Component Costs for Multiple-Hearth Sludge Incineration from Field Data*

WALTER UNTERBERG and GEORGE R. SCHNEIDER Rocketdyne/Rockwell International

Canoga Park, California

ROBERT J. SHERWOOD Envi rotech Corporation

San Francisco, California

ABSTRACT

Field data were obtained from nine operating municipal multiple-hearth sewage sludge incinerators, with unit furnace capacities from 200 to 4500 Ib dry solids/hr. The data yielded normalized cost relationships for capital, labor, material and supplies, hearth replacement , fuel, and power in terms of furnace parameters. Effects of operating schedules and thermal cycling were considered.

INTRODUCTION

A recent study carried out by the authors for the U.S. Environmental Protection Agency was concerned with the development of a computer program for the predesign and cost estimation of optimum multiple-hearth-furnace (MHF) sewage sludge incineration systems [1]. A field study of a number of operating MHF sludge incinerators was p'erformed to acquire data on their design, operation, and component costs.

This paper, after describing MHF sludge incineration, presents the method of normalizing costs by use of economic indicators, and the various cost elements de­rived from field data in the form of equations, graphs, or tables. In this form, the cost information is directly useful for the analysis of existing or the predesign of planned MHF sludge incinerators.

• Work carried out under Contract 14-12-547 sponsored by the Office of Research and Monitoring, EQvironmental Protection Agency. with Dr. J. B. Farrell as Project Officer.

Rabbled-hearth furnaces have been in use for nearly a century, initially for roasting ores. The present air-cooled , multiple-hearth furnace (MHF) is essentially the Herreshoff design of 1 889.lt has been used for sewage sludge incineration since the 1 930's. The multiple-hearth furnace is a unique combustion device. Unlike furnaces designed for the combustion of solid waste materials, this furnace employs no open burning grates. Furthermore, unlike most incinerators, the combustion zone is in the

,central part of the vertical furnace structure and not out­side in another connecting chamber. The advantages of the MHF include simplicity, ease of control, flexibility of operation, and durability .

289

A typical section through a multiple-hearth furnace is illustrated in Fig. 1 taken from Burd [2] . A typical incineration system comprising the furnace and ancillary equipment is diagrammed in Fig. 2 . The furnace proper consists of a number of annular hearths stacked horizon­tally at fixed distances one above the other inside a refractory lined, vertical, cylindrical steel shell. A centrally located cast-iron shaft, which runs the full height of the furnace, supports cantilevered rabble arms (2 or 4) above each hearth. Each arm contains several rabble teeth, which rake sludge spirally across the hearth below the arms as the latter rotate with the central shaft . The sew­age sludge (dewatered to about 25 percent solids) is fed in at the periphery of the top hearth (IN-hearth) and raked by the rabble teeth toward the center to an opening through which it falls to the next hearth (OUT-hearth). On this hearth, the sludge is raked outward to the periphery where it drops to the next IN-hearth below.

: ' . ... . �. ... . . _ .... _ .......... -_ .... ....

_ . ... ,.. , .. ,' .. � ... �;...:. ..... ' . . "

- " , 0 ', 'r oN J .... Vj� ... .... .) 'W \ LvN��

. S '0'

� ' .

- "

-

.---�OOllNG AIR OISCHARGE

...__-.FU)ATING OAMPER

hqr---tC:;':: S L U DG E IN L ET

, I

COOLING AIR FAN

I

C1-C, /oj BUSTlC,�

AIR RETURN

I

-RIIBBLE ARM

DRIVE

FIG.1 TYPICAL SECTION OF MULTIPLE HEARTH INCIN­ERATOR (FROM BURD: "SLUDGE HANDLING AND DISPOSAL," FWPCA REPORT Wpo20-4 [REF. 21)

This in-out process is continued on down the furnace. Thus, the sludge and gas streams move counter-current to one another, the sludge passing down the furnace and finally turning into ash, while the combustion air flows over each hearth as it moves upward, finally exiting as flue gas at the top hearth.

Combustion air generally is of three kinds: 1 ) recycled cooling air that has traveled up through the hollow central shaft and is then, in part, ducted to the bottom hearth, as shown in Fig. I and 2; 2) ambient air from a blower, usually located at a central hearth in conjunction with auxiliary fuel burners (Fig. 2); and 3) ambient air ad­mitted through adjustable ports and doors at various points into the furnace which is slightly below atmos­pheric pressure because of the induced draft (Fig. 2).

Isheirn [3] has given details of the materials and methods of present-day MHF construction. Of particular interest are the design features at the points of maximum temperature in the central hearths. To permit tempera­tures up to 2000 F, rabble arms and teeth are cast of a nickel-chrome alloy, and the vertical furnace wall is provided with a 1 3 .5-inch thickness of insulation. This is made up of 4.5-inch firebrick next to the combustion zone, followed by 9-inch block type insulation next to the outer steel shell. , The effect of rabbling is continually to "plow up" the

solids and break up lumps of material to expose more surface on each hearth to heat and oxygen. In this way, drying, combustion, and heat exchange occur at high rates. Owen [4] has suggested that the incinerator be divided into three zones (indicated in Fig. 1 ): sludge drying, sludge combustion, and ash cooling. These designations refer to the downward progress of the sludge. The cor­responding upward flow of the gases provides the follow­ing pic ture :

1 ) In the bottom ash cooling zone, the entering combustion air is heated above ambient by the ashes.

�cct .. .'J :":�\'iil\g :,:. r '::.::ru:':',,�r _ t

- - , . . • . •

- . ' ... (,Ol�"·G

I

d :, i r

.,::.' . .. ,.� .'

':IJ r ;.:-:I�;ll::·: " 3S ..

,j, . ,

'.';\ ' .. .or :.l:xi I i�lrv

,', n:r:ll T ."u(>l . 'rl.'-

Sl.I:·' "- ..:ooler

'n 0

" .\·nl ('or:-, ion . \ i r from Llowcr*

t .\J j us t.l�le PortS ar.J

(0(\1 in!:> t'h 'If r rro:": 0

; . • 10* >- Disc! . .lq:: ... ·

* [lee::..it· ::(\tor Dr:'.··.· *,\ ��Ur.1P with :�o::.or !)r iv .. •

-r.)(i�':;as

/" I ��uceQ 0 :::raft ?a

\'c t Scrub�er

� �:-:tif' for

Seru:" cr and ?rcc�o!..:r

FIG.2 MHF FLOW AND EQUIPMENT DIAGRAM

290

Scru��er .. ',..'ater

2) In the central sludge combustion zone, the air burns with the sludge such that the product gases are in the 1400 to 1 600 F range.

3) In the sludge drying zone, the gases are cooled by the incoming sludge to a discharge temperature (into the scrubber) of about 800 F.

The economics of MHF sewage sludge incineration have heretofore been studied mainly from an overall cost standpoint. Burd [2] quoted a manufacturer's brochure which gave MHF costs based on the size of the population served, and translated this into a total annual cost per ton of dry solids. Maclaren [5] made an MHF estimate of a similar nature. Studies of MHF sludge incineration have been made for specific communities. Sebastian and Cardinal [6] presented costs for city population equiva­lents ranging from 1 0,000 to 1 ,000,000 (dry solids vary­ing from 360 to 36,000 tons/annum). Quirk [7] developed costs for a city of 100,000 (2530 tons of dry solids/ annum) in some detail. Weller and Condon [8] .discussed MHF selection factors and their application to Kansas City, Missouri (average dry solids 30,400 tons/annum). Mick and Linsley [9] compared actual performance and cost data for the year 1 955 in four cities covering a range in population from 1/2 to 2 million (Buffalo, Cleveland, Detroit, and Minneapolis-St. Paul). The range in dry solids was from 7242 to 84,290 tons/annum. Mick and Linsley pOinted out the difficulties of securing good operation at all times and presented some of the practical considerations.

Burd [2] has stated that a general literature review, presumably based on the references cited above, yielded a range in MHF incineration costs of 8 to 40 dollars/ton of dry solids, exclusive of dewatering or ash disposal. Twenty dollars/dry ton was cited as an average. The trend seemed to be toward a lower cost/dry ton as plant capacity increased.

DATA ACQU ISITION

A field study of a number of operating MHF sludge incinerators was considered the best means of acquiring data on design and operational factors, and on their relationship to costs. The field study was to provide in­formation on the effectiveness of the MHF operation in terms of the following requirements.

