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Investigation of Three Systems to Dry and Incinerate Sludge CHARLES A. HESCHELES, P. E., Consultant and SYDNEY L. ZEID, P. E., Manager, Columbus Office Malcolm Pirnie, Inc. White Plains, New York ABSTRACT The design of a new thermal reduction installation to dry and burn sanitary and industrial sludge*prompted an investigation of available systems before making a final selection. This paper summarizes the results of the evaluation and presents the technical and economic justification for the final recommendations. For the system under design, sludge cake from a" vacuum filter,containing approximately 20 percent solids and 80 percent water, was to be fed directly to the thermal reduction units. At the installation studied there were two I25-tons/day thermal reduction units, one for the sanitary sludge, the other for the industrial sludge,with both units operating 100 h per week. In case of emergency when one unit breaks down,the other unit was to operate full time handling both sani- tary and industrial sludge. Essential to any investigation such as this is a review of various systems available,consideration of operating conditions,evaluation of economics involved,and analysis of comparative factors of design and operation. * Sludge is defined as treatment unit underflow which has been thickened and conditioned prior to vacuum filtration. Sani- tary sludge is a product from the combined primary and secondary treatment tanks underflow in a municipal sewage treatment plant. Industrial sludge is the underflow from clarifiers treating the effluent from various chemical processes at an industrial plant, adjacent to the municipal treatment plant studied. 265 INVESTIGATION OF SYSTEMS TO DRY AND BURN SLUDGE The systems evaluated to dry and burn sludge in- cluded the fluidized bed,the Herreshoff multiple hearth and the rotary kiln. F luidized Bed F urnace The fluidizer consists of a wind box, a constriction plate to support the fluidized bed,a fluidized (sand) bed, a refractory-lined reactor and a gas outlet. The high pressure air from the wind box flows upward through slots in the horizontal constriction plate,enters the sand bed and agitates the sand. Auxiliaries include a high pressure blower,auxiliary fuel burners, gas scrubber,air compressor,etc. At start-up,the unit is preheated by the auxiliary burner to approxima tely 1000 F sand bed temperature. The sludge is fed soon after through an air atomized whirling nozzle. The moisture in the sludge is liberated instantaneously,and is converted into superheated steam. The dry sludge starts to burn in suspension while it drops on the sand bed for final incineration. Both vapor and dry gases leave the fluidizer and flow through a mechanical col- lector and a water scrubber before venting to the atmosphere. Sludges that are not self-sustaining require additional heat provided by auxiliary fuel burners to maintain furnace temperatures of 1300 to 1500 F. The sand bed is maintained in a continuous turbulent motion by the incoming blower air necessary to obtain a high heat transfer rate between sand bed and com-

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Investigation of Three Systems to Dry and Incinerate Sludge

CHARLES A. HESCHELES, P. E., Consultant and

SYDNEY L. ZEID, P. E., Manager, Columbus Office

Malcolm Pirnie, Inc.

White Plains, New York

ABSTRACT

The design of a new thermal reduction installation

to dry and burn sanitary and industrial sludge* prompted an investigation of available systems before making a final selection. This paper summarizes the results of the evaluation and presents the technical and economic justification for the final recommendations.

For the system under design, sludge cake from a" vacuum filter, containing approximately 20 percent solids and 80 percent water, was to be fed directly to the thermal reduction units. At the installation studied there were two I 25-tons/day thermal reduction units, one for the sanitary sludge, the other for the industrial sludge, with both units operating 100 h per week. In

case of emergency when one unit breaks down, the

other unit was to operate full time handling both sani­tary and industrial sludge.

Essential to any investigation such as this is a review of various systems available, consideration of operating

conditions, evaluation of economics involved, and analysis of comparative factors of design and operation.

* Sludge is defined as treatment unit underflow which has been

thickened and conditioned prior to vacuum filtration. Sani­

tary sludge is a product from the combined primary and

secondary treatment tanks underflow in a municipal sewage

treatment plant. Industrial sludge is the underflow from

clarifiers treating the effluent from various chemical processes

at an industrial plant, adjacent to the municipal treatment

plant studied.

