cyclone for separation of glass. The effluent from the liquid cyclone is sent to a screen classifier where removable fiber is mechanically separated. Nonrecoverable organics such as rubber, textiles, plastics, leather, food wastes, etc. are initially dewatered in a thickener. After this initial

5

dewatering, sewage sludge from the adjoining treatment plant can be mixed with the organic wastes for disposal. Final dewatering is accom­plished by a cone press, yielding a cake with a solids content of over 40 percent which is suitable for autogenous combustion. The incinerator is 25 ft (7.6 m) freeboard in size and a cold windbox type. Dewatered solid cake is broken down to small pieces and pneumatically conveyed to a single feed point.

The operation began in June, 1971. In the initial stage of operation, glass softened in the bed and started agglomerating the bed material to very coarse sizes. We were aware of possible problems if excessive glass entered the incinerator and the system was designed to reject the glass prior to incineration.

As it turned out, the liquid cyclone was over­loading, performing poorly, and a large quantity of glass was being fed into the reactor. At some point the fluid bed, which had been agglomerated to very coarse bed material, became sluggish due to poor fluidization, which resulted in poor combustion. Excessive freeboard bur'ning was observed. It was also found that softened glass was plugging the exhaust duct, especially at its entrance area. When the scale grew too big, a chunk of it dropped to the

bed, causing defluidization. These problems occurred in the early plant operation and caused frequent shut-downs.

In addition to the poor performance of the liquid cyclone, the location of the feed point, which was high in the freeboard, was believed to contribute to freeboard burning. Modifications were made accordingly: Installation of a properly selected liquid cyclone and lowering the feed point down to the bed section. Subsequently, the plant has been running without any major problems caused by a process difficulty. An operating time of 14 to 16 months has been achieved before completely replacing the bed with fresh sand. Freeboard temperature, according to a recent report, is maintained at 1650 F (899 C).

A consulting firm ran performance tests on the reactor for the client. The operating data obtained during the tests are excerpted from the report and shown in Table 1.

Feed material % Total solid Dry solid feed

TABLE 1 OPERATING DATA

pulped refuse only 42

H HV of dry solid 9060-9480 Ib/hr (4110-4300 kg/h) 6460-6625 Btu/lb

Fluidizing air flow Auxiliary fuel

( 15,026-15,410 kJ/kg) 14,700 SCFM (416 m3/min.) None

COOLING

WATER

STORAGE

TANK

INTERMEDIATE

STORAGE

TANK

PUMP

Although the plant in Franklin, Ohio is handling wet preprocessed refuse sludge, the refuse, after dry preprocessing, can be thermally disposed of if metal and glass are removed before incineration. The excessive heat of combustion evolved has to be controlled by some means that will maintain the temperature at a manageable level. Water cooling coils can be put inside the bed to extract the necessary amount of heat. Bed cooling coil technology developed for fluid bed reactors in mineral processing is available.

INDUST R IAL WASTE INC INE RAT IO N SYSTEM

Fluid bed incinerators also have been success­fully applied to the disposal of industrial wastes such as:

Pulp and paper mill waste liquor Oil refinery waste sludges Pharmaceutical wastes Spent coffee and tea leaves, and others

The basic features of these systems are similar to those of sewage sludge incinerators. Certain parts of the system, of course, have to be modified to meet the specific needs of a certain application.

