Pyrolyzer Design Alternatives and Economic ... - ACS Publications

10 P y r o l y z e r Design Alternatives a n d E c o n o m i c Factors for

Downloaded via TUFTS UNIV on July 19, 2018 at 22:15:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

P y r o l y z i n g Sewage Sludge i n M u l t i p l e - H e a r t h Furnaces CHARLES VON DREUSCHE and JOHN S. NEGRA Nichols Engineering and Research Corporation, Homestead and Willow Roads, Belle Mead, NJ 08502 While multiple hearth furnaces have been used commercially i n ten large plants for p y r o l y s i s of bark and sawmill waste to charcoal for briquette manufact u r e , used commercially for p y r o l y s i s of paper and p l a s t i c laminates on aluminum foil to recover the foil, (1) and demonstrated experimentally for p y r o l y s i s of RDF alone (2) and for c o - p y r o l y s i s of RDF and sludge,(3) t h i s paper will be l i m i t e d to considering p y r o l y s i s of sewage sludge alone. Many sewage treatment plants are equipped with multiple hearth furnaces for sludge i n c i n e r a t i o n . This transforms an offensive and dangerous waste into a safe and inoffensive ash, and s a t i s f i e s various purposes l i s t e d i n Table I . The need f o r t h i s transformation i s l i k e l y to continue since sewage sludge i s the one most l i k e l y place to f i n d , perhaps as an unrecognized by-product of o r ganic synthesis, organic substances that are found to be dangerous and non-biodegradable, perhaps long a f t e r t h e i r initial appearance. Thermal conversion of the complex organics to simple inorganic form i s the only way to insure that a dangerous substance i s not widely d i s t r i b u t e d as a sewage plant product. Most of the many multiple hearth incinerators now use oil or gas to supply the heat needed to evaporate 2 to 4 pounds of water received with each pound of dry sewage sludge s o l i d s and heat t h i s water along with 100% excess a i r from combustion of the organics to a l e g a l l y required 1400 ° F afterburner exhaust gas temperature.

0-8412-0434-9/78/47-076-191$06.50/0 © 1978 American Chemical Society Jones and Radding; Solid Wastes and Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

SOLID

192

WASTES

A N D RESIDUES

TABLE I Reasons for Thermal Destruction of Sewage Sludge A.

Change Physical Properties 1. 10:1 weight reduction from 30% s o l i d s , 33% ash dry basis to ash. 2. 5:1 volume reduction 3. Sticky slime to powder or granules

100%

B.

Public and Employee Safety and Aesthetics Promoted by E l i m i n a t i n g : 1. Odor 2. P o t e n t i a l odor by decomposition 3. Pathogens 4. P o t e n t i a l pathogen m u l t i p l i c a t i o n i f contaminated. 5. Support for larvae, rodents, etc. 6. Organic Toxins - i n c l u d i n g those which may be u n i d e n t i f i e d or whose t o x i c i t y may be currently unrecognized.

C.

Energy Conserved 1. Less trucking and l a n d f i l l operating f u e l (a) because of volume and weight reduction (b) because nearer s i t e l i k e l y to be acceptable.

D.

Reduced Handling and Disposal Cost 1. Acceptable land disposal may be much nearer 2. Reduced transport quantity and distance from plant 3. No cover d i r t required 4. Leachate control may not be required 5. Smaller land f i l l area to buy or longer l i f e of e x i s t i n g l a n d f i l l

Jones and Radding; Solid Wastes and Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

10.

V O N DREUSCHE

A N D NEGRA

Pyrolyzing Sewage Sludge

193

By considering simultaneous a p p l i c a t i o n of pyr o l y s i s and modern sludge dewatering p r a c t i c e s , i t is possible to accomplish the destruction of sludge organics without the need of any a u x i l i a r y f u e l , and substantial energy w i l l be a v a i l a b l e for recovery. Substitution of refuse derived f u e l i s another means of conserving gas and o i l . However, RDF costs money to prepare from r e f u s e . The cost depends on volume through-put. Large scale uses,needed to j u s t i f y RDF preparation p l a n t s , w i l l no doubt govern plant l o c a t i o n and market p r i c e . A high p r i o r i t y f u e l requirement that j u s t i f i e s pyrolyzing a large amount of RDF for i t s f u e l value i n conjunction with the sludge may on the other hand make c o - p y r o l y s i s very a t t r a c t i v e i n some s i t u a t i o n s . A population of 400,000 could be expected to generate about 800 tons of refuse per day. The corresponding 40 MGD wastewater treatment plant would require about 24 tons RDF per day to make up the f u e l d e f i c i e n cy for p y r o l y s i s of t h e i r sludge i f they are able to dewater the sludge to only 25% s o l i d s . The f a i r p r i c e of the RDF, plus transportation, plus storage and metering costs for t h i s bulky, odoriferous b a c t e r i a l l y decomposable f u e l plus incremental furnace size r e quired to burn the RDF may make t h i s unattractive i n some circumstances. How t y p i c a l the 40 MGD plant i s i n s i z e , since i t w i l l be c a r r i e d through t h i s paper as an example, may be judged by reference to Table I I . The magnitude of energy a v a i l a b l e , energy demand and energy d o l l a r worth may also be judged from this Table. It is foreseen that energy may be recovered i n 3 ways. The hot gas e x i t i n g an afterburner on the pyr o l y s i s operation can be used to generate steam. Energy can be recovered from a 25% carbon/75% ash char produced by p y r o l y s i s . The char so produced need not be consumed i n the sewage treatment p l a n t , and i f i t i s , i t need not be consumed at the instantaneous rate that i t i s generated from sludge. A t h i r d p o s s i b i l i t y for heat recovery i s the generation of methane from d i g e s t e r s , made economical by the opportunity created by improved dewatering plus p y r o l y s i s , to safely destroy the energy depleted sludge produced by digesters without purchased f u e l and to return to the digesters



