Estimating the Heating Value of Sewage Sludge - ACS Publications

Chapter 12

Estimating the Heating Value of Sewage Sludge

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 7, 2016 | http://pubs.acs.org Publication Date: December 23, 1992 | doi: 10.1021/bk-1992-0515.ch012

A New Correlation M . Rashid Khan, Matthew A. McMahon, and S. J . DeCanio Research & Development Department, Texaco, Inc., P.O. Box 509, Beacon, NY 12508

On a dry-basis, the heating value of sewage sludge is greater than that of o i l shale or tar sand. The volatile matter content of dry sludge can be higher than that of the high volatile bituminous coal. Available correlations in the literature, developed for coals, were applied for predicting the experimentally determined heating values. In addition, the sludge compositional data (C, H, S, and ash) were used to develop a new correlation specifically for raw sewage sludge. Compared to the models tested, the new correlation developed in this study for sewage sludge provided a better f i t between the measured and predicted values. Sewage sludge is composed of organic and inorganic materials. The organic portion of the sludge is predominantly composed of C, H, N, and S. A variety of organic and inorganic materials can be found in a waste-water treatment plant (1) . Treatment plants receive tremendous quantities of waste-water containing dissolved and suspended solids from numerous sources including domestic, industrial and urban-offs as well as from storm drainage. The solid residue or sludge, the principal product of primary and secondary treatments, has been traditionally ocean dumped, incinerated or landfilled. However, current federal regulations restrict such traditional practices. The option to dispose of such materials by landfilling also suffers from psychological (e.g., "not-in-my-backyard" syndrome) and genuine environmental concerns (e.g., contamination of ground water or agricultural products and leaching). A recent survey of compositional characteristics of domestic sludges indicate that most sludges can be 0097-6156/93/0515-0144$06.00/0 © 1993 American Chemical Society In Clean Energy from Waste and Coal; Khan, M. Rashid; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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c l a s s i f i e d as "hazardous, " and consequently not s u i t a b l e f o r d i s p o s a l by l a n d f i l l i n g (2) . The i n c i n e r a t i o n technology can s u f f e r from emission and p u b l i c p e r c e p t i o n problems. Keeping the l i m i t a t i o n s o f these a l t e r n a t i v e s i n mind, conversion o f sewage sludge t o c l e a n f u e l s by, f o r example, g a s i f i c a t i o n (which r e a d i l y converts e s s e n t i a l l y a l l the o r g a n i c c o n s t i t u e n t s ) t o s y n t h e s i s gas (CO and H ) f o r power g e n e r a t i o n o r as chemical feedstock, p r o v i d e s an e x c e l l e n t avenue t o u t i l i z e t h i s renewable resource ( 3 ) . The p r o c e s s i n g and the u t i l i z a t i o n o f sewage sludge r e q u i r e s a b e t t e r understanding o f i t s p h y s i c a l and chemical p r o p e r t i e s . In p a r t i c u l a r , the a b i l i t y t o estimate i t s c a l o r i f i c v a l u e would indeed be o f g r e a t importance keeping i n mind t h a t the measured h e a t i n g values o f sludge a r e g e n e r a l l y not r e a d i l y a v a i l a b l e and the r e p o r t e d data can s u f f e r from a r e l a t i v e l y l a r g e experimental variation ( p a r t l y due t o p o s s i b l e b i o l o g i c a l / c h e m i c a l degradation o f samples d u r i n g v a r i o u s treatments). C o r r e l a t i o n s are important f o r j u s t i f i c a t i o n and modeling o f t h e conversion processes now being developed. The c o r r e l a t i o n s between t h e c o a l composition and h e a t i n g value were r e p o r t e d as e a r l y as 1940. Over 20 d i f f e r e n t equations are r e p o r t e d i n the l i t e r a t u r e which enable one t o c a l c u l a t e the h e a t i n g value o f c o a l based on the ultimate/proximate analyses (4-9). However, e s s e n t i a l l y nothing c o u l d be found i n the l i t e r a t u r e t h a t c o u l d be r e a d i l y Appl.ied t o s p e c i f i c a l l y estimate t h e h e a t i n g v a l u e of dewatered sludge. To assess the u t i l i t y of existing correlations (developed f o r c o a l ) , the most widely used equations were t e s t e d f o r d r y sewage sludge. Mott and Spooner (1940) claimed t h a t t h e i r equation w i l l y i e l d h e a t i n g v a l u e s agreeing w i t h i n 200 b t u f o r the whole range o f f u e l s , from peat t o a n t h r a c i t e (4) . We, however, were much l e s s s u c c e s s f u l with t h i s equation f o r dewatered sewage sludge. Mason and Ghandi (1980) developed a c o r r e l a t i o n based on c o a l samples from the Pennsylvania S t a t e U n i v e r s i t y c o a l data base (6) . A comparison o f the experimental r e s u l t s and the p r e d i c t e d values (based on Mason and G h a n d i s equation) was made. Compared t o the equation by Mott and Spooner, t h i s equation (termed Data Base [DB] Equation) d i d a b e t t e r job i n e s t i m a t i n g the h e a t i n g v a l u e o f sludge. V a r i o u s equations r e p o r t e d f o r c o a l i n the l i t e r a t u r e were attempted f o r dewatered sewage sludge. However, l i k e Mott and Spooner's equation, these equations (10, 11) appeared t o be i n a p p r o p r i a t e f o r dewatered sewage sludge.


