Gasification of Solid Waste in Dual Fluidized-Bed Reactors - ACS

38 Gasification of Solid Waste in Dual Fluidized-Bed Reactors M. KAGAYAMA, M. IGARASHI, M. HASEGAWA, and J. FUKUDA Tsukishima Kikai Co., Ltd., 17-15, Tsukuda 2-Chome, Chuo-Ku, Tokyo 104, Japan

Downloaded by CORNELL UNIV on October 15, 2016 | http://pubs.acs.org Publication Date: August 29, 1980 | doi: 10.1021/bk-1980-0130.ch038

D. KUNII Tokyo University, Tokyo, Japan

In Japan, pollution emanating from conventional municipal refuse incinerator has become a problem these past ten or more years. The problem centers on air pollution by HCL and SOx in flue gas which has worsening as a result of the increased plastic content in refuse and water pollution of land-fill sites by disposed incinerator ash. We have developed a new solid waste treatment process to solve these problems. It took about seven years to develop the system which involves circulating sand particles between two fluidized bed reactors. In 1972, after performing a survey of research activities concerning pyrolysis for more than one year, TSK (Tsukishima Kikai Co., Ltd.) made the decision to develop the new solid waste pyrolysis system which is comprised of two fluidized bed reactors. This system has been applied to a cracking process as Kunii-Kunugi Process (1), which has been under development as a national project in Japan, for production of olefins from heavy o i l . Fundamental pyrolysis test using a small, single fluidized bed reactor was performed in the first stage in 1973. Various fundamental data of thermal destruction of solid waste were obtained by this test. In the same year, a pilot plant consisting of dual fluidized bed reactors and auxiliary equipment was constructed. I.D. of the cracking reactor and the regenerator is 150mm and 200mm. About 10 kg/hr of solid waste was fed and pyrolized continuously. Using this plant, a continuous pilot test of the process has been made since 1974. Beside the test using these small units, a mock-up plant test on a larger scale was also made to obtain the engineering know-how for the design of dual fluidized bed reactors. The mock-up plant was operated under normal temperature and the sand was fluidized by the compressed air. The scale up factor from the pilot plant to the mock-up plant was 3 times. Through these test the feasibility of this system was confirmed, so a demonstration plant was constructed in 1975 nearby a pulp and paper mill in Miyagi Prefecture. The Ministry of International Trade and Industry (Japanese Government) granted financial support for the demonstration plant as a very important technology to be materialized promptly. 0-8412-0565-5/80/47-130-525$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.

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The p l a n t has been operated s u c c e s s f u l l y f o r about 7000 hours s i n c e A p r i l 1976, d i s p o s i n g sludge from the pulp and paper m i l l , m u n i c i p a l r e f u s e , waste p l a s t i c and blocks of spent t i r e s .


D e s c r i p t i o n of Process The p y r o l y s i s equipment of the process comprise of the cracking r e a c t o r and the regenerator, as shown i n F i g - 1 . These two r e a c t o r s are f i l l e d w i t h sand which i s used as a heat t r a n s f e r medium. Superheated steam i s blown i n t o the r e a c t o r s through nozzles l o c a t e d at the bottom. The sand goes up through the r e a c t o r s with steam and forms a f l u i d i z e d bed zone. Then i t goes down from one r e a c t o r to the other through the c i r c u l a t i o n pipes mainly by g r a v i t y . Thus the sand c i r c u l a t e s between the two reactors. D i s i n t e g r a t e d s o l i d waste i s fed to the cracking r e a c t o r where i t i s mixed w i t h the hot sand to be d r i e d and cracked. By the cracking r e a c t i o n , organic matter i n the s o l i d waste i s pyrolyzed i n t o three components: f u e l gas, t a r and char. Produced gas and t a r are taken out of the top of the cracking r e a c t o r with steam. Char overflows from the cracking r e a c t o r to the bottom of the regenerator w i t h the c i r c u l a t i n g sand through the c i r c u l a t i o n pipe. In the regenerator, char i s c a r r i e d up by steam from the bottom to the f l u i d i z e d bed zone, where i t comes i n contact w i t h a i r and burns. I f the amount of char i s not suff i c i e n t to maintain cracking and combustion r e a c t i o n , (a phenomenon which occurs when the s o l i d waste has low c a l o r i f i c v a l u e ) , a u x i l i a r y f u e l such as o i l or produced combustible gas must be fed to the regenerator. I n c i n e r a t i o n f l u e gases come out from the regenerator and provide heat to the combustion a i r through the heat exchanger. A f t e r heat exchange, they enter the heat recovery process and the gas c l e a n i n g process. Hot sand overflows from the regenerator to the cracking r e a c t o r , and then again heats up the s o l i d waste. The c i r c u l a t i n g sand i s cooled by cracking r e a c t i o n and reheated by char (and a u x i l i a r y f u e l ) combustion. As mentioned above, s i n c e the cracking zone i s separated from the combustion zone, p y r o l y s i s i s made under the optimum oxygen-free c o n d i t i o n and high c a l o r i f i c f u e l gas can be produced. Inorganic coarse m a t e r i a l goes down and accumulates at the bottom of the r e a c t o r s , then i s discharged p e r i o d i c a l l y through the bottom v a l v e s . Fine i n o r g a n i c p a r t i c l e s such as ash are exhausted w i t h the f l u e gas from the top of the regenerator and are caught by the m u l t i - c y c l o n e and e l e c t r o s t a t i c p r e c i p i t a t o r . A c t u a l Demonstration F a c i l i t y In 1975, a 40 ton/day demonstration plant was constructed nearby a pulp and paper m i l l i n M i y a g i P r e f e c t u r e . The main purposes of the demonstration plant were to check the d u r a b i l i t y of the c o n s t r u c t i o n m a t e r i a l s , to e s t a b l i s h the method of operation and to study some other questionable items which could not be