Pulp and Paper Mill Waste Liquor Incineration

Fluid bed incinerators have been installed in several pulp and paper mills to burn sulfite waste liquor (Fig. 6). The idea of incinerating the waste

HOT WATER

VENTURI­

SCRUBBER

EVAPORATOR

GAS

COOLER

FEED

PUMP PELLET

PRODUCTS

FREEBOARD

FIG.6. FLUID BED INC INERATOR FOR NSSC WASTE L IQUOR

6

liquor is to eliminate a source of pollution and at the same time recover marketable chemicals. The incinerator is slightly different from the sewage sludge incinerator and has a cooling compartment below the fluid bed. This compartment serves the dual purposes of product cooling and heat recovery at the same time. A venturi scrubber installed to capture escaping particulates also functions as the second stage liquor concentrator. The weak liquor is, first, partially concentrated in a multi-effect evaporator. This partially concentrated liquor is then fed into the venturi scrubber evaporator. The direct contact of the liquor with the incoming hot off-gases enables a large amount of water to be evaporated. Atomization in the venturi throat and cyclonic separation retain the particulates in the scrubber. The high solids content sludge is then pumped into the fluid bed for oxidation.

The bed temperature is kept at around 1350 F (732 C). Elements such as sodium and sulfur react chemically to form ash. The ash is produced in a pellet form and is mainly comprised of sodium sulfate and sodium carbonate. The oxidized ash product from an NSSC liquor normally would contain about 65 percent Na2S04 and 35 percent Na2 C03 . The composition of the ash usually varies on a day-to-day and mill-to-mill basis. The combination of the sodium compounds as noted can constitute an eutectic compound which would soften at temperatures above 1350 F (732 C) and cause a serious de fluidization problem.

Ash products are continuously transferred through the standpipe to the cooling compartment, where ash cooling and heat recovery take place by the incoming fluidizing air. If necessary, a portion of the discharged product can be recycled back to the bed for reseeding purposes. When the ash products are to be sold to a kraft paper mill, the sulfate content is increased by adding sulfur to the bed. The products thus produced would. contain 90-95 percent sodium sulfate.

Refinery Waste Sludge Incineration

To ensure the satisfactory incineration of this type of sludge, operating conditions should be carefully selected. Complexity of chemical compositions quite often necessitates a certain variation in the fluid bed operating temperature. When the incinerator handles sludges with insignificant amounts of alkali metal and chloride such as API oil separator sludge, waste activated sludge, and clarifier skimmings, the fluid bed operation is rather simple and relatively free of

7

difficulties that might arise from chemical reactions of sodium. The freeboard temperature is main tained at 1500 F (816 C).

When sludges contain substantial amounts of sodium sulfate and .sodium chloride compounds, the melting point of the eutectics formed can be as low as 1154 F (623 C). The bed temperature must be held below this temperature to avoid eventual defluidization. Consequen tly, incomplete combustion in the bed will necessitate freeboard burning for complete oxidation of the combusti­bles. ExceSSively high temperature in the freeboard can melt the eutectics which become sticky and agglomeration of the ashes in the gas duct and roof has been observed.

Although trouble-free operation may be possible with a limited amount of chlorides, any substantial quantity of chloride (more than 500-600 ppm) causes serious problems. In combination with alkali metal compounds, chlorides could form very low temperature eutectics ( 1134 F or 612 C). At these temperatures combustion can be poor and unburned hydrocarbon would be entrained in the outgoing combustion gas stream. One way of resolving this problem is to add kaolin clay to the bed. It will react with sodium and potassium to form a crystalline high-melting-point compound, thus enabling the bed to be maintained at a high temperature.

When there are insignificant amounts of chloride present in the sludge, the bed temperature is held at about 1300 F (704 C). Some alkali metal compounds formed may still constitute an eutectic compound, which could melt or soften at higher temperatures.

AIR POLLUTION CONTROL

Highly turbulent motion of the fluid bed ensures that the combustible solids make intimate contact with oxygen. The temperature of the fluid bed is always maintained at a minimum of 1300 F (704 C). The result is an instantaneous, complete oxidation of organic and inorganic materials and, typically, no unburned combustibles escape. Complete oxidation is one of the key factors for efficient control of air pollution.

For the control of entrained particulates, a high energy scrubber is employed. Stringent EPA requirements on particulate emissions exclude the use of low energy scrubbers.