265

INVESTIGATION OF SYSTEMS

TO DRY AND BURN SLUDGE

The systems evaluated to dry and burn sludge in­cluded the fluidized bed, the Herreshoff multiple hearth and the rotary kiln.

F luidized Bed F urnace

The fluidizer consists of a wind box, a constriction

plate to support the fluidized bed, a fluidized (sand) bed, a refractory-lined reactor and a gas outlet. The high

pressure air from the wind box flows upward through slots in the horizontal constriction plate, enters the sand bed and agitates the sand. Auxiliaries include a high pressure blower, auxiliary fuel burners, gas scrubber, air compressor, etc. At start-up, the unit is preheated by the auxiliary burner to approxima tely

1000 F sand bed temperature. The sludge is fed soon after through an air atomized whirling nozzle. The moisture in the sludge is liberated instantaneously, and is converted into superheated steam. The dry sludge starts to burn in suspension while it drops on the sand bed for final incineration. Both vapor and dry gases leave the fluidizer and flow through a mechanical col­lector and a water scrubber before venting to the atmosphere. Sludges that are not self-sustaining require additional heat provided by auxiliary fuel burners to maintain furnace temperatures of 1300 to 1500 F. The sand bed is maintained in a continuous turbulent motion by the incoming blower air necessary to obtain a high heat transfer rate between sand bed and com-

bustible. The sand picked up by flue gases is con­

tinuously recovered with a mechanical collector and reused in the fluidizer. Flue gas particulates, recovered in the water scrubber, are sluiced away together with

the ashes from the fluidizer. A diagram of the system is shown in Fig. 1. Flow diagrams of the operation are

shown in Fig. l A and 1 B.

Multiple Hearth

The multiple hearth furnace as the name implies contains a series of circular hearths placed one above the other, enclosed in a refractory-lined steel shell.

Waste material fed at the top of the furnace is

moved around the hearth by means of a rotating rabble arm to an opening through which it is dropped to the hearth below. This operation is repeated from one hearth to another until it reaches the bottom hearth.

The wastes are completely reduced to ashes by the time they reach the bottom hearth, from which they are sluiced away.

The flue gases flow upwards counter to the flow of waste material which moves downward. The wet sludge cake is dried in the first few hearths, then it starts burning and is completely incinerated by the time it reaches the bottom hearth. The multiple hearths are designed for an exit flue gas temperature of 800 F. Auxiliaries include: Rabble arm drive, air blower to cool

rabble arm, induced draft fan, water scrubber, and

auxiliary fuel burners to maintain furnace temperatures. A diagram of the system is in Fig. 2 Flow diagrams of

the operation are shown in Fig. 2A and 2B.

Rotary Kiln F urnace

The rotary kiln is a slow rotating cylindrical

furnace where the material to be processed is continuously

SCRU&&tll. !T"'CK

� , . ,;"" . ...

�U!.L OIL STO��G!

SLUPGE FEEDER (0==--0 \. L

r------r. FEE P -Ir.\ .. \n\S")..\:;-;�7�":-;\ .. \-.>:\\�,,f1.\1 20%�LI[)S) MIXER

Fig. 1 Sludge burning fluidized bed system.

266

Cake Feed

Solids - lbs/hr - 2,080 Water - lbs/hr - 8,320 Combustible - MKBH - 19.0

Vapor Dry Gas

lbs/hr 10,120 35,000

Auxiliary Fuel

MKBH - 11. 8

Fluidizer

Dry Collector

. . ... ... .... ...... 1--... �--,.j

... - - - � .. . .

Fly Ash Recirculation

Ash Discharge

Scrubber

Fig. 1 A Fluidizer system, burning industrial sludge.

Soli0s - lb/hr - 2,080 Water - lb/hr - 8,320 Combustible - !1KBH - 13.3

fluidizer

6 • • • • .. • .. • • • • •

. . . . . . . .. . . .