2

1

Experimental Dewatered sewage sludge samples ( o r i g i n a t i n g i n v a r i o u s treatment p l a n t s o f the country) were d r i e d i n a l a b vacuum oven under N . The d r y samples were c h a r a c t e r i z e d by monitoring the f o l l o w i n g : u l t i m a t e a n a l y s i s (C, H, S, Ν ) , ash content and high h e a t i n g v a l u e . A s e l e c t e d s e t o f 2

In Clean Energy from Waste and Coal; Khan, M. Rashid; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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CLEAN ENERGY FROM WASTE AND COAL

samples were c h a r a c t e r i z e d i n m u l t i p l e l a b o r a t o r i e s which i n c l u d e d the f o l l o w i n g : Huffmann L a b o r a t o r i e s , Inc. (Golden, CO) , I n s t i t u t e of Gas Technology (IGT, Chicago, IL) , and Texaco Research & Development (Beacon, NY) t o ensure t h a t analyses i n v a r i o u s l a b o r a t o r i e s provide comparable r e s u l t s . In g e n e r a l , the data obtained from v a r i o u s l a b s were w i t h i n the v a r i a t i o n allowed by the conventional ASTM g u i d e l i n e s f o r each analyses. A l l analyses f o r a given sample were completed r e l a t i v e l y r a p i d l y t o minimize degradation of samples due t o b a c t e r i a l growth. The data (30 observations i n t o t a l ) were analyzed by u s i n g the S t a t i s t i c a l A n a l y t i c a l System (SAS) package developed by SAS I n s t i t u t e (12). The r e g r e s s i o n program a v a i l a b l e i n t h i s package was a p p l i e d t o develop an e m p i r i c a l model. The Heating Value of Sewage Sludge Compared t o the Various F o s s i l Fuels* The mean h e a t i n g value (gross) o f sludge (based on 30 observations) compared t o v a r i o u s f o s s i l f u e l s i s shown i n F i g u r e 1. The heating value of o i l s h a l e (Green R i v e r formation of Mahogeny zone; Colorado; 33 g a l / t o n , d e s c r i b e d by Khan, and Khan and others [13, 14]) was 3200 B t u / l b . The h e a t i n g value of e a s t e r n Kentucky s h a l e can be s i g n i f i c a n t l y lower than the western shale considered i n t h i s study. The h e a t i n g value of the A s p h a l t Ridge b a s i n t a r sand was l e s s than 2000 B t u / l b ( 13, 14). By c o n t r a s t , the h e a t i n g value of an average sewage sludge i s c o n s i d e r a b l y h i g h e r (6400 b t u / l b ) . The h e a t i n g value of an i n d u s t r i a l b i o s l u d g e observed i n t h i s study t o be g r e a t e r than 9000 b t u / l b . However, no i n d u s t r i a l sludges were