8

399,000 525,000

67 133 175 2)

201,000

28

28

289

125

59

84,000

7

21,000

MAXIMUM * * $/YR.

5

MILLION BTU/YEAR (IN THOUSANDS) AVAILABLE IN * REQUIRED SLUDGE/WTP (NET) PER WTP

* * BASIS $3 PER MILLION BTU.

*BASIS OF EXPECTATION - SLUDGE AT 40% SOLIDS (SEE FIG.

83

180

20 - 50

50 &

267

10 - 20

12761

NUMBER PLANTS

494

d

5-10

5

m

PLANT CAPACITY

EXISTING PLANTS

ESTIMATED ENERGY - SEWAGE SLUDGE (10)

TABLE II

ο

•

Si

CD 0^

10.

V O N DREUSCHE

A N D NEGRA


195

heat they r e q u i r e . Digester gas i s r e l a t i v e l y simple to produce, and to clean for transmission, and i t i s a high BTU gas. It i s not the purpose here to examine i n depth the many a l t e r n a t i v e uses for the surplus energy generated by improved e f f i c i e n c y i n thermal destruction of sludge, nor to question how l i k e l y the actual u t i l i z a t i o n of such energy may be. I t must be noted however that i n many plants where digester gas i s produced, the gas i s wasted. The exhaust gas of i n c i n erator afterburners i s also at 1400 °F and i n larger volume than w i l l r e s u l t under the proposed scheme of improved dewatering plus conversion to p y r o l y s i s ; yet only about 20 p l a n t s , including 12 yet to come on stream, a c t u a l l y practice t h i s recovery (4). It i s believed that p y r o l y s i s , by providing more f l e x i b i l i t y as to the form of recovery, plus the opportunity to store energy as carbon and recover heat from that, at times of peak energy demand, w i l l make such recovery more p r a c t i c a l . Pyrolysis of sludge alone without a u x i l i a r y f u e l has been demonstrated and explored over a t o t a l of 400 hours operation i n a 3 ton per day p i l o t operation u t i l i z i n g two d i f f e r e n t raw sludges and one digested sludge. The p i l o t furnace i s shown i n Figure 1. The program was c a r r i e d out i n the p i l o t plant of Nichols Engineering and Research Corporation for the Interstate Sanitation Commission of New York, New Jersey and Connecticut as part of a program funded by the U . S . Envirommental Protection Agency. Release of the r e port of that program i s expected soon. Meanwhile one f u l l scale u n i t for the p y r o l y s i s of sludge and the recovery of carbon from a 40 MGD i n d u s t r i a l wastewater treatment plant is now coming on stream and the f i r s t municipal contract for two units pyrolyzing sludge alone has been b i d and Nichols expects award of the contract momentarily. In focusing on the p y r o l y s i s of sludge alone, i t is the viewpoint of t h i s paper that i n - p l a n t e l i m i n a t i o n of sludge organics i s the primary aim of the operation. E l i m i n a t i n g the purchase of valuable fuels such as RDF, o i l , gas or c o a l , i s the f i r s t step to economy and conservation. The recovery of heat from use of the char w i l l follow i n an increasing number of


SOLID

WASTES

AND

RESIDUES

a. Ο

3»


10.