1 ) Efficient operating schedule, well integrated with operation of entire wastewater treatment plant

2) Good use of operating manpower, considering operator skill and integration with the rest of the treat­ment plant

3) Effective maintenance schedule to maximize the processing capacity of the MHF

291

4) Hearth and exhaust gas temperature level high enough to prevent odors

5) Hearth, exhaust, and ash temperatures low enough to prevent failure of furnace due to overheating

6) Heatup and cool down transients slow enough to prevent failure of refractory due to thermal stresses

7) Combustion efficiency and percent excess air high enough to keep polluting in exhaust gas below maximum allowable concentrations I t was thought essential to study mUnicipal incineration plants that varied as to number of MHF's, size and capacity of a unit MHF, sludge type and composition, plant operating mode, and manufacture. The scope of the project permitted an in-depth survey at seven loca-

I tions. A selection procedure narrowed the dozens of

,

cities in the U.S. with MHF sludge incinerators down to these seven cities, comprising nine incineration plants:

• Cleveland, Ohio (2 incineration sizes) • Minneapolis-St. Paul, Minnesota (installed 1 938,

195 1 ) • Kansas City, Missouri • Battle Creek, Michigan (2 incineration sizes) • Saginaw, Michigan (installed 1 963) • Hatfield, Pennsylvania • Bridgeport, Pennsylvania

In addition to the in-depth data on the nine basic incinera­tion plants, capital cost and other limited data were ob­tained on another four plants located at :

• South Tahoe, California • Minneapolis-St. Paul (installed 1 965) • Saginaw (installed 1 969) • East Rochester, New York

Overall data on the 1 3 plants mentioned above are presented in Table 1 . Effectively, the incinera tion plants studied in depth (No. 1 through 9) provide the following variations:

• Number of Furnaces • Single-furnace design

capacity • Number of hearths • Outer diameter • Effective hearth area • Total solids in sludge

1 to 4 200 to 4500 lb dry

solids/hr 5 to 9 per furnace 6 to 22.25 feet 85 to 2327 sq ft/furnace 20 to 53 weight percent

The annual utilization of MHF incinerators by capacity and time for all plants studied is shown in Table 2 . The nominal operating schedule , which varied from round the clock down to 1 6 hr/week, is seen to have a large effect on the annual "load factor." This is the product of two quantities, X and Y. The quantity X is the total annual operating hours divided by the number of round-the­clock hours per year. The quantity Y is the actual dry

�nlF No.

1

2

3

4

5

6

7 8

9

10

11 12 13

,

Table 1. Summary of Multiple Hearth Furnace Sewage Sludge Incinerators

B=Bid C -Con trae t.

Hill' First IncinE"rntor Operating No.of

Location Y ('Ill" �U[}" 9 ,

Cleveland 1 U(W 4

Hinn.-SLPaul 11138 , 1951 3,1

Kansas City 11166 3

Cleveland 11140 4

Battle Creek 1966 1

DaUle Creek 1 !l62 1

S agi naw 1 !l63 1

llatfield 1 \/67 1

Dridlleport 1964 1

South Tahoe HJ(l7C I HiJU),-St. Paul 1 \Jli5C 3

Saginaw 1 !J60D 1

East Rochester 1 !)(l3C 1

* Design ** High Oil/Grease Content

�u[F 11 IIi t D." til Ou te r lIe arth Din , No. of Aren, Ft-In lIearths n�

22-3 9 !!3::!7

22-3 tl 20tH

22-3 8 20tH

18-\1 tl 1·1!.!5

18-9 6 10li8

18-11 5 8!10

1(;-9 6 8·15

10- !I 5 230

U-O 6 85

14-3 6 515

22-3 11 28Utl

22-3 6 1500

10-0 5 2�U

Opg. Dry �o 1 id� Dntu Hate �U[}' Period

Yenrs Av�. Hax. 1\l-- Lh/llr LlJ/lIl'

lili-6\1 4lli 7* 7UlO

�tl-litl :lliOO fJOtlO

Li 7 -lHI 4riOO* ·1 800

58-6li 20tl3'� 5370

Iitl-69 2050* 3(;50

62-67 lli�5* 1600

uli-6!J 18tlO 28�U

67-6!J ·100 480

60 200* 336*

Not \100* -

Part G:.!5U* 6775* of Hain 5170-* --

Study ·1t1O� 6UO*

Table 2, Summary of Steady·State Operation Per MHF (Annual Averages)

Actunl Nominal Actual De s ign

Eff. Steady-Stnte Stcudy-Stn tc I Dry Operating ".£lenition Hearth Total Solids Schcuule Area per No. Year Fraction of Fl ow

HBF MllF' , of Fraction of or }'ull 'rime, Hatio No. Location Sq Ft. M1'W' 8 Full Time Perioc.l (X) ( \)

I Cleveland 2327 4 1.0 1966 .260 .723 (24 Ilr " 1967 .223 .6UO 7 Day/Wk) 1 %S -- --

2 Minn.-St. Paul 2084 4 1966 .8S5 .5UO 1 tll;7 .t!SU .670 1 UUtl .855 .S13

4 Cleveland 1425 4 62-65 . 718 .621 7 Saginav 845 I 1966 .851 1.06

lU67 .8'1li . !)U 1U68 .U20 1.01 196U .930 .9U6

3 Kansas City 2084 3 0.714 67-6U .476 .775

I) Battle Creek 1068 1 (24 llr x 1968 .654 1.03 :> Day/Wk) 1 \/69 .666 .937

6 Battle Creek 3aO 1 1964 .597 .8·16 1965 .75U 1.075 1U67 .796 1.012

9 Bridgeport 85 1 (�.357 ,) 1969 .2975 .818 12 x I)

8 Hatfield 230 1 0.095 67-69 . 033 --

(8 x 2 ) .080

292

Av�. Avg. lIe,,,·l.h

1'0 tal LoudJ!;:. Solids, DI'\' I.l, ,

\\' t. % llr n:.'

25* 1.7\l*

35-21 1.73 2CJ 2.1H*

25* 1.-17*

2li 1 . U�*

�li 1.83*

53** 2.22

20 1.7-1

25* 2.35*

15* I . :) 7�

25* 2.2J*

.15'h 3 . :32*

25* 2. utP�

Ac tua1 Tons/Y'

Full- I i IlIt� Dc-

sign 'rOlls/Yr • Lond I'nctor,

(X).(Y)

,1St! · 15·1

.�30

.5�O

.r,V5

.liU 1

.Hli

.UUU • H�17

I)' IG . . -

.U�tj

.369

.(jj5 .li �·l .505 .81Li ./:lUli

• 2!)�!

--

}If 1'.

" u

'" ... on CJ "1'1 " , - ", � .

.

--.-

. 11

N N N

n

N N

N

N

Il Il N

n

solids flowrate in the operating incinerator divided by the design dry solids flowrate.

In one plant (MHF No. 1 ), less than the total of MHF's ran at one time, while in another plant (MHF No. 6) the annual amount incinerated exceeded the nominal value, due to operation at overcapacity and overtime. In yet another plant (MHF No. 3), only 2 of the 3 total units operate simultaneously, so that there is always a standby unit.

ECONOM IC IND ICATORS AND LABOR RATES

All costs were converted to 1 969 dollars since the field visits were made in that year. Adjustments were needed for raw data obtained during the previous three decades on capital equipment and other materials, and for the cost of all types of labor involved. Much effort was devoted to arriving at realistic conversion methods, which are summarized in Tables 3 and 4, standardized to 1 969 . The principal considerations are detailed in the following paragraphs, by column.

Year

1938 9

1 �·IO 1 2 3 4

1945 6 7 8 9

1950 1 2 3 4

1955 6 7 8 9

1960 1 2 3 4

1965 6 7 8

1969

Table 3. Summary of Economic Indicators for Capital

Equipment and Materials

(Standardized to 1969)

�nIF Normal Installed Haiflt.enance �nIF

Capi tal Purts & Cas tin�r'S 0 He f '·· .. e Lory Cost Su lies Cost COsL

(1) 2 3 4 :25.6 30.5 25.2 30.1 25.7 3U.1i

.

27.5 32.1 29.7 34.1 30.3 34.3 31 .0 34.4 31 . 5 34.7 36.8 42.1 44.7 41.8 50.1 51.2 48.8 45.5 54.2 55.5 49.0 46.8 56.8 55.5 51.4 49.1 59.6 57.4 55.0 54.6 65.7 61.4 55.9 56.3 66.5 61.5 57.3 59.0 69.2 62.1 58.6 59.9 71 .1 63.2 60.9 62.3 73.2 64.9 65.8 66.4 79.1 71.0 70.2 69.8 84.2 77 .0 72.2 71.2 85.9 79.4 74.5 73.5 87.4 81.4 76.0 75.0 87.4 82.7 76.6 76.3 86.8 82.3 77.7 78.1 87.5 82.6 78.7 79.7 87.6 82.7 80.4 81.7 88.0 84.0 82.1 83.7 88.9 85.2 85.3 86.8 91.1 87.5 89.0 89.9 . 93.3 91.8 93.8 94.5 96.2 95.5

10U.0 100.0 1UI).0 100.0

293

INSTALL ED CAPITAL EQUIPMENT (Column 1, Table 3)

The cost of installed MHF incinerator systems was considered to be the sum of the fabricated equipment costs (castings, blowers, motors, controls, etc.) and the charges for construction. Castings include the center shaft, rabble arms, teeth, and other structural components. A cost ratio of 60 percent equipment/40 percent construc­tion was taken as typical , based on industry experience. The Average Marshall and Stevens Equipment Cost Index [ 1 0] and the Engineering News Record Construction Cost I ndex [ 1 1 ] were combined in the 60/40 ratio to produce the MHF Capital Cost Index (Col. 1 of Table 3).

NORMAL MAINTENANCE PARTS AND SUPPL IES

(Column 2, Table 3)

The Plant Maintenance Cost Index [ 1 2] was used for updating normal maintenance materials. This index dctually is a I1)ix of labor ar:d materials, but was adopted here for materials alone.

MH F CASTINGS COST (Column 3, Table 3)

The Process Machinery subcomponent of the Equip­ment component of the Plant Cost Index [ 1 3] was used for castings. Of the seven subcomponents, it is the one that appeared most representative of castings.

RE FRACTORY COST (Column 4, Table 3)

The Clay Products components of the Marshall and Stevens Equipment Cost Index [10] was adopted for the furnace brick.