- - --- - - - - -

. -- -- - - - - - .

HHHHH .....

Vapor Dry Gas

Dry Collector

rlv .l\sh Re circulation

Auxiliary Fuel

17.0 t1KBH Ash Discharqe

Fig. 1 B Fluidizer system, burning sanitary sludge.

267

lbs/hr 9,920

34,000

Scrubber

agitated as it moves forward in the kiln. This system

uses two separate independent kilns in the drying and incineration of sludge. The first kiln dries the wet cake using hot flue gases from the incinerator kiln,

while the second kiln incinerates the dried cake. The wet cake is fed to the dryer kiln at the same end that the hot gases enter the dryer -both flow in parallel in the same direction. The hot gases enter the dryer at approxima tely 1 100 F and leave the dryer at approxi­mately 250 F. The dry cake leaving the dryer is split into two streams, one is recirculated with the wet cake fed to the dryer while the other stream is fed to the incinerator. The incinerator kiln operates at tem­peratures of approximately 1600 to 2000 F with the

aid of auxiliary fuel burners, if additional heat is required.

Induced

Draft

fan

Scrubber

fly Ash---/

Slurry

Each operation is independently controlled. The kiln incinerator and kiln dryer maintain independent temperatures. The kiln dryer controls product moisture

by controlling outlet flue gas temperature. Auxiliaries include: independent kiln drives, mechanical collector, dryer, hot gas supply fan, induced draft fan, wet scrub­ber, and automatic controls. A diagram using the parallel flow system is shown in Fig. 3. Flow diagrams using the

counterflow system are shown in Fig. 3A and 3B.

SCOPE OF INVESTIGATION

The investigation of the three furnaces (systems) was made in three stages. The first stage was an economic study, the second stage was a field survey of opera-

f

floating Damper

Air

[ Filter

J - ---t o •

Cake

Fig. 2 Multiple hearth Herreshoff sludge furnace.

26 8

tional problems, and the third stage was the final evaluation of the systems based on the findings in the first two stages.

ECONOMIC EVALUATION

The economic study included capital investment, operating costs, maintenance, etc., of the installations. The results of this study are shown in Table 1. The

results show that the fluidized bed operating costs are higher than those for the multiple hearth or

rotary kiln.

'-'aDor Dry Gas

Ibs/hr 9,540

28,600

800°F

, ---

OPERATION

The survey included field trips to inspect and observe operating installations. As a result of the first stage study, the choice was narrowed down to the remaining two systems, i.e. the multiple hearth furnace and the rotary kiln furnace as outlined below.

MultipkHearth Units

Drying and Burning Sanitary Sludge

The installation studied has been in operation about 10 years and comprises four 150 tons/day Herreshof multiple hearth furnaj::es burning sanitary

r1.

Solids - Ibs/hr - 2,080 Water - Ibs/hr - 8,320 Combustibles - MKBH - 19.0

D - ---I , , I

,- .. -

, 2 L. ___ I

3 ' ,- .... _1 I , 04-- - -1

$' , ,-.-

,

� Ash

Auxiliary Fuel Hl<BH-O

Fig. 2A Multiple hearth system, burning industrial sludge.

26 9

sludge in a municipal sewage treatment plant. The sludge is fed continuously from vacuum filters to the multiple hearth furnaces. The filter cake has a moisture

content of about 70 percent and a solids content of about 30 percent. The flue gas temperature in the drying zone controls the rate of moisture evaporation which in turn controls the sludge drying. An outlet gas temperature of 800 F is the design temperature for good operation. A higher outlet temperature moves the combustion too close to the top of the furnace, while a lower temperature moves the combustion lower in the

furnace releasing a residue not acceptable for land fill. The operation of the furnace is affected by:

1) changes in the amount of air infiltration; 2) varia·

tion in the volatile content of the sludge; 3) furnace draft.control; 4) auxiliary firing control; and 5) operator competence.