i n c l u d e d i n the data base aimed a t developing the new c o r r e l a t i o n . Compared t o e s s e n t i a l l y a l l f o s s i l f u e l s (excluding petroleum based f u e l s ) , sewage sludge has a h i g h e r H/C (atomic) r a t i o (Figure 2) . The mean H/C r a t i o o f sewage sludge was 1.65 (based on 30 o b s e r v a t i o n s ) , c o n s i d e r a b l y higher than t h a t of the bituminous c o a l s (Pitt#8) with H/C r a t i o of 0.89 or a sub-bituminous c o a l (H/C of 0.96 f o r Wyodak c o a l ) . The H/C of the sewage sludge i s comparable t o t a r sand bitumen (with H/C of 1.5). In addition to the elements described above, s i g n i f i c a n t amounts of c h l o r i n e and v a r i o u s v o l a t i l e metals can be present i n sewage sludge. For example, the c h l o r i n e content of one sludge was as high as 0.6% (dry b a s i s ) . Other v o l a t i l e i n o r g a n i c s i d e n t i f i e d i n the t h i s sludge i n c l u d e d the f o l l o w i n g : B e r y l l i u m ( l e s s than 0.02 ppm), Vanadium ( l e s s than 1 ppm), and Manganese (900 ppm). However, t h i s study d i d not c o n s i d e r the r o l e of c h l o r i n e or v a p o r i z a b l e metals on the h e a t i n g value of sludge. V a r i a t i o n s i n the Sewage Sludge Composition. The mean, standard d e v i a t i o n , minimum and maximum v a l u e s f o r the compositional analyses are presented i n F i g u r e s 3 and 4. The v a r i a t i o n s i n the C, Η, Ν and S content i n d i f f e r e n t samples are shown i n F i g u r e 3. The s u l f u r content f o r v a r i o u s sludges ranged between 0.18 and 3.61 percent with a mean


12. KHAN ET AL.



B t u / l b , d r y (Thousands)

Bit C o a l (Pltt#8) S u b - b i t (Wy) Oil Shale (Col#)Tar S a n d (Asp)* Sewage Sludge

Feedstock F i g u r e 1. Feedstocks H/C

Comparison (Dry-basis)

o f Heating

Values

o f Various

(Atomic)

Bit C o a l (Pltt#8) S u b - b i t (Wy) O i l S h a l e ( C o l # ) T a r S a n d ( Α β ρ ) · Sewage Sludge

Feedstock F i g u r e 2. Feedstocks

Comparison (Dry-basis)

o f H/C o f (Atomic) o f V a r i o u s


147

In Clean Energy from Waste and Coal; Khan, M. Rashid; ACS Symposium Series; American Chemical Society: Washington, DC, 1992. 8tdDav

Min

% dry aiudpa

% dry aludQa

M «an

Max

Variations in Sulfur

Variations in Hydrogen

V a r i a t i o n s i n C, H, N, and S i n the Data Set

Max

Variations in Nitrogen

Variations in Carbon

F i g u r e 3. Used

% dry «ludg*

% dry aludpa


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149


Variations in Sludge Heating Value Btu/lb (ThouMndt)