OPERATING LABOR (Columns 1 and 2, Table 4)

In this (and every other labor category) there are regional differences, but their ·consideration was felt to be an unwarranted refinement in view of the other Simplifica­tions made. The national Average Hourly Gross Earnings per Nonsupervisory Worker in Electric, Gas,.and Sanitary Services [ 1 4] were selected. The 1 969 rate was $3.88/hr.

NORMAL MAINT ENANCE LABOR (Columns 3 and 4, Table 4)

As for Normal Maintenance Parts and Supplies, the variation of the associated labor was also represented by the Plant Maintenance Cost Index [ 1 2]. Reference [ 1 5], which dealt with a wage survey in the I ndustrial Chemi­cals Industry, was used to obtain absolute hourly rates.

Table 4. Summary of Economic Indicators for labor

(Standardized to 1969) .

Labor Categories

Hnintenance

Operating unci Castin/;" Refrnetory

Year Index $ Per IIr Index

(1) (2) (3)

1958 62.1 2.41 71.2

9 65.2 2.53 73.5

1960 68.3 2.65 75.0

1 70.6 2.74 76. :1 2 73.5 2.85 78.1

3 76.0 2.05 79.1

4 78.4 3.0·1 81.7

1965 81. 7 3.17 83.7

6 85.0 3.30 86.8 7 88.7 3.41 8Y.!) 8 93.6 3.63 91.5

1969 100.0 3.88 100.0

From the maintenance standpoint , it was felt that the duties, and therefore wages, would be comparable. The Nationwide Average of all Maintenance Skills (varying from janitor to instrumentation repair) for November 1965 was given in [15] as $3.41 /hr which corresponded to a 1 969 rate of $4.07 according to [ 1 2].

CASTING REPLACEMENT LABOR (Columns 3 and 4, Table 4)

This was taken to be identical in all respects to the Normal Maintenance Labor above, in view of similar skills being involved.

REFRACTORY REPLACEMENT LABOR (Columns 5 and 6, Table 4)

This work involves bricklaying and is more highly paid than Normal Maintenance. The National Average Straight Time for Masonry, Stonework, and Plastering [ 1 6] was adopted, with its 1 969 rate of $4.9 1 .

INSTA L LED MHF CAP ITA L COST

In practically all cases, capital charges was the largest item contributing to total MHF cost per annum or per

294

$ Per Ilr Index " .- Per Ilr (4) (5) ( 6 )

2.90 60.5 2.97 2.99 63.7 3.13

3.06 6r..6 3.27 ,

3.11 68.6 :3. :\7 3.18 71.1 ;1. ·1 !I 3.2i> 7�.9 3.f>H 3.33 75.2 a . r. \}

.

3 .. \ 1 78 .. \ 0.H5

:1 • f)·1 8I.9 .\ .O:.! 3.ut> H5.!) ·1 ,).) . --

3.1'5 !1I . 'I ·1 .·1 \}

.1. 07 100.0 ·1 • fit

ton of dry solids processed. The conventional method of financing public works by municipal and other bonds, typically with a 25 -year payoff period, and the custom of soliciting bids for each individual job from qualified vendors are the principal factors in determining the capital charges. It is difficult to establish the exact value for a piece of equipment such as a furnace because con­tracting is usually by competitive bidding. In the present case, all the MHF units examined were built by two vendors: BSP Corporation of San Francisco and Nichols Engineering and Research Company (NERCO) of New York. The conversion of original costs (some dating as far back as 1 938) to 1 969 levels was made according to Column 1 of Table 3 .

Table 5 gives the breakdown of installed capital costs, which include the metal and refractory parts of the fur­nace proper, assembled with the various air blowers, fuel injectors, drive motors, scrubbers, controls, instrumenta­tion, and other accessories necessary to the operation of the furnace. The installed cost does not include consulting engineering fees, sludge dewatering, ash handling and disposal, building, or land. Surprisingly , the field visits with few exceptions did not produce good capital cost data - many capital cost figures were approximate or not known. It was then decided to contact the consulting engineers on the various installations; in all cases co­operation was excellent and files going back decades

Table 5. MH F Installed Capital Cost

�n!F No. Location

1 Clevelantl � Mi/Ul.-S t. Pnu1 2B Minn.-SL. Pnul

3 Kansas Ci ty

4 Cleveland

5 BaLtle Creek

6 Bnttlc Creek

7 Saginaw

8 Hntfie1d

\J Bridgeport

10 SouLh Tahoe •

11 Hi/ln.- St. Puu1

12 Saginaw

13 East Rochester

(U-Uid) No. Contract of

Year MllF's

1 %3 <1

l\.l31l 1050

19G·1

1\138

l\.1GG

1960

11163

1 !Jll7

1%4

U)(17

1065

1 \.16 UB

1 \J63

3 1

3

4

1

1

1

1

1

1

3

1

1

Installed Kan:ias Cost ["gin. (�u!F Soc.

Plu5 1\)(;2 Bui1dillJ,( ) Ref. I I �)

Uni t Mllt' lIearth

Aren,Ft2

2327

2UIl·1 :o!08'1

2081 1-125

1068

800

845

230

85

575

2808

1560

230

Source for Co�L�; Ilelllark:i

(CE-COllsulling Engrs. )

llavens & Emerson (CE)

l' 01 tz ,KiJl:,! , 1?" nd J , Amlc rs 011 ( CE) 1U51 M-SL.P. Snnitury Di.LI·icL ll"porL ( l\)(.h ,uUlual), p.llu

Black & VeaLch (CE)

llnvcJls & EJIlel'son (CE); hull<y cOlilbust-ion air prchcllLer inclutleu.

HcNalllcc, POI' LeI' & Sec1ey (CE); Elaborate conLrols incillued.

Malcolm Pi1'1lie & Co.(CE); EsLu cost.. of Imilding suut.l'acted.

lIuuucll, HoLh & Clu"I, (CI';)

'frucy Lnginccrs, Inc'. (CE ) Gcorg:� B. �h'hUSt Illc. (CF);costs of �Ull" componCH ts a\'nilnhlc.

South Tahoe Puhlic Util. l>isLj I:;s Ld. dc",'a teriJl� etc. cos t..s �\lhll'HC t cd.

T 01 Lz ,l( illg t Duvull ,AndcrRon (CE); Est,l \l(.'\\'at..l!l'.cosLs suhtl'£H:'lt'ti.

�Ie LeaH w"l Ed,ly (Cl::)

Lozil.·r EJI:,!;irtt·t·l'� (CE )

Table 6. Engineering Fee Structure

Correlating Equation: Y = O.5Xo .•••

Fee, Pcrc"ll (. of Installcu COOl

ASC[ Hediall �tissouri Ohio

19Gi Rc r . I 18) Soc. Prof. Soc. Prof. Eng!'s. Engrs.

Complexity: Prc-1\)utl Pn-l %8

Aver- Aboy" Il.pf'. (J c) R�f . I I ") , " Dollars �led.-Higb age A\'g. Hillimwn Hinimwn

(X) *

10,000 12.5 -- -- 13.0 --

25,000 1l. 1 -- -- 12.0 --

50,000 10.0 9.4 12.7 11. 0 --

100,000 9.0 8.25 10.75 10.0 12.0

;,!OO,OOO 8.0 7.3 9.25 8.25 9.75

300,000 6.75 6.85 8.6 7.25 9.0

400,000 5.9 6.5 8.1 7.0 8.63

500,000 5.2 6.25 7.75 6.75 8.4 •

750,000 4.0 5.85 7.25 6.25 7.6

1 Million 3.5 5.6 6.8 5.75 7.2

5 Million -- 4.75 5.8 4.67 5.84

295

Orig. Cost pel' Uni t

�Ull" (1 UOO$ )

37ti

217

380

241

197

225

71

233

81G

630

8U

Se1cc Lt'u Absol ute Etwiuecr-0

ing Fee (�Iis souri

Hillimuru) Dolla,'s

(Y) 1,300

3,000

5,500

10,000

16,500

21,750

28,000

33,750

46,875

57,500

233,500

1!J(j\) Cost pCI' lI"i t

�UlF ( 1 OUUS)

317

253

U2

2(;2

903

IIJ�

yielded desired information and in some cases also cor­rections of field data. One bonus was that the 9 basic units examined could be expanded to 1 3 , for capital cost only, because the installed costs of other recently con­structed MHF sludge incinerators were made available to this program. Hearth area was chosen to be the variable indicative of capacity because it related directly to hard­ware and so to its first cost.

In three of the units, only combined costs of incinera­tion, dewatering, disposal, etc., were available, so estimates were made for the incineration cost alone. Table 5 indicates the data sources and the nature of any adjustments made. The contract dates are the basis for dollar cost conversion.

Figure 3 is a log-log plot of the 13 points (cost vs hearth area). A least-square fit of the points to a power function resulted in an exponent of 0.60, which happens to be the "classical" value for chemical process equip­ment . No doubt, this close an agreement is fortuitous but it does point up the chemical process aspect of incinera­tion. Some inflationary trend was noted with the cost adjustment chosen in that MHF's for which contracts had been signed in or after 1 966 tended to fall above the least-square line, while MHF's ordered before 1 95 1 tended to fall below the line. The equation of the least square line is the power function.

CCl = 5464 (FHA)o.60

For meaning of abbreviated terms, please consult the Glossary at the end of the paper.