' ... ·1 'Jr

� \ scrUbbef

lhs/hr Dr) r '),170 �' :,as 19,330

--

81)()OF

Operator salaries in the municipal field are lower than those paid by industry. Municipalities are faced

with a real problem in hiring and keeping good operators. A good operator watches his furnace con·

stantly, and looks at the instrumentation and necessary ad justments to maintain operation within limits outlined by the manufacturer.

The multiple hearth furnace design has incorporated a limited amount of automation, but relies mostly on the operator's good judgment for satisfactory reo suits. The results of such good judgment are shown in Figs. 4, 5, 6, and 7, which show variations in flue gas temperatures in four operating furnaces over a 24·h period at the time of inspection. The variation in the flue gas temperatures at the various hearths shown in these charts indicate the need for increased automation in the operation of the mUltiple hearth

Cake Feed

Solids - lbs/hr - 2,080 Water - lbs/hr - 8,320 Combustibles - IIKBH - 13.3

r7 0 - --f- __ --, , I

r- --, � I - --�

1 I ,_:.J , .4 , __ -.

5' ,- _...J of •

l Ash

Auxiliary fuel /<IKBH - 3.8

tlLlltiple Hearth furnace

Fig. 2B Multiple hearth system, burning sanitary sludge.

270

furnaces to reduce maintenance, auxiliary fuel con­sumption and particulate emission.

Rotary kiln systems

A combination 225 tons /day stationary grate incinerator and a rotary kiln dryer was used to dry and incinerate sanitary sludge at a municipal treatment plant. The plant is located in a pulp and paper industrial

1

Mechanical Collector

Auxi I iary -_t_---'--_ Fir i ng

Dryer Ki In

Recirculated Dried

community and the sanitary sludge includes a good percentage of pulp from the local pulp and paper

industry.

An insulated rotary kiln sludge dryer 7 ft in diam and 40 ft long reduces the moisture in the sludge from 70 to 20 percent. The capacity of the unit is

6 770 lb /h of wet cake. The dryer is located on the first floor of the filter building where the wet cake is fed to the kiln for predrying before it is conveyed to the

Stack

Induced Draft Fan

........... ..--... .....

-4 __ __

-- .. - -

Feed -�ake !

o

o 0

Pug Mi I I

�_'-. ______ Sludge

Fig. 3 Rotary kiln system.

271

municipal incinerator for burning. The dryer kiln

uses hot gases from the municipal waste incinerator combustion chamber to dry the sludge. The hot gases enter the dryer at 1 200 F, pass through it parallel with the filtered sludge and the cooled gases are re­turned to the municipal waste incinerator outlet ex­pansion chamber at 300 F.

The dryer is equipped with an auxiliary burning furnace to supplement or replace the hot incinerator gases when the incinerator flue gases are not available. The auxiliary furnace has been idle since its installation.

Screw conveyors transfer 75 percent of the dried sludge from the cool end of the rotary dryer to the

lbs/hr Vapor 9,500 Dry Gas 29,800

I 265°F

incinerator. The other 25 percent of dried sludge is re­cycled through the dryer after being mjxed with the incoming wet cake from the filters. This recycling con­ditions and fluffs the moist sludge entering the dryer.

Air jets blow the dried sludge into the municipal incinerator furnace for incineration. The air jets prevent the sludge from caking and smoldering.

Dry ing and burning industrial sludge

The kiln observed was a counter-current flow unit for disposing industrial sludge from three 10,000 lb /h vacuum filters. The kiln was purchased to handle

lbs/hr Vapor 16,500 Dry Gas 50,000

>= 265°F 1 Vapor Dry Gas

lbs/hr 7,000

20,200

Feed Cake

Vapor Dry Gas

Dryer Kiln

lbs/hr 8,580

50,000

f

Solids Water

lbs/hr

2,080 400

Solids - lbs/hr - 2,080 Water - lbs/hr - 8,320 Combustible - m<BH - 19.0

Vapor Dry Gas

Incinerator Kiln

Fig. 3A Rotary kiln system, burning industrial sludge.