V O N DREUSCHE

A N D NEGRA


197

instances. The conversion of e x i s t i n g units to p y r o l y s i s seems eminently p r a c t i c a l but f i r s t some choices must be made between process design a l t e r n a t i v e s leading to d i f f e r e n t products. These choices depend on the i n d i v i d u a l plants requirements, opportunities for energy recovery and the nature of i t s sludge. Pyrolysis Process Description and Operating Modes The P y r o l y s i s Process. The thermal destruction of sludge by p y r o l y s i s i n a multiple hearth furnace i s a process by which s o l i d s are raked along the f l o o r of successive compartments while a larger vapor space exists above the s o l i d s . When l i m i t e d amounts of a i r are properly introduced into the vapor space the oxygen reacts with the organic vapors present and there i s starved a i r combustion (not p y r o l y s i s ) of these vapors. The oxygen does not, i n the proposed designs, p e r s i s t through the vapor space and reach the s o l i d s . The s o l i d s receive heat by r a d i a t i o n . They are shielded from contact with oxygen by a stagnant f i l m of organic vapor or water or both leaving the s t i r r e d bed of s o l i d s . Adding to the protection i s the fact that the l i m i t e d amount of a i r admitted has i t s oxygen consumed r a p i d l y i n the organics r i c h vapor phase. This is p y r o l y s i s with respect to the s o l i d s . Hot products of starved a i r combustion from the middle hearths provide the required heat for drying the wet incoming sludge on the upper hearths. Combustion of the gaseous products i s completed i n an afterburner. The e s s e n t i a l point of the p y r o l y s i s process i n reducing or e l i m i n a t i n g f u e l requirements i s the e l i m i n a t i o n of large excess a i r requirements common to incinerators. The middle hearths of the multiple hearth incinerator receive and burn sludge that has been predried on the upper hearths. Approximately 100% excess a i r i s required to d i l u t e the temperature of combustion of t h i s sludge to the 1800 °F upper l i m i t for which normal multiple hearth construction i s s u i t a b l e , as can be shown by a heat balance around t h i s portion of an i n c i n e r a t o r . P y r o l y s i s eliminates t h i s


198

SOLID

WASTES

A N D RESIDUES

need for excess a i r by postponing the completion of combustion u n t i l a f t e r p a r t i a l l y burned organic vapors have passed over the wet incoming sludge where they w i l l c o o l , pick up water vapor and may p i c k up add i t i o n a l organic vapors, steam d i s t i l l e d toxins, or odors. The organic vapors provide f u e l i n the a f t e r burner to r a i s e the temperature of water evaporated i n the upper hearths to the required 1400 ° F f i n a l combustion temperature. Low Temperature Char Operating Mode, The organics i n the sludge have undergone destructive d i s t i l l a t i o n g i v i n g o f f almost a l l the organic vapors that can be released, when the s o l i d s have been heated to 1200 to 1300 ° F . The process can be c a l l e d complete at that point and the char discharged. This would be low temperature char (LTC operating mode). The product i s a s t e r i l e , b l a c k , charcoall i k e free flowing mixture of powder and granules having a bulk density of approximately 16 pounds per cubic foot and containing 75% ash. It contains a l l of the ash of the feed sludge plus amorphous carbon and some non-putrescible unleachable material akin to the v o l a t i l e content of c o a l . It i s believed that t h i s material could be s a f e l y l a n d f i l l e d s a t i s f y i n g the objectives of Table I. This material has been observed to have retained the l e a d , zinc and cadmium content of sludge to an extent not usual i n other modes of p y r o l y s i s operation nor i n i n c i n e r a t i o n . Allowing the s o l i d s temperature to exceed 1200 to 1300 °F seems to i n i t i a t e v o l a t i l i zation of these substances to a fume which i s d i f f i c u l t or impossible to capture i n most a i r p o l l u t i o n abatement equipment. Under development are methods to recover these metals from the low temperature char. One h a l f percent t o t a l l e a d , z i n c , cadmium content i n the char i s not uncommon for sludge i n i n d u s t r i a l areas (5). Recovery i s the one way to avoid putting the 80,000 pounds per year (more or l e s s ) of these metals, that a 40 MGD plant might have, into either the land, the water, or the a i r .


10.

V O N DREUSCHE

A N D NEGRA


199

High Temperature Char Operating Mode. For high temperature char, the char can be brought to temperatures of 1600 ° F or higher either by allowing a high gas temperature i n the l a s t hearth of the p y r o l y s i s zone, or by very l i m i t e d contact of the char with a i r on a hearth at the end of the p y r o l y s i s zone. The product i s about 25 pounds per cubic foot and contains only ash and carbon. The carbon content can be estimated from the data i n Table I I I . It ranges from 7 to 18% of the organics i n the sludge and w i l l vary from about 10% to about 40% of the product depending on the kind of sludge and the r a t i o of ash to organics i n the sludge. Lack of r e s t r i c t i o n i n applying high gas phase temperatures, i f one need not c a r e f u l l y avoid heating the char above 1300 ° F , allows a greater heat transfer rate throughout the furnace. Thus the furnace for operation i n t h i s mode can be of minimum size for a given throughput capacity. Also r a i s i n g the temperature to t h i s l e v e l does reduce the heating value of the char discharged, making more heat a v a i l a b l e i n the furnace or afterburner. Char Burning Operating Mode. The char r e s u l t i n g from p y r o l y s i s i n the middle hearths can be converted to ash on the lower hearths, (CBA operating mode).An oxygen free atmosphere can be maintained as i n the p y r o l y s i s hearths and the carbon reacted with CO2 and H2O to produce CO and H2. This requires a temperature of at least 1600 °F to obtain p r a c t i c a l reaction rates (6). A l t e r n a t i v e l y a i r , or a i r plus steam (7), can be c i r c u l a t e d across these lower hearths and substant i a l l y vented from the side of these lower hearths to avoid an excess of a i r entering the middle p y r o l y s i s hearths. An excessive amount of oxygen entering the hearths where maximum organic vapor release occurs, r i s k s development of temperatures there i n excess of the 1800 ° F that normal multiple hearth furnace cons t r u c t i o n can withstand. The oxygen containing gases vented from the side of the lower hearths are brought by a separate external duct to the afterburner where these gases serve as preheated a i r for combustion, thus u t i l i z i n g the heat obtained from the carbon to r a i s e the afterburner temperature.