OTHER CAPITA L COST COMPONENTS

COOLING AIR FANS AND COMBUSTION AIR

BLOWERS

It is possible to determine the required capacities of cooling air fans and combustion air blowers given the , dimensions of the flow channels, the cooling rate re-quired to maintain adequate apparatus operating life, the sludge feed rate, and combustion and (including excess air) requirements. Consideration of heat transfer correla­tions, friction factors, thermodynamics, and stoichiom­etry could then be used to predict the blower and fan requirements. However , manufacturers of MHF's have presumably already solved this problem empirically or otherwise with every furnace delivered. A realistic pro­cedure, then, consists of utilizing information on fans and blowers collected during the field visits. However, the field data were incomplete (only half the plants visited responded). The available data points on air/flowrate,

12. � 500 � " • " - •

• � • • � • � " & � • • � • 7. � � - � � -• 200 " �

• � Doto from • • � � � c Table • • .' � •

• • c 0 - f; 13

� 100 NU.IIIerlll" nre HIlF NWllberll

,o �������-L���������-L�� ")0 1 00 2 00 ';00 1 000 2000 iUOO

Errt'ctive Reart.h Area pt'r HlIY (AlA.), 8'1 rt

FIG.3 MHF INSTALLED CAPITAL COST

fan and blower horsepower, and wet sludge rate were straight-line fitted with the following results:

Combustion Air

Blower rated horsepower = 0. 1 5 per daily ton of wet sludge Air flow at 1 6 osi = 1 50 scfm per rated horsepower i.e., air flow = 22.5 scfm per daily ton of wet sludge

Cooling Air

Fan rated horsepower = 0.08 per daily ton of wet sludge Air flow at 8 inches water = 450 scfm per rated horse­power i.e. , air flow = 36 scfm per daily ton of wet sludge

BUILDING COST

. Installed capital cost for an MHF never includes the building, if any, in which the furnace is housed. In many cases, furnaces are installed in existing buildings, or the latter may be enlarged. When entirely new buildings ac­company the furnace, the costs may vary widely. In the 1 965 Minneapolis-St. Paul contract (MHF No. 1 1 , Table 5), one new building 'housed four incinerators with dewater­ing equipment , controls, instrumentation, lockers, lunch room and service facilities. An estimate of the cost of that portion of the building applicable to incinerators alone gave a ratio of building cost to incinerator installed cost of 0.5 1 3. For the Battle Creek 1 960 contraCt (MHF No. 5) the same ratio was estimated as 0.667. No other examples of simultaneous furnace and building construction came to light. Obviously, the cost of a new building is significant compared to the MHF installed capital cost.

296

ENGINEERING FEE

Under the current system of contracting for treatment plant equipment, a fairly consistent fee structure has evolved for consulting engineers, as a percentage of the net construction cost, here taken as the installed cost of the MHF plus building, if any. Table 6 lists percentages recommended in the states of Kansas, Missouri, and Ohio [ 17] and by the American Society of Civil Engineers [ 1 8] . For the present purpose, the Missouri fee structure was selected as being in the middle of the range of fee

,

structures examined. The least-square power law fit for this structure, in the cost range from $ 10,000 to $5 ,000,000 is

EFC = 0.5 (CCI + BGC)°·846

LAND COST

One of the advantages of an MHF (which is built "up" rather than "out") is the small land area occupied. Four available data points were used to establish a ratio of land area to superficial circular area based on MHF outer diameter. This ratio was found to be KLA = 4.938. In all cases, the land area is a small fraction of an acre, and thus a minor cost item.

HEARTH REP LACEMENT MATER IAL, LABOR, AND FREQUENCY

During the life of an MHF, it may become necessary to replace one or more hearths because of their deteriora­tion due to exposure to high temperature and to heating/ cooling cycles. Based on hardware experience , estimates were made for material and labor required to replace the

two components that make up a hearth : castings (e.g., rabble arms and structural supports) and refractory (brick lining of horizontal and vertical hearth surfaces).

Table 7 shows that all replacement expenditures per hearth increase with diameter. Castings are more expensive than refractories from the material standpoint, while the amount and unit cost of refractory replacement labor exceed those for castings (Columns 4 and 6, Table 4). Correlating polynomials for all four hearth replacement expenditure items were fitted to the data in Table 7. The equations are valid for Outer Diameter (HDIA) values from 6 to 23 feet and apply to replacement of one hearth:

On-Site Castings Material

On-Site Refractory Material

Casting Man-Hours

Refractory Man-Hours

CAST = 1 08 1 -220 HDIA + 89.5 (HDIA)l.S

REFR = 458 -1 20 HDIA + 1 4.6 (HDIA)2

CLH = 7.47 -0. 1 05 HDIA + 0.0299 (HDIA)2

RLH = 69. 1 -0. 1 05 HDIA + 0.530 (HDIA)2.2s

Frequency of hearth replacement should preferably be based on the lifetime history of an MHF. Part of a life­time replacement history was available fot the three Minneapolis-St. Paul units commissioned in 1 940, during the period 195 1 to 1 965 . Table 8 compiles the number of hearth castings and refractories replaced during the 15-year period. For the castings, it was assumed that each rabble arm replaced was equivalent to � casting and each tooth to 1 /40 casting. The result is that the equivalent of 44 hearth castings and 1 0 hearth refractories were re­placed during years 1 1 through 25 . With the assumption that the replacement frequency for the first 1 0 years was

Table 7. Hearth Replacement Material and Labor

Castings (One Hearth) Ref rae tory ( One Hearth) Outer

Diameter, Ft On-Site Material, Labor, On-Site Mntcrial, I,abor, (13.5 in Wall) 19G!J $ Man-Hours 19t19 :S Han-Hours

"1IDIAII nCAST" "ClJ I" " IU1,'U" "nul"

10.75 1907 10 913 192

14.25 2695 12 1661 288

16.75 3410 14 2365 3:JG

18.75 44115 16 3542 480

22.25 5511 20 4950 6·10

297

Table 8. 15 Years Hearth Replacement History: Three Minneapolis-St. Paul MHF's

(Operating Since 1939)

Each MHF: 8 Hearths = 20 Arms + 138 Teeth

Cas tings Refractor ...

Rabble Rabble Total ,

Teeth Arms MIIF },- 0 • Hearth Total

Year Heanlt No. }; 0 • Equiv. No. Equiv. Equiv. Hearth" -I l)!) J - All , �Q O.iiO f) 0.50 1.00 , -

-

1 ')'j� I AlJ I f) 0.05 <) 0.50 0.55 - -

1\.153 .UJ �4 0.60 -

1.25 1.85 , ;J

1954 3 iro , � 1

1/2 1 1/� 1 1/4 1 All 81 2.00 6 1.50 3.50

2/2 1 2'3 / 1 2/4 1

1955 l/All 9 0 . 25 2 0.50 0.75

2/All 51 1.25 8 2.00 3.75

3/All 4 0.10 - -- 0.10

1\:15G All 58 1.45 16 8.0 9.45

1/ 1 I 1

1/3,4,6 1

2/1 1 1957 AlJ 55 1.40 12 6.0 7.40

1958 - All -- -- 2 0.5 0.50

1959 All 18 0.50 3 0.75 1.25

1960 All 44 1.10 11 5.00 6.10

1961 1/ 7 0.20 4 1.00 6 . 20

3/ 2 0.05 3 0.75 0.80

1962 3/ 3 0.075 2 0.50 0.58 , 4/ - -- 2 0.50 0.50

1963 1/ - -- 5 1.25 1.25

1964 1/ - -- 2 0.5 0.50

2/ - -- 5 1.25 1.25

3/ - -- 4 1.0 1.00

19G5 1/ 5- 0.125 - -- 0.12

15-Year Totals 42.90 10 ,

--.- . --- .- -

298

only half of that during years 11 through 25 , the follow­ing tabulation results:

Hearths replaced dur ing years 1 1

through 25 ( in 3 M H F 's, each 8

hearth)

Equivalent MH F's replaced during

years 1 1 through 25

Equivalent M H F 's replaced during

years 1 through 1 0 (at half rate of

years 1 1 through 25)

Equivalent M H F 's replaced during

total 25-year l ife

Castings Refractories 42.9

1 .79

0.60

2.39

(CR L)

1 0

0.42

0. 1 4

0.56

( R R L)

Conditions at Minneapolis-St. Paul were relatively "mild", e.g., temperatures below 1600 F, primary sludge with high total solids, no sulfur in the auxiliary fuel, and, per­haps most important, 24 hr/day (HPD) operation, This . meant few cooldown/heatup cycles and consequent low thermal stress. It was decided to use the replacement rates above (rounded oft) for all 24 HPD furnaces, and a rate twice as high (to account for increased cycling) for all furnaces which operated less than 24 hr/day, i .e . :

CRL = 2.5 for HPD = 24 and 5 .0 for HPD less than 24 RRL = 0.5 for HPD = 24 and 1.0 for HPD less than 24

NORMAL MA INTENANCE MATER IA L AND LABOR

Multiple-hearth furnace maintenace has a periodic component (hearth replacement, see previous section) and a continuous component (normal maintenance). The latter involves, for the most part, regularly scheduled in­spection (for wear, corrosion, and failure), servicing (e.g., lubrication of rotating machinery) and adjustment (e.g. , of instrumentation) of all MHF components that may under­go changes as a result of operation and mere exposure to the environment.