Tables 2, 3 and 4 present the results of recent emission tests performed on a fluid bed incinerator

designed to handle a mixture of primary sludge and humus from coil fIlters with 16-22 percent total solids. Dry solids feed rate was 1426-1662Ib/hr (647-754 kg/h) during the tests. The venturi scrubber �p was maintained at 30 in. H20 (7.47 kPa).

TABLE 2 PARTICULATE EMISSIONS

Particulates Test No. Date mg/m3d.g. Ib/ton dry sludge

1 June 9, 1976 2.2 0.060 2 June 9, 1976 1.7 0.046 3 June 10, 1976 0.6 0.014 4 June 10, 1976 2.3 0.054 5 June 10, 1976 1.9 0.044

Note: Federal EPA requires particulate emission of no more than 1.3 Ib/ton of dry sludge.

As shown in Table 2, the particulate emissions were extremely low and would satisfy the emission requirements of any locality.

The average combustion gas composition at the particulate sampling point was measured as shown in Table 3.

TABLE 3 GAS COMPOSITION

% Vol ppm CO. o. (1) N. CO SO. NOx (2) H-C(3) 11 .5 8.0 80.5 N.D. N.D. 183 0.9

N.D.: Not Detectable Note: The gas samples were taken downstream of the

plume suppressor unit. 1. At the upstream of the plume suppressor unit O.

measured 5.7 percent average. 2. Measured by chemiluminescent method 3. Measured by flame ionization technique

The heavy metals content in the entrained particulates was analyzed as shown in Table 4. Due to changes in sludge composition, the heavy metals content may vary on a day-to-day basis.

Test No.

1 2

TABLE 4 HEAVY METALS CONTENT

Date

June 9, 1976 June 9, 1976

",g/m3 Pb Cd Cu Cr Ni Hg 65 9 26 39 180

144 15 6 22 53

WASTE HEAT RECOVERY

According to the purpose or end-use, sensible heat recovery can be classified into three categories: auxiliary fuel saving within the system; steam generation for external use; and the combination of the two. The advantages of heat recovery can casily be visualized by the amount of auxiliary fuel to be saved or the amount of steam that can be generated.

8

AUXIL IA RY FUEL SAVING BY HEAT

RECO VE RY

In this scheme, the heat recovered is used to preheat the incoming fluidizing air, as shown in Figs. 2 and 3. This system offers two advantages: an increase in capacity and, at the same time, a reduction in auxiliary fuel consumption. Tables 5 and 6 illustrate the merits of heat recovery in warm and hot windbox systems compared with a cold wind box system.

TABLE 5 COMPARISON OF DRY SLUDGE DISPOSAL CAPACITIES

Warm Windbox Hot Windbox % T.S. Cold Windbox (600 F or 316 C) (1000 F or 538 C)

15 70 84 94 20 25 30

100 133 172

120 160 205

133 177 228

Suppose an incinerator with no heat recovery has a rated capacity of 100. The same incinerator can burn 20 percent more dry sludge if equipped with a warm windbox and 33 percent more dry sludge if equipped with a hot windbox.

TABLE 6 COMPARISON OF AUXILIARY FUEL CONSUMPTION

Warm Windbox Hot Windbox % T.S. Cold Windbox (316 C) (538 C)

15 100 80-85 67-74 20 100 71-80 51-66 25 100 53-73 21-54 30 100 5-60 0-32

If an incinerator is designed to handle a sludge with 20 percent solids, auxiliary fuel consumption can be reduced by 20-29 percent if the same incinerator is equipped with a warm windbox and by 34-49 percent with a hot windbox, depending on the heating value of dry solids. Higher heating values would contribute to larger fuel savings.

