37
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Pilot Plant Study on Sewage Sludge Pyrolysis T. KASAKURA NGK Insulators, Ltd., Nagoya, Japan M. HIRAOKA Kyoto University, Kyoto, Japan
Sludge incineration process has been increasingly adopted by many municipalities as one of the effective methods for reducing the volume and stabilizing the organic materials of sewage sludge. But recently the incineration process has been pointed out to have several serious problems, namely higher energy consumption, and secondary pollution such as Cr+6 formation in ash and NOx formation in exhaust gas. Some investigators had proposed that "pyrolysis" would have a possibility to solve these problems, and have studied pyrolysis of sewage sludge. The authors had already conducted the laboratory scale study and the preliminary pilot plant study, and proposed that "drying-pyrolysis process" (pyrolysis followed by indirect steam drying of dewatered sludge cake) (Fig.-1) could be one of the most economical and feasible alternatives for conventional incineration process. The authors have further conducted the feasibility study on a continuous system of "drying-pyrolysis process" to evaluate the performance of the process in pilot scale, and to demonstrate its effectiveness as a thermal processing of sewage sludge. This paper presents the results of this pilot plant study. The purpose of this study can be summarized as; (i) To evaluate the performance of drying-pyrolysis process in pilot scale (ii) To demonstrate the effectiveness of drying-pyrolysis process as a thermal processing of sewage sludge (iii) To compare drying-pyrolysis process with other thermal processes of sewage sludge (iv) To show the feasibility of the practical plant of drying-pyrolysis process PILOT PLANT and EXPERIMENTAL PROCEDURE The pilot plant for this study consists of a four shaft indirect steam dryer of paddle type, a four hearth furnace for 0-8412-0565-5/80/47-130-509$05.00/0 © 1980 American Chemical Society Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
=3c
paddle dryer
P y rràytic olytic furnace
H—I Scrubber f o r dryer exhaust gas
Q (^Quenching t a n k Q * Gas' Sampling p o i n t G£, G j , G^,
Residue
Fuel O i l
Θ-
LPG
Γ
Cake feeder DQOQI
Cake
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Waste Gas
Θ
Treated Sewage Water
Waste Gas
—Θ
Tap Water
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37.
KASAKURA AND HiRAOKA
Sewage Sludge Pyrolysis
511
p y r o l y s i s , a c o m b u s t i o n chamber f o r p y r o l y t i c gas ( a f t e r b u r n e r ) , and a h e a t r e c o v e r y b o i l e r f o r c o m b u s t i o n chamber e x h a u s t g a s . The o v e r a l l l o a d i n g c a p a c i t y o f t h i s p l a n t i s a b o u t k t o 5 t o n s per day a s d e w a t e r e d cake w i t h w a t e r c o n t e n t 75 %· ( F i g - l ) F i v e d i f f e r e n t k i n d s o f d e w a t e r e d c a k e s were s e l e c t e d f o r t h e p i l o t t e s t , w a t e r c o n t e n t and h i g h e r h e a t i n g v a l u e o f sample c a k e s were v a r i e d from kG t o 76 %, from 1 , 5 0 0 t o 2 , 9 6 0 k c a l / k g D. S. r e s p e c t i v e l y . ( T a b l e - I ) Cake A; S l u d g e from a combined sewerage s y s t e m sewage t r e a t m e n t p l a n t of a r e p r e s e n t a t i v e c i t y i n Japan. The c a k e was d e w a t e r e d by vacuum f i l t e r a f t e r l i m e and f e r r i c c h l o r i d e w e r e added t o m i x e d p r i m a r y and e x c e s s s l u d g e . Cake B; S l u d g e from a r e g i o n a l sewerage s y s t e m sewage t r e a t m e n t p l a n t c o n t a i n i n g