AVERAGE

8

62

71

59-67

5,

6, 7

48-66

63

12

8

11-17

6-12

18

COMBUSTIBLES - % VOLATILES FIXED CARBON

2, 3, 4

1

SAMPLE NUMBER

26

21

18-24

28-40

19

% ASH

AERATED PRI.

WAS + PRIMARY

ANAEROBIC DIGEST

AEROBIC DIGEST

TYPE SOLIDS

SOME COMPOSITIONS OF VARIOUS SEWAGE SLUDGE SOLIDS TESTED

TABLE III

10.

V O N DREUSCHE

A N D NEGRA


201

The advantage of reacting away the carbon r e s u l t ing from the p y r o l y s i s zone of the furnace i s to avoid a loss of heat from the process i n the form of unburned f u e l value as carbon i n the material produced. The material produced consists e n t i r e l y of ash and has a bulk density of approximately 33 pounds per cubic f o o t . Comparison of modes. Table IV shows the reasons for s e l e c t i n g one mode of operation as compared to another. It should be pointed out that a single furnace sized large enough to react away a l l of the f i x e d carbon, can be operated i n any of the three modes, LTC, HTC, or CBA, by simple v a r i a t i o n i n control settings. This i s important l a t e r when considering the p o s s i b i l i t y of s t o r i n g and refeeding char product. Carbon Y i e l d s and Reaction Rates. It should however be pointed out that maximum energy production from a given quantity of sludge, by reacting away a l l of the f i x e d carbon formed i n p y r o l y s i s , does require a d d i t i o n a l hearth area. The extra hearth area required can be found by estimating the pounds per hour of f i x e d carbon l i k e l y to be formed from the organics i n the sludge, and d i v i d i n g by a rate of r e a c t i o n . Some t y p i c a l carbon reaction rates are shown i n Table V. T y p i c a l f i x e d carbon y i e l d s found for various sludges are indicated i n Table I I I . The importance of t h i s factor may best be shown by example. The 40 MGD plant c a r r i e d through this paper as an example, pyrolyzing 5000 pounds of sludge s o l i d s (dry b a s i s ) per hour at 25% s o l i d s ( a l t e r nately 40% s o l i d s ) requires 1,333 square feet ( a l ternately 833 square feet) to reach the HTC stage of product. This area requirement can be calculated from the usual heat transfer equations i n the form of q (heat transfer to solids)=U(a c o - e f f i c i e n t of t r a n s f e r ) X Area X gas to Solids Temperature D i f ference. Based on a t y p i c a l 12% f i x e d carbon y i e l d and 0.6 pounds reacted per square f o o t , the o x i dation of the 600 pounds of f i x e d carbon formed r e quires that an a d d i t i o n a l 1000 square feet be added



Minimum

Intermediate 25

Maximum 16

L a n d f i l l Volume

Bulk Density

lbs/ft3

Maximum

Intermediate

Minimum

Heat Recovery

lbs/ft3

A l i a s Cr+3

33

lbs/ft

3

A l i t t l e may remain oxidized s t i l l under study

Less

A l l as Cr+3

Less

Chrome as Cr+3

CBA 9 lbs/Hr-SF

Maximum

HTC lbs/Hr-SF.

Heavy Metals Retention

13

10

Loading rate (approx.)

LTC lbs./Hr-SF,

Comparison of P y r o l y s i s Modes of Operation

TABLE IV


0

MODERATE

MODERATE

HIGH

1596

1608

1588

1599

LOW MODERATE HIGH

IN GAS

MODERATE

1742

^CONCENTRATION

0

_°2 0 4 9

LOW

MODERATE

MODERATE

HIGH

MODERATE

HIGH

CONCENTRATION IN GAS* 02 CO9 + STEAM

1823

TEMPERATURE GAS - ° F

_C0

CARBON GASIFICATION RATE

TABLE V

2

16 25 - 30 42 - 47

+ STEAM

(VOL.7o)