TABLE 7 POSSIBLE FUEL SAVINGS

Warm Windbox Hot Windbox HHV, Btu/lb d.s. (316 C) (538 C)

gal/year $/year gal/year $/year 5000 (11630 kJ/kg) 57,000 22,800 95,700 38,280 5500 (12790 kJ/kg) 59,500 23,800 100,000 40,000 6000 (13960 kJ/kg) 62,100 24,840 104,400 41,760 6500 (15120 kJ/kg) 64,800 25,920 108,700 43,480. 7000 (16280 kJ/kg) 67,300 26,920 113,100 45,240 Note: The calculations are based on the disposal of sludges

with 20 percent solids in a 15 ft F BD (4.6 m) incinerator. Average operating period is assumed to be 2500 hr/year. No. 2 oil cost at 4011 /gal.

Table 7 will help to visualize the magnitude of

FEED

EXTERNAL SATURATED STEAM �ATER WATER

USE TREATMENT

100 - 1

STEAM DRJi::: -tz:J PUMP J

.

.f 650°F L\ WASTE

HEAT

BOILER

S LUDGE FEED SCRUBBING

SLUDGE WATER

.I: � �

I

r '- ./

EXHAUST

TRAY COOLIN � WATER 70°

FAN

G

F

EFF LUENT WATER DEWATERING FREEBOARD �� r! H.. HOT WATER

SCREW CONVEYOR '- I" QUENCH WATER QUENCH

�FLUID BED FUEL -..( -----IF NEEDED

SCREW FEEDER

.r 'Y �

./ FLUIDIZING

AIR BLOWER

TANK

VENTURI

SCRUBBER

"' • �

� ASH SLURRY

PUMP

TO

DIS

ASH

POSAL

FIG.7. FLUID BED INCINERATOR WITH WASTE HEAT BOILER

annual auxiliary fuel savings that can be achieved

with heat recovery systems. It is shown that heat

recovery would significantly reduce the annual

operating cost.

STEAM GENERAT I ON IN WASTE HEAT BOILER

The sensible heat of combustion gases can be

extracted in a waste heat boiler to generate steam

(Fig. 7). This unit does not preheat the fluidizing

air. While the hot gases are passing through the boiler, they are cooled down to about 650 F

(343 C) and then go to a scrubber for final cleaning and cooling. An exhaust fan generally is located downstream of the scrubber and draws the gases through the boiler. Boiler tube surfaces are kept

clean by either soot blowing or tube rapping. Some typical steam generation data are shown

in Table 8 for a sewage sludge incinerator. A fluid

bed incinerator has the inherent characteristic of

high exit gas temperatures and low excess air. This

gives a larger hot gas·to-boiler tube 6T and therefore requires less tube surface and a smaller, less expensive waste heat boiler than incinerators with low exit gas temperatures and high excess air req uiremen ts.

%T.S.

15

20

25

30

TABLE 8 STEAM GENERATION FROM

SEWAGE SLUDGE INCINERATION

kg steam/kg dry solid kg steam/kg wet sludge

7.4 1 .1

5.0 1.0

3.5 0.9

2.6 0.8

9

Note: For steam generated at 350 psig (2413 kPa) and

650 F (343 C) exit gas from the waste heat boiler.

The amount of steam generated per total heat input

is essentially constant regardless of the solids

content. As the amount of dry solids in the feed

sludge gets smaller, the ratio of steam generated to

dry solids becomes larger.

COMB INATION OF FUEL SAV I NGS AND STEAM GENERAT ION

When the incinerator is designed to handle

sewage sludges of varying solids content, a combined system as shown in Fig. 8 can be applied. For sludges of high heating value and high solids content, a small amount of heat is extracted in the heat exchanger to preheat the fluidizing air. Most of the sensible heat is extracted in the waste heat boiler. To handle sludges of low heat value and/or low solids content, a large amount of heat is needed to preheat the fluidizing air to a high temperature for auxiliary fuel savings or for

possible autogenous combustion. In this case the sensible heat extracted in the waste heat boiler becomes relatively smaller. Control of the quantity of hot gases flowing into the heat exchanger is accomplished by adjusting the damper located at the off-gas exit of the heat exchanger.