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checked by the small u n i t t e s t . The plant c o n s i s t s of the two r e a c t o r s of 2000mm I.D., feeding system, gas handling system and other a u x i l i a r y equipment. The plant has been operated f o r about 7000 hours and necessary data obtained. During the operation p e r i o d , continuous operation f o r 1200 hours was twice accomplished s u c c e s s f u l l y . The flow sheet of t h i s plant i s shown i n F i g - 2 . M u n i c i p a l r e f u s e i s taken from the p r a c t i c a l i n c i n e r a t i o n plant and crushed by the hammer-type m i l l . Then i t i s stored i n the feed hopper. Pulp and paper sludge i s dehydrated to a moisture content of about 80% and stored i n the sludge hopper. From the hopper, crushed s o l i d waste or sludge i s taken out continuously and supplied to the c r a c k i n g r e a c t o r through the conveyor s c a l e where i t i s weighed. In case of the t e s t f o r mixture of sludge and p l a s t i c waste, crushed p l a s t i c waste i s stored i n the feed hopper and i s taken out and mixed with pulp and paper sludge at the i n l e t of the conveyor s c a l e . A f t e r being supplied to the cracking r e a c t o r , where temperature i s around 700°C, a l l kinds of s o l i d wastes are treated i n the same way. Produced combustible gas i s discharged from the top of the c r a c k i n g r e a c t o r and passes through the cyclones i n which most of the p a r t i c l e s are caught. Residual p a r t i c l e s contained i n the gas are washed out at the scrubbers where the gas i s cooled at the same time and moisture vapor i s condensed. T h i s condensed water i s used as scrubbing l i q u i d and then i s taken out continuously as process waste water. The scrubber c i r c u l a t i n g l i q u i d i s cooled by c o o l i n g water at the heat exchangers condensed water which i s taken out from the scrubbers i s introduced to the t a r decanter where t a r i s separated from water by décantation. T a r - f r e e water i s concentrated i n the evaporator and concentrated l i q u i d i s treated i n other equipments. Clean combustible gas which s t i l l contains hydrogen s u l f i d e i s stored i n the gas holder and i s used as the f u e l gas f o r the regenerator and the a f t e r - b u r n e r . I n c i n e r a t i o n f l u e gas which i s at a temperature of around 800°C i s cooled by a i r f o r i n c i n e r a t i o n at the heat exchanger which i s i n s t a l l e d on top of the regenerator. A f t e r heat recovery, the f l u e gas goes through the cyclone where l a r g e p a r t i c l e s are caught, then passes through the m u l t i - c y c l o n e and the e l e c t r o s t a t i c p r e c i p i t a t o r . In these f a c i l i t i e s almost a l l f i n e p a r t i c l e s are caught. Particle-free f l u e gas i s discharged from the stack. When heat i s i n s u f f i c i e n t because of low c a l o r i f i c value of the s o l i d waste, heavy o i l i s supplied f o r the regenerator as a u x i l i a r y f u e l . Steam f o r f l u i d i z a t i o n i s s u p p l i e d from an o f f - s i t e source. I t i s superheated by the superheater. Operation Data of Demonstration Plant Test M u n i c i p a l Refuse. In Table-X, average values of component of municipal refuse are shown. T a b l e - I I shows the values of chemical a n a l y s i s which were made a f t e r p u l v e r i z a t i o n . As shown i n T a b l e - I I , moisture content i s lower than that of usual average


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CONVERSION OF

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f i g u r e f o r Japanese urban r e f u s e . T h i s i s due to the f a c t that water was evaporated during crushing and storage.