The field survey produced sketchy and incomplete in­formation. In particular, normal maintenance was some­times lumped in with operation or hearth replacement. Frequently, labor and material (the latter called Parts and Supplies) were often reported as a combined dollar ex­penditure. The variables to be correlated were assumed to be expenditures per elapsed calendar time, rather than per operating time, because normal maintenance efforts and environmental deterioration are both related to calenda: time. Physical size, expressed as hearth area was taken as the independent variable, although its influence would be expected, and, in fact, was found to be minor.

Table 9 shows data from six MHF's for normal mainte­nance material and labor, Fig. 4 is a plot for parts and supplies, and Fig. 5 a similar plot for labor, both for

Table 9. Yearly Normal Maintenance Material and Labor

JIearth Par t � IlUt! Su l i es

. N o. . Area A l l Pel' �UU-' )UIF o.f Sq . Ft. /�UIF �oa" 1 s I DG U $/Yr . �o . Lo.catio.n �U!F ' s " FlL\" Year $/Y r .

2 Hinn . -S t . Pnul 1 :!Ot:H 1%5 - r, 92 , , -

3 Kansas C i ty 3 20tH 1 909 5 , OUO

5 Battle Creek 1 1 068

l OU9 3 , 000

6 Bat tle Creek I 890

7 Saginaw 1 8·15 1969 2 , 400

9 Bridgepo.rt 1 85 1969 2 , UOU*

*Estimate

Correla ting Equations :

Parts nnd Suppl ies : AMSY � 570 (FHA)0

.023

, 1969 S per year

Labo.r : ANMM = 120 (FUA)0

.307

, Han-fir per year

299

" A.'I� l' "

� , 1 78

1 , UG7

1 , 500

1 , 5UO

2 , '100

2 , 00U

Ycar

1 !)ij5

1 UtiU

lOU\l

1 \lU \I

I \lti 9

.

/tLlw Da ta

Labo.r

E.xpcndi lurc

$ 1 :! , O!1·1 ( ·1 Hlli." s )

$ 1<1 , UUIJ"

(3 Hill.' , s )

2 , OUU �Ial\-nr ( 2 mu" s )

72U HIUl-Ilr *" (,)1 - " of To. till Labo r ((; . 3 }bll-Ili' /lay)

Hall-I l l' • Yr • •

Pc' l' )11 iJ' 0, \� '.1'1" � '. ,

M':-: 0

I , ·1 73

l , OUU

I , Uilil

720 ·IUU

f- ' I-!----f-----

')00 5 0

9

, 100

, ,

, , I I

, , ,

7 •

(, · 1·5

, I �oo r;oo 1000

, , -

-

-

It; 1-"

-

-

-

Data from -Tublc 9 -Numerals --rrOIl! MIIF

Numbers -

-

I I I I � -L 2000 5000

FIG.4 COSTS OF NORMAL MAINTEN ANCE PARTS AND SUPPLIES

�

� - < -" � --- - � = = " � :: >-

-- � oS � ,- Co - = - � � 0 � 0 0 0: ;..- ,

= �

:woo ,.. " " f-,.. ,..

1000 t-t- ' r r

'j00

r t-t-t-

250 'j0

q ",..

I I I I 100

, , , ,

I I I I 200

, ,

I 'jOO

, , , ,

G

7.

I I I I 1000

, I , , ,

�;

I _

-

.. -

-

-

-

-

Dote from -Table 9

- Numcrlt Is -

or(' MHF -

NumbcT8 -

-

I -L-L-.l i -L �ooo 1)000

FIG.5 NORMAL MAINTENANCE LABOR

annual expenditures as a function of hearth area. Least­square power law fits for these curves are as follows:

Annual Parts and Supplies AMSY = 570 (FHA)o.023, 1 969 Dollars

Annual Labor ANMM = 1 20 (FHA)o.307, Man-Hours

OPERAT ING LABOR

Since this item is usually the second-largest component of the total annual cost (the fust being capital charges) considerable effort was expended in obtaining realistic data. These are listed in Table 10 where the raw data are converted to Equivalent (i .e. , considering supervision) Nonsupervisory Man-Hours per MHF per 24 hour Operat­ing Day (qJMD). In the absence of other information, the nonsupervisory man-hours were increased by 1 0 percent to account for the cost of supervision. Where supervisory man-hours were available, a supervisory hourly rate 1 � times the nonsupervisory rate was adopted.

Figure 6 is a plot of the 1 0 points of Table lO on a qJMD vs FHA log-log graph. The data indicated that there was a minimum operating labor level in "small" plants,

300

independent of the MHF size. This level was taken to be CllMD = 6. These plants, too, operated the MHF on 8-hour shifts (compared to round-the-clock operation for the other plants), contributing to higher specific labor costs. The remaining points (except two) were correlated by a 45-degree line (indicating direct proportionality), which met the minimum qJMD at FHA = 7 1 9. The correlation then becomes

For FHA equal to less than 7 1 9 sq ft : CllMD = 6 For FHA greater than 7 1 9 sq ft : qJMD = 0.008343 (FHA)

For two points which lie below the correlating line on Fig. 6 represent Saginaw and South Tahoe. In Saginaw, the incineration is continuous and requires no auxiliary fuel in view of the 45 weight percent solids content ; at South Tahoe two other MHF's (for lime recalcination and and activated carbon regeneration) are part of the same plant and all three MHF's are operated jointly. Reduced operating labor would be expected in these two plants, and the dashed line parallel to the main line represents a 1 /3 reduction, which is reasonable.

A direct proportion relationship means that no ad­vantage in operating labor cost is gained by having fewer

Table 10. Operating labor

IDill = Non-supervisory Han-hours SHII :I Supervi s o ry Man-bours

MIIF N o .

1

Location

Cleveland

2 M i nn . -S t . Paul

3 Kansas City

4 Cleveland

5 Battle Creek

6 Battle Creek

7 Sagina\l(

8 Ilatfield

9 Bridgeport

10 South Tahoe ,

No . lIearth of Area

HlII" s Sq Ft /HIlF

4 �327

4 :!084

3 2084

4 1425

I 1068

I

I 845

1 230

I 85

I 575

Raw Data (for 21-)�. Opera t i ng Day) 19UU Unl ess Stnt�rI

( 72 NHlI + 8 SHlI ) per Day for 1 �O W ' s

66 NHlI per Day for <I �O lF ' 8 ( 1 !J65 ) 10jt (c s t . ) of (Dowater + Incin. ) Lauol' (80 Equi val ell t IDOl/Day r or :2 fOil'" s )

40 IDOl per Day for 4 WIF ' s ( l U58 )

8 IDnI per Day

8 IDUI per Day

4 IDUI per Day

6 . 4 Nl'UI per Day - No S}Ol

80jt (est . ) o f Total Labor ( 6 . 3 NMlI per Day)-No SHH

$19 . 93/Day at $5.65 Equivalent Hourly Rate

* Equivalent NHlI (Es t . ) • 1l0% of Actual IDOl (For Supervision )

-Supervisory Hourly Rate (Est . ) • 150� o f Non-supervisory Hate

I I I I f-

�o �

.,. !f � .",. f-:-: -c

_ . >. -I • : 0 '" '" 10 -, Q. � o 0 - " -

r-r ,:3 -- r-

.,. '!t ':' 1 - r--

'. " 9 � 0 '; . -�

,H r-" • Q. f-

I I I I 50 100

I I I

R

� I I I

200 ';00

I I I I

6 •

� 7

I O / ....

I I I I 1000

I

.

I

Effective Hearth Area per HIIF' (F1IA ) , g i l ft - - - 1'1(1n18 \d 1 i 1 Luhnr Sn\'ill��

FIG . 6 OPERATING LABOR

301

I I

-1

�

-

-

-

-

-

Data from Tabl. 1 0 -

Numf' rn l s are MIlF -

Nwnbere

I I �OOO

Equival r n l IDIII pC I' �no' per �·l-JIJ· .

Opera t.ill:,!; lJit�·

21 . U�

1 8 . �1C

l G . U

1 1 . lI*

1 :1.8*

1:1. 8*

4 .4*

6 . 1

5 . 0

3 . 5

larger MHF's in a plant, i.e., there is no "large economy size." While at first this appears unrealistic, it should be noted that a recent study of over 1 5 00 U.S. treatment plants by Michel, Pelmoter, and Palange [ 1 9] was inter­preted by Smith [20] to indicate that the total labor (mainly operating) was almost directly proportional to the plant design capacity independent of plant size. Ad­mittedly, this was the result of an overall statistical study of all types of conventional wastewater treatment plants; other overall plant studies (e.g., McMichael, [2 1 ] indicate a savings for larger plants. Obviously, the relation between operating labor and capacity for specific processes will vary with the process.

E LECTR ICAL POWER CONSUMPT ION

Good records are usually kept of electrical power consumption, by kilowatt-hour and dollar. Since power is consumed mainly during operating periods, a measure of cumulative power usage is the cumulative tonnage of treated sludge. The quantity kilowatt-hour per ton dry solids (PDS) was selected as the variable. Table 1 1 shows the data points that are plotted on Fig. 7 on a log-log plot vs FHA. The figure shows that PDS decreases as FHA in­creases. Ten of the 1 1 points were correlated (for FHA up to 2808 sq ft) by the function

PDS = 29.6-6.55 x 10-9 (FHA-2808)3 + 4.94x 1 0-!6

(FHA - 2808)5

The one uncorrelated point is for the Minneapolis fur­naces (No. 2) which employed natural draft and had no wet scrubber, thereby reducing power usage below the level of the forced draft, wet scrubber installations.