ACKNOWLEDGMENT

The author wishes to thank Mr. C. J. Wall for his valuable advice in the preparation of this paper.

•

SLUDGE FEED

EFFLUENT WATER

SLUDGE DEWATERING FREEBOARD

HEATED AIR

BY-PASS

AIR BLOWER TO SCRUBBER COOLER

FIG. 8. FLUID BED INC INERATOR W ITH A IR PREHEATING AND WASTE HEAT BOILER

REFERENCES

[1) Becker, K. P., and Wall, C. J., " Incinerate Refinery Waste in a Fluid Bed," Hydrocarbon Processing, October 1975.

Incineration of Wastes," Chemical Engineering Process, October 1976.

[4) Pledger, W. R., and Gwyn, J. E., "Fluidized Waste Incinerator and Methods," U.S. Patent No. 3,994,244, November 30, 1976. [2) Wall, C. J., Graves, J. T., and Roberts, E. J., "How

to Burn Salty Sludges," Chemical Engineering, April 14, 1975.

[5) Roberts, E. J., and Angevine, P. A., "Fluid Bed Incineration of Wastes Containing Alkali Metal Chlorides," U.S. Patent No. 3,907,674. [3) Becker, K. P., and Wall, C. J., "Fluid Bed

Key Words

Combustion

Disposal

Energy

Fluidized Bed

Incineration

Refuse

Waste Heat

10

Discussion by

Eric H. Smith

Holden, Massachusetts

This illustration [1] is shown in Fig. 1. In an actual bed the particles would be much more randomly positioned than shown. However, the particles would all be about the same size.

Henry Kwon is to be commended for a most interesting paper showing and describing so many noteworthy applications of fluidized bed tech­nology. To supplement this article a diagrammatic illustration of the fluidization phenomena, which all of these applications employ, is hereby sub­mitted.

In Diagram A the particles are at rest and the fluid passes through the bed without disturbing the particles. The LlP increases as the flow increases.

Eventually the fluid commences to lift the particles, the space betweer the particles increases and there is a slight decrease in the LlP. This is shown in Diagram B.

Superfici al vel.

Diagram "A"

!§!§.t] • t t i

( Fixed) Stationary

bed

(8

�p

Fixed

bed

Diagram B Dia gram C

0 0 0 0 0 0 0 0

0 0 00000 0 0 0 0 000000 0 0 0 0 000 0 0

0 0 0..Q.Q.Q 0 0 0 0

t t t t t t

Fluidized (Conveying) bed Transport

1 to 15'/5ec )8 (.30 to 4.5 M / sec)

D i a gram D

Fluidized

bed

Transport

Fluid velocity

FIG.l

1 1

The particles now are in violent motion and the bed "boils". This condition exists over a limited range of superficial velocity. Over this range of flow the toP remains constant.

Finally, in Diagram C the fluid velocity has exceeded the terminal velocity of the particles, the toP drops and the particles are transported and swept out of the vessel.

REFERENCES

[11 Bailie, R. C., 1968 National Incinerator Confer­ence, ASME, New York, 1968.

Questions by

Question 1

Professor A. Buekens

University of Brussels

Does your company conduct tests on the fluidized-bed gasification or pyrolysis of shredded refuse or of plastics? If not, can you comment on the possibilities of such systems, as developed by Ishikawajima Harima and other Japanese companies?

Question 2

For which reason do you feed the sulfide waste liquor into the bed, rather than spraying it into the bed, as Copeland does? And how do you control the particle size of the bed material?

Discussion by

Richard C. Petura

Malcolm Pirnie, Inc.

White Plains, New York

The fluidized bed reactor is reliable and effec­tive for disposal of sludges in an environmentally acceptable manner. The subject paper is very timely and the author is to be complimented in presenting the state-of-the-art aspect for energy recovery with fluidized bed incineration.