Table-I A n a l y s i s of municipal refuse (before p u l v e r i z a t i o n , weight r a t i o on dry b a s i s ) Items Percentage Wood waste F i b e r s and c l o t h Paper Garbage Iron Nonironic metal Glass and s o i l Plastics Total

% by weight 4.9 8.1 48.6 15.1 11.0 1.1 4.9 6.3 100.0

Table-II Chemical and p h y s i c a l a n a l y s i s of p u l v e r i z e d municipal refuse Items Moisture Combustible Incombustible

Chemical analysis (dry basis)

Total C H Ν 0 Total-S Total-CL Ρ Inorganic Total Bulk s p e c i f i c g r a v i t y Net s p e c i f i c g r a v i t y Moisture Ash Ignition loss Fixed carbon C a l o r i f i c value

*W : Wet base,

D :

(W)* (D) (W) (D) (D) (D)

% by weight 45.4 37.6 17.0 100.0 33.40 4.42 1.26 28.05 0.47 1.00 0.30 31.10 100.00 0.34 0.84 45.4 31.1 68.9 9.4 3625 Kcal/kg

Dry base


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M a t e r i a l Balance and Heat Balance. Required heat was mainly s u p p l i e d by i n c i n e r a t i o n of char, and some amount of produced com b u s t i b l e gas was f e d as a u x i l i a r y f u e l to the regenerator, as the amount of char was not s u f f i c i e n t f o r continuous thermal c r a c k i n g . The m a t e r i a l balance around the r e a c t o r s i s shown i n T a b l e - I l l and heat balance i n Table-IV. R a d i a t i o n and convection l o s s i n T a b l e IV i s l a r g e r than that of u s u a l i n c i n e r a t o r s because of the t h i n refractory. I t can be decreased i n case of commercial p l a n t s . Energy balance of the t o t a l plant i s shown i n F i g - 3 .


Table-Ill M a t e r i a l balance of m u n i c i p a l r e f u s e p y r o l y s i s

Solid Waste

Input Items Moisture Combustible Incombustible Sub t o t a l

Kg/Hr 726.4 601.9 271.7 1600 2650 150 1136 5536

A i r feed Combustible gas Steam Total

Output Items Dry gas C. gas* Steam Sub t o t a l Dry gas F. gas* Steam Sub t o t a l Ash Total

Kg/Hr 393 1374 1767 2770 721 3491 278 5536

* C. gas : Combustible gas F. gas : F l u e gas from the regenerator

Table-IV Heat balance of m u n i c i p a l r e f u s e p y r o l y s i s Heat i n χ 10b Kcal/Hr Municipal Refuse Air Combustible Gas Steam Total

%

3.182 0.035

67.4 0.6

0.652 0.863

13.8 18.2

4.732 Steam 1 : Steam 2 :

100

Heat out χ 10b I t e m ^ ^ - ^ ^ Kcal/Hr Combustible 1.812 Gas 1.272 Steam 1 0.471 Flue gas 0.658 Steam 2 0.054 Ash 0.465 Heat l o s s 4.732 Total

% 38.3 26.9 10.0 13.9 1.1 9.8 100

Steam from the c r a c k i n g r e a c t o r Steam from the regenerator



ASH 0.054 ( 1.3 % )

FLUE GAS 1.129 ( 2 7 . 6 % )

COMBUSTION AIR (0.030(0.7%)

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Figure 3.

y

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CONCENTRATED LIQUID 0.000 ( - % )

I

WASTE WATER 0.015 ( 0 . 4 % )

COOLING WATER 370 (33.5 %

COMBUSTIBLE GAS 1.056 ( 25.8%)

6

UNIT : χ l0 Kcal/Hr

Energy balance of municipal refuse pyrolysis unit (Χ 10 Kcal/hr)

RADIATION LOSS 0.465 ( 11.4% )

V

(77.8%)