Variations in Ash Content % dry sludge

Min

Mean

Max

8tdDtv

F i g u r e 4. V a r i a t i o n s i n the Heating Content i n the Data Set Used

Value

and Ash



150


v a l u e o f 1.71 (with a standard d e v i a t i o n o f 1.05 about t h e mean). The oxygen content o f sludge ranges between 3.5 and 27.8% with a mean o f 16.5 (and a standard d e v i a t i o n o f 6.3%). The v o l a t i l e matter content f o r a g i v e n sludge ranged between 45 and 62% (dry b a s i s ) . These v a l u e s a r e s i g n i f i c a n t l y higher than the v o l a t i l e matter content o f a high v o l a t i l e bituminous c o a l (with a v o l a t i l e matter content o f 35%, d r y b a s i s ) . The H/C r a t i o (atomic) f o r the data s e t used ranges between 1.44 and 1.86 with a mean o f 1.65 with a standard d e v i a t i o n o f 0.106. F i g u r e 4 shows t h a t the mean h e a t i n g v a l u e o f t h e sludge was 6409 with a standard d e v i a t i o n o f 816 (based on 30 o b s e r v a t i o n s ) . The measured values f o r t h e sludge ranged between 5261 and 8811 B t u / l b . The h e a t i n g v a l u e and the compositional c h a r a c t e r i s t i c s of sludge are dependent on the nature o f sludge as w e l l as on t h e degree o f d i g e s t i o n (or pretreatment) a sludge has undergone. The minimum ash content f o r t h e sludge was 18.9% w h i l e t h e maximum v a l u e f o r the sludge was 58.68% (the mean was 40.5%). The h i g h e r ash content g e n e r a l l y r e f l e c t s t h a t the sludge has e i t h e r been d i g e s t e d o r h e a t - t r e a t e d t o convert a l a r g e p o r t i o n o f the organic constituents. The sludge composition i s dependent on t h e nature o f pretreatment a given sludge has experienced. F o r example, the sludge c o n d i t i o n e d by a wet o x i d a t i o n process (intermediate pressure, 300-400 p s i ; o x i d i z i n g atmosphere; temperature o f 250-375 F) has a s i g n i f i c a n t l y d i f f e r e n t composition and a lower h e a t i n g v a l u e (Figure 5) compared t o the untreated sludge (lower C, H but a h i g h e r oxygen content compared t o the untreated m a t e r i a l s ; a l l data on d r y b a s i s ) . The compositional d i f f e r e n c e s between v a r i o u s sludges can be s i g n i f i c a n t . For example the d i f f e r e n c e s i n t h e compositional c h a r a c t e r i s t i c s between the Los Angeles (CA) and P a s s a i c (NJ) a r e shown i n F i g u r e 6. I t i s i n t e r e s t i n g t o note t h a t the p y r i t i c s u l f u r i s t h e dominant s u l f u r type f o r s e v e r a l sludges (Figure 7) . The presence o f t h i s l a r g e c o n c e n t r a t i o n o f p y r i t e i s not t y p i c a l o f domestic sludges but suggests t h e formation o f p y r i t i c s u l f u r from o r g a n i c s u l f u r by b a c t e r i a l a c t i o n . Such conversion o f o r g a n i c t o i n o r g a n i c s u l f u r has been r e p o r t e d i n t h e c o a l l i t e r a t u r e . E v a l u a t i o n o f the New C o r r e l a t i o n . The v a r i a t i o n s (between the measured and p r e d i c t e d values) f o r t h e t h r e e models evaluated i n t h i s study a r e summarized i n F i g u r e 8 (discussed i n d e t a i l s i n the f o l l o w i n g s e c t i o n ) . Figure 9 compares t h e measured and p r e d i c t e d h e a t i n g v a l u e s based on the Data Base Model. The disagreement between t h e p r e d i c t e d and measured v a l u e s i n F i g u r e 9 i s much l a r g e r than those shown i n F i g u r e 10 (based on the new c o r r e l a t i o n developed i n t h i s study). F i g u r e 10 compares t h e measured and p r e d i c t e d h e a t i n g v a l u e s c a l c u l a t e d based on t h e c o r r e l a t i o n developed i n t h i s study. The f o l l o w i n g equation d e s c r i b e s t h i s model:


12. KHAN ET AL


151


Percent Dry B a s i s

C

Η

•I

Ν

S

Passaic, NJ (Raw)

VM

HI

Btu/Lb/1000

Ash

Wet Oxidized (IP)

F i g u r e 5. E f f e c t o f Wet O x i d a t i o n on Sewage Sludge Composition (Passaic Sewage Sludge) ; Raw and Wet O x i d i z e d (@ Intermediate Pressure)

