investigation of three systems to dry and incinerate sludge · investigation of three systems to...
TRANSCRIPT
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.