FUEL FOR THERMA L CYC L ING

When MHF's are not operated round the clock, the question arises of how best to schedule the on-off cycles, so that MHF life and operating time are not curtailed to the point of inefficiency. In addition to cycling as part of the operating schedule, at least one annual cold MHF in­spection is advisable. Due to the inability of refractory to sustain tensile loads that may result from thermal stresses, it is important to limit the temperature differences within the material by limiting the heatup and cooldown rates. Although experts differ on details, there is a general con­sensus that somewhere in the temperature range from ambient to operating (say , 70 to 1 500 F) a "soak" period should take -place for reasons of stress equalization within refractories. In this study, 1 200 F was chosen as the soak temperature , and also as the "standby" heating tempera­ture, when the furnace is to be inactive for short time periods, such as overnight, or even weekends. This soaking should be carried out both during healup and cooldown.

The allowable rates of change of temperature are lower for large hearth diameters and furnaces than for small ones, and are usually specified by the manufacturer.

Table 1 1 . Electrical Power Consumption

lIear th Area/MIll' N o .

MllF Sq F t o f N o . Location "}'1lA" MI[F ' s

I C l eve l and 2327 -1

2 Hi IUl . -S t . Paul 20H4 4

3 Kansas C i ty 2()H4 3

4 C l eve l and 1425 4

5 Bat t l e Cree i< I Oti8 1

7 Saginaw 8'15 I

8 Hu t f i e l d 230 1

9 Bridgeport 85 1

1 1 �li rUl . -S t . paul 2808 3

"'Low Va lue : Natul'al Draft - No We t Scrubber

-Es tima t e d frOID lIT' and S l ud�e F l o w Ratings

" 'Es timated from AMl. F l o w R a t e s

-

Data Year

I ()(j 7

l Uli7

' 0 7- ' 0 9

' 0 2- ' 65

l V08

l OG \!

l \)()()

1 \I{\ \ l

1 \)68

302

Ac tual Avg . pe r MIIF l OOO ' � Kllll

p e r Y e a r

9-1 . 7

--

2U;

205

2�U

5/11

--

1 7 . 3

--

- .

- -

Dry So l ids

TOllS IY('cLr I -- - -

3:WO

--

7 2 7 2

10u'!

HOUO

7 (; 20

- -

' ) I )l �)()( - -

I --

KWH

p C I' Ton Dry S o l i d s

- -

�H . t:l

[) . 2"*

2 \) . 7

50 . (;

3 7 . 8

7 1 . 0

9H . ' 1-

7 tl . ,I

:F,.7

1{. t I - ' , , , I r ". f-r

-

"--

50 r

i-- • '"

c � c �

-- •

f-- • -if t-o ,- '" c 0 c

'- • 20 +' • • • -' •

f-_. "-•

" '" -- , � . - '" " 10 • f-"-

if, � --

5 , I , ,

. � , , , -

' . -

"'l J,

I I , -'-

,

I I •

)

2 --'-

, , , '--

-

-

-

-

Data frolD-Tabho 1 1

Numerals -are MHF -Numbers _

-I ..L I I

50 200 500 1000 �OOO ')000 1 0 , 000

Effective Hearth Area p�r HRF (FHA) , aq ft Note : MRF No . �) nnt ti ttpet . h1"CflU8f' plant

I llIet 110 ... ·l·t ... crulll",!" 0 " iutlnct·" tlrilrt fall.

FIG . 7 POWER CONSUMPTION

The field reports showed that the large furnaces, in Minneapolis-St. Paul and Cleveland, are limited to tran­sient rates of 20 to 25 F/hr with soak periods of 1 to 2 days, so that it requires several days to heatup or cool­down a furnace completely. The .allowable temperature change rates increase to 1 50 F/hr for the smallest MHF's examined. The duration of the stated transients affects the steady-state annual MHF operating period and also the fuel requirements during transients. To limit the temperature change rates, it is sometimes necessary to apply heat to an MHF while it is cooling down. In this study, however, it was assumed that a furnace could be "bottled up" (Le., all openings closed off) well enough to avoid the need for heating during cooldown. Moreover, the heat input rate (Btu/hr) was taken to be the same for 1 ) the presoak ambient to 1 200 F and 2) the postsoak 1200 to 1 500 F periods.

A unified "optimum" operational scheme for all MHF's was developed with the help of the field data which sug-

gested that the furnaces should be divided into three operational groups :

(A) (Large Cities) MHF No. 1 , 2 ,4 (also 7)

24 HPD (hours per day) 7 DPW (days per week)

(B) (Intermediate Cities) 24 HPD MHF No. 3 , 5 , and 6 <7 DPW

(C) (Small Cities) 24 HPD MHF No. 8 and 9 Any DPW

First, a Heatup Time (CYT), hours, (also equal to cool­down time) was developed as a function of furnace hearth area (FHA), based on typical temperature change rates soak times. Table 12 shows how CYT values were generated for several FHA levels, and Fig. 8 shows the resulting CYT vs FHA straight-line-segment graph.

Figure 9 depicts the two types of cycle considered : a cold cycle for maintenance inspection for all three groups, and a hot cycle for standby at 1 200 F applicable to groups (B) and (C). Periods during which fuel is used

•

Table 12. MHF Heatup Rates and Times

Note: Heatup Hours = Cooldown Hours, but no fuel used during cooldown

Effective Assumed Ueart.h Alloynll1e

Aren, Sq Ft lien t.up " fllAlI Rate , °1',/llr.

200 :.t. 200 100

600 ,. 200 100

1100 :t 300 15

1100 ,. 300 50

2000 + 25

Hour. tor Portions ot Complete lleatup (0_1 500oF)

0-1200"F . 1200 F Sunk 1200-1flOU 1" (4/9) err (4/9) err _ (1/9) CYT

8 8 2

12 12 3

16 16 4

24 24 6

48 48 12

303

Total Jll!atup,

lIours "CYT"

18

27 3U

54

108

As.umcd IIOUl'S 101' Inspectioll ("/U ) LYf

1"

2·,

32

1M

tiC

1 1 , , , , I I , ,

r--7 - -

q0 -

�

� -:. • •

:.>

I �

" ::: ,-0 .,

" , �

0 • �

�-�--

.' 0

� �

0 0

u 0;0 ·

�

"- I r- I -

, �

- - - -' r d " --

r.; _.J - - - Tuble 1 2 Data

'" 10 � -.

� .c:::':.J Adopted for -

Compute-r . Projtrltm I I I I , , , I I , I I

o 1000 ":1;0(1(1

FIG . 8 MHF HEATUP TIME

1101' CYCIJl •

1500 \ ..

• 1200 -0

No Fuel 11- I1ca Standby· FU('l

-=

�

-0 tup Fuel

•

/9 ::h Night I W'cakcnd � ... 1/9 etc.

,:: 1

Time (CYT uni t s )

COlJ) CYCU, IIcutup Fuel

No Fup 1 .. Standby Fue 1 • 1500 - loLl intcnoncc 0 1200 - Inspection = � d --� • 0

...

0

1--'1 -I· R/9 1/9

FIG . 9 ASSUMED MHF THERMAL CYCLES

at the heatup rate, standby rate, or not at all are indicated on the figure.

Next, the total yearly cycling hours per MHF (YHUH) and yearly hot standby hours per MHF (YSBH) were established for each operational group in terms of CYT, N0F (total number of MHF's in plant) and SN0 (number of standby MHF's). This information is detailed in Table 1 3 which assumed two complete "cold cycle" in­spections per year. To model standby units, it was as­sumed that the load was evenly distributed among all furances on an annual basis. Combination of the YHUH

304

and USBH components in Table 1 3 leads to the following relationships for totaJ hours: .

(A) YSBH = (8/9) (CYT) (N0F-SN0)/(N0) YHUH = ( 1 0/9) (CYT) (N0F-SN0)/(N0F)

(B) YSBH = (N0F-SN0) [(8/9) (CYT) + 1 248 (7-DPW)] /(N0F)

YHUH = (62/9) (CYT) (N0F-SN0)/(N0F) (C) YSBH = [(N0F-SN(,?» /(N0F)] [(8/9) CYT] +

[8736-52 (HPD) (DPW)] YHUH = [(N0F-SN0)/(N0F] (CYT/9)

( 1 0 + (52 DPW)] From the field data, hourly standby and heatup heat re­quire men ts, in.terms of million Btu/hr, were developed as functions of the effective hearth area. Table 1 4 shows the available data, reasonable on heatup and scant on standby. Linear least-square equations are fitted to the standby data, plotted on Fig. 1 0, and the heatup data plotted on Fig. 1 1 :

Standby Heat Requirement, Btu/hr

Heatup Heat Requirement, Btu/hr

SBQ = 3 1 5 (FHA)

HUQ = 1 9 1 3 (FHA)

As might be expected, the standby requirement is a small fraction of the heatup requirement. The sum of the products (YSBH) • (SBQ) and (YHUH) • (HUQ) is the total annual heat requirement for thermal cycling. Division of this sum by the heating value (Btu/lb) of the fuel to be used then provides the annual fuel consumption for thermal cycling. The thermal cycling fuel is in addition to steady-state fuel needed (if any) as determined by the incinerator heat balance.