i n d u s t r i a l w a s t e w a t e r . The c a k e was d e w a t e r e d b y f i l t e r p r e s s a f t e r a n o r g a n i c p o l y m e r was added t o m i x e d p r i m a r y and e x c e s s s l u d g e . Cake C; The same s l u d g e a s c a k e B, b u t t h e c a k e was d e w a t e r e d b y f i l t e r p r e s s a f t e r l o w t e m p e r a t u r e ( I 5 0 - l 6 0 °C) t h e r m a l conditioning. Cake D; S l u d g e from s e p a r a t e d sewerage s y s t e m sewage t r e a t m e n t p l a n t f o r domestic waste. The c a k e was d e w a t e r e d b y f i l t e r p r e s s a f t e r c o n v e n t i o n a l ( l 8 0 - 200 °C) t h e r m a l c o n d i t i o n i n g o f the mixed s l u d g e . Cake E; S l u d g e o f chromium t a n n e r i e s ' w a s t e . The cake was d e w a t e r e d b y vacuum f i l t e r a f t e r a n o r g a n i c p o l y m e r was added t o raw s l u d g e . The d e w a t e r e d c a k e was d r i e d t o t h e w a t e r c o n t e n t o f 30 t o *f0 % i n t h e i n d i r e c t steam d r y e r , and f e d i n t o t h e p y r o l y t i c f u r n a c e t h r o u g h the screw conveyor. Combustion a i r l e s s than the t h e o r e t i c a l amounts f o r s l u d g e i s s u p p l i e d i n t o t h e f u r n a c e f o r p a r t i a l combustion o f c o m b u s t i b l e p y r o l i t i c gas. That i s , p y r o l y s i s was c a r r i e d out by " D i r e c t H e a t i n g " method. O f f g a s from f u r n a c e i s i n t r o d u c e d i n t o t h e c o m b u s t i o n chamber t o b u r n i t s r e m a i n i n g c o m b u s t i b l e s , and t o decompose i t s p o l l u t a n t and o t h e r odor c o m p o n e n t s . E x c e s s a i r i s a u t o m a t i c a l l y c o n t r o l l e d by t h e oxgen c o n c e n t r a t i o n a t t h e e x i t o f t h e comb u s t i o n chamber. Heat o f o f f gas from t h e c o m b u s t i o n chamber i s r e c o v e r e d by the waste heat b o i l e r . Steam g e n e r a t e d i n t h e b o i l e r i s s u p p l i e d t o t h e i n d i r e c t steam d r y e r . When t h e amounts o f steam g e n e r a t e d i s n o t s u f f i c i e n t f o r d r y i n g d e w a t e r e d c a k e , a u x i l i a r y f u e l i s used t o g e n e r a t e t h e a d d i t i o n a l s t e a m . I n t h i s s t u d y , t o make a c o m p a r i s o n w i t h d r y i n g - p y r o l y s i s p r o c e s s , t h e e x p e r i m e n t s on d i r e c t f e e d p r o c e s s e s , i . e . , d i r e c t p y r o l y s i s p r o c e s s and i n c i n e r a t i o n p r o c e s s , were c o n d u c t e d . I n t h e s e p r o c e s s e s , t h e d e w a t e r e d cake was d i r e c t l y f e d i n t o t h e furnace. The p l a n t was o p e r a t e d a t s t e a d y s t a t e f o r a t l e a s t 5 or 6 h o u r s i n a l l r u n s .
Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
THERMAL CONVERSION OF SOLID WASTES AND BIOMASS
512
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Table I .
A n a l y t i c a l r e s u l t of f e d cake.
Cake Run No. M o i s t u r e content (wt%) Ignition loss (wt% on dry b a s i s ) Higher h e a t i n g value ( k c a l / k g on d r y b a s i s )
A 803 75.8
Β 816 74.0
59-5
56.7
2,960
3,290
C 922 45-7
D 819 55.3
Ε 823 76.4
32.0
83.6
64.3
1,500
4,310
4,000
100
90
ο Cake A •
Β
Δ
C
A
D
*
Ε
Decomposition
r a t i o of combustibles
(%)
I g n i t i o n l o s s of residue
20
10
50
10 Loading r a t e of d r i e d Figure 2.
cake (kg'DS/m *hr)
Variation of ignition loss of residue and decomposition ratio of com bustibles with loading rate of dried cake
Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
37.