27 2

lbs/hr

1,580 29,800

Auxiliary Fuel MKBII-O

30,000 lb /h of 30 percent solids sludge, but experience shows that the unit can only burn 16,000 lb /h of 40 percent solids sludge under good control. The filter

cake is chopped while being conveyed to the kiln. The kiln is 7 0 ft long and 1 0ft in diam. The first 30 ft

of the kiln -the dryer -is lined with drying flights and the remainder -the incinerator -has a mono­lithic refractory lining. Sprays of water are located in the drying flights for re-wetting the sludge in case the

burning area moves back into the drying area. The temperature sensed by the thermocouples is controlled at about 1 1 00F. The burning produces a volume re-

Vapor Dry Gas

Ibs/hr 9,200

20,800

duction of about 90 percent. Natural gas is used for start-up and /or for complete burnout as required. The sludge cake has a variable heating value from 2300

to 4800 Btu /lb on a dry basis. A large percentage of the solids is noncombustible fly ash from the power house boilers which accounts for the low heating value of the sludge. The flue gases leaving the kiln a t approxi­mately 4 00 F are pushed by an induced draft fan through the scrubber and afterburner. Ash from the kiln is cooled with sprays and removed by a drag conveyor

ready for hauling away.

265 OF �_--:...:....:.-_, 265°F -

Ibs/hr I lbs/hr Vapor Dry Gas

Feed Cake

22,500 Vapor 13,300 50,000 Dry Gas 29,200

Dryer Kiln Feed Cake

Solids Water

Solids - Ibs/hr - 2,080 Water - Ibs/hr - 8,320 Combustible - �BH - 13.3

Ibs/hr Vanor 1,260 Dry r,as Ill, ROil

201l00P

Incinerator Kiln

Auxiliary Fuel 11KFlH-O

Fig. 38 Rotary kiln system, burning sanitary sludge.

27 3

T ABLE NO. 1

ESTIMATED OPERATING COSTS

Including interest and amortization

SYSTEM Rotary Fluidized t-Iul tiple

Kiln Bed Hearth

Estimated Loading Dry solids lb/year 7, 200,000 7,200,000 7,200,000

O�erating Hours hr/year 3,640 3,640 3,640

r.:stimated Construction Cost $900,000 $ 1, 150,000 $1, 100,000

Annual Cost Interest & Amortization 6.9% 62,000 79,000 76,000

!-1.aintenance 8%. Equipment Cost 29,000 37,000 35,000

Labor & Operation Manpower 60,000 60,000 60,000

Light & Power 10,000 25,000 7,000

Fuel Purchased 6,000 80,000 16,000

Ash Disposal 2,000 2,000 2,000

Supplies & M iscellaneous 15,000 15,000 15,000

Total Annual Cost $184,000 $ 298,000 $ 2 1 1,000

Cost per 1000 lb Solids at estimated loading $ 25.5 $41. 5 $ 29. 5

27 4

The operation of the kiln - 12 hjday, 5 days per week -is not continuous. This type of operation is hard on the refractory. Alternate heating and cooling causes refractory to crack and break loose, resulting in spot repairs and more frequent than normal complete replacement of refractory.

The original brick and mortar kiln lining had a very

short life and was replaced with a castable refractory lining using Z clamps. The castable refractory has

withstood well the stress of alternate heating and cooling.

I 0 Fan Impeller Erosion

The I.D. Fan located ahead of the scrubber had con­tinuous erosion of the open impellers from the high percentage of fly ash in the original sludge. A change

was made to a paddle wheel fan, with blades made of

HEARTH TEIIPERATURES OUR ING A 24 HOUR OPERATI ON

2,000

1,750

1,500

1,250 ... i 0

... ... = ..... e ...

{:. ... 1,000 ...

� ! ..... -

... <.> e ,. ... = ...

750

500

250

HEART H NUMBER

Fig. 4 Chart No. 1, Hereshoff multiple hearth.

275

soft steel, which has reduced maintenance costs con­siderably.

RESULTS OF THE INVESTIGATION

The results of the detailed investigation of three systems to dry and burn sanitary and industrial sludge are described below.