SOLID WASTE 3.182

COMBUSTIBLE GAS 0.652



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Combustible Gas. Component of the combustible gas which i s produced by p y r o l y s i s of municipal r e f u s e i s shown i n Table-V. The produced combustible gas of the demonstration plant has a d i f f e r e n t composition from that of the small p i l o t p l a n t . The former i n c l u d e s more carbon monoxide and l e s s carbon d i o x i d e than the l a t t e r . The d i f f e r e n c e i n gas composition between the two t e s t s seems to come from the f a c t that i n case of the l a r g e r r e a c t o r , supplied s o l i d waste i s heated up promptly and over-cracking i s prevented. Because o f the high carbon monoxide content, the c a l o r i f i c value of the demonstration plant i s higher than that of the small r e a c t o r s . Sampling of the combustible gas f o r chemical a n a l y s i s was made a t the o u t l e t of the gas scrubber. The gas was not d e s u l f u r i z e d , so the c o n c e n t r a t i o n of hydrogen s u l f i d e i s high as shown i n Table-V. When combustible gas i s s u p p l i e d f o r o f f - s i t e p l a n t s as clean energy, d e s u l f u r i z a t i o n ( i ^ S recovery) may be necessary, but i n case of that gas i s used o n - s i t e , e i t h e r f l u e gas d e s u l f u r i z a t i o n a f t e r i n c i n e r a t i o n or H2S recovery can be a p p l i e d depending on the s i t u a t i o n . Further clean gas can be converted to methane r i c h gas (town gas) as a s u b s t i t u t e n a t u r a l gas., In t h i s case, the amount of town gas w i l l be about h a l f that of raw gas, and i t s c a l o r i f i c value i s about 8500 Kcal/Nm^. For example, the amount o f town gas which could be produced from a 450 ton/day municipal r e f u s e p y r o l y s i s plant would be e q u i v a l e n t to the gas consumed by 100,000 people i n Japan.

Table-V Chemical a n a l y s i s of combustible gas Component H2 CO C02 CH4

Other

C2H4 hydrocarbons H2S NH3

HCL HCN N2

Total C a l o r i f i c value

% by volume 19.58 35.84 16.73 14.35 5.68 3.40 0.34 0.75 ppm 15 ppm 0.5 ppm 4.08 100 4716 Kcal/Nm3

Flue Gas. I n c i n e r a t i o n f l u e gas -was continuously analyzed by an automatic a n a l y z e r . The average value of a n a l y s i s are shown i n Table-VI. From oxygen c o n c e n t r a t i o n i t i s c l e a r that char i n c i n e r a t i o n can be made under the c o n d i t i o n of very low excess a i r .


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NOx concentration i s c o n s i d e r a b l y low comparing with conventional i n c i n e r a t o r . P a r t i c l e s i n f l u e gas can be caught e a s i l y by the e l e c t r o s t a t i c p r e c i p i t a t o r , and i t thus e a s i l y passes s t r i c t regu l a t i o n s f o r p a r t i c l e s emission.

Table-VI A n a l y s i s of i n c i n e r a t i o n f l u e Component 0 C02 N2 SOx NOx


2

gas

% by volume 1.9 17.8 80.3 35 ppm 40 ppm

Ash. Two kinds of ash are discharged by t h i s process. One of them i s coarse i n o r g a n i c matter which i s taken from the bottom of the r e a c t o r s , the other one i s f i n e p a r t i c l e ash discharged from the m u l t i - c y c l o n e and the EP. The coarse m a t e r i a l contains very l i t t l e organic m a t e r i a l because i t remains i n the r e a c t o r s f o r a long time. Organic or combustible matter i n f i n e ash i s also s m a l l . I t i s l e s s than two-percent as shown i n Table-VII. Since a l l heavy metals are f i x e d i n ash and are h a r d l y s o l u b l e i n water, the ash can be l a n d f i l l e d without any a d d i t i o n a l treatment. But i t i s p r e f e r a b l e to s o l i d i f y the ash f o r easy handling and minimizing the amount of s o l u b l e heavy metal i n case of commercial plants. S o l u b i l i t y t e s t was made f o r ash by the r o u t i n e method s t i p u l a t e d by the Japanese Environment P r o t e c t i o n Agency. The r e s u l t i s shown i n Table V I I I .

Table-VII A n a l y s i s of ash Items Ignition loss Net s p e c i f i c g r a v i t y Bulk s p e c i f i c g r a v i t y Mean p a r t i c l e s i z e pH v a l u e * * Measured as 10% s l u r r y f o r one hour)

1.83% 2.51 0.53 18.1 μ 9.83 [-] ( I t was

stirred


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Table-VIII S o l u b i l i t y t e s t of heavy metal i n ash Heavy metal


Hexavalent chromium T-cadmium T-lead T-mercury

Concentration ppm

Gasification of Solid Waste in Dual Fluidized-Bed Reactors - ACS

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