KASAKURA AND HIRAOKA
513
Sewage Sludge Pyrolysis
EXPERIMENTAL RESULTS AND DISCUSSION Performance of Drying-Pyr-olysis Process I n d i r e c t steam drying operation was a v a i l a b l e to a l l kinds of dewatered cakes tested and i t s e f f i c i e n c y ( o v e r a l l heat t r a n s f e r c o e f f i c i e n t U) v a r i e d with the kinds of cakes. Cake A Downloaded by UNIV OF MASSACHUSETTS AMHERST on November 5, 2017 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch037
U(Kcal/m
hr °C)
l
k
Q
_
1
5
0
Cake B _
l 6 o
1 7 0
Cake C
Cake D
275
300-360
Cake Ε
220-280
The o v e r a l l heat t r a n s f e r c o e f f i c i e n t (U) based on the input heat was c a l c u l a t e d from the amount of condensed drain which r e l e a s e d from the dryer by the f o l l o w i n g equation. q = U-A ( at) where q i s supplied heat and A i s t o t a l heat t r a n s f e r area. The temperature d i f f e r e n c e ( Δ t) was obtained from the d i f f e r e n c e between cake temperature (95°0 constant) and steam temperature. The operating conditions i n t h i s study were s e l e c t e d on the previous work (Majima et a l . , 1977) and the p r e l i m i n a r y study by the p i l o t p l a n t . P y r o l y t i c furnace was mostly operated under the f o l l o w i n g c o n d i t i o n ; a i r r a t i o f o r combustibles i n sludge 0 . 6 , p y r o l y t i c temperature 900°C, and r e t e n t i o n time 60 minutes. In t h i s study, to evaluate the weight r e d u c t i o n e f f e c t of thermal processing, "Decomposition R a t i o " d i f i n e d as follows was introduced. Decomposition Ratio of Combustibles (%)
K l
lc
100 - l r
;
1
U
U
'
l c ; i g n i t i o n l o s s of fed cake (%) l r ; i g n i t i o n l o s s of residue {%) When the loading rate to furnace was taken under 25 kg D.S./m^«hr more than 95 % of combustibles i n sludge cake was de composed by p y r o l y s i s r e g a r d l e s s of the kinds of cakes. Thus, p y r o l y s i s i s estimated to be as e f f e c t i v e as i n c i n e r a t i o n i n weight r e d u c t i o n of sewage sludge. The r e l a t i o n s h i p between l o a d i n g rate and decomposition r a t i o or i g n i t i o n l o s s i n s o l i d residue i s shown i n Figure 2 . r
Heavy Metals Table 2 shows the behaviour of chromium VI (Cr+6) i n p y r o l y t i c operation. The behaviour of C r ° i n i n c i n e r a t i n g operation on cake Ε i s a l s o shown i n the t a b l e . The r a t i o of C r ° to t o t a l chromium (Cr ^/T-Cr) i s an i n d i c a t o r of the behaviour of chromium compounds i n thermal processings. As i n d i c a t e d i n the table, a part of chromium HE (Cr 3) i n fed cake i s oxidized into Cr 6 d u r i n g i n c i n e r a t i n g operation, whereas a considerable p o r t i o n of Cr 6 i n fed cake i s reduced during p y r o l y t i c operation. +
+
+
Jones and Radding; Thermal Conversion of Solid Wastes and Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
514
THERMAL CONVERSION OF SOLID WASTES AND BIOMASS
Table I I .
+
Behaviour of C r ^ i n p y r o l y t i c + 6
cake
( r u n No.)
+ 6
operation
T
/ - C r Dissolved Cr {%) (mg/ )
Ε Ε
Table m .
Drying-pyrolysis Direct pyrolysis Incineration
Table IV. Process
Dispersion
ratio
Run No. 803 903 904
X
2
-
N.D.
-
l40
gas
Direct pyrolysis A (903) 04 3 290-345 45.6 6.5 111 75 0.04 Ο.76