Ilr . Day s Ope r a- per pe r Type

t i o n a l Day We e k o f

G r o up I[PD DPW Cy cl e

(A ) 2<1 7 C o l d 110 t

(n) 21 <7 C o l d

lIo t

( C ) <24 Any Co l d

fIo t

Table 13. Yearly Cold. Heatup. and Stalldby Hours Per MHF

Assumption: Load evenly distributed among N 0 F on annual basis

.

C o l d lIo u r s 1 I c a tu!, l l o l l " � S I.a I I 0 1 by l I�urs ( O - l :!()lJ , J :!lJl I- i .-,UU''t- (,,-i J :! lJlJ'\') N o Fue l Used)

l Iollr� Cy c l e s It.r s . 1 10 1 1 " " Cyd e � 1''' 1' p e r per per p e r

Cy c l c Y e ur Year Cy c l ,' Y e ur

1 7 2 :11 [, .) - "Y - xy - xy \i () U -•

-- -- N Olle -- --

1 7 2 :I -l 5 . ) - xy - xy V

xy \) V -

1 5 2 n 1 G2 - xy V xy - xy \) V

1 7 2 3,\ 5 2 - xy - xv - xy \) U 9 •

1 52 DPW 5::! 1 [,2 DPW - xy xy - xy 9 \) \) . DPW

-

II" ., . 1 I0llrs

PL' !' p e r

Y " U I ' Cyc l l'

I lJ · 1 - xy - xy V V

NUlie --

I I I ·1 - xy - xy U V r,!,! ( 7 -DI'II' ) - xy U 0·1 . � Y 1 0 'I - xy - xy 9 V 52 :!.[ Y 7 -DI'I'! xy U . DPW ' ( :!.I -IU'D)

C \" ' l l' � I H' I '

Y t ' i t l '

. . -

--

. . -

5 !:!

. ) -

:""} " . -

Sf ) , - DPW

I I . . , .

pt · I '

)'t ' i l l '

8 - X y . V

•

�un('

H - xy V 1 :!·18 y

• ( 7-WI\ ) Ii - xy \J

y[8, ; I \ ; -5�( l U'O ) (01'1. \ �

Abbreviations I x - CYT ; y - (N¢F-SN¢ )/N¢F •

Table 14. Thermal CYCling Heat Requirements Per MHF

� He, IAvg. MIlFJ ill. 'h Oil, Gas , Fue l

Area Gal Cu Ft ,. .] Stand- Heat Qty . l I e at l Ieat "FIlA" Opg . (BtU ) ( Btu ) at, by,

lOt, �� ( Btu ) H a tc , Cy/Yr p e r nl' , N o . Location ISq .Ft . Gal Cu l't 1 " Dny/Yr U n i t 101 i lltu (llr/Cy) lUti Btu

1 ClovelcUlu �:.J27 l OG6 H l , 535 207 . 5 2298 130 . 5 . 734 ( 1 . a9xl05: (GUO)

. , Hiun . - 20H'1 1959 25, 70U JiJ7:.! 1 [; 3 . 97 St. Pl\ul

1�/·��5 \ pel' (GU)

Y; .

3 Kansas City 2084 1969 25, 000 22 1 per 4 . 4 (;u njWk per Week

(880 ) Week ( ::; ) 4 Cl eveland 1425 162-65 204 3 .15>:106 506 55 . 1 . 383

(Avg) II(1 .39xl05) (600) 5 , 6 Battle 1068 , 1969 -- 400/1Ir -- -- . 358 1 4 , 000 12 .54 1 per 1 . 3 9

Creek 800 (896) Cu ' . per 7�ek t;9�� Week ( U )

7 Saginaw 845 19G9 flOO 126 t; 1 . 75 Gal/Cr; per (,�) 1 . 4xlO i ) 1e

8 rr .. tfie1d 230 1969 (HOU 5 . 71 ( 1 � ) U . 475 Cu Fl/Cy per

(9aG ) Cycle 9 Bridgeport 85 1 969 �ijU 1 . 4 ( a ) 0 . 167

Ga1/ C): pr' r 1 . .1xI UV) Cy r. l (�

305

0.8 r------------------r------------------+-----�

" +' -= � " 0.6 r------------------r------------� • • o .. "

.. � = +' ,.. '" J! •

= ... 0 = � " � 0 . 4 '----------------:----t.r--:: = � r 6 •• • ,, :0:

"C c o

... <r.

0.2 1----------:

o o 1000 2000

Data from Table 14 Nwaerah are MBF Number.

JOOO

Effl'ctivl' HNlrtll ArC'll (II..'T MIfF (FIlA ) , RII rt

F I G . 1 0 MHF STANDBY HEAT REQUIREMENT

3 •

_ ,E -, " E • �. Co

, 1------------------r---------7L-----��--------------� "

.. � � .... , '" -

,:: III C

_ 0 ::: .... � '------------------� .... I = -

•• 0. >: "

... '\ • •

1 I-------��------+-----------------_+----------------�

o o 1000 2000

Datu from Table 1 4 Numcr . . l R fire

HHF Numbers

JOOO

FI G.1 1 MHF HEATUP HEAT REQUI REMENT

FUE L FOR STEADY·STATE OPERATION

A basic MHF energy balance matches the combined heat energy contained in the incoming sludge and auxiliary fuel to the sum of the outgoing heat flows. The latter are associated with the exhaust gases (including heat required for evaporation of the water content of the sludge), the ash, the part of the cooling air rejected to the atmosphere , and the furnace heat losses to the environment. The steady-state fuel requirements can then be found when

sludge properties, minimum percent excess air, tempera· tures of the exhaust stream, and some minor parameters are specified .

Another approach to finding steady·state fuel require· •

ments is to' examine the records of operating sludge in· cinerators for historical sludge and fuel flows and properties. The sum of the heat energies released by the sludge and fuel, normalized on the basis of some MHF capacity variable, might be proven of value as an empirical guideline. Of several tried, the quantity Total (Fuel +

306

Table 15. Total MHF Heat Release

a \' .. . Year Wt.%

HIIF or D tujLb Vo l n-No . Location Period Volati les tiles

1 Cleveland 1 966 9 , 530 ·13 . 7 1 % 7 1 3 . 3

2 Minn . -St . Paul 1 9GG' 1 0 , 000 71 .8 Hili 7 U9 . 5

3 Kansas City 67-69 1 0 , 500 59 . 9

4 Cleveland 62-65 1 0 , 070 43 . 9 5 llatt1e Creek 1 9(;8 1 04 • 57 . 0

1 U G U 5'\ . � 6 Battle Creek 1 964 1 04 • 5li . 5

1 965 58 . 4 1 967 G3 . 7

7 Saginaw l !1G6 1 3 , 970 401 .8 1 9G7 ·itl • 7 1 0G8 48 . 2 10G9 51 . 1

8 Hatfield 1 % 9 1 04• ( cst.)

9 Bridgeport 1 969 17000jLb . Dry Sol . (cst. )

I

*As sumed v a l u e

Sludge) Heat Release in million Btu/ton wet sludge ap­peared to be the most useful. Table 1 5 gives average annual fuel and sludge data for all the furnaces visited in the field. The Total Heat Release computed from these data appears in the last column, with values varying be­tween 3 and 7.

The Total Heat Release is between 3.5 and 4.5 for most of the furnaces investigated. Values much above 4.5 are probably excessive (since they are out of step with the current practice) and could indicate either higher exhaust stream temperatures than in most incinerators, or excessive use of cooling or combustion air. The highest value listed in Table 1 5 , 7 .0 for Saginaw, is the result of 1 ) the high calorific value of the sludge volatiles and 2) the high concentration of solids in the sludge. The Saginaw MHF could be operated quite well with a much wetter sludge, thereby reducing filtration costs or could be used as a source of thermal energy for other tasks.

The Kansas City MHF plant also appears to operate at a high heat usage per ton of sludge. Since the Kansas City sludge has about an average solids content and heat re­lease, the reason here would appear to be the use of excess fuel perhaps accompanied by enough excess air to �

maintain a reasonable exhaust temperature. The Cleveland MHF's, on the otherhand, have a Total

Heat Release of about 3. This is quite low and indicates a colder exhaust temperature then the average operating

. ) ,. I ' Z" " 7.IX+Y)"

106 ttu All Fuel Av/!: . Tot. Avg . Total

S o l i (l s , (�r . Avll. ' ) (}'ue � + S1 ud�c ) Per Ton l Oll n tu p('r Wi. . ;� • lOu Dtu pc,. I I I Dry Sol 'ron Dry :-;01 . Sl tulge 'ron Wn t S l ud)!c

8 . 33 5 . 4 8 . 25 4 . 25

1 4 . 1 0 . 375 1 3 . 9 0 . 50 12 . (; 1 . 0

\l . 4 4 . 0

1 1 . 4 4 . 'IG 1 1 . 0 6 . 1 5

1 1 . 3 7 . (;0 1 1 . 7 3 . -15 12 . 7 1 . 18

12 . 5 None 1 3 . 0 1 3 . 2 1 4 . 3 1 3 . 0 --1 4 . 0 14 . 0

�3 . 0 2:.1 . ·1 27 . 5 28 . 2 2\l . 4 2 3 . 4 2U . 0 :20 . 1 :24 . 7 2G . 6 21 . 75 .

1il . 9 '1 j •

·1 7 • .

11l . �O. « ' st.)

1 3 . 5(cst.)