Percent of dry sludge

40 ι

C

Η

Ν

Los Angeles

F i g u r e 6. Composition

Comparison

of

S

Ash

Passaic, NJ

LA

versus

Passaic


Sludge


152


%Var-100x(Predicted-Measured)/Measured

Sludge (This Model)

D B E q (Coal)

M o t t E q (Coal)

Various Equations H i

F i g u r e 8. Data S e t

M e a n of all O b s

M a x i m u m Var (Range)

Composition o f V a r i o u s Models Tested f o r t h e


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153


8900

8400

7900


>7400

6900

6400

DATA BASE EQUATION

5900

5400

4900 4900

5400

5900

6400

6900

7400

7900

8400

8900

Measured Heating Value, Btu/lb

F i g u r e 9. P r e d i c t e d and Measured Heating Values f o r Sewage Sludge (Based on the Data Base Equation, i . e . , L i t e r a t u r e Model) 8900

8400

7900

k

7400

£6900

I 6400

MODEL

DEVELOPED IN THIS STUDY

5900

5400 4900 4900

5400

5900

6400

6900

7400

7900

8400

8900

Measured Heating Value. Btu/lb

F i g u r e 10. P r e d i c t e d and Measured Heating Value f o r Sludge Based on the New Sludge Model In Clean Energy from Waste and Coal; Khan, M. Rashid; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

154


Q (Btu/lb) = 241.89*C + 264.26*H + 236.2*S + 20.99*Ash 4174.68 2

R f o r the model i s 0.99. The parameter measures t h e p r o p o r t i o n o f t o t a l v a r i a t i o n s e x p l a i n e d by t h e r e g r e s s i o n . I t i s c a l c u l a t e d by d i v i d i n g the sum o f squares due t o r e g r e s s i o n by the t o t a l sum of squares. R i s related to c o r r e l a t i o n c o e f f i c i e n t , r , by the f o l l o w i n g i n simple l i n e a r r e g r e s s i o n : r = square r o o t o f R . In a d d i t i o n , r , has the same s i g n as the slope o f the computed r e g r e s s i o n . The F value f o r the model was 650.6. The F r a t i o i s t h e r a t i o produced by d i v i d i n g the mean square f o r the model by the mean square o f e r r o r . I t t e s t s how w e l l t h e model as a whole ( a f t e r a d j u s t i n g f o r the mean) accounts f o r the behavior o f the independent v a r i a b l e . The Ρ v a l u e f o r the model was 0.0001. Ρ d e f i n e s the "observed l e v e l o f s i g n i f i c a n c e . In s t a t i s t i c a l terms, t h e l e v e l o f s i g n i f i c a n c e , alpha, o f a t e s t i s d e f i n e d as the p r o b a b i l i t y o f r e j e c t i n g the n u l l hypothesis ( i . e . , no l i n e a r r e l a t i o n s h i p between the dependent and independent v a r i a b l e s ) , g i v e n the n u l l hypothesis i s t r u e . The P-value g i v e s us the l a r g e s t value o f alpha t h a t would l e a d t o t h e acceptance o f the n u l l hypothesis. In other words, from s t a t i s t i c a l standpoint, the c o r r e l a t i o n developed i n t h i s study f o r e s t i m a t i n g the h e a t i n g value o f sludge i s h i g h l y significant. 2


2

11

Comparison o f Various C o r r e l a t i o n s . Attempts were made t o estimate the h e a t i n g value o f sludge u s i n g c o r r e l a t i o n s widely r e p o r t e d i n the l i t e r a t u r e Appl.icable f o r c o a l (and o i l s h a l e ) . In p a r t i c u l a r , the equation by Mott & Spooner and the Data Base equations were compared with t h e newly developed c o r r e l a t i o n . The percent v a r i a t i o n between the measured and t h e p r e d i c t e d values were c a l c u l a t e d f o r each model by t h e f o l l o w i n g equation: % V a r i a t i o n = 100 χ