The energy crisis has caused all associated with the incineration of wastes to search for new methods of conserving fossil fuel usage. This paper offers potentials for energy conservation within the

12

system and for external use. It is always possible to reduce fuel usage within the system with a gas to air heat exchanger (air preheater). It is not al­ways possible to reduce fuel usage within the sys­tem with a gas to air heat exchanger (air preheater). It is not always possible that there will be a use for steam. The effective use of steam requires a need 24 hr, 7 days a week throughout the year. This requires the fluid bed reactor to operate on a similar basis and in many cases extends the interval between outages for inspection, cleaning the scrubber system, recharging the sand bed, and other maintenance. It is possible that the longer operating periods would contribute to increased maintenance.

The combined system for energy recovery greatly reduces the steam produced by the waste heat boiler. The air preheater usually operates with a gas temperature differential of 450 F (232 C), from 1500 F (816 C) leaving the reactor to 1050 F (566 C) leaving the air preheater. This results in a gas temperature differential of 400 F (223 C), from 1050 F (566 C) entering the waste heat boileL to 650 F (343 C) leaving. While the temper­ature drop through the air preheater and the waste heat boiler are similar, there is far less energy recovery achieved in the waste heat because the specific heat of the gas decreases with lower gas temperatures and the heat transfer rate is sub­stantially decreased.

In applying waste heat boilers to fluid bed incinerators, the author indicates that the high exit gas temperature and low excess air requires less tube surface and a smaller, less ex pensive boiler than incinerators with low exit gas tempera­tures and high excess air requirements. This apparent advantage is substantially reduced be­cause the gas leaving the fluid bed incinerator and passing through the boiler has 100 percent of the ash in the sludge plus the sand carryover from the bed. This high ash or dust loading necessitates special design of the boiler to achieve low gas velocity through it in order to avoid tube erosion. A waste heat boiler applied to the incinerator with low exit gas temperatures and high excess air re­quirements is subject to less erosion potential because 70-80 percent of the ash is discharged from the incinerator and only 20-30 percent is transported in the gas and passes through the boiler.

It is hoped that the author will clarify Table 8 which indicates that more steam is produced at 15 percent total solids than at 30 percent total

solids. The greater quantity of steam that is pro­duced at 15 percent total solids results from the greater gas flow discharged by the fluid bed reactor and available to the boiler for heat recovery. The increased moisture with 15 percent solids requires substantially more supplementary fuel to evaporate the moisture and convert it to superheated gas. The supplemental firing and evaporation of moisture result in the greater gas flow: While it indicates more steam can be recovered from the heat in the gas, it should be remembered that the increased cost for the supplementary fuel may more than offset the value of the steam and thus make the operation with these conditions uneconomical. It would be helpful if the author would tabulate the overall operation of the fluid bed and boiler which would indicate both the fuel used, the steam pro­duced, and the operating costs that are involved.

It is hoped that the author may provide this data in his closure and further increase the value of this paper.

AUTHOR'S REPLY

To Mr. E. Smith

Thanks for your addition. The mechanism of a fluidized bed has been described in many papers

13

and I felt it was not necessary to repeat it in my paper.

To Professor Buekens

Question 1

No, we do not conduct tests on gasification or pyrolysis in the fluidized bed. We feel that it is technically feasible to apply a fluidized bed to gasification or pyrolysis, although some difficulties have to be overcome. Economic justification may be of more concern.

Question 2

a. In some applications a top spraying feed sys­tem may be more advantageous. Our experiences in sulfite waste liquor incineration have demon­strated that an in-bed feed system is more accept­able in most cases encountered. A top spraying feed system may cause unburned hydrocarbon carry-over due to possible pyrolysis of liquor in freeboard.

b. The control of bed material size is achieved by a reseeding system, by steam injection or by controlling chloride content in the feed.

Editors Note: Mr. Petura's discussion was not sub­mitted to Mr. Kwon because of a filing error.