0.40

0.25

0.30

0.30

0.67

0.86

2

RATE CARBON GASIFICATION LB/HR. - F T

204

SOLID

WASTES

A N D RESIDUES

to the furnace s i z e . Similar consideration of hearth area to eliminate f i x e d carbon from s o l i d s accounts f o r the e a r l i e r statement that RDF i s a f u e l that needs hearth area i n a multiple hearth furnace while gas or o i l burners do not. Heat Balance Results Presentation of Fuel Requirements and Energy Recovery P o t e n t i a l . In Figure 2, the f u e l r e q u i r e ments (where any e x i s t ) and the heat a v a i l a b l e f o r recovery, are both plotted against the sludge chara c t e r i s t i c s expressed i n terms of higher heating value per u n i t of water content. It i s water that consumes heat i n the furnace and f u e l value of the organics that must provide the heat or be supplemented with purchased f u e l . The ash hardly matters except that i t c a r r i e s water with i t at any given feed s o l i d s content. It i s not possible to simply r e l a t e f u e l r e q u i r e ment to f i l t e r cake moisture content without assuming a figure f o r sludge ash content and f o r heating value of the sludge organics. Ash content of sludge varies from 20 to 50%. Heating value per pound of organic content varies from 8,000 to 14,000 BTU. Variations over about h a l f as wide a range as that j u s t s t a t e d , occur within many plants from season to season i f not from week to week. The two supplementary scales on Figure 2 show the s o l i d s content to which sludge must be dewatered to provide the corresponding f u e l c h a r a c t e r i s t i c f o r two d i f f e r e n t sludges. The range has been i n t e n t i o n a l l y narrowed to keep the scale on the page. Sludge at 40% s o l i d s shown to range from 3600 BTU/lb. water ( i f i t has 40% ash content and the nonash portion has a heating value of 9000 BTU/lb) to 6000 BTU/lb water ( i f i t has 25% ash content and the non-ash portion has a heating value of 12,000 BTU/lb. Impact of Cake Ash Content, Heating Value of Organics and Water Content. These two supplementary scales c e r t a i n l y emphasize the importance of cons i d e r i n g sludge ash content and heating value of


VON

DREUSCHE

Figure 2.

AND

NEGRA


Heat recoverable and required with respect to sludge characteristics


206

SOLID

WASTES

A N D RESIDUES

sludge organic content when comparing d i f f e r e n t f i l t e r cake moisture contents as feed to a furnace. O i l y or greasy sludges are frequently d i f f i c u l t to dewater to high s o l i d s content, but the high BTU per pound of organics may well r e s u l t i n the wetter cake being equally acceptable for p y r o l y s i s or i n c i n e r a t i o n . High dosages of lime and f e r r i c s a l t s for coagulation before f i l t e r i n g sometimes gives a d r i e r cake than polymer f l o c c u l a t i o n but i f the cake i s not a l o t d r i e r , may increase cake ash content as to r e s u l t i n no improvement i n b u r n a b i l i t y . The graph also makes i t clear that small f l u c tuations i n moisture content plus fluctuations i n ash content and heating value of organics w i l l normally cause considerable v a r i a t i o n i n the f u e l characteri s t i c s of what is being fed to the pyrolyzer or incinerator. This w i l l become important when cons i d e r i n g storage and return of char to the furnace. Ir

,f

Conclusions from Figure 2, r e : P y r o l y s i s . The graph makes i t clear that : A. P y r o l y s i s i n the CBA mode increases recoverable heat by 10% as compared to i n c i n e r a t i o n , only when considering sludge feed of more than 5,000 BTU higher heating value per pound of water. Otherwise recoverable heat i s r e duced 33%, because with lower heating value i n the sludge feed the i n c i n e r a t i o n base of comparison uses purchased f u e l s . B.

The desirable aspect of p y r o l y s i s from an energy economy viewpoint, i s reduction or e l i m i n a t i o n of requirement for a u x i l i a r y fuel.

C.

P y r o l y s i s can be f u e l l e s s (including f u e l l e s s afterburner) with sludge dewatered to the range of 29 to 39% s o l i d s for most sludge compositions as compared to 42 to 52% s o l i d s required for i n c i n e r a t i o n to be f u e l less and provide a 1400 °F exhaust temperature. Being able to dewater sludge to 42 to 52% s o l i d s range i s quite uncommon, i n fact the authors know of


10.

V O N DREUSCHE

A N D NEGRA


207

no commercial piece of equipment that could be depended upon to do t h i s r e g u l a r l y on t y p i c a l sludges, although perhaps the new membrane plate f i l t e r presses w i l l get well into t h i s range on some sludges. A range of 29 to 39% s o l i d s for dewatering both sludges is something one would very confidently expect from 3 kinds of dewatering equipment: recess tray f i l t e r presses, b e l t f i l t e r presses equipped with squeezing devices and most confidently membrane plate f i l t e r presses. It seems apparent that r a i s i n g the sludge f u e l content per pound of water content from the r e s u l t s usually obtained by dewatering to 20 to 30% s o l i d s content i s necessary to make p y r o l y s i s a f u e l l e s s operation; and that a s h i f t from i n c i n e r a t i o n to p y r o l y s i s i s necessary to keep the extent of dewatering required down to a l e v e l that can be p r a c t i c a l l y depended upon from normal equipment. Raising sludge f u e l content per pound of water content depends p r i m a r i l y on improved dewatering of the sludge since one has no c o n t r o l over the heating value of the organic content and l i t t l e or no c o n t r o l over the ash content. Improved Dewatering Much i s r e l a t i v e l y new i n t h i s important area. F l o c c u l a t i o n . Polymer flocculants have become cost competitive with f e r r i c - l i m e coagulation. Some sludges coagulate and dewater best with f e r r i c and lime, some with one polymer, some with another. It i s not easy to be sure the choice of f l o c c u l a n t made during a t e s t period i s s t i l l the best f l o c c u l a n t choice during subsequent weeks of operation when sludge c h a r a c t e r i s t i c s may have v a r i e d . But i n creasingly systems are being designed for easy change from one f l o c c u l a n t to another. The increased f l e x i b i l i t y can r e s u l t i n better f u e l c h a r a c t e r i s t i c s i n the sludge f i l t e r cake i f due allowance i s made for the e f f e c t of f e r r i c and lime on sludge ash content, instead of seeking simply the highest s o l i d s content.