(Predicted-Measured)/Measured

The v a r i a t i o n s (between the measured and p r e d i c t e d values) f o r t h e t h r e e models are summarized i n F i g u r e 8. The model developed i n t h i s study provides a mean v a r i a t i o n o f 0.019% between the p r e d i c t e d and the measured v a l u e s . The maximum v a r i a t i o n between the measured and p r e d i c t e d v a l u e s was never g r e a t e r than 2.83%, based on the new model. In c o n t r a s t , the Data Base Equation p r o v i d e s a maximum v a r i a t i o n o f 10.2% while the equation by Mott & Spooner y i e l d e d a maximum d i f f e r e n c e o f 14.8% between t h e measured and p r e d i c t e d v a l u e s . Summary & Conclusions The f o l l o w i n g c o n c l u s i o n s are d e r i v e d based on t h i s


study:


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155

ο

The heating value of dry municipal sewage sludge i s c o n s i d e r a b l y higher than t a r sand or o i l s h a l e but lower than t h a t of bituminous c o a l . The atomic H/C r a t i o of sewage sludge, however, i s h i g h e r than t h a t of bituminous c o a l , but comparable t o the H/C r a t i o of o i l s h a l e . Some i n d u s t r i a l biosludges can have h e a t i n g value comparable t o t h a t of a low rank c o a l .

ο

The v o l a t i l e matter content of sludge i s higher than t h a t of c o a l , o i l shale or t a r sand.

ο

The compositional c h a r a c t e r i s t i c s (C, H, N, S and ash) of sludge can vary widely among sewage sludge of different origin. Wet o x i d a t i o n of sewage sludge s i g n i f i c a n t l y reduces i t s h e a t i n g value as w e l l as i t s C and H content.

ο

The conventional equations developed f o r c o a l are not r e a d i l y Appl.icable f o r sewage sludge. The equation developed i n t h i s study serve reasonably w e l l f o r e s t i m a t i n g the h e a t i n g value of sludge of v a r i o u s o r i g i n based on i t s a n a l y s i s (C, H, S, and ash).

L i t e r a t u r e Cited 1. 2.

3. 4. 5. 6.

7. 8. 9. 10.

V. T. Chow, R. Eliassen and R.K. Linsley, Editors, "Wastewater Engineering: Treatment, Disposal & Reuse", McGraw-Hill, 1979. Federal Register, "National Sewage Sludge Survey: A v a i l a b i l i t y of Information and Data, and Anticipated Impacts on Proposed Regulations; Proposed Rule," Vol 55., No. 218, 40 CFR Part 503, Nov 9, 1990. Willson, W. G. et. a l , "Low-Rank Coal Water S l u r r i e s for G a s i f i c a t i o n , " F i n a l Report by UNDEERC, EPRI Report No. AP-4262, Nov., 1985. Mott, R. A. and Spooner, C. E., "The C a l o r i f i c Value of Carbon i n Coal," Fuel 19, 226-31, 242-51 (1940). Selvig, W. A. and Gibson, F. Η., " C a l o r i f i c Value of Coal," i n Lowry, H. H. ed., Chemistry of Coal U t i l i z a t i o n 1, 139, New York, 1945. Mason, D. M. and Ghandi, Κ., "Formulas f o r Calculating the Heating Value of Coal and Char: Development, Tests and Uses," Am. Chem. Soc. Div. Fuel Chem Preprints, 1980, 25(3), 235. Boie, W., "Fuel Technology Calculations," Energietechnic 3, 309-16, 1953. Grummel, E.S., and Davis, I. Α., "A New Method of Calculating the C a l o r i f i c Value of a Fuel From I t s Ultimate Analysis," Fuel 12, 199-203, 1933. Neavel, R. C., S. E. Smith, E. J . Hippo, and R. N. M i l l e r , "Interrelationships Between Coal Compositional Parameters," Fuel 65, 3, pp. 312-320, 1986. Wilson, D. L., "Prediction of Heat of Combustion of S o l i d Wastes from Ultimate Analysis," Environmental Science Technology 6, No. 13, 1119-1121, 1972.


156


11.

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RECEIVED August 5, 1992


Estimating the Heating Value of Sewage Sludge - ACS Publications

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