3 . 1 li 2 . 9�

.l . Otl 4 . 06 4 . 88 3 . 1 -1 ·1 . 47 1 1 0 ) . . -

4 . G 7 4 . 03 4 . 1 7 5 . 7'1 G . 1 G . 2 G . 1I2

--3 . 78

0

MHF or perhaps a smaller amount of excess air and cool­ing air than is in use elsewhere.

307

CONC LUSIONS AND RECOMMENDAT IONS

In-depth data from field visits to installations of various sizes with preferably lengthy and well-documented operat­ing histories have proven essential in providing needed in­formation for estimation of component costs of MHF sludge incineration. The cost data remain continually use­ful since they are tied to economic indicators that are published year after year. The estimates derived from this work should be compared with actual costs associated with new incinerators; such feedback should be incorpo­rated in the formulations to reflect Significant changes in the cost structure as advances are made in the field.

AMSY

ANMM

BGG CAST CCI

G LOSSARY

Yearly normal maintenance material cost per MHF, dollars/year Yearly normal maintenance labor per MHF, man-hours/year Cost of incinerator building, dollars On-site cost of castings for one hearth, dollars Installed capital cost, dollars

CLH

CRL

CYT DPW EFC FHA HDIA HPD HUQ

MHF N(/)F 0MD

PDS

REFR RLH RRL

SBQ

SN(/) YHUH YSBH

Castings replacement labor for one hearth, man-hours Complete MHF castings replaced during MHF life (SLF), number Total heatup cycle time, hours Weekly incineration schedule, days/week Engineering fee, dollars Effective hearth area per MHF, sq ft MHF outer diameter ( l 3 .5 -inch wall), feet Daily incineration schedule, hours/day Heatup heat requirement per MHF for 0 to 1 200 F and 1 200 to 1 500 F heating, Btu/hour Multiple hearth furnace Total number of MHF's Daily operating labor per MHF, man-hours/24-hour operating day Electrical power consumption rate, kilowatt­hours/ton dry solids On-site cost of refractory for one hearth, dollars Refractory labor for one hearth, man-hours Complete MHF refractories replaced during MHF system lifetime (SLF) Hot standby ( 1 200 F) heat requirement per MHF, Btu/hour Number of standby MHF units Yearly heatup hours per MHF Yearly standby hours per MHF

REFERENCES

[ 1 ) Unterberg, W., R. J. Sherwood, and G. R. Schneider,

"Computerized Design and Cost Estimation for Multiple-Hearth Sludge Incinerators," Report 1 7070 EBP 07/7 1 , U.S. Environ­

mental Protection Agency, July 1 97 1 . [ 2 ) Burd, R. S., "A Study of Sludge Handling and Disposal,"

U.S. Department of the Interior, FWPCA Report WP-20-4, May 1 968.

[ 3 ) Isheim, M. c., "The Multiple Hearth Furnace," BSP Corporation, San Francisco, June 1 969.

[ 4 ) Owen, M. B., "Sludge I ncineration," Paper 1 1 72 , Journ.

San. Eng. Div., Amer. Soc. Civil. Engrs. , Vol. 8 3, No. SA- I , pp. 1 1 7 2-1 through -25 , February 1 95 7 , and "Sewage Solids

Combustion," Water and Sewage Works, pp. 442-447, October

1 959.

[5 ) MacLaren, J. W., "Evaluation of Sludge Treatment and Disposal," Canadian Municipal Utilities, pp. 23-33, 5 1-59, May

1 9 6 1 .

[ 6 ) Sebastian, F. P. and P. J . Cardinal, "Solid Waste Disposal," Chemical Engineering, Vol. 75, No. 22, pp. 1 1 2-1 1 7 , October

1 968.

[ 7 ) Quirk , T. P., "Economic Aspects of Incineration versus Incineration-Drying," (from Sludge Concentration·Filtration and

Incineration, Continued Education Series No. 1 1 3 , University of Michigan, Ann Arbor, p. 389), 1 964.

[ 8 ). Weller, L. W. and W. R. Condon, "Problems in the Design

of Sludge Incinerating Systems," Proceedings 16th Annual

Conference on Sanitary Engineering, University of Kansas,

January 1 966.

[ 9 ) Mick, K. L. and S. E. Linsley, "An Examination of Sewage

Solids Incineration Costs," Water and Sewage Works, Vol. 1 04,

No. 1 1 , pp. 479-487, November 1957.

[ l O ) Marshall and Stevens Equipment Cost Index (Average

and Clay Products), Chemical Engineering, 5 March 1962, p. 1 25

( 1 9 1 3-1961); 5 May 1 969, p. 1 37 ( 1 950- 1 968); 23 March 1 970,

back flap ( 1 964-1969).

[ 1 1 ) Engineering News Record Construction Cost Index, Business Statistics, 1 967 Biennial, U.S. Department of Commerce,

Office of Business Economics ( 1 9 39-1 966); Construction Review,

January 1 970, p. 42 ( 1 964-1969).

[ 12 ) Plant Maintenance Cost Index, McGraw-Hill Department

of Economics, 330 W. 42nd Street, New York, New York (1 947-1969).

[ 1 3 ) Plant Cost Index (Average and Process Machinery),

Chemical Engineering, 25 April 1 966, p. 188 ( 1 947-1 965 );

5 May 1 969 (1 950-1968).

[ l4 ) Average Hourly Gross Earnings per Non-Supervisory Worker in Electric, Gas, and Sanitary Services, Business Statistics,

1 967 Biennial, U.S. Department of Commerce, Office of Business Economics ( 1958-1966); Monthly Employment and

Earnings, Bureau of Labor Statistics, U.S. Department of Labor ( 1 967-1 969).

[ 1 5 ) "Industrial Wage Survey-Industrial Chemicals," Bulletin 1529, Bureau of Labor Statistics, U.S. Department of Labor, October 1 966.

[ l6 ) Masonry, Stonework , and Plastering (Straight Time Earnings, National Average), Monthly Employment dnd Earnings,

Bureau of Labor Statistics, U.S. Department of Labor ( 1 958-

1969).

[ 17 ) Smith, R., "Cost of Conventional and Advanced Treat­

ment of Wastewater, Jour. Water Poll. Control Fed. , Vol. 40,

No. 9, pp. 1546-1574, September 1 968.

[ 18 ) American Society of Civil Engineers, Civil Engineering,

p. 3 1 , October 1 967.

[ 1 9 ) Michel, R. L., A. L. Pelmoter, and R. C. Palange,

"Operation and Maintenance of Municipal Waste Treatment

Plants," Jour. Water Pollution Control Federation, Vol. 4 1 , No. 3,

pp. 335-354, March 1 969.

[ 2 0 ) Smith, R., "Estimation of Operating and MainteQance

Cost for Wastewater Treating Processes," Internal FWPCA

Memorandum, 30 April 1 969.

[ 2 1 ) McMichael, W. F., "Operating and Maintenance Costs,"

Internal FWPCA Memorandum, 9 August 1 968.

308

AC KNOWLEDGMENTS

This work was sponsored by the Office of Research and Monitoring, Environmental Protection Agency, and was monitored by Dr. J . B. Farrell as Project Officer. Messrs. K. W. Fertig and W. H. Moberley performed the computer programming. Messrs. L. Lombana and R. Stroshane carried out the field visits.

The Field Survey of MHF Sewage Sludge I ncinerators could not have been accomplished without the coopera­tion of the municipal authorities who frequently assisted

•

in furnishing records, and , in particular, the pa tience and courtesy of the plant personnel who provided performance and operational information. Thanks are due to personnel at the fQllowing cities and plants:

• Cleveland Southerly Wastewater Treatment Plant, Ohio

• Minneapolis-St. Paul Sanitary District, Minnesota • Kansas City Big Blue River Sewage Treatment Plant,

Missouri • Battle Creek Sewage Treatment Plant, Michigan • Saginaw Sewage Treatment Plant, Michigan • Hatfield Township Sewage Treatment Plant,

Pennsylvania • Bridgeport Sewage Treatment Plant, Pennsylvania • South Tahoe Public Utility District, California

Especially helpful were Mr. Maurice L. Robins, Super­intendent and Chief Engineer, and Mr. Scott E. Linsley, Assistant Chief Engineer, both at Minneapolis-St. Paul, who sent records covering over two decades of incinerator operation. Additional information was also provided by Mr. Walter Tresville, Director, Cleveland Southerly Plant, and Mr. David Evans, South Tahoe.

Information on installed cost, some going back over 30 years, was obtained from personnel associated with the following consulting firms:

• Havens and Emerson, Cleveland, Ohio • Toltz, King, Duvall, Anderson & Associates, St.

Paul, Minnesota • Black & Veatch, ConSUlting Engineers, Kansas City,

Missouri • McNamee, Porter and Seeley, Ann Arbor, Michigan • Malcolm Pirnie Engineers, White Plains, New York • Hubbell, Roth & Clark, Inc. , Bloomfield Hills,

Michigan • Tracy Engineers, Inc., Lemoyne, Pennsylvania • George B . Mebus, Inc. , Willow Grove, Pennsylvania

,

• Metcalf and Eddy, Inc., Engineers, Boston, Massachusetts

• Lozier Engineers, Rochester, New York Welcome assistance on economic indicators was given by Messrs. Greenwald and Norden, McGraw-Hill Department of Economics, New York and Mr. Robert Ball, Bureau of Labor Statistics, U.S. Department of Labor, Washington, D.C.

309