208

SOLID

WASTES

A N D RESIDUES

F i l t e r i n g Equipment. The recessed plate f i l t e r press and b e l t f i l t e r s that squeeze the cake, both introduced to the sewage treatment f i e l d within the past few years, have r a i s e d cake s o l i d s content from the 2 0 to 3 0 7 o s o l i d s range to the 3 5 to 4 5 % s o l i d s range. Now i n t e r e s t i s s h i f t i n g to expandable membrane f i l t e r presses. This i s a concept patented some years ago by Ciba-Geigy ( 8 ) and licensed to Moseley Rubber Company Limited of England. That company a f t e r several years of developing and improving f a b r i c a t i o n methods for such plates i s now introducing them to sewage works i n England and i n this country ( 9 ) . In these the compartments containing a p a r t l y or f u l l y formed cake i s reduced i n volume, by expanding the face of the trays as shown i n Figure 3 . This squeezes the cake transversely whereas i n the older recessed tray plates pressure i s applied to the cake only by the force of new sludge coming i n through the feed entry p o r t . The older fixed plate presses therefore apply pressure only edgewise to the center of the cake and t h i s pressure i s soon l o s t to f r i c t i o n of the cake against the two adjacent f i l t e r c l o t h s . The cake can be squeezed by the expandable membrane tray for any desired time period and a t h i n cake i s squeezed as hard or harder than a t h i c k cake. The expandable membrane feature can be used to reduce i n i t i a l c a p i t a l cost of f i l t e r presses by r e ducing t o t a l cycle time. T y p i c a l l y , for the standard recess p l a t e , 7 5 % of the sludge volume enters the press i n the f i r s t 2 5 % of the press cycle a f t e r which sludge input rates d r a s t i c a l l y slow. By expanding the membranes with compressed a i r at t h i s p o i n t , a given s i z e press produces 7 5 % of the normal cake weight and with squeezing time allowed may require only 3 0 to 3 5 % of the normal cycle time. This i n essence doubles press c a p a c i t y . Besides producing a d r i e r cake i n normal operation due to the transverse d i r e c t i o n of the squeezing forces and the time for which squeezing forces can be a p p l i e d , and the thinner cake that i s produced, the membrane plate eliminates production of occasional "wetter than normal" cake. An i n completely f i l l e d press has f u l l squeezing pressure


10.

VON

DREUSCHE

AND

NEGRA


Figure 3. Flow scheme expandable membrane filter press


209

210

SOLID

WASTES

A N D RESIDUES

applied to each cake. Also an occasional upset i n coagulation may be l a r g e l y o f f s e t by slow i n i t i a l f i l t r a t i o n giving a thinner than normal cake during the time allowance for press f i l l i n g but that t h i n cake w i l l be squeezed dry while c o r r e c t i o n i s made to coa g u l a t i o n . The cake can be p r o f i t a b l y l e f t to squeeze for as much cycle time as safety factor i n the i n i t i a l i n s t a l l a t i o n of the presses and any under-capacity of actual operation compared to planned operation may a l low i n the f i l t e r c y c l e . F i l t r a t i o n Results with Membrane P l a t e s . The performance of such membrane plates i s described i n a B r i t i s h p u b l i c a t i o n (9) from which Table VI has been borrowed. Conditioned sludge from a single tank was pumped to a standard recessed plate press and s i multaneously to a membrane plate press. Cakes were formed, squeezed and then a d d i t i o n a l sludge pumped i n and squeezed, a procedure which accounts for there being a f i r s t and second feed time and a f i r s t and second squeeze time. Press capacity seems to be about doubled and comparative cake s o l i d s content increased from an average of 26.2% s o l i d s to 34.3% - a change from 2.8 l b s . water/lbs. s o l i d s to 1.9 l b s . water/lbs. solids. Such membrane plates were purchased for the Nichols p i l o t plant press which uses 4 f t . by 4 f t . plates. The main object of t h e i r use was to f a c i l i tate cake preparation for p y r o l y s i s t e s t s . But when only 1 tray was a v a i l a b l e f o r the f i r s t such campaign, i t was i n s t a l l e d i n a stack of regular r i g i d recessed trays and t r i e d . When the diaphrams were expanded they squeezed the cake on e i t h e r side of t h i s one t r a y . Though some s l i g h t sludge movement out of the two squeezed compartments occurred, perhaps preventing f u l l pressure development, the r e s u l t s i n Table VII were obtained. It i s believed that i t i s now possible to depend on dewatering most sludges to 35 to 50% s o l i d s i n a cost competitive system and obtain sludge cake that w i l l generally exceed 5000 BTU a v a i l a b l e per pound of water contained. This i s an important part of the system for pyr o l y s i s of sludge alone since the f i r s t aim a f t e r