F luidized bed furnace

The fluidized bed furnace to dry and burn sludge was ruled out at the present time because of its high capital and operating cost. The system retains many operating and maintenance problems unre­solved. Specifically, the authors had concern regarding the soundness of the fluidized bed structural design

MUln TEIPEUTUIES DUll.' A 24 HOUI O'UATIOI

... •

... --.... --� • ... .... ... u --!

2,DDD -._ . .

1,150 +-----t---:

1,5DD

1,250

1,000

150

500

250

o o

.-.- t -�­

I

I- �� -r-, I

.. .. l- ;.

i �-

t t

HEARTH NUMBER

t

Fig. 5 Chart No. 2, Hereshoff multiple hearth.

276

l

I

r ·l I i

______ I

-_ ... ___ ...• "t ••.

to overcome high temperature stresses, and the effect of the sand recovery system on poilu tion con trol from the flue gases leaving the furnace.

Multiple hearth furnace

In a multiple hearth furnace, the burning and drying are performed in one furnace. The physical and chemical properties of the sludge vary, and the operator does not have the means to realize a change before it completely

2,000

1.750

1.500 -.

� ..

-:. 1,250' _. :.

... -..,. -. 0 ! ... ... =>

:;; :. ... 1,000 ... ... • ... .... ... ... c -... => ...

750

500

250

upsets the burning and drying pattern. The investiga­

tion of a multiple hearth installation operating four furnaces showed that the operator does not correct and control the burning of sludge in a multiple hearth

furnace. The multiple hearth furnace responds slowly to changes made by the operator turning on and off auxiliary burners, ad justing furnace draft, etc. The rate of sludge feeding to the furnace is dependent on the treatment facility production and can only be con­trolled, if more than one furnace is in operation, by

HEARTH TEMPERATURES OUR 1 MG A 25 HOUR OPERATI OM

--= -

_ .

-: --

: �

--_ .

= -1 \.

o L-____ -+ ______ �------�----_+------�----� o

HEARTH NUMBER

Fig. 6 Chart No.3, Hereshoff multiple hearth.

277

prorating the feed among the furnaces. Changes in the burning rate of sludge and changes

in its quality affect the drying operation. These changes further complicate the furnace operating pattern by shifting the burning and drying hearths up or down, causing changes in the temperature configuration.

The multiple hearth furnace is practically dependent on the operator's skill, because of its limited automatic control. This can be observed in the fluctuations of the outlet flue gas temperature from 6 00 to 16 00 F.

Rotary Kiln F urnace

The drying of sludge in a rotary kiln is a very old application. This type of kiln has been used in drying all

kinds of rna terials in the mining and chemical industry. The dried sludge resembles small pellets that can be

easily fed to an incinerator for burning. A percentage of dried sludge is continuously recycled with the incom­ing wet sludge. This is particularly helpful in the material handling and in the drying process. Another

IlEARTH TEIPEU TUIES DUll.' • 24 HOUR OPEIA T I H

... 0

� .... .... ... ... ... • � ... u .... -... = ...

2,000 T--',---I

! 1,750 .l,...... __ • • -- ..... --�,------------t-----I -�-

�- .

1,500 -+ .

.. �

1,250 ---r- . I j;.

t t

1,000

750

500 ---_ ..

--

250

...

-f -t-

..., -- -t-.

__ 1_ �

f ot -t 1 I

-� -

't _

.-

�-. -'1= ;=

t ,.

HEARTH NUMBER

Fig. 7 Chart No.4, Hereshoff multiple hearth.

27 8

feature of the system is the feeding of the wet cake

at the same end of the kiln that the hot gases enter, with both flowing in parallel, in the same direction toward the cold end of the kiln. The emission of volatile gases is thus reduced considerably, and the possibility eJl..ists that the sanitary sludge burning and drying might operate without fume incineration.