TABLE

VI

11.6 12.7 106 106 8.6 8.6 13.0 13.0 10.0 100 11.7 11.9

25.8 28.0 283 28.3 22 0 22.0 31.2 31.2 21.4 21.4 26.0 28.4

621 621 621 621 621 621 621 621 621 621 621 621

180 180 180 180 180 180 180 180 150 150 150 150

mins

Maximum Feed Pressure kPa

Feed Time

8 10 10 10 10 10 10 10 10 10 10 10

Possible No. of Cycles per day

11.0 8.6 10.8 10.2 8.2 10.0 10.0 10.4 10.6 11.5 10.9 11.0

36.7 31.2 35.4 30.9 29.7 34.1 34.3 33.4 34.3 36.3 34.9 40.9

, '

Dry Solids Applied kg

Cake Dry Solids %

ι t

80 55 55 55 55 55 60 60 60 60 60 60

mins

Total Time

1st 45 20 20 20 20 20 30 30 30 30 30 30

mins 2nd "10 15 15 15 15 15 10 10 10 10 10 10

Feed Time

The output consideration is based on a day's operation at the Brockhurst Works ie a shift system with attendance hours 07.30 to 22.00h Monday to Friday.

27 29 30 31 32 33 34 35 36 37 38 39

Test No

Table A — Brockhurst sludge. 1 No. Membrane Chamber 1130mm χ 1075mm χ 32mm

%

Dry Solids Applied kg

Cake Dry Solids

Membrane

Table A — Large test press. 1 No. Recess Chamber 1220mm χ 1220mm χ 32mm

621

ι 10 10 10 10 10 10 10 10 10 10 10

kPa

ι ι

j ' !

io—t

2nd

mins

Squeeze Time

kg 88 86 108 102 82 100 100 104 106 115 109 110

Total Dry Solids per day

Press

1st 15 10 10 10 10 10 10 10 10 10 10 10

Per chamber

Possible No. of Cycles per day

=

90

1st 317 310 310 310 310 310 310 310 310 310 310 310

psi

2nd 310 310 310 310 317 310 310 310 310 310 310 310

Maximum Feed Pressure kPa

5 5 5 5 5 5 5 5 5 5 5 5

system

Maximum Squeeze Pressure kPa 1st 2nd 517 517 414 483 379 434 414 414 448 448 386 483 485 503 448 414 455 455 448 414 414 414 621 586

kg 58 63 53 53 43 43 65 65 50 50 58 59

Total Dry Solids per day

Recess Press

Output comparison — 2 shift

22 25 25 25 25 25 22 22 25 25 25 19

mm

Cake Thickness

52 37 104 92 91 133 54 60 112 130 88 86

Increase obtained by use of Membrane


26.2 34.3

MEMBRANE PLATES

7o SOLIDS

RECESS PLATES (STD.)

TYPE

10

5

PER DAY

MEMBRANE PLATES

VS.

STANDARD RECESS PLATES

FILTER COMPARISON SUMMARY

TABLE VI-A

103

55

DRY SOLIDS (AVERAGE) PER DAY - KG


40.2 37.4

35.8 34.3

2

3

45.7 46.8

29.8 43.5

19

20

LIME-FERRIC CHLORIDE CONDITIONED SLUDGE

34.4

31.9

POLYMER CONDITIONED SLUDGE 1

PRESS RUN

CAKE SOLIDS - % UNCOMPRESSED COMPRESSED CAKE CAKE

TABLE VII NICHOLS PILOT PLANT TEST RESULTS ON FILTER PRESS

214

SOLID

WASTES

A N D RESIDUES

thermal destruction of the organics must be to e l i m i n ate the use of purchased f u e l . Char as Supplemental F u e l . Variations i n sewage sludge dewaterability, i n ash content and BTU per pound of organics, and occasional upsets i n coagulant choice and a p p l i c a t i o n must be expected. Supplementation of the heating value of the sludge i t s e l f , by return of previously produced char i s one way to overcome t h i s problem. For a 40 MGD wastewater treatment plant producing approximately 5000 pounds of sludge s o l i d s per hour, f u e l l e s s operation i n the LTC or HTC mode to produce char would be recommended whenever the sludge has more than 5000 BTU per pound of water contained. This w i l l be most i f not a l l of the time, judging from the conversion scales at the bottom of the graph i n Figure 2 and from the performance expected of the Moseley membrane p l a t e s . Assuming 30% ash i n the feed and 12% y i e l d of f i x e d carbon from organics, the char produced would consist of 600 pounds per hour of c a r bon plus 1500 pounds per hour of ash and be about 140 cubic feet per hour. A 14,000 cubic foot storage b i n f o r example would therefore hold about 100 hours production, or 60,000 pounds of carbon - 900 m i l l i o n BTU of heat value i n easy to store and handleable form. The furnace f o r p y r o l y s i s could be sized at 1833 square feet of hearth area as indicated e a r l i e r i n t h i s paper, to burn away a l l of the f i x e d carbon from the normal feed r a t e . S e l f - s u s t a i n i n g operation would then be maintained through any deviations i n sludge ash content, or heating value per pound of organics contained, or cake moisture content that reduced the f u e l chara c t e r i s t i c of the sludge to as l i t t l e as 3400 BTU per pound of water contained. Extreme high sludge ash content or low heating value of sludge organic content, reducing sludge heating value below 3900 B T U / l b . , could be offset by the return of char from the storage b i n . Low sludge a v a i l a b i l i t y i n r e l a t i o n to heat demand could be met i n the same way.