The industrial facility was designed to dry and incinerate industrial sludge in one single rotary kiln. This op'eration caused fluctuations in the drying and

burning zones, similar to fluctuations in the multiple hearth furnace. As a result of this experience, in­stallations today consider separate individual kilns

for drying and burning. The drying opera tion is con trolled by main taining

outlet gas temperatures from 200 F to 300 F by controlling the rate of flow of hot flue gases from the incinerator to the dryer. Any excess gas from the incinerator is by-passed to the stack. The resulting dry pellets from the dryer are fed to the incinerator

kiln for burning at temperatures from 1600 F to 2000 F.

Comparisons

A summary of questions and answers is detailed in Table 2 for comparison of the multiple hearth furnace

versus the rotary kiln.

SUMMARY OF SURVEY

A summary of the findings in the survey, with special emphasis on the multiple hearth versus the rotary kiln drying and incineration of sludge show the fol­

lowing pertinent facts:

Multiple Hearth

The operation of drying and incineration is in­corporated in one unit.

The various operations cannot be controlled sepa­rately. The resultant opera­tion is a compromise that could produce a question­

able odor control.

Rotary Kiln

Each operation of drying and incineration is an in­

dependent operation per­formed in separate units.

Each unit operation is controllable allowing for variations in sludge qual­ity. The system lends itself to good odor con­

trol.

27 9

TABLE 2 DETAILED ANALYSIS OF

MULTIPLE HEARTH VS ROTARY KILN

A summary of questions and answers are detailed below for comparison of the multiple hearth versus the rotary kiln.

!--Dryer Separate Unit

2--Incinerator Separate Unit

3--After Burner Separate Unit

ANSWER

Mul tiple Hearth

No

No

No

Rotary Kiln

Yes

Yes

Yes

4--Dryer, Incinerator. After Burner Incorporated in one uni t Yes No

5--Puq Mill For dried cake recirculation No Yes

6--Water Scrubber Separate

7--Induced Draft Fan

8--Stack

9--B!ower to cool Rabble Arm

lO-Booster for Gas Recirculation

II-Auxiliary Burners a. Incinerator Furnace

12-Grease Burning

I)-Flue Gas Temperatures a. Leaving Dryer b. Entering Scrubber

l4--Mechanical Drives

l5--Temperature Control

a. Incinerator Furnace

b. Drying

16--Burning Resul ts

l1--Pollution Control

l8--System versatili ty Load Control Response

19--Compliance with Future Stricter Pollution Ordinances

Yes Yes

Yes Yes

Yes Yes

Yes No

No Yes

Yes Yes

Yes Yes

600 -16000F 200 - 3000F 600 -16000F 7000F

a. Rabble Arms a. Incinerator b. Blower b. Dryer c. 10 Fan c. 10 Fan

d. Booster Fan e. Pug Mill f. Screw Conveyor

Dependent on Independent Drying

Dependent on Independen t Drying Operation

Good Good Good Good

Slow Fast and and

Poor Good Yes Yes

20--Elimination of Steam Plume Yes Yes Future Hot Air Temperature

RECOMMENDATION BASED ON THE STUDY

The survey on sludge drying and burning show that three systems are available today to dry and burn sludge.

• Fluidized-Bed Furnace • Multiple- Hearth Furnace • Rotary Kiln Furnace The fluidized bed system has a high capital and

operating cost that cannot be justified at this time. The system still retains unresolved operating and main-­tenance problems.

The selection is then narrowed between two systems, the mUltiple hearth and the rotary kiln. From the standpoint of capital investment and annual operating cost, the differential between the two systems is

280

small enough not to influence their choice on the basis of either cost. The choice has to be made on the basis of performance and maint-eRance.

The two-rotary kiln system is more versatile than the multiple hearth system. The two-rotary kiln system

has independent temperature controls for each ma jor operation - I) the dryer temperature and 2) the incinerator furnace temperature; whereas the multiple

hearth, being one entity, does not have the same flexibility.

With regard to maintenance, the rotary kiln has a

few more units requiring maintenance. That by itself should not be a deterrent in view of the advantages accrued in the operation.

The plant should have good operators irrespective

of the system installed.