10.

V O N

DREuscHE

A N D NEGRA


215

A temporary excessive sludge moisture content, for example change from 40% moisture to 25% moisture for 8 hours would require feeding the furnace half the usual s o l i d s r a t e . This 10,000 l b s / h r . wet cake feed would be pyrolyzed and the carbon burned away i n 1200 square feet leaving 600 square feet for burning of supplemental char added from the storage b i n , 360 pounds per hour of carbon or 5,000,000 BTU/hr. or 2,000 BTU/lb. of dry sludge s o l i d s fed could be added i n t h i s way; maintaining f u e l l e s s operation although reducing energy recovery from exhaust gases from 2750 BTU recovered per ton sludge s o l i d s to 2500 BTU recovered. These numbers are quickly worked out from the graphs and tables given e a r l i e r i n t h i s paper. The char storage b i n could also of course be used for stand-by f u e l to keep the multiple hearth furnace hot when sludge i s not a v a i l a b l e , or to provide start-up f u e l . Conelusions Pyrolysis of sludge alone can be r e l i e d upon to eliminate a l l use of purchased f u e l s . This requires the use of modern dewatering equipment, which now has become commercially a v a i l a b l e . Heat recovery i n the range of 2,000 to 3,000 BTU per pound of dry sludge s o l i d s seems e a s i l y f e a s i b l e . Energy recovery i n the form of increased use of digesters to generate methane, while s t i l l avoiding a f u e l cost to thermally destroy the organics i n the r e s u l t i n g energy depleted sludge may i n some cases be a preferred use. Energy recovery as a char ash mixture to be treated f o r the e l i m i n a t i o n of heavy metals, converted to active carbon, or used as a f u e l at other locations than the sewage treatment p l a n t , may also develop as the preferred form. It i s believed that much work remains to be done i n finding valuable uses, preferably i n - p l a n t , for the carbon ash mixture that can be produced from sewage sludge and for the heat a v a i l a b l e from the gases exhausting from the afterburner. Steady improvement in dewatering equipment and i n thermal destruction energy e f f i c i e n c y w i l l increase the a v a i l a b i l i t y of


216

SOLID WASTES AND RESIDUES

these products and thus b r i n g about increasing e f f o r t s to u t i l i z e them. The p y r o l y s i s of sludge alone i n conjunction with better dewatering p r a c t i c e s , w i l l eliminate a l l f u e l cost; and generate enough steam from the exhaust gas to run the exhaust gas fan (the only major power consumer on the p y r o l y s i s unit)and aeration blowers for the waste activated sludge process, by steam t u r b i n e . It w i l l at the same time provide for the production of char, thus laying a base for eventual heavy metals recovery. This should make i t much more cost e f f e c t i v e than heretofore to carry out the e s s e n t i a l function of converting offensive and dangerous sludge organics to safe and inoffensive inorganics. "Literature Cited" 1. Patent U . S . 3,650,830 (1972) Nichols Engg. & Res. (Mathis, E . A . ) 2.

Nichols Engineering & Research Report, "Pyrolysis of Refuse Derived F u e l " for Aenco, Inc. July 1,1976

3.

S i e g e r , R . B . and Bracken, B . D . , AIChE Symp.Ser., (1977), vol.73 (143-149)

4.

Jacknow, J., "Environmental Aspects of Municipal Sludge Incineration", WEMA, Jan. 1978.

5.

Camp Dresser & McKee, Alexander Potter A s s o c . , Phase 1 Report for I . S . C . of N.Y.,N.J., Conn. June, 1975.

6.

von Dreusche, C . J r . "Process Aspects of Regeneration in a M u l t i p l e Hearth Furnace AIChE A u g . , 1974.

7.

Patent, U.S. 4,046,085 (1977) Nichols Engg. & Res. (Barry, L.T. etal.)

8.

Patent U . S . 3,289,845 (1966) CIBA-GEIGY,(Weber,O.)

9.

Edmondson, B . R . , and Brooks, D . R . , Water Services (May 1977)

10. EPA-430/9-77-011, "Energy Conservation i n Munic i p a l Wastewater Treatment". MCD-32. (March, 1977) APRIL 18, 1978.


Pyrolyzer Design Alternatives and Economic ... - ACS Publications

Recommend Documents